HPGR orhigh-pressure grinding rolls have made broad advances into nonferrous metal mining. The technology is now widely viewed as a primary milling alternative, and there arc a number of large installations commissioned in recent years. After these developments, an HPGR based circuit configuration would often be the base case for certain ore types, such as very hard, abrasive ores.
Though long established in the cement industry, penetration to the hard-rock mining industry was slow , and hampered by high maintenance requirements both for wear surfaces in general, and in particular, high wear on the edge of rolls. HPGRs first made inroads into diamond processing (where rock fracture along grain lines favored a reduction in diamond breakage during comminution), and in the iron-ore industry. Over the course of the past 20 years. HPGR based circuits have become a circuit commonly evaluated, and there are now many circuits in operation.
This industry acceptance has been based on a reduction in the level of overall maintenance effort, an increase in the available size of the units, and the unit operations ability to improve overall comminution efficiency (particularly for harder ore types that can be problematic in a typical SAG circuit). Improvements to wear life and overall availability decreased the overall maintenance effort required. Incorporation of studs on the surface of rolls to allow formation of autogenous wear surfaces, and implementing edge blocks of a long-wearing material for edge protection, have allowed HPGRs to break into the mainstream of mineral processing. These wear-retarding innovations were the focal point of a full-scale trial at Lone Tree (Seidel ct al., 2006). Successful completion of this trial marked somewhat of a turning point in interest in HPGRs for hard-rock applications. Manufacturers have also paid special attention to the bearings, wear surfaces, and the handling of tramp metal through the rolls to improve operational reliability, reduce maintenance, and obtain longer service lives.
The most common HPGRbased circuit involves feeding primary crusher product to a secondary crushing circuit with of cone crushers in closed circuit with screens, followed by tertiary crushing with HPGRs, also operating in closed circuit with screens. The product of these two stages of crushing and screening then passes to secondary milling. In hard-rock metals mining applications, HPGRs are currently in use in tertiary and quaternary crushing applications, as well as in secondary pebble crushing. In many respects, HPGRs replace crushers as a unit operation.
However, from a process standpoint. HPGRs produce a product with substantially more lines (for a given P80) than a crushing circuit. In this regard, the size distribution of an HPGR circuit is much more similar to the product of an SAG circuit than a conventional crushing circuit, reducing the amount of power in the ball-mill circuit required (relative to a crushing circuit).
While HPGRs replace crushing as a unit operation, they represent a much larger installation of power in a given footprint relative to conventional crushers. As such, larger single-line capacities relative to a conventional crushing circuit can he attained. Freeport McMoRans (Freeport) Cerro Verde operation was a ground-breaking installation in that a combination of secondary crushing (using MP1000s), tertiary crushing using HPGRs, and screens replaced what would have been more typically been a large SAG mill feeding a multiple ball-mill circuit (Vanderbeek, 2006). The circuit, commissioned in 2006. was a significant step, and presented an alternative to conventional crushing plants or AG/SAG milling for primary milling applications (see Figure 17.10). Indeed, among the lessons learned for the Cerro Verde circuit w ere techniques to address rolls w ear, maintenance techniques, and elements of the art of operating a comminution circuit of this configuration (Koski et al., 2011).
Newmont Mining Companys Boddington gold project followed on the heels of the Cerro Verde project and, after considerable care and study, selected HPGR comminution, with circuit commissioning in 2009 (Hart et al.. 2011). Using Boddington as a reference for comparing an SAG-based circuit to the selected HPGR-based circuit was widely documented in the literature. The project was commissioned with a similar comminution flowsheet to the Cerro Verde (and also employing four 2.4 x 1.7-m units).
A Cerro Verde expansion used a similar flowsheet as the 2006-commissioned circuit to triple circuit capacity. The expansion circuit includes eight MP1250 cone crushers, eight HPGRs (also 2.4 x 1.7-m units, with 5 MW each), and six ball mills (22 MW each), for installed comminution power of 180 MW. and a nameplate capacity of 240,000 tpd. The expansion circuit was under commissioning and ramp-up in Q4. 2015; combined, the original and expansion Cerro Verde HPGR-based circuits are the largest throughput mill in world.
Like trends in mills, larger equipment sizes continue to evolve. Freeports Morenci operation, for example, commissioned a 3.0 x 2.0-m Metso HPGR (called the hydraulic rolls crusher, or HRC) in an expansion mill circuit in 2014. The circuit has a single-line capacity of over 60,000 tpd. with the single HRC having 11.4 MW of installed power, and operating in conjunction with twin MP1250 cone crushers to feed twin 24 x 40-feet ball mills (26 MW for each of installed power). This single-line capacity approaches that of the larger SAG circuits, with a substantially reduced number of material handling units (feeders, conveyors, screens, chutes) relative to a typical crushing plant, and a more straight-forward plant layout. Notably, the HPGR in this installation (the Metso HRC) made a substantial step forward in process performance with a flanged roll set, w hich eliminates material bypassing the full crushing effect on the edge of rolls, as well as other innovations.
Of note, an HPGR circuits mode of operation is fundamentally different to that of SAG mill. As a largely volumetric machine, the comminution specific energy in an HPGR is a function of the power drawn by the machine at a given rolls pressure setting, divided by the throughput. This has two related effects: firstly, HPGR throughput has relatively little variation based on ore hardness, but it also implies that the specific power input for the HPGR stage is also relatively fixed. As a result, for harder ore types, the product of the HPGR circuit the grind coarsens with harder ore at equivalent throughput. This is typically a positive effect relative to an SAG circuit (where throughput drops with harder ore. but typically achieving an overall finer grind)a coarser grind typically has less impact on revenue (based on a shift on the grindrecover) curve) than a drop in throughput for an SAG circuit. Stated another way, HPGR circuits aremore accommodating of ore variability. Amelunxen et al. (2011) captured this impact well, and converted this variability to NPV estimates relative to an SAG mill circuit (assuming that the SAG was designed based on median ore hardness). Sizing of the secondary milling circuit needs to consider this variability in comminution response in primary milling.
The wear on a rolls surface is a function of the ores abrasivity. Increasing roll speed or pressure increases wear with a given material. Studs allowing the formation of an autogenous wear layer, edge blocks, and cheek plates. Development in these areas continues, with examples including profiling of stud hardness to minimize the bathtub effect (wear of the center of the rolls more rapidly than the outer areas), low-profile edge blocks for installation on worn tires, and improvements in both design and wear materials for cheek plates. As mentioned, the HRC technology takes a different approach through the use of a flanged roll, which in turn also reduces edge wear.
HPGRs typically operate with improved comminution efficiency relative to rotating millsthis effect is typically more pronounced with harder ore types. Also. HPGRs improve observed downstream comminution efficiency. This is attributable to both increased fines generation (which can be corrected for mathematically, as this portion of comminution w ork is actually done by the HPGR. and not downstream unit operation), but also due to what appears to be weakening of the ore. which many researchers attribute to micro-cracking. This effect has been observed by the author in both well-controlled (and fincs-corrected) laboratory tests, and also in plant trials, as well as by other operators and researchers. A typical HPGR-circuit product approaches the lines generation of an SAG-circuit product, both with markedly more fines than a crushing circuit.
Of note is that while the HPGR improves comminution efficiency, the savings in overall circuit power requirements can be reduced or even negated by an increase in conveying and pumping costs relative to large single-line SAG circuits. Put simply, some of the power savings of more efficient comminution is used to transport material through the various unit operations of crushing, HPGR milling, and screening.
Media wear is much less than an SAG circuit in terms of total volume (balls and liners) or as unit consumption in terms of kg/kWh or kg/t. However, although the volume is less, the wear materials are much more highly finishedin economic terms, a high-volume. lower value media is replaced by a low-volume. higher value media. The cost is materials is therefore canceling to a greater or lesser extent. On the other hand, the savings in transport and logistics costs for the reduced volume can be substantial.
A number of trade-off study papers have been published. Very generally, such trade-offs often pit the higher capital cost of an HPGR circuit (with additional unit operations, bells, etc.) to lower comminution energy costs (based on higher comminution efficiency) relative to an SAG circuit. During studies of the Boddington project, comminution power efficiency gain was somewhat offset by increased power for additional conveying and screening units, for an overall net 5% decrease in unit power required for the circuit (Seidel et al., 2006). While the magnitude of the observed power efficiency benefit varies. HPGR circuits demonstrate a consistent benefit, which tends to be more marked for harder ores. Considerations in these trade-off studies also consider the differences in media consumption and overall circuit (not solely comminution) power requirements.
In summary, and relative to an SAG mill primary circuit, HPGRs appear to be most attractive with hard and abrasive ores, and in environments with high power costs. Availabilities are now such that aside from rolls change-outs, which are akin to a mill liner change, the unit rarely controls circuit availability. Overall single-line availabilities comparable to SAG milling can be attained.
HPGR is typically used in a third-stage or fourth-stage crushing application ahead of grinding. You could always try to build a circuit doing 45 m classification, but I suspect your circulating load would be overwhelming. Most HPGR applications in hard rock mining achieve 3000 m to 7000 m product
The largest HPGR Polycom in operation (Figure 5) using a maximum roll diameter of 2.2 meters is processing diamond-bearing rocks in Australia at a maximum feed rate of 600 to 800 mt/h with a top feed size of 150 millimeter (6). This rock material is reduced in one pass to 57% -1 millimeter with a power input of less than 3 kWh/mt.
Polycom HPGR offers particular improvements in the early physical recovery of coarse gold and gold-bearing sulfides through the addition of a PGF Circuit (i.e., Polycom Gravity Separation Flash Flotation).
The Polycom HPGR provides an easy and fast adaptation for throughput and product size through pushbutton changes of the hydraulic pressure. Constant product fineness can be maintained even when variations in ore grindability occur. This is of particular importance to gold operations where variations in particle size of gold and gold-bearing sulfides, silicification, changing rock types or other alteration features present challenges to conventional grinding circuits.
Following are several options for the use of the Polycom high-pressure grinding roll for optimization of gold ore comminution circuits. Specifically the increasing significance of whole ore oxidation treatments of refractory ores will require cost-efficient and optimal liberation of ultrafine precious metals mineralizations.
Additional options for high-pressure grinding roll use in gold ore comminution circuits are illustrated in Figure 11. As indicated by Kapur et al. (1992), high-pressure roll grinding is likely to replace ball mills in increasing numbers in the near future.
Krupp Polysius has developed a rapid and effective test for evaluating a gold ores amenability to high-pressure grinding. All test products are subjected to cyanide leach tests and mineralogical analysis to provide optimal performance data and recommendations for pilot plant work, scale-up and/or plant operation.
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In this chapter an introduction of widely applied energy-efficient grinding technologies in cement grinding and description of the operating principles of the related equipments and comparisons over each other in terms of grinding efficiency, specific energy consumption, production capacity and cement quality are given. A case study performed on a typical energy-efficient Horomill grinding technology, is explained. In this context, grinding circuit is introduced and explanations related to grinding and classification performance evaluation methodology are given. Finally, performance data related to Horomill and high-efficiency TSV air classifier are presented.
Cement is an energy-intensive industry in which the grinding circuits use more than 60% of the total electrical energy consumed and account for most of the manufacturing cost . The requirements for the cement industry in the future are to reduce the use of energy in grinding and the emission of CO2 from the kilns. In recent years, the production of composite cements has been increasing for reasons concerned with process economics, energy reduction, ecology (mostly reduction of CO2 emission), conservation of resources and product quality/diversity. The most important properties of cement, such as strength and workability, are affected by its specific surface and by the fineness and width of the particle-size distribution. These can be modified to some extent by the equipment used in the grinding circuit, including its configuration and control.
Performance of grinding circuits has been improved in recent years by the development of machinery such as high-pressure grinding rolls (HPGR) (roller presses), Horomills, high-efficiency classifiers and vertical roller mills (VRM) for clinker grinding which are more energy efficient than machinery which has been in common use for many years such as tube mills. Energy-efficient equipments such as high-pressure grinding rolls, vertical roller mills, CKP pre-grinders, Cemex mills and Horomills are used at both finish grinding of cement and raw material-grinding stages due to higher energy consumption of conventional multi-compartment ball milling circuits. Multi-compartment ball mills can be classified as:Single-compartment ball millsTwo- or three-compartment ball mills
Multi-compartment ball mills and air separators have been the main process equipments in clinker grinding circuits in the last 100 years. They are used in grinding of cement raw materials (raw meal) (i.e. limestone, clay, iron ore), cement clinker and cement additive materials (i.e. limestone, slag, pozzolan) and coal. Multi-compartment ball mills are relatively inefficient at size reduction and have high specific energy consumption (kWh/t). Typical specific energy consumption is 30kWh/t in grinding of cement. Barmac-type crushers found application as a pre-grinder in cement grinding circuits operating with ball mills to reduce the specific energy consumption of ball mill-grinding stage . An overview of technical innovations to reduce the power consumption in cement plants was given by Fujimoto .
In this chapter, operating principles of high-pressure grinding rolls, Horomill, vertical roller mills, CKP pre-grinders and Cemex mills which are widely applied in finish grinding of cement are briefly explained in addition to the advantages and disadvantages over each other.
The Barmac rock-on-rock crusher has a rotor that acts as a high-velocity, dry stone pump, hurling a continuous rock stream into a stone-lined crushing chamber. Broken rock about 3050mm in diameter enters the top of the machine from a feeder set and is accelerated in the rotor to be discharged into the crushing chamber at velocities of up to 85m/s. Collision of high-speed rocks, with rocks falling in a separate stream or with a rock-lined wall, causes shattering. The product is typically gravel and sand-sized particles. Barmac crushers are available from 75 to 600kW. The product-size distribution can be controlled by the rotor speed . A schematic of a Barmac-type VSI crusher is given in Figure1 .
The material between the rolls is submitted to a very high pressure ranging from 100 to 200MPa. Special hard materials are used as protection against wear, for example, Ni-hard linings to protect the rollers. During the process, cracks are formed in the particle, and fine particles are generated. Material is fed into the gap between the rolls, and the crushed material leaves as a compacted cake. The energy consumption is 2.53.5kWh/t and about 10kWh/t when recycling of the material is used. The comminution efficiency of a HPGR is better than ball mills such that it consumes 3050% of the specific energy as compared to a ball mill. Four circuit configurations of HPGR can be used in grinding of raw materials, clinker and slag such as :Pre-grinding unit upstream of a ball millHybrid grindingSemifinish grindingFinish grinding in closed-circuit operation
Application of HPGR in cement grinding circuits and the effects of operational and design characteristics of HPGR on grinding performance were discussed by Aydoan . HPGR arrangements and semifinish-grinding options are given in Figures3 and 4.
Vertical roller mills have a lower specific energy consumption than tumbling mills and require less space per unit and capacity at lower investment costs. Vertical roller mills are developed to work as air-swept grinding mills. Roller mills are operated with throughput capacities of more than 300t/h of cement raw mix (Loesche mill, Polysius double roller mill, Pfeiffer MPS mill). Loesche roller mill and Polysius roller mills are widely applied in cement raw material grinding. Schematical view of a Pfeiffer MPS mill is given in Figure5 , and a view from inside of a vertical roller mill is given in Figure6.
A cross section of a Loesche mill with a conical rotor-type classifier is shown in Figure7. The pressure arrangement of the grinding rolls is hydraulic. The mill feed is introduced into the mill from above, falling centrally upon the grinding plate; then it is thrown by centrifugal force underneath the grinding rollers. A retention ring on the periphery of the grinding table forms the mill feed into a layer called the grinding bed. The ground material spills over the rim of the retention ring. Here an uprising airstream lifts the material to the rotor-type classifier located at the top of the mill casing where the coarse particles are separated from the fines. The coarse particles drop back into the centre of the grinding compartment for further size reduction, whereas the fines together with the mill air leave the mill and the separator. The separator controls the product sizes from 400 to 40m. The moisture of the mill feed (cement raw material) can amount to 1518%. The fineness of the mill product can be adjusted in the range between 94 and 70% passing 170 mesh. Capacities up to 400t/h of cement raw mix are recorded .
Better product quality can be achieved as compared to the ball mill product due to the better options for separate grinding. For example, in additive cement production, the blast furnace slag has to be ground to Blaine values of 5,000cm2/g. Water demand and setting times are similar to that of a ball mill cement under comparable conditions .
A mill feed arrangement conveys the raw material to the grinding bowl. Two double rollers (representing four grinding rollers) are put in motion by the revolving grinding bowl. The double rollers are independently mounted on a common shaft; they move and adjust themselves to the velocity of the grinding bowl as well as to the thickness of the grinding bed. Thus, rollers are in permanent contact with the grinding bed. A hydropneumatic arrangement transfers the grinding pressure to the rollers. The disintegrated mill feed is shifted to the grinding bowl rim from where a gas stream emerging from the nozzle ring surrounding the grinding bowl carries the material upwards to the separator. The coarses precipitated in the separator gravitate centrally back to the grinding bowl, whereas the fines are collected in the electric precipitator. A raw material moisture of up to 8% can be dried when utilizing the preheater exit gases only. If hot air from an air heater is also supplied, then a raw material moisture of up to 18% can be handled . The power requirement is 1020% lower than a ball mill, depending upon the grindability and moisture content of the raw material . Other types of roller mills such as ball race mill (Fuller-Peters mill) and Raymond bowl-type ring mill are used in coal grinding.
The CKP pre-grinder has been under development by Chichibu Cement and Kawasaki Heavy Industries since 1987. It has been commissioned by Technip under licence since 1993. The system is applied widely for clinker grinding and has also been used on raw material grinding. In operation, material is fed through the inlet chute onto the grinding table centre, spread out to the grinding path by the centrifugal force arising from the table rotation, before being compressed and ground by the rollers. The preground material drops down out of the periphery of the table to the bottom of the casing and is discharged by the scrapers through the discharge chute. Grinding principle of the CKP system is shown in Figure8. Typical CKP application is given in Figure9 .
Main advantages of the CKP pre-grinders are stated by Dupuis and Rhin  as follows:The grinding capacity can be increased up to 120% for some raw materials.Installation is very easy due to the compact design as well as the possibility of installing the CKP outdoors.The energy consumption of the total grinding plant can be reduced by 2030% for cement clinker and 3040% for other raw materials.The overall grinding circuit efficiency and stability are improved.The maintenance cost of the ball mill is reduced as the lifetime of grinding media and partition grates is extended.
F.L.Smidth has developed this cement grinding system which is a fully air-swept ring roller mill with internal conveying and grit separation. This mill is a major improvement of the cement grinding systems known today which are ball mill, roller press (HPGR)/ball mill, vertical roller mill and closed-circuit roller press for finish grinding. Views of mill interior are given in Figures10 and 11. Cemex grinds the material by compressing it between a ring and a roller. The roller rotates between dam rings fitted on the sides of the grinding ring, ensuring uniform compaction and grinding. The mill rotates at a subcritical speed, and scooping devices at both ends of the ring ensure effective internal conveying of the material being ground. The material leaves the scooping devices at various points, which ensures good distribution of the material in the airstream between the air inlets and outlets. The process air enters through two inlets at either end of the mill and leaves through an outlet at either end of the mill. The air passes the falling material and carries the finer particles to Sepax separator, in which the final classification of the product takes place. The oversize particles are returned from Sepax to Cemex for further grinding. Due to this unique combination of internal grit separation and air-swept material conveying to Sepax, no external mechanical conveyor is needed, which makes the installation very compact and simple. The airflow rate through the mill is relatively low, the only lower limitation being the need for sufficient internal grit separation and conveying of the preseparated material to the final classification in Sepax separator .
Main purposes in designing of the ring roller mill (Cemex) can be summarized as follows:To reduce the specific energy consumption of grindingTo reduce the wear on the mill elements by applying pressures on the grinding bedTo reduce the energy consumption of the mill fan by reducing the air consumption in the grinding processSimple mechanical designSimple and compact design to reduce the external mill load recirculationSimple and easy control of product quality and mill operationSimple and easy change of product type
Grinding tests by the F.L.Smidth company have shown that Cemex produces cement which meets the requirements of the standard specifications while enabling substantial savings in grinding energy consumption compared to the traditional ball mill systems. Due to the more energy-efficient grinding process, Cemex ground cement will usually have a steeper particle-size distribution curve than corresponding ball mill cements. Consequently, when ground to the same specific surface (Blaine), Cemex cement will have lower residues on a 32 or 45m sieve and tend to have a faster strength development. Grinding of cement to a lower Blaine value will reduce the specific power consumption . A comparison of typical specific energy consumption of Cemex mill with conventional multi-compartment ball mill grinding and HPGR pre-grinding closed-circuit operations is given in Table1.
Some of the advantages of Cemex mill can be summarized as follows:Up to 40% lower energy costs compared with conventional grinding installations.Low-maintenance cost.Fully air-swept mill installation.Internal conveying and grit separation.No external mechanical conveyor.Low noise level.Well-proven mill components.A third of the grinding pressure of the roller press and moderate grinding pressures.Long life of wear segments.Drying and cooling ability.Compact and simple design.High grinding capacity.Cement quality meets prevailing standards.Same or better strengths than cement from ball mill.
As it was stated in the literature, grinding tests have shown that Cemex produces cement which meets the requirements of standard specifications while enabling substantial savings in grinding energy consumption compared to the traditional ball mill systems. Due to the more energy-efficient grinding process, Cemex ground cement will usually have a steeper particle-size distribution curve than corresponding ball mill cements. Consequently, when ground to the same specific surface (Blaine), Cemex cement will have lower residues on a 32 or 45m sieve and tend to have a faster strength development. When grinding to a 28-day-strength target, Cemex cement can be ground to a lower Blaine value, which further reduces specific power consumption .
Horomill is a ring roller mill which is a joint development by the French plant manufacturer FCB Ciment and the Italian cement producer Buzzi Unicem Group . Horomill can be used in grinding of:Cement raw materials (i.e. limestone, clay, iron ore, etc.)Cement clinker and cement additive materials (i.e. limestone, slag, pozzolan, etc.)Minerals and coal
The Horomill (horizontal roller mill) consists of a horizontal shell equipped with a grinding track in which a roller exerts grinding force. The shell rotates faster than the critical speed which leads to centrifuging of the material. The main feature is the roller inside the shell which is rotated by the material freely on its shaft without a drive. Operating principle is schematically shown in Figure12. Material is fed to the mill by gravity. There are scrapers located in the upper part of the shell. Scrapers cover the entire length of the mill and scrape off the material which falls onto the adjustable panel of the material advance system. Position of the material advance system which is sloping towards the discharge end could be changed in such a way that material could advance slower or faster, and thus it determines the number of passage of material under the roller which means the adjustment of circulating load. Grinding pressures change within a range of 500800bars. Concave and convex geometries of the grinding surfaces lead to angles of nip two or three times higher than in roller presses resulted in a thicker layer of ground material .
As compared to hybrid systems, Horomilling resulted in lower energy consumptions with energy savings of 3050% for the same product quality. Noise generated is lower than conventional ball mill. They are smaller and compact units. Frictional forces in the Horomill grinding are kept at its minimum, and hence wear is due to the lack of differential speed between the material and the grinding ring. Horomill is designed for closed-circuit finish grinding when compared with an HPGR. Bed thickness is two or three times the roll press (HPGR) .
It also has the flexibility of a vertical roller mill in grinding of different materials. A larger angle of nip draws the material bed into the grinding gap and reduces wear as compared to vertical roller mills. The recirculation of material within a vertical roller mill is very high. The recycle ratios are 15 or more, but it is practically impossible to measure the recycle ratios in a mill operating on the airflow principle. Material bed passes many times through the stressing gap, and it is possible to adjust the number of stressing during operation in a Horomill. Also an internal bypass can be implemented if some of the ground material is returned from the mill outlet to the inlet. The external recycle ratio of a Horomill connected in a closed circuit lies between four and eight and is therefore lower than with a roller press (HPGR) and vertical roller mill . A comparison of the angles of nip of material is given in Figure13 . A photograph of an industrial scale Horomill  is shown in Figure14.
Typical industrial scale Horomill grinding and classification closed circuit are given in Figure15. The circuit includes an elevator, a conveyor to the TSV classifier, a finished-product recovery filter at the TSV outlet and an exhauster. The rejects from the TSV classifier are returned by gravity to the mill inlet. The main features of the plant are as follows :Horomill-installed power: 600kW at variable speedHoromill diameter: 2,200mmCircuit nominal rate in CP42.5R cement production: 25t/h at 3,200 BlaineNominal-circulating load: 140t/hTSV classifier for classification
An industrial sampling survey was carried out during CPP-30R (pozzolanic portland cement) production around the Horomill grinding and classification circuit given in Figure16. Sampling points of the circuit are shown in a simplified flowsheet (Figure16). Horomill was closed circuited with a TSV-type dynamic separator in the circuit.
Prior to sampling surveys, steady-state conditions were verified by examining the variations in the values of variables in the control room. When steady-state condition was achieved in the circuit, sampling was started, and sufficient amount of samples were collected from each point as shown in Figure16. Due to the physical limitations, dried pozzolan stream was not sampled. Samples collected after stopping the belt conveyors by stripping the material from a length between 3 and 5m is shown in Table2. The operation during sampling was closed to steady-state conditions. Important variables of the operation were recorded in every 5min in the control room. Average values of the control room data were used in the mass balance calculations. Mass balance calculations were carried out using JKSimMet computer program. Design parameters of the Horomill are presented in Table3.
A combination of sieving and laser-sizing techniques was used for the determination of the whole particle-size distributions for each sample. SYMPATEC dry laser sizer was used to determine the particle-size distribution of subsieve sample of 149m for each sample. Size distribution of +149m material was determined by dry sieving using a Ro-Tap. The entire size distribution for each sample was calculated using the sieving results obtained from the top size (50.8mm) down to 149m and laser results obtained for the subsieve sample of 149m.
Some errors are inevitable in any sampling operation. These errors result from dynamic nature of the system, physical conditions at particular point, random errors, measurement errors and human errors. Mass balancing involves statistical adjustment of the raw data to obtain the best fit estimates of flow rates. In this context, by using the particle-size distributions and the control room data, an extensive mass-balancing study was performed around Horomill#3 circuit. Tonnage flow rates (t/h) and particle sizes of the streams are calculated by JKSimMet mass balance software. The success of the mass balance was checked by plotting the experimental and calculated (mass-balanced) particle-size distributions as shown in Figure17. These results plotted in a 45 line indicate the quality of both sampling operation and laboratory studies.
According to the result of mass balance calculations, if there had been a statistically significant difference between experimental and calculated values (scattering data), the data would have been rejected and not be used for performance evaluation studies. In this research, data obtained as a result of sampling and experimental studies were found to be in a satisfactorily good fit. Mass balance model of the circuit with the calculated tonnage flow rates (t/h) in every stream and fineness as 45m% residue is shown in Figure18.
where F80 is the 80% passing size of the Horomill feed determined as 1.06mm and P80 is the 80% passing size of the Horomill discharge determined as 0.56mm. It means that the ratio of size reduction is 1.88.
Using the F80 (13.21mm) and P80 (0.024mm) size values from the mass-balanced size distributions of the fresh feed and the TSV fine, the ratio of the overall size reduction was calculated as 550.42 by Eq. (1):
Horomill motor power (2,126 kW) is the average operating mill motor power reading from the control room during the sampling survey and used in the calculation. Total fresh feed tonnage is the dry tonnage amount used in the mass balance calculations represented by the TSV fine stream tonnage flow rate which is 100.66t/h. Thus, the specific energy consumption (Ecs) can be calculated by Eq. (4):
The performance of any classifier, in terms of size separation, is represented by an efficiency (TROMP) curve. An example for a classifier is shown in Figure19. It describes the proportion of a given size of solids which reports to the coarse product. Mass-balanced particle-size distributions and tonnage flow rates around the separator were used to evaluate the performance of the separator. Percentage of any fraction in the feed pass to the coarse product (%) is defined as partition coefficient and expressed by Eq. (6):
The d50 size corresponds to 50% of the feed passing to the coarse stream. It is therefore the size which has equal probability of passing to either coarse or fine streams. When this size is decreased, the fineness of the product increases. The operational parameters that affect the cut size are rotor speed and separator air velocity. Cut size for the TSV was determined as 23.33m. The percentage of the lowest point on the tromp curve is referred as the bypass. It is the part of the feed which directly passes to the coarse stream (separator reject) without being classified. Bypass value is a function of the separator ventilation and separator feed tonnage. The bypass value of TSV was 23.29% which indicated a consistent performance for this separator. Fish-hook effect () is the portion of fines returning back into separator reject stream. When there is incomplete feed dispersion at the separator entry, or even within the classification zone, aggregates of fine particles may be classified as coarse particles and thus report to the coarse stream. Fish-hook amount of TSV was 1.58% which also indicated how effectively it is operating:
Usually, for TSV-type separator, it is between 0.55 and 0.7. When the normal range for sharpness (k) parameter is considered, it is found to be not in the normal range . When the normal range for sharpness (k) parameter is considered, it was found to be not in the normal range. The imperfection of separation is defined by Equation 8, and I was calculated as 0.47:
Typical operating conditions for the Horomill and two-compartment ball mill grinding with HPGR pre-crushing and classification circuits are compared in Table4 for the same production type. As can be seen from Table4, Horomill production configuration has resulted in energy savings of 50% as compared to HPGR/two-compartment ball milling configuration .
It was also reported that concrete workability from a portland cement with a 3,200 Blaine which is a Horomill product is equal or better than an equivalent ball mill product. Mortar and concrete strengths are always higher as shown in Figures20 and 21. The closed-circuit recirculation factor is noted as about six in Horomill grinding . A comparison between the grinding systems and conventional ball mills applied in cement grinding circuits is given in Table5. Grinding efficiencies of different systems in grinding of cement to a fineness according to a Blaine of 3,000cm2/g were compared in Table6.
The efficiency of a two-compartment ball mill is defined to be 1.0. This efficiency reflects the power consumption of the mill only and does not include any auxiliary equipment like conveyors and dust collectors nor the separator.
Comparisons between different energy-efficient grinding technologies and applications were presented for production of cement with energy savings. Industrial-scale data related to Horomill and Polysius HPGR/two-compartment ball mill circuit provided insights into the operational and size-reduction characteristics of Horomill and HPGR/two-compartment ball mill-grinding process with indications that Horomill application could produce the same type of pozzolanic portland cement at lower grinding energy requirement. The specific energy consumption figures indicated approximately 50% grinding energy savings in Horomill process.
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The F-series HPGR, developed by FLSmidth, incorporates a combination of engineered solutions and optimised components. Our design is derived from our years of maintaining these machines in the field and aimed at decreasing maintenance downtime, while placing special emphasis on the health and safety aspects of these operations.
Our HPGR is flexible enough to be highly suitable for both brownfield expansions and greenfield installations. The wide variety of ways our customers are already using the machine shows just how helpful the technology is.
Our HPGR is a perfect complementary tool to work in conjunction with, and even enhance the performance of traditional grinding mills and fixed-gap crushers. The HPGR exposes feed material to very high pressure for a short amount of time. The compression typically causes the rock to crack and cleave along the grain boundaries, weakening the rock structure and exposing the ore particles. This high pressure also creates a large amount of fines and causes the formation of microcracks in the larger particles, lowering the Bond work index of the ore and reducing the ball mill power required downstream.
The F-Series HPGR is ourfull size machine used in large mining projects throughout the world. Operation, maintenance and service personnel collaborated to create this modern HPGR design. With it, you get a reduced footprint, convenient and safe handling for heavy components and improved shipping.
We re-designed our legacy roll press to upgrade it and optimise it for mining duty, and the result is our S-Series HPGR. Though this series is meant for smaller projects, plants and retrofits, it is packed with plenty of the same features that are in our larger models. And the smaller size means that your machine can be shipped partially or fully assembled, resulting in a faster installation.
The lower section of our HPGR feed chute, which typically experiences the highest wear, is lined with FLSmidths patented FerroCer Impact Wear Panels. Our wear panels haveproven to greatly outperform conventional wear plates in this application where the feed ore is often large, very hard, and highly abrasive. These panels are easy and safe to install, while providing the longest possible wear life. Learn more about where else in your plant operation you can optimize your durability with FerroCer Impact Wear Panels.
The planetary reducers are shaft mounted to the rolls and are linked together via the torque sharing arm assembly. This saves you money because it eliminates the cost of additional foundation structure and hardware to bear the torque arm reaction forces.
This feature also allows the drive train to be captured and held in place during roll change-out using our (optional) drive retraction cradle, which further optimises the roll change-out time and makes the procedure safer.
Mounting the hydraulic cylinders to the frame with hinges allows maintenance personnel to safely access and inspect these components without getting inside the framework of the HPGR. For any maintenance that required removing the cylinders, overhead access is free and clear of the frame with the hinges in the outboard position. Dual-acting means that no special tools are required to separate the rolls to clear tramp metal. By simply reversing the oil flow, the hydraulic cylinders can be used to retract the rolls and allow the tramp to fall through, getting the machine back online quickly.
The HPGR has a skid mounted tank with a lubrication oil conditioning system that supplies clean filtered and temperature conditioned oil to the roll bearings. A constant flow of oil carries the contaminants and heat that are generated in operation away from the bearings to maximising uptime. The lubrication oil system instrumentation monitors and protects the system. In contrast, conventional grease lubricated units are messy, require frequent relubrication and must be water cooled.
We designed the hydraulic pumping system to the cylinders with advanced skew control (ASC) and use linear variable differential transducers (LVDTs) to monitor the system. As skewing starts to occur in the floating roll, the ASC system adjusts the press force applied to the hydraulic cylinders, keeping the roll assemblies as near parallel as possible during operation, while at the same time maintaining interparticle contact and grinding efficiency over the roll width.
There are two frame styles offered for industrial applications. The S-Series, which incorporates a traditional box frame design, has been optimised for smaller plants. The F-Series, using our express frame, accommodates the need to handle and operate larger and heavier components.
We designed and built our express frame for a faster and safer replacement of the roll surfaces. With the minerals processing industry trending towards larger equipment, we specifically developed the express frame to handle larger, heavier components in a safer and easier manner.
This new frame uses a tapered pin and bushing concept. Removal of the tapered bushing creates more clearance space for inserting and removing pins. A wedge jacking system takes the load off the pins for quick extraction and replacement.
FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.
For extreme efficiency and high equipment availability, look no further than roller press technology and products from FLSmidth. The roller press is a uniquely designed comminution tool, in that feed ore is exposed to extremely high pressure for a short amount of time resulting in a highly effective method of size reduction.
The roller press features two opposing rolls rotating at a relatively low speed. One roll is fixed to the frame of the machine, the other allowed to move against a set of hydraulic rams. Grinding force is provided via hydraulic pumping system and assisted by nitro-charged accumulators. The roller press is designed to operate in choke feed conditions. Feed material is introduced into the gap between the rolls. This action starts to push the moveable roll against the hydraulic rams, compressing the nitrogen in the accumulators. Once the overall system hydraulic pressure equals that in the crushing zone the roll stops moving and the operating gap is established. Reduction in the material is accomplished through the comminution principal known as inter-particle breakage. Adequate automation possibilities are inbuilt in the operation logic of the roller press in order to ensure safety, smooth operation, high performance and high availability.
What does this mean for you? In addition to boosting plant efficiency and availability, the high operating pressure of the roller press produces micro-cracks in the discharge particles. These micro-cracks are unique to roller press operation and offer reductions in grinding work index and metallurgical benefits to the downstream process.
FLSmidth is a world leader in roller press technology having supplied several machines over the last 30 years. Our roller press family is split into two product lines, each uniquely designed to best serve their respective industry:
FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.
Upgrading cement grinding systems offers both process and commercial benefits. The use of a high-pressure grinding roll in semi-finish grinding mode significantly reduces specific energy consumption and improves production capacity. But this type of grinding system requires two types of air classifiers, which usually ties up additional investment and space. Therefore, Germany-based Maschinenfabrik Kppern has developed a compact air classifier that enables grinding system upgrades by integrating only one combined machine. By Florian Kleemann, Kppern Aufbereitungstechnik GmbH & Co KG, Germany.
High-pressure grinding rolls (HPGR) have been used in cement production for more than 30 years. Since the first industrial application for comminution in 1985, these highly-efficient machines have gained in importance with their main applications in the grinding of brittle materials.1 HPGRs were often added upstream to existing cement plants with a ball mill and an air classifier in closed-circuit grinding. This pre-grinding, which partially moves the grinding load from the ball mill into the HPGR, results in savings of specific energy of around 20 per cent compared to simple closed-circuit ball mill (CCBM) grinding.2
For more than a quarter of a century the ThyssenKrupp Polysius POLYCOM high-pressure grinding roll (HPGR) has been in successful operation. More than 65 of the presses are working in highly-efficient clinker grinding systems around the world. By using the POLYCOM as a stand-alone grinding machine (finish grinding circuit) a reduction in specific power consumption of around 50 per cent compared to conventional ball mill circuits is achievable. The high compression stressing applied inside the material bed in a HPGR is distinguished by its significantly-higher degree of energy utilisation in comparison to the frictional and impact stressing of the material being ground in a ball mill. By Dr A Haack and Dr O Hagemeier, ThyssenKrupp Polysius AG, Germany.
POLYCOM works in closed circuit with SEPOL PC the static-dynamic separator. A very compact static cross-flow separator is combined with the highly-efficient dynamic separator SEPOL LM to the newly-established SEPOL PC.
As a result of the continuous optimisation of the overall machine concept, HPGRs POLYCOM can be integrated into both new and existing grinding plants without any problems from either the process technological or plant engineering points of view. Figure 1 presents a typical process flowsheet of a finish-grinding system.
The grinding plant can also be equipped with a ball mill following the POLYCOM. This is called a combi-grinding circuit. The main feature of such a system is the pre-grinding of cement clinker in the POLYCOM and a final grinding in the ball mill. The specific power consumption compared to a ball mill cement grinding is significantly lower and additionally paired with a 30 to 40 per cent increase in output.
Figure 1: The Schretter & Cie cement plant in Vils, Austria.Figure 2: Former system: HPGR in pre-grinding mode.Figure 3: Upgraded system: HPGR in semi-finish grinding mode with the Koppern 2-Stage Koesep air classifier. (Former 2nd generation air classifier still available as fall-back option).Figure 4: Set-up of Koppern 2-Stage Koesep air classifier.
During 2015 the Austrian cement manufacturer Schretter & Cie GmbH & Co KG modernised its grinding process. The former system with a high pressure grinding roll (HPGR) working in a pre-grinding mode was upgraded to a grinding circuit with an HPGR in semi-finish grinding mode. A compact 2-Stage Koesep air classifier, the latest development of Kppern, based in Germany, was installed. The new combined air classifier was implemented during ongoing operation with total plant downtime of just 40hr. One year after re-commissioning of the grinding system it can be noted that the modernisation improved production, stabilised the process and reduced energy consumption. This article reflects experiences made throughout the first year of operation and shows effects with regard to production and energy consumption.
Cement production is among the most energy-intensive procedures in the processing industry. Therefore cement producing companies are always aiming to decrease the energy demand by improving plant technology. With regard to the grinding circuits, mills and air classifiers are the main loads in terms of energy consumption.
By using a high pressure grinding roll (HPGR) in pre-grinding or semi-finish grinding mode, savings of specific energy consumption up to 30% compared to ball mill in closed circuit can be achieved. The state-of-the-art technology is a HPGR in semi-finish grinding mode. For this, two types of air classifiers and thus two machines are usually required, these being one static and one dynamic classifier. In general, air classifiers are proven devices for classification of mineral resources. They are used in grinding circuits together with ball mills, HPGRs and vertical roller mills. Due to the importance of reducing power consumption per tonne of product and simultaneous increase of production of existing grinding systems, research and development is ongoing.1,2
Optimisation of the air classifying system offers process and commercial benefits. Schretter & Cie realised an opportunity to upgrade its existing grinding unit. Prevously it had used an HPGR for pre-grinding in combination with a downstream ball mill in a closed circuit with a dynamic air classifier, for about 25 years. In 2014, the company decided to upgrade the system in cooperation with Kppern. The aims were to reduce both energy consumption and CO2 footprint while increasing production capacity. The operation of the HPGR was changed to semi-finish grinding and, instead of installing the two separate air classifiers, a decision was made to install the newly-developed Kppern 2-Stage Koesep air classifier. This machine combines the two main groups of air classifiers in one compact housing. The HPGR product passes the static part of the classifier before it partially enters into the dynamic part. The ball mill product is fed directly into the dynamic part of the classifier.
Schretter & Cie is a medium-sized company in Austria, which has been processing cement, lime and gypsum since 1899. The plant in Vils produces a wide range of products to different specifications. Due to the high flexibility of production process, the company is able to provide specific products tailored to customers requirements. A lot of special binders and custom-made building materials are available.3
Before Schretter & Cie installed the new 2-Stage Koesep air classifier, the grinding system consisted of an HPGR in pre-grinding mode with partial flake recycle and a ball mill in a closed circuit with a Hischmann 2nd generation air classifier. A simplified flowsheet of the former grinding system is shown in Figure 2.
Schretter & Cie, in cooperation with Kppern, developed a concept to integrate a combined air classifier in the existing plant. The upgraded flowsheet of the cement grinding unit is shown in Figure 3. The HPGR and the ball mill are now both operated in closed circuit with the new 2-Stage Koesep air classifier.
It was a high priority to ensure that the old grinding system could still be used even during the ongoing modernisation project. Finally a strategy was developed that allowed the use of either grinding system. The old separator is still available to provide a fall-back option if necessary. The main advantage of the new semi-finish grinding system is the removal of fines from the grinding circuit after the first comminution step in the HPGR. Hence this material is not fed to the ball mill anymore, which removes the need for a second comminution step for this material. Overgrinding of fine material is reduced, leading to better performance of the ball mill and increased production.
To efficiently separate HPGR product it is necessary to first deagglomerate and coarse-classify the flakes, which is done by means of a proven static cascade separator. Here the fine fraction can reach fineness up to 2500cm/g (Blaine). To further classify the fines leaving the static classifier unit, a dynamic separator is used. Thereby the fine flow is divided into material with the required product fineness and so called fine coarses, which are rejected to the ball mill. The Kppern 2-Stage Koesep air classifier combines these two functions in one machine with an exceptionally compact design.
Maschinenfabrik Kppern GmbH & Co. KG, headquartered in Hattingen, Germany, is specialised in the design and manufacture of machines for the cement and mineral processing industries. One of its latest developments is the 2-Stage Koesep air classifier, which is shown schematically in Figure 4. It is mainly intended for use in combination with a HPGR and a ball mill in cement semi-finish grinding circuits, but can be adapted to finish grinding systems with only a HPGR or even single ball mill grinding units.
Generally, the air classifier consists of two main parts: the static separator and the dynamic separator. Depending on the application, the static separator can be equipped with a different number of cascades. As shown in Figure 4, two cascades are installed at the plant in Vils. The product of the HPGR is fed through the inlet (1) into the static cascadeseparator, where it is crossed by the primary separating air (2). The drops and impacts on baffle plates (3) and guiding plates (4) for disgglomeration. The coarse rejects of the static separator move downwards through the outer cone (5) and are discharged at the outlet (6). This material goes back to the HPGR.
Fines are carried upwards by the airstream and enter the dynamic part of the classifier. This material passes through guiding vanes (7) before reaching the rotating cage (8) driven by a motor (9). Coarse grains are rejected due to higher centrifugal forces, whereas finer particles pass through the rotating cage.
The rejected coarse material of the dynamic separator falls through the inner cone (10) and is discharged as middlings at the outlet (11). This material is fed to the ball mill. The ball mill product is directly fed to the dynamic separator at the inlet (12). All material with the required product fineness leaves the classifier together with the separation air at the outlet (13) as the final product of the grinding system. The set-up of the 2-Stage Koesep is already patented in Germany4 and Europe.5 The system is summarised in Table 1.
After the decision to install the new air classifier in 2014 by Schretter & Cie, Kppern received material for trials with the 2-Stage Koesep pilot plant. After successful tests, the planning and construction of the plant started. The erection of the new building began in December 2014. After delivery of all plant components, including the air classifier itself, fans, material handling equipment and cyclones, the erection commenced in April 2015. Construction work continued until June 2015. Schretter & Cie switched production over to the new overhauled grinding system with the 2-Stage Koesep air classifier in July 2015.
The first priority was to achieve the former product characteristics for each and every cement type. After a trial period of approximately two weeks all requirements for the first cement type had been fulfilled. This was possible due to the easily-adjusted parameters of the separator.
Laboratory results showed that the cements produced with the new 2-Stage Koesep achieve higher strength values throughout the product range. As a consequence it was possible to reduce the Blaine values of the cements, which positively influenced the production rate of the entire grinding system. After final adjustments of the ball mills ball charge gradation at the end of 2015, the plant throughput was increased once again.
The changeover of the different cement types to the new grinding system went smoothly without major problems. Once settings were found after a short period of testing, the same results could be repeatedly achieved with regard to product quality. Furthermore it was found that the Kppern 2-Stage Koesep has a fully reproducible and stable performance. This makes it very easy to switch between different cement types without noteworthy waste production.
During the first few weeks of operation after initial commissioning the capacity of the plant was increased further. Table 2 shows a summary of process data as comparison of the former and new grinding system.
Schretter & Cie upgraded its cement grinding unit by implementation of the new Kppern 2-Stage Koesep air classifier. By converting the former pre-grinding into a semi-finish grinding High Pressure Grinding Roll, the company significantly increased the grinding capacity at the plant in Vils.
Due to the compact design of the 2-Stage Koesep air classifier and a clear-sighted layout of the new material transport equipment, the upgrade of the grinding plant could be erected while the old system was still in full operation. The integration of the new classifier and the switch-over to the new grinding circuit was then realised with just 40hours downtime.
After one year of operation it is proven that the upgrade was worth the effort. The energy consumption of the grinding circuit has been reduced by approximately 13%, whereas the product rate increased on average by 19% over the same period for the cement types shown. To further improve the performance of the air classifier, Kppern continues to conduct research projects at the pilot plant 2-Stage Koesep air classifier at its test facilities at the University of Freiberg in Germany. Test results are evaluated on the industrial air classifier at the Schretter & Cie plant in Vils.
1. Gnter, et al. The application of roller presses for high pressure comminution, Paper presented at the Symposium on GrindingProcesses, Toulouse, France, 14 - 15 February 1996.2. Streicher, C; Flachberger, H. Aufbereitungstechnische Untersuchungen zur Optimierung von Querstrom-Drehkorbsichtern aus dem Hause Christian Pfeiffer - ein Zwischenbericht, In: BHM, 158., 2013, Issue 6, pp. 251-257.3. Schretter & Cie website: http://www.schretter-vils.co.at. Accessed 30 June 2016.4. Gnter, H. et al., Vorrichtung zum Sichten von krnigem Gut und Mahlanlage, DE 10 2011 055 762 B4. 28 August 2014.5. Gnter, H. et al.: Vorrichtung zum Sichten von krnigem Gut, EP 2 785 472 B1. 20 July 2016.
Ivrindi Gold is a new mining and enrichment project in Western Turkey owned by TMAD Mining Industry and Trade. With ample resources available, the company wanted to optimize its output by investing in equipment that could deliver a large throughput. Their feasibility study indicated the heap leach process would be suitable for this site and Ivrindi Gold began searching for the right equipment for the job.
In the heap leach process, gold ore is crushed to a -6 mm product size, drum-agglomerated and stacked onto a leach pad, where it is saturated with a leach solution that dissolves the precious metals and carries them to the pad below and on into the ADR plant. This is a cost-effective process, provided the required particle size is reached and the leach solution can access the gold within the product. Otherwise, you risk wasting valuable materials.
Having established the desired technology, Ivrindi Gold began the process of finding the best possible partner. It wasnt just the HPGR. The contract also included a jaw crusher, cone crusher, screens and other equipment. Ivrindi Gold wanted to work with a supplier who could deliver a full flowsheet solution and one where all parts performed equally well. This significantly narrowed the field.
After looking at proposals from other equipment manufacturers, the decision came down to trust. Who could Ivrindi Gold trust with this project and their future profitability? Having developed a good relationship with our regional team, Ivrindi Gold decided to place their trust in FLSmidth.
It may have seemed quite the leap of faith: the biggest HPGR ever built for a gold heap leach operation; the first F-Series HPGR we had ever built, and the only HPGR operating in hard rock mining in the region. But it was a leap the team at Ivrindi Gold were prepared to take because they trusted us to make a success of it. We committed to being onsite throughout plant erection, start-up and commissioning to ensure we wouldnt let them down.
Not all ores are suitable for HPGR technology and not all ores benefit from crushing to the finer fractions created by the HPGR. In order to identify whether or not your ore and process are a match, you need to undertake extensive lab testing. As with most things, it will likely become a question of cost vs benefits. Does the return on investment outweigh the cost of buying and maintaining additional equipment?
The compressive force of HPGR enables Ivrindi Gold to achieve the desired product fineness of -6 mm, which would be very difficult to achieve with conventional crushers. With that much of the flowsheet decided on, the rest of the equipment could be determined. In all, FLSmidth supplied:
In addition to the 12 months of site service mentioned above, we also provided training for site personnel. A stock of major wear and spare parts is also kept onsite to ensure the plant is well prepared for any eventuality.
The winter of 2018/2019, when this project was being undertaken, was a white-out. Snowfall made access to the site very challenging, and working conditions even more so. Added to this was the fact that this was the very first and also the largest F-Series HPGR we had ever built. The schedule was tight, with no wiggle room built in for development work. And yet the machine was engineered, detailed, manufactured and delivered within the project schedule a testament to our strong, regional project team headquartered in Ankara, Turkey, and the combined operations and engineering efforts of FLSmidth in Austria and the USA. A key factor in this success is that the frame design of the F-Series lends itself to easier shipping and assembly.
Meanwhile, the site team wouldnt let a mountain of snow put progress off course. They were able to get the majority of the HPGR assembled on site in about 30 days, in spite of the poor weather conditions.
FLSmidth has been a consistently reliable and dedicated technical partner, not only in terms of the technology supplied but also in the way they prioritized customer service throughout this project and continue to do so with services and spare parts, says Mr. Hasan Yucel, General Manager of TMAD Mining Industry and Trade.
The growing focus on sustainability is driving new interest in the High Pressure Grinding Roll (HPGR). Using a HPGR as part of a 100% dry comminution circuit will help mines reduce or eliminate water usage where this is possible.
How does the HPGR facilitate this? Well, the HPGR is a dry comminution tool capable of grinding to a very fine (finish) product size. In short, the technology exists to build a dry comminution plant and it increasingly feels like the time to do this is now.
Maybe the answer is to look to the future of dry comminution circuits. Dry classification technology is available now - but dry separation, in most cases, will require a breakthrough in technology. Given this, perhaps the current focus should be on minimising the required water usage and that could mean a focus on dry classification methods.
Dynamic classifiers for example produce a steep product curve, reducing the ultra-fines which become problematic during the dewatering process. Removing the ultra-fines should also help with the flotation/recovery process. All this adds up to reduced water usage, or zero water waste, which is one of the aims of FLSmidths MissionZero sustainability initiative.
FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.
Particle size distributions of obtained samples from several sampling campaigns were determined and raw data were mass balanced before being used in simulation studies. After determination of breakage function, selection function, Bond work index, residence time distribution parameters, and Whitens model parameters for air separators and diaphragms between the two compartments of tube ball mills, performance of the circuits was simulated for given throughputs and feed particle size distribution. Whitens model parameters were determined by GA (genetic algorithm) toolbox of MATLAB software. Based on implemented models for modeling and simulation, optimization of circuits was carried out. It increased nearly 10.5% and 15.8% in fresh feed capacity input to each tube ball mill. In addition, circulating load ratios of circuits are modified to 118% and 127% from low level of 57% and 22%, respectively, and also cut points of air separators are adjusted at 30 and 40 m from high range of 53 and 97 m, respectively. All applications helped in well-operation and energy consumption reduction of equipments.
GEN , ERGN L, BENZER A H. The dependence of specific discharge and breakage rate functions on feed size distributions, operational and design parameters of industrial scale multi-compartment cement ball mills [J]. Powder Technology, 2013, 239: 137146.
TOPALOV A V, KAYNAK O. Neural network modeling and control of cement mills using a variable structure systems theory based on-line learning mechanism [J]. Journal of Process Control, 2004, 14(5): 581589.
DUNDAR H, BENZER H, AYDOAN N A, ALTUN O, TOPRAK A N, OZCAN A, EKSI D, SARGIN A. Simulation assisted capacity improvement of cement grinding circuit: Case study cement plant [J]. Minerals Engineering, 2010, 24(3/4): 205210.
Farzanegan, A., Ghasemi Ardi, E., Valian, A. et al. Simulation of clinker grinding circuits of cement plant based on process models calibrated using GA search method. J. Cent. South Univ. 21, 799810 (2014). https://doi.org/10.1007/s11771-014-2003-7
Protus Nalobile, Jackson Muthengia Wachira, Joseph Karanja Thiongo, Joseph Mwiti Marangu, "A Review on Pyroprocessing Techniques for Selected Wastes Used for Blended Cement Production Applications", Advances in Civil Engineering, vol. 2020, Article ID 5640218, 12 pages, 2020. https://doi.org/10.1155/2020/5640218
Pyroprocessing is an important stage in cement manufacturing. In this process, materials are subjected to high temperatures so as to cause a chemical or physical change. Its control improves efficiency in energy utilization and hence enhances production for good quality assurance. Kilns used in cement manufacturing are complex in nature. They have longer time constants, and raw materials used have variable properties. They are therefore difficult to control. Additionally, the inclusion of various alternative fuels in burning makes the process more complex as the fuel characteristics remain inconsistent throughout the kiln operation. Fuel intensity standards for kilns using fuel oil are very high, ranging from 2.9GJ to 7.5GJ/ton of clinker produced. Grinding of clinker consumes power in the range of 2.5kWh/ton of clinker produced. These and other pyroprocessing parameters make cement production costly. The pyroprocessing process in kilns and the grinding technologies therefore have to be optimized for best processing. This paper discusses the cement manufacturing and grinding processes. The traditional kiln technologies and the current and emerging technologies together with general fuel and energy requirements of cement manufacturing have been discussed. From the discussion, it has been established that the cement manufacturing and grinding technologies are capital-intensive investments. The kiln processes are advanced and use both electricity and natural fuels which are expensive and limited factors of production. The raw materials used in cement manufacturing are also limited and sometimes rare. The calcination of the raw materials requires external energy input which has contributed to the high cost of cement especially to low-income population in the developing countries. Self-calcining materials, in which the pozzolanic materials burn on their own, are potential pozzolanic materials with great potential to lower the cost of cement production. Such materials, as shown from the previous research study, are rice husks, broken bricks, spent bleaching earth, and lime sludge. There is a need, therefore, for research to look into ways of making cement using kiln processes that would use this property. This will be cost-effective if successful. It can be done at micro- and small-scale enterprise.
Cement is the most commonly used binder in concrete production all over the world. Its production is however expensive due to high amount of energy used in its manufacture. It is therefore out of reach of a majority of the world population . The high cost of cement especially in developing countries is mainly due to the high energy demand during the clinkerisation process. During clinkerisation, temperatures in excess of 1450C are employed . This makes the resultant cement unaffordable to low-income earners. This has subsequently led to mushrooming of slums in most parts of Africa.
Researchers have been exploring ways of manufacturing less costly cement using supplementary cementitious materials (SCM). These have materials that include industrial byproducts (fly ash, slag, silica fume, and acetylene sludge lime) and agricultural wastes such as rice husk, palm oil fuel ash, and sugarcane bagasse. These materials have been shown to have cementitious properties and are known as pozzolana . There are two types of pozzolana, natural and artificial. Natural pozzolanas are materials that will react with lime in the presence of water to form cementitious properties in their natural form at ambient temperature. Examples of natural pozzolana are volcanic tuff and diatomaceous earths. Artificial pozzolanas have to be processed before use. The processing includes proper burning and grinding of the materials under carefully controlled conditions to form the amorphous form of silica which is important to their pozzolanic activity. The temperature and duration of burning are important in processing [4, 6, 8]. Calcination and pyroprocessing technologies are therefore very important for the development of low-cost cement.
The wet and dry manufacturing processes are the two main ways of manufacturing cement . The major difference between wet and dry process is the mix preparation method before burning clinker in the kiln. The wet process involves the addition of water to the raw materials to form rawslurry which is thick. In the dry process, the raw materials are prepared by fine grinding and drying. The choice of the process is mainly dependent on the nature of the raw materials available. When the moisture content in raw materials is more than 20%, the wet method is preferred to the dry method . Figure 1 shows a process scheme that would apply to both the dry and the wet processes.
In the past, the wet process was mostly used since homogenization of wet raw materials was easier than that of dry powders. In the wet process, there is an easier control of the chemical composition of the raw materials. This process is however more energy-intensive and hence more expensive compared to the dry process given that the wet slurry has to be evaporated before the process of calcination. The total heat requirements for the dry precalciner kilns are much lower compared to the former wet process kilns. With about 900kcal/kg compared to about 1600kcal/kg for the wet process kilns, the new dry processes are less expensive in terms of heat requirement (about 60% less) . With this reason, many old wet process kilns have been converted into dry process plants. Mostly, semiwet and semidry process kilns are intermediate steps in the conversion. Over the years, wet process plants have been converted to dry ones especially in Europe. By 2004, dry process kilns accounted for 90% of all process technologies used. Figure 2 shows global clinker production per kiln type by the year 2009.
The prime raw material which is limestone is broken into big boulders after blasting in mines . It is then transported by dumpers to a limestone crusher where it is crushed to between 15 and 20mm size. The material is then crushed and piled longitudinally by equipment called stacker/reclaimer. The crushed limestone from pile is transported through the belt conveyer to the hopper. Similarly, other raw materials like clay, bauxite, and iron ore are also transported by a conveyer belt from the storage yard to the respective hoppers. All raw materials are proportioned in requisite quantity through weigh feeders. The proportioned raw materials are then transported by a conveyor belt to the raw mill to be ground into the powder form. After grinding, the powdered raw mix is stored in a raw meal-silo where blending takes place.
Raw material preparation provides a mixture of raw materials and additives that has the right chemical composition and particle size distribution necessary for clinker production . For plants that receive their raw materials already crushed, this stage usually involves grinding (milling), classification, mixing, and storage . Raw material preparation is an electricity-intensive production step requiring about 25 to 35 kilowatt hours (kWh) per ton of raw material, although it could require as little as 11kWh per ton. After primary and secondary size reduction, the raw materials are further reduced in size by grinding. Grinding differs with the pyroprocessing process (kiln type) used. In dry processing, the materials are ground into a powder that can flow in horizontal ball mills or in vertical roller mills. In a ball mill, steel-alloy balls are responsible for decreasing the size of the raw material pieces in a rotating cylinder. Rollers on a round table provide size reduction in a roller mill. Waste heat from the kiln exhaust or the clinker cooler vent, or auxiliary heat from a standalone air heater before pyroprocessing, is often used to further dry the raw materials. The average moisture content in the raw material feed of a dry kiln typically varies between 0% and 0.7% .
Portland cement is mostly made in a rotary kiln . Basically, this is a long cylinder rotating about its axis once every one or two minutes. This axis is inclined at an angle, with the burner being lower at the lower end. Raw mix is fed in at the upper end, and the rotation of the kiln causes it to move gradually downhill to the other end of the kiln. Figure 3 shows a general layout of a rotary kiln .
There are three types of rotary kilns: kiln without preheater, kiln with preheater (PH), and kiln with both preheater and precalciner (PC). Kilns with PH are preferred to kilns without PH as they have lower energy consumption. For this reason, long rotary kiln without PH (long dry kilns) are being replaced over time. Thermal energy requirement is further reduced if a PH kiln is also equipped with a PC. New facilities usually include both PH and PC. A preheater (PH) is series of vertical cyclones in which the material is passed in counterflow with exhaust gases from the rotary kiln so that heat is transferred from the hot gas to the raw meal, which is therefore preheated and even partially calcined before entering the rotary kiln.
The four-stage cyclone preheater kiln system was the standard technology in the 1970s since most plants were built in the range of 1000 to 3000 tons/day production. However, a number of different SP kilns are available. Most common SP kilns have between 4 and 6 cyclone stages. The moisture content of the raw materials determines the number of stages. Where moisture is less than 8.5%, a PH kiln with 4 to 6 stages may be used. The higher the number of cyclone stages, the more the heat recovered. The energy demand of a 6-stage cyclone PH is about 60MJ/t less than the demand of a 5-stage PH, and a 5-stage PH would save approximately 90MJ/t over a 4-stage PH. The addition of a 4th cyclone stage to a 3-stage PH may decrease the energy needs by 250MJ/t, but moisture in the raw materials should not exceed 8.5%. If this is the case, a 3-stage cyclone is preferred as the thermal efficiency will not improve when an extra stage is added. The SP unit has a typical unit capacity between 300 and 4000t/d . In general, a PH tower consists of 1 to 6 cyclone stages, which are disposed one above the other in a tower. The PH kiln performance can be extended using precalcination technology. For the time being, kiln systems with multistage cyclone preheaters and a precalciner are considered to be the state-of-the-art technology for new dry process plants. Precalciner kilns first appeared in the 1970s. The calciner is a secondary combustion device where the total fuel is burnt. In this chamber, about 60%65% of the total kiln emissions are released, while limestone (CaCO3) is decomposed into lime (CaO) and carbon dioxide (CO2). The remainder of the emissions is generated from fuel combustion. As calcination is at least 90% completed when the raw meal is fed into the rotary kiln, the PC technique allows a considerable increase in the clinker capacity. The average capacity of new European plants ranges from 3000 to 5000 tons of clinker per day. However, from a technical point of view, capacities of up to 15,000 tons per day are feasible. Three PH/PC kilns with a capacity of 10,000t/d are currently in operation in Asia. The addition of a PC also reduces the energy requirements. The PH/PC kiln is the most energy-efficient kiln technology. Thermal energy demands for different kilns are listed in Tables 1 and 2. Other kinds of kilns include equipment for semidry and semiwet processes .
For semidry processes, the Lepol kiln (300 to 2000 tons/day)where a travelling grate preheater is installed outside the rotary kilnrequires less thermal energy than a long dry kiln (3002800t/d). In semiwet processes, a filter cake is produced from raw material handling. This cake is either extruded to pellets prior to being fed to the Lepol kiln or loaded into a cyclone SP/PC kiln after being dried to a raw meal in an external dryer. This latter system offers both the lowest heat consumption and the highest clinker capacity (20005000t/d compared to 3003000t/d). If wet raw material preparation is required, a 2-stage PC with the dryer (20005000t cli/d) can provide the lowest thermal energy consumption. The wet slurry is first dried in an integral dryer crusher, after which it is fed to the PH-PC kiln. This modern process is replacing the conventional method which comprises the long wet rotary kiln (3003600t/d) with an internal drying/preheating system .
The fluidized-bed cement kiln (FB) is a recent technique that is emerging. It has been used in Japan since 1989 on a pilot basis. It has a capacity of 20 tons per day. A larger capacity FB of 200 tons/day was developed in 1996 . In China, a pilot kiln with a capacity of 1000t/d is now under construction. Compared to the SP kiln with the grate cooler, the FB kiln could reduce the heat consumption by 1012%, but it is not expected to serve large capacities and is not yet available for the cement industry. Figure 4 shows a fluidized-bed advanced cement kiln system .
The technology offers the following advantages:(i)It reduces emission of CO2 because of the reduction of fuel consumption(ii)It is able to use low-grade coal such as low volatile and low calorific value coal since it has improved burning and heat transfer(iii)It has an improved heat recovery efficiency due to increase in waste heat recovery(iv)Since there are no movable apparatuses, construction and maintenance costs are decreased(v)The system is able to control temperature more tightly and keep longer reaction time, and it enables quality improvement and production of special cement of higher grade(vi)Thermal NOX emission can greatly be decreased since combustion takes place in the fluidized bed without generating flame 
Cement grinding process is the reduction of clinker produced in rotary kilns to a fine form. The clinker has to be ground with the addition of gypsum to get the finish product, cement. The objective is to increase the specific surface of the cement component with a proper particle size distribution and to provide convenient reactivity of cement making it more easily workable when used in concrete .
Grinding systems in the cement industry play an important role in the distribution of the particle size and particle shape . The size and shape of the cement particles affect the reactivity of the clinker. They also affect the dependence on temperature of dehydrating gypsum that is ground together with the clinker. These factors affect the mortar properties of the cement product such as water demand, initial and final setting times, and strength development . Ball mills have been used as the main grinding equipment for finished cement production for over 100 years. Although simple to operate and cost-competitive relative to other technologies, the low efficiency of ball milling is one of the main reasons for the development of more efficient grinding processes in recent years. Vertical roller mills (VRMs), high-pressure grinding rolls (HPGRs), vertical shaft impact (VSI) crushers, and more recently, the horizontal roller mill (HOROMILL) (in which energy consumption is substantially reduced) have resulted in an improvement between 45 and 70% in specific energy related to a typical ball mill .
Ball mills or tubular mills are built with diameters up to 6.0 meters and lengths up to 20 meters; the drive ratings today are as high as 10,000kW with stable operation, and maintenance of a ball mill is relatively simple . The maintenance cost and the capital cost are relatively low compared to other technologies. Due to the high levels of operational reliability and availability, ball mills remain the most frequently applied finishing grinding unit in cement plants. Compared with newer milling devices such as VRM and HPGR, ball mills have the highest specific power consumption and the lowest power utilization (about 3235kWh/ton depending on the material hardness and to fineness between 3,000 and 3,200cm2/g). Most of the energy is lost as a result of heat from the collision of the steel balls among themselves and against the mill walls. Portland cement production is usually finished using a two-compartment ball mill as shown in Figure 5. First compartment or chamber 01 is known as the coarse chamber, and in the second compartment, the material is finely ground . Between the two compartments, there is a classification diaphragm that screens the fine form of the coarse material.
Generally on cement mills, the product is ground dry in a ball mill. It has a relatively wider particle size distribution; hence, it is required to operate the ball mill in a closed circuit with a size classifier with an efficient or sharp cut of the size separator. This happens especially when high levels of fines are generated, when mixtures have low Bond work index, or grinding materials that have a tendency to agglomerate due to overgrinding effect. The circulating loads range from 100% up to 600% that are established based on the grindability of the new feed, the cut size, and the required product fineness in relation to reaching the adequate cement strength. The energy efficiency of dry ball-mill grinding of cement depends on factors such as ball charge fill-ratio, mill length/diameter ratio, size distribution of the ball charge, operating conditions of the air separators, air flow through the mill, production rate, use of grinding aids, and the hardness and fineness of the feed and product (generally referred to as the work index (kWh/t) and the F80 and P80 sizes, respectively).
In high-pressure grinding roll (HPGR), the material is reduced by a highly compressive stress created by two counter-rotating rolls (one fixed and another floating) . This creates a critical fracture process that presses the material into a compact flow area. This flow area is shown in Figure 6. The grinding pressure between the rolls is 50 to 350MPa, and the circumferential speed of the rolls varies between 1 and 2m/s on the grindability characteristics of the feed and the pressure applied to the roll; the compacted cake (consisting of over 70% solids by volume) has a fine fraction below 90m. Up to 40% of these fines must be recovered by deagglomeration of the compacted cake using another deagglomerating device. The specific power utilization is between 14.6 and 19.8 kWh/t at a Blaine area 3,0003,200cm2/g. HPGRs are reported to be 4560% more efficient than ball mills .
Trouble-free operation of an HPGR depends to a great extent on ensuring proper moisture below 3%, and the maximum particle size of the material should not exceed 1.5 to 2 times the gap width. Feed is distributed evenly along the rolls; and foreign material (scats) is not allowed to pass into the rolls and is captured using a magnetic separator system. HPGR is convenient to comminute materials that are not overly fine and have low moisture content . Materials above 3% moisture must be predried before feeding to the rolls. HPGR can be integrated into various circuit configurations in new and existing grinding plants to increase the output of plants that have only ball mills with precrushing before a ball mill .
Vertical roller mills (VRMs) with integrated classifiers have been used successfully for many years in cement plants to grind and dry raw materials simultaneously with moisture contents up to 20% by weight. Their production can be as high as 400 tons per hour, and they have a drive power of 11.5 megawatts . The feed is comminuted by pressure and friction between a horizontal rotating table and 2 to 4 grinding rollers hydraulically pressed against the table as shown in Figure 7. Nowadays, the grinding rollers have diameters as large as 2.5m. The material being ground is carried by pneumatic and mechanical transport to the classifier located in the same housing directly above the grinding chamber. The classifier tailings (oversize rejects) are recycled back into the grinding chamber together with the fresh material. The grinding elements and mill settings are modified to grind harder materials such as clinker and granulated blast-furnace slag. Power use is between 26 and 29 kWh/t when grinding to a Blaine area 3,300cm2/g using a VRM .
Vertical roller mills integrate the grinding, drying, and separation processes into one unit. This integration makes the VRM competitive in terms of specific electrical power consumption compared against other technologies. VRM is 50% more efficient than ball mills when comparing kWh/t used to grind the same product under similar service properties .
The horizontal roller mill or tube has a length/diameter ratio around 1.0 and is supported and driven on axial bearings . A solid single armored grinding roller is pressed hydraulically against the rotating inner drum surface within a cylindrical grinding zone as shown in Figure 8. The pressure is much lower than the HPGR and is comparable to the VRM. No compacted cake is produced that requires further deflaking. The grinding roller is supported on bearings outside the grinding tube. Internal fittings are subjected to heavy wear; however, wear of the grinding elements is still lower with the VRM. Power consumption on the horizontal roller mill when compared against a ball mill is reduced by 10 to 25kWh/t of cement depending on clinker grindability and Blaine specific surface area .
Calcination is the process of changing the chemical composition of a mineral ore by a thermal process or driving off a volatile fraction. Compared to pyrolysis, the absence of oxygen is not necessary in this process . Rotary kilns that are directly heated or those that are indirectly heated are both used for calcination. Calcination is mostly applied in the cement industry. It is carried out in furnaces, also referred to as kilns or calciners, of various designs including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized-bed reactors. The first step in cement manufacturing is calcination of calcium carbonate followed by burning resulting calcium oxide together with silica, alumina, and ferrous oxide after mining and grinding .
Calcination is commonly used in activating supplementary cementitious materials such as clays. Such clay materials are pozzolana. These are materials that are not cementitious themselves but in finely divided form which react with lime at ambient temperatures to form compounds with cementitious properties . For them to attain pozzolanic properties, these clays were activated by calcination at temperature ranges between 600 and 900C . At these temperatures, the crystal structures of their silicates are transformed into amorphous compounds that react with lime at room temperature to form cementitious materials. Not all types of clays can be calcined to present pozzolanic activity. Clays containing high proportions of crystalline minerals such as quartz and feldspar do not produce reactive material . Temperature control is the key during calcination of the clays to avoid formation of crystalline silicate compounds that would otherwise not react with lime at room temperature [37, 38].
Cement production is an energy-intensive process consuming thermal energy of the order of 3.3GJ/ton of clinker produced . Electrical energy consumption is about 90120kWh/ton of cement. Coal has been the key fuel in the cement industry for a long time. A wide range of other fuels such as gas, oil, liquid waste materials, solid waste materials, and petroleum coke have all been successfully used as sources of energy for firing cement-making kilns, either on their own or in various combinations. Since clinker is brought to its peak temperature mainly by radiant heat transfer and a bright (i.e., high emissivity) and hot flame is essential for this, high carbon fuels such as coal which produces a luminous flame are preferred for kiln firing. In favourable circumstances, high-rank bituminous coal can produce a flame at 2050C. Natural gas, which can produce a peak temperature of 1950C and being also less luminous, tends to result in lower kiln output .
Apart from the primary fuels above, various combustible waste materials have been used for calcination in kilns. These alternative fuels (AF) include used motor-vehicle tires, sewage sludge, agricultural wastes, landfill gas, refuse-derived fuel (RDF), and chemical and other hazardous wastes .
Cement kilns are an attractive way of disposing of hazardous materials because of the following:(i)The much higher temperatures in the cement kilns are more suitable to effectively dispose the materials than in other incinerators(ii)The alkaline conditions in the kiln, afforded by the high-calcium raw mix, which can absorb acidic combustion products(iii)The ability of the clinker to absorb heavy metals into its structure
Use of scrapped motor-vehicle tires which are very difficult to dispose by other means is a good example of an alternative fuel. Whole tires are commonly introduced in the kiln by rolling them into the upper end of a preheater kiln or by dropping them through a slot midway along a long wet kiln. In either case, the high gas temperatures of between 1000C and 1200C cause instant, complete, and smokeless combustion of the tire. Alternatively, tires are chopped into 510mm chips, in which form they can be injected into a precalciner combustion chamber. Steel and zinc in the tires become chemically incorporated into the clinker, partially replacing iron that must otherwise be fed as the raw material. High-level monitoring of both the fuel and its combustion products is necessary to maintain safe operation .
Natural pozzolanas such as volcanic tuff and silica fume are used in their natural form. Artificial pozzolanas have to be processed before they are used. One way of processing materials to the pozzolanic form is by incineration of agricultural wastes such as rice husks, rice straw, sugarcane bagasse, and ground nut shells, among others. The incineration of these materials decomposes organic matter in the form of cellulose and lignin leaving an ash rich in silica [42, 43]. When this is done under controlled temperature conditions, the ash is a very good pozzolanic material.
Calcination is another method of processing pozzolana. Materials are heated in a temperature range of 600C900C. Most work reported on calcination of pozzolana has been on a laboratory scale [41, 4446]. An industrial trial of calcination of pozzolanas has been reported . In this case, clays were calcined in a wet-process clinker rotary kiln normally used for clinker production. Several technological parameters were adjusted in order to calcine the material within the optimal temperature range of 750850C and not to exceed 900C. The Ghanian experience has seen production of clay pozzolana at a small-scale level. In this case, the raw clay was ground and mixed with palm kernel shells, which are local agricultural waste. The shells were incorporated as a source of energy and to increase the ash content. They were processed by a vertical draft kiln that required another external of fuel. The clay pozzolana processed at a temperature of 900C was the best . The optimum replacement range of the clay pozzolana with OPC was between 20 and 25%. This achieved the Portland pozzolana cement standard with up to a compressive strength of 24.1MPa and 28MPa at 28th day and 60th day of curing. This prototype plant was very promising. However, the kiln used required external fuel, and it was recommended that a kiln in which agricultural wastes be used as a source of fuel be used.
Self-calcination is a process of activating artificial pozzolana in the case where the materials being processed are the ones that produce the energy required for both calcination and incineration. This can be made possible when a material is pozzolanic but can burn by itself or is blended with another material that cannot burn by itself. In this case, the material produces the required heat for processing, and it processes itself to a pozzolanic form. Typical example is rice husk-clay mixes. Laboratory trials have been done before using Kenyan clay-rice husk mixes .
Agricultural and industrial wastes have been reported as potentially pozzolanic materials when processed under special conditions. Muthengia  reported that broken bricks (BB), spent bleaching earth (SBE), and rice husk (RH) can be processed in blend to make good-quality pozzolana. In this work, the materials were activated by self-calcination. This was possible because the blended mix had RH which burns on its own and SBE which contains residual oils from the industrial filtering process. Incineration of RH alone generates about 15J/g heat , while the calorific value of SBE is about 13.23J/g . In this case, therefore, no external source of energy was required other than small fire for ignition. The materials were incinerated in a special kiln known as the fixed bed kiln (FBK), designed by Ochungo , for incinerating rice husks to form highly pozzolanic rice husk ash. They were left to burn overnight and were collected the following day. They were blended with hydrated lime to form pozzolana-lime cement. Muthengia  showed from this work that the blend in which the pozzolana-lime ratio was 2:1 showed the greatest reactivity. Table 3 shows the compressive strength of pozzolana:hydrated lime cements of various individual pozzolana and the combined blends.
From the table of results of compressive strengths, SBE was a spent bleaching earth from an oil refinery in Nairobi. Broken bricks from two companies in Nairobi were used, with G45-BB from Githurai 45 and CW-BB from Clay Works Manufacturers Limited, also in Nairobi, Kenya. RH was used to activate the BB and the SBE in what was called RH:BB:SBE raw blend. From this work, it was noted that an ideal proportion would be the one with the highest proportion of SBE, then BB, and with the least RH because of pozzolanicity of these materials according to their compressive strength as shown in Table 3. However, this was not possible because when RH is compacted and ignited, its combustion temperatures increase to very high values  which would produce predominantly crystalline silica, which is not reactive. Therefore, a workable mix with a RH:BB:SBE ratio of 5:1:1 was used. Since activating pozzolana makes up for the processing costs of artificial pozzolana, this was seen as the most cost-effective way of processing these pozzolanic materials. The ashed blend consisted of separately activated RH, BB, and SBE. This was less pozzolanic and would be more costly in terms of activation. The most workable and easy-to-process material was therefore the raw blended mix that would self-calcine. This met the requirements of pozzolana-lime cements as specified by the ASTM No 593 part C standard .
Later, Nalobile et al.  optimized the ratio of incinerating the blended materials to a ratio of 10:1:2 (RH:BB:SBE). This was done by varying the amount of broken bricks added to the mixture from none to a maximum of 6kg in a constant mix containing 20kg of RH and 4kg of SBE. The new ratio improved pozzolanicity of the materials in terms of reactivity with lime and compressive strength. Acetylene lime sludge, a waste from the acetylene manufacturing company, was used in place of normal hydrated lime which was used in the previous work. The FBK  was used in this work as the work done by Muthengia et al. . Table 4 shows the compressive strength of the pozzolana-lime cements obtained in this work.
Together with other tests done , this cement conformed to the standard requirements for pozzolana-lime cements as set in ASTM C part 593 . The sample with the highest pozzolanic reaction was used to replace laboratory-made ordinary Portland cement (OPC) at various percentages from 0%, 45%. 46%, 48%, 50%, and 52%, and their compressive strengths were compared with the Kenyan standard . It was observed that replacement of OPC with up to 50% of the pozzolana-lime cements met the required standards . This was therefore seen as a promising venture in making Portland pozzolana cement at a lower cost compared to the current methods.
The FBK used in the aforementioned research was small and laborious to work with. A new kiln which is bigger and more efficient than the FBK needs to be designed so that this pozzolana-lime cement can be tested for its production at micro- and small-scale enterprise. This can benefit poor populations in the third-world countries, especially Africa, and in Kenya. This is because in Kenya, the cost of cementing materials is expensive. This has led to urban poor people living in appalling conditions in slum areas. Most of their houses are made of mud, plastic papers, grass, and old and used corrugated iron sheets. This has led to outbreak of diseases such as pneumonia and cholera. Other conditions such as infestation by jiggers are a common phenomenon in these areas and other rural regions.
From the above discussion, it has emerged that the cement manufacturing and grinding technologies are a large-scale industry with high cost in capital and energy inputs. The kiln processes are advanced and use both electricity and natural fuels which are expensive and limited factors of production. The raw materials used in cement manufacturing are also limited and sometimes rare. The calcination of the raw materials requires external energy input which has contributed to the high cost of cement especially to low-income population in the developing and third-world countries such as those in Africa, Central America, and parts of Asia. The use of self-calcining materials to make cement can be a welcome venture in such cases where the cement raw materials are scarce or the cost of manufacture is prohibitive. Research has shown that it is possible to form pozzolana cement from waste materials such as RH, BB, SBE, and ALS. However, kiln processing of these materials has not been done to test the practicability of production albeit at micro- and small-scale enterprise (MSE). There is need for research into designing a kiln that would be used to process these materials in rural settings where the cost of cement is high, while these materials are a menacing solid waste.
(i)There is a need to develop alternative cement manufacturing plants that will be possible at micro- and small-scale enterprise for affordable production of cement using locally available materials(ii)There is a need for further research to develop a cost-effective and efficient kiln to calcine waste materials such as rice husks, spent bleaching clays, broken bricks, and various clays to make pozzolana cement
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