In December 2016, Gebr. Pfeiffer received an order from the Belgian enterprise Cemminerals NV for the supply of a grinding plant for slag and cement. The plant, which will see a Pfeiffer vertical roller mill of the type MVR 5300 C-6 installed, will be set up in Flanders, in the middle of Europe.
In December 2016, Gebr. Pfeiffer received an order from the Belgian enterprise Cemminerals NV for the supply of a grinding plant for slag and cement. The plant, which will see a Pfeiffer vertical roller mill of the type MVR 5300 C-6 installed, will be set up in Flanders, in the middle of Europe. The mill is guaranteed to achieve capacities of 132 t/h pure slag, ground to 5000 cm/g acc. to Blaine, and of up to 200 t/h CEM II, ground to a fineness of 3500 cm/g acc. to Blaine. The mill main drive is designed for an installed power of 4600 kW, and the SLS 4750 BC high-efficiency classifier, mounted on top of the MVR mill, enables high material fineness degrees of up to 5000 cm/g acc. to Blaine.
The mill is guaranteed to achieve capacities of 132 t/h pure slag, ground to 5000 cm/g acc. to Blaine, and of up to 200 t/h CEM II, ground to a fineness of 3500 cm/g acc. to Blaine. The mill main drive is designed for an installed power of 4600 kW, and the SLS 4750 BC high-efficiency classifier, mounted on top of the MVR mill, enables high material fineness degrees of up to 5000 cm/g acc. to Blaine.
Gebr. Pfeiffers enormous expertise and vast experience in the field of efficient cement and slag grinding convinced the Belgium customer. Sophisticated maintenance concepts and the innovative MVR product in itself, representing the basis for highest availability, were also key factors in the decision to choose the Pfeiffer technology. The mill is slated for commissioning in early 2018.
Gebr. Pfeiffers enormous expertise and vast experience in the field of efficient cement and slag grinding convinced the Belgium customer. Sophisticated maintenance concepts and the innovative MVR product in itself, representing the basis for highest availability, were also key factors in the decision to choose the Pfeiffer technology.
The 13th Global Slag Conference and Exhibition has taken place in Prague, with 120 delegates in attendance from 30 different countries. The 14th Global Slag Conference will take place on 3 - 4 April 2019 in Aachen, Germany.
Adam Smith of Kallanish Commodities started the conference by speaking about trends in the global steel industry. Donald Trump's imposition of tariffs on steel and aluminium imports caused a spike in steel prices in North America at the start of 2018, but these have fallen back after most countries were given waivers from the tariffs. However, an ongoing Trump-led crusade on Chinese intellectual property acquisitions is still dampening market sentiment. The EU exemption from the tariffs ends on 1 May 2018, and European steel association Eurofer fears that around 13Mt of US-bound global steel exports may soon be diverted to Europe instead. The EU is now considering its own external tariffs as well. Two major mergers are in the pipeline, between Tata Steel Europe and thyssenkrupp, as well as the ArcellorMittal takeover of Itailian producer Ilva. Adam suggested that with the EU's trade defence measures, that the mergers may raise steel consumers' sourcing costs (meaning that steel prices are set to rise). The World Steel Association has published forecasts for 2018-2019, suggesting that there will be a mild economic deceleration in the Chinese economy (leading to a fall in steel demand), and that developing economies will once again lead global growth. Developed nations including the EU will attain steady but moderate growth, while India is readying to be a standout growth region for the coming years.
Doug Haynes of Smithers Apex next gave an executive summary of the Smithers report 'The future of ferrous slag to 2027.' Doug suggested that around 50% of global slag production is used in the cement industry, with 32% used in road construction, 10% used in 'other applications, 3% used in embankments and 2% in internal recycling. These figures exclude disposal to landfill. Doug points out that drivers for slag use include construction demand in India, China and Asia, construction market demand, a trend towards sustainability, the technical properties of cement and concrete with slag, environmental pressures on primary rock extraction and the development of processes capable of mitigating Basic Oxygen Furnace (BOF) slag free-lime expansion. However, there are many barriers, including potential decreases in slag production capacity, the financial cost of installing new technology and environmental regulations restricting the use of ferrous slag.
Charles Zeynel of ZAG International next gave an update on granulated blast furnace slag (GBFS), other supplementary cementitious materials (SCMs) and the global cement industry. He started by saying that "after having forecast for years that we are going to run out of slag, the day has finally arrived." He recounted a story of a company that had signed a five-year contract with a Chinese state-owned steel company for significant tonnages of slag: six months later, the price tripled despite the contract terms. But just try and sue a state-owned company in China - you will not get far. The value of slag, he concluded, has increased dramatically. The slag producers have concluded that they want a larger share of the value of slag, so that prices have increased. Charlie pointed out that there is a global oversupply of clinker, but that there is a positive overall outlook for the cement industry due to population growth and urbanisation. Countries are starting to stipulate non-clinker components in cement (some of the emirates in the UAE now stipulate at least 60% fly ash or slag in cement). Due to the continuing closure of coal-fired power stations, particularly in the US and EU, flyash supplies are effectively reducing: slag is the next 'go-to' SCM on the list. There is no current global growth in blast furnace slag production, due to the rise of Electric Arc Furnace (EAF) steel production. Only around 75% of globally-produced blast furnace slag (BFS) is granulated, creating around 250Mt of GBFS, albeit with 150Mt in China, 30Mt in Japan and 70Mt in the rest of the world. China is now auctioning slag supplies for export each week, with the 'winner' having to pay cash up-front. Now around 25Mt of slag is traded internationally, of which 12Mt is Japanese slag, and around 5-6Mt is Chinese slag. Freight rates are rising from their historic lows, and will eventually become a major factor in slag use and pricing. From being a buyer's market, slag is becoming a seller's market.
Michael Connolly of TMS International next gave an update on the regulatory approach to slag and slag products in the USA. In the US around 20Mt of iron and steel slag is produced and marketed each year. Eleven states have now decided that slag is a product rather than a waste, 'if it is not discarded and is distributed for use in commerce.' Industry and the US National Slag Association are busy trying to persuade the remaining states to adopt the same approach to slag. Steel slag is actually the preferred aggregate in high-demand road surface applications, due to its higher coefficient of friction, high resistance to rutting and its high compressive strength, leading to superior skidding resistance. This has led to its use as the track surface for the Indianapolis Motor Speedway, home of the Indy 500. When properly segregated from ladle slag, steel slag is an excellent high-density aggregate ideal for use in concrete. The only drawback is its high density, but when mixes are properly designed using a mix of natural aggregates and slag, a high efficiency mix can be achieved. These mixes usually have a slightly higher strength and durability than mixes with just natural aggregate stone." Steel slag has also been used as an SCM, while TMS also supplies EAF slag to several cement companies as a cement raw feed.
Andrey Korablin of Russian company SmartScrap Ltd, celebrated his birthday by giving an overview of the Russian steel and slag markets. Russia is the fifth largest steel producer in the world, just behind the US, producing around 70Mt, concentrated in the Central Federal District,' around Moscow, in the south west and also around the Urals. The largest producers are Evraz, NLMK, MMK and Severstal. Many slag producers process their own slag, but others use either subsidiary companies or independent processors. Only around 50% of metallurgical waste is recycled. Federal laws are being tightened on waste dumping, while the goal of the Ministry of Industry is to bring the level of municipal solid waste recycling to 80% by 2030, while there are also potential tax breaks for slag use. Road construction and use in the cement industry are the areas, Andrey concluded, with the most potential for increased slag use in Russia.
At the start of the second session, on slag beneficiation, Victoria Masaguer Torres of ArcelorMittal Spain spoke about the use of steelmaking slag in the passive neutralisation of mine waters. The project started a decade ago when a highway construction project allowed the interaction between groundwater and pyritic materials that caused an acid water problem. A laboratory-scale anoxic trial using steel-making slag was used to prove the efficacy of the slag as a neutralisation agent: calcium content of the slag was reduced during the trial, while the pH of the acidic waters was reduced. In a pilot scale trial, 50t of ladle slag was used in one pit, while limestone was used in a control pit. It was found that limestone achieve an acidity reduction of around 90%, compared to around 80% for slag. The slag has a half-life of around two years, and following the exhaustion of the slag as a neutralising material, the pit would be closed and left in place.
Patrick Lecherf of Euragglo next spoke about the agglomeration and briquetting of steel-plant byproducts. Particles are briquetted by first squeezing out the air between particles. Then, if the particles are not self-compacting, by using a binder (such as molasses and lime in the case of steel plant byproducts), the materials are formed into briquettes by being squeezed between patterned rollers. Other binders can be used, such as bentonite, sodium silicate, lignosulfonate, polymers and hardeners, starch and cement. The abrasiveness of the material to be briquetted will determine the frequency of maintenance of the forming rollers.
Amir Shakurov of Ecoslag Recycling of Russia then gave details of molten slag beneficiation using an innovative drum crystalliser. Amir pointed out that a number of different slags have similar viscosity, melting points and enthalpy over a similar temperature range (1500-1800C). The Ecoslag solution is the accelerated cooling and solidification of molten slag (including carbon, stainless, ferro alloys, copper and nickel slags) using a continuously-rotating drum crystalliser to stabilise dicalcium silicate into a stable form. Amir suggested that the slag crystalliser is built of heat-resistant materials that can withstand the wear-and-tear of 100,000 melts. The Ecoslag drum crystalliser has been successfully installed by the Vyksa steel plant in Russia. Amir mentioned a previous Russian invention for slag cooling and granulation in a rotary drum with steel balls and water sprays, which he claimed has been copied by Chinese firms and has now been re-exported around the world.
Jitka Halamov and Ji Py of ArcelorMittal Ostrava, a company that will soon become an independent unit, next spoke on the efficient use of slag resources. Starting in 2012, the company commenced a series of small steps towards improving the recycling rates of its slag materials. Better internal communication, benchmarking, cross-exchange of ideas from other units, logistics optimisation, and some equipment upgrades allowed the beneficiation and significant addition of value to slag.
Global Slag Awards DinnerAt the end of the first day of the conference, delegates gathered on the dining yacht Grand Bohemia for a cruise along the Vltava River, among the fabulous sights of historic Prague. During the cruise the Global Slag Awards 2018 were presented. Global Slag company of the year (slag/slag product producer) was Harsco Metals and Minerals, and the slag user of the year was CRH Group. The supplier of the year for technology, equipment or services was Loesche. The Recoval Dria unit in Belgium was named as the Global Slag plant of the year, and, fittingly, Carbstone blocks (from the same company) were named as the slag product of the year. The Global Slag technical innovation award went to Loesche for its process for steelmaking slag beneficiation. In a popular decision, Dr York Richard was named the Global Slag Personality of the Year. Finally, it was announced that the Global Slag Conference will return on 3-4 April 2019 to the location where it has most recently (2014) achieved the greatest number of delegates - to a location in the very heart of the slag universe: the charming and well-located German town of Aachen.
On the second day of the conference, Phillip Hemple of Gebr. Pfeiffer SE spoke about the MVR vertical slag grinding mill, with MultiDrive. The mill can grind up to 600t/hr of OPC, with total drive power up to 18,000kW. The MultiDrive concept is essentially the substitution of a conventional drive with planetary gear and axial thrust bearing, with up to six decentralised drive units with girth gears and radial thrust bearings. Phillip showed that the vertical roller mill has the lowest long-term maintenance cost. The MVR mill has a parallel grinding gap, leading to symmetrical wear and the ability to turn the rollers. Cement Australia grinds GBFS at Port Kembla with a MVR 6000 C-6 mill, with 5500kW, to 4000-5000 Blaine.
Taking a different view, Dr York Reichardt of KHD Humboldt Wedag spoke about operating results using a roller press for grinding blast furnace slag. York suggested that a strong point of recommendation for the roller press is the roller surface of tungsten carbide studs, which has a guaranteed lifetime of 17,000hr without refurbishment. He also pointed out that the entire plant needs to be wear-resistant, since there is no point in having a roller that lasts 20,000hr but with chutes that wear out after 2000hr: all parts of the plant need to be designed to last when handling an abrasive material like slag. A new series of even larger KHD roller presses will grind up to 280t/hr of OPC and up to 180t/hr slag (to 4200cm2/g Blaine).
Andreas Jungmann next gave a presentation on behalf of Loesche on steel and steel-alloy slag processing. A Loesche vertical roller mill can be used for slag comminution, and Andreas also gave details of a dry density-separation step based on a fluidised bed approach. Such a process has been installed at the Orbix (Recoval/Recmix) stainless steel slag processing plant at Farciennes in Belgium, driven in large part by the economics of metal recovery. Another approach is to modify the molten LD slag (using technology such as from Primetals and FIB), so that the slag has a much lower metal content, making it much more similar in composition to clinker. Ultra-fine grinding can then activate the cryptocrystalline belite phases (which are actually crystals smaller than 1m), to give a 28-day strength similar to OPC.
Fernando Dueas of Cemengal then gave details of the Plug&Grind modular grinding system, featuring an OK VRM from FLSmidth. The Plug&Grind modular mill concept has previously been based on ball mills, and has focused on small to medium capacities, from 60,000t/y up to 400,000t/y ('Plug&Grind Xtreme'). The new vertical roller mill concept uses a three-roller VRM with 1600kW that can grind slag at 4000 Blaine at 60t/hr or 450,000t/y. Such a unit, grinding cement at 500,000t/y and using 1300kW motors, has just been sold to CRH at its cement plant in Dunbar, Scotland.
Andreas Ehrenberg of the FEhS - the Institut fr Baustoff-Forschung e.V. - next spoke about the grindability and reactivity of stored granulated blastfurnace slag. Andreas pointed out that a GBFS stored outside in moist conditions will react and will harden. After initial proof-of-concept laboratory tests, a 3000t test pile was formed and was sampled over 55 months. It was found that, without grinding or activation, the material at the bottom of the slag pile became rock-hard, through the hydration and carbonation of the surface of the slag granules. Material at the top of the pile was not so strongly altered. No significant crystallisation of the granules had occurred during storage: hardening was due to superficial surface hydration, forming calcium-silicate-hydroxide (CSH) reaction products. It was found that the grindability of the slag increased with storage time, requiring only 50% of the specific grinding energy for the same fineness after storage for 55 months. However, this was due to the increased grindability of the reacted surface layer, rather than that of the unreacted interior of the granules. In further tests, it was found that for samples ground to the same Blaine fineness, the particle size distribution was actually coarser, due to the preferential grinding of the softer surface layers of the hydrated granules. Prehydration products are located at the surface of the slag particles, so that the majority of the slag is unaffected by hydration. Andreas concluded that when GBFS is ground to the same particle size distribution, then there is no significant loss of strength despite increasing length of storage time - even for decades.
Next Nick Jones of Harsco and Mark Tilley of Lixivia gave a co-authored paper on a pilot-scale project for the extraction of valuable high-purity calcium products from steel slag. Nick reminded delegates that steel slag is currently used as construction aggregate, for agriculture, for drainage materials and as a feed material for construction and insulation, but he said that Harsco is always looking for approaches to add further value to steel slag. New ideas go through a process at Harsco, from discovery and development to final commercial deployment. Such an idea is Lixivia's Selex process, which is a patented water-based hydrometallurgical process using a recoverable lixiviant (a liquid used to selectively extract a metal from a solid ore) that can selectively separate the complex mixture of minerals and metals that are found in steel slag, which can then be sold onwards. The pilot-scale project concentrated on the production of high-purity precipitated calcium carbonate.Full results will be presented in a future edition of the Global Slag Conference.
The penultimate presentation at the 13th Global Slag Conference was given by David Algemissen of the FEhS, sharing details of the ProInnoLDS project, which was to find new markets for Basic Oxygen Furnace BOF-slag-based products, through the creation of a low-phosphorus hot metal that could be recycled in the steelworks. The problem is an acute one, since 3.1Mt of BOF slag is made each year in Germany, and 10.7Mt each year in Europe. Reducing the basicity of the BOF slag results in a dramatic increase in viscosity, to levels where the reduced slag cannot be poured. Adding 10% sand at 1650C improved the pour ability of the slag, which was further improved by heating to 1750C, but not to finally acceptable levels, and also without producing minerals that would add cementitious properties. Increasing levels of sand addition and adding Al2O3 decreased the basicity, giving a product of low viscosity, high glass content and with adequate cementitious mineralogy, essentially through modifying the chemical composition of the BOF slag to become nearly identical to that of blast furnace slag. The economics of the chemical modification of the slag and reheating required are currently marginal, but may change with the evolving cost of carbon emissions (from the cement industry) over the coming years, to give a economical replacement material for Portland clinker.
The final presentation was given by Jrgen Haunstetter of the German Aerospace Centre (DLR), on the use of slag as a thermal energy storage medium in a concentrated solar power (CSP) unit, as part of the EU-financed research project 'REslag.' Molten salt is commonly used, up to 450C, but regenerator-type storage can be heated using hot air, up to 1300C and slag is being considered as the heat storage medium. Hot air would be passed downwards through a chamber containing the material, heating it through the day, until the sun sets. At this point cool air would be passed up through the medium in the chamber to heat the air before it was sent to the boiler. Ceramic bricks and ceramic honeycombs, refractory materials and packed beds (for example containing slag) can all be used as the thermal storage medium. However, during heating and cooling thermomechanical stresses arise in and between the particles and also between the particles and the walls of the containers. The project was to determine the stresses in slag materials and in the slag containers. After 500 compression cycles at 600C in an experimental rig, no significant damage was found to the slag, suggesting that it could be stable in industrial usage. It was found that perlite refractory bricks were too fragile to be used as an insulating medium at the bottom of such a chamber, but that super-duty firebricks were robust enough for potential use.
At the end of the conference, a number of prizes were awarded. Following a vote by delegates, Nick Jones of Harsco and Mark Tilley of Lixivia won third prize for their paper on a new process to extract valuable minerals from stainless steel slag, while David Algemissen of the FEhS won second prize for his paper on BOF slag modification. However, completing a good day for his organisation, Andreas Ehrenberg won the prize for the best presentation for his paper on grindability and reactivity of slag after prolonged storage. Delegates agreed to meeting again soon, in Aachen in April 2019.
The slag vertical mill was promoted based on energy-saving vertical mill.Reusing steel slag waste as a building material product is a major innovation and an environmental protection project. We can make briquette press adopting steel plant sludge. also we can ultilize steel slag to make a new building material in slag cement plant.It is very popular because of grinding efficiency low energy consumption, compact layout, convenient maintenance. Widely used in cement, electric power plant, building materials, railway and road slag, hard-to-grind materials, quartz, zirconia, sintered bauxite, mullite dry grinding , the rock materials which Mohs hardness between 5-9,the moisture is less than 10%, the finished product water content less than 1%.
The slag vertical mill was promoted based on the market demand of the cement industry from energy-consuming tube mill to energy-saving vertical mill. It has been widely used in foreign countries and started relatively late in China. Steel and cement are all related to the people's livelihood. Reusing steel slag waste as a building material product is a major innovation and an environmental protection project. We can make briquette press adopting steel plant sludge. The steel slag grinding is a new building material in slag cement plant. We are no longer limited only to use Raymond grinding mill to pulverize the slag from steel plant, Raymond roller grinder is a machine that low efficiency, high cost, and appearance of slag vertical grinding mill machine, not only improve the powder grinding efficiency but also following the trend of industrial production, high grinding efficiency, low energy consumption, compact layout, convenient maintenance. Widely used in cement, electric power plant, building materials, railway and road slag, hard-to-grind materials, quartz, zirconia, sintered bauxite, mullite dry grinding , the rock materials which Mohs hardness between 5-9,the moisture is less than 10%, the finished product water content less than 1%.
Fed materials drop on the center of mill grinding bed which is rotating at a constant speed scatter at the grinding bed all round by the continuous centrifugal force to form certain thickness, meanwhile the materials are rolled and smashed by the multiple rollers, the materials gradually move to the fringe of grinding disc , when the scattering materials meet the hot air entering into grinding chamber though van and move upward to separator ,the hot air achieve sufficient heat exchange and moisture is evaporated quickly, the finished granularity is qualified will pass through the powder separator and collected by the powder concentrator then transfer to storehouse and the large granules will fall down to regrind.
Security running,welded arc board sealing, mechanical limiting position to avoid the direct metal friction and collision between roller and mill plate, low pressure hydraulic system operation, roller bearing adopts thin oil concentrated recycling lubrication decreasing the failure of leaking and vibration, it can realize idling startup, all above characters will increase or guarantee our machine can work safety and effectively prolonging the longevity of grinding machine .
Widely used in cement, electric power plant, building materials, railway and road slag, hard-to-grind materials, quartz, zirconia, sintered bauxite, mullite dry grinding, the rock materials which Mohs hardness between 5-9,the moisture is less than 10%, the finished product water content less than 1%.
Firstly, slag vertical grinding mill machine is a industrial mill , it can work in 24hours no breakdown.and its high processing yield,integrated with grinding,drying,classifying and conveying in a system. It is compact and small layout occupying ,lower power consumption,low noise ,low wear-resistance consumption ,all of these are the best advanced characters are compared with the ball mill and Raymond mill.
Generally speaking the vertical mills are Divided into coal grinding mill, limestone, lime ore or cement raw material vertical mill, slag grinding mill, limestone middle hardness materialultra fine powder grinding mill, mini slag or hard materials vertical grinding mill.so which type of mill is the one you want, please let us know your requirement below:
GRMS integrates the functions of crushing, grinding, drying, separating and conveying, with compact structure, covering an area of approximately 50% of that of ball mill system. And it can be laid outdoors, so that investment cost is greatly reduced; it is equipped with PLC/DCS automatic control system, can realize remote control and simple operation.
The horizontal grinding table and conical grinding roller are adopted to ensure the material to form a stable material bed. The energy consumption is low, saving 30% - 40% compared with the ball milling system;
The system is in the state of negative pressure, no dust overflow. Environment is clean, meeting the environmental protection requirements; through the maintenance of the oil cylinder, turning the rocker arm to replace the roller sleeve and lining plate is convenient and fast, so as to reduce downtime losses.
Grinding rollers, rocker arms and other spare parts in the whole system are manufactured by CHAENG originally, ensuring the quality and greatly reducing the customer's worries of no where to find the spare parts.
Special design of the iron removal device can make pig iron recovery rate reach 0.2-0.3%, so that the per-ton production of slag can get higher metal income, and to avoid the metal "enrichment" phenomenon, reduce the consumption of wearing parts and decrease maintenance costs.
The material through the feed tube fell into the center of grinding plate, and then the materials outwards the surrounding area of grinding plate at centrifugal force, to forms a certain thick layer of materials bed, at the same time material was crushed by number of vertical mill rollers. The materials continuous moving to the outer edge of the grinding plate, off the grinding plate materials rising with the hot air which enter from wind ring into the vertical roller mill, through the vertical roller mill shell into the middle of the separator, in this course materials and hot gas got a fully heat exchange, the water quickly evaporates. Separator controls the finished product output size, greater than the specified size are separated and fall back to the plate, while meet the fineness demands are brought through the separator into the finished product warehouse.
Project Overview Project name: Great Wall machinery slag powder demonstration and training base with annual production of 600,000t Project address: Great Wall Road, Mengzhuang Town, Huixian City, Henan Province Contracting mode: General contracting---...
Project Overview Project company: Tangshan Hongyan building material Co., Ltd Project address: Accumulation Area, Coastal Industry, Leting County, Bohai Bay. Contracting mode: General contracting---turnkey project Host configuration: GRMS 46.41,GRMS3...
Considering the dynamic influence of the roll vibration on the lubricant film thickness in the rolling deformation area, nonlinear dynamic rolling forces related to film thickness in the vertical and horizontal directions were obtained based on the Karmans balance theory. Based on these dynamic rolling forces and the mechanical vibration of the rolling mill, a verticalhorizontal coupling nonlinear vibration dynamic model was established. The amplitudefrequency equation of the main resonance was derived by using the multiple-scale method. At last, the parameters of the 1780 rolling mill were used for numerical simulation, and the time-domain response curves of the system's vibration displacement and lubricating film thickness under the steady and unsteady conditions were analyzed. The influences of parameters such as interface contact ratio, nonlinear parameters and external disturbances on the primary resonance frequency characteristics were obtained, which provided a theoretical reference for the suppression of rolling mill vibration.
Hou, Dx., Xu, L. & Shi, Pm. Verticalhorizontal coupling nonlinear vibration characteristics of rolling mill under mixed lubrication. J. Iron Steel Res. Int. 28, 574585 (2021). https://doi.org/10.1007/s42243-020-00528-4
Xiaobing Yang, Bolin Xiao, Qian Gao, "Validating the Use of Slag Binder with 91 Percent Blast Furnace Slag for Mine Backfilling", Advances in Materials Science and Engineering, vol. 2020, Article ID 2525831, 10 pages, 2020. https://doi.org/10.1155/2020/2525831
The use of ground granulated blast furnace slag (GGBFS) is environmentally sustainable and prevalent in the cement industry, but the original alkali-activated slag binder cannot be used for mine backfilling. Few reports have studied slag binders with high slag proportions (>90%) and low-cost activators (solid waste is used) that have higher performance than cement for backfilling. To increase the utilization of slag in the mining industry, this work presents a new slag binder (SB) comprised of 91% slag powder and 9% activator (3% clinker, 5% desulfurized gypsum, and 1% mirabilite). Its performance was evaluated by testing its strength, yield stress, and viscosity, which are three key properties for backfilling. We also investigated its microstructure using SEM, XRD and thermogravimetric analysis (TG/DTG). The results showed that the SB composites have a slightly lower early-age (<3d) strength but a higher long-term strength (>28d). Although the SB backfilling composites had a twofold higher yield stress and nearly the same viscosity as Portland cement, the pressure drop in a pipe was only slightly higher through friction factor modeling. The proposed SB may provide a sustainable binder for the mining industry with better performance and lower cost.
The metallurgical and mining industries produce large quantities of by-products and solid wastes such as slag, waste rock, and tailings, which are generally disposed of in landfills, which raises environmental issues. Blast furnace slag is generated during iron manufacturing, and 95% of it is comprised of four main oxides: calcium, magnesium, silicon, and aluminum . Blast furnace slag has been used for centuries in many fields. For example, some of the first-known uses of blast furnace slag were as railroad ballast, as a concrete aggregate, in bituminous surfaces, asphalt mixtures, pavement structures, unbound base courses, and embankments . Slag is most commonly used in the cement industry due to the similar chemical compositions of the two materials. It is ground to a cement-like grain size known as grounded granulated blast furnace slag (GGBFS) and then used as a cement additive and independent binder [7, 8].
A traditional cement raw material is clinker, whose production is accompanied by large quantities of greenhouse gas emissions, dust air pollution, and excavated clay soil for calcination. The replacement of GGBFS with clinker can address these issues. An academic report found that replacing 45% of ordinary Portland cement with blast furnace slag would result in a 37% reduction in total CO2 emissions . The use of a higher slag proportion in cement can decrease the total energy required for cement manufacturing . Important applications of alkali-activated slag binder and slag composite binder used in mortar and construction concrete have been reported ; however, these binders may not be applicable for mine backfilling, which is vastly different from ordinary concrete.
Mine backfill slurries are mixtures of binder, aggregate, and water that are homogeneously mixed in a surface plant and then transported to fill underground voids by gravity or pumping through pipelines . Compared with normal concrete, mine backfill has special characteristics such as follows: (i) low binder content % (mass percentage of binder/solid mass), which is determined by the geological conditions of the mine and the functionality of the filling body. It usually ranges from 2 to 8% , but in most mines in China, 1020% is used due to complex geological conditions, such as high in situ stresses and crushed wall rock. In ordinary concrete, % is more than 30%. (ii) High water/cement ratio (W/C). The W/C is<1.0 for construction concretes, while mine backfill has a larger W/C of 210 to improve flow through pipeline transport . (iii) Backfill aggregate can be composed of many different materials, such as tailings, waste rock, solid waste, river sand, and Gobi sand. Aggregate also has a large content of fine particles (<75m); for example, fine particles may account for more than 80% of tailings . Construction concrete generally has a rigid fine particle content limit of <5%. The strength of concrete significantly decreases upon increasing the fine particle content . (iv) Tap water may be used for mixing mine backfill, but processed water rich with chloride ions, metal ions, sulfate ions, and many other chemicals are generally used. These ions can have complex effects on hydration reactions .
With these unique characteristics, the slag binders originally developed for building and civil construction may no longer be suitable for mine backfill. Binders are often specially developed and should be validated for use at mine sites. For example, Olivier used four pozzolanic by-products (waste glass, copper slag, wood bottom ash, and coal fly ash) mixed with Portland cement and GGBFS to create low-cost binders for use in mine cemented paste backfill . The cement replacement level of 35%45% was validated. Jiang developed an alkali-activated slag with an activator/slag ratio of 0.3 (slag proportion 77%), and the paste backfill workability and early-age compressive strength were confirmed .
However, there are few reports of slag binder with large slag proportions (>90%) and low-cost activators (solid waste) with a higher performance than cement for mine backfill. In this study, a new slag binder made of 91% slag and 9% activator is presented and examined for mine backfill. Validation tests were performed on the strength and rheological properties (yield stress and viscosity), which are three key properties for backfill. We also investigated the microstructure using SEM, XRD, and thermogravimetric analysis (TG/DTG) to compare the material with ordinary Portland cement type I (PCI) and PCI-slag blended binder to evaluate its performance.
Three kinds of binders were used: PCI (CB, which is used industrially), 50/50 PCI-slag blinded binder (BB, which is widely used in many other mines and literature), and the proposed slag binder (SB). The slag was obtained from JISCO in Gansu Province, China, and ground into GGBFS in a local plant. Many studies have reported that the hydration of GGBFS can be activated by alkali  and sulfate . Here, a combination activation was applied, in which a small amount of clinker was used to generate an alkali environment, desulfurized gypsum (a solid waste by-product of thermal power plants or iron-steel plants) was used as a sustainable sulfate activator, and a very small amount of mirabilite was added to improve the early strength. The SB consisted of 91% GGBFS, 3% clinker, 5% desulfurized gypsum, and 1% mirabilite. The main physical and chemical properties of PCI and GGBFS are listed in Table 1. GGBFS is an acidic slag (M0<1), and the quality coefficient K>1.2 indicates that it has a good hydration property.
To validate the applicability of the SB for mine backfilling, a practical high-density backfill (HDB) slurry of a nickel mine in northwest China was used. A Gobi sand aggregate was obtained from the mining site, which was excavated from the Gobi, and then sieved and milled in a processing plant to eliminate extra-coarse particles and ensure that the maximum grain size was <5mm. The final grain size distribution of Gobi sand is shown in Figure 1, which has a fine particle (<75m) content of 16.5%.
Tap water was used as the mixing water to simulate an actual engineering situation. Although water chemistry can affect some properties of the backfill slurry , this is beyond the scope of this research, which focuses on strength and rheological properties.
The UCS is an important parameter for mine backfilling, because it determines the functionality of the filling mass body, the backfill slurry configuration, the backfilling cost, and mining profitability; therefore, it was used to evaluate the binders in this research. UCS tests were performed on specimens with three different binders for different curing times (1, 3, 7, and 28d) in accordance with the ASTM C109 standard. A microprocessor-controlled electronic universal testing machine LGS100K model was used with a loading capacity of 100kN and an accuracy of 0.5%. The loading rate was 1mm/min during the tests. Before the tests, a high-density backfill (HDB) mixture was prepared using the practical mixing proportion of binder content of %=20% and a solid mass concentration of %=78%. The binder, aggregate, and water were mixed in a mixing machine for 5 minutes and then poured into a cube triple-module with a side length of 7.07cm. The module was sealed by a plastic wrap to prevent water evaporation and then placed into a temperature-controlled chamber at 23C for the designated curing time.
Rheology is another important feature for mine backfilling, since the slurry is transported by pipeline, and the slurry flow in a pipe is dominated by its rheological properties. Thus, it is essential to evaluate the rheological properties when applying a backfill material, which is generally characterized by its yield stress and viscosity; therefore, rheology tests were performed using a Brookfield RST Soft Solids Tester (RST-SST) Rheometer and a Rheo3000 software for the HDB with different binders. The tests were conducted at room temperature using a four-blade vane spindle VT-40-20. The rheometer was set to controlled shear rate (CSR) mode, and HDB samples were sheared at a shear rate range of 0200s1. Before the tests, all materials were homogeneously mixed in a mixing machine for 5min, and then the sample was placed in a 600mL low-form Griffin beaker for measurement. During the test, samples were first sheared at a maximum rate (200s1) for 2min to simulate the shear that occurs in practice during mixing before slurries are directly dumped into a pipeline system without silencing. Then, the sample was sheared from a rate of 200s1 declining to 0s1 over 100s, with data recorded every 10s.
Microstructural analysis, including thermal analyses (thermogravimetry (TG), differential thermogravimetry (DTG)), scanning electron microscope (SEM), and X-ray diffractometry (XRD), were carried out to inspect the HDB properties. TG/DTG and XRD were conducted on the three binder paste samples, which were a mixture of binder and water with a W/C ratio of 1. The SEM samples were made from the cracked cube after UCS tests. To prepare paste samples, the binders and water were mixed using the same procedure and then sealed in a chamber for the desired curing time. Prior to tests, samples were dried in an oven at 40C until mass stabilization and then ground into powders for measurement.
Thermal analyses were carried out using an SDT-Q600 TGA from TA Instruments, which allowed the simultaneous measurement of weight loss, heat flow, and transition temperature changes. The paste samples (about 10mg) were heated in an inert nitrogen atmosphere at a rate of 10C/min up to 800C during tests.
XRD was conducted to acquire the mineralogical and chemical compositions of the HDB. It was performed using a Bruker D8 advance diffractometer. The scanning was carried out over a 2 range of 570 with a step width of 0.02 and a scanning speed of 1/min.
It can be seen that the BB-HDB has the lowest strength for all times, HDB with a blended binder, and slag binder has nearly the same minimum strength after curing for 1d and 3d. Although HDB with PCI binder has the highest early strength (<7d), the SB-HDB shows the highest strength at 7d and 28d. PCI binder has the highest early strength, and partial replacement with slag reduced the UCS. The developed slag binder has a better performance for midterm (7d) and long-term strength (28d).
BB-HDB has the lowest strength at 1d and 3d of 0.41MPa and 1.23MPa. SB-HDB has a slightly higher 3d strength of 1.33MPa, while CB-HDB has the highest 1d and 3d strength of 0.77MPa and 1.97MPa, respectively. The highest strength for CB-HDB was attributed to the rapid hydration rate of its main components (C3A, C3S, and C4AF). The hydration process begins immediately when water is added to produce hydration products, such as hydrated calcium silicate (C-S-H), calcium hydroxide (CH), and ettringite (AFt) . In contrast, slag has a slow hydration rate, and slag particles are only hydrated after an alkali activator reacts with water to produce CH . The active ingredients of the slag create a silicic-rich gel, which then reacts with CH to produce C-S-H gel. Furthermore, slag can react with sulfate activator (gypsum) to generate ettringite ; however, gypsum has a very low solubility in water, which means this hydration process will last for a long time. SEM images were collected to inspect the microstructures of CB-HDB and SB-HDB, as shown in Figure 3.
As illustrated in Figure 3, the main hydration products, such as C-S-H (clusters) and ettringite (bars), were observed in both samples after curing for 3d. CB-HDB was more developed with denser and bigger C-S-H and ettringite clusters, while SB-HDB was less developed with many unfilled voids. Consequently, SB-HDB has lower early-age strength due to the presence of more voids and less-developed hydration products. A TG/DTG analysis of binder paste was conducted for validation, as shown in Figure 4.
Figure 4 shows that there are three DTG peaks or weight loss for both binder pastes. The first weight loss occurs around 100C, which is a result of the dehydration reactions of some hydrates such as C-S-H, carboaluminates, ettringite, and gypsum . The second weight loss occurs near 400C, which is caused by the dehydroxylation of calcium hydroxide . Finally, a third peak is observed near 700C, which is attributed to the decomposition of calcite . Overall, the total weight loss for the CB paste (21.5%) is larger than that of SB (14.5%) after curing for 3days, which demonstrates that the amount of hydration products for CB is larger. Although the third peak of SB is considerably lower, which indicates a lower hydration rate, the first peak for SB is higher, which indicates that SB has a greater amount of C-S-H after curing for 3 days due to the reaction of slag with CH. The extra consumption of CH implies a lower second peak for SB, even though the second peak for both binders is relatively low.
The UCS of backfill cured for 28 days is mainly controlled by the binder content %. It is a crucial parameter for mine design, because it determines the mixing proportion and, thus the backfill cost. In this case, the 28d UCS of the ordinary CB-HDB is 5.27MPa, which is 1.05 times higher than the design requirement (here, a minimum of 5MPa is required for ground support at 28d curing, which is determined in the strength design process due to the extremely high in situ stress load, underhand cut-and-fill mining method requirement, and large portion of coarse Gobi sand used as aggregate). The UCS of BB-HDB is 5.08MPa which is 96.4% of CB-HDB, but the UCS for SB-HDB reached 5.69MPa which is 1.14 times higher than the requirement. This result implies that the combination activation SB has a higher performance than SB during long-term curing, which is beneficial for mining in both mechanical properties and cost. The higher long-term performance is attributed to the second hydration process of slag with alkali and sulfate. This produces complex products, such as ettringite (AFt), gypsum, C-S-H, hydrocalumite, and dolomite. The XRD patterns of the three binder pastes after curing for 28d are presented in Figure 5 to illustrate the difference in hydration products.
It can be seen that SB produces more complex hydration products, but Portlandite (CH) is absent because (i) SB only contains 3 wt% clinker, which generates only a small amount of CH; (ii) the 91 wt% GGBFS in SB can react with CH to generate extra C-S-H gel. Therefore, CH in the SB is completely consumed and absent in Figure 5. Reports have shown that CH does not improve the strength, but it is harmful to the interface transition zone (ITZ), which reduced the strength . In sum, the absence of CH and the extra C-S-H contribute to the higher mechanical performance of the SB. More AFt and dolomite (CaCO3) were detected for the SB due to its complex hydration reaction. Higher mechanical performance is typical of more extrusive paste backfill with ultrafine tailings . The cost of SB comprised of 91% GGBFS which can vary geographically, but in China, it is generally 100 CNY/ton cheaper than ordinary Portland cement. Besides, the binder has more environmental benefits, such as lower greenhouse gas emissions, use of natural raw materials during cement production, and iron and steel industry sustainable development.
The rheological properties of the HDB with different binder agents were evaluated and compared. Similar to most reported materials, the HDB slurry exhibited Bingham plastic flow, as shown in Figure 6. Although there are some unusual points in Figure 6, which are supposed to be caused by the collision of rotational vane spindle (length 40mm and diameter 20mm) and large aggregate particles (maximum diameter 5mm), the Bingham model fitting shows a reasonable result with an adjusted R2>0.9.
From Figure 6, the practical use of CB-HDB had the lowest dynamic yield stress of 5.90Pa, which fully meets the requirement for gravity transport. The yield stress increased to 7.1Pa when half of PCI was replaced by GGBFS in BB-HDB, and it nearly doubled to 11.50Pa for SB-HDB when 91% GGBFS was used. This finding indicates that the backfill slurry with GGBFS has a higher yield stress than without GGBFS, which can be explained by the following two mechanisms: (i) GGBFS has a larger specific surface area and fine particle content than PCI; therefore, the formed cementitious gel has a larger surface area when mixed with water, which will, in turn, capture more free water molecules. Thus, the distance between solid particles is reduced, and flocculation and interparticle attractive forces are enhanced, which increases the minimum external force required to break the microstructure (yield stress) and induce flow; (ii) GGBFS can improve the grain size distribution of the mixtures because of its finer particle size (d50=12.13m. This allows the tiny pores between aggregate particles to be filled, which reduces the porosity and produces more compact mixtures with a higher packing density, which increases the yield stress.
This finding is consistent with many other studies in the literature. References [36, 37] found that the slag grains with more edges produced high shear stresses, and a higher slag percentage produced higher normal shear stresses.
Figure 6 also shows that the viscosity only slightly changed, regardless of the binder agent. They are 0.431Pa/s, 0.455Pa/s, and 0.446Pa/s for CB-HDB, BB-HDB, and SB-HDB respectively. The apparent viscosities were nearly the same as the three HDB during the test, as shown in Figure 7. It can be observed that the apparent viscosity completely overlapped for the three HDB and remained constant at 0.4Pa/s when the shear rate exceeds 50s1. It can be concluded that the binder type does not affect the viscosity in this case.
From the above results, the binder agent only slightly changed the yield stress but had nearly no effect on the viscosity for the HDS; however, it is difficult to determine how large of a change of the viscosity will be significant. The magnitude of the rheological property changes is far from practical use when expressed in terms of yield stress and viscosity. Another widely used practical parameter is the pressure drop or friction factor, which evaluates the pressure loss of the backfill slurry when transported via pipeline. The friction factor is closely related to the yield stress, viscosity, and engineering conditions, such as the pipe diameter, flow state (laminar, turbulent, and transition flow), flow rate (velocity), Reynolds number, and slurry bulk density. Many models can be used to predict the friction factor with high accuracy [38, 39]. To better evaluate the practical applicability of the SB, the pressure drop of the three HDBs will be calculated and compared using a friction factor correlation.
Before selecting the model, the flow state of the HDB under practical conditions should be determined, which is evaluated by the Reynolds number Re as shown in equation (1):where is the slurry bulk density, kg/m3; is the flow velocity, m/s; D is the pipe diameter, m; and p is the plastic viscosity of a Bingham plastic fluid, Pa/s.
In this case, the pipe diameter was 110mm, the flow velocity in the pipe ranged from 2.0m/s to 3.0m/s, the bulk densities of the BB-HDB, CB-HDB, and SB-HDB were 1740.5, 1744.8, and 1736.08kg/m3, respectively, and the plastic viscosities are shown in Figure 8. The calculated Re ranged from 840 to 1336, which falls in the laminar flow regime (Re<2100). Furthermore, HDB behaves as a Bingham plastic fluid as shown earlier; consequently, the BuckinghamReiner friction factor correlation  was used to calculate the friction factor, as shown in equation (2):where f is the friction factor; He is the Hedstrom number and ; and B is the yield stress of a Bingham plastic fluid.
Once the friction factor is determined, the friction head loss can be determined by the DarcyWeisbach equation  using the following equation:where hf is the friction head loss, is the acceleration of gravity (m/s2), and L is the length of the pipeline (m).
According to the described procedure, the calculated pressure drops for the three HDB are shown in Figure 8. PCI has the lowest pressure drop, while the slag binder with 91% GGBFS has a slightly higher pressure drop than HDB (approximately 350Pa/m), even with the doubled yield stress and a small increase in viscosity mentioned earlier. In other words, the flowability of the SB-HDB was not much different than that of PCI.
This work reported a slag binder for mine backfilling with a large slag proportion (>90%), low-cost activator (solid waste), and higher performance than cement. Strength and rheology tests (yield stress and viscosity) as well as microstructure analysis using SEM, XRD, and TG/DTG were conducted to compare its performance with ordinary Portland cement type I (PCI) and PCI-slag blended binder.
The proposed slag binder consists of 91% GGBFS, 3% clinker, 5% desulfurized gypsum, and 1% mirabilite, and it has a lower cost and is more environmentally friendly than cement. The backfill slurry with PCI binder had a higher early strength due to the fast hydration rate of clinker, while the developed slag binder has better mechanical performance after midterm (7d) and long-term (28d) curing, which is beneficial for practical mining applications.
The rheological properties showed that adding slag to HDB produced a higher yield stress, while changing the binder type did not significantly change the viscosity; however, a more practical parameterthe pressure drop calculated from the BuckinghamReiner friction factor correlationshowed that SB-HDB was not significantly different than PCI, which indicates that SB is suitable for mine backfilling.
B. X. and X. Y. conceptualized the study; B. X. was involved in the methodology; X. Y. was responsible for the software; B. X. and X. Y. validated the study; B. X. performed the formal analysis, X. Y. investigated the study; X. Y. collected the resources; B. X. curated the data; B. X. wrote, prepared, reviewed, and edited the original draft and visualized the study; Q. G. supervised the study and was involved in the project administration and funding acquisition.
The authors are grateful for the help of Longshou Mine, Jinchuan Group Co., Ltd. for providing the aggregate, PCI, and blast furnace slag. This study was funded by the National Key Research and Development Program of China (grant no. 2017YFC0602903).
Copyright 2020 Xiaobing Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Slag particles comply with the RosinRammlerBennett distribution model.Ball-milled slag powder has better hydration activity than that of vertical-roller-milled slag powder.The homogeneity coefficient has a great influence on the particle characteristics and hydration activities.
This paper compares and studies the particle morphology, particle size distribution (PSD) characteristics, powder characteristics and hydration characteristics of ground granulated blast furnace slag prepared by a ball mill and a vertical roller mill. Both types of slag particles are found to comply with the RosinRammlerBennett (RRB) distribution model. The degree of sphericity of the blast furnace slag powder particles decreases with the particle size and is lower when produced by vertical roller milling than by ball milling; the differences in particle morphology mainly exist at the 03m particle grade. For the same specific surface area, ball-milled slag powder has a higher angle of repose, natural bulk density and tapped bulk density than does vertical-roller-milled slag powder. Both slag powders are categorized as Coulomb solids, which exhibit cohesiveness and high flow resistance. Ball-milled slag powder has better hydration activity than that of vertical-roller-milled slag powder. The difference in the homogeneity coefficients of the two powders is important to the obvious differences in the particle characteristics and hydration activities of the ball-milled and vertical-roller-milled slags.Get in Touch with Mechanic