Lime products play a key role in the mining and extraction of non-ferrous metals, including aluminum, copper, gold, nickel, cobalt, uranium, titanium and lithium. Lime is also important for the pyrometallurgical refining and smelting of several non-ferrous metals. In addition, lime products are used in effluent and tailings treatment, and in settling, dewatering, filtration, environmental protection and mine safety.
Lhoist offers a wide range of lime products of appropriate purity and reactivity for process optimization and metal recovery. Experts in lime utilization efficiency, we bring innovation and expertise to our customers through customized services. These range from lime product reagent supply to outsourced lime-slaking units for producing milk-of-lime (slaked lime slurry) at customer sites.
Lime utilization efficiency, or consumption performance, in metallurgical processing is dependent on numerous factors other than the CaO content of the lime. This video explains the role of lime slaking and the resulting Ca(OH)2 particle size on lime utilization efficiency. For detailed scientific information on this topic follow this link:https://doi.org/10.1016/j.hydromet.2017.09.001
The mineral separation method of flotation (to produce sulfide mineral concentrates) relies on the principle that different mineral surfaces react differently at different pH levels. By using lime for pH control, differential mineral flotation behaviors can be achieved to concentrate and selectively recover the valuable base-metal sulfides (copper, nickel, cobalt, zinc and lead).
During flotation, pyrite FeS2 the most abundant but least valuable sulfide mineral must be separated from valuable base-metal sulfides, such as chalcopyrite CuFeS2. Without pyrite depression the FeS2 would contaminate and dilute the concentrate product. The use of lime facilitates this separation.
The Bayer process is the principal industrial method for refining bauxite to produce alumina (aluminum oxide). Lime, typically in the form of slaked lime (milk of lime), is used in several components of mineral processing. These include digestion, causticisation, filtration, filter-aid production (tri-calcium aluminate formation) and oxalate removal.
Cyanide is the most commonly-used lixiviant for the recovery of gold into solution, from which it can be recovered and converted into gold metal. Playing a key role in controlling the cyanidation leaching process, lime increases the pH to an effective and safe operational level.
Refractory gold ore generally contains iron sulfide (typically pyrite, FeS2), which needs to be oxidized and removed prior to cyanidation. All the various oxidation methods generate acid, requiring significant levels of lime consumption for acid neutralization, impurity precipitation and pH control (>10.5) for cyanidation. We can maximize lime utilization efficiency for these applications.
Arsenic is a significant and hazardous impurity associated with many copper and refractory gold ores. Often present in acid solutions, it must be removed into a stable solid form. The conversion of soluble arsenic into scorodite FeAsO4.2H2O is considered the most suitable treatment option. It requires the following elements:
Hydrometallurgical processing of copper/cobalt oxide ores entails dissolving the ore with acid. The resulting acidic solution requires several steps to precipitate the dissolved element impurities from solution. Lime is used for various precipitation steps in these processes.
Hydrometallurgical processing of nickel laterites (limonites or saprolites) requires acidic conditions to dissolve the nickel within the laterite ore. This acidic solution is neutralized in several steps using lime, precipitating the dissolved element impurities from solution and recovering valuable base metals (Ni, Co, Cu) as precipitate products.
During the pressure oxidation or autoclave processing of certain ore types, silver may deport to the residue as a co-precipitate in basic iron sulfates, preventing its recovery. A lime boil process can be used: at a high temperature silver-containing precipitates react with lime. This results in their conversion to iron oxides and/or hydroxides and the solubilization of silver for subsequent recovery.
TiO2 production from ilmenite FeTiO3 makes use of sulfuric acid. Limestone and lime are required for neutralizing the resulting effluent and removing dissolved metal impurities prior to re-use or discharge into the environment.
Sulfuric acid is used to leach uranium from certain ore types, often together with an oxidant such as ferric iron. Such leaching occurs either in situ, in heaps or in agitated tanks. After uranium recovery from solution (typically by solvent extraction) the residual, acidic solution bleed requires neutralization and treatment to remove the dissolved metals. Lime is used for both neutralization and precipitation.
Mineral products, such as iron ore and other mineral concentrates, need to comply with tightly-regulated transportable moisture limits that apply to bulk shipping. Too little moisture results in dusting problems. Too much produces fluidization of the material, with possibly disastrous consequences for bulk cargo ships. Lime CaO reacts with free water to form calcium hydroxide, thereby reducing the moisture content of the mineral products. Lime may also be used as a moisture control method for bulk mineral solids in climatic conditions that give rise to wet, sticky ores, causing problems during bulk handling.
Lime can modify the hydraulic properties of facilities for solution and tailings containment. Proviacal DD is a lime-based product specifically designed for hydraulic modification of dikes, dams and tailings facilities. www.proviacal.com
Odor control is important during storage, transport and handling of base metal, sulfide mineral concentrates. Flotation collectors, such as xanthates, are retained in mineral concentrates. Under certain circumstances, these organic compounds may decompose during the stockpiling, transport and handling of base metal concentrates. Decomposition products include gaseous organosulfur compounds that create significant odors. These could cause regulatory authorities to restrict the transportation of these concentrates. Using lime products in the final solid/liquid separation step of concentrate production can significantly reduce odors associated with mineral concentrates.
Milled limestone is used in underground coal mining to prevent and suppress coal dust explosions. It is typically sprayed on mine walls, where it acts as a coal dust binder, preventing the dust from becoming airborne and contributing to the risk of explosion. In addition, bags of limestone are stored at strategic locations in the mine. In the event of an explosion they also explode. The resulting limestone dust dilutes the coal dust concentration, reducing its explosive impact and combustion potential.
The EMGESAL FLUX range consists of high-quality, light metal fluxes tailored for magnesium refining. They remove non-metallic inclusions,provide melt protection and extinguish magnesium fires. Brochure Emgesal - EN
Ghana is very rich in mineral resources. Distribution of main mineral reserves is as follows: gold is about 1.75 billion ounces, which has proven 31.672 million ounces in 1994 and has been mining for over 500 years. Gold in Ghana is still available for mining more than 700 years. Diamond is about 100 million carat, which has proven 8.7285 million reserves in 1994, the fourth in the world. Bauxite is about 400 million tons, proved reserve is 18.9119 million tons in 1994; Manganese reserve is about 49 million tons, which has proven reserve is 4.8917 million tons in 1994, third in the world. In addition, there are limestone, iron ore, andalusite, quartz sand, kaolin, etc.
At the end of 2011, Northern provinces in Ghana found huge reserves of iron ore. In June 2007, the Canadian government announced that they found abundant light crude oil resources in the western province of Western Cape three zones, which preliminary proven reserves are 1.2 billion barrels and had realized the commercial oil production at the end of 2010. In 2012, crude oil production is 26.35 million barrels. Ghana's forest coverage is 34% of the total land area and the main forest concentrated in the southwest. Gold, cocoa and wood are three traditional export products, which are Ghana economic pillar. Ghana is rich in cocoa, which were one of the world's largest producers and exporter of cocoa. Cocoa production accounted for about 13% in the world.
In Ghana, Bauxite is about 400 million tons and proved reserve is 18.9119 million tons in 1994. The reserves of bauxite can be mining for hundred years.Bauxite is actually refers to the composition of main minerals that can be used in the industry. The application field of bauxite has two aspects of metal and nonmetal, which are also the main application fields. Bauxite is the best raw materials for production of aluminum and its dosage is more than 90% of the total bauxite production in the world. Dosage of bauxite in non-metallic proportion is small, but it has very extensive application. For it have widely applications in many fields, it accelerates the development of bauxite processing plant in mining industry.
In Ghana, abundant mineral resources accelerate the development of mining industry. In bauxite quarry site, there are many kinds of bauxite processing plant, such as grinding processing plant of bauxite, crushing processing plant of bauxite and so on. In a reasonable bauxite processing plant, many mining equipment are at demands, such as crusher machines, grinding mill machines, belt conveyors, vibrating feeders as well as some ancillary equipment.
According to the different hardness of minerals, crushers are different types in different processing plant, such as jaw crusher for such materials that hardness is above Mohs 7; impact crusher and cone crusher for material that hardness is under Mohs 6. According to the different required size of finished products,grinding machines have several types, such as ultrafine grinding machines, high pressure and rolling grinding machines, medium speed grinding machines and so on. If clients are not clear about all these types of machines, you can consult our customer service and we will solve your problems as soon as possible.
There are many grinding equipment suppliers of bauxite processing plant in Ghana. Shanghai SBM is one of them, which are professional manufacturers and suppliers of mining equipment in the world. It has traded with more than 170 countries in the world. All their products have been tested by relative department in the international market. They offer the whole set of service for clients, including before and after services.
Halite, the natural form of salt, is a very common and well-known mineral. In Nigeria, the halite materials are distributed in Awe, Abakaliki and Uburu areas. Salt rock is mainly distributed in Benoit state.
According to statistics, Nigeria halite reserves of 15 billion tons, but its salt, caustic soda, chlorine, sodium bicarbonate, sodium hypochlorite and chlorine peroxide products such as domestic demand more than 1 million tons. Nigeria tanneries, food factory, beverage factory, paper mill, bottlers and Oil Company import a lot of these chemical products every year.
Through drying, electrolytic preparing, you can get metallic sodium and chlorine from halite. Sodium metal in the inorganic industry can be used as a raw material for making sodium compounds, and in the metallurgical industry for reduction of titanium, zirconium compounds, such as in the oil refining industry is good desulfurizer.
The whole halite mining production line includes halite crushing stage, grinding stage and other processing stages. Crushing and grinding are the two basic stages for halite processing line. It can process the large scale halite materials into smaller size for the next production stages to get salt or others.
Halite crushing mining stage is the first one in the whole mining production line. It is the basic process in this line. Halite crushing is often divided into 2 procedures, primary quarry crushing and secondary quarry crushing stages. Sometimes, the secondary crushing stage does not need.
SBM has designed and developed halite crusher for production halite with low operating costs. Crushing roller and plates design is a key innovative element and as a result construction and maintenance of the machine is simpler and cheaper than competitive offerings.
Halite impact crusher can be widely used in the halite first and secondary crushing. Simple structure, non-key Connection, high chromium flat hammer, special impact plate, simplified crushing process, and high-efficiency and energy conservation, cubic shape for the final products. The discharge opening can be adjusted.
long service life easy to manage cheap, simple and fast maintenance of crushing roller, crushing plates made of inexpensive material and easily replaced increased productivity adjustable range of output
Halite grinding mill is the single largest user of energy within the circuit. With compact and reasonable structure, the halite grinding machines are highly automatic and can grind or polish multi slabs according to the requirements at one time. With SBM halite grinding mill, you can get the product fineness reaching D97=<5um.
Halite grinding mill has high efficiency and it is environment friendly. The roller and ring are made of special material, which improve the grinding efficiency a lot. For same material and final size, the lifetime of its wearing parts can reach to more than one year, about 2-5 times as long as that of vertical shaft impact crusher and turbo-mill. Besides, there are no rolling bearings and bolts in grinding cavity, so problems caused by bolts shedding or wear of bearings and seal components will never happen.
In halite mining processing production line, choosing the suitable machine is very important for the operators. However, the machine manufacturer who has the strength and ability can produce and manufacture the high quality machine. SBM is such a company which has more than 20 years' abundant production experience for crushing and grinding machine. These machines have been exported to the worldwide clients, such as South Africa, Nigeria, India, Philippines and so on. Choosing the right halite mining production line machine manufacturer is the equal to have success in the whole production stage.
Bauxites having 60% available alumina are relatively rare (pure gibbsiteA1(OH)3has only 65.4% available alumina), and ores having 50% available alumina are considered high quality by most alumina refiners.
Raw bauxite ores sourced from India, Guinea, Ghana, and United States were characterized and compared in terms elemental and mineral composition, fluoride adsorption affinity and capacity, surface area, and equilibrium suspension pH . Fig. 12.4 shows images of each bauxite ore as received (before milling), after milling, and with scanning electron microscopy (SEM).
Fig. 12.4. Images of raw bauxite ores shown (A) as received (before ball milling); (B) after ball milling; and (C) at 8K magnification using a scanning electron microscope (SEM). Geographical origin of bauxite ore samples as presented from left to right in all panels: United States, Guinea, Ghana, and India.
In terms of fluoride removal performance, experiments conducted in synthetic groundwater (prepared using recipes from the British Geologic Survey ) to reduce fluoride from an initial concentration of 10mgF/L to below the WHO MCL of 1.5mgF/L showed that Guinea, Ghana, and USA bauxites performed similarly, with minimum required doses of approximately 9.510.61.0g/L (Fig. 12.5A) . In contrast, India bauxite had a significantly lower performance, with a minimum required dose of 22.81.0g/L. Other studies using different levels of processing, solution matrices, and initial fluoride concentrations indicate that a variety of bauxite ores (from Malawi, Texas, Tanzania, etc.) require higher doses than the samples tested in this study, possibly due to coarser particle sizes (and hence, lower active surface area for adsorption) [31, 3441]. Research indicates that differences in chemical composition of geographically diverse bauxite ores can greatly impact their specific fluoride removal performance.
Fig. 12.5. Characterization of globally diverse bauxite ores in terms of (A) minimum bauxite doses required to remediate 10mgF/L to below the WHO MCL, (B) elemental composition as determined by X-ray fluorescence, and (C) mineralogy as determined by X-ray diffraction patterns. Unlabeled peaks in panel C represent gibbsite. In panel A, averages and standard errors associated with the fluoride probe from duplicate experiments are presented. In panel B, the measurement errors associated with the ED-XRF analysis are shown (but are barely visible in the as-printed figure).
X-ray fluorescence (XRF) data shown in Fig. 12.5B indicates that all four bauxite ores contained approximately 22%29% Al and <2% Ti. Ghana, India, and Guinea bauxites had significant and comparable fractions of Fe (~11%14%). Si was found in all bauxite ores, and its content ranged from 0.5% in Ghana bauxite to approximately 9% in US bauxite. The small differences (3.8%6.9%) in Al content between India bauxite and bauxite from the three other sites are unlikely to cause the greater than twofold difference in the minimum required dose in Fig. 12.5A, as suggested by the similar fluoride removal performance of Guinea and USA bauxites despite their 3.2% difference in Al content. Similarly, the observed differences in fluoride removal performance do not appear to be correlated with the Fe, Si, and Ti contents and phases. The Ca content in most bauxite ores was below the detection limit except in India bauxite, which contained 1.8% Ca. Fig. 12.5C shows the XRD patterns of the four bauxite ores. The main crystalline Al phase in all bauxites was gibbsite, and an additional crystalline Fe phase (hematite) was detected in Ghana, India, and Guinea bauxites. Consistent with XRF results, kaolinite was found only in USA bauxite and calcium carbonate (CaCO3) was found only in India bauxite.
The equilibrium solution pH and composition (24h after bauxite addition) are reported in Table 12.4. India bauxite had a significantly higher equilibrium pH (pH 8.10.1), compared to Guinea, Ghana, and USA bauxites (pH 6.60.1, 6.50.1, and 6.20.4 respectively), which coincided with substantially higher concentrations of Ca and inorganic carbon (3342M Ca and 3989M C, respectively, for India bauxite, compared to 3M Ca and 35M C, respectively, for the other bauxites). These results are in line with the previous XRD and XRF data (shown in Fig. 12.5B and C) and are indicative of the dissolution of CaCO3, which is present only in Indian bauxite and correlated with its lower fluoride removal performance.
Table 12.4. Characterization of a suspension (initially 35mM NaCl) in equilibrium with each bauxite ore in terms of pH, dissolved calcium, and dissolved inorganic carbon (DIC). Averages from duplicate experiments and reported errors are presented as the larger of the range from duplicate tests and measurement errors associated with the analytical equipment used (e.g., pH probe, Ion Chromatograph, and Total Carbon Analyzer)
From Cherukumilli K, Delaire C, Amrose S, Gadgil AJ. Factors governing the performance of bauxite for fluoride remediation of groundwater. Environ. Sci. Technol. 51 (4), 23212328, 2017, doi: 10.1021/acs.est.6b04601 Copyright 2018 American Chemical Society.
Aluminium is the most abundant metal found in the Earths crust (approximately 8%) and is the third most abundant element found on Earth, after oxygen and silicon. Due to its reactive behaviour, aluminium is never found as a pure metal in nature but combined with hundreds of minerals. The chief source of commercially manufactured aluminium today is bauxite. Bauxite is a reddish-brown clay-like deposit containing iron, silicates and aluminium oxides, the latter comprising the largest constituents. At present, bauxite is so plentiful that only deposits containing a content of aluminium oxides greater than 45% are selected to manufacture aluminium. Bauxite derives its name from a small French town called Les Baux, where bauxite was first discovered in 1821. Today, the largest bauxite mines are located in North America, the West Indies, Australia and Northern Europe.
Since bauxite occurs naturally at the surface of the Earths crust, mining practices tend to be straightforward. Surface pits are opened using explosives to reveal bauxite beds. The bauxite ore is then excavated and loaded into trucks or rail cars for transportation to the converting or processing centre. In order to produce commercial-grade aluminium from bauxite, essentially two processes must be employed:
9.1. Illustration of the aluminium manufacturing process encompassing the chemical extraction process (A), electrolysis (B) and alloy casting (C) operations (1. Raw material (bauxite) is processed into pure aluminium oxide (alumina) prior to its conversion to aluminium via electrolysis. This primary step is achieved through the Bayer Chemical Process. Four tonnes of bauxite are usually required in order to generate two tonnes of finished alumina which ultimately produces approximately one tonne of aluminium at the primary smelter. 2. Bauxite feed hopper. 3. Mechanical crusher employed to reduce bauxite particle size and increase surface area for chemical extraction. 4. Input chemical (sodium hydroxide). 5. Input chemical (lime). 6. Aluminium oxide is effectively released from bauxite in the presence of caustic soda solution within the primary reactor (digestion) tank. 7. The aluminium hydroxide is then precipitated from the soda solution. 8. Spent solids/ tailings discard a red mud residue generated as a byproduct of the process. 9. Precipitation tank: aluminium hydroxide is precipitated from the soda solution. The soda solution is recovered and recycled within the process. 10. Drying system (air heater system). 11. Drying system (hot air blower system). 12. Drying system (cyclone fines recovery system): post calcination, the anhydrous end-product, aluminium oxide (Al2O3), is a fine grained free flowing, white powder. 13. Input chemical (aluminium fluoride AlF3). 14. Input chemical (cryolite Na2AlF3). 15. Fuel source (e.g. coke, petroleum and pitch). 16. Molten aluminium: the reduction of alumina into liquid aluminium is operated at around 950C in a fluorinated bath under high intensity electrical current. This electrolytic process (A) takes place in cells or pots', where carbon cathodes form the bottom of the pot and act as the negative electrode. Positive electrodes (anodes) are held at the top of the pot and are consumed during the process when they react with the oxygen generated from the alumina. Two types of industrial anodes are currently in use. All potlines built since the early 1970s use the pre-bake anode technology where anodes manufactured from a mixture of petroleum coke and coal tar pitch (acting as a binder) are pre-baked in separate anode plants. In the Soderberg technology, the carbonaceous mixture is fed directly into the top part of the pot, where self-baking anodes are produced using the heat released by the electrolytic process. 17. At regular intervals, molten aluminium tapped from the pots is transported to the cast house crucible. 18. The aluminium is alloyed in holding furnaces by the addition of other metals (according to end user needs), cleaned of oxides and gases. 19. The liquid metal is then cast into ingots. These can take the form of extrusion billets, for extruded products, or rolling ingots, for rolled products, depending on the way it is to be further processed. Aluminium mould castings are produced by foundries which use this technique to manufacture shaped components.)
The processes of refining and smelting require abundant electrical power and for this reason aluminium production is frequently located in areas where cheap electricity is readily available, e.g. northern Scotland and Scandinavia, where hydro-electric power is used. It is estimated that it takes 4kg of bauxite to produce 2kg of aluminium oxide, which, with the consumption of about 8kW of electricity, produces 1kg of pure aluminium. Due to the high costs associated with aluminium manufacture, metallurgists are continually investigating new approaches to the extraction of aluminium from bauxite in an attempt to reduce overall cost and environmental impact.
Most bauxite ores contain silica contaminants in various forms. Particularly relevant to Bayer processing are forms of silica that yield soluble silica dissolution in the Bayer liquor, which are most commonly clays and quartz. The silica content of a bauxite ore that is capable of dissolution in the Bayer liquor during autoclaving is known as reactive silica. In terms of silica content, the principal defining parameter of a bauxite for Bayer processing is the reactive silica content as weight% SiO2. There is also the nonreactive silica. In low-temperature digestion (~150C), reactive silica equates to silica portion of the kaolinite content of the ore. Kaolinite is presumed to be 100% soluble in Bayer liquor, even at 150C. In high-temperature digestion (>240C), reactive silica equates to kaolinite-silica plus partial quartz-silica. In high-temperature digestion, weight% reactive silica needs to be determined by laboratory testing under digestion conditions.
Another common parameter is the bauxite A/S ratio, which is (aluminum as Al2O3)/(silicon as SiO2). This is for quantifying the silica contamination in relative terms. Obviously, the silica problem is ore related. Ores low in silica will have little reactive silica problem. Ores high in MHA-hydrated alumina (boehmite+diaspore) with high quartz are most problematic. High silica bauxites are considered to be bauxites that contain >8wt% silica. These cannot be processed by conventional Bayer processing but require a sinter process (see Section 3.8).
A significant proportion of global bauxite ore reserves have too high a silica content to be commercially viable for Bayer refining because high silica levels are costly in terms of sodium losses from the Bayer liquor. However, since bauxite ores are a finite resource globally, ways of managing silica in a commercially viable way are a focus of intense research. In future decades, as global bauxite reserves dwindle, silica will become an increasingly acute problem.
Therefore, Bayer refineries are increasingly needing to develop strategies to manage high silica bauxites. The main strategies pursued involve premineral processing to remove silicate minerals from the bauxite ore, engineering the Bayer chemistry to minimize the amount of sodium lost to the Bayer liquor through silica precipitation, and attempts to recover this lost soda from the red mud.
Most bauxite ores are not processed before refining, in terms of beneficiation by mineral processing techniques to remove undesirable impurities such as clay and silica. However, in some cases, clay and quartz may be removed by mineral processing before refining. There is a lot of benefit in beneficiation, particularly in the future as quality bauxites dwindle, and refineries will be increasingly confronted with ores of increasingly high levels of impurity, and so beneficiation of bauxites is likely to become a lot more common in the future. It is always a cost-benefit question. With high-grade ores, there is little need for beneficiation, but with low-grade ores, the cost of beneficiation may be a lot less than the refinery penalty for no beneficiation.
The main problem with silica is its consumption of sodium and thus soda loss (and some aluminum loss) as sodalite precipitation into the red mud (a production loss cost) from the Bayer liquor. Sodium recovery from the red mud has proven to be commercially unviable. This is discussed in Section 3.7. This leaves engineering the Bayer chemistry as a key focus, which will be overviewed briefly in the following sections. Silica contamination is also most undesirable in Bayer-processed alumina used to make alumina ceramics.
Because of its abundance, its high aluminum content, and its ready availability to open-cut mining, the primary commercial mineral source of aluminum for manufacturing aluminum metal is the relatively common aluminum ore bauxite. Bauxite is essentially hydrated alumina in an ore-specific (every bauxite mine is different) blend of its three common mineral forms: gibbsite Al(OH)3, boehmite -AlO(OH), and diaspore -AlO(OH), combined with many impurities, most commonly clay, silica, iron oxide, and titanium dioxide.
A bauxite is often defined by its weight% available alumina as Al2O3. Gibbsite and boehmite are the usual sources of Al2O3 in bauxite ores and diaspore to a lesser extent. The theoretical limit of Al2O3 content for bauxite is 65.4%. This would be for pure Al(OH)3. In practice, an extremely high-quality bauxite will contain, on an equivalent oxides basis, over 50% Al2O3, with a large loss on ignition (mostly, the water of hydration, also some organic content), and a few percent each of SiO2, Fe2O3, and TiO2. In practice, bauxites containing 50% alumina are considered high quality, and some are as low as 30% Al2O3. Of course there are enormous variations between mines, but in terms of bauxite quality, this is a good guideline for typical production quality bauxite. Also of importance is the relative amount of the three phases: gibbsite, boehmite, and diaspore as their dissolution characteristics in the Bayer process are quite different. By far, gibbsite is the cheapest to process.
In 1873, bauxite mining began in Villeveyrac in France. Since then, bauxite mining has grown to a huge scale. By the 1960s, world bauxite production had reached 40 million tonnes. It is now around 260 million tonnes. In 2016, Australia is the world's largest bauxite producer, producing 80 million tonnes. Australia produces about 30% of the world's total bauxite. Australia exports more than 80% of its bauxite, the rest being used primarily for the Australian aluminum industry .
How much bauxite remains on the Earth? In 2012, Australia reportedly had 6.3 billion tonnes of minable bauxite economic demonstrated resources (EDR) and 22% of the world's known estimated 28 billion tonnes of Bauxite EDR [36,37]. This is shown in Table 2.1. While 28 billion tonnes may sound a lot, at 260 million tonnes a year, the world will run out of commercially viable bauxite reserves in about 100years. It is sobering to note that China, currently the world's leading aluminum producer, will consume all of its relatively small bauxite reserves in 15years at current consumption rates, representing an export opportunity for other countries with larger reserves.
It is estimated by the USA Geological Survey  that the world's total bauxite reserves are 5575 billion tonnes, in Africa (33%), Oceania (24%), South America and Caribbean (22%), Asia (15%), and elsewhere (6%). The majority of these are not EDR bauxite, but as bauxite becomes scarce, it will drive up prices and increase the proportion of current non-EDR that becomes viable EDR. It is worth noting that at current consumption, the 5575 billion tonnes stretch out our global depletion date to 210285years into the future. This is a more comfortable horizon, but still very finite.
There are of course other less commercially viable sources of aluminum, such as clay, and potentially some even less accessible bauxite reserves, which are unviable at current world prices. However, in the past, and for the foreseeable future, bauxite extracted by large-scale open-cut mining is the industry norm, since most bauxite deposits are large in area, close to the surface, and in the order of 10m thick, which makes it a very easy ore to mine.
Many of these bauxite reserves have problems with purity, undesirable silica contamination, or low gibbsite proportion. Gibbsite is easy to refine, and it is very difficult to refine boehmite and diaspore. Europe and China have a big problem with low gibbsite proportion, making refining problematic. China has the largest reserve of diaspore bauxite in the world . China and Europe work with major refining challenges. Some Australian bauxite deposits are outstanding; some have problems with high silica content. Nonetheless, in the northern (tropical) states of Queensland and Northern Territory, Australia has some of the world's highest alumina bauxites around the 50% level. Bauxite refining issues will be discussed in Chapter 3 of this book. World economic reserves of bauxite are summarised in Table 2.2.
Usually, the quality of alumina used to make alumina ceramics needs to be of the highest grade possible, that is, chemical grade. Purity is paramount with alumina advanced ceramics. The great majority of bauxite, currently around 85% of global production, is used to make aluminum. As the world leader in bauxite production and alumina production and world number 6 in aluminum production, Australia makes a good case study. It has been active in the aluminum industry for over half a century and has 5 bauxite mines, 6 alumina refineries, and 4 aluminum smelters. In 2011, Australia produced 1.96 million tonnes of aluminum, of which about 90% was exported, creating exports of $3.8 billion, and employing 17,600 Australians in all aspects of the nationally integrated industry: bauxite extraction, alumina, and aluminum production .
It is also notable that while Australia is the world's largest bauxite producer, the world's second largest bauxite exporter, and the world's largest alumina exporter (20.5 million tonnes), it exports most of its bauxite and is only the world's sixth largest aluminum producer behind China, Russia, Canada, India, and UAE . Of the ~60 million tonnes of aluminum produced in the world per year, Australia produces 1.7 million tonnes while China produces more than half at around 31 million tonnes. China is undergoing massive expansion.
In tonnage terms, over 85% of the alumina made in the world today (more than 85 million tonnes a year) is used as feedstock for an aluminum smelter. Aluminum is electrochemically produced from alumina using the Hall-Hroult process. This involves dissolving the alumina feedstock in molten cryolite (Na3AlF6, sodium hexafluoroaluminate) in a carbon-lined pot (cathode) and then using electrolysis via an immersion anode to produce aluminum metal by carbothermic reduction. Depending on the quality of the bauxite, on average globally, 3.6t of bauxite is needed to produce 1t of aluminum metal. About 263 million tonnes of bauxite mined globally, 222 million tonnes (85%) of that used to smelt the world's global output of 60 million tonnes of aluminum. However, in some cases, it can be as little as 1.9t of bauxite per tonne of aluminum.
Aluminum smelting is a very energy-intensive process, and therefore electricity is a major input to the aluminum production process, approximately 15MWhours per tonne, accounting for between 25% and 30% of total operating costs. For example, the Australian Tomago aluminum plant, which employs 950 people at its plant 150km north of Sydney, produces 580,000t of aluminum a year, a quarter of Australia's aluminum production output, and consumes 10% of the electricity production of the State of NSW . Overall, aluminum smelting consumes approximately 13% of Australia's entire electricity consumption. There have been many books and handbooks written on aluminum smelting, and the reader is referred to these for more detail on aluminum smelting. The focus in this book is the ceramic applications of the alumina feedstock.
There are two main types of bauxite ores: lateritic (also known as equatorial) and karst. Lateritic bauxites account for about 90% of the world's EDR minable bauxite and karstic bauxites about 10% . Lateritic bauxites are found in equatorial regions of the world and are much preferable because they are easier to digest in Bayer liquor than karstic bauxites, requiring less concentrated NaOH, lower temperatures, and holding times, as discussed in Chapter 3. Karst bauxites, 10% of the world's bauxite EDR, are found mainly in Eurasia, particularly Eastern Europe and China .
The hydrated alumina polymorphs areGibbsite Al(OH)3common polymorph of aluminum hydroxideBayerite Al(OH)3less common polymorph of aluminum hydroxideNordstrandite Al(OH)3less common polymorph of aluminum hydroxideBoehmite AlOOHaluminum oxy-hydroxide (resistant to Bayer digestion)Diaspore AlOOHaluminum oxy-hydroxide (very resistant to Bayer digestion)
The sources of the red mud waste sediment from bauxite refining are as follows:Goethite (iron oxidered mud). Can be fine grained=slow sedimentation and therefore an alumina contamination risk in bauxite refiningHematite (iron oxidered mud)Maghemite (iron oxidered mud)Magnetite (iron oxidered mud)Ilmenite (iron oxide and titaniared mud)Anatase (titaniared mud). Can deposit scale in the Bayer equipmentRutile (titaniared mud). Can deposit scale in the Bayer equipmentCalcite (calciared mud)Apatite (calciared mud)Crandalite (calciared mud)
As mentioned earlier, lateritic bauxites are not only more common (90% prevalence) but also more easily digested. This is because they are primarily gibbsite with some boehmite. Karstic bauxites, on the other hand, are rare (10% prevalence) and much harder to digest because they contain primarily boehmite and diaspore, the MHA aluminum oxy-hydroxides. Gibbsite, the THA, is comparatively easy to digest in the Bayer liquor, whereas boehmite and diaspore, the MHA, are difficult to digest.
Bauxite and native corundum are the main sources of high-purity alumina. The most common refining process is the Bayer process, which yields -alumina. Various other refining processes have been developed depending on the source of raw materials (Gitzen 1970). Commercially available pure aluminas typically contain 99.599.6% Al2O3, 0.060.12% SiO2, 0.030.06% Fe2O3 and 0.040.20% Na2O, and have densities of 36503900 kgm3. It is also possible to obtain 99.9% pure alumina prepared from ammonium alum. However, for implant work, the American Society for Testing and Materials specifies only 99.5% pure alumina, and less than 0.1% of combined SiO2 and alkali oxides (mostly Na2O).
The crystal structure of -alumina is rhombohedral (a = 0.4758 nm and c = 1.299 nm) and belongs to the space group D36d. Single crystals of alumina have been used successfully to make implants (Kawahara et al. 1980). These are made by feeding fine alumina powder onto the surface of a seed crystal heated in an electric arc or oxyhydrogen flame, and then slowly withdrawing the crystal from the heat source as the fused powder builds up. Alumina crystals up to 10 cm diameter have been grown by this method.
Red mud or bauxite residue as a highly alkaline hazardous waste has been incessantly generated from the beginning of the alumina production industry from bauxite ore in the late 19th century. The worldwide inventory of red mud reached an estimated four billion tons increasing at 120150 million tons annually. This rapid growth rate clearly identifies the development and implementation of upgraded storage and remediation methods as well as large volume utilization of the waste. But safe disposal and recycling of this huge amount of alkaline waste are global issues and are considered as serious challenges to the alumina industries. This review study elucidates, firstly, red mud production process, its chemical and mineralogical nature and other characteristics, and environmental impacts, and then addresses the different methods for the utilization of red mud to maintain a sustainable environment and development.
The term bauxite mainly refers to bauxite with high vanadium content, although various vanadium grades occur. In the Bayer aluminum extraction process, some V, Fe, As, and P impurities exist as their sodium salts, which enter the aluminum solution. The vanadium is enriched in the red mud after primary filtration, up to a concentration of 0.3% V2O5. Na2CO3 is used as a roasting additive, and V2O5 is extracted by oxidizing and roasting the red mud, or via alkali leaching. During aluminum precipitation, V2O5 in the aluminum leaching solution is enriched in the liquid phase, which is then precipitated to obtain a Bayer salt sludge containing 6%20% V2O5. After leaching with water a NaVO3 solution is obtained, which is used in the common vanadium-extraction process.
Alumina powder is made from bauxite, a hydrated aluminum oxide with the formula Al(OH)3, of which there are large deposits in Australia, the Caribbean, and Africa. After crushing and purification, the bauxite is heated at 1150C to decompose it to alumina, which is then milled and sieved:
Zirconia, ZrO2, is made from the natural hydrated mineral or from zircon, a silicate. Silicon carbide and silicon nitride are made by reacting silicon with carbon or nitrogen. Although the basic chemistry is very simple, the processes are complicated by the need for careful quality control, and the goal of producing fine (<1m) powders which, almost always, lead to a better final product. These powders are then consolidated by one of a number of methods.
During the Bayer's process, bauxite is fine milled. As a consequence, the red mud generated has a very fine particle size distribution. The particle size analysis of red mud was carried out using a laser particle size analyzer (MASTERSIZER S, Malvern, UK). Figure14.2 shows the particle size distribution of red mud received from ALCOA, Australia. The characteristic particle diameter X90, X50 and X10 is 56.88, 1.57 and 0.37m. The pH of this red mud was 12.5 and specific gravity 2.88.
The chemical and mineralogical composition of red mud depends largely on the nature of bauxite ore and the extraction process. Thus there is wide variation in chemistry of red mud, as follows: Fe2O3 2060wt%, Al2O3 1030wt%, SiO2 0220wt%, TiO2 028wt%, Na2O 210wt% and CaO 28wt%. It also contains small quantities of minor elements such as V, Ga, Cr, P, Mn, Cu, Cd, Ni, Zn, Pb, Mg, Zr, Hf, Nb, U, Th, Y, K, Ba, Sr, and traces of rare earth elements (Kalkan, 2006; Singh, Upadhayay, & Prasad, 1996; Singh, Upadhayay, & Prasad, 1997; Tsakiridis etal., 2004). Table14.1 shows the chemical analysis of two red muds, NALCO red mud from India and ALCOA red mud from Australia, carried out using a combination of conventional method, atomic absorption spectroscopy and X-ray fluorescence. The NALCO red mud contains a higher amount of iron oxides, whereas the ALCOA red mud is rich in SiO2. Chemistry-wise, silica-rich red mud seems more suitable for geopolymerization and the silica-to-alumina ratio is 2:1, which is the desirable range. Thus, for further studies ALCOA red mud was used.
Like its chemistry, red mud also shows variation in its mineralogical composition. XRD analysis of ALCOA red mud has been carried out using a SIEMENS X-ray diffractometer (Model D500), with CoK radiation and Fe-filter. A scanning speed of 1/min was used and the samples were scanned from 15 to 65C. Figure14.3 shows the mineralogical composition of ALCOA red mud. The main mineralogical phases were identified as haematite (Fe2O3), Goethite (FeO(OH)) and quartz (SiO2). In addition, peaks of cancrenite (Na6Ca2Al6Si6O24(CO3)2) and sodalite (Na8(Al6Si6O24)Cl2) were found, mostly overlapping with each other. Some of the phases, such as haematite and quartz, derived from parent bauxite, whereas some of the peaks, such as sodalite and cancrenite, were formed during the processing stage.
The main raw material for alumina is bauxite (named after Les Baux-en-Provence in France), a mixture of the oxide hydrates and clays (aluminosilicates). Impurities are mostly oxides such as SiO2 and TiO2 and small amounts (ppm) of the strategic compound Ga2O3 and iron oxides which occur as weathering products of low iron and silica bedrock in tropical climatic conditions. The most common mineral constituent of bauxite is gibbsite. Detailed descriptions of bauxite mineralogy are included in a number of text books, e.g., Wells (1984) and summaries can be found in several revisions, e.g., Doremus (1984). Evolution of gibbsite with temperature has been recently studied by neutron thermodiffractometry (Rivas Mercury et al., 2006). Deposits of bauxite exist around the world; the largest ones are found in China, Guinea, Australia, Brazil, and Jamaica.
Purification of bauxite to fabricate aluminum and, to a lesser extent, alumina, is done by the Bayer process. Bauxite consumption per ton of alumina has increased in recent years (from around 2.7 tons in 2005) due to declining quality of bauxite resources and now three tons of bauxite are required to produce one ton of alumina and two tons of alumina are required for the production of one ton of primary aluminum metal. However, the energy efficiency of the Bayer process has improved over that period (International Aluminum Institute 2019, See Relevant Websites section).
Fig. 4(a) shows the geographical share of alumina production in 2018 recognized by the International Aluminum Institute; most alumina was produced in China. The major part of alumina is used for the production of aluminum whereas a small part goes to the alumina chemicals industry (Fig. 4(b)). Nevertheless, the alumina chemicals industry has become a global enterprise in which most of the worlds major aluminum firms participate. Most applications of alumina chemical products are in the field of ceramics, including refractories, cements and abrasives.
Fig. 4. Alumina production by weight in 2018. Total: 64.336 million tons (a) Geographical share. E+C: Eastern and Central; W: West; rest: non-China (b) Metallurgical (Al production) and chemical (Al2O3 production) share. http://www.world-aluminum.org/statistics/alumina-production/#data.
The Bayer process starts by dissolving crushed bauxite in sodium hydroxide under pressure at 300C to form a supersaturated solution of sodium aluminate at normal conditions of pressure and temperature. The insoluble oxides are then removed and the hydrated aluminum oxide is precipitated as gibbsite by seeding, more frequently, or as metastable bayerite by reduction of pH by carbon dioxide. The precipitated low temperature forms, -alumina, are then washed and subsequently dehydrated at 10001200C to fully convert into the stable -alumina phase. This material is named calcined alumina and typically contains 0.10.5 wt% of sodium and calcium oxides. Powders calcined at intermediate temperatures, usually called reactive aluminas, are mixtures of -Al2O3 and transition aluminas. The coarse aggregates made of large alumina single-crystals for the refractory industry (fused alumina) are obtained by fusing and casting reactive alumina powders and crushing the obtained material. Also it can be graded to be used for grinding and abrasives.
The sizes of powders required for the fabrication of high performance ceramics are in the range of microns (m) while the calcined agglomerates have sizes up to 100 m, even though the sizes of the primary crystals can be smaller than 1 m. Therefore, the calcined agglomerates have to be milled down to obtain uniform sized, small particles. A major objective of the calcination step is to obtain soft agglomerates in order to avoid the expensive and polluting process of intensive milling as much as possible. The other main characteristic of the calcined aluminas is the presence of up to 0.5 wt% Na2O as mentioned above. Low soda calcined aluminas are considered when the Na2O is lower than 0.05 wt%. Typical specifications of calcined aluminas are given by Riley (2009).
Available alumina and reactive silica define ore grade and conditions for bauxite refining. Malvern Panalytical s predictive solutions help to efficiently sort and blend bauxite, ensure optimal and profitable extraction of available alumina, and support sustainable and safe waste management (red mud).
Our expertise and solutions range from direct conveyor belt analysis to laboratory analysis and complete automated quality control. We deliver tailored analytical solutions for exploration geologists, mine planners, process engineers, laboratory and quality managers as well as geometallurgists.
Our solutions for real-time monitoring on conveyor belts (elemental and mineralogical) with its fast feedback loops, enable fast counteractions on changing bauxite composition directly in the mine and effective ore sorting.
Optimal bauxite blends with constant composition secure optimal use of caustic soda and other reagents during refining and avoid costly processing of waste material. Control of the moisture content on the conveyor belt using near-infrared (NIR) technology, together with accurate monitoring of the composition of iron ore before shipment, guaranties constant ore quality to avoids penalties.
Our laboratory solutions, tailored to the specific requirements of your mine, provide accurate and fast information whether it is benchtop or stand-alone equipment or completely automated laboratories.
Our expertise in XRF sample preparation, especially to produce high quality glass disks using fusion machines, are the basis for accurate elemental analysis according to international norms. The use of modern technologies such as X-ray diffraction (XRD) to predict available alumina and reactive silica, reduces the need for costly, time intense and unsafe wet chemistry.
Efficiency of alumina refineries, minimal use of energy per ton alumina and highest recovery rates depend on bauxite composition, impurity level of the process liquor and optimal caustic soda consumption. Our portable and flexible elemental analysers are perfectly suitable to monitor impurity levels of bauxite and process liquors directs in the refinery even in remote areas.
Particle size and distribution as well as the crystallographic modification are key variables that define quality of alumina and directly impact the rate of dissolution of alumina in the aluminium smelters. To meet the required alumina specifications our on-line particle size analyzers are designed to operate in rough process environments. Economic benefits are:
A major challenge going forward is to reconcile mans activities on earth with the key pillars of environmental stewardship, sustainability and safeguarding life forms. This applies not least in the manufacturing industries and one example from the global aluminium sector is the issue of bauxite residue otherwise termed red mud.
The vast majority of the worlds bauxite resources include valuable alumina minerals and alumino-silicate clays, which are intimately mixed. Insoluble components of the bauxite are removed by digesting the ore with very hot caustic soda (sodium hydroxide) in the Bayer process.
A waste by product of the Bayer process for producing al oxide from the bauxite ore, red mud contains toxic heavy metals and its high alkalinity makes it extremely corrosive and damaging to soil and life forms, presenting a massive problem for disposal. Toxic dumps and settling pools are a feature alongside all bauxite/alumina plants worldwide, including across Europe, Russia, China, Guinea, Brazil, Jamaica and Australia.
In 2015, annual production of smelter and chemical grade alumina was over 115 million tonnes, which, with the exception of some plants in Russia, Iran, and China, was all produced using the Bayer process. The global average of bauxite residue generated per tonne of alumina is between 1 and 1.5 tonnes; it is estimated that over 150 million tonnes of bauxite residue are produced annually 5-6 million tonnes in Europe alone, and the majority of this waste is being landfilled.
Currently there are about 80 active Bayer plants of which approximately 30 are in China. In addition, there are at least another 50 closed sites, and the combined stockpile of bauxite residue at active and legacy sites is estimated at 3000 million tonnes.
With annual world aluminium production expected to top 60,000,000-tonne barrier this year, the continuously increasing production of red mud as a by-product every year is a serious threat to the environment. For now, technology enables the residue to be stored in a controlled way, but this does not entirely solve the problem. Back in August 2016, two villages in Chinas Henan Province were covered in red mud after a waste pond dam collapsed, releasing 2 million m3 of aluminium production by products.
The toxic waste pond is located in Dahegou Village, home to around 300 people in Henans Luoyang. The lining wall broke and silt mixed with stones swept down the mountainside, submerging the whole village. Although no casualties were reported, the red mud buried many farm and domestic animals. In total, over 400 people were evacuated.
In a similar incident in Hungary in 2010, a waste pond containing red mud collapsed in the city of Ajka, killing 10 people, while 150 others sustained severe chemical burns. The toxic material also destroyed all life in a nearby river,
So, what is the answer? Red mud constitutes such a major environmental and wasted resource problem that leading aluminium companies and organisations are exploring various ways to alleviate the situation.
For Rusal, zero-waste alumina production is one of two major technological challenges for the future, along with a zero carbon-emissions electrolysis process using inert anodes. Rusals involvement in red mud treatment is one of the companys key environmental stewardship programmes. The company has for some years been working together with leading Russian universities to develop new technologies for waste processing and recycling.
As part of this initiative, the company already sells 200-250 kt/yr of red mud generated at its Nikolayev alumina refinery in Ukraine. The refinery adopted an innovative and investment-free mud preparation technology, where the material undergoes unaided separation and drying at the bauxite residue disposal area. This approach was enabled by favourable weather conditions around Nikolayev and by the coarseness of the mud.
In another project, red mud from Rusals refineries in the Urals has been mixed with various additives for ferrous metallurgy applications. The aim is to produce cast iron and dross-based products in new generation two-chamber furnaces. A prototype 500kg/hr furnace has already been built at the Moscow Institute of Steel and Alloys and is undergoing further improvements.
The Urals aluminium smelter (UAZ) has been working on extracting scandium from red mud since 2013. Specialists of the Engineering and Technology Centre (ETC) developed a unique carbonisation-based extraction technology, which does not create an adverse impact on alumina production like acid-based technologies. In 2016, an upgraded pilot unit making scandium oxide from scandium concentrate was commissioned at UAZ. Following earlier improvements, the units current annual productivity has reached 96 kg of scandium oxide. ETC continues with its efforts to reduce the cost of production and deliver a competitive product for the global market.
The scandium oxide output from the unit will be used to produce aluminium-scandium alloys at Rusals smelters. Micro additions of scandium to aluminium alloys greatly enhances properties, and having an in-house source of scandium should create a significant cost advantage for the company, Viktor Mann claims.
Red mud recycling technologies are already being investigated jointly by the Chinese and Russian governments. China has a target to recycle 10% of all of its red mud, and Chalco has already introduced several solutions to recover iron and rare earth metals from red mud waste, so the residues can be used for steel and concrete production. A recent memorandum between UC Rusal and Chalco covered joint R&D projects, including a focus on the red mud problem.
Aluminium producer Alcoa has developed a process that uses concentrated CO2 from industrial gas streams to neutralise the highly alkaline bauxite residue. The process provides direct carbonation saturation with CO2 both locking up the gas and reducing the alkalinity of the slurry to a less hazardous level. The resulting red sand is used to make cement and in road construction.
The new red mud treatment technology has been operating on trial at Alcoas Kwinana refinery for several years. The plant locks up 70,000 t/yr of CO2 and results in direct carbonation of the plants entire residue by-product, which is typically between 2 and 2.5 million dry t/yr. Alcoa says it plans to deploy the technology to nine of its alumina refineries worldwide. Application across Australia alone is estimated to store 300,000 t/yr of CO2 permanently.
In India, major producer Vedanta Aluminium Ltd has commissioned a red mud powder processing unit at its Lanjigarh refinery in Odisha, probably the first of its kind in the alumina industry addressing major environmental hazards. The unique project to produce red mud powder has been commissioned in a fully mechanised and automated plant. According to the company, the process system was developed in-house following continuous research for more than three years.
The new plant delivers advantages such as savings in caustic soda consumption by 10-15 kg per tonne of alumina, minimising land requirement by 50-60%, eliminating containment of wet red mud and associated environmental hazards. The powder can readily be utilised in cement manufacture as well as in other industries, the company claims.
Elsewhere, the focus has been on removing sodium from the residue. In the Bayer process, the alumina and the silicate clays initially dissolve in the caustic soda solution. Sodium in the caustic soda then combines with reactive silica to produce the residue red mud waste, which falls out of the solution taking the sodium with it. The loss of sodium represents a significant cost. This means that the higher the level of reactive silica in the ore, the more expensive it is to refine. Bauxite with more than about 8% reactive silica is generally considered to be uneconomic to process.
In Australia, and other regions of the world, reserves of high quality ore with low silica levels are running out. However, Australia has vast reserves of bauxite containing 8% or more reactive silica and if processing this bauxite could be made economic, mineral reserves would be boosted significantly.
National research outfit, CSIRO has developed a patented process to treat the red mud recovering the lost sodium and also realising other environmental benefits. The residue is treated with sulphuric acid to neutralise it and recover the retained sodium in a solution. This is then subjected to electro dialysis, producing sulphuric acid and sodium hydroxide that can be recycled into the Bayer process. The process is simply bolted on to the current refining process so that it requires no costly changes upstream. It can be applied to some or all of the residue even independently to reprocess red mud already stored. As well as reducing bauxite processing costs, and maximising mineral resources, another significant benefit is that the remaining red mud is less polluting and easier and safer to handle. The process has been proven technically and economically on a lab-scale, and now further trials are to be conducted at a bauxite refining plant as the next step toward commercialisation
China itself warrants separate mention here. As the greatest producer of aluminium and user of alumina, it is dedicating a considerable effort investigating and implementing reuse of bauxite residue, driven largely by national Government initiatives. China has shown the most dramatic change in the last 15 years with alumina production increasing from about 2.5 million tonnes in 2000 to almost 59 million tonnes by 2015 and the associated generation of bauxite residue has grown to some 60 million tonnes a year. The alumina manufacturing routes have historically been very different because of the nature of the indigenous bauxite. Sinter routes or combined Bayer-sinter routes were widespread but are now declining sharply, except for the production of chemical grade aluminas, and the industry has become more dependent on imported ores. This change has significantly altered the characteristics and composition of the bauxite residues produced. Previously, much of the imported bauxite was from Indonesia and Australia but curtailment of exports from Indonesia is changing the nature of the bauxite residue yet again.
Some success in recovering ferrous materials from residues has been achieved in China, particularly at plants in Southern China where the wastes can have an iron content of up to 42% that makes the recovery economically feasible. Notably, magnetic separation techniques have been employed as a first stage of processing to concentrate the iron fraction for reuse in the iron and steel industry.
In 1965, a cement plant was built to re process residue from Shandong Alumina and by the late 1990s over 6 million tonnes of residue were used to make OPC and oil well cements. However, new standards for cement were introduced that restricted sodium level content and the move to imported bauxite led to a reduction in the amount used for cement.
A very strong driving force in China has been government imposed legislation requiring bauxite residue to be reused. Since 2005 there has been a large-scale effort to make use of more of the waste in many areas with the main focus on high-intensity magnetic separation to concentrate iron; and also glass ceramics, bricks, and polymer fillers.
The manufacture of bricks, tiles, and other building materials has been shown to be technically possible by many groups working with a wide variety of bauxite residue sources, using both fired and chemically-bonded methods.
The strongly growing demand for rare earth elements, and the concentration of production in China has led to a renewed interest in the extraction of these materials from bauxite residue. The particularly strong demand for scandium is driven by its emerging use in aluminium alloys as a grain size controller to produce exceptionally high strength but light components.
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