The Process Recovery of Alumina from Lower Grade Bauxite - Research Paper Example

Research on the process recovery of Alumina from lower grade bauxite Bauxite is the ore from which aluminium is obtained and is almost always derived from a parent rock. At the moment there are vast reserves of bauxite in the world. There are two main categories of bauxite: high grade ores (Standard ores) with low levels of silica and low grade ores (non-standard ores) with high levels of silica. The most common method of processing bauxite is by means of the Bayer process. Processing standard ores is less expensive since there is minimal loss of caustic soda during processing while processing non-standard ores is expensive due to high silica levels which is because huge amounts of reagent are consumed. This major reagent in question is caustic soda. Reserves of standard ores are diminishing and this therefore necessitates the search of means of processing lower grade ores at a reasonable cost. In this research paper I will closely investigate and analyse existing processes of the conversion of bauxite into alumina by review of literature available especially with regard to high silica bauxites. The aim of this project is to understand the challenges behind the industries preference of standard grade bauxite ores with corresponding low silica instead of low-grade bauxite with higher silica. The principal method of extracting alumina from bauxite is the Bayer process and this project will deal only with this process exclusively or its variations thereof. Bauxite is processed to form alumina (Al2O3) from which aluminum is obtained. There are several conditions which favour the formation of bauxite and these are good amounts of rainfall, good drainage mechanism through the bauxite deposits and the presence of decaying vegetation. Experiments have been conducted with feldspar identified as the parent of bauxite where only 0.004 mmol l-1 of alumina dissolved at PH 7 and 1.35 mmol l-1 dissolved at PH 4.On the other hand silica showed solubility of 0.55 and 1.46 mmol l-1 at PH 7 and 4 respectively. These solubilities have two implications on the formation of bauxite. When the ground water is acidic in nature alumina and silica will be dissolved. When this solution arrives at a mass of water that is neutral then alumina being scarcely soluble at neutral PH will be precipitated while most of the silica will remain in solution. If the ground water is neutral silica is preferably dissolved while alumina is not and therefore leaving it is behind. Deposits of bauxite have been elevated through geological processes such that these deposits are often found on plateaus. At first the alumina was in form of a gel which over a period of time crystallised into gibbsite[Al(OH)3] which is then converted to boehmite [γ-AlO(OH)] and then diaspore [α-AlO(OH)].As a result bauxites formed during the Cenozoic, Mesozoic and the Paleozoic ages are mainly gibbsite, boehmite and diaspore in nature respectively. There are two broad categories of bauxites which are ; Lateritic or equatorial bauxite Karst bauxite The lateritic bauxite types are mainly obtained from primary aluminosilicate rocks while Karst bauxites mainly from intermingled carbonate and aluminosilicate rocks. It is important to note that lateritic bauxites formed mainly in equatorial climates makes up for about 90% of potential bauxite reserves. (Freyssinet et al., 2005). Effects of weathering and denudation have created a profile in which the aluminous material is found on top of an aluminosilicate base where Kaolinite is the main silicate mineral. Generally the aluminous minerals are to a larger extent gibbsite. Karst bauxites on the other hand are as a result of different weathering conditions compared to bauxites due to the presence of carbonates in the parent rock. Kaolinite still dominates the silicates but other minerals which are more difficult to process such as chamosite may be found. Aluminous minerals are most likely diaspore and boehmite and the differences in mineralogy being due to different levels of exposure to oxidation during weathering. Karst bauxites are predominantly found in Northern Asia and Eastern Europe. (Bardossy, 1982). These two different bauxite types influences the way in which they are processed. Lateritic bauxites are generally easier to digest compared to karst bauxites and are processed by variations of the Bayer method using less extreme conditions of temperatures, caustic concentrations and holding times. Bauxites are mainly located near the ground surface such that open pit mining is used. The major parameters that dictate the quality of the ore are the quantity of alumina, the aluminium hydroxides present and the reactive silica present. The choice of process parameters is determined by the phases of bauxite present and this is established by x-ray diffraction analysis. The other constituents of the ore such as oxides of iron and titanium do not greatly affect the processing of the ore but they do change the quality and quantity of the residue. At the start of processing bauxite is mixed with sodium hydroxide to dissolve alumina. This dissolved alumina is termed as ‘available alumina’. The sodium oxide content of the washed residue is chemically equivalent to the reactive silica. The Al2O3 available in bauxites ranges from about 30-60 percent while reactive silica ranges from about 0.5 to 13 percent. The colour also ranges widely from whitish to black interspersed with brown and red due to iron oxides being predominant. As concerns texture bauxite may either be very hard rock down to a softer earthy nature. Therefore it must be understood that bauxite varies greatly in nature and analyses must be comprehensively carried out in order for geological indications to be verified. It is also important to note that the selling price of bauxite is determined by the available AL2O3 (alumina) and the reactive silica content [Kaolinite (Al2O3.2SiO2.2H2O)].Most times the bauxite is not altered much before processing begins but some ores depending on their nature may be screen washed in order to discard fine particles which typically contain high amounts of reactive silica. As mentioned earlier the Bayer process is the chief method employed world wide for the production of alumina from bauxite. There are variations and/or modifications of the Bayer process according to the type of ore in question but basically the vital steps of the process are the same. These essential steps are: Grinding and digestion Clarification-Liquor purification Precipitation Calcination In the grinding stage the bauxite is reduced by crushers. Initially the bauxite was ground dry by a system of crushers operating in an enclosed circuit with vibrating screens but most modern plants use wet grinding mills in which a part of the solution in process is used to wet or slurry the bauxite. In the mills a combination of crushing and abrasion reduces the size of the ore. Combination of screens and cyclones are used to separate particles according to size where the oversize particles are returned for re-grinding. Silicate materials especially Kaolinite begin to dissolve in the spent liquor 3Al2Si2O5 (OH)4 + 18 NaOH_________6 Na2SiO3 + 6NaAl(OH)4 + 3 H2O ...................EQN 1 In modern mills a recent innovation is used whereby the ground slurry is stored and maintained at high-volume fraction approximately at atmospheric boiling for several hours in order to promote re-precipitation of soluble silica to desilication products or DSP. This stage is termed as pre-desilication. (Roberts,1968) At this stage it is good to clarify some terms used in the bayer process. The Bayer process is a continuous one where the solution used in the digestion of the bauxite is not discarded but is recycled through out the system. This process solution is called liquor. When it is rich in dissolved alumina it is called ‘green liquor’ (or at times ‘pregnant liquor’) and when it is depleted of alumina after precipitation it is called ‘spent liquor’. 6Na2SiO3 + 6 NaAl (OH) 4 + Na2X Na6 [Al6Si6O24].Na2X + 12 NaOH + 6 H2O......EQN 2 The equation above shows the formation of Bayer sodalite in which X represents inorganic anions with the most common being carbonate, sulphate, chloride, hydroxide and aluminate. About 80 to 90 % of reactive silica is converted into DSP during slurry storage. The remaining part is converted during the digestion stage. The digestion stage is where the slurry is mixed with hot NaOH under pressure in order to dissolve alumina. The conditions for digestion are carefully manipulated according to the distribution of the aluminous phase in the bauxite ore. For example if the ore is gibbsite then digestion is operated at temperatures of 140-155 OC. The time of digestion is governed by the kinetics of formation of DSP and not by the dissolution of gibbsite and more so by the reduction of silica which is in solution (the soluble silica).On the other hand predominantly boehmite concentrations are digested at temperatures of 220-270 0C whereby digestion time is optimised to minimize dissolution of quartz which would add reactive silica into the solution at these temperatures. Equations 1 and 2 above show the loss of NaOH from the liquor of roughly 1 mole of caustic soda per every mole of reactive silica. This loss of NaOH is the source of added expense in the economics of processing and is proportional to the content of reactive silica in the ore. Generally bauxites whose reactive silica is greater than 8% by weight are uneconomic. In this digestion stage bauxite is added at given specified proportions in order to achieve a certain desired level of alumina supersaturation. This level is selected such that the supersaturated liquor is stable through the system and also in the ensuing subsequent clarification stage. Clarification is the next major step in the process and it is where the solid residue is separated from the sodium aluminate residue. This must be as thorough as possible since any left over solids will contaminate the product. Settling chambers are used to achieve this but liquid-solid cyclones are more efficient. The sandy fraction may range from 5-50 % of the residue weight but typically with most bauxites it is less than 15 %.The sand is washed and discarded to designated disposal areas. It must be pointed out that the major objective of washing the residue is to minimize losses by recovering all soluble salts especially alumina. This residue that contains insoluble components is referred to in process terminology as ‘red mud’ and washing is achieved by use of a counter-current washing system. Liquor is returned into the circuit but it cannot recover fixed soda from DSP. The final stage in removal of any residual solids in the liquor is filtration. This is achieved by use of pressure filters following which the liquor is ready for precipitation. Precipitation is the next process. The solution leaving the filters is at temperatures of around 375K (1020C) and before precipitation the solution must be cooled to about 345K (720C). Fine alumina hydrate (Al2O3.3H2O) is added as ‘seed’ to crystallise alumina hydrate in solution. The solution that is left is the ‘spent’ liquor mentioned earlier and it is recycled back to digestion. A modification in this process is to cool the solution at an intermediate stage to increase supersaturation so increase the yield.(Tschamper,1981) . The area of the seed is of great importance and the goal is to maximise the amount of seed that is in contact with the solution at any given time. In the absence of seeds it has been observed that nuclei form very slowly while temperatures above 350K also reduce the rate of nuclei formation. (Misra & White,1971) In a nutshell the factors that affect crystallization are temperature, seed size and then quantity and distribution of seed. The final stage is calcination where the alumina hydrate crystals are washed and dried in kilns at temperatures of about 10000C to expel the water of crystallisation.This yields aluminium oxide (Al2O3) which is the raw material for aluminium smelters. As mentioned earlier the time of digestion is governed by the kinetics of the formation of DSP and not by the dissolution of gibbsite. Therefore careful control of formation DSP is vital for any processing plant since it governs the overall efficiency and profitability in production of alumina in that it is associated with reactive silica among other insoluble minerals but reactive silica content is of greater concern. At this stage it is important to look at the structure of DSP in more detail. Bayer sodalite mentioned earlier (refer to equation 2) is just one of the several mineral phases that that precipitate silica from solution and can rightly be termed as ‘desilication products’. Cancrinite is another of the desilication products (DSP) that may form hydrothermally when digestion temperatures exceed 2200C as well as hydrogarnets (silica substituted tri-calcium aluminates) which arise from dissolved lime compounds added to regulate or control liquor amounts or from the bauxite ore itself for example phosphate. Bayer sodalite has a cage structure with net three negative charge counterbalanced by three sodium atoms. When fully occupied the sodalite has a monovalent anion accompanied by sodium in each pocket or cage or either a divalent anion in remaining cages. Breaking down this cage structure is the only way of releasing soda or alternatively removing included ions through openings in the cage structure of sodalite.[smith et al.2008) Cancrinite is similar to bayer sodalite in terms of stoichiometry but has a different structure .Included ions may either be found in cage structures or in large linear channels forming part of its hexagonal structure. Due to location and size of channels in the structure of cancrinite, Calcium carbonate is the preferred compound such that the formula is (Na6 [AlSiO4]6.2CaCO3).This effectively makes cancrinite to have 25 % less sodium than sodalite. The advantage here is that a non-useful anion (calcium) is favoured compared to the valuable sodium. [(Whittington, 1996b)]. In Bayer processing of bauxite reactive silica dissolves in the caustic aluminate solution and then re-precipitates in a series of mainly sodium aluminium silicates. This form part of the discarded red mud residue. It therefore follows from the forgoing discussion that there are three main avenues or strategies to reduce soda losses in the process. 1) Reducing input of reactive silica to the process-search for a kind of pre-treatment that either separates silica from bauxite or perhaps renders silica unreactive during the digestion stages of bayer process. 2) Modify the bayer process to produce lower amounts of soda residue-this may be achieved by either minimising the fraction of silica that becomes part DSP or creating an environment that forms alternative DSPs with lower amounts of soda. 3) Recover soda from DSP-develop a mechanism of re-processing residue to extract the valuable soda and returning it to the cycle through the process liquor or extracting the soda from a concentrate that is high in DSP obtained from the residue. I shall now discuss the technologies either used previuosly or at one time considered for commercial use basing on the three strategies above.This shall include a description of the underlying method or principle, current state of application and then an assesment of the technical and economic challenges to implementation of the same.For some cases the ecological implications shall also be adressed. 1) Reducing input of reactive silica to the process Screening and washing This is effective as long as the silica is concentrated in small particles or fines. The ore is wet screened on vibrating or inclined screens. Materials lower than a certain size are rejected by wet screening which also washes the fine material(attached to larger pisolites) and is high in silica. In Queensland Australia there is Weipa bauxite that is pisolitic ion nature and the silica level is decreasing as pisolite size increases. (Roberts and Dunne, 1980). Bauxite mined is wet screened set at certain pisolite size where undersize particles are rejected. During the screening particles less than 1.7 mm are rejected while the larger are sent to stockpile. In some situations quartz may be targeted for removal. For example Amer and Abdel-Aal (1996) separated a significant amount of quartz (82 %) from bauxite in Egypt by pulverisation an screening using a 63 μm filter screen. This technology is limited to the micro-mineralogy of the bauxite so it cannot be widely adopted. For pisolitic bauxite kaolinite silica is more common on the outer layers of the pisolite and so silica content increases with increasing surface area and hence decreasing size. Surface grinding has been examined by seeking to remove a kaolin rich surface layer in Gove pisolitic bauxite. Owada et al. (2005). If quartz is targeted for removal then the effectiveness of separations depends on size distribution of that mineral and the extent to which it can be separated from valuable aluminous minerals. At Weipa disqualified undersize material is returned to the mine site. If reprocessing (i.e. grinding then screening) is required to free the target minerals then this shall be done as slurry such that the rejected material will bring about major disposal issues. Water is a limiting factor. If screening is done in arid areas then dry screening may be the only option. Dry screening is less effective than wet screening. Gravity separation This technology relies on density separation between silicate matter for example kaolinite and the valuable alumina minerals e.g. gibbsite. This technology is seldom used since the practical differences are small and even it has been considered the effects of density difference have been magnified by the use of hydrocyclones. A study on the optimisation of cyclone equipment with an aim of separating alumina minerals from silica has been done by Gao et al. (2008) Gravity separation however may have potential advantages in other areas. Rao et al. (1996) investigated the possibility of separating some titanium minerals such as rutile and ilmenite from bayer residue with some success. This is because of the high density of titanium minerals compared to sodalite and quartz and geothite. The possibility of adopting gravity separation as a means of reducing silica in lateritic bauxites are remote since the alumina and silica minerals are intimately combined such that separation is almost impossible. On the other hand non-lateritic bauxites such as the diasporic bauxites found in China consist of less intimately combined silica minerals and may be the reason for success with hydrocyclones Gao et al. (2008) Flotation In a suitable medium bauxite can be floated using some chemicals to absorb target minerals which then either cause them to move towards bubbles in a froth and become attached there or conversely use other chemicals to prevent this same process. There are two variations in flotation as indicated above. In the first variation valuable minerals are attracted to froth and subsequently separated from the silica materials to become part of the process solution containing the dissolved alumina. In the second (which is the opposite of the first) the valuable alumina minerals are depressed and are relegated to the tailings. Floating is suitable under two conditions: in diasporic ores where separation of silica minerals and alumina is easy and in the area of particle size of silicate minerals where the preferred size ranges is from about 10-200μm.Hu et al(2008) Initial studies in this area started in Russia by the work of (Andrew et al., 1973 and Ishchenko et al., 1974) but from that time onwards the bulk of this work has been conducted by researchers from China starting from 1979 by the work of Li and Chen(1979) up to now by the work of authors such as Hu et al(2008) and Wang et al(2004).Research in the west does not include the full depth of this vast work perhaps due to language barrier and translation issues and possibly because much of their work appears in patent literature. work in progress Read More