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

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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. Effects of weathering and denudation have created a profile in which the aluminous materials are found on top of an aluminosilicate base where kaolinite is the main silicate mineral. Generally, the aluminous minerals are to a larger extent gibbsite (Freyssinet et al. 2005).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 influence 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.

Mineralogical compositions of lateritic and karst bauxites (Hill and Robson 1981)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. 

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Download file to see previous pages 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 (Bray 2008). The table below summarises the minerals of concern and their formulae. Mineral Formulae Gibbsite Al(OH)3 Boehmite AlOOH Diaspore AlOOH Kaolinite Al2O3.2SiO2.2H2O Chamosite Fe2+Mg1.5AlFe3+0.5Si3AlO12(OH)6 Quartz SiO2 Geothite FeOOH Hematite Fe2O3 Anatose /rutile TiO2 Sodalite Na6[AlSiO4]6.Na2X[where X= SO42-,CO32- or 2Cl-,2OH- Cancrinite Na6[AlSiO4].2CaCO3 Hydrogarnets Ca3Al2(SiO4)n(OH)12-4n(where n 0.6>n>0 for Bayer forms 2.0 THE BAYER PROCESS 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. ...Download file to see next pages Read More