BAUXITE TREATMENT

Abstract: Prior to the digestion of bauxite (which contains silica) in a digestion solution such as caustic soda for the production of aluminium oxide, the bauxite is mechanically activated to induce a mechanochemical reaction in the presence of a reagent such as lime. At least a portion of the silica is rendered insoluble and unreactive to the formation of sodium-aluminium-silicate desilication product (DSP) during subsequent digestion of the bauxite in the digestion solution.

Description: BAUXITE TREATMENT FIELD OF THE INVENTION The present invention relates to a method for treating bauxite which contains silica (hereafter referred to as "silica containing bauxite") as a preliminary step to the extraction of alumina from silica containing bauxite in the Bayer process. The Bayer process was first developed in 1888 and presently accounts for over 90% of the world's alumina production. The process utilises a digestion solution at elevated temperature to digest alumina in bauxite. The digestion solution is typically caustic soda but other solutions in which alumina can be dissolved may be used. For example, the digestion solution may be potassium hydroxide or arnmonium hydroxide. To digest the alumina, the caustic soda solution is typically at a temperature in the range of 150-280°C, with the temperature used being largely dependent upon the nature of the bauxite. The alumina rich liquor phase is separated from undissolved impurities by settling and alumina is recovered from the liquor phase by precipitation of aluminium hydrate crystals. The aluminium hydrate crystals are calcined to produce anhydrous aluminium oxide. The slurry of undissolved impurities resulting from digestion of the bauxite with caustic soda at elevated temperature is commonly referred to as red mud and typically comprises inert iron oxides, titanium oxides and silica compounds. Prior to discharge from the process, the red mud is typically washed with water to recover entrained caustic soda in solution. During digestion, in addition to reacting with alumina in bauxite, the caustic soda also reacts with silica which is typically present in bauxite. The consumption of caustic soda resulting from reaction with silica minerals is a world-wide problem in the alumina refining industry. In bauxite deposits which contain high concentrations of silica minerals, the caustic soda loss associated with reaction with silica can represent a significant fraction of overall alumina production costs. Silica may be present in bauxite deposits in various forms including kaolinite (A1 2 0 3 ,2Si0 2 .2H 2 0) and quartz. In general, kaolinite accounts for the majority of the reactive silica found in bauxite. During digestion, dissolved silica, alumina and sodium combine to precipitate out of solution as a sodium-aluminium-silicate desilication product (DSP) . Each tonne of silica that dissolves from bauxite consumes approximately 1.18 tonnes of caustic soda in forming DSP. The DSP is discharged from the process as a significant component (up to 40% by weight) of the red mud waste product. In addition to consuming significant quantities of soda, DSP formation also leads to significant scaling of process equipment. As a preliminary step to digestion of silica containing bauxite in a digestion solution it would therefore be desirable to treat silica containing bauxite to render silica unreactive to the formation of DSP during subsequent digestion of alumina in the digestion solution. BACKGROUND ART The consumption of caustic soda in the formation of DSP during bauxite digestion and consequential scaling problems have long been recognised. The standard approach to reducing scaling problems has been a pre- desilication process in which the bauxite is reacted with caustic soda at elevated temperature to form DSP in pre¬ treatment tanks prior to digestion. The DSP so formed is precipitated in the pre-treatment tanks and passes out of the process with the red mud. Some DSP scales the pre¬ treatment tanks necessitating frequent descaling of the pre-treatment tanks. Such pre-desilication processes only partially desilicate the bauxite and suffer from a number of disadvantages. Pre-desilication typically removes in the order of 80% of kaolin derived silica and no quartz silica. Processing times range from 6 hours to 18 hours and large quantities of caustic soda are consumed. Ongoing maintenance to de-scale the pre¬ treatment tanks is required. In an alternative approach bauxite is heated to approximately 975°C to break down kaolin to its constituent components followed by leaching with a Na 2 C0 3 solution. The solids are separated from the liquid phase and digested according to the Bayer process while the liquid phase is reacted with lime to precipitate silica. The approach has not proven commercially viable, principally because of the very large amount of energy required to heat the bauxite. Mechanical activation is a process in which mechanical energy is utilised to increase the chemical reactivity of a system. Mechanochemical reactions are induced which result in changes in chemical composition and structure as a consequence of the input of mechanical energy. US patent no. 5328501 teaches a mechanical activation process in which chemical reduction of reducible metal compounds with a reductant is mechanically activated during milling in a high energy ball mill to refine and manufacture metals and alloys. During milling, the energy imparted to the reactants through ball-reactant collision events enables the starting materials to react resulting in the reduction reaction proceeding without the need for high temperatures or melting to increase reaction rates. F. Pawleck, M.J. Kheiri and R. Kammel (1992) "The Leaching Behaviour Of Bauxite During Mechanochemical Treatment", Light Metals, pp 91-95, teaches laboratory scale mechanochemical treatment of bauxite. The paper does not teach a bauxite treatment to be performed prior to the caustic digestion phase of the Bayer process but rather mechanochemical processing of bauxite in the presence of caustic soda as an alternative to the caustic digestion phase of the Bayer process. The teaching of the paper has not found commercial acceptance. SUMMARY OF THE INVENTION The present invention provides a method for digesting silica containing bauxite for the production of aluminium oxide wherein, prior to digestion of the bauxite in a digestion solution, the bauxite is mechanically activated to induce a mechanochemical reaction in the presence of a reagent whereby at least a portion of the silica is rendered substantially insoluble and substantially unreactive to the formation of DSP during subsequent digestion of the bauxite in the digestion solution. DESCRIPTION OF PREFERRED EMBODIMENTS Any reagent which is thermodynamically capable of reacting with the silica to form a compound which does not react with a digestion solution to form DSP and is not dissolved by a digestion solution under the conditions of bauxite digestion may be used. A mixture of reagents may be used. Suitable reagents include calcium, magnesium, barium and manganese compounds. Preferred reagents include oxides and hydroxides such as CaO and Ca(0H) 2 . The digestion solution in which the bauxite is digested following mechanochemical treatment is preferably caustic soda. Preferably, the bauxite is mechanically activated in the absence of caustic soda and/or other digestion solutions to avoid the formation of DSP during mechanochemical treatment. However, it is to be expressly understood that the present invention includes a method in which caustic soda and/or other digestion solutions are present in addition to the reagent during mechanical activation. The particle size of the bauxite may be reduced by grinding or the like prior to mechanical activation of the bauxite. The method of the present invention is suitable for either batch processing or continuous processing of the silica containing bauxite. According to the present invention, the silica containing bauxite can be mechanically activated without any externally applied heat. In the absence of mechanical activation it is believed not to be possible at ambient temperature to render the silica insoluble and unreactive to the formation of DSP during subsequent digestion. Without wishing to be bound by theory, the mechanical activation of silica containing bauxite is believed to increase the reaction kinetics or increase the chemical reactivity of the silica with the result that a chemical reaction or phase change occurs which produces a new compound or phase that is insoluble and unreactive during subsequent digestion. The mechanical activation of silica containing bauxite is believed to generate localised regions of high temperature and pressure. Localised temperatures are believed to be as high as 400°C even though average temperature may only be 40°C-60°C. Preferably, the silica containing bauxite is mechanically activated in a mechanical mill. The expression mechanical mill is to be understood to include ball mills, nutating mills, tower mills, planetary mills, vibratory mills, attritor mills, gravity-dependent-type ball mills, jet mills, rod mills, high pressure roller mills and the like. By way of example, a ball mill is a vessel which contains grinding media which is kept in a state of continuous relative motion by input of mechanical energy. The grinding media is typically steel or ceramic balls. Energy is imparted to the bauxite within a ball mill by ball-bauxite-ball and ball-bauxite- mill collisions with the energy being sufficient to cause mechanical activation of the bauxite in the presence of a reagent as previously discussed. In a preferred embodiment of the present invention, mechanical activation and thermal treatment can be combined by the use of a thermally insulated high energy mill such as an attritor. With such high intensity mills power inputs of the order of 100kW/m 3 can be achieved. The thermal energy generated during milling can result in temperature elevation. Without wishing to be bound by theory, utilisation of the generated heat during milling is believed to substantially increase reaction kinetics through the combined effects of mechanical and thermal activation with the result that milling time and cost may be reduced. Accordingly, process efficiency is increased by utilising the generated thermal energy which would otherwise tend to be lost. At least preferred embodiments of the present invention are advantageous when compared with prior art preliminary treatment processes because: (l) the bauxite can be mechanically activated at ambient temperature; and (2) caustic soda is not consumed. EXAMPLES The ensuing examples are set forth for the purposes of illustration only and are not to be construed as limiting the scope of the present invention in any way. Example 1 A slurry of silica containing bauxite from Weipa, Australia containing 500 grams of solids per litre of water was loaded into a 1 litre capacity horizontal attritor mill containing 3kg of 6mm grinding balls at a charge ratio (grinding balls:bauxite g/g) of 100:1. The bauxite was pre-ground in a ring grinder to 50% passing 45 microns before being slurried with distilled water and loaded into the attritor mill. CaO was added directly to the mill just prior to milling as opposed to being pre- mixed with the bauxite slurry. m he mill was operated with a rotor speed of 600rpm for 0 minutes and following 2chanochemical treatment the slurry was washed from the mill in a manner to ensure 100% recovery of the slurry. The recovered material was dried in an oven at 60°C. The dried samples were analysed by X-ray diffraction (XRD) to identify crystalline phases present and by X-ray fluorescence (XRF) to provide a standard chemical analysis to determine the chemical composition of the dried samples. The amount of unreacted quartz in the dried samples was measured using a standard acid digestion procedure. The mechanochemically treated bauxite samples were digested with caustic soda at 250°C for 10 minutes using standard refinery conditions. After digestion the liquor and resultant red mud were separated by centrifuging. Caustic and alumina in the liquor were measured using standard thermometric analysis. The solids were given a double ammonia wash and then analysed by XRF to measure the amounts of phases present. The unreacted quartz still remaining in the samples was measured using the same standard acid digestion procedure as used for the mechanochemically treated bauxite samples. Tests were carried out on four different bauxite to lime ratios as shown in Table 1. Test No. 4 contained the largest ratio of lime to bauxite and corresponded to approximately the amount of lime theoretically required to convert all of the silica present in the bauxite to a hydrogarnet of composition Ca 3 Al 2 (Si0 4 ) (OH) 8 . In Test No. 1 sufficient lime was used to theoretically convert 26% of the silica to hydrogarnet. TABLE 1 Test No. RATIO % THEORETICAL Lime:Bauxite CONVERSION 1 1:24 26 2 1:16 39 3 1:10 62 4 1:6 104 Table 2 sets out the quartz compositions of the mechanochemically treated bauxite samples measured using the standard acid digestion method. From Table 2 it can be seen that the quartz content of the mechanochemically treated samples was reduced during milling, with the quartz content decreasing with increasing lime content. In Test No. 4, only 10% of the original quartz remained after mechanochemical treatment. TABLE 2 Test No. RATIO % MEASURED Lime:Bauxite QUARTZ untreated bauxite 0 1.20 1 1:24 0.08 2 1:16 0.24 3 1:10 0.16 4 1:6 0.12 Measurements of soda consumption during digestion are set out in Table 3. The decreased soda/silica ratios in the samples mechanically activated in the presence of lime indicate a decreased soda consumption during digestion. The soda consumption decreased with increasing lime content which is consistent with the formation of insoluble iron substituted hydrogarnet, Ca 3 AlFe(Si0 4 ) (OH) 8 , during mechanochemical treatment. TABLE 3 Test No. RATIO SODA/SILICA Lime:Bauxite RATIO untreated bauxite 0 0.68 1 1:24 0.56 2 1:16 0.49 3 1:10 0.39 4 1:6 0.28 Figure 1 illustrates XRD patterns for Test No. 3 before and after mechanical activation in the presence of CaO. Hydrogarnet peaks are clearly present in the mechanochemically treated samples. A peak identification program indicated that the hydrogarnet formed is an iron substituted hydrogarnet, Ca 3 AlFe(Si0 4 ) (OH) 8 , in which some of the alumina has been replaced with iron. This demonstrates that the lime has mechanochemically reacted with silica, alumi ium and iron oxide during milling. Alumina extraction levels of 92-96% for the mechanochemically treated samples were obtained indicating that the mechanical activation of the bauxite does not adversely affect alumina extraction during subsequent caustic digestion. Figure 2 illustrates soda consumption of the mechanochemically treated bauxite against soda consumption of untreated bauxite digested with lime. Since different bauxites and liquors were used in the two tests (Mechanochemically treated vs. Untreated) the soda consumption figures for each test were normalised to a base case, consisting of the respective bauxite sample digested with no lime, so that comparisons could be made. The mechanochemically treated bauxite samples showed a lower soda consumption than the untreated bauxite samples. Figure 2 shows that mechanical activation of bauxite with lime prior to digestion decreased soda consumption by up to 30%. Example 2 A sample of the slurry of silica containing bauxite and distilled water from Example 1 was loaded into a 5 litre capacity vertical attritor mill containing 15kg of 6mm steel grinding balls at a charge ratio (grinding balls:bauxite g/g) of 100:1. Lime was added directly to the mill at a lime to bauxite ratio of 1:6 just prior to iling as opposed to being pre-mixed with the bauxite slurry. The mill was operated with a rotor speed of 600rpm for 60 minutes and following mechanochemical treatment the slurry was washed from the mill in a manner to ensure 100% recovery of the slurry. The recovered material was dried in an oven at 60°C and the dried sample was analysed by XRD. Figure 3 illustrates XRD patterns before and after mechanical activation in the presence of CaO. Hydrogarnet peaks are clearly present in the mechanochemically treated sample. A peak identification program indicated that, as in Example 1, Ca 3 AlFe(Si0 4 ) (0H 8 ) was formed which indicates that different mechanical mills can be used in the present invention.

Claims: CLAIMS 1. A method for digesting silica containing bauxite for the production of aluminium oxide wherein, prior to digestion of the bauxite in a digestion solution, the bauxite is mechanically activated to induce a mechanochemical reaction in the presence of a reagent whereby at least a portion of the silica is rendered substantially insoluble and substantially unreactive to the formation of DSP during subsequent digestion of the bauxite in the digestion solution. 2. A method as claimed in claim 1 wherein the reagent is a calcium, magnesium, barium or manganese compound. 3. A method as claimed in claim 2 wherein the reagent is CaO or Ca(OH) 2 . 4. A method as claimed in any one of the preceding claims wherein the digestion solution is caustic soda. 5. A method as claimed in any one of the preceding claims wherein the bauxite is mechanically activated in the absence of caustic soda. 6. A method as claimed in any one of the preceding claims wherein the bauxite is mechanically activated in the absence of externally applied heat. 7. A method as claimed in any one of the preceding claims wherein the bauxite is mechanically activated in a mechanical mill. 8. A method as claimed in claim 7 wherein the mechanical mill is thermally insulated. 9. A method as claimed in any one of the preceding claims wherein the particle size of the bauxite is reduced prior to mechanical activation of the bauxite. 10. Bauxite treated by a method as claimed in any one of the preceding claims.

Inventors: MCCORMICK PAUL GERARD, PICARO TONY, MCCORMICK, PAUL, GERARD, PICARO, TONY
Applicants: MCCORMICK PAUL GERARD, PICARO TONY, QUEENSLAND ALUMINA LIMITED, UNIV WESTERN AUSTRALIA, MCCORMICK, PAUL, GERARD, PICARO, TONY, THE UNIVERSITY OF WESTERN AUSTRALIA
Application year: 1997
Main classification: C01F7
IPC codes: C01F7