Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
  • C01F7/0613—Pretreatment of the minerals, e.g. grinding

Definitions

• 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.
• silica hereafter referred to as "silica containing bauxite”
• 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.
• the digestion solution may be potassium hydroxide or arnmonium hydroxide.
• 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.
• the caustic soda 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.
• 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.
• DSP sodium-aluminium-silicate desilication product
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Example 1 is 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 is 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 is 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 is 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.
• 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
• 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 .
• sufficient lime was used to theoretically convert 26% of the silica to hydrogarnet.
• 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
• FIG. 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%.
• 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.

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• Life Sciences & Earth Sciences (AREA)
• General Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Geochemistry & Mineralogy (AREA)
• Geology (AREA)
• Inorganic Chemistry (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

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Citations (3)

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• 1996
  • 1996-02-15 AU AUPN8082A patent/AUPN808296A0/en not_active Abandoned
• 1997
  • 1997-02-11 EP EP97902108A patent/EP0880468A4/en not_active Withdrawn
  • 1997-02-11 WO PCT/AU1997/000074 patent/WO1997029993A1/en not_active Application Discontinuation
  • 1997-02-11 BR BR9707543-4A patent/BR9707543A/pt not_active Application Discontinuation

Patent Citations (3)

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