RED MUD PROCESSING

Abstract: Soda and/or alumina values are recovered from sodium-aluminium-silicate desilication product (DSP) found in red mud formed in a Bayer process. DSP is mechanically activated to induce a mechanochemical reaction whereby soda and/or alumina values are solubilised.

Description: RED HUD PROCESSING FIELD OF THE INVENTION The present invention relates to a method for recovering soda and/or alumina values from red mud produced in the Bayer process for extracting alumina from 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. For example, the digestion solution may be potassium hydroxide or ammonium hydroxide. Throughout this specification the expression "a Bayer process" is to be understood to mean a process as described in the preceding paragraph. 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 (Al 2 θ 3 .2Si0 2 .2H 2 0) and quartz. In general, kaolinite accounts for the majority of the reactive silica found in bauxite. During processing, 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 and hence soda and alumina values are lost. It would therefore be desirable to recover soda and/or alumina values from red mud. BACKGROUND ART Various methods have been proposed for recovering DSP soda from red mud and the majority of such processes are based on the reaction of lime (CaO) with DSP at elevated temperatures. Such processes have not found wide commercial use principally because several tonnes of lime are required per tonne of soda recovered and reaction kinetics are such that a significant thermal energy input is required. US patent no. 4044095 teaches a process in which red mud is treated with concentrated caustic soda at high temperatures (in the order of 300°C) in the presence of lime. The DSP in the red mud is converted to a sodium- calcium-silicate and, in a second stage of the process, the sodium-calcium-silicate is converted to calcium- silicate releasing soda for recovery. In addition to soda, alumina is recovered from DSP in the process. US patent no. 938270 also teaches the digestion of red mud with lime at elevated temperature to recover soda from DSP. The processes of US patent nos. 4044095 and 938270 are elevated temperature processes in which the amount of lime consumed exceeds the amount of soda recovered. US 2992893 teaches a process similar to that described above which uses finely divided slaked lime and employs vigorous stirring. The thermal energy requirement of the process is demonstrated by the reaction conditions taught by the patent, namely 30 minutes at 255°C, 6 hours at 90°C and 8 hours at 75°C. AU 88102/82 teaches a process similar to that described in relation to US 4044095 which utilises a single digestion stage at lower caustic concentrations but with a higher rate of lime addition. DSP is converted to an iron substituted calcium-aluminium- silicate of the hydrogarnet type with high soda and alumina recovery. In the Mud Caustication Process (K. Solmar and J. Zoldi, "Lime in the Bayer Process, Present State and Future Trends", TMS Light Metal (1993)) lime is reacted with DSP under atmospheric conditions to liberate soda and form a hydrogarnet. The Mud Caustication Process also requires high levels of lime consumption and relatively long reaction times. As much as 2-3 tonnes of CaO can be consumed per tonne of soda recovered. No alumina is recovered in the Mud Caustication Process and the degree of soda recovery is adversely affected where soluble caustic levels in the red mud slurry exceed about 30 grams per litre. In the "Lime-Soda-Sinter" process red mud is mixed with lime and soda ash prior to being sintered at temperatures in the order of 1000°C. DSP is converted to an insoluble calcium silicate and soluble sodium aluminate which is recovered by leaching the sinter products. US patent no. 4045537 teaches a variation on this process involving the addition of a carbonaceous substance for red muds having a high iron content because iron consumes lime and soda ash. Although Lime-Soda- Sinter processes result in high recoveries of soda and alumina, their industrial application has been limited because of the cost of high temperature operation and the technical complexity of sintering so as to cause frittering or sintering of the particles without substantial fusion or melting. 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. SUMMARY OF THE INVENTION In a first aspect, the present invention provides a method for recovering soda and/or alumina values from DSP formed in a Bayer process, the method comprising mechanically activating the DSP to induce a mechanochemical reaction whereby soda and/or alumina values are solubilised. In a second aspect, the present invention provides a method for recovering alumina values from red mud formed in a Bayer process wherein the alumina values are alumina values other than alumina values derivable from DSP, the method comprising mechanically activating the red mud and recovering the alumina values. DESCRIPTION OF PREFERRED EMBODIMENTS The method according to the first aspect of the present invention is applicable to either the mechanical activation treatment of red mud or a DSP concentrate of red mud. A DSP concentrate may be formed by subjecting red mud to a separation technique, for example, a gravity separation technique or a hydrocycloning technique. During the caustic digestion of bauxite in a Bayer process it is not uncommon for red mud to contain alumina values that have neither been dissolved and separated in the liquor phase nor have been reacted in the formation of DSP. Typically, such alumina values are derived from boehmite and, where red mud is mechanically activated in accordance with the second aspect of the present invention, such alumina values are believed to be recoverable. Red mud may also contain alumina values derived from alumina which was dissolved during digestion but which precipitated from the alumina rich liquor phase prior to separation of the alumina rich liquor phase. Again, such alumina values are believed to be recoverable in accordance with the second aspect of the present invention. The particle size of the red mud/DSP may be reduced by grinding or the like prior to mechanical activation. The methods of the present invention are suitable for either batch processing or continuous processing of red mud/DSP concentrate. According to the first aspect of the present invention, DSP/red mud can be mechanically activated without any externally applied heat with soda and/or alumina values recoverable by conventional washing techniques following conversion into soluble or partially soluble forms. The solubilisation of soda and/or alumina values from DSP is believed to be thermodynamically favoured at ambient temperature but in prior art processes input of thermal energy is believed to have been required due to kinetic limitations. In the absence of mechanical activation it is believed not to be possible to solubilise soda and/or alumina values from DSP at ambient temperature within a commercially feasible period of time. Without wishing to be bound by theory, the mechanical activation of DSP is believed to increase the reaction kinetics or increase the chemical reactivity of the DSP with the result that a chemical reaction or phase change occurs which produces a new compound or phase that can be more readily processed for extraction of soda and/or alumina values. The mechanical activation of ambient temperature DSP 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 recovery of soda and/or alumina values can be enhanced by mechanically activating the DSP in the presence of a reagent. Any reagent which is thermodynamically capable of reacting with DSP to solubilise soda and/or alumina values may be used. Suitable reagents include oxides and hydroxides such as CaO, NaOH and Ca(OH) 2 . Without wishing to be bound by theory, one class of reagents are believed to undergo a cation exchange mechanism with DSP in which sodium is released and replaced with a reagent cation. For example, the reagent cation may be selected from Ca 2+ , K + , Ba 2+ and Mg 2+ . Preferably, the red mud/DSP concentrate 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 DSP within a ball mill by ball-DSP-ball and ball-DSP-mill collisions with the energy being sufficient to cause mechanical activation of the DSP. 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 first aspect of the present invention are advantageous when compared with prior art processes because (1) DSP can be mechanically activated at ambient temperature; (2) where lime is utilised as a reagent, the level of lime consumption is low; (3) processing times are relatively short; and (4) the presence of caustic soda in solution in DSP does not adversely affect recovery of soda/alumina values. 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 red mud slurry containing 470 grams of solids per litre was loaded together with 3kg of 6mm grinding balls into a 1 litre capacity horizontal attritor mill operated with a rotor speed of 600rpm. In some tests the red mud was mechanically activated in the presence of either CaO or NaOH with the mass ratio of red mud:reagent being varied in different tests. The charge ratio (grinding balls:red mud solids g/g) and milling time were varied in different tests. Following milling the slurry was washed from the mill in a manner to ensure 100% recovery of the slurry and the solids were separated from the initial wash liquor by centrifuging. The separated solids were dried in an oven at 100°C, ground with a mortar and pestle and washed with a cold 10% ammonia solution for 10 minutes followed by vacuum filtering and drying at 100°C. The washing procedure was repeated. The milled, washed and dried solids 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 solids. Chemical analyses of the initial wash liquor were carried out on selected samples. Soda recovery was calculated from the total soda content of the feed and product (milled) samples. Soda values in the product were normalised to the feed sample by the Tie element method using Ti0 2 which is unaffected by the milling process and remains constant in the red mud. The corrected soda value was calculated from: (770 2 ) Na i O {eorii =Na a q fpK ) (food) (TiOJ (pro ) The soda removed was then expressed as a percentage of the soda in the feed sample: %Soα-a Recovery - {Na >°^ ~ N *> 0 ^* 100 Similar equations were used to calculate % alumina recovery. Table l sets out the results of a series of ten tests which were carrie^ out on two samples of red mud. The first tample was 1 mud obtained from Queensland Alumina Li m ited's p ;essing plant at Gladstone, Australia and the second sample was a portion of the first sample which had been washed with a 10% ammonia solution and dried to remove any entrained liquor from the Bayer process. The dried second sample was mixed with distilled water to form a 470 grams per litre slurry. The following observations can be made from the results of the ten tests. * The milling of red mud with CaO results in high levels of soda recovery. The recovery of 66% of soda originally contained in the red mud was achieved (see Test Nos. 4 and 7) . In addition to soda, up to 18% of the alumina contained in the red mud was also recovered. * No significant differences in soda recovery were observed between the ammonia washed and unwashed red mud samples. For ammonia washed red mud, a mass ratio of red mud to CaO of 24:1 gave 66% recovery (see Test No. 7) . For unwashed red mud, 66% recovery was obtained in a sample with a mass ratio of 13.4:1 (see Test No. 4) . * Milling with CaO achieved higher levels of soda recovery. In Test No. 4, milling red mud with CaO resulted in 66% soda recovery as compared with 47% recovery obtained in Test No. 5 where the red mud was milled under identical conditions but for the absence of CaO. Similarly, in Test No. 8, 58% soda recovery was achieved as compared with 46% soda recovery in Test No. 10. Alumina recovery was also enhanced when red mud was milled with CaO. * Milling in a NaOH solution (see Test No. 3) resulted in soda recovery similar to that achieved by milling red mud with CaO which indicates that the caustic level in the red mud slurry has no significant affect on soda recovery. Removal of alumina was somewhat higher after milling with NaOH. * Reduction of the charge ratio had no significant affect on the level of soda or alumina recovery (see Test Nos. 1 and 8) . * Reduction of the milling time from 1 hour to 30 minutes using a charge ratio of 40:1 resulted in a decrease in soda removal from 58% to 48% (see Test Nos. 8 and 9) . Typical XRD measurements of (a) the second sample (ie. ammonia washed and dried slurry) and (b) a sample of the milled washed and dried solids from Test No. 7 are illustrated in Figure 1. The XRD patterns show a reduction in peak intensity and a broadening of the diffraction peaks for the milled, washed and dried solids from Test No. 7. Without wishing to be bound by theory, this is believed to be a consequence of extreme reduction in crystal size and the introduction of disorder and crystal defects during milling. The intensities of the DSP peaks relative to the Fe 2 0 3 peaks are substantially reduced during milling. Again without wishing to be bound by theory, this is believed to indicate that amorphisation of the DSP, decomposition or a reaction with other components of the red mud or CaO occurs during milling, thereby enabling soda and alumina to be recovered during subsequent washing. Analysis of the initial wash liquor collected during washing of selected samples is shown in Table 2. The initial wash liquor contains only sodium and aluminium, with the soda recovered as NaOH and Na 2 C0 3 . TABLE 2 Example 2 A red mud slurry obtained from Queensland Alumina Limited's processing plant at Gladstone, Australia containing 580 grams of solids per litre was loaded into a 5 litre capacity vertical attritor mill containing 15kg of 6mm steel grinding balls which was operated with a rotor speed of 600rpm. In some tests the red mud was mechanically activated in the presence of CaO with the mass ratio of red mud:CaO being varied in different tests. The charge ratio (grinding balls:red mud solids g/g) and milling time were varied in different tests. An identical procedure to that used in Example 1 was used for washing and drying the slurry recovered from the mill. Soda recovery and alumina recovery were calculated in the same manner as used in Example 1 and Table 3 sets out the results of a series of 10 tests. The results of Example 2 indicate that different mechanical mills can be used in the present invention.

Claims: CLAIMS 1. A method for recovering soda and/or alumina values from DSP formed in a Bayer process, the method comprising mechanically activating the DSP to induce a mechanochemical reaction whereby soda and/or alumina values are solubilised. 2. A method as claimed in claim 1 wherein red mud containing the DSP is mechanically activated. 3. A method as claimed in claim 1 wherein the DSP is a DSP concentrate formed by subjecting red mud to a separation technique. 4. A method as claimed in any one of the preceding claims wherein the DSP is mechanically activated in the presence of a reagent which is thermodynamically capable of reacting with DSP to solubilise soda and/or alumina values. 5. A method as claimed in claim 4 wherein the reagent is an oxide or hydroxide. 6. A method as claimed in claim 5 wherein the reagent is CaO, NaOH, Ca(0H) 2 or a mixture thereof. 7. A method as claimed in any one of the preceding claims wherein soda and/or alumina values solubilised by the mechanochemical treatment are recovered by a washing technique. 8. A method for recovering alumina values from red mud formed in a Bayer process wherein the alumina values are alumina values other than alumina values derivable from DSP, the method comprising mechanically activating the red mud and recovering the alumina values. 9. A method as claimed in any one of the preceding claims wherein the DSP/red mud is mechanically activated in the absence of externally applied heat. 10. A method as claimed in any one of the preceding claims wherein the DSP/red mud is mechanically activated in a mechanical mill. 11. A method as claimed in claim 10 wherein the mechanical mill is thermally insulated. 12. A method for recovering soda and/or alumina values from red mud formed in a Bayer process or from a DSP concentrate of red mud, the method comprising mechanically activating the red mud or DSP concentrate. 13. Soda and/or alumina values recovered by a method as claimed in any one of the preceding claims.

Inventors: PICARO TONY, PICARO, TONY
Applicants: PICARO TONY, QUEENSLAND ALUMINA LIMITED, PICARO, TONY
Application year: 1997
Main classification: C01F7
IPC codes: C01D1, C01F7