WO2008067594A1

WO2008067594A1 - Removal of impurities from bauxite

Info

Publication number: WO2008067594A1
Application number: PCT/AU2007/001866
Authority: WO
Inventors: Steven Philip Rosenberg; Stephen Charles Grocott
Original assignee: Bhp Billiton Aluminium Australia Pty Ltd
Priority date: 2006-12-07
Filing date: 2007-12-04
Publication date: 2008-06-12
Also published as: AU2007329174A1

Abstract

Impurities are removed from bauxite by leaching a bauxite ore using a leach solution to dissolve impurities from the bauxite ore and form a pregnant leach solution containing the impurities in solution, removing the leached bauxite ore from the pregnant leach solution using a solid-liquid separation process to produce a clarified pregnant leach solution, and concentrating the clarified pregnant leach solution to recover a recycle stream including water for recycling to the leaching process. In addition, the concentrated leach solution can be subjected to an impurity removal process to recover purified leach solution for recycling to the leaching process.

Description

REMOVAL OF IMPURITIES FROM BAUXITE

Field of the Invention

The present invention relates to a process for purification of bauxite by removal of impurities upstream of the Bayer process.

Background to the Invention

Most metallurgical alumina is produced from bauxite via the Bayer process. Typically, run-of-mine bauxite is crushed and transported without further treatment to an alumina refinery where it is milled in contact with recycled caustic liquor. A portion of the aluminous minerals within the bauxite begins to dissolve under these conditions, as do a range of impurities that are also present in bauxite. The nature of these impurities is a function of the geology and mineralogy of the bauxite, but most bauxite will contain silicious minerals (such as clay minerals), organics (such as humic and fulvic acids and their degradation products) and a variety of inorganic minerals from which sulphate, carbonate and various halides go into solution in the caustic liquor used in the Bayer process. Some of the organic impurities present in the bauxite may also degrade during this process, forming carbonate, oxalate and other anions in Bayer liquors.

After milling of the bauxite, it is common practice in most alumina refineries to practice "desilication" or "slurry holding" as the first step of the Bayer process to encourage the removal of some of the dissolved silicious species by precipitation of zeolitic materials referred to as "desilication producf'or "DSP". This process consumes caustic and in some cases alumina, and indeed in many refineries is often the major individual cause of soda loss. Some of the simpler dissolved species are also removed during desilication, as they may be captured within the cage structure of the DSP. Thus, while costly, desilication is an important process in most refineries for the control of dissolved impurities.

After desilication, additional recycled caustic liquor is added and digestion of the bauxite slurry is conducted in the Bayer process at elevated temperatures and pressures to liberate the bulk of the aluminous species as soluble aluminate ions. During this process, further dissolution of impurities occurs, as well as degradation of the organic species present in the bauxite. It is common for some previously unreacted silicious species to dissolve during digestion, and precipitate as DSP. The digested slurry is cooled and the solids separated to create a clarified liquor, and a residue slurry, which is further processed via a washing circuit to recover entrained liquor. The clarified supersaturated aluminate solution is encouraged to precipitate via further cooling and seeding with recycled aluminium trihydroxide. The precipitated aluminium trihydroxide is classified using cyclones or gravity separation, and the product-sized material washed and calcined to form metallurgical alumina. Under-sized material is recycled as seed, and the "spent" liquor that remains after precipitation is recycled to the mills and digestion circuit.

The cyclic nature of the Bayer process results in the accumulation of impurities, which if not controlled, can severely impact on the productivity and product quality of the refinery. The impact of these impurities is well known to those practised in the art, and a variety of processes and techniques have been proposed for use within the Bayer process to remove impurities from one or more of the alumina refinery's liquor streams. Almost invariably, these processes require the impurity to be removed from solution in highly concentrated caustic liquors, which necessitates equipment and processes that are capable of operating in these aggressive environments. The separation of the impurities or the removal of reaction products can also result in the loss of both caustic and alumina values, either in the form of entrained process liquors during solid/liquid separation, or directly through the formation of undesirable side-products. Hence, these in-process impurity removal techniques are frequently expensive to implement, costly to operate, or both.

By contrast, relatively little has been done to remove impurities from bauxite prior to processing the ore via the Bayer process. Physical beneficiation of bauxite is sometimes practised using, for example, spirals, jigs, screens and flotation methods to remove a fraction that contains unwanted or deleterious components such as clay and other silicious minerals, or with a view to reducing the iron content to improve the grade of marginal material. Inevitably, some alumina content is lost when conducting bauxite beneficiation processes. As part of the beneficiation process, the bauxite may be subjected to washing with water. This washing process leads to the removal of water-soluble impurities from the bauxite and removal of impurities which are bound loosely to the bauxite and are thus readily liberated from the surface of the bauxite. Washing with water also helps increase the removal of finely divided minerals such as clay in the form of slime.

Washing with water is not effective in removing impurities which are weakly acidic, have a high molecular weight, are hydrophobic or that are chemically bound to or adsorbed on the bauxite. Consequently, such impurities remain within the bauxite and are only liberated when subjected to the Bayer process within an alumina refinery. It is known at a laboratory scale to wash bauxite in a mild caustic solution to remove some of the impurities present in the bauxite, however to our knowledge this procedure has not been used on an industrial scale. The very large volume of contaminated alkaline washings that would be produced when bauxite is washed with a dilute caustic solution presents a water balance issue that is not easily resolved. These washings cannot be directly re-used as the impurity content would quickly build to a level where it would actually increase the impurity content of the bauxite; storage of these washings would be prohibitively costly, but their release to the environment, even if the residual alkalinity was neutralised, is unlikely to be acceptable under current and pending environmental standards. Due to the relatively low concentration of the dissolved impurities in these alkaline washings, conventional salting-out or impurity destruction processes are likely to be ineffective or prohibitively expensive.

US Patent 4,519,989 describes a process in organic impurities are removed from bauxite by washing the bauxite using an aqueous solution containing dissolved caustic soda, sodium carbonate or mixtures thereof in an amount up to about 80 g/1 total alkali content (expressed in terms of the sodium carbonate equivalent) and filtered to separate the purified bauxite solids from the spent washing solution. A mild caustic solution is used to ensure that little or no aluminium is removed. The process is carried out at an elevated temperature in the range of about 70°C - 110°C. A rinse solution containing degradation products of the organic contaminants is then separated from the washed bauxite and treated to eliminate organic components. The spent washing solution is subjected first to wet oxidation to oxidise the extracted organic sodium salts, ostensibly to sodium carbonate, with simultaneous or subsequent addition of lime to causticise the sodium carbonate, thereby regenerating the wash solution. A principal disadvantage of the process of US Patent 4,519,989 is that any alkali consumed in the extraction of inorganic impurities that are invariably also present in the bauxite cannot be regenerated in this way, with the result that the recycled wash solution quickly becomes ineffective through the accumulation of these impurities. This rapid loss of wash solution effectiveness is compounded by the fact that conversion of the organic sodium compounds to sodium carbonate via wet oxidation is usually incomplete, except under very severe reaction conditions, with the result that organic impurities also tend to accumulate in the recycled wash solution. Ultimately, this leads to the requirement of discarding some or all of the wash solution and replacing it with fresh alkali wash solution. Finally, the volume of spent washings to be treated in this way is considerable, and thus expensive to implement and operate.

It is an object of the present invention to provide an alternative method of treating bauxite to remove impurities upstream of the Bayer process.

Summary of the Present Invention

According to one aspect of the present invention there is provided a process for the removal of impurities from bauxite comprising the steps of: a) leaching a bauxite ore using a leach solution to remove impurities from the bauxite ore and form a leached bauxite ore and pregnant leach solution containing said impurities in solution; b) removing the leached bauxite ore from the pregnant leach solution of step a) using a solid-liquid separation process to produce a clarified pregnant leach solution; and c) concentrating the clarified pregnant leach solution of step b) to produce a concentrated impurity solution and recover a purified recycle stream comprising at least water for recycling to step a).

In one form, the leach solution comprises an alkaline solution that dissolves impurities from the bauxite ore, the total amount of alkali present in the alkaline solution being sufficient to allow leaching to proceed to completion, and the leaching of step a) is conducted at a pH of at least about 9. The leaching of step a) may be conducted at a pH selected from the group consisting of at least 9.5, at least 10, at least 10.5, at least 11, at least 11.5, at least 12 and at least 12.5.

In one form the concentrating of the clarified pregnant leach solution of step c) comprises using evaporation. In another form, the concentrating of the clarified pregnant leach solution of step c) comprises using ultrafiltration. In yet another form, the concentrating of the clarified pregnant leach solution of step c) comprises using nanofiltration. The concentrating of the clarified pregnant leach solution of step c) may comprise using filtration to remove impurities from the recycled stream, wherein the impurities removed using filtration include at least one of humates, sulphates, carbonates, halides and oxalates.

In one form the concentrating of the clarified pregnant leach solution of step c) further comprises using reverse osmosis. Preferably, the concentrating of the clarified pregnant leach solution of step c) further comprises using at least one of ultrafiltration and nanofiltration prior to using reverse osmosis.

In one form, the process further comprises removing suspended solids from the clarified pregnant leach solution prior to the concentrating the clarified pregnant leach solution of step c).

In one form, the process further comprises d) subjecting the concentrated impurity solution of step c) to an impurity removal process to recover purified leach solution for recycling to step a). Step d) may be conducted using at least one of a liquor burner, a causticisation process, wet oxidation, catalytic wet oxidation, salting out, and ion exchange.

To improve leaching kinetics, the process may further comprise e) reducing the particle size of at least some of the bauxite ore prior to leaching of the bauxite ore in step a). The bauxite ore may be reduced to a particle size of no greater than about 1 millimetre prior to the leaching of the bauxite ore in step a). In one form, the recycled stream comprising water that is produced from concentrating of the clarified pregnant leach solution of step c) is combined with the bauxite ore during step e).

In one form, the process further comprises f) feeding the leached bauxite ore from step b) to an alumina refining process. Step f) may further comprise combining the leached bauxite ore from step b) with a liquor to form a slurry that is fed to the alumina refining process.

To mitigate the risk of removal of alumina values from the bauxite, the leach solution of step a) may be saturated with alumina. According to a second aspect of the present invention there is provided a system for removing impurities from bauxite, the system comprising: a leach tank configured to receive and combine a bauxite ore and a leach solution to facilitate the leaching of the bauxite ore and the formation of a pregnant leach solution in the leach tank; a solid-liquid separator oriented to receive the slurry of leached bauxite and pregnant leach solution from the leach tank, wherein the solid-liquid separator is further configured to separate leached bauxite from the pregnant leach solution so as to form a clarified pregnant leach solution; and a concentrating section oriented to receive the clarified pregnant leach solution from the solid-liquid separator, wherein the concentrating section is configured to separate and recover a recycle stream comprising water from the clarified pregnant leach solution and to recycle the recycle stream comprising water to the leach tank.

In one form, the concentrating section separates the recycle stream comprising water from the clarified pregnant leach solution by evaporation. The concentrating section may separate the recycle stream comprising water from the clarified pregnant leach solution by at least one of microfiltration, ultrafiltration, nanofiltration and reverse osmosis to remove impurities from the recycled stream, wherein the impurities removed include at least one of humates, sulphates, carbonates, halides and oxalates.

In one form, the system further comprises an impurity removal section oriented to receive a pregnant leach solution concentrated with impurities from the concentrating section, wherein the impurity removal section is configured to remove impurities from the concentrated pregnant leach solution to form a purified stream and to recycle the purified stream to the leach tank. Preferably the impurity removal section removes impurities from the concentrated pregnant leach solution to form the purified stream using at least one of a liquor burner, a causticisation process, wet oxidation, catalytic wet oxidation, salting out, and ion exchange.

In one form, the system further comprises a size reduction section configured to receive and reduce a particle size of bauxite ore prior to delivery to the leach tank. The size reduction section may reduce particle sizes of the bauxite ore to dimensions no greater than about 1 millimetre.

In one form the size reduction section receives the recycle stream comprising water from the concentrating section and combines the recycle stream with the bauxite ore for delivery to the leach tank. In one form, the system is oriented upstream from an alumina refinery and is configured to deliver the leached bauxite ore to the alumina refinery.

The system may further comprise a slurry tank oriented to receive the leached bauxite from the solid-liquid separator and combine the leached bauxite with a liquor to form a slurry for delivery to the alumina refinery.

According to a third aspect of the present invention there is provided a system for removing impurities from bauxite, the system comprising: a means for leaching a bauxite ore by receiving and combining the bauxite ore and a leach solution to form a pregnant leach solution; a means for separating leached bauxite ore from the pregnant leach solution so as to form a clarified pregnant leach solution; and a means for concentrating the clarified pregnant leach solution by separating a stream comprising water from the clarified pregnant leach solution to form a concentrated impurity solution, wherein the stream comprising water is recycled to the means for leaching.

In one form the system further comprises a means for purifying the concentrated impurity solution to form a purified stream, wherein the purified stream is recycled to the means for leaching. The system may further comprise a means for reducing particle sizes of the bauxite ore prior to delivery to the means for leaching.

The above and still further features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components. Description of the Drawings

In order to facilitate a more comprehensive understanding of the nature of the invention, examples of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic flowchart of a first embodiment of the present invention; and,

Figure 2 is a schematic flowchart of a second embodiment of the present invention.

Detailed Description of the Preferred Embodiments

Specific embodiments of the present invention are now described in detail. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention.

Throughout this specification, the term "leaching" refers to the act of extracting, removing or liberating specific constituents from a solid ore by rendering them soluble. Thus, a leach solution is a liquid that possesses the ability to leach specific constituents from the solid ore or a liquid which contains dissolved reactants that provide this capability.

The term "pregnant leach solution" refers to the leach solution after one or more specific constituents have been leached from the ore.

The term "regenerated leach solution" refers to a pregnant leach solution that has been subjected to a process to reduce the concentration of the specific constituents that have been leached from the ore to allow the leach solution to be recycled and reused in a subsequent leaching operation or within the Bayer process.

The term "impurity" in the context of the present invention refers to any species present in bauxite other than alumina values in the form of, for example, aluminium oxides, hydroxides or oxy-hydroxides. The term "concentrating" in the context of the present invention refers to a process whereby a solution is made richer in impurities, reducing the overall volume of the solution, preferably through the removal of water from the solution.

The term "liquor" is used throughout this document to refer to a solution containing aluminate (Al(OH)₄ ^') ions and hydroxyl (OH^") ions. A "Bayer liquor" is generated by digesting (dissolving) bauxite in a caustic soda solution at elevated temperatures and pressures. The principal constituents of a Bayer liquor are sodium aluminate and sodium hydroxide. All other constituents present in the liquor are considered to be "impurities", the bulk of which are present as soluble sodium salts.

The symbol "A" refers to the aluminium concentration of the liquor and more specifically to the concentration of sodium aluminate in the liquor, expressed as equivalent grams per litre (g/L) of alumina (Al₂O₃ ).

The symbol "C" refers to the caustic concentration of the liquor, this being the sum of the sodium aluminate and sodium hydroxide content of the liquor expressed as equivalent grams per litre (g/L) concentration of sodium carbonate.

The symbol "AJC" is thus the ratio of alumina concentration to caustic concentration.

The term "spent liquor" refers to a Bayer liquor stream after the gibbsite precipitation stage and prior to digestion. A spent liquor typically has a low AJC ratio. The term "green liquor" or "pregnant liquor" refers to a Bayer liquor stream after digestion and prior to precipitation. A pregnant liquor typically has a high AJC ratio.

The term "Free caustic" is the caustic concentration minus the alumina concentration (C- A) with C and A each being expressed as equivalent grams per litre (g/L) concentration of sodium carbonate.

The symbol "S" refers to the soda concentration or more specifically to the sum of "C" and the actual sodium carbonate concentration, this sum once again being expressed as the equivalent g/L concentration of sodium carbonate. Thus, soda concentration minus caustic concentration (C-S) gives the actual concentration of sodium carbonate (Na₂CO₃) in the liquor, in grams per litre (g/L).

The symbol "TOC" refers to "total organic carbon" which includes acetate, oxalate, and high molecular weight organics all present as sodium salts (Na.org), expressed as grams/litre.

The chemical symbol Na₂SO₄ refers to sodium sulphate.

The chemical symbol Na₂C₂O₄ refers to sodium oxalate.

The term "P90" refers to the size of a mesh screen which allows 90 percent of the particles to pass through.

Example 1

The flow chart of Figure 1 illustrates one embodiment of the process for the removal of impurities from bauxite. Bauxite 10 is fed to leach tank 20, in which the bauxite is subjected to leaching using an alkali solution 16 added to the leach tank 20 to dissolve impurities from the bauxite ore and form a pregnant leach solution in which said impurities are present in solution. In the process, a slurry 24 is formed. The slurry in the leach tank 20 is maintained at a level of pH of at least about 9 as described in greater detail below. For example, depending on the type of ore and the type and amount of impurities to be removed, the pH during the step of leaching may be at least 9.5, or at least 10, or at least 10.5, or at least 11, or at least 11.5, or at least 12, or at least 12.5 in various embodiments of the present invention.

Whilst a leach tank 20 is illustrated in Figure 1, it is to be understood that leaching need not be conducted in a tank or other vessel, but could equally be conducted using a heap leaching process, or within a pipeline.

The temperature of the slurry 24 within the leach tank 20 can vary widely, ranging from ambient up to the boiling point of the leach solution. For economic reasons, the temperature of the slurry 24 in leach tank 20 is held in the range of ambient temperature to 60° Celsius with ambient temperature or temperature of the recycled stream 62 or temperature of the alkali solution 16 being preferred, as this reduces the energy requirements of the process. The leach tank 20 is provided with a suitable agitator 22 or other mixing equipment used to encourage mixing of the bauxite 10 with the alkali solution 16 to improve leaching kinetics.

Many of the impurities present in bauxite ores are weakly acidic species, such as humic, fulvic and other organic acids, which cannot be removed by washing with neutral water. Without wishing to be bound by theory, many of these species are rendered soluble by chemical reaction with the alkaline solution to form the equivalent alkali cation salt, and by ensuring that the pH remains in excess of the pKa of the various acidic species (in practise, a pH often of about 9 or greater). It is also understood that some impurities may undergo alkaline degradation, forming simpler, more soluble products. It is further understood that, when the bauxite is leached at a pH of at least about 9, some anionic impurities adsorbed at the surface of the bauxite particles are released into solution due to a change in the surface charge of the bauxite. It is thus important in this embodiment that both the total amount of alkali present is sufficient to allow the leaching reactions to go to completion, and that the pH is maintained at 9 or above during leaching.

The pH in the leach tank 20 can be raised by direct treatment of the slurry 24 by addition of concentrated alkali solution 18, or by adjusting the pH of the alkali leach solution 16 being added to the leach tank 20. Alternatively or additionally, the mass ratio of the alkali leach solution 16 to bauxite 10 may be raised, in which case the pH of the alkali leach solution 16 being added to the leach tank 20 should be higher than 9 to counteract the effects of neutralization of the alkaline leach solution due to the presence of acidic impurities in the bauxite.

The alkali leach solution 16 added to the leach tank 20 may be any suitable liquid capable of leaching impurities from the bauxite. However, in the first preferred embodiment of the present invention, the alkali leach solution 16 used to form the slurry 24 with the bauxite 10 is a sodium hydroxide (caustic) solution. To mitigate the risk of alumina values from the bauxite ore being taken into solution during leaching of the bauxite, it is advantageous to use a caustic solution that is already saturated or nearly saturated with alumina, for example process water from an alumina refinery or refinery process lake water or similar dilute aqueous solution derived from the Bayer process. The same result can be achieved using alkaline leach solutions other than caustic solutions, for example, using an alkaline leach solution containing sodium carbonate, ammonium hydroxide or potassium hydroxide, or mixtures thereof.

The residence time in the leach tank 20 is about one hour, however this will vary depending on such relevant factors as the type of bauxite, the bauxite particle size distribution and the pH of the slurry 24, and additional residence time may be required to achieve the desired extraction of impurities. Whilst only one leach tank 20 is illustrated in Figure 1, a plurality of leach tanks or other means could equally be used or arranged in series or parallel to achieve a required holding time and level of leaching of the slurry. Supplementary dosing of the alkali leach solution 16 may be required for each of the vessels if the vessels are arranged in series, due to on-going reaction of the impurities with the leach solution.

As the impurities are leached from the bauxite 10 in the leach tank 20, the slurry 24 comprises leached bauxite and a leach solution containing impurities that have gone into solution or been entrained therein as suspended solids. The slurry 24 is then directed to a solid-liquid separation section or process 30 to separate a stream of clarified pregnant leach solution 36 from the leached bauxite 32. Whilst a drum filter 30 is depicted in Figure 1, it is to be understood that the separation of the leached bauxite 32 and clarified pregnant leach solution 36 from the slurry 24 may be any suitable device or process known to those skilled in the arts including gravity thickening, pressure filtration, centrifugation and the like.

To reduce the residual caustic content of the leached bauxite 32 and further decrease the impurity content, washing of the leached bauxite 32 within the solid-liquid separation process 30 may be performed using wash water 34. The wash water 34 may be from any convenient source, ranging from fresh water to the alkali leach solution 16.

Leached bauxite 32 may be stockpiled or transported as required, but in the example illustrated in Figure 1, the leached bauxite 32 is directed from the solid-liquid separation device 30 to a bauxite re-slurry tank 40, to which refinery spent liquor 42 (also referred to in the art as "liquor-to-mills" or "liquor-to-digestion") is added to form an alumina refinery feed slurry 46. The alumina refinery feed slurry 46 may be directed to any suitable point in the alumina refinery, such as the desilicators or mills. Agitator 44 is used in the bauxite re- slurry tank 40 to assist in forming the alumina refinery feed slurry 46 and for maintaining the leached bauxite 32 in suspension.

The leached bauxite in the alumina refinery feed slurry 46 has an organic content that has been reduced by up to 20% or more favourably up to 40% or still more favourably up to 80% relative to the normal level of contaminants fed to an alumina refinery when the bauxite is not subjected to the process of the present invention. Since some oxalate normally forms in the Bayer process as a result of reactions of the organics present in the bauxite after it has entered the refinery, as a further consequence, the amount of oxalate that is otherwise formed in the refinery is also reduced. Moreover, adsorbed oxalate is removed from the bauxite during the leaching process. As a result, the oxalate input into the alumina refinery is reduced by up to approximately 20% or more favourably up to 60% or still more favourably up to 90%. Depending on the bauxite, the sulphate content can be reduced up to 50% or more favourably up to 100% compared to the normal run-of-mine bauxite fed to an alumina refinery.

The liquid underflow from the solid-liquid separation process 30 is a stream of clarified pregnant leach solution 36 which is laden with the impurities that have been leached from the bauxite. Depending on the particular impurity species present in a given bauxite ore, the clarified pregnant leach solution 36 contains several grams per litre of TOC as well as a number of other impurities, with for example between 40 to 80% of the extractable organic carbon in the bauxite 10 reporting to the clarified pregnant leach solution 36. Left untreated, the clarified pregnant leach solution 36 would quickly reach a high concentration of impurities and would be unsuitable for re-use as an alkali leach solution 16. To reduce the total demand for water of the process, and to avoid the storage or discharge to the environment of the clarified pregnant leach solution 36, the clarified pregnant leach solution 36 is treated to remove the impurities and regenerate the alkali leach solution 16. With reference to Figure 1, the clarified pregnant leach solution 36 is pumped to a polishing section or process 50 in which residual fine suspended solids are removed. The polishing process 50 comprises a combination of filtration and/or settling apparatus arranged to reduce the level of the residual suspended solids to less than 50 mg/L, preferably less than 10 mg/L or more preferably less than 2 mg/L.

The polished pregnant leach solution 52 is fed to an impurity concentration section or process 60 to generate a recyclable purified regenerated leach solution 62 and a concentrated impurity solution 64. The impurity concentration process 60 could consist of an evaporator and condenser to generate the concentrated impurity solution 64 and the recyclable regenerated leach solution 62 respectively. However, in a preferred embodiment, the impurity concentration process 60 is a membrane filtration system using membrane filtration processes such as microfiltration, ultrafiltration, nanofiltration or reverse osmosis, alone or in combination. The type of filtration process selected depends in part on the molecular weight cut-off (MWCO) of the impurities to be removed from the polished leach solution 52. Using membrane filtration, hydrostatic pressure is used to force the polished leach solution against a semipermeable membrane. A retentate (or reject) stream corresponding to concentrated impurity solution 64 is retained on one side of the semipermeable membrane whilst a substantially clean permeate stream, corresponding to recyclable regenerated leach solution 62 passes through the membrane.

Whilst ultrafiltration is effective for reducing the concentration of high molecular weight organic species such as humates (in excess of approximately 1000 Daltons) within the polished leach solution 52, nanofiltration is preferred in order to remove smaller sized organic and inorganic impurities. Using nanofiltration, the molecular weight cutoff is between 100 and 1000 Daltons, such that rejection of the organic impurities such as humates, as well as some larger inorganic impurities dissolved in the polished leach solution 52, to the retentate stream 64 is very high. The retentate stream 64 from the concentrating section 60 is highly concentrated with respect to humic materials and other impurities such as various organic salts, and also inorganic salts including sulphate, carbonate and oxalate. Between 30% and 90% of the liquid from the polished leach solution 52 will report to the permeate 62, the remainder reporting to the retentate 64. Conversely, between 30% to 95% of the impurities dissolved in the polished leach solution 52 will report to the retentate 64, with the remainder reporting to the permeate 62. The resulting enrichment of the retentate stream 64 is therefore up to five times or more favourably up to ten times or still more favourably up to fifty times. To increase the rejection of low molecular weight impurities reporting to the retentate stream, reverse osmosis may be used, in which the molecular weight cutoff is reduced to less than 100 Daltons. It is advantageous to use at least one of ultrafiltration or nanofiltration as a precursor to reverse osmosis, thus improving the performance and efficiency of the reverse osmosis membranes and reducing the rate at which the reverse osmosis membranes foul or otherwise degrade.

Residual caustic in the polished leach solution 52 will report to both the retentate 64 and the permeate 62. The caustic which reports to the permeate 62 is directly recovered for reuse by recycling the permeate 62 as a portion of the alkali leach solution 16 added to the leach tank 20. Some fresh alkali leach solution 16 will be required to compensate for the volume of caustic reporting to the retentate 64, and to replace caustic consumed during the leaching process. Depending upon the caustic concentration in retentate 64 and the alkali leach solution 16, some additional caustic may be required, which can be provided in the form of a concentrated alkali solution 18 added to the leach tank 20.

The retentate stream 64 contains some residual caustic as well as a high concentration of the sodium salts of the various impurities that have been leached from the bauxite. During the concentration process 60, the volume of liquid in the retentate solution 64 has been greatly reduced compared with the volume of the clarified pregnant leach solution 36 which was separated from the leached bauxite 32 using solid/liquid separation process 30.

Thus the retentate stream 64 is amenable to storage and further volume reduction in, for example, an evaporation pond or evaporator (not shown). However, in a preferred embodiment, the retentate stream 64 is directed to an impurity destruction section or process 70. The caustic and alumina concentration of the retentate stream 64 is low, and the impurity concentration ratio to caustic and alumina concentrations of the retentate stream 64 is relatively high (compared with Bayer process liquors). The impurity destruction process 70 is conducted using a liquor burner, wet oxidation, catalytic wet oxidation, an ion exchange process, a salting out process, the organic impurity removal process described in US Patent 6,555,077, the sulphate/oxalate removal process described in US Patent 6,743,403 or the causticisation process described in US Patent 6,743,403, either singly or in combination. The contents of US Patent 6,555,077 and US Patent 6,743,403 are incorporated herein by reference in their entireties.

The products of the impurity destruction process 70 are a regenerated caustic stream 72 and a by-product stream 74 which may be either discarded or further processed for use elsewhere. When the recovered caustic stream 72 is recycled for use in the leach tank 20, this further reduces the caustic requirements for alkali leach solution 16 and concentrated alkali solution 18.

Test Results

Samples of a South American bauxite, pre-ground to a P90 size of 212μm, were leached with alkaline leaching solutions containing sodium hydroxide, at initial concentrations of O.lmol/L and 1.0 mol/L respectively. The bauxite was mixed with the leaching solutions to produce slurries containing 25-30% by weight of bauxite solids. The slurries were leached in a leach tank in the form of a closed stainless steel vessel equipped with a motor- driven coaxial agitator operating at approximately 500 rpm, with the vessel placed in a thermostatically controlled water bath set at 6O⁰C. Leaching was conducted with a residence time of three hours.

Following leaching, a slurry comprising the leached bauxite and the pregnant leach solution containing impurities was subjected to solid/liquid separation using a centrifuge at 4000 rpm for 35 minutes to produce a leached bauxite and a clarified pregnant leach solution. Samples of the clarified pregnant leach solution were retained for analysis and for subsequent production of the clean recycle stream and concentrated leach solution. The leached bauxite solids were subsequently re-washed with deionised water and air dried at 8O⁰C prior to analysis to determine the residual impurity content. A combination of the leached bauxite solids and clarified pregnant leach solution analyses were used to determine the leaching efficiency of several major impurities from the bauxite. Results of these tests are summarised in Table 1 below:

Table 1

N/A = not available

From the data in Table 1 it may be seen that, for this sample of bauxite, the extractable organic carbon (EOC) content was reduced by up to more than 90%, while the extractable sulphate content was reduced by approximately 70%.

A similar test was performed using a Western Australian bauxite. Results of these tests are shown in Table 2 below.

Table 2

From the data in Table 2 it may be seen that, for this sample of bauxite, the extractable organic carbon (EOC) content was reduced by 36%, while the extractable sulphate content was reduced by approximately 67%.

Separation of the pregnant leach solution from leached Western Australian bauxite was conducted using pressure filtration as the solid/liquid separation process. Tests were performed in a 64mm pressure cell fitted with a disc of woven cloth filter material, with a squeeze cycle pressure of between 10 and 15 Bar, achieving cake moistures of between 17 and 20 weight percent. The use of evaporation as the concentration process to produce a regenerated leach solution and a concentrated impurity solution rich in impurities was demonstrated using process water (lake water) from a Western Australian refinery. 7.5 litres of clarified pregnant leach solution was treated using a low temperature evaporation process to produce 6.5 litres of pure water for use as one component of the regenerated leach solution and approximately 1 litre of a concentrated leach solution amenable to treatment using known impurity removal processes . The results of this step are shown in Table 3 below:

Table 3

N/A = Not Applicable. Concentrations for the Concentrate are calculated values.

The use of a membrane filtration system as the concentrating process to produce a regenerated leach solution and a concentrated impurity solution was demonstrated using the pregnant leach liquor prepared from South American bauxite and 0.1M sodium hydroxide, as well as with process water (also known as lake water to those skilled in the art) from a Western Australian refinery. Membrane filtration was performed as a batch operation using a commercially available 5 cm diameter, 35 cm long spiral wound nanofiltration element, with a feed pressure of 210OkPa. Permeate was removed as the test proceeded, and analysed to determine the separation mass balance across the membrane. For the pregnant leach solution from South American bauxite, retentate was returned to the feed to the membrane filtration process and reprocessed to give a 66% recovery of feed to permeate, while the process water was operated to give 70% recovery of feed to permeate. Results for the South American bauxite and Western Australian process water are shown in Tables 4 and 5 below, respectively. Table 4

N/A = Not available

These results indicate that 84% of the TOC leached from the bauxite is reporting to the Concentrated Impurity Solution. The corresponding results for Na₂SO₄ and Na₂C₂O₄ are 55% and 58% respectively.

Table 5

These results indicate that 67% of the TOC leached from the bauxite is reporting to the Concentrated Impurity Solution. The corresponding results for Na₂SO₄ and Na₂C₂O₄ are 39% and 42% respectively.

To produce the regenerated caustic stream 72, a number of impurity removal processes could be applied. The calculated result of applying the processes described in US 6,743,403 using the Concentrated Impurity Solution produced by evaporation (Table 3 above) is shown in Table 6 below. Table 6

This solution 72 may be returned to the leach tank 20, or alternatively can be directed to the alumina refinery for use within the Bayer process.

Example 2

A second embodiment of the present invention is now described with reference to Figure 2 for which like reference numerals refer to like parts. The flowchart of Figure 2 is the same as that for Figure 1 apart from the addition of a size reduction section or process 12 in which part or all of the bauxite 10 is subjected to comminution in the presence of a liquid. The size reduction process 12 comprises a combination of milling and/or grinding apparatus arranged to reduce the size of the bauxite 10 to improve leaching yield and kinetics.

In one preferred embodiment of the present invention, the bauxite 10 is ground to a P90 of about 1 millimetre where the term "P90" refers to the size of a mesh screen which allows 90 percent of the particles to pass through. It is worth noting that grinding of the bauxite to a P90 of about 1 millimetre would not normally be undertaken prior to transport of bauxite to an alumina refinery, as it is traditionally considered easier to transport coarse material than material that has been finely ground. By way of another example, the bauxite 10 is ground to a P90 of about 300 micrometres to about 400 micrometres and leach tank 22 can be located adjacent to a mine with the leached bauxite slurry 46 being transported via a pipeline or other slurry transfer apparatus to a location adjacent to an alumina refinery. Alternatively, the size reduction process 12 can be conducted at a location adjacent to a mine, with the ground or milled bauxite stream 14 being pumped for tens, hundreds or thousands of kilometres to an alumina refinery, the leach tank 22 being located adjacent to the alumina refinery. In the embodiment illustrated in Figure 2, the purified regenerated leach solution 62 is used as the liquid which is added to the bauxite in the size reduction process 12. In another embodiment of the invention, the size reduction process 12 incorporates a slimes separation, said slimes being circulated directly to the bauxite reslurry tank 40 if the alumina content of the slimes is high, or discarded if not.

Test Results

The advantages conferred by the bauxite size reduction step 12 were demonstrated by comparing the leaching efficiency of several major impurities from a Western Australian

Bauxite. Run-of-mine bauxite was ground to two size distributions, with P90 of lmm and

212μm respectively, and leached with O.lmol/L sodium hydroxide leaching solution. The bauxite samples were mixed with the leach solution to produce slurries containing 25-30% by weight of bauxite solids. The slurries were leached in a leach tank in the form of a closed stainless steel vessel equipped with a motor-driven coaxial agitator operating at approximately 500 rpm, with the vessel placed in a thermostatically controlled water bath set at 6O⁰C. Leaching was conducted for a residence time of three hours.

Following leaching, a slurry comprising the leached bauxite and the pregnant leach solution containing impurities was subjected to solid/liquid separation using a centrifuge at 4000 rpm for 35 minutes to produce a stream of leached bauxite and a stream of clarified pregnant leach solution. Samples of the clarified pregnant leach solution were retained for analysis and for subsequent production of the clean recycle stream and concentrated leach solution. The leached bauxite solids were subsequently re-washed with deionised water and air dried at 8O⁰C prior to analysis to determine the residual impurity content. A combination of the leached bauxite solids and clarified pregnant leach solution analyses were used to determine the leaching efficiency of several major impurities from the bauxite. Results of these tests are summarised in Table 7 below: Table 7

For the lmm material, leaching reduces the Extractable Organic Carbon content by 21%, while for the 212μm material the EOC is reduced by 36%. Leaching reduces the extractable sulphate by 40% for the lmm material, while for the material ground to 212μm the reduction is 67%.

The process of the present invention is conducted prior to the traditional desilication process which is typically the first step after milling in the Bayer process. Accordingly, the leached bauxite, once it is fed to an alumina refinery, can be subjected to desilication in the usual fashion with the desilicated bauxite being subjected to digestion to recover alumina with red mud residue being discarded as waste from the Bayer process in the usual fashion. The key difference between the traditional Bayer process and the present invention is that the impurity burden on the alumina refinery is greatly reduced by ensuring that the bauxite that is fed to the alumina refinery has already been treated to remove a large percentage of the impurities, particularly the organic and soluble inorganic impurities. Accordingly, the process of the present invention takes place upstream of and outside of the Bayer process and indeed, this process can be conducted at a location that is remote from an alumina refinery by hundreds of kilometres and have nothing to do with the refinery in any way. However, it is advantageous for the process to be conducted at a location in the vicinity or at an alumina refinery.

Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the basic inventive concepts. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

In the statement of invention, the description of the invention and the claims which follow, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

What is claimed:

1. A process for the removal of impurities from bauxite comprising the steps of: a) leaching a bauxite ore using a leach solution to remove impurities from the bauxite ore and form a leached bauxite ore and pregnant leach solution containing said impurities in solution; b) removing the leached bauxite ore from the pregnant leach solution of step a) using a solid-liquid separation process to produce a clarified pregnant leach solution; and c) concentrating the clarified pregnant leach solution of step b) to produce a concentrated impurity solution and recover a purified recycle stream comprising at least water for recycling to step a).

2. The process of claim 1, wherein the leach solution comprises an alkaline solution that dissolves impurities from the bauxite ore, the total amount of alkali present in the alkaline solution being sufficient to allow leaching to proceed to completion, and the leaching of step a) is conducted at a pH of at least about 9.

3. The process of claim 2, wherein the leaching of step a) is conducted at a pH selected from the group consisting of at least 9.5, at least 10, at least 10.5, at least 11, at least 11.5, at least 12 and at least 12.5.

4. The process of any one of claims 1 to 3, wherein the concentrating of the clarified pregnant leach solution of step c) comprises using evaporation.

5. The process of any one of claims 1 to 3, wherein the concentrating of the clarified pregnant leach solution of step c) comprises using ultrafiltration.

6. The process of any one of claims 1 to 3, wherein the concentrating of the clarified pregnant leach solution of step c) comprises using nanofiltration.

7. The process of any one of claims 1 to 3, wherein the concentrating of the clarified pregnant leach solution of step c) comprises using filtration to remove impurities from the recycled stream, wherein the impurities removed using filtration include at least one of humates, sulphates, carbonates, halides and oxalates.

8. The process of any one of claims 1 to 7, wherein the concentrating of the clarified pregnant leach solution of step c) further comprises using reverse osmosis.

9. The process of claim 8, wherein the concentrating of the clarified pregnant leach solution of step c) further comprises using at least one of ultrafiltration and nanofiltration prior to using reverse osmosis.

10 . The process of any one of claims 1 to 9, further comprising removing suspended solids from the clarified pregnant leach solution prior to the concentrating the clarified pregnant leach solution of step c).

11. The process of any one of claims 1 to 10, further comprising: d) subjecting the concentrated impurity solution of step c) to an impurity removal process to recover purified leach solution for recycling to step a).

12. The process of claim 11, wherein step d) is conducted using at least one of a liquor burner, a causticisation process, wet oxidation, catalytic wet oxidation, salting out, and ion exchange.

13. The process of any one of claims 1 to 12, further comprising: e) reducing the particle size of at least some of the bauxite ore prior to leaching of the bauxite ore in step a).

14. The process of claim 13, wherein the bauxite ore is reduced to a particle size of no greater than about 1 millimetre prior to the leaching of the bauxite ore in step a).

15. The process of claim 13 or 14, wherein the recycled stream comprising water that is produced from the concentrating of the clarified pregnant leach solution of step c) is combined with the bauxite ore during step e).

16. The process of claim 15, further comprising: f) feeding the leached bauxite ore from step b) to an alumina refining process.

17. The process of claim 16, wherein step f) further comprises combining the leached bauxite ore from step b) with a liquor to form a slurry that is fed to the alumina refining process.

18. The process of any one of claims 1 to 17, wherein the leach solution of step a) is saturated with alumina.

19. A system for removing impurities from bauxite, the system comprising: a leach tank configured to receive and combine a bauxite ore and a leach solution to facilitate the leaching of the bauxite ore and the formation of a pregnant leach solution in the leach tank; a solid-liquid separator oriented to receive the slurry of leached bauxite and pregnant leach solution from the leach tank, wherein the solid-liquid separator is further configured to separate leached bauxite from the pregnant leach solution so as to form a clarified pregnant leach solution; and a concentrating section oriented to receive the clarified pregnant leach solution from the solid-liquid separator, wherein the concentrating section is configured to separate and recover a recycle stream comprising water from the clarified pregnant leach solution and to recycle the recycle stream comprising water to the leach tank.

20. The system of claim 19, wherein the concentrating section separates the recycle stream comprising water from the clarified pregnant leach solution by evaporation.

21. The system of claim 19, wherein the concentrating section separates the recycle stream comprising water from the clarified pregnant leach solution by at least one of microfiltration, ultrafiltration, nanofiltration and reverse osmosis to remove impurities from the recycled stream, wherein the impurities removed include at least one of humates, sulphates, carbonates, halides and oxalates.

22. The system of any one of claims 19 to 21 , further comprising: an impurity removal section oriented to receive a pregnant leach solution concentrated with impurities from the concentrating section, wherein the impurity removal section is configured to remove impurities from the concentrated pregnant leach solution to form a purified stream and to recycle the purified stream to the leach tank.

23. The system of claim 22, wherein the impurity removal section removes impurities from the concentrated pregnant leach solution to form the purified stream using at least one of a liquor burner, a causticisation process, wet oxidation, catalytic wet oxidation, salting out, and ion exchange.

24. The system of any one of claims 19 to 23, further comprising: a size reduction section configured to receive and reduce a particle size of bauxite ore prior to delivery to the leach tank.

25. The system of claim 24, wherein the size reduction section reduces particle sizes of the bauxite ore to dimensions no greater than about 1 millimetre.

26. The system of claim 24 or 25, wherein the size reduction section receives the recycle stream comprising water from the concentrating section and combines the recycle stream with the bauxite ore for delivery to the leach tank.

27. The system of any one of claims 19 to 26 wherein the system is oriented upstream from an alumina refinery and is configured to deliver the leached bauxite ore to the alumina refinery.

28. The system of claim 27, further comprising: a slurry tank oriented to receive the leached bauxite from the solid-liquid separator and combine the leached bauxite with a liquor to form a slurry for delivery to the alumina refinery.

29. A system for removing impurities from bauxite, the system comprising: a means for leaching a bauxite ore by receiving and combining the bauxite ore and a leach solution to form a pregnant leach solution; a means for separating leached bauxite ore from the pregnant leach solution so as to form a clarified pregnant leach solution; and a means for concentrating the clarified pregnant leach solution by separating a stream comprising water from the clarified pregnant leach solution to form a concentrated impurity solution, wherein the stream comprising water is recycled to the means for leaching.

30. The system of claim 29, further comprising: a means for purifying the concentrated impurity solution to form a purified stream, wherein the purified stream is recycled to the means for leaching.

31. The system of claim 29, further comprising: a means for reducing particle sizes of the bauxite ore prior to delivery to the means for leaching.

32. A process for removing impurities from bauxite substantially as herein described with reference to and as illustrated in the accompanying illustrations.

33. A system for removing impurities from bauxite substantially as herein described with reference to and as illustrated in the accompanying illustrations.