WO2011137481A1

WO2011137481A1 - Process for recovery of alumina using tricalcium aluminate

Info

Publication number: WO2011137481A1
Application number: PCT/AU2011/000505
Authority: WO
Inventors: Eric Antoinette Jozef Marie Boom; Daniel Mark Roworth
Original assignee: Bhp Billiton Worsley Alumina Pty Ltd
Priority date: 2010-05-03
Filing date: 2011-05-02
Publication date: 2011-11-10
Also published as: AU2011250647A1; CN103038174B; BR112012027265A2; CN103038174A; AU2011250647B2

Abstract

A process for recovering alumina values from a caustic aluminate solution is disclosed. The process comprises the steps of: a) introducing a source of carbon dioxide to the caustic aluminate solution in the presence of aluminium hydroxide seed crystals to form a first treated stream comprising carbonate ions in solution and aluminium hydroxide in solid form; b) subjecting the first treated stream of step a) to solid/liquid separation to recover alumina values in the form of aluminium hydroxide and produce a first clarified treated stream; and, c) mixing the first clarified treated stream of step b) with tricalcium aluminate to form a second treated stream comprising calcium carbonate in solid form, aluminate ions in solution, and hydroxyl ions in solution.

Description

Process for Recovery of Alumina using Tricalcium Aluminate

FIELD OF THE INVENTION

The present invention relates a process recovering alumina values from a caustic aluminate solution using TCA.

BACKGROUND TO THE INVENTION

The Bayer process has been used to recover alumina values from bauxitic ores for over a century. The process centres on the following reversible equations, for gibbsitic and boehmitic or diasporic ores, respecti vely ( 1 ):

Al(OH)₃ + OH «→ Al(OH)₄ ^" (1 )

AIO(OH) + OH^" + H₂0 <→ Al(OH)₄ ^" (2)

A schematic flowsheet showing a basic implementation of a traditional Bayer Process is illustrated in Figure 1. Blended bauxite ore is first mixed with a portion of the recycled spent liquor and subjected to grinding to reduce particle size. The resultant slurry is then treated via a process known as "desilication" or "slurry holding" to remove soluble silica minerals present in the bauxite, typically in the form of insoluble sodium aluminosilicates.

The desilicated slurry is then mixed with the remainder of the spent liquor and the alumina values of the bauxite extracted via a process referred to as "digestion". In digestion, the conditions are manipulated so as to drive equation (1) or (2) towards the right hand side. During digestion, the free caustic dissolves the aluminous mineral from the bauxite to form a concentrated sodium aluminate solution leaving behind a mud residue of undissolved minerals and impurities, principally inert iron oxides, hydroxides, (oxy)hydroxides, titanium oxides and silicious compounds. The mud residue is often red in appearance due to the presence of the iron minerals and is thus commonly referred to as "red mud". Digestion is favoured by using conditions of high temperature and pressure and these are in turn dependent on the type of ore being treated. The equilibrium expressed in equations (1) and (2) can also be displaced to the right hand side by increasing the concentration of free caustic (hydroxyl ions). After flash cooling, the pregnant liquor is separated from the mud residue in a process referred to as "clarification". The slurry is fed to one or more settling tanks in which the solid particles sink to the bottom and are removed, typically by pumping to the mud washing circuit. The separation of the mud from the concentrated liquor is assisted with flocculants, whilst the "green" (or pregnant) liquor, which is free of all but the finest suspended solids, overflows from the mud settlers.

It is normal for the decanted liquor to then be further clarified by filtration, typically using pressure filters. This so-called "polishing" filtration step is critical in ensuring that the pregnant liquor is free of suspended mud particles that would otherwise result in contamination of the product alumina. Unaided, the cloths employed in these filters would blind very quickly. This occurs because the fine suspended solids in the settler overflow become entrapped within the weave of the cloth, and then proceed to form a dense, highly resistive bed at the filter's surface. To prevent this, it is common practice to supplement the feed to the polishing filter with a "filter aid", which acts to prevent cloth blinding by the continuous formation of a bed of solids which trap the mud particles whilst still allowing the free flow of liquor through the interstices of the bed. An ideal filter aid will be cheap, chemically inert, and of such a size that it can trap the mud particles, and not restrict the flow of liquid, or contribute to blinding of the filter cloth themselves. In most alumina refineries, this role is performed by tricalcium aluminate (also referred to as TCA, C3A or C3AH6).

The mud washing circuit relies on a counter-current decantation process to recover as much sodium aluminate as possible for re-use to minimise loss of alumina and caustic values and to cleanse the mud residue so that it can be disposed of in an environmentally acceptable manner. The washer overflow that subsequently exits the first stage mud washing tank is either directed to the settling tanks, or mixed with the settler overflow liquor to form clarified pregnant liquor. The washed mud residue from the final stage in the mud washing circuit is typically pumped to a mud disposal lake. The counter-current mud washing circuit is fed with wash water, typically fresh water, condensate (condensed steam) or recycled water from the mud disposal lake (known as "lake water"), or combinations of the above. The clarified pregnant liquor which overflows the settling tanks is subjected to filtration as described above, before being sent to the precipitation stage. In the precipitation stage, the equilibrium of equation (1) (reproduced again below) is driven towards the left hand side to form pure Al(OH) , also referred to as "gibbsite".

Al(OH)₅ + OH- Al(OH)₄ ( 1 )

The precipitated gibbsite is separated via hydrocyclones, thickeners or filters. The remaining liquor, after evaporation to remove excess water that has entered the process with the bauxite and various washing steps is referred to as "spent liquor" and will have aluminate ions and hydroxyl ions present in an amount that depends on the temperature, seed surface area and residence time of the precipitation stage. Precipitation is favoured by conditions that increase the supersaturation of the liquor, such as reducing the temperature, addition of gibbsite seed, increasing the concentration of aluminate ions, or diluting the solution. To recover the alumina values and caustic, the spent liquor is recycled to digestion. Thus the spent liquor that is recycled to digestion has dissolved alumina present in it.

The gibbsite crystals formed during precipitation are classified according to size with product grade material being calcined in a rotary kiln or fluidized bed calciner furnace whilst undersized particles are used as "seeds" which aid in the precipitation of gibbsite crystals during the precipitation stage. During calcination, gibbsite is dehydroxylated to form alumina. At the same time, carbon dioxide is generated as a byproduct of the combustion of fossil fuels used to run the calciners. Carbon dioxide is also emitted in the stack gases produced from the power house which is operated to provide electricity to the alumina refinery and may also, in some alumina refineries, be generated by the lime kilns. At this time, there are limited options available to effect any reduction in the emission of greenhouse gases generated during calcination.

The primary goal of the Bayer process is to economically extract the maximum amount of alumina values (Al) from the bauxite fed to digestion into solution and then completely recover this dissolved alumina from the solution in the form of gibbsite during precipitation. The upper limit of the refinery's precipitation yield is set by the difference between the solubility of alumina in a particular liquor at the digestion temperature and the solubility of alumina in that liquor at the temperature used for precipitation. It follows then, that maximizing this difference is a primary aim of most alumina refineries.

One of the main avenues for alumina loss in an alumina refinery is in the liquor that is pumped with the mud residue from the settling tanks into the mud washing circuit. This liquor is supersaturated pregnant liquor having effectively the same concentration of aluminate ions as the pregnant liquor sent to precipitation. The liquor that overflows each stage in the counter- current mud washing circuit becomes progressively more diluted and cooler with wash water. This effectively increases the supersaturation of the liquor, encouraging precipitation of gibbsite in accordance with equation (1). The mud particles in the residue have a high surface area that further encourages such precipitation of gibbsite in the mud washing circuit. Any alumina that precipitates in the mud washing circuit in this manner is lost, as is any dissolved alumina in the liquor reporting to the mud disposal lake. Another avenue for alumina loss is alumina incorporation into TCA filter aid which is utilized during clarification to remove suspended fine solids. Over time, the TCA filter aid becomes contaminated with impurities and becomes "spent". TCA is relatively cheap and simple to produce. Under appropriate conditions, caustic aluminate solutions will react with calcium from a suitable source such as slaked lime to form thermodynamically stable and sparingly soluble TCA. This reaction is utilised most commonly in the alumina industry to produce TCA crystals of a controlled particle size for use as a filter aid. In many alumina refineries, spent TCA filter aid is pumped to the red mud disposal area and can represent a loss of alumina values in the order of 40 to 90,000 tonnes per annum for a typical refinery. In this way, spent filter aid typically represents a significant portion of the "solid alkalinity" that is stored or disposed of by alumina refineries.

There is a need for an alternative method of maximizing the recovery of alumina values in an alumina refinery.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a process for recovering alumina values from a caustic aluminate solution, the process comprising the steps of: a) introducing a source of carbon dioxide to the caustic aluminate solution in the presence of aluminium hydroxide seed crystals to form a first treated stream comprising carbonate ions in solution and aluminium hydroxide in solid form;

b) subjecting the first treated stream of step a) to solid/liquid separation to recover alumina values in the form of aluminium hydroxide and produce a first clarified treated stream; and,

c) mixing the first clarified treated stream of step b) with tricalcium aluminate to form a second treated stream comprising calcium carbonate in solid form, aluminate ions in solution, and hydroxyl ions in solution.

In one form, step c) is conducted at a temperature not less than 50°C. In another for, step c) is conducted at a temperature not less than 50°C and not greater than the atmospheric boiling point of the first treated stream. In yet another form, the first clarified treated stream of step b) is heated such that step c) is conducted at a temperature in the range of 50°C and not greater than the atmospheric boiling point.

Preferably, the caustic aluminate solution is a Bayer liquor. In one form, the Bayer liquor is one or more of; a spent Bayer liquor, an overflow stream from a mud washing stage, or a stream of lake water.

In one form, the aluminium hydroxide recovered in step b) is subjected to screening to produce a source of aluminium hydroxide seed crystals for recycle to step a).

In one form, the aluminium hydroxide seed crystals of step a) are aluminium trihydroxide seed crystals. Alternatively, the wherein the aluminium hydroxide seed crystals of step a) are boehmite or gibbsite seed crystals.

In one form, the process comprises the step d) of subjecting the second treated stream of step c) to solid/liquid separation to remove calcium carbonate and produce a second clarified treated stream.

In one form, the process further comprises the step of treating the calcium carbonate removed in step d) to produce lime. In one form, the process further comprises the step of returning the second clarified treated stream to an alumina refinery at a location downstream of a digester and upstream of a precipitator.

In one form, the caustic aluminate solution has an 'S' concentration of between 20 and 350 g L or between 20 and 100 g L.

In one for, the tricalcium aluminate used in step c) is TCA filter aid or spent TCA filter aid.

According to a second aspect of the present invention there is provided a process for recovering alumina values substantially as herein described with reference to and as illustrated in the accompanying drawings and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a more detailed understanding of the nature of the invention several embodiments of the process and apparatus will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a simplified conceptual flow diagram of a basic implementation of a traditional prior art Bayer Process;

Figure 2 is a simplified conceptual flow diagram illustrating an embodiment of a process for recovering alumina according to the present invention;

Figure 3 illustrates graphically the results of Example 1 as a comparison of C/S over time for both the RWB and AC series of tests;

Figure 4 graphically illustrates the results for the matrix test program of Example 2 showing the final C at a constant 'S' of 17.9 g L at 30°C, 60°C and 95°C;

Figure 5 graphically illustrates the results for the matrix test program of Example 2 showing the final C at a constant 'S' of 37.1 g/L at 30°C, 60°C and 95°C;

Figure 6 graphically illustrates the results for the matrix test program of Example 2 showing the final 'C at a constant 'S' of 56.5 g/L at 30°C, 60°C and 95°C;

Figure 7 graphically illustrates the results for the matrix test program of Example 2 showing the final C at a constant 'S' of 81.4 g/L at 30°C, 60°C and 95°C; and,

Figure 8 graphically illustrates the results for the matrix test program of Example 2 showing the final ^*C at a constant 'S' of 100.7 g/L at 30°C, 60°C and 95°C. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout this specification various terms commonly used in the alumina industry are used. In the interests of clarity, such terms are now defined. The term "liquor" is used throughout this specification to refer to any solution containing aluminate (Al(OH)₄ ) ions and hydroxyl or "caustic" (OH ) ions. In Bayer liquors, the principal constituents are sodium aluminate (NaAl(OH)₄) and sodium hydroxide (NaOH).

Ά' refers to the alumina concentration of the liquor and more specifically to the concentration of sodium aluminate in the liquor, expressed as equivalent g/L of alumina (AI2O3).

'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 g/L concentration of sodium carbonate.

'A/C is thus the ratio of alumina concentration to caustic concentration.

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

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

"Lake water" is the clarified liquor stream that is returned to the refinery from the mud disposal lake (if used) mixed together with collected rainwater. Lake water typically has the lowest A of any liquor stream. The lake water typically has a high carbonate concentration due to reaction of the lake water with carbon dioxide from the atmosphere.

"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, S-C (soda concentration minus caustic concentration) gives the actual concentration of sodium carbonate (Na₂C0₃) in the liquor, in g L.

A Bayer liquor's carbonate impurity level is expressed in terms of the caustic to soda ratio, or 'C/S'. A fully causticised (carbonate-free) Bayer process liquor will possess a C/S ratio of 1.00.

The term "aluminium hydroxide" is used throughout this specification, to refer to crystalline or amorphous compounds consisting of aluminium ions and hydroxide ions. One example of an 'aluminium hydroxide' is 'gibbsite' which is aluminium trihydroxide having the general formula of Al(OH)₃. Gibbsite is also sometimes referred to in the literature as "hydrate" or "alumina trihydrate" or "aluminium trihydroxide" and sometimes expressed using the chemically incorrect formula A1₂0₃.3H₂0. The term 'aluminium hydroxide' is also broad enough to cover 'boehmite' which is an aluminium monohydroxide.

The term 'seed crystals' refers to particles generally with a size smaller than a nominal product size. The function of seed crystals is two-fold. Firstly, the seed crystals promote/enhance the production of gibbsite and secondly, the seed crystals encourage the growth of larger crystals

'Calcite' is calcium carbonate (CaC0₃). 'TCA' is tricalcium aluminate Ca₃[Al(OH)6]₂ which is also commonly written using the formula 3CaO.Al₂0₃.6H₂0, (TCA6) or C3AH6 in cement industry notation. TCA is available as a side-product of causticisation in many alumina refineries. When TCA is made using plant liquors, it may contain impurities by incorporation of anionic impurities present in the plant liquor into the lattice. "Synthetic TCA" is a material that is generated in pure sodium aluminate solutions rather than using plant liquors and is thus pure tricalcium aluminate.

"Spent filter aid" refers to TCA that has been used as a filter aid in an alumina refinery and is the filter aid is dumped on a periodic basis when a cycle of operation of pressure filtration is completed. In this regard, spent filter aid retains the chemical formula of TCA and is likely to be contaminated with fine red mud solids.

"TS" refers to the sum of all sodium salts in solution, expressed as the equivalent concentration in g L of sodium carbonate. The term 'autoprecipitation' refers generally to the growth of aluminium hydroxide not contributing to the production of alumina. "Causticisation" is the term usually used by persons skilled in the art of the Bayer process to describe the process whereby carbonate is removed from a Bayer liquor and replaced with hydroxide through the addition of slaked lime and precipitation of insoluble calcium carbonate. The term "causticisation" as used throughout this specification refers more broadly to any process in which an impurity anion is removed from a liquor and replaced with hydroxide ions.

A process (10) for recovering alumina values according to one embodiment of the present invention is now described with reference to Figure 2. In this embodiment, a source of carbon dioxide (12) is introduced to a caustic aluminate solution (14) in the presence of aluminium hydroxide seed crystals (16) in a first reaction vessel (18) to form a first treated stream (20) comprising carbonate ions in solution and aluminium hydroxide in solid form. The source of carbon dioxide (12) may be introduced to the first reaction vessel (18) using any suitable means, for example a sparging system. The source of carbon dioxide may be of any purity. Suitable sources include carbon dioxide present in the flue or 'stack gas' produced by a power station or calciners or carbon dioxide present in exhaust or "stack gas" produced by a lime kiln.

Without wishing to be bound by theory, the carbon dioxide gas reacts with sodium hydroxide present in the dilute Bayer liquor stream to produce sodium carbonate according to the following reaction: 2NaOH + C0₂ *→ Na₂CO₃ + H₂0 (3)

Alternatively or additionally, under appropriate conditions of pH, the carbon dioxide gas may react with sodium hydroxide present in the dilute Bayer liquor stream to produce sodium bicarbonate. Below pH of 10.5 the formation of sodiumbicarbonate becomes significant and thus the mode of operation for precipitation of sodium carbonate in preference to sodium bicarbonate would be operating above a pH of 10.5. The carbon dioxide also reacts with sodium aluminate present in the dilute Bayer liquor to produce including sodium carbonate and gibbsite according to the following reaction:

2NaAl(OH)₄ + C0₂ «→ Na₂C0₃ + 2A1(0H)₃ (4)

The first treated stream (20) is then subjected to solid/liquid separation in a suitable first solid/liquid separator (22) to recover alumina values in the form of aluminium hydroxide (24) and produce a first clarified treated stream (26). The aluminium hydroxide seed crystals (16) may be added into the first reaction vessel as illustrated in Figure 2 or introduced directly to the caustic aluminate solution (14) upstream of the first reaction vessel (18) as illustrated by the dotted line appearing in Figure 2.

Any suitable aluminium hydroxide seed crystals (16) may be used in the first reaction vessel (18), for example, gibbsite seed crystals separated during sizing of the gibbsite that is precipitated from a pregnant Bayer liquor during the precipitation stage of a Bayer process. Boehmite seed crystals may equally be used when operating at a pH in the range of 10 to 12. Alternatively or additionally, the aluminium hydroxide (24) recovered in step b) may be subjected to a screening operation to produce a source of suitably sized aluminium hydroxide seed crystals as an undersized product stream (28) for recycle to step a) and an oversized product stream (30) which is returned to a suitable location (not shown) in the alumina refinery or sold as a product of the alumina refinery if the oversized product stream (30) is of sufficient quality. If the oversized product stream (30) is considered to be of insufficient quality to be sold as product, the alumina values present in the oversized product stream (30) may be recovered by returning the oversized product stream (30) to the bauxite fed to grinding or milling prior to digestion so that its quality can be improved.

The first clarified treated stream (26) may then be mixed with a source of tricalcium aluminate (32) in a second reaction vessel (34) to form a second treated stream (36) comprising calcium carbonate in solid form, aluminate ions in solution, and hydroxyl ions in solution. Without wishing to be bound by theory, TCA reacts with the sodium carbonate present in the first clarified treated stream of liquor to produce calcium carbonate (as a solid), sodium aluminate (in solution) and sodium hydroxide (in solution) in accordance with the following reaction: Ca₃[Al(OH)₆]₂ + 3Na₂C0₃ «→ 3CaC0₃ + 2NaAl(OH)₄ + 4NaOH (5)

TCA conversion to calcite is more favourable at high temperatures and lower 'S' streams. As described in greater detail in the examples below, the total yield of 'C increases with increasing 'S', however the efficiency of conversion decreases (as shown by the C/S).

For best results in relation to reaction kinetics, the second reaction vessel (34) is operated at a temperature not less than 50°C. Whilst it is possible to operate the second reaction vessel (34) at a temperature greater than the atmospheric boiling point of the first treated stream (36), it is preferable for the second reaction vessel to be operated at a temperature not less than 50°C and not greater than the atmospheric boiling point of the first treated stream (26). It is to be understood that heating of the second reaction vessel (34) may occur by way of direct heating of the second reaction vessel or indirectly by way of heating of one or both of the first treated stream (26) or the source of TCA (32). As described in greater detail in the examples below, addition of spent TCA filter aid to a 1 OOg L synthetic sodium carbonate solution at a temperature in the range of 90°C to 98°C resulted in 83% and 78% conversion of TCA to calcite.

Agitation conditions within the first and second reaction vessels (18 and 34, respectively) are not critical, although the contents of each of the first and second reaction vessels should preferably be completely suspended. Retention times in the first and second reaction vessels (18 and 34, respectively) may vary depending on such relevant factors as the operating temperature, the relative concentrations of the various streams being added and the efficiency of conversions occurring in each vessel. Retention times may thus be in the range of not more than 4 hours to not more than 20 hours. The caustic aluminate solution (14) that is fed to the first reaction vessel (18) may be a spent Bayer liquor stream, for example, a spent Bayer liquor stream from a mud washing circuit or another dilute Bayer liquor, with best performance being obtained with more dilute liquors with an 'S' concentration of between 20 and 100 g L. However, stronger Bayer liquors may equally be used, such liquors having an 'S' concentration of between 100 and 350 g/L. The second treated stream (36) is to remove calcium carbonate, and produce a second clarified treated liquor stream. The second treated stream (36) is subjected to solid/liquid separation using a second solid/liquid separator (38) to produce a second clarified treated stream (40) and a second solids stream (38) which is predominately calcium carbonate as well as any unreacted TCA. The second solids stream (38) may be discarded or washed and calcined to re-generate quicklime for re-use. The second clarified treated stream (40) is enhanced with aluminate ions and hydroxide ions and can be returned to any suitable location within the Bayer process, for example, to the settlers or the liquor polishing filters.

The first and second solid/liquid separators (22 and 38, respectively) can be any suitable solids/liquid separator including one or more gravity settlers, pressure filters, cyclones, or centrifuges, but best performance is obtained using simple filters. Use of TCA in this way allows for the recovery of caustic and alumina values from the mud washing circuit that may otherwise have been lost due to precipitation or discarded in soluble form with the liquor accompanying the mud. The gibbsite solids could potentially be sent back to digestion and recovered into the process, while C0₂ gas is effectively sequestered in the waste calcite solids, reducing greenhouse gas emissions. The waste disposal of spent TCA filter aid is also reduced in scale.

An example of a practical implementation of the schematic flow of Figure 2 is now described with reference to elements of the Bayer Process flow chart illustrated in Figure 1. In a counter-current mud washing circuit (50), the alumina concentration of the washer overflow from the first mud washer (52) is quite high, typically as much as about half that of the pregnant liquor that overflows the settling tank (54) in the clarification stage. Using counter-current decantation, the mud residue is pumped from the first washer (52) to a second washer (56) and so on to the n^th washer (60) while fresh water or lake water (62) is introduced firstly to the last (n-^th) washer (60) in the mud washing circuit (50) and overflows to the n-l* washer (64) and so on up to the first mud washer (52). In the traditional Bayer process alumina values are lost in the mud washing circuit due to precipitation, as well as in soluble form in the liquor accompanying the mud to the mud residue disposal areas. The washer overflow liquor in all stages of the mud washing circuit is, like most liquors, supersaturated with respect to gibbsite. The wash overflow liquor becomes progressively more dilute and cooler with each successive stage of washing resulting in alumina losses due to precipitation onto the mud particles.

In one embodiment of the present invention, the caustic aluminate solution (14) fed to the first reaction vessel (18) is the washer overflow from the n- 1^th washer (64). The present invention is equally applicable to the treatment of a plurality of washer overflow streams each being treated in one or a corresponding plurality of first reaction vessels (18). It is to be understood that the washer overflow liquor from any of the other mud washers could equally be used, however recovery of alumina values is most efficient when the washer overflow liquor is taken from any one or each of the 2nd to n-lth washer(s) (56 and 64, respectively) and least favourable when the caustic aluminate stream (14) is the overflow from the first or last washers (52 and 60, respectively) in the mud washing circuit (50). It is pointless to use the overflow from the first washer (52), as the overflow liquor from the first washer is fed to the settling tank in any event. The alumina concentration of the overflow liquor to the final (nth) washer (60) is generally similar to lake water and is therefore too low to be of practical benefit. In addition, removal of alumina at this stage does little to prevent precipitation of gibbsite further up the mud washing circuit (50).

From the description above, it is apparent that the process of the present invention as used in this embodiment allows for alumina values to be recovered from the mud washing circuit and be delivered in the form of aluminium hydroxide to the precipitation circuit. The flow on advantage of this is a reduction in the occurrence of autoprecipitation in accordance with equations (1) and (2) in the mud washing circuit.

Advantages of various aspects of the present invention are further described and illustrated by the following examples and experimental test results. These examples and experimental test results are illustrative of a variety of possible implementations and are not to be construed as limiting the invention in any way. It will be readily appreciated by persons skilled in the art that there is no one form of the Bayer Process, each alumina refinery having to modify the particular process conditions used depending on a number of factors, most notably the nature of the bauxite being processed. The present invention is thus not limited by the particular number nor type of settling tanks, mud washers, causticisers, solid-liquid separators described in the following examples. Example 1: Conversion of TC A to Calcite

Synthetic TCA was produced in the laboratory and reacted with 60g/L sodium carbonate (Na₂C0₃) at a temperature of 60°C in both a rolling water bath (RWB) and autoclave to compare the kinetics of the reaction and the extent of conversion of TCA to calcite.

When the tests were conducted using the RWB, samples were taken using a 20mL syringe at intervals of 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300 mins, 24, 30, 48, 54 hours. These samples were filtered, cooled and then analysed using titration to determine the 'Α', 'C\ and 'S' concentrations. After 54 hours the remaining mixture was removed from the RWB and filtered. The filtrate was collected and the solids were washed in cold DI water before being analysed on the XRD as a wet sample.

When tests were conducted using an autoclave (AC), lOOOmL of 60g/L Na₂C0₃ was added into the AC which was then sealed and equilibrated to 60°C. 73.09g of synthetic TCA were added to the AC which was then resealed. Samples were taken at the same time intervals as set out above for the RWB test with sampling being concluded at 30hr, not 54hrs. The AC was drained and the slurry was filtered, washed and analysed as above. The results are set out in Figure 3 which shows a comparison of C/S over time for both the RWB and AC series of tests. Results showed substantially complete conversion of TCA to calcite for both series of tests.

Example 2: Matrix Test

A matrix test was conducted to demonstrate the effect of reaction temperature and the concentration of sodium carbonate ([Na₂C0₃]) in the second reaction vessel. The results of these tests have demonstrated that the conversion of TCA to calcite is more favourable at higher temperatures for a given [Na₂C0₃]. Also, at constant temperature, the mass of Na₂C0₃ reacted increases with increasing initial [Na₂C0₃] (i.e. [S]), but the C/S achieved decreases.

The matrix test program is set out in Table 1 below. Table 1: Matrix Test Conditions

A total volume of 300mL of the respective solutions were added to the 500mL polypropylene bottles and added to the water bath at the respective temperatures. Once the bottles had equilibrated, the TCA was added in the respective amounts. Sampling was performed at the respective times and analysed for the Ά', 'C\ and 'S' concentrations. Kinetics Test:

A kinetics test was designed to evaluate the rate of TCA conversion to calcite at the midpoint conditions set out in Table [1]. As such, tests were performed using 40g/L Na C0₃ and 60°C. Samples were taken at 5, 15, 30, 45, 60, 90, 120, 180 and 240min and analysed for the Ά', 'C\ and 'S' concentrations. TCA was charged to 80% of the carbonate consumed in the respective test (80% of the final 'C concentration). The results for the kinetics test are shown in Table 2 below. It can be seen that approximately 50% of the total change in 'C is realised in the first 5min of the 4hr test. This indicates that the reaction rate is very fast. It is expected that this rate would increase with higher temperature and higher starting 'S'.

Table 2: Kinetics Test results

Materials:

12L of TCA slurry was collected from an alumina refinery. The TCA slurry was filtered and washed with hot water. The solids were stored wet in a sealable bag. A wet sub-sample was taken and X-ray diffraction (XRD) performed. Another sub-sample was dried and X-ray fluorescence analysis (XRF) performed. Sodium carbonate solutions were made up by additions of a known standard to 1L volumetric flasks. The standards were analysed to determine the 'C\ and 'S' concentrations.

The results for the matrix test program are illustrated graphically in Figures 4 to 8 which show the final 'C at constant 'S' for three temperatures. The results indicate that TCA conversion to calcite is more favourable at high temperatures and total yield of 'C increases with increasing 'S', however the efficiency decreases (as shown by the C/S).

The conversion of TCA to calcite has been successfully performed in a matrix test program and results indicate that a higher temperature and lower 'S' are favourable for the conversion reaction to take place. The conversion yield appears to be higher with a higher starting 'S', however the efficiency, as measured by the C/S, decreases with higher starting 'S\ A kinetics test performed in a 40g/L sodium carbonate solution and at 60°C showed that the rate of conversion was fast, with approximately 50% of the final 'C reach within 5 minutes.

Example 3: Effect of Agitation

The type of test set out in Example 2 was repeated at 60°C and 40g/l [Na₂C0₃] to determine the effect, if any, of agitation on reaction kinetics. It was established conducting the test using a vigorously agitated autoclave was no faster than the equivalent test using a rolling waterbath.

Now that several embodiments of the invention have been described in detail, it will be apparent to persons skilled in the chemical engineering arts that numerous variations and modifications can be made without departing from the basic inventive concepts. All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the statement of invention and description of the invention 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

Claims defining the Invention:

1. A process for recovering alumina values from a caustic aluminate solution, the process comprising the steps of:

a) introducing a source of carbon dioxide to the caustic aluminate solution in the presence of aluminium hydroxide seed crystals to form a first treated stream comprising carbonate ions in solution and aluminium hydroxide in solid form;

2. The process for recovering alumina values of claim 1 wherein step c) is conducted at a temperature not less than 50°C.

3. The process for recovering alumina values of claim 1 wherein step c) is conducted at a temperature not less than 50°C and not greater than the atmospheric boiling point of the first treated stream.

4. The process for recovering alumina values of claim 1 wherein the first clarified treated stream of step b) is heated such that step c) is conducted at a temperature in the range of 50°C and not greater than the atmospheric boiling point.

5. The process for recovering alumina values of any one of the preceding claims wherein the caustic aluminate solution is a Bayer liquor.

6. The process for recovering alumina values of any one of the preceding claims wherein the Bayer liquor is one or more of a spent Bayer liquor, an overflow stream from a mud washing stage, or a stream of lake water.

7. The process for recovering alumina values of any one of the preceding claims wherein the aluminium hydroxide recovered in step b) is subjected to screening to produce a source of aluminium hydroxide seed crystals for recycle to step a).

8. The process for recovering alumina values of any one of the preceding claims wherein the aluminium hydroxide seed crystals of step a) are aluminium trihydroxide seed crystals.

9. The process for recovering alumina values of any one of claims 1 to 7 wherein the aluminium hydroxide seed crystals of step a) are boehmite or gibbsite seed crystals.

10. The process for recovering alumina values of any one of the preceding claims further comprising the step d) of subjecting the second treated stream of step c) to solid/liquid separation to remove calcium carbonate and produce a second clarified treated stream

1 1. The process for recovering alumina values of any one of the preceding claims further comprising the step of treating the calcium carbonate removed in step d) to produce lime.

12. The process for recovering alumina values of any one of the preceding claims further comprising the step of returning the second clarified treated stream to an alumina refinery at a location downstream of a digester and upstream of a precipitator.

13. The process for recovering alumina values of any one of the preceding claims wherein the caustic aluminate solution has an 'S' concentration of between 20 and 350 g/L.

14. The process for recovering alumina values of any one of the preceding claims wherein the caustic aluminate solution has an 'S' concentration of between 20 and 100 g/L.

15. The process for recovering alumina values of any one of the preceding claims wherein the tricalcium aluminate used in step c) is TCA filter aid or spent TCA filter aid.

16. A process for recovering alumina values substantially as herein described with reference to and as illustrated in the accompanying examples and drawings excluding Figure 1.