WO2005077830A1

WO2005077830A1 - Treatment of alkaline bayer process residues

Info

Publication number: WO2005077830A1
Application number: PCT/AU2005/000182
Authority: WO
Inventors: David John Cooling; Peter Stewart Hay; Laurence Michael Guilfoyle
Original assignee: Alcoa of Australia Ltd
Current assignee: Alcoa of Australia Ltd
Priority date: 2004-02-16
Filing date: 2005-02-11
Publication date: 2005-08-25
Anticipated expiration: 2006-08-16
Also published as: CA2556439C; BRPI0506808A; CA2556439A1

Abstract

A method for the treatment of an alkaline Bayer process residue, the method comprising the steps of: Cooling and dehumidifying a flue gas by contacting the flue gas with a cool water stream and retaining the heat therefrom; Contacting a carbonate solution with the cooled, dehumidified flue gas to form a bicarbonate solution; Heating the bicarbonate solution utilising the heat recovered from the cooled flue gas; Contacting the alkaline Bayer process residue with the heated bicarbonate solution; Recovering a carbonate solution; and Recycling the recovered carbonate solution by contacting the recovered carbonate solution with a cooled, dehumidified flue gas to form a bicarbonate solution.

Description

"Treatment Of Alkaline Bayer Process Residues"

Field of the Invention

The present invention relates to a method for the treatment of residues from the Bayer process. More particularly, the present invention relates to a method for decreasing the pH of alkaline Bayer process residues to render such more suitable for disposal. In one form, the present invention also provides a means for the concomitant treatment of flue gases.

Background Art

The Bayer process is widely used for the production of alumina from alumina containing ores, such as bauxite. The process involves contacting alumina- containing ores with recycled caustic aluminate solutions, at elevated temperatures, in a process commonly referred to as digestion. Aluminate solution is separated from the resulting slurry and cooled.

During alumina extraction, lime and small quantities of chemical reagents are also added to the process. The entrained alkaline solution in the residual slurry contains caustic soda and other soluble compounds resulting from the reaction of the caustic soda with bauxite ore (such as a range of sodium-organic species and dissolved alumina) and also from reaction with air (such as sodium carbonate). Whilst the vast majority of the aluminate solution is removed prior to disposal of the solids, the remaining solution entrained in the residual solids, and the resulting elevated pH, causes difficulties for disposal.

Carbonation has been shown to be an effective means treating residual slurry to reduce its pH and hence the hazardous nature of the residue normally attributed to the high pH and corrosivity of the entrained liquor. Carbonation is the addition of gaseous CO₂ to a residue slurry prior to the deposition of this slurry into a residue storage area. The C0₂ reacts with the alkaline components within the liquor, and if held in contact with the slurry for long enough, the adsorbed and solid forms of alkalinity are also reacted according to the following equations: NaAI(OH)₄ + C0₂ =^ NaAIC0₃(OH)₂ + H₂0 NaOH + C0₂ =^ NaHC0₃ Na₂C0₃ + C0₂ + H₂0 ^ 2NaHC0₃ 3Ca(OH)₂.2AI(OH)₃ +3C0₂ =^ 3CaC0₃ + AI₂0₃.3H₂0 + 3H₂0

International patent application WO 93/16003 (Alcoa of Australia Ltd) describes the direct injection of concentrated C0₂ into thickened residue slurry. However, the effectiveness of this treatment at an operational scale is dependant on the level of solid alkalinity present in the slurry as tri-calcium aluminate, which is slow to react. Adequate retention time in pressurised reaction vessels and the production of sufficient buffering, in the form of bicarbonate and sodium aluminium carbonate, is required to ensure a pH of 10.5 or less is sustained in the final residue deposit.

Whilst the use of concentrated C0₂ adds significantly to the costs associated with the method described in International patent application WO 93/16003, it does allow direct injection of C0₂ into a residue slurry, obviating the need to use elaborate dilute gas contacting systems. The presence of solids and dissolved alumina in the residue slurry to be treated, presents a number of significant issues for the operation of these dilute gas contacting systems.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Disclosure of the Invention

In accordance with the present invention there is provided a method for the treatment of an alkaline Bayer process residue, the method comprising the step of:

Contacting the alkaline Bayer process residue with a bicarbonate solution. A bicarbonate solution is a readily produced form of acidity that obviates the need for the elaborate gas-slurry contacting systems necessary for anything other than concentrated carbon dioxide streams. The bicarbonate solution may be generated by any means. However, one advantage of the invention is that the bicarbonate solution may be generated from a gas stream of relatively low carbon dioxide concentration and may be contacted with the Bayer process residue without the need for elaborate gas-slurry contactor apparatus, which are prone to scaling.

Preferably, the bicarbonate solution is at an elevated temperature.

As discussed in the 'Background Art' section, alkaline Bayer process residue contains solid alkalinity in the form of tri-calcium aluminate. Elevated temperatures facilitate the reaction of tri-calcium aluminate. However, it is difficult and expensive to adequately heat a thickened slurry, so the application of elevated temperatures to the method described in International patent application WO 93/16003 is not generally considered practical. The use of a bicarbonate solution at elevated temperature substantially overcomes this problem.

In one form of the invention, the bicarbonate solution is formed by the step of:

Contacting a solution with carbon dioxide.

Preferably, the solution is a carbonate solution. Economically, the solution is a sodium carbonate solution. Without wishing to be bound by theory, where the solution is a sodium carbonate solution, the reaction of the carbon dioxide with the sodium carbonate solution to form sodium bicarbonate may be represented as follows:

Na₂C0₃ + C0₂ + H₂0 =* 2NaHC0₃

Further, where the solution is a sodium carbonate solution, the reaction of the sodium bicarbonate with the liquid and solid forms of alkalinity in the alkaline Bayer process residue may be represented as follows:

NaAI(OH)₄ + 2NaHC0₃ ^ NaAIC0₃(OH)₂ + Na₂C0₃ + 2H₂0

NaOH + NaHC0₃ = Na₂C0₃+ H₂0 3Ca(OH)₂.2AI(OH)₃ + 6NaHC0₃ ^ 3CaC0₃ + AI₂O₃.3H₂0 + 3Na₂C0₃ + 6H₂0

In a more specific form of the invention, the bicarbonate solution is formed by the step of:

Contacting the solution with a gaseous stream containing carbon dioxide.

In one form of the invention, the gaseous stream may be provided in the form of flue gas.

Typically, flue gas is produced at elevated temperatures, and this conveniently and economically allows the production of a bicarbonate solution at elevated temperatures. Conveniently, the flue gas is provided in the form of a flue gas generated by the Bayer process. A variety of flue gases, such as calciner flue gas, boiler flue gas and kiln flue gas are generated at Bayer process refineries. Conveniently, the gaseous stream comprises calciner flue gas, boiler flue gas or kiln flue gas, or a mixture of two or more of such.

The flue gas may be treated to increase carbon dioxide concentration prior to contact with the solution. Where the gaseous stream containing carbon dioxide is provided in the form of a flue gas, prior to the step of contacting the solution with a flue gas, the present invention preferably comprises the step of:

Cooling the flue gas.

Preferably still, where the gaseous stream containing carbon dioxide is provided in the form of a flue gas, prior to the step of contacting the solution with a flue gas, the present invention preferably comprises the steps of:

Cooling the flue gas and retaining the heat therefrom.

Where the gaseous stream containing carbon dioxide is provided in the form of a flue gas, prior to the step of contacting the solution with a flue gas, the present invention preferably comprises the steps of:

Dehumidifying the flue gas.

Some flue gases have an appreciable water content, and dehumidifying the flue gas may appreciably increase the carbon dioxide concentration. In one form of the invention, the steps of cooling the flue gas and dehumidifying the flue gas are concurrently achieved by the step of:

Contacting the flue gas with a cool water stream.

The step of cooling the flue gas prior to contacting such with the solution enables more efficient conversion to bicarbonate, with efficiencies diminishing at elevated temperatures.

In one form of the invention, where the bicarbonate solution is provided at elevated temperature and method comprises the step of cooling the flue gas and retaining the heat therefrom, the present invention preferably comprises the step of: Utilising the heat retained from the step of cooling the flue gas to produce the bicarbonate solution at elevated temperature.

Thus, in a preferred form of the invention, the flue gas is contacted with the solution at a temperature amenable to the efficient formation of the bicarbonate solution, whilst the bicarbonate solution is contacted with the alkaline Bayer process residue at a temperature amendable to the efficient neutralisation of solid alkalinity in the form of tri-calcium aluminate.

Flue gas typically contains particulates, odour forming compounds, sulfurous compounds and volatile organic carbon compounds. The use of flue gas as the gaseous stream affords the further advantage of reducing the quantities of particulates, odour forming compounds, sulfurous compounds and volatile organic carbon compounds from at least a portion of the flue gas. Thus, where the gaseous stream comprises flue gas, the present invention also provides a means for the concomitant treatment of flue gases.

Carbonation of residue using a bicarbonate solution in the particular embodiment where the bicarbonate solution is generated using flue gas provides the additional benefit of providing a sink for the C0₂ which reduces the atmospheric emission of C0₂ (and other substances) in the flue gas.

Preferably, the pH of the bicarbonate solution is between about 7.5 and 9.0. Preferably still, the pH of the bicarbonate solution is between 7.5 and 8.0.

Preferably, the total alkali concentration of the bicarbonate solution is substantially identical to the total alkali concentration of the alkaline Bayer process residue.

Prior to the step of contacting the alkaline Bayer process residue with a bicarbonate solution, the method may comprise the step of: Thickening the alkaline Bayer process residue. The alkaline Bayer process residue produced by the digestion process contains significant levels of soda and alumina. Thickening the alkaline Bayer process residue reduces soda and alumina levels.

The extent to which the alkaline Bayer process residue is thickened prior to contact with the bicarbonate solution is largely an economic consideration. Higher density results in higher soda and alumina recovery. Higher density also means less caustic in solution to be treated with the bicarbonate.

Preferably, after the step of contacting the solution with a gaseous stream containing carbon dioxide, the method comprises the step of: Recovering a carbonate solution.

In one form of the invention, the step of contacting the alkaline Bayer process residue with a bicarbonate solution is performed in a reaction vessel having an overflow and an underflow, and the step of recovering a carbonate solution comprises the step of: Recovering the carbonate solution as an overflow from the reaction vessel.

In one form of the invention, the reaction vessel is a gravity separation vessel, such as a thickener.

In a preferred form of the invention, where the invention comprises the step of contacting a carbonate solution with carbon dioxide, the carbonate solution so contacted is provided in the form of the recovered carbonate solution.

The recovered carbonate solution is low in alumina, the dissolved alumina having largely precipitated once the alkaline Bayer process residue is contacted with the bicarbonate solution. The low alumina levels substantially reduce scaling issues in any apparatus by which the recovered carbonate solution is contacted with carbon dioxide. ln a specific form of the present invention, there is provided a method for the treatment of an alkaline Bayer process residue, the method comprising the steps of:

Contacting a carbonate solution with a gaseous stream containing carbon dioxide to form a bicarbonate solution;

Contacting the alkaline Bayer process residue with the bicarbonate solution;

Recovering a carbonate solution; and

Recycling the recovered carbonate solution by contacting the recovered carbonate solution with a gaseous stream containing carbon dioxide to form a bicarbonate solution.

In a more specific form of the invention, there is provided a method for the treatment of an alkaline Bayer process residue, the method comprising the steps of:

Cooling and dehumidifying a flue gas by contacting the flue gas with a cool water stream and retaining the heat therefrom;

Contacting a carbonate solution with the cooled, dehumidified flue gas to form a bicarbonate solution;

Heating the bicarbonate solution utilising the heat recovered from the cooled flue gas; Contacting the alkaline Bayer process residue with the heated bicarbonate solution;

Recovering a carbonate solution; and Recycling the recovered carbonate solution by contacting the recovered carbonate solution with a cooled, dehumidified flue gas to form a bicarbonate solution.

The steps of the method may be performed simultaneously in a continuous process.

Best Mode(s) for Carrying Out the Invention

The best method of performing the present invention currently known to the applicant will now be described by way of example only, with reference to Figure 1.

It must be appreciated that the following description of the best method does not limit the generality of the preceding description of the invention.

As shown in the accompanying drawing, a gaseous stream containing carbon dioxide in the form of hot flue gas 10 is introduced into a gas-liquid contactor 12. The hot flue gas 10 is first contacted with a cold water stream 14, and thus cooled and dehumidified. The cooled and dehumidified flue gas is then contacted with a carbonate solution 16, thereby generating a bicarbonate solution 18 of pH of approximately 7.5-8.

Heat recovered from the hot flue gas 10 produces a hot water stream 20, which is then used to produce a heated bicarbonate solution 22 to a temperature of approximately 60°C, by way of a heat exchanger 24. A temperature of approximately 60°C is readily achievable by heat transfer from flue gas, and causes improved reaction of tri-calcium aluminate.

Alkaline Bayer process residue in the form of residue mud 26 from digestion is introduced into a first thickener 28. Thickened alkaline Bayer process residue in the form of underflow 30 from the first thickener 28 is mixed with the heated bicarbonate solution 22 in a second thickener 32, wherein the bicarbonate of the heated bicarbonate solution 22 reacts with the solid and liquid alkalinity of the thickened alkaline Bayer process residue, thereby reducing the alkalinity thereof. The total alkali concentration of the bicarbonate solution 22 is substantially identical to that of the thickened residue mud 30. Carbonated residue 34 from the second thickener 32 is removed as underflow, whilst the overflow 16 is recycled to the contactor 12 in the form of the carbonate solution 16 for generation of the bicarbonate solution 18.

Experimental

To further describe the invention, a series of experiments will now be described. However, it must be appreciated that the following description of those experiments is not to limit the generality of the above description of the invention.

The experiments were conducted in order to determine the feasibility of residue neutralisation using dual superthickeners with stack gas carbonation of the second superthickener overflow. The objective was to produce a carbonated residue slurry, which was considered to be a slurry where the pH has been reduced to below pH 10.0, the majority of the solid alkalinity (primarily tri-calcium aluminate) has been reacted and there is sufficient bicarbonate in the final solution to buffer reversion from any remaining unreacted solid alkalinity

An alkaline Bayer process residue in the form of a sample of superthickener underflow (STUF) was collected from a Western Australian alumina refinery. The STUF had a TC (total caustic concentration expressed as g/L sodium carbonate) of 18.4 g/L, a TA (TA represents total alkali concentration expressed as g/L sodium carbonate) of 23.4 g/L and an aluminate concentration of 7.6 g/L (expressed as g/L Al₂0₃). Further, a carbonate solution in the form of a sample of superthickener overflow (STOF) was collected at the same time. The STOF had a TC of 16.9 g/L, a TA of 24.1 g/L and an aluminate concentration of 8.5 g/L.

The superthickener overflow sample was carbonated, by bubbling through concentrated CO₂, to produce a bicarbonate solution (STOFC), filtered on a filter press and shown to have a TC of -22.34 g/L, a TA of 21.99 g/L and an aluminate concentration of <0.04 g/L. The carbonated super-thickener overflow (STOFC) was then added to the superthickener underflow (STUF) at ratios between 0.8 and 4.0 kL of STOFC per kL of STUF.

Three sets of samples at each mixing ratio were added to a rotating water bath at 60°C, a nominal temperature within the likely range achievable with heat transfer from the flue gas. A sample was taken off at 30 minutes, 6 hours and at 24 hours.

The liquors were analysed for pH TC, TA, Al₂0₃ and the solids for tri-calcium aluminate (TCA6).

Results of the test work are summarised in the following table below and shown graphically on Figures 2 to 4. Figure 2 shows the pH of the slurry after mixing with STOFC. Figure 3 shows the TC of the slurry after mixing with STOFC, TC being the total caustic in solution (a positive number indicates NaOH, a negative number indicates NaHC0₃). Figure 4 shows the alumina concentration of the slurry after mixing with STOFC.

Liquor Alkali TCA6 Al₂0₃ PH (expressed as g/L Na₂C0₃) I NaOH Na₂C0₃ NaHC0₃ Total (g/L) (g/L) (g/Li) (g/L) (g/kg) (g/L) Compositions Prior to Mixing

STOF 16.93 7.20 0 24.13 8.46 13.03

STOFC 0 0 22.34 21.99 <0.04 7.45

STUF 18.42 4.94 0 23.36 1.00 7.56 13.10 Compositions After Mixing Mixing Ratio Time STOFC/ (hours) STUF 0.5 0.8 5.78 15.67 0 21.45 0 4.14 12.17 6 0.8 6.83 14.41 0 21.24 0 3.86 12.36 24 0.8 6.73 14.42 0 21.15 0 3.21 12.44 0.5 1.0 3.35 18.17 0 21.52 0 2.93 11.89 6 1.0 3.87 17.38 0 21.25 0 2.47 12.13 24 1.0 3.93 17.26 0 21.19 0 2.06 12.25 0.5 1.4 0.15 21.55 0 21.70 0 1.44 10.95 6 1.3 0.71 20.76 0 21.47 0 1.09 11.37 24 1.3 0.57 20.85 0 21.41 0 0.79 11.50 0.5 1.8 0 19.24 2.40 21.64 0 0.62 10.40 6 1.7 0 19.95 1.70 21.65 0 0.47 10.57 24 1.7 0 19.85 1.75 21.59 0 0.32 10.61 0.5 2.1 0 17.16 4.28 21.43 0 0.21 10.02 6 2.1 0 17.91 3.77 21.68 0 0.28 10.23 24 2.1 0 17.83 3.82 21.65 0 0.18 10.23 0.5 2.5 0 15.36 6.05 21.41 0 <0.04 9.99 6 2.5 0 16.24 5.35 21.59 0 0.17 10.02 24 2.5 0 15.96 5.64 21.59 0 0.13 10.00 0.5 2.9 0 13.84 7.59 21.43 0 <0.04 9.86 6 2.9 0 14.60 6.92 21.52 0 <0.04 9.85 24 2.9 0 14.29 7.18 21.47 0 <0.04 9.83 0.5 3.3 0 12.52 8.98 21.50 0 <0.04 9.73 6 3.3 0 13.14 8.31 21.44 0 <0.04 9.72 24 3.3 0 13.17 8.22 21.39 0 <0.04 9.73 0.5 3.7 0 11.49 10.03 21.52 0 <0.04 9.65 6 3.8 0 12.07 9.35 21.41 0 <0.04 9.62 24 3.7 0 11.94 9.42 21.36 0 <0.04 9.62 0.5 3.9 0 10.82 10.66 21.48 0 <0.04 9.59 6 3.9 0 11.76 9.67 21.43 0 <0.04 9.60 24 3.9 0 11.48 9.84 21.32 0 <0.04 9.57 The experiments demonstrate that with addition of a sufficient volume of bicarbonate solution in the form of carbonated STOF, the alkaline Bayer process residue (STUF) was 'carbonated' in that the sodium hydroxide, sodium carbonate solution is converted to a sodium carbonate, sodium bicarbonate solution. It can be seen that a pH of less than 10 can be readily attained at appropriate mixing ratios, that the tri-calcium aluminate is fully reacted and, that with a sufficiently high mixing ratio, a high bicarbonate level can be retained with the settled solids providing a significant buffer in the settled mud.

The experiments also demonstrate that the dissolved alumina in the residue slurry (STUF) is precipitated and that the supernatant recovered after settling of the solids (STOF) is low in dissolved alumina, providing a low alumina carbonate/bicarbonate solution for recirculation to the C0₂ contactor, thereby reducing scaling issues.

Data indicates that, at 60°C, the reaction is complete in less than 30 minutes, in that there was no further change by holding for 6 and 24 hours.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

The Claims Defining the Invention are as Follows

1. A method for the treatment of an alkaline Bayer process residue, characterised by the step of:

Contacting the alkaline Bayer process residue with a bicarbonate solution.

2. A method according to claim 1 characterised in that the bicarbonate solution is at an elevated temperature.

3. A method according to claim 1 or claim 2 characterised in that the bicarbonate solution is formed by the step of:

Contacting a solution with carbon dioxide.

4. A method according to claim 3 characterised in that the solution contacted with carbon dioxide is a carbonate solution.

5. A method according to claim 3 characterised in that the solution contacted with carbon dioxide is a sodium carbonate solution.

6. A method according to any one of claims 3 to 5 characterised in that the bicarbonate solution is formed by the step of:

Contacting the solution with a gaseous stream containing carbon dioxide.

7. A method according to claim 6 characterised in that the gaseous stream is provided in the form of flue gas.

8. A method according to claim 6 or 7 characterised in that the gaseous stream comprises calciner flue gas, boiler flue gas or kiln flue gas, or a mixture of two or more of such.

9. A method according to claim 7 or 8 characterised in that the flue gas is treated to increase carbon dioxide concentration prior to contact with the solution.

10. A method according to any one of claims 7 to 9 characterised in that, prior to the step of contacting the solution with the flue gas, the method comprises the step of:

Cooling the flue gas.

11. A method according to any one of claims 7 to 10 characterised in that, prior to the step of contacting the solution with the flue gas, the method comprises the step of: Cooling the flue gas and retaining the heat therefrom.

12. A method according to any one of claims 7 to 11 characterised in that, prior to the step of contacting the solution with the flue gas, the method comprises the step of:

Dehumidifying the flue gas.

13. A method according to claim 12 when dependent on claim 10 characterised in that the steps of cooling the flue gas and dehumidifying the flue gas are concurrently achieved by the step of:

Contacting the flue gas with a cool water stream.

14. A method according to claim 2, comprising the step of cooling the flue gas and retaining the heat therefrom, the method characterised by the step of:

Utilising the heat retained from the step of cooling the flue gas to produce the bicarbonate solution at elevated temperature.

15. A method according to claim 14 characterised in that the flue gas is contacted with the solution at a temperature amenable to the efficient formation of the bicarbonate solution, whilst the bicarbonate solution is contacted with the alkaline Bayer process residue at a temperature amendable to the efficient neutralisation of solid alkalinity in the form of tri-calcium aluminate.

16. A method according to any one of claims 1 to 15 characterised in that the pH of the bicarbonate solution is between about 7.5 and 9.0.

17. A method according to claim 16 characterised in that the pH of the bicarbonate solution is between 7.5 and 8.0.

18. A method according to any one of claims 1 to 17 characterised in that the total alkali concentration of the bicarbonate solution is substantially identical to the total alkali concentration of the alkaline Bayer process residue.

19. A method according to any one of claims 1 to 18 characterised in that the method comprises the step of:

Thickening the alkaline Bayer process residue.

20. A method according to any one of claims 1 to 19 characterised in that, after the step of contacting the solution with a gaseous stream containing carbon dioxide, the method comprises the step of:

Recovering a carbonate solution.

21. A method according to claim 20 characterised in that the step of contacting the alkaline Bayer process residue with a bicarbonate solution in a reaction vessel having an overflow and an underflow, and the step of recovering a carbonate solution comprises the step of:

Recovering the carbonate solution as an overflow from the reaction vessel.

22. A method according to claim 21 characterised in that the reaction vessel is a gravity separation vessel.

23. A method according to any one any one of claims 20 to 22, when dependent on claim 3 characterise in that the carbonate solution contacted with carbon dioxide is provided in the form of the recovered carbonate solution.

24. A method for the treatment of an alkaline Bayer process residue, the method comprising the steps of:

Contacting the alkaline Bayer process residue with the bicarbonate solution;

Recovering a carbonate solution; and Recycling the recovered carbonate solution by contacting the recovered carbonate solution with a gaseous stream containing carbon dioxide to form a bicarbonate solution.

25. A method for the treatment of an alkaline Bayer process residue, the method comprising the steps of: Cooling and dehumidifying a flue gas by contacting the flue gas with a cool water stream and retaining the heat therefrom;

Heating the bicarbonate solution utilising the heat recovered from the cooled flue gas;

Contacting the alkaline Bayer process residue with the heated bicarbonate solution;

26. A method according to claim 24 or 25 characterised in that the steps of the method are performed simultaneously in a continuous process.

27. A method for the treatment of an alkaline Bayer process residue substantially as described herein with reference to Figure 1.