WO2017193162A1

WO2017193162A1 - Methods for the treatment of red mud slurries

Info

Publication number: WO2017193162A1
Application number: PCT/AU2017/050421
Authority: WO
Inventors: Greg Kane; Clive Roscoe
Original assignee: SNF (Australia) Pty Ltd
Priority date: 2016-05-09
Filing date: 2017-05-09
Publication date: 2017-11-16
Also published as: AU2017261841B2; AU2017261841A1

Abstract

The present invention broadly relates to methods for treating red mud slurries, and more particularly to methods for increasing underflow solids in red mud slurries and to methods for accelerating the consolidation of flocculated solids in red mud slurries.

Description

Methods for the treatment of red mud slurries

Field of the Invention

Background of the Invention

The production of alumina from ores such as bauxite is typically performed using the Bayer process. As part of the Bayer process alumina-containing ores are digested with sodium hydroxide at elevated temperature and pressure. The insoluble product generated following digestion is known as "red mud". Amongst other compounds, red mud comprises iron oxide (which is responsible for the red colour), silica, unleached aluminium and titanium oxide. Disposal of red mud is problematic due to its high alkalinity. The total alkalinity of a super thickener feed from an alumina refinery typically ranges anywhere between about 5 gpl and 35 gpl.

Red mud is typically treated by first removing coarse material, such as sand, followed by settling in raking thickeners with the addition of flocculants. The underflow solids are then pumped to settling ponds where dewatering and air-drying take place to provide a residue having about 65% w/w solids. Residue containing this amount of solids may be disposed of by the so-called "dry-stacking" method.

Optimisation of the underflow solids obtained following settling is crucial to minimising the footprint of the settling ponds. A decrease in underflow solids density of only a few percent can substantially increase the settling pond footprint. However, increasing the density of the underflow solids may lead to a concomitant increase in the yield stress of the slurry, thereby reducing the ability of the underflow to be pumped.

Against this background the present inventors have developed a red mud treatment method in which underflow solids may be optimised without compromising pumpability and dry stacking characteristics.

Summary of the Invention

In a first aspect the present invention provides a method for treating a red mud slurry, the method comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry. The method may further comprise separating flocculated solids from liquid obtained following steps (i) and (ii).

In a second aspect the present invention provides a method for increasing underflow solids density in a red mud slurry comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry.

Following steps (i) and (ii) the solids may be capable of being pumped.

In a third aspect the present invention provides a method for accelerating the consolidation of flocculated solids in a red mud slurry comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry.

The method may further comprise separating flocculated solids from liquid obtained following steps (i) and (ii).

In a fourth aspect the present invention provides a method for modulating the rheology of a red mud slurry comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry.

The dextran and the lignosulfonate may be present in a single composition when added to the slurry. In an alternative embodiment the dextran and the lignosulfonate may be added separately to the slurry.

The dextran and the lignosulfonate may be added to the slurry prior to addition of the flocculant. In an alternative embodiment the dextran and the lignosulfonate may be added to the slurry after addition of the flocculant. In a further embodiment the dextran and the lignosulfonate may be added to the slurry simultaneously with the flocculant.

The dextran and the lignosulfonate may be added neat to the slurry, or alternatively may be diluted with water, such as for example lakewater, pond water, sea water or any other suitable source of water.

The dextran may have a number average molecular weight (Mn) between about 1 million and about 100 million, or between about 5 million and about 50 million. In some embodiments the dextran is cross-linked. In some embodiments the dextran is non-newtonian.

The dextran may have a viscosity of about 60,000 cPs at 15% w/w in water at a temperature of about 20 °C.

The flocculant may be a synthetic polymer, for example a water-soluble polymer of one or more ethylenically unsaturated monomers.

In one embodiment the flocculant is an anionic acrylate/acrylamide copolymer.

The dextran may be added to the slurry in an amount between about 0.1 grams per ton and about 30 grams per ton, or in an amount between about 0.1 grams per ton and about 20 grams per ton, or in an amount between about 0.1 grams per ton and about 15 grams per ton, or in an amount between about 1 gram per ton and about 10 grams per ton.

The lignosulfonate may be added to the slurry in an amount between about 0.1 grams per ton and about 10 grams per ton, or in an amount between about 0.1 grams per ton and about 7.5 grams per ton. The methods may be performed in a thickener or a settler.

In a fifth aspect the present invention provides a composition comprising a dextran and a lignosulfonate.

The dextran may be as defined above.

In a sixth aspect the present invention provides use of a composition of the fifth aspect in the treatment of a red mud slurry.

In a seventh aspect the present invention provides use of a composition of the fifth aspect as a conditioning aid in the treatment of a red mud slurry.

Brief Description of the Figures

Figure 1 is a plot of underflow solids (% w/w) versus RMD200 dose.

Definitions

The following are some definitions that may be helpful in understanding the description of the present invention. These are intended as general definitions and should in no way limit the scope of the present invention to those terms alone, but are put forth for a better understanding of the following description.

Throughout this 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 element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Thus, in the context of this specification, the term "comprising" means "including principally, but not necessarily solely".

In the context of this specification the term "about" is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result.

In the context of this specification the terms "a" and "an" are used herein to refer to one or to more than one (i.e to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element.

In the context of this specification the term "dextran" is understood to refer to an a-D-1 ,6 glucose-linked glucan with side chains 1 -3 linked to the backbone units of the polysaccharide.

In the context of this specification the term "lignosulfonate" is understood to refer to water-soluble polymers that are by-products from the production of wood pulp using sulfite pulping.

In the context of this specification the term "flocculant" is understood to mean a compound or composition which when added to a liquid containing finely divided suspended particles, destabilises and aggregates the solids.

In the context of this specification the term "red mud" refers to the insoluble solid material which is a residual product from Bayer process liquor that does not freely dissolve during the digestion stage, or which precipitates as part of the digestion process.

Detailed Description of the Invention

The present invention is predicated on the surprising finding by the inventors that treatment of a red mud slurry with dextran and a lignosulfonate together with a flocculant increases the rate of solids compaction and ultimately delivers higher underflow solids compared to use of the flocculant alone, without compromising pumpability and dry stacking characteristics. For example, it has been found that pre-dosing a composition comprising dextran and lignosulfonate into a super thickener feedline prior to addition of an anionic flocculant increased underflow density by 3.2% w/w compared to use of the anionic flocculant alone.

Accordingly, in one embodiment the present invention relates to a method for treating a red mud slurry comprising adding a dextran and a lignosulfonate to the slurry, and adding a flocculant to the slurry. In another aspect the present invention relates to a method for increasing underflow solids density in a red mud slurry comprising adding a dextran and a lignosulfonate to the slurry, and adding a flocculant to the slurry. In this aspect, "increasing underflow solids density" may be measured relative to the underflow solids density obtained when using the flocculant only.

In yet another aspect the present invention relates to a method for accelerating the consolidation of flocculated solids in a red mud slurry comprising adding a dextran and a lignosulfonate to the slurry, and adding a flocculant to the slurry.

In a further aspect the present invention provides a method for modulating the rheology of a red mud slurry comprising adding a dextran and a lignosulfonate to the slurry and adding a flocculant to the slurry. Without wishing to be bound by theory it is thought that the dextran/lignosulfonate combination alters the rheology of the underflow such that the underflow has properties that would otherwise be associated with an underflow slurry of lower solids density. In other words, the dextran/lignosulfonate combination acts as a slurry conditioning aid.

In the above aspects the methods may further comprise separating flocculated solids from the liquid obtained following steps (i) and (ii). Removal of flocculated red mud from the supernatant liquid may be performed by methods well known to those skilled in the art, for example by sedimentation, centrifugation, filtration, or by use of a washer, thickener or settler.

In still a further aspect the present invention relates to a composition comprising a dextran and a lignosulfonate. The dextran and the lignosulfonate may be as described herein.

In another aspect the present invention provides use of a composition comprising a dextran and a lignosulfonate in the treatment of a red mud slurry.

Typically, in the methods of the invention, the dextran and the lignosulfonate are present in a single composition when added to the red mud slurry. However, the dextran and the lignosulfonate may be added separately to the slurry if desired. For example, there may be application scenarios where there is an inherently high slurry viscosity in a thickener feed which may compromise feedwell flocculation and impact on a range of key thickener performance indicators including dose rates, underflow density, overflow clarity etc. In such scenarios it may be beneficial to add the lignosulfonate earlier in the process so as to reduce the feed slurry viscosity. Typically, the dextran and the lignosulfonate are added to the red mud slurry prior to addition of the flocculant. However, the dextran and the lignosulfonate may be added to the red mud slurry after addition of the flocculant, or simultaneously with addition of the flocculant. The time difference between addition of the dextran and the lignosulfonate and the flocculant may vary depending on process throughput, however in some embodiments the difference will be between about 10 seconds and 10 minutes.

The dextran and the lignosulfonate may be added neat to the red mud slurry, or alternatively may be diluted with any aqueous liquid, such as for example, lake water, pond water, sea water or any other suitable source of water. In some embodiments the dextran and the lignosulfonate are added as part of a single aqueous composition prior to addition of the flocculant. In other embodiments the dextran and the lignosulfonate are added as part of a single aqueous composition after addition of the flocculant.

Dextrans suitable for use in the methods and compositions of the invention may have a number average molecular weight (M_n) between about 1 million and about 100 million, or between about 5 million and about 50 million. The dextrans may be anionic, non- ionic or cationic. In some embodiments the dextran has a viscosity between about 55,000 and 65,000 cPs at 15% solution in water at a temperature of about 20 °C. In alternative embodiments the dextran is non-newtonian. In other embodiments the dextran is cross-linked.

Dextrans suitable for use in the methods and compositions of the invention may be obtained from High Performance Product Engineering, Columbus, GA, USA.

Lignosulfonates suitable for use in the methods and compositions of the invention include those commercially available from Borregaard AS under the trade name PIONERA L-800.

In the methods of the invention the dextran may be added to the slurry in an amount between about 0.1 grams per ton and about 30 grams per ton, or in an amount between about 0.1 grams per ton and about 20 grams per ton, or in an amount between about 0.1 grams per ton and about 15 grams per ton, or in an amount between about 1 gram per ton and about 10 grams per ton.

In the methods of the invention the lignosulfonate may be added to the slurry in an amount between about 0.1 grams per ton and about 10 grams per ton, or in an amount between about 0.1 grams per ton and about 7.5 grams per ton, or in an amount between about 0.2 grams per ton and about 5 grams per ton. The exact amounts of dextran and lignosulfonate employed will likely depend on a variety of factors, including the particular dextran and lignosulfonate used, the flocculant used and the physical and chemical properties of the red mud slurry (i.e., alkalinity amount of suspended solids, mineral composition etc). Those skilled in the art will however be able to determine appropriate amounts of dextran and lignosulfonate to use under any given circumstances using only routine trial and experimentation. In some embodiments the ratio of dextran and lignosulfonate may be varied depending on the particular circumstances as well as the molecular weight and degree of cross-linking of the dextran.

Flocculants that may be used in the methods of the invention include any cationic, anionic, non-ionic or zwitterionic flocculants. In some embodiments the flocculant is a synthetic polymer or copolymer, for example a water-soluble polymer of one or more ethylenically unsaturated monomers. Typically these polymers are anionic with the anionic content dictated by the alkalinity of the red mud. Examples of water-soluble polymers of one or more ethylenically unsaturated monomers include, but are not limited to, polyacrylamides, copolymers of acrylic acid and acrylamide, copolymers of acrylate and acrylamide, copolymers of acrylic acid and acrylamide modified to include a hydroxamic acid moiety and copolymers of acrylic acid and acrylamide modified to include ammonium acrylate. Examples of other flocculants that may be employed in the methods of the invention include, but are not limited to, polyDADMAC, polyhydroxamates, polyamines, polyimides, carboxymethylcellulose, chitosan, guar and polyglutamic acid. Typically, the polymer has a molecular weight of greater than 10 million. In some embodiments the flocculant may also be a dextran. In other embodiments the flocculant may be a flocculant other than a dextran.

The amount of flocculant used will depend primarily on the properties of the red mud slurry being treated and can be empirically determined readily by one of skill in the art. Typically, the amount of flocculant required is dependent on both the nature of the slurry, the type of flocculant used and the target settling rate band required.

Preconditioning the slurry with dextran and lignosulfonate may also aid the overall reduction in flocculant requirements as the settling rate and sometimes clarity will improve, over the "blank", prior to the addition of the flocculant.

The methods of the invention may be performed in a thickener. The use and operation of thickeners in red mud treatment is well known amongst those skilled in the art. The dextran/lignosulfonate combination and the flocculant may be added in various regimens so as to enhance red mud disposal properties. For example, a composition comprising dextran and lignosulfonate may be introduced via a thickener feed line prior to addition of a flocculant. Alternatively, the dextran/lignosulfonate combination and the flocculant could be added simultaneously into a thickener feedwell. As a further alternative the dextran/lignosulfonate combination may be injected into a primary flocculant dilution line. Yet another alternative may involve addition of the flocculant via a thickener feed line followed by injection of the dextran/lignosulfonate combination into the suction and/or discharge side of the thickener underflow pumps.

In some embodiments a composition comprising dextran and lignosulfonate can be used as either a preconditioning, post-conditioning or added simultaneously with flocculants/coagulants or other process additive(s) to a process slurry where the term "slurry" includes:

• Process slurry feeding into a thickener/washer;

• Underflow slurry pumped from a thickener/washer to another thickener/washer in circuit;

• Underflow slurry pumped from a thickener that is subsequently blended with a coarser tailings fraction (e.g. codisposal) and then reflocculated;

• Slurry being pumped to "any" type of filtration, dewatering and or clarification process.

Examples

The invention will now be described in more detail, by way of illustration only, with respect to the following examples. The examples are intended to serve to illustrate this invention and should not be construed as limiting the generality of the disclosure of the description throughout this specification.

Example 1 - Preparation of a composition comprising dextran and lignosulfonate

Form a solution by adding 97.5 kg of HPPE dextran (approximately 15% w/w active in water) Spec 5 -50M molecular weight (available from High Performance Product Engineering, Columbus, GA, USA) to 2.5 kg of a 30% w/v solution of PIONERA L-800 powder (available from Borregaard AS, Norway). Mix the solution using mechanical agitation for a minimum of 30 minutes to ensure solution homogeneity. A longer mixing time may be required, however typically the mixing time required will not exceed 60 minutes.

Example 2 - Laboratory test

The composition prepared in Example 1 (which may be referred to as "RMD200") was used in a bench red mud treatment method together with an anionic flocculant (ALP70VHM obtained from SNF (Australia) Pty Ltd, Lara, Victoria). Red mud was obtained from the Alcoa refinery located in Kwinana, Western Australia.

RMD200 (diluted to 1 .0% w/v using lakewater followed by dilution to 0.025% w/v using lakewater again) was added to the red mud slurry in doses ranging from 20 to 50 grams per ton (g/t), followed by the addition of 29 g/t of ALP70VHM flocculant (this amount may be varied depending on the target settling rate). An increase in underflow solids density from 41 .9% w/w to 45.1 % w/w was observed using 24 hour raked compaction tests and targeting a feedwell settling rate of around 15 m/hr. This represented an increase of around 3.2% w/w above the underflow density of 41 .9% w/w, achieved with the flocculant only, or a relative increase in the order of about 8%. Relevant data are presented below in Table 1 and in Figure 1 .

Figure 1 shows a strong point of inflection from around 30 g/t of RMD200 and a "dose window" of 30 - 45 g/t. Thereafter there is a decline in the underflow, however at 50 g/t RMD200 dose rate there is still a substantial increase in underflow density.

R

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 acknowledgement or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavor to which this specification relates.

It will be appreciated by those skilled in the art that numerous variations and/or modifications may be made to the invention without departing from the spirit or scope of the invention as broadly described. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1 . A method for treating a red mud slurry, the method comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry.

2. The method of claim 1 , further comprising separating flocculated solids from liquid obtained following steps (i) and (ii).

3. A method for increasing underflow solids density in a red mud slurry comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry.

4. The method of claim 3, wherein following steps (i) and (ii) the underflow solids are capable of being pumped.

5. A method for accelerating the consolidation of flocculated solids in a red mud slurry comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry.

6. The method of claim 5, further comprising separating flocculated solids from liquid obtained following steps (i) and (ii).

7. A method for modulating the rheology of a red mud slurry comprising:

(i) adding a dextran and a lignosulfonate to the slurry; and

(ii) adding a flocculant to the slurry.

8. The method of claim 7, further comprising separating flocculated solids from liquid obtained following steps (i) and (ii).

9. The method of any one of claims 1 to 8, wherein the dextran and the lignosulfonate are present in a single composition when added to the slurry.

10. The method of any one of claims 1 to 8, wherein the dextran and the lignosulfonate are added separately to the slurry.

1 1 . The method of any one of claims 1 to 10, wherein the dextran and the lignosulfonate are added to the slurry prior to addition of the flocculant.

12. The method of any one of claims 1 to 10, wherein the dextran and the lignosulfonate are added to the slurry after addition of the flocculant.

13. The method of any one of claims 1 to 10, wherein the dextran and the lignosulfonate are added to the slurry simultaneously with the flocculant.

14. The method of any one of claims 1 to 13, wherein the dextran has a number average molecular weight (Mn) between about 1 million and about 100 million.

15. The method of any one of claims 1 to 14, wherein the dextran has a viscosity of about 60,000 cPs at 15% w/w in water at a temperature of about 20 °C.

16. The method of any one of claims 1 to 15, wherein the flocculant is a synthetic polymer.

17. The method of claim 16, wherein the synthetic polymer is a water-soluble polymer of one or more ethylenically unsaturated monomers.

18. The method of claim 16, wherein the synthetic polymer is an anionic acrylate/acrylamide copolymer.

19. The method of any one of claims 1 to 18, wherein the dextran is added to the slurry in an amount between about 0.1 grams per ton and about 30 grams per ton.

20. The method of any one of claims 1 to 19, wherein the lignosulfonate is added to the slurry in an amount between about 0.1 grams per ton and about 10 grams per ton.

21 . The method of any one of claims 1 to 20, wherein the method is performed in a thickener or in a settler.

22. A composition comprising dextran and lignosulfonate.

23. Use of a composition according to claim 22 in the treatment of a red mud slurry.

24. Use of a composition according to claim 22 as a conditioning aid in the treatment of a red mud slurry.