AU755418B2 - Dextran, starch and flocculant combination for improving red mud clarification - Google Patents

Description

1 -1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT 0: ORIGINAL S Name of Applicant/s: Nalco Chemical Company Actual Inventor/s: Scott Lewis Barham Address for Service: BALDWIN SHELSTON WATERS 60 MARGARET STREET SYDNEY NSW 2000 S Invention Title: "DEXTRAN, STARCH AND FLOCCULANT COMBINATION FOR IMPROVING RED MUD CLARIFICATION" Details of Associated Provisional Application No. PP3704 dated 25 MAY 1998 The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 22354.00 -2- FIELD OF THE INVENTION The invention relates to a method for enhancing flocculation of red mud in the Bayer process and, more particularly, the use of certain additives for increasing the settling rate of red mud and/or improving the clarity of the supemrnatant liquid.

BACKGROUND OF THE INVENTION Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

In the Bayer process for the production of alumina, bauxite ore is pulverised, slurred in water, and then digested with caustic at elevated temperatures and pressures.

The caustic solution dissolves oxides of aluminium, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore (referred to as "red I .mud") are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate product is precipitated out of the solution and oo o collected as product.

In more detail, the pulverised bauxite ore is fed to a slurry mixer where a water slurry is prepared. The slurry makeup water is typically spent liquor (described below) and added caustic. The bauzite ore slurry is then diluted and passed through a digester or a series of digesters where alumina is released from the ore as caustic-soluble sodium aluminate. The digested slurry is then cooled to about 1 10C (about 230'F), typically employing a series of flash tanks wherein heat and condensate are recovered. The aluminate liquor leaving the flashing operation contains from about 1 to about 20 weight percent suspended solids, which solids consist of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solid particles may be removed from the 500058154_I.DOc/BSW 2a aluminate liquor with a "sand trap" cyclone. The finer solid particles are generally separated from the liquor first by settling and then by filtration, if necessary. The slurrya. a a a a.

500058154_.DOCIBSXV -3of aluminate liquor and the finer solids is normally first fed to the center well of a mud settler, or primary settler, where it is treated with a flocculant, and as the mud settles, clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a weir at the top. This overflow from the mud settling tank is passed to the subsequent process steps. If the aluminate liquor overflowing the settler contains an unacceptable concentration of suspended solids (at times from about 50 to about 500 mg of suspended solids per liter), it is then generally further clarified by filtration to give a •filtrate with no more than about 10 mg suspended solids per liter of liquor. The treatment of the liquor collected after the primary settlement to remove any residual suspended solids before alumina trihydrate is recovered is referred to as a secondary S• clarification stage.

The clarified sodium aluminate liquor is seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, AI(OH) 3 The alumina trihydrate particles or crystals are then separated from the concentrated caustic liquor, 15 and the remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.

In another section of the Bayer circuit, the settled solids of the primary settler ("red mud") are withdrawn from the bottom of the settler and passed through a countercurrent washing circuit for recovery of sodium aluminate and soda. The countercurrent washing circuit utilizes two or more washers which receive a mud washer feed slurry from either the settler underflow or other washer underflow, as well as any dilution liquor. As noted above, the red mud does not include any coarser particles removed prior to feeding the slurry to the primary or mud settler.

-4- The at least partial separation of the red mud solids from the pregnant liquor at elevated temperatures by settling or by filtration is expedited by the use of a flocculant.

This initial clarification of the pregnant liquor into a clarified liquor phase is referred to as the primary settler stage. Flocculating agents improve the separation of insolubles by increasing the rate at which the solids settle, by reducing the amount of residual solids suspended in the liquor, and by decreasing the amount of liquor in the settled solids phase. Flocculation performance is highly important in the primary settlement stages.

Red muds are comprised chiefly of iron oxides (at least about 50 weight percent of the red mud solids), together with silicon oxides, calcium oxides, sodium alumino-silicates, titanium oxides and other materials, and commonly represent from about 5 to about *percent of the dry weight of the bauxite ore. Generally these muds are comprised of very fine particles, which hinder the desired rapid and clean separation of red mud particles from the solubilized alumina liquor. If the rate of separation is slow, output is materially diminished and overall process efficiency is impaired. If the separation is not clean, the 15 resultant aluminate liquor will require a more extensive treatment to remove residual solids, and/or the alumina trihydrate recovered will contain levels of impurities that are undesirably high for many end-uses.

The polysaccharides starch and dextran have, for some time, been used in red mud flocculation. For instance, U.S. Patent No. 3,085,853, April 16, 1963, Lesinski et al., uses native dextrans to increase the rate of sedimentation of finely divided solids in aqueous suspensions and thereby facilitate the separation of such solids. Later synthetic polymeric flocculants became more commonly employed for the Bayer process. U.S.

Patent No. 3,390,959 issued July 2, 1968 to Sibert, uses acrylate homopolymers and copolymers which contain not more than 20% of other ethylenically unsaturated polymerizable polar monomers for the Bayer process. Included in Sibert's polar comonomers are acrylamide and diethylvinylphosphonate, among others.

Diethylvinylphosphonate is the diethyl ester of vinylphosphonic acid, and can be hydrolyzed to the monoethyl ester in caustic solution.

U.S. Patent No. 3,397,953, August 20, 1968, Galvin et al., uses a blend of starch :and polyacrylic acid on red mud suspensions, noting that polyacrylic acid alone is not suitable as a flocculating agent. The polyacrylic acids exemplified generally have molecular weights of less than 300,000. The flocculation and sedimentation activity of the blend is exemplified in the primary settler stage of a bauxite process. U.S. Patent No. 3,445,187, May 20, 1969, Sibert, uses synthetic acrylic acid polymer alone to enhance the rate of separation of red mud solids from the aqueous caustic solutions during secondary clarification steps. The synthetic polymer used contains at least about weight percent of the acrylic acid mer unit, and has a molecular weight in excess of 15 50,000, and preferably in excess of 100,000. U.S. Patent No. 3,541,009, November 17, 1970, Arendt et al., uses a combination of causticized or modified starch, a water soluble polymer, and a caustic alkali to enhance the coagulation, sedimentation and/or filtration of aqueous suspensions of solids, including the settling of red mud from Bayer process liquor. The water soluble polymer is derived from at least one olefinically-unsaturated monomer and has a molecular weight in excess of 100,000.

U.S. Patent No. 3,681,012, August 1, 1972, Sibert, uses an acrylic acid polymer most preferably having a molecular weight of at least, 1,000,000, either alone or in combination with starch, for clarification of digested bauxite containing solubilized -6alumina and red mud residues. U.S. Patent No. 4,767,540, August 30, 1988, Spitzer et al., uses a polymer that contains hydroxamic acid groups for the same purpose. U.S.

Patent No. 5,008,089, April 16, 1991, Moody et al., uses a combination of dextran and synthetic anionic polymer for flocculating red mud in Bayer process liquors.

U.S. Patent No. 5,217,620, June 8, 1993, Mahoney et al., uses a combination of pullulan, lactan, rhamsan, or zooglan with a conventional water soluble anionic flocculant for red mud settling.

The synthetic flocculating agents employed for the settling or filtration of red mud are generally water soluble polymers of one or more ethylenically-unsaturated monomers, and have been used together, as noted above, with starch or dextran for aluminate liquor clarification. The synthetic flocculating agents are usually anionic, and the optimum anionic content of such polymer is usually related to the alkalinity of the liquor. In the washing circuit, the early wash liquors have the highest alkalinity and may require a more highly anionic polymer than the later wash liquors.

:15 The present invention seeks to overcome or at least ameliorate some of the disadvantages of the prior art or provide a commercial alternative thereto.

DISCLOSURE OF THE INVENTION In a first aspect, the present invention provides a method for treating Bayer process liquor containing red mud comprising contacting the Bayer process liquor with, in combination, an effective amount of a water soluble synthetic flocculant, dextran and starch prior to separating the red mud from the liquor.

In a second aspect, the present invention provides an agent for treatment of Bayer process liquor containing red mud said agent comprising, in combination, a water soluble synthetic flocculant, dextran and starch in amounts effective to increase separation of the red mud from the Bayer process liquor.

The combination preferably contacts the slurry containing the red mud suspended in Bayer process liquor, or a liquor slurry containing bauxite prior to or during digestion. The dextran, starch and flocculant combination can be added to the Bayer process liquor separately or together provided that in at least one point of the process a combination of all three components are present in the Bayer process liquor. If the three components are added separately, they may be added in any order, but it is preferred to add the starch and polymer (separately or together) prior to the addition of the dextran.

In preferred embodiments, the starch and polymer are added to the process upstream from the point of addition of the dextran.

Once the components of the combination are added, they are mixed sequentially with the Bayer process liquor, and the red mud contained in the Bayer process liquor is removed by sedimentation, centrifugation or filtration.

15 Water-soluble synthetic flocculants which may be used in combination with dextran and starch include, but are not limited to acrylates, homopolymers of acrylic acid, copolymers of acrylic acid and acrylamide and copolymers of acrylic acid and acrylamide modified to contain a hydroxamic acid or acrylic acid moieties. Particularly preferred are ammonium acrylate polymers because of their replacement ratio and apparent synergy. The red mud thus treated may be separated from the liquor phase using a separator selected from the group consisting of settlers, thickeners, centrifuges and filters.

-8- Preferably, the combination which contacts the Bayer process liquor is used in an amount of from about 0.01 to about 10 grams per liter of Bayer process liquor treated.

The combination is more preferably used in an amount of from about 0.1 to about 2 grams per liter of liquor treated. The combination may contact the Bayer process liquor anywhere. For example the combination may contact the Bayer process liquor at a point selected from the group consisting of the primary settler feed, bauxite pretreatment, bauxite digestion and flash tanks. As stated above the dextran, starch and polymer may S" be added to the liquor separately or together. Preferably the starch and polymer are •o.

added together as one solution or separately as far back upstream from the addition of dextran as possible (the further back, the better for clarity reduction). For example the starch and polymer can be added as one solution or separately to a thickener feed line followed by addition of dextran to the feed line just prior to the feedwell or into the feedwell via sparges. Preferably the combination contacts the Bayer Process liquor in the primary settler feed.

15 Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

BRIEF DESCRIPTION OF DRAWINGS The present invention will now be described by way of reference to the following non-limitative examples and drawings in which: Figure 1 is a graph comparing dosage of starch in grams per tonne (GPT) to reduction in clarity and -9- Figure 2 is a graph comparing additions of various constituents in grams per tonne (GPT) and their effect on clarity.

BEST MODE FOR CARRYING OUT THE INVENTION To evaluate the effectiveness of the combination, settling tests were completed in a waterbath with temperature maintained at 96C. Eighteen (18) cylinders of identical mud/liquor characteristics were tested during one experiment.

High Molecular Weight Homopolymer Ammonium Acrylate (hereinafter referred to as Polymer A) with a molecular weight greater than 10 million and supplied by Nalco was diluted in spent liquor to a concentration of 1.5 gpl by introducing the neat polymer to the vortex produced by a cage stirrer at 800 RPM and mixing four five minutes. Dextran (hereinafter referred to as Polymer B) was diluted in lake water to a concentration of either 5 or 10 gpl (0.5 or by gentle mixing by shaking the bottle by hand. Starch was supplied as a 400 gpl solution and diluted with lakewater to 100 gpl and then further diluted with spent liquor to a final concentration of 15 gpl again by shaking the bottle by hand.

Polymer and Starch solutions were added together and the cylinders mixed using a gang plunger which allows six cylinders to be tested at one time. After addition and mixing of polymer and starch solutions the dextran solution was added and further mixing completed using the gang plunger.' Settling rate is presented in m/h and determined by measuring the time for flocculated mud particles to settle from 1000 ml to 600 ml in a 1000 ml cylinder.

Clarity was determined by decanting 250 ml of supernatant from the top of the settled cylinder after 30 minutes, adding 75 ml of ION NaOH and boiling mixture to 10 negate any precipitation of hydrate. After boiling, the liquor was cooled and passed through a turbidity meter and clarity determined and presented in NTU units.

The results of these tests showing the various synthetic flocculant/starch/dextran dosages are shown in Tables 1 and 2.

S..

S 55 *5 *5*

S

S

S

S *5 *S S 555 *S* S. S S St S S S S S S S S S. 55. TABLE 1 Cylinder Plunges After Polymer/ Starch Addition Plunges After Dextran Addition Polymer Polymer Starch Starch Dextra Dose Dose Dose Dose Dose (ml) (gpt) (ml) (gpt) (ml) n Dextran Settling Clarity Reduction Dose Rate in Clarity (gpt) (in/h) (NTU) N% 0 250 500 750 1000 1500 0 250 500 750 1000 1500 0 250 500 750 1000 1500 100 100 100 100 100 100 200 200 200 200 200 200 400 400 400 400 400 400 220 215 210 200 200 155 245 225 215 210 185 145 215 205 160 140 115 a..

a.

a a *a a..

a.

a a a a aaa a.

a a:I*a 00 .aa TABLE 2 Cylinder Plunges After Polymer/ Starch Addition Plunges After Dextran.

Addition Polymer Dose (ml) 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 Polymer Dose Starch Starch Dose Dose Dextran Dose Dextran Settling Dose Rate Clarity Reduction in Clarity (gpt) (ml) (gpt) (ml) 0 250 500 1000 1500 2000 0 250 500 1000 1500 2000 0 250 500 1000 1500 2000 (gpt) 0 0 0 0 0 0 250 250 250 250 250 250 500 500 500 500 500 500 (in/h) (NTU) 235 185 200 215 190 160 210 200 200 140 110 230 195 165 135 -13- The results are also shown in the attached Figure 1. It can seen from the figure that including dextran and polymer in a starch flocculant combination resulted in a marked improvement in clarity.

To highlight the synergistic effect of the various constituents of the treatment agent, further tests were carried out with various dosages of the water soluble synthetic flocculant, starch and dextran. The synthetic flocculant (hereinafter referred to as Nalco 85111) was a high molecular weight ammonium acrylate with a molecular weight greater than 10 million made up at 0.18% solution in lake water. The starch was made S• up to 1.1% in spent liquor as per conventional plant practice. The dextran (hereinafter referred to as Nalco 85711) had a molecular weight of greater than 500,000 and was made up as a 1% solution in lake water.

As with the previous examples, the polymer and starch were added first. The combination of slurry, starch and Nalco 85111 were mixed by plunging 10 times and S0, then the Nalco 85711 dextran added and mixed by plunging a further five times.

15 The clarity tests were conducted in a manner similar to the aforementioned examples, however, the clarity was determined after five minutes to allow the differences in dosages to be more clearly identified.

Results of the tests are shown in Table 3 and figure 2.

*0 0 .0 0.0 TABLE- 3- COMPARATIVE EXAMPLES Eg 85111 Conc (gpl) 1 1.8 2 1.8 3 1.8 4 1.8 1.8 6 1.8 7 1.8 85111 Dose (ml) 1.5 0 1.5 1.5 0 0 1.5 Starch Cone (gpl) Starch Dose 9 9 9 0 9 0 0 85711 Conc (gpl) 1 85711 Dose (ml) 20 20 0 20 0 20 0 Feed Solids (gpl) 40 40 40 40 40 40 40 Floe Dose (gpt) 68 0 68 68 0 0 68 Starch Dose (gpt) 990 990 990 0 990 0 0 85711 Dose (gpt) 500 500 0 500 0 500 0 S rate T600 (secs) 45 300 57 64 600 80 Srate (rn/h) 11.1 1.7 8.8 7.8 0.8 <0.2 6.3 0/F Clarity

NTU

185 275 135 250 1000 370 The synergistic effect of the three constituent making up the treatment agent will be clear from these results and in particular figure 2. As can be seen from this figure, in each example where one of the constituent is left out, there is a substantial reduction in the clarity determined after five minutes. The closest comparative example is Example 4 in which only dextran and the water soluble synthetic flocculant are added to the Bayer process liquor. In this instance, clarity is measured as 135 NTU. This is nearly higher than Example 1 using the proposed treatment agent (clarity 80 NTU). Other comparative examples are between two and 12 times less effective.

.:000: It will be clear to persons skilled in the art therefore that the combination of water soluble synthetic flocculant, dextran and starch provides a significant increase in the effectiveness of separation processes particularly sedimentation, centrifugation and filtration which is unrecognised and hitherto unsuspected from the prior art.

Changes can be made in the composition, operation and arrangement of the method of the present invention described herein without departing from the concept and 15 scope of the invention as defined in the following claims.

Claims (12)

1. A method for treating Bayer process liquor containing red mud comprising contacting the Bayer process liquor with, in combination, an effective amount of a water soluble synthetic flocculant, dextran and starch prior to separating the red mud from the liquor.

2. A method as claimed in claim 1 wherein the red mud is separated from the liquor by a process selected from the group consisting of sedimentation, centrifugation and filtration.

3. A method of claims 1 or 2 wherein the water soluble synthetic flocculant, dextran and starch combination is used in an amount of from about 0.01 to about 10 grams per liter of liquor treated.

4. A method of claims 1 or 2 wherein the water soluble synthetic flocculant, dextran and starch combination is used in an amount of from about 0.1 to about 2 grams per liter S. of liquor treated. 15 5. A method according to any of the preceding claims wherein the water soluble synthetic flocculant, dextran or starch are added separately or together to the Bayer process liquor.

6. A method according to any of the previous claims wherein the water soluble synthetic flocculant and starch'are added together to the Bayer process liquor and one solution and separate from the dextran.

7. A method according to claim 6 wherein the water soluble synthetic flocculant and starch are added together to the Bayer process liquor upstream of the dextran addition to the Bayer process liquor. -17-

8. A method according to any one of the previous claims wherein the water soluble synthetic flocculant, dextran and starch combination contacts the Bayer process liquor at one or more points selected from the group consisting of primary settler feed, bauxite pretreatment, bauxite digestion and the flash tanks.

9. A method according to any of the preceding claims wherein the step of separating the red mud from the liquor is carried out by a separator selected from the group consisting of settlers, thickeners, centrifuges and filters.

10. A method according to any of the preceding claims wherein the water soluble synthetic flocculant is selected from the group consisting of homopolymers of acrylic acid, copolymers of acrylic acid and acrylamide, copolymers of acrylic acid and acrylamide modified to contain a hydroxamic acid moiety and copolymers of acrylic acid and acrylamide modified to contain an acrylic acid moiety. S 11. An agent for treatment of Bayer process liquor containing red mud said agent comprising, in combination, a water soluble synthetic flocculant, dextran and starch in a 15 quantity sufficient to increase separation of the red mud from the Bayer process liquor. 12 An agent as claimed in claim 11 wherein the water soluble synthetic flocculant, dextran and starch combination is used in an amount of from about 0.01 to about 10 g/1 of liquor treated.

13. An agent as claimed in claim 11 wherein the water soluble synthetic flocculant, dextran and starch combination is used in an amount of from 0.1 to about 2 g/l of liquor treated.

14. An agent as claimed in any one of claims 11 to 13 wherein the agent comprises two components, a first component comprising water soluble synthetic flocculant and

18- starch and a second component comprising dextran, the two components being suitable for separate addition to the Bayer process liquor. An agent as claimed in any one of claims 11 to 14 wherein the water soluble synthetic flocculant is selected from the group consisting of homopolymers of acrylic acid, copolymers of acrylic acid and acrylamide, copolymers of acrylic acid and acrylamide modified to contain hydroxamic acid moiety and copolymers of acrylic acid and acrylamide modified to contain an acrylic acid moiety. 16. A method for treating Bayer process liquor containing red mud substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. 17. An agent for treatment of Bayer process liquor containing red mud substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. DATED this 25th Day of May 1999 15 NALCO CHEMICAL COMPANY Attorney: PAUL G. HARRISON Fellow Institute of Patent Attorneys of Australia of BALDWIN SHELSTON WATERS