AU667390B2

AU667390B2 - Trihydrate clarification aid for the bayer process

Info

Publication number: AU667390B2
Application number: AU46114/93A
Authority: AU
Inventors: Robert P. Mahoney
Original assignee: Nalco Chemical Co
Current assignee: ChampionX LLC
Priority date: 1992-09-14
Filing date: 1993-09-03
Publication date: 1996-03-21
Anticipated expiration: 2013-09-03
Also published as: IE930677A1; GB9318210D0; CA2105333A1; GB2270519A; AU4611493A; GB2270519B; IE76299B1

Description

Hi 667390 1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE S P E C IF I CATION FOR A STANDARD PATENT 0 0 0'a Q

ORIGINAL

o 0 a a 0 09 o o a 000 o:*Name of Applicant: 04 o Actual Inventor: 04 00 o 0 o0: 'Address for Service: 0 0 0 0o NALCO CHEMICAL COMPANY Robert P. MAHONEY SHELSTON WATERS Clarence Street SYDNEY NSW 2000 So* Invention Title: 0 "TRIHYDRATE CLARIFICATION AID FOR THE BAYER

PROCESS"

Th' following statement is full description of this invention, including the best method of performing it known to us:- I- -I 'C;r 2

SPECIFICATION

Technical Field of the Invention The present invention is in the technical field of trihydrate clarification, the settling of alumina trihydrate particles, in the Bayer process for the recovery of alumina from bauxite ore.

Background of the Invention In the Bayer process for the production of alumina, bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate product is precipitated out of the solution and collected as product.

SIn more detail, the pulverized 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 S' caustic. This bauxite ore slurry is then diluted and passed through a digester or a series of digesters where about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. The digested slurry is then cooled to about 230 OF, typically -3employing 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 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 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 slurry of aluminate liquor and finer solids is normally first fed to the center well of a mud settler, or primary settler, where it is treated with a 0 o flocculant, and as the mud settles, clarified sodium aluminate solution, referred to as "green" or S" "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 00 00 0 o0 suspended solids per liter), it is then generally further clarified by filtration to give a filtrate with 0 0 0 no more than about 10 mg suspended solids per liter of liquor.

o 0 Theclarified sodium aluminate liquor is seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, AI(OH),. The alumina 0* "0 trihydrate particles or crystals are then separated from the concentrated caustic liquor, and the remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.

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 clarification stage.

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. As noted above, the red mud does not 000000 oO include any coarser particles removed prior to feeding the slurry to the primary or mud settler.

0 00 0a q 0* a aIn the recovery of alumina trihydrate as product in the Bayer process, or for use as 0 P0 GoC 0 0 000 precipitation seed, the alumina trihydrate crystals are generally separated from the liquor in 0o which they are formed by settling and/or filtration. Coarse particles settle easily, but fine particles settle slowly and to some extent are lost product or, if recovered by filtration, blind the filters. The fine particles of alumina trihydrate which do not settle easily often flow 0 0 p s, o 41 0 over the top of the alumina trihydrate settler and are sent back to digestion with the spent liquor.

'"That alumina trihydrate is then digested and precipitated again in a second cycle through the Bayer process, unnecessarily expending energy and reducing the alumina extraction capacity of the spent liquor.

Canadian Patent No. 825,234, October 1969, uses dextran, dextran sulfate and compositions therewith containing anionic salts to improve the flocculation and filtration of I1P1 Il^rn-rl Y~ar~-eYr~^-rr-~~o;fe~iF)ilU~.

C-TIIllr ~li 5 alumina trihydrate from alkaline solutions thereof. U.S. Patent No. 5,041,269, August 1991, Moody et al., uses a flocculant for the recovery of alumina trihydrate crystals comprising a combination ofdextran, or certain other polysaccharides, together with an anionic flocculant polymer including acrylic monomer.

It is an object of the present invention to provide a more effective separation of alumina trihydrate particles from aqueous alkaline media, particularly the fine alumina trihydrate "0 0 particles, and particularly the separation thereof from the alkaline liquor in which they are 'I formed in the Bayer process. It is an object of the present invention to provide a method W a4 whereby the suspended solids retained in the supernatant after settling of the alumina trihydrate from the alkaline liquor of the Bayer process are diminished. It is an object of the present invention to provide a more effective Bayer process wherein the separation of alumina trihydrate S from the alkaline liquor is improved by an improved clarification of the supernatant that forms above the settling, or settled, alumina trihydrate solids. These and other objects of the present invention are described in more detail below.

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i .i -6- Disclosure of the Invention According to a first aspect of the invention, there is provided a method for the improved clarification of a Bayer process alumina trihydrate slurry comprising adding to a Bayer process alumina trihydrate tbhikener feed slurry an anionic water-soluble hydroxamated polymer in an amount effective to improve the clarification of said alumina trihydrate thickener feed slurry upon settlement by reducing the amount of suspended solids in the supernatant formed during said settlement.

The present invention provides a method for the improved clarification of a Bayer process alumina trihydrate slurry comprising adding a water-soluble anionic hydroxamated polymer to a Bayer process alumina trihydrate thickener feed slurry in an amount effective to improve the clarification of the slurry upon settlement. The present invention provides an improved Bayer process wherein the clarification of a Bayer process alumina trihydrate slurry is improved by adding to a Bayer process alumina trihydrate thickener feed slurry a water-soluble anionic hydroxamated polymer in an amount effective to improve the clarification of the slurry upon settlement.

Preferred Embodiments of the Invention In the Bayer process digestion, the aqueous alkaline solution acquires a sodium aluminate content. After primary settlement and secondary clarification the cr-icentrated aluminate liquor (a supersaturated solution) is seeded with alumina trihydrate crystals to induce precipitation. The precipitate settles and is then separated from the spent liquor and further treated.

The precipitated alumina in the form of alumina trihydrate is difficult to separate from the concentrated caustic liquor. The finer crystalline material tends to settle slowly and gives poor supernatant clarities, which results in product or precipitation seed losses.

Further, where

V

iVT O -7recovery of the precipitate includes a filtration step, fine material causes filter blinding. The addition of the hydroxamated polymer iraproves the supernatant clarity, reducing the amount of suspended solids, which are typically very fine suspended solids, in the supernatant upon settlement. The improved supematant clarity minimizes alumina trihydrate losses and improves the subsequent supernatant filtration by reducing filter blinding, or eliminates the need for such subsequent supernatant filtration. It is also believed that the hydroxamated polymer clarification 010.

aid will improve the alumina-caustic liquor separation on a vacuum filter by forming a more 0 porous filter cake.

0 Q0. The liquor from which the alumina trihydrate crystals are to be separated in a commercial Bayer process is commonly referred to as the alumina trihydrate thickener feed, or alumina trihydrate thickener feed slurry. It is the feed to the Bayer circuit station at which the °alumina trihydrate crystals, after formation, are to be concentrated, or thickened, by settlement, after which the supernatant and the alumina trihydrate crystals are separated. In a broader sense, an alumina.4rihydrate thickener feed slurry is an alkaline aqueous slurry in which the solids are predominantly aiumina trihydrate crystals, prior to separation of solids and the aqueous phase by settlement and the like.

The alumina trihydrate precipitate is usually recovered after settlement by filtration and washing of the settled phase after supernatant removal, although it may be subjected to Vi Irrr "r 8 44 r tt 64r 4 1411 4 44( t4 J 4I 1 settlement alone with removal of the supernatant. The recovered trihydrate crystals may then be treated by the usual processes, including calcination, which drives off the water and is sufficient to burn off any organic residues, including the residues of the hydroxamated polymer. The supernatant (spent liquor) is recycled in the conventional manner to the digestion stage.

A commercial alumina trihydrate thickener feed is most commonly a caustic liquor having a pH from about 10 to above 14, a temperaturs between about 75 OF and about 190 °F (from about 24 °C to about 88 and an alumina trihydrate solids content of from about 5 to about 100 grams per liter of alumina trihydrate thickener feed slurry.

The alumina trihydrate thickener feed slurry to which the hydroxamated polymer is added may have a concentration of dissolved organic components up to about 30 g/liter of slurry, and such a slurry typically has a concentration of dissolved organic components in the range of from about 0.1 to about 5 or 10 g/liter of slurry. The solids content of the alumina trihydrate thickener feed slurry outside of the alumina trihydrate solids generally is no more than about 50 mg (.05 grams), and the concentration of solids other than alumina trihydrate solids generally does not exceed a weight ratio of about 1:100 other solids to alumina trihydrate solids.

The alumina trihydrate crystals therefore comprise at least about 99 weight percent of the total solids in the alumina trihydrate thickener feed slurry to which the hydroxamated polymer is added.

E-r~ II~C rOs- -9- The alkalinity of the .luimiina trihydrate thickener feed slurry can range from about 5 to about 400 g/liter of slurry, expressed as sodium carbonate. The alkalinity of alumina trihydrate thickener feed slurries typically are in the range of from about 50 to about 400 g/liter of slurry expressed as sodium carbonate, and often are within the range of from about 200 to about 400 g/liter of slurry expressed as sodium carbonate.

The process is generally carried out with the alumina trihydrate slurry at a temperature of 0 9 at least about 50 °C and often within the range of from about 60 "C to about 80 oC.

o i o The addition of the hydroxamated polymer is made after precipitation has been induced o and preferably while at least a significant portion of the alumina trihydrate solids remain suspended in the liquor, but it is possible to add it after some separation of alumina trihydrate solids has occurred as long as it is added before the fin.l separation of alumina trihydrate 0o 04 o o residuals.

0 The hydroxamated polymer should be added to the alumina trihydrate thickener feed Sslurry in anamount effective to improve the clarification of the supernatant upon settlement. An improvement in supernatant clarification is, of course, in comparison to supernatant clarification 0 for the same process parameters in the absence of the hydroxamated polymer.

The effective amount of hydroxamated polymer can depend on the concentration of alumina trihydrate solids in the slurry, other slurry conditions, and the hydroxamated poiymer's 12 k _nlYY 1 10 anionic charge density and molecular weight. The amount of the hydroxamated polymer added is usually in the range of from about 0.01 to about 20 mg/liter of slurry, preferably from about 0.05 to about 10 mg/liter of slurry, and more preferably from about 0.1 to about 5 mg/liter of slurry. Commonly less than about 0.5 mg/liter of slurry of polymer can be used.

The effective amount of hydroxamated polymer can be from about 0.0005 to about 1 pound of hydroxamated polymer per ton of alumina trihydrate solids in the slurry, and in more preferred embodiment from about 0.005 to about 0.1 pound of hydroxamated polymer per ton of alumina trihydrate solids in the slurry. About 0.1 pound per ton is the maximum needed under most conditions; and more than 1 pound per ton will often be unnecessary.

10 The hydoramic polymer should be sufficiently stable under the process conditions used, the temperatures and caustic conditions described above.

Any water soluble hydroxamic polymer may be used, but the hydroxamic polymer is commonly a polymer containing mer units with hydroxamic acid or salt pendant groups of the Formula I o000 0000 0 000000 0 0 CO O 0 00 0 0 0 **o oD~ 04 00 00 0 0 0 *0 o OO 00 0 o e a -f Formula I C =0 t 40 R" R

_W

11 wherein R is hydrogen or a cation, and R' and R" are independently hydrogen or a substituent to carbon other than hydrogen, for instance a lower alkyl such as methyl.

The hydrcxamic polymers, or hydroxamated polymers, are well known in the art and can be prepared by post-polymerization derivatization from polymers containing pendant reactive groups, such as pendant ester groups, pendant amide groups, pendant anhydride groups, pendant nitrile groups and the like by the reaction thereof with hydroxylamine or its salt at a temperature within the range of from abouL 20 "C to about 100 °C for several hours. From about 1 to about mole percent of the available pendant reactive groups of the precursor polymer may be replaced by hydroxamic groups in accordance with such procedures. Such post-polymerization 10 derivatization may be carried out in a polymer solution or in a polymer-containing latex, o 4 il S including both water-continuous latices and water-in-oil latices (wherein the polymer is substantially associated with the dispersed aqueous phase). When two or more species of pendant reactive groups are present in the precursor polymer, the reaction with hydroxyl amine may favor one or more of such species over the other(s). The molecular weight of the 15 hydroxamated polymer is dependent upon the molecular weight of the precursor polymer.

Hydroxamic acid or salt-containing polymers of very high molecular weights can be prepared by tt using a water-in-oil latex of, for example, polyacrylamide or copolymers of acrylamide with acrylic acid or other suitable comonomers t 1 12 Exemplary of the polymers which may be hydroxamated for use in the present process include acrylic acid ester polymers, methacrylic acid ester polymers, crotonic acid ester polymers, and the like carboxylic acid ester polymers, such as polymers produced from the polymerization of methyl (meth)acrylate, ethyl (meth)acrylate, t-butyl (meth)acrylate, cyclohexyl (meth)acrylate, dimethyl aminoethyl (meth)acrylate, dimethyl aminoethyl (meth)acrylate, methyl crotonate, and the like type of carboxylic acid ester containing mer units, generally wherein the ester moiety is derived from a alcohol, and preferably wherein the ester moiety is derived from a alcohol. The polymers which may be hydroxamated for use Sin the present process also include polymers of maleic anhydride and esters thereof, pendant 10 nitrile containing polymers such as those produced from acrylonitrile, pendant amide containing polymers such as those produced from acrylamide, methacrylamide and the like. The '"hydroxamic acid or salt-containing polymer may be derived from homopolymers, copolymers, terpolymers, or polymers of more varied mer units. Hydroxamic polymers are well known to those of ordinary skill in the art and are specifically disclosed, together with methods for their preparation, in U.S. Patent Nos. 3,345,344, 4,480,067, 4,532,046, 4,536,296, and 4,587,306, and U.K. Patent Application 2171127, hereby incorporated hereinto by reference. Suitable hydroxylamine salts include the sulfate, sulfite, phosphate, perchlorate, hydrochloride, acetate, propionate, and the like.

i Y- l---i-l----l--r*I-UIIII~PI~-mVfCC ~Clj~ -i~lljiVa-~PII-C-rCI~ 13 The hydroxamic acid or salt-containing polymer for use in the present process should generally have a weight average molecular weight within the range of from about 10,000 to about 50,000,000. The hydroxamated polymer preferably has a weight average molecular weight of at least aboutl million, preferably with an IV of from about 5 to about 40 dl/g.

The degree of hydroxamation, that is, the concentration of mer units (polymer segments containing two adjacent backbone carbons) of the Formula I above, may vary from about 1 to about 90 mole percent, and preferably is within the range of from about 5 to about 75 mole percent, and most preferably from about 10 to about 50 mole percent.

Since the alumina trihydrate thickener feed slurry is alkaline, the hydroxamated polymer 0 10 in preferred embodiment is predominantly anionic, although it can also contain nonionic or even 0 a minor amount of cationic mer units, provided that the amount of any such cationic mer units is sufficiently small so that the polymer retains an overall anionic nature and its water solublity.

The anionic mers other than the hydroxamic mer units (referred to herein as the "anionic mer units" unless expressly indicated otherwise) are generally carboxylic acids or sulphonic acids, St 15 and are usually derived from acrylic acid (AA) but can be derived from methacrylic acid (MAA) or a sulfoalkyl acrylamide, such as 2-sulfopropylacrylamide, or the other anionic monomers noted above. The hydroxamated polymer generally is comprised of from about 1 to about 99 mole percent, and preferably from about 15 to 90 mole percent, anionic mer units with I i r i. .r 14 the balance being hydroxamic mer units, optionally together with nonionic mer units. More preferably the hydroxamated polymer is comprised of from about 25 to about 75 mole percent anionic mer units, the balance being hydroxamic mer units, optionally together with nonionic mer units. In even more preferred embodiment, the hydroxamated polymer is comprised of from about 25 to about 75 mole percent anionic mer units, from about 10 to about 40 mole percent hydroxamic mer units and from about 10 to about 40 mole percent nonionic mer units.

The nonionic mer units are usually the polar (meth)acrylamide mer units (AcAm or methAcAm), but others may be used, for instance, mer units derived from vinyl acetate, vinyl o pyrrolidone, butadiene, styrene, alkanolacrylamides such as methylol acrylamide and others.

0 9 o, O O4 c 0 15 Test Method The test method employed for evaluating Bayer process trihydrate clarification with and without a trihydrate clarification aid is as follows. A quantity of alumina trihydrate thickener feed sufficient for the desired number of comparative test runs is obtained fresh from a commercial Bayer circuit, and for each test run a 1000 ml sample thereof ("test sample") is charged to a 1000 ml graduated cylinder. The test samples are each plunged ten times to re-suspend the alumina trihydrate particles, then dosed with the trihydrate clarification aid being used, if any, and again plunged ten times to disperse the trihydrate clarification aid in the alumina trihydrate thickener feed slurry. The plunging is accomplished using a stainless steel rod with a perforated steel disk or rubber stopper attached to the end. The slurry is then left 0 standing, without agitation, for a settlement time period of 30 minutes immediately following the final plunging. At the end of the 30 minute settlement period, a sample of the supernate liquor is collected and the turbidity of such supernate sample is determined with a turbidity meter. The turbidity of the supernate is inversely related to clarification efficiency. The 15 alumina trihydrate thickener feed slurries so obtained fresh from a commercial Bayer circuit will ,4 contain from about 5 to about 100 grams of alumina trihydrate crystal per liter of slurry, and no It ,.more than about 50 mg of other solids per liter of slurry.

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16 Example 1 The Test Method described above was employed to demonstrate the trihydrate clarificatio. performance of a representative hydroxamated polymer according to the present invention. The polymer was an anionic, water-soluble terpolymer comprised of acrylic acid, acrylamide, and acrylhydroxamic mer units in the approximate mole ratios of 2:1:1, derived from the hydroxamation of a high molecular weight acrylic acid/acrylamide copolymer. The weight average molecular weight of the hydroxamated polymer was about 10,000,000. The polymer dosage employed was 0.2 mg of polymer actives per liter of the slurry, and polymer being added as a dilute aqueous solution. A blank or control in which no trihydrate clarification 10 aid was used was also run as a comparison. The test results, in terms of supernatant turbidity 5 are set forth below in Table 1.

Table 1 Dosage of Clarification Supernatant Turbidity Clarification Aid Aid (mg/liter) (NTU) none none 168 i Hydroxamated Polymer 0.2 118 l 17 The hydroxamated polymer employed in the present invention is, as noted above, a water soluble polymer. The water solubility characteristics of the hydroxamated polymer preferably is defined in terms of the fluidity of its aqueous solutions. By "water soluble" is meant herein, and generally, that an aqueous solution of the polymer, at the polymer actives concentration at which it is charged to the aluminum trihydrate thickener feed slurry, is reasonably fluid, and preferably has a viscosity of no more than about 5,000 to 20,000 cps Brookfield, at a pH of between about 6 and about 14, and ambient room temperature (from about 23 to about 26 The addition of the hydroxamated polymer in the form of a dilute aqueous solution facilitates a rapid dispersion of the polymer in the slurry. Such aqueous solutions of the hydroxamated polymer should not 10 be overly viscous, but they also should not be so dilute that unnecessary volume of water is S" added to the alumina trihydrate slurry. For most hydroxamated polymers, an aqueous solution containing from about 0.01 to about 0.5 weight percent of polymer actives is generally reasonable. To further facilitate the rapid dispersion of the hydroxamated polymer in the slurry, in preferred embodiment the polymer is added to the alumina trihydrate thickener feed slurry as an alkaline aqueous solution, for instance having a pH of at least about 9, and more preferably at Sleast about 10, up to about a pH of about 14.

I -Cll' I ai*- arrarrrp~~"l--ur~-a--- c1-rmCira 18 The present invention provides a method for the improved clarification of a Bayer process alumina trihydrate slurry comprising adding to a Bayer process alumina trihydrate thickener feed slurry an anionic water-soluble hydroxamated polymer in an amount effective to improve the clarification of the slurry upon settlement. The present invention also provides an improved Bayer process for the production of alumina wherein bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures, forming an aqueous sodium aluminate solution containing caustic-insoluble constituents, wherein the caustic-insoluble constituents are then substantially separated from the aqueous phase of the aqueous sodium aluminate solution, and the sodium aluminate solution is then seeded with 10 crystals of alumina trihydrate whereby a solid alumina trihydrate product is precipitated out of the solution, subjected to settlement (whereupon a supernatant is formed) and collected as 0 o9 a 000 0 *0 product or recycled as precipitation seed. Such improved process is characterized by adding to 0000 the solution after formation of the alumina trihydrate precipitate, and prior to the complete formation of the supernatant, a water-soluble anionic hydroxamated polymer in an amount 15 effective to improve the clarification of the supernatant upon the settlement of the alumina 0 t trihydrate precipitate.

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19 In preferred embodiment, the alumina trihydrate thickener feed slurry or the solution has an alumina trihydrate solids content of from about 5 to about 100 grams per liter of alumina trihydrate thickener feed slurry or the solution. In preferred embodiment, the alumina trihydrate crystals of the alumina trihydrate thickener feed slurry or the solution comprise at least about 99 weight percent of the total solids in the alumina trihydrate thickener feed slurry or the solution.

In preferred embodiment, the alumina trihydrate thickener feed slurry or the solution has an alkalinity of from about 5 to about 400 g/liter of the slurry or the solution, expressed as sodium carbonate. In preferred embodiment, the hydroxamated polymer is added to the alumina trihydrate thickener feed slurry or the solution when the alumina trihydrate thickener feed slurry 10 or the solution is at a temperature within the range of from about 60 C to about 80 °C.

o In preferred embodiment, the amount of the hydroxamated polymer added to the alumina 0 S0 trihydrate thickener feed slurry or the solution is in the range of from about 0.01 to about 20 mg 0 00 000 o a per liter of the slurry or the solution. In preferred embodiment, the amount of the hydroxamated 00*0 polymer added to the alumina trihydrate thickener feed slurry or the solution is in the range of from about 0.05 to about 10 mg per liter of the slurry or the solution. In preferred embodiment, the amount of the hydroxamated polymer added to the alumina trihydrate thickener feed slurry 6 1 or the solution is in the range of from about 0.0005 to about 1 pound of the hydroxamated o polymer per ton of alumina trihydrate solids in the slurry or the solution.

1' 20 In preferred embodiment, the water soluble hydroxamic polymer is a polymer containing mer units of the Formula I

R'

I

C-C-

I

Formula I C

N-O-R

wherein R is hydrogen or a cation, and R' and R" are independently hydrogen or a substituent to carbon other than hydrogen, for instance a lower alkyl such as methyl.

In preferred embodiment, the hydroxamated polymer has a weight average molecular 10 weight within the range of from about 10,000 to about 50,000,000. In preferred embodiment, 4 the hydroxamated polymer has a weight average molecular weight of at least about 1 million. In S, preferred embodiment, the hydroxamated polymer has an Intrinsic Viscosity of from about 5 to t P 1 about 40 dl/g. In preferred embodiment, the hydroxamated polymer is comprised of from about to aboutJ5 mole percent anionic mer units other than hydroxamic mer units, the balance being hydroxamic mer units, optionally together with nonionic mer units. In preferred embodiment, the hydroxamated polymer is comprised of from about 25 to about 75 mole percent anionic mer units other than hydroxamic mer units, from about 10 to about 40 mole percent K< hydroxamic mer units and from about 10 to about 40 mole percent nonionic mer units.

t c I 21 Industrial Applicability of the Invention The present invention is applicable to the Bayer process for the production of alumina from bauxite ore.

4 I It t i i

Claims

1. A method for the improved clarification of a Bayer process alumina trihydrate slurry comprising adding to a Bayer process alumina trihydrate thickener feed slurry an anionic water-soluble hydroxamated polymer in an amount effective to improve the clarification of said alumina trihydrate thickener feed slurry upon settlement by reducing the amount of suspended solids in the supernatant formed during said settlement.

2. The method of Claim 1, wherein said alumina trihydrate thickener feed slurry has an alumina trihydrate solids content of from 5 to 100 grams per litre of alumina trihydrate thickener feed slurry.

3. The method of Claim 1 or 2 wherein the alumina trihydrate crystals of said alumina trihydrate thickener feed slurry comprise at least 99 weight percent of the total solids in said alumina trihydrate thickener feed slurry. i

4. The method of any one of Claims 1 to 3 wherein said alumina trihydrate thickener feed slurry has an alkalinity of from 5 to 400 g/litre of said alumina trihydrate thickener S 15 feed slurry, expressed as sodium carbonate.

5. The method of any one of Claims 1 to 4 wherein said hydroxamated polymer is added to said alumina trihydrate thickener feed slurry when said alumina trihydrate thickener feed slurry is at a temperature within the range of from 60°C to 80 0 C.

6. The method of any one of Claims 1 to 5 wherein the amount of said hydroxamated polymer added to said alumina trihydrate thickener feed slurry is in the range of from 0.01 to 20 mg per litre of said alumina trihydrate thickener feed slurry.

7. The method of any one of Claims 1 to 6 wherein the amount of said hydroxamated S* polymer added to said alumina trihydrate thickener feed slurry is in the range of from 0.05 to 10 mg per litre of said alumina trihydrate thickener feed slurry.

8. The method of any one of Claims 1 to 7 wherein the amount of said hydroxamated polymer added to said alumina trihydrate thickener feed slurry is in the range of from 0.00023 to 0.45 kilograms (0.0005 to 1 pound) of said hydroxamated polymer per ton of alumina trihydrate solids in said alumina trihydrate thickener feed slurry.

9. The method of any one of Claims 1 to 8 wherein said water soluble hydroxamic polymer is a polymer containing mer units of the Formula I

17131-OO.DOC/mtm nxmrsll-hn~-rramr~"- -ac- n -23 S4-C-c I Formula I C O N -O -R wherein R is hydrogen or a cation, and R' and are independently hydrogen or a substituent to carbon other than hydrogen. The method of any one of Claims 1 to 9 wherein said hydroxamated polymer has a weight average molecular weight within the range of from 10,000 to 50,000,000. 11. The method of any one of Claims 1 to 10 wherein said hydroxamated polymer has a weight average molecular weight of at least 1 million. 12. The method of any one of Claims 1 to 11 wherein said hydroxamated polymer has an Intrinsic Viscosity of from 5 to 40 dl/g. S 0o 13. The method of any one of Claims 1 to 12 wherein said hydroxamated polymer is 15 comprised of from 25 to 75 mole percent anionic mer units other than hydroxamic mer units, the balance being hydroxamic mer units, optionally together with nonionic mer S~ units. 14. The method of any one of Claims 1 to 13 wherein said hydroxamated polymer is comprised of from 25 to 75 mole percent anionic mer units other than hydroxamic mer S 20 units, from 10 to 40 mole percent hydroxamic mer units and from 10 to 40 mole percent nonionic mer units. An improved Bayer process for the production of alumina wherein bauxite ore is S° pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures, forming an aqueous sodium aluminate solution containing caustic-insoluble 4 25 constituents, wherein said caustic-insoluble constituents are then substantially separated from the aqueous phase of said aqueous sodium aluminate solution to form a clarified sodium aluminate solution, and wherein said clarified sodium aluminate solution is then seeded with crystals of alumina trihydrate whereby a solid alumina trihydrate product is precipitated out of said solution and subjected to settlement, whereupon a supernatant is formed, and said aluminum trihydrate precipitate is collected as product or recycled as I "R t precipitation seed, characterized by adding to said clarified sodium aluminate solution Tc^ l- nrmear~ c-~ 24- after at least the formation of said alumina trihydrate precipitate, and prior to the complete formation of supernatant, a water-soluble anionic hydroxamated polymer in an amount effective to improve the clarification of said supernatant upon said settlement of said alumina trihydrate precipitate. 16. The method of Claim 15 wherein said clarified sodium aluminate solution at the time when said hydroxamic polymer is added has an alumina trihydrate solids content of from 5 to 100 grams per litre and said alumina trihydrate solids comprise at least 99 weight percent of the total solids in said solution. 17. The method of Claim 15 or 16 wherein the amount of said hydroxamated polymer added to said clarified sodium aluminate solution is in the range of from 0.01 to 20 mg per litre of said solution. 18. The method of any one of Claims 15 to 17 wherein the amount of said o o hydroxamated polymer added to said solution is in the range of from 0.00023 to 0.45 kilograms (0.0005 to 1 pound) of said hydroxamated polymer per ton of alumina trihydrate solids in said clarified sodium aluminate solution. 19. The method of any one of Claims 15 to 18 wherein said hydroxamated polymer has a weight average molecular weight of at least 1 million. The method of any one of Claims 15 to 19 wher-in said hydroxamated polymer has an Intrinsic Viscosity of from 5 to 40 dl/g and is comprised of from 25 to 75 mole percent anionic mer units other than hydrox. inic mer units, the balance being hydroxamic mer units, optionally together with nonionic mer units. 21. A method for the improved clarification of a Bayer process alumina trihydrate slurry comprising adding to a Bayer process alumina trihydrate thickener feed slurry an anionic water-soluble hydroxamated polymer in an amount of from 0.01 to 20 mg per litre of said alumina trihydrate thickener feed slurry, wherein said alumina trihydrate thickener feed slurry has an alumina trihydrate solids content of from 5 to 100 grams per litre of said alumina trihydrate thickener feed slurry, and wherein the alumina trihydrate crystals of said alumina trihydrate thickener feed slurry comprise at least 99 weight percent of the total solids in said alumina trihydrate thickener feed slurry. s L 17131-OO.DOC/mtm r I 22. A method for the improved clarification of a Bayer process alumina trihydrate slurry, substantially a, herein described with reference to Example 1 but excluding the :ontrol. DATED this 6th day of Septenber, 1995 NALCO CHEMICAL COMPANY Attorney: RUTH M. CLARKSON Fellow Institu..j of Patent Attorneys of Australia of SHELSTON WATERS 0 t0 0 04 0 0 0t 0« o g 17131-00.DOC/mtm Abstract of the Invention In a Bayer process for the production of alumina, the clarification of the alumina trihydrate slurry is improved by adding to the alumina trihydrate thickener feed slurry an anionic water-soluble hydroxamated polymer in an amount effective to improve the clarification of the slurry upon settlement. Q so 0 4 0 II i s,