AU668029B2 - Bayer process Do not seal seling fee to be refunded - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 CO MPLETE SPECIFICAT ON FOR A STANDARD PATENT

ORIGINAL

o o a a o 0 o e o a ai oa os o s o ea a A Name of Applicant: Actual Inventor: Address for Service: Invention Title: Details of Associated NALCO AUSTRALIA PTY LTD A.C.N. 000 424 788 Grant DOBERER SHELSTON WATERS 55 Clarence Street SYDNEY NSW 2000 "BAYER PROCESS" Provisional Application No: PL8698 dated 7th May, 1993 r o sirci c ooio The following statement is a full description of this invention, including the best method of performing it known to us:ic I -2- This invention relates to an improvement in the flocculation of red muds from the Bayer alumina recovery circuit.

In industry, alumina is recovered from bauxite by means of the well-known Bayer process. Such a process involves digesting the ore in an aqueous alkaline liquor such as sodium hydroxide, lime and/or soda. The alumina is extracted into solution as sodium aluminate, leaving an insoluble digestion residue ("red mud") which is required to be separated from the sodium aluminate solution. This is achieved by flocculating the red mud with a flocculating agent in a primary settling stage, o Optionally, the overflow from the primary settling stage is further subjected to a secondary classification stage 15 in which the suspended red mud solids are further flocculated and separated. The red mud from both the primary settling stage and the secondary classification stage is then washed in a plurality of sequential Qo$ 4 washing stages which comprise washing the mud and 20 separating the mud from the liquor by flocculation.

4 44 o After the final washing stage, the red mud is settled from the wash liqior in a lagoon or in a mud stack.

For the above process to be efficient it is 4 desirable that the very fine red mud particles are cleanly separated from the sodium aluminatc solution as quickly as possible. For this to be achieved, it is desirable that the flocculent used increases the L

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r -3settling rate of the red mud particles to thereby provide a dense red mud sediment and a supernatent free of residual suspended solids.

European Patent application 367437, describes a process of separating red mud from Bayer Process liquors by flocculation using as flocculants, dextran and a synthetic anionic polymer. The flocculants were added to one or more stages of the i-wr rocess either together or sequentially. On page 3 of the patent application, it is stated that preferably the dextran is mixed into the process liquor before the addition of the anionic polymer. It was found that better results were achieved if the dextran was vigorously mixed into the liquor first and the synthetic polymer mixed in less vigorously later. It was thought that in such a process, the dextran was acting as a coagulant.

The present inventors have found that the above process can be significantly enhanced if the polysaccharide is added to the process liquor after the addition of the synthetic polymer.

According to a first aspect, the present invention consists in a method for flocculating red muds from a bayer alumina recovery circuit comprising the steps of: i adding to at least one stage of the recovery circuit, an effective flocculating amount of a water soluble synthetic polymer flocculant; and then adding to said stage an effective flocculating amount of the polysaccharide dextran.

The polysaccharide of step is dextran comprising predominantly 1:6-glucose linkages.

6 I ~CL~ 1~I~1~~ -4- According to a second aspect the present invention consists in a method according to the first aspect further comprising adding the polysaccharide in the form of multiple additions.

According to a third aspect the present invention consists in a method according o the first or second aspect further comprising using doses of dextran greater than mg/L and high shear mixing.

The flocculants can be added to almost any stage of the circuit but preferably they are added to the primary settler, to the secondary classification liquor or to one or more of the subsequent washing stages. Most preferably the flocculants are added to the primary settler.

Preferred embodiments of the invention will now be described with reference to the various Examples.

The synthetic polymers suitable for use in the invention include any conventional high molecular weight water soluble polymers lkown and used in the art and are generally polymers of one or more ethylenically-unsaturated monomers such as homoand copolymers of acrylic acid, (meth)acrylic acid, maleic acid, crotonic acid, itaconic S.ll acid, polyacrylic acid, sulphonic acid, acrylamide, meth(acrylamide), methylolacrylamide, polyacrylamide, N-sulfoalkyl(meth)acrylamide and their various monovalent salts. For example homo- or copolymers of acrylic acid and acrylates,

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S 555.5 j? C- I-L copolymers of acrylic acid or acrylate and acrylamide polymers containing hydroxamic acid groups and other polymers, copolymers and terpolymers such as are described in US 3,390,959, US 3,541,009, US 3,716,617, US 4,678,585, US 4,767,540, US 4,678,840 and EP 367,437 can be used and are included within the scope of this invention. The polymer is preferably anionic but may contain some nonionic or cationic mer units, particularly those of the quaternary ammonium salt type. In all cases, the choice of polymer is dependent on which process liquor is used and the alkalinity of the liquor.

For the primary settler stage, preferably the polymer is a sodium polyacrylate or ammonium polyacrylate homopolymer, most preferably, ammonium polyacrylate.

The polysaccharide for use in the invention is dextran. Dextran is generally synthesized by bacterial enzymes to provide a back-bone having glucose units joined predominantly by 1:6-glucose linkages but having a smaller number of non 1:6 linkages.

Such dextrans are described in US 3,085,853, GB-A-1154993 and CA-A-825234.

Preferably, both the polymer and the polysaccharide have molecular weights of at 15 least 50,000, more preferably 500,000 to 10,00)0,000.

00t0 Both the polysaccharide and the polymer are preferably dissolved in water or in a the plant liquor prior to addition to the primary or secondary settlers in order to facilitate rapid dispersal of the flocculants. The amounts added are dependent on the specific bauxite ore being processed, and the prevailing reaction conditions including S2u temperature, pH, and red mud solids content. Typically an aqueous solution of dextran contains between about 0.01 to 2.5 weight% dextran and an aqueous solution of polymer contains between about 0.01 to about 5 weight% polymer. The dextran is typically H6.__ a~una~l r*rmcP3 added at a dosage of 0.1 to 20 rgs per litre of red mud containing solution and the polymer at a dosage of 0.5 to 20 mg/L.

Although the dextran can be added as a single dose, preferably the dextran is added in a series of pulsed doses.

When high doses of dextran are used (10 to 40 mg/L), preferably, high shear mixing is used to mix the dextran through the liquor. Conversely, when low doses are used (1-10mg/L), preferably, low shear mixing is used.

The flocculants are preferably added at an

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-7alkaline pH, more preferably at least 9.

EXAMPLE 1 A series of experiments were conducted to determine the effect of post-addition of dextran to the primary settler stage of a Bayer recovery process in accordance with the present invention compared with that of pre-addition of dextran as described in EP 367,437.

A constant dosage of polymer was added to a graduated cylinder containing settler feed from a typical Bayer process. The settling rate was about 3.50 m/hr.

Dextran was then added, doses being varied between 5 and mg/L. The rate of mixing of both flocculents (by plunging with a disk attached to a rod) was kept o oconstant. The amount of suspended solids and settling 15 rate were then determined. The results are shown in Table 1: 0.00 0 00 o n r 0 0 0 0 0 Added OW) 30 min.

Polyacrylate Polymer (10) 0.16 3 3.5 165 0 Dextran (5)/Polyacrylate- Polymer 0.10/0.16 20/3 3.9 195 -21 Dextran (l0l/Polyacrylate Polymer 0.1010.16 20/3 3.8 150 9 Dextran (20)/Polyacrylate Polymer 0.10/0.16 20/3 3.9 .135 18 Dextran (40)/Polyacrylate Polymer 0.10/0.16 2.0,3 4..3 110 33 Polyacrylate Polymer (10) 0.16 33.2' 155 0 Polyacrylate Polymer (10)/Dextran 0.16/0.10 3/20 3.3 135 13 Polyacrylate Polymer (10)/Dextran (10) 0.16/0.10 3/20 3.6 81 48 Polyacrylate Polymer (1C)/Dextran (20) 0.16/0.10 3/20 4.1 23 Polyacrylate Polymer (10)/Dextran (40) 0.16/0.10 3/20 4.9 17 89 -9- It can be seen from the above table that post-addition of dextran was far superior to pre-addition of dextran. In fact, using 40 mg/L dextran with pre-addition only gave about a 30% reduction in suspended solids, whereas post-addition of dextran showed about a reduction in suspended solids and a comparatively much higher settling rate.

EXAMPLE 2 A second experiment was conducted similarly to Example 1, but in this experiment the effect of increasing agitation (measured as a the number of "plunges") was analysed. The dose of dextran solution was kept at 40 mg/L. The settling rates and the suspended solids were determined. The results are shown in Table 2: o a 0 o A a a* L i; TAB3LE 2.

Flocculant in Conc. of Mixing Settling Rate Suspended V Reduction order of use and Actives in No. of (cml/mm4) Solids as Dose (mg/L) Aqueous Solution Plunges NTU after Added (Ir) 30 min.

Polyacrylate Polymer (10) 0.16 3 3.5 165 0 Polyacrylate Polymer (10)/Dextran (40) 0.16/0.1 3/3 10 86 48 Polyacrylate Polymer (10)/Dextran (40) 0.16/0.1 3/8 9.2 43 74 Polyacrylate Polymer (10)/Dextran (40) 0.16/0.1 3/20 5.0 23 86

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~ilU Ii _riniii-mJ~iirT^ 1 I -1 1 It can be seen from the table that increase in the mixing ratio caused a substantial reduction in suspended solids.

It was further noted that the excessive shear exhibited by the extra mixing did not break down the structure of the flocculents but served to build or rebuild the flocculents which became more shear resistant, sometimes larger and capable of settling more quickly. A distinct slurry/liquor interface was formed.

EXAMPLE 3 A third experiment was conducted similarly to Example 1 but in this experiment multiple additions of "V dextran was used. The settling rates and suspended solids were determined. The results are shown in Table 3: 4 6 4* 4 TABLE 3 Flocculant in Conc. of Mixing Settling Rate Suspended V Reduction order of use and Actives in No of (cm/min) Solids as Dose (mg/L) Aqueous Solution Plunges NTU after Added 30 min.

Polyacrylate Polymer (10) 0.16 3 4.1 170 0

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Ca Polyacrylate Polymer (10)/Dextran (5)/Dextran 0.16/0.1/0.1 3/10/10 4.0 61 64 r Polyacrylate Polymer (10)/Dextran (10)/Dextran (10) 0.16/0.1/0.1 3/10/1C 4.7 22 87 Polyacrylate Polymer (10)/Dextran (20)/Dextran (20) 0.16/0.1f0.1 3/10/10 4.9 15 91 0olyacrylate Polymer (10) 0.16 3 32 155 0 Polyacrylate Polymer (10)/Dextran 0.16/0.10 3/20 3.3 135 13 Polyacrylate Polymer (10)/Dextran (10) 0.1S/0.10 3/20 3.6 81 48 Polyacrylate Polymer (10)/Dextran (20) 0.16/0.10 3/20 4.1 23 85 Polyacrylate Polymer (10)/Dextran (40) 0.16/0.1 3/20 4.9 17 89 0 PolycryatePolyer 10)Dextan 5)/extrn 016/01/01 310/1 4. 6164 I o'n

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PolycryatePolyer 10)Dextan 10)Dextan 10) 0.160.10.1 3/1010 .7 2 8 PolycryatePolyer 10)Dextan 20)Dextan 20) 0.160.10.1 3/1010 .9 5 9 "olycrylte olymr G16 33.2 55 -13- It can be seen from the table that by using two additions of dextran the reduction in suspended solids was improved over the single addition of dextrans particularly at the low level dosages range (1 to mg/L). Best results using dual addition were obtained using dextran dosage ranges between 10 and EXAMPLE 4 A fourth experiment was conducted similarly to Example 1 but in this experiment a different polymer was used. This polymer is an ammonium acrylate polymer.

Dual dosages of dextran were used varying between 1 and mg/l. The amount of mixing was kept low and constant.

The settling rate and suspended solids were determined.

The results are shown in Table 4: It, 4491 6 4 Flocculant in order of use and Dose (mg/L) Polyacrylate Polymer Polyacrylate Polymer Polyacrylate Polymer Polyacrylate Polymer Polyacrylate Polymer Am~monium Polyacrylate Dextran /Dextran Ammonium Polyacrylate Dextran /Dextr Amonium Polyacrylate Dextran /Dextr Ammonium Polyacrylate Dextran /Dextr NN0 0 0 0~~Ade CO 30 min00. 00 0 (10000 0 00 005 16 00 Activestra in)Deta (3o of60.1. (cm/m3 Solid as4 (l0)/exra ()Dxrn() 0.6 .10.1 3 3 3.5 165 (Pol/mera (2.5)/Deta 2) 54 (10)Dxrn(5/ta 3) 0.16/0.1/0.1 3/3/3 3.0 90 an(2.)5exr) (5/eta 5 0.16/0.1/0.1 3/3/3 3 63 Polymer an() 5 0.16/0.1/0.1 3/3/3 4.0 50 Polymer an 0.16/0.1/0.1 3/3/3 4.5 41

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It can be seen from the table that the animionium acrylate polymer is superior to the acrylate polymer at low dextran dosages. Further much less ammonium acrylate polymer was required.

From the above tables it can be seen that by performing the process of the invention the amount of suspended solids is significantly reduced (a ten fold reduction being apparent at high doses) by adding the dextran after the polymer.

Although the above discussion has referred to th-e use of process of the invention with regard to the primary settler, the process can be used in any of the subsequent washing stages including the super thickener (a large vessel at the tail of a conventional washing tail) and/or the mnud lake used for final settling.

Further it is anticipated that the above process will be equally suitable to other conventionally used polysaccharides such as starches and celluloses, and such variations and other modifications apparent to a person skilled in the art are included in the scope of this a 44 4.

TABLE Mixing Number Settling Rate Suspended Solids as *solution) of Plunges (rn/br) NTU after 30 mins 0.6) 5 24.8 182 0.6)/Pullulan 5/5 14.4 97 Flocculent in order Df use and dose (ml of Polyacrylate Polymer( Polyacrylate Polymer( Polyacrylate Polymer (0.6)/Pullulan 5/5 12.4 100 Polyacrylate Polymer (0.6)/Pullulan 5/5 16.5 107 Pullulan (1/Polyacrylate Polymer 5/5 23.5 187 iululan (2)/Polyacrylate Polymer 5/5 24.8 171 Pullulan (3)/Polyacrylate Polymer 5/5 26.3 160

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Claims (15)

1. A method for flocculating red muds from a bayer alumina recovery circuit comprising the steps of: adding to at least one stage of the recovery circuit, an effective flocculating amount of a water soluble synthetic polymer flocculant; and then adding to said stage an effective flocculating amount of the polysaccharide dextran.

2. A method according to claim 1 wherein the dextran is added in successive additions.

3. A method according to claim 2 wherein the dextran is present in an amount greater than 10 mg/L.

4. A method according to any one of the preceding claims wherein the dextran is added while using high shear mixing.. A method according to any one of the preceding claims wherein the water soluble polymer is selected from the group consisting of one or more ethylenically-unsaturated monomers.

6. A method according to claim 5 wherein the ethylenically-unsaturated monomer is selected from the group consisting of homo- and copolymers of acrylic acid or acrylate, 4. acrylamide polymers, (meth)acrylic acid, maleic acid, crotonic acid, itaconic acid, 20 polyacrylic acid, sulphonic acid, acrylamide, meth (acrylamide), methylol-acrylamide, polyacrylamide, N-sulfoalkyl(meth)acrylamide and their various monovalent salts.

7. A method according to claim 6 wherein the water soluble polymer is anionic and d t"S contains some nonionic or cationic mer units. -18-

8. A method according to claim 6 wherein the water soluble polymer is sodium polyacrylate or ammonium polyacrylate.

9. A method according to claim 8 wherein the water soluble polymer is ammonium polyacrylate.

10. A method according to any one of the preceding claims wherein the water soluble polymer and polysaccharide each have a molecular weight of at least 50,000.

11. A method according to claim 10 wherein the molecular weight is between 500,000 to 10,000,000.

12. A method according to any one of the preceding claims wherein the polysaccharide and water soluble polymer are each dissolved in water prior to addition to the recovery circuit.

13. A method according to claim 12 wherein the polysaccharide is present in a concentration of from 0.01 to 2.5% by weight of red mud containing solution.

14. A method according to any one of the preceding claims wherein the polysaccharide is added at a dosage of from 0.1 to 20 mg/L of red mud containing solution. A method according to claim 12 wherein the polymer is present in a concentration of from 0.01 to 5% weight of red mud containing solution.

16. A method according to any one of the preceding claims wherein the polymer is added at a dosage of from 0.5 to 20 mg/L of red mud containing solution. I, o 7 -19-

17. A method according to claim 1 substantially as herein described with reference to any one of the examples, excluding any comparative examples. DATED this 9th day of February, 1996 NALCO AUSTRALIA PTY LTD Attorney: RUTH M. CLARKSON Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS 04 I t:i B fl t 1 !,s P I- ;1 i .I- ABSTRACT The present invention relates to a method for flocculating red muds from a bayer alumina recovery circuit comprising the steps of: adding to at least one stage of the recovery circuit, an effective flocculating amount of a water soluble synthetic polymer flocculent; and then adding to said stage an effective flocculating amount of a polysaccharide having predominantly l:6-glucose linkages or predominantly 1:4-glucose linkages. 0o 00 00 o 0 0000 0 0 4 4 0 00 0 0 0 6 o t Fo: t