AU726700B2 - Improved flocculation in the bayer process - Google Patents

Description

4 11 1 -1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE

SPECIFICATION

FOR A STANDARD

PATENT

ORIGINAL

Name of Applicant: Actual Inventors: NALCO CHEMICAL

COMPANY

James Morton TIPPETT and Graeme John FARQUHARSON Address of Service: Invention Title: SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 "IMPROVED FLOCCULATION IN THE BAYER PROCESS" Details of Associated Provisional Application No. PO 3033 dated 16 October, 1996 The following statement is a full description of this invention, including the best method of performing it known to us:- FIELD OF THE INVENTION This invention relates to the flocculation of red mud in the Bayer process.

BACKGROUND OF THE INVENTION The almost universally used process for the manufacture of alumina is the Bayer process in which bauxite ore is pulverised, slurried in water, and then digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of S. aluminium, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore (referred to as "red mud") are then separated from the *10 aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate is precipitated out of the solution and collected as product.

In more detail, the pulverised bauxite ore is fed to a slurry mixer where a water slurry is prepared. The slurry make up water is typically spent liquor and added caustic.

This bauxite or slurry is then diluted and passed through a digester or a series of digesters where, under pressure and temperature, about 98% of the total available •alumina is released from the ore as caustic soluble sodium aluminate. After digestion, the slurry then passes through several flash tanks where the pressure of the digested slurry is reduced from several atmosphere to one atmosphere and the temperature of the slurry is reduced from a temperature of up to about 250C to about 100°C. The aluminate liquor leaving the flashing operations contains from about 1 to about weight 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. The liquor is fed to a primary settler where it is treated with a flocculant. As the mud settles, clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a well at the top of the primary settler and is collected. This overflow from the primary settling tank is further clarified if necessary and is subsequently seeded with alumina trihydrate crystals to induce precipitation of alumina which is subsequently separated from the concentrated caustic liquor and the remaining 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 ("underflow") and passed through a 10 counter-current decantation washing circuit for recovery of sodium aluminate and soda. -o*o* In the washing train which typically employs 5 washer stages, the settled solids from the primary settler (normally a 6 0 0-800g/l solids slurry) is washed with hot water. Red mud is usually washed a plurality of times by counter-current techniques. Each washer containing suspended red mud and dissolved alumina and caustic are treated with flocculants to promote settling. The composition of the red mud suspension is different at each stage of settlement due to dilution with the washing liquor and is accompanied by a progressive fall in temperature from around 1 00C (primary settlement) to around (final washer stage). The concentration of caustic present in the liquor falls from around 200-250 gpl (as sodium carbonate) in the primary mud settlement down to gpl (as sodium carbonate) in the final wash stage.

In the 1970's it was practice to use a single general purpose flocculant in the washing circuit. By about 1980 with decreasing cost of synthetic flocculants it was realised that the use of washer specific-flocculants specific flocculants for use in specific washers) produced more cost effective performance. If the rate of red mud separation is too slow, output is materially diminished and overall process efficiency impaired.

The use of synthetic sodium acrylate-acrylamide copolymers as a flocculant for red muds has been practiced for many years. These copolymers are synthesised by the copolymerisation of a monomer of acrylic acid (or one of its salts) with an acrylamide monomer. The monomers copolymerise readily in any ratio and powder, bead, emulsion and solution polymers are commercially available.

x CH,-CH HL yCH 2

CH

I 1 1 CONH, CO CO e nf acrylamide

I

NH2 NaO yCH,-CH x0- 1 1 0 CONa sodium acrylate Acrylic homopolymers are highly anionic while acrylamide homopolymers are non-ionic. The ratio of acrylate to acrylamide in the copolymer influences the degree to which the copolymer is anionic, the anionicity increasing as the ratio of acrylate to acrylamide increases.

Many factors affect the efficacy of a copolymer of a particular anionicity as a flocculant in any particular washer. These include the particle size distribution and solids in the washer as well as alkalinity, ionic strength and other factors.

There is an optimum ratio of acrylate:acrylamide in the copolymer to be used as a flocculant for a washer of any particular alkalinity, the effectiveness of the flocculant declining at a higher or lower alkalinity.

In practice therefore, flocculant suppliers manufacture a range of copolymer "grades" having acrylate:acrylamide mole ratios of for example 80:20 grade, 70:30 grade, 60:40 grade, 50:50 grade, and so on. The alumina refinery maintains a stock of several flocculant grades and selects for use in any particular washer of a washer train, that grade which is on average most effective for use in that particular washer having regard to conditions in that washer (particularly the alkalinity).

This method suffers from a number of disadvantages. Firstly, it requires the alumina refinery to maintain stocks of a plurality of grades of flocculant with attendant stock control, handling and storage problems. Secondly, the grade selected in most cases 10 does not correspond to the optimum acrylate-acrylamide mole ratio but the ratio is merely an approximation to the optimum. The alkalinity in any stage of the washer fluctuates and a grade which is optimum for a washer stage in one period may be suboptimum in another. In addition there is an ever present desire for more effective flocculants.

15 An object of the invention is to provide a method which avoids or at least ameliorates some of the disadvantages of the prior art. An object of preferred forms of the invention is to provide a composition which provides improved flocculation performance.

SUMMARY OF THE INVENTION According to one aspect, the invention consists in a process for flocculating red mud in the Bayer aluminium process which comprises the steps of: determining the alkalinity in a red mud settling stage; hydrolysing a polymer or copolymer selected from the group consisting of homopolymers of acrylamide or methacrylamide monomer and copolymers of one of said monomers with a comonomer up to 30% of acrylate, methacrylate, methyl acrylate or an AMPS terpolymer to produce a copolymer hydrolysis product which is effective as a red mud flocculant, varying the hydrolysis conditions in step such that the mole ratio of acrylamide or methacrylamide in the hydrolysis product is at or near the required ratio to produce optimum settling of red mud in said settling stage, and adding the hydrolysis product produced in step to said settling stage.

10 In preferred embodiments of the invention the starting material is an acrylamide homopolymer. The hydrolysis is varied in step by control of one or more hydrolysis variables such as time, temperature and alkalinity to produce an acrylate:acrylamide copolymer.

N*OH

-*CH.-CH-)w N -4 H-CH).CH.-C CO CO CO

NH

2 NH, O0 NaO approx. limit y= 0-9 x=0-1 The hydrolysis is desirably conducted using an alkaline solution obtained from one of the process streams available in the alumina plant and adjusted to a desired alkalinity by dilution or by caustic addition as required. By optimum ratio is meant the ratio which under the alkalinity conditions in the washer produce the maximum rate of settling or achieves a given rate of settling at a minimum rate of addition of flocculant.

-7- In a preferred embodiment, the acrylate is an alkali metal or ammonium acrylate.

According to a second aspect, the invention consists in a process according to the first aspect, wherein the alkalinity in the settling stage is continuously or periodically monitored, and the hydrolysis conditions in step are continuously or periodically varied according to the alkalinity in the settling stage to produce a hydrolysis product having a mole ratio of acrylate to acrylamide at or near the optimum for use in the stage, that is to say which promotes the maximum rate of settling or achieves a given rate of Ssettling at a minimum rate of addition.

In preferred embodiments, alteration of the hydrolysis conditions and consequently 10 the mole ratio is carried out automatically and continuously in response to changes in the alkalinity in the settling stage.

According to a third aspect, the invention consists in a process according to the first or second aspect comprising a washer train having a plurality of settling stages and wherein the steps and are performed separately in respect of each stage so that the mole ratio of acrylamide in the hydrolysis product added to each washing stage is at or near an optimum for that respective stage of the train.

Surprisingly, it has been discovered that a sodium acrylate-acrylamide copolymer manufactured by hydrolysis of an acrylamide under selected conditions is more effective as a flocculant than an acrylate-acrylamide copolymer synthesised by copolymerising the two monomers at the same mole ratios.

According to a fourth aspect, the invention consists in a process for flocculating red mud and which comprises adding to a plurality of washing stages a flocculant comprising a copolymer containing acrylic acid or acrylate units and acrylamide units, said process being characterised in that the copolymer is selected from the group consisting of copolymers manufactured by the hydrolysis of a poly-acrylamide homopolymer (or a copolymer which is predominantly acrylamide) and wherein the mole ratio of acrylate:acrylamide in the hydrolysis product is chosen having regard to the alkalinity in the washing stage.

The invention will now be more particularly described, by way of example only, with reference to various embodiments.

It is known that the amide functionality of polyacrylamide is chemically relatively unreactive but polyacrylamide can be hydrolysed to produce copolymers containing acrylamide and acrylate groups.

However, because the effect of neighbouring anions (and possibly stereochemical considerations) limit the number of anionic acrylate groups which can be introduced a degree of hydrolysis in excess of 90 mole is difficult to achieve in practice with low molecular weights and the effect become more pronounced as the molecular weight of the parent polyacrylamide increases.

-CH,-CH-CHCH-CH-CH-

I I

I

CO CO

CO

I I

I

0 e NH, eO Na e Na

OH

Because of the relative simplicity of copolymerisation of acrylamide monomer with acrylic acid monomer, copolymerisation has largely superseded hydrolysis of acrylamide homopolymers in the manufacture of acrylate-acrylamide copolymers.

According to the present invention, acrylamide is hydrolysed at a temperature, for a time and using caustic of a concentration selected to provide a desired mole of acrylate in the hydrolysis product (acrylate-acrylamide copolymer). The hydrolysis is conducted in the alumina plant and preferably uses one or other of the available caustic streams as a source of caustic for the hydrolysis. As shown in example 1, the choice of conditions to produce a desired mole ratio of acrylate:acrylamide can readily be determined by simple routine experimentation of a non-inventive nature.

Example 1 The hydrolysis of polyacrylamide was monitored by determining the ratio of the infrared C-O stretching frequencies 1660 cm'' for acrylamide and 1560 cm' for sodium acrylate) and comparing this ratio to that found for sodium acrylate acrylamide copolymers of known composition. The alkalinity of samples of partially hydrolysed polyacrylamide solutions were first adjusted to pH 9 by addition of dilute hydrochloric acid. A portion of each of the samples was then evaporated onto AgC1 plates FTIR spectra were obtained on a Nicolet Magna 550 instrument.

A polyacrylamide powder was dissolved at 0.5% w/w in various caustic diluents (10-200 g/ll). The solutions were agitated at various temperatures and for various times in a water bath. The results are summarised in the tables 1-3.

Table 1 Caustic Temperature Time wt Acrylate (oC) (hr) 40 1 0 20 32.5 1 20 47.5 5.75 32.75 62.5 5.75 39.5 40 10.5 10.5

I.

33.5 37.5 40.75 13.75 30 20 I 42.5 I. Table 2 Hydrolysis Temperature 50 0

C

I wt Acrylate lime (hr) 30 g/l caustic 100 g/l caustic t i 200 g/l caustic 53 51 62 69

__I

Table 3 Hydrolysis Temperature 70 0

C

wt Acrylate Time (hr) 30 g/l caustic 100 g/l caustic 200 g/l caustic 1 42 68 62 44 62 71 8 43 59 62 16 54 67 84 22 47 60 79 44 44 58 93 50 70 97 II The data of tables 1-3 can be used to estimate the best conditions for hydrolysis of the polyacrylamide, for optimum red mud settling performance of a particular washing vessel. Thus, hydrolysis of the polyacrylamide powder in 200 g/l caustic at 70 0 C for 8 hr gave a solution of a 62% sodium acrylate acrylamide copolymer. These hydrolysis conditions can be varied to give another copolymer if required for example by a change in red mud slurry conditions, particularly slurry total alkali.

By similar empirical test of a non-inventive nature the hydrolysis product showing the best settling rate performance for a given washer can be selected as exemplified in example 2.

Example 2 Caustic solutions (25-150 g/l) of polyacrylamide powder w/w) were maintained at 70 0 C for 12 hr. The solutions were diluted with 20 g/1 caustic to 0.05% polymer immediately prior to use.

A red mud slurry (total alkali 18 g/l) was obtained from a particular Bayer plant washer vessel. The dose red mud settling rate performances of the diluted hydrolysed polymer solutions were then determined, on the basis of gm of polymer per tonne of mud required to obtain a red mud settling rate of 10 m/hr (Table 4).

Table 4 Caustic gm/tonne for 10 m/hr 100 12- The data of Table 4 show that the best settling rate performance for the particular washer was obtained by undertaking the hydrolysis in 50 g/1 caustic.

In practice of the invention the hydrolysis may be conducted continuously and the hydrolysis conditions selected continuously in response to measurement of the alkalinity in a particular washer. Since the alkalinity in a particular washer will fluctuate with time, this allows a hydrolysis product which is at the optimum mole ratio for use in that particular washer to be added and for the flocculant performance to be maintained at or near the optimum for that particular washer. There can be an hydrolysis reactor associated with each washer producing an optimum flocculant for use in each respective washer or less preferably, flocculants can be produced on a batch basis in a centralised i hydrolysis reactor producing products for each of the washers on a batch basis.

S Surprisingly, it has been discovered that a sodium acrylate acrylamide copolymer which is a product of hydrolysing polyacrylamide, is more effective in red mud settling 15 rate performance than notionally equivalent powder sodium acrylate:acrylamide copolymers, or notionally equivalent latex copolymer, made up (dissolved) under standard plant operating conditions as shown in Example 3.

Example 3 A caustic solution (50 g/1) of polyacrylamide powder w/w) was maintained 20 at 70 0 C for 12 hr, then diluted to 0.05% with 20 g/1 caustic immediately prior to use.

Solutions of sodium acrylate acrylamide copolymer powders 70wt acrylate and acrylate were prepared in 20 g/l caustic solution aged for 2 hr at ambient temperature, then diluted to 0.05% with 20 g/l caustic.

13- A sodium acrylate acrylamide copolymer latex 70wt acrylate was inverted at 1% (as product) in 20 g/l caustic at ambient temperature. The invert was aged for 2 hrs, then diluted to 0.05% (as product) with 20 g/l caustic.

A red mud slurry (total alkali 18 g/l) was obtained from a Bayer plant washer vessel. Dose 10 m/hr settling rate performances are summarised in Table Table Co-polymer gm/tonne for 10 m/hr latex (70wt% acrylate) 125 powder (70wt% acrylate) powder (75wt% acrylate) hydrolysed polyacrylamide The data of Table 5 show that the hydrolysed polyacrylamide gave a better settling rate performance than either of the two powder copolymers. The performance of the latex copolymer was approximately equal to that of the two powders on a polymer actives basis.

Example 4 In this test, the red mud settling performances of various hydrolysed polyacrylamide solutions were compared.

Caustic solutions (25-150 g/l) of polyacrylamide powder w/w) were o maintained at 70 0 C for 12 hr. The solutions were diluted with 20 g/1 caustic to 0.05% polymer immediately prior to use.

14- A solution of sodium acrylate-acrylamide copolymer powder 75wt% acrylate was prepared in 20 g/l caustic solution aged for 2 hr at ambient temperature, then diluted to 0.05% with 20 g/l caustic.

A sodium acrylate-acrylamide copolymer latex 70wt% acrylate was inverted at 1% (as product) in 20 g/1 caustic at ambient temperature. The invert was aged for 2 hrs, then diluted to 0.05% (as product with 20 g/l caustic.

A red mud slurry (total alkali 45 g/l) was obtained from a Bayer plant washer vessel. Dose 10 m/hr settling rate performances are summarised in Table 6.

Table 6 Co-polymer gm/tonne for 10 m/hr latex (70wt% acrylate) powder (75wt% acrylate) hydrolysed polyacrylamide, 25 g/1 caustic hydrolysed polyacrylamide, 50 g/l caustic hydrolysed polyacrylamide, 23 S100 g/1 caustic hydrolysed polyacrylamide, 150 g/1 caustic The data of Table 6 shows that, for this red mud slurry, the best settling rate performance was obtained using the polyacrylamide hydrolysed in 50 g/1 caustic for 12 hr.

It is not clear why the acrylate:acrylamide copolymer made by hydrolysis is more effective than the synthetic copolymer, and without being bound by any particular explanation, the inventors speculate that the arrangement of acrylate with respect to acrylamide groups resulting from the "neighbouring group" effects during hydrolysis results in a more effective flocculating composition than is obtained using synthesised copolymers.

While acrylamide homopolymer is a preferred raw material for hydrolysis, an acrylate:acrylamide copolymer having up to 30% acrylate could be used as the polymer to be hydrolysed. Likewise a methacrylamide homopolymer or copolymer could be hydrolysed to provide the optimum flocculent to be used in a particular washer stage.

As will be apparent to those skilled in the art from the teaching hereof, the invention may be put into effect using any suitable means for monitoring the alkalinity at different stages in the washing train and for controlling the hydrolysis reaction in the desired manner. Although polyacrylamide powder was used in the examples, it will be understood that polyacrylamide latexes and emulsions may be employed.

Claims (5)

1. A process for flocculating red mud in the Bayer aluminium process which comprises the steps of: determining the alkalinity in a red mud settling stage; hydrolysing a polymer or copolymer selected from the group consisting of homopolymers of acrylamide or methacrylamide monomer and copolymers of one of said monomers with a comonomer up to 30% of acrylate, methacrylate, methyl acrylate or an AMPS terpolymer to produce a copolymer hydrolysis product which is effective as a red mud flocculant, varying the hydrolysis conditions in step such that the mole ratio of acrylamide or methacrylamide in the hydrolysis product is at or near the required ratio to produce optimum settling of red mud in said settling stage, and adding the hydrolysis product produced in step to said settling stage.

2. A process according to claim 1 wherein the acrylate is selected from an.alkali 9 metal acrylate or ammonium acrylate.

3. A process according to claim 1 wherein an acrylamide homopolymer is selected in step 2 to produce a sodium acrylate:acrylamide hydrolysis product.

4. A process according to any one of claims 1 to 3 wherein the alkalinity in the settling stage is continuously or periodically monitored, and the hydrolysis conditions in step are continuously or periodically varied according to the alkalinity in the settling stage to produce a hydrolysis product having a mole ratio of acrylate to acrylamide at or near the optimum for use in the stage.

17- A process according to any one of claims 1 to 3 wherein the steps and are performed separately in respect of each stage of a washer train so that the mole ratio of acrylamide or methacrylamide in the hydrolysis product added to each washing stage is at or near an optimum for that respective stage of the train. 6. A process for flocculating red mud and which comprises adding to a plurality of washing stages a flocculant comprising a copolymer containing acrylic acid or acrylate units and acrylamide units, said process being characterised in that the copolymer is selected from the group consisting of copolymers manufactured by the hydrolysis of a polyacrylamide or methacrylamide homopolymer (or a copolymer which is predominantly acrylamide or methacrylamide) and wherein the mole ratio of acrylate:acrylamide in the hydrolysis product is chosen having regard to the alkalinity in the washing stage. DATED this 2nd day of October 1997 NALCO CHEMICAL COMPANY Attorney: PAUL G HARRISON Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS C C C