CA2014262A1 - Recovery of alumina trihydrate in the bayer process - Google Patents

Recovery of alumina trihydrate in the bayer process

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Publication number
CA2014262A1
CA2014262A1 CA002014262A CA2014262A CA2014262A1 CA 2014262 A1 CA2014262 A1 CA 2014262A1 CA 002014262 A CA002014262 A CA 002014262A CA 2014262 A CA2014262 A CA 2014262A CA 2014262 A1 CA2014262 A1 CA 2014262A1
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Canada
Prior art keywords
process according
dextran
liquor
synthetic polymer
polysaccharide
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Abandoned
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CA002014262A
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French (fr)
Inventor
Gillian Mary Moody
Christine Anne Rushforth
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Ciba Specialty Chemicals Water Treatments Ltd
Original Assignee
Gillian Mary Moody
Christine Anne Rushforth
Allied Colloids Limited
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First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=10654726&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=CA2014262(A1) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
Application filed by Gillian Mary Moody, Christine Anne Rushforth, Allied Colloids Limited filed Critical Gillian Mary Moody
Publication of CA2014262A1 publication Critical patent/CA2014262A1/en
Abandoned legal-status Critical Current

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Classifications

    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01FCOMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00Compounds of aluminium
    • C01F7/02Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/14Aluminium oxide or hydroxide from alkali metal aluminates
    • C01F7/144Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
    • C01F7/148Separation of the obtained hydroxide, e.g. by filtration or dewatering
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01FCOMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00Compounds of aluminium
    • C01F7/02Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/14Aluminium oxide or hydroxide from alkali metal aluminates
    • C01F7/144Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01FCOMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00Compounds of aluminium
    • C01F7/02Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/14Aluminium oxide or hydroxide from alkali metal aluminates
    • C01F7/144Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
    • C01F7/145Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process characterised by the use of a crystal growth modifying agent other than aluminium hydroxide seed

Abstract

ABSTRACT

RECOVERY OF ALUMINA TRIHYDRATE
IN THE BAYER PROCESS

In a Bayer process for production of aluminium alumina trihydrate cyrstals are formed in the pregnant liquor and are flocculated using a combination of dextran and synthetic polymer, usually homo- or copolymer of acrylic acid. The process is particularly advantageous where the liquor contains organic components such as humate and/or oxalate.

Description

~ECOVERY OF ALUMINA TRIHYDRATE

The present invention relates to the removal of trihydrate crystals from pregnant liquor in a Bayer process by the incorporation of a flocculant.
The Bayer process for recovering alumina from bauxite is well known and comprises digesting the bauxite in an aqueous alkaline liquor that acquires a sodium aluminate content. The insoluble residue, or red mud, is separated in a primary settler and is then washed and resettled in several washing stages. Usually liquor from a later wash stage is used to wash mud from an earlier wash stage. The wash liquor from the first wash stage and from the primary settler is further purified, for instance further clarified by filtration, and then alumina trihydrate is recovered by precipitation of crystals from the "pregnant" liquor, which is a super saturated solution.
Precipitation of the alumina trihydrate is induced by the addition of seed trihydrate crystals. The precipitate settles and is then separated from the "spent" liquor and further treated. Although some of the precipitate is relatively coarse and settles ~uickly and is easily separated from the supernatent, for instance by filtration, the finer material tends to settle slowly and gives poor supernatant clarities, which result in product losses. Furthermore where recovery of the precipitating includes a filtration step, fine material causes filter blinding.
It is known to improve the rate of settlement as well as supernatant clarity by the use of flocculants.
Flocculants which have been used include water-soluble highly anionic synthetic polymers, for instance sodium polyacrylate, as well as natural cornpounds such as 2~

starches and derivatives and dextrans. We have found that synthetic polymers are not effective for all liquors, indeed in some liquors the addition of a polyacrylate has no effect at all on the clarity of the spent liquor. Although dextran has been found to be a reliable flocculant, it is generally more expensive than synthetic flocculants for equivalent performance, and its performance is often inadequate.
In US-A-3,085,853 the use of dextran a'one as a flocculant for red mud is disclosed.
In CA-~-825234 mixtures of dextran and "anionic salts" are used to flocculate aluminium trihydrate. One example of an anionic salt is sodium polystyrene sulphonate, all of the rest of the salts are small molecules such as inorganic ions or aliphatic carboxylic acid salts such as citrate and oxalate. The addition of polystyrene sulphonate to dextran appears to make the separation slower than using dextran alone.
In a new process according to the invention alumina trihydrate crystals are formed and recovered from a 8ayer process liquor by adding a flocculant to the liquor and the process is characterised in that the flocculant comprises polysaccharide and a high molecular weight water-soluble synthetic anionic flocculant polymer formed from ethylenically unsaturated monomer including acrylic monomer.
The invention is based on the surprising discovery that for pregnant liquor where synthetic anionic flocculants have little or no effect on the recovery of trihydrate, the combination of that polymer with dextran or other polysaccharide gives a significant improvement over the use of dextran alone in the final clarity of the supernatant liquor, improving the subsequent filtration rate, and also improves the rate of settlement. In addition, the invention allows use of a broader molecular - 2~ 2~:

weight ranqe of dextran than was previously possible with the use of dextran alone.
It is found that the process of the invention has maximal benefits when the liquor to which the flocculant is added has a content of dissolved organic components in the range of 0.1 to 30g/1 eg around 4 g/l. We have found that it is these low organic content liquors from which it is most difficult to precipitate the trihydrate crystals and in which synthetic polymers are inadequate.
The process of the invention is generally carried out with the liquor at a temperature of at least 50C.
more preferably in the range 60 to 80C. We have found that in liquors which are subjected to high temperatures the use of conventional polyacrylate alone is inadequate, whereas the use of the flocculant according to the invention gives surprising improvements over the use of dextran alone.
The addition of the dextran/polymer flocculant is generally made after precipitation has been induced by the addition of seed trihydrate crystals. It is alternatively possible to add the f.locculant combination prior to the addition of the seed crystals. The flocculant is mixed into the liquor in the normal manner.
The two components may be added to the liquor simultaneously, for instance they may be added either as a blend, usually an aqueous solution of the two components, or as a solid agglomerate. Alternatively the components may be added sequentially, for instance the polymer may be added first and the dextran second or vice versa. Best results are obtained when the dextran is added prior to the polymer. Usually the components are each added as aqueous solutions. The components are generally supplied for use in the process as solids, optionally a mixture of solids where they are added as a blend, or a pre~formed agglomerate, and are first 4 2~

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usually above 0.5 million and preferably above 1 million) but is usually in the range of 0.1 to 20, preferably 0.2 to 10, and more preferably 0.2 to 5, mg/l. Often less than 1 mg/l synthetic polymer can be used.
The alkalinity of the liquor can range from, for instance, 10 to 300 g/l, expressed as sodium hydroxide.
When, as is often preferred, the dextran is used in the more caustic liquors the alkalinity typically is in the range 100 to 300, often 200 to 300, g/l expressed as sodium hydroxide.
The precipitate is usually recovered by filtration although may be subjected to settlement alone with removal of the supernatant. The trihydrate crystals that are recovered can 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 flocculant. Likewise the supernatant "spent" liquor is recycled in the conventional manner to the digestion stage.
Although this invention has been described in terms of use of dextran ln combination with an anionic synthetic polymer, dextran sulphate, or alternatively other glucans or other homo- or hetero- polysaccharides, may also be useful in the process of the invention.
Useful dextrans or alternative polysaccharides are water-soluble and generally have a molecular weight of at least 50,000 although values in the range 500,000 up to 40,000,000 can be preferred. Best results are achieved with molecular weights above 1 or 2 million or even above 5 million. It is preferable for the material to be a polysaccharide of D-glucose, usually having some linkages, preferably backbone linkages, other than (1-4). The polysaccharide is generally a microbial polysaccharide. Dextran is a polysaccharide of D-glucose which has a backbone linkage other than a(1-4), 2 ~t~ S~

predominantly ~ 6). Commercially available dextrans such as those produced by the fermentation of Leuconostoc mesenteroides or L. dextranicum. Suitable dextrans are also described in US 3,085,853. Amylopectin, which has predominantly a(1-4) linkages between glucose units with side chains joined by ~(1-6) linkages, can be used in place of dextran in the process.
The synthetic polymeric flocculant is a high molecular weight water-soluble polymer, and is preferably predominantly anionic in nature. Non-ionic and cationic polymers are also suitable for use in the invention, however the cationic polymers tend to be only of use on sequential addition with the detran. The preferred anionic polymers are formed from anionic ethylenically unsaturated monomer optionally with non-ionic ethylenically unsaturated monomer. Preferably the monomers are all acrylic monomers.
The anionic monomer is generally a monoethylenically unsaturated carboxylic or sulphonic acid which is usually acrylic acid but can be, for instance, methacrylic acid, 2-acrylamido methyl propane sulphonic acid. The anionic monomer is generally present in the form of a sodium or other alkali rnetal or ammonium salt.
The non-ionic monomer, if present, is usually acrylamide but other non-interfering monomers may be included in known manner. For instance a minor amount of methylol acrylamide units may be included, eg as described in US 3,975,496. Since the pregnant liquor is highly alkaline the content of anionic residues is preferably high.
The polymer is generally a polymer comprising 20 to 100%, usually 50 to 100~ anionic monomer (usually sodium acrylate) with the balance being non-ionic (usually acrylamide). Preferably the polymer is formed from 80 to 100~ anionic rnonomer most preferably being a homopol~mer 2~ 2 of anionic monomer eg polyacryl~te. The homopolymers tend to be of use over a wider range of dextran concentrations than the copolymers, the latter tending to work better at relatively low dextran concentrations.
The polymer usually has a molecular weight in the usual range for flocculatants, eg of at least 1 million, preferably with an intrinsic viscosity in the range 5 to 30dl/g. It is produced by conventional polymerisation processes for instance by aqueous gel polymerisation or by reverse phase polymerisation.
The following examples illustrate the invention; in all of which an equivalent amount of sodium tripolyphosphate to dextran is added:

Example 1 500ml of a simulated pregnant liquor comprising a liquor containing 25g/1 alumina trihydrate and 200g/1 sodium hydroxide is maintained at a temperature of 75C.
Each sample is well mixed using a steel plunger and dosages of components as indicated in the table are added. Mixing is obtained by 10 strokes of a steel plunger. The sample is allowed to stand for iu minutes and a sample of the supernatant is then removed and its turbidity measured in nephelometric turbidity units (NTU) using a standard turbidimeter. The results dre shown in the following table.
The synthetic polymer is a homopolymer of acrylic acid in the form of its sodium salt conventionally used in flocculation of trihydrate crystals. The dextran used was presented in the form of a 50:50 (by weight) mixture of dextran sodium tripolyphosphate (STPP), so that an additive containing dextran contains the same amount by weight of STTP.

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washing with hot deionised water, drying the filter paper and contents at 100C and reweighing.
TABLE _ 5 Additive Dose Level Suspended Solids (mg/l) (g/l) Untreated - 2.99 Synthetic A 0.5 4.73 ~.0 3.84 2.0 3.27 Dextran 0.25 2.20 0.5 1.47 1.0 1.31 Synthetic A:
Dextran 0.25:0.125 2.81 0.5 :0.25 2.09 1.0 :0.5 1.42 0.125:0.188 3.16 0.25 :0.375 1.45 0.50 :0.75 1.28 -0.05:0.225 2.88 0.10:0.450 1.79 0.20:0.900 0.98 The examples shows that the use of a dextran:synthetic blend irrespective oE the ratio of the blend, gives a lower suspended solids and therefore lower 1 0 '2~ ,q~

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TAsLE 3 Additive Dose Suspended Solids (g/tonne) (g/l) . _ _ _ . _ . . _ Untreated - 2.04 Dextran 5 1.75 1.08 0.91 Synthetic A:
Dextran 5~2.5 2.18 10:5.0 1.66 15:7.5 0.91 2.5:3.75 1.60 5.0:7.5 1.07 7.5:11.25 0.93 1.0: 4.5 1.50 2.0: 9.0 1.14 3.0:13.5 0.97 As in Example 2, the results show that the use of synthetic with dextran produces a lower turbidity than generally obtained at the individual dose levels of dextran alone.

Example 4 A simulated pregnant liquor comprising a liquor containing 208g/l alumina trihydrate solids (solids content 208g/l in spent liquor) taken in was maintained at 30C. As in Example 1 the dextran was in the form of a 50:50 dextrans STPP blend and the same synthetic polymers ~4~

were used. The components were added in the dosages shown in Table 4 and mixing achieved by 3 strokes of a plunger. Solids settlement rate was measured and suspended solids measured by filtration of a sample of 5 supernatant which had been removed after allowing 30 minutes settlement.

Product Dose Level Settlement Supernatant (g/t) Rate (m/hour) Suspended Solids (g/l) 15 Untreated - 0.2 1.68 Dextran 4.75 1.7 1.35 12.0 2.2 0.70 Synthetic A: 2.375:3.56 2.5 1.02 20 Dextran 6.0 :9.0 3.0 0.31 _ _ The results show that the synthetic:dextran blend to give a greater settlement rate, and lower turbidity, over 25 the use of equivalent doses of dextran alone.

Example 5 A simulated pregnant liquor comprising a liquor containing of alumina trihydrated solids and 3 approximately 200g/1 alkali (quoted as Na2CO3). The form of dextran used was the same as in Example 1 (dose given as the mixture), as were the synthetic polymers used.
The components were added at a 0.025% solution (prepared in tap water) in the dosages shown in Table 5 35 and mixed using 4 strokes of a plunger. The samp]es were allowed 10 minutes to settle. 100 ml sample of supernatant were taken and filtered through a GFC filter.
The results are quoted as a % reduction in solids retained on the filter as compared with the blank.

Additive Dose % Reduction Over Blank (g/t) Dextran 1.85 51.2 3.7 44.2 5.55 58.1 7.4 61.2 9.25 65.0 Synthetic A; 1.39Ø925 45.5 Dextran 2.78:1.85 54.3 4.16:2.775 59.3 5.55:3.7 62.4 6.94:4.625 61.4 -Synthetic A 3.7 24.2 7.4 12.8 11.1 16.5 14.8 -5.7 18.5 19.4 The results shows all synthetic :dextran samples to give an approximate 10 fold increase in settlement rate over that achieved with the equivalent doses of dextran alone.

Example 6 Lr~

Tests were carried out using the same method as in Example 1 except that STPP was not added to the dextran used.
5 samples of dextran of known molecular weight were compared with the industrial dextran evaluated in all previous examples. Standard industrial grade molecular weight > 3,000,000, from viscosity measurements The samples tested were as follows:-10 (a) Molecular weight 100,000 - 20~,000 (b) 200,000 - 275,000 (c) 500,000 -(d) 2,000,000 (e) 5,000,000 - 40,000,000 Three series of tests were carried out:
(i) Samples (a) - (e) were tested as 100% dextran against a standard sample which contains 50% sodium tripolyphosphate.
(ii) Sample (c) - (e) were tested as 50:50 blends with a synthetic homo-polymer of acrylic acid in the ~orm of its sodium salt, against the standard as in (i) above.

(iii) Sample (b) compared with sample (e) both blended 50:50 with the same synthetic polymer as in (ii) above.

For series (i) and (ii) the results are quoted as an average of the results obtained from four separated dose levels and as % turbidity compared with the equivalent dose level of the standard industrial grade dextran dextran.
For series (iii) sample (b) results were calculated as a % of sample (e) results. Since the relationship between sample (e) and the standard was already 2~k~

established, the relationship between (b) and the standard could then be calculated The dose level used in this example ranged from 0.25 to l.Omg/l Additive % turbidity a 100% dextran237 b " 208 c " 158 ' d " 108 e " 94 b 50:50 Synthetic A: 140 dextran 15 c " 103 d " 89 e " 90 A figure of e.g. 140% indicates that the turbidity is 140% that of the standard.
The above shows that only samples (d) and (e) i.e.
dextran with molecular weight greater than 2,000,00 give the desired result if used alone. However, when blended with a synethetic it is possible to use a lower molecular weight dextran to achieve similar results, and dextrans of a molecular weight greater than 500,000 are acceptable.
Samples (a) and (b) do provide improvements over untreated samples or those treated with synthetic alone but are not as effective as the higher molecular weight procedure.
Therefore, the example illustrates that a wider molecular weight range of dextran can be used according 2~4~

to the present invention when dextran is blended with a synthetic polymer, rather than when used alone.

Example 7 This example serves to show the effect of the presence of sodium tripolyphosphate compared to the presence of synthetic.
Samples (b) and (e) were used in Example 6 (i.e.
dextran free of STPP). These were evaluated adding as 100% dextran, 50:50 blend with sodium tripolyphosphate and 50:50 blend with synthetic as used in the previous example.
The test method used was the same as in Example 1.

Additive Dose (mq/l)Turbidity (NTU) e 0.25 113 0.5 110 e:sodium tri- 0.25:0.25 110 polyphosphate 0.5 :0.5 98 e:synthetic A 0.25:0.25 100 0.5 :0.5 93 2S b 0.25 164 o 5 160 b: sodium tri- 0.25:0.25 164 polyphosphate 0.5 :0.25 160 b: synthetic A 0.25:0.25 157 0.5 :0.5 151 Blending dextran with sodium tripolyphosphate has a marginal beneficial effect for the higher molecular 2~ "~3, ,r, ~ r~ ~-~ct: .~r~ low~ 3C~ r we i, r, r F~
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mABLE 11 Additive Dose mg/l _Turbidity NTU

Dextran 0.25 164 0.5 143 _ Dextran: 0.188:0.125 144 Synthetic A0.375:0.25 134 Synthetic A 0.5 162 1.0 161 -Dextran: 0.188:0.125 162 15 Synthetic B0.375:0.25 144 -Synthetic B 0.5 >170 1.0 >170 Example 12 The substrate was a simulated liquor as in Example 1, but with 100 mg/l sodium humate added.

ddi ive _ Dose mq/l Turbidity NTU
Dextran 0.1'5 162 0.25 153 0.375 151 0.5 142 -Dextran: 0.094:0.063 155 Synthetic A 0.188:0.125 125 0.281:0.188 108 0.375:0.25 102 Dextran: 0.094:0.063 169 Synthetic B 0.188:0.125 161 0.281:0.188 151 0.375:0.25 154 -Example 13 The substrate was simulated liquor as in Example 1, but with 2 g/l sodium oxalate added.

Additive Dose_mg/l Turbidity NTU

Dextran 0.25 161 0.5 142 Dextran:
Synthetic A 0.188Ø125 136 0.375:0.25 108 Dextran:
Synthetic B 0.188Ø125 161 0.375:0.25 145 Example 14 The substrate was a simulated liquor as Example 1 but humate and oxalate were added at 200mg/1 and 2g/1 respectively.

~ABL{~ 1 4 si ~ 1 7 ~
r a. 1 . O ~ 08 2.~

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~;.J~rr~r ~b. i ~
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i , Synthetic G comprises a 50:50 sodium acrylate acrylamide copolymer having a similar molecular weight as A.
A simulated liquor comprising a liquor containing 20% aluminium trihydrate solids in 200g/1 of NaOH is maintained at S0C. The additives were mixed into the liquor in 500cm3 cylinders using four strokes of a plunger. The settlement ratio of the solids was measured between two fixed points. The supernatant turbidity was also measured as described in Example 1 after allowing 10 minutes settlement. The dextran used was in the same form as in Example 1.

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~ ~L'f~r~ ,,- i. f~

1,1 This example serves to show the flocculating ability of further different types of synthetic polymer blended with dextran.
The simulated liquor used and test procedure were the same as in Example 1, except the multistage addition was used.
The synthetic used were as follows (C-K formed by the same process as A and presumed to have similar molecular weights):
Synthetic A Polyacrylate B Polyacrylamide C 95:5 Sodium acrylate : acrylamide D 86:14 " " "
E 77:23 " " "
F 63:37 " " "
G 50:50 " " "
H 35:65 " " "
I 25:75 " " ~
J 10:90 " " "
K 6:94 " " "

The dextran used was in the same form as in Example 1. Table 16 shows the residual turbidity (NTU) at each dose level (mg/l) of additive and can be compared against that achieved using an equivalent dose of dextran alone.

27 2~
TAi .l.

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S ~ ~ 17~ 16:~15?l;!'t~l 1~, P. 1?~ I~a ~g~ i ~9 r~ ~ r; :; 1 C r . ~ . 2 ., ~ A O O

17~ 15~llJi~

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, 5 i.~ n i~lp~V~ rl~ ~rl t~!,r~ ~ s~ ' a~x t~ r r jt~r ~.3i,t'~ ,p~tl;'l!l~'L' ~ i.LII~
?~ y~r~ .~c.~ s~a~;S .~r~l?~t~ orX
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i* 1 ~
, _ .. _ _. . .. , .. _ !

~; I

,. . .

2~

Example 1 was repeated but replacing dextran by amylopectin to assess the flocculation properties of amylopectin on its own.

5Table 17 Additive Dose mq/l Turbidity(NTU) Amylopectin 10 156 These results indicate that amylopectin has flocculant properties, when used in fairly high dose levels. It may be of use in combination with a synthetic flocculant.

Example 18 This example serves show the effect of the flocculant according to the invention on filtration.
Again the dextran used was in the same form as in Example 1, as was the synthetic polymer.
Plant filtration was carried out by rotary vacuum filtration of tertiary (fine) hydrate thickener underflow. No flocculant was added to aid sedimentation.
This was then repeated but this time flocculant was added before the pump transferring slurry from the thickener underflow slurry tank to the bowl of the rotary vacuum filter.
The results obtained are quoted as % increase in throughput compared to that obtained without flocculant (based on m3/hr).

2~

TA_~ 1 8 i 3 . g: 5 . ~ 2~ ,~
.R.5.6 i1.4 5 ~ 4 . 4 .3 . ~ 9 i!

5'~3 'J,'J .l t . !3h ~ t!~ ~t th~ ~ ~~ c~u1 ~n~.
i nc~c~ h~:~3u~ p ~d ~h~,r~o~
.f .L~',~;''Y '~ .t.~at~c~n ~ L~
rJ~r~ Y ~ h~ ?el dt!~r. of t~ ex~r~n ~!5 'i 5 .~

I

Claims (22)

1. A process in which alumina trihydrate crystals are formed and recovered from a sayer process liquor in which a flocculant is added to the liquor to improve clarification wherein the flocculant comprises a polysaccharide and a water-soluble synthetic polymer formed from ethylenically unsaturated monomer including acrylic monomer.
2. A process according to claim 1 wherein the liquor comprises a low organics content.
3. A process according to claim 1 wherein the temperature of the liquor is at least 50°C.
4. A process according to claim 1 wherein the synthetic polymer comprises at least 50% anionic monomer.
5. A process according to claim 4 in which the polymer is a homopolymer of anionic monomer.
6. A process according to claim 4 or 5 wherein the anionic monomer is selected from unsaturated carboxylic and sulphonic acids.
7. A process according to claim 6 in which the monomer is acrylic acid.
8. A process according to any of claims 1 to 4 wherein the synthetic polymer is polyacrylamide homopolymer.
9. A process according to claim 1 wherein the molecular weight of the synthetic polymer is at least 1,000,000.
10. A process according to claim 1 wherein the polysaccharaide is selected from dextran, dextran derivatives and amylopectin.
11. A process according to claim 1 wherein the molecular weight of the polysaccharide is in the range 50,000 to 40,000,000.
12. A process according to claim 11 wherein the molecular weight of the dextran is at least 1,000,000.
13. A process according to claim 1 wherein the amount of polysaccharide added is in the range 0.1 to 10 mg/l.
14. A process according to claim 1 wherein the amount of synthetic polymer added is in the range 0.1 to 20 mg/l.
15. A process according to claim 1 wherein the ratio of polysaccharide synthetic polymer is in the range 1:10 to 5:1.
16. A process according to claim 1 wherein the flocculant comprises a blend of polysaccharide and synthetic polymer powders.
17. A process according to claim 1 wherein the flocculant comprises an agglomerate of polysaccharide and synthetic polymer particles.
18. A process according to claim 1 wherein the polysaccharide and synthetic polymer are added sequentially.
19. A process according to claim 18 in which the polysaccharide is added first.
20. A process according to claim 18 wherein the synthetic polymer is a cationic polymer.
21. A process according to claim 1 wherein the flocculant is mixed in after some alumina trihydrate has precipitated.
22. A process according to claim 1 in which the liquor is filtered after it has been subjected to the process as defined.
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AU632801B2 (en) 1993-01-14
DE69005349D1 (en) 1994-02-03
EP0392779A1 (en) 1990-10-17
ES2047848T3 (en) 1994-03-01
US5041269A (en) 1991-08-20

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