GB2268422A - Dewatering of mineral suspensions - Google Patents

Abstract

An aqueous mineral concentrate having a solids content of 200 to 600 grams per litre is flocculated with a polymeric anionic flocculant formed from ethylenically unsaturated monomers comprising sulphonic monomer and the flocculated suspension is dewatered by means of a pressure belt filter. The flocculant type used appears to give a floc structure that is particularly suited to pressure belt filtration. A preferred anionic flocculant polymer is a copolymer of the sodium salt of acrylamido methyl propane sulphonic acid (AMPS) and acrylamide. Preferred flocculant polymers have intrinsic viscosity of at least 8dl/g.

Description

Allied Colloids Limited Dewatering of Suspensions and Polymers for Use in This Anionic flocculants are widely used for flocculating mineral suspensions. The flocculated suspensions are often dewatered by sedimentation. The normal flocculants are polymers of sodium acrylate, but the use of sulphonate polymers is known (e.g., U.S. 4,342,653 and 4,704,209). In U.S. 4,647,382 a sulphonate polymer is used to make a redispersible cake, but since the cake is redispersible the polymer must be of low molecular weight relative to high molecular weight flocculants.

A particular problem arises when it is desired to dewater an aqueous mineral concentrate having a solids content of 200-600g/l by mixing it with a polymeric flocculant to cause flocculation followed by dewatering it under pressure and shear on a pressure belt filter. This leads to the production of a substantially dry cake that can be removed from the filter.

This process is widely used on coal based slurries, barrel wash effluents, tailings, coal slurries, screen underflows, etc. but it is also used for sand effluents, limestone effluents, china clay, calcium carbonate and the other mineral substrates.

The requirements are that rapid dewatering of the slurry takes place such that sufficient free water is removed prior to pressure being applied when the pressure belts come together. Further dewatering takes place in the pressure zone and as the belts part the filter cake should easily fall away from the cloths. The filter cake usually has a moisture content of 25-45% w/w.

The polymeric flocculant that is conventionally used in such processes in which dewatering takes place ona pressure belt filter is a high molecular weight non-ionic or anionic acrylamide copolymer, i.e., a substantial homopolymer of polyacrylamide or a copolymer of sodium acrylate with acrylamide, or sometimes sodium acrylate homopolymer.

The use of this flocculant tends to give rather large flocs that can cause blinding of the filtration belts or cloths.

It is known to mix into the flocculated system relatively low molecular weight cationic polymers so as to strengthen the flocs and facilitate the separation of the flocculated material from the pressure belts.

According to the invention, we find a better performance can be obtained if the flocculant has high molecular weight and contains sulphonate groups. It seems that this flocculant type gives a floc structure that is particularly suited to pressure belt filtration.

A process according to the invention for dewatering an aqueous mineral concentrate having a solids content of 200600g/l comprises flocculating the concentrate with an anionic flocculant polymer formed from ethylenically unsaturated monomers comprising sulphonic monomer, and dewatering the flocculated suspension by means of a pressure belt filter. It can be a conventional pressure belt filtration system for dewatering aqueous mineral suspensions that have previously been flocculated by use of an acrylamide-sodium acrylate copolymer. Typical pressures during a conventional pressure belt filtration cycle are at least lpsi, preferably at least 1.5psi, more preferably at least 3psi, and can be up to 30psi and above in the later stages of the cycle.

The flocculant polymer is formed from ethylenically unsaturated monomers comprising a monomer that includes sulphonic groups, preferably sulphonate groups. The preferred sulphonate monomer is acrylamido methyl propane sulphonic acid (AMPS, U.S. trade mark) or salts thereof but other sulphonate monomers can, less preferably, be used.

Examples include vinyl sulphonate and allyl sulphonate (but it is necessary to ensure that the flocculant has adequate molecular weight).

The polymer is generally a copolymer of sulphonate monomer with other monomer. The other monomer may be anionic or non-ionic or a blend. The monomers are usually all water soluble. A preferred comonomer is acrylamide and so preferred copolymers for use in the invention are copolymers of sulphonate monomer (generally AMPS) and acrylamide.

Additional monomer can be used for forming the copolymer, and in particular terpolymers with an ethylenically unsaturated carboxylic acid monomer, especially acrylic acid (usually as sodium acrylate) can be used.

The copolymers are generally formed from a monomer mixture containing 5 to 95%, preferably 10 to 60%, often 20 to 40%, sulphonate monomer, preferably the sodium salt of AMPS. The mixture generally contains from 5 to 95%, preferably 30 to 70%, often 40 to 60%, by weight acrylamide. The mixture may also contain from 10 to 50%, preferably 15 to 35%, by weight carboxylic monomer, for instance sodium acrylate.

The polymer preferably has intrinsic viscosity above 8, most preferably above lOdl/g. It is usually unnecessary for it to have intrinsic viscosity above about 25dl/g, but higher values can be used if desired.

Intrinsic viscosity is measured by suspended level viscometer in buffered 1N NaCl at 250C.

The polymer may be made by conventional techniques, for instance gel polymerisation or reverse phase emulsion or bead polymerisation.

We find the use of the AMPS copolymer or terpolymer gives a stronger floc structure than is obtainable with the conventional sodium acrylate-acrylamide flocculants. This improved floc structure reduces the risk of blinding of the belts or other filter elements and allows the attainment of a lower moisture content in the dewatered product.

As a result of the use of the sulphonate monomer, it may be possible to achieve adequate results without the need for subsequent dosing with low molecular weight cationic material, especially if the aqueous dispersion that has been treated with the sulphonate anionic flocculant is subjected to vigorous stirring before cake formation so as to degrade the flocs. However it is generally preferred to flocculate the aqueous mineral suspension with the defined sulphonate flocculant and then to treat the flocculated suspension, often after shearing or other stirring to break down the floc size; with a low molecular weight cationic material. This material has lower molecular weight than the flocculant. It can be a cationic polymer which can be any of the materials that are conventionally regarded as low molecular weight cationic coagulants.Typically they have intrinsic viscosity below 4dl/g, often below 2dl/g and may have molecular weight typically below 1 million. Suitable materials are polyamines, homopolymers of diallyl dimethyl ammonium chloride and copolymers of it with acrylamide (usually in a minor amount) and homopolymers (or copolymers with acrylamide) of dialkylaminoalkyl -(meth) acrylates or (meth) acrylamides. For instance suitable polymers are Mannich reaction products and homopolymers (or copolymers with acrylamide) of quaternised dimethylaminoethyl acrylate or dimethylaminopropyl acrylate (e.g., MAPTAC).

Instead of using a cationic polymer, a monomeric precipitant can be used. Examples are fatty amines, generally in quaternised salt form. An example is quaternary tallow amine.

The dosage of the flocculant (and also the dosage of the cationic material if used) is generally in the range 50 to 600g/tds. The ratio of flocculant to cationic material (if used) is generally in the range 100:0 to 50:50.

The following are examples of the invention.

Example 1 A coal screen underflow of following characteristics was used as substrate.

35.1% w/v solids (weight (g) by volume (1)) 30.1% Ash (on solids) 62.4%-63# (on solids) pH - 7.0 400cm3 samples of substrate were taken in 600cm beakers and stirred mechanically using a gate stirrer. The anionic flocculant was added at 213g/tds (grammes/ton dry solids) stirring continued for 8-10 seconds. The flocculated sample was transferred to the chamber of the belt press simulator and allowed to dewater for 45 seconds.

The top belt was lowered on to the semi-formed cake and a pressure cycle as follows applied.

Pressure Cycle: Time (Sec) Pressure (nisi) 0-15 3 15-30 6 30-45 9 45-53 12 53-60 18 60-68 24 The moisture content of the resultant cake was measured.

Product A = 30:70 NaAc:ACM IV = 12 B = 40:60 NaAMPS:ACM IV = 17 C = 20:20:60 NaAc:NaAMPS:ACM IV = 16 D -= 20:30:50 NaAc:NaAMPS:ACM IV = 19 E = 30:20:50 NaAc:NaAMPS:ACM IV = 20 The following cake moisture contents were obtained.

A = 35.3% B = 33.8% C = 34.0% D = 34.5% E = 34.3% ExamPle 2 The same slurry as in 1 was used. In this case after the anionic flocculant had been added mixing was continued for 10 seconds. A cationic polyamine of IV approx 0.3 was then added at an active dose of 213g/tds and mixing continued for 2 seconds. The following cake moisture contents were obtained.

Product A - 35.2% Product B - 34.8% Example 3 Coal screen underflow as in Example 1 was used.

Anionic flocculant was added at 213g/t and stirring continued for 10-22 sec. Cationic polyamine as in Example 2 was added at the ative dose level quoted and mixing continued for 2-3 seconds. The sample was treated as in Example 1.

Cationic Active Cake Moisture Content % Dose git Product A Product B 213 34.8 34.1 142 34.4 34.6 71 34.5 34.2 35.5 34.6 33.8 14 Cake broke down 33.9 Product B maintains an improved performance as the sose of cationic product decreases.

Example 4 Coal screen underflow as in Example 1 was used.

Anionic flocculant was added at 213g/t and stirring continued for 10-14 seconds. Cationic polyamine as in Example 2 was added at 213g/t and mixing continued for 2-3 seconds. The test parameters used were varied as follows: (i) Volume of sample (ii) Pressure cycle Volume A = 400cm3 Volume B = 600cm3 Volume C = 800cm3 Cycle A = that quoted in Example 1.

Time (Sec.) Pressure Cycle B Cycle C Cycle D 3 0-30 0-45 0-60 6 30-60 45-90 60-120 9 60-90 90-135 120-180 12 90-106 135-158 180-210 18 106-120 158-180 210-240 24 120-136 180-203 240-270

Cycle E is the same as Cycle B but 24psi is maintained until no further dewatering is observed.

Volume Cycle Cake Moisture Content Product A Product B A B 34.7 34.2 A E 34.1 33.3 B A 35.5 34.1 C A 36.5 35.5 B C 34.7 32.9 C D 33.0 32.2 Under all the conditions evaluated Product B provides improved moistures over Product A.

Example 5 A sample of coal screen underflow from the same plant as that used in Example 1 was the test substrate but it had slightly different characteristics as follows.

38.1% w/v solids 33.7% Ash (on solids) 54.0% -63y (on solids) pH - 7.0 Tests were carried out as described in Example 1 except that anionic flocculant was added at a dose level of 198t/tds and stirring maintained for between 20 and 25 seconds. Cationic polyamine was added as in Example 2 at the dose levels quoted and mixing continued for 2-3 seconds.

Anionic flocculants test: Product B Product C Product F 30:70 NaAc:ACM IV = 23.

Cationic Cake Moisture Content (%) Active Dose (g/tds) Product B Product C Product F 198 32.6 32.6 33.3 132 32.1 32.9 33.9 66 32.1 - 33.3 33 31.4 - 13 - - The products containing sodium AMPS provide productsan improvement in moisture content.

Example 6 Tests as described in Example 5 were carried out using Products A & B. In this case the cationic component used was quaternary tallow amine, generally less effective than the polymeric cationic coagulants normally used.

Active Dose of Cake Moisture Content % Cationic Product Product A Product B 198 33.3 32.1 132 Cake Broke Down 31.5 66 Did not press 31.5 33 Did not press Cake Broke Down 17 Did not press Did not press Even with a less effective cationic component Product B continues to work with a lower cationic dose and provides cakes of lower moisture content than Product A.

Claims (13)

1. A process for dewatering an aqueous mineral concentrate having a solids content of 200 to 600 grams per litre comprising flocculating the concentrate with an anionic flocculant polymer formed from ethylenically unsaturated monomers comprising sulphonic monomer, and dewatering the flocculated suspension by means of a pressure belt filter.

2. A process according to claim 1, wherein the anionic flocculant polymer has an intrinsic viscosity of at least 8dl/g.

3. A process according to claim 1 or claim 2, wherein the anionic flocculant polymer is formed from ethylenically unsaturated monomers comprising a monomer that includes sulphonate groups.

4. A process according to any preceding claim, wherein the sulphonic monomer is acrylamido methyl propane sulphonic acid (AMPS) or its sodium salt or vinyl sulphonate or allyl sulphonate, preferably AMPS or its sodium salt.

5. A process according to any preceding claim, wherein the flocculant polymer is a copolymer of sulphonic monomer and acrylamide.

6. A process according to any preceding claim, wherein the polymer is formed from a monomer mixture containing 10 to 60%, preferably 20 to 40%, sulphonate monomer.

7. A process according to claim 5, wherein the copolymer is formed from a monomer mixture containing 30 to 70%, preferably 40 to 60%, acrylamide monomer.

8. A process according to any preceding claim, wherein the flocculant polymer has an intrinsic viscosity from 10 to 25dl/g.

9. A process according to any preceding claim, wherein after flocculation with the anionic flocculant polymer the aqueous mineral concentrate is subjected to vigorous stirring before cake formation.

10. A process according to any preceding claim, wherein after flocculation with the anionic flocculant polymer the aqueous mineral concentrate is treated with a cationic material having lower molecular weight than the flocculant polymer.

11. A process according to claim 10, wherein the cationic material is a cationic coagulant having intrinsic viscosity below 4dl/g.

12. A process according to claim 10, wherein the cationic material is a monomeric precipitant, preferably a quaternised fatty amine.

13. Use of an anionic polymer formed from ethylenically unsaturated monomers comprising sulphonic monomer to flocculate an aqueous mineral concentrate having a solids content of 200 to 600 grams per litre which is to be dewatered subsequent to flocculation, said dewatering being carried out by means of a pressure belt filter.