GB1588016A

GB1588016A - Suspensions of paricles in solutions of polymeric flocculants

Info

Publication number: GB1588016A
Application number: GB2781776A
Authority: GB
Original assignee: Associated Portland Cement Manufacturers Ltd
Current assignee: Associated Portland Cement Manufacturers Ltd
Priority date: 1976-07-05
Filing date: 1976-07-05
Publication date: 1981-04-15
Also published as: FR2357597A1; FR2357597B3

Description

(54) SUSPENSIONS OF PARTICLES IN SOLUTIONS OF POLYMERIC FLOCCULANTS (71) We, THE ASSOCIATED PORTLAND CEMENT MANUFACTURERS LIMITED, a Company organised under the laws of Great Britain, of Portland House, Stag Place, London SW1E 5BJ, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the manufacture of aqueous solutions of polymers capable of acting as flocculants, using inorganic particles as aids to such manufacture.

In one aspect of the invention the solutions produced may be used as solutions of polymeric flocculants containing particles which act as weighting agents. In another aspect the invention concerns the preparation of suspensions of mineral particles in aqueous solutions of polymeric flocculants. Such suspensions are of value in producing flocs of the suspended particles themselves as disclosed in our co-pending U.K.

patent Application No. 53309/74 (Serial No.

1,497,280).

The formation of such flocs is useful, for instance in the clarification of waste water containing suspended particles, and is particularly useful in the paper industry for controlling the 'formation' of the paper and the retention of filler in the paper. The invention further relates to the production of flocs of particulate matter from such aqueous suspensions of the particles.

It is generally accepted that the high molecular weight polymers generally used as flocculants operate by bridging between the individual particles to be flocculated, different segments of one molecule absorbing on different particles. These polymers are most effective when fully solvated by the water and of very high molecular weight. The types of polymers most frequently employed are the partially hydrolysed polyacrylamides, acrylamide-acrylic acid copolymers or cationic polyacrylamides.

Difficulty arises in large scale practice in making solutions of such high molecular weight polymers. Aqueous solutions of such polymers are of such a high viscosity that these can only be handled or pumped at very low solids contents. This means that it is not economic to transport these materials in aqueous solution. The highest molecular weight polymer are therefore supplied as dry powders of such a particle size that they have good powder flow characteristics. In spite of this, even when the particles of solid polymer are added carefully stirred water, there is a strong tendency for them to swell and then cluster together to form sticky masses. These can then adhere to the stirrer or to the sides of the container and it is then very difficult to bring them into solution.

This difficulty becomes the more severe the larger the scale of the operation. Mechanical devices are at present employed to help with this process: an example is the "Eductor" by which a stream of particles suspended in air is injected into a turbulent flow of water. Process plant for carrying out the whole operation continuously or on a large scale is, however, expensive and requires either automatic control or manual operation and attendance, both being expensive.

In U.K. Patent Specification No. 1,497,280 a method of operation is disclosed whereby solid mineral particles, particularly calcium carbonate particles, in aqueous suspension, can be mixed with solutions of anionic polyacrylamide flocculants without the normal flocculating action taking place. A similar principle is employed in the present invention, which is particularly useful in preparing the suspensions of mineral particles in water with surface active agents and polymeric flocculants disclosed in that Specification.

According to the present invention we provide a method of preparing an aqueous solution of a polymer capable of flocculating mineral particles, wherein the tendency to the formation of lumps of swollen polymer is reducked, which comprises adding the particulate solid polymer to agitated water in the presence of particles of said mineral and an effective amount of a surface active substance which substantially inhibits the flocculating action.

The flocculant is added to the agitated water in the presence of the mineral particles, generally mineral powder particles. Thus the flocculant may be added alone to agitated water which contains mineral powder particles in suspension, or the flocculant may be mixed with a dry mineral powder and the resulting mixture added to agitated water which may or may not contain further mineral powder particles in suspension.

It is supposed that the mineral particles stop lump formation by keeping the swelling polymer particles apart, but the invention does not depend upon the correctness of this supposition.

It is known that a mixture of particles of polymeric flocculant with starch granules is much less prone to form sticky lumps when attempts are made to prepare solutions of the polymer.

Tlle starch granules keep the swollen polymer particles apart until sufficient viscosity has been imparted to the water by solution of the polymer to stop the polymer particles clustering together. Mineral particles, however, which are liable to flocculation by the polymeric flocculant, cannot satisfactorily be used on their own in this way. By the addition of suitable surface active material to inhibit such flocculation, the use of such mineral particles can according to the invention provide a solution to what is a major industrial problem.

Solid particles suspended in water, and especially particles of a mineral substance, tend to cluster together to form flocs. This phenomenon is a result of the energy residing at the interface between the solid particles and the water. A lower level of energy is reached when the particles are in contact. This flocculation is easily broken down by shear but when the shear stops, flocs at once re-form.

When the water contains dissolved molecules these tend to absorb at the particle/water interface. According to the nature of the.dissolved molecules these may absorb strongly, in some cases so that most of the molecules can be removed from solution. When this is the case, the properties of the particle/water interface can be greatly changed, in which case the molecules are known as surface active materials. In particular the tendency for the particles to flocculate together can be greatly reduced or greatly increased.

A reduction of flocculation can occur by mechanisms involving electric charges formed at the interface, or by steric effects. The class of materials known as surface active agents is divided into anionic, which produce a negative charge at the interface, cationic which produce a positive charge at the interface, and non-ionic, which produce no charge but operate primarily by steric mechanisms. In the case of polymeric dissolved materials the electrostatic and steric mechanisms can operate simultaneously. In this last case it may be usefully considered that the solid particles are prevented from flocculating together by barriers of polymer and water molecules.

In selecting a surface active material for inhibiting flocculation of the mineral particles by added polymer, consideration is to be given to the intended purpose of the solution of polymer produced. If the polymer is of interest as a flocculant it will, of course, be necessary to employ a flocculation-inhibiting surface active material in the performance of the invention an agent whose inhibiting effect can be eventually counteracted. A case of particular interest in this respect is where the mineral particles used are themselves to be flocculated, and especially in the case where these mineral particles alone or with further mineral filler are to provide flocculated filler (so-called "pre-flocculated" filler) for papermaking, such as pre-flocculated calcium carbonate, for example chalk whiting.

According to a preferred embodiment of the invention the surface active substance inhibiting flocculation is a sufficient amount of a surface active agent such as to impart to the mineral particles in suspension in the water an electrostatic charge measured by zeta potential similar in sign to the charge present on the dissolved molecules of the polymer and of sufficient magnitude to inhibit the flocculating action of the polymer on the mineral powder particles.

The invention is particularly effective in connection with anionic polymers to be dissolved, with which there are used mineral particles to which a negative charge is imparted by means of a suitable surface active agent.

Among the polymers the polyacrylamide polymers and copolymers are of particular interest.

The surface active agent can be dissolved in the water or introduced to the system, if desired by pretreating the mineral particles with the agent before they are mixed with the polymer or dispersed in the water. Mixed surface active substances can be used, for instance to achieve an enhanced electrostatic effect.

It may be noted that the absorption of materials from aqueous solution resulting in flocculation can operate by three mechanisms: (a) the adsorbed molecules can neutralise one of the above-mentioned mechanisms by which flocculation has been inhibited, e.g., by charge neutralisation; (b) a molecule may interact with the solid material by "reverse adsorption", for example a soap may absorb on to a hydrophilic material thus making it hydrophobic so that the particles are strongly flocculated; or (c) a long chain molecule hydrated in solution and so extended can absorb onto more than one particle, thus holding particles together and causing flocculation. This last bridging mechanism is used industrially for many purposes.

In general solutions of the long chain polymers are made in water and such a solution is then added to a suspension of particles to be flocculated. The invention facilitates the making of these solutions. In the case of clarifying large volumes of water, charge dispersal and consequent flocculation can be brought about by dilution.

Examples of surface active materials useful in imparting a charge to mineral particles or otherwise inhibiting flocculation by long chain polymers are: low molecular weight polymers carrying a multiplicity of negative charges such as sodium polyphosphates, sodium polyacrylates, sodium polymethacrylates, sodium polysilicates and condensed naphthalene sulphonates; soaps with a hydrocarbon chain solubilised by a carboxy or sulphonic acid group neutralised by soda; and polymers or oligomers such as starch phosphate.

Mineral substances suitable as the mineral particles used in the invention include not only whiting, which requires relatively little surface active agent to become highly charged, but also for instance ground dolomite and limestone, precipitated calcium carbonate, china clay, titanium dioxide satin white, talc and silica.

The quantity of mineral particles required is readily determined by experiment insofar as it is not predetermined by the amount desired in a final product such as preflocculated filler for paper. As a practical minimum an amount of mineral substance having a surface area at least ten times that of the swollen polymer granules is indicated. The maximum may be a sufficient amount to achieve the desired solubilisation or it may be limited only by the maximum concentration that can be suspended in the water.

The amount of surface active material required is simply that which is effective to inhibit flocculation and is readily established in practice, depending in some cases on the existence of a concentration determinable by experiment at which maximum charge is produced.

The following Examples are given for the purpose of illustrating the invention, Examples 1 to 6 being comparative.

EXAMPLE 1 (Showing that under carefully controlled conditions of mixing and of polymer addition, a solution of the polymer can be prepared in the laboratory without lump formation).

Two litres of tapwater, hardness about 240 p.p.m. measured as calcium carbonate, was mechanically stirred in a 5-litre beaker using a propeller blade mounted off-centre, the diameter of the swept circle being two-thirds that ofthe internal diameter of the beaker and rotating so that a 'down-stream vortex' was formed. It was important that the 'tail' of this vortex did not extend down to the stirrer blade.

Eight grams of anionic polyacrylamide flocculant, Reten 421, (Hercules Powder Co. Ltd., London) ("Reten" is a trade mark) was fed with care as separate particles into the vortex, from an angled channel formed from transparent plastics film, on which the granules of polymer could be clearly seen. A suspension of unclustered granules was formed which on slow stirring for 30 minutes showed no lumps. If the vortex 'tail' led to the stirrer propeller, swollen polymer formed around it. This swollen polymer was slow to dissolve. The conditions of this Example could not be obtained in a large-scale operation.

Examples 2, 3 and 4 show that it is not easy to avoid lumps of polymer granules in preparing a polymer solution.

EXAMPLE 2 With the same mixing conditions as in Example 1, but with rapid addition of polymer as a stream, lumps were formed and persisted after 30 minutes stirring. On standing overnight a gelatinous layer formed at the bottom of the beaker.

EXAMPLE 3 With a smaller propeller stirrer placed centrally and with slow and careful addition of polymer granules, a mass of swelled polymer formed around the stirrer blade. This was not completely dissolved after two hours stirring.

EXAMPLE 4 With the small propeller placed off-centre and rapid addition of polymer, lumps were formed but not on the blade.

EXAMPLE 5 (Showing that the preliminary mixing of polymer granules with particles of a natural chalk whiting does not help matters, because the processes of wetting and dispersion of the mineral particles, of partial solution of the polymer and of flocculation of the particles by dissolved polymer take place concurrently).

Two litres of tapwater were stirred as in Example 1. A mixture of 8 g of Reten 421 and 80 g of natural chalk whiting, Snowcal 6ML (Cement Marketing Co. Ltd.) ("Snowcal" is a trade mark), was prepared by mixing dry by hand with use of some energy. This was added all at once into the off-centre vortex.

After 30 minutes there were lumps of gel associated with whiting particles and after standing overnight the bottom layer was lumpy.

Strong hand stirring did not break up the lumps.

EXAMPLE 6 (Showing that the presence of particles in the water did not help to avoid lump formation).

A suspension of 1000 g Snowcal 6ML, in 500 ml tapwater, was prepared by stirring for 15 minutes as in Example 1, using a large offcentre blade. Dry granules of Reten 421 (0.264 g) were added into the vortex, all at once. A granular flocculated mix resulted. The flocculated nature of the mix was evidenced by the thick state at only 66 per cent solids content. Stirring was continued for a further 15 minutes. On addition of the following deflocculating mixture: 2 Molar Ammonia Solution 5 ml 10 per cent ammonium carbonate solution 1 ml Nopcosant K, surface active agent (Diamond Shamrock Ltd.) 1 g Pyramid No. 8, surface active agent (Crosfield & Son Ltd.) 4 g the mix thinned at once and lumps of swollen polymer could be seen and were retained on a BS 150 mesh sieve. ("Nopcosant" and "Pyramid" are trade marks).

EXAMPLE 7 (Showing the operation of the method of the invention). To 340 ml tapwater, there was added the deflocculant mixture described in Example 6. Granules of Reten 421, 0.264 g, were mixed with 1000 g Snowcal 6ML. This mixture was added to the water, stirred as in Examples 1 and 6, as fast as the vortex would receive the mixture. An apparently smooth mixing, free from lumps, resulted almost at once and after 15 minutes the mixture had all the properties of a mixture made from a separately dissolved and matured polymer solution.

EXAMPLE 8 A result nearly as good as in Example 7 was obtained without pre-mixing the polymer granules and the powder, the powder being previously dispersed in the water.

The following dispersion of Snowcal 6ML was made: Tapwater 340 ml 2 M ammonia solution 5 ml 10 per cent ammonium carbonate solution 1 ml Nopcosant K, surface active agent 1 g Pyramid No. 8, surface active agent 4 g Snowcal 6ML 1000 g To this dispersion, stirred with the larger propeller blade into a down-stream vortex, Reten 421, 0.264 g, was added all at once. An initial granular appearance disappered to give a smooth normal mixing in 30 minutes.

It is clear that a combination of the methods of Examples 7 and 8 can be most satisfactory in practice. The polymer granules are then premixed with part of the mineral particles and the rest of the mineral particles stirred into the solution of surfactant. The mixture of polymer granules and mineral particles is then added to the solution, the mineral particles preventing balling up of the polymer even if added in large doses.

It is apparent that either pre-mixing polymer granules with whiting particles, or more surprisingly, addition of polymer granules to a suspension of chalk whiting particles, enables solubilisation of the granules to occur without lump formation, provided the particles are themselves deflocculated. This discovery enables suspensions of deflocculated chalk whiting particles in solutions of flocculating polymer, such as these disclosed in U.K. Patent Specification No.

1,497,280 to be manufactured in one stirred tank. That Specification discloses only preparation by separate dissolving of the flocculating polymer alone.

It is an advantage of the invention that the inhibition of flocculation described serves to minimise problems which arise from the subjection to shear of polymer-bridged particles which not only tends to destroy the flocculation but also to impair the value of the dissolved poly mer as a flocculant for the same or for other particles.

WHAT WE CLAIM IS: 1. A method of preparing an aqueous solution of a polymer capable of flocculating mineral particles, wherein the tendency to the formation of lumps of swollen polymer is reduced, which comprises adding the particulate solid polymer to agitated water in the presence of particles of said mineral and an effective amount of a surface active substance which substantially inhibits the flocculating action.

2. A method according to Claim 1, wherein the polymer is added to agitated water which already contains mineral powder particles in suspension.

3. A method according to Claim 1 or 2, wherein the polymer is mixed with a dry mineral powder before being added to the agitated water.

4. A method according to any preceding Claim wherein said polymer comprises a polyacrylamide.

5. A method according to any preceding Claim wherein said mineral is a calcium carbonate.

6. A method according to any preceding Claim wherein the surface active substance inhibiting flocculation is a sufficient amount of a surface active agent such as to impart to the mineral particles in suspension in the water an electrostatic charge measured by zeta potential similar in sign to the charge present on the dissolved molecules of the polymer and of sufficient magnitude to inhibit the flocculating action of the polymer on the mineral powder particles.

7. A method according to Claim 6, wherein said polymer is anionic.

8. A method according to Claim 6 or 7, wherein said surface active material is a low molecular weight polymeric surfactant bearing a multiplicity of negative charges.

9. A method according to Claim 6, 7 or 8, wherein the mineral is chalk whiting.

10. A method according to any of Claims 6 to 9, wherein the mineral powder particles alone or together with further mineral filler are subsequently flocculated by counteraction of said inhibition, to provide flocculated filler for papermaking.

11. A method of preparing an aqueous solution of a polymer according to Claim 1, being a method substantially as described in Example 7 or 8.

12. An aqueous solution of a polymer capable of flocculating mineral particles, whenever prepared by a method according to any of Claims 1 to 11.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the mix thinned at once and lumps of swollen polymer could be seen and were retained on a BS 150 mesh sieve. ("Nopcosant" and "Pyramid" are trade marks). EXAMPLE 7 (Showing the operation of the method of the invention). To 340 ml tapwater, there was added the deflocculant mixture described in Example 6. Granules of Reten 421, 0.264 g, were mixed with 1000 g Snowcal 6ML. This mixture was added to the water, stirred as in Examples 1 and 6, as fast as the vortex would receive the mixture. An apparently smooth mixing, free from lumps, resulted almost at once and after 15 minutes the mixture had all the properties of a mixture made from a separately dissolved and matured polymer solution. EXAMPLE 8 A result nearly as good as in Example 7 was obtained without pre-mixing the polymer granules and the powder, the powder being previously dispersed in the water. The following dispersion of Snowcal 6ML was made: Tapwater 340 ml 2 M ammonia solution 5 ml 10 per cent ammonium carbonate solution 1 ml Nopcosant K, surface active agent 1 g Pyramid No. 8, surface active agent 4 g Snowcal 6ML 1000 g To this dispersion, stirred with the larger propeller blade into a down-stream vortex, Reten 421, 0.264 g, was added all at once. An initial granular appearance disappered to give a smooth normal mixing in 30 minutes. It is clear that a combination of the methods of Examples 7 and 8 can be most satisfactory in practice. The polymer granules are then premixed with part of the mineral particles and the rest of the mineral particles stirred into the solution of surfactant. The mixture of polymer granules and mineral particles is then added to the solution, the mineral particles preventing balling up of the polymer even if added in large doses. It is apparent that either pre-mixing polymer granules with whiting particles, or more surprisingly, addition of polymer granules to a suspension of chalk whiting particles, enables solubilisation of the granules to occur without lump formation, provided the particles are themselves deflocculated. This discovery enables suspensions of deflocculated chalk whiting particles in solutions of flocculating polymer, such as these disclosed in U.K. Patent Specification No.

7. A method according to Claim 6, wherein said polymer is anionic.

9. A method according to Claim 6, 7 or 8, wherein the mineral is chalk whiting.