WO1996033138A1

WO1996033138A1 - Aqueous purification processes

Info

Publication number: WO1996033138A1
Application number: PCT/GB1996/000934
Authority: WO
Inventors: Adrian William Nixon; Jason Paul Monckton
Original assignee: Allied Colloids Limited
Priority date: 1995-04-18
Filing date: 1996-04-16
Publication date: 1996-10-24
Also published as: GB9507895D0; AU5340896A

Abstract

The invention provides a process for the recovery of inorganic solids from an aqueous suspension of inorganic solids and polymer latex by preferential coagulation of the inorganic solids. This preferential coagulation is carried out by the addition to the suspension of a cationic polymeric material having cationic content of at least 1, and usually 4, meq/g in an amount of less than 1000 ppm and usually less than 100 ppm. The coagulant is added in an amount such that the ratio of inorganics to organics in the floc formed is greater than the corresponding ratio in the suspension treated by a factor of at least 2, usually above 10. The coagulated solids are recovered. This process is particularly useful in the paper-making industry where the aqueous suspension may be derived from coating colour suspension, in which the inorganic solids are primarily clay or calcium carbonate and the recovered solids can be recycled for use.

Description

AQUEOUS PURIFICATION PROCESSES

The invention relates to a process for the recovery of inorganic solids from suspensions of a mixture of organic and inorganic solids, in particular industrial effluents containing pigment and latex- Many industrial processes involve the use or production of aqueous suspensions of solids. These solids may be inorganic material, organic material or a mixture of the two. Where the solids are a mixture of organic polymer latex and inorganic, in some circumstances it would be desirable to be able to separate the suspended inorganic solids from the latex.

A particular example of such a case occurs in paper- making processes. Many grades of paper are provided with a "coating colour" which is usually white and is an aqueous suspension of solids. The suspended solids are a mixture of inorganic pigment, often clay, with binder and as many as 20 other components, generally organic, including materials such as dispersants and biocides. The binder components are generally synthetic latices such as styrene butadiene rubber (SBR) or polyvinyl acetate. These binder components are usually present in a larger amount than the other organic components.

For storage and application purposes the coating colour is formed as a high solids content (often up to 65%) suspension. The concentrated coating colour suspension is prone to settling out, drying and blockage of the paper- coating machine if its continuous production and use is interrupted. In practice there are on occasion breaks in the paper being coated by the paper-coating machine, resulting in the process of coating the coating colour onto the paper being temporarily suspended. The coating head in which the coating colour suspension is held for coating purposes is immediately washed clean to prevent drying and caking of the suspension held there and possible blockage of the coating machine. This results in dilution of the suspension, for instance to 0.1 to 5%, often around 1%, solids and the production of large amounts of unusable diluted coating colour.

A typical paper mill using coating processes may have to dump several tons of diluted coating colour per day. Disposal traditionally involves discharge as a separate effluent or as part of the mill effluent, often after conventional effluent treatment.

This conventional effluent treatment usually involves separation of substantially all the solids together from the liquor, for instance by coagulating (sometimes referred to as flocculating) all the solids, often using alum as a coagulant. Conventional effluent treatment processes are described in, for instance, US-A-3,141,816 and GB-A- 1,387,744. The water may then be recycled or may be discharged. The coagulated solids obtained when the effluent is diluted coating colour are a mixture mainly of inorganic filler and polymeric latex binder. They cannot be reused and generally have to be dumped, generally landfilled. Effluents containing dispersed pigment and latex are also formed in other industries, such as the paint industry.

It would be desirable to be able to treat the diluted unused coating colour or other suspension of suspended inorganic solids and latex so as to separate the inorganic and latex components to produce an inorganic fraction containing no latex or containing latex in an amount sufficiently low that the latex residues do not cause contamination problems if the inorganic is reused. The separated inorganic filler may then be recycled, for instance to the paper-making process as a filler or as a pigment for the production of further coating colour.

International Patent Publication W093/21381 describes a method in which pigment is allegedly separated from coating colour effluent using a polymeric material. The only material mentioned or exemplified is high molecular weight polyethylene oxide, molecular weight 6,000,000 or greater. This non-ionic, high molecular weight material is said to give preferential separation of pigment materials. However, we have found these polymeric materials to give inadequate selectivity in separation. The invention provides a process of recovery of inorganic solids from an aqueous suspension of particulate inorganic solids and water-insoluble polymer latex, the process comprising preferentially coagulating the inorganic solids and recovering the coagulated solids, wherein the preferential coagulation is conducted by including in the suspension a solution of a cationic polymeric material having cationic content of at least l meq/g in an amount which is less than lOOOppm and which is such that the ratio of inorganic solids to water-insoluble organics in the recovered coagulated solids is significantly greater than this ratio in the suspension, for instance by a factor of at least 2.

Coagulation can occur by a process of aggregating solids by means of charge neutralisation. It is believed that certain materials act as aggregants primarily in this way. Coagulation can also occur by a method, often termed flocculation, which is believed to involve bridging by the coagulant (often called a flocculant) between the particles of the solid to be aggregated by the process. It is believed that certain materials cause aggregation primarily by this mechanism. It is believed that many materials cause aggregation by a combination of both mechanisms. In this specification we will use the term "coagulation". However, this is to be understood as covering all coagulation and flocculation mechanisms for aggregation of suspended solids and is not limited to any one particular mechanism.

The process can be applied to recovery of pigment from a variety of pigment-latex suspensions but is especially beneficial when the suspension is a dilute suspension, for instance having a total solids content of below 10%, and usually below 5%, by weight. The solids content is usually at least 0.1% by weight up to 2% by weight. The suspension is often an industrial effluent containing pigment and latex.

The invention is particularly useful in the paper- making industry, where the suspension of water-insoluble polymer latex and inorganic solids may be diluted unused or effluent coating colour. Other systems in which the invention is useful include effluent containing inorganic pigments from paint factories, for instance the effluent from a factory making water-dilutable emulsion paints. The recovered inorganic component can be reused as pigment or filler.

The polymer latex comprises water-insoluble polymer, especially synthetic polymer, dispersed in the aqueous medium. Generally the polymer is a polymeric binder material for a coating colour or latex paint.

The polymer latex, that is water-insoluble polymer in aqueous dispersion, may be for instance binder such as SBR, styrene-acrylate copolymer, or polyvinylacetate. The suspension can contain other water-insoluble organic materials as well as the polymer latex. Generally these additional organic matreials form a minor component, for instance 5 wt% or less, often below 2 wt%, of total water-insoluble organics and often substantially zero. The invention is particularly useful when the inorganic solids comprise inorganic filler or pigment such as clay, talc or calcium carbonate. Within the term "clay" we include materials such as kaolin clays, including china clay. These usually represent at least 40% by weight, often 70 to 100% by weight of the inorganic filler. Other fillers that can provide some or all of the inorganic component include titanium dioxide, satin white, aluminium hydrate, blanc fixe and calcined clays, especially when they consist predominantly of clay. Coating colour suspensions generally contain 40% or more, often 60% or more, by weight inorganic solids, of clay or calcium carbonate, and the inorganic solids may consist substantially only of clay or calcium carbonate.

As indicated above, the suspension preferably has a total solids (inorganic and polymer plus any other organics) content of less than 10%, more preferably less than 5%, generally in the range 0.1 to 2%, and so the suspension that is to be separated should be diluted to such a concentration (if it is not already at such a concentration) before conducting the process of the invention.

If the suspension of polymer latex and inorganic solids is diluted unused coating colour the clay and/or other pigment recovered may be recycled to the paper-making process for incorporation into the paper-making cellulosic thickstock or thinstock or, if the separation is especially efficient and the recovered solids comprise substantially no water-insoluble polymer, as a starting material for the production of coating colour.

If the suspension is effluent from paint factories containing inorganic pigments then the pigments, which may include those mentioned above, may be separated and re-used either in the paint industry or in other applications.

Generally, the uncoagulated material (containing as much as possible of the water-insoluble polymer and as little as possible of the inorganics) is treated as a waste product, for instance by incineration or dumping. This may be done after separating the material uncoagulated in the process of the invention from the water, usually by a further coagulation (for instance bridging flocculation) step carried out on the supernatant, often using alum as a coagulant.

The object of the process of the invention is to recover coagulated solids having a content of inorganic material as high as possible and a content of water- insoluble polymer as low as possible. The process of the invention allows the recovery of coagulated solids having an inorganic content of greater than 85% or 90% and even greater than 95%, 97% or 99% by dry weight of the recovered solids.

The percentage of inorganic material in the recovered coagulated solids can depend on the proportions of inorganic solids and water-insoluble polymer originally present in the suspension to be treated. Suspensions which are diluted coating colour generally have a content, by dry weight of solids, of between 70 and 90% inorganic filler and pigment, often between 85 and 90%. A content of as much as 10 to 15% or more polymer latex in the coagulated solids generally is not acceptable if the inorganic solids are to be reused. The process of the invention allows preferential coagulation of the inorganic materials so as to reduce the proportion of latex (in the recovered inorganic fraction) to less than half the proportion in the total suspension. The process conditions (i.e., the nature and the amount of the polymer coagulant) are therefore selected so that a larger proportion is coagulated of the inorganic solids than of the organic solids (the majority of which comprises water-insoluble polymer) . Thus the recovered coagulated material contains a higher ratio of inorganic solids to water-insoluble organics than did the suspension treated. At the same time, the supernatant liquid contains a lower ratio of inorganic solids to water-insoluble organics than did the suspension treated. The coagulant used in the invention is selected and is added in an amount such that the ratio of inorganic solids to water-insoluble organics increases by a factor of, generally, at least 2 between the suspension and the recovered coagulated material. Preferably it increases by a factor of at least 4, more preferably at least 10. The invention allows the achievement of an increase in this ratio by a factor of greater than 100 but the ratio is usually below this. It is acceptable that a low amount of inorganic material remains uncoagulated. There must be a balance found between on the one hand coagulating a maximum amount of the inorganic solid but at the same time coagulating too much of the water-insoluble polymer, and on the other hand coagulating substantially no water-insoluble polymer but at the same time allowing too much of the inorganic solids to remain uncoagulated. Preferably at least 50%, more preferably 70% or even more than 80% or 90% of the inorganic solids originally present are coagulated. It is usually unnecessary to coagulate more than 98%, and usually not more than 90 or 95%, of the inorganic solids. Measurement of the content of inorganic solids and of water-insoluble organics of samples can be measured using known incineration methods. These methods can be applied to any or all of the aqueous suspension to be treated, the coagulated solids and the supernatant. A suitable method is as follows:

A sample of the suspension, coagulated solids or supernatant is shaken thoroughly to ensure homogeneous particle distribution. An accurately weighed amount of the sample is put into a clean pre-weighed crucible. The sample is dried in an oven at 100°C for 2 hours. It is then cooled in a desiccator and weighed. The sample is exposed to the same conditions until constant weight is obtained. The sample is then ashed for 2 hours at approximately 570°C. After cooling and placing in a desiccator, the sample is weighed. The sample is replaced in the furnace and ashed for a further 2 hours. The ashing and weighing procedure is continued until constant weight is obtained.

The ash weights are used to calculate the amounts of inorganic solid and of water-insoluble organics in the sample. It is assumed that all organic materials are burnt off during ashing.

The polymeric material used as a coagulant in the process of the invention is cationic and must have a cationic content of 1 meq/g or greater. Preferential coagulation of the inorganic solids, substantially without the latex, tends to increase with increasing cationic content of the polymer. Preferably cationic content is 4 meq/g or greater, more preferably 6 meq/g or greater. Cationic content in meq/g is lOOOx (weight proportion of cationic monomer in the polymer) divided by (molecular weight of the cationic monomer) . Thus a copolymer of 65% by weight DADMAC 35% acrylamide has a cationic content of 4meq/g, DADMAC homopolymer has 6.19meq/g and polyamine epichlorhydrin has 7.27meq/g. Cationicity in meq/g can also be determined by titration, for instance using a Mutek particle charge detector.

It is preferred for achievement of particularly preferential separation that at least 70% or 80% of the monomer units from which the coagulant polymeric material is formed carry a cationic charge. Coagulant polymers in which 100% of the monomers are cationic are most preferred.

Preferably also the polymeric coagulant has intrinsic viscosity (IV) of 3dl/g or below. IV is measured by a suspended level viscometer at 25°C on polymer solution in

1M NaCl buffered at pH 7. Preferably the IV of the polymeric material used is below 2, and often below 1, dl/g as this tends to improve preferential separation. Generally the IV is at least 0.1, usually at least 0.2, dl/g.

Many water soluble cationic polymeric materials can be used as coagulants in the process of the invention. Suitable materials include those formed from units based on diallyl dialkyl ammonium salts, in particular diallyl dimethyl ammonium chloride (DADMAC) ; quaternised amines such as epichlorhydrin diamine reaction products; quaternised imines, such as polyethyleneimine; dicyandiamides; and dialkylaminoalkyl (meth)-acrylamides and -acrylates, as acid addition or quaternary ammonium salts. Particularly preferred are polymers based on DADMAC monomers, generally as homopolymers, or as copolymers with a small amount (e.g. up to 30 mole%) acrylamide or other suitable water soluble non-ionic ethylenically unsaturated monomer. The dose of coagulant polymer necessary to achieve the benefits of the process of the invention is below lOOOppm, i.e., 1000 parts dry weight polymer per million parts by volume (grams per cubic metre) , generally below 500ppm and often below 200ppm, and preferably below lOOppm. The cationic polymer may for instance be added in an amount of 60ppm or less, e.g. 30ppm. The amount is usually at least lOppm. The amount depends, inter alia, on the solids concentration of the suspension. An important feature of the invention is that, in addition to selecting a high cationic content, preferably low IV, cationic polymeric coagulant, it is necessary to select an appropriate low amount. If the amount is too low, inadequate coagulation occurs. If the amount is too high there is an increased tendency towards coagulation of the latex with the inorganic pigment or, at higher doses, there is a tendency towards resuspension of the pigment. Accordingly, for any particular combination of suspension and coagulant polymeric material routine testing should be conducted at a range of low dosages in order to determine the dose that gives the desired improvement in the inorganic:organic ratio, and in particular the desired balance between minimum coagulation of the latex and maximum coagulation of inorganic pigment. The cationic polymeric coagulant can be added as an emulsion or as beads that dissolve in the suspension but is preferably added as an aqueous solution, typically having a polymer content of 0.1 to 10% by weight.

The treated suspension may be allowed to settle and the sludge and supernatant may be separated by decantation. The sludge and supernatant may be separated using a plate or lamellar clarification apparatus. Other conventional ways of separating the aggregated solids from the remainder of the suspension can be used. The supernatant, containing most of the latex and some of the inorganic, if desired can be treated with, for instance, an anionic flocculant of IV above 4dl/g or other suitable material to produce a clear supernatant which can be discharged and a small sludge which can be dumped.

The process of the invention is illustrated by the following examples. Example 1

A 1% clay-latex suspension was prepared, in which the amount of latex was smaller than the amount of clay and which is therefore a simulation of a coating colour effluent. 200 ml samples were placed in plastic beakers. Five additions of each polymer product, each at a different level, were made to separate samples and each was stirred at 500 rp for 1 minute. The samples were left to stand overnight.

After overnight standing 10 is of each supernatant was diluted with deionised water and turbidity measurements were made using the Hach 2100 turbidimeter. (The reason for the dilution of the samples was to ensure all the readings were in the range of the turbidimeter scale) . The height of the sediment was measured. The polymers used were

Polymer A - polyamine epichlorohydrin reaction product of IV 0.4dl/g and 7.27meq/g cationic content

Polymer B - polyDADMAC of IV 0.5dl/g and 6.19 meq/g cationic content Polymer C - polyDADMAC of IV l.Odl/g and 6.19meq/g cationic content.

For comparison alum was also used as a coagulant. Alum is often used in conventional effluent treatment processes to coagulate organic and inorganic material non- selectively. Results are shown in Table 1 below. Data is generally quoted to 2 significant figures. Table 1

Product Addition Supernatant Sediment level (ppm, Turbidity height (mm) by volume of (NTU) suspension) Blank - 915 0.5

Alum 25 840 2.5 40 5 14 50 2 14 75 2 14 100 3 14

Polymer A 25 800 4 40 670 7 50 400 12 75 240 13 100 460 10

Polymer B 25 830 6

50 420 11

75 30 13.5

100 650 7

150 690 4

Polymer C 25 780 5 40 620 9 50 460 10 75 40 14 100 370 11

In this example preferential coagulation of the clay is manifested by a high sediment value (indicating a large amount of coagulated clay) and a turbid supernatant (indicating a large amount of dispersed polymer latex) . On this suspension, optimum addition levels are therefore between 30 and 50 ppm of polymers A, B and C. It will be seen that increasing the dose significantly decreases turbidity, indicating aggregation of the latex into the sediment. Observation and incineration shows that at 30 to 50ppm the ratio of pigment:latex is much higher than in the starting suspension. Example 2

A 1% suspension of clay and latex was prepared, broadly similar to Example 1. 200ml samples were placed in plastic beakers. Four additions of each product were made to separate samples and each was stirred at SOOrpm for 1 minute. The samples were left to stand for 24 hours.

After this 24 hour standing period, samples of each supernatant were taken, diluted with deionised water and turbidity measurements were made using a Hach DR2000 turbidimeter. (The reason for the dilution of these samples was to ensure all the readings were in the range of the turbidimeter scale) .

The height of the sediment was measured.

The polymers used were polymer B and polymer C. For comparison alum was also used as a coagulant.

The results are shown in Table 2 below. Table 2

Product Addition Level Supernatant Sediment Height (ppm by volume Turbidity (NTU) (mm) of suspension)

Blank _ 820 1

Alum 75 440 4 100 240 11 150 6 26 200 1 25

Polymer C 75 330 9 100 350 11 150 280 14 200 200 17

Polymer B 75 340 11 100 340 12 150 240 15 200 90 21

Preferential coagulation can be observed as a high sediment of flocculated clay and a very turbid supernatant due to dispersed polymer latex.

The optimum addition levels were shown to be between 75 and lOOppm of polymers B and C in this particular example. Plant trials can then establish dosage required such that the ratio of inorganic:latex is increased by a factor of at least 2, and often very much more, between the untreated suspension and the coagulated solids.

These examples show that the process of the invention is effective in achieving selective coagulation of inorganic material from latex in a suspension of both and show the advantage of the process in comparison with alum, which is used at present for treating coating colour effluent and which gives non-selective coagulation (as illustrated by the resultant low turbidity) . Tests using a high molecular weight cationic polymeric material having 0.26meq/g cationic content gave poor coagulation.

Treatment of the supernatant with anionic, high molecular weight flocculant gives total flocculation of suspended solids, indiscriminately. Preferential aggregation of inorganic solids was not shown. Accordingly this is a useful post-treatment for the supernatant to produce a clear supernatant and a sludge of the latex and small amount of inorganic. Example 3

This example compares use of the process of the invention with the process described in W093/21381, in which polyethylene oxide is used as a separator.

A 60% solids suspension in water of clay and latex was prepared. The solids comprised 10% organic and 90% clay (inorganic) . From this was prepared a 1% suspension. To 200ml of this suspension was added, with stirring at 500 rpm in a Heidolph stirrer, polymer A at a dose of 5 ppm. After addition of polymer A the coating colour suspension was stirred in the Heidolph stirrer at a speed of 500 rpm for a further minute. This was repeated at various dosages. All of these procedures were repeated for polymer B and a polyethylene oxide having molecular weight greater than 6 million. All samples were left to stand overnight. The following day sediment heights were measured using a ruler. 10ml of each supernatant were diluted using an appropriate dilution factor (established by carrying out various dilutions of the blank to ensure a wide range of readings on the turbidity meter used) . Turbidity of the supernatant was measured using a Hach 200 Turbidity Meter. Results are shown in Table 3. Table 3

Product Addition Supernatant Sediment Level (ppm Turbidity Height (mm) by volume of (NTU) suspension)

Blank - 870 3

Polymer A 5 390 3.5

■1 25 390 4

It 50 390 4.5

It 75 480 7

•1 100 270 10

II 150 280 15

•1 200 210 18

II 250 10 22

Polymer B 5 330 3

II 25 400 4

II 50 360 5

II 75 390 6

II 100 370 9

II 150 350 12

•1 200 230 18

II 250 140 22

Polyethylene 5 400 3 Oxide

II 25 430 2

•1 50 270 4

II 75 360 2

It 100 260 4

•1 150 140 4

II 200 - 5

II 250 80 5

The sediments in the PEO tests had a different character from those in the tests with Polymers A and B and it was difficult to determine their heights.

These results illustrate that both polymers A and B are capable of giving a much better combination of increased sediment and reasonably high turbidity than polyethylene oxide. It seems that polyethylene oxide is much less discriminating than polymers A and B and gives an unusual sediment always containing both the inorganic and latex components.

Claims

CLAIM8 1. A process of recovery of inorganic solids from an aqueous suspension of particulate inorganic solids and water-insoluble polymer latex, in which the process comprises coagulating the suspension by including water soluble coagulant in the suspension and separating the coagulated solids, and in which the inorganic solids are coagulated preferentially by using as the coagulant a cationic polymeric material having cationic content of at least 1 meq/g in an amount which is less than lOOOppm and which is such that the ratio of inorganic solids to water- insoluble organics in the recovered coagulated solids is greater than this ratio in the suspension by a factor of at least 2.

2. A process according to claim 1 in which the aqueous suspension is diluted coating colour which has been obtained from a paper-coating process.

3. A process according to any preceding claim in which the inorganic solids comprise clay and/or calcium carbonate in an amount of at least 40%, by weight inorganic solids, and the amount of inorganic solids in the aqueous suspension is below 90% by weight total solids.

4. A process according to any preceding claim in which the cationic polymeric material is added in an amount of less than lOOppm.

5. A process according to any preceding claim in which the ratio of inorganic solids to water-insoluble organics in the recovered coagulated solids is greater than this ratio in the suspension by a factor of at least 10.

6. A process according to any preceding claim in which the amount of inorganic solids in the aqueous suspension is below 90% by weight total solids and in the recovered coagulated solids is above 95% by weight total solids.

7. A process according to any preceding claim in which the recovered coagulated solids comprise at least 50% by weight of the inorganic solids present in the suspension.

8. A process according to any preceding claim in which the cationic polymeric material has a cationic content of 4 meq/g or greater.

9. A process according to any preceding claim in which the cationic polymeric material has a cationic content of

6 meq/g or greater.

10. A process according to any preceding claim in which the cationic polymeric material has intrinsic viscosity of 3dl/g or below.

11. A process according to any preceding claim in which the cationic polymeric material is a polymer of diallyl dimethyl ammonium chloride (DADMAC) ; an epichlorhydrin diamine reaction product; a polyethyleneimine; a dicyandiamide; or a polymer of dialkylaminoalkyl (meth)- acrylamide or -acrylate, as acid addition or quaternary ammonium salt or mixtures thereof.

12. A process according to any preceding claim in which the cationic polymeric material comprises polyDADMAC homopolymer or a copolymer of DADMAC units and up to 30% by weight acrylamide units.

13. A process according to any preceding claim, in which the coagulated solids are subsequently recycled for use as filler or pigment.

14. A process according to any preceding claim in which, after the separation of the coagulated solids, the supernatant is flocculated by the addition of anionic polymeric flocculant having IV above 4dl/g to produce a sludge containing the latex and some of the inorganic solids and a substantially clear effluent.

15. A process according to any preceding claim in which the water-insoluble organics are the polymer of the water- insoluble polymer latex.