PRIORITY

This application is a U.S. national application of the international application number PCT/F12018/050360 filed on May 15, 2018 and claiming priority of Finnish application 20175441 filed on May 16, 2017 the contents of all of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for producing an aqueous polymer solution according to the independent claim presented below. The invention also relates to uses of the obtained polymer solutions in manufacturing of paper, board or the like or in treatment of sludge or water.

BACKGROUND OF THE INVENTION

Polyacrylamides are commonly used as auxiliary substances in various processes such as in wastewater treatment and paper industry.

The polyacrylamide polymers are often manufactured in a form that is not a solution, for example they can be dry powders, emulsions or dispersions. For the use polyacrylamide polymers should be in the form of an aqueous solution. If water used as dissolving water has high calcium hardness, problems relating to the functionality of the polymers have observed. This kind of dissolving water condition occurs e.g. in many paper and board manufacturing locations, wherein only water with high calcium hardness is available. Problem in dissolving with hard water is that performance of the polyacrylamide polymer solution drops and also viscosity of the polymer solution may change. Cationic polyacrylamides have typically been acidified during manufacturing of the polymers. The acidification has been carried out by using polyprotic acid, such as e.g. adipic acid and/or citric acid. Thus, the anionic groups of the polymer and added polyprotic acids may complex with any multivalent cationic species present in the aqueous solution of the polymer, such as Ca2+or Mg2+ion abundantly present in hard waters, and with cationic groups of polymer. In similar way complex formation may occur between cationic and anionic charges present in the polymer chain, such as in case of amphoteric polymers, but also in case of cationic polymers as typically some of the cationic groups may hydrolyse into anionic groups during manufacture or storage. Complex formation causes the macromolecular chains of the polymer to compress into loops and tails. A copolymer having its macromolecular chains compressed may not benefit to the fullest extent from its molecular weight in applications where high molecular weight and dimensional extension are needed, such as flocculation of suspended solids, thereby providing lower performance than the same polymer in non-complexed form.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce or even eliminate the above-mentioned problems appearing in prior art.

The object of the invention is to provide a method for dissolving water-soluble copolymers in dissolving water having high calcium hardness and/or alkalinity.

It is especially an object of the present invention to provide a method for pH adjustment of copolymer solution when using dissolving water with high calcium hardness and/or alkalinity.

Another object of the invention is to provide an arrangement or a polymer dissolving unit, with which the method of the invention can be carried out in the form of batch processing of the copolymer solution.

In order to achieve among others the objects presented above, the invention is characterized by what is presented in the characterizing parts of the enclosed independent claims.

Some preferred embodiments of the invention will be described in the other claims.

The embodiments and advantages mentioned in this text relate, where applicable, both to the method, the polymer dissolving unit as well as to the uses according to the invention, even though it is not always specifically mentioned.

A typical method according to the invention for producing an aqueous copolymer solution to be added into an aqueous suspension comprising solid particles to be flocculated comprises the steps ofdissolving a copolymer of (meth)acrylamide and at least 1 mol-% cationic monomers, calculated from total amount of monomers, in the dissolving water having calcium hardness of at least 5 mg/l, expressed as Ca2+, whereby an aqueous solution of the copolymer is obtained, andadding acid to the dissolving water before combining with the copolymer and/or to the obtained aqueous solution of the copolymer in an amount sufficient to provide the aqueous solution of the copolymer with a pH of equal to or less than 4.5, wherein the acid comprises monoprotic organic acid and/or mineral acid.

Typically, an aqueous solution of the copolymer obtained by the method of the invention is used for flocculation of suspended solids. The aqueous solution of the copolymer may be used as a retention or drainage aid in manufacturing of paper, board or the like. The aqueous solution of the copolymer may also be used as a flocculant in sludge dewatering or in microflotation in water treatment.

A typical process for flocculating suspended solid particles in manufacturing of a paper, board or the like, or in treatment of sludge or water comprisesproviding an aqueous solution of the copolymer obtained by the method according to present invention, andadding the aqueous solution of the copolymer into an aqueous suspension comprising solid particles to be flocculated.

More detailed, a typical process for flocculating suspended solid particles in manufacturing of a paper, board or the like, or in treatment of sludge or water comprisesproviding an aqueous solution of the copolymer by dissolving a copolymer of (meth)acrylamide and at least 1 mol-% cationic monomers, calculated from total amount of monomers, in the dissolving water having calcium hardness of at least 5 mg/l, expressed as Ca2+, and by adding acid to the dissolving water before combining with the copolymer and/or to the obtained aqueous solution of the copolymer in an amount sufficient to provide the aqueous solution of the copolymer with a pH of equal to or less than 4.5, wherein the acid comprises monoprotic organic acid and/or mineral acid, andadding the aqueous solution of the copolymer into an aqueous suspension comprising solid particles to be flocculated.

A typical polymer dissolving unit according to the invention comprisesa dissolving tank equipped with an agitator,a polymer inlet line connected to the dissolving tank for supplying polymer into the dissolving tank,at least one water inlet arranged in connection with the polymer inlet line and/or directly connected to the dissolving tank,an acid inlet connected to the dissolving tank and/or to at least one water inlet, andan acid storage vessel and an acid dosing pump for supplying the acid from the acid storage vessel to said acid inlet.

It has been surprisingly found that above mentioned problems appearing in the prior art can be alleviated by using acid comprising monoprotic organic acid, such as formic acid, and/or mineral acid to adjust pH of the copolymer solution below the first pKa of the anionic monomer that may be present in the copolymer, or below the second pKa:s of the polyprotic acids that may be present in copolymer composition to be dissolved. The pH may be adjusted for example at least about 0.5 pH unit below said pKa:s.

The pH adjustment by using acid comprising monoprotic organic acid and/or mineral acid inhibits complex structure of the copolymer in water with high hardness since then the complex formation between the cationic groups of the copolymer and any multivalent anionic species present in the aqueous solution of the copolymer, such as Ca2+or Mg2+ion abundantly present in hard waters, and even any anionic charges present in the copolymer itself, may be effectively prevented and also already formed complexes may also be unraveled. The ion complexing of the copolymer decreases when the pH of the copolymer solution is adjusted to be equal to or less than 4.5, such as equal to or less than 4.2, preferably equal to or less than 4.0, especially equal to or less than 3.8, as the amount of ions available for crosslinking decreases significantly, turning into non-ionic form. The lower the pH is compared to the first pKa of the anionic monomer that may be present in the copolymer or below the second pKa:s of the polyprotic acids that may be present in copolymer composition to be dissolved, the less the copolymer is in complexed form, thereby providing better end-use performance of the copolymer such as flocculation. Additionally, the added acid may form ion bonds with the Ca2+and/or Mg2+ions present during dissolving of the copolymer, thereby decreasing complex formation of these ion species with the anionic groups of the copolymer, although this effect is only supplementary and may not be as efficient in preventing the copolymer from complexing as the adjustment of the pH to the indicated level. Thus, the copolymer remains substantially non-complex in the obtained aqueous solution of the copolymer and the functionality of the copolymer remains also in calcium containing waters. The benefit of the present invention is thus that same copolymer dissolved provides a substantially same functionality with the smaller dosage.

A dissolving method according to the present invention may inhibit viscosity decrease of the aqueous solutions of copolymer and so the obtained copolymer solution may have greater viscosity when it is added into an aqueous suspension comprising solids, such as a fibre suspension or water to be treated. However, it should be noted that viscosity of the copolymer solution may decrease with dissolving method according to invention as viscosity may decrease due to increased conductivity too and not only due to ion-complex formation. The dissolving method according to the invention is especially suitable for high molecular weight copolymers such as sludge dewatering or papermaking flocculants without damaging or affecting their performance. The method according to the invention improves significantly flocculation and/or drainage performance and/or retention performance in the situations where water with high hardness and/or alkalinity is used as dissolving water.

The dissolving method according to the present invention can be performed at one stage without adding complexity compared to existing dissolving devices or arrangements.

