Process for scale control using mixtures of polycationic and polyanionic polymers

Claim 1

A process for inhibiting formation of magnesium hydroxide scale or sludge in evaporative desalination units which comprises adding to the saline water being processed in said units an effective amount of a mixture of (1) a polyanionic polymer containing at least about 50 mole percent or repeating units derived from an acrylic acid and any balance of repeating units derived from one or more monomers compatible therewith, the acid units being in the form of at least one member selected from the group consisting of free acid radical, ammonium salt, and alkali metal salts, and (2) a polycationic polymer selected from the group consisting of (a) dimethylaminepolyamine-epichlorohydrin reaction product wherein the amount of said polyamine is from 0 to about 15 mole percent of the total amine content and the amount of said epichlorohydrin is from at least the molar equivalent of the total amine content up to the full functional equivalent of said amine content, (b) poly(dimethyldiallylammonium chloride), (c) quaternarized derivatives of poly(dimethylaminoethylmethacrylate), and (d) poly[oxyethylene(dimethylimino)ethylene(dimethylimino)ethylene dichloride], said polyanionic polymer having a molecular weight in the range of about 500 and about 50,000, said polycationic polymer having a molecular weight in the range of about 1,500 and 500,000, and the molar ratio of said polycationic polymer to said polyanionic polymer based on the average molecular weight of the repeating units therein being in the range of about 2:1 and about 25:1.