Method of improving gypsum slurry filtration in the production of phosphoric acid

A method for improving the filtration of a gypsum slurry in a wet process phosphoric acid manufacturing process which comprises adding to a said gypsum slurry an effective amount of a polymer which consists essentially of about 0.5 to about 99.5%, by weight, of:

• a) an unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid, their salts and mixtures thereof;
• and b) about 99.5 to about 0.5%, by weight, of an unsaturated sulfonic acid selected from the group consisting of 2-acrylamido-2-methylpropyl sulfonic acid, 2-methacrylamido-2-methylpropyl sulfonic acid vinyl sulfonic acid, sulfoalkyl acrylate, sulfoalkyl methacrylate, allyl sulfonic acid, methallyl sulfonic acid, 3-methacrylamido-2-hydroxypropyl sulfonic acid, sulfonic acid acrylate, their salts and mixtures thereof.