Control of metals and dissolved organics in the bleach plant

Abstract

Transition metals are removed from cellulose pulp and treatment fluids in a pulp mill. Medium consistency digested pulp is treated in an acidic stage, and then washed to produce a filtrate. The pH of at least a substantial part of the filtrate is adjusted so that it is greater than 9 (preferably greater than 10.5), while a sufficient carbonate content is provided to keep the calcium content low. Adjusting the pH causes dissolved transition metals in the filtrate to precipitate as solids, and the solids can then be filtered out, e.g. by precoat filtering, or clarification followed by polishing filtering. The greatly reduced transition metal content filtrate is then used elsewhere in the pulp mill, as in a washer following an oxygen delignification stage. The acidic stage may be an EDTA stage, an ozone stage, an acid only stage, etc., and the pulp is preferably treated so that it is bleached in at least one Z stage and one P stage, and preferably also an E (e.g. E.sub.op) stage.