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Abstract

Copper arsenate, substantially free from impurities that will form a sludge in the process for producing wood preservatives such as chromated copper aresenate and having a water content of less than 15%, is prepared by mixing a solution of soluble arsenate and a copper sulfate solution, either or both solutions containing sludge-forming impurities, to form a copper and arsenic-containing solution without precipitating copper arsenate. The pH during the adding is controlled at a value in the range of 1.7 to 2.2 to precipitate impurities. Precipitated impurities are removed, and the copper and arsenic-containing solution is neutralized with a suitable alkaline substance, preferably dilute sodium hydroxide solution, to pH 3.2 to 4.2 to precipitate copper arsenate. The water content of the copper arsenate is reduced by adding to the copper and arsenic-containing solution, either in the mixing of arsenate solution with copper sulfate solution or in the precipitation of copper arsenate, an amount of soluble chromium in the range of about 0.005 to 0.250 gram chromium per liter of solution.

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First Claim