Laporte Industries Limited

Other

Andrew D Broome

Ian K Egerton

Manufacture of arsenic acid by oxidation of arsenic trioxide with hydrogen peroxide is potentially hazardous, especially when making concentrated solutions of arsenic acid as sought by the users. The process also can suffer from substantial wasteful decomposition of hydrogen peroxide. The problems arise from or are exacerbated by the nature of the reactants, the exothermic character of the principal and decomposition reactions and the formation of localized hot spots. The problems can be ameliorated by introducing the reagents with agitation into a body of reaction product and maintaining the reaction mixture at a temperature of not more than 70.degree. C. and preferably 45.degree. to 55.degree. C. By carrying out the process in such a manner, it can accomodate even low grade arsenic trioxide as reagent.

Manufacture of arsenic acid

A copper arsenate composition suitable for incorporation in a copper/chrome/arsenic wood preservative composition is made by reacting copper metal with a source of 5-valent arsenic such as arsenic acid under the oxidizing influence of hydrogen peroxide. The quantity of arsenic acid used may be made consistent with the arsenic to copper ratio required in the final wood preservative composition which may be made up by including a suitable source of chromium with the copper arsenate reaction product.

Wood preservative compositions and a process for their production

Wood preservative compositions containing as active ingredients compounds of copper, of chromium and of arsenic are produced by mixing copper arsenate and sodium dichromate in an aqueous medium into which is introduced strong acid, for example sulphuric acid, effective to achieve a soluble product. The acid addition is preferably effective to reduce the pH to below 0. The composition so produced is a homogeneous paste which may be dissolved in water to working concentration and used for the preservation of wood by impregnation techniques.

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Warrington, United Kingdom

Andrew D Broome