Process for the manufacture of arsenates from arsenious acid or arsenites

摘要

Alkali arsenates, or mixtures thereof with alkali arsenites, are obtained by subjecting an aqueous solution containing arsenic trioxide and alkali to oxidation by intimate contact with air or a gas containing free oxygen under normal pressure in a fine state of division or over a large surface area. Preferably the proportions of alkali and arsenic trioxide are such as correspond to trialkali arsenate, elevated temperatures, say 80--100 DEG C., are used, and oxidation is effected in presence of a catalyst such as a noble metal, a metal having variable valency, or compounds or mixtures thereof. The specified catalyst is copper sulphate. In carrying out the process, the oxidizing gas may be supplied from a fine aeration device such as is used in the yeast industry, or the solution may flow down a tower filled with bodies of large surface area up which the gas is passed. In the latter case, the process may be a continuous one, a part of the treated solution being recirculated, and a part being treated with milk of lime to recover calcium arsenate and reform alkali which is used for dissolving more arsenic trioxide for continuing the process. From the calcium arsenate, by treatment with sulphuric acid, a solution of arsenic acid may be obtained.