GB122170A - Improvements in or relating to Preparing Pigment Zinc-oxide. - Google Patents

Abstract

122,170. American Zinc, Lead, & Smelting Co., (Assignees of Wemple, L. E.). Jan. 9, 1918, [Convention date]. Pigments.-A process for preparing pigment zinc oxide, free from coloured oxides of lead and cadmium and from soluble zinc salts such as zinc sulphate, consists in first producing: by oxidizing in the presence of sulphur a crude zinc oxide containing a small proportion of zinc sulphate and having any lead and cadmium present as sulphates also, and then treating this crude product with a reducing agent such as carbon under such temperature conditions that the zinc sulphate is eliminated or decomposed whilst the lead and cadmium sulphates remain unchanged. The sulphated crude zinc oxide may be obtained as the immediate product of the usual volatilization process if the raw material contains sufficient sulphur; otherwise, sulphur may be added to the raw material, or the volatilized product may be treated with sulphur. A convenient reducing agent is lampblack, mixed with the crude oxide in the proportion of one to two per cent., the mixture being heated for about half an hour at a temperature between 900 and 1200‹ F., preferably in a muffle to prevent contamination with fuel ash. The material is then stirred with free access to air to burn off any remaining excess of carbon and to facilitate the escape of sulphur dioxide and other excluded gases. The stirring and consequent rapid cooling prevent agglomeration of particles and the formation of crystalline compounds. Some grades of crude oxide produced by volatilization furnaces contain appreciable amounts of carbon or other reducing agents, and the necessity for adding a reducing agent for eliminating the zinc sulphate may be avoided by regulating the volatilization process so that the crude oxide contains either sufficient carbon to permit direct heating, or excess carbon adapted for admixture with other grades deficient in carbon. It is possible to use as the reducing- agent oxide of lead or cadmium or other metal having a white sulphate, but such oxides must be used in carefully regulated quantities as a slight excess would discolour the final product. The pigment zinc oxide prepared by this process contains lead solely as insoluble white sulphur compounds, and sulphur in insoluble form in excess of that required to sulphatize the lead present, probably as zinc sulphide or basic zinc sulphate. Methods of testing the colour, fineness, covering power, and oil-absorbing capacity of the pigment are described.