Production of secondary ultramarine from primary ultramarine

摘要

Primary ultramarine is oxidized to secondary ultramarine by carrying out at least the major and later portion of the oxidation by means of sulphur dioxide, in the absence of substantial quantities of oxygen, water vapour and strongly acid gases. The sulphur dioxide is preferably mixed with an inert gas such as nitrogen to act as a carrier for the sulphur vapour formed during the operation and which may finally be recovered by cooling and condensation. The process is applicable to a single-step operation starting from the firing of the raw materials, but preferably is used in a two-step operation with intermediate cooling of the primary ultramarine and ready contact between it and the sulphur dioxide in the second step. In the examples, primary ultramarine is prepared as described in Specification 617,522, the briquets being cooled, broken up and loaded into a fused silica container. This is heated to 400 DEG C. while nitrogen is passed through to displace air after which the nitrogen is replaced by sulphur dioxide, the temperature rising to as high as 466 DEG C. After cooling, the charge is washed, wet-ground and dried. Before oxidation, the primary ultramarine may be leached with water to remove soluble polysulphides.