| 摘要 |
The invention relates to a process for producing calcium sulphate alpha-hemihydrate by dehydration of calcium sulphate dihydrate products which are present as a suspension in aqueous sulphuric acid, wherein the conversion to alpha-hemihydrate takes place, in quantity terms, in two stages at elevated temperature and pressures not exceeding the natural atmospheric pressure, and the alpha-hemihydrate is filtered off from the aqueous sulphuric acid. In order to provide a process, which can be carried out without elevated pressure, for producing alpha-hemihydrate, which very largely avoids the disadvantages described and, in particular, ensures rapid dehydration of the dihydrate to alpha-hemihydrate and prevents formation of anhydrite, it being intended that the conversion can be carried out in simple apparatus without complicated control and monitoring and leads to a constant product quality, and it likewise being intended that the process runs virtually without consumption of acid, it is proposed that the major part of the calcium sulphate dihydrate is converted in the first reaction stage (main reaction stage), the sulphuric acid being held at the boiling point under atmospheric pressure, that the remaining conversion to alpha-hemihydrate is carried out in the following second reaction stage (final reaction stage) at temperatures below the boiling point at atmospheric pressure or in vacuo, before the suspension is filtered, and that the sulphuric acid ... Original abstract incomplete. <IMAGE>
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