Double-hydrated lime

The requirements for highly hydrated dolomitic lime, (also called di-hydrated or double hydrated lime) are specified in [20.15, 20.16]. It is required to contain less than 8 % of free lime, to ensure that it is sound. 

The sulphites of the alkaline earths are prepared either by exposing the moistened, or better, the hydrated oxides, to gaseous sulphurous acid, when this ges is absorbed or tbe oxides or carbonates may be suspended in water, end a stream of sulphurous acid passed through or they may be produced by double decomposition from a soluble alkaline sulphite, and a chloride of the earthy metal. These sulphites are all more or less nearly insoluble in water and alcohol, but are rendered more soluble by an excess of sulphurous acid, in this respect resembling the carbonates. The only practical purpose to which any of these earthy sulphites have been applied, is as disinfectants. For this purpose, a mixture of the sulphites of lime and magnesia is used, Mr. M Dourall seems to have a method of producing sulphites much more readily than any other maker. 

High-calcium and partially hydrated dolomitic limes are hydrated at temperatures approaching 100 °C. This is achieved by adding a sufficient excess of water to moderate the temperature by boiling. In practice, with high-calcium quicklime, approximately double the stoichiometric quantity of water is added, with most of the excess being vented to atmosphere as steam, after absorbing much of the heat of reaction. 

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