Design of Hydrating Plants

Magnesium oxide in quicklime tends to become over-burned and to be slow to hydrate. It can cause expansion of the hydrate. Magnesian and dolomitic quicklimes generally require pressure hydration to produce a non-expansive product (see section 20.8). 

Other impurities in the quicklime cause contamination of the hydrate. Where impurities form a surface layer on the quicklime particles, they may inhibit the hydration process. Such an effect can be overcome by crushing the quicklime. Where the hydrate is used in applications which are particularly sensitive to impurities (e.g. production of drinking water [20.3]), the levels of trace elements in the feed lime may need to be limited. 

Although impurities in water affect the rates of reaction between quicklime and water when producing milks of lime (sections 22.2 and 22.8.2), relatively little has been reported on their effect on the production of hydrated lime. This could be because  

On a pragmatic basis, the acceptability of a proposed water supply may be assessed by using a representative sample in a standard reactivity test and comparing the result with that obtained using distilled water. If the difference is within the normal variability of the quicklime reactivity, the water supply should be acceptable. If otherwise, hydration tests using the water should be carried out to ensure that the required quality of hydrated lime can be produced consistently. 

Quicklime was traditionally hydrated on the small scale by sprinkling water on to a pile of lime until no more water was absorbed. The lime was left for some time to mature and was then passed over fine sieves to remove grit. However, such a product was of poor and variable quality and required a high labour input. 

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