High-calcium quicklime

Ca(OH)2 + heat high calcium high calcium quicklime hydrate... 

Quicklime and hydrated lime are reasonably stable compounds but not nearly as stable as their limestone antecedents. Chemically, quicklime is stable at any temperature, but it is extremely vulnerable to moisture. Even moisture in the air produces a destabilizing effect by air-slaking it into a hydrate. As a result, an active high calcium quicklime is a strong desiccant (qv). Probably hydrate is more stable than quicklime. Certainly hydrated lime is less perishable chemically because water does not alter its chemical composition. However, its strong affinity for carbon dioxide causes recarbonation. Dolomitic quicklime is less sensitive to slaking than high calcium quicklime, and dead-burned forms are completely stable under moisture-saturated conditions. 

The trade association for U. S. and Canadian manufacturers of high calcium quicklime, dolomitic quicklime, and hydrated lime, collectively referred to as lime."... 

Mean apparent density. As the heat treatment of individual lumps in a commercial lime kiln can vary widely, their apparent densities also vary. The mean apparent density is used to characterise the average degree of sintering of a particular quicklime. For a particular high-calcium quicklime, the mean apparent density correlates with the reactivity to water (see Fig. 13.2). 

Reactivity to water is measured by the rate of release of the heat of hydration [13.3, 13.5, 13.6], or by the rate at which an aqueous suspension produces hydroxyl ions (which are neutralised at pH 9.2 by hydrochloric acid) [13.4]. A large number of reactivity tests have been used [13.7-13.11], The relationships between some of those test methods (for a high-calcium quicklime) are given in Fig. 13.1 (N.B., the relationships for other limes may differ significantly from those given). 

Figure 13.1, Approximate relationships between reactivity tests for a high-calcium quicklime. (N.B. results vary with grading of quicklime and source of limestone)...

Figure 13.2 shows how the reactivity of a dense high-calcium quicklime is related to its mean apparent density. 

Figure 13J2. Relationship between reactivity and mean apparent density for a dense, high-calcium quicklime...

Scanning electron micrographs of quicklime demonstrate the dramatic change in structure resulting from the sintering process, for the same high-calcium quicklime (Fig. 15.7). The effects of sintering on lime reactivity are described in section 13.2. 

Light-burned dolomite (or dolime) is produced in either rotary or shaft kilns. The principles are similar to those of making high calcium quicklime. Heat usages are presumably somewhat lower, owing to the lower heat of calcination of dolomite and its lower dissociation temperature. 

The MgO component of high-calcium quicklimes and of dolomitic limes is relatively unreactive to water. Generally less than 25 % of it reacts under normal hydration conditions (equation 20.2) [20.2]. 

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. 

Most slakers are of the detention type, which essentially consist of a tank agitated with an impeller (e.g.. Fig. 22.2). They produce a fluid milk of lime, and generally use between 3.5 and 4 parts by weight of water to 1 part of high-calcium quicklime. For dolomitic quicklimes, the water to lime ratios are lower. 

Category 1 building lime — hydrated 2 building lime — putty 3 hydraulic building lime 4 high calcium quicklime — hydrated lime 5 high calcium quicklime — milk of lime 6 hydrated lime... 

Originally only low reactivity quicklime was used and many producers specify that quality. In the UK, however, suitable quicklimes were not generally available and many of the processes were initially modified to use cement as the sole binder. Subsequently, part of the cement has been replaced, with advantage, by moderate and high reactivity high-calcium quicklimes. 

Ground" refers to pulverised high-calcium quicklime, lump" refers to granular high-calcium quicklime, hydrate" refers to hydrated high-calcium lime and milk" refers to milks of lime % values are by mass. 

Calcium silicon is used in the iron and steel industry as a de-oxidizer and desul-furizer and for the modification of non-metallic inclusions. A high calcium quicklime is required (e.g. 94 to 97 % CaO) with controlled levels of combined CO2 and water. 

The process requires a high-calcium quicklime with low levels of MgO and Si02 [31.25]. The relative merits of lime and caustic soda for the dehydrochlorination stage are discussed in [31.25]. 

High-calcium quicklime is used with a similar chemical requirement to those for steelmaking (see section 27.4.2). As in steelmaking, the required particle size depends on the method of addition/injection of the quicklime. 

A high-calcium quicklime is preferred. Reactivity to water does not appear to be particularly important, but, where blocks or granules are produced, the particle size distribution should be such that it gives the required rheological properties. 

High-calcium quicklime contains mainly CaO and not more than 5 % (m/m) MgO. 

Type N or normal hydrated lime is defined in ASTM specification C-207 (2). It is generally produced by hydrating high-calcium quicklime at ca 100 °C. 

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