High-calcium limestone

Alumina in combination with siUca is present in limestone chiefly as clay, though other aluminum siUcates in the form of feldspar and mica may be found. When present in appreciable quantities, clay converts a high calcium limestone into a mad or argillaceous stone, which when calcined yields limes with hydrauhc properties. Limestones containing 5—10% clayey matter yield feebly hydrauHc limes those containing 15—30% produce highly hydrauHc limes. 

Thermal Properties. Because all limestone is converted to an oxide before fusion or melting occurs, the only melting point appHcable is that of quicklime. These values are 2570°C for CaO and 2800°C for MgO. Boiling point values for CaO are 2850°C and for MgO 3600°C. The mean specific heats for limestones and limes gradually ascend as temperatures increase from 0 to 1000°C. The ranges are as follows high calcium limestone, 0.19—0.26 dolomitic quicklime, 0.19—0.294 dolomitic limestone, 0.206—0.264 magnesium oxide, 0.199—0.303 and calcium oxide, 0.175—0.286. 

Solubility. High calcium limestone is only very faintly soluble in water. In cold C02-free water it is often regarded as insoluble. Between 17 and... 

The neutralising power of lime and limestone and other alkaUes is compared in Table 2 (8). Of all these alkaUes, MgO is the strongest base, followed by CaO. Thus neutralization of a given acid requires less dolomitic limestone or lime than high calcium limestone or lime. 

I he basic processes arc calcination and hydration. Commencing with high-calcium limestone, the reactiuns are ... 

Reaction with aqueous acids. In general, limestones react readily with adds and are used for acid neutralisation. High calcium limestones react readily with dilute hydrochloric and nitric acids at ambient temperatures, whereas dolomite and dolomitic limestones only react readily when the dilute acid is heated. The reaction of limestone with sulfurous acid (formed by the dissolution of sulfur dioxide in water) is the basis of a flue gas desulfurisation process (see section 12.5.2). The reactions with acids which form insoluble or sparingly soluble calcium salts (e.g., sulfurous, sulfuric, oxalic, hydrofluoric and phosphoric acids) are inhibited by the reaction product. 

High-calcium limestone is not widely used in fertiliser formulations, as it can release ammonia from nitrogenous compounds. 

The high calcium limestone and dolomite should both be high in calcium plus magnesium carbonate content and low in silica and phosphorous (Table 11.4 [11.2]). They should be resistant to decrepitation on heating, and produce limes which are strong enough to resist being crushed in the blast furnace. 

The major components of glass are silica sand, soda ash (Na2C03) and limestone. Both high-calcium limestone and dolomite are used (see also the use of quicklime in glass manufacture, section 32.6). 

Dolomitic limestone is usually specified for the production of containers and tumblers, as the MgO content increases resistance to etching by acids and other solvents. High-calcium limestone is generally favoured for flat glass. 

The optimum temperature for operation at atmospheric pressure is about 850°C for both high-calcium limestone and dolomite (N.B., it is understood that the magnesium oxide component does not react with the oxides of sulfur). 

In the sulfite process for making paper pulp, lumps of high-calcium limestone may be used to react with sulfur dioxide to produce a Ca(HS03)2 liquor. This liquor is used to digest the pulp to remove components of the pulp other than the cellulose. Increasingly, however, limestone is being replaced by alternative alkalis (magnesia, ammonia or soda ash), which can be recycled more readily. 

The passage of a limestone particle through a lime kiln can be divided into five stages. The following description refers to high-calcium limestone, but parallels can be drawn with magnesian/dolomitic limestones and dolomite. 

For spheres of dense, high calcium limestone, with a range of diameters. % values are mJm. 

Heat transfer in lime burning can be divided into three stages. To simplify the presentation, the following relates to high-calcium limestone, but the principles also apply to other types of limestone. 

The primary raw materials are purified saturated brine (see section 31.18) and high-calcium limestone. The overall simplified equation is ... 

One non-standard desulfurisation technique, used on a rotary kiln by a producer of calcined dolomite, is to include finely divided dolomite in the feedstone. The fines calcine, become airborne as a result of the action of the kiln internal fittings (i.e., trefoils and lifters), and remove a significant amount of the sulfur dioxide. It is not known whether this technique would be as successful with high calcium limestone, which calcines at higher temperatures than dolomite. Other techniques will no doubt be evaluated, such as the injection of hydrated lime into the back-end of the kiln. The cost-effectiveness of such techniques in relation to alternatives, and their effects on kiln operation, would need to be assessed. 

Calcitic limestone refers to a high calcium limestone, with less than S % of magnesium carbonate. 

Chemical-grade (or -quality) limestone see high-calcium limestone. 

High-calcium lime is quick- or slaked lime produced from high-calcium limestone. 

High-calcium limestone is a general term for limestone consisting of mainly CaCOa (at least 95 % m/m) and having less than 5 % (m/m) MgCOa. 

Ultra-high calcium limestone contains more than 97 % CaCOs. 

McDonald R (ed) (1997) Colour physics for industry, 2nd edn. Society of Dyers and Colourists, West Yorkshire, England. Whitening (1601) n. A finely divided form of calcium carbonate obtained by milling high-calcium limestone, marble, shell, or... 

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