Limestone economic considerations

Drying Fhiidized-bed units for drying solids, particularly coal, cement, rock, and limestone, are in general acceptance. Economic-considerations make these units particularly attrac tive when large tonnages of solids are to be handled. Fuel requirements are 3.3 to 4.2 MJ/kg (1500 to 1900 Btu/lb of water removed), and total power for blowers, feeders, etc., is about 0.08 kWh/kg of water removed. The maximum-sized feed is 6 cm (IV2 in) X 0 coal. One of the major advantages of this type of dryer is the close control of conditions so that a predeterminea amount of free moisture may be left with the solids to... 

The problem with sulfur dioxide containment is that many smelter processing units produce sulfur dioxide concentrations of 1-2% whereas the minimum economic concentration for sulfur dioxide conversion processes (e.g., for sulfuric acid or sulfur production) is 3.5-4%. Thus, the economic solution to containment requires either modification of the smelter process to obtain higher sulfur dioxide concentrations, or capture of sulfur dioxide at relatively low concentrations followed by some means of regeneration of a high concentration of sulfur dioxide (Chap. 3). Throw-away approaches to sulfur dioxide containment, lime or limestone wet scrubbing, are worthy of consideration by small-scale smelters (e.g., Eqs. 13.35-13.37). 

Limestone is extracted by both surface quarrying and under-ground mining operations. However, economic and safety considerations result in almost all limestone being extracted by quarrying, which also recovers a higher proportion of the rock. Some examples of successful limestone mines are given in section 4.3.7. 

The economic haulage distances for lime products are considerably greater than those for limestone. Road haulage, is often economic up to 250 to 400 km. Where appropriate, the use of rail and water transport for large customers enables that range to be extended considerably. 

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