Charging of limestone

All intermittent kilns are thermally inefficient because there is little heat recovery in the preheating and cooling zones. In addition, much of the volatile matter in the fuel distils out of the kiln as smoke rather than burning. Judging the amount of fuel required to calcine a charge of limestone is based partly on previous experience and partly by poking rods into the charge to assess the amount of unburned limestone. 

There are two process routes for making steel in the UK today the electric arc furnace and the basic oxygen converter. The latter requires a charge of molten iron, which is produced in blast furnaces. The raw materials for producing molten iron are iron ore, coking coal, and fluxes (materials that help the chemical process) - mainly limestone. 

Iron is extracted mainly from its oxides, haematite (Fe2Os) and magnetite (Fe304), in a blast furnace (Figures 10.12 and 10.13). These ores contain at least 60% iron. The iron ores used are a blend of those extracted in Australia, Canada, Sweden, Venezuela and Brazil. The blast furnace is a steel tower approximately 50 m high lined with heat-resistant bricks. It is loaded with the charge of iron ore (usually haematite), coke (made by heating coal) and limestone (calcium carbonate). 

Metallurgical coal coal that meets the requirements for use in the steelmaking process to manufacture coke it must be low in ash and sulfur and form coke that is capable of supporting the charge of iron ore and limestone in a blast furnace. 

Probably the rotary horizontal kiln is the most versatile, since it allows a feed of lumps or fines of limestone or marble, or wet or dry calcium carbonate sludges (Fig. 7.1). The main component of this calcination system is a 2.5- to 3.5-m diameter by 45- to 130-m long firebrick-lined inclined steel tube. Heat is applied to the lower end of this via oil, gas, or coal burners [7]. The feed to be calcined is fed in at the top end. Slow rotation of the tube on its axis gradually moves the feed down the tube, as it tumbles countercurrent to the hot combustion gases. In this way, wet feed is dried in the first few meters of travel. Further down the tube, carbon dioxide loss begins as the temperature of the feed rises. By the time the solid charge reaches the lower, fired end of the kiln it reaches temperatures of 900-1,000°C and carbon dioxide evolution is virtually complete. Normally the temperature of the lower end of the kiln is not allowed to go much above this as it reduces the life of the kiln lining. It also adversely affects the crystal structure of the lime product since it produces a dead-burned or overburned lime. Overburned lime is difficult to slake to convert it to calcium hydroxide and raises... 

The production cost of limestone depends on a number of factors. The nature of the deposit can be important massive deposits with little overburden, horizontal strata and consistent physical/chemical properties favour low extraction costs, particularly if linked with a large-scale operation. Selection of appropriate equipment to keep the combined costs of labour, capital charges and other operating costs to a minimum is important to ensure a strong competitive position (see chapters 4 and 5). 

Intermittent mixed-feed kilns are similar in general design, but, instead of additional fuel being added on to the hearth, it is added in layers while the kiln is being charged with limestone. 

The earliest continuous kilns (also known as running kilns or draw kilns) were fired using the mixed-feed principle (Fig. 16.3). Alternate layers of limestone and fuel (wood or coal) were charged into the top of the kiln and lime was removed through the drawing door. 

Batches of limestone are charged alternately to each shaft. The burden is drawn downwards through a preheating/regenerative heat exchange zone, past the fuel lances and into the calcining zone. From there the quicklime passes into the cooling zone. 

The production cost of quicklime is generally dominated by the charges for limestone and energy per tonne of product. As approximately 1.81 of limestone are required per tonne of quicklime produced, it is generally economic to minimise transport costs by building the kilns adjacent to, or within the limestone quarry, rather than near the principal customers. 

Many lime kiln installations incorporate a screen to remove fines produced by breakage before charging the limestone into the kiln. When the particle size of the stone is large, a metal screen deck can produce high sound levels. Use of rubber decking reduces the problem. 

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