Lime kilns about

Carbon dioxide can be used to extinguish fires by smothering them. One form of portable fire extinguisher is a cylinder of liquid carbon dioxide—the gas can be liquefied at ordinary temperatures under pressures of about 70 atm. Some commercial carbon dioxide (mainly solid carbon dioxide) is made from the gas emitted in nearly pure state from gas wells in the western United States. Most of the carbon dioxide used commercially is a by-product of cement mills, lime-kilns, and iron blast furnaces. 

Calcium oxide (quicklime) is prepared on a large scale (18 million tonnes in 1988) by heating CaCOs in lime kilns (equation 31). Calcium hydroxide is called slaked lime. Soda lime is a mixture of NaOH and Ca(OH)2 and is made from quicklime (CaO) and aqueous sodium hydroxide. It is much easier to handle than NaOH. Some of the more important, large volume uses of Lime are summarized in Table 14. Bleaching powder is made by passing Cl2(g) into slaked lime, and about 90 000 tonnes a year are produced. Though bleaching powder is often written as Ca(OCl)2, really it is a mixture (equation 32). 

Calcium carbonate decomposes into calcium oxide and carbon dioxide at about 900 °C. Most lime kilns operate with a maximum temperature of 1300° to 1500°C. Portland cement kilns operate at about 1450° to 1500 °C. 

Perhaps the earliest excavated lime kiln was at Khafaje in Mesopotamia which was dated at about 2450 B.C. A battery of six lime kilns, excavated at a legionary site at Iversheim, Germany, showed that the Romans produced lime in quantity on military sites. The production of lime in kilns was mentioned by Cato in 184 B.C. Pliny the Elder (ca. 17 A.D.), in his Chapters on Chemical Subjects described the production, slaking and uses of lime, and stressed the importance of chemical purity. 

In the absence of calcium hydroxide, quicklime only reacts with carbon dioxide at temperatures above about 300 °C and below 800 °C. This reaction can occur in the cooling zone of lime kilns under abnormal conditions. It results in carbonation of quicklime at the surface of the pores, and can give abnormally low reactivities for a given mean apparent density. 

However, they work well when the feed particles are large. The reason for the relatively shorter length is that the feed is dry with a moisture content the same as granular solids rather than slurry. Applications include lime kilns and lightweight aggregate kilns where the mined stones are crushed to about 1.3-5 cm (0.5-1.5 in.) before feeding them into the kiln. 

Short dry kilns are usually accompanied by an external preheater or pre-calciner (Figure 1.7) in which the feed is dried, preheated, or even partially calcined prior to entering the main reactor (kiln). As a result the thermal load on the kiln proper is reduced. Hence kilns equipped with preheaters or precalciners tend to be short, on the order of 15-75 m (about 50-250 ft) depending on the process. The shorter kilns are those in which the entering feed material is almost calcined. Applications include cement and some lime kilns. Because of the large feed particle size encountered in limestone calcination, modern lime kilns are equipped with preheaters which function as a packed bed of stone with a countercurrent flow of kiln exhaust gas rather than the typical cyclone preheaters in cement kiln systems. 

The chemical-grade ore, containing about 30% chromium, is dried, cmshed, and ground in ball mills until at least 90% of its particles are less than 75 ]lni. It is then mixed with an excess of soda ash and, optionally, with lime and leached residue from a previous roasting operation. In American and European practice, a variety of kiln mixes have been used. Some older mixes contain up to 57 parts of lime per 100 parts of ore. However, in the 1990s manufacturers use no more than 10 parts of lime per 100 parts of the ore, and some use no lime at all (77). The roasting may be performed in one, two, or three stages, and there maybe as much as three parts of leached residue per part of ore. These adaptations are responses to the variations in kiln roast and the capabihties of the furnaces used. 

About 93% of the CaCOs mud is dried and lecalcined in rotary kilns to recover the CaO. Calcium hypochlorite bleaching liquor (p. 860) for paper pulp is obtained by reacting lime and CI2. 

Calcium Oxide (lime, Quicklime, Burnt Lime, Calx, Unslaked Lime, Fluxing Lime). CaO, mw 56.08, white or greyish-white lumps or powd, mp 2580°, bp 2850°, d 3.25-3.40g/cc. SI sol in w with formation of calcium hydroxide and evolution of large amts of heat sol in acids, and insol in ethanol. Coml prepn consists of heating calcium carbonate in kilns at 1000—1100° until all of the C02 is driven off. Lab prepn is by burning calcium carbonate or calcium oxalate at about 800° using a quartz crucible in an electric furnace... 

Silica bricks are manufactured from crushed ganister rock containing about 97 to 98 percent silica. A bond consisting of 2 percent lime is used, and the bricks are fired in periodic kilns at temperatures of 1,500 to 1,540°C (2,700 to 2,800°F) for several days until a stable volume is obtained. They are especially valuable when good strength is required at high temperatures. Superduty silica bricks are finding some use in the steel industry. They have a lowered alumina content and often a lowered porosity. 

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