Overburned lime

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 calcium and magnesium oxides that hydrate after cutting cause expansion of the blocks in the autoclave. While a small expansion can be tolerated, excessive expansion causes the material to exceed the specified dimensions [26.50, 26.51] and can reduce strength through cracking of the mass, or micro-cracking of the walls between the voids. It is, therefore, essential to limit the amounts of overburned calcium oxide and of magnesium oxide, in the lime. 

Grappier Cement. A cement made from fine-ground underburned or overburned slaked lime. 

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