Calcination dead burning

Calcination or dead burning is used extensively to dehydrate cements (qv) and hygroscopic materials such as MgO, and to produce a less water sensitive product. Calcination is also used to decompose metal salts to base oxides and to produce multicomponent or mixed oxide powders for... 

Magnesium oxide. The natural minerals, i.e., magnesite (MgCO ), brucite [Mg(OH)9], etc., after being crushed to predetermined size, are calcined at temperatures varying from 1055 to 2000 K, depending upon whether a caustic or a dead-burned produc t (periclase) is being... 

Calcination temperature is very important in the production process and dictates the particle size, purity and reactivity of the product. A dead-burned, sintered dense microcrystaUine product is obtained at calcination temperature of 1,400 to 1,700°C. A caustic-burned product is obtained when magnesium carbonate or hydroxide is calcined at 600 to 700°C. A hght grade (specific gravity 2.9) highly reactive caustic-hurned magnesia that contains some moisture and carbon dioxide is obtained at about 600°C. A denser form from... 

II Anhydrite 11 insoluble anhydrite inactive anhydrite dead-burned gypsum chemical anhydrite mineral anhydrite Produced hy calcining at 250-1.000 C. Relatively inert. Reactivity depends upon calcining-time-lemperaiure relationship and particle size. 

The basic manufacturing operation is calcining the raw material to obtain the dead-burned magnesite which may be used directly for making monolithic linings, or serves as intermediate product in the manufacture of bricks. It has to be sufficiently dense and water resistant. 

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... 

Accurate Chinese production figures are difficult to obtain however, it is estimated that the total production capacity of magnesia in China is about 2.7 Mt per annum. This comprises of 1.8 Mt of dead-burned magnesia and 900,000 tonnes of caustic-calcined magnesia. The purity of the magnesite ranges from 91 to 98% loss-free MgO content. 

The production of chemical-grade magnesia or light-burned MgO requires careful control of the calcination temperature to achieve the required specific surface area of the finished product. A furnace well suited to this requirement is the multiple-hearths Herreshoff-type. For the production of dead-burned magnesia typically shaft or rotary kilns are employed. See Chapter 5 for furnaces used in MgO production. 

A variety of furnaces are used in the magnesia industry to calcine either magnesite or magnesium hydroxide filter cake. The most commonly used are the multiple-hearth furnace (MHF) and rotary and shaft kilns. MHFs are primarily used to produce reactive lightly calcined magnesia, while rotary and shaft kilns produce hard and dead-burned grades of magnesia. 

Four qualities of calcined dolomite are produced — half-, light-, hard-, and dead-burned. 

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