Burnt Dolomite

Brief Overview

Dolomite is a common rock-forming mineral. It is a calcium magnesium carbonate with a chemical composition of CaMg(CO₃)₂. There are two types of materials often called dolomite, a true chemically uniform calcium magnesium carbonate with the chemical formula CaMg(CO₃)₂, and a dolomitic limestone, which is just an irregular mixture of calcium and magnesium carbonates. Dolomite differs from calcite and aragonite in its crystal structure. In dolomite crystals, layers of carbonate ions alternate with layers of magnesium and calcium ions, rather than only having layers of calcium ions alternate with carbonate ions as in calcite and aragonite. Dolomite crystals usually form transparent to translucent rhombs that are colourless to light-coloured, although crystals may be red to brown if iron impurities are present. Some dolomite crystals also exhibit crystal faces that form slightly curved surfaces, rather than flat planes.

Manufacturing Process

 Dead burnt refractory dolomite is produced in rotary kilns. Generally high-grade dolomite, containing combined impurities less than 3%, is selected for dead burning. As it is difficult to densify high purity dolomite in a rotary kiln, it is customary to use some mineralizers to facilitate dead burning. Iron oxide is a common additive. The manufacturing process varies with the grade of D.B. dolomite desired to be produced. In most of the plants in European countries a typical operation, employing rotary kilns lined in the hot zone with basic bricks and fired with powdered coal, is used. The temperature reached in the hot zone is of the order of 1760ºC. The kilns have continuous gas sampling equipment which measures and records the oxygen, combustibles, and carbon dioxide contents of the kiln exit gases for combustion control. The latter is particularly important in achieving a uniform quality of the product, effective use of dead burning agents and efficient use of fuel in the manufacture of dead burnt dolomite. 

 The dolomite after dead burning is cooled in either rotary or reciprocating recuperative coolers. The air used for cooling gets heated and is again used as secondary air for combustion in the kilns. When D.B. dolomite is manufactured with an additive, it is necessary to use somewhat higher firing temperature in order to shrink the dolomite in a reasonable time-cycle in the kiln. This has been accomplished by improved thermal efficiency in the kiln. 

Metallurgy Industry

Dolomite is used as a flux and for manufacture of refractories for basic open hearth furnaces and steel ladles. Dolomite for use as flux in steel metallurgy should be hard, compact and fine-grained so that it can stand the burden of the batch in the blastfurnace as well as the basic steel convertor. It should not be crystalline nor a saccharoidal texture which gives fritting effect in the furnace. It should be free from phosphorous and sulphur and contains low levels of impurities. Generally, two grades of dolomite are used. One is called blast furnace (BF) grade and the other steel melting shop (SMS) grade.