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Dolomite

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Dolomite Calcium Magnesium Carbonate
CaMg(CO3)2

In England, dolomite has become a useful source for the production of magnesite by reacting calcined dolomite with sea-water.

The History Says

Dolomite is named for the French mineralogist Deodat de Dolomieu. They are found all over the world and are quite common in sedimentary rock sequences. These rocks are called appropriately enough dolomite or dolomitic limestone. Disputes have arisen as to how these dolomite beds formed and the debate has been called the "Dolomite Problem".

The Present Scenario

Dolomite at present time, does not form on the surface of the earth; yet massive layers of dolomite can be found in ancient rocks. That is quite a problem for sedimentologists who see sandstones, shales and limestones formed today almost before their eyes.
DOLOMITE is a double carbonate of calium and magnesium, CaCO3, MgCO3. The mineral was first identified by Count Dolomien in 1791 and named after its discoverer. It is of sedimentary origin and is supposed to have been formed due to chemical action of sea-water containing high percentage of magnesia, on limestone.

Theoretically, dolomite contains:
CaCO3 54.35%
MgCO3 45.65%
In other words, it contains:
CaO 30.4%
MgO 21.7%
CO2 47.9%
In nature, considerable variations in the composition of dolomite relating to lime and magnesia percentages are found. When the percentage of CaCO3 increases by 10% or more over the theoretical composition, the mineral is termed 'calcitic dolomite', 'high-calcium dolomite' or 'lime-dolomite'. With the decrease in percentage of MgCO3, it is called 'dolomitic limestone'. With the variations of MgCO3 between 5 to 10%, it is called 'magnesian limestone', and upto 5% MgCO3 or less it is taken to be limestone for all purposes in trade and commercial parlance.

Dolomite usually contains impurities, chiefly silica, alumina and iron oxide. For commercial purposes, the percentage of combined impurities should not go beyond 7% above which, it becomes unsuitable for industrial use. It is then used only for road ballasts, building stones, flooring chips etc.

Hardness Associated Minerals Chemical/Typical composition Colour characteristics Luster Field Indicators
3.5-4 include
calcite
sulfide ore minerals
fluorite
barite
quartz
and occasionally with gold
white often pink or pinkish and can be colorless, white, yellow, gray or even brown or black when iron is present in the crystal Unlike calcite, effervesces weakly with warm acid or when first powdered with cold HCl pearly to vitreous to dull typical pink color, crystal habit, hardness, slow reaction to acid, density and luster

Industrial Applications

Dolomite is chiefly used as refractory, ramming, and fettling material in steel melting shop, and as fluxing material in blast furnace operation in secondary steel and ferromanganese manufacture. To a lesser extent it is used in the glass industry especially in sheet-glass manufacture. It also finds use in the manufacture of mineral wool.

In England, dolomite has become a useful source for the production of magnesite by reacting calcined dolomite with sea-water. The UK is meeting nearly 50% of her magnesite requirements by this method. Dolomite is also a good source of magnesium metal. The magnesium metal is extracted from dolomite by the well-known fero-silicon process.

Dolomite decomposes completely above 900ºC. The product resulting from this relatively low-temperature calcination is highly porous and reactive and is known as 'calcinated dolomite'. Dolomite is sometimes used both in the raw and calcined form as refractory material for hearth maintenance and for banking door in open hearth furnaces.

For most refractory uses, it is desirable to subject the dolomite to a heat treatment at a high temperature of the order of 1700ºC, to shrink the material thoroughly and render it less reactive. Dead burnt (D.B.) dolomite is sthe term generally used for the refractory made by firing dolomite, with or without additives, at high temperature to produce dense, well-shrunk particles.

In basic converters the bricks employed are generally of D.B. dolomite and sometimes also of D.B. magnesite. Dolomite bricks are kept in the outer lining because it has lower thermal conductivity than magnesite.

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. Some of the means to attain higher efficiency have been the use of insulating brick-back of the basic lining in the hot zone and the optimum utilization of secondary air from the recuperative coolers, in order to pick up as much of the available heat as is possible from the cooling of the product.

There is another product known as 'stabilised' refractory dolomite. It is manufactured by the process similar to that of portland clinker. Dolomite and serpentine with small amounts of suitable stabilising agents, are ground to a slurry in a ball mill. The slurry is fired to a dense mature clinker in a rotaery kiln having a temperature of the order of 1760ºC.

The optimum capacity of vertical or shaft kiln using coke admixed with dolomite for fuel is kept at a minimum of 100 tonnes a day. Such kilns are widely used in the USA and the UK, Scandinavia and other European countries. Great progress has been made in the automation of kilns in the steel-producing countries. D.B. dolomite can stand temperatures upto 2300ºC. It is widely used as a refractory material wherever steel is refined using basic slag. It is used for original hearth installations in the open hearth furnaces as well as for hearth maintenance. These hearths are installed using tar-dolomite ramming mixes and rammed dolomite. Dolomite refractories are also used in electrical furnaces and in cement industry during clinker manufacture.

Specifications

Steel manufacturers prefer dolomite of the following composition for dead burning: 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 blast furnace as well as the basic steel convertor. It should not be crystalline, i.e., it should not have a saccharoidal texture which gives fritting effect in the furnace. Impurities as low as possible are preferred. It should be free from phosphorous and sulphur. Generally, two grades of dolomite are used, one is called blast furnace (BF) grade and the other steel melting shop (SMS) grade.

The dolomite containing insolubles (Al2O3+Fe2O3+SiO2) upto 7%, is used in the blast furnace by most of the steel manufacturers in the country but the dolomite dontaining a maximum of 4 to 5% insolubles is preferred. Silica and alumina contents in dolomite are not regarded deleterious for blast furnaces. They only cause unnecessary increase in the slag. Dolomite has been found as a useful support to limestone in removing sulphur from the iron ore. It also reduces the viscosity of the slag, thus chemical reactions in the furnace. For the steel melting shop the total insolubles below 4% are preferred. The silica content should be as low as possible, in no case above 2% being tolerated.

The ferro-manganese manufacturers in India usually prefer SMS dolomite for fluxing purposes.

In general, the metallurgical industries require dolomite of the following grades for fluxing purposes.
Blast furnace grade SMS and Ferro-Manganese grades
CaO 28 - 30% Min. CaCO3 MgCO3 95% Max.
MgO 18 - 20% Min. Fe2O3 + Al2O3 2% Max.
Al2O3 + SiO2 + Fe2O3
(total insolubles)
7% Max. SiO2 for SMS
for ferromanganese
2% Max.
3% Max.

For use in the colourless sheet-glass industry, the dolomite should contain not more than 0.1% Fe2O3. Total acid insolubles less than 2% are preferred. A higher content or SiO2 or Al2O3 is not regarded deleterious.

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