Dolomite

Infobox on Dolomite
Example of Dolomite
Facts
Origin	-
Stowage factor (in m3/t)	0,6/0,7 m3/t
Humidity / moisture	-
Ventilation	No special requirements
Risk factors	-

Dolomite

Description

Dolomite is a double carbonate of calium and magnesium, CaCO₃, MgCO₃. 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.

Specifications

Steel manufacturers prefer dolomite of the following composition for dead burning:
MgCO₃- 35% Min.
SiO₃- 1% Max.
Fe₃O₃+Al₃O₃ - 1.5% Max.
CaCO₃ - Remainder

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.

In general, the metallurgical industries require dolomite of the following grades for fluxing purposes.

Blast furnace grade
CaO - 28 - 30% Min.
MgO - 18 - 20% Min.

SMS and Ferro-Manganese grades
CaCO₃ MgCO₃ - 95% Max.
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.

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 rotary 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.

Application

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 the 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.

Use; refractory for furnaces, manufacture of magnesium compounds and magnesium metals, as building material, in fertilisers, stock feeds, paper making, ceramics, mineral wool, removal of sulfur dioxide from stack gases.

Shipment / Storage

Dolomite is a light yellow/brown coloured mineral stone which is very hard and compact. Dolomite may sometimes, incorrectly, be used to describe a material consisting of the oxides of calcium and magnesium (dolomite quicklime). Usually shipped in bulk and to be trimmed in accordance with the relevant provisions as required in the IMSBC Code.

No special requirements with regard to stowage, weather precautions or ventilation.

Risk factors

There are no special hazards. This cargo is non-combustible or has a low fire-risk.

Consult the IMSBC Code for further particulars.