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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.
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:
other words, it contains:
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
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.
sulfide ore minerals
and occasionally with gold
||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
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
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
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
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
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
Steel manufacturers prefer dolomite of the following composition for
------------- 35% Min.
--------------- 1% Max.
------- 1.5% Max.
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 (Al2
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.
||28 - 30% Min.
||18 - 20% Min.
||Fe2O3 + Al2O3
+ SiO2 + Fe2O3
||SiO2 for SMS
use in the colourless sheet-glass industry, the dolomite should
contain not more than 0.1% Fe2
. Total acid
insolubles less than 2% are preferred. A higher content or SiO2
is not regarded deleterious.
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