Dolomite

 

Dolomite is a solid solution of carbonates and magnesium. It is an essential material that was used by many industries.  The main use of dolomite in refractory industry ad later as flux. It is used as an ingredient in the production of ceramics, bricks, and glass. In natural dolomite, the ratio of calcium oxide and magnesium oxide is a little large. If the ratio is lesser than the 1.39 value, it was called Magnesia dolomite refractory brick. Now, let's see about Dolomite refractory in this article.

 

For Dolomite,

Dolomite = Calcium carbonate * Magnesium Carbonate

• Magnesium Carbonate = 45% - 65%

• Calcium carbonate = 35% - 55%

For Doloma,

Doloma = Calcium oxide * Magnesium oxide

• Calcium oxide = 30.46%

• Magnesium oxide = 21.87%

• Carbon dioxide = 47.7%

Note: Carbon dioxide leads to crack, so it has to be calcined.

Doloma can be prepared from Dolomite. Doloma is also termed as “Dolime”

Stage 1: Single-stage calcination

Dolomite is subjected to firing at a temperature of 1900 degree Celsius to form Doloma and free lime in a shaft kiln or rotary kiln.

Problem: In Free State, lime is highly hygroscopic in nature. So, this will leads to hydration tendency or hydration problem is called as “Perishing”.

Stage 1 characteristics:

• Bulk density = 2.5 g/cc

• Apparent porosity = 15%

• Hydration tendency of 100%

This is not used for refractory purposes. To overcome this hydration problem in stage 1 of calcination, double calcination takes place in stage 2 which was mentioned below.

Stage 2: Double calcination

Dolomite is lightly calcined at 1000 degree Celsius to form doloma. This hydrated doloma results in the formation of Calcium hydroxide and Magnesium hydroxide. So, it has to be pelletized. The process of pelletization of doloma takes place in pyrolysis at a temperature of 1600 degree Celsius to form pelletized doloma.

Stage 2 characteristics:

• Bulk density = 3.2 – 3.3 g/cc

• Apparent porosity = 0.8%

• Hydration tendency of 1.7%

This stabilized doloma or pelletized doloma can be preferred for refractory material. The process of stabilization or pelletization is done to prevent the perishing problems (because of the presence of free lime) and to make doloma inactive.

Impurities

The impurities involved are ferrite, silica and aluminium oxide. During the process of calcination, these kinds of impurities incorporate with lime to form secondary phases. Apart from these impurities, other impurities include brownmillerite and Dicalcium ferrite. The factors affecting impurities are as follows:

• High-temperature properties

• Hydration tendency

• Bulk density

How to overcome the perishing problem?

• Calcium oxide (free lime) is converted to lime silicate.

• Stabilized phase should have tricalcium silicate to prevent dusting problems.

Types of lime calcium silicate

• Mono calcium silicate = 1540 degree Celsius

• Di calcium silicate = 2130 degree Celsius

• Tri calcium silicate = 1900 degree Celsius

Di calcium silicate undergoes polymorphic transformation such as alpha, beta, and gamma. Alpha is of high-temperature form and gamma is of low-temperature form. The change in alpha to gamma di calcium silicate is accompanied by a volume expansion of 10% which is very high. Breakage happens due to volume expansion which will further lead to dusting problems. Gamma form is retained by the addition of a few percentages of ferrite.

To overcome the perishing and dusting problems, stabilized doloma can be prepared. The preparation of stabilized doloma is given below. 

Dolomite + serpentine combination is subjected to firing at a temperature of 1600 degree Celsius in a rotary kiln. After firing, tri calcium silicate is formed along with periclase, water, carbon dioxide. With the addition of a glassy phase (from extra silica) to the above tri calcium silicate formation, pea-size clinkers are formed, which is sieved and stored finally.

Properties of Dolomite

1. Color: White to buff color (Depending on iron and other impurities)

2. Porosity: 23%

3. Specific gravity: 3.2

4. Pyrometric cone equivalent: 1650 degree Celsius for stabilized dolomite

5. The important property of Dolomite is hydration resistance. It depends upon the following factors:

• Lime content

• Nature of mineralizer

• Concentration and distribution of mineralizer

• Sintering temperature (highly reactive at higher sintering temperature)

• Particle size (medium)

Hydration resistance can be improved by

• Addition of silica, magnesia, chrome, zirconia and ferrite

• Sintering temperature (2100 deg. celsius) and long soaking time

• By using pelletized doloma (stabilized doloma)

6. Hot modulus of rupture (HMOR):

When fusible impurities are present, the HMOR value decreases with an increase in temperature.

7. Chemical activity:

Resist steel but not ferrite (used in transporting vessels). Doloma can’t withstand the action of ferrite.

8. Slag resistance:

Slag resistance is enhanced by the presence of Di calcium silicate. When slag comes in contact with di calcium silicate, it is converted into slag and loosened the structure. Slag attack is easily attacked by acidic slags, which will primarily dissolved lime grains. To minimize dissolution of lime, the grains of doloma should be larger so that it has better hydration and slag attack resistance.

9. Thermal shock resistance:

Di calcium silicate has lower thermal shock resistance than tri calcium silicate due to polymorphic transformation which will lead to a change in volume. Hence, thermal shock resistance is lower.

In tri calcium silicate, less than 4% silica is added to arrest the phase transformation of di calcium silicate to avoid polymorphic transformation. Hence, thermal shock resistance is higher.

For stability purposes, tri calcium silicate is better and for better properties, di calcium silicate is preferred.

Applications of Dolomite:

• The mixture of tar and dolomite is used as monolithic lining in the LD converter.

• It is used for the fettling purposes of open-hearth furnaces and electric arc furnaces.

• At a zone of higher temperature, there are more alkalis were present. So, it is used in the sintering zone of cement rotary kiln.

• Liquid alkalies are highly corrosive. So, it is used in the copper smelter.

Finally, we have seen a topic on Dolomite refractory in a basic and simple manner. I hope you find this information useful.