Limestone and dolomite flux and their use in iron and steel plant

Limestone and dolomite flux and their use in iron and steel plant

Limestone is a naturally occurring mineral. The term limestone is applied to any calcareous sedimentary rock consisting essentially of carbonates.  The ore is widely available geographically all over the world. Earth’s crust contains more than 4 % of calcium carbonate. Limestone is basically calcite which is theoretically composed of exclusively calcium carbonate (CaCO3). When limestone contains a certain portion of magnesium, it is called dolomite or dolomitic limestone (CaCO3.MgCO3). Dolomite theoretically contains CaCO3 54.35 % and MgCO3 45.65 % or CaO 30.4 %, MgO 21.9 % and CO2 47.7 %. However, in nature, dolomite is not available in this exact proportion. Hence generally the rock containing 40-45 % MgCO3 is usually called dolomite. When MgCO3 is less than 40 % but more than 20 % then the limestone is called dolomitic limestone. The chemical composition of limestone and dolomite varies greatly from region to region as well as between different deposits in the same region. Therefore, the end product from each natural deposit is different.  Typically limestone and dolomite are composed of calcium carbonate (CaCO3), magnesium carbonate (MgCO3), silica (SiO2), alumina (Al2O3), iron (Fe), sulphur (S) and other trace elements. These minerals are shown in Fig 1

limestone and dolomite

Fig 1 Limestone and dolomite

The limestone from the various deposits differs in physical chemical properties and can be classified according to their chemical composition, texture and geological formation. Limestones from different sources differ considerably in chemical compositions and physical structures. The chemical reactivity of various limestones also shows a large variation due to the difference in crystalline structure and the nature of impurities such as silica, alumina and iron etc. The varying properties of the limestone have a big influence on the processing method. Hence it is necessary to know comprehensive information of the limestone such as physical and chemical properties. The major use of limestone in industry is shown in Fig.2.

Industrial use of limestone

Fig 2  Industrial use of limestone

Both limestone and dolomite are extensively used in an iron and steel plant in various processes and their use is explained below. Specification of limestone and dolomite for iron making is less rigid. But for steel making limestone should have very low silica (SiO2) and alumina (Al2O3) since these elements will need additional flux to neutralize them which will increase slag volumes. Further additional heat will be required for keeping fluid this amount of additional slag. Also important is the consistency of chemical composition and size fraction. Further limestone used for calcination should have good decrepitation index.

Iron and steel making

Basically limestone is used as a slag former. Dolomite is used as a slag former, slag modifier and as a refractory material. The process of iron making is the reduction of iron ore to produce iron. Iron ore normally contain gangue materials such as silica (SiO2), Alumina (Al2O3) along with sulphur (S) and Phosphorus (P). Removal of these impurities is done by combining the gangue materials with CaO and/or MgO to form slag which consists of low melting point complex compounds such as calcium silicate, calcium aluminate etc. CaO and MgO is charged along with other raw materials in the form of lime stone and dolomite or it is charged through sinter where again fines of limestone and dolomite is used. This limestone or dolomite is first decomposed into CaO or CaO+MgO which then combine with gangue to form slag. Further lime from limestone reacts with sulphur present in the raw materials to form CaS which goes into slag.

During pretreatment of hot metal in desulphurization plant lime is an essential component of the desulphurizing compound. During steel making high basicity of the steel making slag is being maintained with the help of lime. Lime is also used in secondary steel making. In steel melting shop, fettling of the lining as well as patching is done with dolomite based compounds. Calcined dolomite is also used in converters to maintained MgO levels in the steel making slags. It also acts as a slag modifier in case of slag splashing. Burnt dolomite is also used for making refractory bricks for the purpose of lining in steel melting shop.

Flue gas treatment

During combustion of coal and petroleum products there is high level of SO2 emissions. To control these emissions within norms it becomes necessary to treat the flue gases for SO2 and other acid gases. Lime is used effectively for this purpose.

Water treatment

Lime is used to treat waste water for adjustment of pH levels and precipitation of heavy metals in the form of hydroxides, sulphates and phosphates as insoluble salts. It is added as neutralizing agent for acidic water before its discharge.

Sludge treatment

Lime is widely used to coagulate and to stabilize sludge in the sludge treatment units