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Limestone and Dolomite and their Use in Iron and Steel Plant

• satyendra
• May 8, 2013
• 3 Comments
• basic, Calcined dolomite, dolomite, flux, lime, limestone, neutraliziing, Sintered dolomite, slag,

Limestone and Dolomite 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 calcium carbonate (CaCO3) in the form of the mineral calcite. It forms predominantly on the sea floor where material rich in calcium carbonate (‘calcareous’ material) accumulates. This calcareous material can be organic, chemical or detrital in origin.

Types of limestone include (i) bituminous limestone, (ii) carboniferous limestone, (iii) coquina  which is a sedimentary rock composed mostly of fragments of shells, (iv) coral rag, (v) chalk  which is a soft, white, porous sedimentary rock made of calcium carbonate, (vi) fossiliferous limestone, (vii) lithographic limestone, (viii) oolite which is a sedimentary rock formed from ooids, (viii) rag-stone which represents work done with stones that are quarried in thin pieces, (ix) shelly limestone, (x) travertine which is a form of limestone deposited by mineral springs, (xi) marl, and (xii) tufa which is a porous limestone rock formed when carbonate minerals precipitate out of ambient temperature water. Limestones altered by dynamic or contact metamorphism become coarsely crystalline and are referred to as ‘marbles’ and ‘crystalline limestones’. The limestone which is used by iron and steel industry in bulk quantity is a bedded type sedimentary limestone.

The two most important constituents are calcite and dolomite. Limestone frequently contains magnesium carbonate, either as dolomite CaCO3.Mg CO3 or magnesite (MgCO3) mixed with calcite. Such rocks are termed as ‘dolomitic’ or ‘magnesian’ limestone. The mineral calcite is normally white or gray but impurities either within or between the calcite particles can make a limestone brown, yellow, bluish gray, pink, red, green, gray, or even black. Calcite has a specific gravity of 2.710 and hardness of 3 on the Mohs scale. It breaks readily into small blocks. The texture of limestone is clastic or non-clastic. If it is clastic or bioclastic then grains and / or broken or whole shell fragments are visible. If it is non-clastic / chemical then it is crystalline and no clasts are visible. The grain size is variable and can consist of clasts of all sizes. Limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, clay minerals, pyrite, siderite, and other minerals. It can also contain large nodules of chert, pyrite, or siderite. The calcium carbonate content of limestone gives it a property which is frequently used in rock identification. It effervesces in contact with a cold solution of 5 % hydrochloric (HCl) acid.

Calcined lime has a density of around 3.2 tons/cum to3.4 tons/cum, bulk density of around 0.88 tons/cum to 1.2 tons/cum, and pH value in the range of 12-13. Its reacts with in water to form Ca(OH)2 which is hydrated lime. Melting point of calcined lime is around 2570 deg C. Calcium oxide reacts exothermally with water to form calcium hydroxide (CaO + H2O = Ca(OH)2 + 276kcal/kg CaO). Dry hydrated lime is generally produced from high calcium quicklime and contains 72 % to 74 % calcium oxide with 23 % to 24 % chemically combined water.

Limestone chemical purity has a direct effect on the chemical purity of lime which is produced by the calcining of limestone. There are some possibilities for removing the impurities when they are principally located around the stone, but otherwise further processing steps keep the impurities embedded with the finished product. The main impurities in limestone are in the form of clay, silt, or sand, or sometime organic matter. The calcium (Ca) and carbon dioxide (CO2) constitute the calcium carbonate (CaCO3), with impurities including magnesium (Mg), silicon (Si), aluminum (Al), sulphur (S), phosphorus (P), potassium (K), and sodium (Na). A metallurgical grade of limestone needs a minimum of 95 % carbonates (Ca and Mg).

The limestone from the various deposits differs in physical and chemical properties and can be classified according to their chemical composition, texture, and geological formation. Hence, the limestones from different sources also 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 (SiO2), alumina (Al2O3), and iron (Fe) etc. The varying properties of the limestone have a big influence on its use and the processing method. Hence, it is necessary to know comprehensive information of the limestone such as physical and chemical properties before taking a decision on its use and processing.

Limestone is normally extracted from open-air quarries rather than underground mines, because of an outcrop of quality products (Fig 1) in most of the deposits. Nevertheless, it is common that good chemical quality occurs only in certain layers, raising the need to select and separate aggregate types from the chemical stone. Limestone extraction is done by blasting, followed by crushing and screening. Depending on the chemical characteristics, limestone can be washed to remove the surrounding impurities such as clays. Calibrated stones size (Fig 1) of the lime stone depends on its further use. Finer-sized limestone can be used partially as raw material for self-fluxing sinters.

Fig 1 Limestone

Dolomite is a common rock-forming mineral. It is a calcium magnesium carbonate with a chemical composition of CaCO3.MgCO3. It is the primary component of the sedimentary rock known as dolostone and the metamorphic rock known as dolomitic marble. Limestone which contains some dolomite is known as dolomitic limestone. Dolomite theoretically contains 54.35 % CaCO3 and 45.65 % MgCO3 or 30.4 % CaO, 21.9 % MgO and 47.7 % CO2. However, in nature, dolomite is not available in this exact proportion. Hence, in commercial parlance, the rock containing 40 % to 45 % MgCO3 is normally called dolomite. Dolomite rock which contains in addition to dolomite either calcite or a mixture of calcite and magnesite are called ‘dolomitic limestone’. Dolomite deposits are normally associated with limestones. This fact, together with the presence of variable amounts of impurities such as SiO2, Al2O3, S (sulphur) and iron oxides, has an important bearing on the suitability of dolomite for specific applications.

The mineral dolomite (Fig 2), the major constituent of the rock dolomite, also occurs mainly as white or gray crystalline particles, but impurities can impart other colours. Sometimes the crystalline dolomite particles composing dolomites contain a small amount of Fe in the ferrous state which is colourless. As long as the dolomite is protected from the weather, the ferrous iron remains unchanged and has little or no effect on the colour of the stone. However, when the stone is exposed to the weather, the ferrous iron oxidizes or ‘rusts’ and is changed to another compound, a hydrated iron oxide that turns the rock yellow or brown.

Fig 2 Dolomite

The mineral dolomite has hardness on the Mohs scale of 3.5 to 4 and a specific gravity of 2.8 to 2.9. The colour of mineral can be colourless, white, pink, green, gray, brown or black. Its streak is white. Its luster is vitreous or pearly. The diaphaneity can range from transparent to translucent. The cleavage of dolomite can be perfect, rhombohedral, or three directions. Its fracture is conchoidal and tenacity is brittle. Rhombohedral cleavage, powdered form effervesces weakly in dilute HCl acid. Its crystal system is hexagonal.

Calcined dolomite has a density of around 3.5 tons/cum at 20 deg C, bulk density of around 0.85 tons/cum, and pH value in the range of 12-13. Its solubility in water is around 9 %. Melting point of calcined dolomite is around 2800 deg C. Calcium magnesium oxide reacts exothermally with water to form calcium hydroxide (CaO.MgO + H2O = Ca(OH)2 + MgO + 276kcal/kg CaO). MgO also reacts exothermally with water but the reaction is slow.

The mineral impurities in case of limestone and dolomite include (i) other carbonate minerals (e.g. siderite), silica (quartz, chert), clay minerals (kaolinite, illite, and smectite), mineralization (fluorite, galena, and sphalerite), organic matter, and others (pyrite, iron oxides, etc.)

Both limestone and dolomite can be calcined by heating them either in a rotary kiln or in a vertical kiln. During the calcining, the decarbonation reaction takes place. These reactions which take place in the temperature range of 800 deg C to 1,000 deg C in case of limestone and in the temperature range of 700 deg C to 900 deg C in case of dolomite are CaCO3 + heat = CO2 + CaO (high calcium lime), and CaCO3.MgCO3 + heat = 2CO2 + CaO. MgO (calcined dolomite).

Both limestone and dolomite are extensively used in an iron and steel plant in various processes. Specification of limestone and dolomite for iron making is less rigid. But for steelmaking limestone need to have very low SiO2 and Al2O3 since these elements require additional flux to neutralize them which increases the slag volumes. Further additional heat is 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 is required to have good decrepitation index.

These minerals are being used in their natural form or in calcined form. In ironmaking, limestone and dolomite are used in the production of sinter, pellets, and directly in blast furnace as fluxing materials. In case of steelmaking limestone and dolomite are used as lime or calcined dolomite both in the primary steelmaking (basic oxygen furnace, electric arc furnace), and in the secondary steel making processes.

For its use in the ironmaking, the CaCO3 content in limestone is not to be normally less than 90 %. The combined SiO2 and Al2O3 is normally not to exceed 6 % and both the S and P (phosphorus) contents as low as possible. In case of steelmaking, the insolubles in limestone are to be as low as possible but is not to exceed 4 %. Good fluxing limestone is naturally to be low in acid constituents like SiO2, Al2O3, S, and P. Limestone is to be dense, massive, preferably fine-grained, compact and non-fritting on burning.

For many of the  applications in the iron and steel industry there are strict limits on the chemistry of the dolomite used, which mainly needs to be low in SiO2 (frequently less than 1 % or even less than 0.55 %) in addition to low in S (less than 0.1 %) and P (less than 0.02 %).

Specification of limestone and dolomite for iron making is less rigid. But for steelmaking limestone is to have very low silica (SiO2) and alumina (Al2O3) since these elements need additional flux to neutralize them which results into increase in the slag volumes. Further additional heat is needed for keeping fluid this amount of additional slag. Also important is the consistency of chemical composition and size fraction. Further limestone and dolomite used for calcination need to have good decrepitation index.

In iron and steel Industry, limestone and dolomite are used both in ironmaking and steelmaking. Limestone and lime products are used as fluxing material both in ironmaking and in steelmaking. Basically limestone is used as a slag former, while 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 asSiO2, Al2O3 along with S and 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 the S present in the raw materials to form calcium sulphide (CaS) which goes into slag.

Lime addition in the steelmaking is important for protecting the refractories, and maintaining the basicity for assisting in dephosphorization. While oxygen lancing facilitates the removal of carbon from the bath as CO (carbon mono oxide), other elements, including Si, Mn (manganese), and P, also oxidize and are absorbed in the slag layer. Top-blown converters typically use lump lime, while bottom-blown converters can also add pulverized lime through tuyeres. Soft burnt lime having a large specific surface and good reactivity favours dephosphorization and desulphurization. Higher quality lime facilitates a lower consumption of lime and an improvement in productivity during steelmaking.

During pretreatment of hot metal in desulphurization plant lime is an essential component of the desulphurizing compound. During steelmaking high basicity of the steelmaking slag is being maintained with the help of lime. Lime is also used as a desulphurizing agent and as a component of synthetic slag. Limestone powder is one of the most important fertilizers. Hydrated lime (either dry or as a slurry) has a number of miscellaneous applications in the steel plant and further processing of the steel products.  It is normally used in wire drawing, acting as a lubricant as the steel rods or wires are drawn through dies, and in pig casting in which a lime whitewash coating on the moulds prevents sticking.  Lime is used to neutralize the acidic waste pickle liquor in which iron salts are also precipitated.  After pickling, steel products are frequently given a lime bath to neutralize the last traces of the pickling acid adhering to the steel surface. Hydrated lime is also used to provide temporary corrosion protection in the form of a whitewash coating on steel products and to neutralize acidic water in coke oven and by-product plants.

The hydrated lime has several other usages which include water treatment, waste water (effluent) treatment, flue gas treatment, and sludge and sewage 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. Lime is also 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. Lime is widely used to coagulate and to stabilize sludge in the sludge treatment units. Various uses of limestone and lime is shown in Fig 3.

Fig 3 Uses of limestone in iron and steel industry

The long association with the iron and steel industry has made the dolomite mineral as mineral of considerable economic importance. The main uses of dolomite (Fig 4) in ironmaking and steelmaking are (i) as a fluxing material (ii) for protection of refractory lining, and (iii) as a refractory raw material. Dolomite in iron and steel industry is normally used in three forms. These are (i) raw dolomite which is also the natural form of dolomite, (ii) calcined dolomite, and (iii) sintered dolomite.

When dolomite is used as a fluxing material then it is used as either raw dolomite or calcined dolomite. When dolomite is used for the protection of refractories, it is used in calcined form and when dolomite is being used as a refractory raw material, it is used in the form of sintered dolomite.

Fig 4 Uses of dolomite in iron and steel industry

In steelmaking, 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 of various vessels used during steelmaking.