Limestones are used in various stages of metallurgical production. Depending on the magnesium oxide content, they are divided into ordinary fluxing and dolomitized fluxing. Depending on the purpose, limestones are used for the production of pellets, blast furnace production, open-hearth and converter production. They differ in granulometric and chemical composition, compressive strength and water absorption. In the production of pellets, limestone is a fluxing additive. Ordinary or dolomitized limestone is traditionally used to produce pellets with varying degrees of fluxing. The dissociation of limestone during the firing of pellets is an endothermic reaction and occurs with the expenditure of heat for its decomposition. Limestone also has virtually no binding effect, but in some cases its significant addition to the batch (in the production of highly fluxed pellets) allows for the pelletizing process to be carried out with increased moisture content in the concentrate or to reduce bentonite consumption somewhat. It is supplied raw in wagon loads with a moisture content of no more than 3% and a size of 3-25mm in summer and 20-60mm in winter.

     The need for pellet fluxing is one of the blast furnace operators' requirements. The addition of CaO corrects the pellet acidity, giving it the necessary physical and metallurgical properties suitable for use in cast iron production. The introduction of calcium and magnesium containing materials into the charge is necessary to obtain a slag of a certain composition and physical properties (melting point, viscosity, fluidity), which is capable of removing sulfur, phosphorus, and other harmful impurities. Feeding raw limestone into the blast furnace leads to overconsumption of coke, which is more expensive than natural gas, for decomposition of limestone. Therefore, highly fluxed agglomerates with a basicity modulus of 1.2-1.8units and fluxed pellets with a basicity modulus of 0.3-1.0units are used as non-metallic blast furnace charge in order to reduce the feed of raw limestone. In principle, blast furnace operators find a balance between the supply of non-metallic materials of varying degrees of fluxing and the required amount of iron introduced into the iron ore charge for optimal operation of the blast furnace.

       For the production of iron ore pellets of varying degrees of fluxing, various types of limestone are used as a fluxing additive. The formula of ordinary limestone is CaCO3. Dolomitized limestone MgCO3 is characterized by the presence of an increased content of magnesium oxide MgO 5-9.0% in the structure of the material. Less commonly used is dolomite CaCO3·MgCO3 with an MgO content of 9-21%. Magnesite and dunite are also
sometimes used as a source of MgO (over 40%). The use of the first additive is limited by its availability and reserves, the second additive contains a significant amount of SIO2.

    When selecting limestone for pelletizing, the following characteristics are considered - the content of basic oxides (CaO+MgO), moisture content, grain size distribution, content of silicon oxide (SiO2), sulfur (S), phosphorus (P), insoluble residue. In some cases, strength and water absorption criteria are additionally used. These indicators are important, since high water absorption and low strength often lead to the destruction of limestone pieces during transportation, storage and worsen the process of its processing, since the material becomes heterogeneous with an admixture of small wet fractions. The main criterion for choosing limestone for pelletizing is the maximum content of basic oxides (CaO+MgO), this is especially relevant for highly fluxed pellets, since it allows reducing its addition to the batch while maintaining the basicity modulus of the pellets produced.

      In order to assess the applicability of limestone for each metallurgical process (agglomeration, pelletizing, blast furnace, steelmaking, lime kilns), a rating of limestones is compiled, in which a list of specific quality parameters of limestone required for inclusion in the rating calculation is determined for each metallurgical process. The following parameters are taken into account as key parameters when calculating the limestone rating, depending on its grade and method of use in metallurgical production:

         a. mass fraction of (CaO+MgO), %;
         b. mass fraction of insoluble residue (IR), %; for open-hearth and converter production it is taken equal to the mass                          fraction of (SiO2);
         c. moisture content (W), %;
         d. compressive strength (CS), MPa;

     The remaining limestone parameters (mass fractions: MgO, S, P, water absorption, granulometric composition) must comply with the requirements of the technological process.

        Before being fed into the batch, the limestone undergoes the technological stages of crushing (when using a large fraction) and grinding with drying in ventilated ball mills or similar units. When dosing into the batch, the prepared limestone has a moisture content of 0.3-0.7%, the content of class 0.056mm is 84-90%. The insignificant moisture content in ground limestone allows using a concentrate with a higher moisture content in the production of fluxed pellets than in the production of unfluxed pellets. The higher the basicity modulus, the higher the moisture content of the concentrate that can be fed into the batch for the production of green pellets. Thus, the difference in moisture content in the concentrate between the production of pellets with a basicity modulus of 0.12units and a basicity modulus of 1.0units is can be 0.4-0.5% (abs.).