Iron ore pellets are produced with a basicity from 0.08 to 1.2units. The basicity modulus is the ratio of the content of basic oxides to acidic ones (CaO+MgO)/(SiO2+AL2O3) and is designated B4. The formula is often simplified, in which case the basicity is determined by the ratio of calcium oxide to silicon oxide CaO/SiO2 and is designated B2. In this regard, the supplier and the consumer may disagree about the basicity values, since they can be determined by different formulas. Pellets are divided into acidic or non-fluxed with a natural basicity of 0.08-0.12units, in which no fluxing additives are added, partially fluxed 0.12-0.5units, fluxed 0.5-0.8units, and highly fluxed 0.8-1.2units. In fact, there is no clear gradation and each consumer determines for himself what type of product pellets of a particular basicity belong to. The strength of roasted unfluxed pellets is mainly due to the hematite bonds of the polycrystalline structure of the pellet. When adding a fluxing additive with a basicity of 0.12 to 0.8units, a slag binder based on CaO, SiO2 and Fe2O3 oxides is added in addition to the hematite bonds. In highly fluxed pellets, in addition to the slag binder, ferrite phases CaO-Fe2O3 appear, participating in the construction of the pellet structure.

   As discussed above, ordinary (CaCO3) or dolomitized limestone (MgCO3) is used for fluxing pellets in the volumes necessary to obtain the specified basicity. Slaked lime Ca(OH)2 is used less often. The fineness of the binders should correspond to the fineness of the concentrate grinding, so that after the decomposition of the limestone, calcium oxide completely reacts with silicon oxide and hematite. The content of free calcium oxide in the fired pellets should be as low as possible, since it comes into contact with atmospheric moisture (hydrates) and causes a decrease in the strength of the pellets. The higher the basicity, the greater the presence of free calcium oxide in the pellets and the more prone the pellets are to destruction. Manufacturers of fluxed and especially highly fluxed pellets must take into account the time of year, storage periods of pellets in the warehouse, duration and type (land/sea) of logistics, as well as the number of reloads when producing pellets of these types. 

     Ground limestone has virtually no effect on the properties of green pellets and partly serves to stabilize the moisture content of the batch fed to pelletizing due to its moisture content of about 0.3-0.6%. Adding limestone and the formation of a slag binder allows for producing strong pellets at significantly lower firing temperatures. So, if to obtain a strength of 250kg/pellet for unfluxed pellets, a firing temperature of 1280-1320 C is required, then for fluxed pellets the firing temperature can be about 1250-1280 C. This is especially true for the production of pellets from concentrates with a silicon oxide content of less than 3.0%. In general, the lower the silicon oxide content in the concentrate, the greater the need to add limestone to form a liquid phase and create a strong structure for the fired pellet. It is desirable to add limestone to unfluxed pellets in the amount of 0.5-1.0%, even at the expense of some increase in basicity by 0.02-0.05units. 

     Naturally, for concentrates with different iron and silicon oxide contents, the same addition of limestone leads to different basicity, but similar results in terms of slag bond formation and effect on strength properties. For example, for magnetite concentrate with an iron content of 68.20%, the optimal basicity is 0.6-0.8units, for a concentrate of 65.20%, the optimal basicity is 0.3-0.5units. The porosity of pellets with the addition of calcium oxide decreases due to the formation of a slag bond to a certain basicity value for a concentrate of 65.20% 0.3-0.5units, then the porosity increases due to the formation of a large number of macropores, the amount of unbound calcium oxide increases, which significantly weakens the granules even during liquid-phase sintering and, as a result, the strength of the pellets decreases with a further increase in basicity.

   The use of dolomitized limestone does not improve the cold strength of pellets, and in certain cases even reduces it somewhat, but can slightly improve the swelling index of pellets. The addition of basic oxides (CaO and MgO) to the batch in all cases leads to an increase in the basicity of pellets and, as a consequence, to a decrease in the iron content in pellets, an example is shown in diagram. For each type of concentrate, taking into account its chemical composition (iron content and the amount of basic and acidic oxides), this dependence is different. With an increase in the iron content in the concentrate, the increase in iron in pellets with different basicity has a different value. With an increase in basicity, the increase in iron in pellets increases (diagram). This dependence on the increase in iron in pellets is also different for different types of concentrates used. These values in the first and second cases can be predicted by calculating the charge for each type of concentrate used and each type of pellets produced. Such an assessment is necessary both in calculating the efficiency of producing a certain type of pellets in the pelletizing process and in assessing the efficiency of using different types of pellets in the blast furnace process during cast iron production.

      At the pelletizing plant, an increase in basicity leads to an increase in operating costs (the productivity of the firing machine decreases, the consumption of electricity and natural gas increases) and a decrease in the iron content of the pellets. At the next stage of cast iron production, an increase in the content of (CaO+MgO) as the main component of slag leads to an increase in the effective blast furnace process. It often happens that it is impractical to produce pellets of a certain type at the pelletizing process, but their use in the blast furnace process leads to positive values of production efficiency. If this is a vertically integrated production, then the end-to-end production efficiency at all process stages is considered. In any case, the priority is given to the efficiency of blast furnace production, the process with higher added value. Logistics of pellets with a higher iron content is more profitable than the transportation of pellets with a high basicity. With different owners of the pellet and cast iron production process, they seek acceptable conditions for both parties, both in terms of the production of a certain type of pellets and in terms of price categories, taking into account surcharges for iron content and basicity modulus.