To increase the mechanical strength of pellets during firing, boroncontaining materials are used. In particular, a boron - containing additive, calcined colemanite, has found its application. Colemanite has a chemical formula (Ca2B6O11 5H2O) with a melting point of 986 C and forms borosilicate slag bonds at relatively low temperatures (about 1000 C) in the presence of silica iron ore concentrate. Since the strength of pellets largely depends on the presence of a slag bond during pellet hardening, the mechanism of influence on the improvement of the quality characteristics of pellets is the shift of the onset of liquidphase hardening to the region of lower temperatures. 

      As noted earlier, bentonite is the most common binder used in iron ore pelletizing. However, it is considered an impurity due to its high content of SiO2 and Al2O3 oxides. In recent years, numerous alternative organic binders that do not contain impurities have been tested in order to completely replace bentonite or reduce its dosage in the batch. Organic binders produce green and dry granules of good quality. However, they do not provide sufficient mechanical strength to preheated and calcined pellets due to insufficient slag binding.

      Addition of boron compounds to the batch is proposed as a potential solution to this problem in order to overcome the insufficient strength of pellets made with organic binders or to increase the strength when using bentonite and calcined colemanite together. The addition of calcined colemanite increases the strength of green pellets. The improvement in the strength properties of green pellets with calcined colemanite is due to the clayey nature of calcined colemanite and to the fact that during the pelletizing of
the batch, colemanite is hydrated by the moisture of the batch, turning into the sassolite phase (boric acid B2O3 H2O). Fragments of B2O3 crystals turn into intergrowths of needleshaped sassolite crystals penetrating the body of the green granulated pellet. During the drying of pellets, sassolite at a temperature of 180-190 C again loses water, turning into boron oxide. The first borate melt appears at a temperature of, 450 C as a result of melting of boron oxide. The appearance of local volumes of borate solution leads to the development of the liquid-phase strengthening process. In the temperature range of 600-1000 C, the high-borate melt promotes sintering and recrystallization of ore grains in unfluxed pellets and ore and ferrite grains in fluxed pellets. This is especially important for unfluxed pellets containing refractory minerals of host rocks in the composition of the batch, which cannot form liquid-phase melts. At a temperature of 1000-1200 C, along with the primary melt, a silicate melt is formed due to the melting of silicon-containing phases of the iron ore concentrate. Both melts combine, absorbing the flux of the batch, the fine fraction of the concentrate and solid-phase ferrites. The more boron in the melt, the more intense the strengthening and recrystallization of the ore phase, which leads to an increase in the cold strength of the pellets compared to borate-free pellets.

     The main reason for the increase in the compressive strength of roasted pellets associated with calcined colemanite is the physical melting of calcined colemanite at the point of contact of iron oxide grains during heat treatment. The increase in the strength of roasted pellets is 13-30kg/pellet with a dosage of calcined calcium colemanite in the charge from 3kg/ton to 5kg/ton of concentrate, the yield of suitable pellets also increases and the number of pellets with a strength of less than 210kg/pellet decreases. 

     The use of colemanite in such dosages increases the swelling index of pellets from 12% to 17%, but it is within acceptable values for blast furnace operation - no more than 20%. The use of boron oxide increases the pellet strength during reduction due to an increase in the amount of silicate binder, which reduces the formation of cracks in the pellet body, but at the same time reduces the pellet reducibility due to reduced access of the reducing gas to the ore grains. The optimal content of boron oxide is within 0.3-0.5%, which allows increasing the pellet strength during reduction while maintaining sufficient pellet reducibility. Colemanite calcined at a temperature of 500-550 C after removing chemically bound water is suitable for pelletizing. 

     The addition of colemanite during pelletizing is practically not used due to its cost and the need for an additional calcination operation; in any case, it is necessary to calculate the economic feasibility of its use, taking into account the increase in the cost of pellets and the increase in the strength properties of pellets.

         The purposes of using binders and various additives are:
        a. obtaining high-quality raw pellets (compressive strength, drop strength, particle size distribution, shape, type);
        b. improving the strength of dry pellets at subsequent stages of heat treatment;
        c. improving the physical and chemical properties of fired pellets;
       d. binders should be relatively inexpensive and not have a negative impact on the chemical composition of pellets and the                operation of the firing machine;

    All effects on the initial charge materials and raw pellets should be aimed at optimizing bentonite consumption and improving the physicochemical properties of pellets.