When producing pellets on a conveyor firing machine, heating is performed due to the heat of combustion of natural gas (fuel oil) above the surface of the layer, which leads to uneven heating of the pellets along the height of the layer. The strongest pellets are formed at the top of the layer, and less strong pellets are at the bottom of the layer. Solid fuel in the composition of pellets performs two functions: an energy carrier for the release of heat and the creation of the necessary temperature conditions for the thermal treatment of pellets (internal heat source) and as a component of the charge, necessary for the occurrence of physicochemical processes in an individual pellet and the processed layer. The main objective of using solid fuel is to stabilize the strength properties of pellets along the layer height due to more complete firing of the bottom and middle of the layer, reducing the consumption of natural gas and increasing the productivity of the firing machine. Therefore, the choice of fuel is determined by a combination of many factors. As an additional additive to the batch for pelletizing, solid fuel (fossil coals - hard and anthracite,
peat and coke) is introduced, which is constantly used in the production of pellets from hematite concentrates and serves as a source of additional heat for heating the layer and the physicochemical processes of pellet firing. When using magnetite concentrates, heat sources are practically not used inside the layer, since during the oxidation of iron oxides of magnetite to hematite during heating and firing of pellets, an additional amount
of heat is released necessary for the technological process. When adding solid fuel to the charge and firing pellets from magnetite concentrates, cases of sintering are possible due to local excess heat and an increase in the amount of liquid melt. Firing of coal-containing pellets is carried out at reduced firing temperatures (20-50 C) in order to prevent overheating of the middle and lower parts of the layer. It is necessary to find a balance between the consumption of solid fuel and the firing temperature, at which the top of the layer will be fully fired, and the bottom of the layer will not be subject to local melting.

     To optimize the use of solid fuel when using magnetite concentrates, coke nut was tested as a material with the lowest ash and sulfur content and the highest carbon content. The experiment was based on coke nut with an ash content of no more than 13%, with a moisture content of up to 20.0% and a fraction of 8-25mm. Before feeding into the charge, the solid fuel was crushed together with limestone to a fraction of 86-88% class 0.056mm and a moisture content of 1-3%. Optimum results in compressive strength, drum test and abrasion were obtained by adding 2kg of coke nut per ton of pellets. A slight increase in the productivity of the firing machine by 1-2% was also noted. A further increase in the addition of solid fuel led to a decrease in the strength properties of pellets by 8-15kg/pellet due to an increase in the porosity of the pellet layer and an increase in the FeO content by more than 2.0-5.0% due to the occurrence of a reduction reaction involving coke nut carbon in the process. The use of solid fuel for magnetite concentrates may be appropriate if the pellet strength required is up to 250kg/pellet. The further requirement of the sales market to increase the compressive strength above this value is not commensurate with the use of solid fuel, since an increase in pellet porosity leads to its decrease, which has excluded the widespread use of this technology at present when using magnetite concentrates in the charge.

     The next additive to the batch, along with the use of coconut nuts, was the use of activated peat, which was also used as an intra-layer heat source to equalize the temperature of the ingredient along the layer height. Also, the addition of crushed peat, due to the high water absorption of the material, was used to reduce the moisture content of the batch, which led to a decrease in bentonite consumption. However, since with the use of coconut nuts, an increase in the porosity of the fired pellets when using peat led to a decrease in the compressive strength of the fired pellets of less than 250kg/pellet.

     The use of peat also found its limited application due to the high humidity of the source material of about 50-60%, which complicated the process of its preparation before feeding into the batch. A positive feature of using solid fuel is the reduction in natural gas consumption and a slight increase in the productivity of the firing machine by 1-2%.

    The presence of ash and sulfur in solid fuels must be constantly monitored because they add undesirable elements and oxides to the pellet composition and lead to an increase in the content of sulfur compounds in the exhaust gases emitted into the environment. Therefore, it is desirable to use solid fuels with a high hydrocarbon content, low ash and volatile matter content, as well as sulfur and phosphorus.