Zonal structure of pellets

    One of the reasons for the decrease in pellet strength after firing is the formation of zonal pellets, which are destroyed during transportation to the consumer and subsequent use in the blast furnace during reduction. One of the main reasons for the appearance of zonal pellets is the incomplete oxidation of magnetite to hematite in the entire pellet volume due to a violation of the temperaturetime treatment regime and its inconsistency with the processes of converting the mineral components of the charge into the required mineral components of the pellets.

   Oxidative firing of non-fluxed magnetite concentrate pellets with refractory host rock (minerals such as quartz, chlorite, serpentine with a melting point above the oxidative firing temperature of 1350 C) occurs only due to solid-phase sintering based on the reactions of interaction of ore phases and silicates without the formation of intermediate reaction products. The zonal form of such a pellet is characterized by a sufficiently strong hematite shell obtained as a result of magnetite oxidation and an unoxidized core consisting of magnetite grains, which have lower strength due to the lack of their complete sintering. Strengthening in the pellet volume occurs only due to ore binders. During reduction, these pellets are destroyed due to the loss of ore binders due to disruption of grain contact in the hematite and magnetite zone. The presence of low-melting minerals of host rocks in the iron ore concentrate leads to the appearance of a primary low-melting solution throughout the pellet volume at early stages and, with an increase in the firing temperature, the involvement of silicates in the melt.

  Liquid-phase strengthening occurs simultaneously in the hematite and magnetite zones, but has a different nature of formation. In the peripheral zone, hematite is practically insoluble in the silicate melt, so the melt mainly consists of transferred silicate phases of the concentrate. The melt has a high viscosity and partially wets and strengthens small particles of hematite, as a result of which the peripheral zone during cooling consists of hematite and glass, Inside the pellet, in the magnetite core, the primary melt, also formed from silicate minerals of the concentrate that have passed into it, absorbs small particles of magnetite with an increase in the firing temperature, increasing the FeO content in the melt. The presence of divalent iron oxide leads to a decrease in the viscosity of the melt and an increase in its mobility, due to which additional iron particles are drawn into it. The resulting amount of melt allows blocking the magnetite core from the hematite periphery and prevents the oxidation of magnetite to hematite. When the pellet is cooled, the hematite zone with a smaller amount of more refractory melt has virtually no shrinkage, while the magnetite zone inside the pellet has a larger amount of mobile silicate melt, which leads to significant shrinkage. Due to the different shrinkage of the hematite periphery and the central magnetite part of the pellet, concentric cracks appear at the boundary of the formation of the two zones, reaching the hematite outer part and forming a separation zone of the "nut shell" type, which subsequently breaks down into small pieces and dust during transportation and recovery. The magnetite core in most cases is a solid monolithic part and does not break down. Depending on the conditions of temperature-time treatment and oxidation processes of pellets on the firing machine, the peripheral hematite zone and the magnetite core can have different thicknesses along the perimeter of the pellet. Pellets with diameters over 12.5mm are especially susceptible to such zonal division. The larger the pellet diameter, the greater the probability of forming a two-zone pellet structure. Therefore, when producing green pellets and placing them on a firing machine, it is advisable to obtain pellets with a maximum content of class 10-12.5mm. This class is an ideal component for blast furnace processing. For metallization units, it is advisable to feed pellets with 50% of class 10-12.5mm and 50% of class 12.5-16mm. The main reason for feeding pellets of a larger class is to reduce granule sticking (by minimizing the contact area available for sticking), increase the gas permeability of the layer (pressure drop) and increase the yield of good products (since a smaller size range will contain more granules in the size of 6.3-10mm, which will form particles with a content of class -6.3mm during transportation and processing in the furnace). In both the first and second cases, the content of class more than +16mm is not recommended.

     If the strengthening processes during pellet firing were limited to solid-phase sintering only, then, regardless of the presence or absence of zonal separation of pellets during recovery treatment, the pellet is destroyed due to the loss of contact between ore grains.

    During the firing of fluxed pellets, the influence of silicates of the host rocks on obtaining the zonal structure is of lesser importance, since the process of hardening the batch is accompanied by the formation of calcium ferrites. In the obtained zonal pellets, at the stage of solid-phase sintering, in the outer peripheral zone oxidized to hematite, calcium ferrites are formed as a result of the reaction of the obtained hematite and flux (table). In the magnetite core, only the reaction of limestone decomposition occurs. The melt appears in the peripheral hematite shell at a temperature of 1100-1250 C with the simultaneous melting of low-melting silicates and calcium ferrites, and the absorption of large fractions of silicates and ferrite masses by the solution occurs. An iron-silicate melt is formed, in which iron is in the trivalent form. With increasing firing temperature, the iron silicate melts at the boundary of the two zones begins to dissolve the fine fraction of magnetite in the pellet core, with the accumulation of divalent iron. A certain amount of melt penetrates into the pellet core, absorbing flux and silicates of the enclosing rocks of the magnetite core, increasing in volume and isolating the internal magnetite core from the oxidizing gas phase. At the boundary of the peripheral hematite phase and the internal core, an intermediate coarse-crystalline zone of hematite is formed from the melt, the crystals of which are 50-70 times larger than the sizes of hematite crystals in the outer zone. In the cold state, the intermediate zone has the lowest strength and is the boundary of crack formation and peeling, leading to the separation of pellets into two parts of the magnetite core and the outer hematite part in the form of a shell. As in non-fluxed pellets, in fluxed pellets the magnetite core has greater strength and lower recoverability than the outer hematite zone.

   If a large number of two-zone pellets are formed during the firing processб it is very important to know the thermal properties of the minerals that are the sources of the ironsilicate melt and to organize the temperature-time and oxidative regime of heating and firing so that magnetite is maximally converted into hematite in the entire volume of pellets before the melting of these silicates begins. 

     High-quality pellets must have a certain composition and microstructure. In real production conditions, the composition of roasted iron ore pellets is represented by two groups of phases: ore-containing ones, which contain iron and act as a binder under certain conditions, and silicon-containing ones, which provide the strength properties of the finished product. As noted above, iron ore pellets containing ore and silicate binders simultaneously have strength properties in the cold state. Both binders must be in a certain balance, allowing for maximum pellet strength in the cold state. An increase in the amount of silicate phase (for example, when conducting the firing process and holding at temperatures above the optimum) leads to an increase in silicate contacts and, as a consequence, a decrease in ore contacts, leading to a decrease in the cold strength of pellets. Pellets with an ore and silicate binder have high strength during reduction (hot strength), which is not destroyed during reduction-heat treatment until the formation of a metal frame by ore phases during the reduction process. After this, the silicate binder is destroyed and melts. The strongest during reduction is a composition of three phases, for example, hematite, ferrite phase and glass. The sequence of loss of strength by these phases during reduction ensures the hot strength of pellets throughout the reduction process until melting begins. At the initial stage of reduction, when contacts between grains of the ore phase are destroyed, the sources of pellet strength are ferrite and silicate phases. At the next stage of reduction, ferrite phases are destroyed, and only silicate phases are the carrier of pellet strength until the formation of a metal frame from ore and ferrite phases, only after this should the silicate melt and separate. As cold strength increases, hot strength also increases (diagram).

      Along with cold and hot strength, the pellet must have high reducibility, which is ensured by mineral compositions where most of the iron is in the oxide or ferrite form. The transition of iron into silicates reduces the reducibility of pellets. Reducibility strength and reducibility are two mutually related indicators. An increase in the cold and hot strength of raw materials leads to a decrease in pellet reducibility. In any case, it is necessary to seek a balance between these two indicators. An increase in strength and a decrease in reducibility will lead to additional costs for increasing coke consumption, and a decrease in pellet strength to increase reducibility can lead to the destruction of pellets and the formation of fines in the upper part of the blast furnace, disruption of the technological process of iron production and a decrease in the productivity of the unit as a whole. In any case, an optimal pellet in composition should have at least two mineral phases that differ from each other in the rate of reduction, softening and melting points. This condition is necessary so that, at the moment of recovery and weakening of the ore phase, the second phase-bundle is the carrier of strength. Therefore, when choosing the production of a certain type of pellets, it is necessary to take into account the mineralogical, phase and chemical composition of the initial raw material, the planned basicity and quality characteristics of the finished product.

     Figure shows an example of the microstructure of two types of pellets with a basicity modulus of 0.12 and 0.27units taken from two samples for testing low-temperature strength during reduction. Pellets of both samples are characterized by the presence of a hematite structure, a glassy quartz-silicate bond between hematite grains, inclusions of magnetite not oxidized to hematite, and the appearance of calcium ferrites in the pellet sample obtained with the addition of limestone. In pellets with a basicity of 0.27units, the hematite structure is more uniform (there is no pronounced character of grains with sharp edges). The results of studies of the low-temperature strength of both samples showed a higher value of the LTD+6.3mm index in the sample with a pellet basicity of 0.27units – 85.0%, in the sample with pellet basicity of 0.12units – 73.0%. That is, changing the microstructure of pellets with the addition of limestone in a certain proportion and increasing the basicity leads to an increase in the LTD indicator. This is especially important in the production of pellets from high-grade magnetite concentrates with a low silicon oxide content.

    However, to date, determining the quality indicators of roasted pellets by microstructural and phase changes cannot be recommended for general industrial use, since there are no methods regulating certain microstructure indicators and the influence of their values on the value of a particular quality characteristic. This is due to the fact that in the volume of the test sample, the pellets do not have homogeneity in microstructure and phase composition, and for a certain assessment of the quality of the entire sample, it is necessary to conduct microstructural studies of all pellet samples included in this sample, which is a very costly and time-consuming undertaking. These studies are conducted in research laboratories during the development of new or adjustment of existing technologies and production as technological recommendations.