Deposit formation in the grate-kiln process

Abstract: Deposit formation in the grate-kiln process is a challenge for the iron ore pellet production industry. The deposits cause disturbances that affect the production capacity of the pelletizing plant. To prevent or mitigate these occurrences, it is important to understand the deposit formation mechanism during the process, which is the overall goal of this work. The results from this work can be used to enhance the understanding of deposit formation in the iron ore pelletizing industry.In this work, particle and deposit formations were studied both in a full-scale grate-kiln plant (40 MW) and in a pilot-scale pulverised coal-fired furnace (400 kW). The sampled particles and deposits were characterized with scanning electron microscopy equipped with energy dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), laser diffraction (CILAS) and X-ray fluorescence (XRF).In the first part of this work, the initiating step in deposit formation— i.e. particle formation mechanisms— was investigated. Particles were sampled from the transfer chute in a full-scale grate-kiln production plant during combustion of oil and coal in separate firings. The results showed that particles in the flue gas consisted principally of fragments from iron ore pellets and minor ashes from heavy fuel oil and pulverised coal combustion. Three categories of particle modes were identified: (1) a submicron mode consisting of condensed products from vaporized species that had relatively high concentrations of Na and K for both combustion cases, with high concentrations of Cl and S during heavy fuel oil combustion, and high concentrations of Si, Fe and minor P, Ca and Al during coal combustion (2) a first fragmentation mode consisting of both iron ore pellet fines and fly ash particles with a significant amount of Fe (>65 wt %) for both combustion cases, with high concentrations of Ca and Si during heavy fuel oil combustion and high concentrations of Si and Al during coal combustion (3) a second fragmentation mode consisting almost entirely of coarse iron ore pellet fines, predominantly Fe (~90 wt %). The particles in the flue gas were dominated by iron ore fines within the second fragmentation mode, which contributed >96 wt % of the total mass of collected particles.In the second part of this work, short-term deposits were collected at the same location in the grate-kiln as the collection of particles. They were characterized by their chemical composition and microstructure in order to obtain information about the deposit formation. Deposit sampling was carried out during separate combustion firings of oil and coal. A significant difference in the deposition behaviour was observed: deposition during oil firing was negligible compared with coal firing. The deposits from coal firing were mainly fine-grained iron oxide particles embedded in a molten (bonding) phase that comprised mainly of Si, Al, Fe, Ca and O. Moreover, it was found that the prevailing flue gas direction determines the formation of the deposits on the probe and that inertial impaction controls the deposition rate. However, this rate can also be affected significantly by the amount of entrained particles that were present in the kiln.In the third part of this work, two different coals were combusted both in a full-scale grate-kiln plant and in a pilot-scale pulverized coal-fired furnace (ECF). The ECF is designed as a scaled-down grate-kiln for combustion testing. Particle and short-term deposit samplings were carried out in both appliances. Dust originating from iron ore pellets was only present in the grate-kiln as there was no flow of iron ore pellets in the ECF. The results showed that Na, K and Cl contents in submicron mode were higher in the grate-kiln than in the ECF, due to alkali circulation in the grate-kiln. The coarse mode particles (2.6-4.2 μm) sampled from the grate-kiln contained significantly more Fe, which originated from the iron ore pellets. The presence of coarse particles (>6 μm) was substantial (>96 wt % of the total particle mass) in the grate-kiln but insignificant in the ECF. The short-term deposits from the grate-kiln consisted of a variety of particles from both iron ore pellets and coal ash particles embedded in an iron-rich silicate molten phase. Short-term deposits from the grate-kiln were harder and denser compared to the shortterm deposits from the ECF. Short-term deposits from the ECF were porous and consisted of coal ash particles embedded in a silicate molten phase. The molten phase in short-term deposits from the gratekiln had a higher Fe content and a higher CaO/(SiO2+Al2O3) ratio than the molten phase from the ECF short-term deposits. Thermochemical calculations showed that the molten phase in the short-term deposits from the grate-kiln had a lower viscosity compared to the molten phase in short-term deposits from the ECF.