Iron Ore Sinter
Iron Ore Sinter
Iron ore sinter or simply called sinter is usually the major component of a blast furnace iron bearing burden material. Sinter normally consists of various mineral phases produced by sintering of iron ore fines with fluxes, metallurgical wastes and a solid fuel. Coke breeze is normally used as fuel in the sinter mix since it supplies necessary heat energy for sintering of sinter mix. Fig 1 shows a piece of sinter.
Fig 1 A piece of sinter
In sintering, a shallow bed of fine particles is agglomerated by heat exchange and partial fusion of the still mass. Heat is generated by combustion of coke breeze admixed with the bed of iron ore fines, fluxes, and metallurgical wastes (sinter mix) being agglomerated. The combustion is initiated by igniting the fuel exposed at the surface of the bed, after which a narrow, high temperature zone is caused to move through the bed by an induced draft applied at the bottom of the bed. Within this narrow zone, the surfaces of adjacent particles reach fusion temperature, and gangue constituents form a semi liquid slag. The bonding is affected by a combination of fusion, grain growth and slag liquidation. The generation of volatiles from the fuel and flux materials creates a frothy condition and the incoming air quenches and solidifies the rear edge of the advancing fusion zone. The product sinter consists of a cellular mass of sinter mix materials bonded in a slag matrix.
Important factors that affect the granulation efficiency and permeability of the sinter mix are water addition, particle size distribution, ore porosity, surface properties of the iron ore and the wettability of the iron ore. During sintering process, coke breeze increases the temperature of the sinter mix within the sinter bed to achieve partial fusion and diffusion bonding and on cooling the different mineral phases crystallizes and bond the structure together to form strong sinter. Airflow rate and flame front speed in sintering process guides the performance of the sinter process and this parameter mainly depends on the sinter bed permeability. There are other parameters which influence melt formation and subsequent solidification of mineral phases are of great importance in sinter making. The permeability of the sinter bed controls the productivity of the sinter process, as well as the microstructure and other properties of the sinter.
Many reactions may take place during the sintering process. Equilibrium phase relations are however not reached during sintering, due to the flame front that rapidly passes through the sinter bed. This results in the high degree of heterogeneity of the sinter, and the formation of non equilibrium phases that are not expected from thermodynamic considerations.
The composition of the sinter therefore varies from place to place in the bulk material, depending on the nature of the individual ore, flux, and metallurgical waste particles and the extent of reactions between them.
Microscopically it consists of bonding phases, original unaltered ore particles, remaining glassy phases and very small non uniform pores and cracks. Depending on different parameters such as temperature, composition, oxygen partial pressure, time and atmosphere, different phases form in different proportions, while different morphologies develop. The morphology essentially reflects the mode of formation and is related to a particular chemical composition, heating and cooling rate of the sinter. Some of the common minerals and phases present in sinter are hematite (Fe2O3), magnetite (Fe3O4), magnesioferrite (Mg,Fe)3O4, silicoferrite of calcium and alumina (SFCA), with stoichiometries (M14+6nO20+8n and M25O36, where n=0 and 1 and M= Ca, Fe, Al, Si), anorthite (CaAl2Si2O8), calcium diferrite (CaFe4O7), dicalcium ferrite (Ca2(Fe,Al)2O5), dicalcium silicate (Ca2SiO4), SiO2-rich glass, free lime, periclase (MgO) and olivine (Mg,Fe)2SiO4.
Macroscopically sinter has a non uniform structure with large irregular pores. The structure of sinter consists of pores (of varying sizes) and a complex aggregate of phases, each with different properties. It is the combination of these pores and phases, and the interaction between them that determines the sinter properties, but also makes the prediction of sinter properties difficult.
A sinter is regarded as consisting of essentially three types of materials namely (i) original unaltered (primary) material, (ii) original secondary material which is the result of alteration of the structure and shape through recrystallization in the solid state, and (iii) secondary constituents which result from material that has fused or dissolved during sintering. These constituents may either mutually dissolved or may precipitate from the solution.
Two types of bonding are theoretically possible depending on the mineralogical changes. These bonding are as given below.
- Slag or fusion bond – It consists of partial or complete embedding of crystalline constituents in the matrix of a fused glassy melt, the extent depending upon the volume and wettability of the liquid phase. The bond strength depends upon the amount of glass and the amounts and types of the constituents. These depend upon the fuel rate and impurities SiO2, CaO2, CaO, MgO and Al2O3, added or present.
- Diffusion bond – This bond is due to the recrystallization and crystal growth of hematite. Hematite diffusion plays an important role especially above 1250-1300 deg C because of its relatively high surface mobility at this temperature. Such bonds are, however, obtained in Fe rich low silica ores, since the impurities in lean ores form low melting liquids before the ‘diffusion’ temperatures are reached. In addition, very close packing and good contact of particles is necessary for diffusion to operate effectively. The limits of surface mobility are such that a loosely packed aggregate is not expected to acquire a high strength by diffusion bonding alone. Diffusion bonded sinter is more porous, accessible to reducing gases and hence easily reducible.
The nature of the bonding which affects agglomeration is of interest in that this bonding affects the economics of production, transportation, storage, handling and ultimately, the reduction in the blast furnace.
Sinter strength is strongly influenced by the textural relationship between slag glass, slag crystals and oxide phase. If the slag glass covers the oxide particle, the sinter is weakened, whereas, if the slag glass is confined to interstitial bridges to oxide particles, the texture is relatively stronger.
The produced sinter is normally tested for its mineralogy, reducibility index (RI), reduction disintegration index (RDI), tumbler index (TI), and abrasion index (AI).
Mineralogy of sinter
The mineralogical analysis involves the quantification of phases of different morphologies by point counting. During point counting at least hundred points per sample are counted. Each sample is usually counted three times, and average values are reported.
Although the mineralogy of a sinter is principally dependent upon the chemistry and the mineral character of the sinter mix and the maximum process temperature to which the sinter mix was exposed, such factors as time above the solidification temperature, cooling rate, and the sintering atmosphere affect the final mineralogy.
Sinters of low basicity (Cao/SiO2 < 0.5) are generally characterized by grains of primary and secondary hematite and of magnetite bonding by a slag phase. The slag phase is primarily a silicate glass (combinations of FeO, CaO, Al2O3 and SiO2), but contains fayalite (2FeO.SiO2), wollastonite (CaO.SiO2), iron monticellite (CaO.FeO.SiO2) and anorthite (CaO.Al2O3.2SiO2). The formation of fayalite is promoted by high fuel content in the sinter mix. Fayalite is undesirable phase since it adversely affects the reducibility of sinter. If the proportion of the hematite is decreased then the phase rankinite (3CaO.2SiO2) is more likely to be encountered. Some monocalcium ferrite (CaO.Fe2O3) may be encountered, localized around the pores of the sinter and scattered throughout, often growing as a fringe around magnetite.
As the sinter basicity (CaO/SiO2) is increased beyond 1.0, devitrification becomes an important attribute of the glass. A devitrification of the silicate glass bond occurs at basicity levels between 1.0 and 1.2 with the separation of crystallites of dicalcium silicate (2CaO. SiO2). At this level of basicity appearance of a calcium ferrite constituent occurs.
As the basicity is increased to 1.8, the amount of silicate glass decreases, and increasing amounts of dicalcium silicate and calcium ferrites appears. Above basicity of 1.8, the amount of calcium ferrites in the bond is increased, and the proportion of dicalcium silicates decreases to dilution.
In short there is dependence of sinter strength and reducibility on the sinter mineralogy and mineralogical bonding. It is the proper mineralogical assemblage that enables a sinter to withstand degradation during transit, storage, and descent within the blast furnace, yet gives the reduction and structural properties necessary for rapid reduction at the blast furnace. Within the sinter plant, proper mineralogy influences fuel consumption and amount of returns.
Sinter quality is defined as a combination of its cold strength, its degradation during reduction at low temperatures, its reducibility and its high temperature properties that are related to the temperatures at which the sinter starts to soften, melt and drip during reduction at temperatures above 1150 deg C. All of these properties, which are normally evaluated according to standardized tests, are strongly related to the mineralogy, microscopic and macroscopic structure of the sinter. The reproducibility of these tests that are performed on sinter particles to evaluate their quality is therefore low in comparison with that of synthetic samples due to the high degree of variability in phase composition between sinter particles, even when these sinter particles are obtained from the same bulk material.
The good quality of sinter is important for smooth blast furnace operation. The important aspects of sinter quality are described below.
- Size consistency – Consistency in sinter size has a significant effect on blast furnace performance. There is no universally recognized optimum sinter size, but it is generally accepted that fines are detrimental to furnace operation. Fine material lowers blast furnace stack permeability, increases dust losses, and may lower the maximum permissible blast temperature for smooth furnace operation. Sinter that is too coarse is also undesirable, particularly if its reducibility is low and it is poor in strength, thus undergoing physical degradation during furnace processing.
- Sinter strength – During transportation and charging of sinter into the blast furnace breakdown of the sinter must be minimized. This breakdown is related to its cold strength. Strength is of prime importance in assessing the sinter quality, and often it is the most important single index for sinter quality. Several tests have been developed for determining the strength. Virtually all the tests measure the strength of cold sinter. Hot strength of sinter is important but reliable testing method for the determination of hot strength is not available. The most common testing methods for assessing the strength of cold sinter may be grouped into three categories. These are drop or shatter test, impact test, and tumble test or abrasion test. The standard tests universally acceptable for the determination of sinter strength are as per the ISO specifications.
- Reducibility – The reducibility and reducibility index (RI) depends on the ease with which the reduction gases can penetrate the sinter. Reducibility of the sinter therefore depends on the porosity and surface structure, the intrinsic reducibility of the minerals and their assemblages, and the additional surface area generated during reduction as a result of the inherent volumetric changes that take place. Open pores are a measure of the surface available for gas solid contact. Sinter mineralogy has dominant effect on reducibility. Low FeO content has long been used as an index of good reducibility, since FeO reacts with silica to form the difficult to reduce phase fayalite. Fayalite formation can be reduced by the addition of lime, which combines the silica as crystalline of lime and lime-iron, and as non crystalline silicate glasses. Highly oxidized sinters are generally easier to reduce, especially if a large portion of the hematite has been formed by reoxidation of magnetite.
- Reduction degradation index – In the upper part of the blast furnace shaft, the permeability of the burden may be influenced by the breakdown of sinter upon reduction. The reduction disintegration index (RDI) is defined as a quantitative measure of the disintegration of the sinter that could occur in the upper part of the blast furnace after some reduction. Sinter with a high degree of reduction disintegration generates fines in the top of the furnace which affects the flow distribution within the blast furnace.
- Chemistry – The best chemical criterion for sinter is a maximum iron content and minimum gangue content with suitable basicity, consistent with acceptable strength, reducibility, and blast furnace performance. The iron content is dependent almost entirely upon the iron ores fines which are available for the sinter making. Silica alumina ratio in the sinter is to be less than 0.5. The lower it is better. The ratio between CaO and SiO2 determines the sinter basicity. The sinter basicity level should be such that it eliminates direct flux charging in the blast furnace. MgO in the sinter is to be maintained to have proper MgO in the BF slag for making the slag fluid. For high alumina iron ores, MgO in BF slag is to be maintained in the range of 8 to 10 %. FeO in the sinter must be brought down if does not affect the strength of the sinter. Generally it is seen that FeO content of around 8 % is necessary in sinter for maintaining proper strength of sinter.