The properties of a suite of experimental cokes made from Australian coals were studied. Image analysis was used to quantify the ccke pore and pore wall microstructure, while ultra-micro indentation characterised the strength of the coke wall textures. The micro-properties were compared with compact tension measurements of bulk coke strength. The microstructural features of good cokes included a relatively thick coke wall, and a pore size distribution that has small mean pore size combined with a high pore density. It was found that inert maceral derived coke microtextures displayed the highest hardness values. The hardness of reactive maceral derived coke was lower and decreased with increasing mosaic size. For the set of cokes studied, the hardness and modulus were comparable for the same coke microtexture, irrespective of the coal source. Fracture toughness could not be determined for the coarser mosaic textures using the crack measurement approach. For those textures where measurements were possible, fracture toughness for the components was found to be independent of the parent coal.

Productivity during iron ore sintering is a strong function of the gas flow velocity because the descent of the flame front, for a particular ore mix, is primarily dependent on the rate of convective heat transfer.1 Gas velocity is controlled by the suction applied across the bed and the bed properties. According to the Ergun equation, key properties that determine the gas flow across a packed bed of a given height are the bed voidage, the effective or mean diameter of the particles present and particle shape.2 These important bed properties change according to their proximity to the flame front. In the lower bed, away from the front, granules—each composed of particles held together by water bridges—increase in moisture content because of water condensation from the flowing gases.3 At a later stage, closer to the front, the granule temperature rises and all condensed water and water bridges evaporate. On reaching calcination temperatures the particles forming the granules can change considerably in size because of loss in mass and/or cracking.4 Any loss in intra-granule bond strength, leading to the disintegration of granules and particle dislodgment, will result in decreases in granule size distribution and bed voidage. The structure of a bed during sintering is obviously important as it determines bed permeability.3 A full understanding of the structural changes that occur during sintering can only come about through understanding of the effects of condensed water and increasing temperature on granule strength, size distribution and shape and the effect of these parameters on bed packing and voidage. This is, however, a very complex topic and significant time is required to study the contribution of each variable separately. The aim of the present study is to obtain an indirect, bulk measure that is indicative of bed behaviour at the various stages of sintering. It is to be noted that although the dried, calcined and humidified zones may not be very thick, severe degradation of granules could lead to one of these zones controlling the overall bed permeability.

SynopsisIn iron ore sintering the resistance of the bed to air flow is important because it influences machine productivity. It is clear from theory that the high-temperature zone of a sintering bed is the major resistance to air flow and, therefore, the major contributor to the pressure drop across the bed. Hitherto, however, no technique for measuring this resistance has been available so that the factors that control it can be studied. An experimental technique based on the Ergun equation is proposed to measure this resistance, which is expressed as the reduction in air flow after ignition of the bed. Seven ore blends were studied and the results showed that the resistance to air flow is dependent on ore blend composition and the pre-ignition air flow through the bed. The melting zone of a sintering bed is considered to have a controlling influence on its resistance. In this zone it is concluded that the inertial force, which is a function of gas flow rate, has the ability to dilate gas channels and decrease resistance to air flow. More study is required for understanding of the effect of melt property on the size of gas channels. Use of the technique as an aid to comparison of the 'sinterability' of sinter mixes is also discussed.

The ability to predict the amount of fines produced during the transport and handling of lump iron ore is of particular importance to the iron ore supplier. This study combines simple contact and fracture mechanics approaches to derive a breakage equation for lump iron ore fractured in an ISO tumble drum. The tumble drum simulates the forces experienced by iron ores during relatively low impact handling and transport events. The results of tumble drum tests on three Australian iron ores of widely varying mechanical properties showed that the lump to fines ratio's predicted by the simple breakage equation were proportional to those measured experimentally.

Domestic iron ores and sinters from six mills in PRC have been evaluated. A large number of the ores consist of dense magnetite and hematite, and complex iron ores containing minerals including pyrite, pyrrhotite, biotite and siderite. One particular ore was extremely complex containing a high level of fluorite. Based on the sinter samples received, one sinter was obviously produced from predominantly magnetite blends as silicate glass and SFCA of high temperature morphology were the major bonding phases present and the major iron oxide mineral was magnetite. For the plants using predominantly hematite ore in their blends, SFCA is relatively well developed and the sinters also have a very high level of relict hematite. Sinters with this are characterised by high reducibility and a good strength. However, such a sinter structure is not formed automatically when a high level of hematite is present in the blend. The structure of three sinters indicated that when coke rate is high such a structure is not achievable although high temperature morphology SFCA is developed. The sinter produced from ore containing high fluorite is the most reducible but is also extremely weak. The strength of the different mineral phases in the sinters were characterised using indentation techniques. Good correlations were obtained between tumble indices of the sinters and a composite fracture toughness of the major phases. The study also showed that some of the more typical structures found in these plant sinters could be reproduced using a bench-scale furnace under controlled conditions.

Previous studies of iron ore blends containing pisolite are showed that losses in sintering productivity were a result of deteriorating green-bed permeability. Pisolite ores are extremely porous and absorb a proportion of the water added during granulation. Although increased water addition could be used to counteract losses in productivity, the current study examined methods of inhibiting water absorption by pisolite ore as a means of improving productivity. Additives were used to alter the properties of the granulating water. With no increase in mix moisture contents pilot-scale sintering studies showed that sugar-based additives gave productivities that are higher than those obtained for a comparable ore blend containing a hematite ore rather than the pisolite ore. Several techniques of adding the sugar to the sinter mix were examined and found to increase productivity to varying degrees. Theoretical considerations, in some cases supported by experimental results, indicate that sugar-based additives not only inhibited water absorption by porous ore particles but also improved the permeability of the sintering bed. The study also suggested that, contrary to some commonly held views, the highly reactive nature of Yandi ore is beneficial for sintering. Porous ores enabled the formation of a strong sinter at fast flame-front speeds-both good strength and a short sintering time are requirements for high-productivity operation. An important conclusion of the study is that altering the properties of the granulation water is a practical way of improving the sintering performance of blends that contain porous ores, and more work is under way to investigate other additives.

During mining, crushing and handling the size of iron ore lumps decreases. Key factors in determining the amount and size of fines that are produced are the mechanical properties of the ore. In a continuation of earlier work on the same ores the mechanical properties of three Australian iron ores were determined. In particular, fracture toughness was investigated in greater detail by examining its relationship to crack length using the compact tension test. The elastic modulus and strength of the ores were also determined in flexure and compression. The results were interpreted using Weibull statistics, enabling the ores to be ranked according to strength and elastic modulus. These results, together with data on the mechanical response of the ores to stress and observations concerning the effect of mineralogy on cracking, provide a coherent understanding of the mechanisms involved when the ores fracture.

Magnesia minerals are included in most iron ore sinter mixes to provide the MgO requirements of blast-furnace slag. The properties of these minerals have an influence on the eventual sintering properties of the mixes. A detailed study was undertaken of serpentine and dolomite and the use of limestone, olivine and magnesite was also investigated. A consideration of particle properties indicated that as these materials calcine a reduction in size can occur together with cracking and changes in surface properties. These changes could affect the permeability of the sintering bed and sintering performance. Studies of sintering reactions showed that magnesium and calcium, formed from the calcination of dolomite, are extremely mobile before the formation of melt. Evidence suggests that the movement of these oxides into surrounding areas is via a solid-state diffusion process. The diffusion of calcium into the surrounding hematite particles is faster than that of magnesium, and these species result in the formation of dicalcium ferrite and magnetite/mangesioferrite, respectively. The subsequent conversion of magnetite/mangesioferrite, in the presence of calcium, to dicalcium ferrite is observed, and this results in the removal of magnesium from the structure as ferroan periclase. Studies of pilot-plant sinters indicated that reaction mechanisms formulated from the bench-scale studies were occurring in actual sintering processes. The differences in sintering reactions between dolomite and serpentine were considered to be the major cause of differences in the availability of lime for bonding-phase formation. Pilot-scale studies indicated that the sintering properties of a mix containing dolomite could be improved by increasing the basicity of the sinter mix while keeping the MgO level constant.

During mining, crushing and transport iron ore is subjected to forces that cause the material to fracture into fine particles, a process that is referred to as degradation. The toughness and fracture mechanism of three Western Australian iron ores, Yandi, Koolan and Mt Whaleback, were studied with a view to understanding their degradation behaviour. The microstructures of the three ores are significantly different and they were therefore expected to show differing fracture behaviour. The bulk fracture toughness of the specimens was determined by the three-point single-edge notch beam test and the results were interpreted using Weibull statistical analysis. Indentation fracture mechanics was also used to measure the fracture toughness of the different mineral forms in the ores and to provide an understanding the response of these minerals to contact fracture. By correlating the results of the mechanical tests with microscopic observations of fracture patterns it has been possible to gain an insight into the complex fracture mechanisms that occur during degradation.

A study has been undertaken of the sintering behaviour of the complex domestic iron ore concentrate that is used by the Shanghai Meishan Metallurgical Corporation, Nanjing, People's Republic of China. The ore contains significant siderite, magnetite and hematite, and studies carried out with an infrared-image furnace showed that the siderite has a very significant influence on its sintering behaviour. On the basis of the experimental findings and published information it is proposed that decomposition of the siderite starts at around 400 degrees C and results in the formation of carbon monoxide (and/or hydrogen via the water-gas shift reaction), which then reduces the hematite present in the ore to magnetite prior to the initiation of high-temperature sintering reactions. This gaseous reduction process is extremely effective and particles of Australian hematite and pisolite ores, when mixed with the concentrate, also undergo significant reduction. A consideration of the calcination and reduction equations indicates that carbon monoxide is probably not the only gaseous reductant to be generated during the calcination of siderite. As the reduction process is so effective, the subsequent high-temperature sintering reactions essentially involve magnetite and the fluxes, Consequently, the low-temperature forms of the bonding phase silico-ferrite of calcium and aluminium (SFCA) cannot form, even at high sinter basicity. The results obtained with the bench-scale furnace were confirmed on a pilot-scale sintering facility. The pilot-plant sinters consisted mainly of magnetite, high-temperature SFCA and glass, which is typical of sinters formed from mixes that-contain high levels of magnetite ore.

The degradation of ferrous materials under conditions of low-temperature reduction is a complex phenomenon. The onset of the process is linked to the gaseous reduction of hematite to a 'porous' form of magnetite, which results in a volume increase and the initiation of cracks. The subsequent propagation of the cracks and further reduction of hematite, which lead to the eventual breakdown of the sinter matrix, are less well understood. It is proposed that reduction degradation can be viewed as consisting of two distinct sub-processes. In the first easily accessible hematite grains are reduced, resulting in the initiation of cracks and their propagation into the surrounding sinter matrix as more of the grain is reduced. The second involves the further growth and branching of these cracks, the reduction of areas of newly exposed hematite grains along the walls of the cracks providing the energy requirements. In experimental work indentation methods were used to determine the vulnerability of sinter phases to crack propagation, and two cases are presented to demonstrate that decreases in the fracture toughness of mineral phases in the sinter result in severe deteriorations in sinter reduction degradation indices. It is considered that the proposed mechanism furthers understanding of the causes of reduction degradation, views on which are many and varied.

Laboratory studies have shown that pisolitic ore is assimilated by sintering melts more readily than most Brazilian and Australian hematite ores; changes in the process of melt formation and properties of the high-temperature zone are, therefore, generally assumed to occur if these hematite ores are replaced in a blend by pisolitic ores. Pilot-scale studies and results obtained with commercial sinter machines show that when Yandi pisolite is introduced into hematite ore blends sintering productivity often deteriorates. A study was therefore undertaken with the aim of improving the productivity of ore blends containing IS and 30 wt% Yandi. The results indicate that the loss in productivity is caused by a deterioration in green-bed permeability rather than in properties of the high-temperature zone. Altering the size distribution of Yandi pisolite can improve productivity, but currently produced material with a top-size of -9 mm gave the best overall sintering performance. It was found that the productivity of blends containing Yandi can also be improved by increased moisture addition and enhanced green-bed segregation, although these resulted in reduced sinter strength. It appears that the best way of improving productivity with such blends is to increase the size of the fluxes as the sinters so produced have superior strength. In addition, the benefits of adding Yandi to a sinter mix (improved reducibility and low-temperature reduction degradation properties) were not affected by increasing flux size. The study suggests that a modified sintering technology with the use of coarse fluxes should be adopted for ore blends that contain significant levels of Yandi pisolite.

Coke is the most widely used solid fuel in iron ore sintering. Generally coke is blended with the other sinter mix components dry before granulation. Several studies in the literature have shown that, in addition to the properties of coke, e.g., size distribution, the delayed introduction of coke into the sinter mix, e.g., added towards the discharge end of the granulation drum, affected sintering and the sinter quality parameters. This study was initiated with the view to determine if the granulation practice at the sinter plant of The Broken Hill Proprietary Co., Ltd. (BHP), Rod and Bar Products Division (RBPD) could be altered to improve the positioning of coke in the granulated sinter mix. As pilot-scale sintering studies did not indicate positive results, further studies were carried out including the use of longer granulation time, and the use of optical microscopy to characterise the structure of the formed granules. Results showed that the positioning of the finer coke particles depended on when the coke was introduced into the granulating mix. In contrast, the large coke particles remained free, i.e., unattached to other materials, regardless of the coke addition technique. When the fine coke particles were well-embedded into granules their combustion rate was decreased; this broadened the width of the coke combustion zone resulting in lower sinter productivity.

Results that were obtained in an earlier study indicated that during iron ore sintering porous ore particles are easily assimilated by liquid melt. To provide some fundamental understanding of the parameters that control the assimilation process two simple model systems have now been considered: the dissolution of a solid by a liquid and the penetration of a liquid into a porous solid. Laboratory sintering studies that covered a range of experimental conditions were undertaken to characterize the assimilation of large particles of iron ore and flux by melts formed from different mixes of fines. For ores the results indicate that the assimilation process is complicated by the detachment of fines from the surface of the ore prior to formation of the initial melt and that the degree of detachment is dependent on the mass of adhering fines and the surface roughness of the ore particle. The detachment of fines delays the onset of assimilation and, in some cases, reduces assimilation although the penetration of melt into the porous ore particles is extremely fast and effective. Studies in which different sintering temperature profiles were applied suggest that the assimilation of hematite is controlled by the transport of Fe2O3 away from the surface of ore particles. The assimilation of limestone was found to depend more on thermal conditions than on the three magnesia-bearing minerals that were considered. The results also show that changing the chemical composition of the initial melt significantly alters the extent of assimilation of ores and limestone, but, again, the effect on the magnesia-bearing minerals is less marked.

Australia has large reserves of pisolitic limonite and, recently, mining commenced on the Yandicoogina deposit to produce a high-grade sintering ore. The ore contains about 10 wt% combined water, and simulated sintering temperature profiles indicate that progressive dehydration of ore particles occurs in the temperature range 300-700-degrees-C. Studies by optical microscopy show that although the dehydrated ore particles contain numerous cracks, there is no change in the microstructure of the ore because topotactic transformation to hematite results in the formation of compact masses of very fine-grained hematite microcrysts. When the ore is held at around 1200-degrees-C for 1 min significant recrystallization of these microcrysts occurs, resulting in the formation of optically visible, micrometre-sized hematite crystallites. Studies to simulate the behaviour of ore particles during sintering (as nuclear particles surrounded by a standard adhering fines mixture) show that the recrystallization process occurs much earlier, at temperatures of about 900-degrees-C. Further studies indicate that CaO in the adhering fines mixture enhances the recrystallization of the ore nuclei. The results of the investigation indicate that the pseudo-relict hematite in sinters produced from ore blends that contain significant levels of pisolitic limonite consists of dehydrated goethite that has undergone significant recrystallization and grain growth, a phenomenon that is facilitated by the presence of lime derived from the sintering melt.

The behaviour of iron ores and fluxes in the high-temperature zone of a sintering bed is extremely complex. Following a review of current knowledge an attempt is made to define relevant terminology to describe the processes that occur in this part of the bed. A deformation temperature is used to quantify the initial formation of melt, which is produced predominantly by reactions between fine particles in the sinter mix. The subsequent assimilation of larger particles is considered to depend on the reactivity of the melt and the assimilation resistance of these particles.Novel laboratory experimental techniques have been used to quantify these parameters. The results demonstrate that the properties of the raw materials used have an extremely important influence on the initial processes of melt formation and assimilation and also on the nature of the sintering reactions and sinter-bonding phases.

Titanium containing compounds are valuable materials for addition to the blast furnace to extend operational campaigns. However, the addition of these materials via the sinter plant has been known to create problems. This study investigates the effect of adding a titanomagnetite to two sintering ore blends, containing 10 and 20% pisolitic limonite respectively. The addition of up to 2% titanomagnetite did not affect the sintering parameters or sinter quality. Increasing the titanomagnetite levels to greater than 3% also had no significant influence on sintering but a significant deterioration in sinter RDI was recorded. Results also showed that the deterioration in RDI was greater for the blend containing more pisolitic limonite.SEM studies carried out on the product sinters found most of the titanium in the glass phase. The mechanical properties of the glass was characterised using indentation fracture mechanics. Although some difficult was experienced, due to the small and disperse nature of the glass, addition studies using artifically manufactured glass indicated that the fracture toughness of the glass decreased with increasing titanium levels. It is postulated that because glass in the weakest phase in sinter and is widely dispersed any reduction in its fracture toughness will increase the vulnerability of the sinter to crack propagation, thereby increasing sinter RDI. It is also postulated that increasing the pisolitic level increased melt formation and the dispersion of the glass phase, resulting in a further deterioration in sinter RDI.

Pilot scale sintering tests were carried out to determine the importance of coke size in sintering. The use of closely sized coke fractions indicated that as the coke particles decreased in size, the efficiency of the coke combustion process deteriorated, resulting in higher levels of carbon monoxide in the exhaust gases and, consequently, lower sintering temperatures. Coke size also affected the speed at which the flame front descended down the bed. The use of coke with a broader size distribution (-3 mm) showed that the -0.25 mm fraction had a very detrimental influence on sintering time and hence productivity, but not the efficiency of the coke combustion process. Tests also indicated that productivity could be improved by ensuring that coke particles were placed on the outer surfaces of granules to enhance the combustion process.

Studies were carried out to examine the effect of introducing varying levels of a pisolitic limonite (Yandi) into an Australian iron ore blend. Compared with the other ores in the blend Yandi is more porous; its porosity increases further on dehydration, resulting in the formation of an extremely open structure composed of fine hematite grains (pseudo-relict hematite). This unique structure was found to have a considerable influence on the sintering behaviour of the blends. Pilot-scale sintering studies and plant trials showed that sinter plant productivity, sinter reducibility and the low-temperature reduction degradation index (RDI) altered significantly when the level of Yandi in the ore blend exceeded 15 wt%. The improvement in reducibility is thought to be a direct result of increased porosity in the sinter and the presence of the highly reducible pseudo-relict hematite. Substantial improvements in RDI were obtained in spite of lower levels of FeO in the sinter. Laboratory results indicated that, in the presence of serpentine, productivity deteriorated, but when dolomite was used as the source of MgO no decline in productivity was recorded. Plant trials have also shown that operational changes could be made to the sinter plant to avoid potential deterioration of productivity.