Abstract
Sintering is the most economic and widely used agglomeration process to prepare iron ore fines for blast furnace use. Owing to the depleting reserves of traditional high grade iron ore, there have been considerable changes in iron ore resources available throughout the world, especially in steel mills in East Asia. Corresponding to the changes in the availability of iron ore resources, the amount of impurities in iron ore has been slowly increasing. Some of these impurities have been found to have deleterious impacts on sinter quality and sintering performance. In the meantime, an increasing number of large blast furnaces with inner volumes of more than 5000 m3 have been built in East Asia which require more sinter and are often more demanding in terms of the quality requirements of the ferrous materials. Finally, sinter plants are facing increasing pressure due to more stringent regulations regarding their environmental impact. This paper gives an overall review of a variety of technologies developed worldwide to tackle the changing raw material characteristics and mitigate emissions from sintering operations. Given the high sinter usage and volume of their blast furnaces, most of the recent sintering technologies have originated from East Asia, particularly Japan.
Introduction
Owing to the depleting reserves of traditional high grade iron ore, there have been considerable changes in iron ore resources available throughout the world, especially to steel mills in East Asia. Figure 1 shows the change in iron ore sources used by the Japanese steel industry during the period 1971–2007. The usage of Australian iron ore has been increasing steadily over this period to reduce costs. The increase in the usage of Australian iron ore means that the ore resources have changed from hard haematitic to more goethitic ore types, such as pisolitic or Marra Mamba ores.
Change in iron ore sources used by the Japanese steel industries during the period 1971–2007 (Ueshima and Saito 2011)
Corresponding to the changes in the availability of iron ore resources, the amount of impurities in iron ore has been slowly increasing. Figure 2 displays the chemical and physical characteristics of six typical iron ore fines commercially available on the market. Compared with the samples available 10 years ago, the Fe grade has decreased while the gangue, such as Al2O3, SiO2 and LOI, has increased. In addition, iron ore fines tend to be more goethitic as shown by the increasing LOI of the fines. Finally the proportion of fine material has increased considerably for most iron ore fines. The amount of impurities, LOI and fine material have been found to have significant impacts on sinter quality and sintering performance.
Change in Fe grade, alumina content, −0.25 mm adhering fines and LOI of six typical iron ore fines commercially available on the market
In the meantime, an increasing number of large blast furnaces with inner volumes of more than 5000 m3 have been built, which require more sinter and are often more demanding in terms of the quality requirements of the ferrous materials. Moreover, sinter plants worldwide are facing increasing pressure due to more stringent regulations regarding their environmental impact. Therefore, a variety of granulation and sintering technologies have been developed worldwide to tackle the changing raw material characteristics and mitigate emissions from sintering operations.
New coating and granulation technologies
In the past, sintering productivity depended mainly on the combustion efficiency of added fine coke in the raw materials (Sakamoto 2002). However, the introduction of a fine coke coating on the surface of quasi-particles and segregation of coke particles by vertical depth in the sinter bed have reduced the coke consumption remarkably. Therefore, the permeability of the sinter bed has become increasingly important for maximising productivity. This section discusses new coating, granulation and charging technologies to improve or maintain the sinter bed permeability while using higher addition levels of finer ores.
Limestone and coke breeze coating
To improve coke combustion and control melting, JFE in Japan developed the limestone and coke breeze coating technology. Figure 3 shows the process flowsheet for the limestone and coke breeze coating granulation technology installed on the JFE Kurashiki No. 2 sinter machine, as well as a schematic diagram of the granulated particle and sinter structure for each granulation method. The technology controls the melting reactions of the iron ore and limestone by distributing the coke breeze and limestone on the surface of the quasi-particles. This process improves the sintering productivity and sinter reducibility, because it retains finely porous and highly reducible primary haematite relict particles and maintains product yield by forming a strong calcium–ferrite matrix that bonds together the primary haematite relict particles, as illustrated in Fig. 3.
HPS process
The Hybrid Pelletised Sinter (HPS) process, in which conventional processes for production of both sinter and pellets are incorporated, was developed by NKK (now JFE Steel Corporation) to enable use of large amounts of finer iron ores, including pellet feed with high iron content. Figure 4 shows schematically the material flowsheet for the HPS process (Niwa et al. 1993). In contrast to the conventional sintering process, the blended ore, limestone and burnt lime are first mixed and pelletised in the HPS process using disc pelletisers to produce green pellets which are then coated with coke breeze in a coating mixer before charging onto the sinter machine. Based on experimental studies and semi-commercial plant trials, a commercial plant utilising the HPS process with an annual capacity of 6 million tonnes per annum was constructed by remodelling the No. 5 Sinter Plant at the NKK Fukuyama Steel Works. It was confirmed that it was possible to operate the commercial plant at 4.7% silica content and 60% fine iron ore, including pellet feed, at a high productivity of 1.65 t m−2 h−1.
Schematic diagram of a commercial Hybrid Pelletised Sinter (HPS) plant with a capacity of 6 million tonnes/year (Niwa et al. 1993)
Selective granulation
Selective granulation of fine clayish iron ores was developed by Nippon Steel Corporation (now Nippon Steel & Sumitomo Metal Corporation, NSSMC) in Japan to improve the sintering performance of sinter mixtures containing higher proportions of limonitic ores (Haga et al. 1997a, 1997b). In this process, selected clayish ores are first screened at 2–4 mm to take out the fine material which often contains high alumina. The undersized material is pelletised to make pseudo-particles, which are then mixed with the oversized material and other raw materials. Figure 5 shows the conceptual design of the selective granulation method. The green pseudo-particles prepared from clayish iron ores by selective granulation were found to be strong enough to retain their integrity during handling and effectively retain the segregation of the deleterious chemical components from the bond forming material in the raw mix. Therefore, the concentration of Al2O3 in the bond forming material and the amount of fine material decreased, while the Fe2O3, CaO and coke concentrations increased. This led to an increase in melt formation of the sinter products, which was confirmed by the observed increase in the ratio of open pores and pore size. Consequently, the coke consumption, permeability and the reducibility of the sinter were significantly improved by fine particle selective granulation of the clayish iron ores.
Concept of the selective granulation method developed by Nippon Steel & Sumitomo Metal Corporation (Haga et al. 1997b)
Advanced granulation using APD dispersed micro-particles
In response to the change in raw material characteristics, a new granulation technology using a dispersant, or AGIS (Advanced Granulation for Innovation of Sinter ore) process, was developed by Nippon Steel Corporation (now NSSMC) (Ishiyama, Higuchi, Shinagawa and Ooyama 2011). Figure 6 shows a schematic diagram of the AGIS process. The APD dispersant and additives are mixed with the other raw materials using a high-speed agitating mixer. The micro-particles in iron ore are dispersed in water using an anionic polymer dispersing agent (APD), resulting in strengthening of the points of contact between the nucleus and fine particles or between the fine particles. Laboratory granulation and sinter pot test results by Nippon Steel Corporation (now NSSMC) (Okada et al. 2006, Kawachi and Kasama 2009, 2011) showed that the use of APD increased the adhesion strength of pseudo-particles and pellet strength after drying, which consequently improved sintering productivity more than by using burnt lime. Actual plant tests were also carried out, targeting a reduction in burnt lime addition through improved permeability of the sintering bed resulting from intensified granulation of the raw materials using the new dispersant and micro-particle additives. A reduction of 10 kg t−1-sinter in burnt lime consumption was confirmed while maintaining productivity and sinter strength as well as stable operation. The granulation index and therefore the pre-ignition permeability of the granulated material were retained at levels equivalent to those for the base case where a higher addition of burnt lime was applied but without the addition of APD. Furthermore, the total binder consumption, including the 4.3 kg t−1-sinter of micro-particle additives, was reduced by approximately 6 kg t−1-sinter (Ishiyama et al. 2011).
Process flowsheet for the AGIS (Advanced Granulation for Innovation of Sinter ore) process used on the NSSMC Tobata No. 3 Sinter Machine (Ishiyama et al. 2011)
Innovative ore preparation by intensive mixing
The Intensive Mixing and Granulation technology developed by Siemens VAI Metals Technologies enables a high portion of pellet feed, which is normally not suitable for the sintering process, to be used in sinter plants. Two systems have been installed at the sinter plants of Usiminas in Brazil to increase the usage of pellet feed in their sintering process. It was found to provide excellent homogenisation of extremely fine iron ores and burnt lime, which resulted in an increase in machine productivity (Mascarenhas, Silva, Honorato, and Da Silva 2012). Similarly, intensive mixing was used in the NSSMC AGIS process to ensure an even distribution and good contact of APD dispersant and micro-particles in the raw material. High intensive mixing was also tested by Kobe Steel to utilise Marra Mamba ore (Matsumura, Miyagawa, and Yamagata 2005). The granulability and permeability of the ore blend improved considerably by processing the Marra Mamba ore with nucleus particles in a high-speed agitating mixer.
New charging technologies
Compared with the lower region of the sinter bed, more return fines are generated from the upper bed region because this part of the bed is sintered at a considerably lower temperature. Therefore, an appropriate segregation of bonding agents (coke breeze) is desirable to improve the sintering temperature of the upper bed region without over firing the lower part of sinter bed. Similarly, the lower region of the sinter bed is often less permeable compared with the upper bed region (assuming uniform size distribution of feed material) due to moisture condensation in the lower reaches of the bed during sintering and compression under the load of the upper region of the sinter bed. A coarse size distribution of feed material in the lower bed region is therefore desirable.
However, with conventional drum charging systems, only limited segregation can be achieved due to the natural feed flow. An air nozzle to blow air at the point of raw material mix departing from the sloping chute was found to be efficient for achieving particle size and coke segregation due to the density differences (Sasaki, Suga, and Fukuda 1980). To enhance the segregation degree, some sinter machines have tried multiple layer charging methods. However, such a system requires more than one charging unit for a single sintering machine. Therefore, systems such as the Segregation Feeder (SF) (Sasaki et al. 1980), slit bars (Fukuda et al. 1984, Inazumi 2000), slit wires (Takai et al. 1993), and the Intensified Sifting Feeder (ISF) (Honorato and Seshadri 2008) have subsequently been developed to enhance segregation. Figure 7 shows schematic diagrams depicting the size segregation achieved by some of these charging methods. As shown in Fig. 7a, the slit bars system consists of several bars lying vertical to the direction of material flow, which act as a mechanical screen to separate the feed material into oversized and undersized particles which flow onto the sinter strand at different locations to achieve vertical size segregation in the sinter bed. The slit wires system works essentially on the same principle, but it is easier to remove any adhered material by winding the wire using a mechanical reel. The Segregation Slit Wire (SSW) system with a drum chute has been developed and installed on JFE's Fukuyama No. 5 sinter machine (Takeda and Oogami 2008). The SSW charging system introduces a curved screen and chute (Fig. 7b) to improve screening efficiency and enhance particle size segregation further. Figure 8 shows an ISF unit installed at the Usiminas sinter plant in Brazil (Honoroto and Seshadri 2008). Unlike slit bars, the ISF feeder (Inazumi et al. 1988, Inazumi, Fujimoto and Sato 1989, Inazumi, Fujimoto and Sato 1990, Honoroto and Seshadri 2008) consists of a set of bars distributed at different levels and rotating in opposite directions to maintain uniform friction and surface erosion to avoid sticking of the material. These bars also move in the horizontal and diagonal directions to control, respectively, the contact point of the feed material and the distance between the bars and the surface of the mixture. The ISF is reported to be efficient in enhancing particle size segregation in the bed height direction and preventing avalanche phenomena which have led to a decrease in sinter yield due to inhomogeneous charging (Inazumi et al. 1989, 1990). The Magnetic Brake Feeder (MBF) (Oyama, Igawa, Nushiro and Fujii 2000, Takeda and Oogami 2008), which has been developed and installed in JFE's Kurashiki No. 3 sinter machine, is quite effective for concentrating high FeO material on the surface of the sinter bed which is oxidised exothermically during sintering to make up for the heat deficiency in the upper bed region, while simultaneously lowering charging density.
ISF unit installed at the Usiminas sinter plant (Honorato and Seshedri 2008)
New sintering technologies
Stand-support sintering
The sinter cake load on the combustion-melting zone has a great effect on structure formation of the sinter cake especially in the lower part of the sinter bed. It increases the permeability resistance of the sintering bed due to gas channel plugging and also indirectly affects the qualities of the sinter product. A new sintering technique, called ‘Stand-support sintering’ for supporting the sinter cake with bars or plates attached to pallets, has been developed to improve bed permeability under the influence of gravity. With this technique, better bed permeability is achieved in the lower part of the sinter bed as a result of reduced shrinkage due to gravity and suction by supporting the sinter cake load with vertical supporting plates attached to the pallets, as shown in Fig. 9. This technology has been adopted in several sintering machines operated by Nippon Steel Corporation (now NSSMC) since 1996 and has contributed to improved productivity with minimal effect on sinter quality.
Schematic diagram showing the principle of support stand sintering technology and actual plant data (No. 1 Sinter Machine, Kimitsu Steel Works, NSSMC) revealing improvement in the shrinkage behaviour of the bed and gas flow distribution (Higuchi et al. 2006)
MEBIOS process
Designing or controlling the position and size of voids formed in sinter cake is especially significant, because it affects the size distribution of the sinter product. However, an increase in the amount of fine ore, such as Marra Mamba or ultrafine Brazilian concentrate, in the raw material for sinter blends has been found to lower the permeability of the sintering bed and the sinter yield, probably due to the collapse of voids in the sintering bed. While some pre-granulation techniques, such as selective granulation and HPS, have been developed for utilising these fine ores, the development of a sintering process that can control the void structure of the sintering bed to enable use of a large amount of these fine ores is required. The MEBIOS (Mosaic Em-Bedding Iron Ore Sintering) process was recently proposed as part of an ISIJ (Iron and Steel Institute of Japan) research project for production of porous meso-mosaic texture sinter.
The MEBIOS process is a multiple sintering process that arranges dense pre-granulated pellets (called the aging bed), which do not easily deform during the sintering, in the ordinary sinter mixture (called the induction bed), which creates a ventilation route in the sinter bed (Otomo, Takasaki and Sato 2009, Kamijo et al. 2013). As illustrated in Fig. 10, pre-granulated dense pellets are appropriately arranged in the induction bed, which provides an ideal void network under normal sintering conditions. In the research project, a pisolitic ore, which has a lot of coarse particles, was used for the induction bed and a Marra Mamba type ore, which consists of many fine particles, was used for the aging bed (Otomo et al. 2009). The aim of the MEBIOS process is the formation of a ventilation route in the sintering bed by creating a low-density area around the large pellets due to a kind of wall effect and suppression of sinter bed shrinkage due to support of the load by the upper part of the layer of dense large pellets.
Conceptual illustration of the MEBIOS (Mosaic Embedding Iron Ore Sintering) process
Small dry particles charged into the packed sinter bed were found recently to have a similar effect on controlling the bed structure due to friction between the dry and wet particles. Therefore, a process named ‘RF-MEBIOS (Return Fine – Mosaic Embedding Iron Ore Sintering)’ was proposed to utilise return fines as the dry particles, because they are dry when produced by the sintering machine (Matsumura et al. 2013, Yamaguchi, Kamijo, Matsumura and Kawaguchi 2013). Utilisation of return fines as the dry particles eliminates the requirements for drying coarse ore particles. It was demonstrated by sinter pot tests that productivity increased at the same charging moisture content in the sinter mixture. This productivity increase is caused by the higher permeability of the packed sinter bed due to two major phenomena (Yamaguchi et al. 2013). One is the increase in the pseudo-particle size at granulation and the other is the decrease in the bulk density of the packed sinter bed after charging. The former is achieved due to the higher moisture content of the raw materials during granulation, which decreases the proportion of small pseudo-particles (−0.25 mm), while the latter is due to the higher friction in the packed bed composed of dry and wet particles, which decreases the bulk density.
The beneficial effect of RF-MEBIOS on sinter productivity was confirmed in No. 3 sinter plant in the NSSMC Kashima Steel Works (Matsumura et al. 2013). The RF-MEBIOS method has now been installed on three commercial sintering machines (Kashima, Wakayama, and Kokura) belonging to Sumitomo Metals (now NSSMC). In all three sinter plants, a productivity increase has been confirmed (Matsumura et al. 2013).
Super – Sinter™ for energy and CO2 reduction
JFE Steel Corporation has developed the ‘Super-SINTER™ process, ie, Secondary-fuel Injection Technology for Energy Reduction (Oyama et al. 2011). In this process, natural gas is blown in from the upper layer in the sintering bed. As shown in Fig. 11, the ‘Super-SINTER™’ technology extends the zone in which the flame is in the optimum sintering temperature range, leading to the production of sinter with high strength and high reducibility. It also improves the energy efficiency and therefore reduces the solid fuel consumption. As a result, the off-gas CO2 emissions are decreased by about 60 000 tonnes per year (Anon 2010).
Effect of gaseous fuel injection on the temperature distribution, measured by thermography, of a sintering bed at 250 seconds after ignition (Oyama et al. 2011)
Emissions optimised sintering
Emissions Optimised Sintering (EOS®) is a process developed by Outokumpu Technology in the 1990s where the entire sinter strand is housed within an EOS hood and the waste gases from the entire strand are partially recirculated back to the full surface of the strand. Figure 12 shows the difference in principle between the EOS® process and the conventional sintering process. As a result of the partial flue gas re-circulation system, the EOS® process (Anon 2014):
Minimises off-gas volume by 50–60% and consequently cut off-gas cleaning investment costs. Uses the CO content and heat from the recirculated gas as an energy source, and consequently saves energy by up to 20% (in the form of coke) and reduces operational costs. Significantly minimises dust, NOX, SOX, CO and CO2 emissions. Comparison between the EOS and conventional sintering processes (Outotec 2008)
Summary
Owing to the depleting reserves of high grade iron ore around the world, the level of impurities in iron ore is increasing. Some of these impurities have been found to have deleterious impacts on sinter quality and sintering performance. In the meantime, an increasing number of large blast furnaces with inner volumes of more than 5000 m3 have been built, which are often more demanding in terms of the quality requirements for ferrous feed materials. A variety of technologies have therefore been developed in Japan and worldwide to tackle changing raw material conditions and therefore improve or maintain the sinter quality and productivity required by large modern blast furnaces. As more stringent environmental regulations are being implemented worldwide, future research is needed to mitigate emissions from sintering operations.
Footnotes
Acknowledgement
This paper was originally presented at the Iron Ore 2015 conference (13-15 July 2015, Perth, Western Australia) and has subsequently been revised and extended before consideration by Mineral Processing and Extractive Metallurgy with permission from the AusIMM.