EFFECT OF MAGNETITE, HEMATITE AND PELLET SCREENINGS AS FEED IN SINTER PRODUCTION. Abstract

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1 EFFECT OF MAGNETITE, HEMATITE AND PELLET SCREENINGS AS FEED IN SINTER PRODUCTION Mikael Pettersson 1, Peter Sikström 1 and Dr. Volker Ritz 2 1 LKAB, 952, Luleå, Sweden 2 SGA, Grubenstrasse 5, Liebenburg, Germany Abstract During the last years, the availability of coarse, high quality sinter feed has decreased. There has been a trend for lower iron content and increasing amount of gangue elements. In order to maintain sinter quality, the usage of fine sized concentrate or even pelletfeed in the sinter mix has increased. This study compares usage of three LKAB iron ores as an addition to the sinter blend. In this work, concentrates from magnetite and hematite are compared to pellet screenings as raw material in sintering. In order to minimize variations in the production process, the sinter pot at SGA has been used to produce 4 different types of sinter. From the evaluation, it can be seen that compared to the reference case, increasing magnetite content results in a significant decrease in coke breeze consumption, It is also observed that it is possible to keep the FeO content in sinter at a constant level, also at relatively high additions of magnetite in the sinter blend. The addition of magnetite significantly increases the iron content of the sinter mix, allowing for better flexibility in the choice of raw materials. Addition of pellet screenings seems to have a beneficial effect regarding sintering productivity and also sinter quality. No structural differences are found to be caused by the addition of secondary hematite. Hematite concentrate improves the iron content of the sinter blend similar to the addition of magnetite concentrate. Low silica sinter feed improves the possibility to adjust metallurgical properties by addition of other additives. The reference sinter and the three sinters containing LKAB iron ore have been studied in LOM and SEM in order to detect possible structural differences that can be connected to the usage of concentrate or pellet screenings. Qemscan equipment was finally used in order to quantify the mineral phases that have been identified. Usage of high grade iron ore in the sinter mix is beneficial both from an environmental as well as metallurgical point of view. Producing sinter with improved metallurgical properties and higher iron content would enable for a more efficient and further improved blast furnace performance.

2 Introduction The typical burden in European blast furnaces includes a major part of iron ore sinter. The need for high productivity and environmentally friendly processes increases the pressure for the use of high quality raw materials. The recent years development has however shown a decreasing availability of high quality sinter feed [1]. The trend has shown increasing gangue contents, leading to lower iron content and as a consequence, higher slag volumes in the blast furnace. In order to maintain iron content and gangue levels of sinter feed, the sinter ores became finer due to an increased necessity of beneficiation at the mining site. One way to adjust for the decreased sinter quality has for some blast furnace operators been to increase in the usage of pellet in the blast furnace burden. Others have chosen to keep sinter amount constant and instead tried to improve the quality of produced sinter by various means. Keeping quality has in most cases meant that fine ores as concentrate or even pellet feed has been added to the sinter mix. The increased usage of these types of ores have resulted in larger focus on agglomeration, either by the use of intensive mixers, but also usage of various binders [2]. The LKAB production volume is dominated by iron ore pellet, however a significant volume of ore for sinter production is also produced. In this study, three types of LKAB ore suitable for sinter production has been tested compared to a reference sinter. Sinter preparation and experimental equipment The sintering tests were conducted at Studiengesellschaft für Eisenerzaufbereitung, SGA, in Germany. The sinter pot has a diameter of 450 mm and a height of 600 mm. Bed height for the tests was 520 mm, including a hearth layer of 20 mm. In order to improve sintering performance, a rate of 0,8% burnt lime was used during the tests. The target was to produce a sinter which could be regarded as a typical sinter of a European sinter plant. The aim for chemical composition was set according to values listed in Table 1. In total, four sinters were to be produced where three of the sinters would contain addition of LKAB iron ore. Table 1 Aim for chemical composition of reference sinter Element (%) Fe ~56,5 FeO 7,3 ± 0,5 SiO2 5,9 ± 0,2 MgO 1,0 ± 0,2 CaO/SiO2 2,0 ± 0,1 In order to adjust the chemistry, limestone, olivine and sand were also used. For all tests, return fines ratio was set at 0,95-1,05. Raw material In order to reach the chemical specification of a sinter similar to a European reference, a number of commercial available sinter feeds and concentrates were used, taken from the SGA stock. As an addition to the European reference, three LKAB fines are added at a portion of 20% of the total ore mix. The mixtures for the four sinters produced can be seen in Table 2

3 Table 2 Iron ore mix in sinter blends for sinter pot trials. Sinter blend Reference 20% MAF 20% MHF 20% PF Commercial Sinter Fines 1 (%) Commercial Sinter fines 2 (%) BF return fines (%) Commercial Concentrate 1 (%) 8 6,4 6,4 6,4 Commercial Concentrate 2 (%) 7 5,6 5,6 5,6 LKAB magnetite conc. (MAF) (%) 20 LKAB hematite conc. (MHF) (%) 20 LKAB Pellet Screen-off, (PF) (%) All LKAB fines for sintering are to be regarded as high iron and low silica content fines. Addition of these ores will lead to an increased possibility of using fines of lower quality, while maintaining iron content in sinter. In Table 3, the chemical composition of sinter blends can be seen. At 20% addition, especially for magnetite concentrate, the increase in iron content is significant. All LKAB ores however contribute in a positive way regarding iron content. A low silica content can also allow for an increase in sinter basicity, a change which has proven to have positive effects especially by improved sinter strength [3]. Table 3 Chemical composition of sinter blends prior to sinter production Sinter blend Reference 20% MAF 20% MHF 20% PF Fe (%) 63,96 65,26 64,46 64,45 SiO2 (%) 4,22 3,54 3,93 3,81 Al2O3 (%) 0,92 0,79 0,91 0,80 CaO (%) 1,34 1,12 1,19 1,20 MgO (%) 0,23 0,24 0,23 0,38 P (%) 0,03 0,03 0,05 0,03 Mn (%) 0,37 0,31 0,30 0,31 TiO2 (%) 0,07 0,13 0,11 0,10 V (%) 0,00 0,03 0,02 0,02 FeO (%) 3,93 9,14 4,31 3,41 LKAB magnetite and hematite fines are to be classified as coarse concentrates, an increase of these materials can contribute to a finer particle size distribution between one to three millimetres. Addition of pellet screenings on the other hand effect a coarser particle size distribution when added to the reference sinter mix. The respective curves for the four different blends can be seen in Figure 1.

4 Figure 1 Particle size distribution for sinter blends Production of sinter As stated in Table 1, the aim was to produce four sinters with similar chemical composition. The outcome of the production based on chemical analysis can be found in Table 4. The analysis show that the sinter preparation was successful in order to have almost identical types of sinter based on chemical analysis. When using LKAB low silica fines, additional quartzite had to be added in order to reach the desired basicity. Fluxes also had to be added in order to obtain the desired calcium and magnesium analysis of the sinter, hence the same iron content. Table 4 Chemical composition of produced sinter Sinter Reference 20% MAF 20% MHF 20% PF Fe (%) 56,06 56,32 56,16 56,02 SiO2 (%) 5,92 5,88 5,88 5,96 Al2O3 (%) 1,14 0,96 1,07 1,02 CaO (%) 11,7 11,7 11,77 11,9 MgO (%) 1,04 1,03 0,99 1,07 P (%) 0,028 0,027 0,045 0,026 Mn (%) 0,365 0,3 0,285 0,3 TiO2 (%) 0,07 0,124 0,108 0,096 V (%) 0,004 0,003 0,015 0,02 FeO (%) 7,08 7,21 7,81 7,17 From the chemical analysis it is also observed that despite a 20% addition of magnetite to the sinter blend, the FeO level can be kept constant. Sintering results During the tests, both the sintering process and the metallurgical properties of the sinter was evaluated. In Figure 2, the productivity can be seen. From the graph it becomes clear that an addition of 20% of LKAB iron ore concentrate to the sinter blend, has no significant impact on productivity. On the other hand, addition of 20% pellet screenings prove to have a positive effect compared to the reference case. From the results, the addition of 20% pellet screenings increase the productivity by slightly more than 7%.

5 This also indicates that productivity can be kept on a similar level when adding LKAB concentrate to the sinter blend up to at least 20%. During pelletizing of magnetite ore, a significant part of the required energy originates from the oxidation energy during transition from magnetite to hematite. During the sinter tests, it became clear that an addition of magnetite concentrate strongly contributes to a decrease in coke breeze consumption, offering a possibility for decreased CO2 emissions during the sintering process. In Figure 3, the decrease of coke breeze consumption can be seen, which calculates to approximately 10% reduction, compared to the reference sintering test. Reduced consumption of coke breeze has also been reported in earlier papers [4]. Figure 2 Productivity, reference sinter compared to sinter with addition of LKAB iron ore. Figure 3 Consumption of coke breeze Compared to the reference sinter blend the addition of LKAB sinter fines contributes to a lower alumina input, an effect that has proven to be favorable with regard to sinter degradation [4][5]. In Figure 4, the RDI values are presented. It can be seen that for the two sinters with the lowest alumina content, the RDI is clearly better compared to the reference case. For MHF the RDI value is at a comparable level to the reference sinter. LKAB sinter fines have a high iron content and a low amount of contaminating elements. This allows more flexibility regarding choice of other ores in the sinter blend. During the tests, these aspects were not used to full extent since the chemical composition was kept constant. Moreover, it can be seen in Figure 5 that addition of LKAB sinter fines improves the reducibility of the sinters. Figure 4 RDI testing on produced sinter Figure 5 Reducibility for produced sinter In summary it can be concluded from the tests that the addition of 20% LKAB iron ore to the sinter blend can have positive effects to productivity, fuel consumption and sinter quality

6 compared to a reference sinter without LKAB material. Based on which parameter that is of highest importance and priority, the most suitable ore grade can be chosen respectively. Sinter evaluation In order to further evaluate the effects of LKAB iron ore as an addition to the sinter blend, structural studies were conducted. Initially a light optical microscope was used. Further evaluation was then performed in a scanning electron microscope. The last equipment to be used was the qemscan analysis in order to determine the mineralogy of the four sinters. Light optical microscope LOM During initial analysis, it was seen that larger sinter pieces had more varying structure compared to smaller pieces. This led to the conclusion to use pieces of mm for the evaluation. These pieces contained often various types of structures compared to small pieces that could contain only one type of structure. The sinters were molded into epoxy and prepared and polished according to a typical routine. The first material to be evaluated by optical microscopy was the reference sinter. This was found to be relatively uniform in structure with various types of calcium ferrite structures. There were also areas of hematite that could easily be seen. An example of the structures found can be seen in Figure 6. A dominating calcium ferrite phase has also been found in earlier publications[6]. In the sinter produced with addition of MAF, the similar general structure found in the reference material was also seen here. The dominating phase was also in this sinter, a calcium ferrite type structure. Compared to reference sinter, an increase in particles of magnetite with oxidized outer rims and needles as shown in Figure 7 were also found. Figure 6 Sinter structure in reference sinter, H = hematite, CF = Calcium ferrite Figure 7 Magnetite particles in sinter with addition of LKAB magnetite concentrate, MAF. H = hematite, M = magnetite The situation regarding structure in sinter containing MHF was similar. No large deviations regarding structures could be seen compared to what has been observed in the above mentioned types of sinter. Again, a calcium ferrite structure was dominating. The last sinter to be optically evaluated was with an addition of 20% pellet screenings. From the observations that were made, no clear differences could be found compared to the earlier

7 sinters, based on structure. There were no differences found that could be connected to the addition of secondary hematite compared to sinter produced from primary hematite. Scanning electron microscope SEM By evaluating the sinter in a scanning electron microscope, both the structure and the chemical analysis could be analyzed. For the reference sinter it became clear that the calcium ferrite phase was found to be built from a platy network, as seen in Figure 8. From the figure it becomes clear that there are plates oriented in various directions. Platy structures of calcium ferrite phase has also been discussed in earlier publications.[7] In order to determine the difference in color for the needle shaped structure, point analysis was conducted as seen in Figure 9. From the analysis, it can be stated that the darker color needles, in general contain higher calcium concentrations and lower iron content. Figure 8 Platy calcium ferrite structure in reference sinter Figure 9 Calcium ferrite needles in reference sinter From the SEM analysis of sinter with MAF addition, structures and composition of needles were found to be similar to the ones found in the reference case. As for the reference sinter, the dominating structure was of a calcium ferrite type, shown in Figure 10. In this figure, the same platy structure can also be seen. For the different types of sinter, mapping was also conducted in order to see variations over larger areas. As for earlier analysis and structural studies, calcium and iron is spread uniformly over the sinter. As found in the light optical microscope, sinter with addition of pellet screenings showed similar structures to the other sinters evaluated. Area analysis was conducted as a complement to point analysis and mapping analysis as shown in Figure 11.

8 Figure 10 Calcium ferrite phase in sinter with addition of LKAB magnetite concentrate, MAF Figure 11 Analysis of areas found in sinter containing LKAB pellet screenings, PF Qemscan analysis Evaluation continued using the qemscan equipment in order to quantify the mineral phases. At a first glimpse, the mineralogy seems very complex with a large number of different compositions. However, when studied in detail, it becomes clear that the sinter is strongly dominated by a small number of phases. As in the earlier evaluations, the first sinter to be analyzed was the reference. This sinter proved once again to be dominated by a calcium ferrite with varied calcium content. Some areas could also be seen containing magnesium and silica, forming a SFMC phase. The evaluation of sinter containing LKAB materials, MAF, MHF and pellet screenings gave similar results. In Figure 12, the qemscan analysis from sinter containing MAF can be seen. The dominating mineral composition is calcium ferrite at various calcium content. From the overall scan, it is very difficult to see any differences between the different sinter blends. All sinters that were produced have calcium ferrite phase levels at approximately 70%. Figure 12 Qemscan analysis of sinter with MAF addition

9 Evaluation of results from qemscan analysis In Figure 13, the mineral composition of the 4 sinters produced can be found. Only small differences can be seen between the different sinters. One indication is however that the reference sinter has a dominating calcium ferrite with higher calcium content compared to the sinters produced with LKAB ore addition. In the case of LKAB sinter feed, the dominating calcium ferrite phases seem to contain up to 10% calcium. How the metallurgical properties for sinter vary, with aspect to changes in calcium content in calcium ferrite is not yet fully evaluated. Figure 13 Mineral composition of evalutaded sinters. 20% MAF, 20% MHF, 20% PF and reference. Conclusions The high iron content and low amount of gangue elements in LKAB iron ore increases the flexibility in the choice of raw materials for sinter production. High quality iron ore improve the possibilities to adjust the chemical composition of sinter in order to achieve desired properties. Production of sinter with high iron content and low silica can significantly improve the possibility to operate the blast furnace in an efficient and cost effective way. The magnetite ores also help to reduce the consumption of coke breeze which from an environmental point of view can be advantageous. It is also shown that the FeO level of the sinter can be kept constant at a 20% addition of magnetite concentrate. The addition of pellet screenings significantly improves the productivity while maintaining metallurgical properties of the produced sinter. Although two out of the three iron ores are considered as coarse concentrates, productivity at an addition of 20% is generally kept at the same level as for the reference sinter. Based on which parameter that is of highest importance, the choice of LKAB ore can be done respectively, in order to have the largest benefits. Optical evaluation of all the produced sinters indicate high amounts of calcium ferrite which from a strength and reducibility point of view, is desirable. From the minerology evaluation the produced sinters with addition of magnetite, primary hematite and secondary hematite seem to result in similar structures.

10 References [1] Dr Jose Henrique Noldin Jr, Prof Dr Peter Schmöle, Dr Hans Bodo Lüngen. Trends in Ironmaking given the new reality of iron ore and coal resources, Aistech 2015 [2] Luri Pinto Mascarenhas et.al, Implementation of the intensive mixer at sinter plant, 42 nd ICSTI conference 2012, Rio de Janeiro. [3] LH Hsieh, JA Whiteman, Effect of raw material composition on the mineral phases in lime fluxed iron ore. ISIJ international, Vol , pp [4] Javier Mochon et.al, Iron ore sintering Part 2 Quality indices and productivity, Dyna, year 81, no 183 pp Medellin, February 2014, ISSN [5] Yuji Iwami et.al, Effect of oxygen enrichment on mineral texture in sintered ore with gasous fuel injection, ISIJ international, Vol , pp [6] Taichi Murakami, Takeyuki Kodaira and Eiki Kasai, Effect of the reduction of calclium ferrite on disintegration behacvior of sinter under high hydrogen atmosphere, ISIJ international, Vol 55, 2015, pp [7] Min Gan, Xiaohui Fan and Xuling Chen (2015). Calcium Ferrit Generation During Iron Ore Sintering Crystallization Behavior and Influencing Factors, Advanced Topics in Crystallization, Prof. Yitzhak Mastai (Ed.), ISBN: , InTech, DOI: / Available from: