SeqiOlivine Improves the Sintering and Pelletizing Performance of Iron Ore

Transcription

1 SeqiOlivine Improves the Sintering and Pelletizing Performance of Iron Ore Andreas Fredriksson Minelco AB, BOX 952, SE , Sweden Tel.: Key words: Olivine, Dunite, Fosterite, Orthosilicate, Industrial Mineral, Additive, Sinter, Pellet ABSTRACT Minelco, the industrial minerals company, started a couple of years back to investigate the possibility of utilizing an olivine-rich dunite rock at as a source of olivine to different industrial applications. The usage of olivine in industrial applications is predominantly as a slag conditioner and fluxing agent into different metallurgical processes and products, where addition to the blast furnace process, to the sintering of iron ore and iron ore pelletizing are the main areas. In this investigation the usefulness of the olivine mined at for sinter and pellet production was studied. The chemical analysis of the olivine from shows an olivine mineral with high fosterite content. Sintering behaviour and sinter properties shows that the olivine is a high-rated olivine additive. The full-scale tests in a concentrating plant and pelletizing plant (LKAB, Sweden); show that the olivine from is more suitable compared to the alternative material. The grindability was slightly improved with the olivine from. An additional test in an experimental blast furnace owned by LKAB indicates that a ferrous burden with pellet based on olivine from generates e.g. less reducing agent consumption and a higher productivity. The olivine mined at was accordingly verified as a first-rate additive to the production of iron ore based sinter and pellet. Since the second half of 2006, the olivine is a permanent additive to LKAB s pellet production in Sweden. INTRODUCTION In 2003, Minelco started to investigate, in cooperation with junior exploration company, the possibility of utilizing an olivinerich dunite rock at as a source of olivine to different industrial applications. Minelco is the industrial minerals company of the iron-ore-based products supplier, Luossavaara-Kiirunavaara AB (LKAB) in Sweden, and was founded in 1989 originally to explore applications for the high quality magnetite produced at the Swedish mines for non-metallurgical markets. The generic olivine is regarded as an isomorphous series of orthosilicate minerals, with the two mineral end-members fosterite (Mg 2 SiO 4 ) and fayalite (Fe 2 SiO 4 ) in varying amounts. In a more general designation it can be written as (Mg, Fe) 2 SiO 4, or 2(Mg, Fe)O SiO 2. The name olivine is due to its olive-like yellow-greenish colour and the name was established as early as the end of the 1700 century by J. Werner. The occurrence of fosterite is normally in basic and ultrabasic rocks, and in siliceous dolomitic limestones formed through thermal metamorphosis. Olivine is one of the most common mineral types on the Earth and it has also been found on e.g. meteorites, and consequently been named olivinoids 1. The mineral also gives name to the group of minerals with similar crystal structure, the olivine group (e.g. tephroite - Mn 2 SiO 4, monticellite - CaMgSiO 4 and kirschsteinite - CaFeSiO 4 ). The melting point of fosterite is 1890 o C and for fayalite 1205 o C, which means that the olivine is commonly not interesting in metallurgical circumstances if the content of fayalite exceed about 15%. The usage of olivine in industrial applications are predominantly as a slag conditioner and fluxing agent into different metallurgical processes and products, where addition to the blast furnace (BF) process, to the sintering of iron ore and iron ore pelletizing are the main areas. The relative high MgO content and SiO 2 content are regarded as beneficial, as well as the relatively low LOI (loss on ignition) of olivine, with respect to dolomite and limestone. Dolomite and limestone need calcination to produce CaO and MgO and as seen in reaction 1a and 1b, 2 hence also generate carbon dioxide. When regarding the slag chemistry, the extra amount of silica can in low-grade iron ores (with high silicate content) be an unwanted contribution. CaCO 3 + <=> CaO + CO 2 CaMg(CO 3 ) 2 + <=> CaO + MgO + 2CO 2 (1a) (1b) In the BF-slag the magnesia can increase the fluidity range, reduce alkali recirculation (by formation of stable magnesium alkali silicates), and it may also make the slag less sensitive to other chemical impurities and/or temperature variations. Iron ore pelletizing with generic olivine as an additive may reduce swelling, provide a narrow melting range and accelerate the reduction 3. Another area traditionally using olivine is in foundries, as foundry sand. This is due to the olivine s

2 physical/metallurgical properties, i.e. the high melting point (pure fosterite 1890 o C), good thermal conductivity, high heat capacity, low thermal expansion and high green strength 4. This paper will deal with the evaluation of some lab-scale sintering and full-scale pelletizing tests accomplished by LKAB and Minelco, together with an independent research institute, of the olivine mined at in comparison with a reference olivine, and a serpentine. A pilot scale blast furnace test performed by LKAB is also commented. The olivine from formal product brand name is SeqiOlivine, which comes from the name of the location (Seqinnersuusaaq) of the mineralization located 90 km north of the capital of (Nuuk). The Seqinnersuusaaq deposit is a large quite monomineralic and homogenous dunite deposit consisting of about 97% olivine and the deposit is estimated to be over 100 Mt of olivine. A typical chemical composition of the olivine can be seen in Table 1. Table 1. Chemical composition Component wt-% MgO SiO 2 Fe 2 O 3 Cr 2 O 3 CaO Na 2 O K 2 O In previous publications, product properties and characteristics such as geology and mineralogy 5, chemical and physical properties, as well as its feasibility and applicability to different industrial applications 6, have to some extent been illustrated and discussed EXPERIMENTAL Sinter tests made by Studiengesellschaft für Eisenerzaufbereitung (SGA), Germany SGA conducted the sintering tests to evaluate the effect of adding a new olivine grade on sintering behaviour and on sinter properties. The tests were made in five different series, three series with the olivine from in three different size distributions (<1mm, <2mm and <3mm). Additionally, one series was conducted as a reference with a commercially available olivine (<3mm), as well as one series using a serpentine in the size <3mm. Chemical, physical and metallurgical properties of the sinters from all series were tested. Fundamental factors important during laboratory sintering have thoroughly been clarified by Loo and Wong, see reference 7. In three of the sintering series (with the additive group with <3mm in size) additional softening and melting properties were tested in the REAS-test of SGA. Sintering conditions The sintering tests were performed in a pot furnace, with the conditions seen in Table 2, for all the five tests. The sinters were tumbled (70 revs) for return fines generation, and subsequently the return fines were screened at 4 mm. Table 2. Sintering test pot furnace characteristics Pot grate Bed height Ignition time Ignition temperature ( o C) Ignition suction Sintering suction 0.45 m (diameter), 0.6 m (height) 520 mm (incl. 20 mm hearth layer) 90 seconds 1220 o C 400 mm WG 1200 mm WG For adjustment of the chemistry, limestone, olivine or serpentine and quartzite were used % of burnt limestone was added to improve the sintering performance. For each series, initial tests were performed to adjust a balanced return fines ratio of At balanced return fines ratio, the sinter feed moisture is varied to achieve optimum productivity. The chemical target analysis for the final sinter was set as illustrated in Table 3.

3 Table 3. Chemical target analysis for the sintering tests Fe tot (%) About 58.5 FeO (%) 7.5 ± 0.5 SiO 2 (%) 4.6 ± 0.1 MgO (%) 2.0 ± 0.1 CaO/ SiO 2 (%) 1.9 ± Raw materials For all the sinter tests the sinter mix (SM) had a fixed ore mixture as illustrated in Table 4. All ores originate from SGA s stock and their analyses are represented in Table 4. The analyses of the different olivine grades and the serpentine are listed in Table 5. To fit the target analysis (table 3) olivine or serpentine, limestone and quartzite were added into the ore mixture. Coke breeze rate and return fines rate were adjusted to reach a balanced return fines ratio and the target FeO-content of the sinter. Table 4.The sinter mix in percentage per ore and the chemical composition of the sinter feed ore mixture KBF (10% of SM) Carajas SF (35% of SM) Fabrica SF (25% of SM) Itabira SF (15% of SM) Yandi Fines (10% of SM) Robe River Fines (5% of SM) Al 2 O 3 (%) CaO (%) FeO (%) Fe tot (%) MgO (%) SiO 2 (%) LOI (%) Moisture (%) Al 2 O 3 (%) CaO (%) FeO (%) Fe tot (%) K 2 O (%) MgO (%) Mn (%) Na 2 O (%) P (%) S (%) SiO 2 (%) Table 5. The chemical composition, LOI and moisture of the olivines and the serpentine Olivine <1mm Olivine <2mm Olivine <3mm Ref. olivine <3mm Serpentine (serpentine) <3mm LOI (%) Moisture (%) Chemical, physical and metallurgical testing All the sinters were analyzed for their chemical analysis. The physical properties of all the sinters were tested for size distribution and for tumbler strength and abrasion according to ISO When regarding the sinter metallurgical properties, all the sinters were tested for reducibility according to ISO 4695 and ISO 7215, and for disintegration behaviour according to ISO 4696 part 1 and 2. Full-scale process tests in LKAB s plants, Sweden A full-scale test with the olivine from was conducted in a grate-kiln process, illustrated in Figure 1. The grate-kiln process started its production in 1969 with a designed production capacity of 1.8 Mt/yr and in 1979 the process was upgraded to

4 a yearly production of 3.0 Mt iron-ore pellets. Last year, in 2006, LKAB produced through this grate-kiln process 3.8 Mt ironore based pellet, which so far is the production record in million ton per year of the plant. The magnetite raw material are concentrated separately thorough the concentrating plant as illustrated in Figure 1. The full-scale test was performed during two weeks in January 2006 to ensure that LKAB can use the olivine in the production of blast furnace pellets. The fundamental properties such as crushing, grinding, mixing and quotation of the additive were studied and the stability of the pelletizing production, as well as quality of the produced pellet, was ensured. The delivered olivine was about 40% <12mm in size. The additives (olivine, limestone) are crushed, screened and subsequently grinded in a Morgårdshammar ball mill (Ø 40 mm of charge balls), see Figure 1. The effect of the ball mill is aimed at 620 kw (through adjustment of ball charge) and the weight percent solids of the pulp are aimed at about 64-65% in the ball mill. In the comparison of additives, a reference period of about two months was used. Flow explanation Material flow Circulating flow Tailings Rod mill Concentrating plant Magnetic separator Pebbles Pebble mill Magnetic separator Additives section Spiral classifier Ball mill Additives Process water Thickener Spiral classifier Filter Raw material from Kiruna To tailings pond Pellet plant Balling drum Binder Oil Coal mill Figure 1. General flow sheet over the LKAB s plants in Sweden used for full-scale test of the olivine To port

5 Experimental blast furnace test at LKAB, Sweden A pilot scale test was performed in the LKAB owned experimental blast furnace (EBF) in Sweden. Today, 18 campaigns have been successfully conducted for internal product development, customer s trials and development of new technology. The trial results demonstrated that the EBF is a very efficient tool for simulating the commercial blast furnace operations. Trials with focus on the olivine were divided into two periods, namely, a reference period with 100% of one of LKAB s existing commercial products as the ferrous burden, and subsequently a period when 100% of a pellet with the olivine was the ferrous burden. RESULTS Sintering tests The reference test with a commercially available olivine, with a coke breeze rate at 4.85 parts (see Table 6), and a return fines ratio of 25.5 parts, four balanced sintering tests were performed. Coke breeze consumption is calculated between kg/t sinter, productivity between t/m 2 24 h, with the optimum productivity at a sinter feed moisture of 5.9%. Table 6 Example of sintering test total mixture illustrated in parts (pts) olivine <3mm Reference olivine <3mm Serpentine <3mm Sinter mixture (pts) 100,00 100,00 100,00 Water (pts) 9,75 9,55 9,35 Coke breeze (pts) 5,00 4,85 4,85 Return fines (pts) 23,00 25,50 26,00 Sinter feed (pts) 137,75 139,90 140,20 In Table 7, the three sinter tests of the olivine from are shown together with the serpentine. The results of the sintering data for the olivine, e.g. coke consumption, yield and sintering productivity for all the test can be summarized as follows: Table 7. Sintering test data and results for the three different size distributions of the olivine olivine <1mm Greenl. olivine <2mm Greenl. olivine <3mm Serpentine <3 mm Number of tests Coke consumption (kg/t) Best prod.* at moisture (%) * productivity The results indicate a better productivity with increasing size distribution of the olivine. The importance of size of the olivine as additive on the oxidation of magnetite pellets has been published by Forsmo and Hägglund 8. One statement they do is that a better oxidation in the grate due to a coarser olivine improves the low temperature reduction strength (LTD, ISO 13930) in pellets. These sintering results obtained by SGA, indicates increased coke breeze consumption up to 2 kg/t for the tests using the olivine. At the same time, a clearly increased sintering productivity was found, especially for the tests with coarser olivine. In Figure 2, the results of the average productivity of all the five different additives are illustrated as this is regarded as the most relevant comparison between the additives.

6 40 Average productivity (t/m 2 24 h) ,3 34,6 34,9 31,3 33,4 25 olivine <1mm olivine <2mm olivine <3mm Reference olivine <3mm Serpentine <3mm Figure 2. The average sinter productivity for the five additives tested Chemical analysis of the sinters The chemical analysis of the sinters from the test (not shown in this paper) shows that the target analysis was generally well met for all the tests with exception of the sinter using serpentine as an additive. The serpentine in these tests has a different chemical composition than the olivines and due to this, the target MgO-content of 2.0% could not be met, which would have been possible only at increased SiO 2 -content of the sinter. At constant basicity, the Fe-content of such sinter would decrease, and therefore this was not adjusted for. The higher alumina content of the serpentine also meant a higher Al 2 O 3 -analysis of the sinter. Physical properties of the sinters The tumbler strength and abrasion according to ISO 3271 showed only minor difference and are rated as equal. The tumbler tests results with the different olivines as additive showed % >6.3 mm and the abrasion indices were about 5.8% <0.5mm. The sinter with serpentine had slightly lower tumbler strength at about 74.7% >6.3mm and a similar abrasion index as for olivine. The tumbler strength for olivine as additive was about 2.5% higher compared with that of the serpentine. The physical properties of the sintering tests with olivine as additive can be rated as favourable. Metallurgical properties of the sinters The reducibility can be rated as high for all the sinters. R 40 according to ISO 4695 shows tendency to have a trend of reducibility in the following order; reference olivine < olivine < serpentine. No trend was found for the different sizing of the olivine. The test according to ISO 7215 (see Figure 3) shows a clear trend of increasing reduction index (reference olivine < olivine < serpentine). Additionally, the results indicate a trend of increasing reduction indices with increasing sizing of the olivine.

7 80 Reduction index (%) ,5 68, ,7 74,2 60 olivine <1mm olivine <2mm olivine <3mm Reference olivine <3mm Serpentine <3mm Figure 3. ISO 7215 reduction index test Regarding the disintegration tests (ISO ) no clear trends were found. Also some high temperature properties as reduction behaviour during indirect reduction, permeability specific data and melting and dripping specific data were analyzed but not illustrated in this paper. A comment regarding these tests is that no significant difference was found between the materials. The softening, melting and dripping behaviour of the three sinters (produced by the three <3mm materials) are typical for high basicity sinters. Full-scale process tests in LKAB s plants, Sweden The olivine from were subsequently verified by LKAB s research and development personnel to meet all the prerequisites in the pelletizing production, regarding its handling in the mineral processing plant (crushing and grinding), and the other aspects of mixing and quotation. The product quality was also secured through extensive testing. In Table 8, a summary of some measured process data from the additive concentrating plant is shown. The mean energy to grind the olivine is about 27.4 kwh/ton. When comparing the grinding energy per ton and generated fineness, which is about for the olivine and about for the alternative olivine, the olivine needed less energy than the reference olivine to reach equivalent degree of fineness. No significant difference was found in measured surface area of the compared samples. The quotation of the additive have during the test period been about 3.18% of the raw material input, compared to a reference period with about 3.15%. This is regarded as a stable additive quotation. Table 8 Summary of process data from additive concentrating plant Variable Unit N Min Max Mean Median Std. Dev. Grinding energy per ton additive kwh/ton 17 24,69 31,17 27,37 27,7 1,669 Grinding energy per ton add.* and fin.* kwh/(ton, %wt -45 µm) 17 0,3829 0,4917 0,4289 0,4321 0,02958 Feed into the ball mill ton/h 17 19,76 24,65 22,51 22,31 1,238 Quotation of add. ton add./ton raw mat.* 17 0,0285 0,0343 0,0318 0, , * add.=additive, fin.= fineness and mat.=material All though the processes were going on their maximum production capacity, no process deviations with respect to the olivine could be found. The product quality (measured through standardized methods) of the produced blast furnace pellets showed no deviations that could rank the olivine to be a poor additive, or that it otherwise could be different than the alternative olivine from a pellet quality point-of-view. Experimental blast furnace test at LKAB, Sweden The results from the EBF indicated that there were no significant differences between the commercialized pellet and the pellet with the olivine as additive. Therefore, the trial results between the reference period and the test period were very

8 comparable. In general, in comparison with the reference period, the production rate during the test period was slightly higher, and with slightly lower consumption of the reducing agent. The process stability, indicated by the permeability status, top gas utilisation, burden descent etc., was slightly improved. The hot metal quality did not deteriorate with the use of a pellet with the olivine as additive. SUMMARY AND CONCLUSIONS The chemical composition of the olivine from is seen as an olivine of high quality and is similar to the reference olivine. For the serpentine, however, distinctly lower MgO-content was analysed, besides a clearly higher Al 2 O 3 -content and a clearly higher LOI. The sintering results indicated increased coke breeze consumption up to 2 kg/t sinter for the tests using the olivine. At the same time, however, a clearly increased sintering productivity was found, especially for the tests with olivine < 3 mm as additive. The physical properties of the sintering tests with olivine as additive can be rated as favourable. The tumbler strength results are very similar for these tests and up to 2.5 % higher compared to the sinter using serpentine as additive. Regarding metallurgical properties, all sinters resulted in high reducibility. A tendency was found for increasing reducibility using olivine from and serpentine as additive, compared to the reference olivine. No clear trend was found for the disintegration behaviour. One of the main results from the full-scale tests at LKAB s concentrator plant was that the grinding seemed to consume somewhat less energy with the olivine, in comparison to the alternative olivine. This result needs a more thorough indepth investigation with a bigger experimental data bank to be all conclusive, but still it is an indication of a difference between the normally used olivine and the olivine. In the pelletizing production at LKAB s plant in Sweden, the olivine was rated equal to the beforehand used and the product quality was established as satisfactory. The experimental blast furnace test concluded that the production of iron, in pilot scale with a pellet based on the olivine, was not affected by the olivine. The test indicated, however, a positive process stability, a slightly higher productivity and a to some extent lower consumption of reducing agent. The olivine mined at was accordingly verified as a first-rate additive to the production of sinter and pellet. Since the second half of 2006, the olivine is a permanent additive to LKAB s pellet production in Sweden. ACKNOWLEDGMENTS This paper is a summarized view of research conducted by others than the author. In accordance, the researchers are acknowledged for their high-quality work and endurance. Dr Guangqing Zuo, Mr Johan Oja, Mr Tommie Edeblom, Dr Lars Nilsson, Mr Johan Bucht, Mr Peter Mörtlund, Mr Benny Andreasson, Mr Jonas Lövgren and Mr Mikael Pettersson all have given a substantial contribution to the input of data and management of the research. SGA in Germany is acknowledged for their sintering tests. GLOSSARY A symbol of heat BF Blast furnace Dolomite CaMg(CO 3 ) 2 Dunite An igneous, plutonic rock, of ultramafic composition, usually with coarse-grained or phaneritic texture Fayalite Fe 2 SiO 4 Fosterite Mg 2 SiO 4 Fluxing agent A designation of material/additives to e.g. ores or ceramics to facilitate melting, or prevent oxidation. ISO 3271 Tumbler and abrasion test ISO 4596 Reducibility test ISO /2 Disintegration tests ISO 7215 Reducibility test Kirschsteinite CaFeSiO 4 LOI Loss on ignition Limestone CaCO 3 Monticellite CaMgSiO 4 SM Sinter mix Tephroite Mn 2 SiO 4

9 REFERENCES K.K.E. Neuendorf et al, Glossary of Geology, 5ed, American Geological Institute, Alexandria, Virginia, US, 2005 R.L. Bates, Geology of the Industrial Rocks and Minerals, Dover Publications, Inc., New York, 1969 P.W. Harben and M. Kuzvart, A Global Geology: Industrial Minerals, 1996 A.M. Evans, Ore Geology and Industrial Minerals: An Introduction, Blackwell Science Ltd, UK, 1993 Fredriksson, SeqiOlivine from mine to end user, Conference in Mineral Processing, 6-7 Feb., Luleå, Sweden, 2007 Z. Guangqing, Industrial Applications of SeqiOlivine, IM Dinner and Forum, Helsinki, November, 2006 C.E. Loo and D.J. Wong, ISIJ International, vol. 45, no. 4, pp , 2005 S.P.E Forsmo and A. Hägglund, Int. J. Min. Proc. 70, pp , 2003