Experimental Investigation on Direct Reduction Followed by Magnetic Separation for Nb2o5-Bearing Ore

ISIJ International, Vol. 53 (2013), No. 8, pp. 1358–1364

Experimental Investigation on Direct Reduction Followed by Magnetic Separation for Nb2O5-Bearing Ore

Xin JIANG,1)* Fengman SHEN,1) Ligang LIU,1) Xiaogang LI2) and Lin WANG3)

1) School of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning, 110004 China. 2) Baotou Steel Group Mining Research Institute (Limited Liability Company), Baotou, Inner Mongolia, 014010 China. 3) Department of Mechanical Engineering, Shenyang Institute of Engineering, Shenyang, Liaoning, 110136 China. (Received on February 21, 2013; accepted on May 18, 2013)

Recently a special attention is being paid on the ferroniobium production worldwide, especially in China. In present work, direct reduction followed by magnetic separation for Nb2O5-bearing ore is investigated. Reflected light microscope, scanning electron microscope with EDX, and high performance X-ray diffraction were used for fundamental analysis. The experimental results show that, (1) the optimum reduction parameters are 0.9 of C/O, 1 200°C and 20 min. At these optimum parameters, the degree of metallization of ore-coal composite pellet is more than 90%. (2) In direct reduced pellets, metallic iron and slag have gathered respectively, and they can be magnetically separated. Nb and Ti are associated together and inserted in slag, and stay in slag phase after magnetic separation. (3) Based on the process of direct reduction followed by magnetic separation, 7.71% of Nb2O5-enriched slag is obtained, which is a good feed for producing ferroniobium or metallic niobium in electric furnace. The Nb2O5 content in Nb2O5- enriched slag is 1.8 times of the original ore, and the recovery of Nb is about 85%. These experimental results can give some theoretical references for industrial application in future.

KEY WORDS: Nb2O5-bearing ore; ore-coal composite pellet; direct reduction; magnetic separation; degree of metallization.

1. Introduction 25000

Niobium is an important element for MRI (Magnetic 20000 Resonance Imaging) machines, high technology, and the steel industrial. Over three-quarters of the world’s niobium 15000 production currently comes from Brazil, with most of remainder coming from eastern Canada. Recently a special 10000 attention is being paid on the ferroniobium production 1–6) worldwide, especially in China. China has been a top Imports, Metric tons 5000 importer of ferroniobium in recent years, and its imports have increased over the last decade (Fig. 1). 0 In tradional process of ferroniobium production in China 2000 2002 2004 2006 2008 2010 2012 include, (1) the raw mineral is processed primarily by phys- Year ical processing technology to get Nb2O5-bearing concen- Fig. 1. Chinese imports for niobium. trate. (2) The Nb2O5-bearing concentrate is used to produce sinter or pellet. (3) Sinter or pellet is feed into blast furnace (BF) to produce Nb-bearing liquid iron. (4) Nb is oxidized tant factor for producing ferroniobium or metallic niobium. to niobium oxides in converting process (BOF), and Compared with Sinter-BF-BOF route, reduction followed enriched in slag. In this long Sinter-BF-BOF route, particu- by magnetic separation route has the following advantag- larly in BF, some Nb presents in liquid iron, and some Nb es.8–11) (1) At the condition of BF, the reducing potential and oxides present in slag because the selective reduction of fer- reducing temperature are uncontrollable, and they are strong rous oxides and niobium oxides can not be achieved in BF. enough to reduce both ferrous oxides and niobium oxides. So the recovery of Nb is very low (about 72%) and the ener- At direct reduction process, ferrous oxides can be reduced, gy consumption per unit Nb produced is very high.7) but niobium oxides can not be reduced by controlling reduc- As we all know, the grade of Nb2O5 content is an impor- ing potential and reducing temperature. So, direct reduction is an effective process to achieve selective reduction. (2) In * Corresponding author: E-mail: [email protected] BF and BOF, the Nb2O5-enriched slag is liquid. But in mag- DOI: http://dx.doi.org/10.2355/isijinternational.53.1358 netic separation, the Nb2O5-enriched slag is solid, so the

© 2013 ISIJ 1358 ISIJ International, Vol. 53 (2013), No. 8 energy consumption is relatively lower. Therefore, aimed –74 μm is about 76%, and –99 μm is about 90%. The size for producing Nb2O5-enriched slag, direct reduction fol- distribution of this applied ore is easy for pelletization. So lowed by magnetic separation is investigated in present the ore-coal composite pellets are used as the feed material work, which can give the theoretical references for industri- for direct reduction experiments. The goal of direct reduc- al application in future. tion is that the ferrous oxides are reduced to metallic iron, but the niobium oxides are not reduced (based on the ther- modynamic analysis).14) Then metallic iron can be separated 2. Experimental Design with slag by magnetic separation. The niobium oxides pres- 2.1. Raw Materials ent in slag. This slag is named as Nb2O5-enriched slag, in Almost over 90% of Nb2O5-bearing resources in China which the Nb2O5 content is increased. This Nb2O5-enriched are found in Baiyunebo deposit in the Inner Mongolia slag is a good feed for producing ferroniobium, metallic nio- Autonomous Region. In this study, Baiyunebo ore is used as bium, and rare earth etc. The chemical compositions of coal the Nb2O5-bearing raw material. The Nb2O5 content of orig- and bentonite used in present work are listed in Table 2. inal Baiyunebo ore is very low, only 0.4–0.7%. But with the Before the pelletization, the coal and bentonite are ground development of beneficiation technology, the grade of to –74 μm. 12,13) Nb2O5 can be increased to 4.32% (Table 1). From Table 1, in addition to the ferrous oxides and niobium oxides, 2.2. Experimental Procedure there are some SiO2, TiO2, CaO, MgO and some alkalis The key for selective reduction of ferrous oxides and nio- (Na2O and K2O). bium oxides in present work is not only to reduce iron Niobium does not occur freely in nature. It primarily co- oxides and no-reduce niobium oxides, but also the reduced exists in form of pyrochlore, a niobium-rich, complex min- metallic iron particles grow to big size which is benefit for eral containing some of calcium, sodium, titanium, rare the following step --- grinding and magnetic separation. earth and other elements. Pyrochlore is the most important Another key is that the pellet can not melt during the process mineral for niobium extraction and production. Niobium can of direct reduction. If melting, the process probably is diffi- also be found in columbite, which is a niobate of iron, man- cult to be realized in industrial trial. ganese, and magnesium. X-ray diffraction of the applied ore In order to get optimum operation parameters for the is shown in Fig. 2. From the figure, one can conclude that, selective reduction of ferrous oxides and niobium oxides, (1) our applied ore is composed of hematite, magnetite, series of direct reduction experiments were conducted. In quartz, and some complex mineral including Ti, Ca, Na etc. these series of experiments, the effects of following vari- (2) Hematite is the main ferrous oxide, and there is small ables on the reduction of ore-coal composite pellets were amount of magnetite. (3) The co-exist ore containing niobi- studied: um oxides and titanium oxides is the main Nb2O5-bearing (1) The processing temperature (the temperature in the ore. The co-exist ore and the compact structure are the main Muffle furnace) for reduction. Basically, higher temperature characteristics for niobium extraction and production. (4) will result in melting of pellets, and lower temperature will The slag is mainly composed of silicate and quartz. result in low metallization degree and low rate of reduction. The size distribution of Nb2O5-bearing ore is shown in (2) The reduction time. This is related to the reduction Fig. 3. From the figure, the powder of –50 μm is about 50%, temperature, and proper time will result in high degree of metallization and big size of metallic iron, which is benefit Table 1. Chemical composition of Nb2O5-bearing concentrate ore, mass%. 100 TFe FeO Nb2O5 CaO SiO2 MgO Al2O3 MnO 8 35.52 1.6 4.32 2.42 21.87 1.05 0.26 0.31 80 percentage, % Cumulative

FSPNa2OK2OTiO2 Sc2O3 TREO 6 60 0.73 0.73 0.07 2.42 0.18 6.71 0.032 2.72 4 40

Size distribution, % 2 14000 ƹ Ʒ--Fe2O3 ƹ--Ti NbO4 20 Size distribution ƾ ƾ--Fe3O4 --Ca SiO4 ƽ Cumulative percentage Ƶ--TiO2 Ƽ--NaFeO Si2O6P 12000 --SiO2 0 0 ͩ 050100150200 ƽ 1 Ʒƹ Diameter, um ƾ 10000 Ʒ Ƽ Fig. 3. Size distribution of Nb2O5-bearing concentrate ore. Ƶ Intensity Ʒ Ʒ 8000 Ƽ Ʒ ͩ Ƶ ͩ Table 2. Chemical composition of coal and bentonite, mass%. 6000 Pulverized Fixed Carbon Total Carbon Volatile Matters Ash H2O 20 40 60 80 Coal 60.49 75.4 31.23 8.28 6.68

Angle, 2θ(deg.) SiO2 Al2O3 CaO MgO Bentonite 69.74 16.60 2.41 1.14 Fig. 2. X-ray diffraction of Nb2O5-bearing concentration ore.

1359 © 2013 ISIJ ISIJ International, Vol. 53 (2013), No. 8 for the following grinding and magnetic separation. Fig. 5. From the figure, one can conclude that the pellets (3) The amount of carbon addition, in term of the gram- will seriously melt when the temperature is higher than atomic of fixed carbon in the coal added to the gram-atomic 1200°C. According to the chemical composition of ore, the of combined oxygen in iron oxides, which is denoted as C/O dominant oxides in slag should be SiO2, TiO2, and Nb2O5. (mol C/mol O). Generally, the melting point of slag with these oxides should The experimental set-up in Muffle furnace is shown in be very high, but our experimental result is contradictory Fig. 4. The temperature is measured by a thermocouple with it, and the slag melt at 1 300°C. In order to clarify this fixed near to the sample. As an example, the reduction phenomenon, microscopic energy spectrum is used to ana- experimental procedure consists of the following steps: lyze the composition of reduced pellet (Fig. 6). (1) Pelletization. Mix raw materials with proper moisture From Fig. 6, it can be seen that there are three main phas- content → pelletizing → drying → ore-coal composite pellets. es in the reduced pellets: (2) Heat up the furnace to the pre-determined tempera- (1) White phase, which is metallic iron phase and repre- ture. sented by point A. Metallic iron grains have grow to be crys- (3) Place ore-coal composite pellets into a crucible, and put tal stock, which are big enough for magnetic separation. the loaded crucible into the muffle furnace at pre-determined (2) Light grey phase, which is the compound mineral of temperature. titanium oxides and niobium oxides, represented by point B. (4) Keep the furnace at the pre-determined temperature to The melting point of this phase is very high, and the mineral reduce ore-coal pellets for pre-determined time. keeps its original shape with obvious angles and lines. (5) After pre-determined time, the crucible with the direct (3) Deep grey phase, which is the basal body---melting reduced iron (DRI) is taken out of the furnace. And the cru- slag. There are large amounts of SiO2 and small amount of cible is cooled under flowing argon to avoid re-oxidation of Na2O and FeO. The chemical composition of this slag is metallic iron during cooling. shown in Table 3. From the table, SiO2/(SiO2+Na2O) and (6) The degree of metallization (MD) is defined as the SiO2/(SiO2+FeO) are 90.3 mol% and 79.5 mol% respective- percentage of metallic iron in total iron. It’s calculated by ly. The liquidus lines of SiO2–Na2O and SiO2–FeO binary “[(metallic iron) / (total iron)] × 100%”. The metallic iron phase diagram are shown in Fig. 7. It can be seen that, for and total iron are measured by chemical determination. The this kind of slag, some liquid presents at experimental tem- DRI pellets are examined by reflected light microscope and perature (1 000°C to 1 400°C). Higher temperature, more scanning electron microscope. And the formed phases in liquid phase. Therefore, the Na2O and FeO are the main rea- Nb2O5-enriched slag are identified and analyzed by high sons for lower melting point of slag in reduced pellets. performance X-ray diffraction. Therefore, in order to realize the process of direct reduction (7) A CXG-08SD(A) magnetic tube is used for magnetic at solid state, the reducing temperature should not be higher separation. The range of the magnetic field density is 0– than 1 200°C in the laboratory condition. 600 mT. 3.2. Effect of Reducing Temperature on Degree of Met- allization 3. Experimental Results and Discussion The degrees of metallization of ore-coal composite pellets 3.1. Effect of Higher Temperature on Melting of Pellets at different reducing temperature and for different reducing In order to investigate the effect of the reducing temper- time are shown in Fig. 8. One can conclude that, (1) reduc- ature on the reduction and melting of ore-coal composite ing temperature is more important than reducing time. (2) pellets, a series of reduction experiments at different tem- When the reducing time is 25 min, the degrees of metalli- perature were carried out, and the temperature is 1 000°C, zation at 1 000°C, 1100°C, and 1 200°C are 13%, 66%, and 1100°C, 1 200°C, 1 300°C, and 1 400°C respectively. The 90% respectively. The carbothermic direct reduction of iron reducing time and carbon addition are constant, and they are oxide, FeO + C = Fe + CO, is a strong endothermic reaction θ 15) 25 min and C/O=1.0 respectively. (ΔH 298=154 kJ/mol). Therefore, higher temperature can The appearances of pellets after reduction are shown in increase the rate of reduction reaction and the degree of metallization. In order to obtain Nb2O5-enriched slag by magnetic separation following direct reduction, not only the high degree of metallization of pellet is necessary, but also the size of

1) Thermocouple, 2) Air atmosphere, 3) Heating elements (SiC), 4) Crucible, 5) Ore-coal composite pellets, 6) Refractory insulating board Fig. 5. Appearances of pellets after reduction at different tempera- Fig. 4. Experimental set-up in Muffle furnace. tures (25 min).

200 FeKa Point A

150 FeLa 100 Counts

0 02468101214 Energy, KeV

250 500 TiKa SiKa NbKa Point B Point C 200 400

150 300 NaKa MgKa Counts Counts 100 200 AlKa OKa OKa CaKa 50 100 TiKa FeKa 0 0 02468101214 02468101214 Energy, KeV Energy, KeV

Fig. 6. Microscopic energy spectrum analysis for reduced pellet (1 200°C, 25 min, C/O=1.0).

Table 3. Chemical composition of slag. 100

Percentage SiO2 Na2OFeO Mol% 73.21 7.87 18.92 80

40 1200ć 1100ć 1000 20 ć Degree of Metallization, % Degree

0 5 10152025 Reducing time, min

Fig. 8. Effects of reducing temperature and reducing time on degree of metallization.

metallic iron are very small and obscure, look like iron whisker. (2) At 1100°C, the degree of metallization is higher than that at 1 000°C. In Fig. 9(b), the crystal grains of metallic iron grow and become bigger, and the metallic iron phase is obvious (white). (3) At 1 200°C, the degree of met- Fig. 7. Liquidus lines of binary phase diagrams for SiO2–Na2O and allization is highest. In Fig. 9(c), the crystal grains of metal- SiO –FeO. 2 lic iron continue to grow and become bigger and bigger, and these big metallic iron particles are benefit for grinding ore metallic iron particles in reduced pellet is another signifi- and magnetic separation. cantly important influencing factor. The big size of metallic Based on the above analysis, higher temperature (higher iron in reduced pellet is benefit for grinding ore and mag- than 1 200°C) will result in melting of pellet and the direct netic separation. Therefore, the microstructure and the size reduction in solid state can not be realized. But lower tem- of metallic iron in reduced pellets at different temperature perature (lower than 1 200°C) will result in low degree of are observed and analyzed by reflected light microscope, metallization and small size of metallic crystal grains, which which is shown in Fig. 9. is not benefit for the following grinding ore and magnetic From Fig. 9, it can be seen that, (1) at 1 000°C, the degree separation. Therefore, in the lab scale, the optimum reduc- of metallization is lower. In Fig. 9(a), the crystal grains of ing temperature for Nb2O5-bearing ore-coal composite pel-

Fig. 9. Micrographics of pellets after reduction at different temperature. lets’ selective direct reduction is 1 200°C. 100

3.3. Effect of C/O on Degree of Metallization 80 The amount of carbon addition is also an important influencing factor on the degree of metallization of ore-coal composite pellets. Generally C/O is higher, the reducing rate 60 is faster, and the degree of metallization is higher.16) In present work, the effect of C/O on degree of metallization of 40 C/O=0.7 Nb2O5-bearing ore-coal pellets at 1 200°C (optimum reducing temperature based on above discussion) is shown in Fig. C/O=0.8 20 C/O=0.9 Degree of Metallization, of Degree % 10. From the figure, it can be seen that, (1) when C/O is 0.7 C/O=1.0 and 0.8, the degrees of metallization of pellet are lower, only 0 about 80% for 20 min, due to the insufficient reducing 0 5 10 15 20 25 agent. (2) There is no obvious difference in degree of met- reducing time, min allization between C/O=0.9 and C/O=1.0, and they are about 90% for 20 min. (3) Basically, there is no obvious Fig. 10. Effects of C/O on degree of metallization of pellets ° increase in degree of metallization after 20 min. Therefore, (1 200 C). the optimum C/O is 0.9, and the optimum reducing time is 20 min in our lab scale investigation. –200 μm, and the size distribution of powder before magnetic separation is shown in Fig. 12. A magnetic tube is used 3.4. Elements Distribution in Reduced Pellets for magnetic separation. The density of magnetic field in The reduced pellets will be magnetic separated for metal- current work is kept at 90 mT. and the Nb2O5 content in slag lic iron and Nb2O5-enriched slag, so the elements distribu- after magnetic separation is shown in Fig. 13. tion in reduced pellets is necessary to be investigated. The From Fig. 13, one can conclude that, (1) when the carbon optimum parameters based on above analysis are C/O=0.9, addition is lower, C/O=0.7 (No. 071220), the reducing agent reducing temperature is 1 200°C, and reducing time is 20 min. is insufficient and the degree of metallization is lower, so the The elements distribution of reduced pellet at these opti- FeO content in slag is higher and result in the lower Nb2O5 mum parameters is give in Fig. 11. From the figure, one can content in slag. (2) Lower reducing temperature, 1 100°C conclude that, (1) metallic iron and slag have gathered (No. 081120), result in lower degree of metallization, and respectively, and they are obviously two phases, so they can the Nb2O5 content in slag is lower too. (3) C/O=0.9 is good be magnetically separated. (2) Nb and Ti are associated enough for high degree of metallization and high Nb2O5 together and inserted in slag, they are non-magnetic sub- content in slag. So, higher C/O than 0.9 is not required, stances, and stay in slag phase after magnetic separation. (3) because the ash of coal can increase the slag volume and The distributions of Na and Si are similar, that’s why the decrease the Nb2O5 content in slag. In Fig. 14, The X-ray melting point of this slag is very low. diffraction of slag after magnetic separation exhibits that some of niobium oxide is reduced to niobium carbide 3.5. Magnetic Separation (NbC–Nb2C), and some of niobium is still in the form of 10 grams of some selected reduced pellets at different pyrochlore (Ti.4Fe.3Nb.3O2). Therefore, most of niobium is reducing parameters are the feed materials for magnetic sep- enriched in slag after magnetic separation, which is the goal aration tests. The pellets are ground to powder with 80% of of our present work.

Fig. 11. Elements distribution in reduced pellet (C/O=0.9, 1 200°C, 20 min).

8 100 5000

Ʒ--Fe2O3 ƹ--FeTi O3

Cumulative percentage, % percentage, Cumulative --Nb6 5 80 Ƶ--TiO2 ƾ C 6 ƽ--Ti.4Fe.3Nb.3O2 4000 ƽ Ƶ 60 ƾ ƽ 4 Ʒ ƾ 3000 ƹ ƽ 40 Ƶ Internsity ƾ 2 Size distribution, % 20 Size distribution 2000 Cumulative percentage 0 0 20 40 60 80 100 0 100 200 300 400 500 Angle, 2θ(deg.) Diameter, um Fig. 14. X-ray diffraction of slag after magnetic separation. Fig. 12. Size distribution of powder before magnetic separation.

8 7.71%, which is a good feed for producing ferroniobium or metallic niobium in electric furnace. The Nb2O5 content in original Nb2O5-bearing ore is only 4.32%. Therefore, the Nb2O5 content in this Nb2O5-enriched slag is 1.8 times of 6 the original ore. The recovery of Nb is about 85%, which is higher than that of Sinter-BF-BOF route in China (about 72%).

4 content in slag, % content in slag, 5 4. Conclusions O 2

Nb In this study, the direct reduction followed by magnetic separation for Nb2O5-bearing ore is investigated. The main 2 findings could be summarized as follows: 12345 071220 081120 081220 081225 091220 (1) For the selective direct reduction of Nb O -bearing Experiment No. 2 5 ore-coal composite pellets, the optimum C/O is 0.9, the opti- Example: 071220 --- C/O=0.7 (the first two numbers), Reducing mum reducing temperature is 1 200°C, and the optimum temperature is 1 200°C (the middle two numbers), Reducing time is 20 min (the last two reducing time is 20 min in our lab scale investigation. At the numbers). conditions of these optimum parameters, the degree of met-

Fig. 13. Nb2O5 content in slag after magnetic separation, %. allization of ore-coal composite pellet is more than 90%. (2) In direct reduced pellets, metallic iron and slag have Based on above analysis and discussion, the sample gathered respectively, and they can be magnetically separat- 091220 (C/O=0.9, 1 200°C, 20 min) is the best one in our ed. Nb and Ti are associated together and inserted in slag, test range, and its Nb2O5 content in slag is highest, about they are non-magnetic substances, and stay in slag phase

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