Effect of Adding Biomass from Palm Kernel Shell on Phase Transformation and Microstructure during Carbothermic Reduction of Ilmenite

This study investigated the effects of adding pulverized biomass from palm kernel shells as a reductant on phase transformation and microstructure during the carbothermic reduction of ilmenite. The ilmenite concentrate was reduced at a temperature range of 1000–1200°C for up to 3 h in an inert atmosphere. The amount of reducing agents used in this study varied between 6 and 10 wt.%. The reduced samples were studied using an X-ray diffractometer and optical microscope to analyze their phase transformations and microstructures. The results revealed that near-complete dissociation of ilmenite was attained when biomass was added up to 8 wt.%. Complete dissociation of ilmenite occurred, and Ti₃O₅ formed when ilmenite was reduced with 9 wt.% biomass at 1200°C for 2 h. At this point, the microstructural study showed that a significant amount of metallic iron, with an average metallic iron size of 241.5 µm², had formed.

Biomass; Carbothermic reduction; Ilmenite; Palm kernel shell

The world’s rutile reserves are low compared to ilmenite reserves (U.S. Geological Survey, 2021), and their amount is dwindling because rutile is still used in the titanium extraction process. An upgrading process is carried out to increase the titanium content by separating iron from titanium oxide in ilmenite and by increasing TiO₂ levels.

Currently, the upgrading process of ilmenite is carried out using the Becher, Murso, Laporte, Benelite, Austpac, Dunn, Kataoka, Altair, and BHP Billiton processes (Nguyen and Lee, 2019). However, the Becher, Murso, and Laporte processes require high energy consumption that yields high CO₂ emissions and are less effective due to the need to conduct an acid leaching process after pyrolysis. In addition, in the Benelite process, ilmenite is limited as a feed. The Austpac process is less effective because it requires higher acidity for the leaching of the remaining magnetic iron. The Dunn process is also considered less environmentally friendly because of the highly corrosive Cl₂ handling process, while the Kataoka, Altair, and BHP Billiton processes are also considered less environmentally friendly because of the subsequent leaching process (Nguyen and Lee, 2019). The Becher process involves reducing the iron in ilmenite to metallic iron by way of burning coal at 1200°C followed by aeration and leaching to remove the metallic iron (Gázquez et al., 2014). 

Many aspects of the carbothermic reduction process of ilmenite have attracted much research. Tripathy et al. (2012) reduced ilmenite at 1000–1150°C using graphite and coke as reducing agents. According to the findings, graphite is effective as a reducing agent at a reduction temperature of 1150°C, and it can work within a shorter time interval. Meanwhile, coke is more effective when heated to 1000°C. El-Hussiny et al. (2008) reduced ilmenite with breeze coke at 800–1200°C and discovered that reducibility increased with the stoichiometric amount of coke. Furthermore, the rate of reduction increased as the temperature rose. Bhalla et al. (2017) carried out the carbothermic reduction of ilmenite with graphite as the reducing agent at temperatures between 1050°C and 1350°C under an argon atmosphere. They reported that the rate of reduction increased with increases in temperature and decreases in particle size. Ilmenite reduction can be accomplished through a step in which iron oxide is first reduced to metallic iron and then to iron carbide, while titanium oxide is initially reduced to Ti_nO_2n-1 with the possibility of later lower oxide formation. The particle core (original ilmenite grain) retains its composition, with a slight reduction in iron and titanium oxides in the early stages of reduction and is partially reduced in later stages. According to Wang et al. (2008), the reduced ilmenite phase formed by graphite contains iron, ilmenite, rutile, reduced rutile, pseudorutile, and graphite below 1200°C. From 900–1000°C, however, reduced rutile was absent. Above 1200°C, the Ti₃O₅ phase appears alongside iron, rutile, reduced rutile, Fe₃C, and pseudobrookite solid solution.

The utilization of biomass as an alternative carbon source has been studied in many fields, for example, the co-gasification process with blending fuel (biomass and coal) and reductant agents in the carbothermic reduction of minerals. Ahmad et al. (2020) investigated the effect of a torrefied palm kernel shell and Mukah Balingian coal on product yield and gaseous composition in the co-gasification process. They concluded that pretreatment with this biomass mixture resulted in higher gas yields and lower tar and charcoal yields, thereby improving the co-gasification performance. Meanwhile, many researchers have investigated the use of biomass (including palm oil waste) as a renewable carbon source for the carbothermic reduction process of minerals and ores. Using the biomass of palm kernel shell waste as a reducing agent, iron oxide in low-grade iron ore could be reduced completely to magnetite and partially to wustite when up to 30% of the biomass was used in the reduction process (Abd Rashid et al., 2014). Adding up to 30% of sawdust biomass containing 12.5% fixed carbon and 80.7% volatile matter successfully reduced the iron ore to its metallic iron phase at 1200°C (Strezov et al., 2006). Furthermore, Srivastava et al. (2013) investigated the use of fine wood biomass as a reducing agent in the smelting of magnetite concentrate. They found that pig iron nuggets with metallization of 98.10% and a total Fe of 97.16% could be produced at a temperature of 1450°C and a reaction time of 20 min. In addition, Zhang et al. (2017) studied the roasting process of low-grade limonite ore with biomass at temperatures of up to 750°C for 45 min in a vacuum atmosphere. They reported that after dehydration at high temperatures, the iron-bearing materials in limonite ore became ?-Fe₂O₃, which was then reduced to Fe₃O₄ by biomass reduction. Fe₃O₄ will convert to Fe₂SiO₄ at temperatures above 650°C, causing magnetic materials to be reduced and the calcined ore’s magnetism to be weakened. Furthermore, at a temperature of 550°C with a biomass ratio of 15%, the recovery rate and iron grade were 72% and 58%, respectively.

However, published reports on the use of biomass as a reducing agent in the upgrading process of ilmenite are still lacking. Ismail et al. (1982) investigated the decomposition of preoxidized ilmenite using sawdust biomass as a reducing agent at 1100°C for 3 h. They reported that two steps occurred during the decomposition process. The first step was the conversion of Fe³⁺ in ferric pseudobrookite to Fe²⁺ in ilmenite. At this point, the concentration of ferric pseudobrookite decreased over time, while the concentration of ilmenite increased. This indicates that ilmenite was reformed by a recombination reduction mechanism in the early stages of reduction. The second step was the conversion of Fe²⁺ to metallic iron. As a result, the concentration of ilmenite gradually decreased, while the concentrations of metallic iron and rutile increased (suboxides of titanium). Recently, Setiawan et al. (2020) investigated the carbothermic reduction of ilmenite using palm kernel shells as a reducing agent. They found that a reduction temperature of 1200°C using a solar furnace promoted pseudobrookite formation, and a unique line morphology in metallic iron was observed instead of the spherical structure found in samples heated in electric furnaces. They found that the mechanisms behind the carbothermic reduction of complex weathered ilmenite using the biomass of palm kernel shells and graphite as reducing agents were based on the diffusion of oxygen atoms (Setiawan et al., 2021).

Thus, this research aims to investigate the effects of biomass addition during the carbothermic reduction of ilmenite on phase transformation and microstructure evolution. The phase characterization and detailed microstructures during the carbothermic reduction of ilmenite at up to 1200°C for different reaction times were observed and discussed.

    The effects of adding palm kernel shell biomass as a reducing agent on the carbothermic reduction of ilmenite were studied. The following conclusions were drawn: (1) The phase analysis showed the near-complete dissociation of ilmenite at 1200°C for 2 h with the addition of 6 and 8 wt.% biomass. Complete dissociation was reached as more biomass was added. At this point, the phases formed were metallic iron, rutile, pseudobrookite, and anosovite; (2) The morphology of metallic iron consisted of the streak and granule shapes. The particle size of metallic iron was greatly enhanced with increased added biomass. The maximum average grain size of metallic iron formed was found to be 241.5 µm² with the addition of 9 wt.% of biomass at 1200°C for 2 h.

    The authors would like to express their gratitude for the financial support from the Directorate of Research and Public Engagement, Universitas Indonesia, under the PUTI Prosiding Grant 2020 (NKB-1199/UN2.RST/HKP.05.00/2020) and the PUTI Kolaborasi Internasional 2020 (NKB-797/UN2.RST/HKP.05.00/2020). PT Monokem Surya, Karawang, Indonesia is thanked for providing ilmenite concentrate. Dr. Mark I Pownceby (CSIRO Mineral Resources, Australia) is gratefully acknowledged for providing XRF data.

Ahmad, R., Ishak, M.A.M., Ismail, K., Kasim, N.N., Mohamed, A., Ani, A., Radzun, K., 2020. The Effect of Pretreated Palm Kernel Shell and Mukah Balingian Coal Co-Gasification on Product Yield and Gaseous Composition. International Journal of Technology, Volume 11(3), pp. 501–510

Abd Rashid, R.Z., Salleh, H.M., Ani, M.H., Yunus, N.A., Akiyama, T., Purwanto, H., 2014. Reduction of Low Grade Iron Ore Pellet using Palm Kernel Shell. Renewable Energy, Volume 63, pp. 617–623

Bhalla, A., Kucukkargoz, C.S., Eric, R.H., 2017. Solid-State Reduction of an Ilmenite Concentrate with Carbon. Journal of the Southern African Institute of Mining and Metallurgy, Volume 117(5), pp. 415–421

El-Hussiny, N.A., Lasheen, T.A., Shalabi, M.E.H., 2008. Kinetic Reduction of Rosetta Ilmenite with Coke Breeze and Beneficiation of the Product. The Journal of Ore Dressing, Volume 9(18), pp. 8–16

Gázquez, M.J., Bolívar, J.P., García-Tenorio García-Balmaseda, R., Vaca, F., 2014. A Review of the Production Cycle of Titanium Dioxide Pigment. Materials Sciences and Applications, Volume 5(7), pp. 441–458

Ismail, M.G.M.U., Amarasekera, J., Kumarasinghe, J.S.N., 1982. Studies on Decomposition of Ilmenite from Sri Lanka. Journal of the National Science Foundation of Sri Lanka, Volume 10(1), pp. 107–127.

Nguyen, T.H., Lee, M.S., 2019. A Review on the Recovery of Titanium Dioxide from Ilmenite Ores by Direct Leaching Technologies. Mineral Processing and Extractive Metallurgy Review, Volume 40(4), pp. 231–247

Run, H., Xiaodong, L., Qinghui, W., Qinzhi, W., Jinzhu, Z., 2019. Non-Isothermal Reduction Kinetics of Iron During Vacuum Carbothermal Reduction of Ilmenite Concentrate. Metallurgical and Materials Transactions B, Volume 50(2), pp. 816–824

Sarkar, B.K., Samanta, S., Dey, R., Das, G.C., 2016. A Study on Reduction Kinetics of Titaniferous Magnetite Ore Using Lean Grade Coal. International Journal of Mineral Processing, Volume 152, pp. 36–45

Setiawan, A., Rhamdhani, M.A., Pownceby, M.I., Webster, N.A.S., Harjanto, S., 2021. Kinetics and Mechanisms of Carbothermic Reduction of Weathered Ilmenite using Palm Kernel Shell Biomass. Journal of Sustainable Metallurgy, pp. 1-19

Setiawan, A., Shaw, M., Torpy, A., Pownceby, M.I., Harjanto, S., Rhamdhani, M.A., 2020. Solar Carbothermic Reduction of Ilmenite using Palm Kernel Shell Biomass. The Journal of the Mineral, Volume 72(10), pp. 3410–3421

Srivastava, U., Kawatra, S.K., Eisele, T.C., 2013. Production of Pig Iron by Utilizing Biomass as a Reducing Agent. International Journal of Mineral Processing, Volume 119, pp. 51–57

Strezov, V., 2006. Iron Ore Reduction using Sawdust: Experimental Analysis and Kinetic Modelling. Renewable Energy, Volume 31(12), pp. 1892–1905

Suryanarayana, C., Norton, M.G., 1998. X-Ray Diffraction a Practical Approach. New York: Springer Science+Business Media New York, pp. 207–221

Tripathy, M., Ranganathan, S., Mehrotra, S.P., 2012. Investigations on Reduction of Ilmenite Ore with Different Sources of Carbon. Mineral Processing and Extractive Metallurgy, Volume 121(3), pp. 147–155

U.S. Geological Survey., 2021. Mineral commodity summaries 2021: U.S. Geological Survey, p. 200.

Wang, Y.M., Yuan, Z.F., Guo, Z.C., Tan, Q.Q., Li, Z.Y., Jiang, W.Z., 2008. Reduction Mechanism of Natural Ilmenite with Graphite. Transactions of Nonferrous Metals Society of China, Volume 18(4), pp. 962–968

Zhang, K., Chen, X.L., Guo, W.C., Luo, H.J., Gong, Z.J., Li, B.W., Wu, W.F., 2017. Effects of Biomass Reducing Agent on Magnetic Properties and Phase Transformation of Baotou Low-Grade Limonite During Magnetizing-Roasting. PLOS one, Volume 12(10), pp. 1–18