**1. Introduction**

The ironmaking industry is one of the largest fossil fuel-consuming industries globally. Several methods are utilized in the industry, such as the blast furnace/basic oxygen furnace (BF-BOF), direct reduction/electric arc furnace (DR-EAF), smelting reduction/basic oxygen furnace (SR-BOF), and melting of scrap in an electric arc furnace (EAF) [1]. These methods use fuel and reducing agents in coal, coke [1], natural gas, or oil derived from fossil fuels [2]. Consequently, this comprises a considerable contribution to global CO2 emissions. Thus, renewable fuels utilization is an alternative way to reduce CO2 emissions. Biomass is now being heavily explored, as evidenced by multiple studies indicating that biomass can help mitigate CO2 emissions in the ironmaking process. Purwanto et al. [3] found that using charcoal obtained from oil palm empty fruit bunch for sintering low-grade iron ore potentially decreased CO2 emission in the ironmaking process. Furthermore, utilizing

**Citation:** Zulkania, A.; Rochmadi, R.; Hidayat, M.; Cahyono, R.B. Reduction Reactivity of Low Grade Iron Ore-Biomass Pellets for a Sustainable Ironmaking Process. *Energies* **2022**, *15*, 137. https:// doi.org/10.3390/en15010137

Academic Editor: Francesco Nocera

Received: 12 November 2021 Accepted: 20 December 2021 Published: 25 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

torrefied biomass as an ironmaking reductant has the potential to be carbon neutral due to biomass's propensity to adsorb CO2 during its growth phase [4]. Reducing agents required in the iron ore reduction process can be met by biomass's when thermally decomposed into carbon, CO, and H2. This is shown in several reduction studies using charcoal from sawdust, nutshell, and waste biomass [5] and raw biomass, such as pine sawdust [6,7], coconut shell waste [8], and corn straw [9]. The use of biomass in iron ore reduction provides a good interaction between iron ore and biomass. The iron ore plays a good role as a catalyst for the pyrolysis process of biomass into the volatile matter, carbon, and gas [10,11] and, simultaneously, the pyrolysis results become reducing agents that encourage reduction reaction.

Biomass may be used as a reducing agent in various methods. The chemical vapor infiltration (CVI) method utilizes volatile matter and gas from the pyrolysis of biomass to diffuse into iron ore in different chambers. This method stimulates the formation of carbon deposits, and the reduction reaction co-occurs. The CVI method directs the formation of nanoscale distances between carbon-iron ore, which causes the reduction rate to occur faster [12]. Investigation of Cahyono et al. [13] showed that using the CVI method in a sinter plant could significantly decrease coke breeze and CO2 emissions in the ironmaking sector. Another study indicated that the reduction rate of composites with the CVI method was higher than that of a mixture of dehydrated iron ore and coke [14]. Another way for obtaining carbon deposits as reducing agents is to impregnate iron ore with biomass tar and then carbonize it. This triggers a high reduction reactivity [15]. In addition, the biomass is also used as a composite mix with iron ore in the form of pellets or briquettes. Briquettes of a mixture of iron ore and pine sawdust, which reduced at a temperature of 1100 ◦C with a reaction time of 60 min, generated the reduced iron with Fe metal content up to 94.5% [16]. The investigation conducted by Guo et al. [17] showed that the pellet reduction rate with biomass was relatively higher than without biomass at the same temperature. This is caused by the increase in pellet porosity due to dehydrated and pyrolyzed biomass promoting a higher interfacial chemical reaction rate.

The biomass utilization as a reducing agent can be more efficient when it is upgraded to increase the calorific value and volumetric energy density, reduce ash, more accessible handling properties, and diminish moisture content. Several forms of upgraded biomass include a pellet form, charcoal, torrefied biomass, and others [1]. According to Yuan et al. [18], the metallization degree of the reduced iron ore-straw fiber pellets was slightly lower than those of the bamboo char-iron ore pellets and the charcoal-iron ore pellets at 600–800 ◦C. However, it will reach a comparable metallization value to the other two pellets at temperatures above 1000 ◦C. Meanwhile, another study showed slightly different things. The reduction process for iron ore-raw biomass mixture pellets provides a faster reduction rate at a relatively low temperature than composite pellets of iron ore-coke and iron ore-charcoal. In addition, it displays lower apparent activation energy than the other pellets [19]. Several reduction studies using iron ore-raw biomass mixture pellets have been carried out. Rashid et al. [20] used spherical pellets of iron ore-palm kernel shell mixture with a diameter 10–12 μm in their reduction study. The composition of PKS by 30% by weight at a temperature of 900 ◦C resulted in the majority content of reduced pellets being wustite (FeO). Furthermore, the study conducted by Huang et al. [6] used a cylindrical pellet mixture of iron ore-pine sawdust with a Ø 15 mm × 10 mm. The study results demonstrate that the iron ore-biomass ratio that gives the optimum degree of metallization is 1:0.6.

One of the essential parameters considered in the reduction or pyrolysis process is the heating rate. Several studies have analyzed the effect of heating rate changes on reduction reactivity through thermogravimetric analysis (TGA). The study utilized carbonized goethite ore samples [12] and iron ore-biochar pellets [21]. These studies show that increasing the heating rate on the same reducing agent will shift the initial temperature of the weight loss. However, the trend of decreasing weight loss for each heating is almost the same. Eventually, several previous studies that used biomass as a reducing agent in the

iron ore-biomass pellets found it attractive to apply these for manufacturing sponge iron or direct reduction iron (DRI). Based on the analysis of Mousa et al. [1] and Suopajärvi et al. [2], one of the processes in ironmaking is with DR-EAF, which requires the supply of DRI in the process. The DRI of this iron ore-biomass pellet reduction process can be an attractive alternative.

The current study is proposed due to the promising prospects for DRI preparation in the pellets of mixture iron ore-biomass and the lack of information about the effect of the heating rate on the reduction reactivity of iron ore-biomass pellets. The purpose of this study is to find out in more detail how the reduction reactivity in pellets of low degree iron ore and palm kernel shell (PKS) mixtures at various heating rates, the mechanism of the reduction of the pellets at various temperatures and heating rates, and the effect of the temperature and heating rate on the value of the reduction and metallization degrees. Results of the thermogravimetric analysis (TGA) could inform the reduction reactivity of iron ore-PKS composites at various heating rates. The temperature and heating rate were varied during the reduction laboratory experiments. The reduction mechanism was investigated from the X-ray diffraction (XRD) analysis of the reduced composites and the composition of the reduced gas. Finally, the reduced product's degrees of reduction and metallization of the reduced product are determined.

#### **2. Materials and Methods**

#### *2.1. Materials Specification and Preparation*

PT. Meratus Jaya Iron&Steel, South Kalimantan, Indonesia, supplied raw iron ore, with a size ranging from 1 to 3 cm. Meanwhile, palm kernel shell (PKS) as the biomass utilized in the study was provided by PT. Astra Agro Lestari Tbk., South Kalimantan, Indonesia. The iron ore was finely crushed and filtered into particle sizes ranging from 53 to 149 microns. The original ore was calcined at 350 ◦C for 3 h in an air environment with a heating rate of 3.5 ◦C/min before being mixed with biomass. The heating process is designed to reduce the combined water (CW) and increase the sample surface area [9–11,22]. Additionally, biomass powder is obtained by crushing and sifting into particles with sizing of 74 to 149 μm. Eventually, molasses is used as a binder to form iron ore-biomass pellets.

Table 1 shows the composition and phase identification of the original iron ore/goethite as determined by X-ray fluorescence (XRF Epsilon 4, Malvern, UK) and X-ray diffraction (XRD-Bruker D2 Phaser, Billerica, MA, USA).



*TFe*: Total Fe; CW: combined water.

Phase identification of the dehydrated ore was also carried out by XRD analysis. N2 adsorption-desorption measurements were used to determine the iron ore samples' BET surface area, pore-volume, and pore distribution (Quantachrome Instr. Ver. 10.01, Boynton Beach, FL, USA). Furthermore, scanning electron microscopy (SEM) was used to examine the surface structure of original and dehydrated iron ores (SEM-Jeol Jsm 6510 La, Tokyo, Japan). The reduction reactivity of dehydrated iron ore as a raw material for pellets was examined by thermogravimetric analysis (TG-Linseis STA, Selb, Germany). Additionally, components and elements of biomass and molasses were determined using the proximate analysis (Nabertherm N3/R Muffle furnace, Lilienthal, Germany; Heraeus UT 5042 EK, Burladingen, Germany) and ultimate analysis (LECO CHN/S-628/632, St. Joseph, MI, USA). The analysis results are provided in Table 2.


**Table 2.** Properties of the biomass and molasses.

The iron ore-biomass pellet composite is formed with an iron ore-biomass ratio of 7:3 by mass, and the binder used is 9% of the mixture. After mixed ingredients are obtained, pellets for thermogravimetric analysis (TGA) and reduction experiments are formed according to the cylinder diameter and pellet weight required. The pellets for TGA analysis are thin cylindrical pellets with a diameter of 2 mm and a composite weight of 19 mg. Pellet formation utilizes a pellet mold and a pressure of 2 kN (hydraulic pump). At the same time, the formation of cylindrical pellets for the reduction experiments uses a pellet mold with a diameter of 5 mm and a pressure of 40 kN, with a pellet weight of 0.2 g. Subsequently, they are dried in an oven at 105 ◦C for 3 h to expel moisture before being stored in closed storage before use.
