Developing Iron Ore Pellets Using Novel Binders for H2-Based Direct Reduction (1)
1. Introduction
In recent times there has been an outstanding shift from the traditional coke-based iron- and steelmaking process to a more environmentally friendly green H2-based direct reduction process. This shift is primarily motivated by the significant reduction in the carbon emissions achieved by the H2-based technology where the carbon emissions decrease significantly, dropping from 1600 kg CO2 per metric ton of steel produced to just 25 kg CO2 per metric ton of steel produced. However, iron ore pellets will be the exclusive feed material that fits this green transition due to their special properties such as high porosity–better reducibility, uniform chemical composition, better strength, better permeability, enhanced steel yield, and increased metallization rate compared to other feeds such as lump iron ore and sinter. Forecasts suggest that by 2050 global steel demand will see further growth and will have increased by one-third of what it is today (1950 Mt). With the growing demand for steel and the transition towards the green H2-based direct reduction process, there is a significant expected increase in the demand for iron ore pellets. In 2020, the demand for iron pellets was approximately 400 Mt, and it is projected to rise to 540 Mt by the year 2027. This substantial increase in demand reflects the shift in manufacturing preferences and highlights the crucial role that iron pellets will play in meeting the requirements of the evolving steel industry.
Pelletizing is the technique of rolling fine-grained iron ore concentrates with adequate size distribution into small balls of the desired size using a wetting agent and a binder. The most common and traditional binder is bentonite; the wetting agent is usually water. Bentonite consists of alumina and silica and is generally added at a concentration between 0.5 and 1.5% of the iron ore concentrate. This increases the gangue materials of iron ore and consequently decreases the iron content. The addition of 1% bentonite decreases the iron content by about 7 kg/ton of iron ore. Currently, 3.2 Mt of bentonite is used for pelletizing iron for ironmaking. Lime also has to be added while pelletizing with bentonite to adjust the basicity, which increases the amount of slag generated and the CO2 emissions. For every ton of crude steel produced, approximately 500 kg of solid wastes (slag) are generated. One possible solution to reduce the undesirable gangue constituents, including alumina, silica, and calcium, in the final product is to explore alternative binders that can replace or decrease the usage of bentonite.
Several studies and research have already been conducted to test organic binders as alternative binders to bentonite. A combination of boron compounds with organic binders such as carboxymethyl cellulose (CMC), starch, dextrin, and some organic-based binders was tested. It was found that the produced pellets show a good metallurgical and chemical quality when compared with bentonite-bonded reference pellets. Moreover, CMC organic binder was used to produce iron ore pellets, and it showed better mechanical properties and satisfactory metallurgical properties when compared with bentonite. The only problem was with its fired compressive strength and tumbling resistance. Moreover, there are several organic binders markedly accessible for iron ore pelletization such as BASF' s Alcotac® CS and FE-16, which have been successfully used as alternatives for bentonite for iron ore pelletization. Additionally, the polymer-based binder, KemPel™ from Kemira, has shown promising results in partially replacing bentonite without compromising pellet qualities and properties.
According to the authors’ knowledge, organic binders have been seen as a promising replacement for bentonite in iron ore pelletizing. There exist some novel and innovative organic binders, of which some have been specifically designed for iron ore pelletizing. However, relatively little research has been conducted on utilizing new and innovative organic binders in iron ore pelletizing as a substitute for bentonite and examining their impact on the reduction behavior for the next generation of H2-based ironmaking. Working towards more sustainable ironmaking in Sweden, CO2 emission reduction strategies include the adoption of breakthrough low emission technologies, which completely transform the industry. These breakthrough technologies include H2-based ironmaking. Additionally, the use of organic binders during pellet production could provide a sustainable option for decreasing the emissions resulting from mining activities of inorganic binders. Hence, the current work mainly concentrates on the evaluation of using four selected innovative organic binders for magnetite ore pelletization. The study explores the influence of binder type, binder dosage, and moisture content on the characteristics and properties of the pellets. The physical and mechanical properties of the produced pellets were studied before and after the firing of the pellets. Additionally, the reducibility of both the green and fired pellets was explored using H2 gas to check whether the produced pellet fulfilled the prerequisite strength before and after reduction.
2. Materials and Methods
2.1. Materials
Magnetite iron ore concentrate received from Kaunis Iron AB, Sweden, was used in this study. In addition, commercial bentonite was used as a reference binder to compare the performance of selected organic binders in the pelletizing process.
By reviewing prior research studies and collaboration with the expertise of binder developers, four binders, Table 1, were chosen to be investigated for partial and/or full replacement of bentonite in this study.
Table 1. Selected binders for pelletization.
| Selected Binders | Composition | Source |
|---|---|---|
| KemPel | Anionic polyacrylamide | Kemira, Helsinki, Finland |
| Alcotac CS | Modified anionic polyacrylamide | BASF, Heidelberg, Germany |
| Alcotac FE16 | Anionic polyacrylamide | BASF, Heidelberg, Germany |
| CMC | Carboxymethyl cellulose | Commercial product |
2.2. Characterization
The magnetite concentrate and bentonite were subjected to chemical analysis using X-ray Fluorescence (XRF) techniques to determine the concentration of various components present in the sample. Additionally, mineralogical analysis was carried out using a Malvern Panalytical X-ray diffractometer (XRD) in 2θ geometry with Cu tube Kα radiation (λ = 0.154184 nm) and a beam current and voltage of 40 mA and 45 mV, respectively. This analysis aimed to identify the mineral composition of the magnetite concentrate both before and after thermal treatment. Moreover, the particle size distribution analysis of the magnetite ore concentrate was carried out using Retzsch Camsizer XT (Retsch Technology GmbH, Haan, Germany).
2.3. Recipes
The recipes shown in Table 2 were designed to investigate the feasibility of replacing the bentonite partially or fully with the selected organic binders. One reference recipe, namely R1, was designed using 1% bentonite and 99% magnetite. The recipe named K refers to the recipes pelletized using KemPel; similarly, H refers to Alcotac FE 16, C refers to Alcotac CS, and U refers to CMC. The pelletizing conditions were not optimized for all the recipes in this study; however, it could be an interesting topic to be investigated in further studies.
Table 2. Designed recipes for binders screening.
| Recipes | Magnetite (g) | Bentonite (%) | KemPel (%) | Alcotac CS (%) | CMC (%) | Alcotac FE16 (%) |
|---|---|---|---|---|---|---|
| R1 | 2000 | 1 | 0 | 0 | 0 | 0 |
| K1 | 2000 | 0.265 | 0.1 | 0 | 0 | 0 |
| K2 | 2000 | 0.5 | 0.1 | 0 | 0 | 0 |
| K3 | 2000 | 0.2 | 0.1 | 0 | 0 | 0 |
| H1 | 2000 | 0 | 0 | 0 | 0 | 0.05 |
| H2 | 2000 | 0 | 0 | 0 | 0 | 0.1 |
| H3 | 2000 | 0.265 | 0 | 0 | 0 | 0.05 |
| H4 | 2000 | 0 | 0 | 0 | 0 | 0.5 |
| H5 | 2000 | 0 | 0 | 0 | 0 | 0.75 |
| H6 | 2000 | 0.1 | 0 | 0 | 0 | 0.5 |
| H7 | 2000 | 0.1 | 0 | 0 | 0 | 0.3 |
| C1 | 2000 | 0.5 | 0 | 0.04 | 0 | 0 |
| C2 | 2000 | 0.3 | 0 | 0.04 | 0 | 0 |
| C3 | 2000 | 0.4 | 0 | 0.06 | 0 | 0 |
| C4 | 2000 | 0.4 | 0 | 0.1 | 0 | 0 |
| C5 | 2000 | 0 | 0 | 0.1 | 0 | 0 |
| C6 | 2000 | 0 | 0 | 0.5 | 0 | 0 |
| C7 | 2000 | 0 | 0 | 0.75 | 0 | 0 |
| C8 | 2000 | 0.3 | 0 | 0.5 | 0 | 0 |
| U1 | 2000 | 0.25 | 0 | 0 | 0.1 | 0 |
| U2 | 2000 | 0.5 | 0 | 0 | 0.25 | 0 |
| U3 | 2000 | 0 | 0 | 0 | 1 | 0 |
| U4 | 2000 | 0.25 | 0 | 0 | 0.5 | 0 |
| U5 | 2000 | 0 | 0 | 0 | 0.5 | 0 |
| U6 | 2000 | 0 | 0 | 0 | 0.75 | 0 |
2.4. Pelletization and Testing
The magnetite concentrate and respective binders were homogenously mixed using an Eirich mixer according to the recipe composition (Table 2). Then, these mixtures were fed into the bottom part of a laboratory scale pelletizing disc of diameter 35 cm, an inclination angle of 45°, and the rotation speed was set at 50 rpm. Along with feeding the mixed recipe, water was sprayed into the disc pelletizer. When the pellets started forming and attained a specific size, they were taken out and sieved using the mechanical sieving method (Retsch AS200 basic). The sieves were of the sizes 20 mm, 16 mm, 12.5 mm, 10 mm, 9 mm, 6.3 mm, 5 mm, 2.8 mm, 1 mm, and 0.5 mm. The pellets in the size range of 9–16 mm were taken out and kept aside, while both the undersized and oversized pellets were crushed and put back into the pelletizer. When most of the pellets acquired the desired size range, the pelletizer was stopped, and all the pellets were taken out and kept aside in a metal tray for drying in air for 4 h. The moisture content of produced pellets before and after drying was determined by accurately weighing and placing it into a Mettler Toledo moisture analyzer (Mettler-Toledo HB43 Laboratory & Weighing Technologies, Greifensee, Switzerland) equipped with a halogen heating unit. After drying in the air, the pellets were further dried in the oven at 105 °C for 2 h. The dry pellets were then sieved mechanically into different size fractions, weighed, and stored in zip lock bags in a dry location. Next, a set of ten pellets within the size range of 9–12.5 mm were selected for testing their cold compressive strength using a hydraulic compression tester (ENERPAC hydraulic center, Düsseldorf, Germany) and a drop test from a height of 45 cm onto a steel plate. The cold compressive strength (CCS) was determined by placing a single pellet on a fixed metal plate and compressing it with a movable piston at a velocity of approximately 20 mm/min. The compression force values in Newtons (N) were automatically recorded during the test. These tests were conducted in accordance with the ISO 4700–2015. standardTo ensure reliable strength measurements, ten pellets were tested for each CCS and drop measurement, and the results were subsequently averaged. The selected recipes that showed the highest CCS values and drop numbers were subjected to firing under airflow, 5 l/min in a muffle furnace (ENTECH, ECF 20/17, Eurotherm 2408 P4, Sweden). The heating rate was set to 10 °C/minute until it reached the peak temperature of 1250 °C. The residence time at the peak temperature was set to be 20 min. The cooling rate was also set to 10 °C/minute. After the furnace had cooled down to room temperature, the samples were taken out. Moreover, the fired pellets were subjected to CCS and drop test measurements as with dried pellets.
The reduction progress of dried and fired pellets was tracked through non-isothermal thermogravimetric analysis using a Netzsch STA 409 instrument (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany) with a detection limit of ± 1 μg. The schematic and equipment setup are presented elsewhere. A single dried/fired pellet was placed on alumina plate inside the TGA with the thermocouple positioned underneath. The pellet was then exposed to a thermal profile comprising a constant heating rate of 20 °C/min until reaching a temperature of 950 °C and holding at 950 °C for one hour, in which 100 mL/min H2 gas flow was maintained throughout. The reduction extent, RE, was calculated based on the reducible oxygen eliminated from the pellets using the following Equation (1):
where W0 is the initial mass of the pellet, Wt is the weight of pellet at a certain time (t), and x factor is the theoretical reducible oxygen ratio for hematite (0.30) and for magnetite (0.28). The y factor is the concentration fraction of magnetite in the dried pellet (0.91) and of hematite in the fired pellets (0.95). Hematite and magnetite fractions are calculated based on total iron content as affirmed from XRF analysis assuming that all Fe is in the form of magnetite in the dried pellets and hematite in the fired pellets
