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Article

Utilization of Waste Straw Biomass in Suspension Magnetization Roasting of Refractory Iron Ore: Iron Recovery, Gas Analysis and Roasted Product Characterization

by
Yue Cao
1,2,
Yongsheng Sun
1,2,*,
Peng Gao
1,2 and
Wenbo Li
1,2
1
School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China
2
National-Local Joint Engineering Research Center of High-Efficient Exploitation Technology for Refractory Iron Ore Resources, Shenyang 110819, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(22), 15730; https://doi.org/10.3390/su152215730
Submission received: 21 September 2023 / Revised: 20 October 2023 / Accepted: 31 October 2023 / Published: 8 November 2023
(This article belongs to the Section Waste and Recycling)
Browse Figures
Versions Notes

Abstract

:
The straw-type biomass, as a green and alternative reductant for the suspension magnetization roasting (SMR) of iron ores, is proposed. The roasted products are investigated at a roasting temperature of 750 °C, the roasting time of 7.5 min and the biomass dose of 25%. The iron phase results indicate that hematite ores were reduced to magnetite by the biomass, and the magnetization transformation increased from 0.64 A·m2·g−1 to 36.93 A·m2·g−1. The iron ore microstructure evolutions of holes and fissures are detected by SEM-EDS. The biomass pyrolyzed to form CO2, CO, CH4, H2O, H2, C=O, benzene skeleton, C-Hand C-O compounds at 200–450 °C, while the mass loss of the magnetization roasting process occurred at 450–750 °C by using TG-FTIR. The GC/MS results showed that the organic gases preferred to produce the O-heterocycles at 329 °C while the hydrocarbons were dominant at the high temperature of 820 °C for the hematite ore and biomass mixture. The gas composition analysis explained that the reducing gaseous products (CO, CH4 and H2) were used as a reductant and consumed obviously by hematite ore in the SMR process. The innovative utilization of biomass waste was effective for iron recovery of hematite ore and contributes to the reduction of greenhouse gases and the protection of the environment.

1. Introduction

Rapidly growing demand for products of the iron and steel industry has led to fast consumption of iron ores [1]. Currently, high-grade iron ore reserves are being depleted and the utilization of low-grade and refractory iron ore is a suitable way to solve this problem [2]. In China, a large reserve of refractory iron ore lacks appropriate treatments due to its properties such as poor iron grade, complex composition and fine-grained granularity. The suspension magnetization roasting (SMR) process is one of the most efficient and advanced technologies used to deal with refractory ores, transforming weak magnetic iron minerals into strong magnetic magnetite and maghemite via the chemical reaction of the ore in a suspension state at a certain temperature, and then magnetic separation is carried out according to the magnetic differences in the minerals present. As an advanced and semi-industrial technology for refractory ores, SMR technology has been studied by many researchers.Li et al. [3] obtained iron concentrate products, and the Fe grade and Fe recovery from the concentrate were 67.09% and 85.93%, respectively. SMR plays a significant role in treating refractory ore and hazardous wastes. Yuan et al. [4] studied iron-containing red mud and obtained a grade of 72.11% and an iron recovery from concentrate of 98.85% product during reduction roasting. Iron-containing red mud has been researched by Liu et al. [5], and the red mud was roasted effectively at 1000 °C. In addition, microwave heating and roasting is also considered as an effective way to treat minerals [6]. Therefore, SMR technology is an effective method and has advantages in terms of the production of iron ore or refractory ore materials.
Carbon peaking and carbon neutrality goals have been urgently implemented to solve the outstanding problems between resource utilization and the environment to achieve sustainable development [7]. Therefore, GHGs (greenhouse gases) are subject to strict emission restrictions. The exploration, production and utilization of renewable and sustainable biomass resources are indispensable for replacing the consumption of fossil fuels and reducing the emission of GHGs in the future and are also the keys to solving the carbon peaking and carbon neutrality problems. Biomass, including plant residue and manure, is a carbon neutral, environmentally friendly and sustainable resource [8,9]. Fossil fuels are used to generate heat and electricity. However, fossil fuels gradually destroy the environment by continuously emitting gases, accounting for 80% of global GHG emissions [10]. Compared with fossil fuels, bio-based products release fewer GHGs and can prevent harmful sulfur oxides and nitrogen oxides (NOX and SOX) from polluting the atmosphere. In 2010, the biomass reserve of straw-type biomass was 700 Mt and 50% of it could be made available to produce energy equivalent to 180 Mt of standard coal [11]. Moreover, the productions of straw-type biomass in 2025 and 2030 are expected to reach 882.14 Mt and 926.43 Mt, respectively [12]. Because of its advantages in terms of reserves and emissions, biomass has great potential in achieving carbon peaking and carbon neutralization.
There are many useful ways to utilize biomass [13,14,15]. The main methods for biomass utilization are combustion and composting. To promote renewable energy sources and diversify non-fossil fuel consumption, traditional utilization methods, such as burning and composting, are gradually being replaced by the liquefaction and gasification of biomass [16,17,18]. Biomass gasification is an effective way of transforming biomass polymers into combustible gases such as carbon monoxide, hydrogen and low-molecular hydrocarbons via pyrolysis, oxidation, reduction and reforming. Carbon monoxide, hydrogen and low-molecular gases from biomass pyrolysis are just the gas reductanta needed in reducing processes [19]. Using non-condensable and low molecular gases, many reducing methods that use biomass as a reductant have been explored to achieve the reduction of iron ore. The application of biomass as the reductant and fuel in iron ore pellet sintering in a blast furnace is one of the effective methods [20]. Co-pyrolysis technology for biomass and coal is a method to produce coke. In order to promote the application of biomass coke in a blast furnace, it is necessary to gain insight into the influence of biomass coke [21,22]. Thomas et al. [23] found that the reduction effect was better, and the mass fractions of sulfur elements in the iron product were lower compared with traditional reducing agents. Vlaimir et al. [24] discussed the experimental analysis and kinetic modeling of iron ore reduction using sawdust and discovered that the optimal biomass dose was 30%. In addition, the application of biomass in non-blast furnaces mainly played the role of a reducing agent [25,26,27]. The application of biomass is not only used to treat pure iron-bearing minerals but also refractory iron ores. Rath et al. [28] studied the reduction roasting process of refractory ore and obtained roasted products with a grade of 64% and a recovery of 66% when using cow dung. The reduction of Bayer red mud was studied in relation to biomass [29]. Refractory tailings has been reduced successfully [30]. Moreover, the application scale of biomass and ores is gradually developing towards semi-industrialization and industrialization. Iron ore, used as the catalyst, was also one of the applications in terms of biomass pyrolysis [31,32]. Therefore, various and effective ways of utilizing biomass and iron ore will be the trend of sustainable development in the future.
In our previous study, experiments for pure hematite were conducted to investigate the reduction mechanism [33]. In this study, the reducing process of biomass and iron ores using suspension magnetization roasting technology has been acknowledged and studied. The analysis of iron ore products has been considered by many scholars. However, details concerning the gaseous products researched in relation to biomass and iron ore mixtures are scarce. In this study, we studied straw-type biomass pyrolysis and magnetic reduction in the SMR process. The gaseous products pyrolyzed from the straw-type biomass are effective during the SMR process as an alternative and environmentally friendly reductant. The aim of this work is to reveal the rules of gas evolution of those released from biomass and to study the reduction mechanism of iron ores with reductants. The rules relating to gas evolution have been studied comprehensively and in detail through the use of TG-FTIR coupled with GC/MS. A gas analyzer was also used to examine the gas compositions and releasing trends of CO2, CO, H2 and CH4. Hematite changes to magnetite in a stepwise manner, indicating that hematite can be reduced to magnetite and that the use of biomass as a reductant is feasible under suitable conditions based on the results concerning the microstructure evolution of hematite ores and the phase transformation.

2. Methods and Materials

2.1. Materials

With the end of the seasonal harvest stage, there was a lot of straw remaining in the farmland of Liaoning province, China. The storage cost of waste straw biomass is high, and the burning of waste biomass would pollute the air. Therefore, biomass collected from Liaoning province, China, was used as the reductant in the SMR process. Prior to the experiment, the straw-type biomass was cut into 2–3 cm pieces, and the pieces were ground to a fineness of 0.9 cm. Finally, waste biomass particles with a fineness of 0.9 cm were used as the material. The preliminary, elemental and fiber constituent results are shown in Table 1. Hematite ores with a fineness of −0.038 mm, accounting for 80% of those prepared in this study, were supplied by Ansteel, Liaoning Province, China. The results of proximate, fiber constituent and elemental analysis for waste straw biomass and the composition and iron phase analysis for hematite ore are included in Table 1.

2.2. Methods

The SMR system is displayed in Figure 1. This system consists of a gas supply system, a gas mixing system, a suspension magnetization roasting system, a cooling and air-washing system and a drier and gas composition analyzer. For the experiment, N2 gas was carried with a flow rate of 300 mL/min. The carried gases were mixed and adjusted in the gas mixing system. The electric heating furnace was turned on, and a heating rate of 15 °C/min was set. When the roasting temperature reached a specific value from the ambient temperature, the roasting tube was quickly put into the furnace. Straw-type biomass and hematite ores were roasted in a roasting tube. The cooling and air-washing system was arranged between the roasting furnace and gas composition analyzer to cool down the condensable gases and some light bio-oils. The hot stream was cooled down to 50 °C after the first bottle (isopropanol), and then passed through the second bottle (water) where the condensable gases and some light bio-oils are coagulated using an ice–water mixture (0 °C). The non-condensable gases were used as the reductant in the iron ores tube. The gas composition analyzer can detect reducing gases simultaneously.
The ranges of the roasting temperature, roasting time and biomass dose were 400–700 °C, 2.5–15.0 min and 10–35%, respectively. The roasted samples were investigated using X-ray diffraction (XRD) analysis (X pertpro, PANalytical B.V., Almelo, The Netherlands), scanning electron microscopy (ULTRA PLUS, Zeiss, Germany, and SSX-550, Hitachi, Japan) and vibrating sample magnetometry (JDAW-2000D, YingPu, Shanghai, China). The gas products were investigated via the combined method of Thermogravimetric-Fourier Transform Infrared Spectroscopy (TG-FTIR) coupled with gas chromatography-mass spectrometry (GC/MS). Thermogravimetric analysis (TG) was performed using a thermal analyzer (STA 8000, PerkinElmer, Shelton, CT, USA) at heating rates of 5 °C·min−1, 10 °C·min−1, 15 °C·min−1 and 20 °C·min−1 and the flow rate of N2 was 60 mL·min−1. The Fourier-transform infrared spectrometer (Frontier, PerkinElmer, America) data were collected over the range of 4000−600 cm−1 at a resolution of 4 cm−1. The pyrolysis vapors moved from the quartz tube into a gas chromatography/mass spectrometry analyzer (Clarus SQ8 GCMS, PerkinElmer, USA). A GC/MS injector temperature of 275 °C and interface temperature of 300 °C were used. A capillary column named ELITE-1 (30 m × 0.25 mm × 0.25 μm) was used in the process of GC separation. A split ratio of 1:60 was held. The column was first held at 40 °C for 3 min, then the temperature was increased to 180 °C and held for 3 min at a heating rate of 3 °C·min−1. Finally, the temperature was increased to 280 °C at a heating rate of 4 °C·min−1. The operating conditions of MS were an ionization energy of 70 eV and a scan rate of 35–450 amu per second. The GC/MS data were obtained based on the NIST database. The gas composition analyzer (Gasboard-3100, Hubei Ruiyi Automatic Control System Co., Ltd., Wuhan, China) was also used to study the contents of the reducing gases.
The roasted samples (iron ores) were separated using a Davis Tube Tester (RX/CXG50, Tangshan, China) with a magnetic field intensity of 104 KA·m−1. The concentrate of iron ore was collected, and the iron ore tailings were flashed into the tailings collector. All of the samples produced from the magnetic separation were filtered, dried and weighed. The recovery was calculated using the following equation:
ε = β × ( α − θ ) α × ( β − θ ) × 100 %
where α, β and θ are the iron grade of the raw ore, concentrate and tailing, respectively, and ε is the recovery of iron concentrate.

3. Results and Discussion

3.1. Iron Recovery

3.1.1. Effect of Biomass Dose

The effect of the biomass dose on SMR was investigated in the range of 10% to 35%, with a roasting temperature of 750 °C and a roasting time of 7.5 min. The effects of biomass dose on iron grade and recovery in relation to the magnetic concentrate are displayed in Figure 2a. As can be seen from Figure 2a, the grade of the iron concentrate changed greatly, showing a trend of first tending to balance and then declining sharply. When the biomass dose increased from 10% to 25%, the recovery of concentrate increased to 94.49%, while the grade of concentrate also demonstrated a fluctuation of 61.95–63.15%. The grade of concentration dropped rapidly, and the recovery was almost unchanged when the biomass dose increased from 25% to 35%. There were two reasons for this phenomenon. One important reason was the over-reduction that occurred when the dose of biomass increased from 25% to 35%. The other reason was the influence of ash and char during magnetic separation. The separation of biomass, tailings and concentrate was carried out via magnetic difference. The separation of tailings and concentrate was easy, while the process of concentrate and biomass (ash and char) was hard due to the difference in density. A small amount of ash and char floating on the water’s surface showed that it was difficult to completely separate ash and char from the iron concentrate. Under the experimental conditions of low-grade and high-dose biomass, this phenomenon of mixed ash and char had an obvious decreasing influence on the grade of the iron concentrate. The optimum biomass dose was 25%, and the iron concentrate grade and recovery were 62.10% and 94.39%, respectively.

3.1.2. Effect of Roasting Temperature

Temperature is one of the most important factors in the SMRT process. The effects of the roasting temperature on iron grade and recovery in relation to the magnetic concentrate are shown in Figure 2b. With increasing iron grade, a profile of concentrate recovery was discovered. When the roasting temperature increased from 600 °C to 700 °C, the concentrate recovery increased to 96.42% with a rapid increase in grade. However, when the roasting temperature increased from 700 °C to 900 °C, the grade of the iron concentrate decreased to 49.61%. The bonds of the organic compounds of lignin, cellulose and hemicellulose were easier to break at high temperatures in straw-type biomass, which increased the generation rate and the released amount of gases produced via pyrolysis, resulting in increased CO and H2 reducing gases. The maximum reduction rate and reduction degree increased as the reducing temperature increased in the CO atmosphere. Similarly, magnetic transformation had a good reaction rate in the H2 atmosphere at temperatures of 1073–1223 K. Therefore, hematite transformed FeXO well in the temperature range of 600–900 °C. From the perspective of magnetic separation, magnetite (Fe3O4) was the ideal target product and the formation of FeO should be avoided. It should be noted that the release of reducing gas was variable at the temperature range of 600–900 °C, and the released gases were a mixture of H2, CO, CH4 and CO2. When the temperature increased from 600 °C to 700 °C, the recovery from the concentrate increased, indicating the conversion process of Fe2O3 → Fe3O4. Strongly magnetic Fe3O4 would convert into weakly magnetic FeO when the temperature increased from 750 °C to 900 °C. Weakly magnetic wüstite (FeO) would not be separated by the magnetic units in the Davis tube. The optimum roasting temperature was determined to be 750 °C and the iron concentrate grade and recovery were 62.10% and 94.39%, respectively.
The temperature of the biomass and hematite ores was affected by sample purity, reducing gas type and particle size. Pure hematite had a homogeneous structure to reduce, and the interference of impurities could be ignored. Gaviria, J. P. et al. and Ponomar, V. P. et al. [34,35] used pure hematite powder or synthetic hematite as the experimental sample, and the reducing gas (H2) was supplied directly from the gas supply system. Pure hematite could be reduced at a lower temperature of 260–360 °C. Ponomar, V. P. et al. [36] mixed synthetic and natural hematite with carbohydrates (glucose, fructose, sucrose and ascorbic acid) as the roasting sample, and hematite could be reduced at 300–800 °C. In addition, it was found that the reduction of Fe2O3 to Fe3O4 with a reductant of heavy fuel oil occurred at 800 °C to form gaseous pyrolysis products. Ellid, M. S. et al. [37] used pure hematite and the roasting time was set from 30 min to 24 h. High-purity hematite with a fine powder was easily transformed into magnetite over a long roasting time. In the self-reduction system of hematite with siderite, the optimum indexes of pure hematite and actual hematite are quite different [38].
Therefore, the experimental temperature depended on the sample purity, particle size and reducing gas types. In this study, it was reasonable to determine a temperature range of 500–800 °C.

3.1.3. Effect of Roasting Time

The roasting time experiments were carried out under the conditions of a nitrogen atmosphere, a roasting temperature of 750 °C and a biomass dose of 25%. The results of roasting time were studied with time varying between 2.5 and 15.0 min and are displayed in Figure 2c. As can be seen from Figure 2c, with the increase in roasting time, the grade of the iron concentrate increased first and then tended to balance. The recovery of concentrate had the same profile. When the roasting time was 5.0 min, the grade and recovery were 60.93% and 94.49%, respectively. Prolonging the roasting time to 15 min, the iron grade varied between 61.72% and 62.535%. Also, the recovery varied between 94.385% and 95.195%. The reductant reduced the hematite by capturing the oxygen element of the sample and the content of oxygen decreased with the roasting time increasing. The decreasing oxygen dominated the increased reduction degree. Therefore, the reduction process of Fe2O3 → Fe3O4 → FeO occurred in the roasting time experiment. However, the over-reduction did not occur when the roasting time increased from 2.5 min to 15.0 min. The iron grade and recovery maintained balance in the roasting time range of 5.0 min and 15.0 min. The balance curves showed that the reducing gas formed from the biomass was released completely. After careful consideration, the optimum condition was 7.5 min, and the iron concentrate grade and recovery were 62.10% and 94.39%, respectively.

3.2. Gas Analysis during Reduction

3.2.1. TG-FTIR Analysis

TG-FTIR-GC/MS was used to study the gas evolution rules of waste straw-type biomass and hematite ores during the magnetization roasting process. The profiles of TG and DTG (derivative differential analysis of TG) for different heating rates as functions of temperature are shown in Figure 3a,b.
The hematite could not be reduced without a reducing agent or only by increasing the temperature according to the thermodynamic basis of reduction in the temperature range of 200–500 °C. Due to the removal of crystal water in Fe2O3·nH2O, a mass loss of 2.74% was discovered from the TG curve.
For biomass, the pyrolysis process of straw biomass was divided into two stages. The first stage of waste straw biomass mainly involved the evaporation of moisture. The second stage, which involved the major mass loss of waste straw biomass, took place at the temperature of 200–450 °C [39,40]. During the thermal degradation process, a nearly 25% mass loss was discovered for all the heating rates when the temperature was 850 °C.
For the mixture of biomass and hematite, the degradation process could be divided into four stages. The first stage was the drying stage (<200 °C) in which the biomass began to lose water, resulting in a certain amount of weight loss. The second stage was the pyrolysis process of waste straw biomass, mainly in the 200-450 °C range, and the devolatilization of waste straw biomass occurred rapidly due to the mass loss of hemicellulose and cellulose [41]. In this stage, the mass loss was regarded as the main stage of biomass pyrolysis. The third stage (450–750 °C) was the magnetization roasting process. In this stage, the reducing gases produced from waste straw biomass were prepared as a reactant, and hematite could be transformed to magnetite via magnetization roasting when the temperature increased from 450 °C to 750 °C. The fourth stage was the over-reduction process, when the temperature was above 750 °C. In this stage, the mass loss showed an obvious decrease when the temperature increased from 750 °C to 850 °C and the peaks in the DTG curves showed that the rate of weight loss was different from the third stage. The reason for this phenomenon was that the oxygen elements of iron oxides were constantly captured and the reducing process of Fe3O4 → FeXO → Fe occurred. The 2.74% mass loss of hematite ores was caused by the addition of a small amount of limonite into hematite ores and the removal of crystal water caused by the decrease in the curve mass fraction for the curve of hematite.
TG-FTIR analysis could not only keep a record of the mass change in the sample with the temperature during pyrolysis, it could also reflect the relative content of evolved gases based on the FTIR data. Based on the Beer–Lambert law, the absorbance of various evolved gases measured from FTIR had a linear relationship with their concentrations [42]. The main reducing gases (CO, CH4) could be studied using TG-FTIR analysis. However, there were still some materials lacking a dipole moment that could not be detected using FTIR, such as H2 and Cl2.
Figure 3c shows the 3D FTIR diagram of the mixture magnetization roasting process with a heating rate of 15 °C·min−1. Major gases detected via FTIR during the magnetization roasting of a waste straw and hematite ore mixture are shown in Figure 3d. According to the characteristic wave numbers of gas species, H2O (4000–3400 cm−1), CH4 (3000–2700 cm−1), CO2 (2400–2250 cm−1), CO (2250–2000 cm−1), the fingerprint region of CO2 (586–726 cm−1), aromatics (1690–1450 cm−1), alkanes, alcohols, phenols, ethers, lipids (1475-1000 cm−1) were apparently observed in the FTIR spectrum experiment [43,44]. The 2D FTIR rules of volatile gases are shown in Figure 3e. As shown in Figure 3e, it could be seen that all the evolved gases had a peak at about 350 °C. It is worth noting that the absorbance of CO2 increased when the temperature increased from 450 °C to 850 °C and other evolved gases kept an absorbance balance at the same temperature.
The reconstruction diagram of the relationship between thermogravimetric analysis and FTIR is shown in Figure 3f. As can be seen in Figure 3f, the temperature corresponding to the maximum mass loss peak and the second mass loss peak in the DTG curves (15 °C·min−1) were used as characteristic points to study the gases rules in FTIR using an intuitive method. The connections between thermogravimetric analysis and FTIR analysis were described via the characteristic points. The characteristic temperature points of DTG were 329.4 °C and 820.2 °C, respectively.
In the first characteristic point of weight loss (329.4 °C), it was clear that the waste straw biomass pyrolyzed drastically according to the trend of the TG curve. Similarly, there was a strong peak appearing in the DTG curve at the temperature range of 200-450 °C and the maximum thermal weight loss rate was 2.30%/min−1. A large amount of volatile matter was released from the mixture according to the color surface indication of the FTIR analysis. It could be seen from the FTIR diagram that the intensities of CO2, CO, CH4 and organic volatiles increased first and then decreased. In the second characteristic point of weight loss rate (820.2 °C), there was a weak peak standing in the DTG curve in the temperature range of 750–850 °C. The maximum thermal weight loss rate was 0.85%. Compared with the first characteristic point, the weight loss rate of the second point became slower, while the intensities of CO2 and CO at the second point were higher than that at the first point. The intensity of organic volatiles containing C-H, and C-O and C=O decreased more at the second point than that of the first peak. A suitable explanation for the increased CO and CO2 is that a large amount of CO and CO2 was released from the aromatic cyclization process of biomass carbon at high temperatures. It should be noted that CO2 could be produced after the reducing reaction involving hematite. The characteristic temperature points (329.4 °C and 820.2 °C) would be chosen as the GC/MS characteristic points for further gas study and the content will be introduced in the next section.

3.2.2. GC/MS Analysis

The condensable gas compositions of biomass from the thermal decomposition were very complicated and difficult to fully characterize via a single analytical technique [45]. In order to study the gas rules of a waste straw and hematite ore mixture further, and to understand the influence of condensable gases on the SMR process, it was necessary to detect the condensable gases via GC/MS.
The composition and content of organic evolved gases from waste straw biomass at different reaction temperatures are shown in Figure 4. The composition of organic gases was relatively simple and the O-heterocycles specie was composed of 1,6-anhydro-β-D-glucose, accounting for 8.16% [46]. In addition, some small relative contents of hydrocarbons, carbonyls and o-species were detected at the pyrolysis temperature of 329 °C and the relative contents of hydrocarbons, carbonyls and O-species were 2.85%, 2.73% and 3.00%, respectively. When the temperature increased to 820 °C, the species of evolved organic gases increased, and the total content increased rapidly. Hydrocarbons and carbonyls were the most important gas products, with relative contents accounting for 29.63% and 8.99% at 820 °C. The relative content of O-heterocycles decreased with increasing relative content of O-species. The explanations for this phenomenon were the conversion and breaking of the alkyl chains of lignin and the depolymerization of cellulose and hemicellulose [47]. Nevertheless, the secondary reactions of waste straw biomass products contributed to the organic gases [48]. Therefore, the GC/MS analysis indicated that the organic gases preferred to produce O-heterocycles at 329 °C while the hydrocarbons were dominant at the higher temperature of 820 °C for the hematite ore and biomass mixture.

3.2.3. Gas Composition Analysis

The main reducing gas composition analysis conducted through the use of a gas composition analyzer is displayed in Figure 5a–f. As for the pyrolysis process of waste straw biomass, CO, H2 and CH4 were major reducing gases, and CO2 was the important gas during waste straw biomass pyrolysis [49]. Therefore, CO, CO2, H2 and CH4 would be studied in this process. H2, the essential gas product, could be detected using the gas composition analyzer when compared to FTIR analysis. The concentration of CO increased from 8.61% to 20.51% and the concentration of CO2 increased from 13.12% to 17.37% when the roasting temperature increased from 500 °C to 700 °C, as shown in Figure 5a,b. Moreover, the concentrations of CH4 and H2 had increasing profiles, which peaked at 13.23% and 8.53%.
As for the roasting process of a waste straw biomass and hematite ore mixture, the maximum concentration of CO2 increased from 7.35% to 12.03% and maintained a balance range between 12.50% and 11.96% when the roasting temperature increased from 500 °C to 800 °C. Biomass gradually began to pyrolyze and produce reducing gases when the roasting temperature increased from 500 °C to 600 °C, and the gas maximum concentrations of H2, CO and CH4 sharply increased to 7.44%, 6.10% and 7.14%, respectively. In Figure 5d–f, the gas maximum concentration of H2, CO and CH4 decreased when the temperature increased from 600 °C to 800 °C, indicating that more reducing gases reacted with hematite ores at the higher temperatures. The profiles of the H2 curves were considered worthy of greater attention. At 500 °C, the concentration of H2 curves was negligible, as shown in Figure 5a,c. With increased temperature, the maximum concentration of H2 increased from 0.52% to 7.44% at first and then dropped to 3.77%.
It is worth noting that CO was the main product at 700 °C (Figure 5b), but only the third gaseous product in the presence of iron ore (Figure 5e). Compared with the biomass at 700 °C, the reducing gas concentration of H2, CO and CH4 decreased obviously in the SMR process for the mixture at 700 °C, showing that the reducing gases were consumed in the reducing process. Moreover, CO2 was the main product of the mixture at 700 °C due to the consumption of CO for the hematite reduction reaction. Therefore, the reducing gaseous products (CO, CH4 and H2) were used as reductants and consumed by hematite ore in the SMR process.
The production regulations relating to small-molecular pyrolyzed gases from biomass are shown in Figure 5g. In this study, the small-molecule gases (CO2, CO, H2, CH4), which are non-condensed gases, were important gas products during the SMR process [50]. The gases evolved from the waste straw biomass, which consisted of cellulose, hemicellulose and lignin. The non-condensable and small-molecule gases were almost pyrolyzed using three fundamental components [51]. Figure 5g shows the main pyrolysis reaction of cellulose, hemicellulose and lignin of the biomass and small-molecule products with increasing temperature. In Figure 5g, lignin could produce CO gas when the temperature increased from 180 °C to 430 °C by breaking the Cβ-Cγ bond, conversely to the alkyl chains and reorganization groups [52,53]. In addition, the formation of CO for higher temperatures was certainly due to the conversion of the phenol rings, which were known to still be stable at 550 °C [54].. H2 could be generated at temperatures between 500 °C and 800 °C in the case of lignin [55]. The conversion of short lignin substituents in aromatic rings could contribute to the production of CH4 gas [56,57]. As for the pyrolysis of cellulose, CO generation was due to fragmentation and secondary reactions when the temperature increased up to 280 °C [58,59]. In addition, the conversion of short substituents of aromatic rings in cellulose was dominant for CO gas at the temperature range of 500–800 °C [60]. The fragmentation and secondary reactions at low temperature and conversion of short substituents of aromatic rings at high temperatures were the reasons why CH4 gas, in the case of cellulose, could be generated from these two temperature ranges [58]. However, the temperature ranges of H2 and CH4 were similar. The generations of H2 were not only caused by the conversion of short substituents of aromatic rings, but also by rearranging the polycyclic structure during the charring process [61]. Hemicellulose is difficult to extract from the biomass, so xylan is generally used and studied as a model of hemicellulose. In the pyrolysis process of xylan, the fragmentation and secondary reactions of xylan could generate CO gases in the temperature range of 260–380 °C, while the production of CO at temperatures of 500–800 °C occurred due to the conversing the polycyclic structure [45]. The production of CH4 was similar to CO gases at the high temperature range. Rearrangement of the polycyclic structure in the charring process can produce H2 gas at the temperature range of 500–800 °C [62]. The process of CO2 release from lignin, cellulose and xylan was similar, occurring by fragmentation, secondary reactions and bonds breaking. The reducing gases (CO, CH4, H2) and CO2 generated from lignin, cellulose and xylan were prepared as the reductants for the magnetization roasting process. Because of the existence of the reductant, ferromagnetic hematite was converted into strong magnetic magnetite and the reduced magnetite could be separated via the magnetic units of the Davis tube.

3.3. Characterization of Roasted Products

3.3.1. Phase Transformation

Phase transformation analysis of hematite ores mixed with waste straw biomass was detected via XRD, and the results are illustrated in Figure 6a,b. As shown in Figure 6a, XRD analysis was carried out at 800 °C, and the characteristic peaks of magnetite (311, 440) gradually strengthened and became prominent when the roasting time increased to 7.5 min. The characteristic peaks of hematite (104, 110) almost disappeared completely, indicating that the hematite was transformed to magnetite when the roasting time was 7.5 min. When the peaks of magnetite showed a larger area in 15.0 min, magnetite was dominant in the mineral composition and the hematite changed into magnetite completely.
As shown in Figure 6b, the samples were roasted for 7.5 min. The characteristic hematite peaks (104, 300) weakened intensely when the roasting temperature was 700 °C. Conspicuous differences emerged at 750 °C, and the peaks of hematite gradually diminished and finally vanished. However, the peaks intensity of magnetite (311, 440) decreased sharply, the wüstite peaks appeared when the roasting temperature increased from 750 °C to 900 °C. The reason for this phenomenon was that, with the increase in temperature, the reducing gases released from waste straw biomass increased at the same time, which led to an over-reduction reaction. As the reaction proceeded, the phase transformation of iron minerals was gradually reduced in the following order: Fe2O3 → Fe3O4 → FeO.

3.3.2. Magnetic Analysis

The VSM results reflected the magnetism transformation of roasted products (Figure 6c–f). Hematite ores are weakly magnetic minerals, and they can be reduced to magnetite or maghemite, which is a ferromagnetic substance. Magnetization and saturation magnetization would change during the SMR process. Compared with magnetization and saturation, the magnetization of hematite, magnetite and maghemite (γ-Fe2O3) have higher saturation magnetization values. When over-reduction occurred, saturation magnetization decreased, and the magnetization also exhibited the same profile. The reduction effect of hematite ores by waste straw biomass could be characterized by the magnetization and saturation magnetization of the products. When the magnetic field increased to 1000 kA·m−1, the magnetization curves of the roasting time and roasting temperature increased first and then balanced (Figure 6c,e). However, the curve of the raw ore was a straight line with a mean value of 0.64 A·m2·g−1, indicating that the hematite ores had the characteristics of weak magnetic substances. The saturation magnetization of temperature had an increasing curve profile when the temperature increased from 500 °C to 750 °C, while the value of saturation magnetization dropped from 36.93 A·m2·g−1 to 3.15 A·m2·g−1 sharply when the temperature continued to rise to 900 °C, as shown in Figure 6d. The over-reduction reaction would transform magnetite into weakly magnetic wüstite. In Figure 6f, the saturation magnetization of roasting temperature peaked at 7.5 min. The over-reduction reaction occurred, and the value of saturation magnetization was 23.93 A·m2·g−1 at 15.0 min. The results of saturation magnetizations were consistent with the optimum experimental conditions shown in Section 3.1.

3.3.3. Microstructure Characterization

The microstructure evolution of particles was analyzed using SEM-EDS to determine the distribution of the iron phase, the transformation of hematite and the morphology of the biomass, as shown in Figure 7. The products were prepared from mixture samples from 2.5 min to 15.0 min at 800 °C. Figure 7a indicates that the waste straw biomass had a lot of tiny holes on its surface. The surface of hematite ore (point 1) was dense and smooth and there were no fissures, holes or crevices, as shown in Figure 7a. In Figure 7b, with the prolonged roasting time, the hematite was transformed to magnetite, resulting in the atomic ratio of iron to oxygen being 0.70 at point 3. In addition, there was obvious quartz gangue mixed with the reducing samples at point 2. The surface of the iron ores had tiny fissures, and the surface of the waste straw biomass (point 5) became rough and uneven, which appeared as tiny holes, as shown in Figure 7c. The atomic percentage of carbon elements reached 80.59% due to the volatilization of hydrogen and the oxygen elements of biomass. Also, there was a mixture of iron bearing minerals and quartz (point 4), as shown in Figure 7c. As shown in Figure 7d,e, microstructure images with a relatively flat surface of magnetite (point 7) were obtained at 7.5 min, filled with tiny cracks. The sample was also mixed with quartz (point 6). As shown in Figure 7f, when the roasting time was prolonged to 10 min, the cracks and fissures of the iron ores were obvious. Derived from the chemical structural formula of Fe2O3, Fe3O4 and FeO, the ideal atomic ratios of iron to oxygen element for hematite, magnetite and wüstite were 0.66, 0.75 and 1. Compared with the ideal atomic ratio of iron to oxygen element, the magnetite atomic ratio (point 8) of iron to oxygen element was 0.95, showing the trend of over-reduction. In addition, the waste straw biomass had clear holes and the portions of waste straw biomass appeared brittle with conchoidal fractures, as shown in Figure 7g [63]. With the deepening of pyrolysis, the biomass lost its toughness and its structure became fragile. Prolonging the roasting time from 10 min to 15 min, the holes gradually enlarged and connected together to become fissures and some structures of iron ores were destroyed, showing a loose and porous surface, as shown in Figure 7h. Because the atomic radio of iron to oxygen was 1.13, the surface of magnetite (point 9) showed the over-reduction trend. In Figure 7i, the carbonization of waste straw biomass became deeper and the atomic percentage of carbon element was 90.96%. Compared with point 5, the atomic percentage of oxygen element decreased from 17.12% to 5.92% owing to the volatilization of oxygen-containing components. Also, the atomic percentage of K elements increased from 1.07% to 1.22% owing to the production of ash. As for the biomass structure, the shape was distorted and compacted to a certain extent after roasting.

4. Conclusions

The magnetization suspension roasting process using biomass has been investigated. The biomass pyrolysis process and iron ore reducing process are two reaction stages in this process: (1) The biomass pyrolysis process. The degradation process of the mixture was divided into four stages. The devolatilization stage of waste straw biomass at 200–450 °C and the magnetization roasting stage for a mixture at 450–750 °C were two important stages. FTIR analysis indicated that CO2 was the main gas released from the thermal degradation of a waste straw biomass and hematite ore mixture. In addition, the evolved gases, including H2O, CH4, CO, C=O, benzene skeleton and C-H and C-O compounds had a trend of increasing first and then decreasing when the temperature increased from 100 °C to 850 °C. The gas composition analysis showed that the reducing gaseous products (CO, CH4 and H2) were used as a reductant and consumed by hematite ore in the SMR process. The GC/MS analysis indicated that the organic gases preferred to produce O-heterocycles at 329 °C while the hydrocarbons were dominant at the high temperature of 820 °C. (2) The reducing process of iron ore was studied. An iron concentrate with an iron grade of 62.10% and a recovery of 94.39% was obtained under the optimal conditions of a roasting temperature of 750 °C, a roasting time of 7.5 min and a biomass dose of 25%. The phase transformation of the roasted products was gradually reduced in the following order: Fe2O3 → Fe3O4 → FeO. Meanwhile, the saturation magnetization peaked at 750 °C with a maximum value of 36.93 A·m2·g−1. The SEM-EDS analysis indicated that the surface of the iron ores showed more cracks and fissures with the increasing roasting time and the atomic percentage of carbon elements for biomass increased to 90.96% at 15.0 min. In addition, the surface of the hematite ore was reduced gradually.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su152215730/s1, Figure S1. Preparation process of straw powder. Figure S2. The general ion diagrams of straw-type and hematite ores mixture at different characteristic temperature: (a) 329 °C, and (b) 820 °C. Table S1. Peak areas of evolved compounds at 329 °C in SMR process by GC/MS. Table S2. Peak areas of evolved compounds at 820 °C in SMR process by GC/MS.

Author Contributions

Writing—original draft preparation, Y.C.; writing—review and editing, Y.S.; methodology and software, P.G.; project administration, W.L. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by the National Natural Science Foundation of China (52022019).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the SMR system.
Figure 1. Schematic diagram of the SMR system.
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Figure 2. Effects on iron grade and recovery under different conditions for concentrate: (a) effects on dose of straw-type biomass, (b) effects on roasting temperature and (c) effects on roasting time.
Figure 2. Effects on iron grade and recovery under different conditions for concentrate: (a) effects on dose of straw-type biomass, (b) effects on roasting temperature and (c) effects on roasting time.
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Figure 3. TG−FTIR analysis of waste straw biomass with a heating rate of 15 °C·min−1: (a) TG curves of hematite ores and biomass at different heating rates, (b) DTG curves of hematite ores and biomass at different heating rates, (c) 3D FTIR diagram, (d) FTIR spectrum for volatiles with different temperature at 329 °C and 820 °C, (e) evolution of volatile components with increasing temperature and (f) the reconstruction diagram of the relationship between TG and FTIR.
Figure 3. TG−FTIR analysis of waste straw biomass with a heating rate of 15 °C·min−1: (a) TG curves of hematite ores and biomass at different heating rates, (b) DTG curves of hematite ores and biomass at different heating rates, (c) 3D FTIR diagram, (d) FTIR spectrum for volatiles with different temperature at 329 °C and 820 °C, (e) evolution of volatile components with increasing temperature and (f) the reconstruction diagram of the relationship between TG and FTIR.
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Figure 4. The composition and content of organic evolved gases from the waste straw and hematite ore mixture at different reaction temperatures.
Figure 4. The composition and content of organic evolved gases from the waste straw and hematite ore mixture at different reaction temperatures.
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Figure 5. The composition analysis and the source discussion of main reducing gas: (a) composition analysis for biomass at 500 °C, (b) biomass at 700 °C, (c) mixture at 500 °C, (d) mixture at 600 °C, (e) mixture at 700 °C, (f) mixture at 800 °C and (g) the reducing gas source discussion.
Figure 5. The composition analysis and the source discussion of main reducing gas: (a) composition analysis for biomass at 500 °C, (b) biomass at 700 °C, (c) mixture at 500 °C, (d) mixture at 600 °C, (e) mixture at 700 °C, (f) mixture at 800 °C and (g) the reducing gas source discussion.
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Figure 6. Characterization of roasted products: iron phase analysis of (a) roasting times, (b) roasting temperatures and magnetic analysis of (c) magnetization curves for roasting temperature, (d) saturation magnetization for roasting temperature, (e) magnetization curves for roasting time, (f) saturation magnetization curves for roasting time.
Figure 6. Characterization of roasted products: iron phase analysis of (a) roasting times, (b) roasting temperatures and magnetic analysis of (c) magnetization curves for roasting temperature, (d) saturation magnetization for roasting temperature, (e) magnetization curves for roasting time, (f) saturation magnetization curves for roasting time.
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Figure 7. SEM images of a waste straw biomass and hematite ore mixture at different roasting times: (a) raw ores, (b) mixture at 2.5 min, (c) mixture at 5 min, (d) roasted ores at 7.5 min, (e) enlarged images of ore surface at 7.5 min, (f) roasted ores at 10 min, (g) waste straw biomass at 10 min, (h) roasted ores at 15 min, (i) waste straw biomass at 15 min and (j) the corresponding EDS analysis of point 1 to point 10.
Figure 7. SEM images of a waste straw biomass and hematite ore mixture at different roasting times: (a) raw ores, (b) mixture at 2.5 min, (c) mixture at 5 min, (d) roasted ores at 7.5 min, (e) enlarged images of ore surface at 7.5 min, (f) roasted ores at 10 min, (g) waste straw biomass at 10 min, (h) roasted ores at 15 min, (i) waste straw biomass at 15 min and (j) the corresponding EDS analysis of point 1 to point 10.
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Table 1. Raw analysis of waste straw biomass and hematite ore (%).
Table 1. Raw analysis of waste straw biomass and hematite ore (%).
Waste straw biomassProximate analysis 1Fiber constituent analysis
VAMFCLHV 2CHC 3LExtract
78.753.981.8515.4216.5034.2827.285.4832.96
Elemental analysis
CHONPSH/CO/C
45.045.7146.110.460.030.121.520.77
Hematite
ore		CompositionFeOSiO2Al2O3CaOMgOPSLOI 4
Mass fraction1.1149.430.371.593.050.0250.0421.61
Iron phaseFe in carbonateFe in magnetiteFe in hematiteFe in sulfideFe in silicateTFe
Mass fraction0.250.4128.440.300.8130.36
Distribution0.821.3494.220.982.64100
1: V—volatile matter, A—ash, M—moisture, FC—fixed carbon. 2: Lower heating value, MJ/kg. 3: hemicellulose. 4: loss on ignition.
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Cao, Y.; Sun, Y.; Gao, P.; Li, W. Utilization of Waste Straw Biomass in Suspension Magnetization Roasting of Refractory Iron Ore: Iron Recovery, Gas Analysis and Roasted Product Characterization. Sustainability 2023, 15, 15730. https://doi.org/10.3390/su152215730

AMA Style

Cao Y, Sun Y, Gao P, Li W. Utilization of Waste Straw Biomass in Suspension Magnetization Roasting of Refractory Iron Ore: Iron Recovery, Gas Analysis and Roasted Product Characterization. Sustainability. 2023; 15(22):15730. https://doi.org/10.3390/su152215730

Chicago/Turabian Style

Cao, Yue, Yongsheng Sun, Peng Gao, and Wenbo Li. 2023. "Utilization of Waste Straw Biomass in Suspension Magnetization Roasting of Refractory Iron Ore: Iron Recovery, Gas Analysis and Roasted Product Characterization" Sustainability 15, no. 22: 15730. https://doi.org/10.3390/su152215730

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