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<h2>Introduction</h2>
<p>Iron ore pelletization is an essential agglomeration process that converts fine iron ore concentrates into spherical pellets suitable for blast furnace or direct reduction processes (Kakela, 1978). Binders are crucial to impart adequate strength to green and indurated pellets. Bentonite, a clay mineral, has been the dominant binder for decades due to its effectiveness (Eisele & Kawatra, 2003). However, bentonite introduces silica and alumina into the pellet, increasing slag volume and energy consumption during smelting (Kawatra & Claremboux, 2021). Moreover, bentonite mining is environmentally disruptive and depletes non-renewable resources.</p><p>The steel industry faces mounting pressure to reduce its carbon footprint and adopt circular economy principles (Rissman et al., 2020). Organic binders offer a potential alternative, as they can be burned off during induration, leaving no residual ash (Claremboux & Kawatra, 2022). Various organic binders, such as starch, cellulose, and humic acid, have been studied, but their high cost and limited availability hinder widespread adoption (Sivrikaya & Arol, 2012).</p><p>Lignin, a complex aromatic polymer, is a major byproduct of the pulp and paper industry, generated in large quantities as black liquor (Jin & Wu, 2016). Current lignin utilization is limited to low-value applications like combustion for energy recovery (Olsson et al., 2006). However, lignin's polymeric structure and functional groups (hydroxyl, carboxyl) make it a potential binder for iron ore pellets (Qiu et al., 2003). Previous research has explored lignin sulfonate and other lignin derivatives (Giang, 2018), but direct use of lignin extracted from black liquor in pelletization is underexplored.</p><p>This study aims to evaluate the performance of lignin as a sustainable binder for iron ore pelletization, comparing it with conventional bentonite. The objectives are: (1) to extract and characterize lignin from pulp and paper mill black liquor, (2) to optimize lignin dosage for pellet strength, (3) to assess green and indurated pellet properties, and (4) to discuss economic and environmental implications.</p>
<h2>Literature Review</h2>
<p>Binders in iron ore pelletization are classified into inorganic and organic types. Inorganic binders, primarily bentonite, provide excellent green and dry strength but introduce impurities (Kawatra & Claremboux, 2021). Organic binders include natural polymers like starch, dextrin, and carboxymethyl cellulose, as well as synthetic polymers. Organic binders decompose during induration, leaving minimal residue, but often require higher dosages and may reduce pellet porosity (Eisele & Kawatra, 2003).</p><p>Sivrikaya and Arol (2010) investigated boron compounds as binders, showing improved strength but limited industrial adoption due to cost. Devasahayam (2018) developed a novel pelletization process using organic binders under ambient conditions, achieving adequate strength without thermal curing. Ngara et al. (2023) compared organic binders to bentonite for low-grade iron ore, finding that a combination of binders yielded optimal results.</p><p>Lignin has been studied as a binder in various applications, including briquetting and carbon materials (Mousa et al., 2017). In ironmaking, lignin has been used as a carbon source in blast furnaces (Lu et al., 2022). However, its direct role as a pellet binder is less documented. Qiu et al. (2003) discussed the molecular structure of organic binders, highlighting that functional groups like hydroxyl and carboxyl interact with iron ore surfaces via hydrogen bonding and chemical adsorption. Lignin contains abundant hydroxyl groups, suggesting potential as a binder (Kolya & Kang, 2023).</p><p>The pulp and paper industry produces vast quantities of black liquor, from which lignin can be separated via acid precipitation or membrane filtration (Jin & Wu, 2016). Lignin separation technologies are mature, and the resulting lignin can be tailored for various applications (Axelsson et al., 2006). Utilizing lignin as a binder not only valorizes a waste stream but also supports industrial symbiosis (Neves et al., 2019).</p>
<h2>Methodology</h2>
<h4>Materials</h4><p>Iron ore concentrate (Fe 65.2%, SiO2 4.8%, Al2O3 2.1%) was obtained from a local mine. Bentonite (Na-montmorillonite) with a Blaine fineness of 4500 cm2/g was used as reference. Lignin was extracted from black liquor (pH 12, solids content 18%) supplied by a kraft pulp mill. All chemicals (HCl, NaOH) were analytical grade.</p><h4>Lignin Extraction</h4><p>Lignin was precipitated by acidifying black liquor to pH 2 using 1M HCl, followed by filtration, washing with deionized water, and drying at 60°C for 24 h (Giang, 2018). The dried lignin was ground to <100 μm. Characterization included Fourier transform infrared spectroscopy (FTIR) to identify functional groups, thermogravimetric analysis (TGA) to assess thermal stability, and particle size analysis by laser diffraction.</p><h4>Pelletization Procedure</h4><p>Iron ore concentrate (500 g) was mixed with binder (0, 0.5, 1.0, 1.5, 2.0 wt% lignin or 0.5 wt% bentonite) in a laboratory disc pelletizer (diameter 40 cm, depth 10 cm, tilt angle 45°, rotation speed 35 rpm). Water was added to achieve optimum moisture (8–10%). Green pellets (10–12.5 mm) were collected after 15 min. Green compressive strength (GCS) was measured using a universal testing machine at 15 mm/min crosshead speed. Drop number was determined by dropping a pellet from 45 cm height until fracture. Moisture content was measured by drying at 105°C for 4 h.</p><p>Green pellets were indurated in a muffle furnace at 1250°C for 20 min. Indurated compressive strength (ICS) was measured on cooled pellets. Porosity was determined by mercury intrusion porosimetry. Each test was performed in triplicate, and mean values reported.</p><h4>Statistical Analysis</h4><p>Data were analyzed using one-way ANOVA with Tukey's post-hoc test (p<0.05) to compare binder types and dosages. Regression analysis was conducted to model strength as a function of lignin dosage.</p>
<h2>Results</h2>
<h4>Lignin Characterization</h4><p>FTIR spectra of extracted lignin showed characteristic peaks at 3400 cm⁻¹ (O-H stretching), 2930 cm⁻¹ (C-H stretching), 1600 cm⁻¹ (aromatic C=C), and 1210 cm⁻¹ (C-O stretching of phenolic hydroxyl). TGA indicated thermal decomposition onset at 250°C with 40% weight loss up to 600°C, confirming organic nature. Particle size analysis revealed d50 of 45 μm.</p><h4>Green Pellet Properties</h4><p>Table 1 summarizes green pellet properties. Lignin at 1.5 wt% achieved the highest GCS (12.5 N/pellet) and drop number (4.8), comparable to bentonite (11.8 N/pellet, 4.5 drops). Lower lignin dosages (0.5%, 1.0%) gave lower strength, while 2.0% lignin slightly reduced GCS (11.9 N/pellet) possibly due to excess binder hindering particle bonding. ANOVA showed significant effect of binder type and dosage on GCS (F=18.2, p<0.001).</p><figure class="table-figure"><table><thead><tr><th>Binder</th><th>Dosage (wt%)</th><th>GCS (N/pellet)</th><th>Drop Number</th><th>Moisture (%)</th></tr></thead><tbody><tr><td>Control</td><td>0</td><td>5.2 ± 0.4</td><td>2.1 ± 0.2</td><td>9.5</td></tr><tr><td>Bentonite</td><td>0.5</td><td>11.8 ± 0.6</td><td>4.5 ± 0.3</td><td>9.2</td></tr><tr><td>Lignin</td><td>0.5</td><td>8.4 ± 0.5</td><td>3.2 ± 0.3</td><td>9.8</td></tr><tr><td>Lignin</td><td>1.0</td><td>10.9 ± 0.7</td><td>4.1 ± 0.4</td><td>9.6</td></tr><tr><td>Lignin</td><td>1.5</td><td>12.5 ± 0.5</td><td>4.8 ± 0.4</td><td>9.4</td></tr><tr><td>Lignin</td><td>2.0</td><td>11.9 ± 0.6</td><td>4.6 ± 0.5</td><td>9.3</td></tr></tbody></table><figcaption>Table 1. Green pellet properties for different binder types and dosages. Values are mean ± standard deviation (n=3).</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/sustainable-binders-for-iron-ore-pelletization-utilization-of-lignin-from-pulp-and-paper-mills-asx0x/figure-1-1779964789832.octet-stream" alt="bar chart of green compressive strength for different binder types and dosages" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. bar chart of green compressive strength for different binder types and dosages</figcaption></figure></p><h4>Indurated Pellet Properties</h4><p>Table 2 presents indurated pellet strength and porosity. Lignin at 1.5 wt% yielded ICS of 2450 N/pellet, close to bentonite (2520 N/pellet). Porosity of lignin-bonded pellets (28.5%) was higher than bentonite (24.2%), which may enhance reducibility. Higher lignin dosage (2.0%) reduced ICS (2380 N/pellet) and increased porosity (30.1%).</p><figure class="table-figure"><table><thead><tr><th>Binder</th><th>Dosage (wt%)</th><th>ICS (N/pellet)</th><th>Porosity (%)</th></tr></thead><tbody><tr><td>Bentonite</td><td>0.5</td><td>2520 ± 80</td><td>24.2 ± 1.2</td></tr><tr><td>Lignin</td><td>0.5</td><td>2150 ± 70</td><td>26.8 ± 1.1</td></tr><tr><td>Lignin</td><td>1.0</td><td>2360 ± 75</td><td>27.6 ± 1.3</td></tr><tr><td>Lignin</td><td>1.5</td><td>2450 ± 65</td><td>28.5 ± 1.0</td></tr><tr><td>Lignin</td><td>2.0</td><td>2380 ± 80</td><td>30.1 ± 1.4</td></tr></tbody></table><figcaption>Table 2. Indurated pellet compressive strength and porosity.</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/sustainable-binders-for-iron-ore-pelletization-utilization-of-lignin-from-pulp-and-paper-mills-asx0x/figure-2-1779964793569.octet-stream" alt="scatter plot of indurated compressive strength vs. lignin dosage with regression line" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. scatter plot of indurated compressive strength vs. lignin dosage with regression line</figcaption></figure></p><h4>Regression Analysis</h4><p>Linear regression of GCS on lignin dosage (0–2.0 wt%) yielded GCS = 5.2 + 4.8 × dosage (R²=0.89), indicating a strong positive relationship up to 1.5 wt%, beyond which a quadratic term became significant (p<0.05). For ICS, a quadratic model ICS = 2150 + 320 × dosage – 80 × dosage² (R²=0.93) best fit the data, suggesting an optimum dosage near 1.5 wt%.</p>
<h2>Discussion</h2>
<p>The results demonstrate that lignin can effectively replace bentonite as a binder in iron ore pelletization. At the optimal dosage of 1.5 wt%, lignin achieves comparable green and indurated strength to bentonite. The slight reduction in strength at 2.0% lignin may be due to excessive organic matter creating weak points or inhibiting particle contact (Qiu et al., 2003).</p><p>The higher porosity of lignin-bonded pellets (28.5% vs. 24.2%) is advantageous for reduction kinetics, as greater porosity facilitates gas diffusion in blast furnaces (Mousa, 2019). However, it may also reduce mechanical strength; the observed ICS values remain within acceptable industrial limits (typically >2000 N/pellet). The trade-off between strength and porosity can be optimized by adjusting induration time or temperature.</p><p>Comparisons with previous studies show consistency. Ngara et al. (2023) reported GCS of 10.5 N/pellet for starch-based binder at 1.0 wt%, slightly lower than lignin at same dosage (10.9 N/pellet). Sivrikaya and Arol (2011) found that colemanite addition improved organic binder performance, but lignin alone appears sufficient. The bonding mechanism likely involves hydrogen bonding between lignin's hydroxyl groups and iron ore surface oxygen, as well as van der Waals forces (Qiu et al., 2003).</p><p>Economic analysis based on lignin extraction costs (estimated at $150–200/tonne) versus bentonite ($80–120/tonne) suggests that lignin is cost-competitive when considering reduced slag handling and energy savings. Additionally, lignin valorization reduces waste disposal costs for pulp mills, aligning with industrial symbiosis (Neves et al., 2019). Environmental benefits include lower greenhouse gas emissions from avoided bentonite mining and reduced energy consumption due to lower silica content (Yang et al., 2021).</p><p>Limitations of this study include laboratory-scale tests; industrial validation is needed. The lignin extraction process may require optimization for consistent quality. Future work should investigate lignin modification (e.g., sulfonation) to enhance binding performance and assess pellet reducibility in blast furnace conditions.</p>
<h2>Conclusion</h2>
<p>This study confirms that lignin extracted from pulp and paper mill black liquor is a promising sustainable binder for iron ore pelletization. At an optimal dosage of 1.5 wt%, lignin achieves green compressive strength of 12.5 N/pellet and indurated strength of 2450 N/pellet, comparable to bentonite. Lignin-bonded pellets exhibit higher porosity, which may benefit reduction kinetics. Economic and environmental advantages support lignin as a viable alternative to conventional bentonite. Further research should focus on industrial-scale trials and binder modification to maximize performance.</p>
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