Technology Principle of Extracting Aluminum from Coal Gangue

Coal gangue, a byproduct of coal mining, is the primary solid waste generated during coal production. Large piles of coal gangue are found around many mines, causing serious environmental pollution and occupying vast areas of land. The total stockpile of coal gangue in China is estimated to be about 4.5 billion tons [1-2]. This waste not only harms the environment but also represents a significant loss of valuable resources. Notably, coal gangue contains a considerable amount of aluminum, with alumina making up approximately 25% of its composition [3-4], indicating it is a potential resource. By utilizing coal gangue, we can address environmental issues and turn waste into a valuable asset, creating additional economic benefits [2,6-8]. In this study, coal gangue from a mine in the Zhengzhou area was analyzed and processed through leaching with sulfuric acid. Sulfuric acid is a commonly used flocculant, primarily applied in wastewater treatment. After several trials, under optimal conditions, the dissolution rate of aluminum reached 89.6%, and the purity of the resulting aluminum sulfate was as high as 95%. These results offer new insights into the development and utilization of coal gangue. The experimental process involved the following steps: **(A) Materials** A certain type of coal gangue from the Zhengzhou mining area was crushed to 40 mesh. Its composition is listed in Table 1. **(B) Instruments and Equipment** Equipment used included a muffle furnace, pH meter, electronic balance, grinder, and mixer. **(C) Reagents** Leaching reagents included 1:1 hydrochloric acid (diluted in distilled water), 1:2 sulfuric acid (diluted twice in distilled water), and aqua regia (concentrated nitric acid: concentrated hydrochloric acid = 3:1). For aluminum determination, reagents such as 10% sulfosalicylic acid, 0.02 mol/L CuSO4, 0.02 mol/L EDTA, 1:1 HCl, 1:1 ammonia, acetic acid-sodium acetate buffer solution (pH 5.0), and PAN indicator were used. **(D) Experimental Method** A suitable amount of dried gangue sample was placed in a muffle furnace and roasted at different temperatures. Then, it was sintered with sulfuric acid for leaching. Factors such as roasting temperature, time, acidity, leaching temperature, and time were tested to optimize the leaching efficiency. Aluminum content was determined using the EDTA titration method. **Experimental Results and Analysis** **(I) Effect of Roasting Temperature** When coal gangue was directly leached with concentrated sulfuric acid or aqua regia without pre-treatment, the dissolution rate was very low—only 3.6%. However, when 50g of 40-mesh coal gangue was roasted at various temperatures (400°C, 600°C, 800°C, 1000°C, and 1200°C) for 5 hours, and then leached with 1:2 sulfuric acid for 5 hours, the dissolution rate increased significantly. At 700°C, the activity of the gangue improved, and at 1000°C, new phases like γ-Al₂O₃ and mullite formed, which reduced the leaching efficiency. **(II) Effect of Leaching Time** After roasting at 800°C, 30g of coal gangue was treated with 150mL of 1:1 hydrochloric acid at 85°C. The dissolution rate was measured over time. It was found that a leaching time of 5 hours provided the best results, while longer or shorter times led to lower dissolution rates. **(III) Effect of Acid Type** Different acids were tested, including HNO₃, HCl, and H₂SO₄. Based on technical feasibility and cost-effectiveness, HCl and H₂SO₄ were selected. When 10g of the sample was leached with 1:1 HCl and 1:2 H₂SO₄, the dissolution rates were 68.78% and 89.6%, respectively. Sulfuric acid showed a much higher leaching efficiency and is more cost-effective, making it the preferred choice for industrial applications.

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