| China’s development promotes growing demand for energy. As the most abundant resource in domestic, coal is still the most important energy products. Large amounts of ash produced after burning coal will obviously pollute the environment with directly stacking. In fact, coal provides both energy and resources. Thermal power Engineering Institute of Zhejiang University proposed a new technology, coal gradient utilization. It was designed to fully use both energy and resources in coal, including heat, electricity, and gas generation, and furthermore extracting a variety of valuable elements in ash. In general, silicon and aluminum dominate most coal ash. Recovering them not only promotes the coal value, but also protects the environment.Among the methods of Al&Si utilization in coal gangue and its ash, effective ways including the calcined kaolin preparation and Al&Si extraction. The coal gangue and its ash studied were major composed of silica and alumina. Their ratio (Si/Al) is close to the theoretical value of kaolinite (1.18:1). After summarising the Al&Si utilization status, the coal gangue and its ash were conducted to prepare calcined kaolin and to extract Al&Si by innovatively proposing Al&Si-cogeneration through acid leaching and alkali fusion method. Al&Si-cogeneration process was described as following:coal ashâ†'firstly sulfuric acid leachingâ†'first leaching residues (FLR) roasted by mixing with sodium carbonateâ†'roasted product leached by waterâ†'water leaching residues (WLR) secondly leached by sulfuric acidâ†'evaporation and crystallization of two acid leaching solutionsâ†'calcinationâ†'crude alumina leached by alkaliâ†'sodium aluminate solution decomposed by adding aluminum hydroxide seedâ†'aluminum hydroxide calcinationâ†'alumina products.Coal gangue, slag of CFB and washed fly ash provided were evaluated on their potential as calcined kaolin by chemical composition analysis, X-ray diffraction (XRD) analysis and orthogonal study of iron leaching. Results showed that coal gangue can better fulfill the preparation of high-quality calcined kaolin. Further study showed that HCl was suitable for deiron of coal gangue. Optimal iron leaching rate was 54.86% under the condition of 2 mol/L HCl at 40℃ with 2 h and an S/L of 1:3. This caused less effect of aluminum dissolution and promote iron leaching rate. Mossbauer spectroscopy showed that the structural ferric ions and ferric oxides evolved from ferrous ions were difficult to remove by acids. Therefore, iron removal by acids should be achieved prior to calcination, namely this gangue was better for burning after grinding process to prepare calcined kaolin. Product whiteness reached 75.8% under the above conditions.In regarding alumina extraction from coal ash, Al&Si-cogeneration through acid leaching and alkali fusion method was proposed. Calculation through thermodynamic theory proved the feasibility of the process. In the first acid leaching stage, two coal ash (A and B) were studied to obtain optimal leaching conditions, respectively. For A, a better aluminum leaching rate reached 97.34%. For B, an optimal aluminum leaching rate was only 81.72% due to the existing aluminum-bearing crystals. Thus, Al&Si-cogeneration process was conducted on B. After roasting a mixture of the FLR and sodium carbonate, the aluminum-bearing crystals in FLR were completely destroyed while sodium silicate was generated. Experimental study demonstrated an optimal roasting temperature at 860℃, which could break aluminum-bearing crystals in FLR with generating more soluble sodium silicate. Under this condition, silicon dissolution reached 85.98% after roasting product leached by water. In the second leaching step with 5 mol/L sulfuric acid, the aluminum dissolutions from WLR were 99.06% at 100℃. Total aluminum dissolutions were more than 97%, which was obviously higher than traditional acid leaching method.Two acid leaching solutions were then evaporated and crystallized at 115～117℃ to get impurity solid aluminum sulfate. Calcination of this aluminum sulfate tests showed that optimal calcination conditions were stay 2 h at 839℃ with heating rate of 5℃/min, which could ensure fully decomposition of aluminum sulfate with little SO2 generation. Crude alumina from calcination were purified by Bayer process to obtain pure alumina products. Tests study showed that due to high alumina content with few silica, crude alumina was suitable for Bayer process and the effect of NaOH utilization was better. At the molar ratio (ak) of 2.99, optimal aluminum dissolution was 97.25% with caustic concentration 240 g/L in mother liquid,10 min at 220℃ and mixture ratio of 1.75:1 (ak). Al(OH)3 precipitation coefficient reached 43.62% after sodium aluminate solution decomposed by seeding method.Al&Si-cogeneration through acid leaching and alkali fusion method was prove to be good process for coal ash utilization. In the acid leaching stage, sulfuric acid could be circulated by recovery SO3. In alkaline leaching stage, caustic soda could also be circulated by repeated dissolution and decomposition. Thus, the process consumed fewer materials. Because more than 97% aluminum was leached, ont only Al and Si were completely separated, but also byproducts of silica, sodium silicate and high iron slag generated. Therefore, this process is a good economy, high utilization of resources, sustainable and environment friendly industrial technology. |
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