Research Into The Liquid Polyaluminium Chloride (LPAC) Coagulant Prepared By Coal Gangue

The resource reclamation and reuse technology of coal gangue which is the main contamination source in Inner Mongolia Province are major problems for scientists and engineers. Liquid Polyaluminium Chloride (LPAC) coagulant was prepared by coal gangue as its raw materials and acid-leaching-two-step method as its process flow, and Wei Tai Kaolinite Developmental Co., Ltd as its production base and coal washing plants, thermal power plants, water supply corporations as its performance testing bases, the problems of high production cost and complex process were successfully solved through process flow optimization and the performance of LPAC was comprehensively assessed.For more than two years running and process optimization, the outputs of LPAC reached 15000t/a, the process flow were calcining in rotary kiln (T=750-850℃, time=2h), acid-leaching in reaction kettle (T=120℃, C_HCI=14% (m/m), m_{coal gangue}/m_HCI= 1/2.2, time= 2h), basicity-adjusting with limestone powder (regulating Al₂O₃ concentration and basicity to about 9% and 40-60% ), removing the undissolved solid waste by pressure-filtration when the temperature of solution was less than 70℃, filtrate modulating (regulating Al₂O₃ concentration and basicity to about 10% and 60%), packing and putting in storage.By lab trials for water treatment with LPAC, it was shown that LPAC could remove the slime in coal-washing water and the treated water can be reused in coal washing process. LPAC could remove 91.5%, 80% and 83.6% turbidity in high, medium, and low turbidity Huang He river water treatment, and reduced the turbidity form 77NTU, 27.25NTU and 5.4NTU to 6.55 NTU, 4.47NTU and 1.14NTU, respectively. The performance of LPAC was superior to the contrasting coagulants such as solid PAC and liquid PAC which were synthesized with acid-leaching-one-step method.By pilot testing with LPAC, it was shown that LPAC could remove 90.6% turbidity in industrial wastewater treatment and the turbidity reduced from 108NTU to 2.58NTU, the dosage of LPAC was just half of solid PAC which was in use in this plant, so when using LPAC in water treatment process could reduce the residual Al which may be harmful for soil and groundwater. LPAC could reduce the turbidity from 598NTU to less than 3NTU in high turbidity water treatment process. By LPAC coagulation of Huang He river water of low temperature (0.4-0.8℃) and low turbidity (180NTU) and frequently water flow variation (920-2249t/h), dosage 20-40mg/L, the residual turbidity could reduce to less than 10NTU and kept steady. It is shown that LPAC was a kind of Al₁₃Cl₁₅ which played a great role in water treatment process with X-ray Diffraction (XRD) detection. The mechanism of coal gangue calcining process is:Al₂O₃·2SiO₂·2H₂O(kaolinite)(?)Al₂O₃·2SiO₂(metakaolinite) +2H₂O3Al₂O₃·2SiO₂(metakaolinite)(?)3Al₂O₃·2SiO₂(mullite) +4SiO₂ The production and selling process of LPAC are conforming to clean production through environmental impact assessment (EIA).By technical and economic analysis, the production cost of LPAC is 496.99RMB Yuan/t, and of Huang He River with LPAC is 0.0719RMB Yuan/t, the water-treating cost with solid PAC which was synthesized with acid-leaching-one-step method is 0.0969 RMB Yuan/t in the same condition.Liquid Polyaluminium Chloride (LPAC) coagulant which was synthesized by coal gangue as its raw materials and acid-leaching-two-step method as its process flow has a low production cost and a well performance in water treatment process, so this kind of LPAC has notable economic and social benefit, and potential application prospects.