Four processes of electrolytic aluminum overhaul slag treatment
Overhaul slag is a typical hazardous solid waste discharged from electrolytic aluminum production, with a total discharge of millions of tons in China every year. Because overhaul slag contains soluble fluoride and cyanide with high toxicity, if not properly disposed, it will mix with rainwater into rivers, penetrate into underground water sources, pollute surface water sources, and cause great harm to the ecological environment, so the harmless treatment of overhaul slag has become one of the major problems to be solved urgently in the electrolytic aluminum industry. There are four treatment methods of overhaul slag: wet method, fire method, acid method and landfill method.
01 Wet treatment
The overhaul slag of electrolytic cell containing fluorine and cyanide and water-soluble calcium, magnesium and aluminum ion compounds can form calcium, magnesium and sodium salts of hypochlorite in water. They are mixed with water and ball milled for pulping. The cyanide leached from the slurry is reduced and decomposed by hypochlorite, and the fluoride reacts with calcium, magnesium and aluminum ions in the slurry to form non-toxic calcium fluoride (CaF2), magnesium fluoride (MgF2) insoluble in water After aluminum fluoride (AlF3) is precipitated, centrifugal solid-liquid separation or filtration sedimentation tank solid-liquid separation is adopted. The separated water can be reused in the front treatment process, and the precipitated solids can be used as additives or building materials for refractory production. This method is the mainstream trend of dealing with overhaul slag in electrolytic aluminum enterprises in China.
02 Fire treatment
Combustion is an effective method to remove cyanide. When heated to 300 ℃, about 99.5% of cyanide in the lining of the waste tank disappears; When heated to 400 ℃, about 99.8% of cyanide disappears; When heated above 700 ℃, cyanide disappears completely. Fire treatment of electrolytic cell overhaul slag can effectively destroy cyanide and make fluoride escape or convert into relatively insoluble fluoride in the form of HF. Refractory materials are decomposed into inert slag that meets the requirements of environmental protection. The treated materials can be buried or sold as raw materials. Its disadvantages are strict requirements on equipment air tightness, huge investment, large energy consumption and secondary pollution.
03 Acid treatment
Domestic research on overhaul slag treatment started late. The research technology is mainly acid treatment and direct utilization, which is still in the small experimental stage and has not been popularized and applied in batch. The waste tank lining is treated by sulfuric acid method, that is, the waste tank lining is crushed and put into the acidolysis tank filled with water and concentrated sulfuric acid for acidolysis, and the generated gas is repeatedly rinsed with water to recover hydrofluoric acid; The filter residue and filtrate produced after acid hydrolysis can be further treated. The filter residue can be made into graphite powder, industrial aluminum hydroxide and alumina. The filtrate can produce fluoride and sulfate products.
04 landfill
The main processes of the landfill method include waste slag transportation, site selection of the slag yard, design and layout of the slag yard, pretreatment of waste slag before landfill, treatment of leachate, etc. However, the landfill method has the limitations of high site and investment cost, and at present, the relevant national departments have clearly prohibited the approval of new overhaul slag storage slag yard, which does not meet the relevant requirements of national environmental protection, Therefore, it is not feasible at present and in the long run, and enterprises should consider it carefully.
At present, electrolytic aluminum plants at home and abroad attach great importance to the treatment of overhaul slag. China's environmental protection department has also formulated corresponding judgment standards and treatment regulations to realize the harmless and resource-based treatment of overhaul slag. Enterprises should choose appropriate treatment technology according to the production scale and national environmental protection requirements.