The Reduction And Harmless Process Research Of Recycled SPL Of LANZHOU ALUMINUM CO.,LTD

Spent Pot Lining?SPL?is a kind of waste slag produced in the electrolytic aluminum industry.Because it contains excessive fluoride,it was defined as a hazardous waste by the state in 2016.According to the requirements of the national environmental protection department,LANZHOU ALUMINUM CO.,LTD has recycled its SPL that were discharged before 2016.Because the recovered SPL is mixed with a small amount of clay and other waste debris,the resource treatment is very difficult and can only be harmless.LANZHOU ALUMINUM CO.,LTD has developed a wet process.Although it can reduce the concentration of fluoride ions and achieve harmlessness,the SPL has increased by 80%after treatment.In order to remove the excessive fluoride in the SPL and achieve the purpose of reduction and harmlessness of SPL,the following research has been done in this paper:Through research the composition and morphology of the recycled SPL of LANZHOU ALUMINUM CO.,LTD,it was found that the fluoride in the SPL mainly exists in the form of CaF₂,Na₃AlF₆ and NaF,and the fluoride and other substances in the SPL have inclusions and coatings.According to the“solid waste-Determination of fluoride-Ion selective electrode method”?GB/T 15555.11-1995?,the average fluoride ion concentration in the leaching solution of SPL has reached 7064 mg/L,and the pH value is11.67,which is much higher than“Identification standards for hazardous wastes-Identification for extraction toxicity”?GB5085.3-2007?,the standard of hazardous waste is that the inorganic fluoride in the solid waste leachate exceeds 100 mg/L.According to the types of fluoride in the SPL and the research on the existing process,a"water washing-baking"combined defluorination process was proposed,and its feasibility was experimentally studied.Experiments were conducted separately.The experimental results show that the"water washing-baking"method can not only make the fluoride ion concentration reach the national standard,but also reduce the SPL.The process mainly includes three steps of washi ng and dissolving out defluorination,treatment of fluorine-containing waste water,and SPL roasting to fix fluorine after washing.It has the advantages of simple process,low energy consumption and high defluorination efficiency.This article chooses this process to deal with SPL and optimizes its process.Single factor experiments and orthogonal experiments were performed on factors affecting the defluorination effect,such as water temperature,liquid-solid ratio,water time,and particle size in the pr ocess of water dissolution and defluorination.The results show that the defluoridation effect is best when the SPL with a particle size of less than 0.05 mm is added,20 times water is added,and the temperature is maintained at 90?for 1.5 hours.The main component of the removed phase is NaF,and the desalted amount is 25.6%.The factors that have the greatest influ ence on the defluorination effect are particle size,temperature,liquid-solid ratio,and time.Keeping the optimal water-washing process conditions unchanged,the obtained filtrate was added with excess CaCl₂,CaO,Al₂?SO₄?₃ and activated carbon and other defluorination agents to investigate the defluorination effect of different defluorination agents.The experimental results show that adding 5 times the theoretical amount of CaCl₂ can reduce the fluoride ion concentration in the filtrate from 2182 mg/L t o 8.6 mg/L reaching“Integrated wastewater discharge standard”?GB 8978-1996?below 10 mg/L standard.The pH value was reduced from 10.72 to 9.45.After adding a small amount of Al₂?SO₄?₃,the pH dropped to 6.83,which was close to neutral.The filter residue obtained under the same water washing conditions was further subjected to a roasting experiment to fix fluorine,and the effects of CaCl₂ addition amount,roasting temperature,roasting time and other factors on the effect of fluorine fixati on were investigated and a thermodynamic analysis was performed.Experiments show that after adding CaCl₂,CaCl₂ will react with NaF to form CaF₂ which is difficult to dissolve in water to precipitate.The reaction is an exothermic reaction,which is beneficial to the further volatilization of fluoride.The optimal process is:the amount of CaCl₂ added is 0.16g/g,the roasting temperature is 850?,and the roasting time is 2h.The concentration of fluoride ions in the SPL after the combined treatment with"Wash-bake"combined process has been reduced to 98 mg/L,which meets the state's permitted emission standard.The pH dropped to7.68,which was close to neutral,and the weight of the SPL was reduced by28.73%.The purpose of reduction and harmlessness of SPL was successfully achieved,and a new path was opened for the SPL treatment.