Study On Degradation Characteristics Of Cyanide In Spent Potlining

Spent potliner（SPL）was a hazardous waste containing highly toxic cyanide which is listed in the National Hazardous waste list produced by electrolytic aluminum smelting process.With the increasingly stringent discharge standards,the harmless treatment efficiency of SPL needed to be improved at the same time.Therefore,effectively solving the threat of residual cyanide discharge in SPL has become one of the important environmental problems to be solved urgently.In view of the above problems,this paper taked SPL as the main research object,and aimed to develop efficient,fast and cheap decyanation technology.In this paper,based on exploring the occurrence state of cyanide in SPL and the leaching solution of SPL,the degradation characteristics of cyanide were studied systematically by wet method（new chlorine dioxide treatment technology,microwave activated persulfate treatment technology）and fire method（microwave-sensitizer synergistic treatment technology）,and the cyanide removal mechanism was described.These were the main results of the research:（1）Cyanide in SPL mainly existed in the form of sodium cyanide and iron cyanide complex,of which 51.32%was iron cyanide complex and 48.68%was sodium cyanide.Cyanide was present in the leaching solution as CN^-,[Fe（CN）₆]^4-and[Fe（CN）₆]^3-.About half of the cyanide could not be leached by water.（2）A new type of chlorine dioxide preparation was used to treat cyanide on the basis of alkali chlorine method.The new chlorine dioxide preparation had a significant degradation effect on CN^-,but the degradation effect on[Fe（CN）₆]^4-and[Fe（CN）₆]^3-was not ideal.Under the conditions of reaction p H=10.5,T=25℃,reaction time 3 min and molar ratio M_{Cl O2}/M_CN-=2.0,the gold mine cyanide wastewater with total cyanide concentration of 318.02 mg/L and cyani de concentration of 315.28 mg/L could be treated up to the standard.The residual concentrations of total cyanide and cyanide were 0.38 mg/L and 0.05mg/L,and the degradation rates were as high as 99.88%and 99.98%.The mechanism study found that the final degradation products of cyanide were N₂ and CO₃^2-,while Cl O₂ was reduced to Cl^-.（3）The treatment of cyanide by microwave activated persulfate was proposed for the first time.Under the condition that p H value was 10.15,microwave power was 300 W,microwave action time was 180 s,and[oxidant]:[CN^-]_molar was 5,the SPL defluorination leaching solution with a total cyanide concentration of 34.91±0.3 mg/L could be removed up to the standard,the degradation rate was as high as 99.88%,and the remaining total cyanide was less than 0.04 mg/L.In exploring the degradation mechanism of cyanide in MW-PS reaction system,it was found that SO₄^·-and HO·coexisted and contributed to the degradation of cyanide.SO₄^·-played a leading role in cyanide degradation.Under the action of free radicals,the Fe-CN gradually dissociated into CN^-,CN^-was oxidized to CNO^-,and then oxidized to CO₂,N₂ and NO₃^-,and S₂O₈^2-was reduced to SO₄^2-.（4）Microwave-sensitizer synergistic rapid degradation of cyanide in SPL was proposed for the first time.When the mass ratio of SPL to sensitizer was 15:1,the microwave power was 600W,and the action time was 4 min,the total cyanide content in the SPL was degraded to 21.3 mg/kg by 636.80±0.5mg/kg,and the removal rate was 96.66%.And the residual cyanide can not be leached in alkaline conditions.Compared with the traditional muffle furnace,the power consumption cost of this method was only 1/10 of that of the traditional muffle furnace.The rapidity of the microwave heating process made the cyanide degradation process in a state of hypoxia,and the final products were N₂ and CO₂.Sensitizer Fe₂O₃ acted as a catalyst,which reduced the activation energy of cyanide degradation and accelerated the reaction.Among the three efficient and rapid cyanide treatment technologies proposed in this paper,microwave activated persulfate method and microwave-sensitizer synergistic treatment method had a fast reaction rate,were not limited to cyanide types,and had a good application prospect.It not only enriched the theoretical research of wet and fire cyanide removal technology of APL,but also provided a certain reference for the engineering application of cyanide treatment,and provided a new way for the harmless treatment of other cyanide-containing wastes.