Research On Deep Purification Technology Of S^2- And Organic Matter In Red Mud Leachat

Red mud is a solid waste generated during the alumina production process and is currently mainly disposed of by stockpiling.However,red mud heaps produce leachate containing a large number of pollutants under the influence of rainfall leaching and groundwater immersion,which must be purified and treated to meet emission standards before discharge.The red mud leachate from a decommissioned alumina plant in Guizhou is an alkaline and highly saline wastewater with excessive pollution factors such as S^2-,COD,NH₃-N and chroma,which is difficult to degrade biochemically.In this paper,a combination of chemical precipitation,flocculation and catalytic ozone oxidation was proposed for the deep purification and treatment of leachate with excessive S^2-and COD pollutants,and the removal mechanism was investigated.The main results of the study are as follows:（1）Study on the removal of sulphur ions from red mud leachate.Single-factor experiments were conducted to investigate the effects of the type of desulphurisation agent,leachate pH,the excess coefficient of desulphurisation agent,reaction temperature and reaction time on the removal of S^2-,COD and other pollutants from leachate;the kinetic data of the reaction process of removing S^2-from red mud leachate were fitted with the Avrami equation to obtain the limiting link affecting the reaction rate of sulphide precipitation.The results showed that under the optimized conditions of Fe SO₄ as the desulphurization agent,pH 8 of the leachate,Fe SO₄ excess factor of 1.2,reaction temperature of 40℃and reaction time of 9 min,the removal rates of S^2-,COD,ammonia nitrogen,total nitrogen and colour in the red mud leachate could reach 97.55%,16.21%,21.63%,16.32%and 20%,with S^2-meeting the emission standards.The activation energy of the Fe S precipitation reaction was 31.59k J·mol^-1,which indicated that the diffusion link was the limiting link affecting the Fe S precipitation reaction rate.（2）Study on flocculation enhancement of red mud leachate.Single-factor experiments were carried out to investigate the effects of PAC dosage,APAM dosage and the sequence of PAC and APAM dosage on the removal rates of S^2-,COD and other pollutants in leachate;to determine the changes in the Zeta potential on the surface of leachate colloids and to explain the flocculation mechanism of flocculants in relation to the flocculation effect of red mud leachate;and to analyse the organic species and their changes before and after flocculation of leachate by three-dimensional fluorescence spectroscopy.The experimental results showed that under the optimized experimental conditions of 750 mg·L^-1 of PAC,2.1 mg·L^-1 of APAM and the addition of PAC followed by APAM,the removal rates of S^2-,COD,ammonia nitrogen,total nitrogen and chromaticity were increased to 98.57%,43.42%,26.05%,22.58%and 99%respectively.The flocculation mechanisms of PAC and APAM were electro-neutralisation and adsorption-bridging,respectively.The red mud leachate contained mainly tryptophan-like substances,fulvic acid-like substances,humic acid-like substances and dissolved metabolite substances before and after the flocculation-enhanced treatment,with the highest proportion of humic acid-like substances.The combination flocculant had a certain effect on the removal of substances in all four areas,with the best effect on the removal of humic acid-like substances and the less obvious effect on the removal of dissolved metabolite substances.（3）Study on the removal of organic matter from red mud leachate.The differences between ozone oxidation and ozone oxidation catalyzed by different crystalline forms of MnO₂ on the removal of COD,ammonia nitrogen and other pollutants from leachate were compared and the best crystalline form of MnO₂ with the best catalytic activity was selected;XRD,SEM,BET and XPS characterization tests were conducted on different crystalline forms of MnO₂ to elucidate the key factors affecting the catalytic activity of different crystalline forms of MnO₂;single-factor experiments were conducted to investigate the effects of catalyst The effect of catalyst dosage,ozone flow rate,leachate pH,reaction temperature and reaction time on the removal of COD,ammonia and other pollutants from leachate was investigated.The results showed that ozone oxidation was less effective in removing COD and ammonia nitrogen from red mud leachate,and the addition ofα-MnO₂,β-MnO₂,γ-MnO₂ andδ-MnO₂ catalysts could improve the effect of ozone oxidation on the removal of pollutants from red mud leachate,among whichγ-MnO₂ had the best catalytic activity.Under the optimized conditions of 15 g·L^-1catalyst,400 L·h^-1 ozone flow rate,pH 8 of leachate,50℃reaction temperature and80 min reaction time,the removal rates of COD,ammonia nitrogen and total nitrogen in leachate were 95.98%,19.11%and 16.24%respectively.The COD met the emission standard,while the ammonia and total nitrogen failed to meet the emission standard.In summary,the combined process of"chemical precipitation-flocculation enhancement-catalytic ozone oxidation"proposed in this paper could effectively remove S^2-,COD and chroma pollution factors from red mud leachate and met the emission standards.However,the removal of ammonia nitrogen and total nitrogen was poor and failed to meet the discharge standard.