Treatment Of Arsenic-bearing Biological Metallurgical Wastewater By Coprecipitation Of Arsenic With Iron

Arsenic-containing wastewater is the main pollutant generated during the biological preoxidation of gold ores,which contains high concentrations of iron and arsenic.A common technique for immobilization of arsenic from the wastewater is lime neutralization.However,a large amount of amorphous precipitates are formed with high water content and poor stability,therefore it will take high costs for precipitates storage.Both scorodite and arsenic-bearing jarosite are ideal compounds for arsenic fixation with good properties of crystalline structures,high arsenic content and high stability.In view of this,the process of removing arsenic in biological metallurgical wastewater by coprecipitation of arsenic with iron was investigated in this thesis.In this process,the arsenic in wastewater was stabilized because it was fixed as crystalline arsenic-iron precipitate with high stability and low water content,which was of great significance to stable the arsenic and make it harmless.Firstly,the effect of preoxidation with ozone on the treatment of biological metallurgical wastewater by arsenic and iron coprecipitation reaction was investigated.The preoxidation of As(?)to As(?)in wastewater by ozone before precipitation was beneficial to increase the arsenic removal and to improve the stability of precipitates.It was found that after preoxidized by ozone,the arsenic removal ratios were improved from 93.18%at pH=2,97.38%at pH=2-3 and 98.20%at pH=2-4 to 94.78%,99.76%and 99.81%respectively.And the leached arsenic concentrations of the precipitates decreased from 2.62 mg/L,2.45 mg/L and 3.67 mg/L,to 2.21 mg/L,0 and 0.31 mg/L,respectively.Secondly,the effects of additional ferrous ions and gold concentrate were investigated,respectively.With the addition of ferrous ions,the arsenic removal ratio in wastewater and the stability of precipitates were improved.XRD and SEM results showed that the precipitates in wastewater consisted of crystalline arsenic-bearing natrojarosite and a small amount of arsenic-bearing amorphous substances.It was found that the arsenic removal ratios were 99.14%at pH=2,99.99%at pH=2-3 and 100%at pH=2-4.The TCLP tests showed that the leached arsenic concentration was 0.27 mg/L for the precipitates formed at pH=2,and for the precipitates formed at pH=2-3 and pH=2-4,no leached arsenic detected in the TCLP tests.With the addition of gold concentrate at dosage of 20%,98.20%of arsenic was removed when the precipitation was carried out at pH=2.The precipitates consisted of crystalline arsenic-bearing natrojarosite and a small amount of arsenic-bearing amorphous substances,and no arsenic was detected in the leaching solution.When the precipitation reactions of wastewater were carried out at pH=2-3 and pH=2-4,the arsenic in the wastewater was completely removed and the precipitates were arsenic-bearing goethite,which were very stable and no arsenic detected in the leaching solution.Thirdly,the effects of initial ferric ion concentration of the solution on the arsenic removal ratio and the scorodite formation were investigated in As(?)-Fe(?)system.In all the experiments with the additional ferric ions,it was found that there was amorphous ferric arsenate formed in the heating process.The amorphous ferric arsenate was found to be converted into crystalline scorodite in 8 hours if the molar ratio of Fe(?)/As(?)was not higher than 1.6.However,the crystallinity of scorodite decreased with the increase of additional ferric ion concentration,and the stability of the solid products and the arsenic removal ratio of the solution were improved.The results of the TCLP leaching test showed that at Fe(?)/As(?)of 0.8 and 1.6,more than 80%of arsenic could be precipitated as a form of scorodite.The arsenic leaching concentration of the formed solid products were less than 5 mg/L,which could meet the emission standards,so that the solid products may be considered to be stable for safe disposal.Nevertheless,if the molar ratio of Fe(?)/As(?)was higher than 1.6,the amorphous ferric arsenate could not convert into crystalline scorodite after 8 hours.The arsenic removal decreased and the amorphous ferric arsenate was unstable for high arsenic leaching concentration.