Study On Arsenic And Arsenic In Arsenic-containing Iron Solution By Hydrothermal Method

Arsenic is highly toxic,with bioaccumulation and carcinogenicity.Nonferrous metallurgy is the main industry that produces arsenic-containing waste water;the improper disposal of arsenic-containing materials in the smelting process is the main reason for the frequent occurrence of arsenic pollution in China.Scorodite is an ideal arsenic precipitate with high stability,low leaching toxicity and small volume,while lime neutralization produce massive volume of solid waste and lead to secondary pollution.Therefore,it is of great significance to stable arsenic-containing materials and makes it harmless.At the basis of pure arsenic-containing solution,the present paper mainly focused on the research of hydrothermal precipitation of arsenic with oxidation of ferrous.The results are as follows:(1)The potential-pH diagram of Fe-As-H2O system at hydrothermal condition was drawed based on the theory of electrochemistry and thermodynamics.The diagram showed that the theoretical range of iron arsenate at elevated temperature is 0.06<pH<4.54,and the experimental results showed that the crystalline scorodite was available for formation in the theoretical range.(2)The effects of initial arsenic concentration,initial pH,initial Fe/As molar ratio,reaction temperature,reaction time,oxygen partial pressure and stirring speed on the removal of arsenic were investigated under hydrothermal condition.The results are as follows:the purified scorodite can be formed at Fe/As molar ratio below 2,initial pH below 2.0,temperature below 180? and the agitation speed above 200 r/min.The formation of scorodite is irrelevant to the initial arsenic concentration,oxygen pressure and reaction time.The residues contained both scorodite and basic ferric sulfate with the agitation speed 200r/min,initial pH above 1.0 and the temperature higher than 180?.Both scorodite and jarosite were produced at Fe/As molar ratio higher than 2.0.Basic ferric sulfate and jarosite in the precipitate was the main cause of iron and sulfur content increasing and arsenic content decreasing.(3)The optimum conditions for the formation of scorodite by hydrothermal method were as follows:initial arsenic concentration 10g/L,initial pH 1.0,initial Fe/As molar ratio 1.5,temperature 160?,the oxygen partial pressure 0.6 MPa and the stirring speed 500 r/min.Arsenic precipitation rate is up to 97%and larger scorodite containing 31%arsenic can be formed under the optimized conditions.(4)The kinetics of ferrous oxidation in the process of scorodite formation was in accordance with the second order reaction,and its apparent activation energy was 52.5kJ/mol which indicated that it was controlled by chemical reaction.The rate of ferrous oxidation decreased and the particle size of scorodite increased with the increase of initial arsenic concentration and decrease of initial pH,initial Fe/As molar ratio,temperature and agitation speed.