Arsenic Removal And Solidfication Of Arsenic Bearing Dusts Of Nonferrous Metallurgy

Smelting dusts contain appreciable metal values and a large amount of arsenic due to its volatilization. To recover the metal values from smelting dusts, it is necessarily important to eliminate arsenic pollution by removal and solidification of arsenic. Focused on the representative arsenic bearing dust of Imperial Smelting Process(ISP) in zinc metallurgy, the removal and solidification of arsenic from secondary zinc oxide are investigated in this paper and the findings also benefit for treatment of other arsenic bearing smelting dust. The main experimental results are as follows:1. The chemical phase of arsenic in secondary zinc oxide from the production site was systematically characterized. Based on the results, secondary zinc oxide can be classified into three types:Type Ⅰ with As2O3as the main phase of arsenic in comparison to type Ⅱ with zinc arsenite (Zn(AsO2)2) and type Ⅲ with lead arsenate (Pb(As2O6, Pb4As2O9)). The main reason of the diversity of secondary zinc oxide is fluctuation of production atmosphere, which could be caused by the operational condition or the composition change of raw materials.2. Based on thermodynamic analysis of equilibrium solution, a principle process including selective alkaline leaching, catalytic oxidation of As(Ⅲ), arsenic precipitation and cement solidification was proposed. Arsenic in the three types secondary oxide could be selectively leached out in alkaline leaching system. The addition of S2-not only improved the leaching of arsenic, but also reduced the loss of metal values. The oxidation of arsenic could be thermodynamically accomplished by different oxidants, such as air, MnO4-, MnO2, MnOOH and Mn3O4. The pH value had a significant effect on arsenic oxidation. The products of arsenic precipitation with lime varied according to the terminal pH of solution system. The precipitates were converted into Ca3(AsO4)2and Ca5(AsO4)3OH from CaHAsO4·H2O with the increase of pH value, and Ca5(AsO4)3OH should be the optimal precipitate in terms of the removal of As and the stability of calcium arsenate. 3. The results of arsenic leaching from three types of secondary zinc oxide revealed that the leaching of As reached84%and68%in NaOH-H2O leach system for type Ⅰ and type Ⅱ secondary zinc oxide, respectively, and the leaching of Zn and Pd were both less than1%. For the treatment of type Ⅲ secondary zinc oxide, it is suitable to employ NaOH-Na2S-H2O system to leach out more than73%of As and less than0.8%of Zn and0.4%of Pb.4. The results of oxidation of As(Ⅲ) by hydrogen peroxide, air and air with catalyst showed that As(Ⅲ) in the leach solution could be completely oxidized to As(Ⅴ) using1.2times of the stoichiometric amount of hydrogen peroxide with higher cost. Only22.3%of As(Ⅲ) was oxidized to As(V) by air even in8h. Due to the effect of super stoichiometric oxidation to As(Ⅲ) by the catalysis of manganese oxide on the air oxidation of As(Ⅲ), potassium permanganate could be used to remarkably improve the oxidation efficiency of As(Ⅲ) by air. By blowing0.2m3/h air at As/Mn mole ratio of21and pH of13, As(Ⅲ) could be completely oxidized in5h at40℃.5. Arsenic precipitation with lime was greatly influenced by temperature, pH and Ca/As mole ratio. The removal of arsenic from oxidized solution reached99.5%with a Ca/As mole ratio of3at pH12and95℃in2h in comparison to that of67%without As oxidation, and arsenic was mainly precipitated as Ca5(AsO4)3OH. To minimize the formation of lead and zinc arsenate in lime precipitation, the S2-could be used to selectively precipitate Zn and Pb after the pH adjustment of solution to12.6. It not only converted amorphous or low crystalline arsenic species to the high crystalline arsenate calcium by calcinations process, but also promoted the particle to grow and densify, which improved the stability of arsenate calcium. Ca5(AsO4)3(OH) was the only arsenic precipitate in arsenate calcium with high contents of lead and zinc after calcination. Pb5(AsO4)3(OH) existed in precipitate was decomposed into Ca2Pb04and PbO2. It is important to wash precipitate after arsenic precipitation to eliminate remaining arsenic and alkali. The resultant precipitate was then calcined and solidified with a cement/arsenate calcium mass ratio of3. The obtained solid meets the standard of GB508.1-2007according to the analytical method of HJ/T300-2007, leaving only1.5mg/L of As in the leachate.