Sulfated Roasting and Evaporating Selenium by Microwave Heating
Copper anode slime contains many valuable metals such as silver,copper,selenium,tellurium,lead and gold,which is an important raw material for the production of gold,silver,selenium,tellurium and other precious metals and scattered metals.Due to the technology of microwave heating had many advantages,such as selective heating,fast heating speed,high heating efficiency,good heating uniformity and so on,hence copper anode slime of removing arsenic and antimony as raw material was studied the effects of steaming selenium conditions on selenium volatilization rate under different microwave heating conditions of roasting time,roasting temperature and ratio of acid to ore,and the optimum process conditions were also determined.The content of selenium in steamed selenium residue was analyzed by inductively coupled plasma atomic emission spectrometer(ICP-AES).The existing forms and characteristics of main minerals in the sample were analyzed by X-ray diffractometer(XRD),scanning electron microscope and energy dispersive spectrometer(SEM-EDS).The kinetics of sulfation roasting of copper anode slime heated by microwave was studied by the shrinking unreacted core model of gas-solid phase reaction at 673～823 K.The total experimental results showed that the conditions of increasing the roasting temperature,prolonging the roasting time and increasing the acid-ore ratio of acid to ore were beneficial to increase the volatilization rate of selenium,because the increase of above conditions could promote the positive reaction of selenium evaporation reaction.The temperature conditions'results showed that the rate of selenium volatilization increased with the increase of temperature,whereas tended to rise slowly at 823 K,with the greatest rate of selenium volatilization was 94.49％.The time factors'results showed that sulfuric acid was sufficient at the early stage of steaming selenium,which promoted the reaction of steaming selenium.However,when the time of the selenium steaming reached 45 min,the volatilization rate of selenium tended to rise gradually,and the optimal volatilization rate of selenium was 94.49％.The conditions of ratio of acid to ore results showed that when the ratio of acid to ore was relatively low,that was the amount of sulfuric acid was insufficient,hence the reaction of selenium and selenides with concentrated sulfuric acid to form SeO2 was incomplete.With the increase of ratio of acid to ore,the contact area between sulfuric acid and material increased,which could make the steaming selenium reaction proceed rapidly.With the ratio of acid to ore of 0.85,the evaporation selenium reaction was basically complete,and the selenium volatilization rate of 96.83％could be obtained.Under the optimum conditions of air flow rate of 200 L·h-1,roasting temperature of 823 K,roasting time of 45 min and the ratio of acid to ore of 0.85,the selenium content in steamed selenium slag was decreased from 6.43％to 0.17％,and the volatilization rate of selenium reached 96.83％.Selenium mainly existed in the form of CuSe2,CuSe,Cu2Se and NiSe in copper anode slime,and no characteristic peak of selenium compounds was observed in steamed selenium slag,indicating that selenium was volatilized thoroughly.Steamed selenium slag was mainly composed of AgCl,BaSO4,PbSO4,CuO,CuSO4,NiSO4 and other compounds.The selenium volatilization rate obtained at the roasting temperature of 673～823 K and the roasting time of 35～50 min was introduced into the shrinking unreacted core model of the gas-solid phase reaction,respectively,and the relationship between and time t was obtained.The relationship diagrams between lnk(k is the rate constant)and 1000/T(T is the temperature)under different temperatures was obtained according to the slope of each fitting line in the diagram.The apparent activation energy of sulfation and evaporation of selenium by microwave heating of copper anode slime was calculated as 41.830 kJ·mol-1 by Arrhenius formula,and the pre-exponential factor was 3.319,indicating that the process of selenium evaporation was controlled by chemical reaction.The kinetic equation was as follows:1-(1-x)1/3=3.319×exp(-41.830/RT)×t,where x is the volatilization rate of selenium,t is the roasting time,R is the gas constant.
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