The Effect Of Inorganic Additives On The Leaching Of Coal Arsenic By Sulfur Dioxide

Coal is the main energy resource in China.In the process of coal combustion,harmful elements such as arsenic will be released,causing serious harm to the environment.Arsenic in coal can be leached by flue gas containing sulfur dioxide.The leaching process is based on the fact that the coal pyrite can be oxidized by SO₂ and O₂.The arsenic associated with sulfide mineral is leached simultaneously.Meanwhile,the catalytic oxidation between SO₂ and Fe2+/Fe3+ can convert SO₂ to sulfate,thus removing SO₂ from flue gas.To choose suitable additives which can increase leaching rate of arsenic from coal,a bubbling stirred tank reactor was used as absorption equipment.Gases of N2,O₂,and SO₂ were used to simulating flue gas.The effects of different additives at different concentrations were examined on the arsenic leaching rate,valence of arsenic in the liquid,and SO₂ removal efficiency.The effects of different additives were compared.Furthermore,the effects of pH,temperature,and concentrations of SO₂ and O₂ on arsenic leaching were studied.Experimental results show that adding iron sulfate,ferric chloride,aluminium sulfate and aluminium chloride could all increase arsenic leaching rate.The arsenic leaching rate increased with increasing additive concentrations.The variation trend of total iron leaching rate was similar to that of total arsenic.These additives could also effectively increase the SO₂ removal efficiency.Compared with iron sulfate and aluminium sulfate,the main characteristic of ferric chloride and aluminium chloride was that they could convert most of the leached As???to As???,thus reducing the toxicity of arsenic and facilitating the subsequent treatment of the leached arsenic.Adding sodium chloride could also oxidize As?III?to As?V?.Further experiments for the Al Cl3-enhanced process were carried out.The results showed that lower pH value was favorable to the leaching reaction of pyrite,and therefore higher arsenic leaching rate could be achieved.This is possibly due to the fact that the existing forms of ferric iron and arsenic in the slurry are different at different pH values.Temperature experimental results showed that SO₂ removal efficiency,iron and arsenic leaching conversions,and trivalent arsenic concentration all increased with the increase of temperature.This is because the reaction rate and the reducibility of pyrite increase withincreasing temperature.For same reaction time,the iron and arsenic conversions and the trivalent arsenic concentration all increased with increasing SO₂ concentration.SO₂ removal efficiency,iron and arsenic conversions increased with increasing O₂ concentration.But,because of oxygen oxidation,the aqueous concentration of trivalent arsenic decreased with the increase of O₂ concentration.As the O₂ concentration was increased to 10%,the increase of iron and arsenic conversions became smaller with further increase of O₂ concentration.Experimental results also showed that the arsenic leaching process could be described by the reaction-controlled shrinking core model,which is similar to the coal pyrite leaching process.