Where and how are arsenic impurities formed during contact process ??

Dear student

Arsenic oxide is present as an impurity in SO2 gas. While oxidising to SO3 , the arsenic oxide needs to be removes since it interferes with the catalyst (is a catalyst poison) and destroys its catalytic efficiency.

It is removed by passing the gas over ferric hydroxide. 
   As2O3 + 2Fe(OH)3 → 2FeAsO3 + 3H2O



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