Method for the manufacture of mercury free sulfuric acid

In a method of manufacturing substantially mercury-free sulfuric acid by roasting non-ferrous metalsulfide material and then removing dust, cooling, washing and removing mist from the roasted gas to prepare a cool feed gas containing SO.sub.2 and impurities comprising water and mercury, removing said impurities from the roasted gas, converting the SO.sub.2 to SO.sub.3 and absorbing the SO.sub.3 in dilute sulfuric acid, the improvement which comprises:

• continuously flowing a stream of said feed gas into and through first scrubbing means and therein continuously scrubbing said feed gas with a continuously circulating stream of first liquid scrubbing agent consisting essentially of sulfuric acid solution having an H.sub.2 SO.sub.4 concentration of from 30 to 99 wt.% and having a temperature of from 20.degree. to 70.degree. C to absorb impurities comprising water and mercury present in said cool feed gas into said scrubbing agent and to produce a stream of purified feed gas containing SO.sub.2 ;
• continuously removing said stream of said first liquid scrubbing agent from said first scrubbing means, withdrawing a portion of said stream, adding to said withdrawn portion a reagent effective to precipitate mercury present in said withdrawn portion, removing the precipitated mercury from said withdrawn portion, recovering the balance of said withdrawn portion as a purified dilute sulfuric acid solution, and continuing to circulate the balance of said stream of said first liquid scrubbing agent through said first scrubbing means without precipitating and removing mercury present therein;
• continuously feeding said stream of purified feed gas exiting from said first scrubbing means through second scrubbing means and therein continuously scrubbing said purified feed gas with a continuously circulating stream of second liquid scrubbing agent consisting essentially of sulfuric acid solution having an H.sub.2 SO.sub.4 concentration of about 98 to 99 wt.% and a temperature of about 60.degree. to 80.degree. C to absorb into said second scrubbing agent an additional quantity of the mercury present in said purified feed gas whereby to produce an additionally purified feed gas containing SO.sub.2, said second liquid scrubbing agent having a higher concentration, a higher temperature or both higher concentration and higher temperature, than said first liquid scrubbing agent;
• converting the SO.sub.2 in said additionally purified feed gas to SO.sub.3 and then flowing the SO.sub.3 -containing gas through two absorption stages in series and in each of said stages contacting said SO.sub.3 -containing gas with different dilute sulfuric acid solutions to absorb the SO.sub.3 whereby to produce two separate concentrated sulfuric acid solutions, the dilute sulfuric acid solution employed in the first absorption stage comprising the purified dilute sulfuric acid solution obtained by precipitating mercury from the scrubbing agent used in the first scrubbing means, the dilute sulfuric acid solution employed in the second absorption stage comprising a second portion of the scrubbing agent employed in the first scrubbing means and which is not subjected to mercury removal, and including the step of feeding the concentrated sulfuric acid solution from the second absorption stage to said second scrubbing means to serve as said second scrubbing agent therein.

In a method of manufacturing substantially mercury-free sulfuric acid by roasting non-ferrous metal sulfide material and then removing dust, cooling, washing and removing mist from the roasted gas to prepare a cool feed gas containing SO.sub.2 and impurities comprising water and mercury, removing said impurities from the roasted gas, converting the SO.sub.2 to SO.sub.3 and absorbing the SO.sub.3 in dilute sulfuric acid, the improvement which comprises:

• continuously flowing a stream of said feed gas into and through first scrubbing means and therein continuously scrubbing said feed gas with a continuously circulating stream of first liquid scrubbing agent consisting essentially of sulfuric acid solution having an H.sub.2 SO.sub.4 concentration of from 30 to 99 wt.% and having a temperature of from 20.degree. to 70.degree. C to absorb impurities comprising water and mercury present in said cool feed gas into said scrubbing agent and to produce a stream of purified feed gas containing SO.sub.2, said first scrubbing means comprising two scrubbing towers and said feed gas being flowed in series through said scrubbing towers and being scrubbed by separate streams of said first scrubbing agent;
• continuously removing said stream of said first liquid scrubbing agent from said first scrubbing means, withdrawing a portion of said stream, adding to said withdrawn portion a reagent effective to precipitate mercury present in said withdrawn portion, removing the precipitated mercury from said withdrawn portion, recovering the balance of said withdrawn portion as a purified dilute sulfuric acid solution, and continuing to circulate the balance of said stream of said first liquid scrubbing agent through said first scrubbing means without precipitating and removing mercury present therein;
• continuously feeding said stream of purified feed gas exiting from said first scrubbing means through second scrubbing means and therein continuously scrubbing said purified feed gas with a continuously circulating stream of second liquid scrubbing agent consisting essentially of sulfuric acid solution having an H.sub.2 SO.sub.4 concentration of about 98 to 99 wt.% and a temperature of about 60.degree. to 80.degree. C to absorb into said second scrubbing agent an additional quantity of the mercury present in said purified feed gas whereby to produce an additionally purified feed gas containing SO.sub.2, said second liquid scrubbing agent having a higher concentration, a higher temperature or both higher concentration and higher temperature, than said first liquid scrubbing agent;
• continuously removing a portion of the second liquid scrubbing agent from the second scrubbing means and mixing said portion with the first liquid scrubbing agent circulating through the second of said scrubbing towers, and removing a portion of the first liquid scrubbing agent circulating through the second of said scrubbing towers and mixing it with the first liquid scrubbing agent circulating through the first of said scrubbing towers;
• converting the SO.sub.2 in said additionally purified feed gas to SO.sub.3 and then flowing the SO.sub.3 -containing gas through absorption tower means and therein contacting said purified feed gas with dilute sulfuric acid solution to absorb the SO.sub.3 whereby to produce concentrated sulfuric acid solution.