Patent GB-1410032-A (GB)
Title: Method for the manufacture of mercury free sulphuric acid
IPC: C01B17/56
Filed: 1972-09-11
Abstract: 1410032 Sulphuric acid; removing mercury from SO 2  MITSUI MINING & SMELTING CO Ltd and HACHINOHE SMELTING CO Ltd 11 Sept 1972 [10 Sept 1971 28 Feb 1972 29 Feb 1972 24 April 1972] 42020/72 Heading C1A In a method of manufacturing sulphuric acid by roasting non-ferrous metal sulphide material and then removing dust, cooling, washing and removing mist from the roaster gases to produce a cool feed stream of SO 2  containing gas, mercury is removed from the feed gas by passing the cool gas into and through a scrubber, where the gas is brought into contact with a scrubbing agent, which consists of sulphuric acid in a concentration of from 30-99% by weight the rest being water, at a temperature of at least 20‹ C. whereby the mercury content is absorbed into the-scrubbing agent to give a purified gas stream, and a contaminated scrubbing agent; the gas stream is then converted into an SO 3  containing gas stream and absorbed in an aqueous liquid to give sulphuric acid whilst the scrubbing agent is treated to precipitate the mercury and the precipitated mercury is removed from the scrubbing agent. The scrubber preferably comprises a single tower and a stream of scrubbing agent is circulated through it, and from this stream a portion is removed and treated to precipitate and remove mercury therefrom. If desired, the scrubber may comprise a series of towers with a circulating stream of scrubbing liquor through each, when a portion of the stream circulating in each tower is removed and fed to the stream circulating in the preceding tower of series, the portion removed from the first tower being treated to precipitate and remove mercury therefrom. The pressure and temperature of the feed gas may be increased after it leaves one tower and before it enters another, the temperature and concentation of the scrubbing agent in said other tower being higher than that in said one, for example, the temperature and concentration in said other tower may be 70‹ to 100‹ C. and 70 to 99% by weight respectively. The desired temperature increase of the feed gas may be accomplished by means of a heat exchanger after the pressure has been increased. When the scrubbing agent has a concentration of no more than 80 wt. per cent the mercury may be precipitated by adding at least one reagent selected from the sulphides, hydrosulphides and polysulphides of alkali metals and alkaline earth metals, hydrogen sulphide, selenium compounds and sulphur so as to produce the sulphide or selenide of mercury which is then removed by filtration. Copper or silver may be added to the scrubbing agent before or during addition of the precipitating reagent. When the concentration of the sulphuric acid scrubbing agent is more than 80 wt. per cent the scrubbing is treated by adding at least one powdered metal (which term includes Se) selected from Al, Zn, Se and Cu, and Cu or Ag may also be added to the scrubbing agent before or during the addition of this powder. The temperature of the scrubbing agent during use is preferably 20‹ to 70‹ C. After the precipitated mercury compound has been separated from the scrubbing agent, this sulphuric acid solution may be utilized to absorb gas containing SO 3  from conventional catalytic converters, thereby forming concentrated sulphuric acid. Alternatively, the purified scrubbing agent may be removed as product. In a further embodiment, a metal having a greater tendency to ionize than mercury is used to precipitate the mercury from the scrubbing agent, e.g. the metals specified above.