Acid tower, sulfur emissions

Process air in sulfur-burning plants is dried by contacting it with 93—98 wt % sulfuric acid in a countercurrent packed tower. Dry process air is used to minimise sulfuric acid mist formation in downstream equipment, thus reducing corrosion problems and stack mist emissions. 

S02 emissions from sulfuric acid plants are controlled in spray towers. Effluent gases contain less than 0.5 percent S02. The S02 emissions have to be controlled (or recovered as elemental sulfur by, for example, the Claus process). An approach is to absorb the S02 in a lime (or limestone) slurry (promoted by small amounts of carboxylic acids, such as adipic acid). Flow is in parallel downward. The product calcium salt is sent to a landfill or sold as a by-product. Limestone is pulverized to 80 to 90 percent through 200 mesh. Slurry concentrations of 5 to 40 percent have been used in pilot plants. 

Even minor residual emissions from the Onahama smelter are treated. The tail gases from the acid plants are scrubbed with caustic soda to reduce the final sulfur dioxide concentration to about 20 ppm. The converter building is enclosed to prevent escape to the atmosphere of untreated gases that leak from the furnaces. Leaking gases are collected as close to the sources as possible and are scrubbed in a limerock tower before release. Virtually all of the sulfur dioxide from the smelter is reportedly contained (32). 

Waste Characteristics and Impacts Atmospheric Emissions - The process has two emissions to the atmosphere sulfur dioxide and acid mist, both of which are released with the waste gas from the final absorber tower. 

---