Offgas, metallurgical

SO2 in smelting and roasting gas accounts for about 20% of sulfuric acid production. The SO2 is ready for sulfuric acid manufacture, but the gas is dusty. If left in the gas. 

After treatment, the gas contains 1 milligram of dust per Nm of gas. It is ready for drying, catalytic SO2 oxidation and H2SO4 making. 

A major use of sulfuric acid is as catalyst for petroleum refining and polymer manufacture, Chapter 5. The acid becomes contaminated with water, hydrocarbons and other compounds during this use. It is regenerated by  

Most industrial acid plants have three flows of sulfuric acid—one gas-dehydration flow and two H2S04(f)-making flows. These flows are connected through automatic control valves to  

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Fig. 4.1. Flowsheet for cooling, diluting and cleaning metallurgical offgas. Gas temperature is lowered from 1500 to 310 K. S02 concentration is lowered from 10-75 volume% S02 to 8-13 volume% S02. Dust plus vapor concentration is lowered from 10-200 g per Nm3 of gas to 0.001 g per Nm3 of gas. Efficient gas cleaning minimizes plugging of downstream catalyst and equipment. It also minimizes poisoning of the catalyst. It is the key to efficient, continuous long term metallurgical acid plant operation. Mercury-from-gas removal is described in Section 4.5.3.

The first step in treating metallurgical offgas is cooling in preparation for electrostatic dust precipitation. Electrostatic precipitators operate at about 600 K. Above this temperature, their steel structure weakens. Below this temperature, sulfuric acid forms from small amounts of S03 and HzO(g) in the furnace offgas - causing corrosion of the precipitator. 

An alternative method of cooling metallurgical offgas is to pass it through sprays of water. Spray cooling avoids investment in waste heat recovery equipment but wastes the heat of the gas. It also generates acidic waste liquid that must be neutralized and treated for solids removal/recycle. 

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