Offgas, metallurgical cooling

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. 

Fig. 4.5. Dynawave scrubber for removing particulate and soluble impurities from cooled (600 K), partially cleaned furnace offgas (Puricelli et al., 1998 Enviro-chem, 2005). Note the (i) upward sprays in and (ii) the weir atop the downcomer. The sprays clean the gas. The weir continuously overflows water, which wets and cools the downcomer walls. These scrubbers are used for both metallurgical and spent acid regeneration furnace gases. Large scrubber details are downcomer diameter 1 m (Hastelloy G30)...

Fig. 6.2. Dehydration of metallurgical and spent acid decomposition furnace offgas. Dehydration is done after (i) H20(g) has been condensed by gas cooling and (ii) aqueous mist has been removed by electrostatic precipitation (not shown). The gas leaving dehydration contains 50 milligrams of H20(g) per Nm3 of gas. The acid plant s main blower is situated immediately after dehydration.

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