Gas Handling and Cleaning

The net gas output from the sinter machine is at a temperature of 200 to 500°C and contains a significant load of dusts, fumes and volatile materials, including compounds of lead, cadmium, mercury, chlorine and fluorine. The SO2 content can be variable from 2.5 per cent to 6.5 per cent, depending on measures taken to raise its concentration such as tip end gas recirculation. Gas may also be separated from the front end and tip end, giving relatively high SO2 concentrations from the front end and low concentrations from the tip end. 

For the production of sulfuric acid by the contact process gas strength should be above five per cent and preferably closer to seven per cent. This is clearly difficult from lead sinter plants with a single gas pass and either only front gas is used or tip end gas recirculation is applied. Where only front end gas is used for sulfuric acid production, overall sulfur recovery to acid is of the order of 60 per cent the remainder is discharged to atmosphere. 

Gas that is not processed in an acid plant needs to be cooled to below 200°C using an open humidification tower before being cleaned using bag filters to remove dusts and fumes before discharge to atmosphere. In this process it is important to ensure that the gas temperature is not lowered below its dewpoint as this can lead to serious corrosion problems. Although total or partial discharge of sinter gas was commonly practiced in the past it is becoming less acceptable. 

For acid production sinter gas is usually cleaned by drop out of dust in an open chamber or flue followed by hot electrostatic precipitators of the plate and wire type with two or three fields in series. This is followed by wet gas scrubbing and wet electrostatic precipitators to remove mist from the scrubbing operation. A mercury removal scrubber may be included, if mercury levels in the clean gas are high enough to give product acid above 1 ppm mercury. Mercury removal is not always necessary as it is with zinc concentrate roasting. 

Many devices are used for wet gas scrubbing such as spray towers, packed towers, plate scrubbers such as the Peabody scrubber or Venturi scrubber. Scrubbing liquor is circulated through the scrubbing system and becomes acidic due to the absorption of SO3 contained in sinter plant gas. Often the 

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