Smelter gas cleaning one vessel from the quench to the drying tower
Base metals such as zinc, lead and copper are all found in nature bound with sulphur as sulphite minerals: sphalerite (ZnS), galena (PbS), several copper minerals, silver (found as acanthite Ag2S), and, along with other precious metals. Processing of the metals invariably leads to sulphur dioxide (SO2) formation. Other metal processing, such as aluminum from bauxite ore [Al(OH)3] which contains no sulphur will produce SO2, nitrogen oxides (NOx), dust and fluorine compounds(1). Heavy metals such as mercury (Hg), arsenic (As) and cadmium (Cd) are commonly evolved in the smelting process(2). SO2 is typically recovered from smelters, at high a percentage as possible, and returned as feedstock to the sulphuric acid (H2SO4) plant. Produced acid is used internally at the smelter or sold. (C&E News routinely refers to sulphuric acid as the largest volume chemical in the world.) Byproducts and dust from the smelting process must be recovered in the air pollution unit operations avoiding potential consequences of governmental air emission regulation violations including significant penalties (3,4). Several different types of smelter air pollution control technologies are available, such as bag houses to collect coarse dust, contact cooling, wet and dry electrostatic precipitators (ESP) for fine particles collection, venturi scrubbers to collect other particles and some gaseous emissions. Additionally, packed bed scrubbing towers collect both particulate matter as well as gaseous emissions. This discussion covers accomplishing all of the air pollution abatement process steps, within a single process vessel.