This paper is concerned with three trace metals that are sufficiently volatile to occur in the flue gas from coal in the vapor state. These elements are arsenic, selenium, and mercury. Securing their control as particulate matter in an electrostatic precipitator or baghouse may be aided by a process of chemical conversion from a vapor to a solid. This change may not always be necessary because some chemical process may prevent the appearance of each metal in the vapor state or some physical process such as adsorption may obviate the need for chemical conversion. Nevertheless, chemical conversion, if it can be easily achieved, may ensure that a physical deposition process such as electrostatic precipitation or fabric filtration will be an efficient removal process. For arsenic and selenium, the type of chemical process of interest is chemical reduction of the volatile oxides As4O6 and SeO2 to the relative involatile unoxidized elements. Ammonia at the low concentrations found useful for baghouse gas conditioning has been found to be capable of reducing SeO2 to the element, and it also has the potential of reducing As4O6 to the element. For mercury, the oxide is less volatile than the element. Thus, to removal elemental mercury from flue gas, chemical oxidation rather than chemical reduction is conceptually useful. Oxidants of potential value are the elements sulfur and selenium, which may produce the involatile sulfide and selenide.
