A positioning assembly is used for positioning a circuit board on a case. The alloy layer is effectively protected against re-oxidation by a non-corrosive, alkali metal fluoroaluminate layer which forms simultaneously. The supply pipe is movable so that by manipulating the height of the supply pipe in conjunction with discharging particulate matter through the column, the height of the bed of particulate matter in the column can be adjusted so that the retention time of the off gas in the particulate bed is constant. The compounds precipitate in aqueous environments. The power splitter may be replaced by a polarization splitter.