The nature of the various processes requires that considerable ventilation measures be applied to the working areas, especially near of furnaces. All fumes extracted are often rich in gold and silver and cannot be allowed to escape to atmosphere. For this reason is important to consider a comprehensive arrangement of fume hoods and ducts in order to ensure that all dust and gases are removed from the building and treated both to recover valuable elements and to avoid any pollution. It has been established that for appropriate ventilation, some volumes must be considered, for example, 160-180 m3/min melting house; 850-870 m3/min gold refinery; 420-440 m3/min De-Golding area; 130-150 m3/min silver bar casting area and 270-290 m3/min silver anode casting area.
Fumes from the melting house and the two silver casting areas are passed directly to electrostatic precipitators. Gases from the gold refinery and the De-Golding section are drawn through a wet impingement scrubber where most of the base metal chlorides and some of the gold and silver are removed. Any liquor overflow is run into cementation tanks and the sludge delivered to the silver leaching and reduction section for recovery of the gold and silver. The scrubbed gases then pass through the electrostatic precipitators for the recovery of the residual gold and silver particles. The electrostatic precipitators are of the tube and wire type. The wire current is approximately 1 mA at a maximum potential of 50,000 V. Usually, fans are mounted at the top of each precipitator so that the tubes are at a negative pressure of 18-25 mm water gauge with a linear velocity in the tubes of 0.5-0.9 m/s.
The efficiency of the precipitators is monitored by automatic samplers, which are themselves electrostatic precipitators, but operate under more ideal conditions. A higher wire current and a lower linear velocity is employed and the samplers are considered to be highly efficient. From the mass and assay of the dust collected in the samplers, the overall efficiency of the whole network is calculated to be higher than 98% for gold and 97% for silver. In some refineries, 250-300 kg of gold and 1500-2000 kg of silver are collected annually from the precipitated dust, which is treated in the by-products area.