Lead (Pb) and litharge (PbO) are extensively used in the fire assaying of precious metals (PM). It is well known that Pb is the best collector of PM. Fire assaying is extensively used in the determination of PM. However, lead is very toxic. The OSHA Permissible Exposure Limit (PEL) set by the standard is 50 micrograms of lead per cubic meter of air (50mg/m3), averaged over an 8-hour workday. Looking at these stringent standards, it appears that these low exposure levels cannot be achieved without substantial capital expenditure. The fate of lead in fire assaying seems to be the same as that of asbestos. Asbestos possibly is the best high-temperature material on the planet. Lack of proper guidelines for safe handling and bad publicity killed the industry. Many collector metals have been investigated to replace lead but none has been universally accepted. The reasons are obvious. Lead is cheap, has a low melting point and is very easy to handle. Preferential collection of PM in lead and its subsequent separation by simple oxidation of Pb to PbO makes it the metal of choice for fire assaying. Based on engineering principles and the knowledge of skilled fire assayers, laboratories were designed and operated to meet all the OSHA standards for the laboratory environment and safety of the workers. Years of monitoring lead in the laboratory and in the assayers' blood samples proved the successful design. The system was designed to have a minimum of capital and energy requirements. The design and operating aspects of this laboratory are discussed.

Author: Dr. Rajesh K. Mishra