Proposal would limit arsenical dust exposure

Arsenic’s toxic nature had been known for centuries but it wasn’t until mid-Victorian times that its virulence came to be universally recognized. The mineral somehow found its way into the working man’s beer with devastating results. About the same time an enterprising chef discovered that certain arsenical compounds imparted an enticing green color to cake icing. It wasn’t too long before the health authorities of the day determined that arsenic was not a desirable food additive.

Arsenic still causes major headaches for the metallurgist. It is a common component of many gold ores and almost invariably present in those that are refractory. It is a major problem for the smelter where arsenic in base metal concentrates volatilizes and must be captured before the furnace gases can be delivered to the stack.

Today, it is stringently controlled. In Ontario it appears on the province’s “designated substances list,” an unusual and onerous designation for about a dozen compounds.

Mines and mills in the province are exempt from the controlling regulations because arsenopyrite and other primary arsenical minerals behave differently from arsenic encountered elsewhere. In these other workplaces, the maximum allowable exposure to arsenical dusts is 0.2 milligrams per cubic metre over 40 hours, where 0.2 represents the measure of the elemental arsenic in the dust.

There has recently been a proposal to remove the exemption and to limit the exposure of the mine and mill worker to no more than 0.01 milligram per cubic metre per 8-hour shift.

In view of these proposals, Dickenson Mines (TSE) and CANMET (Canada Centre for Mineral and Energy Technology) developed a sampling program to determine what the arsenic levels are for both surface and underground workplaces at Dickenson’s A.W. White gold mine at Red Lake, Ont.

Dickenson’s environmental engineer, David Mchaina, describes the process and gives the results in a paper to be presented at the American Society of Mining Engineers annual meeting Feb. 24-27 in Phoenix, Ariz. In the paper, Mchaina notes the taking of 85 samples from underground, the assay laboratory, the refinery and other locations; 51 were “personnel” samples and 34 “area” samples. Of these, all gave arsenic levels below 0.2 and 90% gave results below the proposed limit of 0.01.

For the “personnel” samples, a battery-powered air pump was attached to the man’s belt and intake air drawn via a small diameter tube through a 7-micron filter. The tube’s inlet was located close to the man’s face to accurately sample the air breathed and was kept there for the duration of the shift. The pump weighed approximately one kilogram, about the same as an ordinary miner’s cap lamp and battery. Air was drawn at the rate of two litres per minute.

“Area” samples were taken by a similar unit but this time the assembly was kept stationary for the eight hours it was in the workplace. It is evident from the results of Mchaina’s survey that if the proposed new limit was to be implemented, it wouldn’t pose any problems to Dickenson.