ENVIRONMENTAL HAZARDS OF METALLURGICAL WASTE PRODUCED BY THE GOLD INDUSTRY

Boron anomalies were recently reported in >30 year old McIntyre gold mine tailings in Timmins, Ontario, Canada (Paliewicz et al., Mine Waste and the Environment, in review). The levels of soluble B greatly exceeded the worldwide quality thresholds and could not be accounted for by mine waste alone. This novel study aims to: 1) assess the B leachability of pyro-metallurgical waste (slag) and 2) validate the hypothesis that large quantities of slag processed by the Timmins GoMill and discarded in the McIntyre Dam #5 until the nineties is the dominant source of B anomalies found in the dam.

Borax (Na₂B₄O₇·10H₂O) is a common flux added during assaying and refining of gold ores. Modern slag samples were acquired from four gold mine operations in Timmins, Ontario as analogues of historic slag. Time-series experiments that lasted from 3 hours to 6 days, using a crush-leach, flask-shake methodology determined the B leachability slag in pure water, indicating that the exchange equilibrium was established in 24 to 48 hours. The effects of the initial pH of the leaching solution and of the rock type (from felsic to ultramafic) on B leachability were also investigated.

All slag samples were light to dark green, holohyaline glasses with the exception of two samples that were partially crystallized. The coarse, acicular, hexagonal crystals were identified via scanning electron microscopy and Raman spectroscopy to be Na-metaborate (NaBO₂), a highly soluble compound. Ion chromatography analyses of leachate solutions indicate that the holocrystalline samples release on average 6.2 ± 0.3 wt% soluble B. The partly crystallized slags released 9.9 ± 0.2 wt% soluble B. The B leachability was not affected by the pH of the leaching solution. Moreover, the final pH of slag leachates was buffered by the soluble slag components to a pH ranging 11 to 13, regardless of the initial pH of the leaching solutions.

Results indicate that soluble B leached from slag far exceeds other potential sources on the tailings (host rock, gangue minerals, etc.). B-rich metallurgical waste should not be discarded without pre-treatment or recycling, because of its high solubility in aqueous environments. Water treatment plants do not commonly measure B, so hazardous B contamination might remain undetected in the vicinity of other tailings with similar practices.