TREATMENT OF GOLD MILLING EFFLUENTS
Herman Erkku, Senior Process Development Engineer
Environment Canada
Burlington, Ontario, Canada
Lynn S. Price, Manager Environmental Control
Falconbridge Nickel Mines Limited
Toronto, Ontario, Canada
INTRODUCTION
In the early 1970s, Environment Canada had established a joint government-industry task
force to identify the best practicable technology(ies) for the removal of undesirable constituents from mine/mill wastewaters.  For gold mining, specific concerns related to the presence
of cyanide, arsenic and other heavy metals such as copper and zinc.
A literature search indicated that only scant information existed with regard to the removal
of objectionable levels of cyanide and arsenic from gold milling effluents.   Nonetheless,
previous research pointed towards the processes of alkaline chlorination and arsenic oxidation,
followed by arsenic precipitation with lime and iron salts to reduce the levels of these contaminants. Performance and cost details, however, were lacking for this seemingly promising
route.
Consequently, a joint government-industry program was initiated in 1976, in conjunction
with Giant Yellowknife Mines Limited, with the general objective of demonstrating the feasibility of removing cyanide and arsenic from gold milling effluents. This program was funded
under Environment Canada's DPAT (Development of Pollution Abatement Technology) program in which industry and government share the costs.
DESCRIPTION OF GIANT YELLOWKNIFE MILLING PROCESS
At Giant Yellowknife Mines Limited, both mine and mill are situated four miles outside of
Yellowknife, Northwest Territories, Canada, 600 air miles north of Edmonton, Alberta.
Mining for gold started in 1948. The mill capacity was initially 250 ton/day and has been increased to 1200 ton/day.
A detailed description of the operations at Giant Yellowknife Mines has been presented by
Foster [ 1 ].  However, for the purpose of this paper, a simplified mill flow sheet is presented
in Figure 1. The gold in this ore is mainly locked in a fine grained arsenopyrite (FeAsS).  Current gold assays are approximately 0.26 oz/ton. The ore is crushed and ground in conventional
circuits. The finely ground ore is then subjected to a bulk sulfide flotation which results in
roughly 100 tons of concentrate containing the gold and a barren waste rock which is discarded to the tailings pond.
The sulfide concentrate is thickened and then roasted in a two-stage fluid bed roasting system.  The roaster calcine, which is withdrawn from the roaster bed and the gas cleaning cyclones, is quenched in water. The calcine is subjected to a two-stage water wash to remove
remaining soluble arsenic. The thickened slurry is then leached in a conventional cyanide
circuit.   A bleed ("Barren Bleed") of roughly 10% from the recirculating barren cyanide
stream permits control of the undesirably build-up of copper and zinc.
The hot gases from the roaster are cleaned in a electrostatic precipitator to recover the entrained fine calcine dust. This dust is quenched, washed and then treated in a special cyanide
leach, using activated carbon to collect and retain the gold.  The cleaned gases from the precipitator are then cooled to condense the arsenic trioxide in the form of a fine powder. This
powder is collected via bag collectors and stored in special underground chambers.
The Barren Bleed and the spent slurry from the treatment of the calcine dust ("Carbon
Plant Barren") both contain cyanide and are referred to as the "Cyanide Streams" for dis-
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