The Mill

Kevin Weston is a mining engineer. He was hired by Inco during development of the Casa Berardi East mine and by all accounts he was a talented engineer. So why is he now the mill superintendent? As Weston explained it, the move was unplanned. With little warning, the mill superintendent’s job became available in late summer.

Casa Berardi needed an interim superintendent, so Weston was approached. He accepted and, because of his performance, the “temporary” tag was eventually dropped.

“It’s a completely different world here (in the mill),” Weston told The Northern Miner Magazine during a tour. “Production miners have a lot of go and energy. These guys (mill personnel) are better if they take their time with things and look closely for problems.” He also discovered that, while the Casa Berardi flowsheet represents a simple process requiring only a few months to understand, “it will take years to master.” When he does need expertise, he consults with staff at Inco’s Sheridan Park Research Centre in Mississauga, Ont.

The 1,200-tonne-per-day mill is a relatively standard carbon-in-leach plant. Without any deliberate effort at blending, East and West mine ores are put through a Traylor jaw crusher. The ores are not identical, however; some gold in the West mine ore occurs as a fine-grain solid solution in arsenopyrite. Mill recoveries run between 87% and 88%. But they could be higher with less fine-grain material running through the circuit, Weston said. Ferric sulphate treatment neutralizes the arsenic in the tails, sampled at seven parts per million (p.p.m.).

Currently, the minus-6-inch crushed material is conveyed to an Allis-Chalmers 5.5×2.7-metre semi-autogenous grinding (SAG) mill charged with 4-inch balls. Second-stage grinding is accomplished by a 4×5.3-metre kvs ball mill, charged with 2.5-inch balls. The final grind product is 80% minus 200 mesh.

Because graphite from the graphitic Casa Berardi Fault often accompanies millfeed, care must be taken to avoid cyanide contamination before leaching. If the process water contains cyanide to any appreciable degree, the graphite, acting like carbon, would adsorb gold. This would result in gold- enriched tailings — the nemesis of gold processors. “The graphite is so fine that it would run right out with the tails,” Weston said. Tailings currently grade 0.5 grams gold per tonne. He is considering treating process water as it enters the mill from the polishing pond rather than trying to rid the tails of cyanide before they go to the ponds. But first, environmental regulators must be satisfied that the mill system is a closed circuit that will never result in any discharges to the surrounding area.

In a separate test, ore was treated by gravity concentration with jigs and tables. But the ore simply wasn’t amenable to this. “We will use it if the (mine) geologists find coarse gold,” Weston commented.

After grinding, the slurry is thickened to 45% solids, and lime and flocculant are added for pulp settling. Thickener underflow reports to a pre- aeration vessel, which helps accelerate the leaching process. Pre-aerated pulp flows by gravity through a series of six leach tanks. Gold loading of carbon runs countercurrent to the leaching cycle at the rate of five tonnes per day. At the end of the 11-day cycle, the carbon is stripped of gold. Weston is studying the merits of oxygen injection into the leach tanks.

The stripped carbon is reactivated in a rotating kiln, while the cyanide- laced slurry from the leach tanks is sent through a cyanide destruction process patented by Inco. This is the Inco SO2/Air Cyanide Destruction Process. Weston said the destruction unit has been a success, although the original equipment has required some modifications. For example, the shafts in the Dorr-Oliver cells required reinforcing. And angle-iron bracing has eliminated motor vibration, which had been a concern. “The unit is taking it (cyanide content) down to fewer than five p.p.m. from 120 p.p.m.,” Weston said. At the stripping stage, the heating unit for stripping (a Chromalox machine) has had some elements burn out prematurely.

The plant features few circuits monitored or controlled by computer. Apart from a few automatic flow-control valves and gauge controls, the mill is a manual operation. “I don’t see any need for it (more process control computerization) now,” said Weston. “We’re running the mill with three operators and it works.” Nevertheless, with talk of expanding the mill to 1,800 tonnes per day, more automated monitoring and control could be needed, he said.