ZINC DIVISION: A Solid Foundation: Brunswick Mine and Mill

Underpinning Noranda’s Zinc Division is the large and venerable Brunswick mine near Bathurst, N.B. One of the world’s largest, this massive sulphide deposit was discovered in 1952. Successful drilling the following year lead to a positive development decision, and production began in 1964. Noranda gained control and responsibilities as operator in 1967. The operation has grown from 4,500 tonnes to 10,000 tonnes of ore daily. During its lifetime, the mine has yielded over 80 million tonnes of ore. Its year-end 1999 proven and probable reserves totalled 37 million tonnes grading 8.64% Zn, 3.40% Pb, 0.38% Cu and 102.44 grams/tonne Ag.

The mine has two active shafts: No.2 service shaft is 945 metres deep, and the No.3 production shaft reaches 1,430 metres beneath the surface. Mining proceeds using bulk open stoping, a much more productive method than the mechanized cut and fill of years past. The haulage levels are trackless thanks to the use of Atlas Copco load-haul-dumpers (LHDs). A major haulage automation project is underway that will improve efficiency and safety.

Many underground mines are implementing paste fill for safety, environmental, and economic reasons. Brunswick is no exception. The use of cemented rockfill was phased out after the new $22.3-million paste fill plant was commissioned in June 1998. The benefits are many: lowered placement costs; tightened filling of stopes to counteract increasing ground stress; shortened filling times to shorten the mining cycle; improved ore recovery by reduced caving and dilution; lengthened mine life; and reduced surface disposal of tailings.

Making paste fill begins with increasing the density of mill tails to 55-60% solids in a thickener. Thickener underflow passes through vacuum disc filters to produce paste of about 80% solids. Filtercake is weighed as it passes over a belt conveyor, then mixed with portland cement by a pair of twin-screw mixers. Paste is discharged to a gob hopper before it is sent underground by gravity flow. It is distributed by a network of 200-mm boreholes and pipes.

The mill relies on differential flotation to make four concentrates containing separately 24% Cu, 53% Zn, 43% Pb and a 55% bulk Pb-Zn. Ore is crushed in either of two gyratory crushers at the mine. It is ground in open circuit and reground in closed circuit with cyclones before flotation. Concentrates are cleaned in individual circuits, thickened and further dewatered using disc filters or Larox pressure filters. Moisture is further removed in oil-fired rotary dryers prior to storage and shipment.

But just because the process is conventional doesn’t mean that it has not been upgraded to state-of-the art technology. Changes made in the mill quickly pay for themselves.

Drying concentrates with pressure filters saves energy. Final moisture contents of 7-9% can be achieved by the Larox units, eliminating the use of oil-fired dryers. Two pressure filters were installed in 1983 and 1985.

Grinding efficiency has been increased by the switch to an ANI 8.53-by-4.27-m semi-autogenous mill, the first of its kind in North America, installed in 1998. Savings accrue by virtue of the mill’s size and design. It has the same throughput as two primary ball mills, and the SAG mill produces a finer grind than the ball mills. There is only a small charge of balls in the SAG mill, which translates into considerably lower grinding media expense.

Beginning in 1981, very large flotation cells were introduced into the mill. Outokumpu OK8, OK5 and OK3 units are found in the copper and zinc rougher and cleaner circuits. When a single large cell replaces a multitude of smaller cells, the reduction in moving parts presents a great savings in maintenance costs. Newer cells are also more easily automated for better control and efficiency.

Those mill tailings not needed for paste fill are impounded upstream from dams in four active ponds. Water is treated before release by raising the pH, allowing thiosalts to oxidize and removing heavy metals as a sludge.