LAC MINERALS’ MACASSA MILL

For most of his working life, Macassa’s mill superintendent, Len Robinson, worked with Merrill-Crowe gold recovery systems. He was a stranger to carbon-in-pulp (cip) technology when it was incorporated into Macassa’s $20-million cip facility that replaced an aged Merrill-Crowe system in 1988. “CIP takes some getting used to,” he said. “You can’t let the carbon get away on you. If you get carbon in the grinding circuit, you can throw out the whole metallurgical circuit.” Today, Robinson feels quite comfortable with his cip mill. It treats up to 600 tons (545 tonnes) per day of ore from the Macassa mine and 750 tons (680 tonnes) per day of tailings from LAC Mineral’s Lake Shore project. The latter averages 0.085 oz. per ton of gold (2.91 grams per tonne) and testwork had shown that grinding to 95% minus 37 micrometres followed by cyanidation for 96 hours gives about 75% recovery. Recoveries now run about 70%, varying with grade. Gold recovery on Macassa averages 95.7%

Crushing (Macassa Ore)

The crushing plant, the same one used with the old mill, operates 24 hours per day producing 600 tons of fine ore (P80 =7750 microns). The crushing plant feed is run-of-mine ore. Actual crushing time, however, is only 12 to 13 hours per day because of erratic ore deliveries. The maximum hourly crushing rate is 90 tons (81.8 tonnes) per hour, hence the crushing plant has ample capacity to operate usually only five days a week and still feed the mill with enough material for the complete 7-day week.

The 2-stage crushing circuit uses a 36×28-inch primary Birdsboro Buchanan jaw crusher in series with a 4 4-1/4-ft. Symons shorthead crusher. The latter operates in closed circuit with a 6×14-ft. screen. Screen undersize is transported on three 24-inch-wide conveyors in series to a new 1,650-ton (1,500-tonne) fine ore bin.

Tailings Reclamation and Re-pulping

Tailings are dredged (with a 31.5-inch basket cutter head) from the reclaim tailings pond and pumped to a screen house plant on site at the Lake Shore tailings project. The trash screen is fitted with a perforated plate that removes coarse wood chips, gravel and other debris before the pulp is pumped to the reclaim surge tank. The coarse material is taken off via a chute for dumping as waste.

The pulp is pumped from the Lake Shore screen house to the Macassa mill at 15% to 26% solids, specifically to a 1,490-cubic-metre surge tank outside, adjacent to the thickener. The tank provides sufficient capacity to allow grinding over 24 hours, while dredging is performed on 12-hour day shifts.

Ore Grinding

Ore is withdrawn from the fine ore bin at 25 to 26 tons per hour. using two variable-speed slot feeders controlled by a weightometer on the primary mill feed conveyor. Primary grinding occurs in an Allis-Chalmers 10.5-ft.-diameter-by-13-ft., 800-hp ball mill operating in closed circuit with four 15-inch Krebs cyclones (one operating and three on standby). Calcium cyanide solution and lime slurry are added to the primary cyclone feed pumpbox.

The cyclone overflow P 80 product size keeps at about 77 micrometers. This material gravitates to the secondary cyclone feed pumpbox for pumping to six 10-inch-diameter cyclones (two operating and four on standby) operating in closed circuit with the secondary ball mill. The secondary mill is an 800-hp, 10.5-ft.-diameter- by-13-ft. unit. Secondary cyclone overflow at 18% to 22% solids and with P 80 size of 30 micrometers flows through a trash screen removing wood chips, plastic, and so on. It is pumped to a (149 Om3) 33-ft.-diameter-by-33-ft. pre-leach tank outside.

Tailings Grinding

Lake Shore tailings are pumped from the reclaim surge tank at 31 tons per hour to a bank of eight 10-inch Krebs cyclones via the grinding mills’ common pump box. The underflow goes to a pulp distributor, which splits the pulp between the 12.5×16-ft., 1,600-hp Allis-Chalmers ball mill (67%) and the 200-hp mpsi tower mill (33%) working in parallel and in closed circuit with the cyclones. This latter mill has a net finished product capacity of about 10 tons per hour.

The product size fluctuates around 95% minus 45 micrometres. The cyclone overflow goes through a 28-mesh trash screen and feeds an outside 65-ft.-diameter, high-capacity thickener (Dorr-Oliver) at around 27% solids. The thickener underflow pulp (50% solids) reports to leaching, while the chemical-free overflow is pumped back to the Lake Shore pond. This is possible because no chemicals are added prior to the first Lake Shore leach tank.

While Robinson has found that the tower mill saves on horsepower, wear parts must be replaced every three months. “It’s a quick job, but the plates are expensive,” he commented.

Ore Pre-leaching, Thickening and Carbon Columns

Experience in the old plant has shown that total gold recovery is improved by pre-leaching at low solids density and that up to 94% of the gold is leached in the grinding circuit and pre-leach tank. Trash screen undersize, therefore, is pumped to a 730-cubic-metre pre-leach tank outside, providing five to seven hours residence time. This tank overflows to a 65-ft.-diameter thickener fitted with a high-capacity feedwell, which thickens the pulp to 50% solids before pumping to the leach circuit.

Thickener overflow containing about four to five parts per million of gold is pumped via a surge tank to three, 5-ft.-diameter-by-15-ft.-high carbon columns in series. Haycarb Yao 6×12 mesh granulated carbon is transferred once a day from column to column countercurrently to the solution flow. The carbon loads to 145 to 175 oz. per ton (5,000 to 6,000 grams per tonne) gold and transfers from the first column to the carbon stripping circuit. Barren solution will overflow from the third column to the mill solution tank for use in the ore-grinding circuit. About 85% of the gold in the thickener overflow will be recovered on to the carbon.

Leaching

Reground tailings are pumped to two outdoor, 40-ft.-diameter-by-45-ft. leach tanks (1,490 cubic metres) in series. Calcium cyanide solution, lime slurry and pure oxygen are also added to the first Lake Shore leach tank. Partially leached ore from the ore thickener goes into three 1,490-cubic-metre leach tanks in series. Pure oxygen is also added in the first leach tank that treats Macassa ore. This configuration provides 77 hours of residence time for tailings and 144 hours for ore. Overflow from the last leach tank of each circuit gravitates through the CIP circuit.

Carbon-in-pulp (CIP)

This circuit comprises six 20-ft.-diameter-by-20-ft. CIP tanks (184 cubic metres) in series in the mill building. Each tank provides two to 2.5 hours residence time and is fitted with a twin-impeller, high-efficiency agitator and a launder screen equipped with three air-swept screen panels per side.

The pulp gravitates through the tanks via the launder screens and finally discharges from CIP tank No. 6, from where it goes to the vibrating carbon safety screen. This screen collects any carbon that escapes through the launder screens and allows the tailings to pass to the tailings pumpbox. Tailings are pumped to the dam via a 6-inch-diameter HDPE pipeline.

At a maximum rate of 1.3 tons of carbon per day is added to tank No. 6. The carbon is advanced through the other tanks using recessed impeller pumps every six hours. The carbon concentration in the pulp (CIP tanks one to six) is maintained at 15 to 20 grams per litre. Pulp from tank No. 1 is pumped to a vibrating washing screen above, and adjacent to, the carbon stripping circuit. Washed, loaded carbon (at 145 to 175 oz. gold per ton, or 5,000 to 6,000 grams per tonne) flows directly to the loaded carbon storage tank while the pulp returns to the CIP No. 1 tank. The carbon pumping sequence is controlled by operator-adjusted timers linked to a programmable “logic controller.” Attritioned fresh carbon and recycled carbon are screened and added to tank No. 6 (carbon column No. 3) via the stripped storage tank eductor as needed.

Carbon fines are captured by way of a screen and are shipped in bulk to Noranda Inc.’s Horne smelter.

Carbon Stripping and Gold Recovery

Washed loaded carbon from CIP tank No. 1 and carbon column No. 1 is educted to the stripping vessel, which has been sized to take up to 3.8 tons of carbon (or one day’s production). Stripping occurs at high pressure (350 KPa) using 12 cubic metres per hour of a solution containing 1% sodium hydroxide and 0.2% sodium cyanide at 140^o (deg) C. The time required to reduce the carbon to a gold loading of about five ounces per ton (171 grams per tonne) is seven hours.

The barren strip solution is pumped from the fresh eluate tank through an in-line electric solution heater to bring it to the required 140^o (deg) C and then to the strip vessel.

Loaded eluate containing 140 to 200 parts per million gold is pumped from the pregnant strip solution tank to the electrowinning cell, which contains 12 cathodes, each loaded with five pounds of wire wool. On average, 160 oz. of gold are loaded on to each cathode and they are removed from the cell once a week.

Barren strip solution from the electrowinning cell containing about five parts per million gold is returned to the fresh eluate tank. Make-up additions of sodium cyanide and sodium hydroxide solutions are added as required from a mixing tank.

Carbon Handling and Regeneration

The carbon requires periodic acid washing and regeneration in order to retain its activity. Acid washing is carried out in a 5% nitric acid solution while regeneration is done in an externally heated horizontal rotary kiln at 600^o (deg) C. Acid washing or kilning are normally performed alternatively after every stripping.

The carbon educted from the strip vessel is directed to the sizing screen, the acid wash tank or the dewatering screen ahead of the regeneration kiln feed hopper. Acid-treated carbon is neutralized with sodium hydroxide and water washed prior to educting to either the regeneration kiln or the sizing screen. Carbon discharging from the kiln is water-quenched and then educted to the sizing screen.

Carbon is transported through the circuit using eductors powered by a dedicated high-pressure water system. Water is pumped from the eductor water tank to each eductor as required and then drained back to this tank from the vessel to which the carbon is delivered. The tank is fitted with a central feedwell, allowing carbon fines that accumulate in the eductor water to settle. The collected fines will then be pumped to a filter press.

Carbon consumption ranges from 0.04 to 0.10 lb. per ton (20 to 50 grams per tonne) of ore treated. The losses will be made up with fresh carbon, which will be educted to the carbon sizing screen from a conditioning tank. A batch 500 kg. of fresh carbon is soaked in water for 24 hours and agitated another two hours in this tank to break off weak corners from the carbon grains, which would otherwise degenerate to fines in the CIP circuit and cause losses of gold. The fines will be removed as sizing screen undersize which will drain to the eductor water tank feedwell.

Gold Refining

Cathodes removed from the electrowinning cell are dismantled on the stripping tables. The wire wool and deposited gold are allowed to dry and this material is fluxed and smelted on an induction furnace and poured into bullion molds. Bullion averages 84% to 85% gold and 10% to 13% silver.

PRODUCTION STATS 1989

Gold output ………………………………….. 82,540 oz.

(1990 target output) ………………………….. 85,000 oz.

Tons milled …………………………………. 165,487 tons

Millhead grade (oz./ton) ……………………….. 0.499

Average mill recovery rate …………………….. 95.68%

Production ost per oz. ………………………… US$304

Current daily mine tonnage ……………………….. 591 tons

Reserves …………………….. 1.5 million tons of 0.51 oz.