BIG FARO MILL FINE-TUNED FOR GRUM ORE

The Faro concentrator was initially a 5,000-tonne-per-day operation in the early 1970s. It has since been expanded several times so that today it can process 14,000 tonnes daily, which, says Ken Ball, Manager of Processing, “is a pretty big plant by Canadian standards today.”

Crushing

Blended mine ore (from Vangorda and Faro) is crushed in the primary crushing plant, an Allis-Chalmers 54×74-inch gyratory crusher, giving a product of minus 150 mm. Screened undersize from the crusher (minus 19 mm) reports directly to fine ore bins ahead of grinding. The screen oversize is stored for subsequent secondary crushing at which time it is conveyed to the secondary crushing plant where two stages of cone crushing (a 7-ft. standard and two, 7-ft. short head) reduce the ore to minus 19 mm before it is conveyed to the fine ore bins ahead of the grinding circuit.

According to Ball, the crushing circuit can crush 20,000 tonnes per day. Planned maintenance is carried out most days in the primary or secondary crushing plants, while the other plant produces part of the grinding circuit feed tonnage.

Grinding

The grinding circuit includes two major circuit flows, each coinciding with a mill throughput expansion. Both circuits have a combination of open circuit rod milling followed by two stages of ball milling; each stage is in closed circuit with cyclones. The ore is ground to 80% minus 43 microns to maximize the lead and zinc sulphide liberation from the gangue in the ore.

During the grinding process a pH modifier (lime), zinc and iron sulphide depressants (SD 200 and sodium cyanide) and a lead sulphide collector (xanthate) are added. Frother (MIBC) is added to the reagent-conditioned slurry just before it enters the lead rougher flotation circuit.

Lead Flotation

Two parallel banks of Outokumpu OK38 flotation cells comprise the main lead rougher circuit. They are divided equally into rougher and scavenger cells, with each concentrate reground in separate 9×12 regrind ball mills. The tails flow to the zinc rougher circuit. Reground scavenger concentrate returns to the lead rougher feed, while reground rougher concentrate reports to the lead cleaning circuit.

Two banks of smaller flotation cells clean the rougher concentrate in classic countercurrent upgrading. Cleaning has recently been extended to five stages from the original three. As a last step, the first stage lead cleaner tails are scavenged in a bank of cells. Concentrate from these cells joins scavenger concentrate for regrinding. The tails flow to the zinc rougher flotation circuit. There are small additions of pH modifier, frother and collector added throughout the cleaning circuit. Lead concentrate, assaying 64% lead at about 79% recovery, is pumped to the dewatering section. Saleable silver and gold metals are contained in lead concentrates and account for roughly 16% of net sales of lead concentrates and 4% of total sales.

Zinc Flotation

The lead scavenger and lead cleaner tails are conditioned with pH modifier (lime), zinc sulphate activator (copper sulphate), collector (xanthate) and frother (Dowfroth 250) before it is subjected to zinc rougher and scavenger flotation in large flotation cells similar to the lead circuit. The zinc rougher and scavenger concentrates flow to the regrind ball mill, where further liberation of the zinc from associated gangue occurs. The concentrate is cleaned once in a bank of large flotation cells with the concentrate passing to columns and the tailing scavenged to recover as much zinc as possible before it is combined with the zinc scavenger tails for discharge to the tailing pond. The zinc cleaner concentrates are subjected to more cleaning in two column cells (rougher and scavenger) in series (Cominco Engineering Services Ltd. was involved in the column cell technology). The retention time is 30 minutes per cell. There are 38 spargers in each cell, and keeping them clean is the biggest challenge, Ball said.

In the column, circuit feed is regulated in two surge tanks. The pH is adjusted, and both columns give final grade product. Column tails are returned to the feed of the first-stage zinc cleaner flotation cells. Column cell set points are computer-controlled on operator input (Fisher and Porter Micro distributed control systems). Zinc concentrates assay 49%-50% zinc at a recovery of 77%.

Operator control is supported by continuous instrument assays from an Outokumpu Courier 300 On-Stream Analyzer (OSA). Eleven streams are sampled to provide data on final and intermediate concentrates, associated circuit tails streams, the final tails leaving the mill, and metallurgical accounting samples. These analyses assist in estimating circulating loads and facilitate problem-solving.

De-watering

Final lead and zinc concentrate streams report to separate, identical dewatering circuits where the concentrate solids are settled in thickeners, dewatered on disc filters and further dewatered by hot air in rotary driers to a final moisture content of less than 7%. Two filters and driers are dedicated to lead concentrates and three of each to zinc. Four driers are currently coal-fired and one is oil-fired.

Recent Developments

The Faro concentrator has proven to be versatile, having had to deal with several ore types from two separate pits and soon, from a third, the Grum. Take, for example, a single factor — the work index (as measured in kWhr/t). The Faro work index is 13.2; Grum, 12.0; and Vangorda, 9.2. The low Vangorda index is one of the reasons throughput has been high. But as the Vangorda feed is depleted, re-adjustments will be made in the mill to handle Grum ore, which has two major ore types — massive sulphides and disseminated sulphides with some carbonaceous quartz. According to Ball, Grum will require four cleaner stages and three regrind stages. “The key thing for Grum is that the lead and zinc circuits need additional regrinding.” Planned mill maintenance has been instrumental in accomplishing low unit costs and high throughput and available operating hours.