THE LITTLE MILL THAT COULD: Est Malartic mill copes with a variety

If ever there were to be a country-wide award for flexibilty in processing plants, for adapting to changing times, then LAC Mineral’s Est Malartic mill would be the run-away winner.

The mill was built in 1938 to treat the ores from East Malartic mine, and a few years later, from the neighboring Barnat. These mines closed in 1981, but that was not the end of milling. Custom ore started arriving, first from Bousqet No.1, then Doyon, later Francoeur and now Bousquet No. 2. Every new modification and installation to keep abreast of technology was carried out within the original mill structure.

But Bousquet No. 2 ore caused radical changes. While the mine in the commercial sense is a gold mine and one of Canada’s richest at that (its 1990 heads averaged 0.40 oz. per ton), the ore is typically base metal. Sulphides are the dominant constituent: the ore ranges from 30% to 50% sulphur. There is an abundance of pyrite, and it is accompanied by bornite and chalcopyrite. There is sporadic sphalerite. Most importantly, there are substantial values in coarse gold. A fifth of the total gold output is, in fact, recovered as a gravity concentrate.

(To avoid misconceptions on the references to ore grades, it should be noted that Bousquet No. 2 proven and probable reserves to December, 1990, averaged 0.25 oz. gold per ton and 0.71% copper over 3.9 million tons. The higher grades milled during 1990 resulted from the mining plan — joint extraction of the boundary pillar with the adjoining LaRonde mine — and the need to maximize revenue in the early production years. The 1990 grades were 0.40 oz. and 1.65% respectively).

Bousquet’s copper content precipitated the major change in the mill’s metallurgy. Copper is a plague in a cyanidation circuit. It consumes cyanide and, what is more, cyanide reacts preferentially with copper when copper minerals are present. Consequently, the basic flow sheet is designed to recover a coarse gold concentrate at the earliest opportunity and then to eliminate as much of the copper as possible as a saleable flotation concentrate. The remaining gold in the flotation tails is then extracted by cyanidation.

Overall gold recovery is 96%, 62% of which reports to the copper concentrate, 21% to gravity, and the balance of 17% to cyanidation. Bousquet mill throughput has recently been increased from 1,350 tons per day to 2,000 tons with 2,500 tons projected for 1995.

The ore is crushed underground to minus 6 inches and trucked 23 miles to the mill. Conventional secondary and tertiary crushing reduce the ore to half-inch. The mill feed passes first to a 9×12-ft. rod mill and then to three 8×8-ft. ball mills in parallel. Classification is by cyclone, with part of the circulating load diverted to two, 36-inch duplex jigs. Jig concentrate is cleaned up on a single Wilfley table and the high-grade product sent to the refinery furnace.

Mill pulp at 60% minus 200 mesh passes to the flotation circuit, which comprises two unit cells in series and a rougher-cleaner circuit. The first unit cell is an Outokumpu 10×10-ft. and the second a 12×12-ft. Maxwell. Both produce a final concentrate. Rougher flotation consists of six, 565-cu.-ft. OK cells, with the rougher concentrate cleaned in four, 105-cu.-ft. OK cells followed by eight, 40-cu.-ft. Denver cells. There is provision for regrinding if the rougher concentrate needs further liberation.

Rougher cell tails grading 0.07-0.08 oz. per ton constitute the feed for cyanidation. The pulp is leached in a bank of six pachucas for 35 hours and the pregnant solution filtered and clarified in preparation for Merrill Crowe precipitation. Metallic copper makes up 90% of the Merrill Crowe precipitate, with gold assaying up to 1,200 oz. per ton. It is not upgraded on the property, but is shipped at the rate of 2,000 lb. per week to custom refiners.

Est Malartic thus produces gold in three different forms: bullion derived from gravity concentrates; copper concentrate carrying up to 18% copper and 2- 5 oz. gold per ton (silver content is higher by a multiple of three); and low-grade Merrill Crowe precipitate.

But that is not the end of the story — Est Malartic’s vaunted flexibility is being tested once again. There could be a fourth saleable product by the end of 1991. Present plans are contingent on the successful completion of research that is now in its final stages and could see the adoption of the AVR process. AVR is the acronym for “acidification-volatilization-recovery,” the technology of which centres on the recovery of cyanide from barren leach solutions. In its basics, the cyanide of the barren is driven off by adding acid and the cyanide, now in gaseous form, is reabsorbed to produce fresh sodium or calcium cyanide for reuse. A secondary reaction precipitates heavy metals, in this case copper, and up to three tons per day of rich copper precipitate will be made available for smelting. The process can be adapted to meet a number of differing parameters, and it is these that are now being studied to determine which adaptation will be the most suitable for Est Malartic.

Basic AVR technology has been known for several years but the number of recorded users is small: it was used at Flin Flon in the 1940s and there are operating systems in Mexico and Tasmania.

LAC and the Canada Centre for Mineral and Energy Technology (CANMET) are working together on the AVR project. Bousqet ore is a heavy consumer of cyanide, using up to 8 lb/ton, and regenerating the greater part will be a major cost saving. In addition, there will be no cyanide passing out with the tailings, and the sulphur dioxide plant, now in use to destroy residual cyanide, will be made obsolete (this is the INCO process). There are benefits all round in the application of AVR and there is the added bonus of a premiumpriced copper by-product.

Est Malartic will be constructing a new impoundment for Bousquet’s tailings. The design will incorporate permanent underwater storage and so eliminate the generation of acidic seepage that would otherwise take place. The new impoundment will be commissioned in 1992. In the meantime, the dams created for the original Est Malartic mine will continue in service.

It is interesting to note that the original Est Malartic ore bodies were strongly carbonatized. The sands and slimes of a generation ago are thus now providing a service that would never have been thought of before — they are the ideal neutralizer for Bousquet’s sulphide tailings.