FALCONBRIDGE IN SUDBURY: SULPHUR REDUCTION AT STRATHCONA

Falconbridge’s first mill on the North Rim was the Hardy (1955), followed by the Fecunis (1957) and finally the Strathcona mill, commissioned in 1968. The earlier mills closed in 1972 and 1975 respectively and today Strathcona handles all the ore from Falconbridge’s Sudbury mines. This includes ore from Fraser, Strathcona, Strathcona Deep Copper, Onaping, Craig, Lockerby, Falconbridge Crown pillar and custom ores. Timmins Nickel is the main contributor in the last category.

For comparative purposes, Inco mills about 11 million tons per year from its Sudbury mines versus three million from Falconbridge, though Inco’s announced head grade of slightly under 1.2% nickel for 1991 (T.N.M., Nov. 2 6/90) is less than Falconbridge’s estimated 1.4%.

A simplified Strathcona flow sheet is shown on page 36 and consists of the following:

* a copper circuit handling Strathcona’s “Deep Copper” ore, which grades a phenomenal 8% copper and also carries important platinum, palladium and gold values;

* the principal circuit recovering a high-nickel, low-copper concentrate;

* sub-circuit scavenger flotation recovering all the remaining sulphides with regrinding of the magnetics to release a nickel-rich pentlandite;

* and finally, the preparation of fill material from the tailings for underground use. Much of the fill is prepared with varying proportions of cement and pumped as a slurry to Strathcona and Fraser mines. The remainder is filtered, stockpiled and trucked dry to the more distant mines as required.

Crushing and grinding circuits are standard. Crushing starts underground where the ore is reduced by jaw crusher to minus six inches. On arrival at surface, it passes through a 3-stage cone-crushing process reducing it to 0.5-inch rod mill feed.

Feed for the Deep Copper circuit, about 25,000 tons per month at present, is ground to 70% minus 200 mesh in a single, closed-circuit ball mill (1,800 hp). Conventional flotation produces a concentrate grading 24% copper and substantial values in gold, platinum and palladium. A portion of the concentrate is passed to column flotation, producing a high-grade, 30% copper product. Until recently, the copper concentrate was delivered exclusively to Inco and Noranda for custom smelting but now goes primarily to Noranda with occassional shipments to Kidd Creek.

The most significant change in nickel metallurgy over the past 15 years has been the drive to reduce sulphur dioxide emmissions from the smelter. The most direct means is to reduce the sulphur content of the concentrate sent to the smelter. This places the onus for improvement on the mill’s metallurgists, and Falconbridge’s metallurgists have delivered commendably in this regard. Improvements in flow sheet design and in equipment have been introduced consistently over the years and nickel concentrate that once was shipped to the smelter with between 50% and 60% sulphur in the ore now contains less than 40%. And further reductions are realizable in the future. The reason for the lower sulphur recovery to concentrate is pyrrhotite rejection. Pyrrhotite is the main sulphur carrier, and pyrrhotite rejection is one of the essentials of the flotation process.

Feed for the nickel flotation circuit is ground to between 50% and 60% minus 200 mesh in two parallel, rod mill/ball mill circuits of 900 and 1,750 hp respectively.

In its barest essentials, the nickel flotation circuit entails producing a relatively high grade of nickel-copper concentrate, from which a high-grade copper concentrate is separated by column flotation.

ATTEMPTS TO UPGRADE THE PROCESS CONTROL SYSTEM ARE FOCUSED ON COLUMN FLOTATION.

The initial recovery of nickel-copper is held at a relatively moderate level to keep as much pyrrhotite out of the concentrate as possible. A large percentage of the nickel consequently remains with the pyrrhotite. The next stage is to take the tailings from the initial flotation and float all the remaining sulphides. The resulting concentrate is then subjected to magnetic separation and the two products, magnetics and non-magnetics, are treated separately.

The non-magnetics carry residual copper values, nickel sulphides and gangue minerals and are upgraded by flotation to produce a low-nickel/low-copper concentrate and a final tailing.

The magnetic product consists primarily of pyrrhotite, but the pyrrhotite carries an important proportion of fine-grained nickel sulphides which must first be liberated before any additional nickel can be recovered. It is ground to 80% minus 325 mesh, the greater part of the nickel sulphides released and subsequently recovered in a flotation concentrate with a large proportion of pyrrhotite rejected as a tailing. However, the finely ground pyrrhotite is susceptible to auto-oxidation and under suitable conditions will generate sulphur dioxide and heat and therefore is unsuitable for backfill underground.

The sulphur content of the feed to the smelter is controlled by mixing the high-grade nickel concentrate from the column flotation step with the low-sulphur nickel concentrate produced from the magnetic product.

Reduction of the sulphur recovery to the smelter concentrate is not achieved without cost; the nickel content of smelter concentrate is now in the 6.5%-to-7.5% range with the copper averaging 3%. Nickel passing into the tailings approximates 0.1%.

New additions to the mill include: column cells, jumbo flotation cells, pressure filtration units and sophisticated process control.

Two column flotation cells designed by Hatch Associates and Falconbridge are two metres in diameter by 13 metres high for the nickel-copper separation and a third is on order. Dorr-Oliver jumbo cells have been on test since January 1990. The four 1,350 cu.-ft. cells will replace 48 100-cu.-ft. original cells. The chief advantage of the units is energy savings.