FALCONBRIDGE IN SUDBURY: THE FINE-TUNED SMELTER

Ever since the Falconbridge smelter was completely updated in the late 1970s — the original pelletizing and sintering plants were replaced with slurry-fed, fluidized-bed roasters; the blast furnaces with two submerged electrode furnaces; and an acid plant was constructed — the focus has been on fine-tuning to reduce sulphur dioxide SO2 stack emissions. (The emissions are now at 70 kilotonnes per year, well below the government-mandated 100 kilotonnes by 1994.)

For example, Falconbridge installed copper finger coolers along the sides of its furnaces to prolong furnace life and, with an eye to the future, to better withstand the higher smelting temperatures required to further reduce SO2 emissions. It is also revamping the cooling towers, increasing fan capacities in the roasters, and incorporating a host of other relatively modest, but important, changes. Otherwise, it is business as usual at the smelter.

The concentrate feed from the Strathcona mill, the only concentrator now supplying the Falconbridge smelter, runs at a grade of 6.5% nickel, 2.7% copper and 0.21% cobalt. The slurried concentrate is thickened to 68% solids by disc filters. It is fed to two 6-metre-diameter fluid bed roasters, one for each electric arc furnace. Each roaster has an 1,000-tonne-per-day capacity. The roaster bed is heated to 690degc. Originally, the roasters achieved a 50% roast, by which one-half the sulphur was oxidized to SO2 and made into sulphuric acid in the acid plant. However, in 1983 Falconbridge improved that performance to 60%. To achieve this, extensive metallurgical pilot-plant and full-plant testwork was required, the purpose being to raise the roasting temperature and up the coke rate to the electric furnaces. Roaster and electric furnace upgrades will continue, according to Gary Kaiura, technical superintendent of the smelter complex.

At Ontario Hydro, Falconbridge is developing smelting technology in a pilot-stage furnace for a higher roast. “Our goal is to capture as much of the sulphur dioxide at the roaster stage in order to redirect it to the acid plant,” Kaiura says.

Calcine is drawn off the bottom of the roasters and reports to the furnaces. About 85% of the calcine leaving the roasters is recovered from the off-gases. Initially, two primary and four secondary cyclones cleaned the gases. Falconbridge, however, has modified the system to gain capacity for higher roasts. The secondary cyclones are now bypassed without sacrificing dust collection efficiency. After the cyclones, the off-gases are cooled to 320deg by atomizing water spray nozzles and passed through four electrostatic precipitators. At least 99% of the dust is removed from the gas stream, with the calcine collected and routed to the furnaces. The cleaned gas stream then enters the acid plant, which has a rated capacity of 1,180 tonnes per day of 93% H2SO4.

The two furnaces, nine metres wide by 30 metres long by 2.7 metres high, are equipped with six Soderberg self-baking electrodes 18 metres long and 1.4 metres in diameter. Calcine is distributed along the lengths of the furnaces through fettling pipes at rates of 42 to 44 tonnes per hour. Coke is added as a reductant. Gas uptakes, one at the matte end and one at the slag end, remove about 2% of the calcine, which is recovered in precipitators.

THE SO2 EMISSIONS ARE NOW AT 70 KILOTONNES PER YEAR, WELL BELOW THE 1994 LIMIT OF 100 KILOTONNES.

All this activity is kept operating at optimum levels from a central control room equipped with a recently installed Foxboro Distributed Control.

In the converter aisle, matte is tapped from the furnaces on average once every hour into ladles that charge the converters. In the converter aisle, Falconbridge is introducing software to aid the skimmer in doing his job. “We’re trying to develop a computer-assisted system that has a metallurgical model of converting that runs in background mode,” says Kaiura. “The system advises the skimmer on what his next move should be.” The company also developed a unique converter slag cleaning process. The process results in much higher cobalt recovery, less reductant to the electric furnaces, higher degrees of roast and economic treatment of secondary recycle metals.

From the converter, the matte is transported to the matte room, poured into molds to solidify and then cooled and broken with a rock hammer and a jaw crusher.

The 79,000 tons (71,688 tonnes) of matte produced in 1990 and grading 52% nickel, 20% copper and 1.8% cobalt were shipped to Falconbridge’s Norwegian refinery via rail to a Quebec port for trans-shipment across the Atlantic.

The SO2 abatement program, launched in 1989, will ensure Falconbridge’s continuing commitment to environmental improvements, Kaiura notes. The capital and the research and development earmarked for the smelter are being committed to increase the capacity of the roasters, the furnaces, and the acid plant to achieve higher degrees of roast.