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(July 25, 2002, Gazette)

A quick course on hydromet technology


The likely scale of the Voisey's Bay development is smaller than anticipated when Inco spoke of constructing a smelter. Instead of using pyrometallurgical (pyromet) processes as in Sudbury, Ont., Voisey's Bay Nickel Co. Ltd. concluded hydrometallurgy (hydromet) would be more appropriate for the reduced quantity of nickel in the ore body.

I should like to try to explain why this is a sensible decision both for the company, an Inco subsidiary, and for the province.

Last November in St. John's, Dr. Chris Fleming of Lakefield Research Ltd. presented convincing arguments to the Newfoundland chapter, Canadian Institute of Mining and Metallurgy. Some readers have evidently not understood his rationale, judging by newspaper and radio phone-in show comments.

Pyromet has a serious environmental problem in producing poisonous sulfur dioxide, and because the need for roasting requires more energy. Hydromet produces no poisonous gases but does generate quantities of waste solids (iron oxides and sulfur) requiring storage in an environmentally responsible manner.

Because of its relatively smaller scale and ability to be built in stages rather than a gigantic pyromet project, a hydromet operation should be able to continue in the province beyond the 30 years' expected life of the Voisey's Bay (VB) deposit.

Unlike pyromet, hydromet has flexibility to handle dissimilar ores from a variety of sources, some outside this province. For Inco, a new plant, less costly to build, with fewer environmental disadvantages and lower energy costs than Sudbury should also be preferable.

The composition of the nickel ore is relevant to a successful extraction process. Voisey's Bay ore is largely pyrrhotite (iron sulfide), the ore containing 59 per cent iron, 34 per cent sulfur. The principle nickel sources in the ore are pentlandite (nickel-iron sulfide) and millerite (nickel sulfide).

Only three per cent nickel, two per cent copper and 0.14 percent cobalt are present. In Inco's view, the nickel and cobalt are the most precious metals in the ore. The problem is to separate them from the large quantities of sulfur and iron. This is generally done in stages which may involve heating or the use of water. When heating at high temperature, the process is called pyrometallurgy (pyromet). When water is employed, it is called hydrometallurgy (hydromet).

Pyromet refers to extraction of metals using a furnace, often at red heat, generated electrically or by burning a fuel such as coke. Steel making from iron ore is the most widely known example. Sudbury nickel sulfide ores are smelted by roasting in air or oxygen at high temperature to oxidize the sulfur present to sulfur dioxide.

Sulfur dioxide is a source of aerial pollution, even though strenuous efforts are usually made to convert it to sulfuric acid and prevent its escape. For reasons of engineering and capital investment, a pyromet plant has to be large. Such a plant for Voisey's Bay would shorten the project life below 30 years. Furthermore, there is excess smelting capacity worldwide.

There are many different hydromet processes. With sulfide ores, one needs to oxidize the sulfur present to get rid of most of it. This is accomplished by pressure leaching in air or oxygen, a process invented by Sherritt Gordon Mines Ltd. in 1954. It is widely used in numerous plants around the world.

The first successful commercial operation was at Fort Saskatchewan, Alberta where nickel was leached from sulfides using air, ammonia and water under 130 psi pressure and 85o C. This is a tried and true process known as the Sherritt ammonia leach process. The ammonia helps to facilitate the nickel dissolution. Inco intends to improve it by not using ammonia, but using oxygen instead of air.

Inco intends to pressure leach with oxygen and water at 110o C. A Russian company at Nadezhda has already done something similar. The oxygen oxidizes most of the sulfur in the sulfides to solid sulfur. Some will be oxidized further to sulfate ions, helping to dissolve the metals. Acid formed will later be neutralized by adding limestone to form gypsum, which separates from solution. At the planned final pH of 3, solid iron oxide forms. Both sulfur and iron oxide can also be filtered off, the residual liquid being used to obtain solid nickel and cobalt by electrolytic plating.

The voluminous solid waste consists of iron oxide, sulfur and gypsum, of lower toxicity than the acids and heavy metals in pyromet slag. Sulfur may be used in road paving, while gypsum is used in gyproc wall board. However, neither they nor iron oxide would be easily recoverable.

If ammonia were used to neutralize the sulfuric acid, instead of gypsum, ammonium sulfate (which could be sold as a fertilizer) would form. Since the slightly acidic solution containing nickel and cobalt is valuable, it is unlikely to be allowed to escape into the environment. No gaseous sulfur dioxide would be produced in the hydromet process. Nor would hydrogen sulfide, which might result if acid extraction were attempted. (Neither oxygen nor ammonia are acidic substances).

Hydromet recovers a larger fraction of the nickel and cobalt from the ore than would pyromet. It is amenable to the smaller throughput of material required to produce the 50,000 tonnes/year that Bob Kelly of the Newfoundland and Labrador Chamber of Mineral Resources mentioned in the Telegram, Feb. 17. At that rate, Voisey's Bay would sustain the process for at least 30 years. Capital costs and energy requirements for hydromet would be less than for a smelter. Lower grade Voisey's Bay ores (from outside the nickel-rich ovoid) or ores from outside Newfoundland & Labrador could be treated.

Hydromet has been demonstrated by Lakefield Research Ltd. as technically viable at pilot plant scale. The prospect of eventually having a new plant, using recent technology, built in stages and the employment that this would bring, surely makes it worthwhile to wait for the scaled-up demonstration plant to prove itself .

Frank R. Smith is honorary research professor in the Department of Chemistry at Memorial University. He taught physical and analytical chemistry and materials science there for over 30 years and has a special interest in hydrometallurgy of iron. In 1997 he participated with others in advising VBN regarding environmental concerns. The opinions expressed herein are the author's alone.