Sulphurous gases: putting pollution into context

Today, man is universally condemned as the environmental hooligan. However, nature too can devastate the global balance.

For example, the latest phase of sulphur dioxide control is on schedule at the Sudbury operations of Inco (TSE). When it comes on line Jan. 1, 1994, the maximum allowable discharge of sulphur dioxide into the atmosphere will be 265,000 tonnes per year.

At Salt Lake City, Kennecott will start building a new smelter in 1993 that will capture 99.9% of the sulphur contained in the company’s copper concentrates.

In Alberta, plants recovering elemental sulphur from sour gas capture 99.4% of the element before the stack gases are discharged into the atmosphere. The above examples are only a few of the scores of plants that must meet the stringent environmental requirements of the western world.

Then, as if to mock man’s good intentions, Mount Pinatubo in the Philippines blew its top. Even China’s burning of nearly a billion tonnes of coal per year is not in the same class; at an estimated 0.5% sulphur content, that amount of coal will generate about nine million tonnes of sulphur dioxide per year.

Mount Pinatubo, according to the American National Oceanic and Atmospheric Administration, ejected 27-36 million tonnes of sulphur-rich gases and ash during the course of its 1991 eruption. There may be more in 1992. Ignoring unregulated sources, smelters and thermal power stations pour out large volumes of sulphurous gases. The sulphur content of the two types of gas is at opposite extremes and their processing is quite different. In a nutshell, smelter gas is a valuable product: it is the raw material for sulphuric acid manufacture. The sulphur of a power station’s furnace gas, on the other hand, is a noxious impurity, and it must be removed. It is an extra expense.

Today’s smelters produce a gas carrying upwards of 50% sulphur dioxide. Coal-oil fired power stations produce a dilute, with less than 3% gas. However, the volume of furnace gases produced by the burning of hundreds of tonnes of coal per hour at a typical city power station is enormous. A correspondingly large and expensive cooling installation is needed to reduce the temperature of the gases low enough so they can be scrubbed in water and the sulphur dioxide dissolved. But that is not all, the resulting weak acid must be neutralized, usually with limestone, and the solid residue hauled to a regulated dump site.

If 100% oxygen were to be fed to a power station’s furnaces, the cooling and scrubbing installation would need to be one-fifth of the size of one using atmospheric air (one-fifth of atmospheric air is oxygen).

A majority of the world’s copper (and nickel) smelters do just that. Most have used oxygen-enriched air or pure oxygen for years, not to reduce the size of their gas treatment plants (one of the benefits) but to make a cleaner, faster and less expensive smelting process. And, they generate a rich sulphur dioxide gas for conversion into sulphuric acid. If it seems that smelters can have their cake and eat it too, that is not the case. Sulphuric acid is produced in volume in North America; it is a low-cost chemical. If industries using the acid are distant from the smelter, transportation costs rapidly erode profit margins that are thin to begin with. A number of sulphide smelters have investigated the possibilities of converting sulphur dioxide into sulphur rather than acid. Elemental sulphur is easily handled, it can be stored indefinitely. It is an innocuous product. The conversion is simple enough in the laboratory but developing an inexpensive, industrial process is another matter.

Cominco (TSE) operated a full-scale conversion plant at its Trail, B.C., smelter during 1936-1943. So did the Orkla Mining company in Norway until 1962. Outokumpu Oy, Finland’s largest mining and smelting company, has developed its own process. Environmental gremlins and high costs have prevented widespread adoption of these processes.

Inevitably the problems will be solved. The incentive is there.