Pollution controllable — at a cost

Press, 27 November 1980, Page 21

Pollution controllable — at a cost

More can be done, but,».

Fluorides are emitted by aluminium smelters because of the electrolytic process used to produce the metal. Unlike iron and other metals, aluminium cannot be produced by melting the ore. The oxide has a very high melting point (about 2000 degrees C), and the metal is volatile at this temperature. An electrolytic method is used instead. The fine abrasive alumina powder is k dissolved in molten cryolite (a double salt of aluminium fluoride and sodium fluoride) and an electric current is passed through the mixture. This deposits aluminium metal at the cathode, the negative electrode, which is formed by the carbon blocks which line the steel pot. The way the ■ surface crust on the molten mixture is broken to let in more alumina, and the manner in which the carbon anode (positive electrode) blocks, are replaced, have a major influence on the amount of fluoride emitted.

The process produces between 18 and 40 kilograms of fluoride for every tonne of aluminium produced, in about equal

proportions of particulate and gaseous fluorides. But in the pot of the least polluting design (the “centrework” pre-bake cell) the alumina is added automatically and the hoods over the pot- do not need to be lifted. Emissions are contained beneath the hoods, and up to 98 per cent can be collected. They are then “dry-scrubbed” — passed through the unused alumina powder for the fluorides and other particles to be collected for recycling. The present smelter at Tiwai Point does not have a dry-scrubbing system of pollution control. It. has simple mechanical collectors which remove 75 per cent of the particulate matter (dust) from the pots, but only a small percentage of the gases. The balance is discharged through the smokestack. The smelter pre-dates the dry-scrubbing technology. New Zealand Aluminium Smelters want to add a third potline at Tiwai Point, and Mr Pullen says they will be allowed to use the same kind of mechanical collector for the new pots. It is considered undesirable to have two kinds of pollu-

tion control equipment in the same plant. “We didn’t feel that it was warranted at this stage,” he says. “But there is provision for requiring dry-scrubbing if it is considered desirable in the future.”

There would have to be environmental evidence that they were needed — evidence of effects on animals, pasture or native vegetation — and the consortium’s operating licence would be modified, making the dry-scrubbing equipment a new condition.

The United States Environmental Protection Agency (E.P.A.) made a comprehensive review of the aluminium smelting industry in the early 19705, and laid down New Source Performance Standards for Aluminium Smelters. It considered the technological options for pollution control available at the time, and estimated their cost-effectiveness. The New South Wales State Pollution Control

Commission did its own evaluation for the Hunter Valley study produced this year, and says that it is satisfied that the technology exists to meet the E.P.A.’s emission standard. It decided that the combination of “centre-work” pre-bake cells with efficient hooding and dryscrubbing of cell gas emissions, supplemented by control equipment to reduce emissions from the anode-baking furnace, can achieve an over-all emission rate as low as 1 kilogram of fluoride for each tonne of aluminium produced.

That is the basis of the E.P.A.’s New Source Performance Standard laid down in 1976. It is also the emission level which the New Zealand Health Department says it will probably apply for a second smelter here.

The second smelter is to have two pot-lines, and later possibly three. Each pot-line comprises about 200 pots, each of W'hich produces one tonne of aluminium a day. If it keeps within the pollution -limit, 400 kilograms of fluoride will escape into the surrounding atmosphere each day from the 400 tonnes of aluminium produced from the first two pot-lines. More could be done to reduce fluoride emissions even further, but at considerable cost to the

smelting company. The E.P.A. calculated that adding roof "scrubbers” to reduce emissions from the whole smelting plant to 0.75 kilograms per tonne of aluminium produced would nearly double the capital cost and more than double the operating cost for the control equipment. This was based on 1972 cost levels,, but the New South Wales commission says the relative costs remain about the same.

Carbon blocks regularly removed from the pots contain cyanide. Smelters have - no economical method of recycling them

yet, and there is no wholly satisfactory way of treating them either. Smelters store them on the site and neutralise the liquids as they leach out. The N.S.W. commission says precautions sh_-’d be taken to ensure that leachate is not generated at all. This can be achieved by requiring the spent carbon pot linings to be stored under cover. The enormous stack of carbon at Tiwai Point is

on a concrete pad surrounded by retaining walls. The leachate is treated with chlorine and then discharged through a pipe just off the beach. It

is monitored by the local catchment board and by the company, Mr Pullen says the cyanide levels have always been within the catchment board’s limits. Although the Washdyke site ■is some little way inland, the leachate would still be discharged to the sea after treatment. Mr Pullen says that the consortium would have to get an easement for a pipeline.

At Tiwai Point, he adds, the smelter company is experimenting with grassing the piles of spent carbon cathodes in an attempt to cut down the amount of leachate. Pollution control equipment represents at least 12 per cent of the cost of a modern smelter, and can be as much as 30 per cent. The amount spent indicates just how seriously fluoride is regarded as a pollutant. But whether a smelter company is forced to put in the extremely expensive control equipment to capture the last traces of pollution is in the end, a political decision. It could mean the difference between having a slightly polluting smelter and having no smelter at all.

“Environmentally there need be no problem,” Mr Pullen says. "The emissions can be controlled. It is only a question of dollars and cents.”

Tomorrow; Latest research questions "safe” fluoride levels; and forecast that rainfall in 1983-86 will provide, serious problems for a second smelter.