INNOVATIONS A Solution to Acid Mist

Acid mist is a serious problem for workers in electrowinning plants. Not only does it cause respiratory problems; it erodes their teeth. To maintain safe working conditions, large amounts of fresh air must b e drawn into these plants and heated, adding significantly to metal production costs.

But an industrial ventilation systems design company, desom Engineering Systems (Downsview, Ont.), is marketing a solution. It is a simple device invented by Prof James Smith and a student, Angela Papachristodoulou, of the University of T oronto. It controls acid mist at the source.

In the electrowinning process, an electric current is passed through an acid solution containing zinc, for example, in an impure state. The current causes the pure zinc to “plate” onto a surface (a cathode) from which it is then removed. In the process, oxygen is produced which forms bubbles that burst on the surface of the solution. This generates an acid mist in the air. Operators currently use either plastic balls or a chemical surfactant to combat the problem. But the plastic balls, which float on the acid solution, are expensive and can stick to the equipment while surfactants reduce t he efficiency of the electric current.

The u of t/desom device (called Electrocaps) is a strip of acid-resistant vinyl that is laid across the top of the solution vessels. It provides a flat surfac e on which the bubbles can coalesce before they burst and trap what little acid mist does form — all without interfering in the production process.

John Davis, president of desom, says Electrocaps has been tested at Kidd Creek ‘s metallurgical facility in Timmins, Ont., for about two years. But since surfa ctants are still used in another part of the circuit, results haven’t been clear cut.

“We estimate that, without energy calculations, Electrocaps can pay for itself in 2-2 1/2 years,” Davis says. “But the energy calculation is w here the real savings are.” By taking the energy required to heat ventilatio n air needed to dil ute acid mist into account, Davis says, Electrocaps can pay for itself in 6-8 mo nths in a typical application.

Because of the wide variety of vessel sizes and shapes, it takes about 10 week s for desom to deliver the product from the time an order is approved.

An order has just been placed by Codelco in Chile and the Impalla mine in Spri ng, South Africa, Davis says. He estimates the market worldwide is in the order of $12 million with a potential repeat business of $2 million per year.