Control of the acidic aerosol resulting from electrowinning operations

CIM Bulletin, Vol. 81, No. 915, 1988

J. VAN DUSEN, A. PAPACHRISTODOULOU and J.W. SMITH ,Department of Chemical Engineering and Applied Chemistry , University of Toronto, Toronto, Ontario

As part of a study to evaluate factors affecting tankroom air quality, the mechanism of aerosol generation from bursting bubbles in electrowinning operations has been investigated. Typically 2 to 3 mg/m2.s are produced in zinc tankrooms. Analysis shows that the rate of production of film drops (geometric mean diameter ~ 2 fim) increases with decreasing bubble size, and that jet droplets (geometric mean diameter —20 fj.m) are not produced with gas bubble larger than ~ 4 mm. Both mechanisms of drop formation and current efficiency are affected by surface active agents, with an over-all reduction in emission of up to 60% to 70% being achievable. A source control system which does not adversely affect electrolyte properties is always preferred, and coalescence of all gas bubbles in to a continuous stream of gas at a single point on the electrolyte surface would eliminate aerosol emissions. This objective was the basis for design of a coalescence device which, in laboratory tests, reduced the emission rate by over 95%. The control device fits over the electrodes in the electrowinning cell and is partially immersed in the electrolyte. A number of changes in the design which resulted in significant improvement in the performance of the device are reported. Field tests have shown that only about 75% of the aerosol below 10 turn can be eliminated by coalescence. The remainder (up to 95% of the total) is controlled by coalescence of droplets, by impaction on the exposed collector surface and gravitational settling. The device does not interfere with operation of the tankroom, handles the effect of surges in power during harvesting, and is simple and inexpensive to maintain.

Keywords: Air quality, Electrowinning, Coalescence, Health, Safety, Environmental control