Researchers have developed a quick and reliable test for assessing the ability of foaming agents to suppress acid mist formed during the industrial electrowinning of zinc.

The test could be used to check the mist-suppressing performance of plant samples and for quality control of foaming agents, according to the Parker Centre.

Caroline Hughes observing the foaming tank
for measuring acid mist suppression.

Bubbles of oxygen and hydrogen are evolved at the electodes during zinc electrowinning from sulfuric acid solutions. When these bubbles rise to the surface they burst, spraying acid droplets into the air.

This acid mist if uncontrolled is a hazard to worker health and can also corrode equipment. Foaming agents are added to produce a foam blanket which smothers mist formation.

Some foam covers are better than others at suppressing acid mist. A Parker Centre team has studied what properties of a foam - such as foam height, structure and coverage of the surface - are important for an effective cover. The work was undertaken for the Pasminco Hobart Smelter which sought to achieve consistent suppression while minimising foaming agent addition as, in general, foaming agents reduce electrowinning efficiency.

The team designed and constructed a novel small-scale foaming tank that mimicked an electrowinning cell. Foaming agents and spent electrolyte samples supplied by Pasminco were added to the glass tank which sat in a water bath to control the temperature. Foam was generated by flowing gas through frits on the tank bottom and the foam height and coverage recorded on video camera.

An air sampler and filter membrane were used to monitor the acid mist. The aerosol formed above the tank is the same as the solution of acid and zinc in the tank. Hence the acid in the aerosol is indicated by the zinc content which is easier to assay. "So we would run the tank without any foaming agent in it and measure how much zinc was emitted, run the tank with foaming agent and measure the zinc emitted and then determine the percentage suppression," said the team leader, Caroline Hughes of CSIRO Minerals.

While you might think the frothier the better, the tank tests showed the height of the foam was a poor measure of mist suppression ability. However, the percentage of the electrolyte surface covered with foam was closely related to the measured suppression.

This means mist prevention can quickly be checked by looking at the foam coverage over the tank surface, providing a rapid laboratory test to ensure the foaming agent in a plant sample is working as it should.

Pasminco Hobart currently uses a mixture of two foaming agents, licorice and tutogen. Both agents by themselves produced 100% foam coverage and acid mist suppression in tank tests. But licorice outperformed tutogen and twice as much tutogen was needed to obtain effective suppression.

Licorice had originally been used on its own at Pasminco Hobart. Tutogen had then been added because it was believed to be more stable than licorice above 38^oC. "We showed that wasn't true," said Hughes. She said that in the tank, both tutogen and licorice suppressed more than 99.7% of the acid mist generated at temperatures between 35^oC and 43^oC. The performance of both dropped sharply on increasing the temperature from 43^oC to 45^oC.

These results suggest the electrowinning temperature could be pushed up a few degrees above 38^oC to decrease cell voltage and power consumption without destroying the foam and causing an acid mist problem.

The team found that licorice bubbles are different from tutogen bubbles, which affects how they work. Hughes said the bubbles in a licorice foam are larger flatter bubbles while a tutogen foam consists of small spherical bubbles. "Licorice bubbles tend to decay away sidewards, merging together to form larger bubbles which maintains foam coverage," she said. "Tutogen bubbles tend to pop upwards into the air, resulting in holes and patchy coverage."

Hughes said this difference means that with licorice alone, a huge height of foam is not needed to be safe from acid mist - one layer of bubbles was sufficient to maintain full foam cover. More tutogen is required if used alone, so that multiple layers form and bubbles are always present below holes.

The researchers also demonstrated that in the foam of a tutogen/licorice mixture, tutogen inhibits the merging of licorice bubbles into larger bubbles. This explains how the presence of tutogen in the mix reduces the possibility of gas explosions which can occur when larger bubbles bring greater amounts of hydrogen and oxygen together.

"A combination of licorice and tutogen, as used at Pasminco Hobart, therefore represents a good compromise, achieving the better coverage properties of licorice while limiting the formation of excessively large bubbles," said Hughes.

By Ros Dilworth, Communications Officer, the Parker Centre.
First published in the Minerals Gazette.