Dr Jie Wu, who has led the research team developing the technology at CSIRO Manufacturing and Infrastructure Technology in the Melbourne suburb of Highett, says the nozzle technology was developed after problems were identified with ‘sparging’ gases into liquids – forcibly blowing gases such as oxygen or nitrogen into a liquid, where some of the gas dissolves.
He says sparging is a common but not particularly efficient technique for dissolving gas into liquids within many industries, including gold production, mineral sands processing, pharmaceuticals manufacturing and brewing.
“When the gas goes through a liquid, only a small amount of the gas is dissolved,” Dr Wu says. “Most of the bubbles just rise to the surface and go into the air as waste.”
By enhancing the rate at which the gas is absorbed into the liquid as bubbles pass through it, the overall efficiency of the process can be significantly improved.
To that end, the CSIRO nozzle makes smaller bubbles, which are absorbed more easily. “If we make them smaller, we have more surface area (compared to total volume) and therefore we can increase the rate of transfer of gas into the liquid,” Dr Wu says.
Creating smaller bubbles, however, requires the gas to be pumped through the nozzle at a high velocity, which can erode the nozzle material. “So our nozzle has a unique design, which does not cause nozzle material erosion while operating at high velocity to produce smaller bubbles.”
Trials of the nozzle started at the Stawell Gold Mine – owned by the Melbourne-based Leviathan Resources – in 2003. The nozzles are being used in ‘leach’ tanks where oxygen is bubbled into a gold ore slurry to help dissolve the gold so it can be recovered.
The slurry contains a highly reactive sulfide mineral. Adding oxygen helps make it less reactive and so reduces the amount of expensive cyanide needed to dissolve the gold.
Results from the trials show the nozzle cut the amount of oxygen needed by up to 25 per cent in some types of slurry – such as those containing basalt. The average reduction was 13 per cent.
The nozzles are likely to save the company more than $35,000 a year in each of the two leach tanks at the processing plant.
Peter Wemyss, senior metallurgist at Stawell Gold Mine, says the nozzle has greatly simplified the oxygen injection system being used at the plant and will pay for itself within three months. He says the nozzle also seems to wear extremely well, with no wear evident on a 55-millimetre nozzle after a year of operation.
The nozzles are being made under licence by Melbourne engineering company Chemical Plant and Engineering Ltd. They are expected to be on the market soon.