More efficient removal of iron and silica impurities and better control of hydrometallurgy through an understanding of the underlying chemical processes are among the outcomes of a 3-year research effort in the Parker Cooperative Research Centre for Integrated Hydrometallurgy Solutions.

Iron and silica impurities in many metal ores, including zinc, copper and gold, cause headaches in a wide range of hydrometallurgical processes. The particles are so fine – between 3-7 billionths of a metre – and precipitate so quickly that they can be a nightmare for filtering, settling and dewatering.

A major focus of the Parker Centre’s research in this area has been on ferrihydrite, which is a precipitated iron oxy-hydroxide. Ferrihydrite is the least crystalline of iron oxide phases and is particularly difficult to filter.

"Our aim was to develop a fundamental understanding of the chemistry of ferrihydrite precipitation, so we could recommend strategies for better control over its formation in specific hydrometallurgical processes," explains project leader Dr Bill Richmond of Curtin University of Technology.

The result is a significant improvement in understanding of the ways ferrihydrites form, leading to potential gains in efficiency in the hydrometallurgical process – including simplifying it and so lowering costs.

"We found we could control the crystal structure of the ferrihydrite and also, to some degree, its aggregation, which is important for removing it," Richmond says. The researchers, in collaboration with other Parker Centre researchers at CSIRO Minerals, have devised a mathematical model to describe the process of ferrihydrite precipitation.

Better control of ferrihydrite precipitation and separation of the target metal also means less water needs be used and discarded from the hydrometallurgical process, improving its environmental sustainability.

Another finding was that zinc sulfides can be used to accelerate the transformation of ferrihydrite to other forms of iron oxide that are more easily removed from the brew.

The research has also had important wider spinoffs. The small size and large surface area of the ferrihydrite particles means they can be used to mop up other undesirables, such as arsenic, cadmium and mercury – and potentially, to lock them up for safe disposal, reducing the environmental impact of hydrometallurgical metal extraction.

This property has also suggested a possible commercial use for ferrihydrite – normally a waste product – as a filter for extracting contaminants such as arsenic from drinking water.

During the research the team also made an important fundamental discovery of a new precipitated iron oxide phase. This has potential use as a catalyst due to its stability.

And finally, the research may even help fight human disease, Richmond says. Because our bodies also make use of ferrihydrites to do things such as maintain iron in the blood, the research is throwing new light on the basic processes involved. This could be important in combating diseases caused by iron imbalance; and a better understanding of how living organisms handle ferrihydrites may in future help to design better hydrometallurgical processes that imitate them.

For further information, contact
Dr Bill Richmond, tel: (08) 9266 3402, e-mail: W.Richmond@curtin.edu.au.

By Julian Cribb, Senior Editor, ScienceAlert.
First published in Australia’s Mining Monthly in January 2006 as part of the Cutting Edge features series.