Bacteria and catalysts
22 May 2003
Bacteria could soon have a major role to play in recycling precious metals from spent automotive catalysts and other industrial wastes.
Scientists at the University of Birmingham have made a patent application for a new process in which the organisms get to work recovering the valuable materials more quickly and cheaply than conventional methods.
In the automotive industry, the technology could help to overcome future shortages and increased prices of platinum group metals (PGMs) as demand outstrips supply. The technology integrates metal removal from spent automotive catalysts and PGM recovery in a simple process.
Research was carried out as part of a major waste minimisation initiative backed by government and industry. The WMR3 programme (Waste Minimisation through Recycling, Re-Use and Recovery in Industry) is spearheading new ways to clean up the environment and make better use of resources. The research project tested a novel reactor in which bacterial action speeds up metal recovery by up to 50%.
'This new technology is a world first and it represents a very significant advance in recovery of platinum group metals,' says Professor Lynne Macaskie, leading investigator at the University of Birmingham's School of Biosciences. 'Bacteria make the process quicker and faster, and that gives us a real edge when it comes to recovering these valuable, strategic metals.'
Today's catalysts last for around 50,000 miles. Increasing numbers are now being scrapped but recovering the tiny content of PGMs is difficult and expensive using conventional recycling technology. In the UK, most spent catalysts are treated in furnaces with other scrap material or shipped overseas for recycling.
To find a better solution, three departments at Birmingham collaborated with industrial partners in the three-year project. Specialists from the School of Biosciences and departments of Chemical Engineering and Metallurgy & Materials Science received support from EA Technology (now called C-Tech Innovation Ltd) and test samples from Degussa. An initial grant of £192,000 and an additional £252,000 for follow-up studies were awarded by the Engineering and Physical Sciences Research Council.
A key advance is a type of electrochemical cell, known as an electrobioreactor, for which patent application has been made. It resembles a hollow pencil, whose outer casing supports the bacteria. When the 'pencil' is immersed in a continuous flow of waste, metal is deposited onto the cell walls of the bacteria. If left to build up, the deposit will drop off to the bottom of the vessel from where it can be recovered.
Tests show the process to be fast and efficient; the process takes only about 15 minutes under typical operating conditions, continuously removing PGMs with approximately 90% efficiency.
Follow-up studies showed that palladium that has been 'biorecovered' in this way performs better as a catalyst than its chemical counterpart. It can even treat pcbs (polychlorinated biphenyls), a major environmental pollutant, which chemically prepared palladium is unable to do.
'We have demonstrated a steady-state, continuous process for recovery of fine-quality PGM that can be used directly as a chemical catalyst,' says Professor Macaskie. 'The results of the WMR3 represent a big step forward. Recycling technology will now be able to keep pace with increased demand.'
Further research is now being carried out by two EU-funded research projects and a Faraday partnership between Birmingham, Imperial College and Cambridge Universities, together with C-Tech Innovation, working with a network of companies with waste problems.
