Detection and prevention of ferrosilicon corrosion in dense medium plants

A suspension of finely powdered ferrosilicon in water is the dense medium used in the gravity upgrading of iron ore, diamonds and other minerals. Ferrosilicon can be lost in the plant through corrosion. The greatest corrosion losses occur when the ferrosilicon is held in sumps during plant shutdowns. Such losses can be catastrophic with complete inventories lost and the production of a. large amount of easily ignited hydrogen gas. Residual oxidised alloy call form a cemented mass, the removal of which can cause a costly delay at start-up. An instrument has been developed for monitoring corrosion by measuring the mixed potential (E-m) of the settled ferrosilicon over time. A rapid increase in the rate of fall of E-m indicates the on-set of active corrosion, which is when ferrosilicon loses its passive layer and starts to react with water to produce hydrogen. Software has been developed incorporating an algorithm that analyses the electrochemical data in real-time to give sufficient warning to operators so that preventative action can be taken to arrest the corrosion process. The monitor was tested on different grades of ferrosilicon over a range of conditions using a small scale laboratory sump. A survey of inhibitors confirmed that sodium nitrite the basis of common proprietary inhibitor products, was the most effective in delaying the on-set of active corrosion in settled beds. Various techniques to arrest the active corrosion process were tested in the laboratory and a combination of aeration and nitrite addition found to be most effective. The combined monitor, alarm system and treatment technique together form a ferrosilicon corrosion prevention system that has been trialed successfully at several sites including BHP - GEMCo, Argyle Diamond Mines, Hamersley Iron - Tom Price and BHP Iron Ore Newman. By being able to detect when active corrosion is about to occur preventative action can be implemented, thus preventing further medium losses and reducing the time taken to remove cemented ferrosilicon at plant start-up.