Chemical Technology May 2015

Monitoring of tailings storage facilities could provide better levels of confidence that the facility is performing as was intended if regu- lar monitoring data measurements were compared to expected values rather than just being checked for changes, according to two young engineers at SRK Consulting (SA). In a presentation to the South African Young Geotechnical Engineers conference in Stellenbosch late last year, civil engineer Andrew Derrick and geotechnical engineer, Waldo Kruger argued that traditional monitoring techniques did not generate a tangible measure of ‘degree of belief’ - es- pecially after a number of highly publicised tailings dam failures in recent years. One of the factors behind this issue was that the regulated hazard profile for tail- ings dams was based on the Department of Water and Sanitation’s protocol for the inspection of water retention dams. “While the depth and volume retained by a water storage dam tends to remain relatively steady, the same cannot be said of a tailings dam,” said Derrick and Kruger. “Tailings dams are constantly changing, and these changes are essentially detrimen- tal; as they get higher, the potential risks increase and there is progressively more chance of operational errors.” These dangers are aggravated by the fact that the dams do not generate revenue directlytofundtheirownmaintenance,andare often treatedasan irritationby ownerswhoare seldom familiar with the geotechnical aspects of these structures, they said. To check on the stability of tailings dams, current monitoring practices include site in- spections, meetings and analysis of material, geometric and climatic data - as well as check- ingon theseepage regimeand thewater table. Standpipe piezometers are installed to Call to make tailings dams safer around 80 ºC, making it possible to leach directly at the mine and hence provide a seamless transition from oxide ore to sulfide concentrates. While existing atmospheric leach processes leach a maximum of 95 % copper in 20-60 hours, the FLSmidth ® Rapid Oxidative Leach process can leachmore than 98%of copper fromconcentrates containing as low as 8 % copper in less than 6 hours; a faster and much simpler process. And that, as Head of Group R&D, Jens Almdal, points out is a giant leap in produc- tivity improvements: “For a copper mine with remaining lowquality copper deposits produc- ing around 200,000 tons of copper per year, our 3% output increase wouldmean an extra USD 40 million per year with a copper price around USD 6 600 per ton. That is indeed

FOCUS ON MINERALS PROCESSING AND METALLURGY

An aerial photograph taken of a tailings storage facility as part of an aerial inspection which forms part of an active monitoring program at the facility

measure water levels within the facilities, and these reflect the efficiency of the drainage system; large changes in the piezometric level, then, may indicate a possible blockage of drains. “This information, however, is only a record of changes within the dam - implying some formofconsistencyorinconsistency,”theysaid. “Thereadingsarenotcomparedtoanexpected value that measures the performance against the design.” So,whilemonthlydrainflowmeasurements are useful for checking on individual drain performance,theinformationdoesnotindicate the level of risk or the probability of failure. “Monitoring programmes should be de- signed in such a way that fieldmeasurements are compared to anticipated performance,” said Derrick and Kruger. “An investigation should be done into the establishment of a database of monitoring information, so that an empirical model could be developed from the data. Expected drain flows could then be obtained using empirical data.” They also argued for an analytical model that could be calibrated against the empiri- cal model and the database; while admitting that this could be a difficult task, they were a significant contribution to profitability in an industry that normally chases 0,1-0,2 % increase in copper recovery by design modifications of flotation machinery,” Jens Almdal states. For copper producers, the new technology could mean a renewed business case for re- maining deposits: several existingmines have lost the ability to make copper concentrates suitable for smelting. The FLSmidth ® Rapid Oxidative Leach technology makes it pos- sible to recover more copper from low grade concentrate. Also, there aremineral deposits yet to be developed that are unsuitable for smelting but with the new technology would be suitable for leaching. The new technology also makes it pos- sible to develop mineral deposits containing

confident that this would allow for a more reliable quantifying of the risks inherent in tailings dams. “In this way, a systems approach could be applied to the monitoring of tailings stor- age facilities, and this would enable owners, regulators and practitioners to re-assess our ‘degreeof belief’ onanongoingbasis - instilling more confidence in our monitoring practices,” they said. Andrew Derrick, civil engineer and Waldo Kruger, geotechnical engineer, SRK Consulting (SA) arsenic for recovery of copper, gold and silver, while complying with stringent environmental air and land pollution regulations. As the new technology operates at atmospheric pressure, a concentrate can be treated at themine location, with complete control over the arsenic-bearing residues generated after leaching. This makes it possible to avoid the potential of arsenic contamination of sea, air and land while en route frommine to smelter. “A lot of existing mines have piles of cop- per concentratewithmore than 0,5%arsenic concentrations which is too high for smelting. Our technology can be used to process these piles of high arsenic concentrate or even make it possible to develop new mineral deposits high in arsenic,” explains Manfred Schaffer, President, Minerals division. Forfurtherinformation about FLSmidth, please visit www.flsmidth.com.

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Chemical Technology • May 2015