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GRAVITY SEPARATION jects in polymetallics, which have not been a traditional gravity-separation area, and with gold sulphides being more reliant on gravity separation. We believe this to be the future, especially in coarser separation," he said. "For instance at Pirquitas [Silver Standard Resources' mine in Argen- tina], we did an operation for silver, tin and zinc pre-concentration ahead of the mill in which we upgraded the ore by 30%–40%," he added. "Applications in polymetallics are starting to in- crease, and certainly in all types of sul- phide recoveries." Gekko has developed its primary gravity-separation system, the Inline Pressure Jig (IPJ), over the last 17– 20 years, Gray told E&MJ;, noting it has become a reliable continuous separator that can recover minerals down to the 100 µm. "The uniqueness of the jig allows it to be used in applications where there is a broad range of sizes to treat, and it is extremely useful in grinding circuits," he explained. "The other focus is the recirculating loads and gangue rejection with this unit." "The key target area is in gangue rejection," he said. "We are not only focusing on the recovery of the valuable minerals into a small concentrate. Much emphasis is being placed reject- ing gangue materials before spending energy on them—discarding them early in the process cuts the costs of energy, time and chemicals used down-stream. Every ton of gangue removed from the system early allows a further fresh ton to be treated, which increases the metal value delivered to the down- stream process for the same level of energy absorbed." According to Gray, Gekko's jigging technology was developed predomi- nantly for production in extremely low- grade, high-tonnage operations and for recovering gold. However, early re- search and development work highlight- ed the potential for high recovery of sul- phides and other heavy materials, lead- ing the company to develop its InLine Spinner to recover free gold from IPJ concentrates that are typically heavy with sulphides and free gold. Designed for heavy concentrate feeds, the spin- ner deliberately exerts relatively low 'g' forces on the feed and has different separation mechanisms compared to other bowl-type centrifugal concentra- tors, he said. "Coarse liberation and gravity is, in our mind, the way of the future, because of its reduced energy con- sumption throughout the process, and the focus on primary pre-concentration and gangue rejection ahead of the milling system," Gray stated. "Gekko's Python technology allows for all the concepts of gravity, pre-concentration and gangue rejection into one package. Developing the Python flowsheet and modular plant allows for a low capital and operating cost system with pre- designed flexibility." Designed as a compact, modular processing system, Python consists of a process train encompassing coarse and Air Classification Enhances Gravity-Separation Effects In the most recent edition of its customer magazine Results, Metso Minerals reported on two applications where its air- classification technology has helped customers achieve prod- uct-quality results by using centrifugal force to enhance grav- ity effects. In the first, Arizona-based Salt River Materials Group uses Metso centrifugal air classifiers to produce fine fly-ash for use as a feed material in the local ready-mix con- crete and other markets. The raw fly-ash obtained from coal-fired power stations con- tains a range of particle sizes, while the company's customers need coarse particles removed. Salt River now has three plants in operation, with a combined capacity of around 1.45 million mt/y, using classifiers that are designed for separations in the range 20–100 µm (635–140 mesh). Metso notes that wear in the classifiers is minimal, since there are no moving parts, that adjustments to the cut point can be made by fine-tuning the airstreams, and that the concept is suitable for other industri- al-mineral and fine-grinding applications. 56 E&MJ; • NOVEMBER 2012 In the second case study, Luck Stone Corp. uses Metso gravitational inertial classifiers to produce finely graded engineered sand from rock produced at its quarries in the eastern U.S. Faced with the disadvantages inherent in wet processing systems, the company evaluated a number of dry-processing routes before selecting this type of classifi- er. Metso explains that it now offers three classifier sys- tems: gravitational classifiers, which produce 0.15– 1.65 mm (12–100 mesh) separations and are suitable for coarse industrial minerals; centrifugal classifiers, which produce 0.02–0.15 mm (100–600 mesh) separations and are suitable for industrial mineral, mining, fly-ash and cement applications; and gravitational inertial classifiers, which produce 0.063–0.3 mm (50–230 mesh) separations and are used for precise engineered-sand applications. By using these, and Metso air classifiers, Luck Stone now pro- duces engineered sand to meet both asphalt and concrete industry specifications. www.e-mj.com fine crushing, screening, gravity and/or flotation separation, concentrate han- dling and a tailings-disposal system. By pre-concentrating underground, the company says, transport volumes can be reduced and costs cut. In the most recent edition of its Footprint newslet- ter, Gekko describes an updated version of the Python that has been re-engi- neered to handle gold recovery from surface waste-rock dumps. Gold Fields has been a long-term user of its equip- ment, according to Gekko, with an underground Python system at its Kloof mine in South Africa, and plans to use a surface Python to recover gold from its KDC East waste-rock dumps. Challenges and Solutions Gray identified several challenges being faced when working with gravity separa- tion technology. Critical to success, he said, is getting the testwork right. It is essential to ensure the samples are rep- resentative due to the generally low number of samples tested in most pro- grams. The success of testwork to the application is dependent on the reliabil- ity of the sampling to provide an accu- rate representation of the orebody. A second area of concern is the skill level at mine sites. Gray noted that there is an inherent need to ensure that the skill level and education of the oper- ators is taken into account, and that there is a focus during the commission- ing process to allow for adequate opera- tor training time. This is critical to the success of the plant, he said, adding