Engineering & Mining Journal

NOV 2012

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FLOTATION Conventional Technology A wide range of bubble sizes, online control of froth level, turbulence-free operation and no need for a blower or compressor comprise a user-friendly system Pneumatic Flotation Challenges By Evren Ören, Lutz Markworth and John van der Heever The Pneuflot pneumatic flotation system was introduced in 1987, with the first in- stallation at Pittston Coal Co. in Pennsyl- vania for coal flotation. Since then, the tech- nology has been widely used in coal flotation for treating fine coal slurries, and also for industrial minerals, ferrous minerals, non- ferrous metals such as copper, lead, nickel and zinc, and for precious metals—plat- inum, gold and silver. The same size of equipment, with no changes in design, has been used in different applications all over the world, with the process eminently capa- ble of handling material sizes of 80% pass- ing 45, 180, 350, 500 or 1,000 µm. Initial market conditions, with high demand for steam coal and from coking- coal plants, were the driver for the former Humboldt Wedag Coal & Minerals Tech- nology—today's MBE-CMT—to concentrate on the energy industry, with a number of processing plants having been commis- sioned in China, India, Australia and Europe. Then in 2008, Pneuflot technology was introduced on an industrial scale to the potash industry in South America. This application proved that efficiencies were higher than with conventional agitator cells when treating coarse potash, with a com- plete bank of cleaner cells being replaced with a single Pneuflot cell. The technology has since been supplied to a North American producer for a similar application. Pneuflot's strong potential has also been recognized by iron ore producers in South America that have been evaluating systems for recovering highly abrasive, ultra-fine magnetite ore by removing silica gangue. These users have achieved better selectivi- ty, lower wear and lower energy consump- tion compared to conventional technologies. What Makes Pneuflot Different? In a conventional agitator-flotation system, bubbles and the opportunities for collisions between bubbles and particles are generat- ed by mixing. Air is normally introduced by a blower, although self-aspiration is also possible. The technology is widely accept- ed by operators, and small cells can be supplied by local manufacturers. It is less sensitive to feed variations, but changing conditions—in particular where base-metal minerals are being processed—make con- trol more difficult, especially with self-aspi- rating systems. A disadvantage is that wear and slimes generation can be high because of the high shear forces, while turbulence from the mixing process reduces the selec- tivity, especially for fine fractions. Column-flotation technology does not use an agitator, with the feed being intro- duced from the middle of a deep tank by gravity, and air being injected from the bot- tom of the cell with the help of a compres- sor. The particle-air collision time in a sin- gle cell is very low, so selectivity is very high. Multiple cells are used to get an acceptable yield. Some new technologies, such as recycling the underflow, can help cut the number of cells needed, but there are still constraints on throughput. Columns are sometimes used together with agitator cells to collect particles that can- not be recovered by the agitator technolo- gy. However, changing conditions can still cause problems, especially in relation to the gravity feed system. Pneuflot laboratory-scale test pneumatic flotation machine has a 50-liter-capacity conditioning agitator tank and can handle feed rates of 300–400 liter/h, while its pilot-plant version has a 2-m3 agitated feed/conditioning tank. 60 E&MJ; • NOVEMBER 2012 A pneumatic flotation cell operates on the principles of mixing the air and pulp in a continuous stream, and ensuring that cor- rectly sized bubbles are fed into the pulp as it enters the cell. This maximizes the num- ber of particle and bubble collisions. There are no rotational parts as in agitator cells, and unlike column cells, no further air injection is needed in most cases. In addition, all of the parts that are exposed to friction are made from special rubber and ceramic materials, which pro- vide much better wear behavior and long service intervals (sometimes up to eight years). Its ability to produce a very wide range of bubble sizes (0–1,000 µm) gives Pneuflot another advantage in that the www.e-mj.com

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