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
By Evren Ören, Lutz Markworth and John van der Heever
The Pneuflot pneumatic flotation system was introduced in 1987, with the first installation at Pittston Coal Co. in Pennsylvania for coal flotation. Since then, the technology 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—platinum, 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 capable of handling material sizes of 80% passing 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 Technology—today's MBE-CMT—to concentrate on the energy industry, with a number of processing plants having been commissioned 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 complete 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 selectivity, lower wear and lower energy consumption 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 generated by mixing. Air is normally introduced by a blower, although self-aspiration is also possible. The technology is widely accepted 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 control more difficult, especially with self-aspirating 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 selectivity, especially for fine fractions.
Column-flotation technology does not use an agitator, with the feed being introduced from the middle of a deep tank by gravity, and air being injected from the bottom of the cell with the help of a compressor. The particle-air collision time in a single 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 cannot be recovered by the agitator technology. However, changing conditions can still cause problems, especially in relation to the gravity feed system.
A pneumatic flotation cell operates on the principles of mixing the air and pulp in a continuous stream, and ensuring that correctly sized bubbles are fed into the pulp as it enters the cell. This maximizes the number 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 provide 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 same equipment can be used for all flotation stages, from rougher through cleaner to scavenger. The technology is widely accepted by coal, potash and iron ore producers, with trials having taken place for other minerals. However, pumping and feed-sump designs need to be done properly to ensure a successful application.
Following successful applications in ferrous metals, there has been increased interest in Pneuflot from the base- and precious-metals industries. The technology consistently delivers higher selectivity compared to conventional agitator-flotation systems, and higher recoveries in fewer stages than column flotation. Because of this, MBE-CMT has been receiving more requests from its customers for pilot-plant testing for base- and precious-metal applications, following on from bench-scale testwork.
Before an industrial-scale flotation plant can be designed, laboratory-scale testwork and—where possible—semi-industrial pilot testwork is needed. The pilot plant can be installed anywhere in an existing circuit as a 'plug and play' system, and the results generated can be scaled up directly to a full-scale application.
Responding to its customers' requests, MBE-CMT now offers a full on-site testing program. The Pneuflot laboratory machine has a 50-liter-capacity conditioning agitator tank and can handle feed rates of 300–400 liter/h, while the pilot-plant version has a 2 m3 agitated feed/conditioning tank. Ore-based testwork typically uses a feed concentration of 400 g/liter, with 20 kg of feed material being needed for each test run on the laboratory machine, rising to 400–800 kg for the pilot-scale unit.
Scaling Up for Industrial Applications
Since the early 1990s, Pneuflot systems have been commissioned successfully for industrial minerals, non-ferrous ores, iron ore, rock salt and potash. MBE-CMT produces industrial-scale machines in sizes ranging in diameter from 800 mm to 6 m. Slurry feed rates of 10–1,400 m3/h (4–560 mt/h of dry solids) can be processed in a single cell, depending on its diameter. The number of cells required for each stage of the flotation process (rougher, scavenger and cleaner) is calculated from the testwork results, with the company designing the flowsheet accordingly.
Pneuflot technology works very effectively at lower operating costs than for column or mechanical-agitator flotation, producing higher yields and metallurgical recoveries, especially when used on ultra-fine feed material. Turbulence in a Pneuflot cell is comparatively low, and the bubble size range in the self-aspirated aerator can be reduced by changing the feed-slurry rate—using a frequency converter on the motor's power supply.
Treating the final tails from existing flotation plants can present major challenges that a Pneuflot system can help to overcome. For example, using a Pneuflot unit has upgraded a 0.2-g/mt gold tailings feed to 5 g/mt gold at 2% yield and 18% metallurgical recovery. Similarly, the technology has been used to treat ultra-fine platinum ore grading 5.1 g/mt (0.15 oz/ton) that could not be processed using conventional technology to give 20 oz/ton in the final concentrate.
Proving its Case
MBE-CMT's sister companies and the main research institutes have successfully installed laboratory testwork-scale Pneuflot machines in Brazil, South Africa and Russia, with MBE-CMT holding a large database of lab- and semi-industrial-scale test results for a variety of different minerals. As an example of the testwork that the company can undertake, one of its customers asked for its help after experiencing high gold losses in a copper oxide flotation circuit. In the testwork, the samples were ground sequentially, then run through flotation.
In the first run, the ore was treated using a Humboldt Wedag-type agitator flotation machine, which recovered 30 g/mt of gold after one rougher stage (d100 = 71 µm) followed by two cleaner stages. Metallurgical recovery was 47.3%. The same type of ore was then run through a lab-scale MBE-CMT Pneuflot cell, using the same reagent regime. A rougher and two scavenger stages produced a concentrate grading 32 g/mt gold at and 92% metallurgical gold recovery, with subsequent semi-industrial pilot tests having confirmed the results of Pneuflot lab testwork.
Testwork using pneumatic flotation from deposits in North America, Brazil, Chile, Russia, South Africa, Namibia, Kazakhstan and China is reporting results that should offer food for thought for operators in the base- and precious-metals industries. Technical advantages such as the wide range of bubble sizes, online control of the froth level and turbulence-free flotation make Pneuflot a technical leader.
Evren Ören is a process development manager and Lutz Markworth is a senior process engineer with MBE Coal & Minerals Technology GmbH in Germany. John van der Heever is a process and pro-
duct-development manager for MBE Minerals SA in South Africa.