September 2013

Past the airy nothings

Finer grinding and tenacious froths are pushing pump manufacturers to redesign and reinforce the best practices in efficient froth pumping

By Eavan Moore

Froth flotation does a fine job of separating minerals from gangue, but problems surface when moving the products of that process. The tiny air bubbles in mineral froth stop up centrifugal pumps easily, coalescing at the pump’s inlet in a condition called airlock. The initial onset of airlock reduces the suction the pump can produce; as excess air builds, it blocks the pump inlet completely, causing the pump to falter until enough pressure builds up to burp out the air bubble. By that time, the pump sump may have overflowed. In addition, losses from the effect of froth on the pump force it to operate at a higher speed to produce the same pressure. As a result, froth pumps are often limited in the amount of pressure they can produce before the onset of cavitation, which can wear out pump parts.

Dealing with froth is increasingly important as mines see more tenacious froths, substances that hold tightly to their air content despite efforts to disperse it en route. “Froths from fine-grained concentrates [below 20 microns] are far harder to handle,” says Peter Woodall, principal engineer at Xstrata Technology. “Such froths are becoming more common as ores become finer-grained and need more regrinding.”

In 2011, GIW Industries announced changes to its horizontal centrifugal pump design in an attempt to mitigate this problem. Centrifugal pumps use a spinning vaned impeller – a fan with blades – to create low-pressure zones. As long as that pressure is sufficiently lower than the atmospheric pressure pushing on the slurry as it enters, slurry will be drawn through the vanes and toward its next stop in the process. An ideal pump design would suck the slurry, but not the air that makes the froth.

GIW drew on concepts pioneered by its parent company KSB in biological processing and the handling of viscous liquids to develop a design that delays the onset of airlock, produces more pressure, and vents air through strategically placed holes in the impeller, explains Robert Visintainer, the company’s vice-president of engineering and R&D. The main pumping vanes are of a design that disturbs the airlock while it produces more pressure at the same pump speed, allowing heads of 50 metres before the onset of cavitation. In addition to the first circle of vanes, which draws the slurry into the eye of the impeller, a set of vanes at the back of the impeller creates a second zone of lower pressure that selectively draws air away from the slurry to be vented out. The air passes through a de-aeration chamber before leaving the pump. GIW found that while the air venting system alone did not delay the onset of airlock, it held pressure losses constant at a level sufficient to keep the pump working.

Weir Minerals developed a similar design. The venting system Weir has dubbed CARS – continuous air removal system – is also based on a secondary set of vanes and has holes at the back of the impeller shroud to let air escape. Weir’s design uses slightly different vanes and puts the vent exit as close to the shaft centerline as possible, instead of sending it out at a tangent (as GIW’s pump does), because it was found to be the best way of keeping slurry out of the air stream.

Michael Bootle, senior design engineer at Weir, suggests the ability to vent air effectively may allow for the use of higher specific speed froth pump designs to generate higher head. These designs, which feature oversized inlets and open impellers of smaller outside diameter for shorter vane passageways of greater width, minimize the likelihood of “blocking” the impeller eye or vane passageway with an air bubble. In non-vented form, they have shown the ability to handle higher air volumes more efficiently than similarly sized standard slurry pumps and are sized based on a modified “froth volume factor” (a function of percent air volume rather than a “traditional” froth factor). They have, however, been limited in their ability to generate higher head, as the corresponding higher speed of these smaller diameter impellers promotes separation of air and slurry and the formation of an air bubble in the impeller eye and possible airlock. Intermediate, partial solutions to this problem with non-vented designs has been the development of larger diameter, lower specific speed froth impellers with elongated main pumping vanes extending into the eye and secondary splitter vanes to lower pump speed, minimize air and slurry separation, and enhance suction performance. With the introduction of venting systems, separation of air and slurry associated with higher speed may become an advantage, says Bootle. “If the venting is highly effective in removing air, it could be more desirable to separate as much air from slurry as possible,” he explains, “so that pump performance is improved and less air is pumped to subsequent pumps or down the pipeline.”

Alternative approaches

These horizontal froth pumps provide high-volume flow but they have competition from longstanding vertical models. Jan Andersson, technical director of slurry handling solutions at Metso, says vertical froth pumps outperform horizontal ones in many applications by centrifuging the aerated slurry in a conical tank while increasing the vortex with special impeller shaping that separates air from solids and liquids. “Airlock happens on horizontal and vertical pumps with the inlet facing downwards,” says Andersson. “On vertical pumps with the inlet facing upwards, it will not happen.” Instead, air rises along the vertical shaft through the open vortex and exits at the top.

FLSmidth has a different take on pump development. The company does not put resources towards developing new froth pump designs; instead, says Kenny Don, applications engineer at FLSmidth, traditional approaches to handling froth are all that are needed. “You can try to remove the air, but if you size the pump based on the froth factor flow (effectively oversizing the pump) and optimize the effective level of your sump, there’s no need,” he points out. FLSmidth Krebs’ millMAX pump range handles the majority of mineral froths without resorting to expensive pump designs.

Oversizing is done by taking empirical observations of the foam volume and tenacity of a given slurry stream, converting them into a “froth factor” between one and six, and multiplying the desired volume flow rate by the froth factor to get a design flow rate. Don says FLSmidth’s slurry pumps, if accurately oversized and fitted with a flared inlet to minimize pressure losses from friction, can outperform specialized froth pumps. He adds that FLSmidth has a special advantage: a patented adjustable wear ring that prevents slurry from recirculating back to the suction side of the pump and interfering with its performance.

Smaller-scale operations have other options than centrifugal pumps. Bill Hancock, owner of Zeroday LLC, notes that peristaltic hose pumps move both air and slurry by rhythmically squeezing them through a tube, avoiding airlock completely. These pumps can handle tough froths, while using less power and lowering maintenance costs. Higher upfront capital costs and volume constraints are their chief limitations. The largest of the models sold by Zeroday operates at a continuous rate of up to 462.3 gallons per minute (gpm). By contrast, Weir Minerals’ horizontal froth pump goes up to 14,000 gpm.

Hancock says that improved peristaltic hose pumps, which use rollers as the component that compresses the tube, only recently made inroads into North America. Gliding on greased bearings, the rollers are an alternative to shoe compressors, which generate friction, require higher torque, and put more stress on the hose.

The root causes

The pump itself is only one comparatively easy part of the froth handling challenge, points out Joe Pease, Xstrata Technology’s CEO. The entire plant needs to be designed to handle both expected and unexpected froth problems. That was the approach taken at GlencoreXstrata’s McArthur River zinc mine in Australia, where 50 per cent of the concentrate recovered is finer than 2.5 microns and the froth is extremely tenacious. As throughput increases, McArthur River is in a continual process of tweaking its methods for de-aerating froth – from sumps to pipes to pumps. “You need to get the ‘boring’ stuff right too,” says Pease. “The good news is that you can design to handle difficult froths, and it doesn’t cost that much. But it is important to design it in up front. Give yourself room to make changes.”

Pump manufacturers, too, are looking at how they can de-aerate slurry before it reaches the pump. Andersson says that is Metso’s current focus. At GIW, a third round of froth pump testing this year will focus on conditions inside the sump and how they affect what reaches the pump.

“Hopefully, this will contribute to better methods for managing the froth upstream,” says Visintainer. “There is plenty more to be learned here.”

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