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Recent advances in magnetic separator designs and · PDF fileRECENT ADVANCES IN MAGNETIC SEPARATOR DESIGNS AND APPLICATIONS 65 By feeding directly on to the magnetic roll, rather

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  • RECENT ADVANCES IN MAGNETIC SEPARATOR DESIGNS AND APPLICATIONS 63

    Existing technologyMagnetic separation equipment for minerals processinggenerally falls into three basic categories: low, medium andhigh intensity, based on the relative magnetic field strengthemployed to accomplish separation. Low intensity magneticseparators (LIMS), are generally wet separators and arecommonly used for concentration of magnetite, or forscalping of ferromagnetic materials. The LIMS are highcapacity units and simple to operate, so they serve theserelatively straightforward applications well. Mediumintensity magnetic separators are most commonly dry, rare-earth based drum magnetic separators (RED) with commonapplications in highly paramagnetic mineral applicationsincluding ilmenite, chromite and garnet processing. TheseRED designs, similar to LIMS, are robust and typicallyoffer high capacity and simple operation. High intensitymagnetic separators may be wet or dry. High intensity drymagnetic separations are carried out with induced-roll(IRM) or rare-earth roll (RER) magnetic separators, thoughthe latter is far more common (Dobbins, Sherrell 2009).The RER is known for efficient separations with weaklyparamagnetic minerals, albeit it at somewhat lower capacitythan low and medium intensity magnetic separators. RERunits are most commonly employed in cleaning zircon,silica sands and a variety of other industrial minerals. Wet,high intensity magnetic separators (WHIMS) generate highfield strengths and pass slurry through matrix arrangementsto collect the magnetic fraction. WHIMS are traditionallyapplied in mineral sands to collect ilmenite in the earlystages of a flowsheet, and have a host of other commonuses including iron ore (haematite) beneficiation.

    Room for improvementFeed characteristics

    Finer particle sizes A common problem in minerals processing is the efficientrecovery of mineral values in the fines fraction, commonly

    thought of as

  • HEAVY MINERALS 200964

    magnetic fines to travel and adhere to the magnetic surface,under the belt, which can be detrimental to both belt lifeand separation efficiency. While there, this material cancause premature belt wearing, and, as it builds in thickness,increase the distance from the magnet to the particles beingseparated on the outside of the belt. While RER machinesare relatively low maintenance, belt wear issues related tofines within the feed can become costly, and in dustyapplications, belt replacements can average near 40 per centof annual RER maintenance costs (Dobbins and Sherrell,2009).

    Static charging

    Static charge build-up between the surface of the RER beltand fine particles can also be detrimental to separationefficiency. When feed travels the length of the belt to themagnetic zone, particles rub together creating a staticcharge (triboelectric charging) that can cause the fine non-magnetic/non-conductive particles to adhere to the beltsurface and interfere with separation efficiency (Figure 1).Ionizers can be used to disrupt this charging phenomenon,but in many cases do not completely eliminate theseparation efficiency impact. (Dobbins and Sherrel, 2009).

    Feed bed make-up

    Compaction of the feed bed on the RER belt can lead tonon-magnetic particles becoming held within the bed ofmaterial, and segregation of the feed bed can lead to finerparticles reporting to the bottom of the material bed (Figure 2). Both compaction and segregation reduceseparation efficiency by preventing the release ofnonmagnetic particles from the belt and thus increasingentrainment of non-magnetic particles into the magneticstream.

    WHIMS concerns

    Particle collection capacity

    During WHIMS processing, the balance betweenhydrodynamic drag force and magnetic attraction force tiltsin favour of the drag forces, so maintaining adequatecapacity requires a higher magnetic field gradient to attractweakly magnetic particles. As the magnetic force increases,its effective range decreases, so more points of collectionare required to maintain effective particle collection. Withthe commonly used grooved matrix plates, even at 12grooves per inch (a common size for particles near 100 m), there simply are not enough collection points toeffectively overcome the opposing physical forces withinthe magnetic zone. The only combative solution is to reduceseparator throughput to avoid large reductions in separationefficiency.

    Matrix plugging

    In some cases, to overcome capacity issues, different matrixmaterials (e.g. steel wool) are used to increase the numberof collection sites. Unfortunately, more collection pointsmeans smaller gaps within the matrix. Once the matrixvolume exits the magnetic field, the traditionally reliedupon forces of gravity and water flushing to free magneticparticles are often not enough to overcome matrix pluggingby larger particles, or matrix blocking caused byconglomerations lodged as lumps into the empty spaces ofthe matrix. Additionally, magnetic flux leakage in

    conventional electromagnet designs can generate asufficient residual magnetic field to inhibit magneticparticle release, even while it is in the washing position andoutside of the magnetic zone. These particles can reside inthe matrix volume until the magnetizing current is turnedoff. A blocked or partially blocked matrix leads to areduction in throughput and efficiency, and will result ineventual machine shutdown for aggressive matrix cleaning.

    Particle misplacementMatrix material configurations that allow for morecollection points also have stronger magnetic forces. Whenthese matrices are magnetized and collecting multipleparticles at each high-gradient trapping site, non-magneticparticles can also become trapped. This entrainment occurswhen fine non-magnetic particles are held close to thematrix by magnetic particles and prevented from flowingthrough and reporting to the non-magnetic product. Particleentrainment can lead to a reduction in grade and recovery,potentially causing an increase in recirculating loads orrequiring additional stages for cleaning or scavenging(Dobbins and Hearn, 2007).

    Recent technology advances

    High efficiency rare-earth roll separator

    Feed systemThe proprietary feed system on an Outotec high efficiency(HE) rare-earth roll is optimized to handle standard feeds orfiner feeds (

  • RECENT ADVANCES IN MAGNETIC SEPARATOR DESIGNS AND APPLICATIONS 65

    By feeding directly on to the magnetic roll, rather thanfurther back on the belt, belt residence time is reduced. Thisreduction in residence time minimizes any tribocharging, orstatic build-up, of non-magnetic materials, thus preventingthem from sticking to the belt. Additionally, a lowerresidence time means the feed bed stays fluid andhomogenous entering the separation zone allowing non-magnetic material to more easily leave the magnetic roll.Optimum position of the feed point can be determinedduring laboratory testing.

    Dust controlSeveral remedies to the dust dilemma have been employedin RER processing over the past few years, such as de-dusting of the minerals prior to separation and adding purgeair to the volume between the magnetic roll and the idlerroll. the new HE RER has taken these remedies a stepfurther: The entire frame, hopper and door systems arespecifically designed to contain fine particles inside themachine. Internally, the units are equipped with a tunable,per-stage dust extraction design, as well as purge air in thecassettes to minimize dust issues. The combined result ofthese improvements is a much cleaner and safer operationalenvironment, improved part life and sustained separatorperformance.

    Process dataTwo heavy mineral samples were tested with the conditionsshown in Table I. The ilmenite sample showed similarseparation results between the two feed points at loweryields (Figure 4). At higher yields, the separation improvedwhen the sample was fed directly on the magnet roll withless zircon reporting to the magnetic product. The zirconsample also showed improvement when feeding directly onthe magnet roll compared to traditional belt feeding with areduction in both the % Fe2O3 (from 0.074% to 0.048%)and % Al2O3 (from 0.59% to 0.48%) at similar yields(86%). The % TiO2 in the zircon non-mag product wasunchanged at 0.084%.

    FluxForcehybrid rare-earth magnetic separatorThough the new HE RER design is a great improvementover conventional RER units, some problems still exist.Static charging issues have been greatly reduced and dustissues are much more controlled, but these remedies do notcompletely address the machine design drawbacks,especially when finer particles, which are becoming morecommonplace, are present. The use of belted roll

    technology, and the openness required of these designs, issimply not ideal for all applications, but, until recently, noalternative existed.

    Outotec (USA) Inc. has completed the first prototype of anew design that provides a solution to some of these moredifficult applications. The developmental FluxForceTMhybrid rare-earth magnetic separator is a cross betweenrare-earth roll and drum magnetic separator (Figure 5). TheFluxForceTM utilizes an ultra thin and flexible shell thatcomes in direct contact with a smaller diameter magneticroll, enabling the belt and idling roll assembly to beeliminated. This magnetic roll is similar to those used in thetraditional and HE RER units, offering the same magneticstrength. The ends of the unit are sealed like an REDdesign, so no dust can collect on the roll. In effect, theFluxForceTM takes the best of both designs and mergesthem into one machine. This allows RER level fieldstrengths to be applied to applications where previoustechnology was unacceptable due to contamination frombearing wear and belt material.

    Enhanced FluxForceTM feeding arrangementThe stepped nature of RER machines, brought about bythe