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Separation magnetism

Figure 1. Suspended electromagnet. The magnet is suspended over the conveyor belt to

remove ferrous tramp from the coal prior to crushing.

Daniel Norrgran and Jaisen N.Kohmuench, Eriez, US, examine

new developments in coalpreparation equipment

Increasing energy demands andthe generation of new power plantcapacity worldwide have escalatedthe demand for coal. New levels ofcoal production have been reached inrecent years. Accordingly, there have

been new developments in processequipment focused on increasing therecovery of coal as well as providing andmaintaining a high level of productivity.

Eriez has made several advances inequipment specic for coal preparation

plants. The following are some of the

major developments in coal processingequipment.

Suspended electromagnets to

remove tramp iron and protect

downstream equipment from

damage and unscheduled

maintenance.

Wet drum magnetic separators

for the continuous recovery of

magnetite in the heavy media

circuit.

Hydraulic separators for the

upgrading of fine coal in the -2 mm

l

l

l

(-10 mesh) range.Column flotation cells for the

recovery of fine coal -0.150 mm

(-100 mesh).

Suspended magneticseparators for trampmetal removalThe practice of using magnetic separatorsto collect ferrous metal from process

streams has had a long history in themining industry. The rst application

and the most prevalent application today

l

I

REPRINTED FROM WORLD COAL APRIL 2007 www.worldcoal.com


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is the removal of ferrous tramp fromconveyor belts. This is a straightforwardapplication suspending an electromagnetover the conveyor belt. A typical exampleis illustrated in Figure 1. The magnetremoves tramp metal that representsa potential hazard to downstreamcrushers, mills, pulverisers and grinders.The magnet can also be mounted overfeeders or chutes.

There have been signicant

developments in the designand manufacture of suspendedelectromagnets. The 1220 mm to 1520mm (48 to 60 in.) wide coal conveyor

belts once considered large have evolvedto between 1830 mm and 2440 mm (72 to96 in.) wide. Suspended magnets haveincreased in size and magnetic eld

strength accordingly.

Suspended magnets -componentsThe electromagnet consists of anelectromagnetic coil with a cylindricalsteel core positioned in a steel housing.

The coil magnetically induces the steelcore, which in turn projects a magneticeld for the collection of tramp metal.

The coil and core are submerged intransformer oil to dissipate heat. Thecomponents are shown in Figure 2.

A power supply is used to energisethe magnet. The power supply convertsthe alternating current input to directcurrent output to energise the coils.

Suspended magnets -magnetic circuit designThe coil design and the subsequentpower draw are the two rst order

variables in the design of theelectromagnet and the projectedmagnetic eld. The coil must be

sufciently large in combination

with minimised power to allow forconvection cooling. Furthermore, thisdesign extends the life of the conductorinsulation and subsequently the life ofthe electromagnet.

Magnetic circuit modelling andoptimisation of the electromagnetis now carried out using multi-dimensional nite element analysis.

This methodology provides anaccurate depiction of the magnetic eld

conguration. The modelling provides

analytical assessment of the power input,

the magnetic eld strength and thethermodynamic properties.

Suspended magnets -magnetic feld strengthThe suspended electromagnet is typicallycongured with a cylindrical steel core

that is induced to project a magneticeld downwards onto the surface of

the conveyor belt. The magnetic eld

extends from the core out to the edgesof the magnet box. This magnetic eld

conguration approximates the cross-

section prole of a conveyor belt runningon idlers as shown in Figure 3.

Suspendedelectromagnet selectionThe foremost factor in electromagnetselection is the burden depth of thematerial on the conveyor belt. Notethat the belt speed, belt width, capacity,and bulk density all are factors in thematerial burden depth on the conveyor

belt. The burden depth and subsequent

idler height determines the suspensionheight of the magnet and consequentlythe effective magnetic eld strength

required at the conveyor belt surface.

Figure 4. Positioning of suspended electromagnets. In-line position is over the trajectory

discharging the head pulley. Cross-belt is over the conveyor belt prior to the head pulley.

Manual clean or self-cleaning magnets may be utilised. Pictured are self-cleaning magnets.

These magnets utilise a cross belt to discharge collected ferrous tramp.

Figure 5. Manual cleaning electromagnet 3810 mm (150 in.) wide designed for a 3050 mm

(120 in.) wide conveyor belt. The magnet was installed over the head pulley at a 40 in.

suspension height. This magnet weighs 59 t and operates at 35 kW.

Figure 2. Major components of a

suspended electromagnet.

Figure 3. Typical magnetic eld

conguration of a suspended electromagnet.


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The suspension height is the distancebetween the face of the magnet and theconveyor belt.

Suspended electromagnets aremounted in one of two positions over aconveyor belt as shown in Figure 4. Inposition 1, the magnet is mounted justover the stream of material leaving thehead pulley. This position utilises thefull potential of the magnet as it reactswith the material in suspended trajectory.Tramp metal is easily pulled throughthe suspended burden. Furthermore,the ow of material is directed towards

the magnet face. Collection of the trampmetal from the material ow does not

necessitate a change in direction. Atconveyor belt speeds of less than 350ft/min., the suspended trajectory of thematerial is minimal and becomes near

vertical. In this case the magnet must beshifted to a position approaching directlyover the head pulley.

In position 2, the magnet is mountedover the conveyor belt prior to the headpulley. This position requires highermagnetic eld strengths to attract the

ferrous component, shift the direction ofmomentum, and pull it through the bed

of material.Suspended electromagnets are

available as a manual cleaning style ora self-cleaning style. Manual cleaningmagnets are best suited where only small

amounts or occasional pieces of trampmetal are encountered. The magnet mustbe periodically turned off in order toremove tramp iron accumulation fromthe magnet face. Self-cleaning magnetsemploy a cross-belt running aroundthe magnet face to provide continuousremoval of collected tramp iron.

Experience dictates that the operatingmagnetic eld strength or the magnetic

eld strength at the belt surface should

generally be in the 500 to 600 gauss rangefor adequate ferrous collection. As the

magnet width increases to accommodatethe belt width, the magnetic eld

strength also increases. This is simplythe case that a wider magnet has a largercore and coil. By design, the larger coilhas more amp-turns and operates at ahigher magnetic eld strength.

Suspended magnets- high capacity conveyorbeltsVery large electromagnets have been

developed to accommodate increasingconveyor belt widths and capacities.Suspended magnets ranging up to3810 mm (150 in.) wide and appropriate

for conveyor belt widths up to 3050 mm(120 in.) are in operation. A largeelectromagnet is shown in Figure5. Electromagnets for high capacityconveyor belts require a thoroughassessment of the specic application.

These electromagnets are designed andengineered for continuous operationtaking into account the requiredmagnetic eld conguration as well as

the thermodynamic properties.

Wet drum magneticseparatorsOver the years, the perception ofwet drum magnetic separators hastransformed. Innovations in bothmagnetic circuit design and materials ofconstruction have been applied to wetdrum magnetic separators to maximise

magnetite recovery and minimise wearand maintenance. A wet drum magneticseparator that was once viewed as anecessary ongoing expense may now

be viewed as an avenue to signicant

cost savings to the operation. Wet drummagnetic separators commonly used inheavy media coal circuits are il lustratedin Figure 6.

Wet drum magneticseparators - feed

factors affecting drumperformanceIn a wet drum separator, the magneticforce acting on a ferromagneticparticle is predominately opposed byhydrodynamic drag force. This feature,when properly applied, provides thevehicle of separation washing awaythe non-magnetic particles while theferromagnetic particles are collected inthe magnetic eld. The hydrodynamic

drag force is also responsible for anylosses of ferromagnetism. The sizing

of the wet drum magnetic separator isprimarily based on the slurry volumeand the magnetic loading.

Wet drum magneticseparators -componentsMagnetic circuit modelling andoptimisation of permanent magneticcircuits is now carried out using multi-dimensional nite element analysis. The

input is a scale design of the magnetic

circuit; the output is a contour plot ofthe generated magnetic eld intensity

and the magnetic eld gradient. This

methodology provides an accurate

Figure 6. Wet drum magnetic separators

for heavy media application. Self-levelling

counter-rotation tank style.

Figure 7. Finite element analysis and

modelling of a wet drum magnetic element.

Contour plot details magnetic eld intensity

and magnetic eld gradient.

Figure 8. Wet drum magnetic separator

performance. Magnetite loss as a

function of feed rate.


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depiction of the magnetic eld

conguration as shown in Figure 7. The

North American industry standard isthe Eriez 950 gauss Interpole magneticelement.

The counter-rotation wet drumtank style is preferred for heavy mediaapplications. The drum rotates againstthe slurry ow in the counter-rotation

tank style. Any magnetite that is notimmediately collected will pass throughto a magnetic scavenging zone. The shortpath that the magnetic material must

be conveyed between the feed entrypoint and the magnetics discharge lip,combined with the magnetic scavengingzone, results in high magnetiterecoveries.

Essentially all wet drum tanks used inheavy media applications have levelling

spigots and a full width overow thatmust be maintained during operation.A deviation in the overow may result

in inefciencies in the performance and

the loss of magnetite. A modication

of the counter-rotation wet drum tankrepresents a recent development intechnology for heavy media wet drummagnetic separators. The self-levellingtank has no discharge spigots to adjust ormonitor and maintains a constant slurrylevel at any ow rate.

Wet drum magneticseparators - sizing andseparator parametersThe purpose of the wet drum magneticseparator is to recover magnetite in theheavy media circuit. When properlyapplied, the magnetic loss to the wetdrum non-magnetic efuent will be

reduced to less than 0.25 g of magnetite/l of efuent. This generally equates to a

magnetite recovery in the 99.8 to 99.9%range. In order to meet this performance,

a number of conditions must be satised,including magnetic eld strength,

tank design, and various feed slurryparameters.

Slurry volume is one primary factorin sizing wet drums. Magnetite recoveryis directly related to the unit capacity orow rate through the separator. As the

ow rate increases, the slurry velocity

and consequently the uid drag force

increases, which tends to detach orwash away magnetite particles from

the opposing magnetic eld.Hydraulic capacity is measured asm3/hr of slurry/m of drum width.

Magnetic loading or the amount

of magnetite in the feed slurry is theother primary factor in sizing wetdrum magnetic separators. Any givenwet drum magnetic separator has thecharacteristic of removing a limitedamount of magnetite based on thediameter of the drum, peripheral speed,and the magnetic eld strength. Magnetic

loading is measured as tph of magnetite/m of drum width.

In heavy media applications, themagnetic loading is based on a valuewhich will give the minimum acceptableloss to the non-magnetic product. Muchhigher loadings are possible, but willresult in progressively exceeding themagnetic tailings concentration goalof 0.25 g of magnetite/l of effluent. Athigh levels of magnetics in the feedslurry, a reduction in the feed rate

or unit capacity of the separator willbe necessary to achieve acceptablemagnetite recoveries.

The slurry sizing parameters as wellas the magnetic loading parameters forthe0.9 m and 1.2 m diameter wet drummagnetic separators are provided inTable 1.

Wet drum magneticseparators -performanceExtensive sampling of wet drum magneticseparators in heavy media applications

has indicated that the industry standardfor magnetite losses is 0.25 g ofmagnetite/l of non-magnetic product.Figure 8 demonstrates the typicalperformance of a wet drum magneticseparator operating in a coal preparationplant employing a heavy media circuit.The wet drum magnetic separator was 0.9m diameter with a counter-rotation styletank. This separator was isolated in thedilute heavy media circuit. The feed ratewas incrementally increased with samples

of the non-magnetic product taken at eachlevel. The magnetite level was measuredin each non-magnetic product sample.

Figure 11. Results of CrossFlow full-scale

evaluations versus washability data.

Figure 12. 4.6 m (15 ft) diameter CoalProcolumn otation cell with an adjustable

wash water system. The wash water removes

entrained clays from the coal froth.

Figure 10. Pilot-Scale results versus

washability. Comparison of the CrossFlow

separator to spiral concentrators.

Figure 9. Schematic diagram of the

CrossFlow hydraulic separator.


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CrossFlow hydraulicseparatorHydraulic separators are frequently usedin the minerals processing industry toclassify ne particles according to size,

shape or density. Although many typesof hydraulic separators exist, a devicethat has been gaining popularity inrecent years is the teeter-bed or hindered-

bed separator. The Eriez CrossFlowseparator has been developed as a newgeneration of teeter-bed separator. Itincorporates several design features toimprove process performance (separation

efciency and capacity) and reduce

operating costs (power consumptionand water usage). A schematic of theCrossFlow is provided in Figure 9.

Conventional hydraulic teeter-bedseparators inject feed slurry directly intothe teeter-bed. This causes turbulencein the separation zone resulting ina detrimental impact on separatorperformance. The CrossFlow design usesan improved feed delivery system thatgently introduces the feed slurry acrossthe top of the separator. This transitionalfeed system delivers the ow to the full

width of the separator so that the slurryvelocity, and anyassociated turbulence,is minimised. Thisfeed system alsominimises the effect

of feed variations andallows for a constantteeter-water velocitythroughout theseparator.

CrossFlowseparator- comparisontestingCentralAppalachia strip

coalTest work wasconducted using a pilot-scale CrossFlowseparator to treat coal from a US stripoperation. The material treated wasrelatively high in rock with a feed gradeof approximately 40% ash. Operatingin parallel with the CrossFlow was asingle re-pulping test spiral. Feed wassupplied to both units from an existingslurry distributor. Tests were conductedto create grade/recovery curves. Theresultant data is presented in Figure

10. As shown, the CrossFlow separatoroperates very close to the washabilitycurve. At maximum separationefciency, a product containing 12%

ash was produced at a mass yield of

78% and combustible recovery of 92%.In contrast, the spiral operated furtherfrom the washability data, providing asignicantly higher ash product for the

same combustible recovery.While spirals offer many advantages,

including high combustible recovery,they often suffer from misplacement ofcoarse rock to the clean coal product,as illustrated by the higher spiralproduct ash values presented in Figure10. Additionally, coal spirals typicallyoperate at a high specic gravity cut

point.

CrossFlow separator -production scaleSoutheast Appalachian coalA full-scale CrossFlow was installedand commissioned at a Kentucky coal

preparation plant. Performance data ispresented in Figure 11. The unit workedwell upon start-up and produced a cleancoal product with an ash content ofapproximately 10%. This was achievedat a combustible recovery of better than90%.

Three site evaluations were conductedin an effort to optimise separatorperformance. During the initial testperiod (set 1), the separator was simplysampled throughout the test period.

Figure 11 shows that the combustiblerecovery regularly exceeded 90% withproduct ash values ranging between 8%and 10%. During a subsequent effort(set 2), the objective was to improverecovery while maintaining productgrade. Evaluations were conducted whilerunning the unit at a high bed pressurewhile varying the elutriation water rate.

Test set 2 resulted in an incrementalimprovement in combustible recoveryas presented in Figure 11. The averagecombustible recovery and yield

improved by nearly 2%. The data setgenerated during this follow-up testingsuggested that additional performanceimprovement could be realised byfurther increasing the bed pressure.Generally, an increase in teeter-bed levelwill increase the effective gravity cut-point of the separator, thereby increasingcombustible recovery.

This approach was investigated ina third test (set 3). Again, as seen inFigure 11, the increase in bed density

improved the combustible recoveryby an additional 2%. In total, thissystematic approach resulted in anaverage increase in product mass yield

Table 1. Wet drum magnetic separator sizing parameters for heavy media application.

Counter-rotation style tank. Heavy media sized (Grade E) magnetite. Feed slurry at less than

15% solids as magnetite.

Drum diameter Sizing parameter

Hydraulic Magnetic loading

0.9 m

36 in.

90 m3/hr/m drum

120 gpm/ft drum

16 million tph/m drum

5.5 tph/ft drum

1.2 m

48 in.

130 m3/hr/m drum

170 gpm/ft drum

28 million tph/m drum

9 tph/ft drum

Figure 14. Traditional column otation circuit treating by-zero

coal. The feed to the column is typically -0.150 mm (-100 mesh).

Figure 13. Comparison of column otation

cell results to a conventional mechanical

otation cell.


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of over 4% or 5.4 short tph (5.9tph). For this installation, thisprocess improvement resulted in arevenue increase of more than US$2 million per year(5.4 short tph x 7000 hr/yr x US$55/t).

CrossFlow separator- parametersCrossFlow separators are sized

based on the feed solids rate.Capacity is generally rated astph of feed per m2of cross-sectional area. Material in the 2mm x 0.150 mm (10 x 100 mesh)size range provides excellentseparation response. The typicalcapacity for coal cleaning is20 to 30 tph feed/m2(2 to 3

tph/ft2) of cross-sectional area.CrossFlow separators have

been manufactured up to 3 m2.Typically a3 m2CrossFlow separator can treat180 to 270 tph of feed.

Column otationRecovering coal nes has typically

been considered problematic

due to high processing costs and poorotation performance. However, the

economical potential for ne coal

recovery circuits continues to grow as thevalue of coal continues to climb.

Traditionally ne coal otation has

been conducted utilising conventionalmechanical cells. These cells typicallyhave a low prole and are arranged in

series as a bank. While conventionalotation cells are well-proven and are

used extensively throughout the minerals

industry, their primary drawback for coalrecovery is the appearance of ne clay

in the clean coal product. In contrast toa conventional mechanical cell, columncells utilise an effective method offroth washing. This approach virtuallyeliminates the hydraulic entrainmentof clays resulting in high combustiblerecovery while maintaining maximumproduct grades.

Column otation -

descriptionUnlike a conventional mechanical cell,feed slurry enters a column cell andis distributed throughout the cross-

section of the cell. In thecolumn cell, the slurrymoves counter-currentto a rising swarm of ne

bubbles that are generatedby an air sparging device.Particles that attach to the

bubbles are carried up thecolumn and are dischargedto the product launder.Particles that do not attachsettle down through thecolumn and report to theunderow tailings stream.

The additional benet of

column otation is the

ability to apply wash waterto the froth column andeliminate clays from theproduct stream. An example

of an industrial-scale frothwashing system is presentedin Figure 12.

The primary advantageof column otation cells

is the ability to providea superior separationperformance comparedto conventional otation

processes. This capabilitycan be easily illustrated bythe test data summarised in

Figure 13, which comparesthe column otation technology with

an existing bank of conventional cells.The data for the column cells tend tofall just below the separation curvepredicted by release analysis which isan indication of the ultimate otation

performance for a given coal. In addition,the data from both a laboratory (5 cmdiameter) test column and a full scale(3 m diameter) column fall along nearlyidentical recovery-grade curves. Thisnding suggests that the recovery-

grade response obtained using smalltest columns can be used to condently

predict the performance of full-scalecolumns.

Column otation - circuittypesThere are two circuits commonly utilisedfor ne coal otation. This includes 1)

the traditional by-zero circuit treatingminus 0.150 mm (-100 mesh) feedmaterial and 2) the deslime circuit

treating nominally 0.150 x 0.045 mm(100 x 325 mesh) feed material.

In the by-zero approach, minus 1 mmfeed is sent to large diameter classifying

Figure 16. Microcel sparging system with essential design features.

Figure 17. Typical plant performance

compared to release analysis (by-zero circuit).

Figure 15. Deslime column otation circuit with additional

classication. The feed to the column is typically -0.150 mm + 0.045

mm (-100 mesh +325 mesh).

Figure 18. Plant performance compared to

release testing (deslime circuit).


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cyclones. Typically, these cyclonesare congured to make a cut-point of

approximately 0.150 mm (100 mesh).This circuit is illustrated in Figure 14.The overow stream of the cyclone is

sent directly to otation.

The second type of circuit, whichhas been gaining popularity, especiallywhen producing coal for the steammarket, is the deslime circuit. In thiscircuit, a secondary bank of 150 mm (6in.) cyclones is used to further classifythe otation feed at approximately 0.045

mm (325 mesh) in an effort to rejecta large portion of the ultra-ne clay

and/or coal particles. This circuit isillustrated in Figure 15. This approachcan be advantageous when the feed stockcontains little combustible material in thenest size classes.

Each circuit has certain advantages.The by-zero circuit will always provide

the maximum product tonnes. On theother hand, the deslime circuit hascontinued to gain popularity due toits simplicity and ease of operation. Inaddition, the removal of the ultra-ne

material results in a higher otation

capacity, reduces the number and/orsize of columns required, and yields aproduct with lower moisture.

The ability to consistently maintaina high level of performance in a columnotation cell is a result of a relatively

deep froth and efcient froth washing.Column systems must also maximisethe rate of bubble surface area movingthrough the column in order to providesufcient capacity. As such, the air

sparging system is perhaps the mostimportant component in a columnotation cell. Details related to the

specic design features of various air

induction and sparging technologieshave been presented in the literature. Thesparging systems recommended by Eriezare presented as follows.

SlamJet SpargertechnologyTwo different air induction systemsare used in the Eriez CoalPro columnotation cells. One system is the

Canadian Process Technologies SlamJet.

The other is the Microcel.The SlamJet sparger system uses a

series of removable air-lances whichinclude a large, single orifice locatedat the end of the sparger. High velocityair is injected into the column cell tocreate and disperse fine bubbles. This

technology incorporates a self-closingmechanism that eliminates the chancesof back-flow of slurry into the aerationsystem. The SlamJet technology is bestapplied on deslime applications. Thisis due to the high aeration rates andslightly larger bubble size afforded bythis bubble generation approach.

Microcel technologyPerhaps the most common spargingtechnology used in the coal industry is

the Microcel system. This technologywas conceived after research showedthat the rate of otation could be

enhanced through the use of smaller airbubbles. The small (

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