Magnetite Standards and Testing Procedures (1)

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Magnetite Heavy Media Standards and Testing ProceduresM. W. Mikhaila; D. G. OsbornebaCoal Preparation Section. Fuel Processing Laboratory, Coal Research Laboratories, Canada Centre for

Mineral and Energy Technology (CANMET), Devon, Alberta bCoal Preparation and Handling. PTKaltim Coal of Indonesia,

To cite this ArticleMikhail, M. W. and Osborne, D. G.(1990) 'Magnetite Heavy Media: Standards and Testing Procedures',International Journal of Coal Preparation and Utilization, 8: 3, 111 121

To link to this Article DOI 10.1080/07349349008905179URL http://dx.doi.org/10.1080/07349349008905179

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Coal Prepururion. 1990 Vol. 8. pp. I I 1-12Photocopying permitted y license onlyMinister of Supply and Servim Canada. 1990.Printed in the United Kingdom

M ag ne t i te Heavy M ed ia : S tandardsand Test ing ProceduresM . W MIKHA IL and 0 G. O S B O R N E tReceived Murch 20 1990, infinul form April 6 . 1990

Magnetite heavy media pr- are armrate. efficient and ey y to control separators. capable of produc-ing high quality clean coal. ,Magnetite recovery circuit operation and magnetite co ~ u m p ti o n rc the mainre sons for high capital and 'op rat ing costs. Ovcr the last two decades. several co un tri a have k e n activelyinvolved in establishing magnetite specifications for coal heavy media circuits and in developing workingstandards for t a t magnetite.

his paper reviews media recovery circuits. operating parameten for magnetic separators. magnetiteconsumption and specifications for heavy medium magnetite. Itdiuu methcds and instntmmts usein the determination of those physical and chemical properties of magnetite. and rhcological prop eni n ofmagnetite suspensions which arc relevant to the application of heavy media separators. Thep w alsooutlines the development of the international standards for magn etite and m agnetire suspensions fati ng.Key w r r Magnetite. heavy medium. huvy medium control, heavy medium recovery. internationalstandards.

INTRODUCTIONThe importance of heavy medium circuits in comparison with other cleaning proces-ses has increased sicnificantlv in the last t wo dec ade s T he reason is the need for sham .efficient and easy-to-con trol systems to meet the deman d for high quality clean coal.The heavy medium bath handles coarse coal, usually in the lOmm size range, andthe heavy medium cyclone clean coal in the SM 5mrn range. Most of the recentlydesigned preparation plants include the heavy medium cyclone circuit to clean the0.5 mm coal because of its accuracy in the sep aratio n of fines. Also, recent attem ptsto clean the -0.5 mm coal by heavy medium cyclo ne have shown promisin g results.'The use of heavy medium for fine coal processing increases the consumption ofmagn etite and dem and s certain specification s of fine magnetite to achieve optimum

*Coal Preparation Section. Fuel Proe atin g Lab ontory . Coal Rcsearch Laboratories. Canada Centre forMineral and Energy Technology (CANMET ). Dcvon. Albe rta'*Coal Preparation and Handling. T K al tim C d f Ind on es ia .


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M W MlKH lL and D SBORNE

separation. More efficient magnetic separators have been introduced along withimproved rinsing capacity an d elaborate media recovery circuits to minimize magne-tite losses. Magnetite resources are abundant as primary deposits, particularly inCanada. However, the market for magnetite for heavy medium use is limited andcannot support the operation of a magnetite mine. As a result, magnetite presentlyused for heavy medium is a by-product of existing metal mines or is reclaimed fromtailings.Several countries have been'actively involved, in the last two decades, in establish-ing magnetite specifications for heavy m edium applications. These countries includeAustralia. Britian, Can ada, German y, Netherland . South Africa and U.S.A. M any ofthe tests used were comm on but some were specific to certain cou ntries. The recentlyavailable I S 0 stand ard on the subject is based on experience gained over many yearsby several countries. The objective of this paper is to review the standards andstandardized testing procedures utilized to characterize the m agnetite and the heavymedia prepared with the use of such a megnetite. Since the characteristics of themagnetite recirculated in a coal preparation plant depends on the media recoverycircuits? such circuits are also reviewed in this paper. Th is paper is based on a detailedstudy carried out by Kilborn Engineering (Vancouver) on behalf of CANMETentitled Characteristics of Can adian M agnetite Resources for Optim um Utilizationin Coal Heavy Media Processes .

MEDIUM CIRCULATION CONTROL AND RECOVERYEfficiency of separation in heavy media circuits depends upon magnetite mediarheology and stability. This subject is discussed in anoth er paper.' Following theseparation in the cyclone or vessel, the medium ad hering to the p rodu ct is rinsed awaywith water. The diluted medium is then directed to the recovery system which inc bd esthe following:1 Rinsing water (recirculated within the pliant).2. Mag netite rinsed from the products.3. An overRow or bleed from the heavy media slurry feed.4 Fresh magnetite to compensate for loss.

The discharge of the recovery system circuits of concentrated medium (magnetiteplus water) and non-magnetic tailings is a very dilute slurry, which is bled off aseffluent and utilized for rinsing water or sent to tailings thickener. There are threealternative media recovery circuits (recommended by the Dutch State Mines, nowStamicarbon) used universally in heavy medium plants:I. Single-stage magne tic separation an d magnetite thickening (Figure 1) which is

~ s e d hen the feed to classifying cyclone contains a magnetite concentration ofless than I00 g/l (1.08 g/cm 3).2. Two-stage magnetic separation with cyclone thickening prior to the secondarymagnetic separation (Figure 2) which is applicable when the dilute medium

concentration i; in t he rang e of 250 g/l (I . 19 glcm').


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MAGNETITE HE VY MEDIA

FIGURE Media r ov q system employing magnctitc thickener

3. Three-stage magnetic separation with cyclone thickening before the second andthird m agnetic separato rs (Figure 3) which is used when a m edium with a relativedensity too low to effect the required separation is fed to a magnetic separator.Recommended (by Stamicarbon) operating parameters for magnetic separatorsare:

Concurrent drum separator with permanent ceramic magnets.250 g/l maximum feed concentration to the se para tor.Maximum solids content (magnetic and no nma gnetic) less than 16 by volume.Minim um flux density of 750 Gauss (75 milli tesla) in air at a distance of 50m mfrom the drum.The relative density of recovered magnetite of 2.1 g/cmJLosses of magnetite not to exceed 0.3 g/l.The density of the medium has to e kept constant within certain limits byautomatic means in order to obtain the required optimum separation. T he density ofthe medium entering the separating vessel, or cyclone, is usually measured by a

Qd

FIGURE Media recovery system employing magnetic xpardton


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M W MIKHAIL and D G OSBORNE

LI LI IFIGUR 3 Media recovery syst m for low field concentration

nuclear density gauge. There are a number of design alternatives for locating thesensor which measu res the correct set) med ium den sity in the recovery circuit. In anycase, it is essential to have a control circuit that responds quickly to changes inmedium density.M G N E T IT E C O N S U M P T I O NThe consum ption loss) of magne tite is influenced by several factors, of which themost important are:a) The fineness of the m agnetite. Finely grou nd magnetite exhibit better stability butmay also show higher viscosity; generally magnetite fineness improves heavy

medium separation efficiency but since finer panicles a re m ore difficult to recover,it may lead to higher losses.b) The fineness of the treated coal. Increased surface area of fine coal paniclesincreases the am oun t of m agnetite atta che d t o coal particles which is difficult torinse off.c) Th e efficiency of the magnetic sepa rator. Fac tors such as Gau ss rating, positionof permanent magnets, feed volume, percentage of magnetics in the feed influencethe efficiency.d) The design of the magnetite recovery circuit - single, two or three-stage separa-tion and use of dilute m edium cyclones versus thickeners.e) General poo r conditions of ope ration and maintenance problems.

generally accepted figure of magnetite losses for European. eastern U.S. andSouth African co al preparation ope ration s would be 0.15 kgltonne of fine coal1 0 4 . 6 mm) and 0.05 kgltonne for coarse coal + 10 mm ). However, fo r C anadiancoals reported losses are mu ch higher 0.75 to 3.0 kgltonne ). This is largely attribut-able to the presence of a high propo rtion of finer panicles due to coal friability) wherethe average particle size of the 10-0.6mm fraction is 1.8 mm. It is wo nh noting tha tmagnetite consumption should be quoted on raw coal fed to the heavy medium


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M GNETITE HE VY MEDI Scircuit. The amount of medium adhering to the products on a sieve bend plus drainscreen can be calculated from the following empirical equation:'

where Q medium in [/tonne productd =av era ge particle size of product in mm (weighted average size from screenanalysis)p average relative density of product

Mass of solid M ) n g llitre of pulp (m edium) is calculated as follows:

where p density of magnetitep density of pulp.

S P E C I F I C A T I O N S F OR H E A W M E D I U M M A G NE T IT EThe following brief and general specifications, based on the British coal miningindustry standards and accepted world-wide are:J.6.'a) Particle-size distribution Maximum 5% by mass larger than 45 pm and 30%by mass sma ller than 10 pm.b) Relative density 4.9 to 5 2 g/cml.C Magnetic conte nt Not less than 95% by mass.It is also usually required that magnetite moisture content be less than 10%.ME THO DS U SED IN THE DETERMINATION O F MAGNETITEPHYSICAL PROPERTIESP a r t i c l e sizes1 Dry Screening: An autom atic sieve shak er is operated w ith 20 3 of magnetite for10 minutes using three screens with apertures of 53, 45 and 3Spm.1 Wet Screening: A wet vibrating sieve apparatus (NCB design) can be used toseparate the same amount of magnetite as dry screening ( 2 0 9 . A 1 5p m screencan be included to cover a wider range than dry screening. Th e sample is wettedbefore screening and gently sprayed during the process. The minus Z5pm is

screened over lOum using a Fritsch Analysette sieving ap par atu s with alcohol a sthe suspended medium because of its low surface tension.3. Cyclosizer: A Warman Cyclosizer with five cyclones in series is used with wateras the med ium with magnetite sample. Th e precision of separation improves withthe increase in elutriation time, but 10 minutes is often adequate.4 Sedim entom eter Size Analyzers:


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6 M W MIKH IL and D G OSBORNEa) Sedigraph: It is based on gravitational x-ray panicle size analysis. Th e magnetitesuspension is pumped upward in a sample cell for detection by x-ray. The x-raydetector gives signals as a function o f the Stokes diam eter of magnetite suspension inthe sedimentation (sample) cell. The range of the application is 1 to 50pm.b) Photosed imentom eter Size Analyzer: It is based on concen tration measurementsof a suspension settling unde r gravity, panicle size is calculated using Stok es equation.Mag netite has to be demagnetized by heating to ab ove 600C prior t o testing. The sizerange of application is from 2 to 100pm.5. Microsco pic Size Analytical Techn iquesa) Optical Microscopy: It measures the diameters of particles using a calibrated fibremicrometer eyepiece. The range of panic le~siz emeasurement is 1 to 150pm but theaccuracy is dependent on operator's skill. More recent automatic microscopes withimage analyzers can be used to accurately determine the size.b) Laser Size Analyzer: The size distribution of particles is determined in thesuspension flowing through a sam ple cell which is illuminated by a co ntinu ous laserbeam. The suspended particles scatter the light which is collected by a series of lensesand a rotary optical filter. The measured panicle size range is usually flexible anddependent on using certain detector head.M a g n e t i c c o n t e n t an al yz er :1. Mag netic Chute: It includes a sloping and hinged non-magnetic tray containinga permanent magnet which holds the magnetite in a flowing slurry. The hingedtray is lifted away from the m agnet and the magnetic particles are washed off for.analysis.2. Davis Tube: It is a laboratory un it used specifically for determining the ferro-magnetic a nd non-magnetic fractions with field intensity ranging from 400-800Gauss that would give consistent results. It includes a sloping non-magneticframe which contains a permanent magnet unit above which a hinged tray isfitted. The magnetic panicles ar e held by the magnet when slurry is poured in tothe tray. The magnetics are then removed from the tray.for analysis.3. I S 0 Method: Th e ap par atu s consists of a vertical glass tube (8 mm in diameter)with two permanent magnets at the centre of the tube and both the tube andmagnets (IS0 8833). The apparatus is filled with the magnetic suspension. Thecon tents are then drained i nto a lOOml beaker while magnets are moved u p anddown to capture magnetite panicles. This magnetite is later on washed intoanother beaker after the tube is drained.D e n si ty m e a s u r e m e n tI Bechman Pycnometer: The apparatus functions on the basis that a sampledisplaces an equal volum e of air. Two cylinders of the same volume a re used, one

s a reference and the other for measurement, each having an identiel movablepiston. The measured volume of a sample of known mass is then used to calculatedensity.2. Density Bottle: A 50ml capacity density bottle is used to measure the volume ofdistilled water displaced by a 20 g magnetite sam ple ( IS 0 8822).A summary of the above procedures and equipment is shown in Table 1.


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'MAGNETITE HEAVY MEDIATABLE l

Methods used in the determination of the physical propenia of magnetiteMethod or Equipment LimitationsPmpefly

Panicle size Dry screening Only for panicles larger than 5 pmWet screening Many variables introduced. forpanicles above 5 pmCyclosizcr Time


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Stability

M . W MIKHAIL and D . G SBORNETABLE 2

Method used in the determination of the rheological prope nia of magnnitePropeny Method or Equipmen t LimitationsViscosity Stormer virom etcr Reliability only achieved bycustom modificationDeVaney Shelton Undefined shear raw doesconsistometer not allow precise rhmlogicalmeasuremenuCapillary virom eter Non homogm eous movementof panicla affensflour n capillaryRotational viswmetn Commercial un ts am notsuitable to work withwdimenting systems

S Index ube Operator influcncr very significantDRL U rube apparatus Not developed to commercialu=ee

Settling Tests N o standardized methodavailableand time produces the settling rate. The slope ob&ed during the free-settling zonerepresents the settling rate in mm/min with typical rates of 10-20mmlmin.Magnet ic properties testing:Magnetite's m agnetic property is not a rheological property, however, it influences therheology and stability of the magnetite slurry.Satmagan Balance: It measures the deflection in the mass of a magnetic materialupon .being placed in a magnetic field. The susceptibility of a certain amount ofmagnetite in a glass tube is measured by a deflector indicator when the sample isexposed to magnets located at certain distance from the sample.Vibrating Sample Mange tometer: It measures magnetic susceptibility saturationmoment, remnance and coercive force.Demagnetization testSimon -Carves Dem agn etiza tion M ethod:' A slurry of 1.6 relative density is placed ina 100cc grad uated m easuring cylinder and tho roughly apitated. After the settling rateis measured. the cylinder is magnetized ip a field of 6 5 kA/m. and the settling rateremeasured. After that, the cylinder is demagnetized and the process repeated.A summ ary of the rheolopic procedures and equipment" is shown in Tables 2 an d3.

MAGNETITE CHEMICAL ANALYSISChemical analysis is used to distinguish magnetite from other sources of iron andincludes:a. Determ ination of the total iron content using I S 0 2597 volumetric me thod .b. Determ ination of the ferrous iron content by dissolving the sample in hydrochlor-ic acid unde r nitrogen a nd titraring ferrous iron from the resultant solution usingpotassium dichromate.


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MAGNETITE HE VY MEDITABLE

Methods w to characterite mapetic propertiesMethod or Equi pment Limitations

Magnetic Satmagan balanceSurepdbditylCoerciveF o r a Vibrating mag-netometerThennogravimetricEaton apparatus

Demagnetization Simon-Carvemethod

Single-point me surementof susceptibilityNot developed to commercialgeMesures hysteresis curve onlyand requires calibrationReference is HgCo SCN)SO414 which must ecarefully preparedOperator influence. butreproducibility isreasonable

More detailed chem ical analysis would only be required fo r new sources of mediato determine the following: SiO,, AI,O,, TiO,. P, S, Ba, Cd, Zn. Ca. Mg Ni, CO, CU,Au and Ag usually by x-ray fluorescence analysis.

DEVELOPMENT O F INTERNATIONAL ST AN DA RDS FOR MAGNETITETESTINGIndustrial experience and practices by several coal producing countries have beenused to develop standard procedures to test magnetite for coal preparation purposes.Th e International Stan dard I SO/D IS 8833 (submitted in November 1986) specifiesthe following properties for testing:a. Moisture content.b. Particle size distrib ution .c. Magnetic con tent.d. Funda mental magnetic properties.e. Relative density.f. Tota l iron content.g. Ferrous iron content.

All of the above properties may need to be determined for a new deposit but fora known supply of magnetite a. b. c. and e are usually sufficient. The followin g isa summ ary of tests performed on sam ples from several mazn etite deposits in Canad a(a, b. c, and e) most of which are in compliance with suggested IS0 standards:a. Moisture Content: Total moisture content can be determined either directly(100g sample) or in two stages (I kg sample) in compliance with the 6 0 .8 8 3 3metho d, where the free moisture and air-dried moisture con tents are determinedseparately. The latter method is, however, mainly applicable to large quantities

I kg sample). Samples are dried in laboratory circulated air-oven a t tem-peratures of 105-1 IOC.


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120 M. W lKHAlL and D. G. OSBORNEb. Particle Size Distribution: The size analysis should comply with I S 0 2591, wherethe plus 38 pm fractions are wet screened and the minus 38 pm fraction is screenedusing a sub-sieve centrifugal classifier or a Warm an Cyclosizer.c. Magnetic Conte nt: The magnetic chute with a fixed magnetic field strength(perman ent ceramic magnets) is recommended (i.e. similar to draft I S 0 8833 testmetho d). The multiple pass procedure (usually two passes) ensures total recoveryof magnetic material.d. Relative Density: The set pycnometer method on which the volume of liquid isdisplaced by a known m ass of sample (draft I S 0 8833).

P R O P O S E D R & D W O R KDeterm ination o f the rheological properties of a magn etite suspension is the principalarea that requires further research as indicated in most major coal producing mun-tries. Tw o main subjects are investigated:a. Medium rheology in relation to p anic le size distribution, fundam ental magnetiteproperties, the presence of non-magnetic materials such as clay minerals andviscosity reducing chemical additives.b. Medium stability, the methods for its determination and effect of chemical aidson the stability.

Wh ile (a) is being specifically directed to wards sop histicated sc ientific investigationsrequiring advanced viscometric methods. (b) is mainly concerned with obtaining areliable method for quantifying the effect.'There are several methods to determinemedium stabiliry F5 ndex, Simon Carves settling method an d the Czech oslovakianJ-Index); these tests, however, are very empirical and therefore prone t o opera tor b ia sAs a consequence, all methods currently employed should be comprehensively ev-aluated before a suitable method is adapted for standard use.R e f e r e n c e sI D. G. Osborne. Fine Coal Cleaning by Gravity Methods: A Review of Current Pramice , w lPrtpararion 2.207-24 1 (1986).2. Characteristics of Canadian Magn etite Res our ce for Optimum Utilization in Coal Heavy McdiumProeessts, Pan 1. Survey of Mametic Medium Coal Cleaning , CAN MFT w nt ra n repon prep redby Kilborn ~ ng inec rine 1986)-3 Characteristics of Canadian Mag naitc Resources for Optimum Utilization in Coal Heavy MediumTesting and Analysis Stages . CANMET wntran repon prepared by Kilbom Engineering (1986).4 T . J. Napier-Munn, The ERffl of Dense Media Viscosity on Separation EBcirncy , C o al P r r p a r qrion this issue.. Densc Medium Coal Preparation . S u n o n a m Ltd. Publications (1968).6. T. E. Abrey, A Note on the Use of Magne tite in Coal Prepa ration in Great Britain ; Mining ndMarrrials Engineering 25-29 (1972).7. Specification for Magnetite , The NCB Code of P r a n ia for Coal Pre par at~o n Draft)(1985).8. C C. Graham. The Application of Magnetite for Coal Rrpara tion in Australia . F ~ n tntmnadonalSME-AIME Fall Meeting. Honolulu. Hawaii (1982).9. T. Allen. Panic le Size Measurement . 2nd Edition. Chapman and Hall. London (1978).10. R. Bosold. D etermination of Magnetics Content of Mag naite Standard Pr od ur e . Pcnekc.Homer City Coal Laboratory (1983).


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MAGN ETITE HEAVY MEDIA 121I I N. F. Schulz. Determ ination of the Magnetic Separation Characteristic with the Davis MagneticTubc . Truns Soc. .Winins &n ineering. pp. I ? 16 1964).12. The Testing of Mag netite for Ux n Coal Preparation . Draft Inrernntional S ta n d a h ISOIDIS

8833 (1986).13. L. Jonker. The D evelopment of Standard R oc ed um for the Evaluation of Magnetite or Uw nHeavy Medium Separation . Mintek Repon. No. MIU. Republic of South Africa (1984).14. H. F. Yancy. M. R. C m r and M. Sukaski. Viscosity. IU Mc asu me nr and Impoflance in mwMedium Cleaning of the Fine Sizes of Coal . 3rd Inte rnatio nal Coal Prc para tion Congress. Liege(1958). Paper FI, pp. 584.IS. 8 Klein. S. J. Partridge and J. S. Laskowski. Rheolonv of Unstable Mineral Susansions . a l--Prepararion. this issue-16. K. Kam orniczyk . D. Gleisbcrg and M. Reia. Test M ethods to Determ ine Viscosity. Stability andCorrosion Behavior of H nv y Medium Suw nsio ns Consisting of Ferrosilicon, Ferrochmme. andMagnetite . Knapsack A. G. Cologne, &netall (1973). -17 C. C. Grah am and R. Lamb. Coal Preparation in D en x Media Rheology: 4 Review of Measure-ment and Control . A CIRL Research Repo rt (Australia). P.R. 82-3 (1 981).

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Magnetite Standards and Testing Procedures (1)