THE AMERICAN MINERAI,OGIST, VOL. 41, JULY AUGUST, 1959
THE I\,IAGNETIC SEPARATION OF SOI'IE ALLUVIAL I,IINERALS IN I'IALAYA*
B. H. FnNrant, Minerals Eramination Diaision, GeologicalSurttey D epartment, F'ederotion of M al,aya. Assrnlcr This paper presents the results of a seriesof magnetic separationswhich have been in- vestigated {or a number of minerals occurring in X{alayan alluvial concentrates.The pur- pose of the investigations was to establish,by the isolation of individual mineral species,a reproducible and reliable method for the identification and quantitative estimation of minerals in alluvial concentrates examined by the Geological Survey in Malaya In par- ticular was sought the isolation of columbite from ubiquitous ilmenite. All the separations were made on the small, highly sensitive Frantz Isodynamic Model L-1 laboratory separa- tor, The minerals which have been successfully separated include ailanite, anatase, andalu- site (and chiastolite), arsenopyrite, brookite, cassiterite,columbite, epidote, gahnite, garnet (pink), ilmenite, manganeseoxide (51.6/e Mn), monazite, pyrite, rutile, scheelite,siderite, staurolite, thorite, topaz, tourmaline, uranoan monazite, wolframite, xenotime, and zircon.
PnocBpunp When using an inclined feed, the Frantz Isodynamic separator (see Figs. 1(A) & (B)) hasthree inherent variables. These are the field strengLh (current used),the sideslope, and the forward slope.
5;6s $lope wdrd 511)Pe
(A) (B)
Irc. 1. Diagrammatic representation of side slope and forward slope.
The field strength is increasedby means of a rheostat which raises the current from zero in stagesof 0.05 amps. to 1.4 amps. Early in the investigationsit was decidedthat stepsof 0.1 amp. would be sufficiently gradual. The side slope can be varied continuously from 0 to 90o and can also be given a reverseslope, as much as 90", for the diamagneticseparation of minerals such as zircon. Results of a few separations made by my
+ Published by permission of the Director, Geological Survey, with the authority of the Minister of Natural Resources. Government copyright is reserved. 738 MAGNETICSEPARATION OF ALLUVIAL MINERALS 739 coileagueswhen the separator was firsl installed showed that, for the purpose of these investigations, side slopesvarying by lessthan 5o would produce no significant differentials, and further, that side slopesgreater than 15oshowed more promise, as regardsthe separationof columbite from ilmenite, than did slopesof less than 15o.Accordingly a seriesof tests was carried out at side slopesof 15o,20",25", and 30o. The forward slope and a fourth, independent, variable, the rate of feed, control the rate of flow of the sample.The steeperthe slope,the faster is the flow. The rate of flow of a sampleis not critical provided that it is slow enoughto allow the mineral to remain long enoughin the magnetic field for the magnetic pull exerted by the separator to be fully effective. A forward siopeof 15owas chosenfor, and maintained throughout, the investigation. Much more critical is the uniformity of grain size of the sample,as the larger grains tend to "suppress" the finer material. For the purpose of simple identification and the estimation of mineral percentagesin an alluvial concentrate a grain size of 0.4 mm. or 0.0166 inch (36 mesh B.S.S.)is considereda satisfactoryupper limit. With larger grainsdiffi- culty is experiencedin distinguishing amongst the opaque minerals and between the opaque minerals and the translucent minerals such as gahnite, cassiteriteand rutile. The grain sizeof 0.2 mm. or 0.0083inch (72 mesh B.S.S.) was selectedas a suitable lower limit. This gave uni- formity to the sample and provided finer material for a parallel seriesof tests to be conducted, thereby ascertaining whether a differencein grain size gives rise to any significant differencein results. For each mineral the same sample was used for all the separations, thus eliminating variations independent of the change in side slope. Each sample, averaging 2 to 3 gms., was dried before being passed through the separator. After separation the fractions were examined under the microscopeand the weight percentagesrecalculated to allow for any impurities, which were estimated visually.
AssBsslmNr on RBsur,rs On the basisof the separationscarried out the mineralstested can be divided into two groups, the "magnetic" and the "non-magnetic." The "non-magnetic" minerals are those that are not attracted at a value of 1.4 amps. giving the maximum field strength of the separator.They are anatase,andalusite (and chiastolite),arsenopyrite, brookite, cassiterite, pyrite, rutile, scheelite, topaz, and zircon. The "magnetic" minerals are given in Tables 1 to 8. A study of these tables shows that, for the purpose for which the investigations were efiected, side slopesof 20' (Tables 3 and 4) and 30" B. H. FLINTER
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E ,',4vva TIAGNETIC SEPARATION OF ALLUVIAL MINERALS dl Fi zi .N-N I I 1 il l f o F,l a a a") .. b .4.. z F <, .oF a t'l NN - z; Hts CNHN a< o 6.iJ; H> $$r z i,i u) aa dca € ooo4c. PH .lo oi o. -i€; -+r o\ t4a Il tr N O+ SO \O jiD ;F: o z 8fri;r fi O O\@ r€ N\O Fi No.l - @ I F p^.E ?.€ 6 KB .e 5..e* e iE.:g ri Frc. 2. A plot of data from Table 5. Side slope 25', forward slope 15"; grain size -36+72 mesh B.S S. (Tables 7 and 8) can be disregarded,because there is too much over- lapping of the mineral species,particularly columbite and ilmenite, and that at a side slope of 30" (and therefore above) effective separation cannot be achievedbecause the magneticattraction at the lower amper- agesis not strong enoughto overcomethe effectof gravity. Ilowever, side slopesof 15" (Tables 1 and 2) and 25'(Tables 5 and 6) give convenientbreaks at amperagesof 0.3, 0.5 and 0.8 and 0.4, 0.7 and 1.0 respectively,and theseseparations are summarizedin Fig. 2. Aithough most of the minerals tested give rise to some variation due to grain size,in nearly all casesthese variations becomenegligible over a suitable range of amperages.For a side slope of 15" and amperagesof 0.3, 0.5 and 0.8, only four minerals (gahnite, staurolite, thorite and tourmaline) show a marked variation, and for a side-slopeof 25o and amperagesof 0.4,0.7 and 1.0,the only two mineralsshowing a marked variation are gahnite and tourmaline. For this reason,and becauseof the better separationof coiumbitefrom ilmenite and monazitefrom xeno- time, a sideslope of 25ois preferableto one of 15". ExcrprroNs Occasionallyan anomalousresult is encountered.The most common causeis the presenceof a partial coating of iron oxide, which can be removed by treating the minerai with hydrochloric acid, when it will behave normally. A less frequent cause is the presenceof very fine in- MAGNETICSEPARATION OF ALLUVIAL MINERALS 749 clusionsof a more magneticmineral. Notable examplesof this are zircon and monazite and, in columbite areas,cassiterite (due to inclusionsof the columbite). Both these causesresult in the mineral becoming more magnetic. It is known that, from certain localities in Malaya, cassiteriteand ilmenite will give anomalousresults. Cassiteritefrom these localitiesis attracted at all fractions, from 1.4 amps. to 0.1 amps. and is also at- tracted by a horse-shoemagnet. This magnetic cassiteriteis found in areas of major iron-ore development. No inclusions are discernibleat high power in transmitted light, but it is thought that this apparently inherent magnetism,which can be destroyed by heating to a tempera- ture of a few hundred degreesCentigrade, is due to micro-fine exsolution lamellae of magnetite. Ilmenite, on the other hand, becomesless magnetic. This is due to its alteration to "arizonrte." The greater the degree of alteration, the less magnetic is the ilmenite. The maximum current so far required to attract Malayan material (at a side slope of 25") is 0.7 amps. Rutile is often quoted as being magnetic.Magnetic rutile is commonly encounteredin Malaya. Exhaustivetests have shown that the magnetism is due to the ionic substitution of Ta/Nb (and therefore Fe). These Ta/Nb-rutiles range,in Malaya, from non-magneticmateriai to material attracted at a current strength of 0.7 amps. dependingon the amount of substitution present. The increasein magnetism is accompaniedby an increasein opacity. Pure rutile is non-magnetic. TnB Esrrlr.qTroN ol M.qcNrrrc BBn.cvron rnolr KwowN RESULTS The results given in Tables 1 to 8 representthe mass magnetic sus- ceptibilities of the minerals tested. The mass magnetic susceptibility (Kv) of a mineral is given as (1): magnetic susceptibility density An approximate value for Ka can be obtained from the equation a* : \Y:\ 1o-6c.g.s. where o:the angle of slide-slope and 1:the current in amperes,provided that Kn is much less than 1 and that determinations are made at amperageslower than 1.3, as saturation sets in above this value. An important application of this rule is that, once the Ka value for 750 B. H. FLINTER Tesrc 9. Mass Mecvorrc Suscnprrsnrrv (K;a) rN 10-6C.G.S. Side Slope 15' 25" 0.1 5r7.64 684.04 845.2+ 1000.00 0.2 129.41 171.01 2rt.3l 250.00 0.3 57.52 76.00 93.92 111.11 0.4 32.35 42.75 52.83 62.50 0.5 20.7r 27.36 33.81 40.00 0.6 14.38 19.00 23.48 27.78 0.7 10.56 t3.96 17.25 20.41 0.8 8.09 10.69 13.21 15.63 0.9 6.39 8.44 t0.44 12.35 1.0 .).16 6.84 8.45 10.00 1.1 4.28 J.O.) 6.99 8.26 1.2 3.59 4.75 5.87 6.94 l.J J.tro 4.05 5.00 5.92 1.4 2.64 3.+9 4.31 5.10 any mineral is known, the current at which it is attracted can be calcu- lated for any other side slopeor becauseof any compositionalvariation afiecting the specificgravity. In order that the information contained in this paper can be so used, the Ku values for the various amperages, at side slopesof 15",20",25o and 30o,are given in Table 9. The application of this relationship to the results given in the tables shows that the most significant factor influencing the magnetic suscepti- bility of a mineral is the percentage of ferrous iron (Fe2+) present in its composition.Thus ilmenite and siderite,both with a high percentageof Fe2+(and a fairly low density) have similar high values of Ky. The pink garnet, with its high mass magnetic susceptibility, is a highly ferroan variety (almandite). The increasedamperage needed to attract ilmenite when it is altering to "arizonite" can be foreseenbecause of the de- creasedvalue for K,44resulting from a loss in Fe2+content whereasthe specificgravity remains similar. The columbite/tantalite mineral seriesis very interesting. Although no tantalite has been separatedit would presumably be attracted at higher amperagesthan columbite becauseof its much higher specific gravity, and it should be possibleto obtain, from the current used, the tantalum/niobium ratio of the mineral. Researchin hand on the Ta/Nb- rutile minerais indicates that a relationship exists between the ferrous iron content, the ratio of tantalum to niobium and the specific gravity. With the Ta/Nb-rutile minerals,however, a variable amount of substi- MAGNETIC SEPARATION OF ALLUVIAL MINERALS 751 tution is possible,with correspondingly variable amounts of ferrous iron, which give rise to an added complication which must be taken into ac- count. Cor{cLusroN The effectiveseparation of mineralsaccording to their massmagnetic susceptibility depends principally upon a fine balance between gravity (controlledby variation in the side slope),and field strength (controlled by the current used). Other factors such as forward slope,rate of feed, and grain size, are subsidiary provided they fall within certain fairly broad limits. For the routine examination and estimation of the approximate per- centagesof minerals in Malayan concentratesit has been shown that electromagneticseparation is effective and that the best separationis achieved by using a side slope of 25o and current strength of 0.4,0.7 and 1.0 amps. on material lessthan 36 mesh B.S.S. In order of decreasingmass magnetic susceptibility the minerals sepa- rated are siderite, garnet (pink), ilmenite, wolframite, columbite, manganeseoxide (51.6/6 Mn), allanite, xenotime, epidote, staurolite, tourmaline, gahnite, uranoan monazite, monazite and thorite, with anatase,andalusite (and chiastolite),arsenopyrite, brookite, cassiterite, pyrite, rutile, scheelite,topaz, and zircon being non-magnetic. RBrcnrlqcr (1) H. H. Hnss, 1956 "Notes on operation olFrantz Isodynamic Magnetic Separator." Pamphlet published by S. G. Frantz Co., Inc., p. 8. Manuscript receiled.October 15, 1958.