In the present study,   formation and occurance of beach sand deposits of India and   recovery   of heavy minerals, viz., ilmenite , rutile, zircon,monazite, sillimanite and garnet have been reported. Separation   processes adopted for concentration of heavy minerals at   different plants

of Indian Rear Earth Limited (IREL) have been discussed

Mukherjee, T K

Nair, N R

English

eprints@NML

ISBN : 81-87053-53-4Processing of Fines (2)Eds. : P. Bhattachur- v^vtr , R. Single sand N. G. Gosti-amiO NML Janrshedlnrr-831 007. Iudin, 2000. pp. 93-99Various aspects of fines in separation andprocessing of beach sand minerals in IndiaN. R. NAIR and T. K. MUKHERJEEIndian Rare Earths Ltd. (IREL), Mumbai, IndiaABSTRACTIn the present study, formation and occurance of beach sand depositsof India and recovery of heavy m inerals , viz., ilmenite , rutile, zircon,monazite, sillimanite and garnet have been reported. Separationprocesses adopted for concentration of heavy minerals at different plantsof Indian Rear Earth Limited (IREL) have been discussed..Key words : Beach sand minerals, Gravity separation , High tension separation,Magnetic separation, Froth- )1CM011'INTRODUCTIONIndian Rare Earths Ltd. (IREL), a public sector undertaking under the De-partment of Atomic Energy is engaged in separation of six heavy minerals (HM),namely; ilmenite, rutile, zircon , monazite, sillimanite and garnet from beach sanddeposits of Kerala, Tamil Nadu and Orissa. The raw sand exploited by IREL areavailable in the form of beach washings thrown by the sea j ust next to the shoreline and as inland deposits formed over the year below and above the groundlevel in areas away from the shore line.The beach washings are rich in heavy minerals and less in fines due torepeated washing and concentration by wave action during the rough sea sea-sons. The inland beach deposits occurring little farther inside the land is formedover years by natural forces acting along the beach. They are leaner in heavyminerals and contain more amount of finer minerals (fines). The general charac-teristics of these deposits are summarised in Table 1.Over the years, the quality of beach washings has come down drastically. Thelimited availability of these seasonal accruals has Forced IREL to depend on theinland beach deposits as the primary source of feed material.93n MIYYIY^lu1 ^I i IY a Yhlt :ftK Y I^IYY i^n1 N r4„ I i ^NYMMI4ii ^Yld ltr i ail i 111 ^IY ti^ i I YInM WYaY^ !YW HM1^lYY4tItsN. R. HAIR Bind T. K. M(_: KHER_Jh:HTable I : Characteristics of beach ,vaiul depositsDeposit HM% 17(Fines Occurrence MiningMethodProcessing methodBeach 50-70 <5% Alone the Surface ConcentrateWashings shore line scrapping upgradation.D1'V processi ngInland Inland Pre-concentration.Beach 15-40 5-?0%c Depth 7-9 Dredging ConcentrateDeposit ntctres up«radation,Dry processingDEPOSITSThe inland deposits have a complex mineral assemblage compared to thebeach washin-s. The heavy minerals are finer in size. The major gangue mineralquartz. is of wider size range and the content of fine quartz is relatively higherin the inland deposits. A typical size distribution of beach washing and inlanddeposits are -liven in Fig. 1 for comparison.Size Dist % of MK Beach washing & Inland depositss. ,rIt nW 1 4_.# A. VWW &0.4#4a€witooLfa^rwrFi-. I St:- distrihuaio ir cif hcrerch ti ollirr and urlcnacl der)us11vThe size distribution curves indicate that the inland deposits contain moreamount of fines and has a wide 1-W1ge of particle size distribution compared tobeach washings.94N. R. NAIR and T K. MUKHERJEEMETHODSDredging the raw sand and desliming is the initial operation in recovering thevaluable heavy minerals.Gravity SeprationThe deslimed sand is pre-concentrated to 90% HM using spirals located in afloating/land based concentrator. The rejects mainly silica sand is used for refill-ing the mined area. The pre-concentrated heavy minerals is then upgraded toabout 98% HM in Concentrate Upgradation Plant (CUP) by further stages ofspiral/wet table operation. Due to the significant developments in wet gravityseparation techniques many types of spirals are available to perform various dutyconditions as per the mineral assemblage, size etc. Spiral concentrators havebecome universally acceptable because of its simplicity of operation, less main-tenance requirement and ability to handle large throughput.Closed circuit tests were conducted using spirals with the representative bulksamples from inland deposits to assess the behaviour of the fine particles andmaximise recovery of heavy minerals. The size distribution in the bulk samplefrom inland deposits and that of ilmenite and quartz in the feed to spiral wereas indicated in Fig. 2.•aea..,&.t. 41Fig. 2 : Size distribution in the bulk .vnntpleassn--W- CLEWTZHydrocyclone was used to reject the slimes in the sample thus presenting afeed material of eltective size range coarser than 40 micron . The recovery curves(Fig.:) indicate that there is loss of fine ilmenite in rejects while fine silica ismostly reporting to the concentrate.95i'IIIIY'WWIMIM ^ri11Yl ':YMiilIY^IININi^^ltrl/ Irl9. - I :J ,Y tYi^YIYMMI U Y1N^MMM^III^iMwYY^11I1111MYi w iiN. R. A4lR and I: K. A1liKftFR.IF_FRoughing Operation Sim Dist %10090e0loot 7060403020010 100 1000MicronsFig. 3 : Roughing e)peration si,.e elistributin/t I "c)-+- 0mv M C®nant*s 1a Mimi in TslteEven after repeated cleaning of the concentrate using spirals, certain amountof fine silica is still retained in the concentrate product. The study. thus indicate,that while there is toss of fine heavy mineral in the rejects, the fine silica reportto the concentrate. The concentrate containing fine silica, was tested in a hin-dered settling classifier, namely Floatex Density Separator (FIBS) and it .wasfound the rejection of line silica in the overflow was satisfactory.The gravity concentration process has its limitation in treating particles in thefine size range. Factors like low mass, high surface area, poor momentum. vis-cosity effect etc., result in ineffective separation of fine particles by Gravityconcentration process. In other words, the efficiency of gravity concentrationimproves with increase in particle site. To reduce the sire effect and to make therelative motion of particles specific gravity dependent, closely sized feed isdesirable.The development of Multi-gravity separator, kelsey jig have to sonic extent,helped to overcontc the problems of fine mineral processing in gravity separationprocess. The economic minerals of finer and ultra fine sires can he effectis elyseparated in Multi Bras ity separator. The principle of the system consists ofwrapping of the horizontal concentrating surface of a conventional shaking tableinto a cylindrical drum and rotating it. This can cxcri a force of many time,greater than the normal gravitational pull on the particle in the Bowing film. Thieshelps to enhance the separation process of fines. Similarly, in e<-tse of kelsev lie.it is possible to separate the minerals having difference in specific gravity as loss'as 0.5, which is otherwise difficult with conventional gravity separators.96N. R. NAIR and T. K. MUKHLRJEEFor separation of individual minerals contained in the final concentrate, thebeneficiation processes adopted by IREL in its plants are high-tension separa-tion, magnetic separation, wet tabling, flotation process etc. Particle size distri-bution extending over a wide range, presence of fine particles, slime coating overthe particles etc., cause difficulties in the beneficiation process, as briefly dis-cussed in the following paragraphs.High-Tension SeparationThe heavy mineral concentrate when subjected to this process is separatedinto two fractions - conducting and non-conducting, which are further processedto obtain the individual mineral product. In high-tension separator (HTS), whenhigh D.C. voltage is applied, the air surrounding the electrode gets ionized andmineral particles are charged- The conducting minerals after acquiring the chargeimmediately dissipate the charge to the grounded revolving roll and follow thetrajectory due to the centrifugal force. The non-conducting minerals, slow indischarging the charge, is pinned to the roll and are wiped out by the help of A.C.electrode and a mechanical wiper brush. In mineral sand industries, high-tensionroll separator and electrostatic plate separators are widely used to separate theconducting and non-conducting fraction. Particle size influences separationbehaviour in an HT separator as the surface charge on a coarse grain is lower inrelation to its mass than on a fine grain. Thus, a coarse grain is more readilythrown from the roll surface, and the conducting fraction often contain coarsenon-conductors. In the same way the non-conducting fraction often contains fineconducting particles because they are most influenced by surface charge. A plate/screen plate separator, where the "lifting effect" by a static electrode is utilisedfor separation can help in such cases to remove small amount of non-conductorsfrom a predominantly conducting feed (plate separator) and small amount ofconductors from a predominantly non-conducting feed (screen plate separator).In an electrostatic separator fine grains are most affected by the lifting force andso fine conductors are preferentially lifted to the electrode whereas in a high-tension separator effective separation of fine non-conductors from coarse con-ductors takes place. Thus, a combination of these separators are used in most ofthe flow-sheets.Magnetic SeparationIn mineral san heneficiation. magnetic separators of different magnetic inten-sities are used depending on the nature of the feed material and the operationalrequirement. These include induced roll magnetic separator. lilt roll magneticseparator. cross belt magnetic separator etc., to separate magnetic and non-mag*-netic minerals. In this process, the equipment is operated at a particular rollspeed and magnetic intensity. The presence of fine to ultra fine non-nmagnetic97N. R. NAIR and T K AfL-`KHER.1Efmaterial in the feed causes problem in magnetic separation as well. In order toseparate maximum magnetic mineral, a particular rpm and magnetic intensity isapplied to the equipment which in turn. entraps the finer non-magnetic particlesin the magnetic fraction. If the non-magnetic particles arc to he completelyseparated, then the rpm of the roll is to he increased to develop more centrifugalforces on the fine particles and in doing this more magnetic minerals report tothe non-magnetic fraction, thus reducing the recovery of magnetic minerals. Lifttype induced roll magnetic separators, where the feed is led to the bottom sideof the roll using a vibratory feeder, is effective in picking up fine niafgneticparticles, to get a good magnetic product.The introduction of wet high intensity magnetic separator (WHIMS) and rareearth drum magnetic separator (REDMS) providing high magnetic field gradienthave improved the scope of magnetic separation of fines. Fig. 4 indicate" thcdistribution of magnetic separated using REDMS.RIMt -,Arc D t % of tlmcaiteFig. 4 Site clisttihlrtrun o/ i1mcnile sepcualed irn', Ri:fl 1.5The recovery of finer ilmenite is high in Mag-i and Mug-I1 fraction with verylow loss of fine ilmenite in the non maLg circuit. The advantages in using high=radient (REDMS) in beach sand industry will help in the separation of finerItt^ gneiics/non nia^=netic minerals.Froth FlotationIn the froth flotation circuit too, the Fine minerals Cause proces problems.Apart loin the difficulties of conditioning to he done at higher solid percentage.there arc number oh faetor_s contributing to this problem, such as high specific9 RN. R. NAIR and T K. MUKHERJF_F_surface area of particle, low specific mass, morphology of grains etc. The hydro-philic finer particles (unwanted/gangue) very often report to the froth in flotationcell which needs further cleaning to improve froth grade. In sillimanite flotationcircuit., the finer zircon, quartz and rut.ile have a tendency to report in sillimanitefroth due to entrapment, thus, requiring further cleaning of froth.Column flotation can be used to effectively handle the fine particle process-ing for beneficiation of sillimanite in beach sand mineral processing unit. Gen-eration of fine air bubble, large residence time of particle and air bubble due tocounter current flow of particles and air bubble, long cleaning zone, deeper frothbed and wash water arrangement to clean froth are some of the important factorsthat contribute for the better performance of column flotation while dealing withfine particles. In single stage operation, the grade and recovery of minerals incolumn cell is higher than the conventional cell.CONCLUSIONIn mineral processing, fine minerals cannot be avoided. Fine size materialsoften report in middlings and tailings as the technology for treating fine sizedminerals is not well developed. Most mineral processing treatment includinggravity separation, flotation, magnetic separation, electrostatic separation have alimiting minimum particle size beyond which the separation is not effective. Theinherent difficulty in recovering very fine particles is to be properly addressedand it is expected that the new ideas and improved technology will enable themineral engineers and allied technologists in meeting the challenges of finemineral processing.REFERENCES1. Weiss, N.L., (1985), SME Mineral Processing Handbook, American Institute ofMining. Metallurgical and Petroleum Engineers, Inc., New York.2. Wills. B.A., (]988), "Mineral Processing Technology", 4th edition, Pergamon Press,Oxford.3. Ansari, M.l.. (1997). "Fine Particle Processing - A difficult problem for mineralengineers" Processing of Fines, NML Publication. Jamshedpur, p. 93.99

Various aspects of fines in separation and

processing of beach sand minerals in India

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Various aspects of fines in separation and processing of beach sand minerals in India

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