1 - Mines Magazine - Colorado School of Mines

the conductivity of a mineral, at a given temperature,may vary as much as one thousand fold forspecimens taken from different locations. Nevertheless,many minerals can he selectively chargedhy careful, control of charging time, according toEquation E-7. Even if the conductivity of theparticles is known and can he controlled, the problemof selectivity making a separation using inductiveconduction as a charging mechanism will not becompletely solved because, in addition to the surfacecharge due to inductive conduction, there will benearly always a surface" charge due to (a) particleparticlecontact electrification, and (h) particlemetalcontact electrification. We note that there isno selectivity of the conductive particles with respectto the sign of the charge they bear when theyare charged by this electrification mechanism. Thisexaggerated example also illustrates the importanceof making a particle-particle contact electrificationseparation at the condition of minimum surface conductivity.Charging by Mobile IonsIf, hy some means or other, a small portion of asolid siu-face is given a surface charge, the chargetends to eventually spread evenly over the entiresurface. If the solid is a good electrtcal conductor,the redistribution of electric charge is almost instajitaneous.If the solid is a good dielectiic, forexample, dry pure NaCl, KCl, or quartz (at roomtemperature) the redistribution of the same chargewill be very slow; it may take several weeks. Bysubjecting dielectric materials to an atmosphere ofmobile ions, their surface is made temporarily electricallyconductive. The following simple laboratoryexperiment illustrates the principle of using mobileions to selectively separate a conductive mineralfrom a dielectric mineral by electrical forces.Place a single layer of a mixture of a good insulatoraud a good conductor (quartz and galena,for example) on a grounded metal plate (see Figure13. Place a second plate with a charge + Q infront of the fii-st. Next, plaj^ the flame of a groundedlamp over the sm-face of the minerals (see Jeans'^'*^.The entire surface of the minerals on the, first platewill then have a total charge of — Q distributedover its surface. (Instead of using "Jeans' lamp"we may, of course, substitute any other conveniention source.) After the minerals have been charged,remove thc plate that held the minerals and invertit. It will be found that the PbS particles, beingreasonably conductive, will rapidly share theircharge with the earthed plate and will fall from thcplate. The quartz particles are not capable of losingtheir charge and are held to the plate by theirown image force. Electrostatic image theory ismerely a method of solving Laplace's or Poisson'sequations by inspection of symmetry' conditions.+ Q7Charging*• a *-rFig. 13Por a detailed discussion of image theory see anyphysics text, for example, Sommerfeld*^^'*.A practical variation of the above experimentis the electrical separation of conductors from insulatorsusing' corona discharge as a source of mobileions. Charging hy corona has heen studied in detailby Loeb'^^', Lowe*^"", Lucas'^^*, and others.The electrification mechanism is due to both ion diffusion,and to ion bombardment. The important factorsapplicable to electrical concentration of mineralsare:1. The limiting charge Q on a spherical particleof radius r is proportional to the field B, andto the square of the radius. (See Pauthenier(20) •2. If appreciable corona is required, the dischargeelectrode should be negative because the flashover voltage is higher (Delassale'"''"')-The following photograph shows a modern Carpcocorona-type separator used at the Colorado Schoolof Mines {Figure 14).Rg. 14This separator has proven quite effective for theseparation of electrical conductors from electricalinsulators on an industrial scale.* Thc nonconductingparticles are held to the roll by their imageforces and are mechanically or electrically removedon the back side of the roll. Thc mechanical forcesutilized in this type of separation are obvious.Power RequirementsWith thc exception of the corona-discharge typeseparator the power requirements for electrical concentrationof minerals is (contrary to popular opin-* The Carpco separator and similar- separators such asSutton, Sutton and Steele, can also' be used to charge hyinductive conduction or to provide a suitable field for contactelectrification separations by replacing the fine wireelectrode with a large cylindrical electrode.r- ihII'Discharging26 THE MINES MAGAZINE • JANUARY, 1960•ion) extremely small. By way of ilhistxation, considerthe separation of a ton of —48 +65 meshquai-tz and phosphate particles in au external fieldof 4X1'^'' newt,ons/coulomb using free fall electrodesspaced 12.7X10"^ meters (5 inches apai-t).Let us assume that the particles will move half ofthe distance and that their initial velocity in thedirection of the potential gradient is 0. Pai"ticleparticlccontact electrification will give a ratio ofcharge to mass of about^ =m9X10-"The horizontal force F = QEthus by Newtonian mechanicsQE = md^xdt^coulombKg.where x is the horizontal distance the particle movesdui-ing time tthus cPxdt^thus dxdtC, =0orx=^1.8t2 + C2Cs^Om9Xio--'kg meters~sec.^This energy change has, of course, been obtainedat the expense of the external electric circuit; thatis to say, each particle requires the expenditure of5 X 10"' kg meters^sec^5 X 10"^ joules to traversethe electric field.The average time t^ required for this expenditureof energy is2x(vr — Vo)Thus the power required was1.9 X 10"^ sec.A ton of —48 -j-65 mesh particles contains907.8 kg= 41.8X10" particles.21.7 X 10-" kgparticleThus the poAver required per ton41.8X10=* particles2.63 X 10-^ wattsXparticle= 1.10kw.This corresponds to approximately 1 cent-per-tonpower cost at an industrial power rate. In otherwords, the power cost associated with leakage losses,lighting, materials handling, etc., are far greaterthan thc true power requirements for separating aton of material by the mineral-mineral contact electrificationprocess.Future of Electrical Concentration of MineralsThe writer feels that the future of electrical concentrationof minerals will depend almost entirelyon the rate at which our knowledge of solid-statephysics and surface physics can be increased. Manycombinations of minerals as received from the mineor washing plant can not he selectively electrifiedby the methods outlined in this paper. It is probahle,however, that most combinations of minerals couldbe made to lend themselves to economical electricalconcentration if the metallurgist had sufficientknowledge of solid-state physics. The tremendousstrides made by surface and solid-state physicists inthe past few years indicate that similar studies bypersons interested in minerals beneficiation willlead to tremendous practical accomplishments.AcknowledgmentsThe data contained in this paper were obtained fromthe work done by the writer at the Colorado School ofMines and at the Central Research Laboratory of InternationalMinerals and Chemical Corporation at Skoltie, Illinois.Bibliography1. Seitz, F., 1940, Modern Theory of Solids, McGraw-Hill Boole Co., Inc.2. Kittel, D., 1953, Introduction to Solid State Physics."Wiley and Sons.3. and 4. Mott, N. P., and Gurney, R. W., 1946, ElectronicProcesses in Ionic Crystals.5. Hatfield, H. S., Inst, of Mining and Metallurgy Bulletin,1924, vol. 3, Bull. 233, "Dielectric Separation."6. Henry, P., 1953, Brit. J. of Applied Physics, Supplementto S31.7. Adam,, N. K., 1949, The Physics and Chemistry ofSurfaces, Oxford TJniv. Press, 3rd edition, p. 300.8. Coehn, A., 1928, Handb. der Physik, 13, 332.9. Zwikker, C, 1954, Physical Properties of Solid Materials,Interscience Publishers, Inc., N. Y., p. 253.10. Beach, R., 1947, Electrical Engineering, vol. 66, p.325.11. Mott, N. F., and Gurney, R. W., 1946, ElectronicProcesses in Ionic Crystals, Chap. 2.12. Wagner, P. E., Nov. 1956, J. of Applied Physics, vol.27, no. 11.13. Zwikker, C, op. cit.14. Leverenz, H. W., 1950, Luminescence of Solids,"Wiley and Sons, p. 47.15. Bardeen, J., 1947, Phys. Rev., 56, p. 717.16. Shockley, "W., 1939, Phys. Rev., 56, p. 317.17. Mott, N. F., and Guerney, R. "W., op. cit.18. Viek, P. A., Oct. 1953, Sci. Progr. 41, p. 642.19. Leverenz, H. "W., op. cit.20. Leverenz, H. W., op. cit.21. Leverenz, H. W., op. cit.22. Mott, N. F., op. eit.23. Gurney, R. W., op. cit.(Coniiniied on page 33.)THE MINES MAGAZINE • JANUARY, 1960 27