1 - Mines Magazine - Colorado School of Mines

mixture was water-washed, dried, and heated in aglass container; after which it was poured into anexternal electric field of about 4 X 10^* volts permeter. There was, of course, some electrificationdue to mineral-glass contact, but if we consider therelative surface areas involved, the electrificationdue to the glass can be neglected.The average surface charge density on —48 +60mesh of quai-t.z-Florida phosphate particles was computedby photographing the trajectories of the particlesin a known, uniform, external electric field.Thc maximum charge was of the order of 1 X 10""coulomb per square meter. Since the maximumcharge in air could he about 26 X 10"" coulomb persquare meter it appears'that only approximately4% of the theoretical charge was obtained hy particle-particlecontact electrification. The low chargingefficiency probably is due to the impossibilityof making contact {even with considerable movementto effect repeated contact) because of theroughness and re-entrant angles of the surface involved.Particle-particle contact can not he assui-edmerely hy increasing agitation because one of theoperating difficulties assoeiated with particle-particlecontact electrification is the prevention of theformation of a common surface on all minerals. Acommon surface tends to form hy virtue of attritionduring the material handling stages of the process.This undesirable phenomena ])laces a rather imposingrequirement on thc design of a commercialflow sheet.Experimental TechniquesThe metallurgists faced with the problem of designingcommercial electrical beneficiation processesmust fii'st determine the conditions that will lead tooptimum selective charging of the minerals in theore to be concentrated. There are many sophisticatedexperimental techniques that can be used includingthe determination of glow cui-ves {Leverenz*^^')optical absorption studies (Mott'^s) and Gumey*^"'")aud X-ray and electron diffraction techniques. Aless didactic but more direct api:>roach to the problemis disclosed by using some of thc experimentalset-up used at the Colorado School of Mines shownin Figures 6, 7, S, 9, and 10.w Fig, 6Figure 6 depicts a chamber used for contactelectrification tests. The ambient temperature isadjusted through the use of thermostatically controlledsti-ip heaters mounted on the upper part ofthe chamber. Ambient pressure and chemical compositioncan be controlled by fillmg the chamberto the desired pressure from compressed-gas cylindereof known chemical composition. Humidity canbe controlled by standard H2S04-n20 solutions withinthe chamber.• Fig. 7^1A contaet-break-contact apparatus is shown inFigure 7. A metal cup E is held in a horizontal positionby means of a pin G opposing the motion of aspring, P. Contact between a mineral and the metalcup is made by placing thc grains in the cup. Exchange-chargeis determined hy allowing one ofthe two contacting substances to fall into a Paraday])ail, and the resulting potential rise between thepail and thc earth is then measured with a vacuumtubeelectrometer*. When the pin G is retractedas is shown in Figure 8, the cup moves to a verticalposition, thus permitting the grains to fall into theParaday pail A. In the ease of contact between twominerals, we merely substitute one of the mineralsfor the metal cup. A cross-sectional view of thecontact-break-contact mechanism including an auxiliaryhead for studying contact electrification in thejiresence of an external electric field is shown inFigure 9. The central system is shown in Figure 10.-w Fig. 8* Ttie electrometer used is a Keitliley model 200A vacuumtube electrometer with air input circuit consisting: o£a resistance greater than lOi^ ohms shunted hy a capacitanceoi: 6 nfif.24 THE MiNES MAGAZiNE • JANUARY, 1960Grounded shieKtfElectrometer• Fig. 9 Fig. 12Shielded cableCharging by Conductive InductionLet us consider a solid particle resting on anearthed conductor in the presence of an eieetricfield as is shown in Figure 11.rvFig.The particle rapidly develops local siu-facecharges by induction, but, if the external field isuniform and the particle has no initial charge, thepai-ticles experience no electric force. If the particleis conductive, it will become charged to thesame potential as the metal plate upon which it isresting, with a total charge Q = CpVp, whence itwill experience the electrical force F = QE; whereC|, is the capacitance of the particle and Vp is thepotential difference between the metal plate andthe charging electrode. If the particle is a dielectric,we cannot, in general, speak of its potential becausethc potential varies from point to point on the surfaceof the dielectric. A particle with finite conductivitywill eventually obtain a total chargeQ = CpVp, but the time required may be very large.The problem can be approximated if we consider thefollowing as an equivalent circuit:liFiq.cLet R- = the effective particle surface resistance.The value of Ii will depend on the temperature ofthe sample, its previous chemical and temperaturehistoi-y and, in many cases, on the polarity used inthe concentrating circuit. It may be possible tofavoi"abiy alter the effective resistance of a mineralby heating the mineral in the presence of the vaporof one of its constituent components to produce astoichiometric abnormality, or by introducing im-iOFaraday CupGrounded Chamberpurities into the mineral such that it will behaveas a semiconductor. It is also possible to alter theeffective resistance of a. mineral by irradiation withelectromagnetic energy of a suitable energy. Undergiven conditions, however, one can assign an effectivevalue of R- and consider that the charging mechanismwill behave as follows; neglecting inductanceone can Avrite:oriR-\-where idQdt_dQdtQ_ _C„RIf we assume the particle to have an initial chargeQ„ = 0,VpRthen Q = CV„ - CV,. e - RC RC(E-7)The practical interpretation of the analysis is(a) the charge Q at time t is proportional to thecapacitance of the particle and to the potential ofthe charging electrode, and (b) the time requiredto charge the particle to a given fraction of its finalcharge is proportional to the particle capacitanceand its resistance. Por example, the particle chargesto 63% of its final charge, in time t — RC.Unfortunately, attempts to tabulate the electricalconductivity of minerals are almost hopeless becauseTHE MiNES MAGAZiNE • JANUARY, 1960 25