1 - Mines Magazine - Colorado School of Mines

^-undamentals of'Llectrical Concentration of MineralsIn principle, tlie electrical concentration of mineralsis quite simple. The operation is merely thatof utilizing' the forces acting on charged or polarizedbodies in an electric field to effect a selective sortingof the mineral species introduced into an exteiD.aleieetric field. In actuality, the method of effectinga separation is frequently interestingly difficultdue to our lack of knowledge of the relativelynew science of solid-state physics that is associatedwith the problem. In fact, it can be said that themost important recent contributions to the understandingof the "art" of the so-called electrostaticprocess of concentrating minerals have been thc indirectcontributions by solid-state physicists among•whom may be mentioned Seitz*^', Kittel'^', Mott*-"**,and G-ui-ney'"".Much of the notorious confusion and the nonreproducibleresults involved in '' electrostatic' '*separations of minerals stem from the inability toisolate all the factors that influence a selective separation.A discussion of some of these factors constitutesthe subject of this paper.In an electrical beneficiation process for solidsubstances the major electrification mechanisms are :Contact electrificationElectrification by inductive conductionEleeti-ification by virtue of mobile ionsEach of the above mechanisms gives rise to asurfa.ce charge density on the solid particles. Ifthe charged particles are in a uniform, external electricfield they will experience an electric force such,thatP=QEP — force in newtonsQ = the total charge on the particle in coulombs(1)E = field strength in -—;—•meterIf a separation is made in air (STP) the maximumvalue of the surface charge density is about 3 X ^^'^coulomb/meter^ and the maximum value of E is3 XIO*^ -^""^^^ • Thus, a spherical particle of radiusmeter* The term electrostatic process is somewhat a misnomerbecause the process usually is time dependent. It is generallyused by metallurgists to describe any mineral beneficiationprocess that utilizes an external electric fieid.20ByJAMES E. LAWVER. '43THEAUTHORDr. Lawver attended the Colorado School of Mines,receiving his engineering degree in non-ferrous metallurgyin 1943. He obtained his doctorate at Mines in 1956 inextractive metallurgy. At this time, he also did some workin the fields of mathematical physics and instrumentalanalysis.He has also attettded numerous courses in specializedfields of mineral processing such as X-ray diffraction, electrondiffraction, electron microscopy, and operations research.His experience record includes that of a mining engineerat the Braden Copper Cojnpany i?t Chile, SouthAmerica, and zuith the Bureau of Strategic IVarfare inBrazil. He is a registered engineer in the state of Floridawhere he is curretitly employed by the International Mineralsand Chemical Corporation as a research specialist.Dr. Lawver holds 26 domestic and foreign patents.r cm. charged to its maxunum surface charge densitywdl experience an electrical force P in a maximumuniform, electric field E,p = 47r(r X 10-^)^ X 26.6 X IO"*' X 3 X I0»r^ newton~ 10= 1 X lOV dynes = .022 r^ poundsIt is interesting to compare the magnitude of thcabove force to the force of gravity acting on thespherical pai-ticle with radius r cm. and a specificgravity sP(gravity)=^r3R,X980Whence the ratiowhich is about 4 XP electrical 2.5P gravity B rH r-^ dynesThus a fully charged 28 mesh (.0589 cm.) quartz particlecould experience a maximum electric force of2.65X^589 = ^''-^ times its -weightTHE MINES MAGAZINE • JANUARY, 1960In practice the usable maximum electric field isabout 80% of theoretical value and the value ofthe surface charge density is about 5% of the theoreticalmaximum, thus the above value becomesP electricalP gravity0.1s r (em.)Por completeness it should be noted that in additionto this force, the particles may experience two otherforces in a non-uniform eleeti-ic field, viz., the forcedue to a permanent dipole having a moment perunit volume of P(4)where Fp = grad(P-E; (5)and the force that arises from the energy changein- the eieetric field due to the entrance of a solidwith a value of relative peianittivity different fromthat of the medium in which the separation is made,where F