1 - Mines Magazine - Colorado School of Mines

transfer on only ^^^zr" X 100 = 0.003% of tlicavailable ions. A similar result is obtained by calculatingpossible electron transfer across a metalsurface. Altbougli Coehn's rule is reported to havebeen verified for more than 400 substances, it is oflimited value to the metallurgist beca.use of the difficultyin determining the effective relative permittivityof the surfa.ce la.yers (which is often differentfrom thc substratum) of two substances that are tomake contact. The theory of contact electrificationis a very complex subject, aud the present theorieswill mevitably be changed as more exact knowledgeis obtained concerning the ionic and electronic propertiesof solids. No two triboelectrification seriesarc identical; and no extensive literature search isrequired to find conflicting data of contact electrificationtests, even,with rega.rd to the sign of theresulting charges.For example, when an insulating crystal siich asNaCl or KCl is contacted against a metal at roomtemperature, there should be no electrons in the conductionband of the iusulator so the salt could notbecome positively charged by losing electrons to themetal. If wc take a value of —4ev as the work fmietionof the metai (most metals have a work functionnear this vahie) and the value of —0.5ev for thelowest electron energy in the conduction band ofNaCl or KCl (Mott and Gurney'"'), we see thatthe highest occupied electron state in the metal isin an energy state lower than the lowest conductionstate iu the salt, (sec Fig. la and Ih) therefore, wewould not expect the metal to lose electrons to theinsulator (see Figure la).FORBtODEN_22BANDSV = 0*=-4evFIG, 1-0TYPICAL ENERGY LEVEL DIAGRAM OF APERFECTFIG. l-b.INSULAtORTypical Energy Level Diogrom Of A MetalYet, real crystals of NaCl or KCl usually will hecomecharged negatively hy sliding contact againsta metal. "Wagner'^^* however, reports a positivecharge on both NaCl and KCl, upon contact withnickel. The negative charge on the insulator is explainedby Zwikker points out that local heating is also an importanteffect in electrification due to ion transfer.t Leverenz(20) ascribes tlie trapping of X-ray-excited electronsin KCl crystals to sites of missing or displaced CIions in the crystals.FIG. 3EFFECT OF HEAT TREATMENT ON ELECTRiCALSEPARATION OF SYLVINITE ORE» to M> 40 90 to ro ao 90 loo% N.O nCOVMT IN CONCENTRATE300 SOO TOO 900PREHEATTEHPEHATUitE.'F.In some cases it is also possible to alter thc surfaceof one of the mineral species in the ore by selectivereagentizing or other chemical treatmentusing methods similar to those in froth flotation.In many simple cases it is only necessary to havediscrete surfaces in order to obtain suitable pai'ticleparticiecontact electrification. The treatment isthen simplified to that of desliming (removal of acommon surface from the mineral species in the ore).Por example, Florida quartz contacted against Floridaphosphate rock will charge the quartz highlynegatively and the phosphate rock equally positivelyover a temperature range from about —40° C to300° C. Sea sand contacted against silica gel willcause the quartz to he positively charged and thesilica gel negatively charged (at 30° C).FIG. 5TYPICAL ELECTRICAL SEPARATION OF FLA.PHOSPHATE ROCK USING PARTICLE-PARTSCLECONTACT ELECTRIFICATION CHARGING MECHANISM1004050 60 70 aO 90 100V. RECOVERY OF PHOSPHATE MINERAL IN CONCENTRATEFigure 5 shows the mineral distribution of asingle pass for a separation of quartz from Plroidaphosphate rock at a temperature of 150° C. TheTHE MINES MAGAZiNE • JANUARY, 1960 23