Document RJBaDk04DYv7zj188BqDK9XMB

rr <O-n ation trode ery to Journal o f Electrostatics, 10(1981) 177--182 Elsevier Scientific Publishing Com pany, Amsterdam -- Printed in The Netherlands 177 TRIBOELECTRIC PHENOMENA IN MINERAL PROCESSING. THEORETIC FUNDAMENTALS AND APPLICATIONS M. CARTA, G. ALFANO, P. CARBINI, R. CICCU, C. DEL FA' Istituto d Arte Mineraria dell'Universit di Cagliari, Italy. Centro Studi Geo minerari e Mneralurgici del Consiglio Nazionale delle Ricerche presso l'Univer sit d Cagliari ingle ac ter- 9) and gust) . L j INTRODUCTION , Surface-based ore processing methods, amongst which electric separation, are gaining increasing interest in mineral technologv. The Institute of Mining and Mineral Preparation of the University of Cagliari is engaged in this field with a broad research programme which started many vears ago, (ref,1,and references cited therein#2-6). 1 Three lines have been followed: - theoretic investigation in order to define suitable models for the chemico-phvsical processes underlving the behaviour of minerals under varving environmental ! conditions; - experimental tests, mainly with oure soecies, for the control and a better de finition of the models adopted; - finally, on the basis of the indications of theorv and aoolied research, the solution of real processing oroblems at both laboratorv and nilot niant scales, starting from crude ores. As far as the theoretic studies are concerned, a unified model has been propo sed as the result of the broad research carried out; this model is suitable for the description of the main surface-based phenomena involved in mineral processing A large series of experimental checks have demonstrated its reliabilitv in most cases. In particular, the Fermi level has been assumed as the outstanding parameter for the condition between the behaviour of the various minerals in electric sepa ration and flotation and their surface enerqy prooerties fully described bv the Fermi-Dirac statistics. The indications of the model are to be considered as verv helpful for the im provement of the techniques and for the achievement of better practical results, as demonstrated by the applications, some of which are illustrated and discussed 1 later in the paoer. 1 0304-3886/81/0000--0000/$02.50 1981 Elsevier Scientific Publishing Company k Ifi | s ;.l 178 THEORETIC FUNDAMENTALS A simple model for the description of the interphase energy exchanges has first been studied in order to explain the triboelectric charging phenomena of mineral particles. The experimental research dealt with the measurement of the work func tion of the different minerals and with the detection, on a statistical basis, of the electric contact charges acquired by the single grains. The work function was determined by means of the measurement of the contact po tential difference with respect to gold, assumed as the reference metal, using the modified Kelvin methods (rotating or vibrating condenser). The contact potential measurements were carried out on specimens of natural crystals of different charac teristics and origin, properly cut and smoothly polished. The data obtained with the methods adopted showed a satisfactory reproducibility. Electric contact charges acquired by the single grains of the different mineral species after sliding on vibrating chutes made of various metals, ceramic materials glass and plastics, were detected using a Faraday well connected to a high input -13 -14 impedence electrometer. With the apparatus set-up, charges as small as 10 -10 coulomb could be measured. It was revealed that the sign of the electric charges depend on the work function of the mineral with respect to that of the contact sur face. More precisely, experimental results allowed to demonstrate the theoretic hypothesis regarding the transfer of electrons in triboelectric charging from the mineral to the "rubbing medium" and viceversa: the mineral is positively charged if its work function is higher, and negatively charged if it is lower than that of the contact surface. 3esides natural minerals, the model was also checked, after modification of the surface energy structure of the same minerals, with treatments such as X or Y ir radiation, ionic or electronic bombardment, thermal processes. This fact opens very interesting possibilities in order to assume the optimal conditions for a selective triboelectric charging of the different mineral species to be separated. Particularly important is the effect of temperature, which can be described by resorting to the energy band model in semiconducting solids. The necessary data regarding the electrophysical parameters involved In the sim plified band model were collected for the various minerals by measuring the elec tric conductivity 0 and the Seebeck coefficient X as a function of temperature. The gradient of the conductivity curve (Log d versus reciprocal absolute tempera ture) in the intrinsic field gives the width of the forbidden band Eg while the sign and value of x in the so-called "transition zone" reveal the kind of conduc tion (n-type or p-type) and give the position of the Fermi level Ef with reference to the upper edge of the valence band. Within the limits of the proposed goal, the model adopted, although simplified, is still very useful for a rough understanding of the phenomena involved in elec tric separation and in flotation. Moreover, on the basis of the indications made has first mineral :>rk funcJasis, of Dntact pousing the itential int characled with it mineral : materials Jh input r 13 -lo-14 charges intact suri<y `,Iic t . the charged an that of Lon of the or y ir- optimal al species Lch can Lds. n the si'.r :he elecrature. temoera.le the : conducreference umplified. in elecm s made i available, it is possible to forecast the results obtainable in the processing of a crude ore containing two or more mineral components; the most suitable external actions for the improvement of the separations results can also be identified. CHECK OF THE MODEL The reliability of the model was ascertained on three mineral samples of fluo rite, barite and calcite. The above indicated electrophysical parameters were sy stematically measured under different conditions in order to find the desired cor relations with the separation results working with pure species or with synthetic mixtures of them. At increasing temperatures, the Fermi level diminished continuously for n-type semiconductors like the fluorite and calcite minerals examined, as was expect>d, from a position corresponding to the energy levels of the "donor"states to the mid dle of the forbidden band. The opposite happened for the barite mineral examined for which conductance was p-type owing to the presence of "acceptor" states. It is to be pointed out that minerals of the same species can be either n-type or o-type according to the impurities contained, and to the presence of distortions natural or induced, in the crystal lattice. As the position of the Fermi level varies with temperature, the behaviour of the mineral grains in triboelectric separation alters accordingly. This was fully demonstrated by the experiments carried out using a cyclone as the charging device in order to achieve better contacts under the action of the centrifugal force. INDUCED MODIFICATIONS As demonstrated, the energy structure of minerals in surface layers can be modi fied through external actions in order to create the most favourable conditions for the surface-based mineral processing methods. Such a possibility is a step of outstanding importance for the improvement of the techniques available and for the set-up of new ones. It is difficult to illustrate in a short space the different experimental re sults obtained. However, besides the effects of temperature, the modifications in duced by mechanical actions, which produce distortions of the crystal lattice, gi ving rise to localized energy levels inside the forbidden band, are also worth men tioning. Therefore it is necessary to control with particular care the grinding phase before separation, in fact, as it is well known, the kind of grinding, wet or dry, and the particular environmental conditions during the mineral comminution affect to a great extent the final results. A further modification can be accomplished by introducing doping elements into the crystal lattice; however, the practice of this technique is not yet satisfac tory as regards the application in the field of ore dressing. Very promising are the possibilities offered by the various techniques of irra- 180 diation in the surface-based mineral processes, notably in electrostatic separa tion. In fact, new energy properties can be acquired and maintained by the mineral grains, subjected to such actions, for a given time. It is thus possible to differ entiate the behaviour of minerals which, practically, cannot be separated under normal conditions. Similar effects are produced by ionic or electronic bombard ment at high field intensity. In general the different kinds or irradiation and bombardment give rise to "ac ceptor" states thus increasing the tendency of the mineral substances to acquire negative contact charges. However, it is possible to take advantage of the differ ent "decay" times, which are characteristic of each mineral, in order to attain the most favourable selectivity conditions. The environment moisture is of great importance in triboelectric charging phe nomena. In fact, it is evident that the adsorption of water molecules on the min eral surface modifies the energy structure thus influencing the exchange processes. Moreover, the water or vapour coatings increase the surface conductivity causing an easier leakage of the triboelectric charges. INDUSTRIAL RESULTS Following the indications of the basic research, many problems regarding the processing of crude ores by electrostatic seoaration could be satisfactorily re solved . The separation tests were carried out with the "Electrociclojet" and the "Tri boelectric cyclone separator", both designed and built at the Institute's work shop on both laboratory and pilot plant scales. In the case where the surface elec tric properties of natural minerals are already well differentiated under normal conditions, good separation results can generally be achieved through the appro priate selection of the material on which contact charging takes place. As demon strated, the work function of such materials must be intermediate between the work functions of the mineral species to be separated. Some examples are reported below: a) Upgrading of a barite crude ore with a quartzeous gangue. Suitable charging mat erials: perspex, grs. A two stage separation with the triboelectric cyclone sepa rator gave a 97% BaSO^ concentrate with a recovery higher than 95%, the BaSO^ con tent in the reject being around 9%. Similar results were obtained with the Electro ciclojet starting from a feed material finer than 0.5 mm including the finest si zes; the concentrate grade was higher than 98% BaSO^ with a recovery of about 96% and very low losses in the refuse. b) Upgrading of a fluorspar ore with a comp Lex ganaue. Suitable charging material: stainless steel. The separation, with one cleaning and two scavenging st:ages was carried out with the Electrociclojet; a final product having a CaF^ content higher than 97% was obtained with a recovery or about 90%. araineral differder rd- o "acuire -ifferain ' phemin- cesses. .sing th^ "Tri<rke elec-rmal -pro iemone work 1 ng mat sepa)4. coniiectrot siJt 96% erial: : was higher , c) Cleaning and pyrite removal from coal. Suitable charging material: Stainless steel. The problem is of outstanding interest owing to the increasing consumption of coal in power stations. The reduction of the sulphur content below fixed limits is mandatory for ecological reasons. Tests have been carried out on laboratory and semindustrial scales on various samples of run of mine. The problem was satisfac torily solved as far as the removel of the free pyrite is concerned, whereas very little could be done for the elimination of the organic sulphur and of the finely dispersed pyrite particles. Anyway, it is to be pointed out that, notwithstanding the difficulty of the problem, inorganic sulphur was lowered to below 1.0% for the majority of the samples tested, with bearable losses. At the same time also the gangue minerals (calcite, quartz, silicates) were rejected in the refuse, owing to their well diferentiated properties with respect to the main coal maceris. Therefore, a low sulphur, low ash clean coal could be obtained with a single operation. For instance, a 0.8% Sulphur, 5% ash clean product was obtained with a high re covery from a West Virginian raw coal with a single stage separation in the Electrociclojet; a further cleaning stage was sufficient to lower the Sulphur content to below 0.5%. Similarly, from a West German raw coal having 20.3% ash and 0.82% pyritic sul phur, a 0.33% inorganic sulphur, 5% ash clean product was separated by effectively combining selective comminution and triboelectric separation. In the case where the characteristics of the natural comoonents are not suffi ciently differentiated, it is practically impossible to achieve a satisfactory se paration. In this case it is necessary to resort to suitable external actions in order to induce a selective modification of the surface electric properties. A very interesting way is represented by the effect of temperature. The follo wing examples are worth noting: a) Electric separation with triboelectric charging of quartz from feldspar contai ned in a beach sand. Tests were carried out using a stainless steel cyclone for triboelectric charging. Both the feed material and the cyclone waLls were heated to temperatures around 100C. Besides the separation of a quartz product with a grade higher than 90% it was also possible to separate sodic from ootassic feld spar with interesting efficiencies. b) Upgrading of a Tunisian lean phosphate. The solution of this interesting pro blem is rather difficult owing to the very complex mineralogical structure of the ore. Preliminary tests of triboelectric charging clearly demonstrated the great sensitivity of the effect of temperature. A further series of systematic separa tion tests using a stainless steel cyclone for triboelectric charging revealed that both the concentrate grade and the recovery vary according to the temperature. The optimum conditions could be found within a temperature interval ranging from ISO to 200C; at higher temperatures the recovery continues to increase but the 182 grade becomes unsatisfactory. According to the setting of the main geometric and operational parameters of the separator, Ca(PO^>2 grades higher than 75% could be obtained working with the Kalaa Djerda run of mine, an increase of about 10 % points with respect to the feed grade. CONCLUSIONS The values of the electroohysical parameters of the various mineral components present in a raixure, can be correlated with the separation results achieved in a high intensity electrostatic field, after triboelectric charging. The correlations revealed allow a clearer explanation from the standpoint of solid state physics, of the mechanisms involved in surface phenomena. Moreover, new and particularly beneficial techniques of ore dressing have been proposed; although these techniques were successfully tested with relatively sim ple separation problems, the future progress forecasted could render their appli cation very profitable also for the treatment of difficult ores. REFERENCES 1 M.Carta, R. Ciccu, C. Del F, G. Ferrara, M. Ghiani, P. Massacci, Improvement in electric separation and flotation bv modification of energy levels in surfa ce layers, X Int. Processing Congress, I.M.M., London, April, 1973,pp.120 -148 2 M.Carta, R. Ciccu, C. Del F, G. Ferrara, M. Ghiani, P. Massacci, Progress' nel la conoscenza delle caratteristiche elettrofisiche e strutturali della fluorite e dei minerali con essa associati, Giornata di Studio sulle Fluoriti Italiane, Ass.Min.Subalpina, Torino, December, 1972, oo.38-52. 3 R. Ciccu, C. Del F, L'influenza di alcune tecniche di modifica del livello ener getico superficiale dei minerali sui processi di separazione elettrostatica. Pubblicazione della Facolt di Ingegneria dell'Universit di Cagliari, Italy, June, 1972. 4 M. Carta, R. Ciccu, C. Del F, M. Ghiani, Etude exprimentale de la ohisiaue de la surface des minraux et le traitement des minerais. Collogue Minerals et Mineraux, S.I.M., Nancy, October, 1978. 5 R.Ciccu, C. Del F9, G. Alfano, P. Carbini, L. Curreli, P. Saba, Some tests of electrostatic separation gangue. Seminar on Beneficiation of Lean Phosphates with carbonate gangue, XI Int. Min. Processing Congress, Istituto di Arte Mine raria dell'Universit di Cagliari, Cagliari, May, 1974, p p . 220-241. 6 M. Carta, C. Del F, R. Ciccu, L. Curreli, M. Agus, Technical and economical problems connected with the dry cleaning of raw coal and in particular with py rite removal by means of electric separation, VII Int. Coal Preparation Congress Bureau of Mines, Australia, Sydnev, May, 1976, pp. 220-248.