Bán Mária

Novák Mihály

English

University of Szeged

0 THE CHANGE OF THE ELECTRODE POTENTIAL OF SPHALERITE-PYRRHOTITE SYSTEMS AND ITS ROLE AT THE WEATHERING OF SULPHIDE ORE DEPOSITS (Preliminary Report) by MICHAEL NOVAK and MARIA BAN Institute for Mineralogy and Petrography, University of Szeged, Hungary INTRODUCTION The outcrop of sulphide ore deposits and their part beneath the sur-face, respectively, is exposed to the effect of chemical and physical fac-tors of the weathering. The study of the oxidation processes taking place in' the oxidation zone is essential also from the point of view that the cementation zone, often representing economically important and valu-able mineral enrichment, forms in the deeper part of the ore body due to the action of metalbearing aqueous solutions seeping down from the oxidation zone. The processes taking place a t . the weathering of sulphidic ore de-posits earlier were interpreted as simple chemical reactions, the electro-chemical properties of the single ores, 'however, were only less taken into consideration. The relatively few referring data of the earlier lite-rature are comprised by H. Schneiderhôhn [1924] in his comprehensive wortk dealing with the mineralogy of oxidation and cementation zone, respectively. The examinations mentioned above have dealt partly with the solu-bility relations of the single ores and the interaction referred to the solubility due to the contact of the ores, partly with the problem of the electromotive force series of the sulphide ores. It is plausible that the solubility relations of the single ores are essential at thé weathering of the ores and the formation of the oxida-tion and cementation zone. In the change of the solubility properties, however, the differences among the electrode potentials of the single ore minerals play an important role. 59 THE ELECTROMOTIVE FORCE SERIES OF SULPHIDE ORE MINERALS FROM THE POINT OF VIEW OF THE INTERPRETATION OF THE WEATHERING Recently, examinations regarding to the electromotive force series of these ores are carried out by Rechenberg [1951]. He used as elec-trolyte, not destilled water as Gottschalk and Buehler [1910], but solu-tions of different concentration and pH. On the basis of his examina-tions, he stated that apart from minimal deviations the sulphide ore mi-nerals in a given e. m. f. series can be arranged and the sequence of the minerals in this series is determined by structural properties. Rechenberg's opinion: the sequence according to the potential, recalcu-lated to the normal hydrogen electrode, is analogous with the normal potential series of the metals. The question is, whether i t would be possible to establish a defined potential series of general validity referring to the sulphide ore minerals or not, and further, how far would such a series 'be applicable to inter-pret the weathering relations of single sulphidic ore deposits. It is pointed to by Grasselly [1953] that while the potential series of the metals means the series according to the normal potentials, measured at exactly defined conditions, the normal potential of natural sulphide ore minerals can not be so simply established. The sulphide ores approach only exceptionally the ideal composition containing nearly always mechanical and/or isomorphous impurities in varying quantities, further, they are mostly in contact not alone with self-ionic aqueous solutions and the ion-concentration is also changeable. It seems to be more reliable studying the weathering of sulphide ore deposits that not so much the establishing of a defined potential series of general validity for sulphide ore minerals is essential, as the establishing of the potential series valid for ' the given, deposit under the given circumstances, putting up this series by taking into account of the actual circumstances of the given deposit such as its, mineral association, the actual composition of the ore minerals, their mechanical inclusions and isomorphous im-purities, the pH and the composition and concentration of the aqueous solutions 'being in contact with the ores. According to the above-mentioned, the potential of the single ore minerals gives some explanation regarding their solubility relations only in the case if these potential values are not taken • into conside-ration in general, but are referred to the environment wherein the ores occur, because also the same ore mineral in an other occurrence can show partly an other potential value, partly also at the same potential value a different behaviour can be observed owing to the different en-vironmental conditions. Recently, Noddack, Wrabetz and Herbst [1955a, 1955b, 1956] have treated the electrode potential of natural and artificial sulphides not so much from geochemical but mostly from physico-chemical viewpoint. According to their examinations most of the natural and artificial sulphides follow the Nernst's equation and in a galvanic cell, building with their own metal, they represent the positive pole. On the basis of the measured potentials, a defined electromotive force series is given 60 I also by these authors. It is to be noted that in the series both natural and artificial sulphides are present. The authors pointed to that the results are strongly influenced by stoichiometrical deviations and by the presence of oxygen either on the surface of the electrodes or in the electrolyte. The effects influencing the electrode potentials of the sulphide ore minerals are derived partly from the. properties of the ore minerals themselves, partly from the properties of the environment. Thus, from the point of view of the electrochemical processes playing an essential role at the weathering, among others, also the changes in the com-position of the ores, being caused either by mechanical inclusions or by isomorphus components, are to be taken into account. THE EFFECT OF THE CHANGE OF THE Zn/Fe RATIO ON THE CHANGE OF THE ELECTRODE POTENTIAL OF SPHALERITES The sphalerite-pyrrhotite system seemed to be very suitable to study the potential-influencing role of the isomorphous components, since partly the iron-containing sphalerite is common ore mineral of hydrothenmal sulphide deposits, partly as in the sphalerite the zinc can -be substituted by iron in considerable amounts and this solid solution in given cases may remain stable also by normal temperature. The iron content of the sphalerites depends upon the pressure and tem-perature at the time of its formation. The 'higher the formation tempera-ture the greater amount of the zinc can be substituted by iron in the lattice of the sphalerite. The question." is whether the pyrrhotite content and its change in the sphalerites causes changes in the electrode potential of sphalerite, and if it does, in what direction this change takes place. The purpose of the examinations was to establish the connection between the change of the pyrrhotite content and the electrode potential .of the sphalerites, the direction of the probable change of the electrode po-tential and further to. establish whether this change is unequivocal or not with the change of the iron content. ' The measurements were carried out by Orion pH electrometer. The electrodes were sphalerite specimens, from various localities. The Zn and Fe content of the samples were determined. Electrolyte was ZnS04 solution in various concentration. Reference electrode: n calomel electrode. The potential values in the Tables are those measured vs. n calomel electrode. At the grinding and polishing of the surface of the ore sections care was taken to have possibly identical surface quality' in the case of each sample. The polished ore sections were also micro-scopically examined to find the part on the surface where it contains no or relatively few inclusions and thus to find the part designed' to take up the cell containing the electrolyte. The locality, the, zinc and iron content and their ratio, respec-tively, as well as the measured potentials (vs. N. C. E.) of. the samples are comprised in Table 1. ' 61 Table 1. Locality Zn o/o Fe «/o Potential in mV Zn/Fe 0,01 m ZnSO. pH 2,5 Rodna {Oradna, Rumania) -Her ja (Kisbánya, Rumania) Rodna (Oradna, Rumania) Gyergyóholló (Rumania) Gyöngyösoroszi (Hungary) Dognacea (Dognácska, Rumania) Bañská Stiavnica 48 42,8 54 60.3 63 52.4 14,4 12 11,9 4,2 2 1,7 3,3 3,5 4,5 14,4 31 31 — 138 — 78 .— 105 — 6 0 + 230 + 485 (Selmecbánya, Czechoslovakia) Picos del Europa (Spain) 63 66 1,8 0,33 35 200 + 380 + 620 The data — despite of the relatively few samples — offer the possibility to answer the question. It can be stated that in the poten-tial values of the sphalerites significant différencies prevail presumably depending upon the iron content. The potential is more positive at low iron content whereas at higher iron content the potential is less positive or even negative vs. the normal calomel electrode. The potential be-comes more positive with increasing Zn/Fe ratio. However, at the same time can not 'be exactly stated that the change of the potential would be determined only and in every case, predominantly by the change of the iron content. There were samples, the potential of which, despite of the similar iron and zinc content and Zn/Fe ratios, respectively, were different. Slight or more consi-derable individual différencies in potentials of the samples with simi-lar iron and zinc content, however, do not change the tendency that the sphalerites with higher iron, content, referring to higher formation temperature, posses in general rather negative potential whereas the sphalerites formed at lower temperature containing less iron, show an increasing positive potential with the decreasing iron content versus the normal calomel electrode. The change of the potential of the sphalerite samples is in con-nection besides the iron content with other factors too, as e. g. the différencies in granular structure of the specimens, the quality and quantity of the foreign inclusions at the surface of the polished ore section. The potential of the various sphalerite samples, is co-controlled by these factors too. It is also to be taken into consideration that the iron content of the sphalerites can be present not only as FeS in solid solution, but also as unmixed pyrrhotite lamellae or drops in the spha-lerite. In the latter case the presence of several micro local elements at the surface of the polished ore -section is to be taken into account and also these exert an influence on the average potential of the sample. That is, two sphalerite samples with the same iron content can posses different potential depending upon whether the iron, as solid solution or unmixed pyrrhotite inclusions, is present. 62 The electrode potential of sphalerites of different iron content changes between —138 mV and + 620 mV versus the normal calomel electrode. THE POSITION OF SPHALERITES FROM DIFFERENT LOCALITIES IN THE POTENTIAL SERIES OF' SULPHIDE ORE MINERALS It was already mentioned that several authors have determined a defined potential series for the sulphide ore minerals. From some point of view, however, may be questionable the suitability of putting up of such a potential series of natural sulphide ore minerals, considering this series as generally valid, and further, the Tightness of the interpretation of the weathering of different sulphide deposits on this basis. What does the position of single ore minerals in the potential series mean practically from the point of view of the weathering? It means that in the case of a polymineralic ore vein, the ore minerals of higher potential, in contact with ore minerals of lower potential, are rather protected from dissolution, whereas the ore minerals of lower potential behave anodic, i. e., they will dissolve and their solubility is greater than that of which could be established in the case of pure ore minerals in no contact with others. Arranging the sphalerites from different localities on the basis of their electrode potential into the potential series proposed either by Recheriberg [1951] or Wrabetz, Noddack and Herbst [1955b], it can be stated that sphalerite samples can be ranged into both series in dif-ferent positions, and the electrode potential and thereof the position in the series, further the behaviour of the different sphalerite speci-mens in contact with the other members of these series, roughly, is de-termined — with the reservations mentioned above — by the iron (pyrrhotite) content depending upon the locality, the conditions of the formation. In our opinion the comparison with the potential series according to Noddack, Wrabetz and Herbst [1955b] seems to be rather reliable, though in this series 'besides the natural sulphides also artificial sul-phides are involved, but the potentials are in self-ionic solutions meas-ured, whereas the potential series according to Rechenberg [1951] con-tains alone natural sulphides, but the potentials are measured in solu-tions of foreign ions. The fact that the various sphalerite specimens examined, in dif-ferent positions can be ranged into the potential series considered of general character by the authors mentioned, may mean the following. 63 Table 2. The position of sphalerite specimens f rom different localities in the potential series of sulphide ore minerals according to Rechenberg and Noddack, Wrabetz and Herbst Potential series according to Noddack, Wrabetz and Herbst Sphalerites, locality • potential, iron content Potential serie? according to Rechenberg AgaS (art.) 579 mV CuS (art.) 319 mv ZnS (nat.) 219 mV Cu2S (art.) 199 mV FeS2 (nat.) 139 mV PbS (nat.). 89 mV ' MnS (art.) — 121 mV Picos del Europa 620 mV; 0,33% Dognacea 485 mV; 1,7% Bañská Stiavnica 380 mV; 1,8% Gyöngyösoroszi 230 mV; 2 % Gyergyóholló — 60 mV; 4,22 % Her ja — 78 mV; 1 2 % Rodna — 138 mV; 14,4% Pyri te Marcasite . Chalcopyrite Sphaleri te Pyrrhoti te Galena Argentite 377 mV 351 mV 277 mV 178 mV 117 mV 114 mV — 5 mV The potential values a re referred to normal calomel electrode. While in the case of ideal or approximately ideal pure sulphide electrodes, among others in the case of ZnS electrode, a well defined potential value, and on the basis of such potential values,' a defined potential series can be established, in the case of natural sulphide ore minerals as' here, that is, in that of natural sphalerites of different lo-calities and of different iron content, can not be generally spoken about the electrode potential of the sphalerite, because this potential value due to the action of several factors may be changed witihin wide limits. This consideration can. not be limited only to the sphalerites, but it can be applied also to the natural sulphide ore minerals in ge-neral. In the nature the- minerals, whose composition is close to the ideal ones are rather rare and the contamination of the ores by diffe-rent foreign components is common. These mechanical and/or iso-morphous impurities also in the case of the same mineral may be dif-ferent depending upon the locality, the conditions of the formation and therefore, the same mineral occurring, however, in different localities. 64 having different electrochemical properties, from the point of view of weathering a diverse behaviour may show.,. Thus considering the series of the sulphides, according to Noddack, Wrabetz and Herbst as well as Rechenberg in Table 2, if the potential series is considered as the potential series of the sulphides and the position of each mineral therein and the conclusions to be drawn are treated only in general, it can be stated that the sphalerite in both series is ranged approximately into the middle and the galena has a less positive potential than the sphalerite in both series, that is, in contact with galena the sphalerite will be protected from dissolution, whereas the solubility of galena becomes more considerable than if it were in no contract with sphalerite. Inspite of all, however, can not be supposed that in contact of the two minerals mentioned above, always the galena tends to dissolved and the sphalerite will be protected as the adverse case is possible too. Considering namely, the series of the sphalerites from different loca-lities and assuming that the composition and thereof also the potential of galena varies between closer limites than that of the sphalerite, it can be assumed that even the sphalerite with higher iron content hav-ing thus a less positive potential than the galena, will dissolve in con-tact with the latter. So, it is presumable that sphalerites with higher iron content, e. g. sphalerites from Rodna or Herja, in contact with galena or pyrite, being of anodic character in this mineral-association, will rather dissolve. There again, the sphalerites with low iron con-tent, e. g. sphalerites from Picos del Europa or Dognacea, in a mineral-association of sphalerite-pyrite-chalcopyrite-pyrrhotite-galena will be protected and rather the other members of this series will be dissolved. Thus, on interpreting the weathering processes of given sulphide deposits on the basis of the electrochemical properties of the single sul-phide ore minerals, not the electrochemical properties of the single sulphides in general, but always the actual conditions and circum-stances of the given occurrence are to be taken into account. The further examinations are in progress. REFERENCES \ Grasselly, Gy. (1953): Electrochemical Examination of the Oxidation Processes of Sulphide Ores. — Acta Miner. Petr. Univ. Szegedinensis, 7, 47—64. Noddack, W. und K. Wrabetz (1955a): Über das elektrochemische Verhalten eini-ger Schwermetallsulfide. — Z. fü r Elektrochemie, 59, 96—102. Noddack, W., Wrabetz, K. und W. Herbst (1955b): Die elektrochemische Span-nungsreihe der Sulfide. — Z. fü r Elektrochemie, 59, 752—755. Rechenberg, H. (1951): Untersuchungen über die elektrochemische Spannungsreihe der Erzmineralien. — N. Jahrb. f. Miner. Monatshefte. Schneiderhöhn, H. (1G>24): Die Oxydations- und Zementationszone der sulfidischen Erzlagerstätten. — Fortschr. d. Miner. Krist. und Petr. 9, 115. Wrabetz, K. (1956): Über galvanische Zellen mit Sulfidelektroden. Z. für . Elektro-chemie, 60, 722—731. 65 

The change of the electrode potential of sphalerite-pyrrhotite systems and its role at the weathering of sulphide ore deposits : preliminary report

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The change of the electrode potential of sphalerite-pyrrhotite systems and its role at the weathering of sulphide ore deposits. Nóvák, M. and M. Bán : (preliminary report)

The change of the electrode potential of sphalerite-pyrrhotite systems and its role at the weathering of sulphide ore deposits. Nóvák, M. and M. Bán : (preliminary report)

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