In each roasting run 30 g of the antimonite ore sample was divided ... to P4O6 molecules and based on the As4 tetrahedron. Above 567°C the oxide ... reaction is given by the empirical formula represented by Wagner[21] as: (4). The values ...
J ChimPhys (1997) 94, 620-634 © Elsevier, Paris
Thermodynamic behaviour of complex antimonite ore for electrodeposition of metal value LHMadkour Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt (Received 9 November 1996; accepted 26 July 1996)
ABSTRACT This paper outlines the hydrometallurgical processing and the various possible approaches for the recovery of metallic antimony from its antimonite sulphide oreby electrolysis. Thermodynamic calculations have been made to find the optimum conditions for roasting and/or sulphate roasting of Wadi Abu Quraiya antimonite ore, The antimonite ore (Sb2S3) is roasted directly without concentration and large savingsin reagent and chemical processes are expected. Asuitable roasting temperature of 1200K for 4 hours has been determined. Metallic antimony is cathodically electrodeposited with a high current efficiency at current densities of 350 to 500 A/m2 dependingonthe composition of leach liquor solutions. The results of the electron microscopy investigation confirmed by metal value data in the ASTMcards are in good agreement with the chemical analysis. Key words: Antimony sulphide ore; roasting; hydrometallurgical treatment; antimony deposition. RÉSUMÉ Cet article décrit le traitement hydrométallurgique et les diverses méthodes possibles pour extraire par électrolyse l’antimoine du minerai de sulfure d’antimoine. Des calculs thermodynamiques ont été faits pour définir les conditions optimales du grillage. Le minerai est grillé directement sans préconcentration, ce qui conduit à des économies substantielles. L’antimoine métallique est déposé à la cathode avec un très bon rendement. Les résultats de microscopie électronique analysés à l’aide des fiches ASTM sont en bon accord avec l’analyse chimique. Mots clefs: minerai de sulfure d’antimoine, traitement hydrométallurgique· électrodéposition d’antimoine.
— 621 —
I, INTRODUCTION The antimonite (Sb2S3) mineralization located at Wadi Abu Quraiya in the Central Egyptian Eastern Desert is the main ore mineral and occupies about 40%t[1] of the total mineral constituents. A few minute grains of pyrite, chalcopyrite and sphalerite are sometimes observed as inclusions in the silicates and a fewgoethite grains are also present. Antiinonite is generated in the epithermal zone and usually oxidized into various oxidest[2’3] of yellowish-white coloration (senannontite (Sb203), cervantite (Sb204) and stibiconite (H2Sb203)). The oxidation[4] of antiinonite takes place in the presence of water and enough oxygen. The antiinonite crystals are often cracked and fractured, the fractures are filled by the alteration products of antiinonite (cervantite) in longveins. These veins may be strongly folded as shown in Fig. 1.
Figure 1. Cervantite showing characteristic dissolutionfeaturres (X 1700)
Hydrometallurgical treatment based on leaching and precipitation rather than smelting will playan important role in meeting the requirements for the treatment of complex[5] ores. The thermochemistry of roasting of complex ores by controlling the temperature and equilibrium gas composition for solid ore has been discussed in a number of reviewsand research reports[6,9]. The aim of this work is economically to find the optimum temperature conditions for roasting or controlled sulphate roasting of the antimony sulphide (Sb2S3) ore in order toavoidusing excess quantities of concentrated acids consumed for direct leaching the native ore and consequently recovery of metallic antimony fromthe leach roasted liquorbyelectrolysis.
— 622 —
II. EXPERIMENTAL Sampling Mineralized horizon ore was finely powdered to 100%minus 1.0 mmand driedbefore the roasting process. The antimonite ore was subjected to mineralogical, chemical, spectral and X-ray diffraction analyses as described in the previous paper[10]. Thermal analysis This was carried out by means of a Du Pont instruments 990 thermal analyzer D.T.A. 1200°C cell with OC-Al2O3 as reference. The powdered sample (lg) was heated at the rate of 20°C min-1up to 600°C and range 10 mV cm-1. Hydrometallurgical treatment The roasting experiments on the Wadi Abu Quraiya ore, were carried out at Central Metallurgical Research and Development Institute (CMRD1) at Tabbin (Egypt) ina6 cm diameter and 100 cm height stainless steel reactor. The flow rate of air for fluidization was measured with the help of rotameter. A pressure indicator was introduced between the rotameter and the compressed air line, so that back pressure developed during roasting can be observed. The material ore was fed along withthe fluidising air and the rate of feed of the material was the same as to that ofroasted product collected in the cyclones. The reactor was externally heated before the start of the experiment and the reaction temperature was thereafter controlled manuallyby controlling the flow of the combustion gas. In each roasting run 30 g of the antimonite ore sample was divided equally into three boats for roasting at temperatures ranging from 400°C-1000°C for 4 h. The roasted ore products of the three boats resultingat each roasting temperature were separately leached directly in 30%HC1, 10%H2SO4 and 25% NaOH leach liquors respectively with continuous stirring. The solutionot decomposed ore was filtered, washed with distilled water and finally collectedina measuring flask for antimony electrodeposition. Electrolysis system The electrolytic cell design and general experimental procedure were the same as described elsewhere[1-1 6] All chemicals used were of Analar quality and were used without further purification.
—623 — III. RESULTS AND DISCUSSION The mined ore analysis is given in Table 1. The DTA of the investigated sample of antimonite ore is shown in Fig. 2. The sample shows a very broad endothermic trough at 59°C. This is assigned to loss of humidity water content or physically combined water from the crystal boundaries. Further temperature rise displays four successive exothermic peaks. The peak at 204°C represents the first stage of oxidation of stibnite (Sb2S3) into pentoxide (Sb2O5) . It can easily be seen that the native sample displays an intense broad exothermic peak covering the temperature range 294-432°C, peaking at 388°C. This is due to the most stable oxide Sb2O4[17]. Further temperature rise displays a small peak at 443°C where Sb2O4 loses oxygen and forms the trioxide Sb2O3. The last peak at 557°C is due to phase changes corresponding to another solid formwith a polymeric structure which is in accordance with the observation reported inliterature[18’19] Table 1. Chemical and spectral analyses of antimonite ore sample
Element Sb Fe Pb Zn As Cu Mn Cr Stotal
Moisture Ni B Ti Sn Ag Co Si02 L.O.I.*
Content % 65 0.5 0.05 0.03 2 0.01 0.003 0.005 6.67 0.84 0.0005 0.001 0.0001 0.0001 0.00001 0.00005 18.74 5.86
*Loss on ignition in weight percent at 1100°C.
Mol/mol Sb 1
0.017 4.5 x 10-4 8.6 x 10-4 0.05 3xl0-4 1.02 x 10-4 1.8 x 10-4 0.39 0.0874
—624 —
Figure 2. DTAfor the native antimonite ore sample.
Antimony trioxides and pentoxides are both converted into Sb204iftheyare taking up or losing oxygen respectively. At a suitable temperature (e.g. 750°C)the oxygen pressure of Sb204 is less than that in the atmosphere, while that of Sb2O5is greater. Sb205 must therefore give up oxygen, but it does not decompose beyondthe Sb204 stage. Sb2O3 , the oxygen pressure of which is naturally less thanthat of Sb204, must take up oxygen at this temperature, especially as this reaction proceeds exothermally. Sb2O4 is stable over a wide temperature range, though it seems tohe able to give mixtures with Sb2O3 and Sb2O5 near the limits of this temperature interval, and for this reason is not now used in the quantitative estimation of antimony. The properties of antimony tetroxide Sb2O4 are intermediate between those ofSb2O3 and Sb2O5. It may be regarded as an antimonous salt of antimonic acid; quadrivalent antimony is probably not present in it, since the compounds of quadrivalent antimony are very deeply coloured. Thermodynamic calculationsfor equilibrium roaster gas compositions: The solid form of antimony trioxide Sb2O3 is stable[18] up to 570°C and abovethis temperature there is another solid form with a polymeric structure. The trioxideof antimony is of structural interest, because it shows a transition fromthe molecular lattice characteristic of covalent compound to an ionic lattice. The cubic formof Sb4O6 is similar to arsenic oxide which contains As4O6 molecules, similar in structure to P4O6molecules and based on the As4 tetrahedron. Above 567°C the oxide Sb4O6is converted to the macromolecular[19] valentinite form containing infinite chains
—625 — Antimony pentoxide Sb2O5loses oxygen on mild heating, to give the trioxide Sb2O3. When either oxide of Sb is heated in air at about 900°C[18] white power of stoichiometry SbCA is formed. It has been found that SbO2exists in two structurally different but related fonns[20] Both forms contain a 1:1 mixture of SbIII and SbVand thus the formula is usually written Sb2O4, but should be written as SbO8. This view is supported by the crystal structure of Sb4O8. Above 900°C it loses oxygen and forms thetrioxide. Important reactions that take place when the antimonite ore (Sb2S3) is roasted can be representedby the following equations: ( 1) (2)
(3)
Reaction (1) is strongly exothermic and for all practical purposes during roasting, the equilibrium shifts to the right, with the formation of antimony trioxide (Sb2O3) and S02. Normally the heat evolved in this reaction is enough to sustain the necessary thermal requirements of the roaster. The higher the temperature, the faster the reaction, and the conditions that are available in fluo-solid roaster, giving through mixing of the gasphase with the solids, proves an added advantage. Reaction (2) is of far more importance for sulphate roasting, since the partial pressure of S03inthe furnace atmosphere, whether higher or lower than the equilibriumpartial pressure, decides the presence or absence of sulphates in the calcine. In an oxidizing atmosphere and at lower temperatures, more SO3 is formed. At higher temperatures S02 is more stable; over 700°C, especially in presence of metallic oxides, the reaction rate is higher and more SO3 will decompose to give SCANevertheless some SO3 will always be present and the roaster gases contain almost equal proportions of SO2 and s03. The relation of equilibrium constant (K) with absolute temperature (T) for the reactionis given by the empirical formula represented by Wagner[21] as: (4)
The values obtained for equilibrium constant (K) for different temperatures (T) have beenutilized in following calculations. The fonnation of metallic sulphates depends on the equilibrium constant for reactions ofthetype:
— 626 —
(3)
Since Sb203 and Sb2(SO4)3are solids, their activities can be taken as unity andthus the values of Kdepends on the partial pressure of S03. If the S03partial pressure in the furnace atmosphere is more than the equilibrium pressure of S03for reaction(3). then more of the oxides formed in the reactor according to reaction (1) would react to formthe sulphates according to reaction (3). In order to arrive at the temperature conditions for selective sulphation, it is possibleto theoretically study the effect of the varying proportions of 10, 15, 20, 30, 40 and50 mol of air/mol Sb or 6, 10, 15, 20, 30, 40 and 50 mole of enriched air containing30% oxygen/mol Sb at the roasting temperatures of 700 - 1200 K on the roaster gas composition. The various amounts of oxygen required to formthe different types of antimony oxides can be calculated as: (1 ± 0.25) mol of oxygen are required to convert 1moleof antimony into its different oxides. Since Sb02 is of remarkable stability and infact itis the most stable oxide below 900°C it is the ultimate product of heating (inair)the trisulphide Sb2S3, the trioxide Sb203 or the pentoxide Sb205 above 300°C. Above 900°C Sb02 loses oxygen and forms the trioxide. Sb203. Depending on theabove stability of Sb02, it is possible to theoretically study this case of formation of SbO2 oxide which needs only one mole of oxygen. Theoretical requirements of oxygen to convert all the elements into their oxides froma quantity of ore sample containing 1mole of Sb can be calculated fromTable 1togive 1.4704 mole of 02. Fromthe stoichiometry of the various reactions shown above, it is possible to studythe effect of the varying proportions of either (10-50) mol of air/mol Sb or (6-50) mol of oxygen enriched air /mol Sb. Thus, it is possible to arrive at a material balance for the various gases in the roaster once the proportion of air to ore feed is known. Taking into consideration the equilibrium: (2)
If x is the moles of S03formed the equilibriumconstant is given by:
—627 — (5) Thevalue of Kfor any particular temperature can be obtained. Substituting this value of K in equation (5) we can get the value of x for any particular temperature, then the values of partial pressure of gases or their molar percentage in the roaster gases can be arrived at. Fromthis calculation we deduced that the molar percentage of SO2 falls by changing the feed of air to Sb from 10 to 50. The changes in SO3 percentage are not much affected; the increased amount of oxygen available means more SO2 is converted into SO3, thus compensating for any effect on SO3 percentage due to dilution. The advantage of oxygen enrichment is that a higher SO2 content in the roaster gas can be achieved with a smaller volume of air. The use of oxygen enrichment, particularly where the sulphide content is low, may also result in theautogenous roasting of the ore. In the case of using (6 - 50) mol of oxygen enriched air/mol Sb at the above named roasting temperatures (700 -1200 K) we obtained that SO3 content is high at lower temperatures whereas SO2 content is high at higher temperatures. If the temperatures were lowered, a stage would be reached when the solid oxides present in the calcine would start absorbing SO3 forming the various sulphates. The temperatures at which such reactions would start can be determined by plotting the variation of the decomposition pressures of the various sulphates, against temperatures. Fig. 3 gives the decomposition pressures of the various possible sulphates. The values were calculatedfromthe log Kvalues for the various reactions as given by Kellogg[7]. The decomposition pressure for the two zinc sulphates normal as well as basic two copper sulphates and the ferric sulphate have been plotted by dotted lines as a function of temperature in Fig. 3 and the points of inter-section of these lines with that representing roaster gas SO3 composition, represents the temperature up to which the various sulphates indicated are stable in the roaster atmosphere. The decomposition pressures for the various sulphates of lead are much less than the other sulphates and these have not been plotted. It is observed from Fig. 3 that at 905 K, the ferric sulphate starts decomposing to formferric oxides, while all other sulphates at this temperature are quite stable. The nonnal sulphate of copper decomposes into its basic sulphate at 1010 K and the basic sulphate into cupric oxide at 1060 K. Normal sulphate of zinc starts decomposing at 1040 Kto its basic sulphate and the latter is stable up to 1135 K when the ZnO starts forming. The antimony sulphate starts decomposing to form antimony oxide at 1180 K. Thus between 900 Kand 1000 Kthe decomposition of ferric sulphate takes place, while the copper, zinc, lead and antimony sulphates remain stable.
— 628 —
Figure 3. Equilibrium gas compositions on roasting of Wadi Abu Quraiya antimonite ore ni 1atm. with oxygen enriched air 30% oxygen 6 mols of air/mol Sb content.
—629 — Resultsofthe roasting and leaching experiments Aseries of continuous roasting experiments were carried out using calculated amount of air to make a ratio of 10 moles of air per mole of antimony at temperatures ranging from673 K-1273 K.for 4 h. The rate of feeding of the ore was uniformduring the run. The hot roasted product (30 g) obtained in each roasting run which was divided equally into three boats was separately leached directly in 10%H2SO4, 30%HCI and 25%NaOHrespectively at 60°Cwith stirring. Cationic compounds of SbIII are mostly of the so-called antimonyl ion, (SbCT), although some of the (Sb3+) ion, such as Sb2(S04)3, are known. Antimony salts readily formcomplexes with various acids in which the antimony forms the nucleus of ananion. The Sb2(SO4)3 obtained after the roasting process decomposes in water:
In H2SO4 acid leaching, the species present vary markedly with the acid concentration[22], namely.
Incaseof hydrochloric acid leaching antimonyl chloride is obtained:
The slum was filtered and the leached liquor in each case was analysed[23] for antimony. The process used consists in bringing the antimony contained in the ore into solution as antimonyl ion SbO+ This is earned out by roasting the ore into Sb2(SO4)3 together with the oxide Sb2O3, followed by leaching process in either 30% HCl and/or 10% H2SO4 and/or 25% sodium hydroxide. The impurities can be reduced or eliminated with the precipitation of ferric hydroxide. Any ferrous iron present is first oxidisedto the ferric state by hydrolysing the ferric sulphate.
—630 — At the beginning of leaching, the solid to liquid ratio (S/L = 1:10) by weight. No auxiliary heating is required, as the temperature of the pulp is upwards of about 333K. due to the heat fromthe added calcine, exothermic reactions and the heat of hydration. The pulp remains in the neutral leach for about 1h. Towards the end of the neutral leach, the ferric sulphate in the mother liquoris hydrolyzed to form insoluble basic salts. The rate of leaching increases as the temperature rises, due to an increase in the rate of diffusion and the rate of chemical reactions between the acid used and the solid antimony compounds. The agitation of the pulp, consisting of solid particles and leachant, speeds up diffusion. The acid leach step destroys the antimony silicates to formcolloidal silicic acid:
The antimonites[17] show exactly similar variations in the forms of the salts : fromsalts of the acid HSbCh precipitation gives salts of such acids as:
The alkali antimonites are more highly hydrolysed than the arsenites, so that theyare decomposed by water with the deposition of solid Sb2O3 and are soluble onlyin excess alkaline NaOH liquor. The composition of solid antimonites is dependent toa large extent on the temperature of roasting as well as on the excess of base. The isolated alkali salts are mainly metantimonites, e.g. NaSbO2. The antimonates arevery similar to the antimonites, but the formation of salts of the meta-acid HSbO3ismost common. The percentage dissolution of (SbO+) at different roasting temperatures for 41ns shown in Table 2.
—631 — Table 2. Percentage dissolution at different roasting temperatures for 4h followed by acid and/or alkalie leaching processing.
%SbO+ Temperature (K)
30%HCl
10%h2so4
25%NaOH
673 773 823 923 1073 1180 1200 1273
36.1 50.8 63.7 55.3 49.7 76.4 88.2 84.6
21.4 45.8 55.5 49.8 58.4 67.8 79.8 73.7
32.6 49.7 60.4 53.6 47.9 72.3 81.5 80.1
It is observed that the percentage of antimony ions dissolved increases from36%at 673Kto 63.7%at 823 Kand thereafter decreases due to phase changes corresponding toanother solid form with a polymeric structure which is insoluble in water and also very resistant to acids. Above 1173 K Sb2O4 loses oxygen and forms the trioxide Sb2O3, which it is soluble in concentrated solutions of HC1, H2SO4 and alkalies. Thus, fromthe results obtained, the optimumconditions for roasting the antimonite ore areat about 1203 Kfollowed by direct leaching in either 30%hydrochloric acid and/or 10%sulphuric acid and/or 25% NaOH solution as shown in Table 2. Hence, the optimum values for the roasting process fromexperimental measurements (Table 2) are in good agreement with those obtained above fromthermodynamic calculations andas plotted in Fig. 3. Electrodeposition of antimony Metallic antimony can be recovered fromthe obtained chloride, sulphate and/or alkali antimonites leach liquor respectively by electrolysis. The amount of SbO and impurities which pass into solution depends on the composition of the starting mineral, its granulation, roasting temperature and on the free acid or alkali content of the lixiviating solution. The factors affecting current efficiency are the opposite of those governing applied voltage. The least energy consumption can be obtained when an optimumbalance between all the factors involved is struck. It is not possible to extract
—632 — all the antimony ions present in the original ore leach liquor because a certain proportion remains unattacked, and because another part remains trapped inthesolid residue; this is gelatinous in nature due to the presence of silicic acid and Fe2O3.xF2O. The effect of the concentration of the metal ion in the leach ore solution was studied during the deposition of antimony. Smooth, adherent deposits of silvery white antimony were obtained at lowconcentration. The results showthat there is acritical concentration of SbO+ at which one obtains maximum rate of deposition. This concentration is (0.30 - 0.50) g per 100 cm3 of total volume of the bath at current density of 500 A/m2with current efficiency (Q%= 75%). Also, deposition takes place in the absence of complexing agents, so the effectiveness of the complexing agent in rendering differences between the reversible potential of antimony and its standard potential is important as it aids deposition[24]. Furthermore, complexing agents have an important role in ensuring the presence of sufficiently small SbCCion concentration at the cathode so that this concentration may be suitable for the reduction andthe smooth deposition of the element. At lowcurrent density only a thin laser of antimony was deposited. At higher current densities (> 1500 A/m2) a non-adherent and randomly oriented deposit of antimony over the surface of the cathode was obtained which is in accordance with the observation reported in literature[25]. This can alsobe inferred from the lower current efficiency obtained. The optimum current density required for the metallic antimony electrodeposition from its various leach liquor chloride, sulphate and alkalie antimonites was found as 400 A/m2, 500 A/m2and350 A/m2respectively coincide well with results given elsewhere[10]. CONCLUSION The polymetal complex antimonite ore Wadi Abu Quraiya considered in Egypt asa rather rich source of antimony is subjected to mineralogical, chemical, spectral, X-ray and differential thermal analyses. Roasting or controlled sulphate roasting is appliedto the antimony sulphide ore (Sb2S3) in order to saving reagents rather than larger leach solutions consumed, if the native ore is directly leached without roasting. Thermodynamic studies have been made to find the optimum conditions for sulphate roasting, either in air or in the presence of enriched 30% oxygen air. Theresults obtained fromthe experimental treatment at different roasting temperatures are ingood agreement with theoretical thermodynamic values, which indicated that a maximum dissolution of SbO+ could be obtained at roasting temperature of 1200 Kfor4h. followed by leaching the roasted ore in either 30%HCl, or 10%H2SO4 and/or 25% NaOH at 333 K. The extents of dissolution of 88.2%, 79.8%and 81.5%of SbO+were
—633 — the maximum for using the respective leach solutions HCl, H2SO4 and NaOH. Metallic antimony is cathodically deposited at (350-500 A/m2) current density with a high current efficiency (Q%= 75%) fromthe above named leach liquors. The results of the electron microscopic investigation confirmed by metal value data given in the ASTMcards coincide well with results given by chemical analysis. REFERENCES [1] Salem 1A(1989) The occurrence of stibnite mineralization at Wadi Abu Quraiya. Central Eastern Desert, Egypt Mineralogical Society· of Egypt Conference . [2] El-Sharkawi MA, Saleh AG(1973) The occurrence of cervantite in stibnite veins at Umm Quraiya and Atud districts Eastern Desert U.A.R. Bulletin Fac Sci Cairo Univ 46, 423 - 428. [3] Lindgren W (1933) Mineral deposits. Me Graw-Hill book Company, Inc. New York-London, 930 p. [4] ParkCF, MacDiarmid RA(1975) Ore deposits Freeman and Co, San Francisco, 530 p. [5] Viswanathan P V, Yedavalli B V S, Srinivasan S R, Bhatnagar PP(1968) Symposium on recent development in non-ferrous metals technology, vol. II copper. [6] Smithson Jr G R , Hanway Jr J E (1962) Bench scale development ofa sulphation process for complex sulphide ores. Trans. Metallurgical Society· of A.I.M.E. , 224, 827 p. [7] Kellogg H H (1964) A critical Review of Sulphation Equilibria Trans. Metallurgical Society ofA.I.M.E. , 230, 1622-1634 p. [8] Taha F, Afifi S E. Madkour L H (1982) J. Tabbin Institutefor Metallurgical Studies (T.I.M.S.) 47, 1-19. [9] Madkour L H (1985) Thermodynamic studies on sulphate roasting for zinc electrowinning from carbonate ore J Chem Tech Bioteclmol 36, 197-211. [10] Madkour L H. Salem I A (1996) Electrolytic recovery of antimony fromnatural stibnite ore J. Hydrometallurgy (in press). [11] Afifi S E, Madkour L H (1984) Electrolytic deposition of metal values from Umm. Samiuki polymetal ore Egypt J. Chem 27, 3, 275-296. [12] Madkour L H (1985) J. Chem Tech Biotech 35, A3, 108-114. [13] Madkour LH et al. (1986) J. Electroanal Chem 199, 207-210. [14] Madkour LH (1995) J. Erzmetall 48, 104-109. [15] Madkour LH (1995) Indian J. of Chemical Technology 2, 343. [16] Madkour L H (1996) Electrodeposition of lead and its dioxide from Egyptial carbonate ore residuumJ. Bulletin of Electrochemistry 12, 3-4, 234-236. [17] Thome PC L, Ward A M (1939) Inorganic chemistiy, 3rd English edn. Gurney and Jackson London,700-728 p.
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