CC BY
151
Lidiya А. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
Metallurgy and Mineral Processing
UDC 669.53
EXTRACTION OF COPPER, COBALT AND NICKEL IONS FROM AQUEOUS SOLUTIONS BY EXTRACTANT CYANEX 272
Lidiya A. VOROPANOVA, Viktoriya P. PUKHOVA
North Caucasus Mining and Metallurgical Institute (State Technological University), Vladikavkaz, Russia
The extractant CYANEX 272, which active component is di(2,4,4-trimethylpentyl)-phosphinic acid (C8H17)2POOH, is effective for extraction of copper (II), cobalt (II) and nickel (II) ions. The extraction of metal ions using di(2,4,4-trimethylpentyl)-phosphinic acid as an extractant is carried out due to the formation of an organophosphorus complex with wide pH range: copper at pH > 2, cobalt at pH > 3, and nickel at pH > 5. They are extracted with an organic phase: copper at pH = 3-7, cobalt at pH = 4-7, and nickel at pH = 6-9, and precipitate in the organophosphorus compound: copper at pH > 7, cobalt at pH > 8, and nickel at pH > 10. The possibility of separation of copper (II) and cobalt (II) is insignificant, the stripping of copper (II) and nickel (II) happens at pH = 4-6, and the stripping of cobalt (II) and any of nickel (II) - at pH = 5-6.
The obtained results of ion extraction of the investigated metals can be used not only for processing of technological solutions, but also for purification of effluents from industrial enterprises and mine waters, heap and underground leaching solutions, etc. from the ions of studied metal ions.
Key words: aqueous solution; extraction; extractant; copper; cobalt; nickel
How to cite this article: Voropanova L.A., Pukhova V.P. Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions by Extractant Cyanex 272. Journal of Mining Institute. 2018. Vol. 233, p. 498-505. DOI: 10.31897/PMI.2018.5.498
Introduction. The extraction of cobalt from nickel solutions by alkyl phosphinic acids is largely covered in literature. It is known that to extract cobalt ions from aqueous solutions of sulphates and nickel chlorides it is efficient to perform it with CYANEX 272 extractant, the active component of which is di (2,4,4-trimethylpentyl)-phosphinic acid (C8Hi7)2POOH [1-13]. Metals are extracted by means of a cation exchange mechanism. The CYANEX 272 extractant can be totally mixed with aromatic and aliphatic solvents and is very resistant to heating and hydrolysis, it shows a high selective ability to separate cobalt from nickel, both in sulfate and chloride media, has low ability to extract calcium ions, which excludes gypsum contamination that may appear in the liquid extraction cycle.
It is known that cobalt is extracted selectively from nickel salt solutions within a narrow range: pH « 5. However, the extraction of copper, cobalt and especially nickel in their joint presence in solution, and the extraction of copper and cobalt impurities from solutions of macro quantities of nickel salts has been little studied. The processes of interaction of the extractant with solutions of salts at pH > 7 have not been studied.
Materials and methods of research. The objectives of this study are to find the optimal conditions for the selective and combined extraction of copper, cobalt and nickel by extracting the CYANEX
272 extractant and reextraction from solutions having complex composition, as well as to study the conditions for extracting copper and cobalt impurities from solutions with macro quantities of nickel salts, including usage of kerosene as diluent extractant, and study of the processes of interaction of the extractant with solutions of salts at pH > 7.
Crystalline hydrates of salts CuSO45H2O, CoSO47H2O and NiCl37H2O were used to prepare solutions. The extraction was carried out with stirring and constant pH adjustment, NaOH and H2SO4 solutions were used as neutralizers. The experiments were carried out at room temperature.
Figure 1 shows the scheme of units for extraction and reextraction.
o
, IAAA/N
3-
Fig. 1. The scheme of extraction units 1 - mixer; 2 - beaker; 3 - thermal element; 4 - separatory funnel; 5 - initial solution; 6 - extractant; 7 - extract; 8 - raffinate
5
7
8
4
^Lidiya A. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
D
2000
1000 -
D i
800
400 -
0 ■
pH
Cu
-■— Co -A- Ni
7 pH
Fig.2. Dependency of distribution coefficient D from solution pH of salts CuSO4, CoSO4 and NiSO4: a - individual solutions; b - mixed solutions
A beaker with the initial solution containing extractable ions and the extractant was placed on a thermal element to heat the mixture to a predetermined temperature. For better mass transfer, the contents of the beaker were stirred with a mixer for a preset amount of time. The mixture was then transferred to a separatory funnel, where it was settled and separated into extract and raffinate. After settling they were poured into separate containers through the lower outlet of the separatory funnel and analyzed.
The extraction results were evaluated by the residual concentration of C (in grams per cubic decimeter) of metal ions in the raffinate, the distribution coefficient D = the extraction of
metals in the organic phase (in percent by weight of the initial phase), and the separation coefficient P = Dyej / DMe2.
Results and discussion. Figure 2 shows the dependency of the distribution coefficient D on the pH of the aqueous solution of salts CuSO4, CoSO4 and NiSO4.
The concentration of initial solutions by the sum of metals was, mg/dm3: 400-550 for individual solutions and 660-770 for mixed solutions of metal salts. The set pH was maintained for 0.1-1.5 hours, afterwards it changed insignificantly. The initial aqueous-oil emulsion contained 5 % by weight of the extractant (ratio of aqueous and organic phases A:O = 19:1). The volumes of the organic and aqueous phases changed insignificantly in comparison to initial ones: the volume of the organic phase remained unchanged and the water content was 0.8-1.0.
In Table 1, according to Fig.2, the best extraction results from individual solutions of salts and from mixed solutions of CuSO4, CoSO4 and NiSO4 salts obtained in 0.5 h.
Table 1
The best extraction results from individual and mixed solutions of CuSO4, CoSO4 and NiSO4 salts with CYANEX extractant 272
b
а
0
3
5
Extraction results
Metal of initial solution, mg/dm3 PH Residual concentration, mg/dm3 Distribution coefficient D Extraction, % by weight from initial solution
Extraction of individual solutions
Cu 395 4 6 1232 98.5
Cu 395 5 3 2483 99.2
Cu 395 6 3 2483 99.2
Co 546 5 7 1463 98.7
Co 546 6 6 1710 98.9
Co 546 7 10 1018 98.2
Ni 533 6 12 825 97.7
Ni 533 7 17 577 96.8
Ni 533 8 58 156 89.1
^Lidiya A. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
End of Table 1
Metal Concentration of initial solution, mg/dm3 Extraction results
pH Residual concentration, mg/dm3 Distribution coefficient D Extraction, % by weight from initial solution
Extraction of mixed solutions of salts
Cu 168 4 21 133 87.5
Cu 168 5 5 619 97.0
Cu 168 6 3 1045 98.2
Cu 168 7 3 1045 98.2
Co 276 5 11 458 96.0
Co 276 6 10 505 96.4
Co 276 7 22 219 92.0
Ni 241 6 73 44 69.8
Ni 241 7 78 40 67.8
Ni 241 8 83 36 65.5
In alkaline medium at pH > 7 from individual solutions metals precipitate in the organophos-phorus compound in the gel-like form. The solution was separated from the precipitate through a medium density filter. The filtration was done fast. The filtrate was analyzed for the metal content.
At pH = 8-11, the blue precipitate was formed in the solution of copper sulfate. At pH = 12, a gel-like precipitate of brown color is formed. The best precipitation results were obtained at pH = 12: the minimum residual concentration C = 5 mg/dm3 Cu (II), extraction 98.7 % by weight.
At pH = 8-9 the solution of cobaltous sulphate has noticeable blue precipitate. The best precipitation results were obtained at pH = 8: the minimum residual concentration C = 74 mg/dm3 Co (II), extraction 86.4 % by weight.
During extraction of nickel at pH = 7-9, the organic phase had the form of a lump with higher density and viscosity. The solution was filtered to remove light turbidity.
At pH = 10-12, the green gel-like precipitate is formed. When settling for more than 24 hours, the ratio between the volumes of clarified aqueous phase and precipitate was 2:1. The best precipitation results were obtained at pH = 12: the minimum residual concentration was C = 1 mg/dm3 Ni (II), extraction 99.8 % by weight.
The separation factors ß of copper, cobalt and nickel ions, calculated from the data in Fig.2 and Table 1, are given in Table 2 and in Fig. 3.
400
200
pH
b ß 80
60
40 -
20
4
5
6
pH
Fig.3. Separation factors p of copper, cobalt and nickel ions during extraction from sulfate solutions of their salts (extractant - CYANEX 272): а - from individual solutions; b - from mixed solutions of salts
а
ß
0
0
^Lidiya A. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
Table 2
Separation factors p of coper, cobalt and nickel ions during extraction from sulfate solutions of their salts
(extractant - CYANEX 272)
Extraction PH P
From individual solutions From mixed solutions
Cu/Co Cu/Ni Co/Ni Cu/Co Cu/Ni Co/Ni
4 30.0 246.4 8.2 8.3 33.3 4.0
5 1.7 496.6 292.6 1.4 77.4 57.3
6 1.5 3.0 2.1 2.1 23.8 11.5
7 - - 1.8 4.8 6.1 5.5
According to the data in Tables 1 and 2 and Figures 2 and 3 the following conclusions can be drawn:
1. The extraction of metal ions using di(2,4,4-trimethylpentyl)-phosphinic acid as reagent is carried out due to the formation of an organophosphorus complex with wide pH range:
• copper is formed at pH > 3, cobalt - at pH > 4, nickel - at pH > 5. They are extracted at organic phase:
• copper - at pH = 3-7, cobalt - at pH = 4-7, nickel - at pH = 5-9. They are precipitated as a part of organophosphorus compound:
• copper - at pH > 7, cobalt - at pH > 7, nickel - at pH > 10.
2. Selective extraction of studied ions from aqueous solutions of sulfates during extraction is possible with the following sequence in terms of pH values:
• from individual solutions
Ion Cu Co Ni
pH 4-5 5-6 6-8
• from mixed aqueous solutions of salts
Ion Cu Co Ni
pH 4 5 6
3. The possibility of separation of copper (II) and cobalt (II) is insignificant, and stripping of copper (II) and cobalt (II) from nickel (II) occurs at pH = 5.
4. The extraction of copper (II), cobalt (II) and nickel (II) ions when they are combined in sulfate solutions is much lower than during extraction from individual sulfate solutions.
Table 3 shows the results of reextraction of copper, cobalt and nickel ions from the extracts obtained in the experiments presented in Table 1.
Table 3
Results of extraction of copper, cobalt and nickel ions form extract
Extraction pH Reextraction pHinit ! a & e o tí S w w tí Metal concentration, mg/dm3 Coefficient of distribution D between organic and aqueous phases P Extraction in reextract, % by weight from initial solution
In the initial extract, V = 10 cm3 In reextract, V = 50 cm3
Cu Co Ni Cu Co Ni Cu Co Ni Cu/Co Cu/Ni Co/Ni Cu Co Ni
4 1 0.84 2793 - - 525.6 558.6 0 0.3
5 1 0.87 3097 5035 - 581.4 1003.2 155.8 0.3 0.02 - 15 - - 93.8 - -
6 1 0.87 3135 5054 3192 562.4 1005.4 572.4 0.6 0.03 0.6 20 1 0.05 89.6 99.6 89.6
7 0 1.10 4788 4826 3097 463.6 955.6 562.4 0.6 0.05 0.5 12 1.2 0.1 48.5 99.0 90.7
From the data in Table 3 it follows that reextraction is effective at pH < 1, when extraction was carried out at pH 4-6, and at pH < 0, when extraction was carried out at pH 7-8.
^Lidiya A. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
100
80
£ 60
40 -
20
pH
b ß 80000
60000
40000
20000
4
4,5
ß 1
300
200
100
pH
Cu Co Ni
Cu/Co Cu/Ni Co/Ni
pH
Fig.4. Results of batch extraction of metal ions from mixed sulfate solutions of salts in relation to solution pH:
а - extraction, b - separation coefficient
а
0
0
0
4
Table 4 and Figure 4 show the extraction results, which were carried out at a ratio of organic and aqueous phases of O:A = 1:19, a stepwise increase in the pH of the solution during the extraction according to the 4 ^ 5 ^ 6 scheme and subsequent reextraction of each step at pH = 1 and the ratio O:A = 1: 5. The extraction time was 30 minutes and reextraction - 24 hours.
Table 4
Results of batch extraction of metal ions from aqueous sulfate mixed solutions of salts
Concentration, mg/dm3
Extraction pH Reextraction pHend Initial solution, V = 190 cm3 Reextractant, V = 50 cm3 Raffinate
Cu Co Ni Cu Co Ni Cu Co Ni
4 0.17 168 323 176 520.6 558.6 0 30.9 176.0 176.0
5 1.30 168 323 176 38.0 646.0 239.4 21.0 6.1 113.0
6 1.34 168 323 176 11.4 19.0 266.0 18.0 1.0 42.9
7 1.36 168 323 176 11.4 3.8 0 15.0 0.3 42.9
Extraction pH D ß Total extraction in reextractant, % by weight from initial solution
Cu Co Ni Cu/Co Cu/ Ni Co/Ni Cu Co Ni
4 84.3 15.9 0.001 5.3 84300 15900 81.6 45.5 0.01
5 133.0 987.1 10.6 0.1 12.5 93.1 87.5 98.1 35.8
6 158.3 6118 58.9 0.03 2.7 103.9 89.3 99.7 75.6
7 193.8 20437.7 58.9 0.01 3.3 347.0 91.1 99.9 75.6
It follows from the data in Table 4 and in Figure 4 that, when the content in the initial solution (mg/dm3) is 168 Cu, 323 Co, 176 Ni, at pH = 4 of copper 82 %, at pH = 5 of cobalt 98 % and at pH = 6 of nickel 76 %.
Selective extraction of non-ferrous metals can be accomplished by fractional extraction with a gradual change in the solution pH and maintaining the changed pH value at each batch extraction; extraction of predominantly copper ions is carried out by extraction at pH = 4, cobalt at pH = 5, and nickel at pH = 6-7.
Table 5 and Figure 5 show the results of batch extraction of copper (II) and cobalt (II) ions against the background of the nickel (II) macro component. The extraction was carried out at a ratio of the organic and aqueous phases of O: A = 1:19 and a stepwise increase in the pH of the solution during the extraction process according to the scheme 4^5^-6 at the extraction time for each step no more than 2 hours, the subsequent reextraction of each step at pH = -0.5, the ratio of O: A = 1: 5 and 2 days reextraction.
The composition of raffinate (extractant CYANEX 272, extraction time at each stage was not more than 2 hours) is the following: concentration, mg/dm3: Cu-35, Co-148, Ni-55300; composition, % by weight: Cu - 0.06, Co - 0.27, Ni - 99.67.
^Lidiya A. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
80
40 -
4
5
pH
400 -
200
4
- -5
pH
Cu Co Ni
Cu/Co Cu/Ni Co/Ni
Fig.5. Results of batch extraction of metal ions from aqueous mixed sulfate solutions of salts against the nickel (II) micro component in relation to solution pH: a - extraction, b - separation coefficient
b
a
ß
0
0
Table 5
Results of batch extraction of metal ions from aqueous sulfate mixed solutions of salts against the nickel (II) macro component, extractant CYANEX 272
Reextraction pHend Concentration, mg/dm3
Extraction pH Initial solution, V = 190 cm3 Reextractant, V = 50 cm3 Raffinate
Cu Co Ni Cu Co Ni Cu Co Ni
4 5 6 -0.37 -0.07 -0.06 286 286 286 416 416 416 55950 55950 55950 270 460 205 228 485 293 613 982 751 215.1 94.1 40.0 356.1 228.4 151.4 55782.2 55502.4 55334.6
Extraction pH D ß Total extraction in reextractant, % by weight from initial solution
Cu Co Ni Cu/Co Cu/ Ni Co/Ni Cu Co Ni
4 5 6 6.3 38.7 116.9 3.2 15.6 33.2 0.06 0.15 0.21 2.0 2.5 3.5 105 258 556.7 53.3 104 158.1 24.8 67.1 86.0 14.4 45.1 63.6 0.3 0.8 1.1
From data in Table 5 and Figure 5 it follows that when the initial solution has (mg/dm ) 286 Cu, 416 Co, 55950 Ni, at pH = 4 we extract 25 % of copper, at pH = 5 of cobalt 45 % and at pH = 6 of nickel 1 %, the ratio of metals in raffinate after extraction allows to have nickel with purity of 99.7 %.
100
m 80
60
£
ö o
40
W
20
4
ß
1000 -400 -
600 -400 -
200 0
Cu Co Ni
Cu/Co Cu/Ni Co/Ni
pH
pH
b
a
0
5
5
3
4
3
Fig.6. Results of batch extraction of metal ions from aqueous mixed sulfate solutions of salts against the nickel (II) macro component in relation to solution pH: a - extraction, b - separation coefficient. Extractant 30 % solution of CYANEX 272 in kerosene
^Lidiya A. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
From the data in Tables 4-5 and Fig. 4-5 it follows that a significant increase (by more than 2 orders of magnitude) in the concentration of nickel ions in the solution reduces the recovery of copper and cobalt.
Table 6 and Figure 6 show the extraction of copper (II) and cobalt (II) ions against the background of the nickel (II) macro component, extraction was carried out at a ratio of the organic and aqueous phases of O: A = 1: 6 (the extractant used 30 % solution of CYANEX 272 in kerosene) and a stepwise increase in the pH of the solution during the extraction according to the scheme of at the
extraction time for each step no more than 2 hours, and subsequent reextraction of each step at pH = -0.5 , the ratio O: A = 1: 5 and the time of reextraction - 3 days.
The composition of raffinate (extractant - 30 % solution of CYANEX 272 in kerosene) is the following: concentration, mg/dm3: Cu - 0; Co - 100; Ni - 59900; composition, % by weight: Cu - 0; Co - 0.17; Ni - 99.83.
Table 6
Results of batch extraction of metal ions from aqueous sulfate mixed solutions of salts, extractant - 30 % solution of CYANEX 272 in kerosene, extraction time at each step is not more than 2 hours
Concentration, mg/dm3
Extraction pH Reextraction pHend Initial solution, V= 190 cm3 Reextractant, V= 50 cm3 Raffinate
Cu Co Ni Cu Co Ni Cu Co Ni
3 -0.60 278 266 61600 72 52 420 259.9 252.4 61476.8
4 -0.70 278 266 61600 75 96 500 61.7 227.4 61353.6
5 -0.95 278 266 61600 16 440 3360 17.8 111.2 60491.2
6 -1.00 278 266 61600 40 74 1800 16.1 91.8 59998.4
Extraction pH
D
Cu
Co
Ni
Cu/Co
Cu/ Ni
Co/Ni
Total extraction in reextractant, % by weight from initial solution
Cu
Co
Ni
0.4 21.0 87.7 97.6
0.3 1.0 8.4 11.4
0.01 0.02 0.11 0.16
1.3 21.0 10.4 8.6
40 1050 797.3 610
30 50 76.4 71.3
6.8 77.8 93.6 94.2
5.1 44.6 63.2 65.5
0.2 0.4 1.4 1.8
ß
From data in Table 6 and Figure 6 it follows that when the initial solution has (mg/dm3) 278 Cu, 266 Co, 61600 Ni, at pH = 4 we extract 78 % of copper, at pH = 5 of cobalt 58 % and at pH = 6 of nickel 2.6 %, the ratio of metals in raffinate after extraction allows to have nickel with purity of 99.8 %.
From the data in Tables 5-6 and Figs. 5-6 it follows that the use of kerosene as a diluent increases the extraction of copper and cobalt.
Conclusions
1. Extractant CYANEX 272 is effective for the extraction of copper (II), cobalt (II) and nickel (II) ions.
2. The possibility of copper (II) and cobalt (II) separation is insignificant, and the stripping of copper (II) and cobalt (II) from nickel (II) occurs at pH = 5.
3. The obtained results of ion extraction of the investigated metals can be used not only for processing of technological solutions, but also for purification of effluents from industrial enterprises and mine waters, heap and underground leaching solutions, etc from the ions of studied metal ions.
Acknowledgment. The research was carried out with the financial support of the RFBR and the Ministry of Education and Science of the Republic of North Ossetia-Alania within the framework of the scientific project N17-48-150696/17.
^Lidiya A. Voropanova, Viktoriya P. Pukhova
Extraction of Copper, Cobalt and Nickel Ions from Aqueous Solutions
REFERENCES
1. Vol'dman G.M., Zelikman A.N. Theory of hydrometallurgical processes. Moscow: Intermet-Inzhiniring, 2003, p. 464 (in Russian).
2. Gindin L.M. Extraction processes and their application. Moscow: Nauka, 1984, p. 144 (in Russian).
3. Naboichenko S.S., Ageev N.G., Doroshkevich A.P. et al. Processes and apparatuses of non-ferrous metallurgy. Ural'skii tekhnicheskii universitet. Ekaterinburg, 2005, p. 700 (in Russian).
4. Kasikov A.G., Sharando M.A., Bagrova E.G., Serba N.V. Patent 2617471 RF. Method for extracting cobalt from sulfate solutions of nickel and cobalt. Opubl. 25.04.2017. Byul. N 12 (in Russian).
5. Voropanova L.A., Krutskikh E.Yu., Titukhina V.N. Patent 2203969 RF. Method for selective extraction of copper, cobalt and nickel ions from aqueous solutions. Opubl. 10.05.2003. Byul. N 13 (in Russian).
6. Voropanova L.A., Kisiev N.T., Gagieva Z.A. Patent 2604289 RF. Method for purification of nickel electrolyte from iron (III), cobalt (III) and copper (II) impurities by extraction. Opubl. 10.12.2016. Byul. N 34 (in Russian).
7. Radushev A.V., Gusev V.Yu., Bogomazova G.S. Extraction of copper (II) and nickel (II) from acid solutions with hy-drazides of naphthenic acids. Sovremennye problemy khimii i tekhnologii ekstraktsii. 1999. Vol. 2, p. 189-194 (in Russian).
8. Kotukhov S.B., Greiver M.B., Yur'ev A.I., Bol'shakov L.A. Extraction purification of sulfate nickel electrolyte from impurities. Tez. dokl. XIII Rossiiskoi konferentsii po ekstraktsii, 19-24 sentyabrya 2004 g. Moscow, 2004. Part 2, p. 229-230 URL: https:www.fbr.ru.rffi/ru/projeckt_search/0_254342 (date of access 10.08.2017) (in Russian).
9. Gharabaghi M., Irannajad M., Azadmehr A. Separation of nickel and zinc ions in a synthetic acidic solution by solvent extraction using D2EHPA and Cyanex 272. Physicochemical Problems of Mineral Processing. 2013. N 49 (1), p.233-242.
10. Ocana N., Alguacil F.J. Cobalt-manganese separation: The extraction of cobalt (II) from manganese sulphate solutions by Cyanex 301. J. of Chemical Technology and Biotechnology. 1999. Vol. 73. N 3, p.211-216.
11. Sole K.C., Hiskey J.B. Solvent extraction of copper by Cyanex 272, Cyanex 302 and Cyanex 301. Hydrometallurgy. 1995. Vol. 37, p. 129-147.
12. Tait B.K. Cobalt-nickel separation: the extraction of cobalt (II) and nickel (II) by Cyanex 301, Cyanex 302 and Cyanex 272. Hydrometallurgy. 1992. Vol. 32, p.365-372.
13. Mihaylov I., Krause E., Colton D.F., Okita Y., Duterque J.-P., Perraud J.-J. The development of a novel hydrometallurgical process for nickel and cobalt recovery from Goro laterite ore. CM Bulletin. 2000. Vol. 93. N 1041, p.124-130.
Authors: Lidiya A. Voropanova, Doctor of Engineering Sciences, Professor, lidia_metall@mail.ru (North Caucasus Mining and Metallurgical Institute (State Technological University), Vladikavkaz, Russia), Viktoriya P. Pukhova, Candidate of Engineering Sciences, Engineer, puhova-vikochka@mail.ru (North Caucasus Mining and Metallurgical Institute (State Technological University), Vladikavkaz, Russia).
The paper was received on 11 September, 2017.
The paper was accepted for publication on 8 May, 2018.
