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118
УДК 544 © D. Erdenechimeg, B. Oyun, P. Solongo, B. Maralmaa
INFLUENCE OF SULPHITE IONS ON KINETICS OF GOLD ANODE DISSOLUTION IN ALKALINE AND ACID THIOUREA SOLUTIONS
Electrochemical methods were applied to study the anodic behavior of gold in alkaline thiourea solution in the absence and presence of ^SO3. Gold is highly soluble in acid thiourea in the presence of Fe (III)oxidant. But, gold is difficult to dissolve in alkaline thiourea solution because of the elemental sulfur passivation produced by the irreversible decomposition of thiourea. However, addition of ^SO3 accelerates the selective dissolution of gold in alkaline thiourea solution. ^2SO3 additive improves the stability of thiourea in alkaline media, enhances the anodic polarization current of gold greatly, and does not change the passivation states of the associated elements of gold in alkaline thiourea solution. During dissolution of gold in alkaline thiourea solution the co-elements of gold do not show any hindrance. Identification of the optimal condition of gold dissolution process in alkaline thiourea was carried out. It is the solution of 0.5 mol/l of ^2SO3; 0.1 mol/l of thiourea in temperature of 323 К, and pH value of 12.5. In kinetic study, gold dissolved in alkaline thiourea solution with ^jSO3 and the activation energy was 26.79 kJ/mol and it's respective potential was 0.24 V.
Keywords: electrochemical methods, selective dissolution of gold
Д. Эрдэнэчимег, Б. Оюун, П. Солонго, Б. Маралмаа
ВЛИЯНИЕ ИОНОВ СУЛЬФИТА НА КИНЕТИКУ АНОДНОГО РАСТВОРЕНИЯ ЗОЛОТА В ЩЕЛОЧНОМ И КИСЛОТНОМ РАСТВОРАХ ТИОМОЧЕВИНЫ
Электрохимические методы были применены для изучения анодного поведения золота в щелочном растворе тиомоче-вины в присутствии и отсутствии N2SO3. Золото высоко растворимо в кислотной тиомочевине в присутствии окислителя железа (III). Но, железо трудно растворяется в щелочном растворе тиомочевины из-за пассивации элементарной серы, полученной в процессе необратимого разложения тиомочевины. Тем не менее добавка O3 ускоряет селективное растворение золота в щелочном растворе тиомочевины. Эта добавка улучшает стабильность тиомочевины в щелочной среде, значительно повышает анодное поляризационное течение золота и не меняет состояние пассивации связанных элементов золота в щелочном растворе тиомочевины. Во время растворения золота в щелочном растворе тиомочевины со-элементы золота не создавали помех. Была проведена идентификация оптимального состояния процесса растворения золота в щелочной тиомочевине. Это раствор 0,5 моль/л Nа2SO3; 0,1 моль/л тиомочевины при температуре 323 К и значении pH 12,5. В кинетических исследованиях золото растворялось в щелочном растворе тиомочевины с добавлением ^2SO3, энергия активации составляла 26,79 kJ/моль и ее соответствующий потенциал был 0,24 V.
Ключевые слова: электрохимические методы, селективное растворение золота
Since the initial report of thiourea as a complexing agent of gold dissolution, considerable research has been directed towards the use of thiourea as an alternative ligand to cyanide for gold leaching [1, 4, 7-9, 12]. The gold has good solubility in alkaline thiourea solution, but the main problem is corrosion of equipment used in the process [2]. Within the last 10 years the studies of gold dissolution in alkaline thiourea solution have been carried out by Li Chai and Masazumi Okido [2] to solve this problem.
The product of thiourea decomposition - formamidine disulfide plays a very important role of oxidant in gold dissolution, another product is thiourea acting as a ligand complex ion with gold ion directly. Formamidine disulfide, however, further decomposes irreversibly to elemental sulfur, which causes thiourea to be consumed excessively [4]. This increases the consumption of thiourea [2, 4]. However, formamidine disulfide is evolved as the decomposed product of thiourea and plays an important oxidant role in gold dissolution in alkaline thiourea solution mechanism (in reaction 1). Some additive substances are used to prevent formamidine disulfide in alkaline media from decomposing irreversibly during the gold dissolution (reaction-2) [3, 12].
NH2(NH)CSSC(NH)NH2 + 2H2O + 2e- = 2CS(NH2) + 2OH- E0 = 0.42V (1 ) SO42- + H2O + 2e- = SO32- + 2OH- Eo= -0.033V (2)
NH2(NH)CSSC(NH)NH2+SO3-+H2O=SO4-2+2CS(NH2)2 (3)
Thiourea consumption depends on the thiourea/oxidizing agent concentration ratio and nature of the oxidizing agent used in acid media [11]. Iron (III) sulfate is a useful oxidant for dissolving gold in acid thiourea solution [1].
In the present work, the polarization behavior of gold electrode and the electrochemical aspect of gold dissolution are studied in order to compare gold dissolution in alkaline and acid thiourea solutions.
Experimental
Measurements are made by "Linear sweep voltammetry" method using Bipotentiostat/Galvanostat ^stat-400 equipment. Anodic polarization curves were measured using a typical three-electrode system, including working electrode, platinum wire as the counter electrode, and a saturated calomel electrode. By working electrode used the boards with squire Au (S=3.6 cm2). In software Dropview 2.1 of equipment Bipotentiostat selected parameters are
starting potential of -0.2 V, finishing potential of 0.8 V, scan rate 10 mV/sec [5, 6]. The electrode polarization curve in relation to potential was obtained. Solution was controlled by pH- meter adding 0.5 mol/l H2SO4 or NaOH.
Result and discussion
Effect of Na2SO3 on gold dissolution in alkaline thiourea solution. Anodic gold electrode polarization curve is obtained in both cases with or without Na2SO3 (Fig 1). When Na2SO3 is added, the sharp rise of current density from 0.1 V shows effective dissolution of gold in comparison with the solution without Na2SO3.
The pH effect for gold dissolution in thiourea solution. The effect of pH on the anodic polarization curves for the gold electrode in 0.1mol/l thiourea solution containing 0.5 mol/l Na2SO3 is shown in Fig. 2. The increase of pH over 12.5 has shown the decrease of gold dissolution potential. This indicates that gold dissolves in the alkaline media faster than in the acidic media in case the solution contains thiourea and sodium sulfite.
Fig. 1. The effect of Na2SO3 on gold dissolution in alkaline thiourea solution, 1-with Na2SO3, 2 - without Na2SO3
Fig. 2. The effect of pH on the anodic polarization curves for the gold electrode
Fig. 3. Temperature effect on anodic current of Au electrode in alkaline thiourea
Fig. 4. Temperature effect on anodic current of Au electrode in acid thiourea
The temperature effect for gold dissolution in acid and alkaline thiourea solution. The effect of temperature 294, 313, 323 and 333 K on the anodic current of gold electrode in 0.1mol/l thiourea solution with pH-12.5 in presence of Na2SO3 is studied. The temperature effect is high (Fig 3). In temperature of 333 K the anode polarization is the highest but respective potential curve has bended at the point of 0.5 V potential. It shows that decomposition of gold-thiourea complex takes place if the potential is higher than 0.54 V and the temperature is above 50oC (323 K). Considering this gold dissolution is more favorable at 323 K in alkaline thiourea solution with presence of Na2SO3 (Fig. 3).
In Fig. 4. the effect of temperature 293, 303 and 313 K on the anodic current of gold electrode in 0.05 mol/l thiourea solution with pH 2 is shown. The gold dissolution was dependent on the condition, and temperature effect was poor in acid solution. Depending on the temperature effect on the dissolution the respective region can be determined [10]. The gold dissolution rate in acid thiourea solution can be determined by the last region or by diffusion region [1]. To determine the gold dissolution rate in alkaline thiourea solution and its respective region curve is built in 0.54 V and 0.24 V potentials (Fig. 5). Activation energy of each potential is estimated by Arrhenius' equation (1).
k=Aexp(-E/RT) (1)
Calculated activation energy in 0.24 V potential is 26.79 kJ/mol and the activation energy in 0.54 V potential is 17.39 kJ/mol. It shows that the dissolution rate of gold can be determined in kinetic regions when potential is 0.54 V. When potential is 0.24 V activation energy is between 20 kJ/mol - 40 kJ/mol [11] and the dissolution rate of gold can be determined in both of diffusion and kinetic regions.
3.5 3 2.5
!s 2
1.5 1 0.5
2.8 3 3-2 3.4 3.6
1/T (10"3K)
Fig. 5. Relntion between ln i vs 1/T
Gold is difficult to dissolve in alkaline thiourea solution in the absence of Na2SO3 due to the passivation by elemental sulfur. The addition of Na2SO3 to alkaline thiourea solution makes the selective dissolution of gold possible. The additive Na2SO3 enhances the anodic current of the gold electrode drastically and prevents thiourea in alkaline media from decomposing irreversibly.
The current research provided a basis for the selective extraction of gold from scrap using alkaline thiourea solution, to which Na2SO3 has been added. The activation energy of reaction calculated respectively are 26.79 kJ/mol, 17.39 kJ/mol at the potentials of 0.24 and 0.54 V. The dissolution rate of gold can be determined both in diffusion and kinetic regions. The optimal concentration of acid thiourea to gold dissolution is determined as 0.05 M by electrochemical method and the optimal temperature is 303 K (30oC). The anodic current of gold dissolution in alkaline thiourea is more than the gold dissolution in acid thiourea solution.
References
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11. Maslenitskii I.N., Chugaev L.V. Metallurgy of pressures metals. - Moskow, 1987. - P. 74-109.
12. Tremblay L., Deschenes G., Ghali E., McMullen J., Lanouette M. Gold recovery from a sulphide bearing gold ore by percolation leaching with thiourea // International journal of minerial processing. - 1996. - V. 48. - P. 225-244.
Erdenechimeg Dolgor, doctor of chemical sciences, professor, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, e-mail: Erdenechimeg_d@num.edu.mn
Oyun Batsuh, postgraduate student, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, e-mail: Oyun10@yahoo.com
Solongo P., postgraduate student, Depatment of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, e-mail: solongo_sun@yahoo.com
Maralmaa Byambaa, master, Department of Chemical Technology, School of Chemistry and Chemical Engineering, National University of Mongolia, e-mail: Maralaa_b@yahoo.com
Эрдэнэчимег Долгор, доктор химических наук, профессор, отделение химической технологии, Школа химии и химической инженерии, Национальный университет Монголии, e-mail: Erdenechimeg_d@num.edu.mn
Оюун Батсух, аспирант, отделение химической технологии, школа химии и химической инженерии, Национальный университет Монголии, e-mail: 0yun10@yahoo.com
Солонго П., аспирант, отделение химической технологии, Школа химии и химической инженерии, Национальный университет Монголии, e-mail: solongo_sun@yahoo.com
Малармаа Буямба, студент, отделение химической технологии, школа химии и химической инженерии, национальный университет Монголии.
КОНЦЕНТРИРОВАНИЕ ИОНОВ ЗОЛОТА КОМПЛЕКСООБРАЗУЮЩИМИ СОРБЕНТАМИ
НА ОСНОВЕ 1-ВИНИЛ-1,2,4-ТРИАЗОЛА
Исследованы сорбционные свойства сетчатых сополимеров 1-винил-1,2,4-триазола и 1-винил-1,2,4-триазола со стиролом, сшитых дивинилбензолом, определены сорбционная емкость, коэффициент распределения, избирательность, возможность использования.
Ключевые слова: сорбция, сорбционная емкость, коэффициент распределения, избирательность.
Sorbtion properties of copolymers 1-vinyl-1,2,4-triazole and 1-vinyl-1,2,4-triazole with styrene cross-linked divinyl benzene were investigated; sorption capacity, distribution coefficient, selectivity,utilization are determined.
Keywords: sorbtion, sorbtion capacity, distribution coefficient, selectivity.
Прямое определение элементов в реальных объектах, несмотря на высокую чувствительность современных методов анализа, затруднено как низкими содержаниями, так и составом матрицы, связанным с влиянием макрокомпонентов на аналитический сигнал. Сорбционное концентрирование с использованием комплексооб-разующих сорбентов характеризуется эффективностью и позволяет решать задачи определения малых количеств элементов в образцах сложного состава, агрессивных технологических растворах. Анализ научной литературы показывает, что поиск эффективных сорбентов для извлечения благородных металлов является актуальной задачей. Многочисленные исследования опубликованы в ряде монографий и обзорных статей [1-6]. В качестве сорбентов предлагаются соединения различных классов: группа сорбентов, выпущенная под торговой маркой Полиоргс, волокна, модифицированные силикагели, полиорганосилоксаны и т.д. [7-10]. Высокую сорб-ционную активность по отношению к благородным металлам проявляют комплексообразующие сополимеры на основе 1-винил-1,2,4-триазола с различными сомономерами или сшивающими агентами [11, 12]. Актуальное значение имеют работы, связанные с поиском достаточно несложных схем синтеза и использованием доступных реагентов.
Целью данной работы является изучение сорбционных свойств сополимеров 1-винил-1,2,4-триазола (ВТ), 1-винил-1,2,4-триазола со стиролом (СТ), сшитых дивинилбензолом (ДВБ), и возможности их аналитического применения.
Сетчатые, не растворимые в органических растворителях, кислотах и щелочах сополимеры С1 и С2, характеристики которых представлены в табл. 1, получали радикальной сополимеризацией:
УДК 546.42
© Л.П. Шаулина, Т.Г. Ермакова, Н.П. Кузнецова, Г.Ф. Прозорова
L.P. Shaiilina, T.G. Ermakova, N.P. Kuznetsova. G.F. Prozorova
œNCTNTRATION OF GOLD IONS WITH COMPLEXING SORBENTS ON THE BASIS OF 1-VINYL-1,2,4-TRIAZOLE
С1
С2
