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versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.54 n.4 Concepción dic. 2009
J. Chil. Chem. Soc., 54, N°4 (2009), págs. 334-338.
COPPER ELECTROWINNING BASED ON REACTIVE ELECTRODIALYSIS
GERARDO CIFUENTES (1), JAIME SIMPSON (1), FRANCISCO LOBOS (1), LEONCIO BRIONES (2) AND ALEJANDRO MORALES (3).
(1) Departamento de Ingeniería Metalúrgica, Facultad de Ingeniería, Universidad de Santiago de Chile, Avenida Libertador Bernardo O'Higgins 3363, Casilla 10233, Fono: 56-2-7183224, Santiago, Chile. e-mail: gerardo. cifuentes@usach. cl
(2) Departamento de Ingeniería Mecánica, Facultad de Ingeniería, Universidad de Santiago de Chile.
(3) Departamento de Ingeniería Metalúrgica, Universidad Católica del Norte. Avenida Angamos 0610, Antofagasta, Chile.
This paper shows an alternative to the classic electro winning copper process using a cell based on reactive electrodialysis, which consists of two compartments separated by an anionic membrane, one of which contains the copper sulphate catholyte, where the cathode is located, and the other compartment contains the ferrous sulfate anolyte, where the anode is located. In this type of cell the anodic reaction is the oxidation of iron (Fe2+ to Fe3+), which consumes less energy than the classic water oxidation. Moreover, this new anodic reaction does not produce acid mist.
Key words: Copper, electrowinning, electrodialysis.
The conventional copper electrowinning process, in spite of its present extensive use, has some drawbacks that over the last 20 years have been attemped to be overcome1, but there are few implementations at the industrial level. These deficiencies are related mainly to the low electrolyte flow, whose direct consequence is a low efficiency of the mass transport phenomena. Furthermore, the specific surface area of the electrodes is low, and therefore the amount of mass deposited by chemical reaction is also low, and finally, among the drawbacks, there is a high energy requirement2 (see Table 1.1) because the dominant reaction is the anodic decomposition of water, whose
water, thereby having as a direct consequence a low cost due to energy savings and less impact on the work areas due to the elimination of acid mist.5 One of the most promising reactions is the transformation of ferrous to ferric ion mentioned above.6,7 One way of preventing the reverse reaction (from ferric to ferrous) from taking place at the cathode and thereby affecting the faradic cathodic efficiency, consists in using two electrolytes (anolyte and catholyte) instead of only one, retaining the conductivity between them.8,9,10,11,12 This is possible using an electrodialysis membrane to separate those electrolytes in order to obtain specific electrode reactions. This process is known as reactive electrodialysis (RED). In this context it must also be considered that the sum of the resistances of the catholyte, the anolyte, and the membrane is not greater than the decrease achieved by changing the anodic reaction as well as by decreasing the corresponding overpotential.13
where I is the current fed and t is electrolysis time, usually in amperes and hours, respectively. M is the copper deposited on the cathode.
Most industrial electrolytic processes in the copper speciality work under mass transfer control. If diffusion, Jp is the transport mechanism, the flux equality condition becomes:
where i is the current density, n is the electrons transferred, and F is Faraday's constant. Thus, using Fick's law of diffusion, one has:
There are studies related to the process of electrodialysis associated with copper, but they are still confusing and contradictory.14,15,16,17 An important aspect is that related to operational flow, which causes great expectation because if work can be done with greater flows, higher limiting currents would be obtained as a consequence of higher current density, leading to the same copper production with a much smaller infrastructure.1819 Reactive electrodialysis applied to the electrowinning of copper is a vision of the future in the present.
This paper attempts to demonstrate that copper electrowinning based on reactive electrodialysis can be a good alternative process compared to the present electrowinning process in terms of specific energy, cell voltage, and current density.
Figure 2.1 presents the experimental scheme used in this work.
The effective catholyte and anolyte volumes in the compartment were 275 cm3 of each, adding up to a total volume of 550 cm3, because both electrolytes were recirculated with peristaltic pumps at flows of 700, 800, 900 and 1,200 cm3/min. The electrolyte's working temperatures were 40, 50 and 60 °C.
The cathode consisted of a 4 cm2 copper sheet and the anode was made of platinum, with their back sides insulated with teflon. The cathode current densities used were 300, 450 and 600 A/m2. A 2x2 cm Ionac MA3475 anionic membrane was used as the separation medium between the two electrolyte compartments. This membrane was attached to the separating frame by means of rubber gaskets, as shown in Figure 2.2. The composition of the anolyte was 190 g/L of H2S04 and a 1M concentration of Fe2+ (as FeS04-7H20). The composition of the catholyte was 190 g/L of H2S04 and 30 g/L of Cu2+ as CuS04-5H20).
Electrolysis time for all the tests was 2 hours.
RESULTS AND DISCUSSION
The results are presented in the following tables and graphics.
Effect of electrolyte flow:
From all the results, except at 600 A/m2, it is seen that as electrolyte flow in the cell increases, there is a decrease of the corresponding potential difference. This can be explained graphically with an Evans diagram, shown in Figure 3.1, where for a fixed current density, for example, cell potential decreases as electrolyte flow (Q¡) in the cell increases.
In fact, if we work at a constant temperature of 40 °C and a current density of 300 A/m2, see Figure 3.2, we get a cell potential of 1.9 V for 700 cm3/min, 1.2 V for 800 cm3/min, 0.9 V for 900 cm3/min, and 0.8 V for 1,200 cm3/min, all of them lower than the conventional cell potentials shown in Table 1.1.
Moreover, in Figure 3.2 it is seen that at 600 A/m2 the specific energy (W) is practically constant at different electrolyte flows, because for this system Vcdl and the deposited copper are constant too (see Equation (3)). On the other hand, at 300 and 450 A/m2 the value of W decreases when electrolyte flow increases, and this effect is directly proportional to the Vcell decrease described above, because T|aand r|cin Equation (2), particularly r|Dof each one, Equation (7), decreases because the concentration gradient and 8 at the electrodes also decrease, and therefore J is favored (see Equation (6)).
Figures 3.3 and 3.4 show that specific energy and V cell show a slight decrease with respect to the result of Figure 3.2 at 40 °C, and at higher temperature, particularly at 60 °C as shown in Figure 3.4, produced the lowest specific energy and cell potential values of all the current densities tested.
Effect of Temperature:
When the work was done at constant electrolyte flow and the temperature increase, a decrease of the cell potential is seen that increases as the flow decreases, showing that as the forced convection conditions in the proximity of the membrane improve, polarization due to concentration close to it is minimized, thereby decreasing the cell's energy expenditure.
Effect of current density:
As expected, an increase in current density implies an increase of the system's cell potential (see Equation 2), but this increase is less significant at an electrolyte temperature of 60 °C at all electrolyte flows, and particularly at 1,200 cmVmin V cell is minimum (see Figure 3.12) compared to the results shown in Figures 3.9 to 3.11.
This behavior is associated with the fact that at high temperatures and high flows mass transport by diffusion and convection improves with respect to mass transport associated with migration, but it should not be so important because it is in the presence of acid support electrolytes in both the catholyte and the anolyte. Obviously, this behavior must reach an extreme characterized by the limiting diffusion current at each temperature and the electrolyte flow in particular.
The average faradic cathodic efficiency for all the tests was 98.68%, i.e., no change was detected in the current efficiency under all the conditions tested.
Using reactive electrodialysis, cell potential and specific energy are decreased with respect to the traditional copper electrowinning system, with a maximum six-fold difference between both processes.
In all the tests the average current efficiency was 98.68%, i.e., no change was detected in the current efficiency under all the conditions tested. The parameters studied here confirm an important aspect of this study is that related to operational flow, which causes great expectation because if work can be done at greater flows, higher limiting currents would be obtained as a consequence of a higher current density, leading to the same copper production with a much smaller infrastructure, high current efficiency, and low specific energy.
The authors thank Refinería Electrolítica de Ventanas of CODELCO Chile (Corporation Nacional del Cobre de Chile), and acknowledge the support of the Comisión Nacional de Investigación Científica y Tecnológica under FONDEF project D97I2035.
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(Received: July 22, 2008 - Accepted: May 20, 2009).