Preliminary study of the kinetics of leaching copper powders obtained by grinding circuit boards of computers. Effect of thiosulfate, concentration and temperature.
Mirna Alejandra Mesinas Romero, Isauro Rivera Landero, María Isabel Reyes Valderrama, Eleazar Salinas Rodríguez, Juan Hernández Ávila and Eduardo Cerecedo Sáenz.
e-mail: [email protected]
Autonomous University of Hidalgo State, Academic Area of Earth Sciences and Materials. Carr.
Pachuca-Tulancingo Km 4.5, Mineral de la Reforma, 42184, Hgo. México.
Abstract. Today the society requires new technologies and equipment. This is because the society is becoming more demanding of products and services, making that the amount of wastes both electric as from electronics, increases in an alarming way, being this kind of wastes difficult of recycling in an easy and economical way. The above is due to the complexity of these kind of residues and the lack of adequate techniques of recovery, because these components are made with a variety of materials that difficult its primary separation. Some of these components are nowadays considered as “urban mining resources” because the amount of metals present in these are quite attractive from economic point of view, as is the case of copper, which is present in important amounts.
The present work is related with the study of the effect of thiosulfate, concentration and temperature, over the rate of leaching of the copper present in the ground circuit boards of computers. It was used a mesh 50 from Tyler® series due to the good uniformity of particles obtained. Leaching was carried out in a glass reactor of 500 mL of capacity, mounted on a hot plate with magnetic stirring, coupled to a pH meter to control of OH– concentration. The reagent for leaching is used, was the O2–S2O32-, and evaluating thiosulfate concentration was done in a range from 0.075 to 0.5 M, and the temperature range was from 289-338 K, to determine the global rate of leaching of copper. According the results found, it was determined that under certain work conditions such as: temperature of 338 K, [S2O32-] = 0.5 M, pH
= 10, 750 min-1, 5g of sample and 180 minutes of reaction time, it was possible to get up to 97.35 % of recovered copper. From the obtained results, it was plotted the metal concentration vs. time, getting so straight lines in certain time periods, with which was possible to calculate the experimental constant of rate, kexp; and besides was plotted the log [S2O32-] versus log kexp to determine the order of reaction that for this case was of n = 0.05 in the range of 0.075 to 0.5 M of S2O32-. To determine the activation energy of the system, Ea, it was used the Arrhenius equation, getting so an Ea = 17.4 kJ·mol-1which is indicative of a control by diffusion or transport, for the reaction of leaching of copper.
Keywords: Printed circuit boards, copper, thiosulfate, e-waste, leaching
Introduction
The main economic engine to carry out the recycling of printed circuit boards of computers, is the value contained in the metal fractions [1], in general, this type of waste is made up: 40%
metal, 30% plastics and 30% ceramics, of which, the metal fraction is composed of: copper (20%), iron (8%), tin (4%), nickel (2%), lead (2%), zinc
(1%), silver (0.2%), gold (0.1%) and palladium (0.005%) [2], while the remaining 60-62% is non-metallic materials [3], and although the percentages are economically attractive, the heterogeneous and complex nature of these materials are the main barrier to overcome in the process of recovery of these metal fractions from the printed circuit boards of computer [4].
Multiple studies have been carried out in order to make a thorough characterization of the printed circuit boards of computer; found to consist of a polymeric layer, a metal layer of Cu, Zn, Ni and a gold substrate [5]. And although the chemical composition of these components varies according to the year of manufacture, copper is one of the metals that is always present in high quantities in this type of wastes; likewise, the recovery of copper from this waste is interesting because of the degree of purity that can be obtained, and in all cases, is far superior to that found in copper ores [6].
Various investigations have been developed, which is studied the recovery of valuable metals from printed circuit boards of computers, using physical and mechanical methods of separation, hydrometallurgy, pyrometallurgy, biotechnology among others [6], [7].
In UAEH, they have been conducted kinetic studies in mining tailings, using as a basis the O2-S2O32¯ system and adding an oxidizing agent such as Cu2+ or Zn2+ in order to assess the rate of leaching of the silver contained in this type of tailings, thereby achieving up to 95 % silver leached in the system when Cu2+ is used as oxidizing agent and up to 97 % of leached silver when Zn2+ is used. These studies are proposed as an alternative to the processes in which it is used as complexing agent cyanide [8] [9], within this framework, it has also been proposed and studied the leaching of metals present on the pins of the printed circuit boards of computers using the O2- H2SO4 acidic medium, thus dissolving base metals such as Cu, Zn and Ni, thereby obtaining the solid gold present in these pins with a purity of 99.99 %, as shown by the characterization done by SEM-EDS [10].
It is for the above, this paper proposes the use of a leaching system of the type O2-S2O32¯, for the recovery of copper from waste ground of printed circuit boards of computers. This system has the main feature of being able to impact less on the environment, in comparison to acid leaching which is the most employed process for copper recovery from oxided copper minerals [11].
Methodology
The characterization of copper generated by the grinding of the printed circuit boards of computers, was performed using a JEOL Scanning Electron Microscope JSM-6300;
likewise, it is characterized with an X-ray diffractometer Equinox, with a sweep time of 5 minutes for each sample, and to index the diffractogram obtained, the MATCH version 1.1 program was used. In order to analyze the content of copper in the original sample it was necessary to dissolve 0.1g of the sample in a solution of concentrated HNO3, the solution obtained was decanted into a 200 mL volumetric flask with distilled water calibrating dilutions and standards prepared with identical matrices and analyzed by atomic absorption AAS.
Basic dynamic leaching (medium O2-S2O32-
) Leaching experiments of copper contained in the ground printed circuit boards of computer, were conducted in a 0.5L reactor, on a hot plate with automatic temperature control and magnetic stirring, Thermo Scientific brand. The pH of the solution was continuously measured with a Dual Star PH/ISE with electrode and ATC probe, Thermo Orion brand.
For this study, distilled water is used, and 5g of sample previously sieved to mesh 50 (Tyler series), 0.5 M sodium thiosulfate pentahydrate of brand J.T. Baker and 1 atm pressure of oxygen, injected into the solution; the pH was kept constant by adding to the reactor a 2M solution of NaOH. Aliquots were taken at predetermined time intervals. All experiments were executed in a timely reaction of 180 minutes.
Atomic Absorption Spectrometry
The aliquots obtained from the various leaching experiments conducted were analysed by elemental analysis with a Perkin Elmer atomic absorption spectrometry equipment, model 200 AAnalyst; in order to assess the concentration of copper present in the leach liquor.
Results and discussion
Figure 1, shows a general image of copper powder obtained from grinding of the printed circuit boards of computer, and this copper
powder was sieved with different meshes (in aperture size), selecting that get on mesh 50 due the particles found were the most adequate for the target of this work of basic dynamic leaching, because of the good uniformity presented for the particles in comparison with the others found in the rest of meshes used.
Figure 2, shows the micrograph obtained by SEM-SE of the particles mentioned, where can be observed the morphology and average size of the particles, it also can be observed the presence of bright parts, which corresponds to the plastic fraction that remained after grinding of the printed circuit boards of computer.
Figure 1. Copper powder obtained from the grinding of printed circuits boards of computer.
Figure 2. Micrograph showing a general image of the particles found in mesh 50 (270 µm)
(SEM-SE).
In figure 3, it can be observed an EDS spectrum done to these same particles, showing the presence of copper as majority element, and also it can be seen some contents of iron, aluminium and nickel, with some non-metallic elements such as silicon and calcium. This semi- quantitative and local analysis reveals a chemical composition that is characteristic of the metallic fraction involved in these wastes. To have accurate data of copper content present in the original sample the steps were followed in the methodology section and found to have a 71.96%
copper present in our study raw material. And to have accurate data of copper content present in the original sample, the steps were followed in the methodology section and found to have a 71.96% copper in the raw material of study.
Figure 3. EDS spectrum done to the powders obtained by grinding.
Figure 4, shows the X-ray diffraction spectrum obtained from the same copper powders and this confirms the presence as a major element present in sample to copper, showing 3 characteristics peaks that correspond to the copper PDF [00-003-1005].
Basic dynamic leaching (medium O2-S2O32-) To choice a kinetic model and determine so the controlling stage of copper disolution process, were carried out some experiments, and and the results found are according to the model of spherical particles of constant size with an unreacted core and by diffusive control.
Figure 4. X-ray diffraction spectrum of copper powders.
The most useful variable that describes the evolution of solid during conversion, is X, that is an adimensional number that corresponds to the reacted fraction (ec.1):
Xτ
X = Xt (1)
where X is the copper fraction that has already reacted, Xt is the amount of copper in solution at time t and Xτ is the amount of copper at end of reaction.
Effect of the (S2O32-)´s concentration
The effect of the thiosulfates´ concentration was studied under the following experimental conditions: T=318 K, 500 mL, 750 min-1, PO2=1 atm, sample weight 5g, pH=10, 180 minutes, evaluating the effect of thiosulfates´
concentration in the range of [0.075M-0.5M].
Figure 5, shows the leached copper fraction by testing the model used in this work by diffusive control [1–3(1–XCu)2/3+2(1-XCu)] [12], [13] for all the concentrations of thiosulfates used. Straight lines are the slopes from where the experimental rate constants (Kexp) were calculated.
It can be seen that into the concentration range used it was possible to get an order of reaction near to zero (figure 6), which is
indicative that the obtained rates are determined by a different factor from concentration of reagents [14], where oxygen can act like principal oxidant into the system, leading so a low effect of thiosulfates for copper dissolution.
Figure 5. Copper leaching. Effect of the [S2O32-] concentration.
Figure 6. Copper leaching. Effect of [S2O32-]
concentration (order of reaction).
Effect of temperature
The study of the effect of temperature was carried out under the following experimental conditions: [S2O32-] = 0.5M, 500 mL, 750 min-1, PO2 = 1 atm, pH=10, 5g of sample, 180 minutes
and varying the temperature in the range of 289 K up to 338 K.
Figure 7, shows the effect of temperature on reaction rate for copper leaching. It can be seen a variable behavior of the experimental rate constants which could be due to the instability of the system oxygen–thiosulfate, as a response by the increasing of temperature and for this reason, the possible decomposition of the thiosulfate [15] [16] [17] [18].
Plotting the values of ln Kexp from figure 7, against the reciprocal of the temperature, a straight line with a negative slope is obtained.
The above allows determines the activation energy, Ea, of the system, which is calculated according to the Arrhenius equation (ec.2).
T R A E
k ln a 1
ln = − (2)
Figure 7. Copper leaching. Effect of temperature.
The graphical representation shown in figure 8, pointing that the activation energy for this system is, Ea = 17.4 kJ·mol-1 in the range of temperatures studied; however, it would be necessary evaluate the effect of the partial pressure of oxygen and the stirring rate to determine if these variables could have some effect on the reaction rate of copper leaching,
which would help to determine the real controlling stage of the system.
Figure 8. Copper leaching. Effect of temperature (activation energy).
Conclusions
• Characterization conducted to powders obtained by grinding of the printed circuit boards of computer, reveals that copper is present largely as metallic element.
• The chemical analysis of the original sample reveals a 71.96% copper present in the raw material of study.
• The order of reaction with respect to the concentration of thiosulfate was n=0.05, suggesting that the rates are determined by a different factor reagent concentration
• The activation energy calculated in the temperature range of 289-338 K was of Ea = 17.4kJ·mol-1, the fact of obtaining a value below 20 kJ·mol-1, suggests that the controlling step of the reaction rate of leaching of copper in the system O2-S2O32-, is the transport of oxygen to the solid-liquid interface, however, it is necessary to study of the effect of oxygen partial pressure and effect of the stirring rate in order to confirm that there is some effect on the reaction rate in the leaching of copper
• Under certain experimental conditions of study, such as: [S2O32-] = 0.5M, 500 mL, 750 min-1, PO2=1 atm, pH=10, 5g of sample, t = 240
min and 338K, it is possible to get up to 97.35%
of copper in the leached solution.
Acknowledgement
Authors want to thanks to the National Council of Science and Technology (Consejo Nacional de Ciencia y Tecnología, CONACyT) of the Mexican government, by the support (370508), thanks also goes to the Academic Area of Earth Sciences and Materials (AACTyM) of the Autonomous University of Hidalgo State, (UAEH, México) and to the company
“Corporación de Valores Reciclados S.A de C.V”, who gave the material for this study.
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