2.1 Background of indium
Indium, which discovered by Ferdinand Reich and Theodore Richter[7], was be identified as a by-product of a wider range of commercial ores including tin (stannite and cassiterite), copper (polymetallic ores), zinc (sphalerite), and lead (galena)…etc.[8] Indium originally would be extracted from zinc and lead ores because of the primarily economic reasons. In addition, indium was found to be a minor component in zinc ores, but market supply was mainly depended on the requirements of zinc market. Furthermore, indium appears widely dispersed on the earth surface, is estimated to be 0.05 ppm for the continental and 0.072 ppm for the oceanic crust, respectively. [4] Because the abundance of indium is so imbalance, the supply around the globe is in terms of geography and political policies.
In the early 1990s, a sudden rush of research for ITO, the potential of thin film technology would be discussed widely. The ITO films consisted of 90%
In2O3 + 10% SnO2. Surge in demand for indium caused the price of In increasing. [4] World production of indium was 173 tons in 1996. (66 tons from the European Union, 40 tons from Canada, 25 tons from China, 20 tons from Japan, 18 tons from the Community of Independent States, and 4 tons from Peru.)[6] The indium was used in making bearing alloys, germanium transistors, rectifiers, thermistors, and photoconductors. Indium could be plated onto metal and evaporated onto glass to form a mirror as good as those made of silver with higher resistance to atmospheric corrosion photocells. In the Fig.2-1, it
presented the consumption situations of indium for some countries. Russia, Canada, and China would be the three large consumption countries in the early 1996.
Table 2-1 The consumption of indium for various countries [6]
Tin and indium have many familiar properties, such as melting point, boiling point, density, crystal structure, and atomic diameter…etc. The crystal structure of indium is tetragonal. (a: 325.23 pm, b: 325.23 pm, c: 494.61 pm α:
90.000°, β: 90.000°, γ: 90.000°) The Table 2-2 showed the comparison with In and Sn.
Table 2-2 The comparison of the two elements
Tetragonal 2.9 -0.136
2.2 Refinement background and applications of indium
Indium is a rare element and ranks 61st in abundance in the Earth’s crust at an estimated 240 parts per billion by weight. [9] It is about three times more abundant than silver or mercury. Indium is available in several forms including bar, pieces, powder, nanosize activated powder, rods, shot, sheet and wires; it is a very soft, silvery-white metal with a brilliant luster. In the section 2-1, indium was usually discovered predominantly in zinc-sulfide ore mineral, the sphalerite. The recent applications for indium are as follows (until the end of 2007): [9]
1. ITO targets (70%)
2. Electronic and semiconductor (12%)
3. Solders and low-melting-point alloys (12%) 4. Research (6%)
Let’s firstly focus on the third point, the applications for low-melting-alloy system. After developing lead-free solders for electronic application, several
indium-base systems including tin-indium, tin-zinc-indium, tin-silver (-antimony)-indium, and tin-bismuth-indium were presented. These researches achieved the potential applications in communications and transportation in the industrial laboratories. Indium is also used as a strengthening agent for lead solders and as the base material for many low-melting-point solders due to its flexibility (over a greater temperature range) and the character of low- melting- point. [10-13] In addition, while the WHO claimed to stop using lead solders, indium became the maintenance for lead-free solders.
Since the cost of fuel is getting high in the recent ten years, the substitution was found increasingly. Especially, the solar cell researches were turned into a hot issue recently; they are the fundamental source and would not be tired out for a long time.
When the energy on the earth would be exhausted one day, some experts predicted the solar cell industry would increase the requirement of copper-indium-diselenide.[14] The development of solar cell from indium, mostly solar cell was made of silicon wafer in the past. Using the silicon wafer as the substrates of solar cell had some advantages, such as low cost of equipment, high yields from process, fast producing rate, and good translation efficiency. Therefore, Si wafers would be predicted as the main stream for the solar cell market. Recently, many countries in the world tried best in the thin-film solar cell because of the lack of poly-silicon. A thin-film solar cell was made by sputtering multiple layers, which could induce photoelectric effect on the substrate of metal, glass, and plastic…etc, and its thickness is only several micrometers. This new material is much thinner than the original silicon wafer, and the consumption of silicon wafer would be decreased. There are two semiconductor compounds, CIS (copper indium diselenide) and CIGS
(copper indium gallium diselenide), for producing thin-film solar cell. These semiconductor compounds have wide absorption range and good stability.
Nevertheless, there are two disadvantages for CIS and CIGS. One is the lifetime; traditional solar cell could use about 20 years, and the life time of thin film solar cell is shorter than 10 years. Because the translation efficiency of thin film solar cell was lower than that of silicon wafer, the large area for thin film solar cell would be required. The other is the cost; costs of thin film solar cell are lower than that of the traditional solar cell, but the equipments for sputtering costs are much higher. Therefore, total cost of fabrication process for thin film solar cell would not be declined. For these reasons, there is still much dilemma for developing the CIS and CIGS thin-film solar cell. The developments for all kinds of thin films solar cell would be depicted as following Fig.2-1. The light-blue line represented the CIGS films. In the future, developing new sources and reducing expenses of indium are other important issues.
Fig. 2-1 The forecasts for products quantities of thin film solar cell [14]
2.2.1 Refinement background of indium
The American government established an association named the Indium Corporation (ICA) at 1934 to investigate potential uses of the element indium.
The association offered over 72 years of indium refining, processing, fabricating, and application engineering experience. [4] As the demand for indium was increasing, more countries were investing lots of efforts to find efficient way for recycling indium. All we want to do is not only extracting them from ores, but also refining out other impurities from primary indium.
Base metal smelters were improving the extraction process of indium from ores
& minerals, which concentrating with low indium contents. In addition, these smelters are now more aggressively seeking and purchasing indium containing concentrates from various sources. Refining capacity has been increasing on a worldwide. In 2004, Korea Zinc installed a brand new extraction and refining processing line at their zinc smelter. Dowa Mining increased their indium capacity in Japan. Others have installed refining lines to purify crude indium into higher purities. [4]
Recycling became very significant for the first time in 1996 [6] and affected the market and prices. Recycling of indium could expand rapidly if the current price of indium is sustained or continues to increase. The current processes of extracting indium were shown in the figure 2-2. Many countries had an aggressive recycling program that made up for any shortfalls in domestic production and imports of indium. [4]
Fig. 2-2 The current extracting process and applications of indium [4]
2.2.2 Methods of refinement high purity indium from ITO wasted materials In the section, three methods, which could be used to refine indium, would be introduced. The United States Patent No.060580 offered some methods, such as electrolysis, vacuum distillation and zone purification…etc.
2.2.2.1 Electrolysis
In the methods of electrolysis, the metallic indium would be obtained a purity of 99.99 % (4N), and contained at least 0.5 ppm each of impurities such as Si, Fe, Ni, Cu, Ga, and Pb. The purification from waste compound semiconductors had the problem that large equipment and prolonged time was needed to separate and recover indium. Even if the United States Patent showed the production of 4N-purity indium, pure In couldn’t maintain for a long period because of an abnormal increase in the Si content of the final. [5]
2.2.2.2 Vacuum distillation method
In the method of vacuum distillation, the pure indium would be attained by two-step thermal process. [5] Indium in an “indium feed” was evaporated and then condensed for recovery in the first thermal purification step. In the second thermal purification step, the product of the first step would be separated from impurity elements of lower vapor pressure and the recovered indium was heated. Basically, In could evaporate away impurity elements of higher vapor pressure at higher temperature. As a result, not only the impurities elements with lower pressure than indium would be found, but also those with higher vapor pressure could be separated in an efficient manner to yield indium with a purity of 99.99% or higher. The central concept of the method was using the different vapor pressures to separate indium from other metals. In addition, some elements with higher vapor pressure than indium are P, S, Cl, P, Ca, Zn, As, Cd, and Pb…etc; and those with lower vapor pressure are Al, Si, Fe, Ni, Cu, and Ga…etc.
2.2.2.3 Zone purification method
In the zone purification method, the purified indium mass had to be cut and there was a potential hazard of contamination [5]; hence, the purification process inevitably suffered from a limited and lower throughput. Furthermore, when the purified indium was cast into an ingot, impurities might enter to cause contamination.
2.2.2.4 Hydrometallurgy and pyrometallurgy method
Among all methods were introduced before, the hydrometallurgy and pyrometallurgy would be the better choices. The hydrometallurgy offered
simple and fast experimental procedures, and the whole process could be finished just for three hours. Comparing other methods of recycling indium, the shortest times also required 15 days. [4] The multiple procedures of other methods made the whole process inefficient and the costs of recycling were getting high. Moreover, the hydrometallurgy with higher chemical selections was suitable to use in the ITO wasted solution. The reducing agents liked Zn and Mg was put in it, and the indium would be replaced gradually. The extra reactions were overlooked by the theorems of electrochemistry except the possible reactions in Zn, Mg, and In. In the research, the hydrometallurgy was first used to recycle the raw indium from the ITO wasted solution. After that, the raw indium was refined by the method of pyrometallurgy, it could avoid other impurities during the process. Therefore, the purity of indium would be attained for a long period. The coherent compositions could be gotten after annealing under the melting point of indium, 156℃. In the chapter three, the detail procedures would be mentioned.
2.3 Thermodynamics theorems and chemical analysis for recycling indium
Many chemical reduction methods were used to extract the indium out from the ITO wasted solution. First, the metals ions with the lower electromotive force could be replaced by the metals with the higher electromotive force from the solution. A few elements which were more activated than indium would be chosen to be the reducing agents. From the pourbaix diagrams, the relative reduction potential of each element could be found, some suitable elements could be selected from the diagrams. Indium
would be deionized to these ions, In+, In2+, In3+, and InOH2+ when it existed in acidic solutions. Indium would be the oxides, In2O3, when it existed in neutral solutions. Finally, indium would be deionized to InO2- as it existed in base solutions. If we want to know the chemical reactions about the atoms, ions, and oxides in aqueous solutions, the electrochemical theorems by the “Nernst’s equations” [15] would be defined as follow:
K nFln E RT
E= o − (2-1) E: reduction potential
E°: standard reduction potential R: gas constant
T: temperature
n: the number of interaction electrons F: Faraday constant
K: the equilibrium constant of ions
The possible electrochemical reactions of indium in the solution were shown in the Table 2-3, and the simpler pourbaix diagram was established according to it. Based on the information of pourbaix diagram of In [16], the metal indium would be the stable In3+ ions in acidic solutions. In neutral solution, indium would be covered by an oxide layer on its surface; and the oxide would cut off any reactions between indium and aqueous solution.
Indium would be precipitated by the compounds like InCl3 and InF3 in the HCl (hydrochloric acid) and HF (hydrofluoric acid), respectively. Both precipitate compounds would not be dissolved in acidic solutions. Through controlling the pH value and electrovoltage of the chemical reactions, indium would be ions in aqueous solutions. Furthermore, the metal indium also could be precipitated on
)
cathode electrode in the cell by the electrolysis. The specific ions like In3+
would be replaced in aqueous solutions at the difference of oxidation voltages of Zn, Al, and Mg, which are -0.76V, -1.67V, and -2.34V. All the voltages are lower than the indium voltage of -0.34V. Therefore, if we put Zn, Mg, and Al plates or powder in aqueous solutions, the indium could reduce ions to metal.
The oxidation voltages of above-mentioned:
)
Another factor to influence the oxidation potential of compound is the concentration of metal ions in solutions. The relationship of oxidation potential and ion concentrations between each element was depicted in the figure 2-3.
The magnesium was at the last one and with the greater ability of oxidation than other elements. Tin is the first one and with a lower oxidation potential than indium; therefore, tin was still retained in the ITO wasted solution. As above mentioned, tin and indium have many familiar properties; and a new separation method would be created to extract tin from indium. Eq.2-6 is the oxidation voltage of Sn:
)
Fig. 2-3 Variation of the oxidation potentials of each element with the relationship of ion concentration
Table 2-3 The possible reactions of indium in the research. [16]
No. Reaction Equation
1 +
Ion concentration (log M) Oxidation
-5 −
In addition, the principle of electrolysis was introduced to refine indium.
There are many methods, such as electrolysis, vacuum distillation, and chemical replacement could be used to recycling indium. The equipment of electrolysis was easier than other methods so it would be chosen for us. The
“first” indium by chemical replacement would be defined as the raw metal indium, with a purity of 95%~99%. The indium with a higher purity about 99.9% would be attained after repeated refining, and the kind of indium was so called “fine” indium. Through the electrolysis process, the raw metal indium could turn into fine metal indium with a purity of 99.999% (5N) at least.
Before electrolysis, these impurities metals such as aluminum, tin, and zinc…etc should be removed. The processes of electrolysis were done by twice or three times, and the high purity indium would be obtained gradually. In the procedures, the high-purity indium plate (about 5N) was used as cathode electrode, and the raw metal indium plate from vacuum pyrometallurgy was used as anode electrode. The electrolyte was the In2(SO4)3 or InCl3 solution.
The electrochemical reaction was shown as following [29]:
In(pure, anode)︱In2(SO4)3,H2SO4,H2O(electrolyte)︱In(raw, cathode)
All parts of the electrolyte would deionize in:
In2(SO4)3 → 2In3++ 3SO42- (2-7) H2SO4 → 2H++ SO42- (2-8)
H2O → H++ OH- (2-9) Some electrochemical reactions through external voltages would work possibly.
There are three reactions at the anode electrode were showed as follows:
In → In3+ + 3e- (-0.343V) (2-10) 2H2O → O2 + 4H+ + 4e- (1.229V) (2-11) SO42- → SO3 + 0.5O2 + 2e- (2.42V) (2-12)
Some impurities, such as Sn, Bi, Ni, Cu, and Sb, have higher oxidation potential than indium in the anode electrode. Therefore, no compounds would precipitate at the cathode electrode during electrochemical reactions. However, one reaction about aluminum would occur, the oxidation potential of Al was lower than that of In and the compound Al(OH)3 would be produced in base solution. As a result, the effect of aluminum at the anode electrode could be ignored. The possible reactions were occurred at the cathode electrode:
In3+ + 3e- → In (0.343V) (2-13) H+ + 2e- → 0.5H2 (0V) (2-14) The standard oxidation potential of hydrogen is lower than indium, and indium would be over-potential and produce In3+ by the hydrogen ions. Some hydrogen gas was out in the process of electrolysis. The ion In3+ would be reduced and precipitated metal indium on the cathode electrode.
2.3.1 Reagents of indium in the research
The method of replacement was the fast one in recycling indium. A model was used to explain the processes of replacement. Each element with different oxidation potential and the element with lower oxidation potential could be replaced by higher ones. The chemical equilibrium reaction [29] could be expressed like that:
M1n++M2 → M2n++M1 (2-15) From above reaction, the ion M1n+ was the original ones in the solution, and the element M2 is the replaced metal added in. The M2n+ was the metal ions replaced from the solution, and the metal M1 was the element precipitated after replacement. For example, two elements like Zn and In would be used as
The oxidation potential of Zn:
Zn → Zn+2 + 2e- -0.763V (2-16) The oxidation potential of In:
In → In+3 + 3e- -0.342V (2-17) The oxidation potential of zinc was lower than that of indium, and indium from ITO wasted solution would be replaced by metal zinc. Then it would be dissolved, the metal indium was precipitated underneath the solution. The chemical equilibrium formula was shown:
Inn+ + Zn → Znn+ + In (2-18)
Except the element zinc, there were many elements with lower oxidation potential than indium, such as Li, K, Ca, Na, Mg, and Al, whose oxidation potentials are -0.32V, -2.924V, -2.87V, -2,71V, -2.34V, and -1.67V, respectively.
[17] These elements all were the reducing agents in the research. There were two reasons of choosing the elements Zn and Mg as the reducing agents. One is the cost; the Zn and Mg powder are cheaper than other elements. The other is the oxidation potentials; Zn and Mg are with lower oxidation potentials than indium. The element like Ca was also with the lower oxidation potentials than indium, but Ca still was unsuitable to as the reducing agent. In the next section, the reason would be introduced.
2.3.2 Pourbaix diagram
The pourbaix diagram could be defined by the thermodynamic theorems in aqueous solution. The ordinate axis of this diagram is the reduction potential E(volts), and the abscissa axis is pH value of solution. A pourdaix diagram was separated into four quadrants: upper left side is acid oxidizing zone, left underneath is acid reducing, upper right side is basic oxidizing, and right underneath is basic reducing. The four quadrants would be used to present the state of some compounds, or the states we wanted also could be observed from the diagram. In the figure 2-4, the pourbaix diagram of Ca [16] would be introduced. The interphase compound (CaH2) would be produced between the Ca2+ and Ca, and itformed at a wide range of pH value. Therefore, it was hard to ignore the appearance of the interphase compound during the reaction Ca→ Ca2+ +2e−.
Fig.2-4 The pourbaix diagram of Ca
The pourbaix diagram of In was depicted in the figure 2-5. The In3+ would be the ions which existed in the ITO wasted solution. The reaction of In+3 + 3e-
→In was worked by controlling the fixed electro-voltage. The ion state of In3+
→In was worked by controlling the fixed electro-voltage. The ion state of In3+