Home Appliance Makers’ Effects of Recycling of Rare Metals
Environmental Engineering and Analysis Center
Environmental Protection Group, Sharp Corporation
The demand for liquid crystal displays is sharply expanding because they can save driving power and resources. In the future, an increase in both liquid crystal display production and liquid crystal displays with a larger screen area is expected.
Materials such as indium and the liquid crystal used for liquid crystal displays will be used in large quantities. Liquid crystal is a very expensive material. Indium is a rare metal. Particularly, indium sharply expands in application to transparent conducting thin films and the demand causes a tight supply. Furthermore, it is important for LCD panel makers to ensure indium resource availability because it is a rare metal.
Because of these backgrounds, recovery of indium from liquid crystal display panels for recycling is expected. The establishment of technologies is required to effectively deal with the massive generation of waste liquid crystal display panels and to recover the indium applied to liquid crystal display panels.
2. Recycling technology for indium
There is a method to recycle indium from scrap. First the indium is eluted from the ITO target (indium tin oxide) by using hydrochloride and sulfate. Then impure metal ion is removed by using the sulfide process, the hydroxide method, the electrochemical displacement-deposition method, the solvent extraction method or the electrowinning process. Finally, the indium is refined with electrolytic refining after separation and recovery of the indium.
However, sulfide and hydroxide processes need many alkaline chemicals to adjust pH when indium is separated and recovered. With the electrochemical displacement-deposition method, a large volume of metallic resources such as zinc are consumed. For the solvent extraction method, a difficult problem to solve is that liquid waste disposal is not straightforward. These recycling technologies for indium are mainly to recover indium from high concentration scrap having adsorbed onto sputtering equipment or an unspent ITO target. Actually, a technology to recover indium from etching waste liquid and both used and defective liquid crystal panels during the manufacturing process has not yet been established.
The recycling technology for indium developed at this time is a simple recycling technology with less environmental load using an anion-exchange resin that can adsorb indium.
2.1 Adsorption mechanism of indium
Indium forms an indium chloro complex that has anion properties composed of indium and chloride ions in a hydrochloride-based solution. That is, when a hydrochloric acid solution containing indium contacts an anion-exchange resin, indium adsorbs onto the resin. When an anion-exchange resin that has adsorbed indium contacts water, the decrease in chloride ion concentration in the solution causes a ligand displacement from a chloride ion to a water molecule and the indium forms the indium chloro complex and is cationized. The cationized indium has a decreased ability to adsorb onto the anion-exchange resin and desorbs (Fig. 1). That is, when the chloride ion concentration in the solution containing indium is high, indium adsorbs to the anion-exchange resin. On the other hand, if the concentration is low, indium does not adsorb.
Therefore, the hydrochloric acid concentration of a solution that has contacted an anion-exchange resin having adsorbed indium was continuously measured. Based on changes in the hydrochloric acid concentration, the solution is separated into a solution with recovered hydrochloric acid at a high concentration and a solution with recovered indium at a high concentration but with a low hydrochloric acid concentration. This also has the benefit of less depletion of the anion-exchange resin because this method can conduct absorption and desorption onto an anion-exchange resin by taking advantage of the indium complex.
(a) Adsorption mechanism
Fig. 1 Adsorption and desorption mechanisms of indium
2.2 Recycling flow of indium (Fig. 2)
Shown in Fig. 2 is the recycling flow chart of indium taking advantage of the adsorption mechanism of indium onto an anion-exchange resin.
1)Dissolution of indium
Liquid crystal panels are crushed into pieces less than 10 mm in size and the still existing ITO is dissolved out in a hydrochloride-based acid. This dissolution of ITO using a crushing liquid crystal panel is being promoted. Impurities such as glasses and films in the solution is filtered and removed. The obtained hydrochloric acid solution containing indium contains not only indium and tin applied to transparent electrodes, but also impure metal such as aluminum used in the electrodes of liquid crystal panels as well.
2)Adsorption of indium
As soon as a hydrochloric acid solution containing indium and impure metal is passed through a column filled with an anion-exchange resin, indium adsorbs onto the anion-exchange resin together with tin. On the other hand, impure metal such as aluminum passes through. Therefore, the solution passing through the column is recovered as a high concentration hydrochloric acid solution with no indium. Specifically, the concentration of hydrochloric acid in the solution passing through the column is measured with an electrical conductivity meter. Hydrochloric acid recovered in high concentrations is separated and retained. The recovered hydrochloric acid solution is reusable as hydrochloric acid to dissolve indium from liquid crystal panels. Also, appropriate liquid waste disposal is performed so that a column preparation solution of low concentration indium and hydrochloric acid in an early adsorption process is neutralized as a solution that can be discharged.
3) Recovery of indium
Water passing through a column filled with an anion-exchange resin having adsorbed indium can allow for indium to be recovered by being separated from the anion-exchange resin. With a decrease in hydrochloric acid concentration, an indium chloro complex will transform into an indium aqua chloro complex, so that indium can be desorbed from the anion-exchange resin.
The solution covered indium decreases the concentration of aluminum that does not adsorb onto the anion-exchange resin. Additionally, the solution contains indium and tin that does not only adsorb onto the anion-exchange resin but are also applied to transparent electrodes.
Since the solution recovered indium contains indium and tin, pH must be adjusted within a range of 1.5 to 2.5 by adding sodium hydroxide to the solution. Afterward, the indium solution is recovered after tin is deposited as tin hydroxide and a solid-liquid separation of it is performed.
After tin separation, when the pH of the solution recovered indium is adjusted within the range of 4.5 to 5.5, indium is deposited as indium hydroxide and sludge of highly-pure indium hydroxide is obtained. Incidentally, 94% as a component content of indium sludge was obtained.
Rare metal is an indispensable resource for high-functional electrical and electronic equipment. But there are some concerns about future securing of the resources. Reflecting such a situation, Tsujiguchi et al. have been working on studies to recover indium from liquid crystal panels whose production is sharply increasing and verified highly effective indium recovery by proving tests, using process defective liquid crystal panels discharged from LCD panel plants.
I am indebted to Mr. Akifusa Onishi at the AQUA TECH CO. LTD for being associated deeply with the present study.