- The liquid crystal panel is a subsection of the LCD display. They consist of a very thin layer of a nematic liquid crystal mixture enclosed between two transparent parallel glass substrates held apart by solid spacers and glued together around the edges. Before the LCD is assembled the inner surfaces of the glass substrate are often coated with a passivation or barrier layer (silica) in order to prevent the diffusion of ions from the substrates in to the nematic mixture.
- LCDs incorporate a regular alternating pattern of red, green and blue picture elements (pixels) formed using dyed colour filters. However, these filters can absorb up to two thirds of the light passing through, resulting in too dark an image and poor viewing quality. Therefore, LCDs with colour filters are operated in transmission mode or a transfection (combined transmission and reflection) mode with a powerful back light.
- Electrodes made of a transparent conducting material, usually indium tin oxide (ITO) are deposited on top of these layers. Another thin barrier layer intended to prevent diffusion of ions into the nematic mixture may be deposited on top of the electrodes. This is followed by an alignment layer, which will be in direct contact with the nematic mixture in order to induce a homogeneous orientation of the director in the required plane geometry.
- The two glass substrates are then assembled and glued together leaving a hole so that the evacuated cell can be filled with a nematic liquid crystal under positive pressure, cleaned and then sealed.
- Dr. O’Donoghue is currently under taking a EPA funded desk based study reviewing the rare earth content within LCDs. Results to date show that variations in the generic structure of the liquid crystal panel are possible giving rise to variations in the material volume content of different LCDs. Indium tin oxide (ITO) is a solid solution of indium (III) oxide In2O3 (90-95%) and tin (IV) oxide SnO2 (10-5%) [He et al.,2014]. Indium consumption in ITO accounts for over 70% of its total consumption and the greatest demand on ITO is reported as LCDs [He et al, 2014]. Currently, the price is around the 540 USD/kg (Strategic metal report, 2014). Table 1 lists reported indium concentrations per kg of the glass panel.
|Table 1 List of reported indium quantity in LCDs|
|Author||Indium reported mg/kg|
|Wang et al., (2013)||102 mg per kg of glass panel|
|Ruan et al., (2012)||0.37 mg per gram of circuit board|
|Hasegawa et al., (2013)||380 to 410 mg per kg of glass panel|
|Yoshida et al., ( 2014)||321-395 mg per kg of glass panel|
|Lee et al., ( 2013)||260.7 mg per kg|
- Depending on the manufacturer and the year of production, the indium material content with in LCDs can vary. Therefore, understanding the liquid crystal panel materials and structural variations possible and its effect on subsequent recycling and recovery options is important. Of interest will be the indium location, thickness, pattern structure and presence of other materials in the liquid crystal panel which may have an effect on the chosen recovery process for indium. The author is currently undertaking these studies.