Are we moving towards more efficient, more effective methods of recycling PV waste? The experts at Loser Chemie GmbH certainly believe so…
“If something isn’t done, everything will transform into a mouse grey mud”. That’s the way Boulding formulated his point of view concerning the fact that human waste leads to entropy bigger than the natural sustained process of mixing, which happens on Earth anyway.
Humans have always turned to deposits of the highest concentrations and will be forced to work with lower concentrations in future. Currently, when people in many places are considering how several materials from raw material deposits can be found, a promising way is by developing recycling. In doing so there is a good chance of producing even more without using up natural resources.
However, let’s not forget that energy is also a resource. Every process of recycling expends energy – and that already begins from the gathering process and transport. Later, some chosen examples will be subsequently explained (e.g. challenges and key questions according to economic and ecologic processes of recycling) and furthermore, which method of resolution could lead to success.
Except for photovoltaic technologies based on silicon, there are actually plenty of other PV waste materials based on thin layer technologies with CIS, CIGS, CdTe and GaAs. Indium, selenium, tellurium, gallium are all metals which have to be imported by many industrial countries. The economic significance of recycling PV modules and the associated re-use of these metals is an important topic now, but the dissipative use of many special materials in such products (thin film PV modules, as well as in mobile phones, flat-screen monitors, etc.) complicates the recycling process and gives a return on recycling flows only up to a certain extent.
By recycling a tonne of thin film PV production waste containing cadmium telluride, 200g of tellurium could be conserved in an on-road test. This substance had a chemical purity of 99.99% and the price was 20Ä that year.
The recycling of a tonne of thin film PV waste is not representative at that level. The focus on containing metals all too often leads to understanding that the reprocessing is uneconomical. Nevertheless, a certain energy ability with relevant costs is worth considering at the moment. If you involve synergies, use universal procedures, and if you strive for a recycling technique that is able to transform every part of waste into products, you will have a good chance to establish economical, successful recycling. Loser Chemie GmbH tries to make a contribution with such recycling techniques to solve the problems outlined above.
Silicon cell waste from the PV-Industry
The price for aluminium increased between 2001 and 2005, had high price volatility over the period of 2006-2008, which was followed by a sharp decrease in 2009 and a relatively strong increase which has continued to this day.
As a producer of a big amount of aluminium salts which are used for water treatment we were forced to substitute the raw materials. We were clear that the target substitution only could be one way out of the dilemma if we were able to substitute our raw materials by unused materials because it is obvious that we won’t gather anything of long duration if we substitute our raw materials by ‘already used’ raw materials because these will be in higher demand.
So we were seeking alternative raw materials, particularly waste which contains aluminium which can be recycled as close as possible to 100%. Broken silicon cells are one example of many items which are partly printed with aluminium and silver pastes. This material also provides synergies because you can reuse all components. For our own production, many kinds of aluminium were of interest to us but the possibility to conserve an interesting material by selective removal and recycling of the aluminium-contacts opened up long-term prospects to establish waste-free recycling.
The material “only” consists of silicon and silver contacts and, because of the separation of silver from silicon, could be generated three valuable separate components made from waste. We apply a very simple method for that: on the one hand we don’t want to produce new waste, while on the other hand we want to produce more saleable products.
By the use of a fresh solution of aluminium chloride and water we found a way to remove the aluminium on the back side of the silicon solar cell for simultaneous recovery of a valuable product. During the reaction, poly-aluminium-hydroxide-chloride [Aln(OH)mCl3n-m] ->Aln(OH)mCl3n-mB ] is formed, which is usually produced by pressure digestions of aluminium hydroxide and concentrated hydrochloric acid at high temperatures.
The chemical reaction between the back contact aluminium and the depleted aluminum-chloride solution is very simple and works better than the usual treatment with sodium hydroxide. The next step was that the remaining silver was dissolved and the silicon material was washed and dried.
The applied technology for this treatment works without the formation of nitrous oxide gas which is expected during the application of nitric acid. We use a kind of a transport system which dissolves silver from waste with a very low concentration of Ag by the addition of a stoichiometric amount of hydrochloric acid relative to the amount of silver.
It is decreased by simultaneous release of the silver salt. A further essential advantage of this procedure is the environmental impact of this solvent because it is biodegradable according to the OECD Principles 301A. Figure 1 shows a schema which illustrates how the continuous extraction and concentration of the metal works on the example of silver. Treated parts of silicon resulted from that recycling process. A melting process had to be made with these parts.
A universal wet chemical recycling method for CIS-, CIGS-, CdTe- and GaAs thin film PV waste
Cadmium, selenium, tellurium, gallium, molybdenum and indium are some of the most important elements which are used in these types of thin-film solar cells. We are aware of the limits of the availability of these elements respectively, and the current difficulties in the industry to have a reliable supply of them.
So the recycling of this waste is an valid topic, especially with regard to the coming end-of-life flow of waste. At present, thin-film solar modules are available as glass-glass-standard-modules. The thin film materials are usually deposited in several steps on a glass plate as a substrate and are structured. It is known that the rare metals usually constitute only about 1% of the mass of a photovoltaic module, with its value by itself and is related to the large amounts of PV modules it is still significant.
A legitimate question for this waste is: what will happen to the remaining materials? On closer inspection it turns out that the main amount of waste is high quality flat glass which was coated with the semiconductor layers (see also figure 2). If the simple detachment of the photoactive layers work, high-quality glass will remain, as shown in figure 3, which flat glass producers as a raw material welcome highly because energy can also be saved in addition to the raw materials for glass production energy.
Dissolving the semiconductor layer uses the same procedure is used as described above for silver. After a short contact time, the metals enter into solution and it can be filtered. After classification and control by X-ray fluorescence, the cullet can directly be used in flat glass production. So the recycling method for the matrix and the main waste of PV-scrap is developed, which makes recycling at the point of attachment of the security of rare metals such as indium, gallium and tellurium, economically viable at the end.
The polymetallic solutions which result from this process are a raw material for further processing by the potential of increasing the concentration of specific industries and they can be supplied. We extracted exemplary indium hydroxide from one of these solutions to produce pure indium, see figures 4 and 5.
