A PROCESS TO RECYCLE THIN FILM PV MATERIALS
Robert E. Goozner, William F. Drinkard, Mark O. Long and Christi M. Byrd
Drinkard Metalox, Inc., 2226 N. Davidson Street,
Charlotte, NC 28205
26TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE
ANAHEIM CALIFORNIA

 29 SEPTEMBER - 03 OCTOBER 1997

 ABSTRACT

 INTRODUCTION

 There is wide scale interest in the commercial potential and wide scale use of cadmium and cadmium telluride (CdTe) and copper indium diselenide (CIS) photovoltaic modules. These type of thin film devices have demonstrated advantages which include good performance, the ability to be manufactured by various methods and apparent environmental stability. There are currently several efforts underway in the United States and abroad to manufacture thin film PV modules and commercialize them. The use of these modules would entail the use of toxic metals such as cadmium, selenium and (potentially) lead in their manufacture. The manufacture of thin film photovoltaic modules, as in the manufacture of any article, must address the associated environmental safety and health issues, as has been reviewed by P.D. Moskowitz et al. [1]. Government agencies, industry groups and private citizens will be placing greater emphasis on the requirement that emerging technologies will not endanger health, safety or the environment. This problem will become more acute in the future as large scale thin film photovoltaic production generates appreciable waste streams and superannuated photovoltaic modules. Cadmium and selenium continue to be regulated because of their toxicity. Since these metals are regulated in other industries, national standards must also be met in the manufacture of thin film photovoltaic modules.

An important problem in the field of photovoltaic (PV) technology is that there has been no process specifically designed to remove and recycle the metals in an environmentally benign fashion. The range of metals which can be present in thin film PV devices may include cadmium, copper, lead, gallium, indium, selenium and tellurium. The substrates they must be separated from are glass, plastic or similar low cost substrates.

The results in Table 1 show that the levels of Cd and Se are within current TCLP limits, although the Cd level for the CdTe cell is close to the limit. Repeated testing of CdTe cells will probably yield some results above the TCLP limit. An elevated level of lead was observed for the CIS cells. Further examination showed that this was from a lot of hand soldered prototypes, indicating that the method of mounting the PV laminate in the finished module (especially if lead containing solder is used) should always be evaluated. As a comparison, the TCLP results for Silicon PV cells showed TCLP levels slightly above the EPA limits of 5 mg/L for lead when both the laminate and complete module are tested. Although there are a number of ways that lead can be removed by leaching and precipitation processes, the utilization of lead free solder and solder free electrical connections should be considered for the manufacture of PV modules.

METAL RECOVERY PROCESSES

 Alternative processes have been developed for the recovery of metals from scrap and superannuated CdTe and CIS modules. These processes are presented below.

CdTe CELL PROCESS

 Figure 1 shows a process to recover metals from scrap CdTe modules.

Figure 1. CdTe Cell Recycling Process Flow Diagram

The treatment process in Figure 1 is to treat the CdTe cells with a nitric acid based lixivant. This lixivant selectively oxidizes and solubilizes the Cd and Te while leaving the SnO₂ conducting layer intact on the glass substrate. This will enable the reuse of the substrate in the manufacture of PV cells. Separation of the substrates from the lixivant separates out the Cd and Te. The lixivant is reused to process cells until a high Cd and Te loading is attained. Metal loadings in excess of 100 grams per liter are achievable. The subsequent treatment of the pregnant lixivant is to electrolyze the material with DC current. Proper selection of the current and electrode materials enabled the precipitation of tellurium metal on the cathode while leaving the cadmium in solution. 

CIS CELL PROCESS

The processing of CIS cells can be considered to be more challenging due to the wider assortment of metals in these cells. The metals present in CIS cells can include copper, indium, selenium, cadmium, zinc and additionally lead from the electrical connections. This different assortment of metals (as compared to CdTe cells) led to the development of the recycling process outlined in Figure 2.

The treatment process in Figure 2 is to treat the CIS cells with a nitric acid based lixivant. This lixivant selectively oxidizes and solubilizes the Cu, In, Se, Zn and other metals from the substrate while leaving the SnO₂ conducting layer intact on the glass substrate. The EVA plastic from the laminate will be hydrolyzed and float to the top of the lixivating solution where it can be retrieved for disposal, since the EVA plastic passes TCLP and requires no further treatment. The subsequent treatment of the pregnant lixivant is to electrolyze the material with DC current. Proper selection of the current and electrode materials enabled the two stage deposition of metal: the first being a Cu/Se mixture and the second being the residual cadmium. Electrolysis results for the separation of Cu/Se from Cd are shown in Table 2.

 
The results in Table 3. shows the selective separation of Cu/Se and Cd on the cathode while leaving the Zn and In solution. In full scale operation the electrolysis would be performed in a trough configuration having a series of electrodes, whereby the Cu/Se would plate out on the electrodes nearest the solution inlet and the Cd would plate out on the electrodes nearest the solution outlet.

Oxidation and distillation of the Cu/Se mixture yields a pure SnO₂ product. Decomposition of the lixivant will yield a mixture of indium, zinc and residual metal oxides which can be sold to a refiner or further treated via solvent extraction.

FURTHER WORK

Additional work is being performed on a combined CdTe/CIS recovery process and developing technology to combine these processes with current industrial selenium electrochemistry and cadmium processing technologies.

A pilot plant demonstration of the process on several barrels of PV waste will be performed during 1998.

                                                 ACKNOWLEDGMENT
 

                                                        REFERENCES

2.  40CFR Ch.1 Pt.261, App.II (7-1-91 Edition).