Photovoltaic module recycling: Thermal treatment to degrade polymers and concentrate valuable metals

Abstract

This work investigated the thermal treatment to separate and concentrate economically valuable materials from laminates of crystalline silicon photovoltaic modules (i.e., photovoltaic modules without the aluminum frame and the junction box). Chemical characterization of the metal content was performed by X-Ray Fluorescence (XRF). The polymers of the backsheet were also characterized by Fourier Transform Infrared Spectroscopy (FTIR). The influence of the atmosphere (oxidizing and inert) on the decomposition of the backsheet was investigated by Thermogravimetric Analysis (TGA). Moreover, non-comminuted samples were tested for 4 thermal time lengths (30, 60, 90, and 120 min) in the furnace under ambient air. The degradation of the polymers was measured and 3 material fractions were obtained: silicon with silver and residual polymers (SS), glass and copper ribbons. Furthermore, there was no statistical difference between the mass losses of the samples submitted for 90 (13.62 ± 0.02 wt.%) and 120 min at 500°C (p-value = 0.062). In the SS fraction, silver was 20 times more concentrated than in the ground photovoltaic laminate and 30 times more concentrated than high silver concentration ores. The SS fraction (about 6 wt.%) also presented low copper concentration and a high concentration of lead (hazardous metal). About 79 wt.% glass was obtained, as well as 1% copper ribbons (55.69 ± 6.39% copper, 23.17 ± 7.51% lead, 16.06 ± 2.12% tin). The limitations of the treatment and its environmental impact are discussed, and suggestions for industrial-scale application are given.