Copper extraction and recovery from alkaline copper quaternary and copper azole treated wood using sulfuric acid leaching and ion exchange or electrodeposition

Abstract

Removing copper from treated wood waste would allow the waste to be reused instead of being disposed of in landfills. For that reason, a sulfuric acid leaching process for removing copper from wood waste treated with alkaline copper quaternary (ACQ) and copper azole (CA) was tested at pilot scale. This process, consisting of three 160 min leaching steps at room temperature using a 0.13 N H2SO4 solution followed by three water rinsing steps, allows the removal of 95% of the initial copper present in the treated wood (1890 ± 100 mg kg−1 for ACQ-treated wood and 1190 ± 170 mg kg−1 for CA-treated wood). This study focuses on Cu recovery and revalorisation by ion exchange and electrochemical deposition. Acidic leachates obtained from ACQ- and CA-treated wood contain 99 ± 22 mg Cu L−1 and 124 ± 24 mg Cu L−1, respectively. One chelating resin, Dowex™ M4195, is identified for efficient Cu recovery from acidic leachate and allows ≥76% recovery with a maximum exchange capacity of 0.51 mmol Cu g−1 for ACQ-leachate and 0.39 mmol Cu g−1 for CA-leachate calculated using the Langmuir isotherm. Continuous ion exchange treatment allows Cu recovery as a CuSO4 solution when using H2SO4 as the eluent, or as a Cu-MEA solution for reuse in wood treatment when using monoethanolamine (MEA) as the eluent. Both eluents are able to solubilize ≥99% of the Cu trapped by the resin with a final concentration factor between 40 and 50. In contrast, electrochemical treatment allows for up to 92% Cu deposition at 10 A after 90 min. However, a comparison of the value of recovered Cu and the cost of the electricity required for deposition encourages the use of a lower current (3 A and 90 min). Economic analyses of both processes are presented for a plant with a capacity of 100 000 tons of treated wood per year (ttw yr−1). Accounting for direct, indirect, and capital costs, the treatment of waste wood and the recovery of wood and Cu can generate a profit of 37 US$ ttw−1 using ion exchange and 65 US$ ttw−1 using electrochemical treatment.