Preparation and extraction of reducing agents from spent coffee grounds for leaching of lithium-ion battery black mass

Abstract

Spent coffee grounds represent an accessible source of organic compounds with reducing activity and can potentially be used in the hydrometallurgical processing of lithium-ion batteries. This work investigates the preparation of spent coffee grounds, the extraction of reductively active components, and the application of the resulting extract in the leaching of NCA-containing black mass. Convective, vacuum, and combined drying regimes, particle-size distribution, and three extraction methods—maceration, Soxhlet, and ultrasonic extraction—were compared. The combined drying regime provided a final moisture content of 0.5–0.8 % and high reducing capacity of the main particle size fractions of spent coffee grounds (250–1000 µm). The highest specific yield of reductively active components was obtained by maceration at a solid-to-liquid ratio (S:L) of 1:40 (37.6 ± 1.5 mg ascorbic acid equivalents (AAE)·g^-1); at S:L = 1:10, Soxhlet extraction gave the highest yield, whereas ultrasonic extraction produced an extract of comparable concentration in 30 min instead of 240 min. After concentration, the reducing capacity of the ultrasonic extract was 18.7 mg AAE·mL^-1.

When 2 M citric acid with the spent coffee grounds extract was used, the leaching efficiencies reached 73.5 ± 4.3 % for Ni, 81.2 ± 5.6 % for Co, and 98.9 ± 2.5 % for Li, compared with 35.1 % and 33.4 % for Ni and Co in citric acid alone. In terms of efficiency, the citric acid–spent coffee grounds extract system was inferior to the citric acid–H₂O₂ system (90.4 % for Ni and 91.4 % for Co), yet it provided substantially higher Ni and Co recovery compared with citric acid without a reductant. The results show that a liquid extract prepared in advance from spent coffee grounds can serve as a biogenic reductant in the leaching of the studied NCA-containing black mass with 2 M citric acid.