Heap leaching of a Cr/Ni-rich sludge was validated by VITO in a 1 m³ fully automated setup with recirculation of the leaching solution and continuous pH and redox adjustment.
Approximately 300 kg of sludge was granulated and placed in the cubitainer on top of an inert sand layer to avoid clogging. Based on the laboratory experimental work (in WP3), the test was started with water adjusted to pH 4 with H2SO4.
After 7 weeks of operation, the oxidizing agent Na2S2O8 was added to increase the redox potential of the solution up to 700 mV and speed up the leaching process. Due to the end of the METGROW+ project, the pilot was stopped after 2,5 months before the full completion of the leaching process. The results were in line with the laboratory test results, proving that the process can be upscaled. With the developed leaching process, approximately 80% Cr and 100% Ni can be selectively recovered from the Cr/Ni sludge without dissolution of Fe. Sorption experiments showed promising results for the recovery of Cr and Ni out of the leachate streams.