Abstract:

Nickel is a key metal for the rapid electrification required to limit the effects of climate change. However, nickel mining operations in Sudbury generate thousands of tonnes of pyrrhotite-rich tailings daily. These tailings are stored underwater to slow the formation of acid rock drainage but remain a persistent and long-term liability for mining companies. Further, approximately 0.6% of the tailings are composed of retained nickel leaving behind substantial value that is currently being lost. Bioleaching is a low-energy process that uses iron-oxidizing bacteria to enhance the oxidative dissolution of iron-sulfide minerals. When bioleaching is applied to tailings, the nickel locked in the pyrrhotite is released into solution allowing it to be recovered.

Previous work proposed a multi-stage benchtop bioreactor configuration inspired by Metso’s BIOX^® process to bioleach nickel from pyrrhotite tailings. This approach demonstrated effective nickel liberation while also converting the majority of sulfides to elemental sulfur, which allows downstream separation to reduce acid generation potential.

Building on the benchtop demonstration, the present work investigates the scale-up to a continuous 30 L pilot-scale system. The pilot plant was operated under process conditions informed by benchtop studies, including a fixed pH of 2, operation at 30°C, and minimal nutrients. The system was inoculated with a mixed culture of microbes enriched from legacy pyrrhotite tailings and subsequently adapted to fresh tailings collected from a mill in Sudbury, Canada. The first pilot campaign started by increasing the bacterial culture density in a batch system then slowly increasing the flow rate to achieve a residence time of 10 days. This was held at steady-state to assess comparability with the benchtop tests, and then the flow rate was progressively increased to identify process limitations.

Throughout pilot operation, the oxidation process and extent, nickel leaching efficiency, resource consumption, and microbial community dynamics were monitored. This presentation discusses the observed operational trends, key challenges, and lessons learned in the construction and operation of the pilot relevant to the feasibility of using bioleaching to treat pyrrhotite tailings waste.

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Prof. Radhakrishnan Mahadevan