Date of Award

2026

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering (PhD)

Administrative Home Department

Department of Chemical Engineering

Advisor 1

Timothy Eisele

Committee Member 1

Lei Pan

Committee Member 2

Snehamoy Chatterjee

Committee Member 3

Robert Handler

Abstract

Nickel is increasingly recovered from low-grade laterites and mine tailings, where it occurs in trace amounts locked inside iron-bearing minerals. Reductive bioleaching can free this nickel under mild, low-energy conditions, but the same microbial reaction also dissolves the far more abundant iron, giving a leach solution with a high iron-to-nickel ratio that is costly to purify. This dissolved iron, not the difficulty of releasing nickel, is the real barrier to a workable process. This research develops and tests a way to control iron during leaching itself, using low-temperature, carbon dioxide enhanced reductive bioleaching. Because carbon dioxide is more soluble in cold water, operating near freezing raises the dissolved carbonate enough to hold the released iron in the solid as iron carbonate, while nickel stays in solution because its carbonate is far more soluble and organic ligands keep nickel dissolved. The work defines this selectivity window thermodynamically and uses the Fe/Ni mass ratio to measure it, with a fermented cattail lixiviant and a cold-tolerant metal-reducing consortium. A factorial study showed that nickel release is governed mainly by the lixiviant and the microorganisms, and that near-freezing operation does not lower nickel recovery. On iron-rich tailings at bench scale, low-temperature operation cut the leachate iron-to-nickel ratio by 6.2 times, from 13.87 to 2.25, while nickel recovery held near 45%, with carbon and iron mass balances confirming that the suppressed iron stayed in the solid as a carbonate phase. The process was then scaled about 600 times, to a 12 kilogram pilot run over 205 days, where the selectivity held: about 40 to 45% of the nickel was leached while only about 0.05 percent of the iron entered solution, lowering the leachate iron-to-nickel ratio from about 98 in the feed to about 0.06. This low-iron solution was electrowon directly to high-purity nickel metal with no separate iron-removal stage. The study establishes temperature-controlled carbon dioxide solubility as a new way to manage iron at the source during nickel bioleaching, suited to cold-climate reprocessing of low-grade, iron-rich tailings.

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