Author ORCID Identifier

https://orcid.org/0009-0008-4534-2676

Semester

Spring

Date of Graduation

2025

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mining Engineering

Committee Chair

Qingqing Huang

Committee Co-Chair

Deniz Tuncay

Committee Member

Deniz Tuncay

Committee Member

Deniz Talan

Abstract

Critical minerals (CMs) are essential for U.S. national security and technological development. The U.S. heavily depends on imports of these minerals from other countries, making supply chain disruptions a likely scenario. In consequence, finding alternative sources to conventional mining for these elements has become a topic of interest in recent years. Acid Mine Drainage (AMD), once solely viewed as an environmental problem, has emerged as a potential alternative source for CMs. AMD is a naturally occurring process where surface and underground water interact with sulfur-bearing minerals in the presence of oxygen and microorganisms. This interaction causes the water to become acidic due to the generation of sulfuric acid, triggering a chain reaction in which the acidic water leaches minerals from the rock, thereby increasing both the acidity and the amount of dissolved elements. It has been identified that mining processes can accelerate the generation of AMD since more sulfur-bearing minerals are exposed during these activities. The remediation of AMD has been extensively studied, and various active and passive treatment methods have been proposed. Furthermore, research into the potential for recovering valuable minerals through remediation has been ongoing. Several processes have been suggested to treat AMD; the most effective techniques typically involve treating the AMD sludge resulting from active treatment methods and separating the elements of interest through selective leaching, selective precipitation, and solvent extraction. However, the high operational costs associated with reagent consumption have made it difficult for these processes to be implemented on a large scale. This research investigates the development of a process that combines ion exchange, a more cost-efficient method, with selective precipitation for recovering critical metals (Ni, Co, Zn, and Mn) from AMD. The proposed methodology begins with the characterization and selection of the AMD source. Once the feedstock AMD is selected, most impurities, such as iron and aluminum, are removed using caustic precipitation, which removes 66% of iron and 96% of aluminum at a pH value of 5.00. After the impurity removal stage, a two-stage ion exchange process was developed using Ambersep M4195 to selectively separate and enrich nickel in the first stage and zinc and cobalt in the second stage. These ion exchange processes were optimized through systematic single-variable testing, where the optimized variables included AMD pH, AMD contact time, sulfuric acid concentration, and volume during resin rinsing, as well as sulfuric acid concentration and volume during resin elution. The optimal AMD pH during the nickel enrichment stage was 1.3, and the achieved nickel enrichment factor was 122.9. During the separation and enrichment of zinc and cobalt, the optimal pH value for AMD was 3.5, resulting in enrichment factors of 83.2 for zinc and 80.4 for cobalt. After the two stages of ion exchange, three enriched streams are obtained: a nickel-enriched sulfuric acid solution, a zinc and cobalt-enriched sulfuric acid solution, and the discharge AMD enriched with manganese and other impurities. Selective precipitation processes were specifically developed to partially separate zinc from cobalt and manganese from impurities. Sodium sulfide (Na2S) was selected as a selective reagent to precipitate zinc sulfide (ZnS). The optimal parameters for this precipitation were determined to be a maximum precipitation pH of 8.2 and an S2-/Zn2+ ratio of 2.0:1.0. Precipitation of 96% for zinc and 26% of cobalt co-precipitation was achieved, with the purity of the zinc solids calculated to be 38% (w/w). Sodium hydroxide (NaOH) was utilized to precipitate nickel and cobalt products at a pH of 11.00 with a precipitation percentage of 99% for both elements, yielding solids with a nickel purity of 41% (w/w) and a cobalt purity of 30% (w/w). Oxidative precipitation was chosen for separating manganese from impurities using ammonium persulfate (APS), (NH4)2S2O8. To optimize the recovery and selectivity of this process, the precipitation time, initial pH, temperature, APS/Mn2+ ratio, and APS concentration were studied using systematic single-variable testing. The optimal conditions identified were a temperature of 80°C, a precipitation time of 2 to 3 hours, an initial pH of 3.5, an APS/Mn2+ ratio of 1.6:1.0, and an APS concentration of 0.6 M. The manganese precipitation percentage was nearly 100%, and the purity of the solids was calculated to be 34% (w/w) manganese. Following the removal of the critical minerals of interest, the remediated AMD has a pH value between 7.0 and 7.5 and a concentration of less than 1 ppm for all heavy elements initially present.

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