Material Characterization
The geologic types, amounts, and even locations of copper mine tailings from Arizona’s 100+ years of mining are only partially known, as is their critical metal content. To evaluate these, we will employ two tools: examination of available documentary records to assess rough amounts of tailings and ore produced and ore characteristics, coupled with state-of-the-art field characterization at multiple scales to estimate volumes, tailings and water composition, and critical mineral content.
There are no data nor adequate framework for this challenge, hence we will build a database on copper mining and tailings in Arizona with data compiled from many sources including USGS reports, production records, and other available documents to identify what was mined (original ore characteristics), when it was mined, and approximately how much ore and tailings were produced. For tailings currently being produced, we will work with our multiple current industry partners to assess their types, amounts, characteristics, and critical mineral recovery potential. These data on the types, volumes, and compositions of copper mine tailings around Arizona, will provide the crucial background context for, and a check on, what has to be more far more selective field and lab characterization.
This characterization and assessment will be Task 1 of the project.
Remote Sensing
Task 1 field characterization will be carried out at multiple scales and with multiple methods. At the largest scale, team members expert in remote sensing will use existing airborne and satellite multispectral and hyperspectral infrared imagery combined with historic topographic data to locate copper mine tailings facilities across the state and estimate their volumes. This will produce a hyperdimensional spatial dataset from which we can extract tailings’ surface mineralogy, water content, and dissolved metal content. Linked with the production records, geologic information, and the database produced by the ongoing ABOR Abandoned Mine Lands project, this will yield a first-order estimate of the volumes, locations, and geologic types of tailings in Arizona.
Our team will conduct high-resolution UAV-based scans of the sites using hyperspectral infrared, magnetometric, and ground-penetrating radar. This will produce a second spatial dataset mapping out tailings mineralogy and surface water composition, with billions of hyperdimensional data
points per tailings facility scanned. This will reduce the number of samples needed to adequately represent the heterogeneity of tailings, which (if drilled and sampled by usual industry procedures) would run to many millions of dollars. Consequently, we will use this high-resolution hyperspectral and geophysical imagery, along with samples from current (hopefully future) partners and from geologic proxies for the same deposits in order to characterize the diversity of minerals and related substances that are in tailings. For any realistic assessment of future recovery, it is essential to understand original geologic material, how it changed over time as a deposit is mined, and how tailings evolve post-deposition due to aging and interaction with water, air, and microbes.
Extraction Testing
Samples taken during Task 1 will be analyzed for overall composition and mineralogy down to the nanoscale. Particular focus will be on their content of metals on the 2022 US critical minerals list that are present in copper ores (antimony, arsenic, bismuth, cobalt, gallium, germanium, indium, manganese, nickel, tellurium, zinc, platinum-group metals, rare-earth elements, and titanium). Analysis will identify their content and species in tailings waters, colloids, and solids. Using the results, we will scope out which critical metals are present in recoverable form(s) in each type of tailings in order to focus extraction testing most effectively. This testing stage will be known as Task 2.
Extraction testing will evaluate conventional (1-2) and novel (3-4) techniques for tailings reprocessing, chosen for potential to recover numerous critical metals from diverse tailings types at low operating cost.
(1) Flotation, which uses distinctive surface properties of metallic minerals, is routinely applied to separate them from nonmetallic species.
(2) Controlled gravity and magnetic fields are common means of separating metallic and related (e.g. rare earth-bearing) minerals from the rest of the rock and may work for tailings, depending on mineralogy.
(3) Dissolved metals can be recovered from liquids by solvent extraction, a multi-step technique common in copper mining but untested for tailings, which contain different, less concentrated metals.
(4) Direct single-step extraction from solution, without solvent cycles, is a novel technique for extracting low concentration critical metals.
Co-PI Maier has invented a method of directly extracting rare earth elements from acid mine drainage, which will be modified and applied for other critical metals from tailings. We will also test direct extraction via selective adsorption onto granular solids, a new technique under study for lithium.
Each sample will be divided into multiple subsamples so that different tests can be performed on the same material (enabling comparisons in Task 3). Team members will test each method and combinations of methods on subsamples of each material collected in Task 1. Emphasis will be on assessing how each method performs by technical criteria, i.e. what percentage of the critical metal content in the sample it can recover, how pure the product is, and what reagents and equipment cost, as inputs to Task 3.
Techno-Economic Analysis
The team will analyze the economic and social feasibility by integrating the results of the project to identify the best method for potential large-scale deployment and economic and social constraints. The team will compare each method according to technical (amount of metal obtained, consumption of chemicals, equipment requirements), economic (value of metal, estimated cost of process scale-up, value from decreased hazard), and social (potential issues related to facility design, environmental footprint) criteria for all processes tested. Using this analysis, we will identify the most promising method(s) of economically reprocessing copper mine tailings to recover critical metals from the tailings sampled and tested. Using the estimated volumes of various tailings types, and recovery results, we will calculate rough order-of-magnitude estimates for the amount of economically recoverable critical metals in past and present Arizona copper mine tailings and outline next steps for field pilot study.