Development of a Carbon-Negative Process for Comminution Energy Reduction and Energy-Relevant Mineral Extraction through Carbon Mineralization and Biological C Fixation

ARPA-E Control Number 2707-1555, University of Kentucky, Honaker Development of a Carbon-Negative Process for Comminution Energy Reduction and Energy-Relevant Mineral Extraction through Carbon Mineralization and Biological Carbon Fixation Lead Organization: University of Kentucky, Lexington, KY Principal Investigator: Rick Honaker, Ph.D. Technical category: 1 and 2 Proposed Funding Requested: $5,000,000 Federal and $1,250,000 Cost Share Project Duration: 36 months Concept Summary The proposed concept utilizes supercritical CO2 (sCO2) to pretreat the CO2 reactive minerals in the host rock and tailing material associated with energy-relevant minerals prior to grinding. Subsequently, CO2 saturated liquid will be utilized during grinding in a pre-treatment mill to further assist mineral carbonation by exposing CO2 to fresh surfaces. By converting the CO2 reactive mineral from silicates to carbonates, the bond work index of the ore reduces from 18-20 kWh/t to 10-12 kWh/t, which significantly reduces the specific energy consumption by nearly 50%. The energy-relevant minerals will then be recovered using the HydroFloatTM separator, which is a novel fluidized-bed flotation system developed specifically to recover coarse particles containing small amounts of exposed hydrophobic surfaces that cannot be recovered using a conventional flotation system. This will allow mineral particles discharged from the pretreatment mill to be in a much coarser size range (1 – 0.15 mm), which significantly reduces the comminution energy needed for mineral recovery. The proposed work will also look into enhancing the hydrophobicity of the targeted minerals through advanced chemistry to improve the recovery of low-degree liberation particles. The energy-relevant mineral recovered from the HydroFloat can be fed into a ball mill and then recovered using a conventional flotation system. Since the load of the material fed to the ball mill is significantly reduced due to the rejection of non-valuable minerals by the HydroFloat, the energy consumption per ton of the energy-relevant mineral recovered is reduced substantially. Alternatively, for energy-relevant sulfide minerals (i.e. chalcopyrite and spherite), the mineral recovered from the HydroFloat can be directly leached through a bioleaching system using bacteria such as acidithiobacillus ferrooxidans. The bacteria will selectively dissolve sulfide minerals dispersed in gangue material. The CO2 released from carbonate ore will be captured and consumed in the bioleaching system by serving as a nutrient for bacteria growth, which leads to biological carbon fixation. Overall, the proposed process achieves carbon negative emissions. Innovation and Impact Comminution liberates valuable minerals from gangue minerals by concentrating stresses around the boundaries between the valuable and non-valuable minerals. Sufficient liberation is a prerequisite for obtaining high-grade mineral concentrates from ores at high recovery values. As such, comminution dictates the efficiency of overall mineral processing circuits. Comminution consumes up to 4% of electrical energy globally, and about 50% of mine site energy consumption1. Therefore, it is critical to develop innovative technologies to reduce the energy consumption of comminution without impairing efficiency. During the comminution process, ore particles are broken by impact, compression, and/or attrition. Ore particles are easier to comminute when fractures (flaws) exist, which can be classified into interfacial fracture, interphase fracture, and 1 Jeswiet J., Szekeres A., 2016. Procedia CIRP, 48, 140-145. 1 ARPA-E Control Number 2707-1555, University of Kentucky, Honaker inner phase fracture. The three types of fractures are defined as cracking along the boundary between two different rocks, the boundary between two different mineral phases, and a boundary in a pure, single-phase mineral, respectively. Technologies that promote the formation of fractures, preferably interphase type, will reduce comminution energy consumption by increasing the particle size needed to achieve the required degree of liberation. Ore particles consist of different types of minerals, some of which are CO2 reactive and some are CO2 inert. Thus, different minerals in the same ore react differently with CO2, leading to the generation of flaws and fractures in the particle. Based on this fact, we propose to use CO2 to reduce comminution energy. This concept can be incorporated into existing comminution circuits in three different ways: Figure 1. Comminution energy reduction by ex-situ (pretreatment prior to grinding), in-situ carbonation. (concurrent with grinding), and ex-situ plus in- situ (Figure 1). Several findings from prior studies suggest the viability of using CO2 to reduce ore comminution energy: (1) CO2-reactive mineral surfaces rapidly carbonated with wet supercritical CO2 (sCO2) in less than 1 hour2; (2) carbonation creates lattice defects in CO2-reactive minerals, and lattice defects are more likely carbonated relative the whole mineral3; and (3) mechanical grinding while carbonation can increase the carbonation yield and improve particle size reduction4. The main purpose of comminution in mineral processing is to liberate the valuable mineral particle from the gangue material to allow physical separation. Conventional froth flotation is a commonly applied technology to separate particles based on their surface hydrophobicity characteristics, which typically requires particles to be ground to a top size of 150-200 µm to achieve 80% liberation. To overcome this limit, a novel separator HydroFloatTM was developed which integrates the froth flotation technique within a fluidized bed technology. Due to low turbulent conditions, coarse particles containing poorly liberated valuable minerals with less than 10% hydrophobic surface exposure can be floated with the assistance of fluidized water (Figure 2). This technology has been Figure 2. Recovery of particles with different successfully applied to industrial minerals with several exposed target mineral surface areas. 5 full-scale units demonstrated to recover particles larger than 3 mm. Results show that sulfide mineral recovery increased 15 times for the 1x0.85 mm particles by using a HydroFloatTM separator compared to a conventional flotation cell.5 As such, the amount of the material that is fed to the downstream grinding circuit can be reduced 2 Schaef et al., 2013. Environmental Science & Technology, 47, 174-181. 3 Kwak et al., 2010. The Journal of Physical Chemistry C, 114(9), 4126-4134. 4 Julcour et al., 2015. Chemical Engineering Journal, 262, 716-726. 5 Miller et al., 2016. IMPC Conference paper. 2 ARPA-E Control Number 2707-1555, University of Kentucky, Honaker substantially thus reducing the overall comminution energy needed to recover nearly 100% of the targeted mineral. In addition, the unliberated minerals may occur as micro dispersed crystals in the particle which may require specifically designed collectors to enhance the surface hydrophobicity by crosslinking the micro dispersed crystal sites on the particle surface, which improves recovery of energy relevant mineral finely dispersed in the CO2 reactive host rock. Recently, mineral leaching with the assistance of microorganisms has been intensively studied due to its great potential in metal extraction processes that can lead to sustainable mining and metallurgical processes. Bioleaching has been demonstrated and utilized in an industrial scale to extract valuable material from mine tailings, low-grade ore, mineral concentrate, etc. It can be achieved through direct contact and indirect contact bioleaching. For instance, the leaching of low- grade sulfide minerals such as chalcopyrite is usually retarded by the formation of a passive layer consisting of element sulfur, copper polysulfide, or iron sulfate precipitates. Direct bioleaching using Acidthiobacillus spp. bacteria is a great alternative to resolve the issue. The bacteria reacts with the sulfur existing in sulfides minerals like chalcopyrite which releases copper into the solution. Acid production also occurs through oxidation of ferrous to ferric and sulfur to sulfates, which further enhances the kinetics of the metal extraction process. 6 Previous research showed that by temperature optimization and the dissolved oxygen level in the bioreactor, the acid generation process can be expedited to two days. The advantage of using bioleaching to extract energy relevant minerals from sulfides and carbonated ore is the role of CO2 in bioleaching process. A study showed that concentration of CO2 greatly improves the growth and activity of bioleaching microorganisms by improving the iron oxidizing bacteria to increase the leaching kinetics of Cu from copper sulfide mineral7. The carbon fixation in the bioleaching system is demonstrated in Figure 3. This Figure 3. Carbon fixation in enables the re-capture of CO2 from leaching the carbonates and the bioleaching system. 7 carbonated gangue mineral associated with energy-relevant minerals. Proposed Work The project proposes to test the concept on two different feedstocks: chalcopyrite ore from the Bingham Canyon ore and bastnasite ore from Mountain Pass, which is the only rare earth mine in the United States. Rio Tinto’s Bingham Canyon resource contains a significant amount of plagioclase assemblage CO2 reactive mineral in the host rock while the Mountain Pass deposit contains 10-15% of CO2 reactive minerals. The proposed work will be conducted over a three-year project period with the first two years focused on laboratory and bench-top testing followed by pilot-scale testing of the favorable portions of the concept in the third year. Project tasks include: Task 1: Supercritical CO2 and saturated CO2 liquid treatment. The task will assess the degree of mineral carbonation by optimizing the sCO2 treatment condition and CO2 liquid-assisted grinding operation. Task 2: Comminution energy assessment. The task will assess the grindability alteration of ore after sCO2 treatment, mineralogical determination, and liberation 6 Zhou et al., 2018. Minerals, 8, 596 7 Guezennec et al. 2018. Hydrometallurgy, 180, 277-286. 3 ARPA-E Control Number 2707-1555, University of Kentucky, Honaker Targets State of the art Technology Proposed Technology Carbon ? Carbon mineralization by CO2 ? The kinetics of carbonation using Mineralization injection in rock supercritical CO2 is much faster. ? Olivine: 0.05 ton of CO2 ? Olivine: 0.075 ton of CO2 uptake per uptake per ton of ore (10- ton of ore per hour. micron grain size) per year. Comminution ? Bond work index (Wi) for ? Reduce Bond work index of ore by silicate ore is typically 18-20 40-50% through carbonation using kWh/t (i.e. andesite, olivine.) sCO2 and liquid CO2. ? Innovative mill design to ? Using Hydrofloat to recover energy optimize charge and slurry relative mineral at low degree of motion in mill. liberation and reduce the load feeding ? Optimize stirred speed, pulp into the comminution circuit density, fluid viscosity, and ? Reduce energy consumption of grinding media etc. grinding by >50% Mineral ? Froth flotation typically ? Hydrofloat increases recovery of Recovery require particle size of 100– unliberated energy relative mineral 200-microns. by >50% ? Using bioleaching to enhance metal recovery from low-grade minerals. Carbon Carbonate mineral ? Bio-assisted Carbon Fixation Re-Capture ? Roast and air carbon capture ? Enhanced bacteria growth degree of valuable/gangue minerals using a series of characterization tool (i.e., SEM, Micro CT, etc.). Task 3: HydroFloatTM testing. This task will focus on evaluate and optimize mineral separation efficiency to further reduce comminution energy and improve unliberated mineral recovery using innovative chemicals under various liberation characterization and surface exposure. Task 4: CO2 fixation in bioleaching. The task will assess the mass transfer of CO2 in bioleaching reactors, and evaluate the effect of CO2 on bacteria growth, CO2 fixation rate, and bioleaching kinetics. Task 5: Pilot testing of the favorable concept components in the third year will be performed adjacent to mining processing operations. Task 6: TEA and Life Cycle Analysis. Team Organization and Capabilities The team consists of academic experts from the University of Kentucky (UKY), Virginia Tech (VT), University of Utah (UU), and West Virginia University (WVU). Industrial partners include Eriez Manufacturing (equipment), and Solvay (chemicals). The combined team has a vast array of extractive metallurgy equipment at bench-scale and pilot-scale as well as the analytical tools needed to complete the project tasks. Dr. Rick Honaker (Professor, UKY) will serve as PI with over 35 years of extractive metallurgy and large federally funded project management experience. Dr. Mike Free (Professor, UU) will lead efforts on bioleaching and carbon fixation in bacteria. Dr. Xinbo Yang (Assistant Professor, UKY) will focus on solution chemistry, mineral liberation, process design, and scale-up. Dr. Wencai Zhang (Assistant Professor, VT) will lead carbonation and surface chemistry efforts. Dr. Qingqing Huang (WVU) will work on comminution energy assessment. 4 ARPA-E Control Number 2707-1555, University of Kentucky, Honaker APPENDIX 1: TECHNO-ECONOMIC ASSESSMENT The schematic of the proposed process in this study is shown in Figure 4. Since the primary and secondary crusher, rod mill, ball mill, and flotation unit are typically used in a preexisting mineral beneficiation process, the scope of this techno-economic assessment primarily focuses on the operational costs associated with the supercritical CO2 treatment system. The potential economic gain of the proposed process is the savings on energy reduction on comminution and mineral recovery improvement. The primary operating cost of the Figure 4. Flowsheet sCO2 system that will be developed in this project to pretreat ore particles is associated with the electricity used in the pumps and heaters. There is limited information reported for the techno-economic analysis of this kind of application on carbon mineralization. However, based on existing TEA studies on the application of sCO2 in the food industry, the operating cost is estimated to be as low as $1/ton depending on the operating pressure, temperature, and residence time for treatment. The residence time reduces exponentially with increased particle surface area when the flaws and fractures occur at the boundary of minerals during treatment. The specific energy (Eg, kWh/ton) requirement for the comminution process is a function of the Bond work index (Wi), feed and product particle size expressed as F80 and P80 which is the size at which 80% of the material is passing. Assume 50% of the ore is CO2 reactive minerals, and 50% of the ore is non-CO2 reactive minerals. Assume the Wi for non- reactive ore is 15 kWh/t. Converting the carbon reactive mineral from silicates to carbonates, reduces the bond work index from 20 kWh/t to 10 kWh/t. As such, the overall Wi of the ore is reduced by 28%. Combining the benefits of the sCO2 with those of the HydroFloat as shown in Figure 5, the proposed Parameters Conventional HydroFloatTM technology can potentially reduce the energy cost F80 (micron) 12000 12000 from $1.6/ton to $0.28/ton (@$0.1/kWh). P80 (micron) 100 1000 Production of particles smaller than 10 microns to Wi (kWh/t) 17.5 12.5 achieve a targeted liberation point commonly results Eg (kWh/t) 16 2.8 Energy Cost $1.6/ton $0.28/ton in recovery loss. Using the HyroFloat and /or Figure 5. Comminution cost reduction. bioleaching can minimize the loss. Assuming the feed grade of copper is 0.5%, increasing the recovery of copper from 80% to 100% will result in 1 kg of additional Cu recovered per ton of feed ore. At a Cu price of $10.4/kg, additional product revenue of $10.4/ton of feed may be generated using the proposed process. A similar result can be realized for the processing of bastnaesite. In addition, other elemental byproducts may be generated in the process (Ni, Nb, etc). 5