AOI 1: Low Temperature Plasma Treatment for Enhanced Recovery of Highly Valued Critical Rare Earth Elements from Coal

Grants and Contracts Details


Statement of Project Objectives AOI 1: LOW TEMPERATURE PLASMA TREATMENT FOR ENHANCED RECOVERY OF HIGHLY VALUED CRITICAL REES FROM COAL Area of Interest (AOI) 1 A. OBJECTIVES The principle objective of this project is to develop a novel process using low-temperature plasma treatment integrating with hydrometallurgical process to recover rare earth elements (REEs), especially highly valued REEs (i.e. scandium and critical REEs), from coal and coal byproducts. Low temperature plasma (LTP) will be utilized to pretreat the feed stock sample to liberate, release, oxidize and/or activate the rare earth elements from organic matter in coal or minerals that are embedded in coal. The LTP technique will be integrated with current state-of-the-art REE recovery processes including leaching, solvent extraction and selective precipitation to obtain a product containing ? 2% of total REEs on a dry whole mass basis. The high-valued REEs in LTP treated coal is expected to be recovered at a less demanding leaching condition (i.e. leaching pH, temperature, etc.). The project will be conducted in laboratory scale with a benefit of providing key information leading to overcome challenges in larger scale operations and further maturation of applying the low temperature plasma treatment technique in rare earth production from coal and coal byproducts. B. SCOPE OF WORK The proposed scope of work comprises nine tasks with a few subtasks to be performed during one budget period of 18 months. The proposed project will initially optimize the operating parameters of the plasma treatment process (i.e. power, temperature, treatment time, etc.) based on the resulting changes of exposed surface area, pore size, micro structure, degree of oxidation, etc. The second stage will focus on the integration of the LTP treatment technique with lately investigated flowsheet to recover REE from coal using hydrometallurgical processes including leaching, solvent extraction and precipitation. The optimized leaching condition will be examined by utilizing different chemicals, varying leaching temperature and pH conditions. The project intents to examine the technique universality by evaluating the enhancement on REE recovery after LTP treatment from multiple feedstocks in coal mines across the nation. The economic feasibility of implementing LTP treatment in the state-of-the-art REE recovery processes will be evaluated and optimized towards a promising larger scale operation. C. TASKS TO BE PERFORMED Task 1.0 - Project Management and Planning This task shall include all work elements required to maintain and revise the Project Management Plan and to manage and report on activities in accordance with the plan. It shall also include the necessary activities to ensure coordination and planning of the project with DOE/NETL and other participants. These shall include, but are not limited to, the submission and approval of required National Environmental Policy Act (NEPA) documentation. Task 2.0 - Sampling and Characterization of Proposed Feedstocks Subtask 2.1 - Sample Collection and Documentation A Sampling and Characterization Plan will be developed to provide details related to the collection, shipping and storage of all targeted samples. For planning purposes, it is expected that coarse refuse samples will be collected from various operating coal preparation plants in the Illinois basin including those mining in the Kentucky No. 9, 11, and 13 coal seams and the Illinois No.5 and 6 coal seams. Coarse refuse from the Kentucky No.13 coal seam has already been evaluated and found to be qualified with a total REE content greater than 300 ppm on a dry, whole mass basis. Field sampling based on ASTM standards will be used to ensure that representative samples are collected. If warranted, modifications to the sampling program will be made based on Gy's sampling rules to avoid "nugget" effects associated with REE sample collection. All the samples collected from the mine sites will be stored in a vacuum and low temperature environment to minimize samples' oxidization. For each sample, all pertinent information will be documented including geographic location, site description, etc. Each source of coal will be then subjected to standard proximate and ultimate analyses. Elemental analyses will be also conducted using ICP- based bulk analysis to establish the baseline concentrations of REEs. The final selection of coal byproduct sampling sites will be made in consultation with DOE per the Sampling and Characterization Plan. For planning purposes, it is expected that work will initially focus on the acquisition of samples from (i) the Blackhawk Mining complex (Leatherwood plant) in eastern Kentucky where the Fire Clay (Hazard No. 4) seam is processed, (ii) the Alliance coal complex (Dotiki plant) in Clay, Webster Country of Kentucky where the West Kentucky No.13 seam is processed. Various splits of coals from these sites have all been shown to meet DOE's minimum REE content of 300 ppm (whole coal basis). XRD and XRF analyses will be conducted to identify and quantify the major elements and minerals associated with each feedstock. In addition, the forms of sulfur (i.e., organic sulfur, pyritic sulfur, and sulfate sulfur) will also be quantified for each collected sample based on the ASTM D2492 standard procedure. Subtask 2.2 - Leachability Characterization Rare earth elements associated in coal samples of different density fractions may have distinct leachabilities due to the variation in rare earth mineralogy generated from different geochemical and physical activities. Previous rare earth liberation study of the Fire Clay middlings indicates that REEs are more enriched in the ultra-fine ash materials disseminated in the sample. As such, a detailed liberation study will be performed for the collected samples. Specifically, the samples will be initially pulverized and/or ground to a certain particle size followed by float-sink or flotation tests to remove the ash materials. The obtained low ash materials will be undergone pulverization and/or grinding again to liberate finer ash materials. The liberated ash materials will be utilized as feed stocks for leachability characterization study. The leaching tests will be performed using trace-metal grade sulfuric acid at various pH values (e.g., 5.0, 3.0, 2.0, 1.0) and temperatures (e.g, 20, 40, 60, 80oC) in a three-neck round bottom flasks with a constant solid concentration of 1 wt.%. The leachability characterization results will be compared with the outcomes of the low temperature treated materials to obtain an in-depth understanding of the plasma treatment. Subtasks 2.3 - Resource Assessment A review of the available data from USGS, Kentucky Geological Survey, and other sources will be made with respect to the resources of the Fire Clay and other potential REE-enriched coal refuse. Additional samples from the current mining operations at Blackhawk Mining will be used to establish a quantitative estimate of the total resource of REEs in the coal middlings that are the focus of this project. The most recent resource assessment for the Fire Clay coal includes mine production data through 2014 which will suffice for the proposed work. Task 3.0 - Low Temperature Plasma Treatment Subtask 3.1 - Parametric Study The effect of oxygen plasma treatment on REEs leachability from coal will be evaluated under various conditions using a benchtop plasma unit. The low-temperature plasma (LTP) process partially ionized oxygen to react with coal from the particle surface to deeper layers with time. utilizes radio frequency (RF) discharge to generate plasma in an oxygen rich chamber under vacuum environment, therefore, the oxidation kinetics of coal sample is a function of operation conditions (i.e. temperature, RF power, gas flow rate, etc.), sample characteristic (i.e. particle size, exposed surface area, mineral compositions, etc.). A PE-100RIE unit manufactured by Plasma Etch, Inc. will be the ideal instrument to perform the LTP experiments upon approval of the project. The instrument incorporates an aluminum vacuum chamber which is resistant to plasma etching with 12"x14"x12" processing room. Uniform plasma is generated through a 300W 13.56 MHz direct contact RF power supply with variable power settings, oxygen gas flow controller, and water cooling temperature controller. The collected samples will be pulverized into different particle sizes using a lab-scale crusher and/or attrition mill. Pulverized samples of different particle size will be treated in the plasma unit under various conditions for different periods of time. A systematic parametric study will be designed using an appropriate design of experiments model to investigate the effects of the abovementioned major factors and interactions on the REEs liberation and oxidation performance. The resulting improvement on REEs leachability and leaching selectivity will be used to quantify the act of LTP on REEs recovery from coal. The optimized plasma treatment condition will be used for REE leaching Subtask 3.2 - Sample Characterization The LTP pre- and post-treated sample obtained in subtask 3.1 will subject to a series of characterization analyses to evaluate the modification of sample properties. Proximate analyses will be conducted in a thermogravimetric analyzer following the ASTM standard procedure to determine moisture, volatile matter, ash content, and fixed carbon contents, plus the sulfur content analysis using a LECO sulfur analyzer. Particle size of the pre- and post-treated coal sample will be measured using a CILAS laser particle size analyzer. Surface area and pore size distribution will be measured using a TriStar 300 analyzer. Scanning electronic microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (SEM-EDX) analysis together with particle size, surface area, and pore size distribution data will provide a detailed information regarding the microscopic structures change of the treated materials. The surface functional groups and oxidization status will be analyzed using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). XPS is a surface- sensitive quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state with a detection limit < 300 ppm. The surface chemistry characterization will be utilized to analyze the status of REEs on surfaces of the treated material. The bulk mineralogy and composition will be achieved using X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. All the above-mentioned characterization study will provide an in-depth understanding of the enhancement achieved on REEs recovery after oxygen plasma treatment. The fundamentals will also promote future commercialization of the novel process for REE recovery from coal. Task 4.0 - Leaching Subtask 4.1 - Water Leaching Tests Water leaching is normally utilized to treat solid samples with high solubility. In the commercial rare earth industry, insoluble rare earth concentrates (e.g., monazite, xenotime, and bastnaesite) will be roasted to transform into more soluble forms (e.g., oxides and chlorides) followed by the water leaching process to dissolve the REEs into liquid solution which will be further processed using hydrometallurgical techniques. LTP technique utilizes the oxygen plasma to oxidize the carbon associated in coal in mild conditions. In this case, REEs existing as cationic species adsorbed onto clay mineral surfaces and/or bond with organic matters will be oxidized into rare earth oxides which can be easily dissolved in water. Preliminary results indicated that the presence of pyrite in coal generated acid during pyrite oxidation process which favored the dissolution of RE oxides without consuming additional acid. As such, the treated materials generated in the Task 3.0 will be undergone water leaching at different temperatures (20, 40, 60, and 80oC) and solid concentrations (1%, 5%, 10%, and 20%). Both the leaching residual and leachate will be analyzed for REE content for mass balance purposes. The solid loss and leachate pH, Eh, and conductivity will also be measured to provide indications regarding the solid dissolution behavior and the leachate chemistry. Subtask 4.2 - Salt Leaching (Ion-Exchange) Tests Previous study indicates that the treated materials finely disseminated in the coal organic matrix are mainly clays (i.e. kaolinite and illite) and silica. As such, the free status rare earth species occurred in coal will be likely adsorbed onto negatively charged clay surfaces through electrostatic interactions. Cations such as Na+ and NH4+ which have lower hydration energies are required to displace the REEs from clay surfaces. Salt leaching (ion-exchange) has been commercially utilized to process the ion-adsorption clays, mainly distributed in south China, for recovering REEs. In the current proposal, salt leaching tests will be performed for the treated materials generated in the Task 3.0 using different agents (e.g., sodium chloride, ammonium chloride, and ammonium sulfate). The effects of agent dosages, solid concentrations, and solution temperature will be investigated. Significance of the above parameters will be evaluated using a 2-factor test program, after which a 3-factor test program will be utilized to determine the interactions among the most significant parameters. Subtask 4.3 - Acid Leaching Tests Acid leaching technique utilizes more intense conditions to enhance the leaching performance of solid samples. Significant acid leaching study has been conducted in the University of Kentucky for recovering REEs from coal refuse. It has been proven that REEs associated in coal and coal byproducts can be selectively leached out using 1.2 M sulfuric acid with more than 80% of recovery. Acid leaching will be utilized to treat the materials obtained in the Task 3.0 for more efficient rare earth recovery. Similar to the water leaching and ion-exchange tests, acid leaching will be performed in a three-neck round bottom flask (1 liter) which may be immersed into water bath to control the leaching temperature. Three different organic acids, i.e., sulfuric, hydrochloric, and nitric acids, will be utilized to leach the samples at different solution pH (e.g., 3, 2, 1, and 0). The effects of solid concentrations, leaching temperature, and reaction time will also be evaluated. The water, salt, and acid leaching tests will provide an optimum scheme for recovering REEs from low temperature plasma treated materials. The criteria for an excellent recovery process includes a number of factors, i.e., the highly-valued rare earth recovery, contaminant dissolution, operating cost, radioactive elements (Th and U) release, and environmental issues. As such, for each test, individual rare earth, dissolved solid, and radioactive elements contents will be measured using proper techniques. An overall consideration of the above factors will be made for judging the leaching performance. Task 5.0 REE Concentrate Production 5.1 Selective Precipitation A detailed parametric study will be conducted regarding rare earth precipitation from the leachate to produce a final product with 2-10 wt.% of REEs on a dry, whole mass basis. Performance of three precipitants including oxalic acid, sodium sulfate and ammonium hydroxide will be examined. The effects of several factors including solution pH values, precipitant dosages, feeding speed, and feeding mode (positive and reverse feeding), precipitation temperature, stirring speed, and aging time will be evaluated through single factor experiment, for which one parameter will be changed while the other factors remain constant. The optimization of selective precipitation includes two steps: 1) factors playing dominant roles will be identified first using a 2-level factorial design; 2) based on the identified factors, the interactions between the factors will be determined using a Box-Behnken design with each factor varied over three levels. Box-Behnken results will be utilized to generate a response surface which can be utilized for optimizing the precipitation process. 5.2 Solvent Extraction Another promising technique to generate products with high REE contents from aqueous systems is solvent extraction. However, it is less economical to directly use solvent extraction to treat a liquid solution with low REE concentrations. As such, selective precipitation can be utilized to pre-concentrate REEs followed by re-dissolution in sulfuric acid solution with pH of 2.0 followed by solvent extraction. Raffinate generated from the solvent extraction process will be introduced back to the leaching process to take advantages of its acidic condition. A complete solvent extraction circuit may contain three different steps, i.e., extraction, scrubbing and stripping. In the extraction process, maximum amounts of REEs are expected to be transferred to the organic phase together with minimum contaminants. Di-(2-ethlhexyl) phosphoric acid (D2EPHA) has been proven to be an efficient extractant for REE recovery from the leachate generated from coal refuse. As such, the D2EPHA is proposed to be utilized in current study and the factors that will be evaluated include extractant contents in organic phases, organic/aqueous ratio, loading time, and solution pH. The contaminants reporting to the organic phase together with REEs will be removed using a scrubbing process, in which different molarity of acids, interaction times, and temperatures will be applied to achieve high selectivity. REEs in the organic phase finally will be stripped out using 5 M of sulfuric acid. The same parametric and optimization procedures as the previous section will be utilized. REEs will be precipitated out from the stripping solution followed by roasting to obtain final rare earth oxides. Task 6.0 Environmental Studies Low temperature oxygen plasma treatment is a slow oxidization process accompanying with the transformation of carbon into carbon dioxide. As such, the environmental issues associated the process are minimal and environmental studies will focus on the subsequent processes, i.e., leaching, selective precipitation, and/or solvent extraction. Solid residue of the leaching tests will be undergone static leaching tests to monitor the long-term releasing behavior of the elements associated in the solids. Specifically, the solid residue will be rinsed initially to remove any remaining chemicals such as acid and ammonium cations followed by filtration. The filter cake will be mixed with deionized water in a proper solid/liquid ratio and the slurry will be sealed in a jar. Slurry pH, Eh, and electrical conductivity will be measured once per week and the overall process will last up to 12 months. Finally, the slurry will be filtered and full spectrum elemental analyses will be conducted for both the filter cake and filtrate. In addition, the solid residues will also be mixed with acid neutralization minerals such as calcite for static leaching tests. The results will provide some indications regarding the remediation schemes. Radioactive elements (i.e, Th and U) may be reported into the leachate together with REEs and thus causing environmental issues in the subsequent steps. As such, all the products including both liquid and solid generated from the selective precipitation and solvent extraction processes will be analyzed for Th and U contents to identify their distribution behaviors. Proper schemes will be proposed for the isolation of radioactive elements from REEs and the disposal of the products with enriched Th and U based on the environmental study. Task 7.0 System Design In this subtask, a conceptual design will be developed for the proposed pilot-scale circuit that is capable of meeting the desired recovery and quality levels for each of the selected coal feedstocks. Specific work elements to be completed under this subtask will include (i) preparation of a summary report that summarizes all test data, assumptions, engineering estimates and design criteria used in the development of the selected design, (ii) identification of the most appropriate circuit configurations, processing strategies and unit operations for meeting the project objectives, (iii) development of detailed material balances that specify the expected solid, liquid flow and component (e.g., ash content, REEs, etc.) rates based on particle size analyses, acid leaching data and selective precipitation data and other characterization, laboratory and bench-scale test data, (iv) preparation of a detailed listing of required unit operations including equipment type, unit size, throughput capacity, reagent/chemical requirements, power requirements, air/water requirements, operating limitations, vendor cut-sheets, etc., and (v) construction of a detailed flowsheet that clearly illustrates flow rates, solids contents, particle size distributions, REE concentrations, etc., for all primary and recycled process streams. Task 8.0 Economic & Market Analyses Design criteria for the flowsheet will initially focus on the development of circuits which offer the best overall performance in terms of REE recovery and grade. However, as the development progresses, economic considerations will begin to play a role in the flowsheet design. In this subtask, an economic analyses will be performed to evaluate overall commercialization potential of the proposed circuit design. Items to be addressed in the study will include (i) a summary of total capital costs for the full-scale commercial installation of the proposed circuitry and any required ancillary operations, (ii) a listing of expected operation and maintenance costs including electrical power, reagents, and other consumables, and (iii) a preliminary cost-benefit analysis that specifies the expected rate of return and payback period on the capital investment. Industrially accepted cost estimation procedures and vendor/fabricator quotes will provide the basis for the economic evaluations. The effort will require several stages of circuit design, cost analysis and circuit modification prior to completing the design of optimal process flowsheet for REE recovery. Task 9.0 Summary Report After completing Tasks 1-10, a Draft Final Report will be prepared by the Principal Investigator with input and assistance provided by the various project participants. The report will include a summary of all major experimental data, engineering analyses, computations, test results, major findings, technical deficiencies and recommendations for further work. The draft report will be submitted to DOE for review and comment 30 days prior to the end of project activities. After review by DOE, the draft report will be revised and submitted to DOE for final approval. Information contained in the Final Report will be used to establish the feasibility of rare earth commercial recovery from coal with assistance of low-temperature plasma treatment process. DELIVERABLES This project is expected to provide scientific and technical data that can be used to develop advanced physical and chemical separation technologies for extracting REEs from coal-based feedstocks. Specific scientific and technical deliverables for this project include: 1. A systematic characterization of qualified coal samples (REEs+ Y+ Sc >300 ppm on a dry, whole mass basis) from coal preparation plants that process coal mined from the Illinois Basin, Central and Northern Appalachia Basin, and Powder River Basin. 2. Analyses on the physical and chemical properties modification of coal provided by the low-temperature plasma treatment including variation of surface area, pore structure, ash content and volatility, etc. 3. Investigation on the degree of liberation and oxidation of REEs associated in coal through mineralogy, mineral composition, surface energy configuration, conversions between functional group, etc. 4. Optimization on the operation conditions of the low-temperature plasma treatment with a goal of maximizing the REEs recovery, by investigating the effects of plasma RF power, temperature, gas flow rate, retention time, etc. 5. Comparison of REEs recovery from plasma treated coal, especially the recovery improvement of high-valued REEs (i.e. Scandium and critical REEs), by aqueous leaching processes including water leach, ammonium salt leach, and acid leach. 6. Final rare earth products with over 2% of REEs by weight on a dry, whole mass basis will be generated by pH controlled selective precipitation along with necessary solvent extraction processes. 7. Environmental study to identify the potential environmental issues associated with the leaching processes and identify the potential optimum reclamation schemes after leaching process on mine sites. 8. Economic analyses of the low-temperature plasma aided leaching process to investigate the feasibility of rare earth recovery from coal driven by the advanced recovery on high- valued rare earth elements. 9. Progress on maturation of the plasma treatment technique on bulk material for an advanced REEs recovery from coal and the potential of technology commercialization by investigating and rising above the challenges on system up scaling. Period and final reports will be submitted in accordance with the "Federal Assistance Reporting Checklist (FARC)" and the instructions accompanying the checklist. In addition to the reports specified in the FARC, the Recipient must provide to the DOE Project Officer with the documents listed in Table 8. A listing of specific deliverables associated with project milestones is provided in the Project Management Plan for this proposal. Interim accomplishment data/report providing updated technical progress towards meeting SOPO/FOA objects should be provided every six months. The reports must be envisioned to be one page or less and to offer a succinct update on project performance to date, emphasizing any accomplishments, which can be made publicly available on DOE's EDX website. In addition, state-specific, county-specific and site-specific information regarding the proposed sources of coal and coal by-products should be provided in the Sampling/Characterization (Task 2.0) report. Any analytical results, which include characterization of the REE concentrations in the original feedstock material and/or separated/extracted pre-concentrate or high purity fractions, are to be presented in all the deliverables. All the sample REE contents will be reported on both a dry whole sample basis and a dry ash basis. Analytical results must be provided on an individual elemental basis (i.e., dysprosium metal, Dy), as well as on an oxide basis (i.e., dysprosium oxide, Dy2O3). Similarly, compositional phase identification of the original feedstock and resulting pre-concentrate or high purity fractions must be provided. Report Submission Date Task 1.0 - Project Management Plan 1 month after award (as per Project Officer) Task 2.0 - Sampling and Characterization Plan 1 month after award Task 2.0 - Sampling and Characterization 2 months after sampling and characterization plan Task 3.0 - Low-temperature plasma treatment optimization 4 months after sampling/characterization work Task 4.0 - Leaching experiments 4 months after low-temperature plasma treatment optimization Task 5.0 - REE concentrate production 3 months after leaching experiments Task 6.0 - Environmental Studies Due at the end of the project Task 7.0 - System Design Due at the end of the project Task 8.0 - Economic & Market Analyses Due at the end of the project Task 9.0 -Summary Report Due at the end of the project BRIEFINGS/TECHNICAL PRESENTATIONS The Recipient shall prepare detailed briefings for presentation to the FPM at the Project Officer's facility located in Pittsburgh, PA or Morgantown, WV. The Recipient shall make a presentation to the NETL Project Officer/Manager at a project kick-off meeting held within 90 days of project start date. At minimum, annual briefings shall also be given by the Recipient to explain the plans, progress, and results of the technical effort. A final project briefing prior to the end of the period of performance of the project shall also be given. In addition, if the application is selected for award, additional briefings/presentations consistent with the budget, schedule, and scope of the project shall be provided per the DOE requirement.
Effective start/end date11/16/178/31/19


  • Department of Energy: $322,352.00


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