Evaluation Of Rare Earth Element Extraction From North Dakota Coal-Related Feed Stocks

The rare earth elements consist of the lanthanide series of elements with atomic numbers from 57-71 and also include yttrium and scandium. Due to their unique properties, rare earth elements are crucial materials in an incredible array of consumer goods, energy system components and military defense applications. However, the global production and entire value chain for rare earth elements is dominated by China, with the U.S. currently 100% import reliant for these critical materials. Traditional mineral ores including previously mined deposits in the U.S., however, have several challenges. Chief among these is that the content of the most critical and valuable of the rare earths are deficient, making mining uneconomical. Further, the supply of these most critical rare earths is nearly 100% produced in China from a single resource that is only projected to last another 10 to 20 years. The U.S. currently considers the rare earths market an issue of national security. It is imperative that alternative domestic sources of rare earths be identified and methods developed to produce them. Recently, coal and coal byproducts have been identified as one of these promising alternative resources. This dissertation details a study on evaluation of the technical and economic feasibility of rare earth element recovery from North Dakota lignite coal and lignite-related feedstocks. There were four major goals of this study: i) identify lignite or lignite-related feedstocks with total rare earth element content above 300 parts per million, a threshold dictated by the agency who funded this research as the minimum for economic viability, ii) determine the geochemistry of the feedstocks and understand the forms and modes of occurrence of the rare earth elements, information necessary to inform the development of extraction and concentration methods, iii) identify processing methods to concentrate the rare earth elements from the feedstocks to a target of two weight percent, a value that would be sufficient to leverage existing separation and refining methods developed for the traditional mineral ore industry, and iv) develop a process that is economically viable and environmentally benign. To achieve these overall goals, and to prove or disprove the research hypotheses, the research scope was broken down into three main efforts: i) sampling and characterization of potential feedstocks, ii) laboratory-scale development and testing of rare earth element extraction and concentration methods, and iii) process design and technical and economic feasibility evaluation. In total, 174 unique samples were collected, and several locations were identified that exceeded the 300 ppm total rare earth elements target. The results showed that on a whole sample basis, the rare earths are most concentrated in the clay-rich sediments associated with the coal seams, but on an ash basis in certain locations within certain coal seams the content is significantly higher, an unexpected finding given prior research. At Falkirk Mine near Underwood, North Dakota three coal seams were found to have elevated levels of rare earths, ranging from about 300 to 600 ppm on an ash basis. Additionally, exceptionally high rare earths content was found in samples collected from an outcropping of the Harmon-Hansen coal zone in southwestern North Dakota that contained 2300 ppm on an ash basis. The results dictated that extraction and concentration methods be developed for these rare earth element-rich coals, instead of the mineral-rich sediments. This effort also found that that at a commercial-scale, due to non-uniformity of the rare earths content stratigraphically in the coal seams, selective mining practices will be needed to target specific locations within the seams. The bulk mining and blending practices as Falkirk Mine result in a relatively low total rare earths content in the feed coal entering the Coal Creek Power Station adjacent to the mine. Characterization of the coal samples identified that the predominant modes of rare earths occurrence in the lignite coals are associations with the organic matter, primarily as coordination complexes and a lesser amount as ion-exchangeable cations on oxygen functional groups. Overall it appears that about 80-95% of rare earths content in North Dakota lignite is organically associated, and not present in mineral forms, which due to the weak organic bonding, presented a unique opportunity for extraction. The process developed for extraction of rare earths was applied to the raw lignite coals instead of fly ash or other byproducts being investigated extensively in the literature. Rather, the process uses a dilute acid leaching process to strip the organically associated rare earths from the lignite with very high efficiency of about 70-90% at equilibrium contact times. Although the extraction kinetics are quite fast given commercial leaching operations, there is some tradeoff between extraction efficiency and contact time. However, at shorter contact time there is improved rare earths selectivity that results in a more concentrated product due to limiting extraction of unwanted impurities. There is also a significant difference in the extraction kinetics for the more valuable heavier molecular weight rare earths, which are much faster than the light rare earths. The testing showed that in a one-step process consisting of leaching for two hours with 0.5M sulfuric acid at 40°C, a rare earth concentrate of about 1.4 weight percent rare earths could be achieved with about 70% total rare earths extraction, while also producing a residual coal byproduct that has superior qualities to the feed coal, such as reduced ash content. This represents a concentration factor of 24 over the feed coal. The target of two weight percent rare earths could be achieved by a number of secondary processing methods, such as pH modification or forced air oxidation to selectively precipitate impurities from the rare earths-containing solution. The process developed in this study is simple, highly effective, low cost and novel, with several differentiating benefits compared to methods being developed in the literature. These are made possible by the unique properties of North Dakota lignite coals and the weakly-bonded organic association of the rare earth elements. Key differentiators include the use of the raw coal as the feedstock, the ability to use a mild leaching process, and not needing extensive physical beneficiation processes prior to rare earths extraction. The process is environmentally benign and was demonstrated to be economically viable at the current market conditions. Due to the use of the raw coal as the feedstock, the process can be advantageously integrated with any number of coal utilization processes to augment economics, lower costs and maximize efficiency and synergies. This study evaluated a configuration of rare earths extraction combined with activated carbon production co-located at a combined heat and power facility, and was shown to have highly attractive economics even at small scales representing a first-of-a-kind demonstration system

Leaching Behavior of Rare Earth Elements in Fort Union Lignite Coals of North America

Fort Union lignite coal samples were subjected to a series of aqueous leaching experiments to understand the extraction behavior of the rare earth elements (REE). This testing was aimed at understanding the modes of occurrence of the REE in the lignite coals, as well as to provide foundational data for development of rare earth extraction processes. In a first series of tests, a sequential leaching process was used to investigate modes of occurrence of the REE of select lignite coals. The tests involved sequential exposure to solvents consisting of water, ammonium acetate and dilute hydrochloric acid (HCl). The results indicated that water and ammonium acetate extracted very little of the REE, indicating the REE are not present as water soluble or ion-exchangeable forms. However, the data shows that a large percentage of the REE were extracted with the hydrochloric acid (80-95wt%), suggesting presence in HCl-soluble mineral forms such as carbonates, and/or presence as organic complexes. A second series of tests was performed involving single-step leaching with dilute acids and various operating parameters, including acid type, acid concentration, acid/coal contact time and coal particle size. For select samples, additional tests were performed to understand the results of leaching, including float-sink density separations and humic acid extraction. The results have shown that the majority of REE in Fort Union lignites appear to be associated weakly with the organic matrix of the coals, most likely as coordination complexes of carboxylic acid groups. The light REE and heavy REE exhibit different behaviors, however. The extractable light REE appear to have association both in acid-soluble mineral forms and as organic complexes, whereas the extractable heavy REE appear to be almost solely associated with the organics. Scandium behavior was notably different than yttrium and the lanthanides, and the data suggests the extractable content is primarily associated as acid-soluble mineral forms

Advanced High-Temperature, High-Pressure Transport Reactor Gasification

The U.S. Department of Energy (DOE) National Energy Technology Laboratory Office of Coal and Environmental Systems has as its mission to develop advanced gasification-based technologies for affordable, efficient, zero-emission power generation. These advanced power systems, which are expected to produce near-zero pollutants, are an integral part of DOE's Vision 21 Program. DOE has also been developing advanced gasification systems that lower the capital and operating costs of producing syngas for chemical production. A transport reactor has shown potential to be a low-cost syngas producer compared to other gasification systems since its high-throughput-per-unit cross-sectional area reduces capital costs. This work directly supports the Power Systems Development Facility utilizing the KBR transport reactor located at the Southern Company Services Wilsonville, Alabama, site. Over 2800 hours of operation on 11 different coals ranging from bituminous to lignite along with a petroleum coke has been completed to date in the pilot-scale transport reactor development unit (TRDU) at the Energy & Environmental Research Center (EERC). The EERC has established an extensive database on the operation of these various fuels in both air-blown and oxygen-blown modes utilizing a pilot-scale transport reactor gasifier. This database has been useful in determining the effectiveness of design changes on an advanced transport reactor gasifier and for determining the performance of various feedstocks in a transport reactor. The effects of different fuel types on both gasifier performance and the operation of the hot-gas filter system have been determined. It has been demonstrated that corrected fuel gas heating values ranging from 90 to 130 Btu/scf have been achieved in air-blown mode, while heating values up to 230 Btu/scf on a dry basis have been achieved in oxygen-blown mode. Carbon conversions up to 95% have also been obtained and are highly dependent on the oxygen-coal ratio. Higher-reactivity (low-rank) coals appear to perform better in a transport reactor than the less reactive bituminous coals. Factors that affect TRDU product gas quality appear to be coal type, temperature, and air/coal ratios. Testing with a higher-ash, high-moisture, low-rank coal from the Red Hills Mine of the Mississippi Lignite Mining Company has recently been completed. Testing with the lignite coal generated a fuel gas with acceptable heating value and a high carbon conversion, although some drying of the high-moisture lignite was required before coal-feeding problems were resolved. No ash deposition or bed material agglomeration issues were encountered with this fuel. In order to better understand the coal devolatilization and cracking chemistry occurring in the riser of the transport reactor, gas and solid sampling directly from the riser and the filter outlet has been accomplished. This was done using a baseline Powder River Basin subbituminous coal from the Peabody Energy North Antelope Rochelle Mine near Gillette, Wyoming

Subtask 7.4 - Power River Basin Subbituminous Coal-Biomass Cogasification Testing in a Transport Reactor

The U.S. Department of Energy (DOE) National Energy Technology Laboratory Office of Coal and Environmental Systems has as its mission to develop advanced gasification-based technologies for affordable, efficient, zero-emission power generation. These advanced power systems, which are expected to produce near-zero pollutants, are an integral part of DOE's Vision 21 Program. DOE has also been developing advanced gasification systems that lower the capital and operating costs of producing syngas for chemical production. A transport reactor has shown potential to be a low-cost syngas producer compared to other gasification systems since its high-throughput-per-unit cross-sectional area reduces capital costs. This work directly supports the Power Systems Development Facility utilizing the Kellogg Brown and Root transport reactor located at the Southern Company Services Wilsonville, Alabama, site. Over 3600 hours of operation on 17 different coals ranging from bituminous to lignite along with a petroleum coke has been completed to date in the pilot-scale transport reactor development unit (TRDU) at the Energy & Environmental Research Center (EERC). The EERC has established an extensive database on the operation of these various fuels in both air- and oxygen-blown modes utilizing a pilot-scale transport reactor gasifier. This database has been useful in determining the effectiveness of design changes on an advanced transport reactor gasifier and for determining the performance of various feedstocks in a transport reactor. The effects of different fuel types on both gasifier performance and the operation of the hot-gas filter system have been determined. It has been demonstrated that corrected fuel gas heating values ranging from 90 to 130 Btu/scf have been achieved in air-blown mode, while heating values up to 230 Btu/scf on a dry basis have been achieved in oxygen-blown mode. Carbon conversions up to 90% have also been obtained and are highly dependent on the oxygen-coal ratio. Higher-reactivity (low-rank) coals appear to perform better in a transport reactor than the less reactive bituminous coals. Factors that affect TRDU product gas quality appear to be coal type, temperature, and oxygen/fuel ratios. During this series of tests, a previously tested baseline Powder River Basin (PRB) subbituminous coal from the Peabody Energy North Antelope Rochelle Mine near Gillette, Wyoming was mixed with 20 wt% biomass. Two types of biomass were used - wood waste and switchgrass. Gas and particulate sampling at several locations in the riser provided information on coal devolatilization and cracking chemistry as a function of residence time, transport gas, and mode of operation. The goal of these tests was to compare the operating data and sample chemistry of the coal-biomass mixture to the PRB coal, with a focus on Fischer-Tropsch coal-to-liquid production in oxygen-blown mode. Data are to be provided to DOE to determine kinetic rates of devolatilization and tar cracking

JV Task - 129 Advanced Conversion Test - Bulgarian Lignite

The objectives of this Energy & Environmental Research Center (EERC) project were to evaluate Bulgarian lignite performance under both fluid-bed combustion and gasification conditions and provide a recommendation as to which technology would be the most technically feasible for the particular feedstock and also identify any potential operating issues (such as bed agglomeration, etc.) that may limit the applicability of a potential coal conversion technology. Gasification tests were run at the EERC in the 100-400-kg/hr transport reactor development unit (TRDU) on a 50-tonne sample of lignite supplied by the Bulgarian Lignite Power Project. The quality of the test sample was inferior to any coal previously tested in this unit, containing 50% ash at 26.7% moisture and having a higher heating value of 5043 kJ/kg after partial drying in preparation for testing. The tentative conclusion reached on the basis of tests in the TRDU is that oxygen-blown gasification of this high-ash Bulgarian lignite sample using the Kellogg, Brown, and Root (KBR) transport gasifier technology would not provide a syngas suitable for directly firing a gas turbine. After correcting for test conditions specific to the pilot-scale TRDU, including an unavoidably high heat loss and nitrogen dilution by transport air, the best-case heating value for oxygen-blown operation was estimated to be 3316 kJ/m{sup 3} for a commercial KRB transport gasifier. This heating value is about 80% of the minimum required for firing a gas turbine. Removing 50% of the carbon dioxide from the syngas would increase the heating value to 4583 kJ/m{sup 3}, i.e., to about 110% of the minimum requirement, and 95% removal would provide a heating value of 7080 kJ/m{sup 3}. Supplemental firing of natural gas would also allow the integrated gasification combined cycle (IGCC) technology to be utilized without having to remove CO{sub 2}. If removal of all nitrogen from the input gas streams such as the coal transport air were achieved, a heating value very close to that needed to fire a gas turbine would be achieved; however, some operational issues associated with utilizing recycled syngas or carbon dioxide as the transport gas would also have to be resolved. Use of a coal with a quality similar to the core samples provided earlier in the test program would also improve the gasifier performance. Low cold-gas efficiencies on the order of 20% calculated for oxygen-blown tests resulted in part from specific difficulties experienced in trying to operate the pilot-scale TRDU on this very high-ash lignite. These low levels of efficiency are not believed to be representative of what could be achieved in a commercial KRB transport gasifier. Combustion tests were also performed in the EERC's circulating fluidized-bed combustor (CFBC) to evaluate this alternative technology for use of this fuel. It was demonstrated that this fuel does have sufficient heating value to sustain combustion, even without coal drying; however, it will be challenging to economically extract sufficient energy for the generation of steam for electrical generation. The boiler efficiency for the dried coal was 73.5% at 85% sulfur capture (21.4% moisture) compared to 55.3% at 85% sulfur capture (40% moisture). Improved boiler efficiencies for this coal will be possible operating a system more specifically designed to maximize heat extraction from the ash streams for this high-ash fuel. Drying of the coal to approximately 25% moisture probably would be recommended for either power system. Fuel moisture also has a large impact on fuel feedability. Pressurized gasifiers generally like drier fuels than systems operating at ambient pressures. The commercially recommended feedstock moisture for a pressurized transport reactor gasifier is 25% moisture. Maximum moisture content for a CFB system could be approximately 40% moisture as has been demonstrated on the Alstom CFB operating on Mississippi lignite. A preliminary economic evaluation for CO{sub 2} was performed on the alternatives of (1) precombustion separation of CO{sub 2} in an IGCC using the KBR transport gasifier and (2) postcombustion CO{sub 2} capture using a CFBC. It appears that the capture of CO{sub 2} from the high-pressure IGCC precombustion system would be less costly than from the low-pressure postcombustion CFBC system by a factor of 1.5, although the cost difference is not directly comparable because of the model input being limited to a higher coal quality than the Bulgarian lignite. While the decision to pursue precombustion removal of carbon dioxide has been technically proven with the Rectisol{reg_sign} and Selexol{trademark} processes, General Electric and Siemens have not sold any gas turbine systems running on the high-hydrogen syngas. They have successfully demonstrated a gas turbine on syngases containing up to 95% hydrogen. The technological hurdles should not be too difficult given this experience in the gas turbine industry

Sampling and Analysis at the Vortec Vitrification Facility in Paducah, Kentucky. Semiannual report, November 1, 1996--March 31, 1997

The Vortec Cyclone Melting System (CMS) facility; to be located at the U.S. Department of Energy (DOE) Paducah Gaseous Diffusion Plant, is designed to treat soil contaminated with low levels of heavy metals and radioactive elements, as well as organic waste. The primary components of Vortec`s CMS are a counter rotating vortex (CRV) reactor and cyclone melter. In the CMS process, granular glass forming ingredients and other feedstocks are introduced into the CRV reactor where the intense CRV mixing allows the mixture to achieve a stable reaction and rapid heating of the feedstock materials. Organic contaminants in the feedstock are effectively oxidized, and the inert inorganic solids are melted. The University of North Dakota Energy {ampersand} Environmental Research Center (EERC) has been contacted to help in the development of sampling plans and to conduct the sampling at the facility. This document is written in a format that assumes that the EERC will perform the sampling activities and be in charge of sample chain of custody, but that another laboratory will perform required sample analyses