Phase 1 Study

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EXECUTIVE SUMMARY

Description and Objective of Research:  The demand for rare earth element and other strategic metals (REM) in the world is rapidly increasing due to their use in a variety of high technology applications. More than 100,000 tons of rare earth oxides (REO) are being used worldwide on an annual basis. However, we have less than 10% of the world reserve of rare earth element (REE) mineral deposits in the U.S. More than 60% of rare earth oxide (REO) was imported from outside (mostly from China) in the year of 2013. However, China’s recent institution of export quota has significantly reduced the supply of REMs to the U.S. Hence, new ways of producing REM and/or finding substitutes for those metals in the high-tech applications need to be invented/discovered in a timely manner. Coal Combustion Byproducts (CCBs), which are discarded as waste materials in most cases, are known to contain REEs (in low concentration) that could be extracted using a suitable process flowsheet. This may be a worthwhile route to pursue considering the short supply of REMs and the abundance of coal ash in thousands ash ponds in the US. On an annual basis, more than 110 million tons of Coal Combustion Byproducts (CCBs) are being produced while satisfying our nation’s need for electricity (ACAA, 2014). However, less than 50% of these CCBs are being successfully used in various applications and the remainder is being dumped as waste materials in the landfills and ash ponds. At SIUC, we completed a P3-Phase II grant looking into the new bulk application of fly ash, which may significantly increase the beneficial use of the CCBs.
The overall goal of our P3-Phase I study was to investigate various extraction methods of REMs from coal ash generated from the coal combustion process and to develop a novel, sustainable process flowsheet(s) to produce REMs from coal combustion byproducts. The specific objectives to accomplish the overall goal are:

  1. To characterize coal ash samples collected from multiple regions of the country and investigate the feasibility of extracting rare earth minerals and elements using combined physical and chemical separation techniques.
  2. To envision a process flowsheet(s) for the rare earth and other strategic minerals recovery from coal ash, which could vary based on the characteristics of REMs present in coal fly ash and bottom ash generating in different regions of the US.

Summary of Results (Outputs/Outcomes):
We have completed nearly half of the project work so far. Based on the results we have obtained we are very hopeful about the technical feasibility of the concept. If the concept is economically viable, it will increase REE reserve of the US by nearly six-fold (from 1.8 million ton to 11.8 million ton) and make US less dependent on REE imports. REEs are at the heart of many high-tech applications, such as consumer electronics, wind turbine, solar panels etc. So qualitative benefits to people, prosperity, and the planet are understandable by the commercialization of the proposed concept.

Phase I study results may be summarized as follows:
Coal Ash Characterization: The initial characterization work completed on fourteen coal samples (obtained from US DOE’s coal bank maintained at Penn State) of different coal ranks, from lignite to anthracite, originating from all over the country indicated a maximum coal ash REE content of more than 700 ppm for an anthracite coal sample. REE contents of some coal samples of the same rank but different seam and geographic locations showed a great degree of variation in their REE contents. Majority of the REEs present in coal ash were found to be of the light REE (LREE) category. A maximum of 27% of the more valuable heavy REE (HREE) was found in a low volatile bituminous coal sample. The concentration of other strategic minerals, with the exception of lithium, were extremely low, below 10 ppm in most cases.

Ash Leaching Tests: Using the ash sample having the highest REE content, a total of 32 high temperature leaching experiments were conducted. A 4x2x2 factorial design was utilized by varying the molarity of the HNO3 acid solution at four levels, solid content at two levels and leaching time at two levels.  The ICP analysis of the leachate samples indicate that the highest recovery of LREEs (about 90%) is achieved at the highest molarity of the acid solution, lowest solid content and longest retention time. However, the highest recovery of HREEs (about 94%) needed only an intermediate level of acid molarity. In fact, the HREE recovery decreased at the highest molarity of 6M.

Solvent Extraction, Scrubbing and Precipitation-Stripping Tests: Using the best leaching condition for REE grade, nearly 2 liters of leachate were prepared to serve as the feedstock for the solvent extraction test series. Using a 23 full-factorial design, a total of sixteen solvent extraction tests have been completed. In this test series, we used a 1:1 mixture of tributyl phosphate (TBP) and Cyanex 572 as the organic extractants and Kerosene as the diluent for all SX test along with a retention time of 10 minutes. The three key factors varied during this test series included Organic/Aqueous ratio, pH, and solvent to diluent ratio. The resulting test samples are still being analyzed in a commercial lab for their REE and other impurity mineral contents.  Several scrubbing tests using hot water and precipitation-stripping using oxalic acids have also been completed, but we are still waiting for the test data.

Conclusions:

  • It can be inferred that the REE contents of the higher rank coals, such as anthracite and low volatile bituminous coal are generally higher than the lower rank coals, such as lignite and high volatile-C bituminous coal. However, no specific correlations could be established.
  • More characterization study using XRD and SEM will be needed to identify the specific major minerals associated with the REEs in coal ash. The resulting information will be useful for the Physical Preconcentration study to be conducted later in this project.
  • Because of the extremely low concentration of other stragetic elements like Ge and Ga, in the coal samples of all ranks, we decided to restrict our attention to REEs only for this test program. However, the process flowsheet developed for REEs may be equally applicable for other strategic minerals recovery in future.
  • We will study one other coal sample to have a better understanding of the optimum operating conditions and their relations to coal ranks and other specific characteristics.
  • Our goal is to approach a grade of 2% REE in the final calcined powder product starting from an REE concentration of ~500 ppm in the feed coal ash.

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