Recovery of rare earth elements and compounds from coal ash
Rare earth elements are recovered from coal ash. The coal ash with rare earth elements can be treated with a mineral acid to form an aqueous mineral acid solution. The aqueous mineral acid solution can be extracted to form an organic solution that includes the rare earth salts. The organic solution can be mixed with water to form an aqueous solution that includes the rare earth salts. The rare earth elements are separated from the aqueous solution.
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of and priority to U.S. provisional patent application no. 61/625,292, filed Apr. 17, 2012, the entire contents of which are incorporated by reference herein and owned by the assignee of the instant application.
FIELD OF THE INVENTION
The invention relates generally to recovery of rare earth elements and/or compounds from coal ash.
Rare earth elements can be fundamental to emerging green energy technologies in the United States (e.g., permanent magnet motors for wind turbines and disk drives, hybrid car batteries, compact fluorescent lighting, and/or displays in all types of consumer/defense electronics), as well as other usages such as industrial catalysts for refining heavier crude oil, automobile catalytic converters, and/or as alloying elements. Presently, rare earth elements can be obtained through mining.
Coals from certain regions of the world can be particularly rich in rare earth elements, approaching a total concentration of about 1000 parts-per-million (“ppm”). The combustion of coal in power plants for energy generation concentrates non-volatile minerals in the ash by about ten times, to about 10,000 ppm, or on the order of approximately 1%. Coal ash can be the product of burning coal. Coal ash can be comprised of fly ash and bottom ash. Fly ash can be ash that rises with flue gases. Bottom ash can be ash that is found at the bottom of a furnace. Fly ash can be collected before the flue gases reach chimneys of power plants.
A method to extract rare earth elements from coal is desired. The United States alone produces on the order of 100 million metric tons of fly ash annually. Accounting for process yield and variability in rare earth element content, if rare earth elements are extracted from coal ash, a reasonable fraction of currently available fly ash (e.g., about 10-15%) can be adequate to meet rare earth elements demand in the United States.
SUMMARY OF THE INVENTION
Advantages of the invention include recovering rare earth elements from coal ash. Another advantage of the invention includes economical production of rare earth elements and/or compounds from alternative, non-mineral raw materials. Another advantage of the invention is the ability to build reliable production capabilities and/or supply chain for rare earth elements and/or byproducts. Another advantage of the invention is processing fly ash to recover rare earth materials, particularly heavier rare earths, more economically and energy-efficiently per kilogram of rare earth elements than from processing and extracting mineral resources. Another advantage includes the beneficiation of coal ash, which is an abundant waste material, for recovering economically useful and marketable industrial materials that include rare earth elements as a significant component. Another advantage includes energy efficient extraction of rare earth elements, which can save energy use by about 75% relative to conventional mining per unit weight of rare earth elements produced. Yet another advantage is the accompanying carbon dioxide (CO2) emission can be lower than mining by about 75%. Still another advantage includes production of environmentally beneficiated ash cake, which can be free of hazardous elements.
In one aspect, the invention involves a method of recovering rare earth elements from coal ash. The method involves treating coal ash that contains rare earth elements with a mineral acid to form an aqueous mineral acid solution. The aqueous mineral acid solution is extracted to form an organic solution that includes the rare earth salts. The organic solution is mixed with water to form an aqueous solution that includes the rare earth salts. The rare earth elements are recovered by separating the rare earth elements from the aqueous solution.
The aspect described above can include one or more of the following features. In some embodiments, the mineral acid is nitric acid. In various embodiments, treating coal ash can include heating the mineral acid to approximately 90° C. and exposing the coal ash to the mineral acid for at least one hour. Exposing the coal ash can also include additional heating of a resulting solution formed when exposing the coal ash to the mineral acid in order to generate a more concentrated mixture.
In various embodiments, extracting the aqueous mineral acid solution includes mixing aqueous mineral acid solution with tributyl phosphate and kerosene and removing the organic solution from the aqueous mineral acid solution such that the rare earth salts are substantially removed along with the organic solution
In some embodiments, extracting the aqueous mineral acid solution can be done by performing a dry extraction, a liquid extraction, or combination of dry and liquid extraction. In various embodiments, the dry extraction is done by performing continuous extraction of rare earth salts with tributyl phosphate. The dry extraction can be a Soxhlet extraction. In various embodiments, the dry extraction can include diluting the organic solution with kerosene.
The liquid extraction can include mixing a volume of tributyl phosphate and kerosene equal to a volume of the aqueous mineral acid solution and removing the organic solution from the aqueous mineral acid solution. In some embodiments, the liquid extraction can include mixing a volume of tributyl phosphate and kerosene that is a predetermined amount greater than a volume of the aqueous mineral acid solution and removing the organic solution from the aqueous mineral acid solution. The predetermined amount can be 9 times the volume of the aqueous mineral acid solution.
In some embodiments, mixing the organic solution includes performing multiple cycles of mixing the organic solution with water until a concentration level of rare earth salts in the aqueous solution is below a predetermined threshold.
In various embodiments, separating the rare earth elements can include ion exchange. Ion exchange can either be anion exchange or cation exchange. In some embodiments ion exchange separation results in rare earth element mixtures. The rare earth element mixtures can be suitable to be converted to mixtures of rare earth oxides and rare metals for various applications such as catalyst, magnets, and phosphor applications.
In some embodiments, the coal ash can be exposed to a magnetic treatment such that the coal ash separates into a substantially magnetic portion and a substantially non-magnetic portion of the coal ash.
In various embodiments, extracting the aqueous mineral acid solution forms a second aqueous mineral acid solution. The second aqueous mineral acid solution can be distilled to recover mineral acid.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating the principles of the invention by way of example only.