APPARATUS AND METHOD FOR POWER GENERATION AND VALUABLE ELEMENT RECOVERY FROM COMBUSTION BY-PRODUCTS

Abstract

An apparatus for heat and power generation includes a combustor, operating at a temperature of 500-900 C., a leach tank downstream from the combustor, producing a valuable element solution from combustion by-products and a valuable element recovery apparatus configured to recover the valuable elements from the valuable element solution.

Claims

1. An apparatus for heat generation and enhanced recovery of valuable elements, comprising: a combustor operating at a temperature of 500-900 C.; a leach tank downstream from the combustor, said leach tank (a) receiving combustion by-product from the combustor, (b) extracting the valuable elements from the combustion by-product and (c) producing a valuable element solution; and a valuable element recovery apparatus configured to recover the valuable elements from the valuable element solution.

2. The apparatus of claim 1, wherein said valuable element recovery apparatus includes: at least one of a thickener and a filter press downstream from the leach tank and adapted for removing residual solids from the valuable element solution and thereby generating a pregnant leach solution.

3. The apparatus of claim 2, wherein the valuable element recovery apparatus further includes: a solvent extraction and precipitation apparatus downstream from the at least one of the thickener and the filter press and adapted to concentrate valuable elements in the pregnant leach solution and then selectively precipitate the concentrated valuable elements; and a roaster, downstream of the solvent extraction and precipitation apparatus, adapted to produce a valuable element oxide mix.

4. The apparatus of claim 1, wherein said combustor is a fluidized bed combustor.

5. The apparatus of claim 4, wherein heat generated by the fluidized bed combustor is delivered to a steam generator and used to turn a turbine and generate electric power.

6. The apparatus of claim 5, wherein said combustor is operated in an absence of alkaline additives used to capture sulfur.

7. The apparatus of claim 6, wherein said combustor is operated at temperatures between 500-750 C.

8. The apparatus of claim 1, further including a sulfuric acid plant adapted for producing sulfuric acid from exhaust gases of the fluidized bed combustor and delivering said sulfuric acid to said leach tank for use in extracting the valuable elements from the combustion by-product.

9. A method for power generation and valuable element recovery from coal-based materials, comprising: combusting the coal-based materials to generate heat at a temperature of 500-900 C.; using the heat to produce steam and generate electric power; leaching the valuable elements from combustion by-products of the coal-based materials into a valuable element solution; and recovering the valuable elements from the valuable element solution.

10. The method of claim 9, wherein the recovering includes: concentrating the valuable elements into a pregnant leach solution; precipitating the valuable elements from the pregnant leach solution; and roasting the valuable element to produce a pure valuable element oxide mixture.

11. The method of claim 10, including combusting the coal-based materials in an absence of alkaline additives used to capture sulfur.

12. The method of claim 11, including producing sulfuric acid from exhaust gases generated during the combusting of the coal-based material.

13. The method of claim 12, including using the sulfuric acid for the leaching of the valuable elements.

14. The method of claim 13, including: using at least one of a thickener and a filter press to remove residual solids from the valuable element solution and produce the pregnant leach solution; using solvent extraction in the concentrating of the valuable elements in the pregnant leach solution; and using staged precipitation, selective precipitation or staged and selective precipitation in the precipitating of the valuable elements.

15. The method of claim 9, including completing the combusting of the coal-based materials at a temperature of 550-850 C.

16. The method of claim 9, including completing the combusting of the coal-based materials at a temperature of 550-800 C.

17. The method of claim 9, including completing the combusting of the coal-based materials at a temperature of 600-750 C.

18. The method of claim 10, including completing the combusting of the coal-based materials at a temperature of 550-800 C.

19. The method of claim 10, including completing the combusting of the coal-based materials at a temperature of 600-750 C.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0019] The accompanying drawing figures incorporated herein and forming a part of the patent specification, illustrate several aspects of the apparatus and method and together with the description serve to explain certain principles thereof.

[0020] FIG. 1 is a schematic representation of the first possible embodiment of the apparatus.

[0021] FIG. 2 is a schematic representation of a second possible embodiment of the apparatus incorporating components for the production of sulfuric acid from sulfur in the exhaust gases discharged from the combustor.

[0022] FIG. 3 is a graphic illustration of the leaching characteristics of REEs from the fly ash and bottom ash samples as described in the Experimental section that follows.

[0023] FIG. 4 is a group of graphs illustrating the effects of calcination temperature on acid leaching recovery of REEs from the West Kentucky No. 13 (WK13), Fire Clay (FC), and Illinois No. 6 (IL6) coals as described in the Experimental section that follows.

[0024] FIG. 5 is a group of figures illustrating effects of calcination temperature on acid leaching recovery of HREEs from the West Kentucky No. 13 (WK13), Fire Clay (FC), and Illinois No. 6 (IL6) coals as described in the Experimental section that follows.

[0025] FIG. 6 includes two graphs illustrating the effects of calcination temperature on acid leaching recovery of Fe from West Kentucky No. 13 (WK13) and Illinois No. 6 (IL6) coals as described in the Experimental section that follows.

[0026] FIGS. 7A-7D illustrate the effects of calcination time at 600 C. on the ash contents of the 1.4-1.8 SG fraction of each coal source as described in the Experimental section that follows.

[0027] FIG. 7A is a graph showing percentage of ash content in a given time for different coal samples.

[0028] FIG. 7B illustrates percentage REE recovery in a given time for the WK 13 coal sample.

[0029] FIG. 7C illustrates percentage REE recovery in a given time for the FC coal sample.

[0030] FIG. 7D illustrates percentage REE recovery for a given time for the IL6 coal sample.

[0031] Reference is now made in detail to multiple possible embodiments of the apparatus and method as illustrated in the accompanying drawing figures.

DETAILED DESCRIPTION

[0032] Reference is now made to FIG. 1 which schematically illustrates a first possible embodiment of the apparatus 10 used for heat generation by means of the combustion of coal-based materials and the enhanced recovery of valuable elements, including rare earth elements (REEs), from combustion by-products including fly ash and bottom ash.

[0033] As illustrated in FIG. 1, the apparatus 10 includes a combustor 12 for combusting coal-based materials and generating heat. In at least one particularly useful embodiment of the apparatus 10, that combustor is a fluidized bed combustor (FBC) as shown in FIG. 1. FBCs are a well-developed technology that provide a number of overall benefits including fuel flexibility, lower NO.sub.x, emission and high combustion efficiency.

[0034] The combustor 12 is particularly adapted for operating at a temperature of between 500-900 C. In some embodiments of the apparatus 10, the operating temperature is 550-850 C., in others 550-800 C. and in still others 600-750 C. Operating temperatures above the indicated ranges tend to reduce valuable element/REE recovery and should be avoided in order to improve recovery efficiencies. A cyclone 14, connected to the flue gas exhaust port 16 of the combustor 12, recovers particulates from the flue gas exhausted from the combustor 12.

[0035] The apparatus 10 includes at least one leach tank 18 downstream from the combustor 12. The leach tank 18 is adapted to receive combustion by-product, including fly ash and bottom ash, from the combustor 12 and ash particulates recovered from the flue gas by the cyclone 14. Delivery of the combustion by-product to the leach tank 18 may be by conveyor or other appropriate means. The leach tank 18 is also adapted for the extraction of valuable elements from the combustion by-product into a solution using an acid reagent such as sulfuric acid, nitric acid or hydrochloric acid. More particularly, a valuable element solution is produced in the leach tank 18 in a manner known in the art. That valuable element solution is then processed in a valuable recovery apparatus 20 configured to recover valuable elements from the valuable element solution.

[0036] In the illustrated embodiment, the recovery apparatus 20 includes a thickener 22 and a filter press 24 downstream from the leach tank 14. More specifically, in the illustrated embodiment, the thickener 22 is immediately downstream from the leach tank 18 and the filter press 24 is downstream from the thickener. A filter feed tank 26 is provided between the thickener 22 and the filter press 24 to ensure that solution is fed into the filter press at the desired rate for most efficient processing. In other possible embodiments, the apparatus 20 only includes the thickener 22 or only includes the filter press 24. Those skilled in the art recognize that any number of methods may be utilized to achieve solid/liquid separation.

[0037] The thickener 22 and/or the filter press 24 function to separate residual solids from the valuable element solution and recover a valuable element enriched, pregnant leach solution. More particularly, in the illustrated embodiment shown in FIG. 1, the recovery apparatus 20 also includes a solvent extraction and precipitation apparatus 28. The pregnant leach solution is discharged from the thickener 22 to the downstream solvent extraction and precipitation apparatus 28 for further processing that will be described below. At the same time, the solid waste rich solution is discharged from the bottom of the thickener 22 to the filter feed tank 26. The solid waste rich solution is metered from the filter feed tank 26 to the filter press 24 where the solid wastes are removed from the solution. The solid wastes are discarded from the filter press 24 while the solution is recycled back to the leach tank 14. Alternatively any portion of the solution recovered in the filter press 24 may be directed by the valve 30 to waste water treatment.

[0038] In the illustrated embodiment, the solvent extraction and precipitation apparatus 28 is adapted to receive the pregnant leach solution from the thickener 22, further concentrate the valuable elements and recover those valuable elements by staged precipitation or selective precipitation. One solvent extraction and precipitation apparatus 28 useful for this purpose is disclosed and described in detail in copending U.S. patent application Ser. No. 16/534,738, filed on Aug. 7, 2019 entitled CONTINUOUS SOLVENT EXTRACTION PROCESS FOR GENERATION OF HIGH GRADE RARE EARTH OXIDES FROM LEACHATES GENERATED FROM COAL SOURCES, the full disclosure of which is incorporated herein by reference.

[0039] As illustrated in FIG. 1, such a solvent extraction and precipitation apparatus 28 may include a plurality of mixer and settler units 32a-32c connected in series and a precipitator 34. As should be appreciated, the valuable element depleted solution is recycled from the plurality of serially connected mixer and settler units 32a-32c, corresponding symbolically to loading, scrubbing and stripping respectively, back to the leach tank 14 or to waste water treatment while the concentrated valuable elements are delivered to the precipitator 34. The concentrated valuable elements are then delivered from the precipitator 34 to the roaster 36. The roaster 36 heats the concentrated valuable elements to produce a pure valuable element oxide mix from which it is possible to recover pure valuable elements with further processing in accordance with processing steps known in the art. The roaster 36 is also a part of the valuable element recovery apparatus 28.

[0040] FIG. 2 illustrates an alternative embodiment of the apparatus 10 that is identical to the embodiment of the apparatus illustrated in FIG. 1 except for the addition of the sulfuric acid plant generally designated by reference numeral 50. For purposes of economy of description, the identical structures presented in the two embodiments illustrated in FIGS. 1 and 2 are identified by the same reference numerals.

[0041] In the FIG. 2 embodiment, the coal-based materials are combusted in the combustor 12 in the absence of alkaline additives of the type used in the art to capture sulfur. As a result, sulfur compounds are present in higher concentrations in the flue gas exhausted from the combustor 12. The sulfuric acid plant 50 recovers and converts those sulfur compounds from the flue gas into sulfuric acid in a manner known in the art. That sulfuric acid is, in turn, delivered to the leach tank 14 to leach valuable elements in the extracting of those valuable elements from the combustion by-products. Thus, the combustion process in the combustor 12 that produces the heat for electric power generation also advantageously produces the sulfur precursors necessary to produce sulfuric acid to support the leaching and recovery of the valuable elements from the combustion by-product.

[0042] The apparatus 10 shown and described in FIGS. 1 and 2 are also useful in a new and improved method of power generation and valuable element recovery from coal-based materials. That may be broadly described as comprising the steps of: (a) combusting coal-based materials to generate heat at a temperature of 500-900 C., (b) using the heat to produce steam and generate electric power, (c) leaching valuable elements from combustion by-products of the coal-based materials into a valuable element solution and (d) recovering the valuable elements from the valuable element solution.

[0043] The recovering step may include the steps of concentrating the valuable elements into a pregnant leach solution, precipitating the valuable elements from the pregnant leach solution and roasting the valuable elements to produce a pure valuable element oxide mixture.

[0044] Optionally, the method may include: (a) combusting the coal-based materials in an absence of alkaline additives of the type used to capture sulfur, (b) producing sulfuric acid from the exhaust gases generated during combustion and (c) using the produced sulfuric acid for leaching the valuable elements. This is the method of the embodiment illustrated in FIG. 2.

[0045] The method may also include the steps of: (a) using at least one of a thickener and a filter press to remove residual solids from the valuable element solution and produce the pregnant leach solution, (b) using solvent extraction in the concentrating of the valuable elements in the pregnant leach solution and (c) using staged precipitation, selective precipitation or staged and selective precipitation in the precipitating of the valuable elements. The precipitation of valuable elements is described in further detail in copending U.S. patent application Ser. No. 16/185,120, filed on Nov. 9, 2018 and entitled LOW-COST SELECTIVE PRECIPITATION CIRCUIT FOR RECOVERY OF RARE EARTH ELEMENTS FROM ACID LEACHATE OF COAL WASTE, the full disclosure of which is incorporated herein by reference. In some possible embodiments of the invention, the solvent extraction step is eliminated and the pregnant leach solution is only subjected to precipitation of the valuable elements.

[0046] In one or more of the many possible embodiments of the method, the combusting of the coal-based materials is at a temperature of 550-850 C. In other possible embodiments, the combusting of the coal-based materials is at a temperature of 550-800 C. In still other embodiments, the combusting of the coal-based materials is at a temperature of 600-750 C.

[0047] Experimental

[0048] Leaching of FBC Ash Containing Lime

[0049] The results were obtained from acid leaching tests performed on FBC ash samples are shown in FIG. 3. Similar to the calcined coal samples in the next section, the leaching kinetics of the ash samples was also found to be characterized by a quick release within the first 5 min. A maximum recovery value of around 80% occurred with the FBC1 bottom ash, while relatively lower percentages (60%) were recovered from the other ash samples. Leaching results of the calcined coal samples indicated that the pretreatment temperature is critical for REE recovery and excessive calcination (750 C.) impairs acid leaching performance (see FIG. 4). The maximum REE recovery occurred with the FBC1 bed ash material, which was exposed to a lower temperature (790 C.) relative to the FBC2 ash materials (870 C.). This temperature is near the complete decomposition temperature for clay materials.

[0050] The FBC2 unit received feed from the Illinois No. 6 coal seam, and tests conducted on the 1.4 SG float and 1.4-1.8 SG fractions of a sample collected from the same seam revealed that a calcination temperature of 900 C. reduces REE recovery to values approaching 30% (see next section). The leach recovery values achieved for the FBC2 ash materials were around 60% (see FIG. 3) which indicates that the temperatures and associated conditions in the unit did not result in the transformation of the mineral forms into a state having a significantly reduced solubility. Based on these findings, an FBC unit used to produce electricity and assist in the recovery of critical elements from coal-based sources should be operated at a temperature that is no more than 750 C. in order to maximize REE recovery.

[0051] Calcination Tests to Simulate not Adding Lime to FBC Unit

[0052] To lower the acid consumption for the recovery of REEs a proposed embodiment will combust coal without the addition of lime. To determine the efficacy of this embodiment acid leaching tests were performed on both the original and calcined coal samples to compare the REE leaching characteristics. As shown in FIG. 4, REE recovery was significantly improved by calcination at high temperatures. By calcining at 600 C. for 2 h, 80% of the REEs were leached from the West Kentucky No. 13 and Fire Clay coal sources, which is slightly higher than the values obtained from the Illinois No. 6 coal source (i.e., 75%). However, without any calcination, the REE recovery was relatively low; for example, only 25% of the REEs were leached from the original West Kentucky No. 13 1.4 SG float sample. As shown in FIG. 5, HREE recovery from the 1.4 SG float samples was also increased significantly by calcining at 600 C. for 2 h, which is similar to the total REEs. However, for the 1.4-1.8 SG samples, the increase in HREE recovery is 30-40% and lower than the total REEs (see FIGS. 5 and 4, respectively).

[0053] REEs in low-ash coals can be classified into organic (in forms of organic compounds and/or ionic species in the carbon matrix) and inorganic (ash-forming mineral matter dispersed within the organic matrix) associations. There are two factors that may contribute to the enhanced recovery after calcination: (1) the organically and inorganically associated REEs were liberated after calcination, which might be more leachable (e.g., REEs on the surfaces and entrapped within the inner layer of microdispersed clays), and (2) high-temperature calcination transformed the REEs into ore leachable forms such as rare earth oxides. After calcination at 600 C. or higher for 2 h, the organic matter was completely removed from all the coals as indicated by proximate analysis data. The findings of a previous study reported by Mardon and Hower showed that REEs are relatively non-volatile when coals are burned in boilers (>1000 C.). As such, all the organically associated REEs stayed in the coal ashes, most likely in the form of an oxide. However, in high-rank coals, the organically associated REEs represent a small fraction of the total REEs, that is, less than 10% for LREEs, and thus the enhanced recovery was more likely due to the mineralogy changes caused by calcination. It has been reported that a portion of the REEs is associated with clays in coal. Calcination of clays, especially kaolinite, causes dehydration, which significantly increases their surface area. This effect may provide another explanation for the enhanced recovery and agrees with the XRD characterization findings.

[0054] As shown in FIG. 4, REE recovery was significantly decreased when the coals were calcined at 900 C. relative to 750 and 600 C. This finding may be due to the pozzolanic nature of clays after calcination using temperatures around 900 C., which reduces the surface area and encapsulates the associated REEs. In this case, strong acids would be needed to dissolve the clays to recover the encapsulated REEs. Much higher temperatures (>1200 C.) are utilized in pulverized coal fired combustion (PCC) boilers. As such, the calcination test results on the coal samples at 900 C. highlighted the difficulty of REE recovery from the conventional PCC fly ash and bottom ash sources. FBC units utilize coal significantly at lower temperatures, typically 750-900 C., with a few units operating at or slightly below 700 C. Under these conditions, fusion of associated coal minerals in the FBC unit is low, which benefits the REE recovery from the corresponding fly ash and bed ash materials.

[0055] The acid leaching kinetics of REEs from the calcined samples was characterized by a quick release within the first 5 min of the leaching process (see FIGS. 4 and 5). For example, 70% of the REEs were leached from the West Kentucky No. 13 samples in the first 5 min. Solvent extraction is normally used to concentrate REEs from the leachate. As reported by Honaker et al., Fe is the most difficult contaminant in the leachate due to the relatively high complexing ability of Fe3+ with the extractant, di-(2-ethylhexyl) phosphoric acid (D2EPHA). As shown in FIG. 6, although the overall Fe recovery from the calcined samples increased after calcination, the recovery was relatively low at the beginning of the leaching process. Within the first 15 min, Fe recovery from the calcined samples was lower than that from the non-calcined samples. The differential leaching kinetics between the REEs and Fe benefits the technical and economic feasibility of producing a high-grade concentrate using solvent extraction. An additional benefit of the calcination process is that a portion of the Fe is converted to hematite, which has lower solubility.

[0056] The effects of pretreatment time on REE leaching recovery from the coal sources are critical since the residence time in an FBC unit is limited, varying from several seconds to several minutes. As shown in FIG. 7A, the majority of the organic matter was combusted after 10 min of calcination at 600 C. (ash contents of >75%). When the calcination was increased to 30 min, combustion of the organic matter was nearly complete. As such, the effects of organic matter, such as physical encapsulation of rare earth minerals, on REE leaching are negligible. When the calcined samples were leached at 75 C. using 1.2 M HCl, nearly equal leaching recovery values and kinetics were observed between the coal samples calcined at 10, 60, and 120 min (see FIG. 7B-D). This indicates that a minimum of 10 min of calcination or exposure to similar conditions in an FBC unit is needed to realize the desired leaching characteristics.

[0057] The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings.

[0058] For example, the illustrated embodiment includes a single leach tank 18 downstream from the combustor 12. Multiple leach tanks may be provided, arranged in series and/or parallel. Another example, in the illustrated embodiment, the pregnant leach solution (PLS) is subjected to solvent extraction to recover the valuable elements such as REEs. It should be appreciated that processes other than solvent extraction may be used for this purpose. Such alternative processes include, but are not necessarily limited to, the concentration of the valuable elements/REEs by selective precipitation using chemical additives and/or pH adjustments, selective redissolution of the precipitate followed by selective precipitation using oxalic acid and pH control.

[0059] All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.