Patent Application
20240209475
The invention concerns recovery of rare earth elements and creating a new source of rare earth minerals. In particular, the invention involves recovery of REEs from wastewater, particularly acid mine water, or leachate from coal ash piles or coal refuse or gob piles, or water from pools in deep mines or from coal ash ponds or from other mining operations, using Sulfur Modified Iron (SMI), The invention also encompasses removal of contaminants from water using SMI, wherein REEs are removed as a byproduct.
Sulfur Modified Iron has been used for some years to remove metal contaminants from water. Sometimes the contaminated water is in groundwater, sometimes in drainage ponds, sometimes in leachate, sometimes in industrial process water. The contaminants have included selenium, nitrates, chromium, cadmium and arsenic and compounds of these substances. The contaminant metals are adsorbed and/or absorbed (chemisorbed) by the SMI and fixed to the SMI. SMI is effective in removing small (low level) concentrations of these metal contaminants from water, often to a level below detectable limits. Normally the SMI with contaminants fixed to the particles is disposed of safely without risk that the contaminants will be released back into the environment.
Rare earth metals if present in the water are also adsorbed/absorbed (chemisorbed) to the SMI and fix (bind) to the SMI particles. Rare earth metals found in low concentrations in water include cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu); gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium (Y). The water may have picked up the rare earth metals by leaching through rock at a mining site or at other locations, such as coal ash piles, coal ash ponds, gob piles or refuse piles from deep mining.
The REEs are identified as rare because it is unusual to find them in large concentrations; they are not actually rare. Conventional rare-earth recovery methods require an expensive, difficult and messy extraction process that generates large volumes of contaminated waste. The conventional mining and extraction processes require mining ore from mineral deposits in rock, which is crushed into a powder, dissolved in powerful chemical solutions and filtered. The process is repeated multiple times to retrieve rare earth oxides. Additional processing and refining separates the oxides from their tight bonds and further groups them into light rare earths and heavy rare earths.
In usable form, these elements are necessary components of many modern technologies. Many of them are used in cellular phones, computers, televisions, magnets, batteries, catalytic converters, defense applications and many more segments of modern society.
As an example, acid mine drainage from abandoned mines, or active mines, is one of the biggest industrial pollution sources in Appalachian streams. SMI has been used effectively to remove contaminants from these sources. While reducing unwanted contaminants, the SMI also adsorbs/absorbs and fixes to the SMI rare earth metals that may be present in this water. The SMI concentrates these rare earth metals while cleaning millions of gallons of mine drainage, so that the rare earth metals can be cost-effectively extracted. Coal and the water coming from coal mines can contain all 17 of the rare earth elements, with a substantial amount of the heavy rare earths that are particularly valuable.
Pursuant to the invention, water expected to contain rare earth metals is collected at a site, illustrated herein with a coal mining site, either a surface mine or a deep mine. Such sites will also typically contain selenium, in various oxidated compounds, as discussed in the applicant's U.S. Pat. Nos. 9,272,934, 9,427,706, 9,878,922 and 10,059,613.
In one aspect of the process of the invention, the REE-containing runoff water at a surface coal mining site is caught in detention ponds. In the above-cited patents remediation of the pond water to remove selenate down to permissible levels for discharge to lakes and streams was performed by treatment with Sulfur-Modified Iron (SMI) in an upflow reactor vessel. The SMI will also contain REEs, which will be a by-product of essentially the same process. In some instances the REEs can be the primary product of the SMI process.
After a prefiltering step to remove suspended and colloidal solids, the runoff or pond water is pH-adjusted as needed and pumped through the SMI reactor in upward flow, for a specified detention time in the SMI. The treated water can then be run through an oxidation tank and filtered to remove dissolved iron that is a residual from the SMI process. The process preferably is run in continuous flow.
After a period in which the SMI becomes essentially saturated and less effective and no longer removes appreciable amounts of the contaminant and/or REEs, the spent SMI is removed and processed to separate out rare earth metals, which can be done using existing extracting technologies.
As in the previous patents, the system preferably includes provision for “fluffing” the SMI medium periodically, to expand the bed of SMI with a rapid flow of water through the medium. Not a backflush, this fluffing opens up the spaces between SMI particles, reversing compacting that has occurred, and refreshes the active surfaces to extend the life of the bed. This can be done once or more per day, for about ten to twenty minutes, at, for example, about ten times normal (service) flow rate. Fluff flow is in the same upflow direction as normal service flow. This fluffing cycle is useful in any SMI reactor column removing metals or other substances from water, not limited to coal mine runoff. As an alternative the SMI medium can be as a continuous moving medium, as discussed further below.
The process of the invention encompasses water from deep coal mines as well as surface mines. Acid mine drainage water can be extracted from deep mine pools far beneath the surface, where reclamation has covered all traces of the mine at the surface. In addition, the process can be applied to gob piles or refuse piles from deep coal mines, some of which include millions of cubic yards of rocks, clay and other refuse separated from coal brought out of the mines, and these refuse piles contain REEs.
It is an object of the invention to provide a process and system which effectively and efficiently extract REEs from water, particularly acidic water at, for example, a coal mining site. This can be as a byproduct of a process to remediate contaminants from the water or from refuse or ash piles, or as a primary product. Other objects and advantages of the invention will be apparent from the following description of preferred embodiments, considered along with the accompanying drawings.
Permits for surface mine operators in the U.S. require that the rainfall runoff drainage from the surface mine site be controlled so that no runoff sediment is carried down into creeks, streams or lakes. The rainwater runoff flowing into these ponds from coal mines has aggressively pulled dissolved minerals out of coal and rocks, minerals such as sulfates (e.g. sodium sulfate and magnesium sulfate) and selenium, typically in the selenate form, these minerals being highly soluble. The runoff ponds tend to concentrate the dissolved minerals even further due to evaporation from the ponds. The selenate is difficult to remove from an aqueous solution. Incidentally, the runoff water typically also contains rare earth elements (REEs), i.e. rare earth metals, which are not deleterious to health or environment and occur usually in parts per billion in the water. This is particularly true of acid mine drainage water, and also gob piles and refuse piles from deep coal mining and leachate water from those piles.
As noted above and in the above-referenced patents of the applicant, one particularly deleterious mineral typically leached from rocks in certain geographical areas is selenium, usually in the form of selenate. Examples are sodium selenate, calcium selenate and magnesium selenate. The selenate ion is SeO42−.
Although selenium is a mineral needed in the diet of humans in very small quantities, it has toxic effects to aquatic life at higher concentrations. Certain standards for discharge of runoff water from disturbed or reclaimed land place a limit of five parts per billion (ppb) selenium content for water that will be allowed to flow into streams and lakes. This is imposed whenever human activity is responsible for concentrating the level of selenium. In contrast, runoff water which concentrates in runoff-catching ponds at a surface mining coal site can have twenty to thirty parts per billion selenium and occasionally more.
Similarly, REEs present in the leaching water or other mine waters are at relatively high concentrations. See the article “Overview of Rare Earth Element Investigations in Acid Waters of U.S. Geological Survey Abandoned Mine Lands Watersheds” (Philip L. Verplanck, D. Kirk Nordstrom and Howard E. Taylor, U.S. Geological Survey, Boulder, Colorado, January 1999).
The acidity of the filtered water is checked (and preferably is constantly monitored) at a tank 20, and if needed, pH is adjusted in this tank, which can be on a continuous-flow basis. An automatic system that monitors pH of the pond water and automatically adds acid for pH correction may be included in the tank or chamber 20. The preferred range of pH for treatment in the invented process is below pH 7, and preferably in the range of about 4.0 to about 6.5, more preferably about 4.0 to about 5.5, particularly for REE removal. Preferably pH is not below about 4.0. At a pH greater than about 7 or 8, calcium can be precipitated during the treatment, which has a negative effect on treatment. If the water is too acidic the iron in the SMI could be attacked. In one preferred embodiment the acidity of the water exiting the adjustment tank or zone 20 is about pH 6 for optimum selenium removal, although preferably lower for REE removal. This may require addition of acid (sulfuric acid in a preferred embodiment) in the tank or zone 20, since the runoff water at a location such as the Appalachian Mountains is typically in the range of about pH 7 to 7.5, although acid mine water is often much more acidic. Since the chemical reactions taking place in the SMI reactor tend to increase the pH to above 8.0, the acid addition is required to avoid impairing the life of the SMI. If the water is too acidic a base can be added, such as CaCO3. For REE recovery, as noted below, the preferred pH range may be about 4 to 5.5, or 4 to 6.5.
The prefiltered runoff water, at a pH in the desired range, proceeds to a reactor column 22, which may involve another pump (after flow through a valve 25, the purpose of which will be explained below). The column 22 comprises a vertical reactor vessel through which the water is pumped to flow preferably from bottom to top as indicated; upflow assures proper and even contact with the reactant and helps keep the mass of reactant particles “open” rather than compacted. The vessel is filled with sulfur-modified iron or SM1 as noted above, the SMI being chemically reactive to remove selenate and some other materials from water by adsorption, as discussed in the patents cited above, incorporated herein by reference. The depth of SMI in the reactor vessel should be limited, preferably no more than about six feet high (and preferably lower), to avoid compaction of the SMI particles. Openness is needed for best contact and treatment. In a reactor column having a diameter of about 2.0 feet and a SMI bed depth of about 40 inches, the water will take about eight minutes to flow through the SMI bed (and additional time to exit at the top of the vessel). This is a preferred dwell time for contact with the SM1, or preferably a range of about 3 to 15 or 3 to 10 minutes empty bed contact time, usually 8 minutes being sufficient. The water flows out through a line 26 and a valve 27, explained below.
In a practical installation, a reactor vessel should have an inside diameter in a range of about 3 to 6 feet.
In the effluent of the reactor, indicated at the top of the reactor at 26, the treated water is low in selenium, below permissible limits, and will contain some residual calcium, magnesium and other minerals that may be picked up via leaching, but the water also carries some additional dissolved iron taken on from the SMI.
Iron can be removed from the treated effluent using oxidation and filtration. For example, in a tank 28, the treated water can be subjected to bubbling aeration to oxidize iron in the water, creating an iron oxide precipitate. Oxidation can also be effected by chlorination in a tank such as shown at 28.
The entire process of the invention can be, and preferably is, carried out at atmospheric pressure (disregarding slight hydraulic pressure from pumping and in the reactor column).
As noted above, while removing deleterious substances from the water, the process of the invention also removes and produces rare earth metals as an important, valuable by-product. The SMI when spent is subjected to separation processes to extract the REEs. The spent SMI normally will be removed from the column then processed to accomplish the REE extraction and recovery, as indicated in the blocks 33 and 34. Methods for such processing are known to skilled persons in the chemistry field and can involve heat and/or solvent extraction. See, for example, for a discussion of extraction processes, the article wvutoday.wvu.edu/stories/2018/07/18/wvu-opens-new-research-facility-to-extract-valuable-rare-earths-from-acid-mine-drainage.
As mentioned above, periodically the reactor column, i.e. the contact bed within the reactor 22, is subjected to “fluffing” to loosen and open the bed to prevent or break up compacting that has occurred, to extend the life of the medium. In
During the fluff cycle the valve 23 can remain open, slowly admitting water into the holding tank 34 while this tank is essentially drained by the fluff pump 36. The holding tank provides storage for the surge of fluff water flow that occurs during the fluff cycle. Note that the tank 34 is vented.
At the end of a fluff cycle, the valve 27 is opened, the valve 25 is opened to deliver service water flow via the pump 24, which is reactivated, and the fluff pump 36 is shut off. However, the holding tank 34 must be refilled, which can be done simultaneously with normal service flow through the reactor, and the valve 23 remains open (or is reopened if it has been closed during fluff).
The reactor 22 is shown in greater detail in
As shown in
The drop pipes 54 are of a length to place the high-flow fluff exit nozzles near the bottom of the reactor tank, and this may be in essentially the same level as the service flow diffusers as described above.
The headers 52 and 45 in one preferred embodiment extend through the walls of the reactor tank 22 as indicated in the drawings, and are welded to the walls in sealed relationship.
As noted above, the contact bed 50 of SMI medium is expanded greatly during the fluffing cycle, such that it occupies usually twice or more its normal volume in the tank. During the fluff cycle the fluffing water exits the tank via an overflow weir 65. Water level during fluff is indicated at 66 in
In normal, service flow, the water level will be approximately at the indicated level 70 in
As noted above, the SMI acts by absorption/adsorption. What appears to happen is that the selenate ion is pulled apart. The selenium of the selenate ion is believed to combine partly with the sulfur and partly with the iron and iron oxide (and possibly iron hydroxide) of the SMI; both selenium-sulfur and selenium-iron compounds exist in nature, usually together with other mineral elements and oxides. In fact, powdered iron alone (ZVI) has been used in the prior art to remove selenium, but as compared to the process with SMI, the use of powdered iron requires about three to ten times more iron than does the SMI process. Also, many times more iron becomes dissolved in the treated water with the use of powdered iron rather than SMI.
The SMI in a reactor vessel is expected to remain active and effective for a considerable period of time, even one year or more, when the reactor is operated at about 4-5 gallons per minute per square foot SMI. Much shorter periods of time can be used for batches of water. When the SMI has lost most of its effectiveness, it is replaced and the spent SMI can be subjected to known REE recovery techniques and can be melted down to retrieve the iron for re-use.
The following examples represent tests conducted on approximately 200 gallons of acid mine water recovered from a pond below an abandoned, shut-in coal mine in northern West Virginia. The water consisted of surface water which was rainwater leached through rock.
A small scale SMI reactor was operated in Madison, West Virginia in June 2020. It consisted of an 8 inch diameter, 7 foot high steel pipe with a flanged bottom and an open top. A 6 inch long, 1½ inch diameter wedge wire pipe was placed inside the pipe at the bottom of the reactor perpendicular to the pipe wall to act as a diffuser for the influent mine water. Sulfur Modified Iron (SMI) was put into the pipe to a depth of 40 inches and water passed upflow through the SMI bed at a rate of 0.6 gallons per minute. The time of contact of water with the bed was approximately 14 minutes and the flow flux was about 2 gallons per minute per square foot.
The raw pond water had an initial pH of 3.5. It was adjusted, for three different tests performed on the 200 gallon sample divided into three portions, to respective pH of 4.28, 4.51 and 5.25. The adjustment was made by adding sodium hydroxide to the water. Note that some mine waters will need acidification, or no adjustment at all, to exhibit a pH in the range of about 4 to 5.5, or more broadly, 4 to 6.5.
The SMI process described above was used on a sample of about 65 gallons of acid mine water recovered from the West Virginia mine noted above, as part of the 200 gallons taken from the mine pond.
In this example the acidity of the raw water was adjusted to pH 4.28. This was done by adding NaOH to the raw water, which was originally more acidic, at about pH 3.5.
The pH-adjusted raw water was tested for rare earth elements (REEs), sixteen of which were detected. Metal analysis was by EPA200.8 method and the instrumentation for these elements. Table A shows the content of each of the sixteen REEs in the raw, pH-adjusted water. See the lines labeled “Ex. 1 Raw”. The amounts of the REEs are shown in micrograms per liter, i.e. parts per billion. Note that thorium, uranium, sulfate and chlorine are also shown with detected amounts. The same water, after treatment in the SMI column as described above, was tested for the same REEs post-treatment, as shown in the rows labeled “Ex. 1 Post”. The table shows very small residual amounts of each of the REEs post-treatment.
Table B shows the same data recompiled so that the REEs are listed in a column at left. Each REE's data is presented in a row, with results of Examples 1, 2, and 3 spread across the table.
As shown in Table B, the total REE content of the raw water, including all of the sixteen REEs, was 685.033 mcg/liter of water. Post-treatment, the residual of REEs collectively was down to 2.460 mcg/liter. This is effectively a 100% recovery of the REEs, collected on the SMI particles.
The same raw acid mine water was used, approximately 65 gallons of the total sample of about 200 gallons, for each of Examples 2 and 3. In Example 2 the water was adjusted to a pH of 4.51; in Example 3, to 5.25. Again, the raw pH-adjusted water was measured for content of each of the sixteen REEs listed, the amounts in mcg/liter being shown in Table B. The pH has an effect on detection level, thus the differences in values for raw water REE content among the three examples.
The tables show the results of SMI treatment in Examples 1, 2 and 3. Sixteen REEs were detected in the sample water, as well as many other elements noted in the table. The results in all three examples show essentially 100% recovery of total REEs, by comparing the initial tested content of each REE with the post-treatment content. In each of the three examples recovery is greater than 99%, and in most cases greater than 99.5%. Recovery rates for individual REEs ranged from 99.4% to 99.8%. The REEs are reported in mcg/liter, i.e. parts per billion (ppb). Some REEs show a higher recovery at lower pH, some show the opposite. The results are subject to testing accuracy. In general the results of the three examples show extremely effective REE recovery across the range of the three different acidities.
Note that the initial pH for the tests in Examples 1, 2 and 3 were 4.28, 4.51 and 5.25, respectively. The change in pH due to SMI treatment varied in accordance with initial pH. In Example 1 the increase was from pH 4.28 to pH 6.23; in Example 2, from 4.51 to 5.58; and in Example 3 from 5.25 to 5.66.
It is also noted that the SMI withdrew considerable proportions of other metals shown in Table C: aluminum, calcium, cobalt, iron, magnesium, sodium, nickel, silicon and zinc. In the case of iron, the amount of iron actually increased after SMI treatment, since iron from the SMI is picked up in the water. This effect became greater with higher pH, as seen in Examples 2 and 3. Sodium content changed little post-treatment, while calcium and magnesium increased, with the increase being greater at lower pH. The table also shows recovery of thorium, uranium, sulfate and chlorine. Sulfate increased post-treatment at the lower pH, and chlorine increased in all three examples.
The important end result is that nearly 100% of all sixteen REEs present in the raw water was recovered, captured by the SMI.
Any of the REEs, or all of them, can be recovered from the SMI after a period of service of the SMI, which can be when the SMI is essentially saturated and significantly less effective. This can be by well known conventional methods. For example, see the article “Rare Earth Elements Removal Techniques from Water/Wastewater: A Review”, published in Desalination and Water Treatment, October 2018, describing techniques for withdrawing REEs from waters. In the SMI process the SM1 has pulled off other substances around the REEs. The pure elemental REEs stick to the SMI, which is unique in REE recovery.
One method for REE recovery is to first treat the SMI using a strong acid, e.g. HCl. This separates the iron and breaks down the SMI and leaves zero-valent REEs, as well as other recovered metals, back in solution, including at least some of the iron. This is a highly concentrated solution of REEs. With known REE recovery techniques, such as described in the article referenced above, the REEs are economically recovered. The process of the invention essentially creates a new ore, with very high concentrations of REEs. Other recovery techniques for extracting REEs can be used by those skilled in the art to remove REEs from the spent SM1. These include solvent extraction, electrowinning, nanofiltration, ion exchange resins, differential pH, and acid washing, as well as a number of new techniques being developed currently for extracting REEs from solid materials, e.g. rocks.
The process of the invention thus provides a new and efficient method and system for producing rare earth elements, which can be a byproduct of a process for removing deleterious substances from water, e.g. selenium, or which can be a primary product of an SMI process. As noted above, ZVI could be used in place of SMI, but at significantly lower effectiveness and efficiency. The invention encompasses use of ZVI for extracting REEs.
As explained in referenced U.S. Pat. Nos. 9,427,706 and 10,059,613, a continuous moving medium can be used as the reactor vessel, so that fluffing is not required. The descriptions of those patents relating to continuous moving medium are incorporated herein by reference.
In these mines there typically is a considerable amount of mine water. The mine water is usually acidic, sometimes quite acidic, and usually will contain some of the rare earth elements. In the procedure and system shown in
As indicated in the drawing, mine water is pumped to the surface, then, as noted at 86, the withdrawn mine water passes through processes preferably as shown in
The process of the invention applies to other aspects of waters from coal mines. In deep coal mining practices of the 19th century, coal was removed by hand, from coal “seams”, which often have clay below the seam and slate or shale just above the seam. With the extraction of coal comes a considerable amount of these other materials, as they are inevitably scraped off along with coal from the seam. The seam itself could be thin, a matter of inches, or several or many feet thick. At the surface the coal is cleaned to remove the waste material. The waste material from such an operation is put into a “gob pile”.
In more recent years with coal extracted by machines from the seams, additional material such as clay, shale and/or slate has still been extracted along with the coal, and again this waste material must be separated from the coal, and has been discharged to a pile called a “refuse pile”.
The gob piles and the refuge piles typically contain REEs. Many of these gob piles and refuse piles still exist near former or active coal mining operations. The piles are out in the open, and may be vast in size, sometimes over one-half mile in length and 150 to 200 feet high, as in parts of northern West Virginia. Such refuse piles are currently treated in an ongoing program to ameliorate high content of minerals prior to discharge to a river or stream. All deep mining operations have or have had these piles. Pursuant to the invention these gob or refuse piles can be treated by heap leaching, with an acidic water or simply by rainwater, to collect REEs in the leach water.
With some of these vast volumes of refuse, rain falls through, creating a heap leach without the need for additional water. Sometimes the refuse piles have been covered with sheet plastic or other rain-blocking material, which can be removed to allow the water leaching. The leached rainwater will exit the elongated refuse pile at a side and can then be directed to a plant for processing according to the invention.
In surface coal mining, as discussed above, overburden rocks and other materials are above-ground and rainwater will leach through these and pick up many minerals, some deleterious, but also including REEs, This rain runoff often is collected in ponds, also discussed above. In testing with SMI in 2013-2014, the selenium content of such mine pond water was greatly reduced in selenium, using a SMI reactor column as described above. Recently, piles of the spent SMI used in the 2013-2014 testing were located and were tested for REE content. They were found to have significant amounts of many of the REEs. Recovery of REEs from such sources of spent SMI is also an aspect of this invention. Heap leaching with acidic water can be performed to gather the REEs from the SM1 into solution.
In
The now atmospheric-exposed unconsolidated overburden receives precipitation and it runs off the windrowed spoils (surface runoff). It also leaches down through the spoils. Some of the broken rock in the overburden contains iron pyrite. As the broken iron pyrite on the surface of the overburden spoils sees precipitation and atmospheric oxygen, the surface water runoff becomes low in pH. When the iron pyrite within the overburden spoils becomes inundated with leaching moisture, the moisture typically finds release to the open atmosphere where it quickly becomes acidic like the surface runoff. Both situations are known as “acid mine drainage”—AMD.
This low pH water (AMD) dissolves rare earth elements into solution and they are carried away hydraulically, and if not interrupted would then flow into lakes, ponds and streams and rivers. This phenomenon is known as acid mine drainage. The Will Scarlet mine in Illinois is an example of such overburden found to contain concentrations of rare earth elements.
The drawings show, very schematically, the use of a hydroauger or “hydrauger” drilling system to place a drain, i.e. a long pipe with holes or slots, deeply into a refuse pile for drainage of leached water or acidic water from the pile.
The hydroauger system is one in which a drill bit 108 at the tip of the drilling rig is left in place in the refuse pile. Typically a drill casing 107 supports the drill bit at the far end, the drill casing acting as a drilling shaft, somewhat smaller than the drill bit 108. Additional lengths of drill casing/shaft are successively added at the rig 100, until a desired drilling distance has been achieved.
In a typical hydroauger drilling system, water and compressed air are pumped through the drill shaft/casing as drilling proceeds, so as to wash the drill cutting debris back out of the boring. This can involve an air motor and a water pump.
The schematic view of
The hydroauger system has been used fairly widely for stabilizing hills, by emplacing a series of the hydroauger drains, sometimes an extensive network of the drains, to withdraw water from hillsides that are unstable or may become unstable under conditions of heavy rainfall. The goal typically is to prevent slides by sufficient drainage of water from the hill. In the current invention, however, the withdrawn water bears REEs and is the important commodity. The water is collected and processed in accordance with the invention, via one or more SMI reactor columns to withdraw the rare earth elements onto the SMI, for retrieval through processing of spent SMI.
As explained above, ZVI could be used in lieu of SMI, but with less ideal results.
In
The drain pipe 110 collects water that is leached through the refuse pile, such as from rain or from water (or acidic water) spread over the pile.
As indicated, the drain pipe 110 leads to a water withdrawal pipe 114, to deliver the REE-laden acidic water to an SMI column for processing as described above. The water remains acidic until it reaches the SMI column, at which point its pH will be adjusted if needed, to the desired range for SMI treatment.
In another aspect the invention concerns recovery of rare earth elements (REEs) from surface coal mining, also known as strip mining.
U.S. Pat. No. 11,230,753 describes recovery of REEs from acid coal mine water of several different types. Water can be from ponds that receive acidic water, i.e. rainwater, that has leached through the overburden rocks removed in a coal mining operation, or from deep pools of acid mine water from underground mines, or from gob files or refuse piles that are built up from years of underground coal mining. In such gob or refuse piles the coal has been separated (by hand tools or by machines) from surrounding rock, the surrounding rock being discarded and forming the piles. By the process in that patent, water, either from a deep mine pool of acid water or from water leached through overburden or from leaching through gob piles or refuse piles, is treated in a reactor column containing sulfur-modified iron, or SMI (or alternatively zero valent iron). In this process the SMI particles capture, by adsorption and/or absorption, nearly all REEs present in the water. Once the SMI is essentially saturated with REEs and other substances picked up from the water, the SMI is removed from the column and processed to recover the REEs. Certain of the rare earths are critical to semiconductor fabrication and other manufacturing, and they have strategic importance. U.S. Pat. No. 11,230,753 is incorporated herein by reference in its entirety.
According to the current invention, enhancements are described for obtaining the REE-rich leached water from coal mining operations.
An important aspect of the invention concerns surface coal mining wherein a series of “cuts” are made through coal bearing ground, these deep cuts being in parallel strips and in a series or succession, such that once a cut or strip has been completed and coal removed from beneath the overburden, forming a ridge of spoils to the side, the next parallel cut is made and its spoils are used to fill in the previous cut. This is illustrated in
The invention optimizes recovery of coal mining waters bearing high content of REEs, in some cases unusually high content. Another aspect of the invention applies microfiltration of the acidic, REE-bearing water prior to treatment in the SMI column, to enhance REE recovery.
When the final cut has been made, as at 130, the final spoils pile or ridge at 116n lies adjacent to a deep trench at the cut 130.
This final cut at 130 fills with water, typically rainwater draining into the cut, some from the surface as at 132 but much having leached through the series of ridges or spoils 116a, 116b, . . . 116n.
Indicated in
Pursuant to the invention a boat or float of some kind is placed at the surface of the lake 130. As shown in
The drawing indicates the water can first be put through prefiltration at 140. After that step, in one important embodiment of the invention, membrane filtration of the water is conducted at 142. The permeate is shown exiting membrane filtration at 144, while removed solids or filtrate are rejected as at 146.
The ultrafiltered water at 144 may then be adjusted as to pH, in a pH adjustment tank indicated at 148. This will normally be needed, since the acid mine water may be originally at a very low pH, and a preferred pH range for the water, prior to SMI treatment, is about pH 4 to pH 5.5. Adjustment of the pH in the tank 148 can be by addition of NaOH.
As described in U.S. Pat. No. 11,230,753, fully incorporated herein by reference, the pH-adjusted water can be directed to a holding tank 150, and optionally can be treated with a biocide, as indicated at 152. Beyond the holding tank 150, the water is moved, preferably via a pump 154, to an SMI reactor 156. The reactor column is fully described in U.S. Pat. No. 11,230,753, and it may include a “fluffing” cycle occasionally as needed, to loosen and remove compaction in the particles of SMI within the reactor, extending the effective duration of the SMI material. Optionally, fluffing water, which is a periodic high-velocity charge of water up through the reactor, can be conveyed back to the final cut lake 130, via a pipeline indicated at 160.
After treatment in the SMI reactor, exiting water 162 can be treated with various steps prior to being discharged to streams or lakes. This is described in the aforementioned patent.
Importantly, the spent SMI, after a period of time in which the SMI becomes less effective at withdrawing REEs, is withdrawn from the reactor tank 156 as indicated by the dashed line 164, and the SMI is treated to extract REEs, by conventional processes, as indicated at 166.
By the invention the SMI extraction system is provided with a very rich source of REEs, from the bottom, most-acidic layer of water in a final cut lake, this leach water originating from a surface coal mine, which can be a strip mining operation.
In one aspect of the invention, the recovery of REEs via the SMI reactor can be significantly enhanced in some cases by use of membrane filtration, as at 142. This is particularly the case where a significant level of sulfates is present in the mine water, since sulfates can inhibit the SMI reaction. In fact, regardless of the source of the water to be treated, i.e. whether from a final cut lake or from leached rainwater through overburden, or from deep mine pools of acid mine water, this step can enhance recovery in the case where significant sulfates are present in the water.
The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.
This application is a national filing of PCT/US2022/013563, filed Jan. 24, 2022, which claimed benefit of patent application Ser. No. 17/211,730, filed Mar. 24, 2021, which claimed benefit of provisional patent applications Ser. Nos. 62/994,214, filed Mar. 24, 2020, and 63/047,830, filed Jul. 2, 2020. This application also claims benefit of provisional application No. 63/409,545, filed Sep. 26, 2022.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/013563 | 1/24/2022 | WO |
