This will be a rather involved dispatch; and we apologize in advance for it's length and complexity.
However, it centers on what should be recognized as a matter of strategic importance for the United States of America, which is:
Securing a domestic supply of Aluminum, all raw ore of which, according to the US Geologic Survey, via:
http://minerals.usgs.gov/
we now import, through the recycling, the reprocessing, of Coal-fired power plant Fly Ash.
In an earlier dispatch, now accessible via the link:
West Virginia Coal Association | Iowa Mines Metals from Coal Ash for the USDOE | Research & Development; concerning, in part:
"United States Patent 4,397,822 - Process for the Recovery of Alumina from Fly Ash; 1983; Inventor: Marlyn Murtha, Iowa; Government Interests: The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-82 between the U.S. Department of Energy and Ames Laboratory (at Iowa State University); Abstract: An improvement in the lime-sinter process for recovering alumina from pulverized coal fly ash is disclosed. The addition of from 2 to 10 weight percent carbon and sulfur to the fly ash-calcium carbonate mixture increase alumina recovery at lower sintering temperatures. This invention relates to a method for the recovery of alumina from pulverized coal fly ash. More specifically, this invention relates to an improvement in the lime-sinter process for the recovery of alumina from pulverized coal fly ash";
we documented that the USDOE had financed the development of an improved technology for extracting alumina, that is, Aluminum Oxide, the basic commodity Aluminum ore, from Coal-fired power plant Fly Ash.
That, of course, followed the USDOE's earlier assessment, as seen in:
USDOE Says Coal Ash Could End Aluminum Ore Imports | Research & Development; concerning the report: "Economic Metal Recovery from Fly Ash; 1981; Oak Ridge National Laboratory, USDOE; Abstract: Although most coal combustion ash produced in the United States is discarded as a waste, results are presented to show that fly ash can be an economical source of Al2O3, Fe2/O3, and possibly several other metals, many of which are presently being imported";
that Coal Ash could serve as an economical source of Aluminum Oxide.
Such Al2O3, Aluminum Oxide, is the Aluminum source extracted from the Aluminum ore known as bauxite; which Al2O3 is then typically electrolyzed for the production of pure Aluminum.
More can be learned via:
aluminium (US: aluminum); concerning: "Extracting Aluminum Ore from Bauxite; Aluminum is too high in the electrochemical series (reactivity series) to extract it from its ore using carbon reduction. The temperatures needed are too high to be economic. Instead, it is extracted by electrolysis. The ore is first converted into pure aluminum oxide by the Bayer Process, and this is then electrolysed in solution in molten cryolite - another aluminum compound. The aluminum oxide has too high a melting point to electrolyse on its own."
The point being that, the typical process for extracting Aluminum metal from the Aluminum Oxide ore requires the addition of another Aluminum compound, "cryolite", which acts as a sort of "flux", a material which enables the use of lower temperatures, and, thus, lower voltages and amperages, i.e., less energy, in the standard electrolytic extraction of Aluminum metal from Aluminum Oxide.
Cryolite is a mineral of limited natural occurrence, and, as seen in:
Cryolite - Wikipedia, the free encyclopedia; "Cryolite (Na3AlF6, sodium hexafluoraluminate) is an uncommon mineral identified with the once large deposit ... on the west coast of Greenland, depleted by 1987. It was historically used as an ore of aluminium and later in the electrolytic processing of the aluminum-rich oxide ore bauxite. The difficulty of separating aluminium from oxygen in the oxide ores was overcome by the use of cryolite as a flux to dissolve the oxide ... (since) it can dissolve the aluminium oxides sufficiently well to allow easy extraction of the aluminium by electrolysis. Now, as natural cryolite is too rare to be used for this purpose, synthetic sodium aluminium fluoride is produced from the common mineral fluorite";
it has become so rare, but still remains so necessary for the economical extraction of Aluminum from Aluminum Oxide, that technologies have been developed to synthesize Cryolite from other minerals.
More can be learned via;
http://ibm.nic.in/cryolite.pdf
Since "Synthetic cryolite is (now) used as electrolyte in the reduction of alumina to aluminum", and, since we can, as in the "United States Patent 4,397,822 - Process for the Recovery of Alumina from Fly Ash", obtain the Alumina from Coal Ash, we see herein, that, again according to the Ames Laboratory at Iowa State University, we can also make the needed synthetic Cryolite, rather serendipitously, by synthesizing it, too, from the Aluminum compounds in Coal Ash.
As seen in excerpts from the initial link in this dispatch to:
"United States Patent 4,362,703 - Process for Preparing Cryolite from Fly Ash
Date: December, 1982
Inventors: Mustafa Boybay and Turgut Demirel, Ames, Iowa
Assignee: Iowa State University Research Foundation
Abstract: Cryolite (Na3AlF6) as a source of aluminum is prepared from coal fly ash by reacting the aluminum oxide (Al2O3) in fly ash with phosphoric acid to form aluminum phosphate, which is then converted to sodium aluminate by reaction with sodium hydroxide, and the sodium aluminate is converted to cryolite by reaction with hydrofluoric acid (HF), or equivalent reagent providing H+ and F- ions. Aluminum is thereby obtained from fly ash in a preferred form (as cryolite) for use in producing metallic aluminum.
(The required "phosphoric acid" is cheap, abundant, and can be made several different ways using some inexpensive raw materials. If you like "Coke"(R) or "Pepsi"(R), you've drunk a lot of it. The "sodium hydroxide", perhaps better known as"lye", too, is cheap, abundant and can be made from some pretty common stuff.)
Claims: The process of preparing cryolite (Na3AlF6) from a coal fly ash containing aluminum oxide as one of the principal constitutents, comprising:
(a) reacting aqueous phosphoric acid (H3PO4) with said fly ash to convert a substantial portion of the aluminum of said fly ash to aluminum phosphate (AlPO4), thereby obtaining a first reaction mixture containing a substantial amount of AlPO4 ;
(b) adding aqueous sodium hydroxide (NaOH) to said first reaction mixture and reacting said NaOH therewith to form a second reaction mixture, the amount of NaOH added being sufficient to convert said AlPO4 to sodium aluminate (Na3 AlO3);
(c) separating the undissolved solids from said second reaction mixture to obtain a solution containing said Na3AlO3 ;
(d) adding hydrogen ions (H+) and fluoride ions (F-) to said Na3AlO3 contaning solution to form cryolite as a precipitate, said addition being continued to a final pH below 9.0; and
(e) separating the precipitated cryolite from the residual solution.
The process ... in which said hydrogen and fluoride ions are added as aqueous hydrofluoric acid (HF).
The process ... in which said hydrogen and fluoride ions are added as aqueous hydrochloric acid (HCl) and sodium fluoride (NaF).
(As seen above, a number of other reagents are required, or can be used, in this process. And, we don't want to brush too quickly by that issue, since, somewhat unfortunately, the supply of some of those reagents is problematic. Even though all of them are inexpensive commodity chemicals, most of them are, somewhat unsurprisingly, manufactured and supplied by overseas, and perhaps unreliable, sources, such as China and India; who manufacture them from fairly common minerals such as fluorite, aka "fluorspar", and apatite. The manufacturing process requires the use of sulfuric and hydrochloric acids. We do have significant deposits of the basic minerals in North America, a few developed commercially in Mexico and Canada; and, the potential exists to mine some in the US; and, to produce more as byproducts of processing phosphate rock into fertilizer, as is done in Florida. The needed hydrochloric and sulfuric acids are commodity chemicals which we could arrange to make more of. The supply scenario is complex, which is why we're not providing links to reference documents. The point is, that, while we don't, in the US, now have abundant, or excess, supplies of the needed reagents, we could arrange to make them fairly easily, and to do so by relying on imports of some needed raw materials from trusted neighbors like Mexico and Canada, if we didn't want to establish new mining operations in some areas of the US where deposits do occur. Further, even though the supplies are available, they are finite. But, if worse comes to worse, they can be synthesized; and, some of them can even be regenerated in a designed component of the Aluminum production process itself. As we said, it's complicated; but, definitely "do-able". We're just not equipped and qualified to explain it all succinctly in the context of this report.)
Description and Background: Coal fly ash is produced as a by-product in the burning of pulverized coal. It is recovered from the flue gases. Fly ash is therefore a low cost material which is available in large quantity, but relatively few large scale commercial uses have been developed. With the increasing burning of coal to produce electrical energy, the amount of available fly ash can be expected to continually increase, and therefore processes using fly ash as a starting material will be increasingly important. The principal constituents of fly ash are the oxides of aluminum, iron, calcium, and silicon (Al2O3, Fe2O3, CaO, and SiO2).
The relative proportions of the metal oxides in fly ash vary with the type of coal being burned. In general, however, coal fly ash as produced will contain 15% or more by weight of aluminum oxide, and some fly ashes will contain as much as 28% Al2O3. For example, in the United States, the burning of bituminous coal produces a fly ash having an average aluminum oxide content of about 22%, the range being from about 16 to 27% Al2O3. The iron oxide content is usually much higher in bituminous coal fly ashes than with fly ashes produced from other types of coal, such as subbituminous lignite. Bituminous coal fly ashes may contain from 12 to 22% Fe2O3. However, the iron rich particles of the fly ash can be separated to enrich the aluminum oxide content, such as by magnetic separation. Upgraded fly ash produced by initial removal of the iron oxide may contain as much as 18 to 21% Al2O3.
(Keep in mind, relative to the above statement "coal fly ashes may contain from 12 to 22% Fe2O3", as we've documented in previous reports, including the above-cited:
West Virginia Coal Association | Iowa Mines Metals from Coal Ash for the USDOE | Research & Development; which also concerned:
"United States Patent 4,386,057 - Recovery of Iron Oxide from Coal Fly Ash; 1983; Inventors: Michael Dobbins and Marlyn Murtha, Ames, Iowa; Assignee: The United States of America; The U.S. Government has rights in this invention pursuant to Contract No. W-7405-ENG-82 between the U.S. Department of Energy and Ames Laboratory; Abstract: A high quality iron oxide concentrate, suitable as a feed for blast and electric reduction furnaces is recovered from pulverized coal fly ash";
that: Coal Fly Ash can also be seen and treated as a valuable source of Iron ore.)
It is known that fly ash can be leached with strong mineral acids to solubilize the aluminum, such as by leaching the fly ash with sulfuric acid. It is also known that fly ash can be reacted with phosphoric acid to convert at least part of the aluminum oxide to aluminum phosphate. A process has been proposed using this reaction as the first step in producing Al2O3 or Al(OH)3 and dibasic ammonium phosphate.
(The non-aluminous byproduct "ammonium phosphate" could serve in the follow-on commercial production of fertilizer.)
Most of the recent research and development work on producing aluminum from fly ash has centered on the extraction of alumina by a lime-soda sinter process. After removal of the iron-rich magnetic fraction of the fly ash by magnetic separation, the upgraded fly ash is sintered with lime (or calcium carbonate), and soda ash. For efficient use on a production basis, this process may require the milling of the fly ash in admixture with the calcium carbonate and sodium carbonate, and then pelletizing or nodulizing of the ground mix prior to treatment in the sintering furnace at high temperatures. Following the sintering, cooling and further grinding are required prior to the leaching with a sodium carbonate solution, and additional filtration and precipitation steps are required to recover the solubilized alumina. Therefore, a need has been recognized for an alternative process for recovering aluminum from fly ash, but, as far as is known, it has not been suggested that the alumina might be recovered in the form of cryolite rather than as alumina (Al2O3).
Summary: Cryolite is prepared from a coal fly ash containing aluminum oxide as one of the principal constituents by first reacting the fly ash with aqueous phosphoric acid to convert a substantial portion of the aluminum of the fly ash to aluminum phosphate. This produces a first reaction mixture containing a substantial amount of aluminum phosphate. To this mixture there is added aqueous sodium hydroxide, which is reacted therewith to form a second reaction mixture. The amount of sodium hydroxide added is sufficient to convert the aluminum phosphate to sodium aluminate. The undissolved solids are separated from the second reaction mixture to obtain a solution containing the sodium aluminate. To this solution there is added aqueous hydrofluoric acid (or equivalent), which is reacted therewith to form cryolite as a precipitate at an alkaline pH. The precipitated cryolite is separated from the residual solution. The preferred operating conditions for these process steps, and additional steps which may be employed, are described below.
The process of the present invention is applicable to any coal fly ash containing aluminum oxide (alumina, Al2O3) as one of the principal constituents. Preferably, however, the fly ash starting material contains at least 12% alumina by weight. In embodiments where the fly ash is first processed to remove an iron-rich magnetic fraction, such as by a magnetic separation procedure, the fly ash material for treatment with H3PO4 will desirably contain at least 15% Al2O3. The fly ash may be used in the particulate form obtained by the burning of the coal, and does not need to be further ground prior to use in the process of the present invention. In general, the fly ash starting material will contain from about 12 to 27% alumina together with substantial amounts of iron oxide (Fe2O3) calcium oxide (CaO), and silicon dioxide (SiO2).
In the first step of the chemical reaction sequence of the present invention, the fly ash starting material is reacted with aqueous phosphoric acid (H3PO4). Preferably, the phosphoric acid is used in a concentrated form, such as aqueous phosphoric acid containing at least 40% H3PO4 by weight. In a desirable embodiment, the aqueous phosphoric acid contains 30% or less water by weight, such as 70 to 90% H3PO4.
The amount of the phosphoric acid reactant employed should be sufficient to convert a substantial portion of the aluminum (as alumina) of the fly ash to aluminum phosphate (Al3PO4). In a desirable embodiment, the amount of phosphoric acid to be employed with a particular fly ash is calculated on the basis of acidification value for fly ash. More specifically, the acidification value is calculated for the particular fly ash, which value is referred to herein as the Fly Ash Acidification Mole or FAA Mole. The FFA Mole is computed as two (2) times the total moles of Al2O3 and Fe2O3 in the amount of fly ash used plus 2/3 (0.67) times the moles of CaO therein. Using this calculated value, fly ash may be treated with a molar amount of H3PO4 equal to 0.6 to 2.0 times the Fly Ash Acidification Mole.
(We include the above complexities in our excerpts just to demonstrate for you how thorough the science for, the understanding of the processes for, extracting valuable materials from Coal Ash has, in certain unpublicized and unrecognized circles, become.)
The reaction of the fly ash with the phosphoric acid can be carried out under relatively mild conditions, such as 20 to 40C. at atmospheric pressure. More broadly, the reaction may be carried out under atmospheric pressure and at temperatures ranging from about 15C to 100C.
(The process does not, in other words, require a lot of energy or expensive, high-pressure equipment.)
By carrying out the reaction, the particulate fly ash may be mixed as a solid phase with the liquid phosphoric acid, suitable mixing equipment being employed. Where highly concentrated phosphoric acid is employed, as preferred, such as phosphoric acid having a concentration of 70% H3PO4 or greater, the initial reaction slurry, will become thicker and will tend to form aggregates or lumps, as the reaction proceeds. Therefore in the latter stages of the reaction, it will be desirable to employ solid phase-type mixing apparatus, such as apparatus performing both mixing and grinding, to prevent the reaction mixture from setting up to a solid cake. This may also be prevented by having more water present in the reaction slurry, such as by using more dilute phosphoric acid. However, since additional water is added in the next step of the process, it is believed desirable to minimize the amount of water present in the first step, although this is not essential for the desired conversion of the alumina to aluminum phosphate. The reaction time will depend on the temperature employed, but usually, the reaction will be sufficiently completed in about 1 to 3 days at ambient temperatures (15-25C). By heating the reactants, the reaction time can be shortened to 1 to 3 hours. Upon the completion of the first reaction, as described above, and depending on the amount of water present, part of the converted aluminum phosphate may be in solution and part as a solid phase. However, in the first step of the process, it is not essential to obtain all of the aluminum phosphate in solution, and, in fact, as explained above, it is believed desirable to minimize the amount of water present, which will result in a substantial part of the aluminum phosphate being present as a solid in the first reaction mixture.
Instead of hydrofluoric acid, which is a relatively expensive reagent, the required H+ and F- ions, as provided by HF, can be added to the reaction mixture in the form of an acid plus a water-soluble fluoride salt. For example, a strong mineral acid can be used as the H+ source (HCl, H2SO4, HNO3).
(We can easily make plenty of cheap hydrochloric, sulfuric or nitric acid, the "HCl, H2SO4, HNO3" as above, to provide the needed "H+" ions.)
The fluoride salt may be a water-soluble alkali metal or an alkaline earth metal fluoride. Sodium fluoride (NaF) is particularly desirable since it does not form precipitates, and is an available low cost reagent.
Calcium fluoride (CaF2) is also an available low cost reagent, but the Ca compounds may precipitate requiring removal. In general, however, acids and fluoride salts are equivalent reagents to HF as source of the H+ and F- ions for the desired reaction."
---------------------
Again, the "Calcium fluoride (CaF2)" is, simply, the mineral "fluorite", which we can easily import from Canada and Mexico. However, if we do want as much domestic content as possible, there are fluorite deposits which have been mined, and which could be redeveloped, in a number of states, including Missouri, Oklahoma, Illinois, Kentucky, Colorado, and others. The point is, we can pretty easily get all we need; it's not that big of a deal.
There is a good bit more to the full disclosure; and, it would be easy to get lost, as we perhaps to a certain extent have, in the complexities.
The "take away", however, is this:
Aluminum is a commodity metal of immense commercial and strategic importance.
We currently import all of the Aluminum ore, Aluminum Oxide, from which we make virgin Aluminum metal in the United States of America.
We could, instead, extract all of the Aluminum Oxide we might want, as via the previously-reported "United States Patent 4,397,822 - Process for the Recovery of Alumina from Fly Ash", from the solid residua left by our essential use of Coal in the generation of electrical power.
The most economical industrial processes for extracting and refining Aluminum metal from the Aluminum Oxide ore require the use of a "flux" known as Cryolite.
We can, as herein, via the process of our subject, "United States Patent 4,362,703 - Process for Preparing Cryolite from Fly Ash", manufacture that strategic material from Coal-fired power plant Fly Ash, as well.