Energy demands by the industrialized world are continuing to rise, while the rate of new oil discoveries is falling. Within the next 30 years, available petroleum supplies will fail to meet demand, and oil will no longer be able to serve as the world's major energy source. Other energy sources such as geothermal, solar, and fusion are unlikely to be sufficiently developed to serve as replacements for oil. Coal, on the other hand, exists in relative abundance in the United States, and if it can be adapted for use in existing plants which have been engineered for petroleum use, it can serve as an inexpensive substitute for, and successor to, the more expensive oil fuels in use today. In order to be used as an oil substitute, however, the coal must be converted to a fluid state exemplified by the finely-ground leached coal product of this invention, so that systems burning fuel oil, diesel fuel, and other petroleum products can be adapted to its use with minimal equipment modification. The coal must also be cleaned, or purged of its mineral matter (ash precursor) content, to permit its use without fouling, damaging or reducing the efficiency of the combustion equipment, to reduce or eliminate the requirement for post combustion gas clean up, and to increase fuel value per pound for efficient handling and use; and its sulfur content must be reduced to minimize off-gas cleanup, so as to meet environmental pollution standards.
It is known that coal may be cleaned of its mineral matter by an acid leach. While efforts have been made to utilize HF and HCl to clean coal by dissolving away its ash constituents, known methods are cumbersome and expensive. Additionally, the methods directed to cleaning coal via the acid leach have primarily related to small scale coal processing. The problems involved in large scale processing, such as manufacturing plants dedicated to processing coal as a petroleum product substitute, have not been adequately addressed. In a large commercial operation, the coal processing steps must be consolidated and simplified for economic cost considerations in order to compete as an alternative for oil and gas.
U.S. Pat. No. 4,169,710 assigned to Chevron describes a process for the use of concentrated hydrogen halide, such as hydrogen fluoride, as a comminuting agent for raw coal. The patent also discloses the use of the hydrogen halide to dissolve and remove ash and sulfur from raw (unground) coal. This patent mentions that the hydrogen halide may be purified and recycled; however, no procedure for doing so is disclosed. The Chevron patent does not disclose the use of finely-ground, hydrogen fluoride/hydrogen chloride-purged coal as a substitute for fluid fuels or other forms of finely-divided, highly purified hydrocarbons.
European Patent Application No. 80300800.2, filed Mar. 14, 1980, and published Oct. 1, 1980, under Publication No. 0 016 624, by Kinneret Enterprises, Ltd., discloses a coal de-ashing process utilizing liquid or gaseous hydrogen fluoride to remove silica and/or aluminum bearing mineral matter and other reactive materials from substances, such as coal, which do not react with hydrogen fluoride under the same conditions. The hydrogen fluoride is recovered as a gaseous product at several stages. In the Kinneret process, hydrogen fluoride in gaseous form contacts the coal, which is first ground to -200 mesh. The unreacted gas is then separated by density methods and recycled. An aqueous solution of 20-30% hydrogen fluoride is then used to leach the formed fluoride minerals away from the coal, and hydrogen fluoride gas is recovered from this solution at raised temperatures and pressures, simultaneously causing the crystallization of aluminum, calcium, magnesium, and manganese fluorides. Other minerals including titanium, potassium, and sodium fluorides remain in solution. The heavy gas fraction resulting from the hydrogen fluoride gas treatment of the coal is contacted at elevated temperatures and pressures with water in two subsequent stages to remove sulfur and silicon dioxide and produce gaseous hydrogen fluoride in both cases for recycle. The Kinneret publication discloses the comminution of a coal prior to treating with hydrogen fluoride to remove mineral content, it does not disclose a procedure for producing a product suitable as a liquid fuel substitute or other applications as discussed above.
Bureau of Mines Report of Investigations No. 5191, "Coal As A Source of Electrode Carbon In Aluminum Production," (Feb., 1956) at page 7 discloses the use of froth flotation followed by hydrofluoric-hydrochloric acid leaching, using a boiling solution containing 5 parts of the combined acids to 95 parts water. At page 29, the use of a 2.44 Normal solution of hydrofluoric-hydrochloric acid is used to leach coal at boiling temperatures. There is no teaching or suggestion that milder, even ambient, temperatures can be employed nor is there a discussion regarding large scale operations and/or the need to regenerate the mixed acids.
U.S. Pat. No. 4,083,940 to Das discloses the use of a 0.5-10% hydrofluoric acid solution in combination with an oxidizing agent such as nitric acid, to purify coal to electrode purity (0.17% ash). A gaseous oxygen-containing material is bubbled through the mixture during leaching to provide additional mixing action and oxidation.
U.S. Pat. No. 3,961,030 to Wiewiorowski et al. describes the use of a 10-80% hydrogen fluoride solution to leach clay for the recovery of aluminum. Hydrogen fluoride is recovered for recycle by the addition of water and heat to aluminum fluoride. The recovered hydrogen fluoride can be dissolved in water and recycled in aqueous form.
U.S. Pat. No. 2,808,369 to Hickey describes the treatment of coal with fluoride salts, and with hydrogen fluoride gas, after first heating the coal to effect a partial devolatilization.
Other patents which describe methods to clean coal include U.S. Pat. No. 4,071,328 to Sinke, describing the removal of FeS from coal by hydrogenation and contact with aqueous hydrogen fluoride. U.S. Pat. Nos. 3,870,237 and 3,918,761 to Aldrich disclose the use of moist ammonia for in situ treatment of coal to fragment the coal and facilitate the separation of inorganic components. U.S. Pat. No. 3,863,846 to Keller, Jr., et al. describes an apparatus and method for the utilization of anhydrous ammonia as a coal comminuting agent.
One of the major disadvantages of coal cleaning processes not adequately addressed in the prior art is regeneration of the spent acid leach liquors and capture and reuse after regeneration of substantially all fluorine values throughout all processing circuits. HF is an expensive reagent, so that its use is uneconomical unless it can be recycled. There are known methods of producing both HF and HCl, typically involving treating a readily available and inexpensive source of fluoride or chloride, e.g. CaF.sub.2 or NaCl, to produce the desired acid. For example, U.S. Pat. No. 4,120,939 describes a process for the production of hydrogen fluoride gas from the reaction of calcium fluoride particles with sulfuric acid formed in situ from sulfur dioxide and steam.
While there are known methods of producing HF and HCl, regeneration of spent HF/HCl from industrial streams presents new difficulties not encountered in production from pure reagents. Additionally, HCl and, particularly, HF are corrosive pollutants and recycling the spent acid liquor reduces the cost of environmentally acceptable disposal. HF and HCl have a wide variety of uses in commercial processes. The acids are used in chemical, refining, metallurgical and for hydrometallurgical processes for leach of ores and concentrates and for pickling of metals. In addition, HCl is often used in the processing of ores as a chlorination agent.
Known methods to regenerate HF and HCl from industrial waste, including gaseous as well as aqueous liquid streams containing metal halides, generally utilize the methods of pyrohydrolysis or sulfation depending upon the source and thus the constituents of the waste stream. U.S. Pat. No. 4,325,935 to Krepler relates a method of producing hydrofluoric acid from a solution of heavy metal fluorides by contacting with water vapor at elevated temperature and pressure. There is no teaching as to sulfation of the waste.
Most methods of disposing of HF involve removal of HF from gaseous streams by in-line scrubbing with lime water, an aqueous calcium hydroxide system. In this system, insoluble CaF.sub.2 is formed from the contacting of the HF with the aqueous slurry of CaO. Commercial operating plants utilizing HF generally provide such an in-line gas scrubbing system which captures HF expelled from various points in the process. The CaF.sub.2 sludge is not usually treated to recover and regenerate the HF. Although this scrubbing system may prevent environmental HF pollutant problems, it does represent a loss of HF and requires the mining of more fluorspar, CaF.sub.2, to replace the loss.
Pyrohydrolysis involves subjecting the industrial wastes to high temperature in the presence of water vapor to convert some metal halides to the halogen acid (HF or HCl) and the corresponding metal oxides. However, the specific ability to regenerate the acids and the process steps and parameters involved are almost wholly dependent upon the character and complexity of the starting waste stream. Moreover, the level of halogen recovery depends in large part on the susceptibility of the particular metal halides to conversion. For example, Si, present in aqueous acid waste liquors as fluorosilicic acid, H.sub.2 SiF.sub.6, will pyrohydrolyze according to the following formula: EQU H.sub.2 SiF.sub.6 (aqueous).fwdarw.2HF+SiF.sub.4 +(H.sub.2 O gas) (i) EQU SiF.sub.4 +2H.sub.2 O.fwdarw.4HF+SiO.sub.2. (ii)
However, to achieve total fluoride recovery, the pyrohydrolytic conditions involve heating the liquor to temperatures around 1000.degree. C., at ambient pressure, and contacting the liquor with a stoichiometric excess of water vapor. In addition, calcium and magnesium halides from aqueous feed solutions will not pyrohydrolyze to their respective oxides at any reasonable temperature, e.g. below about 1200.degree. C. U.S. Pat. No. 3,511,603 to Yaws teaches a method for the production of anhydrous hydrogen fluoride from aqueous fluorosilicic acid by decomposing the fluorosilicic acid, fluorinating a metal oxide of iron, copper, nickel, or chromium with the aqueous hydrogen fluoride, and then defluorinating the metal oxide for recycling and producing the anhydrous hydrogen fluoride. The defluorination step involves contacting the metal fluoride with steam at an elevated temperature. U.S. Pat. No. 3,852,430 to Lienau describes a process of regenerating a halogen halide, in particular HCl, and the corresponding metal oxides from the potash industry and titanium ore processing waste streams. This patent teaches preconcentrating the aqueous solution prior to subjecting the waste stream to pyrohydrolysis.
Calcium and sodium halides are generally treated by sulfation to produce the halide acid and the corresponding metal sulfate. Sulfation involves the contacting of certain metal halides with sulfur dioxide, oxygen and water vapor at elevated temperatures to produce the halide acid and the corresponding metal sulfate. Generally, sulfation is taught to occur at lower temperatures than pyrohydrolysis. None of the prior art references teach regeneration of HF and HCl or mixtures thereof by the competing reactions of a complex aqueous leach solution subjected to both pyrohydrolysis and sulfation. Moreover, none teach that pyrohydrolysis and sulfation can be achieved in a single reactor under one set of conditions to produce the metal oxides and metal sulfates and the corresponding HF and HCl gas.
It is apparent that there is a need for a method of regenerating HF and HCl and mixtures thereof from complex industrial waste streams utilizing a single regeneration unit. The difficulty presented, however, is that when multiple metal halides, i.e. different metal fluorides and/or different metal chlorides, are present in the spent aqueous leach liquor, there are competing, simultaneous reactions during both pyrohydrolysis and sulfation because the metal halides consume common reactant(s) (H.sub.2 O during pyrohydrolysis and H.sub.2 O, SO.sub.2 and O.sub.2 during sulfation), and produce a common product (HF/HCl). Additionally, the equilibrium constants for the reaction of each metal halide differ and thus the temperature necessary to drive one reaction toward HF/HCl production may cause another reaction to convert back to the halide salts.
None of the known references suggest that pyrohydrolysis and sulfation can be achieved at the same time in a single reactor under one set of conditions to produce the metal oxides and metal sulfides and the corresponding HF and HCl gas. Thus, one part of the present invention advantageously teaches methods of producing HF and HCl and mixtures thereof, while producing an environmentally acceptable calcine, suitable for disposal without additional treatment. The methods of the present invention have applicability to a variety of commercial industries using these acids in their processes.
The present invention also solves the problems of producing a clean coal, suitable for use as an alternative fuel source, by providing an integrated and simplified system of manufacturing such a coal economically. None of the references teach or suggest an overall system for cleaning coal wherein substantially all of the fluorine values throughout the process except for that reporting to waste as MgF.sub.2 are recaptured and converted to HF, wherein a mixed acid leach is used and regenerated in substantially the same ratio of HF to HCl and wherein the entire process requires only inexpensive CaF.sub.2 and NaCl as halide make-up reagents. The purged coal of the present invention, when finely-ground, is usable not only as a substitute for petroleum fuels, for example, as a coal water mixture, but may also substitute for activated carbon, or as a feedstock for carbon black, electrode carbon, and various chemical processes.