PROCESS FOR UPGRADING COAL

Abstract: In a process for reducing the sulfur and ash contents of coal, coal is sequentially contacted with fused alkali metal caustic, wash water, and acid. Contacting the coal with the caustic produces water-soluble compounds. Sufficient wash water is used to reduce the temperature of the caustic treated coal and dissolve the bulk of the water-soluble compounds before the water-soluble compounds convert to water-insoluble compounds that precipitate on the caustic-treated coal. Caustic removed from the coal by the water is recovered as anhydrous caustic for again contacting coal.

Description: This invention is directed to a process for reducing the sulfur and ash contents of coal. The United States, with nearly one-half trillion metric tons of coal reserves, has the largest total coal reserves in the world. Increased coal utilization has been hampered by environmental constraints, such as restrictions on sulfur dioxide, nitrogen oxides, and particulate emissions. There is a need for new technology to meet those environmental constraints at costs acceptable to coal users. U.S. Patent 4,545,891 describes a process for upgrading, inter alia , coal where both sulfur and ash are to be removed, which process comprises mixing the coal containing substantial amounts of both sulfur and ash, with fused alkali metal caustic to remove more than 79% by weight of the sulfur from the coal and then washing the desulfurized coal with an aqueous solution of a mineral acid to remove over 90% by weight of the ash, the process being carried out at atmospheric pressure. The coal can be washed with water prior to treating it with the acid to remove any residual sulfur compounds and to recover the alkali caustic. U.S. Patent 3,166,483 describes a process for reducing the sulfur content of coking coal which comprises treating the coal for a period of up to 45 minutes in a molten alkali metal caustic bath at 200° to 450°C to form an alkali metal sulfide which is soluble in the bath, separating the coal particles from the bath, and re-using the separated bath material. The separated coal particles may be washed substantially free of bath material. We have now developed an improved process for reducing the sulfur and ash contents of coal, which process is based upon the discovery of the criticality of the period for which the caustic-treated coal is subjected to water washing and of the temperature used in this step. By means of this improved process, mined coal can be processed to yield a product coal containing less than 0.1% ash and less than 0.5% sulfur. Further, most of the caustic used in the process can be recovered and recycled for reuse. This is important for both economic and environmental reasons. According to the present invention, there is provided a process for reducing the sulfur and ash contents of a feed coal containing sulfur and mineral matter, which process comprises treating the feed coal in a reaction zone with fused alkali metal caustic at an elevated temperature to remove sulfur and mineral matter from the feed coal and to give a mixture of (i) treated coal and (ii) water - soluble compounds comprising alkali metal, sulfur and mineral matter, washing the mixture of (i) and (ii) with water in a water wash zone, and separating the treated coal from spent wash water, the separated coal having lower sulfur and ash contents than the feed coal, characterised in that the residence time of the treated coal in the water wash zone is less than 2 hours and the temperature in the water wash zone is maintained at less than 93°C (200°F). The temperature in the water wash zone should be maintained at less than 93°C (200°F) and the residence time in the water wash zone should be short, that is less than 2 hours, so that the water-soluble compounds derived from the sulfur and ash present in the feed coal should remain soluble. If the temperature is too high and/or the residence time is too long, these water-soluble compounds convert to water-insoluble compounds that precipitate on to the caustic-treated coal. Preferably at least 80%, and more preferably at least 90% by weight of such water-soluble compounds are dissolved in the wash water. Optimally, substantially all of the water-soluble compounds are dissolved in the wash water. The temperature in the water wash zone is preferably maintained at from 60°C (140°F) to less than 93°C (200°F), and more preferably less than 82°C (180°F) to minimize precipitation of the water-soluble compounds. An effective amount of wash water for dissolving the water-soluble compounds and cooling the coal is an amount from 1 to 20, preferably from 2 to 10, and more preferably from 3 to 6, parts by weight wash water per part by weight caustic-treated coal. It has been found that the bulk, i.e. at least 50%, of the water-soluble compounds can be dissolved in the wash water if the residence time in the water wash zone is less than 2 hours, and preferably less than 1 hour, and most preferably about 1/2 hour. With a sufficiently short residence time for the coal and a sufficiently low temperature in the water wash zone, at least 70%, preferably at least 80%, and more preferably at least 90% by weight of the silicon and aluminum in the feed coal are removed by the water wash step. Most preferably substantially all of the silicon and aluminum are removed in the water wash step. Likewise, preferably at least 70%, more preferably at least 80%, and most preferably at least 90% by weight of the sulfur that is removed from the feed coal is removed by the water wash step. Preferably the water wash zone comprises at least two separate countercurrent stages in series, with the caustic-treated coal and the water-soluble compounds being introduced into the first stage and the wash water being introduced into the last stage. The coal and the wash water pass through the stages countercurrently. Preferably each stage comprises a mixing zone for mixing incoming coal and water and a separation zone for separating the mixture into washed coal and water. Preferably there are least five of these stages. The washed coal is separated from the spent wash water, the separated coal having a sulfur content lower than the sulfur content of the feed coal and an ash content lower than the ash content of the feed coal. The separated coal is then preferably treated with acid, such as sulfuric acid, to remove additional mineral matter. The product coal preferably contains less than 0.1% ash and less than 0.5% sulfur. Regeneration of the caustic can be accomplished by treating the spent wash water with a calcium containing material to yield an aqueous caustic and a calcium carbonate precipitate. By removing substantially all of the water from the aqueous caustic, a substantially anhydrous alkali metal is produced for recycle to the reaction zone. By this process, the bulk of the alkali-metal that reacts with the mineral matter and sulfur is washed from the coal with the wash water rather than the acid. This is advantageous because it is much easier to regenerate the caustic from spent wash water than from a spend acid solution. An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawing which schematically shows a preferred process embodying the present invention. A process according to the present invention as shown in the accompanying drawing generally comprises a coal-caustic reaction zone 10, a water wash zone 20, an acid wash zone 75, a caustic recovery zone 85, and a spent acid treatment zone 120. Coal-Caustic Reaction Zone Feed coal 8 is fed to a coal-caustic reaction zone 10. Preferably the feed coal 8 is upgraded before entering the coal-caustic reaction zone 10. For example, the coal can be crushed and sized to a dimension of less than about 9.525mm (3/8 inch). Preferably the feed coal 8 is physically cleaned by well known methods such as water washing, flotation separation, etc., to about 10% ash and from 2 to 4% sulfur. Although high ash and/or high sulfur content coal can be crushed and used directly in the process, the process is more efficient with cleaned coal. The sulfur in the coal is usually in the form of pyritic and organic sulfur. The ash-forming mineral matter in the coal usually comprises clays, shales, and pyrite, but also comprises lesser amounts of minerals containing substantially every chemical element known. The feed coal 8 is combined with make-up alkali metal caustic 12 in the coal-caustic reaction zone 10. Caustic materials suitable for this invention include alkali metal caustics. The alkali metal can be selected from Group IA metals of the periodic table, with the hydroxides of sodium and potassium being the preferred alkali metal caustic since they can easily be regenerated. Sodium hydroxide is usually used because of its low cost and availability. Alternatively a mixture of sodium hydroxide and potassium hydroxide can be used. The caustic 12 can be introduced as a dry powder or molten. The coal-caustic reaction zone 10 is maintained at an elevated temperature at which the caustic material is in a fused or molten state. When the coal contacts molten caustic, sulfur and mineral matter are removed from the coal and become dissolved or suspended in the caustic. The sulfur dissolves in the caustic mainly as alkali-metal sulfides. The ash-forming mineral matter dissolves in the form of aliminates silicates, ferrites, and the like. It is believed that among the reactions that occur are the following qualitative reactions: (1) Al₂O₃ SiO₂ + H₂O + NaOH --> NaAl(OH)₄ + NaSi(OH)₅ (2) FeS₂ + NaOH --> Fe x (OH) y + Na₂S (3) Coal - S + NaOH --> Coal(OH)₂ + Na₂S Equation (1) shows the reaction of the major coal mineral component, alumino silicate, with molten caustic to form sodium salts of aluminum and silicon. Equation (2) shows the reaction of the other major coal mineral component, iron pyrite, with molten caustic to form soluble iron hydroxides and sodium sulfide. Equation (3) shows the reaction of the organic sulfur content of coal with sodium hydroxide to form oxygenated coal and sodium sulfide. The temperature in the coal-caustic reaction zone 10 is important. Preferably the temperature is from 280°C to 425°C. At temperatures less than about 280°C, the extraction of ash and sulfur is slow and incomplete. At temperatures higher than about 410°C, the coal can lose a substantial amount of its volatiles in coking reactions. The preferred temperature is from 325 to 400°C, and more preferably at about 370°C. Reactor pressure does not appear to have a significant effect on the coal-caustic reaction. Therefore the reaction can be carried out at atmospheric pressure. A small positive pressure can be maintained in the coal-caustic reaction zone 10 to help the reactants flow downstream. A small positive pressure also works to keep air out of the reaction zone 10. The reaction can take place in an inert atmosphere such as under a nitrogen blanket, which can be introduced concurrently or countercurrently with the coal. The residence time in the coal-caustic reaction zone 10 can be as short as 5 minutes and still give some sulfur and mineral matter extraction. Typically the reactor residence time is from 1 hour to 4 hours. However, when a microwave heat source is used, the residence time can be reduced to within the range of from 1 to 5 minutes. Preferably the mass ratio of caustic to coal is from 1 to 20 parts by weight caustic per one part by weight of coal. In some applications, the amount of alkali metal caustic fed to the coal-caustic reaction zone 10 needs to be sufficient to form a free flowing slurry with the coal. For those applications, a caustic/coal mass ratio of at least about 4:1 is used because at lower ratios, the coal-caustic mixture is not fluid. At ratios less than about 4:1, the mixture in the reaction zone 10 is "putty-like". Preferably the caustic to coal mass ratio is less than about 20:1 for an economical process and to have a small reactor size. The smaller the ratio, the smaller the reactor size. A typical caustic/coal ratio used in reactor 10 is no more than about 10:1. Reactors of various designs can be used in the coal-caustic reaction zone 10. There can be a single reactor, or multiple reactors set up in stages. The flow of caustic and coal can be cocurrent or countercurrent. Because coal is less dense than the molten caustic, the coal tends to float to the top of the reaction mixture. It is thus necessary to have good mixing in reaction zone 10 to ensure efficient and sufficient contact between the coal and the fused caustic. A suitable reactor for use in the coal-caustic reaction zone 10 is a rotary kiln reactor 14 as shown in the drawing. For this type of reactor, low ratios of caustic to coal can be used, on the order of 1 to 3 parts by weight caustic per one part by weight feed coal. An effluent mixture 15 comprising spent fused caustic and caustic treated coal exits the coal-caustic reaction zone 10. The spent fused caustic contains impurities such as sulfides, carbonates, and mineral matter in solution or in a suspended form. These impurities include water-soluble compounds comprising alkali metal, mineral matter, and sulfur. The caustic treated coal has reduced sulfur and mineral matter contents compared to the feed coal 8. When the caustic to coal mass ratio in the effluent mixture is greater than about 4:1, a portion of the spent caustic can be separated from the mixture 15 for recycle to the reactor 10. Separation can be effected by well known methods for solids/liquid separation, including pressure filters, vacuum filter, or a quiet zone with a separator. In the quiet zone, the effluent mixture 15 is left relatively undisturbed and the coal floats to the surface of the molten spent caustic due to the difference in densities between the coal and the caustic (the specific gravity of coal is 1.2-1.3 and the specific gravity of caustic is about 1.8 at reaction temperatures). In the separator the coal on top is skimmed or decanted off. It is desirable to maintain the sulfur concentration in any recycled spent fused caustic at from about 1 to 2%, and mineral matter concentration at from 3 to 5%. Higher sulfur and/or mineral matter concentrations in the recycled spent fused caustic adversely affect the extraction efficiencies in the coal-caustic reaction zone 10. A small ratio of spent fused caustic to coal is preferred in effluent stream 15. As will be explained later, a substantial portion of the caustic material in this stream is stripped of impurities and regenerated to produce clean caustic for reuse in the reaction zone 10. By limiting the amount of spent fused caustic in the effluent stream 15, it is possible to reduce the equilibrium concentration of ash and sulfur in the caustic in the reaction zone 10. Water Wash Zone The mixture of caustic treated-coal and spent fused caustic in the effluent stream 15 is then contacted with water in a water wash zone 20. This separates the spent fused caustic from the caustic-treated coal, producing water-washed coal 21 and a caustic-rich spent wash water 22. Proper operation of the water wash zone 20 is important to the effectiveness of the process according to this present invention. This is because the water-soluble compounds formed in the reaction zone 10 can revert to insoluble compounds, particularly at high temperatures. When these compounds insolubilize, they precipitate on the coal and are discharged from the water wash zone 20 with the water-washed coal 21 rather than the spent wash water 22. These precipitated compounds are either not extracted from the coal, or need to be extracted from the coal in the acid wash zone 75. To regenerate the acid for reuse in the process, it would then be necessary to remove the precipitated compounds from the spent acid. It is more expensive, more energy-consuming, and more difficult to remove the precipitated compounds from spent acid than it is from the spent wash water 22. Accordingly, it is important to operate the water wash zone to avoid such precipitation. In particular, the sodium salts of aluminum and silicon formed in reaction (1), although initially soluble in water or aqueous caustic, quickly revert to insoluble alumino silicate at elevated temperatures, if allowed to stand for excess amounts of time, or if provided a nucleating surface such as lime or calcium carbonate. Similarly, the sodium sulfide produced in reactions (2) and (3) precipitates upon standing, heating of the liquid, or addition of a nucleating surface. Accordingly, the water wash zone is operated to maintain short residence time and low temperatures. The residence time of the coal in the water wash zone 20 is less than 2 hours and is preferably less than 1 hour, and is optimumly about 1/2 hour. The temperature throughout the water wash zone is less than 93°C (200°F) and preferably less than 82°C(180°F). As will be discussed below, the water added in the water wash zone 20 is removed in order to generate clean dry caustic for reuse in coal-caustic reaction zone 10. Water removal consumes energy. Therefore it is preferable that a minimum amount of wash water be used, subject to the requirement that the temperature in the water wash zone 20 be sufficiently low to minimize precipitation of the water-soluble compounds formed in the reaction zone 10. The amount of water used is preferably from 1 to 20 parts by weight water per part by weight feed coal, more preferably from 2 to 10, and most preferably from 3 to 6 parts by weight water per part by weight coal. As shown in the drawing, preferably a countercurrent staged water wash system is used. In each stage, coal is slurried with water, and the slurry is separated into wet coal and a liquid, the coal being sent to the following stage and the liquid to the prior stage. The final wash stage can use essentially pure water, such as a condensate 106 from a water removal zone 104 (described below) and make-up water 24. Optionally, dilute aqueous caustic generated in other parts of the process, such as a side stream 107 taken off from stream 102 from a caustic regenerator 94, can be added to the wash water in any wash stage but the final stage. Stream 107 contains 5 to 10% aqueous caustic and is destined for the water removal zone 85, as will be explained later. Bypassing the water removal step saves energy. A preferred water wash zone 20 as shown in the drawings include seven sequential stages 27A, 27B, 27C, 27D, 27E, 27F, and 27G. The make-up water 24 and recycle water 106 added to the system are added to the last stage 27G and the caustic-treated coal 15 is added to the first stage 27A. Each stage comprises a mixing zone for mixing incoming coal and water and a separation zone for separating the mixture into wash coal and water. The mixing zones are not shown in the drawing, but generally comprise a stirred tank. Preferably the separation zones for the first two stages 27A and 27B are rotary drum vacuum filters. The separation zones for the last five stages 27C-27G are centrifuges. The temperature of the coal tends to be the highest in the first stage 27A because of the high temperature of the coal and the caustic effluent 15 exiting the reaction zone 10, and the exothermic heat of solution resulting from dissolving the caustic in water. To maintain the temperature in the first stage below 93°C (200°F), and preferably below 82.2°C(180°F), cooling is preferably used in the first stage. This cooling can be provided by an evaporative condenser used on the first stage mixing vessel. Alternatively, the first stage mixing vessel can be provided with a pump-around loop which extracts a water-coal-caustic slurry from the vessel, pumps the slurry through a water coded heat exchanger, and then the slurry can be reintroduced into the vessel. It has been found that the combination of (1) short residence time in all the vessels, less than about 1/2 hour, (2) temperatures maintained below 82.2°C(180°F),and (3) stirring to keep the coal solids in a homogenous dispersion prevents water-soluble mineral matter from redepositing on the coal, and thus allows the mineral matter to be removed from the coal with the wash water. Substantially all of the alumina and silica originally present in the coal can be extracted in the water wash step, with substantially no deposition of these minerals on the coal. By proper operation of the water wash zone, at least 70%, preferably at least 80%, and more preferably at least 90% by weight of the silicon and aluminum in the feed coal are removed in the water wash step. Most preferably substantially all of the silicon and aluminum are removed in the water wash step. Likewise, preferably at least 70%, more preferably at least 80%, and most preferably at least 90% of the sulfur that is removed from the feed coal is removed by the water wash step. In other words, for every ten parts of sulfur removed from the feed coal, preferably at least about seven parts are removed by the wash water and no more than about three parts are removed by the acid wash. As the coal moves down the stages of the countercurrent wash system, it contains less and less ash. The ash content of a feed coal 8 can be decreased by about 40% from 11% ash to only about 7.5% ash through the water-wash step. Moreover, what is measured as "ash" is predominantly sodium or sodium and potassium oxides and iron oxide. The water-washed coal 21 contains only small quantities (only parts per million) of silica and alumina. Pressure does not appear to affect the water wash performance. To obviate the need for costly pressurized equipment, preferably the water wash is conducted at atmospheric pressure. Foaming can be a problem in the water wash system 20. Preferably a defoamer that is effective in alkaline solutions is used, such as the defoamers typically used for latex paints and coating systems. A preferred defoamer is Foamkill® 608 from Crucible Chemical Company of Greenville, South Carolina. The spent wash water 22 can contain about 40% to about 60% by weight alkali metal caustic, including alkali metal sulfides that were present in the spent fused caustic/coal mixture 15. Alkali metal sulfides have a solubility of 1% or more in fifty percent aqueous caustic at elevated temperatures. The effluent spent wash water 22 also contains some water-insoluble mineral matter. The water-washed coal 21 preferably has a free caustic content of no more than about 5% by weight. More preferably, the free caustic content is less than about 1% by weight of the coal. The water-washed coal 21 also contains chemically bound alkali present in the caustic-treated coal. Water washing does not remove the chemically bound alkali metal species unless an economically infeasible number of water washes are used. Acid Wash Zone The water-washed coal 21 is passed to the acid wash zone 75 where the coal 21 is contacted with a strong mineral acid such as sulfuric acid to form acid-washed coal 77, the product of this process, and spent acid 78. The acid wash can be effected at room temperature and pressure. The residence time of the coal in the acid wash zone 75 is at least 10 minutes and can be as much as 20 minutes to insure that the bulk of the bound alkali on the coal is removed. The acid wash can be effected in one stage or in multiple stages, either cocurrently or countercurrently. As shown in the drawings, preferably three stages 79A, 79B, and 79C are used. Each stage comprises a mixing vessel (not shown) and a separator, which preferably is a centrifuge separator. Concentrated acid 80 and the water washed coal 21 are fed to the first stage 79A and dilution water 81 is fed to the third stage 79C. The acid-washed coal 77 is removed from the third stage 79C and the spent acid is removed from the first stage 79A. The acid extraction and treatment of the coal principally occurs in the first stage 79A, and the second 79B and third 79C stages are used primarily for water washing the acid off of the acid-treated coal. The acid-washed coal 77, which is the product from the process, contains substantially no free caustic. The acid used can be an organic acid, or sulfuric, sulfurous, nitric, or hydrochloric acid. Preferably, concentrated sulfuric acid 80, about 97%, is introduced into the first stage 79A of the acid wash zone 75, with the dilution water 81 added to the third stage 29C raising the pH in the first stage 79A 1 to 2. The efficiency of acid removal of bound alkali from the coal 21 is pH dependent. The removal is more complete at lower pH's. Preferably the mixture in the acid wash zone 75 is maintained at a pH of no more than about 2, and preferably at about 1. The sulfuric acid used can be generated from sulfur recovered from the wash water 22, using sulfur removed from the feed coal. This process generated sulfuric acid is preferred because of its low cost. The quantity of acid 81 and water 81 used is an amount sufficient to form a free flowing slurry. Preferably from 5 to 10 parts by weight diluted acid is used in the first stage 79A per part by weight coal. The product coal 77 typically has an ash content (mineral matter plus bound alkali) of less than about 0.1% by weight, and preferably contains less than about 0.5% by weight sulfur. Caustic Recovery Zone The caustic-rich spent wash water 22, containing from 40 to 60% by weight alkali metal caustic and insoluble mineral matter, enters a caustic regeneration zone 85 for regenerating the caustic for use in the coal-caustic reaction zone 10. A caustic insoluble solid 93 is added to the spent wash water 22 in a regeneration vessel 94 in the caustic regeneration zone 85 to initiate precipitation of mineral matter and sulfides from the supersaturated spent wash water 22. Preferably the caustic insoluble solid is a calcium containing material such as slaked lime; limestone; or quick lime produced by thermal decomposition of limestone. The amount of calcium salt 93 added is preferably in the range of 1/4 weight to one weight of salt per weight of dissolved mineral matter and sulfide. The precipitation reaction works best with these concentrations. Calcium carbonate, and silicon, aluminum, and sulfur compounds are precipitated, and the precipitate 95 is removed by well known solid/liquid separation methods such as filtering in a centrifuge filter 98. A storage vessel 96 can be provided between the regeneration vessel 94 and the centrifuge 98. Regenerated dilute aqueous caustic 102 is discharged from the centrifuge 98 and is fed to a water removal zone such as an evaporator 104. A portion 107 can be used as wash water in the water wash zone 20. A clean substantially anhydrous alkali metal caustic exits the water removal zone 104, is stored in a vessel 108, and is returned to the reaction zone 10 for reuse. It can be provided as a solid or liquid. Spent Acid Treatment Zone When the acid used in the acid wash zone 75 is sulfuric acid, the following steps can be taken to treat the spent acid 78 in the spent acid treatment zone 120. The spent acid 78 contains a small amount of alkali metal sulfates and dilute sulfuric acid. It can be neutralized with lime 122 in a spent acid neutralization zone 124 to form a precipitate 126 comprising calcium sulfate, and iron, sodium, potassium compounds. The precipitate 124 can be separated and discharged for disposal by methods such as landfill. The waste water 128 contains a small amount of alkali-metal sulfates, and can safely be discharged. Alternatively, the spent sulfuric acid can be electrolyzed to produce dilute caustic and regenerated sulfuric acid. Advantages of the Process The present invention has important advantages over prior art methods using fused caustic to treat coal for removal of sulfur and mineral matter. The invention allows for recycle of most of the caustic used. Recycling of caustic is important for both economic and environmental reasons. Make-up caustic can be expensive. Disposal of waste caustic material can also be costly and difficult from an environmental protection standpoint. The process of this invention allows recycle to be effected economically. As discussed above, it was discovered that water-soluble mineral matter, including water-soluble compounds containing caustic, can be effectively removed from the caustic treated coal with wash water. This result is achieved by obtaining short residence times and relatively low temperatures in the water wash zone. Without this feature of this invention, the bulk of the alkali would have to be removed from the caustic-treated coal with an acid such as carbonic acid. Not only is this expensive, requiring a carbonic acid wash step in addition to a sulfuric acid step, but regeneration of a clean caustic from sulfuric acid wash effluent is much more difficult and expensive than regeneration of caustic from a water wash effluent. Effective recycling of caustic achieved with this invention reduces pollution problems. For example, if the sulfuric acid wash removed the bulk of the caustic from the caustic-treated coal 15 rather than the water wash, the solid calcium sulfate in stream 126 for disposal would contain soluble alkali metal compounds, which could leach from the solids sulfate after disposal. But if most of the alkali is removed from the caustic-treated coal in the water wash zone 20, the spent acid stream 78 contains very little soluble alkali species. In turn, the calcium sulfate stream 126 also contains less alkali metal compounds, and the leaching problem is abated. Therefore it is important to remove as much of the alkali as possible at the water wash step. The process of this invention produces either a crushed coal product or a coal water mixture suitable for pumping. The process is especially attractive because the product coal can be used both to replace oil in boilers, turbines and diesel engines, and as an alternative to flue gas desulfurization for coal fired utilities and industrial boilers. The process serves as a strategic response to potential interruptions in imported oil supplies, and provides for the inevitable shortfall in oil production as world oil reserves begin to run out toward the beginning of the 21st century. Moreover, all of the steps of the process can be conducted at substantially atmospheric pressure. No expensive pressurized equipment is needed. EXAMPLE 140 grams of a caustic/coal mixture containing 40 grams of Pittsburgh No. 8 coal and 100 grams of a 50:50 mixture of NaOH and KOH were prepared. The mixture was heated in a tube nickel reactor (3 inches diameter x 12 inches high) at 370°C for 60 minutes in a nitrogen atmosphere. The reaction time was sufficient to remove essentially all of the sulfur and ash from the coal. Two dissolution experiments were conducted wherein 50 grams each of the reacted caustic/coal mixture was placed in 500-milliliter Ehrlenmeyer flasks and 100 milliliters of water was added to each flask. The mixtures were stirred with magnetic stirrers and treated independently as described below in Conditions 1 and 2: In Condition 1, a cooling system was used to maintain temperature between 82.2 and 93.3°C (180 and 200°F) for approximately 20 minutes. A small amount of makeup water was added to maintain the original volume. At the conclusion of the 20 minute dissolution period, the mixture was immediately filtered through Whatman No. 541 filter paper contained in a 2.5 inch diameter Buchner funnel attached to a 500-mL filtration flask. A 10-mL aliquot of the filtrate was taken and sent to Warner Laboratories for silica and alumina analysis. In condition 2, the mixture was allowed to rise in temperature due to the heat of solution of caustic to 93.3 to 99°C (200 to 210°F) and stirred for approximately 120 minutes. A small amount of makeup water was periodically added to maintain the original volume. The mixture was immediately filtered as for Condition 1 and a 10-mL aliquot of filtrate was taken and sent to Warner Laboratories for analysis of silica and alumina. The data in Table I gives the percentage of silica and alumina present in the filtrates from Conditions 1 and 2. Theoretically, if all of the alumina and silica remains in the water wash and none redeposits on the coal, the filtrate would contain 0.66% silica and 0.32% alumina. The extent to which the actual analyses drop below the theoretical figure indicates the extent to which the alumina and silica redeposit on the coal during the wash. Such a process of redeposition leads to coal containing more ash. The date shows that under Condition 2, approximately half of the silica and approximately 4/5 of the alumina failed to stay in the filtrate and thus redeposited on the coal. This experiment therefore shows that the coal ash deposits back on the coal during the water dissolution step unless the mixture is cooled and the residence time of the coal in the water is limited.

Claims: A process for reducing the sulfur and ash contents of a feed coal containing sulfur and mineral matter, which process comprises treating the feed coal in a reaction zone with fused alkali metal caustic at an elevated temperature to remove sulfur and mineral matter from the feed coal and to give a mixture of (i) treated coal and (ii) water - soluble compounds comprising alkali metal, sulfur and mineral matter, washing the mixture of (i) and (ii) with water in a water wash zone, and separating the treated coal from spent wash water, the separated coal having lower sulfur and ash contents than the feed coal, characterised in that the residence time of the treated coal in the water wash zone is less than 2 hours and the temperature in the water wash zone is maintained at less than 93°C (200°F). A process according to claim 1, characterised in that the temperature in the water wash zone is maintained at from 60° to less than 93°C (140° to 200°F). A process according to claim 1 or 2, characterised in that the residence time of the treated coal in the water wash zone is from one half to less than 2 hours. A process according to any of claims 1 to 3, characterised in that from 1 to 20 parts by weight of wash water are used per part by weight of treated coal. A process according to claim 4, characterised in that from 2 to 10 parts by weight of wash water are used per part by weight of treated coal. A process according to any of claims 1 to 5, characterised in that it comprises the additional step of treating the separated coal with acid to remove additional mineral matter from the separated coal. A process according to any of claims 1 to 6, characterised in that the water wash zone comprises at least two separate countercurrent stages in series, the treated coal being introduced into the first stage and the wash water being introduced into the last stage, and the coal and wash water passing through the stages countercurrently. A process according to any of claims 1 to 7, characterised in that the spent wash water contains carbonates and in that the process comprises the additional steps of recovering substantially anhydrous alkali metal caustic by the steps of: (i) treating the spent water with a calcium containing material to give an aqueous caustic and a calcium carbonate precipitate; (ii) separating the aqueous caustic from the precipitate; and (iii) removing substantially all the water from the aqueous caustic to yield substantially anhydrous alkali metal caustic. A process according to any of claims 1 to 8, characterised in that the mineral matter in the feed coal comprises compounds of aluminium, silicon, and sulfur, and the residence time of the treated coal in the water wash zone is sufficiently short so that at least 70% by weight of the aluminium in the feed coal and at least 70% by weight of the silicon in the feed coal are present in the spent wash water separated from the treated coal.

Inventors: HART, WALTER D., MCCLANATHAN, LOREN C., MEYERS, ROBERT A.
Applicants: TRW INC.
Application year: 1990
Main classification: C10L9
IPC codes: C10L9, C10L10