Patent Grant
10562038
The instant invention pertains to methods of cleaning fine coal of its impurities in aqueous media and removing the process water from both the clean coal and refuse products to the levels that can usually be achieved by thermal drying.
Coal is an organic material that is burned to produce heat for power generation and for industrial and domestic applications. It has inclusions of mineral matter and may contain undesirable elements such as sulfur and mercury. Coal combustion produces large amounts of ash and fugitive dusts that need to be handled properly. Therefore, run-of-the mine coal is cleaned of the mineral matter before utilization, which also helps increase combustion efficiencies and thereby reduces CO2 emissions. In general, coarse coal (50×0.15 mm) can be cleaned efficiently by exploiting the specific gravity differences between the coal and mineral matter, while fine coal (approximately 0.15 mm and smaller) is cleaned by froth flotation.
In flotation, air bubbles are dispersed in water in which fine coal and mineral matter are suspended. Hydrophobic coal particles are selectively collected by a rising stream of air bubbles and form a froth phase on the surface of the aqueous phase, leaving the hydrophilic mineral matter behind. Higher-rank coal particles are usually hydrophobic and, therefore, can be attracted to air bubbles that are also hydrophobic via a mechanism known as hydrophobic interaction. The clean coal product reporting to the froth phase is substantially free of mineral matter but contains a large amount of water. Wet coal is difficult to handle and incurs high shipping costs and lower combustion efficiencies. Therefore, the clean coal product is dewatered using various devices such as cyclones, thickeners, filters, centrifuges, and/or thermal dryers. In general, the cost of dewatering increases with decreasing particle size and can become prohibitive with ultrafine particles, e.g., finer than 44 μm. In such cases, coal producers are forced to discard them. Large amounts of fine coal have been discarded to numerous impoundments worldwide, creating environmental concerns.
Many investigators explored alternative methods of cleaning fine coal, of which selective agglomeration received much attention. In this process, which is also referred to as oil agglomeration or spherical agglomeration, oil is added to an aqueous suspension while being agitated. Under conditions of high-shear agitation, the oil breaks up into small droplets, collide with coal particles, spread on the surface, form pendular bridges between different coal particles, and produce agglomerates. Nicol, et al. (U.S. Pat. No. 4,209,301) found, that adding oil in the form of unstable oil-in-water emulsions can produce agglomerates without intense agitation. The agglomerates formed by these processes are usually large enough to be separated from the mineral matter dispersed in water by simple screening. Further, selective agglomeration gives lower-moisture products and higher coal recoveries than froth flotation. On the other hand, it suffers from high dosages of oil.
The amounts of oil used in the selective agglomeration process are typically in the range of 5 to 30% by weight of feed coal (S. C. Tsai, in Fundamentals of Coal Beneficiation and Utilization, Elsevier, 1982, p. 335). At low dosages, agglomerates have void spaces in between the particles constituting agglomerates that are filled-up with water, in which fine mineral matter, e.g., clay, is dispersed, which in turn makes it difficult to obtain low moisture- and low-ash products. Attempts were made to overcome this problem by using sufficiently large amounts of oil so that the void spaces are filled-up with oil and thereby minimize the entrapment of fine mineral matter. Capes et al. (Powder Technology, vol. 40, 1984, pp. 43-52) found indeed that the moisture contents were in excess of 50% by weight when the amount of oil used was less than 5%. By increasing the oil dosage to 35%, the moisture contents were substantially reduced to the range of 17-18%.
Keller et al, (Colloids and Surfaces, vol. 22, 1987, pp. 37-50) increased the dosages of oil to 55-56% by volume to fill up the void spaces more completely, which practically eliminated the entrapment problem and produced super-clean coal containing less than 1-2% ash. However, the moisture contents remained high. Keller (Canadian Patent No. 1,198,704) obtained 40% moisture products using fluorinated hydrocarbons as agglomerants. Depending on the types of coal tested, approximately 7-30% of the moisture was due to the water adhering onto the surface of coal, while the rest was due to the massive water globules trapped in the agglomerates (Keller et al., Coal Preparation, vol. 8, 1990, pp.1-17).
Smith, et al. (U.S. Pat. No. 4,244,699) and Keller (U.S. Pat. No. 4,248,698; Canadian Patent No. 1,198,704) used fluorinated hydrocarbon oils with low boiling points (40-159° F.) so that the spent agglomerants can be readily recovered and be recycled. These reagents are known to have undesirable effect on the atmospheric ozone layer. Therefore, Keller (U.S. Pat. No. 4,484,928) and Keller, et al, (U.S. Pat. No. 4,770,766) disclosed methods of using short chain hydrocarbons, e.g., 2-methyl butane, pentane, and heptanes as agglomerants. These reagents also have relatively low boiling points, allowing them to be recycled.
Being able to recycle an agglomerant would be a significant step toward eliminating the barrier to commercialization of the selective agglomeration process. Another way to achieve this goal would be to substantially reduce the amount of the oils used. Capes (in Challenges in Mineral Processing, ed. by K. V. S. Sastry and M. C. Fuerstenau, Society of Mining Engineers, Inc., 1989, pp. 237-251) developed the low-oil agglomeration process, in which the smaller agglomerates (<1 mm) foimed at low dosages of oil (0.5-5%) are separated from mineral matter by flotation rather than by screening. Similarly, Wheelock et al., (U.S. Pat. No. 6,632,258) developed a method of selectively agglomerating coal using microscopic gas bubbles to limit the oil consumption to 0.3-3% by weight of coal.
Chang et al. (U.S. Pat. No. 4,613,429) disclosed a method of cleaning fine coal of mineral matter by selective transport of particles across the water/liquid carbon dioxide interface. The liquid CO2 can be recycled. A report shows that the clean coal products obtained using this liquid carbon dioxide (LICADO) process contained 5-15% moisture after filtration (Cooper et al., Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, 1990, August 12-17, 1990, pp. 137-142).
Yoon et al. (U.S. Pat. No, 5,459,786) disclosed a method of dewatering fine coal using recyclable non-polar liquids. The dewatering is achieved by allowing the liquids to displace surface moisture. Yoon reports that the process of dewatering by displacement (DBD) is capable of achieving the same or better level of moisture reduction than the nal drying at substantially lower energy costs, but does not show the removal of mineral matter from coal.
It is an object of the invention to provide a method of cleaning fine coal suspended in water of its mineral matter and simultaneously dewatering the clean coal product by displacing the water adhering to the surface of coal with, a hydrophobic liquid. It is also an object to remove the water entrapped in between the fine particles by subjecting the particulate material to a high-shear agitation in a gaseous phase. In this invention, fine coal refers to coal containing particles mostly smaller than 1 mm in diameter, but the most significant benefits of this invention can be realized with fine coal containing particles less than 0.25 mm.
According to the invention, a hydrophobic liquid is added to an aqueous medium, in which fine coal is dispersed, and the suspension (or slurry) is agitated. Addition of the hydrophobic liquid can take place when the suspension (or slurry) is being agitated. The hydrophobic liquid is chosen so that its contact angle on the coal surface, as measured through the aqueous phase, is larger than 90°. Use of such a liquid allows coal particles to be engulfed (or transported) into the hydrophobic liquid phase, leaving hydrophilic mineral matter in the aqueous phase. The amount of the hydrophobic liquid to be added should be large enough so that all of the recoverable coal particles can be engulfed (or immersed) into the hydrophobic liquid phase. The coal particles engulfed into the hydrophobic liquid phase are essentially dry because the water in contact with the hydrophobic surface is displaced spontaneously by the hydrophobic liquid during the process of engulfment. However, the dewatering by displacement (DBD) process has a problem in that significant amounts of the process water can be entrained into the organic phase in the fowl of water drops stabilized by hydrophobic coal particles. It is well known that particles with contact angles larger than 90° stabilize water drops in oil phase forming a water-in-oil emulsion (Binks, Current Opinion in Colloid and Interface Science, vol 7, 2002, pp. 21-41). It has been found that much of the water entrained into the hydrophobic liquid phase is present as large globules.
As noted by Keller et al. (Coal Preparation, vol. 8, 1990, pp. 1-17), large globules of water are also formed in conventional oil agglomeration processes, in which the amounts of oil added to aqueous slurry of fine coal are in the range of 5 to 56% by volume (a similar range may be used in the practice of the instant invention; however, other ranges might be used, e.g., 5 to 56% by weight, more than 20% by volume or weight, less than 20% by volume or weight, etc). Obviously, the water-in-oil emulsions are still being formed during oil agglomeration processes, which may be an explanation for the high moistures of the clean coal products obtained from these processes.
The hydrophobic liquid containing dry coal particles and entrained water as water-in-oil emulsion is phase-separated from the aqueous phase containing hydrophilic mineral matter. In one embodiment of the present invention, the hydrophobic liquid is transferred to a size-size separator, such as screen, classifier, and/or cyclone, to remove the globules of water from the dry coal particles. The smaller size fraction (e.g., screen underflow) consists of the dry coal particles, while the larger size fraction (e.g., screen overflow) consists of the water globules stabilized by coal particles. If the dry coal yield is low, depending on the efficiency of the size-size separation and the size of coal, the larger size fraction can be re-dispersed in water and subjected to another set of agitation and screening to recover additional coal. In a continuous operation, the larger size fraction may be returned to the feed stream to allow the misplaced coal particles to have another opportunity to be recovered. In this embodiment, the larger globules of water can be readily removed. It would be difficult, however, to remove the smaller droplets stabilized by finer coal particles using the currently available size-size separation technologies, making it difficult to obtain effectively dry coal particles containing less than 1% moisture. If such low moistures are not desired, one can increase the cut size of the size-size separation step, e.g., by increasing the screen aperture, to obtain higher moistures, e.g., 5 to 10% by weight. The clean coal product which is now substantially free of mineral matter and surface moisture may then be subjected to a process, in which a small amount of residual hydrophobic liquid is recovered and recycled.
In another embodiment, the water droplets (or globules) are broken up using an appropriate mechanical means such as ultrasonic vibration so that the hydrophobic coal particles are detached from the water droplets (or globules) and dispersed in the hydrophobic liquid. The organic liquid phase in which the coal particles are dispersed is separated from the aqueous phase in which hydrophilic mineral matter is dispersed, and then subjected to appropriate solid-liquid separation means such as settling, filtration and/or centrifugation. The recovered hydrophobic liquid is recycled. The small amount of the hydrophobic liquid that may be adhering onto the surface of the hydrophobic particles (or solids) obtained from the solid-liquid separation step is also recovered and recycled using processes that may involve vaporization and condensation.
In still another embodiment, the hydrophobic liquid, in which dry coal and water globules are dispersed, is subjected to a solid-liquid separation using a centrifuge, filter, roller press, or other suitable separator. In this embodiment, the water-in-oil emulsions become smaller in size by expression and drainage, leaving only very small droplets of water trapped in between particles. In the instant invention, the entrapped interstitial water is released by disturbing the cake structure, in which the small droplets are entrapped, by high-shear agitation. The tiny water droplets may vaporize or exit the system. Thus, a combination of the solid-liquid separation involving expression and drainage and the additional step involving high-shear agitation allows the moisture contents to be reduced to less than 8% by weight, the levels that can usually be achieved by thermal drying. The extent of the moisture reduction can be achieved by controlling the process of high-shear agitation in terms of agitation intensity, duration, and devices employed.
The hydrophobic liquids used in most of the embodiments of the instant invention are recovered and recycled. Bulk of the liquid is recovered without involving phase changes, while only the small amount of the residual hydrophobic liquid adhering onto the surface of hydrophobic particles (e.g., coal) is recovered by vaporization and condensation. If the liquid has a boiling point below the ambient, much of the processing steps described above are carried out in pressurized reactors. In this case, the small amount of the residual hydrophobic liquid can be recovered in gaseous form by pressure release, which is subsequently converted back to liquid before returning to the circuit. If the boiling point is above the ambient, the hydrophobic liquid is recovered by evaporation. Thermodynamically, the energy required to vaporize and condense the recyclable hydrophobic liquids disclosed in the instant invention is substantially less than that required to vaporize water from the surface of coal particles,
It has been found that the high-shear dewatering (HSD) process can also be used for the clean coal product obtained by a process not involving the DBD or oil agglomeration process described in the instant invention, e.g., flotation. It is necessary, however, that the clean coal product be dewatered by filtration, centrifugation or any other method to produce a cake in which small droplets of water are trapped in between the coal particles. The HSD process can also be used to remove the water from a filter cake formed by hydrophilic particles such as silica and clay.
It is, therefore, an object of the invention to remove inorganic mineral matter from fine coal and simultaneously remove water from the product using a hydrophobic liquid. The invention may be practiced with different types of coal including without limitation bituminous coal, anthracite, and subbituminous coal.
It is another object of this invention to further reduce the moisture of clean coal product to the extent that they can be dried without using excessive heat.
It is still another object of the invention to further reduce the moisture of the particulate materials obtained using dewatering methods such as filtration, centrifugation, or expression, by subjecting them to high-shear agitation.
It is still another object to recover the spent hydrophobic liquid for recycling purposes.
These and other objects of the invention will be fully understood from the following description of the invention in reference to the figures attached hereto.
Two hydrophobic entities in an aqueous environment are attracted to each other. This is a phenomenon known as hydrophobic interaction. Thus, with reference to
The process of dewatering by displacement (DBD) may be depicted schematically by
dG/dA=γ12−γ13 [1]
where γ12 and γ13 are the interfacial tensions at the coal/hydrophobic liquid and coal/water interfaces, respectively. For the displacement process to be spontaneous, dG/dA must be less than zero.
γ12−γ13=γ23 cos θ [2]
in which γ23 is the interfacial tension between water and hydrophobic liquid. By combining these two equations, one obtains the following relationship:
dG/dA=γ23 cos θ<0 [3]
for the spontaneous displacement (dewatering) of water from the surface of coal. According to this relation, the free energy change becomes negative when θ>90°.
We measured the contact angles of n-alkanes on the polished surface of a bituminous coal sample from the Moss No. 3 coal preparation plant, Virginia. As shown in
The process described above can be used to simultaneously remove both the mineral matter and water from the coal particles dispersed in water. However, it has not been previously recognized that the process has an inherent problem of entrapping water into the clean coal products, as is the case with the selective agglomeration (or oil agglomeration) processes. We have already discussed two mechanisms of entrapping water: one is the entrapment of water in the void spaces formed between the particles constituting agglomerates, and the other is the formation of water-in-oil emulsions. The former may be addressed by using larger amounts of oil as suggested by Keller et at. (Colloids and Surfaces, vol. 22, 1987, pp. 37-50), while the latter can be addressed as disclosed in the present invention.
It is well known that colloidal particles with contact angles (θ), measured through the aqueous phase, that are close to 90° can readily adsorb at an oil-water interface and produce oil-in-water or water-in-oil emulsions (Kinks, Current Opinion in Colloid and Interface Science, vol. 7, 2002, pp. 21-41). For spherical particles, water-in-oil emulsions are formed when θ>90°, while oil-in-water emulsions are formed when θ<90°. The energy (E) required to detach a spherical particle of radius r from an oil/water interface, whose interfacial tension is γ23, is given by
E=πr2γ23(1 ±cos θ) [4]
The sign in the bracket is negative for the removal of particles into aqueous phase and positive for removal into oil phase. Eq. [4] suggests that if θ is slightly less than 90°, the particles will be held at the oil/water interface and stabilize oil-in-water emulsions. If θ is slightly above 90°, however, the particles will be held at the interface forming water-in-oil emulsions. In this regard, it is not surprising that Keller et al. (Coal Preparation, vo. 8, 1990, pp. 1-17) reported the observation of “massive water globules”, which was responsible for the high moisture contents of the clean coal products obtained from the selective agglomeration process. This was probably one of the reasons that Keller et al. explored the possibility of using the clean coal products as feedstock for coal-water slurry manufacture.
Eq. [4] suggests also that if coal particles have a high contact angle, the detachment energy (E) becomes small and hence they remain dispersed in oil phase. As shown in
Binks et al. (Langmuir, vol. 17, 2001, p. 4708) suggested that Janus particles, i.e., bifacial particles consisting of hydrophilic and hydrophobic surfaces, should improve the stability of the emulsions stabilized by “solid surfactants”. Glaser et al. (Langmuir, vol. 22, 2006, p. 5227) showed actually that Janus particles reduce the tension (or excess free energy) at the water/oil interfaces substantially and thereby create favorable conditions for the formation of stable water-in-oil emulsions. Therefore, for cleaning a run-of-the-mine fine coal containing significant amounts of Janus particles (or composite particles), it would be difficult to avoid the formation of water-in-oil emulsions, with a consequence of high. moisture products.
Due to the presence of the entrained water, the clean coal products obtained in conventional oil agglomeration processes exhibit high moisture contents, typically in the range of 30-55% by weight. In the instant invention, methods of removing the entrained water have been developed so that the moisture can be readily reduced to substantially lower levels. In one embodiment, the globules of water are removed using a size-size separation method selected from those including but not limited to screens, classifiers, and cyclones. These methods can remove the globules of water that are considerably larger than coal particles.
In another embodiment, the water drops stabilized by hydrophobic coal particles are broken up by appropriate mechanical means such as ultrasonic vibrator, magnetic vibrator, grid vibrator, etc., so that the coal particles are dispersed in the hydrophobic liquid, while the water drops free of coal particles drain into the aqueous phase. The organic phase in which coal particles are dispersed are then phase separated from the aqueous phase in which mineral matter is dispersed. The former is subjected to appropriate solid-liquid separation, while the latter is drained off. The hydrophobic liquid recovered from the solid-liquid separation step is recycled. The clean coal particles obtained from the solid/liquid separation step are substantially free of surface moisture. However, a small amount of the hydrophobic liquid may be present on the coal surface, in which case the coal particles may be subjected to a negative pressure or gentle heating to recover the residual hydrophobic liquid as vapor, which is subsequently condensed back to a liquid phase and recycled.
In still another embodiment, the drops (or globules) of water are removed using a solid-liquid separation method selected from those including but not limited to filters, centrifuges, and presses. It is believed that much of the entrained water globules are expressed and/or drained during the solid-liquid separation process, leaving behind only the interstitial water droplets entrapped in between the particles constituting a filter cake. The filter cake is then subjected to a high-shear agitation to dislodge the entrapped water droplets from surrounding coal particles and release them to the vapor phase in which they can readily vaporize due to the large surface-to-volume ratio and higher vapor pressure due to large radius of curvature. Some of the released water droplets may exit the system into the atmosphere.
The process of cleaning coal by selective agglomeration requires high-intensity agitation. Nicol et al. (U.S. Pat. No. 4,209,301) stated that high-speed stirrers capable of providing greater than 10,000 r.p.m. are needed to observe phase inversion, i.e., completion of coal agglomerates. It was shown also that the phase inversion is observed after 8 minutes of agitation at 6,000 r.p.m, while it takes 18 minutes at 3,000 r.p.m. In contrast, in the present invention, neither high-speed agitation nor long periods of agitation is necessary. A gentle agitation is usually sufficient, although high energy input in the form of strong agitation or long agitation time has no harmful effect.
It has been found that the HSD process can be used not only for drying hydrophobic coal fines but also for drying hydrophilic mineral fines (e.g., minerals in reject 306, 407, and 506 in
The hydrophobic liquids that can be used for the processes described in the present invention include hydrocarbon oils, which include aliphatic and aromatic hydrocarbons whose carbon numbers are less than 18. For the dewatering by displacement (DBD) process, shorter-chain n-alkanes and alkenes, both unbranched and branched, and cycloalkanes and cycloalkenes, with carbon numbers of less than eight may be used so that the spent hydrocarbon oils can be readily recovered and recycled. Liquid carbon dioxide is another hydrophobic liquid that can be used for the DBD process.
When using longer-chain alkanes and alkenes, recycling may be difficult. Therefore, in these instances only small amounts of the reagents are preferably used as agglomerants. The reagent costs can be reduced by using the hydrophobic liquids from unrefined petroleum sources. For the DBD process, ligroin (light naphtha), naphtha and petroleum naphtha, diesel fuel, and mixtures thereof may be used. For selective agglomeration, small amounts of kerosene and heating oils whose carbon numbers are in the range of 12-18 may be used.
The DBD and selective agglomeration processes are ideally suited for separating hydrophobic particulate materials (e.g., high-rank coals) from hydrophilic materials (e.g., silica and clay), with the resulting hydrophobic materials having very low surface moistures. The processes as described in the instant invention can also be used for separating one-type of hydrophilic materials from another by selectively hydrophobizing one but not the other(s), For example, the processes can be used to separate copper sulfide minerals from siliceous gangue minerals by using an alkyl xanthate or a thionocarbamate as hydrophobizing agents for the sulfide minerals. Further, the DBD concept can be used for non-thermal drying of fine coal or any other particulate materials after appropriate hydrophobization.
A volume of pentane was added as a hydrophobic liquid to the coal slurry placed in a 350 ml glass separatory funnel. The coal slurry was received from the Moss 3 coal preparation plant, Virginia, at 15% solids by weight. With a stopper in place, the material in the funnel was agitated vigorously by handshaking for 4 minutes and let to stand for phase separation, Coal particles agglomerated (or were engulfed into the hydrophobic liquid) and formed a layer on top of the aqueous phase. By opening the stopcock at the bottom, the aqueous phase was removed along with the mineral matter dispersed in it. The hydrophobic liquid remaining in the funnel was agitated again for a short period of time and let to stand. It was found that large globules of water surrounded by coal particles settled at the bottom, By opening the stopcock, the water globules were removed. This procedure was repeated several times until no visible water globules could be detected. The coal sample left in the funnel was removed, and the pentane was allowed to evaporate completely before analyzing the sample for moisture content. As shown in Table 1, the clean coal product still contained 25.9% moisture, indicating that smaller droplets of water were still present in the form of a water-in-oil emulsion with hydrophobic coal particles acting as a
solid surfactant.
In another test, the clean coal product obtained in the mariner described above was screened at 60 mesh. It was found that the screen underflow assayed only 2.4% moisture, while the screen overflow assayed 58.2% moisture. This example demonstrated that the high moisture content of the clean coal product was due to the presence of the globules of water stabilized by hydrophobic coal particles, which could readily be removed by a size-size separation step to reduce the moisture content substantially.
Another test was conducted in the same manner as described in Example 1 on a fine coal sample (100 mesh×0) from the Cardinal coal preparation plant, West Virginia. This sample was much finer than the one used in Example 1, with 80% of the material finer than 44 μm. In this example, 800 ml of the slurry at 4.3% solids was placed in a 1 liter separatory funnel along with 200 lb/ton of pentane as a hydrophobic liquid. After agitation and settling, the aqueous phase containing mineral matter was drained off, and the pentane mixed with coal particles was left behind in the funnel. The excess pentane was allowed to evaporate, and the clean coal product analyzed for ash and moisture. As shown in Table 2, the ash content was reduced from 35.6% in the feed to 3.7% with a combustible recovery of 83.7%, but the moisture was as high as 48.7%. The high moisture content was again due to the entrainment of the water droplets stabilized by coal particles.
The procedure described above was similar to the method of dewatering disclosed by Yoon et al. (U.S. Pat. No. 5,458,786), who reported that the moisture of a Pittsburgh coal sample was reduced to 3.6% using liquid butane as hydrophobic liquid. However, the low moisture value reported was due to a sampling error. In U.S. Pat. No. 5,458,786, the aqueous phase was drained until the “mixture of butane and coal began to come out of the tubing”. It appears now that by the time the drainage process was stopped, most of the water globules settled at the phase boundary had already been drained out. The phase boundary could not be seen because the test was conducted in copper tubing. Also, the mechanism of hydrophobic particles stabilizing water-in-oil emulsions was not known at the time. Yoon et al. failed to
recognize the difficulty in sampling under such circumstances.
The same coal sample used in Example 2 was subjected to another test under identical conditions, except that an additional step was taken to remove the entrained globules of water and obtain low moisture products. The additional step involved the use of a screen to separate the water droplets from the dry fine coal particles obtained by the DBD process depicted in
matter from a fine coal slurry generated at an operating coal preparation plant and the entrained water from the clean coal product.
A volume (600 ml) of the fine coal slurry (100 mesh×0) from the Cardinal plant was placed in a 1-litter separatory funnel, and pentane was added in the amount of 20% by weight of coal. With the stopper in place, the funnel was vigorously agitated by hand for 2 minutes, and the mixture was allowed to stand for phase separation. The aqueous phase containing mineral matter was removed from the bottom, and the pentane and coal mixture removed from the top. During this procedure, the mineral matter was substantially removed from coal, and most of the pentane evaporated away from the clean coal product. However, the moisture content remained as high as 52.2%, as shown in Table 4, mostly due to the entrained water globules stabilized by hydrophobic coal particles. The clean coal product was dewatered by a horizontal basket centrifuge to reduce the moisture content to 18.2%. The centrifuge product was then fed to a squirrel-cage fan by means of a vibratory feeder. The exit stream from the fan was collected in a small home-made bag house. The collected coal sample assayed 1% moisture, as shown in the table. Thus, the method disclosed in this example produced a dry coal with 1% moisture with the ash content reduced from 36.7 to 8.6% with a 90% combustible recovery. The ash content could have been reduced further, if the clean coal product was re-pulped and cleaned again before the centrifugation and high-shear dewatering (HSD) steps commenced.
During the centrifugal dewatering step, the water droplets were reduced in size but still filled the void spaces in between the coal particles. The tiny droplets of entrapped water were then separated from the coal particles by the high-shear agitation in air. The tiny water droplets exited the system and/or evaporated quickly without applying heat due to the high curvature and/or the large surface area-to-volume ratio of the water droplets.
The Cardinal coal sample was treated with 200 lb/ton of pentane in the same manner as described in Examples 2 and 3. The clean coal product was dewatered by means of a vacuum filter rather than a centrifuge as in Example 4. The filter cake was then fed to a squirrel-cage fan to further reduce the moisture to 1.7%, as shown in Table 5. The ash content of the product coal was relatively high due to the entrainment of mineral matter. In a continuous process, this problem can be readily addressed by installing an appropriate agitator or implementing a two-step process.
The fine coal sample from the Cardinal plant was subjected to two stages of agglomeration using a total of 360 lb/ton of pentane. The clean coal product was dewatered using a vacuum filter, and the filter cake dried using a squirrel-cage fan in one test and an air jet in another to obtain 1.4 and 2.1% moistures, respectively. Both of these devices were designed to provide high-shear agitation in air to dislodge the small droplets of water from the fine coal particles that had been dried by the displacement mechanism depicted in
A coal sample from the Trans Alta fine coal impoundment, West Virginia, was screened at 100 mesh, and the screen underflow assaying 24.9% ash was treated with pentane (20% by weight of coal) to obtain a clean coal product assaying 8.1% ash and 57.1% moisture with 92.4% recovery. The high product moisture was due to the presence of the water globules stabilized by hydrophobic coal particles. The clean coal product was dewatered using a laboratory-scale horizontal basket centrifuge to reduce the moisture to 21.4%. The centrifuge product was then subjected to a high-shear agitation provided by a squirrel-cage fan to obtain 0.9% moisture. The recoveries for the centrifugation and high-shear agitation were not determined.
A nominally 100 mesh×0 coal sample assaying 36.8% ash was obtained from the Litwar coal preparation plant, West Virginia. A size analysis of the sample showed that 7.8% of the material was coarser than 150 μm and 80.1% was finer that 44 μm. It was cleaned of its ash-forming mineral matter by froth flotation rather than using the DBD or the selective agglomeration processes described in the foregoing examples. A Denver laboratory flotation machine with a 4-liter stainless steel cell was used. The flotation test was conducted with 3 lb/ton diesel oil as collector and 1.2 lb/ton MIBC as frother at 2.6% solids. The froth product was subjected to another stage of flotation test without using additional reagent to obtain a clean coal product with 4.2% ash and 8.3% solids. The product was vacuum-filtered using 5 lb/ton of sorbitan monooleate as a dewatering aid. The filter cake containing 19.6% moisture was then subjected to a high-shear agitation provided by a squirrel-cage fan to further reduce the moisture to 0.9% by weight.
A copper ore sample was ground in a ball mill for 8 to 20 minutes and the mill products were subjected to a series of flotation tests. A composite of the reject materials at 10% solids was dewatered to 15,6% by means of an air pressure filter at 20 psi. The filter cake was then subjected to a high-shear agitation in a squirrel-cage fan to further reduce the moisture to 0.7% as shown in Table 9. In another test, the composite reject material was conditioned with 5 lb/ton
of a cationic surfactant (Armeen C) at 30% solids and subsequently with 3 lb/ton of sorbitan monooleate before vacuum filtration. The filter cake containing 17.5% moisture was then subjected to the high-shear dewatering process to further reduce the moisture content to 0.6% as shown in Table 9.
In this example, a coal sample from the Pinnacle fine coal impoundment, Wyoming County, West Virginia, was tested for the DBD process. The coal sample was a cyclone overflow from a pond recovery plant containing mostly −44 μm materials, which assayed 38% ash by weight. In the plant, the ultrafine coal was not being processed due the difficulties in both recovery, and dewatering. In this example, a volume of the coal slurry was added to a kitchen blender and diluted to approximately 3% solids with tap water. The amount of coal in the mixer was approximately 20 g. After adding 20 ml of pentane to the mixer, the slurry was agitated at a
high speed for 45 seconds and then agitated for another 5 minutes at a low speed. During this time, coal particles agglomerated by the hydrophobic liquid, while mineral matter remained dispersed in the aqueous phase. The slurry was then poured over a 30-mesh screen to remove the dispersed mineral matter as underflow. Most of the +30 mesh material, except the largest of the water droplets stabilized by coal particles, was transferred to a stack of screens consisting of 50 and 70 mesh screens. The +50 and −70 mesh fractions assayed 9.8 and 3.2% moistures, respectively. Table 10 shows the composite results of the test, showing that the product moisture and coal recovery can be controlled using size-size separation devices such as screens.
The coal sample used in this example was the same as in Example 10, A volume (1 liter) of coal slurry containing approximately 40 g of coal was added to a kitchen blender (mixer). After adding 0.5 liter of pentane to the mixer, the mixture was agitated at a low r.p.m. The agitated slurry was slowly transferred to a 1-inch diameter phase separator, which was made of a ¾-inch diameter glass column with a 9-inch height. At the base of the column, an ultrasonic probe was installed to provide a mechanical energy to dislodge the coal particles from the surfaces of the water drops, which tended to congregate at the phase boundary between water and oil due to gravity. The column was also equipped with an overflow launder at the top to collect the clean coal product semi-continuously. With the application of the ultrasonic energy, it was possible to dislodge the coal particles from the water droplets and allow them to be more fully dispersed in the oil phase. Water was then introduced to the base of the settling column to flood the organic phase into the launder, while the aqueous phase was removed from the bottom. The collected coal and ash products were weighed and analyzed for ash and moisture to obtain
the results shown in Table 11. As shown, the instant invention produced 94.3% recovery of combustible materials, with the product coal assaying 3.9% ash and 0.54% moisture.
This application is a divisional application of U.S. Non-provisional application Ser. No. 13/576,067, which is a national stage completion of PCT/US2011/023161 filed on Jan. 31, 2011, which claims the benefit of U.S. Provisional Application No. 61/300,270, filed on Feb. 1, 2010, the disclosures of which are incorporated herein by reference.
This invention was made with government support under Grant No. DE-FC26- 05NT42457 awarded by the US Department of Energy. The government has certain rights in the invention.
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| Number | Date | Country | |
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| Child | 15786079 | US |
