Process for agglomerating coal
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of agglomerating coal particles with hydrocarbon oil, and more particularly to a method for reducing the amount of hydrocarbon oil required to form coal-hydrocarbon oil agglomerates. 
     2. Prior Art 
     Heretofore, it was known that coal particles could be agglomerated with hydrocarbon oils. For example, U.S. Pat. No. 3,856,668 to Shubert issued Dec. 24, 1974, and U.S. Pat. No. 3,665,066 to Capes et al issued May 25, 1972 disclose processes for recovering coal fines by agglomerating the fine coal particles with oil. U.S. Pat. No. 3,268,071 to Puddington et al issued Aug. 23, 1966 and U.S. Pat. No. 4,033,729 issued July 5, 1977 to Capes disclose processes for beneficiating coal involving agglomerating coal particles with oil in order to provide a separation of coal from ash. While these processes can provide some beneficiation of coal, improved ash and pyritic sulfur removals would be desirable. 
     The above U.S. Pat. No. 4,033,729 to Capes et al relating to removing inorganic materials (ash) from coal significantly notes that pyritic sulfur has proven difficult to remove because of its possible hydrophobic character. This disclosure confirms a long standing problem. The article &#34;The Use of Oil in Cleaning Coal&#34; Chemical and Metallurgical Engineering, Vol. 25, pages 182-188 (1921) discusses in detail cleaning coal by separating ash from coal in a process involving agitating coal-oil-water mixtures, but notes that pyrite is not readily removed in such a process. In such a process, beneficiation of coal would be greatly improved if pyrite sulfur removal could be enhanced. 
     While it is known that hydrocarbon oil agglomeration can be useful in recovering coal particles and/or beneficiating coal, the large amount of hydrocarbon oil required in these prior art coal agglomeration processes has detracted from their usefulness. It would be especially advantageous if the amount of hydrocarbon oil could be reduced in forming hydrocarbon oil coal agglomerates. 
     SUMMARY OF THE INVENTION 
     This invention provides a method for reducing the amount of hydrocarbon oil required to form coal-oil agglomerates comprising the steps of: 
     (a) combining a first coal fraction comprising predominately coarse particles, and a second coal friction comprising predominately fine coal particles; and 
     (b) agitating a slurry of the combined coal fractions, hydrocarbon oil and water to form coal-oil agglomerates. 
     It has been discovered that less hydrocarbon oil is required to agglomerate coal particles comprised of a predominately coarse fraction and a predominately fine fraction. The desirable result is that coal-oil agglomerates reduced in oil content are formed. Surprisingly, these coal-oil agglomerates reduced in hydrocarbon oil content can have a size similar to conventional coal-oil agglomerates which when initially formed require a higher oil content. 
     In another aspect of this invention, a method for beneficiating coal involving this improved agglomeration process is presented. 
     In another aspect of the invention, a conditioning agent is employed which renders pyrite more amenable to separation on agglomerating coal particles with hydrocarbon oil. In this aspect of the invention, an improved method for beneficiating coal is presented. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND ITS PREFERRED EMBODIMENTS 
     In its board aspect, this invention provides a method for reducing the amount of hydrocarbon oil required for forming coal-oil agglomerates involving the steps of: 
     (a) combining a first coal fraction comprising predominately coarse particles, and a second coal fraction comprising predominately fine coal particles; and 
     (b) agitating a slurry of the combined coal fractions, hydrocarbon oil and water to form coal-oil agglomerates. 
     This invention involves the discovery that coal-oil agglomerates formed by agitating a mixture of coarse and fine coal particles, hydrocarbon oil and water can form coal-oil agglomerates reduced in oil content. 
     While not wishing to be bound by any theory as to why the desirable results of the invention are obtained, it is theorized that if the coal particles are predominately of the same particle size that the agglomerates can have interstitial voids into which hydrocarbon oil is incorporated and retained by capillary action. The result is that such coal-oil agglomerates can have a high oil content. If a fine coal fraction is available during the agglomeration process, the coal fines can occupy the interstitial voids formed by coarser particles reducing the void space available to hydrocarbon oil. The desirable result is that coal-oil agglomerates reduced in oil content are formed. 
     The coal fraction comprised of predominately coarse coal particles will preferably be comprised of particles such that the weighted size average of the coarse coal particles (d coarse) is more than four times the weighted size average of the fine coal particles (d fine), i.e., 
     
         d coarse/d fine)&gt;4 
    
     Weight size average means the average diameter of the coal particles relative to the amount by weight of coal particles of a particular size. 
     More preferably, the ratio of d coarse to d fine is greater than 6. 
     Generally, it is preferred that from about 60 to about 80 percent by weight of coal particles to be agglomerates be coarse coal particles, and from about 20 to about 40 percent, by weight, fine coal particles. More preferably from about 65 to 75 percent of the coal particles are coarse, and from 25 to 35 percent are fine. 
     While the coarse and fine coal particles employed in this invention can be provided by a variety of known processes, a very suitable method involves, for example, separately grinding or crushing coal to a predominately coarse particle size and a predominately fine particle size. 
     A very suitable coal particle size which is amenable to agglomeration and beneficiation is minus 24 mesh, for example minus 24 mesh and at least 70% on 300 mesh, preferably minus 50 mesh and at least 70% on 200 mesh. It is very desirable, therefore, that the coarse particles have a size distribution characterized by this range. 
     Suitable coals which can be employed in the process of this invention include brown coal, lignite, subbituminous bituminous (high volatile, medium volatile, and low volatile), semi-anthracite and anthracite. In addition, coal refuse from wash plants which have been used to upgrade run-of-mine coal can also be used as a source of coal. Typically, the coal content of a refuse coal will be from about 25 to about 60% by weight of coal. Particularly preferred refuse coals are refuse from the washing of metallurgical coals. 
     In accordance with this invention, the coarse coal particles and the fine coal particles are combined and formed into coal-oil agglomerates. The coal particles can be combined in a variety of ways. For example, coarse and fine coal particles can be mixed together in dry form by tumbling, or aqueous slurries of coarse and fine coal particles can be mixed together. 
     Coal-oil agglomerates can be readily formed by agitating a mixture of water, hydrocarbon oil and the coal particles. 
     The water content of the mixture is not critical and can vary within wide limits. Generally from about 30% to 95% water, and more preferably from about 40% to 90% water, based on the weight of coal, will be employed. There should be sufficient hydrocarbon oil present to agglomerate the coal particles. The optimum amount of hydrocarbon oil will depend upon the particular hydrocarbon oil employed, the size of the coal particles, and the coal-oil agglomerate size desired. Generally, the amount of hyrocarbon oil employed will be from about 5% to 45%, preferably 5% to 25%, by weight, of coal. 
     Suitable hydrocarbon oils for forming coal-oil agglomerates are derived from petroleum, shale oil, tar sand and coal. Especially, suitable hydrocarbon oils are light and heavy refined petroleum fractions such as light cycle oil, heavy cycle oil, heavy gas oil, clarified oil, kerosene, heavy vacuum gas oil, residual oils, coal tar and other coal derived oils. Mixtures of various hydrocarbon oils can be quite suitable; particularly when one of the materials is very viscous. 
     The hydrocarbon oils are hydrophobic and will preferentially wet the hydrophobic coal particles. When the mixture of water, hydrocarbon oil and coal is agitated, the hydrocarbon oil wets (become associated with) the coal particles. These hydrocarbon wet coal particles will collide with one another under suitable agitation forming coal-oil agglomerates. In general, the size of the coal-oil agglomerate is generally at least about 2 to 3 times, more generally at least 4 to 10 times, or more, the average size of the coal particles which make up the coal-oil agglomerates. 
     Agitating a mixture of water, hydrocarbon oil and coal particles to form coal-oil agglomerates can be suitably accomplished using stirred tanks, ball mills or other apparatus. An apparatus which provides a zone of shearing agitation is especially suitable for agitating the mixture. 
     The process can be suitably conduit at temperatures from ambient to 200° F., for example from about 50° F. to 150° F., preferably 50° F. to 100° F., and at pressures sufficient to maintain the liquid state of liquids employed. 
     When coal-oil agglomerates are formed in this manner, the coal particles generally take up substantially all of the hydrocarbon oil present forming coal-oil agglomerates of a size characteristic at the given conditions and oil level employed. At a given coal particle size (and other conditions being equal), increasing the amount of oil provides coal-oil agglomerates of increased size. 
     In forming coal-oil agglomerates, a principal goal is to form coal-oil agglomerates of a size such that the agglomerate can be readily recovered, i.e., preferentially separated from water and minerals (e.g., ash and pyrite) associated with the coal. The desired size of the agglomerate can vary depending on the separation technique which is employed. In order to conserve the valuable hydrocarbon oil, the amount of oil (and agglomerate size) should be as small as possible to provide the desired separation. 
     The resulting coal-oil agglomerates in the water slurry can be recovered by separating, for example, by using suitable screens or filters. This separation step also allows for removal of some of the mineral matter, for example, ash, such that the coal is beneficiated. 
     In an especially preferred aspect of the invention, the coal particles containing ash and iron pyrite mineral matter employed are contacted with at least one conditioning agent which renders pyrite more amenable to separation from the coal particles on forming coal-oil agglomerates. In this preferred aspect of the invention, coal particles are contacted with a promoting amount of at least one conditioning agent capable of modifying or altering the existing surface characteristics of the pyrite under conditions to effectuate alteration or modification of at least a portion of the contained pyritic sulfur. This altered or modified pyritic sulfur is preferentially rejected to the aqueous phase such that recovered coal-oil agglomerates are coal-oil agglomerates wherein the coal exhibits reduced sulfur and ash content. 
     The process of forming coal-oil agglomerates reduced in oil content can be used to recover aqueous slurries of coal fines, and can also be employed to beneficiate coal. 
     The preferred method of beneficiating coal in accordance with this invention involves the following steps: 
     (a) reducing coal size to form a first coal fraction of predominately coarse particle size, and a second coal fraction of predominately fine particle size; 
     (b) combining the first coal fraction comprising predominately coarse particles, and the second coal fraction comprising predominately fine coal particles; 
     (c) contacting an aqueous slurry of the combined coal particles of step (b) with a promoting amount of at least one conditioning agent capable of modifying or altering the existing surface characteristics of the pyrite under conditions to effectuate alteration or modification of at least a portion of the contained pyrite; 
     (d) contacting the slurry of coal particles with hydrocarbon oil to form coal oil agglomerates; and 
     (e) recovering coal-oil agglomerates wherein the coal exhibits reduced iron pyrite and mineral content. 
     While the above method is preferred, the coal particles could be contacted with the conditioning agent prior to combining the coarse and fine fractions, i.e, coal could be beneficiated in a process comprising: 
     (a) reducing coal size to form a first coal fraction of a predominately coarse particle size, and a second coal fraction of a predominately fine particle size. 
     (b) contacting an aqueous slurry of the coal particles reduced in particle size of step (a) with a promoting amount of at least one conditioning agent capable of modifying or altering the existing surface characteristics of the pyrite under conditions to effectuate alteration or modification of at least a portion of the contained pyrite; 
     (c) combining the first coal fraction comprising predominately coarse particles, and the second coal fraction comprising predominately of fine coal particles; 
     (d) contacting the slurry of coal particles with hydrocarbon oil to form coal-oil agglomerates; and 
     (e) recovering coal-oil agglomerates wherein the coal exhibits reduced iron pyrite and mineral content. 
     An amount of conditioning agent is employed which promotes the separation of pyrite from coal. Generally, from about 0.01% to 15%, preferably from about 0.5% to 5%, by weight of coal, of conditioning agent is employed. 
     Preferably the amount of conditioning agent is based on the ash content of the coal. From about 0.05% to 30%, preferably 0.05% to 10%, and most preferably from about 1% to 10%, by weight, ash is employed. 
     Preferably, the coal is contacted with the conditioning agent in aqueous medium. The contacting is carried out at a temperature such to modify or alter the pyrite surface characteristics. For example, temperatures in the range of about 0° C. to 100° C. can be employed, preferably from about 50° C. to about 100° C., and still more preferably from about 20° C. to about 35° C., i.e., ambient conditions. Temperatures above 100° C. can be employed, but are not generally preferred since a pressurized vessel would be acquired. Temperatures in excess of 100° C. and pressures above atmospheric, generally pressures of from about 5 psig to about 500 psig, can be employed, however, and can even be preferred when a processing advantage is obtained. Elevated temperatures can also be useful if the viscosity and/or pour point of the agglomerating oil employed is too high at ambient temperatures to selectively agglomerate coal as opposed to ash and pyrites. 
     Examples of useful conditioning agents include inorganic compounds which can hydrolyze in water, preferably under the conditions of use, and the hydrolyzed forms of such inorganic compounds, preferably, such forms which exist in effective amounts under the condition of use. Proper pH and temperature are necessary for some inorganic compounds to exist in hydrolyzed form. When this is the case, such proper conditions are employed. The inorganic compounds which are hydrolyzed or exist in hydrolyzed form under the given conditions of contacting (e.g., temperature and pH) can modify or alter the existing surface characteristics of the pyrite. Preferred inorganic compounds are those which hydrolyze to form high surface area inorganic gels in water, such as from about 5 square meters per gram to about 1000 square meters per gram. 
     Examples of such conditioning agents are the following: 
     I. Metal Oxides and Hydroxides having the formula: 
     M a  O b .xH 2  O and M(OH).xH 2  O, wherein M is Al, Fe, Co, Ni, Zn, Ti, Cr, Mn, Mg, Pb, Ca, Ba, In or Sb; a, b and c are whole numbers depending on the ionic valence of M, and x is from 0 to about 3. 
     Preferably M is a metal selected from the group consisting of Al, Fe, Mg, Ca and Ba. These metal oxides and hydroxides are known materials. Particularly preferred are aluminum hydroxide gels in water at pH 7 to 7.5. Such compounds can be readily formed by mixing aqueous solutions of water soluble aluminum compounds, for example, aluminum nitrate or aluminum acetate, with suitable hydroxides, for example, ammonium hydroxide. In addition, a suitable conditioning agent is formed by hydrolyzing bauxite (Al 2  O 3 .xH 2  O) in alkaline medium to an alumina gel. Calcium hydroxide represents another preferred conditioning agent. Calcined calcium and magnesium oxides are also preferred conditioning agents. Mixtures of such compounds can very suitably be employed. The compounds are preferably suitably hydrolyzed prior to contacting with coal particles in accordance with the invention. 
     II. Metal aluminates having the formula: 
     M&#39; d  (AlO 3 ) e  or M&#39; f  (AlO 2 ) g , wherein M&#39; is Fe, Co, Ca, Mg, Ba, Ni, Pb or Mo; and d, e, f, and g are whole numbers depending on the ionic valence of M. 
     Compounds wherein M&#39; is Ca or Mg, i.e., calcium aluminates and magnesium aluminates are preferred. These preferred compounds can be readily formed by mixing aqueous solutions of water soluble calcium and magnesium compounds, for example, calcium or magnesium acetate with sodium aluminate. Mixtures of metal aluminates can very suitably be employed. The compounds are most suitably hydrolyzed prior to contacting with coal particles in accordance with the invention. 
     III. Aluminasilicates having the formula: 
     Al 2  O 3 .xSiO 2  wherein x is from about 0.5 to 5. 
     A preferred aluminasilicate conditioning agent for use herein has the formula Al 2  O 3 .4SiO 2 . Suitably aluminasilicates for use herein can be formed by mixing together in aqueous solution a water soluble aluminum compound, for example, aluminum acetate, and a suitable alkali metal silicate, for example, sodium metasilicate, preferably, in suitable stoichiometric amounts to provide preferred compounds set forth above. 
     IV. Metal silicates wherein the metal is calcium, magnesium, tin, barium or iron. 
     Metal silicates can be complex mixtures of compounds containing one or more of the above mentioned metals. Such mixtures can be quite suitable for use as conditioning agents. 
     Calcium and magnesium silicates are among the preferred conditioning agents of this invention. 
     These conditioning agents can be prepared by mixing appropriate water soluble metal materials and alkali metal silicates together in an aqueous medium. For example, calcium and magnesium silicates, which are among the preferred conditioning agents, can be prepared by adding a water soluble calcium and/or magnesium salt to an aqueous solution or dispersion of alkali metal silicate. 
     Suitable alkali metal silicates which can be used for forming the preferred conditioning agents are potassium silicates and sodium silicates. Alkali metal silicates for forming preferred calcium and magnesium conditioning agents for use herein are compounds having SiO 2  :M 2  O formula weight ratios up to 4:1, wherein M represents an alkali metal, for example, K or Na. 
     Alkali metal silicate products having silica-to alkali weight ratios (SiO 2  :M 2  O) up to about 2 are water soluble, whereas those in which the ratio is above about 2.5 exhibit less water solubility, but can be dissolved by steam under pressure to provide viscous aqueous solutions or dispersions. 
     The alkali metal silicates for forming preferred conditioning agents are the readily available potassium and sodium silicates having an SiO 2  :M 2  O formula weight ratios up to 2:1. Examples of specific alkali metal silicates are anhydrous Na 2  SiO 3  (sodium metasilicate), Na 2  Si 2  O 5  (sodium disilicate), Na 4  SiO 4  (sodium orthosilicate), Na 6  Si 2  O 7  (sodium pyrosilicate) and hydrates, for example, Na 2  SiO 3 .nH 2  O (n=5,6,8 and 9), Na 2  Si 4  O 9 .7H 2  O and Na 3  HSiO 4 .5H 2  O. Examples of suitable water soluble calcium and magnesium salts are calcium nitrate, calcium hydroxide and magnesium nitrate. The calcium and magnesium salts when mixed with alkali metal silicates described hereinbefore form very suitable conditioning agents for use herein. 
     Calcium silicates which hydrolyze to form tobermorite gels are especially preferred conditioning agents for use in the process of the invention. 
     V. Inorganic Cement Materials. 
     Inorganic cement materials are among the preferred conditioning agents of the invention. As used herein, cement material means an inorganic substance capable of developing adhesive and cohesive properties such that the material can become attached to mineral matter. Cement materials can be discrete chemical compounds, but most often are complex mixtures of compounds. The most preferred cements (and fortunately, the most readily available cements) are those cements capable of being hydrolyzed under ambient conditions which are the preferred conditions of contacting with the coal in the process. 
     These preferred cement materials are inorganic materials which when mixed with a ratio of water to form a paste can set and harden. Cement and materials used to form cements are discussed in Kirk-Othmer, Encyclopedia of Chemical Technology, 2D. Ed., Vol. 4 c. 1964 by John Wiley &amp; Sons, Inc., Pages 684 to 710 being incorporated by reference herein. Examples of cement materials include calcium silicates, calcium aluminates, calcined limestone and gypsum. Especially preferred examples of cement materials are the materials employed in hydraulic limes, natural cement, masonry cement, pozzolan cement and portland cement. Such materials will often include magnesium cations in addition to calcium. 
     Commercial cement materials, which are very suitable for use herein, are generally formed by sintering calcium carbonate (as limestone), or calcium carbonate (as limestone) with aluminum silicates (as clay or shale). Preferably, such materials are hydrolyzed to use as conditioning agents. 
     With some coals, the material matter associated with the coal may be such that on treatment under proper conditions of temperature and pH the mineral matter can be modified in situ to provide the suitable hydrolyzed inorganic conditioning agents for carrying out the process. In such cases, additional conditioning agents may or may not be required depending on whether an effective amount of conditioning agent is generated in situ. 
     The conditioning agents suitable for use herein can be employed alone or in combination. 
     The coal particles are preferably contacted with the conditioning agent in an aqueous medium by forming a mixture of the coal particles, conditioning agent and water, and the conditioned coal particles are subsequently agglomerated with oil in accordance with this invention. 
     Suitable conditioning agents are disclosed in U.S. patent application Ser. No. 944,452, filed Sept. 21, 1978 commonly assigned, the entire content being incorporated by reference herein. 
     The coal-oil agglomerates of the invention reduced in oil content, and preferentially beneficiated can be recovered in a variety of ways. 
     Preferably the recovery is a separation effected by taking advantage of the size difference between coal-oil agglomerates and unagglomerated mineral matter. For example, the coal-oil agglomerates can be separated from the water and liberated ash and pyrite, etc., by filtering with bar sieves or screens, which predominately retain the coal-oil agglomerates, but pass water and unagglomerated mineral matter. When this technique is employed, coal-oil agglomerates of a size suitable for ready filtering must be formed. 
     Often it is desired to use small amounts of oil to form coal-oil agglomerates. Small amounts of oil, however, provide small coal-oil agglomerates. Small coal-oil agglomerates (aggregates and flocs) can be more desirably separated by taking advantage of the different surface characteristics of the coal-oil agglomerates, and ash and conditioned pyrite, for example, employing well known froth flotation and/or skimming techniques. 
     The process of this invention provides coal-oil agglomerates reduced in hydrocarbon oil content which are suitable for separation using any of these techniques. The desirable result is that reduced amounts of hydrocarbon oil can be employed in beneficiating coal. 
     While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims.