The present invention relates to the preparation of fuels * and, more particularly, to fuel preparation processes which _% are unique in that they can be employed to produce coal-type fuels which have an extremely low (<1.0 wt %) ash content and essentially no pyritic sulfur.

DISCLOSURE OF THE INVENTION AND BACKGROUND ART

In general, this novel, and economically important, result is obtained by milling or otherwise comminuting raw coal until it has been reduced in particle size to ca. 250μm x 0 ⁽ μm equals micrometer or micron) . The raw coal is then slurried in an aqueous liquid, typically clean water; and comminution of the raw coal is continued until the raw coal has been resolved into separate, particulate phases of coal and mineral matter. After this comminution step is completed, a large amount of ^' an agglomerating agent is added to the slurry with agitation; agitation of the slurry is continued until the coal particles have dissociated from the mineral matter and aqueous phases of the slurry and coalesced into agglomerates of product coal; and the agglomerates are recovered from the slurry ⁽ there is virtually 100 percent recovery of the carbonaceous material in this separation) .

A product coal with an even lower ash content than is available from following the steps identified above can be produced by redispersing the product coal agglomerates in clean water and repeating the agglomeration and collection steps. This sequence can be repeated as many times as wanted although it is presently believed that the benefits obtained by pro- _φ ceeding beyond the third collection step will in general not justify the expense of doing so.

No additional milling is required in the second product coal recovery stage (dispersion, agglomeration, and recovery steps) just discussed or in subsequent repetitions of this sequence of steps. Consequently, the elimination of additional

-2- ash afforded by the second (and any subsequent) stages can be effected inexpensively and with only modest expenditures of energy.

Still another technique that can be employed to reduce the ash content of the product coal obtained in the initial (or a subsequent) agglomeration and separation of the product is an acid leach of the product coal.

All of the above-discussed process steps can be carried out at ambient pressure and at ambient temperatures (preferably 70 + 10°F (21.1 + 5.6°C) ) .

The process described above can be used to prepare fuels which can compete directly with Bunker C and residual crude oils and synthetic coal fuels which have been successfully em¬ ployed to fuel gas turbine engines. The flame characteristics of these novel fuels lie between those of flames obtained by burning natural gas and No. 2 fuel oil, respectively.

Specifically, product coals with ash contents of substan¬ tially less than 1.0 weight percent have been produced by the foregoing process with demonstrated repeatability from a number of quite different coals. These fuels typically have the following characteristics:

Particle Size down to 4μm x 0

Ash down to 0.22 wt % ^'

Moisture below 5 wt %

BTU/lb in the range of 15,000

BTU Percent Yield approaching 100%

Small particle size is an important contributing factor to the usefulness of a coal-type fuel. The process described above is eminently capable of generating such fuels as is shown by the foregoing tabulation.

As indicated above, the raw coal being processed into a low ash fuel as disclosed herein is preferably first milled or comminuted while in a "dry" state, formed into an aqueous slurry, and then subjected to further size reduction. Unexpectedly,

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-3- it has been found that this is economically advantageous while the efficiency of the process is not adversely effected by the dry milling contrary to what is stated in U.S. patent No. 4,186,887 which was issued February 5, 1980, to Douglas V. Kelle Jr., et al and which discloses an agglomeration type coal re¬ covery process which, in certain respects, is like the fuel preparation process described herein.

The raw coal is reduced to a top size of ca. 85 percent 250 microns x 0 by dry milling, as indicated above, and subsequently ground to an ultimate top size of- 30μm with a particle size of 85 percent 15μm x 0 being preferred. In some cases the size distribution of the comminuted raw coal limits the maximum degree of ash reduction. The finer the particles the more mineral matter that can be separated.

Another technique that I can advantageously employ to increase the efficacy of the novel fuel preparation process described above involves the addition of milling aids in small a- ounts to the raw coal in the second of the comminution steps. Such additives perform one, or both, of two important functions .promotion of particle dispersion, which results in more efficient milling, and protection of freshly exposed particle surfaces against oxidation. This facilitates the subsequent interaction between the coal particles and the agglomerant and thereby promotes more efficient separation of the coal from the mineral matter and liquid phases of the slurry when the separation and agglomeration of the coal particles is carried out.

The particular additives that are employed depend upon the particular coal being cleaned. Additives that have been employed to advantage include: 1,1,2-trichloro-l,2,2-tri- fluoroethane? OT-100, a dioctyl ester of sulfosuccinic acid marketed by American Cyana id as an ionic surfactant; Surfynol 104E, a tertiary acetylenic glycol marketed by Air Products and Chemicals, Inc. as a nonionic surfactant; and Triton X-114, an octyl phenol with 7-8 oxide groups marketed by Rhom & Haas Co. as a nonionic surfactant.

_4_ Coal particle surface protection is obtained by adsorbing monolayers of the milling additive onto the surfaces of the coal particles in the second (wet) of the milling steps. This requirement can be met by introducing the milling additive into the raw coal slurry at a rate of one-three pounds of additive per ton of coal, depending on the particle size distribution of the raw coal and the molecular ^• area of the additive.

Dispersion of the coal particles in the liquid carrier in the second of the milling steps can also be promoted in many cases by maintaining the pH of the slurry in the range of 6-10 during that step. This can be accomplished by adding a basic material such as sodium hydroxide to the slurry in an amount that increases the pH of the slurry to the desired level.

Reductions in ash content to the levels envisaged herein require an agglomerating agent of particular character; viz., one that has an exceptionally high interfacial tension with water (at least 50 dynes/cm and the higher the better) and a reasonably low viscosity. Agglomeration of -the product coal particles in the disclosed fuel preparation process involves attachment of the agglomerant to the particles of coal liberated in the milling steps and the formation of liquid agglomerant bridges between the particles making up each agglomerant. If the interfacial tension between the agglomerant and the aqueous phase of the coal slurry is not at least 50 dynes per cm, microspheres (or bubbles) of water and mineral matter can fill the voids between and around the coal particles making up the agglomerates. This undesirably increases both the moisture and ash content of the product coal. By using an appropriate amount of an agglomerant having an interfacial tension with water of the magnitude identified above, however, the filling of the voids with agglomerant and the ejection of water and mineral matter from those voids into the main body of the slurry can be insured.

- -

Suitable agglo erants for my purposes include such diverse compounds as pentane, 2-methylbutane, 1,1,2-trichloro-l,2, 2- trifluoroethane, and trichlorofluoromethane. Essentially pure compounds are required as even small amounts of impurities markedly lower the interfacial tension of the agglomerant with respect to water.

The agglomerant forms stable, monolayer films on the coal particles, rendering the particles more hydrophobic relative to the water phase. The amount of agglomerant needed to achieve a monolayer film can be readily calculated from the area of the coal particles and the area of the specific agglomerant molecules. Similarly, the amount of agglomerant required to achieve separation of up to essentially all of the product coal with low ash contents (typically below one percent) from the mineral water slurry can be calculated using as a first approximation the packing of ideal spheres and the change of the agglomerant film thereon to determine that point where the agglomerant attached to the coal particles jus ^* t, but completely, fills all of -the voids between all of the coal particles, yielding a minimum area for the high energy interfacial contact between the agglomerant and the water in the raw coal slurry.

In the case of 1,1,2-trichloro-l,2,2-trifluoroethane, ca. 0.19 wt % of the agglomerant based on dry coal weight will suffice to form the stable monolayers on the coal particles whereas 45 wt % or more of the agglomerant will have to be dispersed in the diluted raw coal slurry to completely fill the voids between the coal particles making up the product coal agglomerates. Separation over a sieve bend can be readily achieved, and most often the optimum reduction of ash in the product coal (depending on the coal and the size distribution) can be observed, when very near 55 wt % agglomerant has been dispersed on the coal particles. Agglomerant in excess of 65 wt based on dry coal results in partial or complete separation of one slurry containing liquid agglomerant and product coal from a second slurry of water with mineral matter.

Petroleum fractions such as Varsol, kerosene, and gasoline are occasionally reported as having interfacial tensions with water in the range of 50 dynes/cm. However, these cuts usually contain acids, ketones, and unsaturated and other compounds that effectively lower this value. Consequently, these and comparable cuts such as light hydrocarbon oils heretofore proposed as agglomerants can not be used to reach the coals of the present invention — the generation of a product from raw coal which has minimal ash and pyritic sulfur at recovery rates approaching 100 percent.

One important advantage of the novel agglomerants employed in the practice of the present invention, aside from their high interfacial tensions with water, is that they have a boiling point below that of water. This is particularly im¬ portant when agglomeration and separation of the product coal is followed by redispersion of the coal particles in clean water, reagglomeration, and separation. Redispersion requires that the concentration of agglomerant with respect to the solids in the agglomerates be reduced in the presence of an aqueous carrier. That cannot be accomplished if the boiling point of the agglomerant is above 100°C as the carrier will boil off before the agglomerant is evaporated if heat is added to the mixture of agglomerates and water to strip off the agglomerant.

The exemplary agglomerants identified above all have boiling points in the range of 30-50°C. Agglomerants in that boiling point range are especially desirable as they remain liquid under most ambient conditions but can be dissociated from the product coal and the water-mineral matter phase of the slurry with only modest expenditures of energy. This is important as the cost of the large volume of agglomerant used in a commercial scale operation requires that ^" essentially all of the agglomerant be recovered and recycled.

Another advantage of the preferred class of agglomerants is that they have viscosities of less than one centipoise. This is important because, as a consequence of their low vis¬ cosity, those agglomerants can be easily and therefore economi¬ cally dispersed in the slurry in a manner that will produce

the requisite encapsulation of the coal particles by the agglom¬ erant. Specifically, the transport of the liquid agglomerant from the water-solids-agglomerant mixture to the product coal occurs by the impact of dispersed agglomerant on the coal particles and the subsequent wetting of the coal particles by the agglomerant. This process, which tends to homogenize the agglomerant distribution over all of the particles, requires that the viscosity of the agglomerant be below 1000 centipoises and the process becomes increasingly more efficient as the viscosity decreases below that maximum value.

Another advantage of the agglomerants I employ in addition to their efficacy is that they do not react with coal which is important for the reasons discussed in U.S. patent No. 4,173,530 issued November 6, 1979, to Smith et al.

Several advantages of the novel fuel preparation processes described herein have been described above. Another is that they can be employed to produce fuels from raw coals ranging from sub-bituminous through bituminous to anthracite as well as from lignite which has above and will hereinafter also be included in the term "coal" for the sake of convenience.

High ranked, unoxidized coals have a natural hydrophobicity which can be treated by the agglomeration type separation process as described above.

Partially oxidized coals and coals of lower rank, however, lack this natural hydrophobicity to at least some extent be¬ cause of their oxygen content. Hydrophobicity to the desired extent can be induced in such coals by using a surfactant to modify the naturally hydrophilic surfaces of the coal and, in effect, transform it into a hydrophobic coal that responds to the process in the same manner as one that is naturally hydro¬ phobic.

The surface active agent, which may be ό ^' leic acid or one of its soluble salts, is preferably mixed with the slurry prior to the separation and agglomeration of the product coal particles in an amount sufficient to produce a monolayer of surfactant on the coal. The carboxylic acid (or comparable) group

of the surface active agent attaches to the polar surface of the coal ^' , allowing the molecule to establish an apparent coal surface which is repulsive to water because of induced hydro¬ phobicity but possesses a strong attraction to the agglomerant. This allows the lower rank or partially oxidized coal particles to be dissociated from the mineral matter and aqueous phases of the slurry and then agglomerated in the same manner as un- oxidized coals of higher rank.

Excess surfactant must be avoided, however, as the excess will significantly reduce the interfacial energy between the agglomerant and the water in the slurry, causing an increase in the ash content of the product coal agglomerates. To avoid this same undesirable result, care must be exercised to avoid the use of surfactants that would render the surfaces of the mineral matter particles in the slurry hydrophobic.

Strong Lewis bases can also be employed to induce hydro¬ phobicity in partially oxidized and lower ranked coals. Lewis bases can be combined into a single molecule with a hydrophobic, organic chain or ring. The Lewis base moiety of the molecule attaches the latter to the coal particles, and the organic fractions of the compounds form a monolayer of additive that renders the entire surface of each,coal particle hydrophobic. Those surfaces accept the agglomerating agent in a manner identi¬ cal to that characteristic of an unoxidized, high ranked coal.

Lewis base-containing molecules that can be employed for the purposes just described are those of the formulas R-OH, R^-NII.,, R-NH ₂ , and R,N where R is an organic chain or ring with more than four hydrocarbons.

An alternative to inducing hydrophobicity is to increase the agglomeration time for partially oxidized and/or lower rank coals. Unoxidized, high rank coals can be completely agglomerated in periods of <5-15 seconds. By increasing the time to minutes, many oxidized coals can also be successfully agglomerated although others cannot because agglomeration time increases with the state of oxidation, reaching infinity for a fully oxidized coal.

It was pointed out above that U.S. patent No. 4,186,887 discloses a process having some similarities to the novel fuel preparation processes disclosed herein. There are also significant differences.

For example, the fuel preparation processes described herei differ from the coal recovery process described in U.S. patent No. 4,186,887 in that there is no milling during the coal re¬ covery phase of the process* in which the coal particles are dissociated from the mineral matter and aqueous phases of the slurry in which they are found and then coalesced into product coal agglomerates. This is significant because it has been found that wear — for example, of the balls in a ball mill — by prolonged milling continued into the recovery phase can re¬ sult in enough worn away material being agglomerated with the coal to significantly increase the ash content of the latter.

The novel fuel preparation processes disclosed herein also differ significantly from the coal bene iciation process described in U.S. patent No. 4,186,887 in that the addition of the agglomerant to the coal slurry and the subsequent dis¬ sociation of the coal particles from the mineral matter and aqueous phases of the slurry and coalesence of those particles into agglomerates are preferably carried out separately.

As discussed above, the essentially complete separation of the coal particles from the associated mineral matter achieved by the fuel preparation processes described herein requires that a monolayer of the agglomerant be adsorbed on the surface of each coal particle. This can most efficiently be achieved in a different unit than the subsequent separation of the product coal from the slurry because the dispersion of the agglomerant is a kinetic process requiring a finite period of time. By carrying out this step separately, one ^' can insure that the wanted dispersion of the agglomerant is completed before the separation of the product coal from the agglomerant is attempted.

OBJECTS OF THE INVENTION ¹⁰

From the foregoing, it will be apparent to the reader that one primary object of the present invention resides in the pro¬ vision of novel, improved coal-type fuels and ^" in the provision of novel processes for producing those fuels.

Another primary and therefore important object of the present invention resides in the provision of coal-type fuels which are competitive with the heavier grades of petroleum- based fuels.

An additional primary object of the present invention resides in the provision of coal-type fuels which can be em¬ ployed to fuel gas turbine engines.

Still other important, but more specific, objects of the invention reside in the provision of coal-type fuels which: have an extremely low ash content; have a high BTU content; have a particle size distribution that permits them to be burned efficiently; are well within the specifications established by major consumers of such fuels.

Yet other specific but nevertheless important objects of the present invention reside in the provision of novel processes for producing fuels from raw coal which: are capable of producing coal as characterized in the preceding objects; are capable of making such coals available at competitive costs on commercial scales; can be carried out in equipment that is relatively uncom¬ plicated, that only needs low maintenance, that is simple to operate, and that can be made available with only a modest capital investment; can be employed to produce fuels from virtually any coal ranging from lignite through sub-bituminous to anthracite; are non-polluting and energy efficient; can be carried out at ambient temperature and pressure; are capable of recovering up to on the order of or exceeding 95 percent of the coal from the raw coal that is processed.

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11

Still other important objects of the invention reside in the provision of processes with the attributes described above that can be used to produce product coals for use other than as fuels and in the provision of such product coals with the desir¬ able attributes identified above.

Additional, important objects and advantages of the inventi and other novel features thereof will be apparent to the reader from the foregoing; from the appended claims; and as the ensuing detailed description and discussion of the invention proceeds in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIGURE 1 is a schematic diagram of a plant in which the prep aration of a low ash product coal can be carried out in accord with the principles of the present invention;

FIGURE 2 is a graph showing the effect of the interfacial tension between the agglomerant and water on ^' the ash content of a low ash coal prepared in accord with the principles ^' of the presen invention; and-

FIGURE 3 is a graph showing the effect of the energy density of the agglomerant upon the ash content of a low ash coal prepare in accord with those principles.

PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to the drawing. Figure 1 schematically depicts a plant 10 in which raw coal can be converted to a low ash coal having the characteristics discussed above in accord with the principles of the present invention. As is apparent from the drawing, plant 10 is relatively uncomplicated. This makes it eas

to operate, inexpensive and simple to maintain, and available with a relatively low capital investment.

In terms of process steps, the first major component of plant 10 is a feeder 12 which transfers the raw coal being pro¬ cessed to a dry grinder 14 which may be, for example, an impact mill, ball mill, race mill or the like. Dry grinder 14 is employed to reduce the raw coal to a size consist typically about 85 percent 250 microns x 0.

From dry grinder 14, the pulverized raw coal is transferred to a slurry batching vessel 16. Here, the raw coal is mixed with clean water to form an aqueous slurry having a solids content in the range of 20 to 70 wt %. The particular weight percent that is employed depends on the coal and is adjusted to optimize the efficiency of the milling process.

The raw coal slurry is transferred to a slurry storage tank 18 from batching vessel 16. This tank provides a capacitance in the system; i.e., it permits plant 10 to be operated continuously notwithstanding the fact that several steps in the process are carried out in batch fashion as will become apparent herein¬ after.

From raw coal slurry storage tank 18, the slurry is trans¬ ferred to a wet grinder 20 where the raw coal is reduced to a particle size distribution preferably on the order of 95% 30-15 microns x 0 although the smaller top size is preferred because, as discussed above, this results in a fuel which can be more efficiently burned.

The wet grinder may be, for example, a ball mill, stirred ball mill, vibratory mill, roll mill, etc.

I consider it neccessary that a minimum of 20 wt % water based on the weight of the slurry be maintained in mill or wet grinder 20. Lower amounts do not provide a sufficiently large body of liquid to hold the mineral matter in suspension, which is a requisite to efficient and uniform grinding. The maximum concentration of aqueous liquid permitted in wet grinder 20 is that at which comminution of the raw coal becomes inefficient (typically ca. 70 wt %) .

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t- t ro

3

i

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14 the concentration .of solids is reduced during agglomeration, the efficiency of ash reduction is increased.

Also, like storage tank 18, surge tanks 24 and 26 provide capacitance in the fuel preparation system. This provides independence of operation between the milling circuit just dis¬ cussed and the next-to-be discussed circuit in which the product coal particles are separated, agglomerated, and recovered from the slurry. This circuit isolation is desirable because, in the event of malfunction of any of the interconnecting components, the subsequent stages can operate for a substantial period of tim without interruption of subsequent unit processes.

Referring again to the drawing, the more dilute, raw coal slurry is transferred alternately from surge-tank 24 and surge tank 26 to mixer 28 where selected agglomerant is added to and mixed with the slurry in a ratio of 45 to 60 wt % based on the dry weight of the coal in the slurry.

The just specified minimum amount of agglomerant is that which I have found necessary .to effect efficient agglomeration of the coal particles liberated in the milling steps. Concentration above . the stated maximum are undesirable because the excess addi tive forms a film through which substantial numbers of the mineral matter particles may not have sufficient energy to escape resulting in their being trapped in the coal agglomerates and raising the ash content of the product.

High shear mixers have been employed to distribute the agglo erant but are not required as long as the mixer will homogenously disperse the agglomerant within the raw coal slurry in a manner insuring that monolayers of the agglomerant are formed on the sur faces of the product coal particles. High shear mixers do have the advantage that the dispersion of the agglomerant can be effected in a very short period of time.

Mechanically, the formation of the agglomerant monolayers on the product coal particles takes place through particle-partic impact until a liquid film has been formed on each particle.

Probably, these films are of equivalent thicknesses on all of the particles when equilibrium is reached. This will typically require about one minute when a high shear mixer is employed to disperse the agglomerant.

From mixer 28, the slurry is transferred to separator 30 which may be a rotating drum or spheroidizer. Here, the disso¬ ciation of the product coal from the mineral matter and aqueous phases of the slurry and the formation of product coal agglomer¬ ates initiated in mixer 28 are continued and the agglomerates dimensionally stabilized; and water is expelled from the agglom¬ erates, contributing to the quality of the product. Thus, sepa¬ rator 30 serves as a polishing unit for mixer 28. The residence time of the slurry in separator 30 will typically be only a few minutes.

Thus, the just-discussed carrying out of the agglomerant dispersion and product coal agglomeration steps in two different process units is an important feature of the present invention because it permits the conditions in each of- these two units to be optimized for the steps carried out therein.

A fraction of the product coal agglomerates are recovered and discharged directly from separator 30 as indicated by line 32 in the drawing. The remainder of the agglomerates and the aqueous and dispersed mineral phases of the slurry are discharged to a static sieve bend 34. Here, the remainder of the product coal agglomerates are recovered while the water and mineral matter are discharged into a refuse circuit shown schematically in the drawing and identified by reference character 36.

The product coal recovered from separator 30 and sieve bend 34 may have an ash content of less than one percent and a moisture content of σa. 20 percent.

Coal with the specifications described in the preceding para¬ graph is a directly usable, superior fuel. However, the ash content of the fuel can be reduced even further and its useful¬ ness increased by applying the principles of the present invention

16

To accomplish further ash reduction, the agglomerates recovered from separator 30 and sieve bend 34 are transferred to a disper¬ sion tank 38 equipped with a heater 39 where they are mixed with sufficient clean water to reduce the concentration of solids to on the order of not more than about 30 to 10 wt %.

At the same time, the concentration of the agglomerant is lowered to 20-30 wt % based on the weight of the solids in the slurry, typically by evaporating part of the agglomerant from the slurry. Heater 39 may be employed to supply any thermal energy necessary for this purpose that is not available from the ambient surroundings.

Upon being reduced to the level or concentration just iden¬ tified, the agglomerant becomes incapable .of .bonding together the particles of product coal making up the agglomerates. Those par¬ ticles consequently dissociate and disperse in the aqueous carrie freeing and dispersing in the aqueous carrier of the slurry any particles of mineral matter that may have been entrapped in the agglomerates in the initial coal recovery and agglomeration step.

From tank 38, the aqueous slurry of redispersed coal particl and liberated mineral particles is transferred to a mixer 40 whic may be of the same character as the mixer 28 discussed previously Here, sufficient agglomerant is mixed with.the slurry to again increase its concentration to the 45 to 60 wt % of agglomerant based on dr,y coal weight required for efficient agglomeration and recovery of the product coal.

The aqueous slurry of redispersed coal particles, freed mineral particles, and agglomerant is next transferred to a separator 42 which may duplicate separator 30. After agglomera¬ tion, and stabilization of the agglomerates is completed, a fract of the coal particle agglomerates are separated and discharged di ectly from the separator as indicated by line 44. The remainder the agglomerates, together with the additional mineral matter dissociated from the coal in separator 42 and the aqueous carrier are passed over a static bend sieve 46, the coal being discharged to line 44 and the water and mineral matter to refuse circuit 36.

17

At this stage, the product is eminently suitable as a fuel as it will typically have a heat content approaching 15,000 BTU _.while the ash content of the product will typically have been reduced another two-thirds from 3 to 1 percent to 1 to 0.3 wt % based on the dry weight of the product. The moisture of the product coal can be controlled from 10 to 40 wt % by way of the process parameters. Additional moisture can be removed by pass the agglomerates through wringer rolls (not shown) although thi will typically not be necessary.

The combined .fractions of product coal agglomerates from separator 30 and separator 42 are processed seriatim through an evaporator 48 and a stripper 50. The agglomerant is recovered the agglomerated coal particles in these units; circulated to a agglomerant recovery system 52 where it is freed of non-condens gases and condensed; and then returned to agglomerant storage t 54, all as described in above-cited patent No. 4,173,530 which i hereby incorporated herein by reference. The mixture of water and dispersed mineral matter in circuit 36 may be transferred to an agglomerant scrubber (not shown) which reduces the agglomeran content of the" refuse from about 100 ppm to less than 10 pp . Thereafter, the agglomerant is combined with that recovered from the product and elsewhere in system 10.

The slurry passes to a conventional thickener (also not sho where the water is clarified and recycled. The now semisolid refuse is transferred to a landfill, for example.

The examples which follow describe tests illustrating var¬ ious novel facets of my novel fuel preparation processes.

EXAMPLE I

As suggested above, perhaps the most important advantage of the novel fuel preparation process described herein is the extre low ash content that can be obtained. This was demonstrated by the following head-to-head test between the foregoing process an the coal beneficiation process disclosed in U.S. patent No. 4,186,887.

-18-

A s-ample of raw coal from the Pittsburg seam (Bethlehem Marianna Mine Wo. 58) was separated into two fractions. One was treated as specified in Example I of U.S. Pat-ent No. 4,186,887 -and the other by the following- procedure.

The raw coal was dry milled to 250μm x 0 in a hairmer mill and mixed with tap water to a 30 wt % solids concentration. The pH of the resulting slurry was adjusted to 8 by adding sodium hydroxide, -and the slurry was then ground in a laboratory ball mill for 16 hours. The resulting slurry was ronoved frcm the ball mill and diluted to 10 wt % solids. The diluted slurry was placed in a Waring Blender and 50 wt % (based on dry coal) of l,l,2-trichloro-l,2,2-triflx>roethane was added with the blender running to separate and agglomerate the coal particles. Upon agglomeration (30-60 sec) the contents of the blender were removed and passed over a sieve bend which retained the coal agglomerates and allowed the mineral matter-water slurry to pass.

The particles making up the product coal agglomerates were redispersed by adding sufficient wat-er to produce an aqueous slurry with a solids content of ca. 10 wt % and allowing agglomerant to evaporate until the agglomerates could be seen to have dissociated. Agglomeration of the redispersed particles and separation of the agglomerates that formed were effected using the procedure described above; and the sequence of redisper¬ sion, agglomeration, and separation of the agglomerates was repeated.

The agglomerates obtained in the third separation step were dried and analyzed. The following data was obtained:

TABLE I

Raw Coal Product, U.S. Patent Product Analysis No. 4,186,887 Technique Present Technique

Ash, Wt % 5.71 2.38 0.89 t/tøM. ETU 3.99 1.60 0.59 % Reduction 59.9 85.2

Total Sulfur, Wt % 1.24 0.90 0.82 #/MYLBTU 0.87 0.61 0.54 % Reduction 29.9 37.9

BTU/lb 14,301 14,842* 15,093*

BTU/lb (MM ¹ )** 15,167 15,204 15,229

BTU Yield, % >95 >95

fe w &i

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xi.O following the procedure described in Example I were further processed with a nitric acid leach. The tests involved a Blue Gem seam product coal with an ash content of 0.4 wt % and Pittsburg seam product coals with ash contents of 1.1 and 0.9 wt

The acid leach was carried out by refluxing dry samples of coal in 4 normal nitric acid for 30 minutes, recovering the resi¬ due, and drying and ashing it according to ASTM procedure D3174-73.

Results of the tests are tabulated below.

TABLE 2

Seam Ash Content

. Acid Leached Product Coal Product Coal

Blue Gem 0.4 0.08

Pittsburg 1.1 0.43

Pittsburg 0.9 0.38

EXAMPLE III

To demonstrate the effect of milling time (wet) on the ash content of fuel prepared in accord with the principles of the present invention, coal from the Blue Gem seam having a particle size distribution of 63μm x 0 was placed in a laboratory ball mil for various periods of time to effect different particle size reduction and to produce different average particle sizes (define as 50 wt % of the particles finer than the average particle dia¬ meter) . That raw coal was milled in water at a concentration of 30 wt % solids.

After milling, the 30 wt % solids slurry was diluted to 10 wt % and placed in a Waring Blender. About 50 wt % of 1,1,2-tri- chloro-l,2,2-trifluoroethane (based on dry raw coal weight) was added and mixed with the slurry until agglomerates of coal par¬ ticles were formed (about 15-45 seconds) . The agglomerates were separated on a sieve bend, the coal collecting on the surface of the sieve bend and the water (plus mineral matter) passing throug the sieve bend.

21 The results are shown in Table 3 below .

TABLE 3

Product Average Particle Size Distribution Diameter (Microns ) Ash (Wt

Raw coal, 63μm x 0 20 0 . 97 4 hours milling, 32μm x 0 5. 7 0. 58 8 hours milling, 23μm x 0 3. 5 0. 52 16 hours milling, llμ x 0 2. 5 0. 49

EXAMPLE IV Another above-discussed, demonstrably effective technique that can be employed in the fuel preparation processes disclosed herein involves the use of a basic material in the second (wet) of the milling steps to adjust the pH of the slurry being treate in that step to, and to maintain it at, a pH in the range of 6-1 This was shown by a test which was conducted with the same coal and procedure as discussed in Example I except that the pH was adjusted as indicated in Table 4 below in which the data obtaine from the test are tabulated.

TABLE 4 pH of Slurry Product Coal, Wt % Ash

1st Collection 2nd Collection 3rd Collect

Acidic pH <3 1.78 1. 29 1. 17

Basic pH >8 _. 1.19 0. 72 0. 68

Neutral pH -1 1.32 1. 04 0 . 95

EXAMPLE V

That the amount of milling, and, consequentially, the degre of particle size reduction, has marked effect on the ash content of the product coal generated by the techniques disclosed herein was also demonstrated by tests employing commercially beneficiate 1.5 inch x 0 coal from the Illinois No. 6 (Herrin) seam located a the Old Ben Coal Company No. 21 Mine, Franklin County, Illinois.

In accord with the present invention, that raw coal was dry

OMPI

22 ground to a size consist of 250 microns x 0 and formed into a 30 wt % slurry with water. The coal was then milled and agglom¬ erated as described in Example III, and the product coal agglom¬ erates were recovered and analyzed.

The results are tabulated in Table 5 below.

TABLE 5

Illinois No. 6 Seam

Product Coal . Ash Product Coal Ash

Milling Regime (Wt % ) Sample A (Wt %) Sample B Raw Coal (250μm x 0) 6. 36 6.82 2 hours milling 3. 04 3.07 4 hours milling • 1. 75 2.36 8 hours milling 1. 13 1.22 6 hours milling 1. 23 1.19

EXAMPLE VI

As discussed above, the use of an agglomerant which has a high interfacial tension with water is essential to the success of the process described herein. This was demonstrated by repeat ing the procedure' described in Example I using two different coal and a variety of agglomerating agents or agglomerants. The resul appear in Tables 6 and 7 below.

22A

TABLE 6

Peerless Coal (Raw Coal: 37μm x 0, 14.88 Wt % Ash)

Interfacial Tension Product Coal

Agglomerant With Water "(dynes/cm) Wt* % Ash

No. 6 Fuel Oil 32 6.5

Benzene 35 4.5

Gasoline 45 2.6

Carbon Tetr'achloride 45 2.8

Kerosene 50 2.6

Mineral Oil 50+ 2.8

Pentane 51.3 2.8

Trichlorofluoromethane 52.8 ^' 2.5

1,1,2-Trichloro-l,2,2-tri- fluoroethane 51.1 2.0

2-Methyl butane 50.1 2.0

23 TABLE 7

Ulan Coal (Australia; Raw Coal Ash 10.55 Wt %)

Wt % Ash, Product Coal Interfacial Tension 1st 2nd 3r

Agglomerant With Water Collection Collection Colle

No. 6 Fuel Oil 32 dyne/cm 6.53*

Benzene 35 4.55 2.40 1.8

Gasoline 45 2.85*

Carbon Tetra- chloride 45 2.88

Kerosene 50 2.44

Mineral Oil ^' 50+ 2.40*

Pentane 51.3 2.51

2 Methyl Butane 50.1 2.01

1,1,2-Trichloro-

1,2,2-trifluoro- ethane 51.1 2.04 1.08 0.8

*SubseQuent collections proved impossible as there is no way to effect further coal dispersal after one collection.

The foregoing data confirm ^' that the use of an agglomerant having a high interfacial tension with water is important in pro¬ cessing coal by the procedures described herein. The data also show that this requirement exists independently of the particular coal being treated.

That a high agglomerant-water interfacial tension is a requisite to the generation of a low ash coal by agglomeration type separation is also demonstrated by the data tabulated graphically in Figure 2, in which the ash content of a product coal generated from a Peerless seam sample is plotted against interfacial tension. The raw coal was processed essentially as described in Example I.

EXAMPLE VII The concentration of the agglomerant employed in the practice of the invention disclosed herein can be varied considerably as long as the amount used meets the criteria specified above. This was confirmed by repeating the procedure described in Example I substituting various weight percents of 1,1,2-trichloro-l,2 _/ 2*r trifluoroethane based on the weight of the raw coal for that used in the test described in the earlier example. The results are,

24 tabulated below and compared with those reported in Example I.

TABLE 8

Wt % of Agglomerant Produce Coal Wt % ASh, Based on Dry Coal Weight Second Agglomeration Step

40 1.07

50 (Example I) 0.93

60 1.16

70 1.18

80 2.31

EXAMPLE VIII

To demonstrate the advantages of redispersion and reag- glomeration, a Pittsburgh seam coal with an ash content of 4 wt and a Blue Gem seam coal with an ash content of 3 wt % were processed as described in Example I (three agglomerations, ini¬ tial and two following redispersion of collected agglomerates) .

Samples of the product coal were taken from each agglomera¬ tion before the next subsequent agglomeration step (a fourth col¬ lection did not significantly alter the ash in the product coal) .

The results of the tests are set forth in Table 9.

TABLE 9

Mill Product Coal Wt % Ash

. Time First Second Third

Run Coal Additi.ve 3 (hours) Collection Collection Collection

1 PGH ¹ None 8 1.70 1. .16 1.05

2 PGH A 8 • 0. .95

3 PGH B 8 1.44 0. ,84 0.88

4 PGH C 8 1.41 0. .93 0.85

5 PGH D 8 1.11 0. .64 0.59

6 PGH E 23 1.10 0. .66 0.52 to cπ

7 PGH F 23 • 1.26 0. .60 0.59

8 PGH G 8 1.34 0, .86 0.73

9 PGH H 8 1.26 0. .82 0.72

10 PGH I 16 0.83

11 BG ² None 4 0.69 0.55

12 BG None 16 0.64 0.40

1. PGH ■= Pittsburgh Seam

2. BG = Blue Gem Seam

3. Identified in Table 10

26

^• TABLE 10

Concentration

(Wt % Based on Weight

Code Additive of Raw Coal)

A NaOH pH adjusted to 8

B NaCN-NaOH 1.4 lbs/ton, pH adjusted to

C NaCN-Na ₂ C0 ₃ 1.4 lbs/ton, pH adjusted to

D Citric Acid-NaOH 0.7 lbs/ton, pH adjusted to

E Citric Acid 1.4 lbs/ton

F Citric Acid-NaOH 1.0 lbs/ton, pH adjusted to ^'

G Citric Acid-NaOH 1.4 lbs/ton, pH adjusted to

H Citric Acid-Sodium 2 lbs/ton

Dithionate

Methanol 3 lbs/ton

The data in Table 9 above are significant for several dif¬ ferent reasons.

First, they show that substantial, additional amounts of ash can be separated by dispersing product coal agglomerates and repeating the agglomeration step and/ also, by repeating the dispersion and agglomeration sequence of steps.

Also, the data confirm that milling additives can have a marked effect on the ash content of the product, coal. This, additionally, underscores the importance of carefully matching the additive to the specific coal being processed into fuel by the procedures disclosed herein.

Furthermore, the data clearly demonstrate that the fuel preparation process in question is eminently capable of pro¬ ducing fuels which have sufficiently low ash ^' contents (ca. 0.5 wt % or less) to make them competitive with petroleum-based fuels.

EXAMPLE IX

To demonstrate the wide ranging utility of the novel fuel preparation processes described herein, the procedure described in EXAMPLE I was repeated using as feedstocks bituminous coals with considerable different morphologies and compositions

27 than the Pittsburg seam Coals employed in the EXAMPLE I tests and an anthracite coal. The variations in the EXAMPLE I pro¬ cedure utilized to optimize the procedure for the different feedstocks are identified and the results of the tests tabulated below.

TABLE 11

Coal Procedural Modifications t % . Ash

Anthracite (Ta aqua, PA) Raw Coatl 7. 98

Mill <D pH < 3; Product Coal ϋ. 76

Bituminous (Ulan, Australia) Raw Coal, 10. 55

Product Coal 0. ,84

Bituminous (Peerless Seam) Raw Coal 14. ,88

Product Coal 0. .95

Bituminous (New Zealand) Raw Coal 1. .00

Product Coal 0, .22

•

29

EXAMPLE X

It was pointed out above that compounds in which a Lewis base is combined with an organic chain or ring can advantageousl be employed to impart hydrophobicity to the surfaces of partiall oxidized or lower ranked coals, thereby increasing the number of sites available to the ^' agglomerant to the level possessed by an unoxidized, higher rank coal and that, as a consequence, the agglomeration type coal separation processes described herein can proceed as efficaciously in the former cases as it does in the latter. This novel aspect of the present invention was demonstrated in tests which were conducted as described in EXAMPLE I with the modifications identified in TABLE 12 below in which the results of the tests are also presented.

TABLE 12

Lewis Base

Ash, Raw Additive (2 lbs /ton Ash, Produ

Coal Coal (Wt %) of Coal) Agglomeration Time Coal (Wt %

Ulan (Aus¬ 10.5 None 3.5 mins 0 . 95 tralia)

Ulan (Aus¬ 10.5 Surfonyl 104E 15-20 sees 0. 95 tralia)

Peerless Seam 14.89 None 1.5 mins 0 . 98

Peerless Seam 14.89 Triton X-114* 15 sees 0. 98

Ohio No. 9 24.0 None Instantaneous 1 . 8

(Freshly

Milled)

Ohio No. 9 24.0 None Could not be aggl (aged 2 wks) ated

Ohio No. 9 24.0 Surfonyl 104E Instantaneous 1. 8 (Aged 2 wks)

*An octylphenoxypolyethoxy ethanol marketed by Rohm and Haas Co.

31 High interfacial tension with water was identified above as one of the important characteristics that an agglomerant must possess to be efficacious in the processes disclosed herein. That parameter can be safely relied upon to choose suitable agglomerants; but, at least theoretically, the more fundamental, but related, relationship of importance appears to be the free energy of mixing of the agglomerant with the water in the aqueous, raw coal slurry. The free energy of mixing between two liquids (AF ) can be determined by the Scatchard-Hildebrand free energy of mixing equation (which follows) :

ΔF _m = ⁽ x _{1 1} ÷ x ₂ ^v 2 ⁾ ^ ^δ l ^{+ δ} 2 ² ~ ^{2Φ δ} l ^δ 2 *1*2 ^{+ R (x} ι ^lnx ι ^{+ x}

where x = mole fractions of liquids (1) and (2) v = molar volumes (1,2) = volume fractions (1,2)

2 δ -= energy densities of liquid (1) and (2) φ ^* -= interaction parameter varying from 0.54 for highly immiscible systems to 1.0 for very similar systems; e.g., water-alcohol, etc.

R — gas constant

T = absolute temperature

The first and last terms of the foregoing equation are constant for a given slurry or system as are the interaction parameters and the energy density of water. Consequently, the energy of free mixing in the processes described herein is determined by the energy density of the agglomerant, which therefore becomes the controlling factor in determining the efficacy of an agglomerant in a particular slurry.

A plot of the square root of the energy density (δ 2) of the agglomerant versus the wt % ash in the recovered product coal is a monatomic curve which decreases from a high, ash-high energy density value for No. 6 Fuel Oil to a low ash-low energy density value for 1,1,2-trichloro-l,2,2-trifluoroethane.

32

The free energy mixing equation more accurately identifies the requisite relationship between water and an efficacious agglomerant because there appear to be properties of liquids othe than high interfacial tension — such as low mutual solubilities of the agglomerant in water and of the water in the agglomerant that are also significant. These other properties are all taken to account in the interaction parameter (Φ) in the free energy of mixing equation set forth above. Nevertheless, that interfacial tension remains a valid practical criteria for selecting an agglomerant is apparent because the interaction parameter and the energy densities involved in the Scatchard-Hildebrand free energy of mixing equation have the same origin as those employed in deriving the equations for interfacial energies (see R.J. Good and E. Ebling, "Generalization of Theory for the Estimation of Interfacial Energies", Chemistry and Physics of Interfaces II, Amercian Chemical Society, Washington, D.C., 1971, pp. 71-96) .

The novel'processes disclosed herein have been identified as methods for preparing low ash fuels for the most part. This was done for the sake of convenience and is not intended to limit the scope of the present invention as the low ash coal generated by the present invention can equally well be used for other purposes. For example, it has properties which make it an unparalled feedstock for coal gasification processes (see, for example, U.S. patent No. 4,034,572 issued December 8, 1981, to Wiese et al) .

Also, many modifications may be made in the process without exceeding the scope of the invention. For example, mill 20 can be replaced with a two-stage milling system consisting of a ball mill for reducing the raw coal from some top size larger than 1/4 inch to at least 100 mesh (150μm x 0) and possibly to 200 mesh (74μm x 0) . The product from this mill is sized in a device such as a centrifuge.

33

The 15μm x 0 overflow from the centrifuge is transferred to mixer '28, and the +15μm material is cycled through an attritor (stirred ball mill) . The output from the attritor is discharged into the centrifuge (in such an arrangement the attritor serves to quite rapidly reduce the 100 x 15μm recycled raw coal to 15μm x 0) .

As a second example, mixer 28, separator 30, and static sieve bend 34 may be.replaced by a cyclone circuit where the 30 to 70 wt % slurry is diluted with more water and agglomerant under vigorous mixing conditions (pumping turbulence) and the agglomerated coal (specific gravity-^^1.45) separated from the water-mineral phase in a cyclone. Several tests of this device have demonstrated its efficiency in the preparation of low ash coals. For example, the preparation of 0.5 wt % ash Blue Gem seam coal has been carried out in this manner.

The invention may be embodied in still other specific forms without departing from the spirit or essential characteristics thereof. The embodiments of the invention disclosed above and in the drawing are therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention is instead indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims ar intended to be embraced therein.