US3655350A

US3655350A - Coal pellet and a method of manufacturing same

Info

Publication number: US3655350A
Application number: US399A
Authority: US
Inventors: Ronald W Utley
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1970-01-02
Filing date: 1970-01-02
Publication date: 1972-04-11
Anticipated expiration: 1989-04-11

Abstract

A coal pellet which is resistant to hydration and has good compressive strength and resistance to impact, containing fine particles of coal, and a coal tar pitch binder having a softening point of about 90* F. to about 190* F. and having a moisture content not greater than 10 percent. The coal pellet is produced by spraying coal tar pitch heated to a temperature of about 300* F. to about 600* F. onto fine particles of coal having a moisture content of about 12 percent to about 30 percent in a mixing vessel, pelletizing the resultant mixture and drying the pellets to the desired moisture content.

Description

United States Patent Utley [151 3,655,350 [451 Apr. 11, 1972 [54] COAL PELLET AND A METHOD OF MANUFACTURING SAME [72] Inventor: Ronald W. Utley, Bethlehem, Pa.

[73] Assignee: Bethlehem Steel Corporation [22] Filed: Jan. 2, 1970 [2]] Appl. N0.: 399

Related [1.8. Application Data [63] Continuation-impart of Ser. No. 732,449, May 27,

1968, abandoned.

[52] US. Cl. ..44/10 R, 44/19, 44/23 [51] lnt.Cl ..C10l5/00,Cl015/l6 [58] Field of Search ..44/23, 10, 24, 19

Primary ExaminerC. F. Dees Attorney-Joseph J. OKeefe [5 7] ABSTRACT A coal pellet which is resistant to hydration and has good compressive strength and resistance to impact, containing fine particles of coal, and a coal tar pitch binder having a softening point of about 90 F. to about 190 F. and having a moisture content not greater than 10 percent. The coal pellet is produced by spraying coal tar pitch heated to a temperature of about 300 F. to about 600 F. onto fine particles of coal having a moisture content of about 12 percent to about 30 percent in a mixing vessel, pelletizing the resultant mixture and drying the pellets to the desired moisture content.

12 Claims, 3 Drawing Figures WA TEAPK'OOF INDEX PATENTEDAPR 11 I972 SHEET 3 [1F 3 INVENTOR Rona/0 14 Z/f/ey COAL PELLET AND A METHOD OF MANUFACTURING SAME CROSS-REFERENCES TO RELATED APPLICATION BACKGROUND OF THE INVENTION Raw coal is generally prepared for commercial use by crushing, grinding and sizing to remove refuse at the mine. In the process, a percentage of the coal becomes so fine that when the coal is dried the fine particles are blown out of the stack and thereby pollute the atmosphere at or near the drying plant. In the case of coking coals, the fine particles although sufficiently low in sulfur and ash-forming constituents to be used in making coke either smother the burning action in the ovens or are blown out causing a pollution problem.

Attempts have been made to agglomerate these fines by mixing them with cold emulsions of petroleum tar pitches or coal tar pitches, briquetting and then heating. Another way is to heat such mixes to relatively high temperatures to make fairly fluid pastes which are then briquetted. However, the pitches become sticky when heated making it difficult to briquette or extrude the pastes. The briquettes or extrusions so formed also adhere to each other causing large lumps to fon'n. Agglomerates thus formed give ofi pungent noxious blackish or brownish vapors when heated which are highly objectionable. The pitches break down, forming gases which result in porous agglomerates which are weak in structure.

Attempts to pelletize the fine coal particles mixed with binders such as lignin or bentonite have not been commercially successful because the pellets have insufficient strength to be handled when they contain over 3 percent moisture. These pellets are not resistant to hydration and cannot be stored. Attempts to drying these pellets to below 3 percent moisture to obtain sufl'rcient strength for handling is costly and hazardous. Costly, because handling the pellets during drying is extremely difficult due to an almost complete loss of strength as the moisture content is decreased and drying times must be increased to obtain the necessary low moisture levels necessary for an increase in the strength level wherein the pellets can be handled with reasonable care. Hazardous, because at the long drying times at the drying temperature required to obtain the low moisture contents necessary for strength, the coal is susceptible to spontaneous combustion or explosion.

It is the primary object of this invention to provide a coal pellet which is resistant to hydration and has good compressive strength and resistance to impact at relatively high moisture contents.

It is a further object of this invention to provide a coal pellet which does not require drying to below 5 percent moisture content.

It is another object of this invention to provide a coal pellet which does not lose its strength when it is dried below percent moisture content.

It is a still further object of this invention to provide a method for producing a coal pellet which is simple, inexpensive and non-hazardous.

SUMMARY OF THE INVENTION Broadly the invention is directed to a coal pellet produced by mixing wet fine particles of coal and a coal tar pitch binder of a suitable softening point, in a suitable vessel, pelletizing the mixture and drying the resultant pellets to remove a major portion but not all of the moisture therein. The coal pellet will not lose its strength when dried below 10 percent moisture content.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a graph comparing the impact strength of the coal pellets of the invention and pellets containing a bentonite binder.

FIG. 2 is a graph comparing the compressive strength of the coal pellets of the invention and pellets containing a bentonite binder.

FIG. 3 is a graph comparing the hydration resistance of the coal pellets as measured by the waterproof index of the invention and pellets containing a bentonite binder.

DESCRIPTION OF THE PREFERRED EMBODIMENT Fine particles of coal, which may be as fine as 100 X 0 Mesh Tyler Sieve Size, having a moisture content of not more than about 30 percent are charged into a mixing vessel, for example, a mixer-muller. A coal tar pitch binder having a softening point of about F. to about 190 F. as determined by ASTM D36-66T is heated to about 300 F. to about 600 F. and may be sprayed onto the particles of coal. A sufi'rcient amount of the coal tar pitch binder is sprayed into the vessel to obtain a mixture which contains about 0.5 percent to about 5.00 percent coal tar pitch and about percent to 99.5 percent coal fines. It is preferred to have a mixture which is about 2.5 percent to about 5 percent coal tar pitch. The mixture is mixed for a time sufiicient to obtain a uniform product. The mixture is then passed to an apparatus for pelletizing, for example, a balling cone or a disc pelletizer, to form pellets, preferably within the size range of one-eighth inch to three-fourths inch diameter. The pellets are then dried in a suitable drier, for example, a fluid bed drier. The pellets have the necessary strength and hydration resistance at moisture levels below about 10 percent, to resist degradation during handling and to be stored for long periods of time, for example, 3 months. It is preferred to reduce the moisture level to between about 2 percent and 5 percent. The desired moisture level may be obtained by heating the pellets within a temperature range of about 200 F. to about 1,000 F. in a conventional pan or fluid bed drier for a sufficient length of time, for example, in a pan drier at 300 F. for 45 minutes or in a fluid bed drier with an inlet gas temperature of about 750 F. for 45 seconds. Unexpectedly, the green pellets, that is, the pellets which have not been subjected to a drying step, have sufiicient strength to resist degradation during the drying operation thus virtually eliminating dusting. As a result, pollution of the surrounding atmosphere during drying is no longer a problem.

Referring now to the drawings, FIG. 1 compares the impact strength of pellets containing 97.25 percent wet coal fines, 2.50 percent of coal tar pitches having three different softening points and 0.25 percent lignin and pellets containing 97.25 percent coal fines and 2.75 percent bentonite. The impact strength test consists of dropping the pellets onto a steel plate from a height of 18 inches and recording the number of times the pellets are dropped without breaking or spalling. Curves a, b and c represent pellets made with coal tar pitches having softening points of 96 F., F. and F. respectively. Curve 11 represents pellets made with a bentonite binder. Increasing the moisture content of the pellets above about 10 percent increases the irnpact strength as determined in the drop test. However, this increase is misleading because the pellets are too soft and become flattened. Therefore, such increase in moisture content is undesirable and the maximum allowable moisture that may be permitted is about 10 percent. It will be noted that the resistance to impact of all the pellets decreased as the moisture content decreased (reading from right to left on the graph). However, the resistance to impact of the pellets containing bentonite dropped to a lower level than any of the pellets made with the coal tar pitches. Note, that at 10 percent moisture content, pellets represented by curves a, b and c had resistance to impact of about six drops, eight drops and seven drops respectively while the pellets represented by curve at had a resistance to impact of only one drop. While the resistance to impact decreased slightly at 8 percent moisture, the decrease was not significant. Continued drying to lower moisture contents increased the resistance to impact of all the pellets. At a moisture level of 3 percent the resistance to impact of the pellets represented by curves a, b, c and d was about seven drops, about nine drops, about eight drops and about 3.5 drops respectively. The pellets containing the coal tar pitches had better resistance to impact than the pellets containing bentonite. It will be noted that there is a dramatic increase in the resistance to impact in the pellets containing bentonite when they are dried to below 2 percent moisture content. However, to dry the pellets to this moisture level considerably increases the danger of explosion and fire in the drier and increases the cost of the operation making it economically unfeasible to obtain this degree of dryness. It will be noted that although the pellets containing coal tar pitch have sufficient strength at moisture contents as high as 10 percent to be usable, it is preferred to remove sufficient moisture to obtain a moisture level of about 2 to 4 percent.

The compressive strength of the bentonite pellets, curve d, is compared to pellets containing the coal tar pitches curves a, b, and c in FIG. 2. As noted on the graph, curve a loses its strength progressively until its strength is virtually nil at a moisture content of about 9 percent. On the other hand, the pellets of the invention, curves a, b and c, progressively increase in strength as the moisture content decreases. The compressive strength of the pellets represented by curves a, b, and e have as much strength and more at 9 percent moisture content than the pellets represented by curve d have at 3 percent moisture content. This progressive linear increase in strength as the moisture content decreases is entirely unexpected and results in the achievement of the invention, namely, the production of a coal pellet having good resistance to hydration, good compressive strength and resistance to impact. The result is achieved by the use of coal tar pitches having a softening point in the range of about 90 F. to about 190 F. As in the impact strength tests, the bentonite pellets have little compressive strength until the moisture level is about 2 percent and the compressive strength increases rapidly below this level whereas the pellets containing the coal tar pitches have sufficient compressive strength at moisture levels as high as 10 percent, although it is preferred to reduce the moisture level to about 2 to 4 percent.

As will be noted in FIG. 3, the bentonite pellets, curve d, have virtually no resistance to hydration as determined by a waterproof index test at any moisture level whereas the pellets containing the coal tar pitches, namely curves a, b and c, have good resistance to hydration at moisture levels as high as 6 percent. At moisture levels below about 4 percent, the resistance to hydration is excellent.

The waterproof index test consists of placing 400 grams of pellets containing a known amount of moisture and of a known size, for example one-half inch in diameter, in a container. The bottom of the container has a plurality of apertures of a size large enough to allow particles less than one-eighth inch in diameter to pass therethrough. The pellets are sprayed with 2 gallons of water per minute for minutes from a height of 15 inches to simulate a rainfall of 70 inches of water. The pellets are then dried and weighed. The percent of solid material remaining in the container is the weatherproof index number.

As seen in the drawings, the pellets having a moisture content below about 10 percent and containing a coal tar pitch with a softening point of 120 F curve b, have the best combination of properties. The pellets with moisture content below about 10 percent but containing coal tar pitches with softening points of 96 F. and 150 F., curves 0 and c respectively, have substantially equivalent properties but these properties are not as good as the pellets represented by curve b. However, all of these above mentioned pellets have superior properties to coal pellets with moisture contents between 2 percent and 10 percent made with bentonite, curve d in the drawings.

All tests upon which the curves are based were conducted on pellets having a diameter of 0.50 inch.

The compressive strength tests were conducted on a standard soil test apparatus.

Of course, it must be realized that the time of drying, the temperature of drying, the size of the pellets to be dried and the moisture content of the pellets are interrelated. At high temperatures less time is required to obtain the desired moisture level in pellets of the same size and moisture content than at lower temperatures. Smaller pellets may be dried in less time than larger pellets at the same temperature. Pellets having high moisture levels must be held for a longer time at the same temperature level to obtain the desired moisture level.

It has also been found that coal tar pitches having a softening point below F. when added to the wet fine-particles of coal do not produce pellets having sufficient strength to resist degradation during drying and handling. They may break up during drying causing dust to be carried into the atmosphere. Coal tar pitches having a softening point above 190 F. may increase the compressive strength of the pellets, however, the pellets become brittle and shatter upon initial impact and cannot be handled by the usual commercial methods of conveying and shipping.

It should be understood, in this specification and claims, that wherever percentages are referred to such percentages are by weight.

In a specific example of the invention, 80 pounds of coal fines having a moisture content of 18.5 percent and a particle size of 28 mesh Tyler Sieve Size were placed in a high speed mixer. A coal tar pitch having a softening point of F. as determined by ASTM D36-66T was heated to 400 F. and sprayed into the mixer to form a final dry mix of 97.75 percent coal by weight and 2.25 percent coal tar pitch by weight. A small portion of lignin liquor was added so that the final mix contained 0.25 percent lignin. Lignin tends to bring the water in the coal fines to the surface to aid in drying but does not necessarily add to the strength of the pellets. The mix was pelletized in a standard 18 inch disc pelletizer to form pellets of 0.5 inch diameter. The pellets were dried in a fluid bed drier with an inlet gas temperature of 750 F. for 40 seconds with a space velocity of 20 feet per second to a moisture content of 2.0 percent moisture. The pellets were subject to a drop test in which the average number of drops without failure when hot was eight and when cool was seven. The compressive strength when hot was 6 pounds and when cool was 9.4 pounds. The resistance to hydration as determined by the waterproof index test was 98.

I claim:

1. A method for manufacturing balled coal pellets which are resistant to hydration and which have good compressive strength and resistance to impact and which do not lose their strength during drying, comprising:

a. charging fine particles of coal having a moisture content of not more than about 30 percent moisture into a mixer,

b. spraying a coal tar pitch having a softening point of about 90 F. to about 190 F. at a temperature of about 300 F. to about 600 F. onto the coal particles and operating the mixer to produce a mix consisting essentially of about 99.5 percent to about 95.0 percent fine coal particles and about 0.5 percent to about 5.0 percent coal tar pitch,

c. balling said mix to form pellets, and

d. drying the balled pellets to a moisture content not in excess of about 10 percent.

2. The method of claim 1 in which the coal tar pitch of step (b) has a softening point of about 1 10 F. to about F.

3. The method of claim 1 in which the pellets are dried in step (d) to a moisture content of 2 to 5 percent.

4. The method of claim 2 in which the pellets are dried in step (d) to a moisture content of 2 to 5 percent.

5. A balled coal pellet which is resistant to hydration and which has good compressive strength and resistance to impact and which does not lose strength while being dried, said pellet consisting essentially of about 99.5 percent to about 95.0 percent fine particles of coal and about 0.5 percent to about 5.0 percent coal tar pitch having a softening point of about 90 F. to about F., and a moisture content not in excess of about 10 percent.

6. The coal pellet of claim 5 in which the coal tar pitch has a softening point of about 1 10 F. to about 130 F.

7. The coal pellet of claim 5 in which the moisture content of the pellet is about 2 percent to about 5 percent 8. The coal pellet of claim 6 in which the moisture content of the pellet is about 2 percent to about 5 percent.

9. A balled coal pellet which is resistant to hydration and which has good compressive strength and resistance to impact and which does not lose strength while being dried, said pellet consisting essentially of about 99.5 percent to about 95.0 percent fine particles of coal and about 0.5 percent to about 5.0 percent coal tar pitch having a softening point of about 90 F.

to about 190 F., about 0.25 percent lignin, and a moisture content not in excess of about 10 percent.

10. The coal pellet of claim 9 in which the coal tar pitch has a softening point of about 1 10 F. to about F.

l l. The coal pellet of claim 9 in which the moisture content of the pellet is about 2 percent to about 5 percent.

12. The coal pellet of claim 10 in which the moisture content of the pellet is about 2 percent to about 5 percent.

Claims

2. The method of claim 1 in which the coal tar pitch of step (b) has a softening point of about 110* F. to about 130* F.
3. The method of claim 1 in which the pellets are dried in step (d) to a moisture content of 2 to 5 percent.
4. The method of claim 2 in which the pellets are dried in step (d) to a moisture content of 2 to 5 percent.
5. A balled coal pellet which is resistant to hydration and which has good compressive strength and resistance to impact and which does not lose strength while being dried, said pellet consisting essentially of about 99.5 percent to about 95.0 percent fine particles of coal and about 0.5 percent to about 5.0 percent coal tar pitch having a softening point of about 90* F. to about 190* F., and a moisture content not in excess of about 10 percent.
6. The coal pellet of claim 5 in which the coal tar pitch has a softening point of about 110* F. to about 130* F.
7. The coal pellet of claim 5 in which the moisture content of the pellet is about 2 percent to about 5 percent.
8. The coal pellet of claim 6 in which the moisture content of the pellet is about 2 percent to about 5 percent.
9. A balled coal pellet which is resistant to hydration and which has good compressive strength and resistance to impact and which does not lose strength while being dried, said pellet consisting essentially of about 99.5 percent to about 95.0 percent fine particles of coal and about 0.5 percent to about 5.0 percent coal tar pitch having a softening point of about 90* F. to about 190* F., about 0.25 percent lignin, and a moisture content not in excess of about 10 percent.
10. The coal pellet of claim 9 in which the coal tar pitch has a softening point of about 110* F. to about 130* F.
11. The coal pellet of claim 9 in which the moisture content of the pellet is about 2 percent to about 5 percent.
12. The coal pellet of claim 10 in which the moisture content of the pellet is about 2 percent to about 5 percent.