US3805713A - Disposal of ash from coal preparation plant tailings - Google Patents

Abstract

The ash in coal preparation plant tailings is converted into inert water-insoluble pellets by continously delivering the tailings in the form of a slurry of water and particles of combustible material and ash to a combustion zone in which a fluidized bed of those particles is maintained. Self-sustaining combustion of the combustible material in the bed is established and maintained at a temperature that causes the ash particles to agglomerate to form said pellets, which are removed from the bed at a rate that maintains the desired pressure drop across the bed.

Description

United States Patent 1 Notary et a1.

Apr. 23, 1974 1 DISPOSAL OF ASH FROM COAL PREPARATION PLANT TAILINGS Inventors: Joseph A. Notary, Monroeville;

Donald E. Metheny, Pittsburgh, both of Pa.

Heyl & Patterson, Inc., Pittsburgh, Pa.

Filed: May 22, 1973 Appl. No.: 362,808

[73] Assignee:

US. Cl /7 S, 110/28 .1, 110/165 R Int. Cl. F23d 19/00 Field of Search llO/7 S, 8 R, 28 J, R,

[ 56] References Cited UNITED STATES PATENTS 3/1964 Sage et al. 110/7 5/1967 Albertson et al. 110/8 8/1968 Tada 110/8 3/1969 Godel 110/28 Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm-Brown, Murray, Flick &

Peckham [57] ABSTRACT 9 Claims, 1 Drawing Figure 400/ 7/0/1441 COAL F/NES 14 4 75R 35 WPUIQ DUST COLLECTOR T/I/CA/ENER 4 peas 5475 PATENTEUAPR 23 191+ ENSQ ww wkm3wq$ ww v I mNm v QWEQSQ m ll DISPOSAL OF ASH FROM COAL PREPARATION PLANT TAILINGS The waste or tailings from coal preparation plants is in the form of a slurry of water and solid particles. The solids in the slurry are made up of coal particles, ash particles and particles of ash and coal combined. The ash is any solid material that has no BTU values and is mostly clay. Getting rid of such coal plant tailings safely without polluting the environment is a serious problem. In the past, the tailings have often been simply run out onto the land, where rain has washed them into streams and rivers that they clogged with silt. More recently, coal preparation plant tailings have been confined in lagoons or dammed up streams, but it has been found that this is hazardous because the dams and the lagoon walls sometimes break and suddenly release their contents and cause flooding. If the ash in the tailings could be disposed of satisfactorily in a manner that would not pollute the environment, getting rid of the combustible material and the water would be relatively easy. The ash has been the problem..

It is among the objects of this invention to provide a method of disposing of the ash from coal preparation plant tailings, which is simple, inexpensive, continuous and non-polluting of the environment, and which simultaneously disposes of the water and combustible material.

Apparatus for carrying out this process is illustrated diagrammatically in the accompanying drawing.

Referring to the drawing, coal preparation plant tailings, consisting of a slurry of water and particles of combustible material and combined combustibles and ash, are conducted from the plant through a conduit 1 to a large collector or thickener 2 provided with an outlet in its bottom. This outlet is connected by a pipe 3 to a constant speed pump 4 that pumps the slurry through a pipe 5 to a control sump 6. The density of the slurry can be measured by a conventional instrument 7 and if the density falls below a desired value some of the slurry can be returned to the thickener through a pipe 8 leading from the control sump. A slurry with a solids content between 35 percent and 45 percent by weight has been found to be best as will be explained later, the solids make-up being in the neighborhood of about 50 percent coal and 50 percent ash by weight.

The bottom of the sump has an outlet connected by a pipe 10 to a variable speed pump 11 that delivers the slurry through a pipe 12 into the side of a furnace 13, which serves as an agglomerator as will be explained presently. In the lower part of the furnace there is a grate 14 provided with a multiplicity of very small openings through it. Projecting into the furnace beneath the grate is a start-up burner 15 that may burn oil or gas. Also connected with the furnace beneath the grate is a pipe 16, to which air is supplied under pressure. This air flows upwardly through the openings in the grate in order to form a fluidized bed of particulate material on the grate.

Before this apparatus is put in operation, a bed of inert granular material, such as sand, is spread on the grate. Then the burner 15 is lit and air is delivered to the furnace below the grate through pipe 16. The air flow is sufficient to cause the sand to form a fluidized bed above the grate. The burner continues to operate until the temperature of the sand is high enough to ignite coal particles. At that time the pumps are started in operation in order to feed the slurry into the furnace above the grate. The solid particles in the slurry mix with the sand and become a part of the fluidized bed. As soon as the combustible material in the bed ignites and starts to burn, the burner is turned off. After that the combustible material in the bed is depended upon to maintain combustion in the furnace, thereby reducing the cost of operation. in other words, burning ofthe fluidized bed is self-sustaining. The high temperature in the furnace causes the slurry water that is delivered to it to immediately vaporize and flow up and out through the stack 18 connecting the top of the furnace to the side of a dust collector 19. The top of the dust collector has an outlet for the water vapor and combustion gases, which can pass into the atmosphere without contaminating it.

It is a feature of this invention that the furnace is maintained at such a temperature that the ash particles in the fluidized bed, including those from which coal fines are burned, agglomerate by forming hard pellets that may be any size up to about a quarter of an inch across. Why this agglomeration takes place is not fully understood, although surface fusion of the particles seem to play a part, but the pellets that are thereby formed not only are inert but they also are insoluble in water. Furthermore, they will not break up into fines again and wash away when rained upon. They can be used for landfill or dumped into old mines. They even can be used as aggregate in the making of building blocks and the like. There are many possible uses for the pellets.

One way of controlling the temperature in the furnace when the solids content of the slurry fed into it is not below about 35 percent by weight is to adjust the speed of pump 1 1 to increase or decrease the feed. Another way is to operate the pump at constant speed and divert some of the slurry back to control sump 6 if a reduction in feed is desired in order to reduce the temperature in the furnace. On the other hand, if the solids content of the slurry is below about 35 percent by weight, burning in the furnace will not be selfsustaining without some help. This help can be furnished by heating the fluidizing air in a heat exchanger or preheater 21. The hotter this air entering the furnace, the higher the temperature in the furnace. The temperature of the preheater, when used, can be controlled by a conventional temperature measuring instrument 22 inserted in the upper end of the furnace or in the stack and operatively connected with the preheater in any well-known manner. Preheating is not required when the solids content is above about 35 percent by weight, because in that case the material will furnish enough BTU s to sustain combustion. If the solids content of the slurry delivered to the furnace is above about 45 percent by weight, too much heat is produced and the feed must be cut back. This means that the equipment then will not be operating at its optimum capacity.

It has been found that for best results the furnace temperature should be between about l,400 and 1,700 F. In this range the ash particles will form pelletsand the bed will remain fluidized. A minimum temperature of about l,400 F is required in order to sustain combustion in the furnace. On the other hand, if the temperature starts to rise above about l,700 F, the character of the fluidized bed changes as the ash starts to fuse together into large lumps and the bed becomes defluidized.

For removing the pellets from the agglomerator, an inclined pipe 24 extends from just above the grate down through it and out the side of the agglomerator to a conveyor 25 or other means for carrying the pellets away. The pellets in the fluidized bed will flow into the upper end of this pipe and slide down through it and onto the conveyor. Since the pressure drop across the fluidized bed should be maintained as constant as possible for proper operation, the outlet pipe for the pellets is provided with a throttling valve 26 to control the flow of pellets away from the bed at a rate that will maintain the desired pressure drop across the bed. This valve is operated in accordance with the pressure drop of the fluidizing air across the bed. If, for example, the pressure drop starts to fall, the change, through the agency of pressure-sensitive control device 27 connected with the inside of the furnace by tubes 28 and 29, will cause the throttling valve to operate to reduce the size of the passage through it in order to slow down the escape of pellets from the fluidized bed.

Dust deposited in the dust collector can be removed through a pipe 31 and a three-way valve 32 that delivers the dust either to the pellet removal pipe 24, whereby the dust mixes with the pellets and is carried away, or back to the furnace. if it happens that the tailings include such a small proportion of coal particles that it is difficult to maintain combustion in the agglomerator, additional fine particles of coal can be added to the slurry in sump 6 through a three-way valve 35 and a pipe 36. Or, by turning the valve, the coal fines can be delivered from the valve to another pipe 37 that will supply them to the fluidized bed in dry condition.

If the fusion point of the ash in a given collection of tailings is so high that the ash particles do not agglomerate in the furnace, their fusion point can be lowered by adding to the slurry in sump 6 a material that will lower the fusion point of the ash. This material can be supplied through a pipe 39 from a hopper 40. Soda ash is suitable for this purpose, but other low melting point inorganic compounds can likewise be used.

It will be seen that the process disclosed herein provides a continuous method of converting the ash in coal preparation plant tailings into an inert product that not only is non-polluting but also can be useful. At the same time, the combustible material in the tailings is burned up and the water passes off as vapor, so the en tire composition of the tailings is disposed of. Operation of the apparatus for carrying out the process is relatively inexpensive, especially since after the initial start-up by the burner the rest of the burning is autogenous with the tailings supplying their own fuel.

According to the provisions of the patent statutes, we have explained the principle of our invention and have illustrated and described what we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

We claim:

1. The non-polluting method of disposing of the ash that forms part of a slurry of water and particles of combustible material and ash constituting the tailings from a coal preparation plant, comprising continuously delivering the slurry to a combustion zone, maintaining a fluidized bed of said particles in said zone, establishing self-sustaining combustion of the combustible material in said bed and maintaining it at a temperature that causes the ash particles therein to agglomerate into inert water-insoluble pellets, and removing said pellets from said bed at a rate that maintains the desired pressure drop across the bed.

2. The non-polluting method recited in claim 1, including maintaining said fluidized bed by continuously forcing air up through it, preheating said air, and controlling said preheating to control said temperature.

3. The non-polluting method recited in claim 1, including controlling said temperature by controlling the rate of said delivery of slurry to said combustion zone.

4. The non-polluting method recited in claim 1, including controlling said temperature to confine it to a range between about l,400 and l,700 F.

5. The non-polluting method recited in claim 1, including first introducing a fluidized bed of inert granular material into said combustion zone, heating that bed to a temperature high enough to ignite said combustible material, then starting delivery of said slurry to the combustion zone, and stopping said heating when said combustible material in the bed ignites and supports combustion in the bed.

6. The non-polluting method recited in claim 1, including maintaining said slurry with a solids content between about 35 percent and 45 percent by weight.

7. The non-polluting method recited in claim 1, including collecting said tailings in a collector for said delivery to said combustion zone, and maintaining the desired density of the slurry in the collector by returning part of the slurry thereto before it reaches said combustion zone.

8. The non-polluting method recited in claim 1, including collecting dust leaving said fluidized bed, and returning the dust to said combustion zone.

9. The non-polluting method recited in claim 1, including adding to said slurry as it is being delivered to the combustion zone a material that lowers the fusion point of said ash.

Claims (9)

1. The non-polluting method of disposing of the ash that forms part of a slurry of water and particles of combustible material and ash constituting the tailings from a coal preparation plant, comprising continuously delivering the slurry to a combustion zone, maintaining a fluidized bed of said particles in said zone, establishing self-sustaining combustion of the combustible material in said bed and maintaining it at a temperature that causes the ash particles therein to agglomerate into inert waterinsoluble pellets, and removing said pellets from said bed at a rate that maintains the desired pressure drop across the bed.

2. The non-polluting method reciteD in claim 1, including maintaining said fluidized bed by continuously forcing air up through it, preheating said air, and controlling said preheating to control said temperature.

3. The non-polluting method recited in claim 1, including controlling said temperature by controlling the rate of said delivery of slurry to said combustion zone.

4. The non-polluting method recited in claim 1, including controlling said temperature to confine it to a range between about 1,400* and 1,700* F.

5. The non-polluting method recited in claim 1, including first introducing a fluidized bed of inert granular material into said combustion zone, heating that bed to a temperature high enough to ignite said combustible material, then starting delivery of said slurry to the combustion zone, and stopping said heating when said combustible material in the bed ignites and supports combustion in the bed.

6. The non-polluting method recited in claim 1, including maintaining said slurry with a solids content between about 35 percent and 45 percent by weight.

7. The non-polluting method recited in claim 1, including collecting said tailings in a collector for said delivery to said combustion zone, and maintaining the desired density of the slurry in the collector by returning part of the slurry thereto before it reaches said combustion zone.

8. The non-polluting method recited in claim 1, including collecting dust leaving said fluidized bed, and returning the dust to said combustion zone.

9. The non-polluting method recited in claim 1, including adding to said slurry as it is being delivered to the combustion zone a material that lowers the fusion point of said ash.

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