In the mining of coal, oil shale, ore, or the like, particularly from underground mines, a high pressure transfer device is desirably used to facilitate conveyance of the particulate material from the mine site to a transportation utilization area. This is commonly provided, such as shown in U.S. Pat. Nos. 3,982,789 and 4,082,368, by feeding the particulates into a hopper connected to the high pressure transfer device, screening large particles out of a particle-liquid slurry passing into the high pressure transfer device while withdrawing liquid and fine particles from the device through a low pressure discharge line, and conveying the particles out of the high pressure transfer device by the introduction of liquid under high pressure into the transfer device, the liquid and entrained particles under high pressure exiting the device through a high pressure discharge line and passing to the utlimate destination.
Prior apparatus and procedures for transporting particulates in the manner described above have been successful, however under some circumstances there is a tendency for so much water to pass through the transfer device into the high pressure discharge line that the concentration of solids in the high pressure discharge line is lower than desired for ultimate utilization of the particles in the slurry. For instance, the concentration of particles in water in the low pressure feed line is generally about 50%, but the concentration in the high pressure discharge line can be as low as 25-30%, a concentration of about 45% or more (e.g., 50%) being desirable. The introduction of excess water into the high pressure device from the high pressure feed line results since it is necessary to ensure forceful washing of all the particles out of the high pressure transfer device, and since pathways can be established during rotation of the rotor of the high pressure transfer device that provide a much lower resistance to liquid flow than the particle-filled pocket into which liquid passage is desired.
According to the present invention, a method and apparatus are provided for continuously conveying particulate material that recognizes the concentration reductions that can occur in the high pressure discharge line, and corrects them. According to the present invention, a portion of the liquid in the high pressure discharge line is withdrawn (while under high pressure) to increase the solids concentration in the high pressure discharge line to the value desired. Withdrawal can be accomplished utilizing an in-line screen having a plurality of peripheral openings, a settling tank, or a hydrocyclone. The withdrawn liquid is returned to the pressure feed line between the high pressure pump and transfer device, and the pressure of the returned liquid is boosted (by a pump or the like) so that it corresponds generally to the pressure of the liquid in the high pressure feed line. A solids concentration measuring device, such as a gamma radiation absorption device, measures solids concentration. The measurements obtained are used to automatically control the amount of withdrawn liquid so that the concentration in the high pressure discharge line is at the desired level.
Conventional systems and methods for conveyance of particulate material also can have pump damage and other problems as a result of build-up of fines in the low pressure lines. This can particularly become a problem when a center screen is used in the high pressure transfer device in order to prevent damage to the transfer device rotor and/or housing. Utilization of such a center screen is disclosed in copending application Ser. No. 123,947, filed Feb. 25, 1980. According to the present invention, fines removal is accomplished by pumping the liquid and fines in the low pressure discharge line to a screen assembly located vertically above the high pressure transfer device and feed hopper, and then allowing the screened liquid to pass by gravity flow into the hopper.
An arrangement such as described above for fines removal also facilitates control of the level in the hopper in a simplified manner. Typically, level control--which is necessary--is accomplished utilizing a sensor which controls a valve disposed in the low pressure discharge line, or a line for feeding liquid into the hopper. According to the invention, the necessity for such a sensing device and associated valve is eliminated, providing a much simpler form of level control. According to the invention, an overflow is provided from an area of the hopper near (but spaced from) its top, this area communicating with the gravity flow from the screening assembly. A stepped arrangement of horizontal pipes is provided. Further, in order to prevent contamination of the overflow line if an upset condition should occur (since the overflow is desirably connected to the sourceof liquid for the high pressure feed line), a vertically extending screen is provided in the hopper adjacent the overflow.
It is the primary object of the present invention to provide a method and apparatus facilitating the efficient conveyance of hard particulate material of non-uniform size. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.