Question from July's Chemical Processing
A vibrating screen separates pulverized lumps of coal from dust (Figure 1). The screen is fed from a lock-hopper above. The fine particles must be separated because they could become carry-over for a gasifier (entrained-flow); a pelletizer scheme is being tested to re-use this material. The lumps are then fed to a screw conveyor to the gasifier. On a vibrating screen, solid is supposed to be spread evenly across a mesh screen. The rocking motion and the slight slope cause the solid to flow towards the discharge and into either the pneumatic conveyor pick-up pipe or the hopper for the screw conveyor. This isn’t happening. Instead, coal is building up on the screen and blocking flow from the lock-hopper above. The screw conveyor hopper is plugged and the pick-up pipe is plugged. Adjustments to the slope and the rocking motion have been increased to no avail. Any suggestions?
Modify the screen feed
I've seen similar problems on other screeners, especially the models that "rock" rather than vibrate. It was a little confusing how the screw and pick-up pipe could both plug if the screener is being blocked by a build-up. However, if I assume that the screen size has been chosen correctly and isn’t broken, the conclusion would be that the coal particles are wedging themselves in the openings and accumulating. This build-up will sluff off periodically causing plugging. When the sluff occurs, large coal particles come crashing down on the screw causing a bridge to form or overloading the screw. The fines are released in clumps, as well, and fall into the pick-up hopper, which also gets overloaded. This should be easy to observe and report back. Most likely, the release of solids from the lock hopper is the main cause of the overload. This can be solved by placing a sheet of metal on top of the feed portion of the screening deck. Sometimes a perforated plate can be used if the amount of fines is very heavy. The plate will spread the flow of solids over the screen and provide a lateral velocity that will keep the larger coal pieces from sticking on the screen. The fines will settle onto the surface of the plate and gently fall through the screen; eliminating the clumps to the pick-up pipe.
Tom Blackwood, director of technology
Healthsite Associates LLC, Ballwin, Mo.
Replace the valves
I see two problems: surge flow from the lock hopper valves and poor feeding at the discharge of the vibrating screen. Obviously costs must be a consideration, especially downtime. First check the design calculations, if possible, for flow from the vibrating screen. Next, inspect the discharge of the vibrating screen feeding the hopper below and the pick-up point for the pneumatic conveying system. Look at the bottom part of the hopper connection spool checking for the angle and for dead space where solids could bridge. If the inspection confirms problems, consider modifying the hopper connecting spool first to increase the angle of repose and reduce dead space (Figure 2).
The angle for the vibrating screen outlet should be at least 70o from horizontal. Since the physical properties of the coal may be different than those developed for the design re-run the Jenike test. This test was developed to measure the internal and wall friction for solids in silos (refer to: http://www.engineering.uakron.edu/~chem/fclty/chase/Solids/SolidsNotes10%20Hopper%20Design.pdf).
If modifying the hopper fails, consider a more expensive modification of the process. If there is sufficient room, replace the lock hopper valves with a feeder screw conveyor. Sometimes, lock hopper valves are installed because there is a space restriction; if so, consider deflector plates to reduce dust formation or a side-entry feed to reduce the height of the screw conveyor.
The feeder should use a variable pitch or conical screw to provide even flow from the bottom of the mill storage tank. A conical screw will have a higher torque and cost. The enclosure should be tight and other precautions e.g., nitrogen purge, are necessary to avoid the dangers of an explosive dust. Off-set bearings may perform better with fine dust and make sure grease fittings are accessible and bearings are greased daily.
I believe that fine dust is being pulled into the coal hopper below the vibrating screen. Depending on the pull of the vacuum system, the pneumatic conveying system could be creating negative pressure inside the vibrating screen. This could be holding the coal to the screen and contributing to plugging the line. It is probably not possible to reduce the vacuum from the system, so the flow of solids must be controlled.
To improve the feeding of the pneumatic conveyor from the vibrating screen, add a rotary air lock, or rotary valve (RV) and a standard-pitch screw conveyor. You may want to consider a nitrogen purge here, depending on the clearance and design of the RV and screw. Because of the nature of the dust, the enclosure must be tight. Flow of the coal to the hopper can also be improved.