Classifying – separating – and assorting solids – Sorting special items – and certain methods and apparatus for... – Separating means
Patent
1993-09-13
1995-01-03
Bollinger, David H.
Classifying, separating, and assorting solids
Sorting special items, and certain methods and apparatus for...
Separating means
B07B 1305
Patent
active
053778480
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a roller screen for separating finer and coarser material fractions of bulk material from each other, especially for separating sawdust from wood chips, the roller screen comprising a plurality of successive rollers rotating about parallel axes of rotation, the upper surfaces of the rollers providing a path for the material to be screened; at least one drive means for rotating the rollers; and a means for feeding the material to be screened to the infeed end of said path. The roller screen according to the invention can be used e.g. in the production of cellulose for screening wood chips used as process feed.
Known apparatuses for screening bulk material include disc screens and various roller screens. A disc screen consists of successive shafts to which discs are attached at regular intervals in such a manner that the discs on successive shafts interdigitate. The shafts rotate at the same rate and to the same direction. The upper surface of the discs functions as a conveyor of the material to be screened and between the discs there is a gap determined by the grain size desired, the finer fraction falling through the screen at the gap while the coarser fraction on passes on to the discharge end of the screen. Previously known disc screens are described e.g. in Finnish Patent Application 780,685 and Finnish Published Specification 70,379.
A roller screen comprises rotating rollers instead of disc shafts. Between the rollers is a gap determined by the grain size desired, the finer material fraction falling through the screen at the gap while the coarser fraction passes on to the discharge end of the screen on the upper surface of the rollers.
Drawbacks of disc screens are a great demand of power and heavy wear of discs, which are due to the large contact surface of the discs and the material to be screened. A disc screen is ill-suited for separation of particulate sawdust of a grain size of less than 3 to 5 mm from wood chips since the particles falling between the discs may be of any length although their thickness is not greater than the gap between the discs. This results in loss of pulpwood, which is useful e.g. in a process for producing cellulose, together with the sawdust removed from wood chips.
The same drawback is also found in known roller screens since the particles falling through the gap between the rollers may be of considerable length in the axial direction of the rollers although their thickness is not greater than the gap.
The object of the present invention is to remedy the above drawbacks by improving the construction of the known roller screen. This is achieved with the roller screen according to the invention, which is characterized in that there are radial and essentially axial grooves on the surface of the rollers, the grooves forming teeth onto the surface of the rollers, and that the teeth of each roller interdigitate with the radial grooves of the adjacent roller, whereby the teeth and radial grooves form slots between them for the particles of the material to be screened.
The invention is based on the idea that the surface of the rollers is grooved in such a manner that slots of the grain size desired are formed between the rollers, the slots "closing" momentarily when the rollers rotate in such a manner that the particles of the wrong size do not pass through the screen.
Thus, in the roller screen of the invention only the particles that are of the grain size selected or smaller than that pass through the screen at the slots that are precisely of the grain size selected. Elongated particles that are bigger than the grain size desired cannot pass through the screen. This is the most significant advantage of the roller screen of the invention in regard to the known apparatuses. In addition, the power demand of this roller screen is smaller and the wear of the screening means is not as heavy as in a disc screen since the contact surface between the screening means and the material to be screened is small. Furthermore, the construction of the roller screen is
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Jokinen Mikko
Kurki Timo
Bollinger David H.
Consilium Bulk Babcock Oy
Jackson Robert R.
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