Classifying – separating – and assorting solids – Sifting – Discharging
Patent
1983-09-07
1984-10-30
Miles, Tim
Classifying, separating, and assorting solids
Sifting
Discharging
209273, B03B 556
Patent
active
044798725
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method in screening fibre suspensions, preferably for removing coarse rejects in a pressure screen, and an apparatus for carrying out the method.
In producing fibre suspensions for pulp and/or paper manufacture, undesirable coarser particles such as splinters, pieces of wood, twigs, and larger bunches of fibres are obtained as a result of the imperfection of the production process. Furthermore, other extraneous matter (pollutants) may be present, e.g. plastics, wire and the like which accompany the raw material. The latter is particularly applicable when using waste paper as starting material. These pullutants must be removed from the system at an early stage in order to avoid operational disturbances caused by them in later stages of processing the fibre suspension, e.g. when cleaning in hydrocyclone separators or vortex cleaners and fine screening. The most usual method earlier of separating these pollutants was screening in an open screen.
It is now, however, a desire to use entirely closed systems to as large an extent as possible, particularly for the main flow line. In using a pressure screen as a coarse screen, the entering coarse particles must not lead to operational interruptions, e.g. because these larger pieces rejected in the coarse screen fasten in the reject conduit and block it.
The object of the present invention is to provide a method in coarse screening with a pessure screen in which blocking (plugging) of the screen reject conduit and reject outlet is prevented, and also an apparatus for carrying out the method.
This object is achieved in accordance with the invention by a special implementation of the reject conduit and by regulating the flow through the reject conduit by affecting the flow in the accept conduit.
The reject conduit is given an orientation in space such that in operation the upper level of the reject is higher then the level of the screen inlet. In its end facing away from the screen the reject conduit is open to the atmosphere. Accordingly, the pressure in the reject outlet depends on the reject level in the conduit, which has a diameter and a radius of curvature in the zone of direction change sufficiently large that no objects, especially elongate objects, e.g. twigs can fasten. The reject conduit is preferably a pipe without a valve and other means, which affect the inside diameter of the pipe or its cross-sectional area and can form flow obstructions on which the reject can fasten.
There is a flowmeter for controlling the flow through the reject conduit, preferably a magnetic flowmeter, which is arranged in the reject conduit. When the reject flow deviates from a criterion value, which may be predetermined and constant or a function, e.g. of the flow through the inject conduit or the fibre content of the incoming suspension, the flow through the reject conduit is changed to come into line with the criterion value. This is accomplished by actuation of a valve in the accept conduit, since the reject conduit lacks flow regulating means, and a change of the accept flow automatically leads to change flow in the reject conduit.
The invention will now be described by means of a nonrestricting embodiment and with the aid of the drawing, the sole figure of which schematically illustrates a screen with associated conduits and control means.
The screen 10 shown in the Figure is a pressure screen described in the Swedich Patent Specification No. 343 621, and has stationary screening means and rotating blades or foils. A fibre suspension coming from a non-illustrated fibre source, e.g. a blow tank, is supplied to the inlet 1 of the screen 10 via a conduit 11 and a pump 8 arranged therein. The incoming suspension is divided into an accepted and a rejected fraction in the screen 10, which leave the screen 10 through outlets 2 and 3, respectively. The rejected fraction or reject contains prime fibres as well as coarse particles and is taken via a conduit 13 to an open screen 20, which is at a higher level than the screen 10. In the open screen 20
REFERENCES:
patent: 3410409 (1968-11-01), Rittenhouse et al.
patent: 4042503 (1977-08-01), Justus
patent: 4126542 (1978-11-01), Carlsmith
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