WO1991000387A1

WO1991000387A1 - Method of purifying pulp of contaminations heavier than it and a separating apparatus

Info

Publication number: WO1991000387A1
Application number: PCT/FI1990/000166
Authority: WO
Inventors: Timo Vuorinen
Original assignee: Oy Tampella Ab
Priority date: 1989-07-03
Filing date: 1990-06-21
Publication date: 1991-01-10
Also published as: FI893237A7; AU5826390A; FI893237A0

Abstract

The invention relates to a method of purifying pulp and to a separating apparatus. According to the method, the pressure difference across the hydrocyclone (1) is kept substantially constant by raising the pressure of the pulp conducted to the hydrocyclone (1) by a pump (3) preceding the hydrocyclone (1) and by returning part of the purified pulp discharged from the hydrocyclone to the inlet channel (2) of the pulp before the pump (3) so that substantially a constant amount of pulp flows through the hydrocyclone (1) in a unit of time. The separating apparatus comprises a pump (3) mounted in the inlet channel (2) of the pulp, a return channel (12) running from the outlet channel (11) to the inlet channel (2) before the pump (3) and provided with a valve (13), and a pressure difference regulator (17) provided with pressure sensors (14, 16) and arranged by means of the sensors (14, 16) to measure the pressure difference across the hydrocyclone (1) and to adjust the valve (13) so that the pressure difference remains substantially constant.

Description

Method of purifying pulp of contaminations heavier than it and a separating apparatus

The invention relates to a method of purifying pulp of contaminations heavier than it, wherein the pulp is conducted through at least one hydrocyclone, whereby the contaminations separate from the pulp due to the influence of centrifugal force. The invention relates further to a separating apparatus for the separation of contaminations heavier than fibres from the pulp, said apparatus comprising at least one hydrocyclone, through which the pulp is conducted, and in which the contaminations are separated from the pulp due to the influence of centrifugal force, an inlet channel for conducting the pulp into the hydrocyclone, and an outlet channel for discharging the purified pulp from the hydrocyclone.

The function of the hydrocyclone is to purify pulp of contaminations heavier than fibres, such as different kinds of rivets, studs, stones or the like, in such a manner that as few fibres as possible are simultaneously removed. This is effected by intro¬ ducing the pulp tangentially into one end of the hydrocyclone so that it circulates along the walls of the hydrocyclone as a spirally circulating flow, whereby heavy contaminations move to the periphery while the flow proceeds towards the conically con¬ verging other end of the hydrocyclone, and dis¬ charging the pulp from the middle of the hydrocyclone through an outlet pipe along the axis thereof. Due to the pressure of the incoming flow of pulp, lighter contaminations and fibres move along the length of the cyclone to the discharge flow, but heavy co¬ ntaminations remain on the periphery due to the influence of centrifugal force, moving towards the reject end of the hydrocyclone, ending up in a reject chamber which is formed at the reject end of the hydrocyclone and from which it is removed in differ¬ ent ways at regular intervals. In known solutions, the heavy material, or reject, to be removed rotates with the rotational motion, particularly at the narrow end of the hydrocyclone, where the flow final¬ ly changes direction so as to return. Part of the reject may then drift with the acceptable pulp on- wards. To avoid this, the reject must be removed often, and therefor3 acceptable fibre material is lost in vain.

It is necessary for the separating capacity of the hydrocyclone that there is a sufficient rotation- al motion inside it to make heavy material move properly to the periphery and to maintain the rota¬ tional motion along the whole hydrocyclone. There have been attempts to accomplish this in the known solutions e.g. by providing a separate rotor in the hydrocyclone to ensure the rotational motion. Without a rotor, for instance when the flow of pulp varies, the separating capacity of the hydrocyclone varies and may in some cases even vanish. Finnish Patent No. 54,863 discloses a hydrocyclone, the object of the invention being to improve the separating capacity by certain dimensioning of the hydrocyclone. The problem with this solution is that the dimensioning functions with a certain rate of flow, but the separating capacity of the hydrocyclone is correspondingly dependent on the changes in the rate of flow, and thus it cannot be regulated to function steadily.

The object of the present invention is to provide a method of purifying pulp of contaminations heavier than it and a separating apparatus by which the separating capacity can simply and easily be maintained at a desired level and by means of which variations in the separating capacity can be avoided when the consumption of pulp changes. The method according to the invention is characterized in that the pressure of the pulp is raised by a pump prior to the hydrocyclone to such a high degree that the pressure of the purified pulp discharged from the hydrocyclone is higher than that of the pulp flowing to the pump, that part of the purified pulp that is discharged is returned to the unpurified pulp con¬ ducted to the hydrocyclone at the suction side of the pump, and that the flow of the pulp to be returned is regulated in such a manner that the pressure differ¬ ence across the hydrocyclone remains substantially constant.

The separating apparatus according to the invention is characterized in that it comprises a pump mounted in the inlet channel for raising the pressure of the pulp so that the pressure of the purified pulp discharged from the hydrocyclone is higher than that of the pulp flowing to the pump, a recirculation channel running from the outlet channel to the inlet channel to the suction side of the pump and provided with a control valve for returning part of the purified pulp to the inlet channel, pressure sensors for detecting the pressure of the pulp intro¬ duced into the hydrocyclone and discharged therefrom, and a regulating device which is arranged to adjust the control valve in such a manner that the pressure difference across the hydrocyclone remains substan¬ tially constant.

The essential idea of the invention is that a substantially stable pressure loss is effected between the inlet and outlet channels of the hydro- cyclone. The pulp is pumped to the hydrocyclone so that the pressure of the pulp discharged from the hydrocyclone is higher than that of the pulp flowing to the pump. Part of the purified pulp discharged from the hydrocyclone is returned to the inlet channel before the pump so as to keep the pressure across the hydrocyclone substantially constant. Con¬ sequently, substantially the same amount of pulp flows through the hydrocyclone in a unit of time irrespective of how much of the pulp is further led to a papermaking machine or the like.

The invention is illustrated in greater detail in the attached drawings, in which

Figure 1 is a schematic view of the separating apparatus according to the invention, and Figure 2 is a schematic view of the method according to the invention applied to several paral¬ lel hydrocyclones.

Figure 1 shows a separating apparatus with a hydrocyclone 1. Pulp is conducted thereto through an inlet channel 2. The inlet channel 2 leads to the hydrocyclone 1 via a pump 3 driven by a motor 4. The pulp is introduced into the hydrocyclone 1 tangen- tially in a manner known per se so that it moves as a spiral flow along the outer surface of the hydro- cyclone towards its other end. At the other end, or reject end, of the hydrocyclone, after its conical part and in the middle of the cyclone, parallel to its axis there is a tubular part 5 with a closed bottom, and below it there is a reject outlet channel 6 transverse to the axis of the hydrocyclone. The channel 6 is connected via a valve 7 to a reject chamber 8, which is, in turn, connected by a valve 9 to a reject outlet channel 10. At the inlet end of the hydrocyclone, centrally in relation to its lon- gitudinal axis, there is a pulp outlet channel 11, further connected by a connecting channel 12 through a valve 13 to the pulp inlet channel 2. A pressure detecting sensor 14 is mounted in the outlet channel 11, and a second pressure detecting sensor 16 is mounted in the connecting channel 15 between the pump 3 and the hydrocyclone 1, these sensors being con¬ nected to a pressure difference regulator 17. The pressure difference regulator 17 is, in turn, arranged to adjust the valve 13 so as to keep the difference between the pressures detected by the sensors 14 and 16, i.e. the pressure difference across the hydrocyclone, substantially constant.

When the apparatus is operating, pulp is con¬ veyed through the inlet channel 2 to the pump, where its pressure is raised so that the pressure of the pulp discharged from the hydrocyclone is higher than that of the pulp flowing to the pump 3. The pulp flows to the reject end of the hydrocyclone 1, where the heavy contaminations drift around the central tube 5 due to the rotational motion and are dis¬ charged to the outlet channel 6 and further through the valve 7 to the reject channel 8. From the reject chamber 8 the contaminations are discharged to the reject outlet channel 10 by closing the valve 7 for a while and by opening then the valve 9 so as to allow the contaminations to be discharged from the reject chamber 8. After that, the valve 9 is closed, and the valve 7 is opened to allow the reject to re-enter the chamber 8. At the open end of the central tube 5 the pulp turns to flow to the centre of the hydrocyclone towards the outlet channel 11, through which it is further conducted to the following step. As the central tube prevents the flow at the end of the hydrocyclone from the periphery to the middle of it, the contaminations circulating along the periphery cannot move to the return flow but are discharged to the reject outlet channel 6 due to the influence of centrifugal force and gravity. The sensors 16 and 14 measure the pressure difference across the hydro- cyclone 1, i.e. the pressure loss caused by the hydrocyclone to the pulp flowing through it. In order to keep the pressure difference constant and thus to stabilize the operation of the hydrocyclone, part of the pulp discharged from the hydrocyclone is con- ducted to the connecting channel 12 and through the valve 13 to the inlet channel 2. Thus a so-called short circulation is formed, whereby the same amount of pulp can be conveyed through the hydrocyclone 1 irrespective of how much of it is actually conducted through the outlet channel 11 onwards. The pressure difference regulator 17, to which the sensors 16 and 14 are connected, adjusts the valve 13 in such a manner that the flow of pulp through the hydrocyclone and thus the pressure loss across the hydrocyclone remains constant irrespective of the amount of pulp used.

Figure 2 shows an embodiment in which three hydrocyclones la - lc are connected to a pump 3. Pulp is conducted to said hydrocyclones through a common inlet channel 2 and after purification led onwards therefrom through a common outlet channel 11. In this embodiment the pipe leading to each hydrocyclone is provided with a control valve 18a - 18c, by means of which the operation of each hydrocyclone can be controlled, if necessary. In this case, pressure difference sensors 14 and 16 are mounted between the pump 3 and the valves 18a - 18c in the inlet channel 2 and in the common outlet channel 11. A short circulation channel 12 is led in the manner described above from the outlet channel 11 through a valve 13 to the inlet channel 2 before the pump 3. The pressure difference regulator 17 keeps the pressure difference between the pump and the outlet channel 11 constant across all the hydrocyclones. On account of variations in the manufacture, there are slight differences between the different hydrocyclones. If necessary, these differences can be levelled by adjusting the valves 18a - 18c. Likewise one of the hydrocyclones can, if necessary, be separated from the line by closing the corresponding valve.

In the specification above and in the drawings the invention is described merely by way of example, and the invention is in no way restricted to that. The invention can be applied in many ways within the scope of the claims, and it can be used in hydro¬ cyclones mounted both vertically and horizontally. The connecting channel, or recirculation channel, running from the outlet channel to the inlet channel can be mounted either immediately adjacent to the hydrocyclone or at a distance therefrom, as long as it is connected between these channels. The pump 3 can be mounted either as a fixed part of the hydro¬ cyclone or at a distance therefrom, as long as the pressure difference measuring is effected between the pump and the hydrocyclone and correspondingly in the outlet channel at points which are not essentially affected by other apparatuses connected with the supply of pulp, and the pulp to be recirculated is conducted to the inlet channel before the pump at the suction side thereof.

Claims

Claims :

1. A method of purifying pulp of contaminations heavier than it, wherein the pulp is led through at least one hydrocyclone (1), whereby the contamina¬ tions are separated from the pulp due to the influ¬ ence of centrifugal force, c h a r a c t e r i z e d in that the pressure of the pulp is raised by a pump (3) prior to the hydrocyclone (1) to such a high degree that the pressure of the purified pulp dis¬ charged from the hydrocyclone (1) is higher than that of the pulp flowing to the pump (3), that part of the purified pulp which is discharged is returned to the unpurified pulp conducted to the hydrocyclone (1) to the suction side of the pump (3), and that the flow of the pulp to be returned is regulated in such a manner that the pressure difference across the hydrocyclone (1) remains substantially constant.

2. A separating apparatus or separating heavier contaminations from pulp, comprising at least one hydrocyclone (1), through which the pulp is led, and in which the contaminations are separated from the pulp by the influence of centrifugal force, an inlet channel (2) for conducting the pulp to the hydrocyclone (1), and an outlet channel (11) for con¬ ducting the purified pulp away from the hydrocyclone (1), c h a r a c t e r i z e d in that it comprises a pump (3) mounted in the inlet channel (2) for raising the pressure of the pulp so that the pressure of the purified pulp discharged from the hydrocyclone (1) is higher than that of the pulp flowing to the pump (3), a recirculation channel (12) running from the outlet channel (11) to the inlet channel (2) at the suction side of the pump (3) and provided with a control valve (13) for returning part of the purified pulp to the inlet channel (2), pressure sensors (14, 16) for detecting the pressure of the pulp introduced into the hydrocyclone (1) and discharged therefrom, and a regulating device (17) connected to the pressure sensors (14, 16) and arranged to adjust the control valve (13) in such a manner that the pressure difference across the hydrocyclone (1) remains sub¬ stantially constant.

3. An apparatus according to claim 2, c h a r - a c t e r i z e d in that it comprises at least two parallel hydrocyclones (la - lc), that a pump (3) is connected to an inlet channel common to all of said hydrocyclones (la - lc), that the inlet channel between each hydrocyclone (la - lc) and the pump (3) is provided with a valve (18a - 18c), that all of said hydrocyclones (la - lc) are connected to a common outlet channel, that a recirculation channel (12) is mounted between the common outlet channel and the common inlet channel, and that pressure sensors (14, 16) are mounted between the pump (3) and the valves (18a - 18c) and in the common outlet channel.