SE470315B - Methods to control the screening process when screening cellulose-containing pulp suspensions - Google Patents

Description

... du * J (JJ ... z- (fl conventional strainers' ability to function at low reject extraction. If the pulp concentration in the last part of the screening zone becomes too high, the strainer plugs. This limitation of the strainer's function at low reject extraction also limits When the energy supply is no longer sufficient to fluidize the pulp, the local flow of the screen means is reduced and only dewatering takes place, whereby the enrichment of pollutants ceases. This is one of the reasons why screening often has to be done in several steps. in order to achieve a sufficient enrichment of contaminants in the reject flow.

The best efficiency in a screening process (separation or fractionation) is obtained when a suitable balance is maintained between the strength of the fiber network, which depends on the pulp concentration and the type and quality of the pulp, and the energy supply which is mainly determined by the screen rotor speed. This balance can be difficult to maintain because the fiber network strength is not easily measurable. This balance is particularly critical when screening with a high pulp concentration,> 3%, since in this concentration range even small variations in the pulp concentration result in relatively large changes in the fiber network strength.

Conventionally, the screening is carried out with a large excess of energy to ensure the fluidization of the pulp and thus the screening process. A known way of reducing this unnecessary energy consumption to some extent is to control the supply of pulp suspension by means of the pressure drop across the screen means. If this pressure drop decreases, the mass inflow can be increased and vice versa. However, this control is far from sufficient to optimize energy input.

According to the present invention, the above problems are solved by continuously measuring and rejecting the reject concentration to control the incoming pulp concentration and / or the energy supply to the screening process. The more detailed features of the invention>> ~: approx.

Because the reject concentration is a measure of the thickening in the screen, this concentration can be used to regulate the mass concentration before the screen, for example by controlling the supply of dilute water, so that the screen's efficiency and working concentration are maximized. Alternatively, for the same reason, the reject concentration can regulate the energy supply, suitably by controlling the speed of the screen rotor. In this case, the energy requirements of the screen are minimized at the highest possible separation efficiency.

The method according to the invention leads to savings in energy consumption and more efficient utilization of the silo devices.

In many cases, the number of steps in a multi-step screening process can be reduced while maintaining screening results. Investment costs and space requirements are thus reduced. Furthermore, both the capacity and the flexibility of the screening process are improved.

The invention will be described in more detail in the following by means of an exemplary embodiment which is illustrated by a flow chart in the figure.

Connected to a screen device 1, preferably a pressure screen, is a supply line 2 for incoming pulp suspension, an outlet line 3 for the acceptance fraction and an outlet line 4 for the reject fraction. The screen device 1 can be of conventional construction with a stationary screen member, a separate rotor element provided with some form of pulsing member being rotatable in the screen device. Alternatively, the screen means may be a rotatable screen drum.

A meter 5 is arranged for measuring the material concentration in the reject line 4. This meter 5 can be of a conventional type, for example a shear force meter or an optical concentration meter.

From the meter 5 a signal 6 is emitted for controlling a valve 9 in $> \ il CD Od ... s CH a supply line 8 for dilution water. This line 8 is connected to the pulp supply line 2 before the screen device 1.

Alternatively, the meter 5 may emit a signal 7 for controlling the speed of the rotor of the screen device 1.

It is of course conceivable to use both of these control systems with signals 6 and 7, respectively, at the same time, but normally it is sufficient to use one of them.

The pulp concentration is suitably controlled so that the reject concentration is substantially kept constant at a predetermined value, for example in the range 4.8-5.2% if the incoming pulp concentration is about 3.2% and the acceptance concentration about 3.0%. In order to keep the reject concentration constant at, for example, 5.0%, the incoming pulp concentration may need to be regulated within 3.0-3.4%, which can easily be achieved by using the valve 9 to regulate the supply of dilution water from the line 8.

If instead the speed of a rotatable rotor element or a rotatable screen member is to be regulated, it may be relevant with the above material concentrations with a speed control within 5 to 10%. For speeds in the range 400-600 rpm, for example, a control of 30 rpm may be required. This can be accomplished with conventional control equipment.

In addition to the above measures for controlling the screening process, it may be appropriate in a manner known per se also to keep the reject flow constant at a predetermined level.

The invention is of course not limited to the described embodiment but can be varied within the scope of the inventive idea.

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Claims (6)

Patent claims A method of controlling the screening process for screening cellulose-containing pulp suspensions in a screening device (1), where incoming pulp suspension (2) is divided by means of a screening means into an acceptance fraction (3) and a reject fraction (4), characterized in that the material concentration in the reject fraction (4) is continuously measured with a meter (5) and transmitted to a signal (6,7) which is used to control at least one of the pulp concentration incoming mass and energy supply to the screening process. 2. A method according to claim 1, characterized in that the control of the process parameters is performed so that the material concentration in the reject fraction is substantially constant. 3. A method according to claim 1 or 2, characterized in that the output signal (6) of the meter (5) is used for controlling the incoming pulp concentration by regulating the supply of spawning water (8) to the incoming pulp suspension (2). 4. A method according to any one of claims 1 - 3, characterized in that a separate rotor element is arranged in the silo device (1) and that the output signal (7) of the meter (5) is used for controlling the energy supply by regulating the rotor element. speed. 5. A method according to any one of claims 1 - 3, characterized in that the screen means of the screen device (1) is rotatable and that the output signal (7) of the meter (5) is used for controlling the energy supply by regulating the speed of the screen member. 6. A method according to any one of the preceding claims, characterized in that the flow of the reject fraction is kept at a constant level.