DE102012212685A1 - Process for the removal of impurities from an aqueous pulp suspension - Google Patents

Abstract

The invention relates to a method for controlling or regulating a flotation (1) which is operated to remove impurities from a pulp suspension (S). The flotation comprises at least one flotation cell (5), by means of which a purified accepts (7) and a flocculent floating floatation foam (3, 4), rejects, which flows from the surface of the paper pulp suspension (S) into a foam collecting channel (8, 9), be formed by blowing an air stream (4) into the fiber suspension (S). The volume flow of the outflowing flotation foam (3, 4) from the flotation cell (5) is controlled or regulated by the change in the level of pulp suspension in the flotation cell (3) by a first regulator (8). The control is inventively improved in that the air volume of the air flow, which is the flotation cell (3) supplied to the ventilation, controlled or regulated by at least one second controller (9) that the composition of the effluent flotation foam (6, 7) accordingly the requirements of a given characteristic value is set.

Description

The invention relates to a method for controlling a flotation which is operated to remove impurities from a pulp suspension.

The flotation plant comprises at least one flotation cell, which is aerated by means of an air flow and by means of which a purified accept and a pollutant-containing floating flotation foam, which flows from the surface of the paper pulp suspension into a foam collecting channel, are formed.

The volume flow of the outflowing flotation foam from the flotation cell is controlled or regulated by the change in the level of pulp suspension in the flotation cell by a first controller.

Methods for the removal of impurities or for flotation of impurities from an aqueous paper pulp suspension are known from several patents, z. B. the DE 10 2004 020 339 or the EP 1 388 370 A1 , well known.

Such a method is used to precipitate a portion of the suspended contaminant particles contained therein from a pulp suspension. In a flotation, air is blown into the pulp suspension, thereby forming a foam or foam carpet containing the substances to be separated. A typical application of such a method is the treatment of a paper pulp suspension obtained from recovered paper, in which the ink particles are already detached from the fibers, so that they can be floated.

The flotation process makes use of the fact that the pulp is hydrophilic while the ink particles are hydrophobic. The hydrophobic particles adhere to the foam formed or to the foam lamellae, thus forming a foam carpet containing the protective particles which floats on the pulp suspension surface.

In addition to the ink particles there are a variety of other substances that are hydrophobic and therefore can be separated by flotation of the pulp, such. As adhesives, plastic particles and possibly also resins.

In addition, however, the foam also contains a certain proportion of liquid and fibrous materials, the proportions of which vary depending on the residence time of the foam in the flotation cell.

From a certain foam thickness, the foam containing the contaminants, the reject, overflows and is collected in a foam collecting channel.

From the DE 10 2008 031 411 A1 For example, a method of flotation is known whereby the level of pulp suspension in the flotation cell is controlled. Thus, the overflow of the flotation cell and thus the amount of effluent flotation foam or reject can be regulated.

Since the flotation foam, which is formed in a single flotation, in most cases still contains a considerable proportion of fibers, the process is carried out in several successive flotation stages. In this case, the overflow, so the flotation foam, each supplied to an upstream flotation stage as an inlet in the next stage. Usually one then speaks of a first and second flotation stage or of a primary and secondary stage. To regulate the amount of rejects, the level of pulp suspension in the cell is changed.

In a further embodiment, a first and second flotation or primary and secondary flotation is used, each consisting of several flotation cells. In this case, the secondary flotation is supplied to the overflow of the primary flotation. The passage, that is to say the pulp suspension purified in the secondary stage, can then be added again to the feed of the first flotation stage. Alternatively, the passage of the second stage may also be added to the passage of the first stage.

Plants that operate according to such methods have proven themselves, because they lead to low material losses to a good quality of the accepts thus formed.

All systems have in common that the overflow or the reject is regulated by raising or lowering the suspension level in the flotation cell.

Thus, in other flotation cell types, by opening or closing the reject valve, the internal pressure of the cell is changed in such a way that the suspension level in the flotation system can be regulated indirectly.

With a change in the suspension level, however, the foam height and thus also the residence time of the foam in the flotation plant changes. Thus, the foam changes its composition as well as its density, ie its mass fraction per volume, so the regulation can not follow a linear relationship between volume - to mass flow.

This means that as the suspension level increases, more reject or overflow is produced, decreasing the foam height in the cell or the residence time of the foam in the flotation cell. This ultimately means that the solids content, ie the mass fraction, decreases in the foam. Despite higher volume flow thus no significant mass flow change occurs. In contrast, increases the residence time with a lower suspension level, thus increasing the foam thickness and thus its composition. Thus, the liquid has more time to drain from the foam blade, the mass flow through the reduced drain remains unchanged.

This is a strong limitation of the control concepts available on the market today because the operating window is severely restricted.

One of the objects of the invention is to propose a method which improves the control and thus achieves a more constant quality of the accept to be produced.

This object is achieved by the method having the features of claim 1.

According to the invention, a method of the initially mentioned type is proposed, in which the air volume of the air stream which is supplied into the flotation cell for ventilation is controlled or regulated by at least one second controller such that the composition of the outflowing flotation foam is adjusted in accordance with the requirements of a predetermined characteristic value becomes.

The additional control of the air flow ensures that the generation quantity of the flotation foam can be adjusted and thus a more constant quality of the accept to be produced is achieved.

Due to the foam channel control by means of the first controller, the required amount of reject (foam) is accordingly provided by raising or lowering the suspension level. By the second control loop, the amount of air and thus the resulting amount of foam is adjusted accordingly. That is, as the suspension level of the cell increases, the amount of air is increased to produce more foam, reducing the suspension level to its original level. The reverse case applies accordingly. This ensures that the foam composition (solids content) is kept constant. On the one hand, this has the advantage that the overflow behavior remains virtually constant (ratio of mass flow to volume flow) and, on the other hand, the tracking of the foam volume significantly expands the operating window of the control.

The characteristic value can preferably be formed from the ratio of mass flow to volume flow of the effluent flotation foam.

Furthermore, in the formation of the characteristic value, the production quantity of the flotation and / or at least one quality value of the suspension feed into the flotation can be taken into account.

However, the characteristic value can also be selected on the basis of or taking into account a predetermined quality value for the accepted product. The quality value of the accept may be the degree of whiteness and / or the ash content.

The measurements of the quality values of the accept can also take place in a subsequent process step, such as after the disc filter, done.

Furthermore, the level of the pulp suspension and / or the foam density in the flotation cell can be measured and these values forwarded to the regulators.

Furthermore, it is possible to measure in the foam collecting channel the level of the flotation foam and / or the foam density in the foam collecting channel and to forward these values to the regulators.

In one embodiment, a plurality of flotation cells may be connected in series to a flotation stage. The flotation foams can then flow off into a common foam channel and a measurement of the foam level and / or the foam density can take place.

In a further embodiment, the flotation may consist of at least two flotation stages. These are connected in such a way that the flotation foam of the first flotation stage is fed into the inlet of the second flotation stage, wherein at least the first flotation stage and / or the second flotation stage consists of a plurality of flotation zones, the flotation foam of which flakes off into a respective foam collecting channel. The flotation foam of the second flotation stage is removed and the accepts fed to the inlet and / or the accepts of the first stage. At least in a part of the flotation cells, the volume of air which is supplied into the flotation cells for ventilation, controlled or regulated by at least the second controller such that the composition of the discharged Flotationsschaums is set according to the requirements of a given characteristic value.

However, the air volume can also be regulated in each cell or it can be provided to regulate the volume of air for at least one group of cells.

• – Drehgeregelte Pumpe
• – Regelventil auf der Druckseite der Pumpe
• – Direkte Luftmengenregelung am Belüftungselement

The air volume control can be carried out by at least one of the following device:

• - Rotary controlled pump
• - Control valve on the pressure side of the pump
• - Direct air flow control on the ventilation element

• – indirekt über das Suspensionsniveau
• – indirekt über Differenzdruckmessung
• – mittels Radarmessgerät
• – mittels Ultraschallmessgerät

Furthermore, the foam thickness measurement can be carried out by at least one of the following methods and / or devices:

• - indirectly via the suspension level
• - indirectly via differential pressure measurement
• - by radar gauge
• - by means of ultrasonic measuring device

Alternatively, it is conceivable that the foam density measurement is carried out by the removal and the weighing of a sample and / or by means of an automatic measuring device.

With the help of the method according to the invention, it is possible to intervene by control processes of the two controllers in the operation of the system that the economically and technologically optimal operating condition can be achieved immediately even with changes, especially the raw materials or requirements. In this way, it is possible for the first time to even out the effects that result from different foaming behavior of the raw materials and / or chemicals. It may happen that, due to a different composition of the raw materials, which is often the case especially with recycled paper raw materials, the foam is looser or more compact, which has so far led to deviations of the operation of the optimal point.

Further features of the method according to the invention and further advantages of the invention will become apparent from the following description with reference to the figures.

In these show:

1 Flotation cell for carrying out the method

2 a process schematic of an embodiment

In 1 is the section of a flotation cell schematically shown in side view 5 shown. This flotation cell 5 may be a cell in a flotation plant with one or more flotation stages. The pulp suspension S as well as the air L for aerating becomes the flotation cell 5 over the ventilation pipe 24 fed.

The resulting by the ventilation flotation foam 3 flows over a weir 23 into the foam collecting channel 8th the flotation cell. In the case of several cells connected in series of a flotation stage, a common foam collecting channel can also be used for several cells 8th . 9 be provided, in which sets a liquid level. The air L, used to ventilate the flotation cell 5 can be supplied through the ventilation tube, for example, from the interior of the flotation cell 5 be sucked off.

Here, the control of the amount of air by means of a rotationally controlled pump, a control valve on the pressure side of the pump or a direct air flow control on the ventilation pipe.

The cleaned accepts 7 the cell 5 is via a pump 22 pumped out of the bottom of the cell. If the flotation cell is not the first or last cell of a flotation stage, pumps are mostly used that pass, accept 7 , an upstream flotation cell to the vent tube 24 lead the downstream flotation cell.

The first controller regulates due to the level transmitter 13 measured levels the control valve 21 such that the height of the suspension level in the flotation cell 5 is kept at a predetermined value. Thus, the amount of foam or the foam volume is regulated, which over the weir 23 flows into the foam gutter. In other cases, the capacity of the pump 22 through the regulator 20 be changed, for. B. if their speed is adjustable.

The second controller 50 regulates the air volume of the air flow, that of the flotation cell 5 is supplied for ventilation. The control or regulation is due to the composition of the effluent flotation foam 3 . 4 , which is set according to the requirements of a given characteristic value.

The characteristic value is preferably the ratio of mass flow to volume flow of the effluent flotation foam 3 . 4 but can also be based on a given quality value for the accepts 7 to get voted.

The mass flow is measured by means of a continuous foam density measurement 51 determined at the outlet of the foam collecting channel 8th from flotation foam 3 . 4 is flowed through.

Furthermore, in the foam collecting channel 8th the level of the flotation foam and / or the foam density in the foam collecting channel are measured and these values are sent to the regulators 20 . 50 to get redirected. All signal paths are drawn by dashed lines.

In 2 is a process schematic of a possible embodiment with two flotation levels 1 . 2 shown. In order to simplify the presentation, the presentation of the second rule was omitted, as it 1 can be seen. Basically there is the possibility of all cells 5 to equip with such air flow control. But it is also possible to provide only individual cells or cell groups with an air flow control.

The paper pulp suspension S is first a first flotation 1 fed. The apparatus design of such a flotation is known per se. It can have one or more flotation cells 5 contain, which are flowed through by the suspension successively or in parallel. In the example shown here are four flotation cells connected in series 5 the first flotation stage 1 drawn. The contaminant-containing flotation foam formed therein 3 is in a foam collecting channel 8th collected, possibly deaerated (not shown) and a second flotation 2 fed.

Also the second flotation stage 2 contains one or more flotation cells similar to those of the flotation stage 1 could be. In each of these is a flotation foam 4 formed into the foam collecting channel 9 flows. For such systems, it is typical that the flotation foam 4 The second stage contains much more contaminants and far fewer fibers than the flotation foam 3 the upstream flotation stage 1 , The one in the second flotation stage 2 made pass 6 (Acceptance) is in the inflow of the first flotation stage 1 led, especially if he is not the quality of the accept 7 the first flotation stage 1 Has.

The level in the flotation cells 5 as well as in the foam collecting channels 8th . 9 , is by means of a level transmitter 13 measured and this value to the regulator 20 . 50 forwarded, the z. B. can be part of the process control system. The adjustment of the level is then carried out by signals of the controller 20 to the fuel pump 22 for the accept 7 or a control valve 21 in the accepts stream. Alternatively, it is possible to influence the inflow amount of the pulp suspension S for flotation, so z. B. via the feed pump 44 or a control valve 41 ,

For the air volume control as well as for the foam level control, the foam level in the foam collecting channel 8th measured, z. B. by a level transmitter 13 . 13 ' , Higher level normally means larger foam discharge at the stowage device 10 , If this outflow is to be changed, for example, the flow-through open cross section of the storage device 10 be set differently, thereby increasing the level of foam in the foam collecting channel 8th changes. The corresponding signal of the level transmitter 13 ' then causes the regulator 20 the suspension level in the flotation cell 5 raises or lowers and thus the amount of in the foam collecting channel 8th outflowing flotation foam 3 to the desired value. Thus, by the method, the amount of the foam collecting channel 8th exiting flotation foam 3 set and changed if necessary. Accordingly, in the second flotation 2 (or more, if any) to proceed, so at the foam collecting channel 9 with stowage device 11 , Analogous to the first flotation stage 1 , the level in the flotation cell is controlled by a level transmitter 13 measured and by signals to the pump 22 ' for the pass 6 or to the appropriate control valve 21 ' (alternatively: inlet pump 44 ' or corresponding control valve 41 ' ).

The regulators 20 and 50 can z. B. also a quality signal of a quality sensor 12 process, the z. For example, the whiteness of the accept 7 measures (or a quality sensor 12 ' z. B. for the whiteness of the feed), and accordingly change with the aid of the invention, the amount of flotation foam to be deducted and / or the amount of air.

In the combination of these two control processes (adjustment of the level in the flotation cell and change the amount of air blown in) is a great advantage of the method according to the invention. Deviations can be compensated immediately, d. h., depending on requirements, the foam composition is optimized for short or longer service life.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102004020339 [0004]
• EP 1388370 A1 [0004]
• DE 102008031411 A1 [0010]

Claims (15)

Method for controlling or regulating a flotation ( 1 ) which is operated to remove impurities from a pulp suspension (S), the at least one flotation cell ( 5 ), by means of which a purified accept ( 7 and a pollutant-containing floating flotation foam ( 3 . 4 ), Rejekt, which from the surface of the paper pulp suspension (S) in a foam collecting channel ( 8th . 9 ) flows, can be formed by a stream of air ( 4 ) is blown into the fiber suspension (S), wherein the volume flow of the outflowing flotation foam ( 3 . 4 ) from the flotation cell ( 5 ) by changing the level of pulp suspension in the flotation cell ( 3 ) by a first controller ( 8th ) is controlled or regulated, characterized in that the air volume of the air stream, that of the flotation cell ( 5 ) is supplied for ventilation, by at least one second controller ( 50 ) is controlled or regulated such that the composition of the effluent flotation foam ( 3 . 4 ) is set according to the requirements of a given characteristic value. A method according to claim 1, characterized in that the characteristic value is formed from the ratio of mass flow to volume flow of the effluent flotation foam. A method according to claim 1 or 2, characterized in that the characteristic value is formed taking into account the production quantity of the flotation. Method according to at least one of claims 1 to 3, characterized in that the characteristic value is influenced taking into account at least one quality value of the incoming suspension in the flotation. A method according to claim 1, characterized in that the characteristic value is selected on the basis of a predetermined quality value for the accepts. A method according to claim 5, characterized in that the quality value of the accept is the degree of whiteness and / or the ash content. A method according to claim 2, characterized in that the level of the pulp suspension and / or the foam density in the flotation cell are measured continuously, and that these values are applied to at least one of the controllers ( 20 . 50 ) to get redirected. Method according to claim 2 or 7, characterized in that in the foam collecting channel ( 8th . 9 ) the level of the flotation foam and / or the foam density in the foam collecting channel are measured, and that these values are sent to the regulators ( 20 . 50 ) to get redirected. Method according to claim 1, characterized in that a plurality of flotation cells ( 5 ) to a flotation stage ( 1 . 2 ) are connected in series in series and the flotation foams in a common foam channel ( 8th . 9 ) flow away. A method according to claim 1, characterized in that the flotation consists of at least two flotation stages, which are connected in such a way that the flotation foam ( 3 ) of the first flotation stage ( 1 ) into the feed of the second flotation stage ( 2 ), wherein at least the first flotation stage ( 1 ) and / or the second flotation stage ( 2 ) from several flotation cells ( 5 ), whose flotation foam in each case a foam collecting channel ( 8th . 9 ), the flotation foam ( 4 ) of the second flotation stage ( 2 ) and the accept ( 6 ) the inlet ( 41 ) and / or the accept ( 7 ) is supplied to the first stage, wherein at least in a part of the cells, the volume of air flowing into the flotation cells ( 5 ) is supplied to the ventilation, by means of at least the second controller ( 50 ) is controlled or regulated such that the composition of the discharged flotation foam ( 3 . 4 ) is set according to the requirements of a given characteristic value. Method according to claim 10, characterized in that the volume of air in each cell ( 5 ) is regulated. A method according to claim 10, characterized in that the volume of air for at least one group of cells ( 5 ) is regulated. A method according to claim 1, characterized in that the air volume control is performed by at least one of the following device: - Rotary controlled pump - Control valve on the pressure side of the pump - Direct air flow control on the ventilation element A method according to claim 1, characterized in that the foam thickness measurement is carried out by at least one of the following methods and / or devices: - indirectly via the suspension level - indirectly via differential pressure measurement - by radar gauge - by means of ultrasonic measuring device A method according to claim 8, characterized in that the foam density measurement by the Taking and weighing a sample and / or carried out by means of an automatic measuring device.

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