The present invention relates to an arrangement and a method in connection with a static mixer, when a liquid or gaseous substance, such as a chemical, is mixed into a medium formed of solid and liquid substance, especially to fiber suspensions being generated during processing of wood or other vegetable-originating substance. To this kind of suspensions belong the fiber suspensions of the pulp and paper industry, such as chemical pulp and mechanical pulp suspensions, as well as pulp suspensions of paper production.

For mixing chemicals and gases into fiber suspensions, dynamic mixers are used, which typically are provided with a rotating rotor or a corresponding member for effecting the mixing, and static mixers. In the latter, a stationary member guiding the flow, typically some sort of throttling, has been arranged in the flow channel, where the flow rate is increased and the static pressure is decreased. Chemical is introduced into a lower static pressure zone or it can be introduced upstream of the point of throttling. In a static mixer, by throttling the flow, i.e. decreasing the cross-sectional area, an increase in the flow rate i.s achieved, and due to the throttling and the shape of the flow channel a strong motion mixing the suspension, even turbulence is generated, whereby the introduced chemical will mix into the actual flowing medium. Static mixers are typically provided with, in addition to or alternatively to the throttling, flow barriers arranged in the flow channel for generating turbulence.

Fiber suspension is a demanding material flow in view of mixing, because in order to obtain a good mixing result the fiber network (fiber flocks) are to be decomposed. In the mixing, a plug flow is to be decomposed, fiber flocks are to be broken into micro- flocks and preferably into individual fibers, whereby the bleaching chemical is made to be distributed in the vicinity of the individual fibers. Traditionally, with medium consistency fiber suspensions high-capacity fluidizing chemical mixers have been used, wherein the rotor of the mixer generates the turbulence required for the mixing. Although modern fluidizing chemical mixers are reasonably small, intensifying energy consumption creates needs to decrease the amount of energy used for mixing chemicals. The operation of static mixers is based on utilizing the pressure loss taking place in the apparatus and/or dividing the suspension flow into partial flows and combining them in the flow direction so that the concentration differences upstream of the mixer will be equalized.

European patent 1469937 (WO 03/064018) describes an apparatus for admixing a gas or a liquid into a material flow. In this apparatus, a tube with a circular cross section is provided with a chamber for the material flow. The chamber has an inlet part, the cross section of which later changes from circular to oval while the area remains unchanged and an outlet part, the cross section of which later changes from oval to circular while the area remains unchanged.. Gas or liquid is fed into the material flow at the narrow- est point of the apparatus, which is provided with e.g. small circular holes around the chamber. The change of the material flow from laminar to turbulent state takes place when the minimum height of the oval cross section is defined in a proper way. The gas or the liquid is added in the turbulent zone. For adding steam into a fiber suspension, direct heat injection heaters are used. In those the steam is admixed directly into a flowing fiber suspension to be heated, whereby the heating takes place quickly. Although direct steam injection heaters are efficient, fiber suspensions with flocking matter tend to clog the heater,- if the suspension is to flow through bends and turns. US-patent 6361025 describes a direct steam injection heater that is designed for viscose material flows, such as fiber suspension, and in which the steam is introduced into the suspension flowing axially through a tube. In the construction according to US-patent 6361025 steam feed takes place in a cylindrical perforated part mounted through the device. The direct steam injection heat exchanger of the above-mentioned US-patent is advantageous as a steam feeding device, because the steam is fed via several small holes into the by-flowing pulp. As long as the pressure drop across the small open steam holes is adequate, the flow of the steam into the suspension remains even. When the velocity of the steam is adequately high, homogeneous condensing of the steam is ob- tained due to high turbulence caused by the steam feed. The steam condenses evenly, as the condensing takes place near the feed point.

When the pressure loss generated in a static mixer is utilized for effecting the mixing, the mixing result typically varies depending on the pressure loss. The pressure loss changes according to flow rate, consistency and pH, and thus production or operation disturbances affect the mixing result. If a static mixer is based on dividing and combining partial flows, the mixing result is not proportional to the generated pressure loss. A possible problem in this type of devices may be clogging of the partial flow channels and clear deterioration of the mixing efficiency, in addition to an increased pressure loss. Based on the above, when treating fiber suspensions the static mixer should break the fiber network of the suspension to an adequate extent, and preferably even fluidize the through-flowing suspension, and the mixing result should not be dependent on the generated pressure loss, and partial clogging of the device should not affect the mixing result. A problematic situation for static mixing is a situation where the mixer is sub- jected outside the optimal conditions for a long period of time, whereby the feed conduits and/or feed openings for the chemicals, and in the worst case the fiber suspension feed channel may get clogged. Then there are little possibilities for opening the cloggings, because the mixer does not have movable members for that purpose. In the designing of a device for mixing fiber suspension attention is to be paid to the possibil- ity of ensuring the functioning condition of the device even if the suspension has been thickened e.g. due a disturbance situation at the mill. This means that when the mixer is taken into use it will reach an adequate operational level at the same time as the chemical feed is initiated. If a pressure loss generated in the device is utilized for generating turbulence in static mixers, but it is still desired to limit the extent of the pres- sure loss, the chemical feed is to be as even as possible with respect to the flow cross- sectional area.

In addition to steam or other gas, it is necessary to introduce into the fiber suspension flow also one or more liquid chemicals, such as bleaching chemical, which has to be distributed and mixed efficiently into the fiber suspension in order to ensure adequately quick and efficient reactions between the suspension and the chemical.

A problem with static pulp mixing has in prior art often been a narrow operating point and uncertainty of operation in fiber suspension consistency ranges of 6-16%. In pulp production the output of the fiberline may vary so that momentarily it is 40-120% of nominal production, whereby it may be complicated to make a static mixer, which receives its mixing energy first of all from the flow of pulp and the flow resistance generated by various types of means, to operate efficiently within this flow/output range. If the apparatus is big and the construction is open, it is not possible to generate such mixing flow or turbulence in the pulp that would allow efficient distribution of chemical into the pulp slurry. Further, if the construction of the mixer causes pressure fluctuation in the mixing point, it also directly affects the feeding of the chemical. In the fiberline, the mixed chemical is typically a bleaching chemical, steam or a pH- regulation chemical. The feeding of chemical into the fiberline is effected by means of pressure difference and a regulation valve. The regulation is successful and the chemical feed is even, if the pressure difference between the chemical line and the pulp line is of a similar value, whereby also the flow through the regulation valve is even. It is known that the flow of the chemical through the regulation valve is directly proportional to the pressure difference over the valve and to the opening of the valve. If the pressure in the pulp line varies due to the above described problems, it auto- matically causes also unsteady flow of the chemicals. The same problem arises if the flow at the mixing point is uneven, whereby more chemical will be passed into the slowly flowing pulp than into quickly flowing pulp.

Publication JP 01-314795 discloses an arrangement where oxygen is added into pulp. Pulp discharged from a drop leg is pumped into a diffuser tube mixer mounted between the pump and a regulation valve, and oxygen is added into the pulp in the mixer. The operational safety of this apparatus should be improved, because adequate control of the flow is not possible in exceptional situations. Neither is it possible to ensure homogeneous feed flow into the mixer. However, it would be important that both the suspension and the chemical flow evenly into the mixer for obtaining a good mixing result.

As the penetration of a chemical into the fiber suspension in static mixing is based on uniform pressure difference between the feed member and the pulp flow, a problem arises as, on one hand, flow resistances should be introduced in the pulp flow for effecting the mixing, but on the other hand, the flow resistance should not cause problems in the flow region. An object of the present invention is stabilizing the chemical mixing in a static mixer application. Thus, the aim is to create as uniform flow conditions as possible, independent of process disturbance situations, such as changed production rate, or corresponding.

The present invention relates to an arrangement for feeding chemical into a fiber suspension in a fiber suspension transfer line, said arrangement comprising in the fiber suspension flow direction a pump, a closing valve, a static mixer apparatus whereto a chemical feed line is connected, and a regulation valve. Preferably the arrangement in the pulp transfer line, such as pulp feed line, comprises a MC (medium consistency) pump feeding pulp, a chemical feed line and a static mixing apparatus and a regula- tion valve for regulating the pumping of medium consistency pulp, which valve is located downstream of the mixing apparatus. A closing valve is located downstream of the pump. The regulation valve can be either a standard type of valve or its construction can be modified to correspond to the mixing requirement for generating additional turbulence. Typically it is a ball valve or a segment valve. It is essential that the flow of pulp through the regulation valve and thus also through the mixing member is controlled by means of the motion of the regulation valve.

The present invention also relates to a method of feeding chemical into a fiber suspen- sion in a fiber suspension transfer line. Fiber suspension is pumped and treating chemical added therein in a static mixer, and the flow in the transfer line is controlled by means of both a closing valve upstream of the mixer and a regulation valve downstream of the mixer. The fiber suspension is a fiber suspension of pulp and paper industry, preferably with a consistency of 6-16%.

According to an embodiment, the chemical feed line of the static mixer is provided with a conduit for introducing an auxiliary medium, such as dilution liquid, into the chemical flow. The purpose of this is to continuously ensure an adequate flow amount from the chemical line into the mixer. The static mixer of the process is dimensioned such that operates optimally within a certain pulp production range. If the situation requires decreasing the pulp production amount, a static mixer will more easily be drifted away from this optimal range than a dynamic mixer, because the amount of chemical required decreases and thus also the flow from the chemical line into the mixer decreases. This results in deterioration of the mixing result. As described in the above, the chemical is typically added into the pulp flow via openings. If the amount of chemical required is decreased, the number of the openings and/or the flow therethrough decreases, whereby the mixing result is deteriorated. By introducing additional liquid into the chemical line, adequate continuous flow can b e maintained from the chemical line and thus also an adequate pressure difference for obtaining efficient mixing. The amount of additional liquid required is typically so low and limited in time that it does not disturb or dilute the overall process.

Changes in the process cause fluctuation in the amount of required chemical. The above described embodiment of the invention ensures even flow of fluctuating chemi- cal amounts and the conduits (e.g. openings) do not get clogged, when a constant pressure difference is maintained between the chemical flow and the pulp flow. Thus, the mixing can be stabilized independent of changes in the pulp production amounts. E.g. at the fiberline of a chemical pulp mill the amounts of chemical change essentially when the bleaching line produces pulp from different sorts of trees or with different target brightness values. The solution according to the present invention can preferably be applied for all pulp treatment chemicals. Typically the chemical to be added is an acid, alkali (e.g. NaoH), chlorine dioxide, peroxide, chelates. The medium added into the chemical can be any substance that is suitable for the chemical. It can comprise a suitable liquid fraction from the mill, such as washing filtrate or water from a pulp dryer. With some chemicals, the medium to be added can be steam as well.

According to an embodiment, the rotational speed of the pump is adjusted based on pressure measurement downstream of the pump. According to an embodiment, pressure measurement is arranged between the closing valve and the mixer.

According to an embodiment, mixing motion, even turbulence, is caused in the fiber suspension flow by means of a regulation valve.

An advantage of the present invention is that it allows solving following problems in the operation of a static mixer:

- pulsation and finally clogging of the pulp line caused by the throttling members of a static mixer,

- decreased tolerance for pulp consistency fluctuations caused by the throttling members of the static mixer,

- narrower operational range of the throttling member of the static mixer compared to that of a dynamic mixer,

insufficient turbulence, and

- constant pressure is maintained during the mixing.

In order to get a better understanding of the present invention and its practical application, reference will now be made, by means of examples only, to the appended figures, in which

Fig. 1 illustrates an arrangement according to an embodiment of the invention for mixing a chemical into a suspension and leading the suspension into a reactor, Fig. 2a and 2b illustrate a static mixer that can advantageously be used in connection with the present invention. Figs. 2a and 2b are side -views showing a longitudinal cross-section of the mixer,

Figure 3 is a cut view along section A-A of the embodiment of Figure 2, and

Figure 4 is a cut view along section B-B of the embodiment of Figure 2.

The arrangement illustrated in Figure 1 is in the pulp transfer line connected to a MC (medium consistency) pump 102 feeding pulp from a drop leg 1 12, a chemical feed line 104 and a static mixing apparatus 106 and a regulation valve 108 for regulating the pumping of medium consistency pulp, which valve is located downstream of the mixing apparatus. The regulation valve 108 can be either a standard type of valve or its construction can be modified to correspond to the mixing requirement for generating additional turbulence. Typically it is a ball valve or a segment valve. It is essential that the flow of pulp through the valve and thus also through the mixing member 106 is controlled by means of the motion of the regulation valve. Additionally, pressure measurement, flow measurement and a closing valve 1 10 can be arranged between the MC-pumping equipment 102 and the regulation valve 108 immediately downstream of the MC-pump and the MC-pump can be provided with rotational speed regulation. Downstream of the regulation valve 108 the pulp is led into a treatment space, such as a reactor, e.g. a pulp bleaching tower (not shown).

In the embodiment presented herein the regulation valve 108 as a dynamic member promotes the mixing and ensures as stabile a pressure difference as possible between the pulp line and the chemical line. Simultaneously it is possible to decrease to a reasonable extent the pressure loss of one or more static mixers in the pulp line.

Technically the regulation valve 108 downstream of the mixing member receives its set value either from the surface of the drop leg 1 12 upstream of the MC-pump by means of measurement 1 14, from flow measurement or corresponding external measurement. Then the regulation valve downstream of the mixing member 106 ensures a steady flow of pulp between the pump and the valve, whereby the flow amount is steady. Simultaneously the pressure difference of the regulation valve is utilized as mixing energy and thus the energy of the mixer and the valve are combined into one unity. Be- cause part of the pumping energy that is utilized in the mixing is consumed in the regulation valve (which is a dynamic member), its operation as a moving member efficiently prevents all kinds of clogging and pulsation effects around the mixing member after the pumping. Thus, the pump-mixing member-regulation valve form a dynamic system, due to which the conditions in the mixing zone remain well controlled. Further, the valve is required in the system in every case, so that the solution according to the invention increases functionality.

According to an embodiment, the chemical feed line 104 of the static mixer is provided with a conduit 118 for introducing an auxiliary medium, such as dilution liquid, into the chemical stream. The purpose of this is to continuously ensure an adequate flow amount from the chemical line 104 into the mixer 106. The static mixer of the process is dimensioned such that it operates optimally within a certain pulp production range. If the pulp production amount decreases, a static mixer will in some stage be drifted away from this optimal range, because the amount of chemical that is required decreases and thus also the flow from the chemical line into the mixer decreases. This results in deterioration of the mixing result. By introducing additional liquid into the chemical line 104, adequate continuous flow can be maintained from the chemical line and thus also an adequate pressure difference for obtaining efficient mixing. The chemical line 104 is provided with a flow meter 116 that is connected to a valve 120 in the medium line. A certain flow amount F _crit is determined and a flow greater than that is to prevail in the chemical line 104 to the mixer 106. When the flow of the chemical decreases below this value (e.g. due to a decrease in the production rate of the mill), valve 120 opens and medium is fed from line 188 into the chemical stream so that the flow in line 104 increases above value F _crit .

The adjustability of the mixing can be increased by providing either the chemical pumping or MC-pumping 102 with rotational speed control. If MC-pumping is provided with rotational speed control, according to an embodiment the set value for the rotational speed is determined based on pressure measurement downstream of the pump. Together with the operation of the regulation valve this ensures both constant pressure and as steady a flow as possible in the mixing zone. This further promotes homogene- ous distribution of chemicals into the fiber suspension. On the other hand, if the chemical pumping is provided with rotational speed control, then the aim is to keep the pressure difference constant between the chemical line and the pulp line, whereby the penetration of chemical from the mixing member into the fiber suspension is as homogeneous as possible. Adjusting the rotational speed stabilizes the pressure to a level where pumping energy is optimized while the pressure is stabilized. The closing valve 110 downstream of the MC-pump 102 is connected to apparatus safety and mill safety, but in some cases it can also be provided with actuators capable of control and it can be used for regulating e.g. the pressure at the mixing point, if needed. Combination of two valves, a closing valve upstream of the mixer and a regu- lation valve downstream of the mixer improves operational safety. For example, flow of chemical into the direction of the pump can be prevented independent of the situation.

The arrangement presented in the above is advantageous in connection with all static mixers and significantly promotes the succeeding of static mixing and positive effects provided thereby include:

- energy of static mixing comes from pumping energy. By locating a regulation valve controlling the flow downstream of the chemical mixing member, the pressure loss re- quired by the mixing and the regulation can be combined and the operating functions of one apparatus can be added.

- Steady flow conditions around the mixing member are created, and

- The flow resistances around the mixing member can be designed more freely and thus ensure a steadier flow of pulp.

Thus, by rearranging existing devices in a novel way around static mixing it is possible to both use energy more efficiently, but, first and foremost, to stabilize the mixing conditions and simplify the plant's systems. Additionally, the static mixing is here controlled by means of a dynamic actuator, i.e. regulation valve, whereby the conditions of the mixing operation as a whole can be controlled and a good operation point of the static mixing is obtained within a wide production range.

It is essential that the arrangement is provided upstream of a fiber suspension treatment vessel, and thus said members form an entity.

Figs. 2-4 illustrate a static mixer that can preferably be used in connection with the embodiments according to the present invention. The mixing apparatus comprises a cylindrical tubular body 12, which defines a space that acts as a flow channel for the fiber suspension in the mixing apparatus. It has a suspension inlet 14 and a suspen- sion outlet 16 with flanges 18 and 20. The longitudinal axis of the tubular body is marked with X. The suspension flows essentially in the direction of the axis X. The mixing apparatus 10 is attached at its flange 18 to the inlet piping for incoming fiber sus- pension and at its flange 20 to the discharge piping for fiber suspension exiting the mixer (not shown).

The tubular body 12 is provided with a tubular feed member 22 that extends into the flow channel transversely against the longitudinal axis X of the tubular body and also against the flow direction F of the suspension. The feed member has a cylindrical wall 24 with through openings 26 for leading a substance from the member into the suspension flow channel. Openings 26 in an adequate number are provided in both the circumferential and axial direction of the feed member wall. The openings 26 are lo- cated in the circumferential direction preferably only on a predetermined portion of the wall. Openings 26 are preferably provided only on those parts of the wall that are directed towards the inner surface 30 of the tubular body and thus towards the protrusions 28 therein. Openings are preferably not provided in the parts of the wall that are located towards the flow direction of the suspension, i.e. upstream and downstream.

The apparatus comprises protrusions 28 that are arranged on the inner surface/inner circumference 30 of the tubular body in the region of the feed member. Thus, the protrusions are located at the feed member so that the height of the flow channel can be lowered. In the tubular body the cross-sectional area of the flow channel remains es- sentially the same upstream and downstream of said protrusions 28 in the flow direction of the suspension. By limiting the height of the flow cross-sectional area between the tubular feed member and the body of the apparatus by means of a protrusion, the chemical is made to distribute evenly into the by-flowing suspension and simultaneously the velocity of the by-flowing suspension is increased to a desired level. It has been found that the mixing efficiency is influenced by the height of the channel between the feed point 22 and the body of the apparatus, which height in Figure 2 is marked with a letter H. The height H of the channel depends on the pressure difference between the chemical line and the suspension line and preferably H is k *d, where k is in the range of (0,004 -0,012)/100 [m/ka] and d [ka] is the pressure differ- ence between the feed line for the substance and the flow channel for the suspension (dp= p2-p1 ; p2 and p1 have been illustrated in Fig. 4 and Fig. 2b, respectively). This way, the feed member divides the flow channel for the suspension into two parts having an equal height, H, or different heights. The length of the protrusion 28 in the longitudinal direction X of the tubular body is preferably at least the length of the diameter of the feed member 22. When seen in the flow direction of the suspension, the cross section of the protrusion is typically a seg- ment of a circle. In the direction of the circumference of the tubular body the protrusion extends to a certain distance, which is determined for each case mainly by the height H required for the flow channel. The protrusion may be an integral part of the tubular body construction, as illustrated in Figure 2, or it may be a separate part that is separately attached on the inner circumference of the tubular body. In the latter case the attaching be effected even afterwards or protrusions can even be replaced due to wear or due to a desire to change their size.

An essential characteristic of the mixer is the decreasing of the flow cross-sectional area downstream of the feed point by means of a throttling member, such as a throttling plate. In Figure 2a the throttling plate 32 is positioned at a distance, L, from a plane T that passes through the center point of the feed member and is perpendicular to the flow direction of the suspension, for narrowing the flow cross-sectional area, and the distance L is a*H, where a=3-8 and H is the shortest distance of at least one protrusion 28 arranged on the inner surface of the tubular body from the outer surface 24 of the cylindrical wall of the feed member 22. Further, when studying the effect of the decrease in the flow cross section on the mixing efficiency it has been found that it has two substantial factors relating to the throttling point. The decrease in the cross-sectional area is to be 40...70% and it is to be asymmetrical with respect to the center line X of the apparatus. At least 60% of the change in the cross-sectional area is to be on one side of the center line X. In the em- bodiment illustrated in Figure 2a the outlet 34 for the suspension is located mainly above the center line X of the apparatus.

A mixing chamber 36 is formed in the space between the feed member tube (feed point) and the throttling, the length L of which chamber is preferably a ^* H, where a is between 3 and 8 and H is the height of the channel at the feed point, as described in the above. In the parts of the flow channel on both sides of the feed member tube, favorable conditions generate suspension jets. Throttling 32 after the feed member limits these jets generated on both sides of the feed member tube, thus intensifying the mixing operation. In the mixing chamber 36 the flow turbulences homogenize the concen- tration differences of the chemical or corresponding substance. Making the throttling asymmetric and thus intensifying the mixing is highly advantageous. In Figure 2a the height of the protrusion 28 is constant, i.e. the distance of its planar outer surface from the inner circumference of the tubular body does not change in the longitudinal direction of the protrusion (in the direction of the longitudinal axis X of the tubular body). The outer surface may also be referred to as guiding surface of the pro- trusion, because it guides the flow of the suspension and thus assists the mixing operation.

As mentioned in the above, the throttling member 32 can at its simplest be a plate. In that case it has a circular opening 34, but more preferably the opening is elliptic or a combination of a circular and elliptic shape or a rectangle or a combination of a circle and a rectangle with rounded corners. Figure 2b illustrates another embodiment. The throttling member can also be a part 38 of the flow channel having a length R of 0.02- 2.0*D, where D is the diameter of the tubular body upstream of the throttling. When the length R of the throttling member is 0.2-2.0*D, the throttling channel 38 preferably widens in the flow direction F of the suspension (opening 34a) so that the cross- sectional area of the throttling channel is at its smallest in the part 40 of the throttling member that is closest to the mixing chamber 36.

Figure 3 illustrates a mixing apparatus seen from the outlet for the suspension. Lo- cated foremost are a flange 20 and a throttling plate 32, wherein the outlet 34 for the suspension flow is elliptic. Between the feed member 22 and the planar outer surface of the protrusion 28 a flow channel 52 is formed having a height H. Most preferably the opening 34 is elliptic or a combination of a circular and elliptic shape or a rectangle or a combination of a circle and a rectangle, where the corners are rounded.

Figure 4 illustrates in side view the longitudinal section (longitudinal axis Z) of the feed member 22 as mounted transversely in the tubular body 12, through which the suspension flows axially. The longitudinal axis Z of the feed member is transversely against the longitudinal axis of the tubular body 12. One end 40 of the cylindrical wall 24 of the feed member 22 is attached to the tubular body 12, while the other end 42 of the cylindrical wall is open. The end 40 of the cylindrical wall of the feed member extends in the axial direction in the form of a flange-like basic plate 44 that is attached to a flange 62 extending from the tubular body 12. A conduit 46 and a flange 48 are connected to the tubular body 12 in the direction of its radius, to which flange a feed pipe (not shown) for chemical or other substance is connected. The open end 42 of the feed member sits in the inner part of said conduit 46. Through the opening 42 of the open end chemical (arrow 50) is led into the interior of the feed member 22, the wall of which feed member is provided with through openings 26, via which the chemical is led into the suspension in channel 52.

Inside the cylindrical wall 24 of the feed member 22 a closing member 54 is provided, whereto a shaft 56 or corresponding is connected, which in its turn is connected to an actuator (not shown) for. moving the closing member around the longitudinal axis Z of the feed member. According to an embodiment, the closing member is formed of a cylindrical wall 58 provided with at least one opening 60. Figure 4 illustrates two openings 60, which are set to face openings 26 of the feed member so that a required amount of chemical can flow and get mixed into the suspension. Thus, the closing member is used to cover a desired number of openings 26 for regulating the amount of chemical.

The openings 26 in the feed member can be holes or slots. It has been discovered that for liquid chemicals the diameter of an individual hole is preferably bigger than 2.0 mm, more preferably 3-6 mm. If the chemical is fed through slot-like openings instead of holes, the width of the slot is to be more than 2.5 mm, more preferably 3-6 mm. The length is preferably 20-40 degrees in the direction of the circumference of the cylindrical wall of the feed member.

When two dynamic actuators are provided in connection with a static mixer, one in the fiber suspension flow (a regulation valve) and one in the chemical flow (e.g. the above- mentioned closing member 54 for regulating the feed openings for the chemical), the mixing apparatus can be controlled within a very wide operational range. The possibil- ity of diluting the chemical further increases the operational range.

The above presented arrangements are suitable for all types of static mixers, but are advantageous also with a dynamic mixer. This is because the regulation valve can also act as a closing valve, whereby investment costs are reduced.

The present invention is not limited to the above presented embodiments, but various modifications are possible within the scope defined by the claims.