Method and apparatus for mixing a first medium to a second medium and a bleaching process applying said method

The present invention relates to a mixing of gas into a medium. The method and apparatus in accordance with the present invention are especially applicable in the bleaching plants of the wood processing industry for mixing gaseous bleaching chemicals into pulp and to bleaching process of pulp, in which the mixing method and apparatus in accordance with the present invention are applied. An excellent application is mixing ozone-containing gas into a fiber suspension flowing in a pipe line and an ozone bleaching process. The previously known methods and apparatuses have not been able to mix satisfactorily large volumes of gas, about 50% of the total volume of the flow, into the medium flow. In the method in accordance with the present invention the mixing is carried out in a strong shear force field efficiently and uniformly, whereafter the fiber network of the medium is allowed to form rapidly and in a controlled manner so that gas is not allowed to separate in the flow as bubbles, but remains in the plug flow in the fiber network.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to mixing a first medium into a second 
medium. The present invention especially relates to mixing gas to a 
medium, but it may as well be applied, for example, for mixing liquids, 
since mixing of gas has considerably higher requirements than the others 
both on the mixers and mixing methods. The method and apparatus in 
accordance with the present invention are especially suitable for mixing 
gaseous bleaching chemicals, such as oxygen or ozone, used in the bleach 
plants of the wood processing industry, and for the pulp bleaching process 
applying the mixing method and apparatus in accordance with the present 
invention. An excellent application is mixing ozone-containing gas with 
fiber suspension flowing in a pipe and an ozone bleaching process. 
The main objective of the present invention is to develop a method of and 
an apparatus for mixing large volumes of gas into a medium. Further, since 
the chemical to be added may be extremely rapidly reacting, such as ozone, 
said preconditions set great demands on the method and apparatus to be 
developped. 
In most of the modern bleaching plants very often large volumes of gas are 
desired to be mixed into a medium consistency fiber suspension, which 
means that the consistency of the fiber suspension is approximately 10-18% 
and it must be possible to mix a large volume of gas therewith. In other 
words during the mixing process approximately 40 to 80% of the medium is 
fiber suspension and approximately 20 to 60% gas, the proportion of the 
gas most usually being approximately 30 to 50%. It is difficult to have a 
uniform feed of such a large gas volume and to reach a good mixing result, 
because gas is separated due to local pressure differences to areas of 
lower pressure, if possible. The non-uniform mixing results on the 
increase of chemical loss, which further results in a non-uniform 
bleaching and in poorer runnability of the process. 
The use of above mentioned ozone as a bleaching chemical in bleaching will 
become more and more popular in the future. There is an ongoing transition 
from pilot testing to applications in a mill scale, which leads to even 
higher demands on the apparatus due to the characteristic behavior of 
ozone. Ozone may be produced and used with the modern technique only in 
very small proportions, whereby most (usually more than 90%) of the 
chemical to be mixed with the pulp to be bleached is in fact inert carrier 
gas compared with the ozone. The result is, of course, that the volume of 
the gas to be mixed is large. Another significant point is that ozone 
reacts very rapidly with the material in the fiber suspension. Thus the 
mixing must be at the same time both very quick, efficient and also 
uniform in result. Since the ozone immediately reacts with all fibrous 
material it encounters, the ozone-containing gas may not be allowed to 
meet only a particular portion of a suspension for a single moment, 
because it will result in a very uneven bleaching. According to the 
present technology ozone is not at all a selective chemical and it reacts 
equally efficiently both with the fibrous material and the lignin to be 
removed or bleached. In other words, if the ozone dosing for a portion of 
the suspension is excessive, the ozone quickly causes damage also in the 
suspension, resulting, of course, in poorer quality of the bleached pulp. 
Thus the mixing must be very uniform right from the beginning. Due to the 
non-selectivity the ozone cannot also not be overdosed and also not for 
the reason that ozone is an expensive chemical. 
Ozone may be industrially produced only in relatively dilute mixtures. In 
other words only 5 to 10% of the gas to be fed for the bleaching, is ozone 
the rest operating merely as a so called carrier gas. The carrier gas is 
in most cases either oxygen or nitrogen. Therefore, approximately 10 to 20 
times the volume of the ozone carrier gas must be fed and mixed although 
relatively small volumes of ozone are otherwise sufficient for the 
bleaching. 
Some prior art mixers in the use of cellulose industry as well as their 
applicability in efficient and uniform mixing of large volumes of gas are 
studied more in detail below. 
U.S. Pat. No. 4,416,548 illustrates an embodiment, in which the gas to be 
mixed is introduced into the frontside of a cylindrical rotor of an 
apparatus slightly resembling a centrifugal pump to a point where the pulp 
flowing axially along the suction duct is divided fan-like into a radial 
flow bringing the gas therewith to the rim of the cylindrical rotor. The 
flow turns there again axial, flowing, for example, between the pin-like 
members stationary mounted on the rotor housing and on the outer rim of 
the rotor to a spiral discharge chamber of the apparatus. The operation of 
the apparatus is based on that the pin-like members of the rotor pass the 
members of the housing very close generating a very strong shear force 
field mixing the chemical effectively to the pulp. The apparatus has two 
significant defects or disadvantages considering the purpose of the 
present invention. Firstly, the gas is fed into the center of the rotor 
into the relatively slowly flowing pulp, which, for example, when feeding 
ozone, results in a local overdose and damage of cellulose in the 
particular portion of the pulp. In order to be able to bleach the whole 
pulp amount, some kind of an overdose should be fed into the mixer, even 
with a risk of damaging the cellulose. Secondly, there is the disadvantage 
that subsequent to the efficient "pin mixing zone", pulp is allowed to be 
quickly discharged into a wider space, a spiral. Consequently, a zone of 
lower pressure is generated, in which the gas in the pulp is easily 
separated by the centrifugal force from the fiber suspension, which is 
still in a fluidized stage. Thus when the consistency of the pulp 
increases and the pulp forms a plug flow in the discharge channel gas is 
entrained therewith in large bubbles. As a result therefrom the ozone 
which possibly still has not reacted in the gas would still react only 
with the fibers defining the gas bubbles. 
FI patent 76132 illustrates a construction which to some extent resembles 
the arrangement in accordance with the U.S. publication. The apparatus, 
however, is evidently a centrifugal pump, the impeller vanes of which are 
arranged two-piece in such a way that it is possible to fit a number of 
feed and mixing pins for the chemicals between said parts. Thus the feed 
of chemicals to a strong field of shear forces, is carried out in an 
orthodox manner but the pulp is discharged from the mixing zone to the 
spiral of the centrifugal pump, in which the pulp is, as known, subjected 
to intense centrifugal forces, due to which gas is separated from the pulp 
to form its own layer. The result is the same as above. 
As a third example of the prior art is an arrangement disclosed in U.S. 
Pat. No. 4,305,894 comprising an axial flow pump, a rotor thereof and 
mixing means for gas. The liquid, with which the gas is intended to be 
mixed, is drawn by the rotor to a cylindrical suction duct, into which gas 
is fed immediately after the rotor in the flow direction through a pipe 
surrounding the axis of the rotor. A stationary blade is mounted on the 
outer surface of the pipe immediately after said gas feed to generate 
together with a liquid, strongly circulating in the duct, a field of shear 
forces in the liquid so that gas is mixed with the liquid. Additional 
efficiency to the mixing may be brought about by adding ribs or like on 
the wall. The apparatus does not, however, guarantee a uniform mixing, 
because the diameter of the suction duct is rather large and the gas is 
fed into the center of the flow. It is not possible to ensure that the gas 
in the apparatus would be able to flow into contact also with the liquid 
flowing on the outermost layer of the flow, but it is assumed that the gas 
is mixed with the liquid flowing relatively close to the axis and the 
liquid in the outermost area of the suction duct remains without gas. When 
ozone is used, the non-uniform mixing results in non-uniform bleaching 
result and damage of cellulose because of a local overdose. 
A fourth prior art publication worth mentioning is DE 2920337, which 
generally describes the utilization of the fluidization and different 
applications thereof, giving an example of mixing liquid or gas into fiber 
suspension. Said embodiment is illustrated primarily in FIGS. 1-4, of 
which the construction of FIG. 2 comprises a cylindrical rotor positioned 
substantially axially in the flow channel, the outer surface of the rotor 
as well as the inner wall of the flow channel being provided with 
protrusions. The chemical, gas or liquid, to be mixed is introduced into 
the rotor through the shaft of the rotor, from the surface of which the 
chemical is fed to a relatively narrow fluidization zone between the rotor 
and the wall of the flow channel. It may be assumed of said construction 
that the mixing of gas into the suspension is uniform, but said apparatus 
still has some significant defects. Firstly, the illustrated rotor is 
rather short, which as such is an orthodox arrangement considering the 
energy consumption and also when the volume of the gas to be mixed is not 
very great or when the intention is to give no time to the chemical to 
react with the suspension. However, what is obtained with the mixer 
described above is a relatively high circumferential speed of the 
suspension causing the separation of the gas in the area immediately 
subsequent to the rotor when centrifugal force forces the suspension to 
the wall of the flow channel and the gas flows to the center of the flow. 
Said separation tendency in the arrangement according to the publication 
is so strong also because the axial cross-section of the rotor is 
rectangular, whereby the pulp being discharged from a rather narrow 
fluidization zone arrives to a large flow channel. There a local zone with 
a strong low-pressure effect (i.e. a big pressure difference) is generated 
and thus gas can readily separate as large bubbles to the center of the 
flow. 
A second embodiment illustrated in DE publication 2920337 (FIGS. 3 and 4) 
comprises a similar rotor, which is mounted to the flow channel 
transversely in such a way that the suspension must pass through a narrow 
gap to the backside of the mixer rotor, where a throttling point of the 
flow channel is arranged. The mixing process corresponds to the previous 
embodiment, but is shorter of its duration. The problem is that the 
suspension is allowed to be discharged from the narrow fluidization zone 
into a quickly widening channel, whereby gas is able to separate as large 
bubbles from the suspension. 
SE patent document 462 857 discloses a mixer for mixing bleaching agent 
with pulp. The pulp is tangentially introduced into the mixer housing 
within which a rotor rotates. The rotor comprises a substantially radial 
plate provided at its outer edge with an annular mixing member having ribs 
or grooves lying in a substantially radial plane. In a corresponding 
manner the stationary housing portions facing said mixing member lie in a 
substantially radial plane and are provided with ribs or grooves. The pulp 
together with the bleaching agent travels radially inwardly through the 
gap between said mixing means and the housing and is axially discharged 
from the mixer. A characterizing feature of the mixer of the SE 
publication is that the pulp is discharged through a narrow annular gap 
into a wide space to be discharged from the mixer. 
As is seen from the prior art review above only few previously known 
apparatuses are able to mix a gaseous chemical quickly and presumably also 
relatively uniformly into the fiber suspension. Yet, until now, it has 
always been mixing of relatively small volumes of gas and/or slowly 
reacting chemicals into the pulp. Thus it is in a way logical that no 
arrangements in accordance with the prior art can ensure the mixing of 
large volumes of gases into the pulp so that the gas would also remain 
uniformly mixed with the pulp subsequent to the mixing. 
The objective of the present invention is to eliminate the disadvantages of 
the prior art apparatuses and to ensure that also large volumes of gases 
are uniformly mixed with the pulp and that they remain mixed with the pulp 
also when the pulp is discharged from the mixer to a flow channel or a 
reaction vessel. 
It is possible to further design the mixing apparatus in accordance with 
the present invention to be applied for carrying out the actual ozone 
bleaching in such a way that the carrier gas and the possibly excessive 
ozone are removed by means of the same apparatus and possibly returned by 
means of a second mixing apparatus to the next bleaching stage or step, 
whereby the term "displacement bleaching" may well be used. Said 
displacement bleaching refers to a bleaching, in which a very rapidly 
reacting chemical is fed into pulp and mixed with the pulp to be treated 
in a relatively long axial distance (the distance is, of course, affected 
by the reaction speed of the chemical). The idea, however, is that, when 
the untreated pulp reaches the mixer, it is mixed with a certain amount of 
chemicals (in this example ozone), which immediately reacts, leaving no or 
hardly any reactive chemical in the gas-pulp mixture, only so called 
carrier gas. When new chemical mixture is fed in the same mixer and mixed 
into the pulp it pushes the "old" carrier gas from thereahead, which 
justifies the use of the term "displacement bleaching". 
Characterizing features of the method in accordance with the present 
invention are 
to feed gas in the first phase into a fluidized medium flow in a narrow 
mixing channel; 
to dampen the shear force field of the gas-medium-suspension in the second 
phase and to allow a plug flow to be formed in the medium, whereby the gas 
remains uniformly distributed in said plug flow. 
It is again a characterizing feature of the apparatus in accordance with 
the present invention that the apparatus comprises a mixer body, an inlet 
channel for medium, a substantially annular mixing channel, a discharge 
channel and a rotor rotatable in the mixing channel and that the rotor is 
tapering in the direction of the flow. 
It is a characterizing feature of the bleaching process applying the method 
in accordance with the present invention 
to feed the bleaching chemical into an annular mixing channel; 
to mix the bleaching chemical into a medium which is in a fluidized state; 
to further agitate the medium allowing the chemical to react with the 
medium; 
to discharge the medium from the mixing channel to the discharge channel; 
and 
to remove residual chemical from the medium in the discharge channel and/or 
in the end portion of the mixing channel. 
The method and apparatus in accordance with the present invention are 
described more in detail below, by way of example, with reference to the 
accompanying drawings, in which

DETAILED DESCRIPTION OF THE DRAWINGS 
FIGS. 1 and 2 illustrate a preferred embodiment of an apparatus in 
accordance with the present invention, comprising bearing, drive and 
sealing unit 12 and a mixing portion 14. The above mentioned unit 12 may 
be considered to be of conventional construction, the details of which are 
neither shown nor described here. The mixing portion 14 comprises a rotor 
body 16, an inlet opening 18 and inlet channel 20 for pulp both mounted in 
said body 16, a mixing channel 22, a discharge channel 24 and a shaft 26 
connected to the drive unit and a rotor 28 connected to the end thereof. 
The inlet channel 20 for pulp may be radial, but it may as well be 
tangential either in the rotational direction of the rotor 28 or 
preferably feeding pulp against said direction, as particularly pointed 
out in FIG. 2. The mixing channel 22 is substantially cylindrical, or more 
accurately annular, and it is surrounded in the embodiments shown in the 
drawings by two gas feed rings 30 mounted to the rotor body 16, said rings 
being preferably, for example, of sintered metal, ceramics or even very 
finely perforated metal plates so that the size of gas bubbles is as small 
as possible when gas is fed into the pulp. Of course, it is possible for 
the number of the feed rings 30 to vary from the above mentioned. As can 
be seen from the drawings, four gas inlet conduits 32 are arranged to the 
wall of the mixing channel, through which conduits the treating chemical 
is uniformly fed to an annular chamber 34 outside the feed rings. At least 
two inlet conduits 32 are preferably required to ensure that the feed of 
the gas from the ring to the mixing channel is sufficiently uniform, 
though some applications may function well enough with only one inlet 
conduit. Preferably, although not necessarily, the walls of the mixing 
channel are provided with axial ribs 36 in addition to said feed rings 30 
to intensify the mixing effect. 
The rotor 28 mounted on the shaft 26 is substantially cylindrical in the 
mixing channel portion and is provided with axial ribs 38 according to the 
drawing, the purpose of which is to generate such an extensive shear force 
field together with the ribs 36 possibly mounted on the wall of the mixing 
channel 22 so that even large volumes of gas are mixed into the pulp 
uniformly. One substantial requirement for the efficient and economic 
operation of the rotor is that distance between the walls of the rotor 28 
and the mixing channel 22 is not too large or that the distance between 
the ribs 38 of the rotor 28 and said wall or the ribs 36 arranged 
according to the drawing on said wall is not very long. 
FIG. 3 illustrates a preferred embodiment for ribs 38' and 36' of both 
rotor 28 and the wall of the mixing channel 22, which according to the 
drawing are incontinuous so that both ribs 36' and 38' are cogged, either 
so that they are formed by a row of protrusions attached on the wall 
surface of the rotor/mixing channel or that they are formed of continuous 
ribs 36' and 38', which have protrusions and lower portions, in other 
words recesses therebetween. In another embodiment the protrusions of the 
counter surfaces are interlaced in such a way that said protrusions may, 
if as strong shear force field as possible is required, fit in the recess 
of the counter rib. In some cases it is preferable to arrange said ribs 
inclined and ascending relative to the axis, whereby they accelerate the 
pulp both tangentially and axially. This kind of alignment of ribs may, of 
course, be applied also to the continuous ribs shown in FIG. 1. 
A substantial and important feature relative to the operation of the rotor 
is the length of the mixing portion, the mixing zone, in other words the 
length of the area, in which the pulp is subjected to such an amount of 
shear forces that the mixing is effective and uniform. The length of said 
area is affected by, for example, following: 
volume of gas being mixed; 
whether the reaction chemical, for example ozone, is desired to react 
practically speaking in the whole of said area; 
energy consumption. 
Of said features at least the second and third are counter features, 
because the reaction of ozone requires some time which again requires a 
long, energy-consuming mixing zone. It is the objective of the arrangement 
in accordance with our invention that the length of the mixing portion of 
the rotor 28 is as small as possible without risking the efficiency and 
uniformity of the mixing. 
A substantial portion in the mixer in accordance with the present invention 
is a slowly tapering tip portion 40 of the rotor, preferably it may be of 
its shape "a reverse Laval-nozzle" or at least very much resembling it for 
two reasons. Firstly, if the chemical to be mixed is slowly reacting, for 
example oxygen, whereby the chemical-pulp mixture is to be fed subsequent 
to the mixing into a reaction vessel, said mixture must be supplied from 
the mixer to the vessel so that the large volume of fed gas does not 
separate, as in the prior art apparatuses, from the fiber suspension 
immediately subsequent to the mixing zone. Said alternative is discussed 
partially when describing FIG. 1 and especially FIGS. 4-6. Secondly, if 
the chemical to be mixed is rapidly reacting, as ozone is supposed to be 
at least according to some sources, the actual bleaching reaction takes 
place in the mixer itself, whereby the residual chemicals and/or the 
carrier gas may be separated from the mixture utilizing the mixer. Said 
alternative is discussed in connection with FIGS. 7 and 8. 
In the embodiment of FIG. 1 a discharge channel 24 is conically widening, 
whereby as from an annular mixing zone/mixing channel 22 the flow channel 
steadily widens. The shape of the preferably vertical discharge channel 24 
resembles a Laval-nozzle, in other words the walls are slightly curved of 
their cross-section, whereby the flow remains attached to the walls and 
does thus not cause separation of gas from the suspension. It is also 
possible to arrange the walls of the discharge channel 24 straight, in 
other words conical, whereby the total value determining the angle of the 
extension should preferably be below 8.degree.. The purpose of the 
expansion of the flow channel when treating the mixture of slowly reacting 
chemicals and pulp is that the fluidized suspension having shear force 
fields forms fiber networks as quickly as possible, said fiber networks 
binding small gas bubbles therein before they have time to accumulate to 
larger bubbles or even to form a gas core in the center of the flow. 
It is substantial for the dampening of the fluidization and the generation 
of the fiber networks to slow down the circumferential speed of the 
circulating flow generated by the rotor 28 in the mixing zone as quickly 
and efficiently as possible. For this purpose the surface of the tapering 
tip portion 40 of the rotor is in one of the embodiments polished very 
smooth, so as not to be able to generate any turbulence close to it or not 
to maintain the circulating movement of the pulp. In order to be able to 
slow down the speed of the flow quickly, the surface of the discharge 
channel 24 in FIG. 1 is provided with low ribs 42 (axially shown in the 
drawing) for stopping the circulating flow. As for the optimal operation 
of the apparatus, it is very important that said low ribs 42 are 
accurately directed so that they steadily turn the flow of the pulp 
circulating more or less in the direction of the rim gradually to an axial 
flow without generating turbulence to the surface layer of the suspension. 
Another requirement for the ribs is that, unless their direction is 
optimal, they may not generate turbulence in the surface layer, in other 
words pressure changes, which bring about separation of gas in the areas 
of lower pressure. In such a case decelerating ribs would have to be used 
that are very low at the first end. The height of the ribs is tended to 
keep so low that the ribs only prevent the circulation of the fiber layer 
accumulated on the surface of the flow channel thus facilitating the 
formation of the fiber network. When the circulating movement is stopped 
the direction of the flow changes gradually to axial, the fibers stick to 
each other and the fiber network begins to form. Then the height of the 
decelerating ribs may be slightly increased in accordance with FIG. 1, 
whereby a larger and larger portion of the fiber flow joins the fiber 
network quickly forming a plug flow. Of course, in some cases also axial 
ribs 42 may be used, as in FIG. 1, whereby the height of the ribs has to 
be determined more accurately than with the spirally wound ribs. The 
purpose of said embodiment of the present invention, which relates to 
mixing of slowly reacting chemicals, is to form the plug flow so quickly 
that especially the gaseous chemicals do not have time to accumulate as 
large bubbles, but they remain uniformly divided in the fiber network. 
It must be noted that the mixing channel does not have to be defined by two 
cylindrical surfaces. It may, for example, be defined by two conical 
surfaces or one conical surface and one cylindrical surface. Also the 
direction of the taper of the cones may be either to the same direction or 
to opposite directions. An embodiment worth mentioning is a construction 
in which the sectional area of the surface of the mixing channel widens to 
the flow direction, whereby it is logical to arrange the outer wall of the 
channel to expand conically and to leave the rotor cylindrical from the 
mixing channel portion. It is, however, possible to arrange both members 
conical, and even at the same opening angle, whereby the increase of the 
cross-sectional area is caused by the mere increase of the radius of the 
channel. Of course, for example, the conicalness of the rotor, as well as 
that of the mixing channel itself, may be arranged to extend only to a 
portion of the length of the mixing channel. Also other than cylindrical 
and conical surface forms are possible, but their use is restricted by 
practical requirements in the manufacture. By arranging the mixing channel 
widening it is sometimes possible to leave the discharge channel 
cylindrical. This may also come into question, for example, in such cases 
when relatively small gas volumes are mixed into a dilute pulp at low 
rotational speed of the rotor, whereby the shear force field easily 
dampens merely by decreasing the cross-sectional area of the rotor in the 
discharge channel portion. 
FIGS. 4a and 4b illustrate another way, alternative to ribs 42 on the wall 
of the flow channel, to decelerate the circulating movement of the fiber 
suspension in the discharge channel 24. Blade-like members 52 are arranged 
to the joint flange of the discharge channel 24 and a pipe attached 
thereto to extend towards mixing channel. The purpose of said members 52 
is, of course, to decelerate the circulating movement of the pulp in the 
discharge channel so as to form a fiber network as quickly as possible. 
The members 52 must, however, be formed very carefully, so as to prevent 
the fiber suspension being discharged as a spiral flow from the mixing 
channel 22 from forming a vortex around the members, whereby a gas bubble 
would easily be formed as a result of a phenomenon corresponding to the 
cavitation/cavitating. FIG. 4b especially illustrates, how the blade-like 
members 52 in a preferred embodiment are located in the discharge channel 
24. The drawing also illustrates a preferred cross-sectional form of the 
blades, which preferably does not form any pressure field around it to 
facilitate the separation of gas from the pulp. The direction of said 
members may either be substantially parallel to the direction of the axial 
flow in the discharge channel, curved in such a way that they turn the 
flow axially or also possibly bent in such a way that they are positioned 
always perpendicular to the circulating flow. 
FIG. 4a yet discloses ribs 36 and ribs 38 of the rotor on the walls of the 
mixing channel. According to a preferred embodiment the ribs 38 of the 
rotor do not extend as far in the flow direction as the ribs 36. In other 
words, the effect of the rotor rotating the pulp may be terminated 
earlier, whereafter the circulating movement of the pulp in the mixing 
channel lasts some time to maintain the pulp in a fluidized state. At the 
same time the circulating movement slows down facilitating the 
deceleration of the circulating movement in the discharge channel. It has 
also been noted to be advantageous if the ends of the ribs are inclined, 
the height of the ribs decreasing to zero. The purpose thereof is to 
prevent the discharge of gas bubbles possibly accumulated behind the ribs, 
in other words to the so called lee side, full sized into the flow. By 
decreasing the height of the ribs the strongly turbulent pulp breaks the 
bubbles with the inclined edge of the rib, whereby the gas mixes better 
into the pulp. 
FIGS. 5a and 5b disclose a third alternative to increase friction surface 
in the area of the discharge channel 24. Members 54 and 56 formed of 
cylindrical or slightly conical surfaces are added in the embodiment of 
the drawing inside the discharge channel 24, the purpose of said members 
being only to increase the friction surface slowing down the circulating 
movement of the suspension, in other words generating a shear force field 
decelerating the circulating movement. Said surfaces may be arranged in 
the distance of 10 to 50 mm from each other, whereby their effect is most 
efficient as a decelerator of the circulating movement and a generator of 
a fiber network. In some cases it is possible to add in the middle of the 
members 54, 56 a stationary shaft to increase deceleration in the pulp 
circulating in the center of the discharge channel 24. The members 54, 56 
are attached preferably from their wider edge to the joint flange 50 of 
the discharge channel 24. It is, of course, possible to support said 
members from their opposite ends to the wall of the discharge channel in 
order to eliminate a possible vibration, as is done by bars 58 in FIG. 6a. 
FIGS. 5a and 5b disclose a way of arranging said members 54 and 56 inside 
the discharge channel 24. It is, however, possible to arrange all members 
to begin from the same axial level either from the end of the rotor 28, 
prior or subsequent thereto. It is also possible to increase the distance 
of the members from each other in the discharge end also so that a number 
of the members do not extend as far as the others, for example, so that 
only every other member extends to the joint flange 50. It is also 
possible that said members 54 and 56 are not exactly rotationally 
symmetric, but are formed of slightly waved material, whereby their 
surface forms a considerably higher friction than the smooth cylinder or 
cone. 
FIG. 6 discloses yet another efficient way of decelerating the circulating 
movement of the fiber suspension. The drawing relates to one of the 
embodiments, in which the flow direction is forced to change from parallel 
to the rim of the mixing channel to axial flow parallel to the pipe line 
subsequent to the mixer. Said change is brought about by arranging spiral 
strips or blades 70 at a slightly ascending angle of 4-10 grades from the 
first end of the discharge channel 24 or even prior to it to the wall of 
the discharge channel 24 (two strips placed opposite to each other shown). 
The rise of the spiral strips 70 is increased relatively quickly, whereby 
the movement parallel to the rim changes to axial. It is, of course, 
essential that the angle must be increased steadily and local lower 
pressure zones in the flow must be avoided. The number of the spiral 
strips also determines a successful deceleration of the flow. If the 
number of the spirals is very low, the rotational speed does not 
decelerate enough or the deceleration brings about local areas of lower 
pressure so that gas is allowed to separate from the flow. In the 
performed experiments it has been noted that the number of the spiral 
strips should vary between 3 to 10 according to the diameter of the 
discharge channel and also according to the rotational speed of the rotor 
and the volume of gas and consistency of the pulp. 
Yet another way of decelerating the rotional movement of the suspension 
flowing from the mixing channel is to arrange one or more plates with 
openings perpendicular to the shaft or at least in an angled position 
relative to the axial direction in the discharge channel, whereby the 
kinetic speed of the suspension parallel to the rim quickly slows down 
when flowing through the openings. 
It is a characterizing feature of another embodiment of the present 
invention primarily relating to the mixing of rapidly reacting chemicals, 
the reaction thereof in the mixer and the discharge of residual 
chemicals/gases and/or carrier gases from the mixer, illustrated in FIG. 
7, that the tip portion 40 of the rotor is provided with gas discharge 
openings 44, through which excessive gas in the fiber suspension may be 
drawn or guided away. In some cases it is also possible to perforate a 
portion of the rotor surface 28 in the mixing channel area to intensify 
the discharge of gas. Said portion locates at the discharge end of the 
mixing channel. The illustrated embodiment may be applied, for example, in 
ozone bleaching, in which a mixture of ozone and carrier gas is mixed into 
the pulp, whereby at least the carrier gas or a portion thereof may be 
separated already in the tip area of the rotor. Also the excessive volume 
of ozone may be removed by utilizing the embodiment of the drawing, if the 
reaction time of ozone is considered to be sufficient in the mixing zone. 
Gas in the illustrated embodiment is led beforehand from the interior of 
the rotor 28 in a manner known per se, for example, through the 
gas-separating centrifugal pumps along the shaft of the rotor either to 
further treatment, cleaning or to be utilized, for example, in some other 
bleaching stage. It is also a characterizing feature of the 
above-mentioned embodiment of said construction alternative using ozone 
that the flow speed of the fiber suspension in the mixing zone is 
appropriate to the ozone, which is the chemical to be supplied, to react 
properly with the fiber suspension in the mixing zone. When practically 
speaking all of the ozone has reacted, the gas that is left, being mainly 
carrier gas, may be removed through the tip portion of the rotor as 
efficiently as possible. In the arrangement in accordance with said 
embodiment, the smoothness of the surface of the rotor is not important. 
It is actually better for the gas separation that the surface of the rotor 
is at least to some extent uneven, rough or even provided with small 
blades, ribs or like. 
FIGS. 8a, 8b, 8c, and 8d disclose an apparatus in accordance with a 
preferred embodiment of the present invention more in detail. The drawings 
illustrate in a way the rotationally symmetric deceleration members 
already described in connection with FIGS. 5a and 5b, which are here 
referred to as 74 and 76. In said embodiment a pipe 78 with a relatively 
small diameter is mounted in the middle of said members, extending right 
from the tip portion of the rotor 28 to the joint flange 50 and further to 
the outside of the apparatus. Moreover, the tip portion 40 of the rotor 
does not have to be polished anymore, but rough to some extent, as was 
described in connection with the description relating to the previous 
drawing. The purpose of the rotor is first of all to generate with the tip 
portion 40 some turbulence around the rotor so that gas tends to separate 
as a thin layer therearound. Therefrom the gas flows further towards the 
tip of the rotor to the area of the smallest diameter, wherefrom it is 
discharged along pipe 78. The pipe 78 is, when required, connected to a 
vacuum source or to some other appropriate apparatus (not shown). The 
objective of this embodiment is to enable a rapid bleaching so that a 
gaseous chemical is led by the rotor 28 to a fluidized pulp layer, allowed 
to flow through said layer and to react with the lignin of the fibers and 
the residual gas is separated by the tip portion 40 of the rotor. At this 
stage and depending on the reaction speed of the chemical also the 
displacement bleaching described above may come into question. The 
apparatus operates, for example, in such a way that when gas accumulates 
forming a bubble around the tip portion of the rotor it rises as the 
lightest to the pipe and fills the pipe 78. By adjusting the volume of gas 
being discharged from the pipe the fibers may be prevented from flowing in 
to the pipe 78. A way (FIG. 8b) to complicate the flow of the fibers to 
the pipe 78 is to provide the tip portion of the rotor with an open 
portion or with an axial recess 41, which of course is filled with gas, 
because the relatively high axial flow speed of the fiber suspension 
carries the suspension past the recess and the fiber suspension is also 
affected by a considerably strong centrifugal force, which is due to the 
rotational speed. FIG. 8c yet illustrates a way of discharging gas from 
the top of the rotor. The top of the rotor 28 is provided with an opening 
for leading the gas into the rotor and out therefrom via a known route. 
FIG. 8d illustrates yet a way of generating a local shear force field to 
the top of the rotor intensifying the gas separation, by mounting small 
blades, ribs or like to said tip area. By combining said embodiment, for 
example, with the deceleration members shown in FIG. 8a an embodiment is 
obtained, in which a fiber network is allowed to be formed of the 
suspension in the outer layers of the discharge channel, but gas is 
intentionally separated from the inner flow layer. 
Besides the embodiments shown in the drawing, also other arrangements are 
possible in the separation of gas. It is possible to cut the rotor so that 
the tip thereof remains blunt, whereby gas is easily separated to the 
discharge side of the rotor. If required blades mounted as an extension to 
the rotor to intensify the circulating movement of the pulp may be used to 
intensify the gas separation. 
FIGS. 9a and 9b illustrate two preferred applications of an apparatus and a 
mixing method in accordance with the present application. In FIG. 9a two 
mixers 10' and 10" in accordance with the present invention are connected 
in series in such a way that the pulp to be treated is introduced via a 
flow channel 80 into the mixer 10", led therefrom via a flow channel 82 
into the mixer 10' and discharged therefrom to a pipe line 84. According 
to an embodiment at least the mixer 10' is preferably either similar to 
FIG. 7 or FIG. 8 (shown here). In other words a rapid bleaching is carried 
out in the mixer 10' as previously described in FIG. 7 and the separated 
residual gas is removed along a flow channel 86 from the mixer 10' and 
also from the so called second bleaching step. The gas is brought along 
the flow channel 86, for example, to a venturi-pipe-type mixer 88, in 
which the residual gas is drawn into and mixed with fresh bleaching 
chemical arriving along a flow channel 90 to the second mixer 10" and 
further introduced into the mixer 10". If so desired or required, the 
second mixer 10" may be a gas-separating mixer, whereby the residual gas 
is further to be led to one of the further treatment or further 
utilization stage. It is characteristic of the described process that the 
volume of gas to be introduced into the mixer 10' second in the flow 
direction of the fiber suspension is greater than the volume of gas to be 
supplied to the mixer 10" first in the flow direction. As can be seen, the 
process in question is a so called counter current bleaching, in which a 
mixture of "clean" bleaching chemical and a "used" chemical removed from 
the second bleaching step is supplied to the first bleaching step, in 
other words to the mixer 10", whereby, of course, the amount of active 
chemical in the first stage is smaller. " Clean and non-used", fresh 
chemical is again fed to the second step. 
FIG. 9b discloses a reverse bleaching process, in which fresh chemical is 
fed to the first mixer 10" and the residual gas, which is separated 
subsequent to the mixer 10" or in the mixer is mixed with fresh chemical 
and fed into the pulp in the mixer 10' of the second step. 
Of course, it is possible that more bleaching steps of said type are 
arranged subsequently, whereby the return of the residual gas may also be 
arranged either only to the first step or always to the step preceding the 
removal or possibly also to the last step or the step following the 
removal. The way how this is done, depends on the amount of chemical fed 
to each step, the chemical contents, the chemical being fed as "fresh", 
etc. 
FIG. 10 yet illustrates a process within the scope of the present 
invention. Number 90 refers to a conventional mass tower, which as such 
may be a bleaching or storage tower, from which the medium or high 
consistency pulp is preferably supplied by a fluidizing centrifugal pump 
92 to a second fluidizing centrifugal pump 94 provided with a gas 
discharge and to the suction side of which in the effective range of the 
fluidizer the bleaching chemical is fed from the pipe 96. Residual gas, 
consisting either completely of a bleaching chemical or, for example, when 
ozone is used, of both the bleaching chemical and a so called carrier gas, 
is separated in the pump 94. The separated gas is led via a pipe 98 into 
an inlet pipe 100, for example, as described in the previous embodiment. 
The chemical mixture from the pipe 100 is fed to a mixer 102, which may be 
a mixer in accordance with the present invention separating gas. The gas 
to be separated is led from the mixer 102 in a previously known manner to 
another mixer 104, from which the pulp to be treated is discharged to a 
vessel 106, in which the bleaching chemicals are allowed to completely 
react. It is worth noting that the above described embodiment is 
completely exemplary. The basic idea is that now for the first time in the 
history a centrifugal pump is suggested to be used not only to mix a 
chemical to a pulp, but also to act as a "bleaching vessel" so that the 
ability of the centrifugal pump to separate large volumes of gas from pulp 
is utilized to separate the residual gas from the pulp. At the same point, 
as an alternative, bleaching can be carried out in three subsequent 
mixers, as three step bleaching. Further, alternatively, pulp is fed to 
the fluidizing centrifugal pump 94 operating as a gas separator by another 
fluidizing centrifugal pump 92, whereby the pressure in the feed of the 
pump 94 facilitates the separation of the gas from the pulp. 
Moreover, a method worth mentioning is to bleach pulp more efficiently than 
before by ozone. As mentioned earlier, ozone may be used due to practical 
reasons only in relatively low consistencies (5-10%). It has been, 
however, noted that due to an efficient, rapid and uniform mixing it is 
possible to use also higher ozone contents in the mixer in accordance with 
the present invention. Such is achieved by adding a semi-permeable 
membrane into communication with said porous surface, by means of which 
the ozone may partially be separated from the carrier gas. Thus greater 
ozone contents are obtained to the mixing zone, which leads to a 
considerably more efficient bleaching result. 
Except for said ceramic, sintered or finely perforated gas feed rings it is 
possible to use in the gas feed quite a different technique. The gas inlet 
rings are provided with relatively large holes having a diameter of 1-3 
mm, of which gas is injected in thin sharp jets to the pulp circulating in 
the mixing zone. By using said technique it is possible to achieve a more 
efficient penetration of gas into the pulp and the flow-through of gas 
from the pulp layer is accelerated. 
As for the ozone bleaching, yet a method worth mentioning is to improve the 
total economy of the system. It is known that the ozone is generally 
manufactured in atmosperic conditions or slightly pressurized from oxygen, 
which is brought to the mill in a pressurized form. Immediately prior to 
the bleaching, in other words the feed to the mixer, the pressure of the 
ozone (and the carrier gas) is raised to 7-15 bar by an expensive 
compressor. It has been noted, however, that it is completely possible to 
manufacture ozone in pressurized conditions from pressurized oxygen so 
that the obtained ozone remains pressurized throughout from the 
manufacture to the mixing, whereby said compressor is not necessary. 
As seen above a new kind of a method and apparatus for mixing gas into a 
medium and a bleaching process applying the method are developed. Although 
several different apparatus variations are disclosed above, they are only 
meant to exemplify and clarify without any intention to restrict from what 
is given in the patent claims, which alone determine the scope of 
invention. Thus also many other alternatives are within the scope of 
invention. The above description also concentrates on the bleaching 
process and especially a bleaching process that uses ozone. However, the 
method and apparatus may as well be applied for mixing of gaseous 
chemicals or additives and also of liquid chemicals or additives. As for 
the detailed bleaching process described above also other bleaching 
processes may come into question. It is, for example, logical that the 
residual gas from one mixer may be led entirely to another bleaching step 
and not necessarily to the preceding or subsequent bleaching step as in 
the above described example. It is also possible that the oxygen to be 
separated as residual gas from the ozone bleaching is led to an oxygen 
bleaching stage or step and not mixed with a high ozone-containing gas.