NO974158L - Vanndispersjonsdyse - Google Patents

Description

The present invention relates to a method according to the preamble of claim 1, for producing microbubbles in a dispersion water nozzle which is used in connection with a high-pressure flotation method (DAF), in which method a water stream that enters the dispersion water nozzle and contains a gas dissolved in a state under pressure, such as for example air, C02, N2, O2 or the like, is treated, where the dispersion water nozzle is equipped with a throttle device having at least two sections. The invention also relates to an apparatus according to claim 9, for implementing the method. The throttle device is a whole consisting of the elements of the dispersion water nozzle that make up throttle devices and which actively participate in the operation of the throttles.

In high-pressure flotation, the liquid to be floated, e.g. lumpy waste water, supplied water under pressure, usually clarified waste water containing gas, e.g. air, which is dissolved in the water under pressure. The commonly used expression for dispersion water is water that is saturated with air under pressure. This high-pressure flotation method, in which dispersion water under pressure and the liquid to be floated, e.g. wastewater to be cleaned, is brought to react with each other, is usually called DAF (dissolved water flotation).

Dissolved air flow is known in itself, and is described for example in US patent 5 154 351. This patent publication also describes a construction of a dispersion water nozzle.

Dissolved air flotation is based on Henry's law, ie the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas. In dissolved air flotation, this is used by dissolving air in water at a pressure of about 2 to 8 bar, preferably about 3.5-6 bar. This water, saturated with air, is led to a flotation clarification tank through pressure reduction parts. When the pressure in the water flow is suddenly lowered, the air is released as small bubbles. The treated water contains solid or colloidal lumps. The lumps are caused to react with fine microbubbles present in the dispersion water, and the combination (a lump plus microbubbles) is caused to rise, in a controlled manner, to the water surface to be collected there

with appropriate means.

As mentioned in US 5 154 351, the air bubbles (microbubbles) used in dissolved air flotation must be of the correct size. If the bubbles are too large, their rate of rise becomes too high, causing turbulence to break down the clumps, and large bubbles will not stick to the clumps. On the other hand, if the bubbles are too small, their rate of rise will be too slow. In practice, microbubbles with a diameter of around 100 µm have proven to be suitable, e.g. for cleaning effluents.

In US 5 154 351, it is also established that with nozzles based on needle valves, and also with labyrinth valves, a good operational efficiency is achieved, i.e. the proportion of bubbles with the correct size is large. A problem involved with nozzles based on needle valves, and with labyrinth valves, is that they become blocked.

On the other hand, the operating efficiency of nozzles based on diaphragm valves and ball valves is poor. A large proportion of bubbles produced are either too small or too large to achieve a sufficient flotation effect. In order to achieve a certain flotation effect, a larger amount of dispersion water is required with these nozzles (diaphragm and ball valves) than with nozzles that have better operational efficiency and are based on, for example, needle valves and labyrinth nozzles.

US 5,154,351 proposes to solve the problems of needle valves and labyrinth nozzles by using a certain ball valve design, so that, for example, blocking problems typical of the above needle valves and labyrinth nozzles can be eliminated, while maintaining the bubble-forming properties of the nozzles at least same level as prior art nozzles based on needle valves or labyrinth nozzles.

The volume flow of dispersion water that passes through the above-mentioned types of nozzles, both through those according to the prior art and those described in the aforementioned patent publication, is of a very low level, perhaps in the order of a few dozen liters per nozzle for one minute.

However, the device described in US 5,154,351 provides an elegant solution to the blocking problems involved with prior art needle and labyrinth valves. It is also proposed that the device according to this invention maintains at least the bubble formation properties according to the previously known valves. In the device according to the aforementioned invention, the equivalence in quantity, ie the volume flow of dispersion water to be treated, corresponds to the capacity of known types of valves. According to US 5,154,351, the diameter of the circular opening used as a throttle device is generally in the order of 2.5 to 3.5 mm, depending on the pressure and overall dimensioning of the nozzle. The volume flow passing through prior art dispersion water nozzles, including that described in US 5,154,351, is in the order of 10-20 l/min. The dispersion water nozzles according to US 5,154,351 have two sections.

An aim of the present invention is to avoid disadvantages in connection with prior art and to produce an improved formation of microbubbles in a dispersion water nozzle. The device (method + apparatus) according to the invention facilitates the production of microbubbles of optimal size, in a controlled manner, in a dispersion water nozzle used in connection with dissolved air flotation, in quantities varying from a few to a dozen liters per minute to several litres, even over 1,000 l/min, for each dispersion water nozzle. The dispersion water nozzle refers to all equipment, in which the inflow of dispersion water is treated to produce microbubbles. In the following, the word nozzle will be used as a synonym for the word dispersion water nozzle, unless otherwise stated.

What is mainly characteristic of the method according to the invention is what is described in the characterizing part of claim 1. What is mainly characteristic of the apparatus according to the invention is what is described in the characteristic part of claim 9.

An aim of the invention, i.e. improved formation of microbubbles in a dispersion water nozzle, has been achieved by using the following basic idea for the invention. The method according to the invention, to produce microbubbles of optimal size, where their quantity per nozzle is several dozen times the amount in the prior art, uses the surprising discovery that the quality and quantity of the above-described microbubbles can be successfully combined in a single nozzle, when at least the first pressure reduction, i.e. throttling of the dispersion water flow, is carried out by using at least one swath-like, preferably curved throttle opening in the throttle. More preferably, the throttle opening or openings in at least the first throttle are also uniform or nearly uniform in width.

The shape of other throttles, their location within the dispersion water nozzle itself, and pressure reductions etc. effected by them are also significant aspects in producing improved formation of microbubbles in a single dispersion water nozzle.

The housing for the dispersion water nozzle means, in connection with this invention, the part of the dispersion water nozzle where the throttles that produce the pressure reduction of dispersion water are arranged. The through-flow opening in the housing applies to both an inlet for dispersion water that flows into the nozzle for treatment therein, and an outlet for dispersion water that has been treated and is to be emptied from the nozzle. The flow opening which is potentially arranged in the nozzle device is given a name according to the basic shape of the throttle, e.g. in this case, it is called a flow-through orifice of a sphere. This through-flow opening in the throttle serves as a cleaning opening, through which the maximum volume flow of dispersion water that passes through the nozzle flows. The function of this maximum volume flow is to remove blockages that may have accumulated in the throttle openings in the throttles and/or near them.

Use of the method according to the invention will thus effectively prevent blocking of the nozzle. Another aim of the invention is to produce an apparatus for implementing the above-mentioned method, in which apparatus the amount of dispersion water to be treated varies from a few dozen liters per minute to several hundred liters per minute, even over a thousand liters per minute for one nozzle. The nozzle according to the invention produces microbubbles of optimal size, in a controlled manner, in a wide stability range of volume flow, and as mentioned above, depending on the size of the nozzle, significantly larger quantities per nozzle than before. The volume flow per nozzle, and thus also the total number of microbubbles produced, is naturally determined by, in addition to the pressure, also the total dimensioning of the nozzle. An essential feature of the device according to the invention, for producing microbubbles in a dispersion water nozzle, is also that contaminants which accumulate in the nozzle and possibly cause blockage, can to a certain extent be removed during the process run, i.e. without interrupting the operation of the dispersion water nozzle. This is based on the above-mentioned wide stability range of the volume flow, which is a specific feature included in the device according to the invention. If, however, an increase in the volume flow within the stability range of the dispersion water nozzle is not sufficient to give the desired result, i.e. that the blockages do not disappear, the dispersion water nozzle can be turned, according to a device according to prior art, to a cleaning position, with which even large contaminants such as causing blockage can be effectively removed from the dispersion water nozzle. In this case, however, it is necessary to interrupt the operation of the nozzle, i.e. the production of microbubbles for the dispersion water to be treated.

Implementation of the method according to the invention is preferably carried out by using a dispersion water nozzle, where the throttle device comprises a piece which in itself is previously known, and which is rotatable in relation to at least one axis and mounted rotatably around the axis of rotation, in a housing which is equipped with a flow opening (flow openings for inlet and outlet flows) at right angles to the axis of rotation, and at least two throttle devices, of which at least the first is slot-like and of which only one is a flow opening for the throttle. The position of the flow opening and the throttles determine the function of the dispersion water nozzle. If the flow opening in the throttle is completely or essentially open, i.e. you are not within the stability range for forming microbubbles in the nozzle, the flow opening serves as a cleaning opening. On the other hand, if it is open to a certain extent, i.e. one is within the stability range of the volume flow in the nozzle, the open part forms part of the first throttle and the last throttle. Constructions of the throttles shall be described in more detail below, through examples, in connection with the explanation of the accompanying drawings. The throttle device in the nozzle device according to the invention thus comprises at least two throttles. The apparatus for implementing the method according to the invention does not have to be designed for use in connection with only one ball valve, for example a cylindrical basic solution is also conceivable.

Especially when the ball valve chamber between the first and the last (n) throttle is enlarged, it has been found advantageous, with regard to optimal formation of microbubbles, to add one or more throttles to this space, preferably in the form of a so-called impact plate. These impact plates are arranged, with regard to the formation of microbubbles, at an advantageous angle in relation to the dispersion water flow which is directed towards the impact plate. The abutment plate can also be an abutment surface, i.e. the interior of the ball valve can be so shaped, for example by using an additional piece or machining, that an abutment surface is formed therein, which abutment surface is at an advantageous angle with respect to the flow direction of the dispersion water. When a stop plate/stop surface is placed inside the ball valve, i.e. n = 3, it is preferably at an angle of about 90° or close to 90° in relation to the dispersion water flow that hits it. In this case, the bubbles produced in the first section are broken down into microbubbles of optimal size.

Dispersion water nozzles which handle a larger volume flow thus preferably have two or three sections, and they are placed in connection with a ball valve which is known in itself. Especially in the first pressure reduction, an edge of the throttle opening is of great importance for the quality of the microbubbles that are formed. A throttle opening which is curved and uniform in width has been found to be particularly advantageous in the first pressure reduction. The number n of throttles can be greater than 2 or 3.

Tests have also shown that the shape of the flow opening in the throttle, which causes the pressure reduction, is of great importance to the size and size distribution of the bubbles that are formed, as well as the amount of microbubbles accepted. A track-like shape has been shown to be beneficial for both the quality of the size distribution of the microbubbles. A curved track has proven to be particularly advantageous. This is probably due to the fact that when nucleating acceptable microbubbles, it is particularly important how close to a fixed edge, i.e. an edge of the track, the nuclei are. When it is desired to process a dispersion water stream flowing in a channel with a round cross-section in a pressure reduction device in connection with a ball valve, i.e. to throttle, a curved track shape is the natural alternative when the edge line for the throttle opening will be as long as possible combined with a track of a certain width. As mentioned above, it has been found that a curved throttle opening of uniform width is particularly advantageous, in connection with a first pressure reduction. However, the exact cause and effect of the above-described behavior in connection with the formation of microbubbles has not yet been found on the basis of tests carried out in connection with this invention.

The method and apparatus according to the invention are further described below, through examples, and with reference to the drawings, which illustrate some devices according to the invention, but it is not intended, however, to limit the invention to these devices. The devices described here are advantageous for treating, for example, water from various processes in the pulp and paper industry, such as, for example, raw water, various process waters and waste water. In these cases, the water to be treated contains either solid or colloidal lumps. The lumps are made to react, in a controlled manner, with fine microbubbles formed in the dispersion water nozzle, and these combinations (a lump + microbubbles) react with each other and are forced to rise in a controlled manner and undisturbed, to the surface of the flotation vessel, from where they then is collected by suitable means. The reaction between the lumps and microbubbles here means a physical impact, in which the lumps and microbubbles stick to each other.

In the drawings, figure la illustrates a throttling device in a dispersion water nozzle according to the present invention, seen from the upstream direction, figure 1b illustrates a throttle device in the same dispersion water nozzle, seen from the downstream direction, figure 2a is a side view in section of a dispersion water nozzle in figure 1 in its operative position , figure 2b is a side view in section of a dispersion water nozzle in figure 1 in its cleaning position, figure 3a illustrates a throttle device of another dispersion water nozzle according to the present invention, seen from the upstream direction, and figure 3b illustrates the same throttle device of a dispersion water nozzle, seen from the downstream direction.

Figure 1 illustrates a throttle device according to the invention, an operative position. Figure la illustrates a throttle device of a dispersion water nozzle with two or more sections according to the invention, seen from the upstream direction, and figure lb illustrates a corresponding device seen from the downstream direction. The device described here is constructed in connection with a ball valve. According to its basic principles, the microbubbles are formed in dispersion water flowing through the throttles 1 and n of the throttle device by using shaped pieces 5, 6 which are adapted in connection with both edges 7, 9 of the through-flow opening in a ball valve 4, which in itself itself is known, and the housing 12 of the ball valve, so that the ball valve is rotated around its axis of rotation to a position in which the pieces 5, 6 which are mounted in connection with the housing and the edges 7, 9 of the through-flow opening 22 which is opened, together form groove-like, curved throttle openings 8, 10, each of which has an even and suitable width.

An inlet side end plate 1 of the throttle device, shown in figure la, is fixed on the outlet side end plate 11, shown in figure lb, with a bolted connection 2. The ball 4 can be rotated to different operative positions by means of a double-acting adjusting arm 3. The operating principle of the adjusting arm 3 is explained in more detail below, in connection with the explanation of Figure 2. In Figure 1a, the shaped piece 5 and the edge 7 of the ball form a groove-like, curved throttle opening 8 of uniform width, on the inlet side. Correspondingly, in figure 1b, the shaped piece 6 and the edge 9 of the ball form a groove-like, curved throttle opening 10 of a uniform width, on the outlet side. The reference number 14 denotes the through-flow opening in the ball valve on the inlet side, and the reference number 15 denotes the through-flow opening in the ball valve on the outlet side. In this application, the throttle openings in the throttles 1 and n are located near the centers of the flow openings 14, 15. This location of the throttle openings, and especially the location of the throttle n, keeps the flow of microbubbles uniform in the cross-section of the dispersion water channel on the outlet side. In this way, the following risk is effectively avoided. If the microbubble stream travels long distances near the channel wall, there is a risk that the wall acts, to a detrimental extent, as a coalescence/nucleation center for large bubbles, so that the proportion of acceptable microbubbles in the stream of dispersion water is reduced.

Figure 2a illustrates a cross-section of the throttle device in Figure 1, when n = 3, and the throttle device is in its operative position. Figure 2b shows a cross section of the throttle device when it is in its cleaning position. In Figure 2, the direction of inflowing dispersion water is shown by an arrow V. In Figure 2a, the housing for the throttle device is designated by the reference number 12. Figure 2a shows both the operative position 16 of the stop plate as well as its outermost adjusted position 17, while the position until ball 4 is unchanged. When the stop plate 6 is in its operative position, it serves as a throttle between sections 1 and n. Microbubbles are thus formed in the dispersion water flowing through the throttles 2 and so on to (n-1) (in this application (n-1) = 2, and there is only a stop plate) in the throttle device in the dispersion water nozzle. The bubbles are formed using the stop plate 16 located inside the ball valve, which is recognized in itself, and which forms part of the throttle device, by rotating the stop plate in relation to the axis of rotation of the ball valve in such a way that the stop plate 16 will be, in terms of formation of microbubbles, at an appropriate angle to the flow of dispersion water impinging on it. In this case, the impact plate is preferably perpendicular or nearly perpendicular to the inflow of dispersion water. The direction of the stop plate 16 is, however, adjustable between the extreme positions as shown in Figure 2a, without the need to change the position of the ball 4. In this device, the stop plate 16 serves as another throttle for the throttle device, and (n-1) = 2. A edge of the throttle opening is formed at an inner surface 19, 19' of the ball 4, and the other edge at the end 18, 18' of the stop plate 16. The end 18, 18' of the stop plate is preferably of a curved shape, corresponding to the shape of the inner surface 19, 19' in the ball 4, so that the throttle opening 20, 20' is correspondingly in the form of a curved groove. If, for example, the shape of the stop plate 16 is a rectangle, the shape of the throttle opening 20, 20' is a corresponding segment. The upper part of the segment is thus formed by the inner surface 19, 19' of the ball 4.

Figure 2b illustrates the stop plate 16 in the cleaning position. In the cleaning position, the stop plate 16 is preferably parallel to the axis 21 of the through-flow opening 22 in the ball. When the throttle device, which is a ball valve in this device, is in the cleaning position, as much dispersion water as possible will flow as freely as possible through the ball 4. In this way, particles or colloids which close the dispersion water flow and which may have collected in the inlet side flow opening 14 become in the ball valve and/or the outlet side flow opening 15 in or near the ball valve, or in a throttle, effectively flushed away. At the same time, particles or colloids that may have accumulated in the interior of the ball valve are also effectively flushed away. Figure 2 also provides a schematic illustration of the operating principle of the double-acting adjustment arm, which belongs to the arrangement of the dispersion water nozzle. When the ball valve has been rotated to a certain position, it is possible to move the stop plate 16 back, by about 45° in the opposite direction to the direction of rotation, without changing the position of the ball 4 in the ball valve. This tolerance is necessary in the application according to Figures 1 and 2 because the operation of the dispersion water nozzle, in its various operative positions, is thereby made as efficient as possible. When the dispersion water nozzle is in the operative position, e.g. according to figure 2a, the impact plate is preferably at an angle of about 90° in relation to the inlet flow of the dispersion water. When the dispersion water nozzle is in the cleaning position according to Figure 2b, the stop plate 16 is preferably parallel to the axis 21 of the through-flow opening 22 in the ball. When, for example, the ball 4 in the ball valve which forms part of the throttle device for the dispersion water nozzle in figure 2a is rotated towards the closed position, up to its operative position, the stop plate 16 has simultaneously been rotated to its operative position, i.e. to a position which is in a angle of about 90° in relation to the inlet flow of dispersion water, which is denoted by the arrow V. When the throttle device is to be changed to the cleaning position, the ball 4 is rotated in the opposite direction compared to before. Thus, when the double-acting adjustment arm 13 is turned, the stop plate 16 is first turned to a position 17, and only then will the ball 4 itself begin to rotate together with the stop plate. This rotation of the ball 4 and the stop plate 16 continues until the cleaning position shown in Figure 2b is reached. Then, a volume flow of dispersion water that is as large as possible will pass through the dispersion water nozzle as freely as possible. And now both the flow opening 22 in the ball 4 and the stop plate 16 are parallel to the dispersion water flow V. Figure 3a illustrates the principles of another throttle device according to the invention, which throttle device has two or more sections. Similar to figure 1, these figures illustrate the throttles 1 and n of a throttle device in an operative position.

Figure 3a illustrates a throttle device for a dispersion water nozzle, seen from the upstream direction, and Figure 3b a corresponding device seen from the downstream direction. In this application, microbubbles are formed in dispersion water flowing through the throttles 1 and n of the throttle device for the dispersion water nozzle, by rotating the ball 4 in a ball valve, which is known per se, around the axis of rotation, so that the edges 7, 9 of the flow-through opening 22 in the ball of the ball valve, which edges are shaped, for example by machining and which edges themselves form part of the first and second nozzle (n), and produce throttle openings which are, with regard to the formation of microbubbles, suitable in width, track- similar, preferably curved, each of equal width. In Figure 3a, the inlet-side flow opening 14 in the ball valve and the edge 7 of the ball 4 together form an inlet-side throttle opening 8, which is groove-like, curved and uniform in width. Corresponding to Figure 3b, the outlet-side flow opening 15 in the ball valve and the edge 9 of the ball 4 form an outlet-side throttle opening 10, which is uniform in width, track-like, and curved.

If the number of throttles n = 3 in the dispersion water nozzle and the nozzle is arranged in connection with a ball valve, the movement of the second throttle, which in this case is the stop plate 16 inside the ball valve, can also be adjusted so that its position can be kept unchanged or close to unchanged even through the position of the ball valve, i.e. the position of the throttles 1 and 3 is changed within the limits of the volume flow as defined by the stability area of the dispersion water nozzle. In this way, it is ensured that cleaning of the ball valve can be carried out by increasing, to a certain extent, the volume of current passing through the ball valve, without any need for a total interruption of the production of microbubbles, i.e. without the need to rotate the ball valve to its proper cleaning position. During the time the ball valve must be in the cleaning position, there is of course no production of microbubbles in the nozzle.

In the application according to figure 3, it is also possible to use a stationary stop plate. In the device shown in Figure 3, the ball valve, when in its operative position, is completely or almost completely in the same position as an unmachined ball valve would be in its closed position. The operative position in this application corresponds to the position in which the through-flow opening 22 of the ball 4 is perpendicular or nearly perpendicular to the axis 21 of the inlet and outlet through-flow opening 14, 15. In the operative position, the stationary stop plate is perpendicular or nearly perpendicular with the inflow of dispersion water, and in the cleaning position, correspondingly parallel or almost parallel to the axis 21 for the inlet and outlet openings 14, 15. The explanation of Figure 3 also describes the operation of the throttles 1 and n. In Figure 3a, the inlet side forms flow opening 14 on the ball valve and the edge 7 of the ball 4 together with an inlet-side throttle opening 8, which is groove-like, curved and uniform in width. Correspondingly, in Figure 3b, the outlet side flow opening 15 of the ball valve and the edge 9 of the ball 4 together form a throttle opening 10 which is groove-like, curved and uniform in width, and which is located on the downstream side.

Tests have shown that the shape of the first section of the throttle device on the nozzle, as well as the pressure reduction that takes place in the first section, is of high importance. As long as the first section of the throttle device is groove-like and preferably curved in shape, and most preferably even in width, wide variations are allowed for the shape of the throttle openings in the other sections, while maintaining the overall production of microbubbles in the dispersion water nozzle on an acceptable level. In the best dispersion water nozzles, however, groove-like, preferably curved throttle openings are used in each section. For example, if the stop plate 16 is rectangular in shape, the throttle openings 20, 20' correspond to a segment-shaped groove. The upper section of the segment is in this case formed at the inner 19, 19' of the ball 4.

Essential for correct operation of the throttle device according to the invention is also the relationship between pressure reduction ions that occur at different throttle sections. For example, a throttle device with three sections arranged in connection with a DN 50 type ball valve has given good results with the following total pressure reductions in the throttle sections. The first section about 70%, the second section about 20%, and the third about 10%. The pressure reductions should also take place as quickly as possible to prevent the harmful association of microbubbles with each other.

With larger dimensions of throttle devices, compromises are inevitable, at least on some points. If the total volume flow per nozzle, however, is of the order of at least 1000 l/min, a small amount of poor quality bubbles may be allowed due to the total amount of acceptable microbubbles per nozzle is very large compared to nozzles according to prior art.

Research has produced good results, for example in the following conditions: - "optimal volume flow" for the formation of acceptable microbubbles around 300 l/min. - ball valve type DN 50 - dispersion water pressure around 5.5 bar

- parts of the first, second and third sections

of total pressure reduction (%) first section around 70%

second section around 20% third section around 10%

- shape and width of the throttle opening 1,

curved groove of uniform width, width about 5-5.5 mm

- shape and width of throttle opening 2,

segment roof about 7 mm

- the shape and width of the throttle opening 3

curved groove of equal width, width about 15 mm

It is also a characteristic feature of the device according to the invention that it is not particularly exposed to changes in the volume flow, on the contrary. For example, a nozzle arranged in a DN 50 type ball valve functions well within a very large range of volume flow of about 175 to 325 l/min, where the optimum volume flow is about 300 l/min. This property has a very beneficial effect on the management of the flotation process itself.

When nozzles according to prior art, e.g. those used in connection with the flotation process associated with, for example, the treatment of waste water, become dirty for one reason or another, and thus tend to become blocked, this blocking of the nozzles will cause the following chain reaction. A dirty nozzle is unable to produce as many acceptable microbubbles as before. The smaller the number of acceptable microbubbles in the flotation process, the poorer the quality of the purified liquid, i.e. the raw material for the dispersion water. The poorer the quality, i.e. the dirtier the dispersion water, the easier the nozzles are blocked and so on, until the whole process has to be interrupted and the nozzles cleaned.

When using the device according to the invention, the course of action in a similar dilemma would be as follows. If dirt accumulates in the nozzle and its capacity decreases from its optimal level, i.e. the number of acceptable microbubbles is less than it should be, the operation of the nozzle, i.e. the forming process of microbubbles, can be affected even during the execution of the process, in the device according to the invention. The nozzle can be cleaned so that the volume flow passing through it is increased from its optimum level, by opening the throttle opening within the adjustment range defined by the dimensions of each nozzle. Use of the dispersion water nozzle continues at the maximum volume flow that produces acceptable microbubbles until the nozzle is clean. When the operations take place within the adjustment limits of each nozzle size, the above cleaning work can be carried out without disturbing the flotation process, i.e. without interrupting it. After the cleaning work, it is then possible to return to the dimension of the throttle opening, i.e. the amount of volume flow that results in an optimal production of acceptable microbubbles.

If the nozzle is strongly blocked, it is turned over to the cleaning position according to Figure 2b for a short time, so that the greatest possible volume flow passes through it, which ensures the removal of blockages in the latter. If blockages occur in the device according to the invention, they will most likely occur in the first throttle, where the groove is smallest. Since, e.g. in the device described above, the very edges of the flow opening in the ball valve either in whole or in part form the first throttle, it is easy to clean this throttle of impurities that cause blockage, using one or the other of the cleaning methods described above.

When the device according to the invention is used in conjunction with smaller ball valves, e.g. DN 15 or DN 32, it has been noted that the use of the basic principle, i.e. the slot-like opening of the first throttle, is a sufficient property to produce acceptable microbubbles in the dispersion water nozzle constructed in connection with the above identified ball valves. For example, the nozzle of type DN 15 functions well within the volume flow range of 0-50 l/min, when the inlet side throttle, i.e. the first throttle in a two-section nozzle, is shaped in such a way that a shaped piece is placed on the edge of the flow opening, so that the first throttle is in accordance with that shown in figure la. The second throttle is then designed, not by another shaped piece together with the edge of the flow opening in the ball, but only a flow opening of the ball, where the said flow opening is turned to an inclined position. Thus, the flow opening which is slightly open will form another, i.e. the last throttle. In this arrangement, the dispersion water flowing through the first throttle will hit the wall of the flow opening, where the flow opening is in an inclined position, and is then emitted from the dispersion water nozzle through the last throttle identified above. Also in this arrangement, the general principle applies that the opening in the first throttle is smaller than in the second or subsequent throttles, in other words the dimensional ratios of openings in the throttles are 1 < 2 < (n-1) < n. This same ratio is on the other hand, indicated by the proportion of pressure reductions occurring in the various throttles of the total pressure reduction for the entire nozzle. Good results in total pressure reduction (%) in the DN 15 type nozzle have been received in the two sections, for example as follows: the first section around 75%, the second section around 25%.

In a nozzle of the DN 32 type, the device according to figures 1 and 2 can be used successfully, but without a separate stop plate, i.e. only as a nozzle with two sections. The nozzle of type DN 32 works well with a volume flow in the range from around 60 to around 150 l/min. Good results in total pressure reductions with nozzle type DN 32 have been received in two sections, e.g. as follows: values in the first section around 75%, in the second section around 25%, i.e. the same as with a nozzle of type DN 15.

In small nozzles, e.g. such as DN 15 or DN 32, the forming range of acceptable microbubbles is not as sensitive to changes as with nozzles with larger volume flows. Therefore, a specific optimal area size has not been established with these small nozzles, in contrast to larger nozzles.

When the size of the dispersion water nozzle is reduced, acceptable microbubbles can be produced by using the basic principle of the present invention alone, i.e. a track-like first throttle in connection with a dispersion water nozzle having at least two sections. Therefore, when using these small nozzles, it is possible to simplify the structure of the dispersion water nozzle without affecting the quantity and quality of the acceptable microbubbles.

The device according to the invention offers a completely new way of constructing an equipment for forming microbubbles in connection with flotation processes. As mentioned above, by using the device according to the invention, it is possible to achieve a production of several hundred litres, even over a thousand litres, per dispersion water nozzle. Despite the large volume flow, the quality of the microbubbles that are formed is acceptable. For the designer, this naturally means that the design and construction of flotation equipment for a certain capacity becomes much easier. With one nozzle, it is possible to replace even several dozen nozzles based on the conventional technique, and to save long distances in the pipe constructions.

Control of the flotation process is easier than before due to the fact that the propensity for blocking of a single nozzle according to the invention is in itself much lower than with conventional nozzles. If the nozzle has to be cleaned, this can be done very quickly, so that the production of microbubbles in the nozzle, alongside the entire process, does not need to be interrupted. The invention also offers excellent potential for further development of the entire flotation process. For example, in processes with nozzles of larger volume flow of about 100-1000 l/min or more, the flotation process can be controlled with regard to dispersion water nozzles, so that the total pressure reduction that occurs in a single dispersion water nozzle is measured with a sensor element. If deviations from the limit values are detected, when the deviations are large enough, or the nozzle or its surroundings are blocked, the control system starts to open these throttles in the opening direction from their optimal operating positions. If necessary, they are opened to their upper limit of the nozzle stability range, until the blockage is discharged from the nozzle. Then, the control system returns the throttles back to their optimal operating position. If the blockage is too large to clear while driving, the control system will move the dispersion water nozzle to its actual cleaning position so that it can be cleaned finally. After this, the throttles are again moved to their optimal operating position.

The above identified invention is described with reference to a few preferred embodiments used in the cleaning of waste water, particularly in the pulp and paper industry. The invention is thus described on the basis of some preferred embodiments, without any intention to limit the invention. There are several other fields in which the invention can be used, e.g. cleaning of raw water and concentration of ore, etc., and, as will be apparent to one skilled in the art, many alternatives and optional modifications of the construction are possible within the scope of the invention as defined by the claims.

Claims (17)

1. Method for producing microbubbles in a dispersion water nozzle used in dissolved air flotation (DAF), in which method a water stream entering the dispersion water nozzle and containing a gas dissolved under pressure, such as air, C02 , N2 , O2 or the like, is processed, where the dispersion water nozzle is equipped with a throttling device having at least two sections, CHARACTERIZED IN THAT microbubbles are produced in the dispersion water that flows through at least the first throttle of the throttling device in the dispersion water nozzle, in which said microbubbles are produced by means of at least one trace- similar throttle opening. 2. Method according to claim 1, CHARACTERIZED IN THAT it produces microbubbles in the dispersion water that flows through at least the first throttle of the throttle device in a dispersion water nozzle, where the microbubbles are produced by at least one curved, track-like throttle opening (8). 3. Method according to preceding claim, CHARACTERIZED IN THAT microbubbles are produced in the dispersion water which flows through at least the first throttle of the throttle device in the dispersion water nozzle, where the microbubbles are produced by at least one track-like throttle opening which is uniform or nearly uniform in width. 4. Method according to preceding claim, CHARACTERIZED BY the fact that the microbubbles are produced in a throttling device which has a section, in which n is at least three, and the throttling device is arranged in connection with a ball valve which is known in itself. 5. Method according to claim 4, CHARACTERIZED IN THAT microbubbles are produced in the dispersion water that flows through the nozzles (1) and (n) of the throttle device in the dispersion water nozzle, where microbubbles are produced by means of shaped pieces (5, 6) arranged in connection with the edges (7, 9) of the through-flow opening (22) in the ball valve, which is known in itself, and which forms part of the throttle device, and in connection with the housing (12) of the ball valve, so that the said ball valve is rotated around its axis of rotation to a position in which both the shaped pieces arranged in connection with the housing and the flow opening, which move to an open position, together form throttle openings (8, 10) which are of suitable width, groove-like, and preferably curved in shape. 6. Method according to claim 4, CHARACTERIZED IN THAT microbubbles are produced in the dispersion water that flows through the throttles (1 and n) in the throttle device of the dispersion water nozzle by rotating the ball valve, which is known in itself, around its axis of rotation, so that the inlet side flow opening (14 ) in the ball valve and the machined edge (7) of the ball (4) together form the inlet side first throttle opening (8), which is groove-like and preferably curved, and correspondingly form the outlet side flow opening (15) and the machined edge (9) ) of the ball (4) together with the outlet side throttle opening (10) of the last (n) throttle, which throttle opening is groove-like and preferably curved. 7. Method according to claim 5 or 6, CHARACTERIZED IN THAT microbubbles are produced in dispersion water that flows through the throttles (2 - (n-1), (n > 2)), by the throttle device in the dispersion water nozzle, by a strike plate or strike surface arranged inside in the ball valve, which is known per se and which forms part of the throttle device, by arranging the said stop plate or stop surface at a suitable angle, with regard to the formation of microbubbles, preferably around 90° or close to 90°, in relation to the dispersion water flow that hits it. 8. Method according to claim 4, 5, 6 or 7, CHARACTERIZED BY adjusting the volume flow of dispersion water flowing through the throttles (1 and n) in the throttle device for the dispersion water nozzle within the stability range of said volume flow, by rotating the ball valve, which forms part of the throttle device, and with regard to the formation of microbubbles, is in its optimal position, towards its open position, so that contaminants that become loose, e.g. from the first throttle (1) which has a smaller opening or from other throttles, is able to pass through the dispersion water nozzle without any problem, after which the ball valve is rotated back to its optimal position. 9. Dispersion water nozzle for use in connection with dissolved air flotation (DAF), for implementing the method according to any one of the preceding claims, which dispersion water nozzle is equipped with a throttling device for producing microbubbles in the water stream entering the nozzle and containing a gas, which has been dissolved in a state under pressure, for example air, CHARACTERIZED IN THAT the throttle device comprises a piece (4) which is known per se and which is rotatable around at least one axis, where the piece is rotatably mounted around its axis of rotation and placed in a housing (12) equipped with a through-flow opening at right angles to the axis of rotation, at least two throttles, of which at least the first is slot-like (8) and only one through-flow opening (22) in the above-mentioned piece, where the piece is rotatable around at least one axis, where the aforementioned flow-through opening is placed in the piece so that dispersion water entering the throttle device can be passed through the inlet side through-flow opening (14) in the housing (12) to the through-flow opening (22) in this piece (4), and further through the outlet-side through-flow opening (15) at the other end of the housing, away from the throttle device. 10. Dispersion water nozzle according to claim 9, CHARACTERIZED IN THAT the throttle device is arranged in connection with a ball valve which is known in itself, and in that the shaped piece (5) is arranged in connection with the inlet-side flow opening (14) in the ball valve's housing (12) and the edge (7) of the through-flow opening (22) of the ball valve together forms a first, groove-like, preferably curved, throttle opening (8) on the inlet side, and correspondingly the shaped piece (6) arranged in connection with the outlet-side flow opening (15) of the ball valve housing (12) and the edge (9) of the flow opening (22) in the ball valve together form a groove-like, preferably curved throttle opening (10) of the last throttle (n) on the outlet side. 11. Dispersion water nozzle according to claim 9, CHARACTERIZED IN THAT the throttle device is arranged in connection with a ball valve which is known in itself, and in that the inlet-side through-flow opening (14) in the ball valve and the machined edge (7) of the ball (4) together forms a first, groove-like, preferably curved throttle opening (8) on the inlet side, and correspondingly the outlet side flow opening (15) in the ball valve and the machined edge (9) of the ball (4) together form a preferably curved throttle opening (10) of the last throttle (n) on the discharge side. 12. Dispersion water nozzle according to claim 9, CHARACTERIZED IN THAT it has n sections, where n is at least 3. 13. Dispersion water nozzle according to claim 12, CHARACTERIZED IN THAT it is arranged in connection with a ball valve which is known in itself, in that the edge of the flow opening (22) in the ball valve itself forms, either completely or partially, the first and the last (n) throttle and that, in connection with the through-flow opening (12) in the ball valve, there are parts, such as stop plate/stop plates (16) or stop surfaces, which cause throttling in sections 2 - (n - 1). 14. Dispersion water nozzle according to claim 13, CHARACTERIZED IN THAT the throttle device comprises shaped pieces (5, 6) arranged in the housing (12) so that it is possible to form microbubbles in the dispersion water that flows through the throttles (1) and (n) of the throttle device in the dispersion water nozzle , when the ball valve, which is known per se and which is part of the throttle device, is rotated about its axis of rotation to a position in which the shaped pieces (5, 6) are arranged in connection with the housing and the edges (7, 9) of the flow opening in the ball valve , which flow opening moves towards its open position, together forming throttle openings (8, 10) which are of a suitable width, groove-like and preferably curved. 15. Dispersion water nozzle according to claim 13, CHARACTERIZED IN THAT the edges (7, 9) of the flow opening (22) in the ball valve, which ball valve is part of the throttle device, are so shaped, for example machined, that they themselves form part of the first throttle and the last (n) throttle, so that it is possible to form microbubbles in the dispersion water stream flowing through the throttles (1) and (n) in the throttle device of the dispersion water nozzle, by using the shaped edges (7, 9) of the through-flow opening ( 22) in the ball valve, which is known per se, where the ball valve is part of the throttle device, when said ball valve is rotated around its axis of rotation to a position in which the shaped edges of the flow opening form throttle openings (8, 10), which with regard to formation of microbubbles, is of suitable width, groove-like and preferably curved. 16. Dispersion water nozzle according to one or more of claims 9-15, CHARACTERIZED IN THAT the throttle opening in at least the first throttle is track-like, and preferably curved in shape. 17. Dispersion water nozzle according to claim 16, CHARACTERIZED IN THAT the throttle opening in at least the first throttle of the throttle device is also uniform or nearly uniform in width.