The present invention relates to a method in accordance with the preamble of claim 1, for producing microbubbles in a dispersion water nozzle used in connection with a high-pressure flotation method (DAF) , in which method a water flow entering the dispersion water nozzle and con¬ taining gas which has dissolved at a pressurized state, such as e.g., air, C0 ₂ , N ₂ , 0 ₂ or the like, is treated, the dispersion water nozzle being provided with a throttle means which has at least two sections. The in¬ vention also relates to an apparatus in accordance with claim 9, for implementing said method. The throttle means is a whole composed of those elements of the dispersion water nozzle which constitute throttles or which are actively participating in the operation of the throttles.

In high pressure flotation, the liquid to be flotated, e.g., flocculated waste water, is supplied with pressur- ized water, most usually clarified waste water containing gas, e.g., air, which has dissolved in the water at a pressurized state. The generally used term for dispersion water is water saturated with air under pressure. This high pressure flotation method, in which pressurized dispersion water and the liquid to be flotated, e.g., waste water to be cleaned, are caused to react with each other, is generally called DAF (dissolved air flotation) .

The dissolved air flotation is in itself known, and it is disclosed, e.g., in US patent 5,154,351. This patent publication also discloses a dispersion water nozzle construction.

Dissolved air flotation is based on Henry's law, i.e., the solubility of a gas in a liquid is directly propor¬ tional to the partial pressure of the gas. In dissolved air flotation, this is utilized by dissolving air into

water at a pressure of about 2 to 8, preferably about 3.5 to 6 bar. This water, saturated with air, is passed to a flotation clarification basin through pressure reduction members. When the pressure in the water flow is suddenly lowered, air is released as small bubbles. The water which is treated contains solid or colloidal floes. The floes are caused to react with fine microbubbles contain¬ ed by the dispersion water, and the combinations (a floe + microbubbles) are caused to rise, in a controlled man- ner, to the surface of the water to be collected there¬ from with suitable means.

As stated in patent US 5,154,351, the air bubbles (micro¬ bubbles) utilized in dissolved air floatation have to be of a correct size. If the bubbles are excessively large, their rising speed is too high, whereby turbulence is brought about which disintegrates floes, and large bubbles do not adhere to floes. On the other hand, if the bubbles are too small, their rising speed is too slow. In practice, microbubbles having a diameter of about 100 μm have proved to be suitable, e.g., in cleaning of efflu¬ ents.

In patent US 5,154,351, it is also established that, with nozzles based on needle valves, and also with labyrinth valves a good operating efficiency is achievable, i.e., the proportion of bubbles of 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 the bubbles produced is either too small or too large in order to obtain an adequate flota- tion effect. In order to obtain a certain flotation effect, a larger amount of dispersion water is needed with these nozzles (diaphragm and ball valves) than with

nozzles having a better operating efficiency and being based on, e.g., needle valves or labyrinth nozzles.

US patent 5,154,351 suggests to solve the problems with needle valves and labyrinth nozzles by using a certain ball valve construction so that, e.g., blocking problems typical to the above-mentioned needle valves and laby¬ rinth nozzles can be eliminated, however maintaining the bubble-forming properties of the nozzle at least on the level of prior art nozzles based on needle valves or labyrinth nozzles.

Typically, the volume flows of the dispersion water pas¬ sing through the above-mentioned types of nozzles, both through those of prior art and those disclosed in said patent publication, are of a very low level, maybe of the order of some dozens of litres per nozzle in a minute.

The arrangement disclosed in patent 5,154,351, however, solves quite elegantly the blocking problem involved with previously known needle and labyrinth valves. It also suggests that the arrangement in accordance with that invention maintains at least the bubble-forming prop¬ erties in accordance with previously known valves. In the arrangement of said invention, the equivalence in amount, i.e., the volume flow of the dispersion water to be treated corresponds to the capacity of known types of valves. In accordance with patent US 5,154,351, the diam¬ eter of the circular opening used as a throttle is gen- erally of the order of about 2.5 to 3.5 mm, depending on the pressure and overall dimensioning of the nozzle. Thus, the volume flows passing through prior art disper¬ sion water nozzles, including the one disclosed in patent US 5,154,351, are of the order of about 10 to 20 1/min. The dispersion water nozzle in accordance with US patent 5,154,351 has two sections.

An object of the present invention is to avoid drawbacks related to prior art and to provide an improved formation of microbubbles in a dispersion water nozzle. The arran¬ gement (method + apparatus) according to the invention facilitates producing of optimum-size microbubbles, in a controlled manner, in a dispersion water nozzle used in connection with dissolved air flotation, in amounts vary¬ ing from a few dozens of litres per minute to several hundreds of litres, even to over a thousand litres per minute, per each dispersion water nozzle. The dispersion water nozzle refers to all the equipment, in which the dispersion water inflow is treated in order to provide microbubbles. The word nozzle will be used below as a synonym for the word dispersion water nozzle, unless otherwise stated.

It is mainly characteristic of the method according to the invention what is disclosed in the characterizing part of claim 1. And it is mainly characteristic of the apparatus of the invention what is disclosed in the char¬ acterizing part of claim 9.

An object of the invention, i.e., improved formation of microbubbles in a dispersion water nozzle, is achieved by utilizing the following basic idea of the invention. The method according to the invention, for producing micro¬ bubbles of optimal size, their amount per nozzle being even several dozen or more times over the amount in prior art, utilizes the surprising discovery that the quality and the 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 effected by using at least one slot-like, preferably curved throttle opening in the throttle. Most preferably, the throttle opening/throttle openings of at least the first throttle is/are also uni¬ form or nearly uniform in width.

The form of other throttles, their layout in the disper¬ sion water nozzle itself, and pressure reductions etc. effected in them, are also significant aspects in provid¬ ing improved formation of microbubbles in a single dis- persion water nozzle.

The housing of the dispersion water nozzle means, in con¬ nection of this invention, the portion of the dispersion water nozzle where throttles producing the pressure re- duction of dispersion water are arranged. The flow- through opening of the housing refers to both an inlet for dispersion water flowing into the nozzle for treat¬ ment therein and an outlet for the dispersion water which has been treated and is to be discharged from the nozzle. The flow-through opening potentially arranged in the throttle means itself is named in accordance with the basic shape of the throttle; e.g., in this case, it is called a flow-through opening of a ball. This flow- through opening of the throttle serves as a cleaning opening, wherethrough the maximum volume flow of the dispersion water, passing through the nozzle, flows. The function of this maximum volume flow is to remove blocks possibly accumulated in the throttle openings of the throttles and/or in the vicinity thereof.

Use of the method according to the invention thereby efficiently prevents blocking of the nozzle. A second object of the invention is to provide an apparatus for implementing the above-identified method, in which appar- atus the amount of dispersion water to be treated varies from some dozens of litres per minute to several hundreds of litres per minute, even to over a thousand litres per minute per one nozzle. The nozzles according to the in¬ vention produce microbubbles optimal in size, in a con- trolled manner, in a wide stability range of the volume flow and, as stated above, depending on the nozzle size, considerably larger amounts per nozzle than before. The

volume flow per nozzle and thereby also the total number of microbubbles produced is naturally determined by, besides the pressure used, also the overall dimensioning of the nozzle. An essential feature of the arrangement according to the invention, for producing microbubbles in a dispersion water nozzle, is also that impurities accu¬ mulating in the nozzle and possibly causing blocks may, to a certain extent, be removed during the process run, i.e., without interrupting the operation of the disper- sion water nozzle. This is based on the above-mentioned wide stability range of the volume flow, which is a spe¬ cific feature included in the arrangement according to the invention. If an increase in the volume flow within the stability range of the dispersion water nozzle is not, however, sufficient for giving the desired result, i.e., the block does not disappear, the dispersion water nozzle may be turned, in accordance with the arrangement of prior art, to a cleaning position, whereby even large impurities causing blocks may be efficiently removed from the dispersion water nozzle. In this case, it is however necessary to interrupt the operation of the nozzle, i.e., production of microbubbles to the dispersion water to be treated.

Implementation of the method in accordance with the in¬ vention is preferably effected by utilizing a dispersion water nozzle, the throttle means whereof includes a piece which is in itself previously known and which is rotatable in relation to at least one axis and mounted rotatably around the axis of rotation, in a housing pro¬ vided with a flow-through opening (flow-through openings of inlet and outlet flows) perpendicular to the axis of rotation, and at least two throttles, at least the first whereof is slot-like and only one whereof is a flow- through opening of the throttle. The position of the flow-through opening of the throttle determines its func¬ tion in the dispersion water nozzle. If the flow-through

opening of the throttle is completely or substantially open, i.e., we are not within the stability range for forming microbubbles in the nozzle, the flow-through opening serves as a cleaning opening. On the other hand, if it is to a certain, allowable extent open, i.e., we are within the stability range of the volume flow of the nozzle, this open portion constitutes a portion of the first throttle and of the last throttle. Structures of the throttles will be described in greater detail below, by way of example, in connection with the explanations of the accompanying drawings. Thus, the throttle means in the nozzle arrangement in accordance with the invention includes at least two throttles. The apparatus implement¬ ing the method of the invention need not be designed for use in connection with a ball valve, only; e.g., cylin¬ drical basic solutions are also feasible.

Especially when the ball valve chamber between the first and the last (n) throttle has been enlarged, it has been found advantageous, in view of optimal formation of mic¬ robubbles, to add one or more throttles to this space, preferably in the form of a so-called impact plate. These impact plates are arranged, in view of formation of mic¬ robubbles, at an advantageous angle relative to the dis- persion water flow directed against the impact plate. The impact plate may also be an impact surface; i.e., the interior of the ball valve may be so shaped, e.g., by using an additional piece or by machining that an impact surface is formed therein, which impact surface is at an advantageous angle in view of the direction of flow of the dispersion water. When one impact plate/impact sur¬ face is disposed inside the ball valve, i.e., n = 3, it is preferably at an angle of about 90° or nearly 90° relative to the dispersion water flow hitting it. In this case, the bubbles produced in the first section are bro¬ ken to microbubbles optimal in size.

Dispersion water nozzles handling a larger volume flow have thereby preferably two or three sections, and they are disposed in connection with a ball valve which is in itself known. Especially, in the first pressure reduc- tion, an edge of the throttle opening is of great sig¬ nificance to the quality of the microbubbles being formed. A throttle opening which is curved and uniform in width has been found to be especially advantageous in the first pressure reduction. The number n of the throttles may be larger than two or three.

Tests have also revealed that the shape of the flow open¬ ing of the throttle, which causes the pressure reduction, is of great significance to the size and size distribu- tion of the bubbles being formed as well as to the amount of accepted microbubbles. A slot-like shape has been found advantageous for both the quality and the size distribution of the microbubbles. A curved slot has been found to be especially advantageous. This is probably due to the fact that in nucleation of acceptable micro- bubbles, it is especially important how near a solid edge, i.e., an edge of the slot, the nuclei are. When it is desired to treat a dispersion water flow advancing in a cross-sectionally round duct in a pressure reduction means arranged in connection with a ball valve, i.e., to throttle, a curved slot shape is a natural alternative when the edgeline of the throttle opening should be as long as possible when combined with a slot of a certain width. As mentioned above, a curved throttle opening having a uniform width has been found especially advan¬ tageous, in connection with the first pressure reduction. However, exact cause-and-effect relations for the above described behaviour in connection with formation of mic¬ robubbles have not yet been found on the basis of tests performed in connection with this invention.

The method and apparatus according to the invention are further described below, by way of example, with refer¬ ence to the accompanying drawings, which illustrate a few arrangements according to the invention, to which ar- rangements it is not, however, an intention to solely limit the invention. The arrangements described herein are advantageous for treating, e.g., waters of various processes in the pulp and paper industry, such as, e.g., raw waters, various process waters and waste waters. In these cases the water to be treated contains either solid or colloidal floes. The floes are caused to react, in a controlled manner, with fine microbubbles formed in the dispersion water nozzle, and these combinations (a floe + microbubbles) reacted with each other are caused to rise, in a controlled manner and undisturbed, to the surface of the flotation basin, wherefrom they are then collected with a suitable means. Reaction between floes and micro- bubble here means the physical action, in which floes and microbubbles adhere to each other.

Fig. la illustrates a throttle means of a dispersion water nozzle in accordance with the present invention, seen from upstream; Fig. lb illustrates a throttle means of the same disper- sion water nozzle, seen from downstream;

Fig. 2a is a sectional side view of a dispersion water nozzle of Fig. 1 in its operating position; Fig. 2b is a sectional side view of a dispersion water nozzle of Fig. 1 in its cleaning position; Fig. 3a illustrates a throttle means of a second disper¬ sion water nozzle according to the present invention, seen from upstream;

Fig. 3b illustrates the same throttle means of a disper¬ sion water nozzle, seen from downstream.

Fig. 1 illustrates a throttle means in accordance with the invention, in an operating position. Fig. la illus-

trates a throttle means of a dispersion water nozzle with two or more sections in accordance with the invention, seen from upstream, and Fig. lb illustrates a correspon¬ ding means seen from downstream. The arrangement dis- closed herein is constructed in connection with a ball valve. In accordance with its basic principle, micro¬ bubbles are formed in the dispersion water flowing through the throttles 1 and n of the throttle means by using shaped pieces 5, 6 fitted in connection with both the edges 7, 9 of the flow-through opening of a ball valve 4, which is in itself 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 fitted in connection with the housing and the edges 7, 9 of the flow-through opening 22 of the ball valve, which is opening, together form slot-like, curved throttle openings 8, 10 each being of a uniform and suitable width.

An inlet-side end plate 1 of the throttle means, shown in Fig. la, is attached to an outlet-side end plate 11, shown in Fig. lb, with a bolted joint 2. The ball 4 may be rotated to different operating positions by means of a double-acting adjusting lever 3. The operating principle of the adjusting lever 3 is explained more in detail below, in connection with the explanation of Fig. 2. In Fig. la, the shaped piece 5 and the edge 7 of the ball together form a slot-like, curved throttle opening 8 having a uniform width, on the inlet side. Corresponding- ly, in Fig. lb, the shaped piece 6 and the edge 9 of the ball together form a slot-like, curved throttle opening 10 having a uniform width, on the outlet side. Reference numeral 14 denotes the flow-through opening of the ball valve on the inlet side and reference numeral 15 denotes the flow-through opening of the ball valve on the outlet side. In this application, the throttle openings of the throttles 1 and n are located in the vicinity of the

centres of the flow-through openings 14, 15. This loca¬ tion of the throttle openings, and especially the loca¬ tion of the throttle n, keeps the microbubble flow uni¬ form in the cross section of the dispersion water duct on the outlet side. In this way, the following risk is effi¬ ciently avoided. If the microbubble flow advances rather long distances in the vicinity of the duct wall, there is a risk of the wall functioning, to a harmful extent, as a uniting/nucleation centre, for big bubbles, whereby the proportion of acceptable microbubbles in the dispersion water flow is reduced.

Fig. 2a illustrates a cross section of the throttle means of Fig. 1, when n = 3, and when the throttle means is in the operating position. Fig. 2b shows a cross section of the throttle means when it is in the cleaning position. In Figs. 2, the direction of the inflowing dispersion water is shown with an arrow V. In Fig. 2a, the housing of the throttle means is denoted with reference numeral 12. Fig. 2a shows both the operating position 16 of the impact plate as well as its utmost adjusted position 17, whereas the position of the ball 4 is unchanged. When the impact plate 16 is in the operating position, it serves as a throttle between sections 1 and n. So, microbubbles are formed in the dispersion water flowing through the throttles 2 ... (n-1) (in this application (n-1) = 2, and there is only one impact plate) of the throttle means in the dispersion water nozzle. They are formed by using the impact plate 16 disposed within the ball valve, which is in itself is known, and which constitutes a portion of the throttle means, by turning the impact plate relative to the rotation axis of the ball valve in such a manner that the impact plate 16 will be, in view of formation of microbubbles, at a suitable angle relative to the disper- sion water flow hitting it. In this case, the impact plate is preferably perpendicular or nearly perpendicular to the dispersion water inflow. The direction of the

impact plate 16 is, however, adjustable between the utmost positions shown in Fig. 2a, with no need to change the position of the ball 4. In this arrangement, the impact plate 16 serves as a second throttle of the throttle means, and (n-1) = 2. One edge of the throttle opening is formed by the inner surface 19, 19' of the ball 4 and the other edge by the end 18, 18' of the impact plate 16. The end 18, 18' of the impact plate is preferably curved in shape, corresponding to the shape of the inner surface 19, 19' of the ball 4, whereby the throttle opening 20, 20' is correspondingly a curved slot in shape. For example, if the shape of the impact plate 16 is a rectangle, the shape of the throttle opening 20, 20' is correspondingly a segment. The upper part of the segment is thereby formed by the inner surface 19, 19' of the ball 4.

Fig. 2b illustrates the impact plate 16 in the cleaning position. When in the cleaning position, the impact plate 16 is most preferably parallel with the axis 21 of the flow-through opening 22 of the ball. When the throttle means, which is a ball valve in this arrangement, is in the cleaning position, as much dispersion water as poss¬ ible flows as freely as possible through the ball 4. In this way, particles or colloids which obstruct dispersion water flow and which have possibly been accumulated in the inlet-side flow-through opening 14 of the ball valve and/or in the outlet-side flow-through opening 15 of the ball valve or in the vicinity of them, or in some throttle, are efficiently flushed away. At the same time, particles or colloids possibly accumulated in the interior of the ball valve are also efficiently flushed away.

Fig. 2 also gives a schematic illustration of the operat¬ ing principle of the double-acting adjusting lever, belonging to the dispersion water nozzle arrangement.

When the ball valve has been rotated to a certain posi¬ tion, it is possible to move the impact plate 16 back, by about 45° in the direction opposite to the rotating dir¬ ection, without changing the position of the ball 4 of the ball valve. This tolerance is necessary in the appli¬ cation in accordance with Figs. 1 and 2 because the oper¬ ation of the dispersion water nozzle in its various oper¬ ating positions is thereby made as efficient as possible. When the dispersion water nozzle is in the operating position, e.g., in accordance with Fig. 2a, the impact plate is preferably at an angle of approx. 90° relative to the dispersion water inlet flow. When the dispersion water nozzle is in the cleaning position in accordance with Fig. 2b, the impact plate 16 is most preferably parallel with the axis 21 of the flow-through opening 22 of the ball. Thus, e.g., when the ball 4 of the ball valve which constitutes a portion of the throttle means of the dispersion water nozzle of Fig. 2a, has been rotated towards the closed position, up to its operating position, the impact plate 16 has at the same time been rotated to its operating position, i.e., to a position which is at an angle of about 90° relative to the disper¬ sion water inlet flow, which is denoted with arrow V. When the throttle means has to be changed to the cleaning position, the ball 4 is rotated in the opposite direction than previously. Hereby, when the double-acting adjusting lever 3 is turned, the impact plate 16 is first turned to a position 17, and only thereafter, the ball 4 itself starts rotating together with the impact plate. This rotation of the ball 4 and the impact plate 16 is con¬ tinued until a cleaning position shown in Fig. 2b has been reached. Then, a volume flow of dispersion water which is as large as possible passes through the disper¬ sion water nozzle as freely as possible. And now, both the flow-through opening 22 of the ball 4 and the impact plate 16 are parallel with the dispersion water flow V.

Fig. 3a illustrates principles of a second throttle means in accordance with the invention, which throttle means has two or more sections. Like Figs. 1, these Figures illustrate throttles 1 and n of a throttle means in an operating position. Fig. 3a illustrates a throttle means of a dispersion water nozzle, seen from upstream, and Fig. 3b, a corresponding means seen from downstream. In this application, microbubbles are formed in the disper¬ sion water flowing through the throttles 1 and n of the throttle means of the dispersion water nozzle, by rotat¬ ing the ball 4 of a ball valve, which is in itself known, around its axis of rotation, whereby the edges 7, 9 of the flow-through opening 22 of the ball valve ball, which edges have been shaped, e.g., by machining them, and which edges in themselves constitute a portion of the first and the last throttle (n) , provide throttle open¬ ings which are, in view of formation of microbubbles, suitable in width, slot-like, preferably curved, and each of them being uniform in width. In Fig. 3a, the inlet- side flow-through opening 14 of the ball valve and the edge 7 of the ball 4 together form an inlet-side throttle opening 8, which is slot-like, curved and uniform in width. Correspondingly, in Fig. 3b, the outlet-side flow- through opening 15 of the ball valve and the edge 9 of the ball 4 together form an outlet-side throttle opening 10, which is uniform in width, slot-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, moving of a second throttle, which is in this case the impact plate 16, within the ball valve can be also so adjusted that its position may be kept unchanged or nearly unchanged, even though the position of the ball valve, i.e., the position of the throttles 1 and 3 were changed within the limits of the volume flow defined by the stability range of the dispersion water nozzle. By this measure, it is ensured that cleaning of the ball

valve may be effected by increasing, to a certain extent, the volume flow passing through the ball valve, without any need for total interruption of the production of microbubbles, i.e. without any need for rotating the ball valve to its actual cleaning position. During the time the ball valve has to be in the cleaning position, there is naturally no production of microbubbles in the nozzle.

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

Tests have revealed that the shape of the first section of the throttle means of the nozzle as well as the pres¬ sure reduction taking place in the first section are of high significance. As long as the first section of the

throttle means is slot-like and preferably curved in shape, and most preferably uniform in width, larger vari¬ ations are allowable for the shapes of throttle openings in the other sections, yet maintaining the total produc- tion of microbubbles of the dispersion water nozzle at an acceptable level. However, in the best dispersion water nozzles, slot-like, preferably curved throttle openings are used in each section. For example, if the impact plate 16 is a rectangle in shape, the throttle opening 20, 20' is correspondingly a segment-shaped slot. The upper section of the segment is in this case formed by the interior 19, 19' of the ball 4.

Essential to proper operation of the throttle means in accordance with the invention is also the ratio between pressure reductions occurring at various throttle sec¬ tions. For example, a throttle means 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 1st section about 70%, the 2nd about 20%, and the 3rd about 10%. The pressure reductions should also take place as quickly as possible in order to prevent harmful uniting of micro¬ bubbles to each other.

With greater dimensions of throttle means, compromises are inevitable at least in some points. If the total volume flow per one nozzle is, however, of the order of at least a thousand litres per minute, a small amount of bubbles of poor quality may be allowed because the total amount of acceptable microbubbles per one nozzle is, however, very large in comparison with prior art nozzles.

Researches have given good results, e.g., in the follow- ing conditions:

- "optimum volume flow" for formation of acceptable mic¬ robubbles about 300 1/min

- ball valve type DN 50

- dispersion water pressure about 5.5 bar

- portions of the 1st, 2nd, and 3rd sections of total pressure reduction, (%) 1st section about 70%

2nd section about 20% 3rd section about 10%

- shape and width of throttle opening 1, curved slot of uniform width width abt 5 to 5.5 mm

- shape and width of throttle opening 2, segment ceiling about 7 mm

- shape and width of throttle opening 3, curved slot of a uniform width width about 15 mm

It is also a characteristic feature of the arrangement of the invention that it is not very susceptible 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 volume flow range of about 175 to 325 1/min, the optimum volume flow being about 300 1/min. This property has a highly advantageous effect on the control of the flotation process itself.

When nozzles of prior art, e.g., those used in connection with the flotation process related to e.g. waste water treatment, become dirty, for one reason or the other, and thereby tend to become blocked, this blocking of nozzles causes the following chain reaction. A dirty nozzle is incapable of producing as many acceptable microbubbles as before. The less the number of acceptable microbubbles at the flotation process, the poorer the quality of the clarified liquid, i.e. the raw material of the dispersion water. The poorer in quality, i.e., the dirtier the dis¬ persion water is, the more easily the nozzles become

blocked, etc. until the whole process has to be inter¬ rupted and the nozzles be cleaned.

When employing the arrangement of the invention, the course of action in a corresponding dilemma would be as follows. If dirt accumulates in the nozzle and its capac¬ ity deteriorates from its optimum 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 influenced even during the run of the process, in the arrangement of 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 limits defined by the dimensions of each nozzle. Use of the dispersion water nozzle is continued with the maximum volume flow producing acceptable microbubbles until the nozzle has become clean. When the operation takes place within the adjustment limits of each nozzle size, the above-men- tioned cleaning work can be done without disturbing the flotation process, i.e., without interrupting it. After the cleaning measure, it is then possible to return to the dimension of throttle opening, i.e., the amount of volume flow, which results in an optimum yield of accept- able microbubbles.

If the nozzle is badly blocked, it is turned to a clean¬ ing position in accordance with Fig. 2b for a short time, whereby the largest possible volume flow passes there- through, which ensures removal of blocks at the latest. If blocks appear in the arrangement according to the invention, they most probably come up in the first throttle, where the slot is the smallest. Since, e.g., in the arrangements described above, the edges themselves of the flow-through opening of the ball valve either totally or partly constitute the first throttle, it is quite easy to clean this throttle of the impurities causing the

block, by either using either one of the cleaning methods described above.

When the arrangement according to the invention is used in connection with smaller ball valves, e.g., DN 15 or DN 32, it has been noted that utilization of the basic prin¬ ciple, i.e., the slot-like shape of the first throttle, is a sufficient property in order to provide acceptable microbubbles in dispersion water nozzles constructed in connection with the above-identified ball valves. For example, the DN 15 type nozzle functions well within the volume flow range of 0 to 50 1/min, when the inlet-side throttle, i.e., the first throttle of a two-section nozzle, is so shaped that, on the edge of the flow- through opening is disposed a shaped piece, whereby the first throttle is in compliance that shown in Fig. la. The second throttle is then formed, not by a second shaped piece together with the edge of the flow-through opening of the ball, but merely a flow-through opening of the ball, said flow-through opening being turned to an oblique position. Thus, the flow-through opening which is slightly open actually forms a second, i.e., the latter throttle. In this arrangement, the dispersion water flow¬ ing through the first throttle then hits the wall of the flow-through opening, said flow-through opening being in an oblique position, and is thereafter discharged from the dispersion- water nozzle through the above-identified latter throttle. Also in this arrangement, the general principle applies that the opening of the first throttle is smaller than that of the second or of subsequent throttles; in other words the dimensional ratios of the openings of the throttles are 1 < 2 < (n-1) < n. This same ratio is, on the other hand, indicated by the pro¬ portions of pressure reductions occurring in various throttles of the total pressure reduction for the whole nozzle. Good results in total pressure reductions (%) of the DN 15 type nozzle have been received in the two sec-

tions, e.g., as follows: the 1st section about 75%, the 2nd about 25%.

In a DN 32 type nozzle, the arrangement according to Figs. 1 and 2 can be used successfully, but without a separate impact plate, i.e., only as a nozzle with two sections. The DN 32 type nozzle functions well in the range of volume flow of about 60 to 150 1/min. Good results in total pressure reductions with DN 32 type nozzles have been received in the two sections, e.g., as follows: values: the 1st section about 75%, the 2nd about 25%, i.e., the same as with DN 15 nozzle.

In small nozzles, e.g., such as DN 15 or DN 32, the form- ing area of acceptable microbubbles is not so sensitive to changes than with nozzles with larger volume flows. Therefore, a distinct optimum area has not been estab¬ lished with these small nozzles, unlike with larger nozzles.

When the size of the dispersion water nozzle diminishes, acceptable microbubbles may be provided by utilizing the basic principle of the invention alone, i.e., a slot-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 affec¬ ting the amount and quality of the acceptable micro¬ bubbles.

The arrangement according to the invention offers a com¬ pletely new way of constructing an equipment for forming microbubbles in connection with flotation processes. As stated above, by utilizing the arrangement of the inven- tion, it is possible to achieve a production of several hundreds of litres, even over a thousand litres, per one dispersion water nozzle. In spite of the large volume

flow, the quality of the microbubbles formed is accept¬ able. To the designer, this naturally means that the design and construction of flotation equipment of a cer¬ tain capacity will become a lot easier. With one nozzle, it is possible to replace even dozens of nozzles based on the conventional technique and to save long distances in ducting constructions.

Control of the flotation process is easier than before because the susceptibility to 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, it is very quick to do, even so, that the pro¬ duction of microbubbles in the nozzle, let alone the entire process, need not be interrupted. The invention also offers excellent potential for further development of the whole flotation process. E.g., in processes with nozzles of large volume flows of about 100 to 1000 1/min or more, the flotation process can be controlled with regard to dispersion water nozzles, so that the total pressure reduction occurring in a single dispersion water nozzle is measured with a sensing element. If deviations from the limit values are detected, which deviations are big enough, or if the nozzle or its surroundings has become blocked, the control system starts to open these throttles in the opening direction from their optimum operating position. If necessary, they are opened up to the upper limit of the stability range of the nozzle, until the block is discharged from the nozzle. There- after, the control system returns the throttles back to their optimum operating position. If the block is too large to be removed during run, the control system will move the dispersion water nozzle to its actual cleaning position, whereby it will be cleaned at the latest. After this, the throttles are again moved to their optimum operating position.

The above-identified invention has been described with reference to a few preferred embodiments used in cleaning of waste waters, especially in the pulp and paper indus¬ try. The invention has thereby been described on the basis of some preferred embodiments thereof, with no intention to limit the present invention. There are sev¬ eral other fields in which to apply the invention, e.g., cleaning of raw water and concentration of ores, etc. , and, as is evident to a person skilled in the art, many alternative and optional modifications of the structure are feasible within the inventive scope defined by the accompanying claims.