The present invention concerns a nozzle which is used in pressure-water flotation clarifiers to release the air dissolved into the water under pressure as bubbles of a size suitable for the flotation process.

The process to which the invention is related is in itself known, and it is based on Henry's law, i.e. the solubility of a gas in a liquid is proportional to the partial pressure of the gas. In pressure-water flotation this is utilized by dissolving air into water at a pressure of about 2 to 8 bars. This water, saturated with air, is passed to the flotation clarification basin through pressure reduction members. When the pressure in the water flow is suddenly lowered, air is released as small bubbles. The air bubbles adhere to the solid particles to be removed and raise them to the surface of the water to be clarified.

The function of the pressure reduction member, for which the designation dispersion water nozzle is used in connec¬ tion with the present patent application, is:

- to lower the pressure in the dispersion water flow,

- to produce bubbles of specified size (about 100 μm) out of the air that is released from the water on reduction of pressure,

- to distribute the air bubbles in the water to be clarified as uniformly as possible.

The main parts of the dispersion water nozzle are a throttle part that operates as the pressure reduction part as well as an equalizing part, such as an expansion member.

The invention -is expressly concerned with the construction of a flow throttle operating as a pressure reduction part.

As is known in prior art, needle valves, diaphragm valves, globe valves, or fixed holes made into metal or plastic have been used as a throttle.

As an expansion member, tube parts or labyrinths made into plastic or metal are used.

Nozzles of different types have different properties in operation. The air bubbles produced must be of correct size. If the bubbles are excessively large, the rising speed of the bubbles is excessively high, turbulence is produced which disintegrates flocks, and large bubbles do not adhere to the flocks. On the other hand, if the bubbles are too small, their rising speed is too slow.

With nozzles based on needle valves as well as with labyrinth nozzles, a good operating efficiency can be obtained, which means high proportion of bubbles of correct size. On the other hand, the efficiency of nozzles based on diaphragm valves and globe valves is poor. Of the bubbles produced, a high proportion consists of either too small or too large bubbles to produce an adequate flotation effect. In the case of such valves, for a certain flotation effect a larger amount of dispersion water is required than with nozzles of higher efficiency.

However, it is a problem in nozzles based on needle valves and,in labyrinth nozzles that they are blocked. Impuri¬ ties, aluminium hydroxide deposit, etc. block the nozzles in the course of time. Cleaning of the nozzles is time- consuming and causes interruptions in production. What is concerned in the blocking of a nozzle is expressly blocking of the throttle part.

By means of a dispersion water nozzle in accordance with the present invention, an essential improvement has been obtained for this problem of blocking, while the bubble formation properties of the nozzle have, however, been maintained at least at the level of the prior-art needle- valve-based or labyrinth-based nozzles.

According to the basic idea of the invention, as the throttle part in the dispersion water nozzle, a member is used which is characterized in that it is a body which is rotationally symmetric in relation to at least one axis, and which is mounted rotatably around the axis of rotation in a housing provided with a flow opening perpen¬ dicular to the axis of rotation, and which throttle member is provided with two flow ducts, which are of different cross-sectional flow areas and which are perpendicular both to the axis of rotation and to each other.

The particular features of the invention come out from the accompanying patent claims.

The invention will be described with the aid of the accompanying drawing, wherein

Figure 1 is a sectional side view of a nozzle in accord¬ ance with the invention,

Figure 2 is a sectional top view of a nozzle in accordance with the invention in the flushing position,

Figure 3 is a top view of a nozzle in accordance with the invention in an operating position, and

Figure 4 is a top view of a nozzle in accordance with the invention in an alternative operating position.

For the pressurized dispersion water saturated with air, the nozzle is provided with an inlet duct 1. The inlet duct terminates in a wide flow opening which passes into the throttle member in the nozzle. In the embodiment shown, the throttle member consists of a closing member 2 of a globe valve, which is placed in a conventional valve housing provided with a flow passage. The closing member 2 comprises a conventional through flow duct 7 of a globe valve. For the purpose of rotating the closing member, the throttle includes a spindle member 3.

According to the basic idea of the invention, a second through flow duct 6 has been formed additionally into the closing member, the cross-sectional flow area of said duct 6 being smaller than the flow area of the flow duct 7. This additional flow duct has been made into the closing member as substantially perpendicular to the main flow duct 7 so that, when the closing member is rotated by means of the spindle 3, it is possible to select either one of said flow ducts 6 and 7 to the through flow posi¬ tion. If the closing member 2 is rotated by 90° from its dispersion-operation position shown in Fig. 3 to the wide through-flow position shown in Fig. 2, any impurities present in the expansion part are flushed efficiently out of the nozzle. When the closing member is rotated further by 90° in the same direction, the flow direction in the flow duct 6 of smaller diameter is reversed, whereby any impurities present in this duct are washed off efficiently.

According to a particular feature of the invention, the flow duct 6 has been produced so that the duct portions placed at opposite sides of the closing member 2 are not placed facing each other. In this way, a turbulent run is obtained for the dispersion-water flow (see Fig. 1) through the closing member 2, said turbulence causing a loss of energy in the flow, and thereby promoting the

release of the air dissolved in the water. A crosswise positioning of the opposite portions of the flow duct 6 can be accomplished by shifting the portions of the flow duct 6 in relation to each other in the direction of the flow duct 7, or by shifting them in the direction perpen¬ dicular to said direction, possibly by using both of said shifting directions. One possibility to disturb the flow running through is to make the flow duct 6 portions at a little angle in relation to each other, diverging from the parallel alignment. The angular deviation may be combined with the above shifting of position.

The opposite portions of the flow duct 6 may also have different cross-sectional flow areas, in which case the nozzle is operated in the position in which the flow area at the inlet side is smaller than the flow area at the outlet side. With flow ducts 6 of circular section, the ratio of the diameters of the duct portions may be, e.g., such that the diameter of the inlet-side duct is 3 mm and the diameter of the outlet-side duct is 4 mm.

Generally speaking, the diameter of a circular opening used for the flow duct 6 is of an order of about 2.5 to 3.5 mm, depending on the pressure and the overall dimen¬ sioning of the nozzle that are used.

When a throttle with these dimensions is used, the follow¬ ing dimensions may be used for the expansion member that follows after the throttle: length about 40 mm and diameter about 9 mm.

In the embodiment described herein, the throttle member 2 has been described as a ball, but corresponding operations can also be achieved by means of a cylindrical piece.

Fig. 3 shows an example of the operating tolerance of the closing member 2 that is shown within which the flow

opening 6 remains completely open. The position tolerance is 17° to both sides of the centre line. In a position that exceeds this tolerance, an operating situation as shown in Fig. 4 is reached, which shows an operating advantage of the nozzle in accordance with the invention, i.e. the possibility of operating with partial load.