More particularly, it relates to a method and apparatus for mixing gas, usually air, with a liquid, e.g, efflu- ent, and dissolving the gas in the liquid.

Many different methods and apparatus are known which are used for this purpose. However, reference is here made to one prior art apparatus only. It is disclosed in Finnish patent 86381. Reference is here also made to a theory disclosed in that patent publication, for dissolving gas in a liquid. It is taught, among other things, in the publication that the solubility of a gas in water is di- rectly proportional to the pressure of the gas and in- versely proportional to the temperature, with certain coefficients. Thus, it can be established that, by rais- ing e.g. the pressure of the liquid several atmospheres, the volume of dissolving gas may be correspondingly increased, in comparison with normal atmospheric pressure conditions. An increase in the temperature lowers the solubility to correspond OOC (Kelvin temperature +2730K), which is correspondingly revised with the prevailing temperature ratio, i.e., if the conditions are +200C, the solubility has lowered from the 0-degree condition by a ratio 273/293, i.e., to a 0.9317406-fold value. Each gas has a coefficient of its own, readable from technical tables, which coefficient also influences the solubility value. Solubility may be given in volume units of gas per volume unit of liquid (Ncm3/cm3) or in volume units of gas per weight unit of liquid (Ncm3/g).

In practice, the most usual uses are related to, e.g., dissolving of air in water, for example in connection with effluent treatment or in aerating lake and pond waters. An essential role is played here by the oxygen of

air, about 20% of the air being oxygen. Oxygen provides, e.g., living conditions of fish in water, and the oxygen content of water should be at least 4 to 5 mg/l. Usually the oxygen content is and it should be over 6 mg/l. Oxy- gen is consumed by organic compounds which have ended up in water and which oxydate and decompose, causing water- courses to overgrow and become eutrophic. To prevent such a course of events, effluents are normally handled in water purification plants where solids are removed as completely as possible and, finally, organic residuals are oxydated, i.e., treated biologically. This procedure often requires plenty of oxygen to be dissolved in water.

Many different methods exist, which may be used for this purpose. The most usual method is to use pressurized air produced by a compressor and to blow it to the bottom part of a waste water basin, through shattering nozzles arranged in connection with the bottom. The smaller the bubbles are, the faster the solubility of air. Therefore, production of extremely small air bubbles with the shat- tering nozzles is aimed at. This requires extra pressure in air blowing. This pressure is in principle wasted for breaking up the air in water, since the solubility is only influenced by how deep down below the liquid level the shattering nozzles are disposed. The method is there- fore not economical, even though it is widelyY used as it is technically easy to realize. Besides being uneconomi- cal, it also has a further drawback, i.e., nozzles becom- ing clogged by impurities in compressed air.

Another way of mixing oxygen with water is to use vari- ous, large mixers. In these devices, water is lifted to fly in large quantities, in the form of drops, in the air, whereby the airdraft being simultaneously formed comes into contact with the drops. As a result, oxygen dissolves in the treated water. This method is used, for example, for treating effluents in the wood processing

industry. However, in spite of large quantities of treated liquid, it cannot be considered an efficient method in terms of energy economy.

One way is to use a swiftly rotatable rotor within the liquid and supply pressurized air to the rotor, either by using self-admission or some other way. The rotor then mixes this air with the liquid, shattering the air effi- ciently. Both high and low efficiencies have been report- ed.

The equipment disclosed in the Finnish patent 8638,1 is based on a pump where the gas to be dissolved is mixed with liquid in such a manner that the suction opening of the pump is provided with a separate inlet conduit for gas, whereby the suction effect produced by the impeller draws the required volume of gas to the impeller and fur- ther into the pump housing. A pressurized outlet pipe of the pump is provided with a pressure mixer unit where liquid and gas are then thoroughly mixed with each other when they are flowing under pressure through the mixer unit to a separator of excess gas.

In this prior art arrangement, liquid flows through a valve and under control thereof, to a suction conduit of the pump. It is typical of an arrangement like this that a conventional centrifugal pump cannot pump such liquid the suction side flow whereof has been controlled in a manner described above. The suction conduit leads the flow to the impeller which is in the pump housing. The suction conduit 5 is provided with a pipe, for leading the gas flowing therethrough directly to the impeller.

The gas flow is in this case best controlled with a con- trol valve. On the pressure side of the pump, connected to the outlet thereof, there is arranged a pressure mixer unit, and after that a control valve. The outlet flow from the control valve is so controllable that the

required pressure is obtained in the mixer. It is also possible to include a pressure gauge control which is known per se, in this arrangement.

When the inlet flow to the impeller is suitable or throttled to a required extent, the gas flow will be absorbed by the liquid and entrained therewith to the impeller. As soon as the gas volume is suitable and the pressure side has been adjusted, either by the load of the piping or by the valve, the flow will be made up of liquid saturated with gas. If and when the pressure of this flow is reduced, for example, to a free atmospheric pressure, the excess gas will be separated from the liquid as molecular bubbles which are ready to adhere to solids, oil, greases, flocs, dregs, or corresponding par- ticles which together rise to the surface. This phenom- enon, i.e, gas release may be utilized in many different applications, for example, flotation.

As air contains four times more nitrogen than oxygen in proportion and as the solubility of nitrogen in water is approximately half of the solubility of oxygen, a big portion (about 70%) of the nitrogen will remain in the liquid in a gaseous form. Depending on circumstances, this portion may either be left in the liquid as bubbles or removed by a separate gas separator. The gas separator arrangement may be known per se, but it is essential to this prior art arrangement to use a controllable valve, for selecting the pressure range in which the gas accu- mulated in the upper part of the gas separator is released. This pressure range is lower than the counter- pressure in the pump which is generated by the valve or the piping arranged thereafter.

The equipment described above seems, however, unnecessar- ily complicated for such a simple task as mixing of air with a liquid. In the first place, the equipment

described needs a valve on the suction side of the pump, for regulating the flow of liquid entering the pump.

Correspondingly, a separate suction conduit with a con- trol valve is needed for the gas to be mixed. However, the pressure mixer unit with a control valve and gas separator, arranged on the pressure side of the pump is the most complicated means of this prior art equipment. A conventional centrifugal pump is out of the question in this case because it is incapable of pumping gaseous liquid.

The basis of the present invention is to simplify the structure of both the gas mixing device and other equip- ment possibly arranged in connection therewith, and to use a centrifugal pump if possible.

As for other equipment arranged in connection with the gas mixing device, it is to be noted that the equipment in accordance with the above identified Finnish patent is suggested for use in aerating/oxydating of lakes and ponds and also for use in aerating/oxydating of effluents of the wood processing industry. It is also worth while noticing that it is necessary, when the equipment in accordance with said patent publication is used, to have been made sure that an even flow of liquid enters the inlet side of the pump. In other words, the suction side of the pump has to be provided with a specific buffer tank, separately built if necessary, for ensuring a suf- ficient flow of liquid.

The present invention provides a simple mixing device, and neither the inlet nor the pressure side thereof calls for any special equipment, but it may be arranged direct- ly in the process. The equipment disclosed in the above- identified patent, for example, requires a separate mixer to be arranged after the pump, just like the other gas mixing devices which are known to us.

The characteristic features of the method and apparatus in accordance with the present invention will become apparent from the accompanying claims.

The method and apparatus in accordance with the invention will be described more in detail in the following, with reference to the accompanying drawing, in which Fig. 1 is a schematic illustration of an apparatus ac- cording to a preferred embodiment of the invention.

In accordance with Fig. 1, the apparatus according to the present invention mainly comprises a mixing device 12, an inlet pipe 10, an outlet means 14 for separated gas, and a pressure pipe 16. The apparatus described above func- tions so that liquid and gas flow freely via the inlet pipe 10 to the mixing device 12. The apparatus is able to function even if the ratio of gas to liquid is arbitrary.

Thus, it is possible that e.g., lake water to be aerated is taken from the lake surface through a pipe so that half of the cross-sectional area of the inlet pipe is below the surface and the other half on the surface. In other words, the apparatus operates without any inlet pressure, in atmospheric conditions. The mixing device 12 is a pressure-raising mixer which attempts to pump the liquid entered through pipe 10 to the pressure pipe 16.

Since it is a characteristic feature of the invention, e.g., that also a large volume of gas enters the mixing device via inlet pipe 10, that the pumping capacity of the mixing device is preferably dimensioned for a larger flow of liquid than possibly can enter the device, and that the pressure pipe or at least the flow thereinto is preferably adjusted in accordance with the liquid flow entering the mixing device, the following things will happen. Because a relatively small liquid flow enters the mixing device in view of the capacity thereof, the mixing device 12 is capable of generating such a pressure that a small amount of liquid passes to the pressure pipe. In

this case, however, part of the liquid remains circulat- ing inside the pump at the same time as the rotor of the mixing device also pumps gas, which has entered the mix- ing device, to a housing of the device. Thus, there is both gas and liquid in the same pressurized space in the mixing device, whereby gas is dissolved in liquid to such an extent which is possible in the prevailing circumstan- ces.

A suitable delay is arranged in the mixing device in the manner described above, in order to give gas time to dis- solve in the liquid. Factors having effect on the delay are naturally the capacity of the mixing device with respect to the incoming liquid flow, the dimension of the pressure pipe of the mixing device, and potential control with a valve, etc.

Another application of the invention is an arrangement in which gas is not actually dissolved in liquid, but it is mixed with the liquid as small bubbles. In this case, the pump housing need not be arranged with a higher pressure required by dissolving, but correspondingly a higher volume flow. An arrangement of this kind is especially suitable e.g., for treating wastepaper pulp, in which treatment ink and other particles removable with flota- tion are removed that way. The invention can naturally be applied to other uses of flotation as well.

Since the ratio of the liquid introduced into the mixing device to the gas introduced is practically arbitrary, the mixing device is provided with gas separating means, for removing surplus gas accumulated in front of the rotor. If too much gas accumulates in the mixing device, it will become filled with gas and can no longer manage to raise the pressure and consequently to dissolve gas.

The mixing device may, for example, be arranged to treat

the liquid flow entering the flotation plant or the dis- persion water circulation of the flotation plant.

A preferred embodiment of the invention worth while men- tioning is a centrifugal pump, which is capable of separ- ating gas and is applicable to be used as a mixing device. In other words, it is a pump having means, in connection with the impeller, for removing gas from the pump. Said means may include, e.g., holes or openings arranged in the pump impeller, through which holes or openings gas is led to the rear side of the impeller, and a vacuum means, most usually a liquid ring pump, which is either mounted on the same shaft as the impeller or pro- vided with a separate drive and disposed outside of the pump. A pump suitable for this purpose is disclosed, e.g., in European patent publication 0 478 228.

A still further preferred application of the present invention is to add various chemicals needed in the pro- cess, such as flocculation chemicals and dispersing agents to the inlet flow of the liquid entering the equipment according to the invention.

As is appreciated from the foregoing description, an apparatus which is much simpler and easier to operate than prior art has been developed for mixing gas with liquid. It is also worth while mentioning that it is a characteristic feature of a preferred embodiment of the invention that the pressure pipe of the equipment may be connected with, e.g., the flotation tank, whereby the same means is simultaneously used for mixing air with liquid, mixing various flotation chemicals with liquid, separating surplus gas, and pumping the liquid to the flotation tank. It is understood that the invention is by no means intended to be limited to what has been described above as preferred embodiments thereof, but the actual scope of the invention is defined by the accom- panying claims, alone.