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Title:
A FLOTATION TANK, A TANK MODULE AND ITS USES, A FLOTATION PLANT, A METHOD OF REPLACING THE FLOTATION TANK, AND METHODS OF MAINTENANCE OF THE FLOTATION PLANT
Document Type and Number:
WIPO Patent Application WO/2016/181024
Kind Code:
A1
Abstract:
A flotation tank (1) comprises a self-supporting tank (2). The tank (2) is made of thermoplastic polymer. The tank (2) has a lower tank part (3) and a tapered upper tank part (4) which is narrower than the lower tank part (3). The tank (2) has a mouth (5) at the upper end of the upper tank part (4) and an overflow lip (6) at the periphery of the mouth (5). The flotation tank (1) further comprises an overflow receptacle (7). The overflow receptacle (7) is made of thermoplastic polymer and connected to the tapered upper part (4) of the tank (2) beside the overflow lip (6) for receiving, collecting and discharging an overflow that overflows from the tank (2) over the overflow lip (6), when in use.

Inventors:
TÄHKIÖ, Pekka (Matinkatu 24 A 13, Espoo, FI-02230, FI)
VAARNA, Valtteri (Fleminginkatu 5 C 73, Helsinki, FI-00530, FI)
LUUKKONEN, Matti (Töölönkatu 24 B 29, Helsinki, FI-00260, FI)
Application Number:
FI2015/050327
Publication Date:
November 17, 2016
Filing Date:
May 13, 2015
Export Citation:
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Assignee:
OUTOTEC (FINLAND) OY (Rauhalanpuisto 9, Espoo, FI-02230, FI)
International Classes:
B03D1/14; B03D1/16; B29C41/04; B65D88/02; B65D90/20; E04H7/02
Domestic Patent References:
2011-09-01
2010-12-16
2012-04-12
Foreign References:
US20120111436A12012-05-10
US2142010A1938-12-27
CN201702028U2011-01-12
US20080257894A12008-10-23
US5890616A1999-04-06
FR2815021A12002-04-12
Other References:
SANTANDER, M. ET AL.: "Adsorbing colloidal flotation removing metals ions in modified jet cell", MINERALS ENGINEERING, vol. 24, no. 9, 29 April 2011 (2011-04-29), pages 1010 - 1015, XP028375348, ISSN: 0892-6875
Attorney, Agent or Firm:
PAPULA OY (P.O. Box 981, Helsinki, FI-00101, FI)
Download PDF:
Claims:
CLAIMS

1. A flotation tank comprising

- a self-supporting tank, the tank being made of thermoplastic polymer, the tank having a lower tank part and a tapered upper tank part which is narrower than, the .lower tank part, the tank having a mouth at the upper end of the upper tank part and an overflow lip at the periphery of the mouth, and

- an overflow receptacle, the overflow receptacle being made of thermoplastic polymer and connect¬ ed to the tapered upper part of the tank beside the overflow lip for receiving, collecting and discharging an overflow that overflows from the tank over the overflow lip, when in use,

2, The flotation tank according to claim 1, wherein the overflow receptacle and the tank are connected to each other by welding,

3, The flotation tank according to claim 1 or 2, wherein the overflow receptacle surrounds circumferen- tially the mouth of the tank, 4, The flotation tank according to any one of the claims 1 to 3, wherein the overflow lip comprises a separate lip member, and the lip member is connectable to the upper part of the tank at a desired height position to obtain a suitable overflow height for the overflow,

5. The flotation tank according to any one of the claims 1 to 4, wherein the overflow receptacle comprises a chute for collecting the overflow, when in use, and at least one outlet at the lowest part of the overflow receptacle, and the chute is inclined towards the outlet.

6. The flotation tank according to claim 5, wherein an angle (a) of inclination of the chute is 5° - 30°, more preferably 8° - 20% most preferably 10° - 15°,

7. The flotation tank according to any one of the claims 1 to 6, wherein the tank has a wall thickness which is 5 - 30 mm. 8. The flotation tank according to any one of the claims 1 to 7, wherein the volume of the tank is 0.5 - 20 m3, more preferably 1 - 15 m3, most, preferably 1 - 8 m3. 9. The flotation tank according to any one of the claims 1 to 8, wherein the cross-sectional shape of the lower tank part is rectangular when the volume of the tank is greater than 8 m3. 10. The flotation tank, according to any one of the claims 1 to 9, wherein the cross-sectional shape of the lower tank part is circular when the volume of the tank is at most 8 m3. 11. The flotation tank according to any one of the claims 1 to 10, wherein the cross-sectional shape of the lower tank part, being rectangular the mouth is rectangular or circular. 12. The flotation tank according to any one of the claims 1 to 10, wherein the cross-sectional shape of the lower tank part being circular the mouth is circu¬ lar . 13. The flotation tank according to any one of the claims 1 to 12, wherein the thermoplastic polymer is polyethylene or polypropylene.

14. The flotation tank according to any one of the claims 1 to 13, wherein the thermoplastic polymer is polyethylene .

15. The flotation tank according to any one of the claims 1 to 13, wherein the thermoplastic polymer is polypropylene . 16, The flotation tank according to any one or the claims 1 to 15, wherein the tank and the overflow receptacle are rotational molded parts,

17. The flotation tank according to any one of the claims 1 to 16, wherein the area to volume ratio between area of the mouth and volume of the tank is 0,15 - 0,4/m.

18. The flotation tank according any one of the claims 1 to 11, wherein flotation is froth flotation,

19. A tank module comprising

- a self-supporting framework having an inner space, and

- at least one flotation tank according to any one of the claims 1 to 16 is disposed, in the inner space .

20. The tank module according to claim 18, wherein the tank module includes at least two successive flotation tanks which are in fluid communication with each oth¬ er, the flotation tanks each having a lip member which is adjusted at a different height position in relation to the lip member of the other flotation tank,

21. A flotation plant comprising - a tank module according to claim 19 or 20, and

- a drive module, the drive module being removably stacked and aligned on tap of the tank module and. including at least two drive units for the equipment immersed in the feedstock subject to flotation; the equipment performing the flotation action; the drive module being a rigid and self-supporting unit capable of being transferable and hoistabie as an in- tegral entity.

22, The flotation plant according to claim 21 , wherein flotation is froth flotation. 23. The flotation plant according to claim 22, wherein the flotation plant comprises gas adding equipment for adding gas to the feedstock subject of flotation.

24. The flotation plant according to claim 23, wherein the flotation plant comprises gas adding equipment for adding- gas to the stream of the feedstock subject of flotation before entering the flotation tank,

25. The flotation plant according to claim 23, wherein the flotation plant comprises gas adding equipment for adding gas to the feedstock subject of flotation in the flotation tank.

26, The flotation plant, according to claim 25, wherein the gas adding equipment includes a rotor inside the flotation tank.

27, The flotation plant according to claim 25, wherein the gas adding equipment includes a hollow drive shaft rotatable by the drive unit, and the rotor is connected, to the drive shaft.

28. The flotation plant according to any one of the claims 21 to 27, wherein the feedstock subject of flotation is slurry or pulp.

29. The flotation plant according to any one of the claims 21 to 28, wherein the flotation plant comprises mixing equipment .

30. The flotation plant according to claim 29, wherein the mixing equipment includes a rotor inside the flotation tank,

31. The flotation plant according to claim 30 wherein the mixing equipment includes a εϊ-ator inside the flotation tank.

32. The flotation plant according to claim 31, wherein the flotation tank has a bottom, and the stator is connected to the self-supporting framework through the bottom.

33. Use of the tank module according to claim 20 or 21 for separating material by notation based on differences of buoyancy properties of substances.

34. Use of the tank module according to claim 20 or 21 for separating solid material by froth flotation based on differences of hydrophilic properties of substanc¬ es .

35. Use of the tank module according to claim 20 or 21 for concentrating ore by froth flotation,

36. Use of the tank module according to claim 20 or 21 for flotation of substances containing abrasive material.

37. Use of the tank module according to claim 20 or 21 for froth flotation of ore containing pyrite, silica, chromite . 38. A method of replacing the flotation tank according to any one of the claims 1 to 18 in. a tank module according to any one of the claims 19 or 20, the method comprising steps of

removing the flotation tank out from, inside the self-supporting framework, and

- installing another flotation tank into the self- supporting framework,

39. The method according to claim 38, wherein the steps of removing and installing the flotation tank are implemented by lifting the flotation tank,

40. A method of maintenance of a flotation, plant according to any one of the claims 21 to 32, in which method an uppermost module is subject of maintenance, and the uppermost, module is hoisted, up and transferred aside from the top of the lower module and the uppermost module is replaced by a another uppermost module which is placed on top of the lower module.

41. A method of maintenance of a flotation plant according to any one of the claims 21 to 32, in which method a lower module, which is underneath an uppermost module, is subject of maintenance, and the upper- most module is hoisted up from the top of the lower module and. transferred aside for gaining access to the lower module,

42. The method of maintenance according to claim 41, wherein while the uppermost module is away from the top of the lower module, maintenance operations are performed for the lower module.

43. The method of maintenance according to claim 41, wherein while the uppermost module is away from the top of the lower module, the lower module is replaced by another lower module.

Description:
A FLOTATION TANK, A TANK MODULE AND ITS USES, A FLOTATION PLANT, A METHOD OF REPLACING THE FLOTATION TANK, AND METHODS OF MAINTENANCE OF THE FLOTATION PLANT FIELD OF THE INVENTION

The present invention relates to a flotation tank. Further, the invention relates to a tank module. Further, the invention relates to a flotation plant. Further, the invention relates to uses of the tank module. Further, the invention relates to a method of replacing the flotation tank. Further, the invention relates to methods of maintenance of the flotation plant . SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a flotation tank. The flotation tank comprises a self-supporting tank. The tank is made of thermoplastic polymer. The tank has a lower tank part and a tapered upper tank part. The tapered upper tank part is narrower than the lower tank part. The tank has a mouth at the upper end of the upper tank part and an overflow lip at the periphery of the mouth. Further, the flotation tank comprises an overflow receptacle. The overflow receptacle is made of thermoplastic poly ¬ mer and connected to the tapered upper part of the tank beside the overflow lip. The overflow receptacle receives collects and discharges an overflow that overflows from the tank over the overflow lip, when in use.

The technical effect of the invention is that as the tank is a self-supporting unit and the overflow receptacle is connected to the tank, this assembly can be handled, hoisted and transferred together as one inte ¬ gral entity. The tank has a rigid monocoque structure that is able to hold its form and forms a rigid sup ¬ port for the overflow receptacle. The tapered upper part gives stiffness for the entire flotation tank. The tapering at the upper part also provides an opti- mal flow pattern inside flotation tank, when in use.

In this application the following definitions apply regarding flotation. Flotation involves phenomena related to the relative buoyancy of objects. The term flotation includes all flotation techniques. Flotation can be for example froth flotation, dissolved air flotation (DAF) or induced gas flotation. Froth flotation is a process for separating hydrophobic materials from hydrophilic materials by adding gas, for example air, to process. Froth flotation could be made based on natural hydrophilic/hydrophobic difference or based on hydrophilic/hydrophobic differences made by addition of a surfactant or collector chemical. Gas can be add ¬ ed to the feedstock subject of flotation (slurry or pulp) by a number of different ways. In one embodiment gas can be added to the stream of feedstock subject to flotation before it is fed to the flotation tank. In one embodiment gas can be added to feedstock subject to flotation in the flotation tank. In one embodiment gas adding equipment can include gas dispersing equip ¬ ment at the bottom of the tank. In one embodiment gas system equipment can include a feedstock (slurry or pulp) jet for jetting the feedstock to air. In one embodiment gas adding equipment includes a rotor inside the tank. In one embodiment gas can be added under the rotor In one embodiment gas is added by a pipe ending under rotor. The pipe can be inside the flotation tank. The pipe can go through the bottom of the flota ¬ tion tank. In one embodiment the rotor takes gas from the surface of sludge by vortex. In one embodiment is added by axis of the rotor. In one embodiment mixing equipment is arranged for mixing the slurry/pulp. Mix- ing equipment could be for example a pump or a rotor. When the mixing is made by pump, the feedstock subject of flotation could be taken from one part of flotation tank and put back to another part of flotation tank. When mixing is made by the rotor, the rotor is inside the flotation tank. In one embodiment mixing equipment can include a rotor inside the flotation tank. In one embodiment mixing equipment can include a stator in ¬ side the flotation tank. The stator is for boosting mixing and to diffuse air to the feedstock (slurry or pulp) subject to flotation.

In one embodiment of the flotation tank, the overflow receptacle and the tank are connected together by welding. The technical effect is that the flotation tank has a good structural stiffness which makes maintenance easy as manageability during maintenance is good. In one embodiment of the flotation tank, the overflow receptacle circumferentially surrounds the mouth of the tank. The technical effect is that the flotation tank has a good structural stiffness which makes maintenance easy as manageability during maintenance is good.

In one embodiment of the flotation tank, the overflow lip comprises a separate lip member, and the lip mem ¬ ber is connectable to the upper part of the tank at the region of the mouth at a desired height position to obtain a suitable overflow height for the overflow. The technical effect is that the overflow heights of identical tanks can be made different with an adjusta ¬ ble lip member. Maintenance is easy as the flotation tanks can be identical and the required overflow height can be adjusted by a suitable height position of the lip member for each tank. In one embodiment of the flotation tank, the overflow receptacle comprises a chute to collect the overflow, when in use, and at least one outlet at the lowest part of the overflow receptacle, and the chute is in ¬ clined towards the outlet. The technical effect is that the flow of overflow by gravity is ensured by the inclination . In one embodiment of the flotation tank, the overflow receptacle has a straight bottom.

In one embodiment of the flotation tank, an angle of inclination of the chute is 5° - 30°, more preferably 8° - 20°, most preferably 10° - 15°. The technical ef ¬ fect of the angle of inclination especially for the angles 8° - 20°, most preferably 10° - 15°, is that the chute does not become blocked and does not wear excessively. Need for maintenance is thereby mini- mized.

In one embodiment of the flotation tank, the tank has a wall thickness which is 5 - 30 mm. The technical ef ¬ fect of the wall thickness within this range is that the tank will not be too heavy so that it can be changed easily but yet it is stiff enough so that it can be easily installed. The tapering of the tank at its upper part makes it stiff so that the tank is stiff despite the relatively thin wall. This makes maintenance easy.

In one embodiment of the flotation tank, the volume of the tank is 0.5 - 20 m 3 , more preferably 1 - 15 m 3 , most preferably 1 - 8 m 3 . The technical effect is that the tanks can be changed easily as they are not too big and heavy. The tanks are still big enough so that a significant volume of capacity can be subjected to maintenance by changing a few tanks. The maintenance operations can be easily made for tanks which are not too big and heavy. In one embodiment of the flotation tank, the cross- sectional shape of the lower tank part is rectangular when the volume of the tank is greater than 8 m 3 . The technical effect is that such great tanks can be sup ¬ ported by sidewalls of the self-supporting framework in the inner space of which the tanks are installed in a tank module. The wall of the tank can be supported against the sidewall of the framework so that the framework bears loads exerted by the hydrostatic pres ¬ sure of the liquid filled inside the tank.

In one embodiment of the flotation tank, the cross- sectional shape of the lower tank part is circular when the volume of the tank is at most 8 m 3 . The tech ¬ nical effect is that the round form gives the required stiffness for the tank up to this size class.

In one embodiment of the flotation tank, the cross- sectional shape of the lower tank part being rectangu ¬ lar the mouth is rectangular or circular. The tech- nical effect of the circular mouth is that it stiffens the structure of the flotation tank. Maintenance is easy as the flotation tanks to be changed have suffi ¬ cient stiffness. In one embodiment of the flotation tank, the cross- sectional shape of the lower tank part being circular the mouth is circular. The technical effect of the circular mouth is that the entity formed by the tank and the overflow receptacle together is stiff to ena- ble easy handling, lifting and maintenance. In one embodiment of the flotation tank, the thermo ¬ plastic polymer is polyethylene or polypropylene. The technical effect of these materials is that they are very resistant to abrasive wear. Especially, when the flotation tank is in use it may accommodate a rotating rotor for gas adding and/or mixing, the mixing of the feedstock subject of flotation by a rotor causes the feedstock (which can be very abrasive) to flow against inner surface of the tank wall and thereby causes se- verely abrasive wear conditions. Need for maintenance is minimized.

In one embodiment of the flotation tank, the thermo ¬ plastic polymer is polyethylene.

In one embodiment of the flotation tank, the thermo ¬ plastic polymer is polypropylene.

In one embodiment of the flotation tank, the tank and the overflow receptacle are rotational molded parts. The technical effect is that with rotational molding it is easy to create stiff tank forms from abrasive resistant plastics materials so that the tank may have a form which can create a good flow pattern for the feedstock subject of flotation during flotation process. Also other manufacturing techniques are possi ¬ ble, for example in one embodiment of the flotation tank the tank and/or the overflow receptacle may be 3D printed parts.

In one embodiment of the flotation tank, the area to volume ratio between area of the mouth and volume of the flotation tank is 0.15 - 0.4/m. The technical ef ¬ fect is that maintenance is easy as the maintenance tools can be brought into the tank via the mouth. The mouth is not too big so that it would not provide structural support and stiffness. The mouth having the above-mentioned area to volume ratio enables easy maintenance as the required tools can be brought into the tank via the mouth while the tank has a sufficient stiffness .

In one embodiment of the flotation tank, flotation is froth flotation.

According to a second aspect of the invention, the in- vention provides a tank module comprising a self- supporting framework having an inner space, and at least one flotation tank according to the first aspect of the invention is disposed in the inner space. In one embodiment of the tank module, the tank module includes at least two successive flotation tanks which are in fluid communication with each other, the flotation tanks each having a lip member which is adjusted at a different height position in relation to the lip member of the other flotation tank.

According to a third aspect of the invention, the in ¬ vention provides a flotation plant comprising a tank module according to the second aspect of the inven- tion, and a drive module. The drive module is remova ¬ bly stacked and aligned on top of the tank module and includes at least two drive units for the equipment immersed in the feedstock subject to flotation, the equipment performing the flotation action. The drive module is a rigid and self-supporting unit capable of being transferable and hoistable as an integral enti ¬ ty.

In one embodiment of the flotation plant, flotation is froth flotation. In one embodiment of the flotation plant, the flota ¬ tion plant comprises gas adding equipment for adding gas to the feedstock subject of flotation. In one embodiment of the flotation plant, the flota ¬ tion plant comprises gas adding equipment for adding gas to the stream of the feedstock subject of flota ¬ tion before entering the flotation tank. In one embodiment of the flotation plant, the flota ¬ tion plant comprises gas adding equipment for adding gas to the feedstock subject of flotation in the flo ¬ tation tank. In one embodiment of the flotation plant, the gas add ¬ ing equipment includes a rotor inside the flotation tank .

In one embodiment of the flotation plant, the gas add- ing equipment includes a hollow drive shaft rotatable by the drive unit, and the rotor is connected to the drive shaft.

In one embodiment of the flotation plant, the feed- stock subject of flotation is slurry or pulp.

In one embodiment of the flotation plant, the flota ¬ tion plant comprises mixing equipment. In one embodiment of the flotation plant, the mixing equipment includes a rotor inside the flotation tank.

In one embodiment of the flotation plant, the mixing equipment includes a stator inside the flotation tank. In one embodiment of the flotation plant, the flota ¬ tion tank has a bottom, and the stator is connected to the self-supporting framework through the bottom. According to a fourth aspect of the invention, the invention provides use of the tank module according to the second aspect of the invention for separating ma ¬ terial by flotation based on differences of buoyancy properties of substances. For example there is buoyan- cy difference when organic material is separated from aqueous material.

According to a fifth aspect of the invention, the invention provides use of the tank module according to the second aspect for separating solid material by froth flotation based on differences of hydrophilic properties of substances. Solid materials separated by froth flotation could be oil sands, carbon, coal, talk, industrial minerals and mineral particles. The minerals may include industrial minerals and ore. Froth flotation to solid material could be made based on natural hydrophilic/hydrophobic difference or based on hydrophilic/hydrophobic differences made by addi ¬ tion of a surfactant or collector chemical or other chemical.

According to a sixth aspect of the invention, the invention provides use of the tank module according to the second aspect for concentrating ore by froth flo- tation. An ore is a type of rock that contains suffi ¬ cient minerals with important elements including met ¬ als that can be economically extracted from the rock. Metal ores are generally oxides, sulfides, silicates, or metals such as native copper or gold. Froth flota- tion of ore could be made based on natural hydro ¬ philic/hydrophobic difference or based on hydro- philic/hydrophobic differences made by addition of a surfactant or collector chemical or other chemical.

According to a seventh aspect of the invention, the invention provides use of the tank module according to the second aspect for flotation of substances contain ¬ ing abrasive material. The abrasive mineral may be, for example, pyrite, silica, chromite. The drive mod ¬ ule being hoistable and transferable as one unit to gain access to the tanks enables that the tanks can easily be maintained or replaced when they are outworn and are at the end of their life. This is important especially with the use in connection with abrasive material. Use of the flotation plant which is easy to maintenance is effective when flotation is made to abrasive material.

According to an eighth aspect of the invention, the invention provides use of the tank module according to the second aspect for froth flotation of ore containing pyrite, silica, chromite. Use of the tank module which is easy to maintenance and has preferably tanks made from PE or PP is effective when flotation is made to ore containing pyrite, silica, chromite. PE and PP are durable against the ore containing pyrite, silica, chromite .

According to a ninth aspect of the invention, the invention provides a method of replacing the flotation tank according to the first aspect of the invention in a tank module according to the second aspect of the invention, the method comprising steps of removing the flotation tank out from inside the self-supporting framework, and installing another flotation tank into the self-supporting framework. In one embodiment of the method, the steps of removing and installing the flotation tank are implemented by lifting the flotation tank. According to a tenth aspect of the invention, the invention provides a method of maintenance of a flota ¬ tion plant according to the third aspect of the inven ¬ tion, in which method an uppermost module is subject of maintenance. The uppermost module is hoisted up and transferred aside from the top of the lower module and the uppermost module is replaced by another uppermost module which is placed on top of the lower module.

According to an eleventh aspect of the invention, the invention provides a method a method of maintenance of a flotation plant according to the third aspect of the invention, in which method the lower module, which is underneath an uppermost module, is subject of mainte ¬ nance. The uppermost module is hoisted up from the top of the lower module and transferred aside for gaining access to the lower module.

In one embodiment of the method, while the uppermost module is away from the top of the lower module, maintenance operations are performed for the lower module .

In one embodiment of the method, while the uppermost module is away from the top of the lower module, the lower module is replaced by another lower module.

The embodiments of the invention described hereinbe ¬ fore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. An appa ¬ ratus, a method, a composition or a use, to which the invention is related, may comprise at least one of the embodiments of the invention described hereinbefore.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to pro ¬ vide a further understanding of the invention and constitute a part of this specification, illustrate em ¬ bodiments of the invention and together with the de ¬ scription help to explain the principles of the inven- tion. In the drawings:

Figure 1 is an axonometric view of a flotation tank according to a first embodiment of the invention, Figure 2 is a cross-section II-II from Figure 1,

Figure 3 is a side view of the flotation tank of Fig ¬ ure 1 , Figure 4 is a front view seen in a direction IV-IV from Figure 3,

Figure 5 is an axonometric view of a flotation tank according to a second embodiment of the invention,

Figure 6 is cross-section VI-VI from Figure 5,

Figure 7 is an axonometric view of a flotation tank according to a third embodiment of the invention,

Figure 8 is a side view of the flotation tank of Fig ¬ ure 7 ,

Figure 9 is a cross-section IX - IX from Figure 8, Figure 10 is a cross-section X-X from Figure 11 of a tank module according to a first embodiment of the in ¬ vention, Figure 11 is a cross-section XI-XI from Figure 10,

Figure 12 is side view of a tank module according to a second embodiment of the invention, and Figure 13 shows an elevation view of a flotation plant according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although flotation is disclosed in the following by reference to froth flotation, it should be noted that the principles of according to the invention can be implemented regardless of the specific type of the flotation, i.e. the flotation technique can be any of the known per se flotation techniques, such as froth flotation, dissolved air flotation or induced gas flo ¬ tation

Figures 1 to 9 show three examples of flotation tanks 1. Referring to all shown embodiments the froth flota- tion tank 1 comprises a self-supporting tank 2. The tank 2 is a rigid structure which is able to hold its form while it is handled, hoisted and transferred. The tank 2 is made of thermoplastic polymer. The tank 2 has a lower tank part 3 and a tapered upper tank part 4. The tapered upper tank part 4 is narrower than the lower tank part 3. The tank 2 has a mouth 5 at the up ¬ per end of the upper tank part 4 and an overflow lip 6 at the periphery of the mouth 5. The froth flotation tank 1 also comprises an overflow receptacle 7. The overflow receptacle 7 is also made of thermoplastic polymer and connected by welding to the tapered upper part 4 of the tank 2 beside the overflow lip 6. The overflow receptacle receives, collects and discharges an overflow that overflows from the tank 2 over the overflow lip 6, when the froth flotation tank is in use. The overflow receptacle 7 circumferentially sur- rounds the mouth 5 of the tank 2. The overflow recep ¬ tacle 7 and the tank 2 are welded together at their upper edges along the periphery of the mouth 5 in order to make a tight connection. As shown in Figures 1 and 2, additional weld holes 11 can also be made to the upper tank part 4 near to the upper end. The edges of the weld holes 11 can be welded to the inner wall 12 of the overflow receptacle 7 in order to secure the connection. Preferably the weld holes 11 have a diame ¬ ter 50 - 100 mm.

The overflow lip 6 comprises a separate lip member 8. The lip member 8 is can be connected, e.g. by bolted joints, to the upper part of the tank 2 at the region of the mouth 5 of the tank 2 at a desired suitable height in order to obtain a suitable overflow height.

The overflow receptacle 7 comprises a chute 9 to col ¬ lect the overflow, when in use, and at least one out ¬ let 10 at the lowest part of the overflow receptacle 7, and the chute 9 is inclined towards the outlet 10. The angle a of inclination of the chute 9, in relation to horizontal, is 5° - 30°, more preferably 8° - 20°, most preferably 10° - 15°, in order to ensure continu ¬ ous flow of the overflow by gravity.

The thermoplastic polymer material of the tank 2 and the overflow receptacle 7 is preferably polyethylene or polypropylene. The tank 2 and the overflow recepta ¬ cle 7 may be rotational molded parts. Preferably the tank 2 has a wall thickness which is 5 - 30 mm. The volume of the tank 2 is 0.5 - 20 m 3 , more preferably 1 - 15 m 3 , most preferably 1 - 8 m 3 . Figures 1 - 6 show two embodiments of froth flotation tanks both having the lower tank part 3 which has a rectangular cross-sectional shape.

Preferably the lower tank part 3 of the froth flota ¬ tion tank 1 has a rectangular cross-sectional shape when the volume of the tank 2 is greater than 8 m 3 . In the embodiment of Figure 1 the mouth 5 of the tank 2 is rectangular. In the embodiment of Figure 5 the mouth 5 of the tank 2 is circular. Due to its form the circular mouth 5 is stiff by nature.

Figure 7 shows an embodiment wherein the cross- sectional shape of the lower tank part 3 is circular and the mouth 5 is also circular. As shown in Figure 9, preferably the tank 2 has a circular cross-section, i.e. the tank 2 is cylindrical when the volume of the tank 2 is at most 8 m 3 . The overflow receptacle 7 is also circular.

Figures 10 - 12 show examples of a tank module 13. The tank module 13 comprises a self-supporting framework

14 which has an inner space 15. In this example four flotation tanks 1 are disposed successively in a row in the inner space 15 of the self-supporting framework

15 of the module. The flotation tanks 1 may be those which are shown and disclosed in relation to Figures 1 to 6. The tanks 2 of the flotation tanks 1 are in flu- id communication with each other. Each flotation tank 1 has a lip member 8 which is adjusted at a different height position in relation to the lip member 8 of another neighboring flotation tank 1. Figure 12 shows a tank module 13 in which there are cylindrical flotation tanks 1 according to Figures 7 to 9 arranged in the inner space 15 of the self- supporting framework 14.

Figure 13 shows an illustration of a flotation plant. The flotation plant comprises a tank module 13 and a drive module 16. The tank module 13 includes four flo ¬ tation tanks 1 as disclosed in relation to Figure 10.

The drive module 16 is removably stacked and aligned on top of the tank module 13. The drive module 16 in ¬ cludes four drive units 17 for the equipment immersed in the feedstock subject to flotation, the equipment performing the flotation action. The drive module 16 is a rigid and self-supporting unit capable of being transferable and hoistable as an integral entity.

The flotation plant comprises gas adding equipment for adding gas to the feedstock subject of flotation. The gas adding equipment for adding gas to the feedstock subject of flotation are in the flotation tank 1. The gas adding equipment includes a rotor 18 inside the flotation tank 1. The gas adding equipment includes a hollow drive shaft 19 rotatable by the drive unit 17, and the rotor 18 is connected to the drive shaft 19.

The flotation plant also comprises mixing equipment which includes the rotor 18 inside the flotation tank 1. The mixing equipment also includes a stator 20 in ¬ side the flotation tank 1. The flotation tank 1 has a bottom 21. The stator 20 is connected to the self- supporting framework 14 through the bottom 21.

Maintenance of the flotation plant is easy and can be performed quickly. If an uppermost module, the drive module 16, in the stack of modules is subject of maintenance, the uppermost module 16 is simply hoisted up and transferred aside and is replaced by another uppermost module 16. If the tank module 13, located underneath the uppermost drive module 16 is subject of maintenance, then the drive module 13 is hoisted up from the top of the tank module 13 and transferred aside for gaining access to the tank module. While the drive module 16 is away from the top of the tank mod ¬ ule 13, maintenance operations can be performed for the tank module. Alternatively, the tank module 13 can simply be replaced by another tank module 13.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The in ¬ vention and its embodiments are thus not limited to the examples described above, instead they may vary within the scope of the claims.