FIELD OF THE INVENTION

The present invention relates to a flotation tank. Further, the invention relates to a tank module. Fur- ther, the invention relates to a flotation plant. Further, the invention relates to uses of the flota ^¬ tion tank. Further, the invention relates to a method of replacing the flotation tank. Further, the invention relates to a method for manufacturing the flota- tion tank. Further, the invention relates to a method of replacing the flotation tank. Further, the invention relates to a method for manufacturing the tank module. 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 self-supporting tank being made of thermoplastic polymer by rotational molding, the self-supporting tank having a lower tank part and a tapered upper tank part which is narrower than the lower tank part, the self-supporting tank having a mouth at the upper end of the upper tank part and an overflow lip at the periphery of the mouth.

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 flo- tation (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 or other chemical. Gas can be added to the feedstock subject of flo ^¬ tation (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 equipment at the bottom of the flotation tank. In one embodiment gas adding equipment can in ^¬ clude a feedstock (slurry or pulp) jet for jetting the feedstock to air. In one embodiment gas adding equip ^¬ ment includes a rotor inside the flotation tank. In one embodiment gas can be added under the rotor. In one embodiment gas is added by a pipe ending under ro ^¬ tor. The pipe can be inside the flotation tank. The pipe can go through the bottom of the flotation tank. In one embodiment the rotor takes gas from the surface of sludge by vortex. In one embodiment gas is added by axis of the rotor. In one embodiment mixing equip ^¬ ment is arranged for mixing the slurry/pulp. Mixing 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 flotation tank comprises an overflow receptacle, the overflow receptacle being made of thermoplastic polymer and connected to the tapered upper part of the self- supporting tank beside the overflow lip for receiving, collecting and discharging an overflow that overflows from the self-supporting tank over the over-flow lip, when in use.

In one embodiment of the flotation tank, the overflow receptacle and the self-supporting tank are connected each other by welding. The technical effect is that the flotation tank has a good structural stiffness.

In one embodiment of the flotation tank, the overflow receptacle circumferentially surrounds the mouth of the self-supporting tank. The technical effect is that the flotation tank has a good structural stiffness,

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 self- supporting 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 flotation tanks can be made different with an adjustable lip member.

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- feet 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 . In one embodiment of the flotation tank, the self- supporting tank has a wall thickness which is 5 - 30 mm. The technical effect of the wall thickness within this range is that the flotation 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 self-supporting tank at its upper part makes it stiff so that the self-supporting tank is stiff despite the relatively thin wall. In one embodiment of the flotation tank, the volume of the self-supporting tank is 0.5 - 20 m ³ , more prefera ^¬ bly 1 - 15 m ³ , most preferably 1 - 8 m ³ . The technical effect is that the flotation tanks can be changed eas ^¬ ily as they are not too big and heavy. The flotation tanks are still big enough so that a significant vol ^¬ ume of capacity can be subjected to maintenance by changing a few flotation tanks. The maintenance opera ^¬ tions can be easily made for flotation 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 self-supporting tank is greater than 8 m ³ . The technical effect is that such flotation tanks can be supported by sidewalls of the self- supporting framework in the inner space of which the flotation tanks are installed in a tank module. The wall of the flotation tank can be supported against the sidewall of the framework so that the framework bears loads exerted by the hydrostatic pressure of the liquid filled inside the self-supporting tank.

In one embodiment of the flotation tank, the cross- sectional shape of the lower tank part is circular when the volume of the self-supporting tank is at most 8 m ³ . The technical effect is that the round form gives the required stiffness for the self-supporting 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 technical effect of the circular mouth is that it stiffens the structure of the flotation tank.

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 self- supporting tank and the overflow receptacle together is stiff to enable easy handling, lifting and mainte- nance.

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 flotation tank wall and thereby causes severely abrasive wear conditions.

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 self- supporting tank and the overflow receptacle are rota ^¬ tional molded parts. The technical effect is that with rotational molding it is easy to create stiff flota ^¬ tion tank forms from abrasive resistant plastics mate ^¬ rials so that the flotation 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 possible, for example in one embodiment of the flotation tank, the self- supporting 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/m - 0.4/m. The technical effect 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 method for manufacturing a flota ^¬ tion tank of the first aspect of the invention. The method comprises at least following steps:

fabricating a mold composed of two mold halves, comprising a first mold half and second mold half, said mold halves having walls, the inner surface of which corresponds to the outline of the flotation tank according to the first aspect of the invention;

- pouring a charge of a polymer resin material powder inside the first mold half when the second mold half is installed on the first mold half closing the mold;

- rotating the mold around two perpendicular axes;

heating the mold while it is rotating around the two perpendicular axes and melting the pol ^¬ ymer resin material and causing dispersing and sticking of the polymer resin to the wall of the mold as a layer; and

- cooling the mold and separating the mold halves from each other.

According to a third aspect of the invention, the in ^¬ vention provides a tank module comprising a self- supporting rectangular frame, said frame comprising a horizontal bottom, vertical side walls and vertical end walls, and at least one flotation tank according to the first aspect of the invention disposed inside the self-supporting rectangular frame. According to a fourth aspect, the invention provides a tank module comprising a self-supporting rectangular frame, said frame comprising a horizontal bottom, ver- tical side walls and vertical end walls, and said frame being designed to withstand hydrostatic pressure of a liquid filled in the tank module, a self- supporting tank arranged inside the tank module, said self-supporting tank being designed to provide protec ^¬ tion for the tank against chemical and/or physical stresses caused by said liquid, an overflow receptacle near an upper end of the tank for receiving froth created during flotation, a discharge pipe connected to the overflow receptacle for discharging the froth from the overflow receptacle. The self-supporting tank and the overflow receptacle are rotational molded parts.

The advantage of the invention is that the tank can be arranged as a light and economical structure by rota ^¬ tional molding. The shape of the self-supporting tank can be designed by flow simulation or flow modeling so that it is optimized for the flow inside the tank. By optimizing the shape of the self-supporting tank the flow will be guided towards the overflow receptacle and not against structures inside the tank. By opti ^¬ mizing the tank it is possible to eliminate or at least alleviate wearing of the tank and particles building up inside the tank or near the bottom of the tank. The self-supporting tank may be a separate part which can be easily replaced by a new one if the self- supporting tank is worn or broken. Maintenance is easier with a rotational molded part as the part can be fixed for example with patches. This means longer life time for the whole flotation tank and decreases maintenance time used in case of shutdown incident caused by the wearing of the self-supporting tank and the overflow receptacle. By using rotational molding in the manufacturing process of the self-supporting tank and the overflow receptacle, the manufacturing costs for the flotation tank are lower than as for example for a welded metal tank. The flotation tank does not require any painting inside which decreases costs and extra work.

In one embodiment of the tank module, the overflow re- ceptacle is arranged to have an angled bottom to guide the froth towards an outlet of the overflow recepta ^¬ cle. In one embodiment of the tank module the overflow receptacle is equipped with one or more outlets for a connection pipe to supply the froth from the overflow receptacle to the discharge pipe. In one embodiment of the tank module, the first outlet is pointing towards the end walls and/or second towards the bottom of the self-supporting tank. In one embodiment of the tank module, the outlet of the discharge pipe is towards the bottom of the self-supporting tank. In one embodiment of the tank module, the self-supporting tank is arranged with first over flow lip and/or second over flow lip on the upper part of a vertical part of a wall of the self-supporting tank for flowing the froth over the self-supporting tank to the overflow receptacle .

In one embodiment of the tank module, the tank module comprises a tank cover for each self-supporting tank for enabling gas tight structure of the flotation tank. The benefit of this feature is that when the flotation tank has the tank cover it is possible to add other equipment on the flotation tank easier. Another advantage is that when the flotation tank has the tank cover it is sealed by the cover and possible toxic or dangerous gases are kept inside the flotation tank and they are not released into the air.

In one embodiment of the tank module, the tank cover is a rotational molded part. By having a rotational molded tank cover the benefit is that it can be de ^¬ signed and manufactured to have optionally features that are later brought to use when these features are needed. For example blind slots can be reserved for other equipment that is optional and not always mount ^¬ ed on the flotation tank or used in the flotation pro- cess.

In one embodiment of the tank module, the tank cover comprises polyethylene or similar plastic polymer ma ^¬ terial which is suitable for rotational molding.

In one embodiment of the tank module, the tank cover comprises a first through hole for a flotation machinery . In one embodiment of the tank module, the tank cover comprises a second through hole for a suction pipe and/or a third through hole for a froth camera and/or a fourth through hole for a level sensor. The second through hole enables that the suction pipe can be pro- vided to the flotation tank. The benefit of the suc ^¬ tion pipe is that it removes the toxic and dangerous gases or fumes that may have been created during the process. The third through hole is needed for the froth camera which detects and measures the size of bubbles in the froth. The benefit of the froth camera is that the process can be controlled and the machin ^¬ ery can be adjusted according to the bubble size. The benefit of the fourth through hole is that it enables that the level sensor can be provided in the flotation tank. The benefit of the level sensor is that it can be used to continuously measure the level of the liq ^¬ uid inside the flotation tank and it may give a warn ^¬ ing and/or it may give info to the logic controlling the flotation process.

In one embodiment of the tank module, the tank module comprises at least two identical self-supporting tanks and at least two identical overflow receptacles. In one embodiment of the tank module, the tank module comprises at least two identical self-supporting tanks and at least two identical overflow receptacles and a tank cover for each self-supporting tank. The benefit of these embodiments is that the flotation tank has a modular structure. Modular structure means that the parts inside the frame comprise plurality of parts that are identical to each other. In one embodiment of the tank module, the tank comprises several identical self-supporting tanks, several identical overflow re ^¬ ceptacles and/or several identical tank covers inside the frame. In one embodiment of the tank module, sev ^¬ eral flotation tanks comprising a self-supporting tank, an overflow receptacle and a tank cover, wherein the flotation tanks are arranged inside the frame and then connected together. These features decreases the manufacturing costs of the tank assembly as many of the parts are the same and same time the assembly of the tank is simplified.

In one embodiment of the tank module, the tank module comprises at least one baffle between each self- supporting tank to provide accurate placing of the tank cover and/or which comprises lifting members. In one embodiment of the tank module, the lifting members are arranged to lift the tank module up.

In one embodiment of the tank module, the tank module comprises at least two flotation tanks. In one embodi ^¬ ment of the tank module, the flotation tank comprises a self-supporting tank, an overflow receptacle and a tank cover.

In one embodiment of the tank module, the neighboring flotation tanks of the at least two flotation tanks are in flow communication with each other. In one embodiment of the tank module, flow communication be- tween the neighboring flotation tanks is arranged via cut-outs on the sides of the self-supporting tanks of the flotation tanks. In one embodiment of the tank module, at least one of the self-supporting tank and the overflow receptacle comprise polyethylene or similar plastic polymer mate ^¬ rial which is suitable for rotational molding. In one embodiment of the tank module, polyethylene or similar plastic polymer has substantially chemical resistance against the liquid filled inside the flotation tank. Substantially chemical resistance means that the mate ^¬ rial may withstand the chemical stresses caused by the liquid which is filled in the flotation tank.

In one embodiment of the tank module, the tank module comprises one or more overflow receptacle connection pipe to supply the overflow from the overflow recepta ^¬ cle to the discharge pipe. In one embodiment of the tank module, the connection pipe connects to the out ^¬ let of the overflow receptacle.

In one embodiment of the flotation tank, at least one self-supporting tank is arranged to guide the fluid flow towards the overflow receptacle. The benefit of this embodiment is that the flow is directed towards the overflow receptacle and not towards some struc ^¬ tures inside the self-supporting tank. The benefit of this is that the wearing of the self-supporting tank is decreased.

According to a fifth aspect of the invention, the invention provides a method for manufacturing a flotation tank of the first aspect of the invention. The method comprises at least following steps:

- making cut-outs by cutting the cut-outs in ^¬ to at least two neighboring self-supporting tanks providing flow communication between the two neighboring self-supporting tanks; and

-connecting the neighboring self-supporting tanks to each other from the cut-outs by welding.

In one embodiment of the manufacturing method of the tank module, the frame is manufactured of metal. In one embodiment of the manufacturing method of the tank module, the frame is manufactured to form a rigid structure.

In one embodiment of the manufacturing method of the tank module, polyethylene or similar plastic polymer is manufactured to have substantially chemical re- sistance against the liquid filled inside the flota ^¬ tion tank.

In one embodiment of the manufacturing method of the tank module, cut-outs are cut into the self-supporting tanks to provide flow communication between the neighboring self-supporting tanks and/or a first hole for an inlet pipe and/or a second hole for an outlet pipe is/are cut into the self-supporting tank(s) . In one embodiment of the manufacturing method of the tank module, the neighboring self-supporting tanks are connected together. In one embodiment of the manufactur ^¬ ing method of the tank module, the inlet pipe and/or the outlet pipe is/are connected to the self- supporting tank(s) . In one embodiment of the manufac- turing method of the tank module, the overflow recep ^¬ tacle connection pipe is connected to the first outlet or to the second outlet of the overflow receptacle.

According to a sixth aspect of the invention, the in- vention provides method of replacing the flotation tank according to the first aspect of the invention in a tank module according to the third aspect of the in ^¬ vention, 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.

According to a seventh aspect of the invention, the invention provides a flotation plant comprising

- a tank module according to the third aspect of the invention, and

- a drive module, the drive module being removably stacked and aligned on top of the tank module and in ^¬ cluding at least two drive units for the equipment im- mersed 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 hoistable as an integral en ^¬ tity .

According to an eight aspect of the invention, the invention provides a method for maintenance of a flota ^¬ tion plant according to the seventh aspect of the invention. In which method an uppermost module is sub- ject 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.

In one embodiment of the method for maintenance of the flotation plant, in which method a lower module, which is underneath an uppermost module, is subject of maintenance, and 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 for maintenance of the flotation plant, wherein while the uppermost module is away from the top of the lower module, maintenance op ^¬ erations are performed for the lower module.

In one embodiment of the method for maintenance of the flotation plant, wherein while the uppermost module is away from the top of the lower module, the lower mod ^¬ ule is replaced by another lower module.

According to a ninth aspect of the invention, the invention provides use of the flotation tank according to the first aspect of the invention for separating material 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 tenth aspect of the invention, the in- vention provides use of the flotation tank according to the first aspect of the invention for separating solid material by froth flotation based on differences of hydrophilic properties of substances. Solid mate ^¬ rials separated by froth flotation could be oil sands, carbon, coal, talk, industrial minerals and mineral particles. The minerals may include industrial miner ^¬ als and ore. Froth flotation to solid material could be made based on natural hydrophilic/hydrophobic dif ^¬ ference or based on hydrophilic/hydrophobic differ- ences made by addition of a surfactant or collector chemical or other chemical.

According to an eleventh aspect of the invention, the invention provides use of the flotation tank according to the first aspect of the invention for concentrating ore by froth flotation. An ore is a type of rock that contains sufficient minerals with important elements including metals that can be economically extracted from the rock. Metal ores are generally oxides, sul ^¬ fides, silicates, or metals such as native copper or gold. Froth flotation of ore could be made based on natural hydrophilic/hydrophobic difference or based on hydrophilic/hydrophobic differences made by addition of a surfactant or collector chemical or other chemical . According to a twelfth aspect of the invention, the invention provides use of the flotation tank according to the first aspect of the invention for flotation of substances containing abrasive material. The abrasive mineral may be, for example, pyrite, silica, chromite. The drive module 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 a thirteenth aspect of the invention, the invention provides use of the flotation tank according to the first aspect of the invention for froth flota ^¬ tion of ore containing pyrite, silica, chromite. Use of the flotation tank which is easy to maintenance and is preferably 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 con ^¬ taining pyrite, silica, chromite.

The tank described here has advantages over the known solutions. The manufacturing of the flotation tank with rotational molding is more economical and faster than manufacturing it with the traditional way. This saves costs from the manufacturing process and also makes the repairing of the flotation tank easier. The flotation tank is not sensitive to corrosion and does not require painting or sand blasting as it is made of polyethylene or similar plastic polymer. Due to the low weight of the flotation tank, the flotation tank is easier to handle. Rotational molding gives more freedom of designing the shape of the self-supporting tank and the complete flotation tank.

As a manufacturing method of the flotation tank, rotational molding is fast, less costly and less vulnera ^¬ ble to poor craftsmanship. 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 embodiments of the invention and together with the de- scription help to explain the principles of the inven ^¬ tion. In the drawings:

Figure 1 is a cross section illustration of an example of a flotation tank module without one side wall of the frame on the front side of the tank; and

Figure 2 is a simplified illustration of an example of a flotation tank; Figure 3 is a cross-section of a flotation tank module with four self-supporting tanks illustrated on the front side of the flotation tank module;

Figure 4 is a schematical illustration of an example of a tank module being the lower module equipped with a drive module being an upper module; Figure 5 is an axonometric view of an example of a flotation tank;

Figure 6 is a cross-section II-II from Figure 1;

Figure 7 is an axonometric view of an example of a flotation tank;

Figure 8 is cross-section VI-VI from Figure 5;

Figure 9 is an axonometric view of an example of a flotation tank;

Figure 10 is a side view of the flotation tank of Fig- ure 7;

Figure 11 is a cross-section IX - IX from Figure 8;

Figure 12 is a cross-section X-X from Figure 11 of an example of a tank module; and

Figure 13 shows an elevation view of an example of a flotation plant. 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 flotation Figure 1 illustrates an example of a froth flotation tank module 13 without one side wall 3a' of the tank so as to show all the parts included in the tank. Fig ^¬ ure 1 illustrates one example of froth flotation tank module 13 comprising a self-supporting rectangular frame 1', said frame comprising a horizontal bottom 2', vertical side walls 3', 3a' and vertical end walls 4', 4a', and said frame 1' being designed to withstand hydrostatic pressure of a liquid filled in the tank. A self-supporting tank is 5' arranged inside the flo ^¬ tation tank module 13, said self-supporting tank 5' being designed to provide protection for the tank 5' against chemical and/or physical stresses caused by said liquid, an overflow receptacle 6' near an upper end of the tank 5' for receiving froth created during flotation, a discharge pipe 7' connected to the over- flow receptacle 6' for discharging the froth from the overflow receptacle 6'. The self-supporting tank 5' and the overflow receptacle 6' are rotational molded parts . The flotation machine is used for floating hydrophobic particles from aqueous slurry that contains these par ^¬ ticles such as ore or precious metals. Figure 4 illus ^¬ trates a tank module 13 being a lower module equipped with a drive module 16 being an upper module compris- ing flotation machinery 10' and with some additional components. Figure 4 illustrates that the froth flota ^¬ tion machine comprises a self-supporting tank 5' that comprises a wall 24' that forms vertical side walls and a bottom for the said self-supporting tank 5'. In Figure 4 the self-supporting tank 5', overflow receptacle 6' and tank cover 8' are illustrated as a sec ^¬ tional view and the frame 1' is illustrated without the end walls 4', 4a'. Figure 4 further illustrates that in the froth flotation machines the machinery 10' comprises an air distribution and mixing device 25', which is arranged in the middle part of the self- supporting tank 5' to distribute air into the slurry for forming froth and mixing the slurry in the froth flotation tank 1. The air distribution and mixing device 25' includes a rotor part 26' that comprises air distribution apertures 27'. A drive shaft 28' is ar- ranged so as to extend vertically in the froth flota ^¬ tion tank, the rotor part 26' being attached to the lower end of the drive shaft 28'. The drive shaft 28' comprises a hollow interior 29' which constitutes a flow channel for conducting flotation air to the air distribution apertures 27' of the rotor part 26'. The hollow interior 29' is illustrated as partial section ^¬ al view in Figure 4. Usually an electric motor 30' is provided in the froth flotation machinery 10' and the electric motor 30' is arranged for rotating the drive shaft 28'.

Figures 1 to 3 are described without the machinery 10' and its part i.e. the rotor part 26', drive shaft 28', electric motor 30' etc. because they do not belong in the tank assembly.

Figures 1 and 2 illustrate that the overflow recepta ^¬ cle 6' is arranged to have an angled bottom 18' to guide the froth towards one or more outlets 19', 19a' of the overflow receptacle 6'. The overflow receptacle 6' is equipped with one or more outlets 19', 19a' for a connection pipe 17' to supply the froth from the overflow receptacle 6' to the discharge pipe 7'. The first outlet 19' is pointing towards the end walls 4', 4a' and/or second outlet 19a' towards the bottom 2'. By using two outlets 19', 19a' which are pointing in different direction it is possible to select either one of the openings 19', 19a' to be connected to the discharge pipe 7'. This feature is advantageous be- cause it enables a modular structure of the overflow receptacle 6' as the same part can be used for both purposes. The outlet of the discharge pipe 7' is to ^¬ wards the bottom 2' of the tank.

Figure 2 illustrates that the tank comprises a tank cover 8' for each self-supporting tank 5' for enabling gas tight structure of the flotation tank 1. Tank cover 8' enables that there is possible to add other equipment on the flotation tank 1. If the liquid consists of toxic or other dangerous material the tank cover 8' enables that the dangerous and toxic gases or fumes are not in contact with the working atmosphere in the facility where tank is used.

Also the tank cover 8' is a rotational molded part. The tank cover 8' comprises polyethylene or similar plastic polymer material which is suitable for rota ^¬ tional molding. The suitable material for the tank cover 8' is selected also according to the liquid which is to be filled in the tank.

Figure 4 illustrates that the tank cover 8' comprises a first through hole 9' for a froth flotation machinery 10'. The drive shaft 28' of the froth flotation machinery 10' is placed through the first through hole 9 ' .

The tank cover 8' comprises a second through hole 11' for a suction pipe 12' and/or a third through hole 13' for a froth camera 14' and/or a fourth through hole 15' for a level sensor 16'. The second through hole 11' enables that the suction pipe can be provided to the tank. Suction pipe 12' removes the toxic and dan ^¬ gerous gases or fumes that may have been created dur ^¬ ing the process. The third through hole 13' is needed for the froth camera 14' which detects and measures the size of bubbles in the froth. Froth camera 14' en ^¬ ables that the process can be controlled and the ma- chinery 10' can be adjusted according to the bubble size. The fourth through hole 15' enables that the level sensor 16' can be provided in the tank. The lev ^¬ el sensor 16' can be used to continuously measure the level of the liquid inside the tank and it may give a warning and/or it may give info to the logic control ^¬ ling the froth flotation process.

The self-supporting tank 5' is arranged with a first over flow lip 23' and/or a second over flow lip 23a' on the upper part of the wall 24' of the self- supporting tank for flowing the froth over the self- supporting tank 5' to the overflow receptacle 6'. The overflow receptacle comprises a chute 9. When flota- tion machinery 10' is operational and flotation pro ^¬ cess is operation the froth will start to froth over the over flow lips 23', 23a' and further to the over ^¬ flow receptacle 6'. This feature enables the froth and the particles in the froth to be collected to the overflow receptacle 6' and to the chute 9.

Figures 1 and 3 illustrate that the tank comprises at least two identical self-supporting tanks 5', 5a' and at least two identical overflow receptacles 6', 6a'. If the tank cover 8', 8a' is needed it is provided for each self-supporting tank 5', 5a'. In case of figures 1 and 3 four pes of each previously mentioned items are provided inside the frame 1'. The tank module 13 may have a modular structure which means that the parts inside the frame 1' comprise a plurality of parts that are identical to each other. In this case there can be several identical self-supporting tanks 5', 5a', several identical overflow receptacles 6', 6a' and/or several identical tank covers 8', 8a' in- side the frame 1'. The size of the frame 1' can be de ^¬ termined according to the quantity of the self- supporting tanks 5', 5a'. It is possible to arrange at least one baffle (not shown) between each self-supporting tank 5', 5a' to provide accurate placing of the tank cover 8', 8a' and/or which comprises lifting members. The lifting members (not shown) are arranged to lift the tank up. For example lift hooks (not shown) can be arranged to the lifting members. Figure 3 illustrates that the neighboring self- supporting tanks 5', 5a' are in flow communication with each other and figure 2 illustrates in detail that flow communication between the neighboring self- supporting tanks 5', 5a' is arranged via cut-outs 20', 20a' on those sides of the neighboring self-supporting tanks 5', 5a, which are against of each other. This feature is enabled when there are at least two self- supporting tanks 5', 5a' provided in the tank module 13.

The self-supporting tank 5' and/or overflow receptacle 6' comprises polyethylene or similar plastic polymer material which is suitable for rotational molding. The suitable material for the self-supporting tank 5' and overflow receptacle 6' is selected also according to the liquid which is to be filled in the flotation tank 1. Polyethylene or similar plastic polymer has sub ^¬ stantially chemical resistance against the liquid filled inside the tank. Substantially chemical re- sistance means that the material may withstand the chemical stresses caused by the liquid which is filled in the flotation tank 1. This means that the material has certain features that for example prevent the liq ^¬ uid to corrode the material in such a way that the surface of the flotation tank 1 is destroyed in short time . Figure 1 illustrates that the tank comprises one or more overflow receptacle 6' connection pipe 17' to supply the froth from the overflow receptacle 6' to the discharge pipe 7'. The connection pipe 17' con- nects to the outlet 19', 19a' of the overflow recepta ^¬ cle 6 ' .

At least one self-supporting tank 5' is arranged to guide the fluid flow towards the overflow receptacle 6'. This enables that the flow is directed towards the overflow receptacle 6' and not towards some structures inside the self-supporting tank 5' which decreases the wearing of the self-supporting tank 5'. A method for manufacturing a froth flotation tank module 13 comprising a self-supporting rectangular frame 1', said frame 1' comprising a horizontal bottom 2', vertical side walls 3', 3a' and vertical end walls 4', 4a', and said frame 1' being designed to withstand hy- drostatic pressure of a liquid filled in the flotation tank 1, one or more self-supporting tank 5', 5a' arranged inside the flotation tank module 13, said self- supporting tank 5', 5a' being designed to provide pro ^¬ tection for the tank 5', 5a' against chemical and physical stresses caused by said liquid, one or more overflow receptacle 6', 6a' near an upper end of the self-supporting tank 5', 5a' for receiving froth created during flotation, one or more discharge pipe 7', 7a' connected to the overflow receptacle 6', 6a' for discharging the froth from the overflow receptacle 6', 6a'. In the method the self-supporting tank 5', 5a' and the overflow receptacle 6', 6a' are manufactured with rotational molding. Rotational molding is an established mass-production method which is more economical than painting or coat ^¬ ing with rubber. In rotational molding the manufactur- ing is performed in a mold. The surfaces of the mold can be made very smooth and all the edges can be rounded . The method of rotational molding can be described as following: the method comprises fabricating a mold composed of two mold halves, comprising a first mold half and second mold half, said mold halves having walls, the inner surface of which corresponds to the outline of the self-supporting tank 5' or the overflow receptacle 6 ' or the tank cover 8 ' . In the method the mold is supported to be rotational about two perpen ^¬ dicular axes. In the method a charge of a polymer res ^¬ in material powder is poured inside the first mold half. In the method the second mold half is installed on the first mold half to close the mold. In the meth ^¬ od the mold is rounded and rotated around the two per ^¬ pendicular axes. In the method the mold is heated while it is rotating around the two perpendicular axes to melt the polymer resin material and to disperse and stick it to the wall of the mold as a layer having a substantially even thickness and forming the wall 24' of the self-supporting tank 5' or a wall of the overflow receptacle 6' or the tank cover 8'. In the method the heating of the mold is stopped and the mold is cooled. After this the mold is opened by separating the mold halves and removing the self-supporting tank 5' or the overflow receptacle 6' or the tank cover 8' from the mold. If needed some finishing operation may be needed on the self-supporting tank 5' or the overflow receptacle 6' or the tank cover 8'.

The frame 1' is manufactured of metal to form a rigid structure .

The self-supporting tank 5', 5a' and/or overflow receptacle 6', 6a' is/are manufactured of polyethylene or similar plastic polymer which is suitable for rotational molding. Polyethylene or similar plastic poly ^¬ mer is manufactured to have substantially chemical re ^¬ sistance against the liquid filled inside the tank.

Cut-outs 20', 20a' are cut into the self-supporting tanks 5', 5a' to provide flow communication between the neighboring self-supporting tanks 5', 5a' and/or a first hole 21' for an inlet pipe 22' and/or a second hole 21a' for an outlet pipe 22a' is/are cut into the self-supporting tank(s) 5', 5a'. The neighboring self- supporting tanks 5', 5a' are connected together e.g. by welding with a plastics extruding equipment. The inlet pipe 22' and/or the outlet pipe 22a' is/are con- nected to the self-supporting tank(s) 5', 5a'. This means that if the tank has only one self-supporting tank 5' the inlet pipe 22' and outlet pipe 22a' are both connected to the same self-supporting tank 5'. If the tank has only one self-supporting tank 5' the cut- outs 20', 20a' are not cut into the self-supporting tank 5'. If the tank has for example two or more self- supporting tanks 5', 5a' the inlet pipe 22' is con ^¬ nected to the first hole 21' of the first self- supporting tank 5' and the outlet pipe 22a' is con- nected the second hole 21a' of the second self- supporting tank 5a'. If the tank module 13 has more than two self-supporting tanks 5', 5a' the inlet pipe 22' is connected to the first hole 21' of the first self-supporting tank 5' and the outlet pipe 22a' is connected the second hole 21a' of the last self- supporting tank which is the case in figures 1 and 3. Also the overflow receptacle 6' connection pipe 17' is connected to the first outlet 19' or to the second outlet 19a' of the overflow receptacle 6' e.g. by welding with a plastics extruding equipment. Figures 5 to 11 show three examples of flotation tanks 1. Referring to Figure 5 to 10 the froth flotation tank 1 comprises a self-supporting tank 2. Figures 5 to 11 show three examples of froth flotation tanks 1. The self-supporting tank 2 is a rigid struc ^¬ ture which is able to hold its form while it is han ^¬ dled, hoisted and transferred. The self-supporting tank 2 is made of thermoplastic polymer. The self- supporting 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 self- supporting tank 2 has a mouth 5 at the upper end of the upper tank part 4 and an overflow lip 6 at the pe- riphery 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 self-supporting tank 2 beside the overflow lip 6. The overflow receptacle receives, collects and dis ^¬ charges an overflow that overflows from the self- supporting tank 2 over the overflow lip 6, when the froth flotation tank is in use. The overflow receptacle 7 circumferentially surrounds the mouth 5 of the self-supporting tank 2. The overflow receptacle 7 and the self-supporting 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 diameter 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 self-supporting tank 2 at the region of the mouth 5 of the self-supporting 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 self- supporting tank 2 and the overflow receptacle 7 is preferably polyethylene or polypropylene. The self- supporting tank 2 and the overflow receptacle 7 may be rotational molded parts. Preferably the self- supporting tank 2 has a wall thickness which is 5 - 30 mm. The volume of the self-supporting tank 2 is 0.5 - 20 m3, more preferably 1 - 15 m3, most preferably 1 - 8 m3. Figures 5 - 8 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 self-supporting tank 2 is greater than 8 m3. In the embodiment of Figure 5 the mouth 5 of the self-supporting tank 2 is rectangular. In the embodiment of Figure 7 the mouth 5 of the self- supporting tank 2 is circular. Due to its form the circular mouth 5 is stiff by nature. Figure 9 and 10 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 11, preferably the self-supporting tank 2 has a circu- lar cross-section, i.e. the self-supporting tank 2 is cylindrical when the volume of the self-supporting tank 2 is at most 8 m3. The overflow receptacle 7 is also circular. Figures 12 show an example 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, for example, those which are shown and disclosed in rela ^¬ tion to Figures 5 to 8. The tanks 2 of the flotation tanks 1 are in fluid communication with each other. Each flotation tank 1 has a lip member 8 which is ad- justed at a different height position in relation to the lip member 8 of another neighboring flotation tank 1.

Figures 13 and 14 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 flotation tanks 1.

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 is 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.

Referring to Figure 14, a froth flotation plant is built to a modular three-storeyed structure. The froth flotation plant has been assembled from self- supporting modules that when assembled together form a modular froth flotation plant having superimposed storeys. The self-supporting modules are transferable and hoistable as integral units and stackable on top of each other to form the storeyed structure. The modules from which the froth flotation plant has been built are removably stacked on top of each other to form the three-storeyed structure having a first storey I at the bottom, a second storey II in the middle above the first storey I and a top storey III above the second storey II. The foundation module 22 is a rigid and self- supporting unit that can be transferred and hoisted as an integral entity. The tank module 13 and the drive module 16 are stacked on top of the foundation module 22. The flotation plant may be arranged on top of pillars 23.

The flotation plant illustrated in Figure 14 comprises two flotation units A in succession. The successive flotation units A are in fluid communication with each other. The flotation plant may comprise a conditioner 46 for conditioning the feedstock subject of flotation before flotation operations. The conditioner 46 is disposed to feed the conditioned feedstock to a first one of the flotation units A in the direction of flow in the succession of the flotation units A.

One of the technical effects is that as the modules are self-supporting units they can be transported, transferred and hoisted as integral entities. The mod- ules can be assembled and furnished at the site of manufacture, e.g. in an engineering workshop, and then transported to the site of installation as integral entities. During transportation, hoisting and use the equipment furnished into the modules are well protect- ed inside the self-supporting framework which acts as a delivery package and thereby eliminates need for separate transportation packages for the equipment. At the site of installation the modules may be placed on top of each other to form the complete flotation unit or a plant having several flotation units in succes ^¬ sion. Installing the flotation units at the installation site can be made safely and quickly with a small amount of labor and in short time. Capacity of the flotation plant can be easily increased by adding mod ^¬ ules to a flotation unit or by adding flotation units. The modules and various equipment contained therein is accessible and easily replaceable for maintenance. Af ^¬ ter the dedicated plant lifecycle, the dismantling of the unit/plant can be made quickly.

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.