BACKGROUND OF THE INVENTION The invention relates to a flotation separa- tion method and system known by the English designa- tion Separation In Froth (SIF), designed for concen- tration of coarse minerals and for typical separation functions in recirculation technology. The SIF method and a preferable SIF froth flotation apparatus are de- scribed e. g. in specification WO 00/51744.

From literature it is known that the practice of conveying the slurry directly to froth was first introduced be Malinovsky, V. A. (1961) "Selective re- covery of the hydrophobic and hydrophobized particles and some surface active agents by separation in froth". DAN SSSR 141,420-423. Furthermore, specifica- tions US 3,434, 596, US 4,274, 949, US 4,469, 591 and US 3,815, 739 describe SIF devices.

The'Separation In Froth (SIF) method is based on separation of materials in froth phase. An essen- tial feature in the method is that ground feed mate- rial treated with chemicals is passed in a SIF flota- tion apparatus directly into froth. Thus, hydrophobic particles are caught in the froth and drift with it out of the flotation apparatus, whereas hydrophilic particles pass through the froth to the slurry layer

below it. In this way, an overflow to be removed with the froth phase and an underflow from the solution be- low are obtained. Separation thus occurs completely in the froth phase, where the dwell time is generally of the order of only 10 s.

The SIF method differs substantially from conventional flotation methods, wherein the contact between particles and bubbles is generated in the slurry and the ascension of particles together with air bubbles to the froth layer is a critical stage in respect of grain size. By the SIF method, it is possi- ble to separate particles considerably coarser than the conventional flotation fineness, depending on the surface properties and the particle form and density of the material. Depending on the mineral, the maximum grain size in concentration is about 3 mm. In mineral concentration, the SIF method is advantageously appli- cable for treatment of the coarse fraction in the classification (screen, cyclone) in grinding circuits and in general in cases where the material is pure ground already in a coarse grain size.

The SIF method has the advantage that the processes can be substantially simplified, thereby achieving savings in grinding energy because a large grain size is acceptable and in chemical consumption because the method uses smaller amounts of flotation chemicals than conventional processes. Furthermore, due to the short dwell time in the SIF apparatus, the process is fast.

In the SIF method, it is preferable to use a high slurry density, which is generally of the order of 50-70%. Therefore, the slurry is generally pre- thickened e. g. by means of a spiral classifier. In some cases the slurry density is sufficiently high by nature, requiring no pre-thickening.

The problems in the prior-art SIF method are related to the preparation of a thick slurry with flo-

tation chemicals. The purpose of preparation is to bring the chemicals to the surface of the minerals.

Previously known is the use of a drum-type preparator in the SIF process. The use of a drum-type preparator has been resorted to because the mixing properties of conventional preparators are generally not applicable for use in the case of the aforesaid high slurry den- sity. This is because, due to the high local mixing energy, they tend to develop sludge, which has many adverse effects on the operation of the SIF process.

For this reason, the only suitable method so far has been to perform the preparation using a drum-type mixer, in which the formation of sludge is insignifi- cant due to a low speed or rotation.

The drum of a rotary drum mixer is usually a cylindrical container arranged to be rotated about its substantially horizontal center axis. The interior wall of the drum is provided with fixed mixing vanes to achieve a better mixing effect. The rotary drum mixer is a continuous-action mixer. The slurry to be mixed is fed into the drum via one end and delivered from the other end. The slurry flow pattern in the preparator is a so-called plug flow pattern.

A disadvantage is that the rotary drum mixer is a device that requires a large floor area. This is a problem especially in the production areas of an ex- isting flotation plant where the processes are renewed to use the SIF method but the buildings are not to be expanded for this purpose. In addition, the drum of a drum-type preparator can be filled only partially be- cause usually about two thirds of its inside space has to be left empty. Thus, the about 30-% degree of fill- ing of the drum is poor. A further problem is that the preparation time is relatively long.

OBJECT OF THE INVENTION The object of the present invention is to overcome the above-mentioned drawbacks.

A specific object of the invention is to dis- close a froth flotation method and an apparatus wherein the preparator takes up as little space as possible.

A further object of the invention is to dis- close a froth flotation method and an apparatus wherein the slurry preparation time is as short as possible.

BRIEF DESCRIPTION OF THE INVENTION The method of the invention for separation in froth is characterized by what is disclosed in claim 1. Further, the apparatus of the invention for separa- tion in froth is characterized by what is disclosed in claim 9. The use of a helical rotor mixer is charac- terized by what is disclosed in claim 19.

According to the invention, in the prepara- tion stage of the method, the slurry is brought into a flowing motion with an axial vertically circulating flow pattern.

According to the invention, the preparator in the apparatus is a helical rotor mixer.

The invention has the advantage that the pre- parator producing the aforesaid flow pattern, in prac- tice the helical rotor mixer, being mounted in an up- right position, only takes up little space. Existing flotation plants can be adapted for the SIF process without building additional space. The degree of fill- ing of the preparator used is high because the con- tainer of the helical rotor mixer is preferably filled completely with slurry to be prepared. The dwell time of slurry in the preparator is short, allowing the

preparation stage of the SIF process to be substan- tially accelerated.

In an embodiment of the method, the axial vertically circulating flow is produced by a helical rotor mixer having a double helix rotor provided with two spiral tubes of round cross-section twisted around a vertical center axis of rotation at a constant ra- dial distance.

In an embodiment of the method, the helix an- gle of the spiral tubes is selected to be 15°-50°.

In an embodiment of the method, the particle size of the coarse-grained material is selected to be at most about 3 mm.

In an embodiment of the method, the slurry is formed to a slurry density of 50-70%.

In an embodiment of the method, the slurry is thickened before preparation.

In an embodiment of the method, the mixing efficiency is adjusted by varying the speed of rota- tion of the helical rotor, thereby changing the flow velocity of the circulating flow.

In an embodiment of the method, the speed of rotation of the helical rotor is so adjusted that the flow velocity is at most 2.0 m/s, preferably at most 1.0 m/s.

In an embodiment of the apparatus, the prepa- rator comprises a container whose interior space is defined laterally by a cylindrical vertical side wall and below by a planar bottom. A double helix rotor is arranged centrically in the interior space of the con- tainer. A power means is provided to rotate the double helix rotor at a predetermined speed of rotation.

Mounted on the side wall inside the container are a number of elongated vertical flow inhibitors protrud- ing from the side wall towards the center axis of the container.

In an embodiment of the apparatus, the double helix rotor comprises a vertical shaft, which is con- nected to the power means. Two identical spiral tubes of round cross-section are secured to the shaft by means of supporting arms opposite to each other mutu- ally symmetrically at a radial distance from the shaft.

In an embodiment of the apparatus, the helix angle of the spiral tubes is 15°-50°.

In an embodiment of the apparatus, the rotor diameter equals 0.5-0. 8, preferably 0.65-0. 7 times the inner diameter of the container.

In an embodiment of the apparatus, the diame- ter of the double helix rotor equals 0.5-0. 8, pref- erably 0.65-0. 7 times the inner diameter of the con- tainer.

In an embodiment of the apparatus, the spiral tubes twist around the vertical shaft through 1/2, 5/8,2/3, 3/4,7/8 or 1 turn.

In an embodiment of the apparatus, the diame- ter of the spiral tubes equals 0.04-0. 07 times the rotor diameter.

In an embodiment of the apparatus, the flow inhibitor has a width equaling 1/12-1/9 times the inner diameter of the container.

In an embodiment of the apparatus, the flow inhibitor and the container wall are separated by a circumferential clearance of a width equal to 0.01- 0.04 times the inner diameter of the container.

In an embodiment of the apparatus, the number of flow inhibitors is 3-12 pcs, preferably 6-8 pcs.

LIST OF FIGURES In the following, the invention will be de- scribed in detail with reference to embodiment exam- ples and the attached drawing, wherein

Fig. 1 presents a diagram representing the principle of a prior-art SIF process, Fig. 2 represents a SIF process according to an embodiment of the SIF process of the invention, and Fig. 3 presents a vertical section of the preparator used in the SIF process in Fig. 2.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 represents a prior-art SIF process wherein a coarse-grained material having a particle size of the order of about 0.1-3 mm is first thick- ened by means of a thickening device 17, e. g. a spiral classifier, to a slurry density of 50-70%. The thickened slurry is then fed into a preparator 1, where the slurry is prepared substantially without forming sludge in it, in other words, without causing the particles to be ground into finer fractions. As stated above, the preparator 1 used in prior art is a rotary-drum preparator with a preparator drum rotating about a horizontal axis. From the preparator drum the slurry is passed via a feeder 3 into a flotation sepa- rator 2, which contains a froth phase and a liquid phase under the froth phase. The slurry is fed into the flotation separator 2 so that it comes directly into the froth phase, with the result that hydrophobic particles are caught in the froth and can be removed from the device as a froth overflow by first removal means 4. Hydrophilic particles sink through the froth into the liquid phase under it and can be removed from the device by second removal means 5.

The SIF process according to the invention presented in Fig. 2 is similar to the process illus- trated in Fig. 1, reference being made to the descrip- tion thereof, but differs from it only in respect of the preparator 1. Here, preparation is performed by means of a compact preparator using a helical rotor, wherein the helical rotor sets the slurry in vertical

axial flow. A feature characteristic of this mixing arrangement is that the helical rotor has a relatively large size in relation to the preparator. The rotor itself covers 25-55% of the effective volume of the whole preparator and preferably 35-45% of this vol- ume. Effective volume'refers to the volume remaining within the flow inhibitors. This size is so large that, rotating in a lifting direction, the mixer pro- duces a circulating flow moving upwards at the circum- ference of the preparator and downwards in the central part. The circulating flow runs heavily directly to- wards the bottom and, assisted by the lower supports of the helical mixer, turns towards the circumference while simultaneously diverging in every direction, to turn upwards again here near the bottom. Similarly, having reached a level near the surface, the circulat- ing flow turns towards the center and here from near the surface further downwards. According to our inven- tion, by using a large lifting-action mixer having a structure as described below and a preparator adapted to the size and use of the mixer, very uniform mixing extending throughout the entire slurry volume of the preparator is achieved.

The mixing intensity is adjusted by varying the helical rotor mixer's speed of rotation, which has a direct effect on the flow rate of the circulating flow and therefore on the turbulence appearing in it.

The intensity of uniform mixing can be adjusted within wide limits, which is due to the size and structure of the mixer, among other things. There is no actual low limit to the flow rate, while the high limit lies be- tween 1.5-2. 0 m/s. However, the remixing time is quite short even at considerably lower flow rates. In most cases of preparation, it is appropriate to reduce the circulating flow to a rate below 1.0 m/s, down to a rate between 0.2 m/s and 0.6 m/s, which has been found to produce a sufficient mixing effect. The mix-

ing is improved by the fact that there appear in the circulating flow cross flows produced by the support- ing arms extending from the shaft of the mixer to the helix tubes.

In SIF preparation it is important to avoid sludge formation, i. e. grinding of solid matter as a result of locally intensive mixing. As explained later on, the helical rotor according to our invention which is used as a mixer has expressly a structure that minimizes the grinding effect. The rotor is con- structed from round tubes and shaped as a symmetrical double helix, by the selection of whose helix angle it is possible to influence the mixing angle meeting the slurry. In addition to what was stated above, espe- cially the relatively large size of the helical rotor greatly reduces the grinding of the slurry expressly because no locally intensive mixing occurs. Mixing continues to be uniform and controlled even when the size of the preparator is increased, which would not be possible if a smaller mixer rotating in a limited preparator space were used. This can be established by way of example by observing how the circumferential speed of the helical mixer increases as the preparator size increases when the diameter of the helical mixer equals 70 % of the diameter of the preparator. When the circumferential speed lies between 2.2 m/s and 4.0 m/s in a 10-m3 preparator and between 2.5 m/s and 4.5 m/s in a 50-m3 preparator, a range of 3.0 m/s to 5.2 m/s is sufficient in a large 300-m3 preparator. Let it be stated that in the aforesaid preparator volumes e. g. circumferential speeds of 3.0 m/s, 3.5 m/s and 4.0 m/s give the same average mixing intensity when the preparator volumes are taken into account in their entirety.

In preparation it is important that the con- tents of the preparator be thoroughly mixed in a uni- form manner to ensure that no part of the supply is

passed through the preparator as an incompletely mixed cross flow. In such a case the preparation is insuffi- cient, and consequently the preparation chemicals have not been applied correctly to the intended mineral surfaces. According to our invention, the prevention of the occurrence of this situation has been ensured by introducing controlled surface feed for the slurry to be prepared and the preparation chemicals. The most preferable method is to feed the slurry and chemicals from above to the surface of the preparator, the slurry to one side and the chemicals to the other side, the feed points being disposed symmetrically or nearly symmetrically relative to each other. The best feed points are located in the vicinity of the verti- cal flow inhibitors, near the inner edge of these and preferably obliquely relative to the radial direction of the preparator against the direction of rotation of the helical mixer. At these points, the surface flow is particularly intensive and is directed in a spiral fashion towards the center. Using this feed arrange- ment, the supplies meet at the center of the prepara- tor and are drawn down by suction from the surface, being simultaneously mixed together in the flow to- wards the bottom. In fact, this feed method improves the performance characteristics of the preparator from the performance efficiency of a conventional"back- mixed"reactor because no part of the feed can cross directly or nearly directly out of the preparator. Let it be further stated that, already for technical rea- sons of mixing, the number of vertical flow inhibitors is mainly 6-8 pcs, which allows several chemicals to be fed, if necessary, in the vicinity of adjacent flow inhibitors.

A fully developed surface flow of correct type is achieved when the helical rotor does not ex- tend to the surface. It is desirable that the upper ends of the rotor should extend to a surface distance

of 30-90 cm from the cover of the container, depend- ing on the size of the preparator. Let it be stated that the bottom clearance of the helical rotor is of the order as the surface distance, yet often smaller, but not less than half the said surface distance. Fig.

3 gives a more detailed illustration of the structure of the helical rotor mixer 1.

Fig. 3 shows the preparator 1 used in the SIF process in Fig. 2. The preparator 1 comprises a con- tainer 6, whose interior space 7 is defined laterally by a cylindrical vertical side wall 8 and below by a bottom 9. A double helix rotor 10 is disposed centri- cally in the interior space 7, to be rotated by a power means 11. From the side wall 8 of the container 6 there extend towards the interior space 7 a number of elongated vertical flow inhibitors 12.

The double helix rotor 10 comprises a verti- cal shaft 13, which is connected to the power means 11. In addition, the rotor 10 comprises two identical spiral tubes 14,15, i. e. the aforesaid helix tubes, which are of round cross-section and fastened to the vertical shaft 13 by means of supporting arms 16 oppo- site to each other symmetrically relative to each other at a radial distance from the shaft. The diame- ter of the double helix rotor 10 is relatively large in relation to the diameter of the preparator, gener- ally 0.5-0. 8 times the preparator diameter and pref- erably 0.65-0. 70 times the preparator diameter.

In the example case in Fig. 3, both of the mutually symmetrical helix tubes 14,15 ascend through half a turn around the shaft 13, this helix angle be- ing between 30°-40°, which is advantageous to use when solid matter is to be flotated. However, the height of the preparator can be increased in relation to its diameter, the height of the helical rotor being simultaneously increased according to the recommended surface distance mentioned above. It is preferable to

keep the helix angle within the above-mentioned range and correspondingly to continue the twist of the two helix tubes about their axis. Depending on the shape of the preparator, the helix tubes 14,15 twist around the shaft 13 e. g. through 5/8,2/3, 3/4,7/8 of a turn or one complete turn, the height of the preparator be- ing simultaneously increased to nearly double relative to its diameter in relation to the example presented in Fig. 3. The height of the preparator can also be reduced in a corresponding manner by introducing heli- cal rotors with helix tubes twisted e. g. through 3/8 of a turn about their axis.

When the risk of sludge formation is small, it is possible to use intensive mixing, in which case an alternative solution will be to use a rotor with helix tubes at a steeper helix angle, e. g. between 40° - 50°. For example, if helix tubes twisted through 3/8 of a turn about their axis are used, preparation will be performed in this category. In this case, more ef- ficient preparation will be achieved by using a large number of vertical inhibitor vanes, e. g. 8 pcs.

The diameter dh of the helix tubes 14,15 of the rotor is preferably 0.04-0. 07 times the diameter of the rotor itself. The same applies to the lowest helix tube supporting arms, which follow the bottom profile of the preparator at a constant distance. The other supporting arms, the spacing of which has been determined on the basis of stiffness considerations, are disposed at a rising angle towards the shaft, preferably at an angle of 60° relative to the shaft.

The uppermost supporting arms are correspondingly in- clined at a falling angle relative to the shaft, pref- erably likewise at an angle of 60°, to ensure that the arms do not extend to a level higher than the upper ends of the helix tubes. In a half-turn helical mixer, the number of supporting arms is generally 3-5 pcs per helix tube. A usable angular spacing as seen from be-

low is e. g. 0°, 30°, 70°, 110° and 150°, in which case the number of supports is five for each helix tube.

The width of the vertical flow inhibitors 12 is 1/12-1/9 of the diameter of the preparator, pref- erably of the order of 1/10 of said diameter. The ver- tical flow inhibitors are mounted near the inner sur- face of the cylindrical side wall 8 of the container 6 so that a circumferential clearance having a width s of 0.01-0. 04 times the preparator diameter, pref- erably 0.02 times the preparator diameter. The number of flow inhibitors 12 is 3-12 pcs, preferably 6-8 pcs.

In the embodiment presented in Fig. 3, the spiral tubes 14,15 comprise a first spiral tube 14, which has an upper end 18 and a lower end 19, and a second spiral tube 15, which has an upper end 20 and a lower end 21. The upper end 18 of the first spiral tube 14 and the upper end 20 of the second spiral tube 15 are at the same first horizontal plane T1 on oppo- site sides of the vertical shaft 13 of the rotor. The lower end 19 of the first spiral tube 14 and the lower end 21 of the second spiral tube 15 are at the same second horizontal plane T2 on opposite sides of the vertical shaft 13 of the rotor. The upper end 18 of the first spiral tube 14 and the lower end 21 of the second spiral tube 15 are at the same first vertical line Ll. The lower end 19 of the first spiral tube 14 and the upper end 20 of the second spiral tube 15 are at the same second vertical line L2, which is on the opposite side of the vertical shaft of the rotor in relation to the first vertical line L1.

EXAMPLE EXPERIMENT 1. Rotary drum mixer as preparator in SIF pro- cess

The feed material in the process was waste from a phosphate concentrating plant, from which the coarse fraction had been separated by classifying in a cyclone. The solid matter content of the slurry was about 60%. The thick slurry was conveyed to a feeder and through it further into a SIF apparatus. The feed rate in the process was 29.5 t/M2 h, expressed in rela- tion to the active froth area of the SIF cell. The dwell time in the preparator drum was 8 min.

The results of the process were as follows: Mass % P205 content (%) Feed 100. 0 1. 02 Concentrate 13. 5 5. 95 Waste 86. 5 0. 25 2. Helical rotor mixer as preparator in SIF process The feed material in the process was waste from a phosphate concentrating plant, from which the coarse fraction had been separated by classifying in a cyclone. The solid matter content was 65%. The thick slurry was conveyed to a feeder and through it further into a SIF apparatus. The solid matter feed rate was 38.7 t/m2h. The effective dwell time was 8 min.

The results were as follows: Mass % P205 content (%) Feed 100. 0 0. 56 Concentrate 13. 0 2. 88 Waste 87. 0 0. 21 Example experiments 1 and 2 show that, in the SIF process provided with a helical rotor mixer, the

P205 content was somewhat lower than in the process provided with a rotary drum mixer (0, 21% vs. 0,25%).

However, the P205 content of the product going into fi- nal waste is the most important value in respect of the total process, so even a small change in the P205 content of the waste is significant. In addition, it is to be noted that in the example with a helical ro- tor mixer the P205 content of the feed is only about half the P205 content of the feed material fed into the rotary drum mixer.

The dwell time in the helical rotor prepara- tor was 4.5 min vs. 8 min in the rotary drum prepara- tor. The result shows that the mixing efficiency of the helical rotor preparator is better than or at least as good as in the rotary drum preparator.

The invention is not limited to the embodi- ment examples described above; instead, many varia- tions are possible within the scope of the inventive concept defined in the claims.