Abstract:
In the illustrative embodiments of the invention disclosed, an installation for mixing and separating two non-miscible liquids includes, inter alia, an improved pump structure for raising the liquid dispersion from the mixing tank to the inlet level to the decanting tank, a low flat chute for conveying the dispersion from the mixing tank to the remote end of the decanting tank, height-controlling means for controlling the level of the interphase within the decanting tank, and provision for recycling the lighter liquid phase to the mixing tank.

Description:
This is a continuation of application Ser. No. 777,290 filed Mar. 11, 1977, now abandoned. 
    
    
     The invention relates to the general method of mixing and separating two non-miscible liquids. 
     As is known, the method consists in producing a sufficiently fine dispersion of one liquid in the other to facilitate exchange, and then in separating the two non-miscible liquids by decantation. Usually, one of the liquids is an impure solution of a given product, whereas the other liquid is an organic solvent which is either specific to the product in question or acts as a dilution aid for an ion exchanger specific for the product in question. Usually the complete installation comprises an extraction set formed by juxtaposing a number of mixers and decanters in which an impure solution of the product flows in counter-current with the organic liquid, followed by a re-extraction set of identical kind in which the organic liquid flows in counter-current with a pure solution collecting the product, so that the organic liquid can be used in a closed circuit. 
     Each mixer and decanter in the set mainly comprises a mixer receiving the two liquid products at its base and agitating them to obtain the desired dispersion, followed by a decantation tank in which the mixture flows slowly and gradually separates owing to the non-miscibility of the liquids and the instability of the dispersion. It is clear that, for a given flow rate, the size of each mixer depends on the residence time of the dispersion before moving to the decantation unit, the time depending on the efficiency of the exchanges, which increases with the fineness of the dispersion. On the other hand, the size of each decanter also depends on the residence time of the dispersion through it, the time being dependent on the rate of coalescence of the dispersion, but this rate decreases when the fineness of the dispersion increases. Consequently, there is always an optimum size for the droplets of the dispersion, which determines the size and consequently the minimum cost of the total installation. 
     The optimum size can be obtained with various kinds of rotary agitators if a suitable rotation speed is adopted. However, the device comprises a number of mixing and settling tanks; in order to supply them with the various liquids, circulation pumps have to be provided. These not only increase the cost of the installation but also introduce shearing, turbulence and additional agitation, with the result that the droplets may not have the optimum size. In many cases, to simplify the installation, use is made of rotary agitators adapted to produce a negative pressure near their axis, thus automatically causing the liquids to flow. However, in the case of such suction agitators, if the diameter and rotation speed are given a suitable value for pumping, they are not often suitable for the dispersion, which usually has an excessive fineness. The result is a considerable increase in the size of the associated decanter. 
     SUMMARY 
     One object of the invention is to provide a mixing and separating installation of the previously-mentioned kind which obviates the aforementioned disadvantages, i.e. reduces the total bulk of the installation to a much lower value than the conventional value, and likewise greatly reduces the length of the pipes required and the number of auxiliary components. 
     According to the invention, each mixer is constructed by disposing a known agitator at the base of the vertical rotary shaft and by disposing a low-turbulence lifting pump at the top part of the same shaft. This avoids introducing additional agitation and also ensures that coalescence begins as a result of the centrifugal force in a conical stream flowing without turbulence. 
     According to another feature of the invention the flow, after being thus subjected to a first pre-coalescence, is conveyed to the end of the decanter remote from the associated mixer by a low, wide spout where the liquid flows slowly and is subjected to a second pre-coalescence, and a very efficient distributor comprising a grid extending over the entire height and entire width of the decanter is disposed between the decanter and the inlet compartment into which the spout discharges, the grid being made up of parallel strips which are substantially horizontal or preferably slightly inclined in the direction favouring coalescence. 
     The pump, which can be used in the installation according to the invention or for any other application, has a large delivery and a low lifting height and is of very simple construction and very rugged. It operates at low speed, using little power, and introduces practically no turbulence or shearing into the liquid. 
     According to the last-mentioned feature, the pump comprises an upwardly flared stationary frusto-conical bowl in combination with a rotor comprising an axial shaft bearing three radial trapezoidal blades, which are preferably curved at their bottom leading edge. The minor base of the truncated cone opens below the lower level of liquid whereas the major bases of the cone has an edge forming an overflow from which the raised liquid flows to the higher level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features of the invention will be clear from the following description of some embodiments, given by way of example and shown in the accompanying drawing, in which: 
     FIG. 1 is a diagrammatic vertical section through a mixing and decantation stage; 
     FIG. 2 is a larger-scale partial cross-section of the decanter, at the distribution grid; 
     FIG. 3 is a diagrammatic perspective view showing a number of successive stages incorporated in a single extraction or re-extraction set; 
     FIG. 4 is a general diagram of the complete installation; 
     FIG. 5 is an axial vertical section of a pump according to the invention; 
     FIG. 6 is a sectional along VI--VI of FIG. 5; and 
     FIG. 7 is a graph showing part of the characteristic curve of the pump. 
    
    
     DETAILED DESCRIPTION 
     A mixer 1, shown on the right of FIG. 1, in conventional manner comprises an axially vertical, cylindrical tank 2, into which two ducts 3, 4 open and supply the aqueous phase and the organic phase respectively. Tank 2 is filled with a mixture of the two liquids up to level 5, and a known agitator 6 rotates at the bottom of the mixture. The structure, dimensions and rotation speed of the agitator are freely chosen to obtain the optimum droplet size in the dispersion of one liquid in the other. Usually the aqueous phase is dispersed in the organic phase, but the opposite may be the case in certain applications. 
     According to the invention, a cone pump 8 is mounted above agitator 6 and on the same driving shaft 7. Pump 8 mainly comprises a frusto-conical stationary wall 9 in which three radial trapezoidal blades 10 rotate, the blades being mounted on a shaft 7. In order to prevent the turbulence produced from agitator 6 from being directly transmitted to the inlet of pump 8, the pump preferably has a bottom or end portion 11 extending and forming a deflector, whereas the dispersion of the two liquids reaches the base of cone 9 through lateral inlets. 
     One of the main advantages of the pump is that it operates at low speed, and more particularly can easily be adapted to the cone diameter and angle so as to operate at the rotation speed of shaft 7, which has been chosen only in dependence on agitator 6 to obtain the optimum fineness of the dispersion. Of course, the lifting height is the part of the cone which extends out of the liquid. Another feature of the pump is that its characteristic is very flat and therefore self-regulating, i.e. large variations in the total flow rate of the two liquids arriving at the base of the mixer produce only slight variations in the height of the liquid level 5. Finally, the main feature of the pump is that it induces very little turbulence in the conical stream of liquid, inside which there is an appreciable centrifugal force. Consequently, the pump produces no shearing or additional dispersion of the mixture, on the contrary, the mixture is already subjected to initial coalescence during its rise, i.e. the droplets of the discontinuous phase begin to grow in size. This assures an optimum exchange between the two phases, resulting in predecantation so that the rest of the installation can be correspondingly reduced in size. 
     The mixture leaving the mixer is conveyed in conventional manner to the inlet of a decanter 12. However, instead of conveying the mixture in conventional manner, directly to that inlet 13 of the associated decanter which is nearest mixer 1, the mixture is conveyed according to the invention to the other end 14 of the decanter, i.e. remote from mixer 1. Furthermore, instead of using a conventional pipe, having a circular section and a reduced diameter in which the liquid flows at considerable speed, use is made according to the invention of a horizontal, wide, low spout 15 having a rectangular cross-section, in which the liquid flows slowly and thus undergoes a second predecantation, the increase in size of the droplets being increased by the thinness of the liquid stream and the slowness of the flow, and also by the length of the spout, due to the aforementioned inversion of the supply. 
     Decanter 12 is an ordinary rectangular tank which is conventional except that the liquid flows through it in the opposite direction from usual. In order further to improve the decantation, the end compartment 16 into which the flared end 17 of spout 15 opens (FIG. 3) is separated from the rest of the tank by an efficient distributor comprising a grid 18 extending over the entire width and height of the tank. The grid is made up of parallel wall elements which are substantially horizontal or preferably inclined in the direction favouring the coalescence and holding-back of the discontinuous phase, depending on whether it is more or less dense than the continuous phase. 
     FIG. 2 is a detailed view of an embodiment of a grid of the aforementioned kind, which can e.g. comprise a stack of superposed, partly overlapping rectangular wall elements 19, secured together by appropriate means. 
     The two phases in decanter 12 are progressively separated in conventional manner but with much greater efficiency owing to the two predecantations which have occurred in pump 8 and spout 15 and have substantially increased the size of the droplets of the discontinuous phase. Owing to the aforementioned set of features, the residence time in decanter 12, which determines its volume, can be about 3 times as small as the normal time, e.g. 5 minutes instead of 15 minutes, so that the size of the decanter can be proportionally reduced, thus greatly reducing the cost of the installation, the building containing it, and auxiliary devices such as fire-preventing means and the like. 
     At end 13 of the decanter the top or less dense phase, usually the organic phase, flows at a constant level over a conventional threshold 20. In order automatically to adjust the height of the interphase 21, use can be made of a device shown in the centre of FIG. 1, comprising a cylinder 22 in which a piston 23 moves in sealing-tight manner and is prolonged by a central chimney forming an overflow at its top. A duct 24 from the base of the decanter opens at the base of cylinder 22, whereas a duct 25 opens into cylinder 22 above piston 23 but below the overflow in all positions of the piston. The height of the piston can be adjusted by a suitable means so as automatically to adjust the hydrostatic pressure in duct 24, thus determining the height of the interphase. 
     Mixing and decanting units such as shown in FIG. 1 are used in the form of a set, usually of four elements. FIG. 3 is a diagram of part of such a set, the continuous lines denoting ducts conveying the aqueous phase and the broken lines indicating ducts conveying the organic phase, the arrows indicating the direction of flow. More particularly, mixer 1, disposed at the centre of the group in FIG. 3, is connected via spout 15 to the oppositely-disposed decanter, and duct 24 coming from decanter 12 conveys the aqueous phase via cylinder 22 to the mixer disposed at the right of the drawing, whereas pipe 26 from decanter 12 conveys the organic phase to the mixer disposed at the left. The same feature is repeated in each component of the set, so that the aqueous phase supplied by pump 27 flows through the entire set from left to right, whereas the organic phase supplied by pump 28 flows through the same set from right to left, i.e. in counter-current with the aqueous phase. 
     As can be seen, the invention can greatly reduce the size of all the pipes in the set, since the pipes all come from the decanter surface 13 near the mixer, and not from the opposite surface, as in the conventional manner. More particularly if, as is frequently the case, it is required to recycle the organic phase, it is sufficient to provide a short connection 29 (shown with the associated regulating valve in FIG. 3) between pipe 26 coming from decanter 12 and the base of the associated mixer 1. 
     By way of example, FIG. 4 is a general diagram of an installation comprising an extraction set 30 and a re-extraction set 31, each set comprising e.g. four stages of the kind shown in FIG. 1. In each set, the stages are mounted in series and in counter-current as shown in FIG. 3. More particularly, the aqueous phase containing the product for extraction arrives at 27 and leaves at 32 whereas the organic phase coming from reservoir 33 is introduced at 28 and leaves via a pipe 34 conveying it directly to the inlet of the re-extraction set 31, from which it emerges through a pipe 35 which conveys it back to reservoir 33, so that the organic phase operates in a closed circuit. In the re-extraction set, another aqueous phase coming from reservoir 36 is conveyed by a pump 27a to the re-extraction set 31, where it flows in counter-current from the organic phase and comes out at 37. 
     By way of example, the first aqueous phase can comprise an impure solution containing inter alia a given metal ion, the organic phase can be kerosene containing in solution an ion exchanger specific for the ion which is to be extracted, and the second aqueous phase can be a solution of an appropriate salt, usually a sodium chloride brine, producing a reverse ion exchange in the ion exchanger. The final product is a pure solution of the desired metal ion at 37. 
     The pump 8 is shown in greater detail in FIGS. 5-7. 
     As FIG. 5 shows, pump 8 comprises a stator formed by a frusto-conical, axially vertical, upwardly flared wall 9. A rotor is mounted in the stator and comprises a vertical shaft 7 disposed coaxially with the stator and bearing three substantially trapezoidal blades 10 extending from shaft 7 to wall 9, leaving a small clearance in between. 
     The bottom end 38 of wall 9 opens below the equilibrium surface 5 of the liquid to be raised, and the top edge 47 of wall 9 forms a circular overflow for discharging the liquid, which flows naturally to the top level 40, a threshold being provided to prevent flow in the reverse direction when the pump stops. Preferably the assembly is covered with a protective hood 41 preventing any spraying of liquid. Two bearings 43, 44 holding the vertical shaft 7 can be disposed on hood 41 and on a holder 42 secured on the hood. A rotary deflector 45 may, if required, be disposed on shaft 7 below the bottom bearing 43 to protect it against sprayed liquid. Preferably, the bottom leading edge 46 of blades 10 is curved to the front with respect to the direction of rotation, as shown in FIGS. 5 and 6. 
     When shaft 7 is rotated by a motor (not shown), with or without a reduction gear, the liquid above level 5 in bowl 9 is rotated, thus becoming hollowed at its centre and rising along walls 1 to the periphery. When the rotation speed becomes sufficient, the liquid rising up the walls reaches the top edge 47 and the pump begins to deliver. Normally, shaft 7 must be driven at a speed above this minimum speed, which mainly depends on the dimensions of the device, but the minimum speed is always relatively small, i.e. a few tens to few hundreds of r.p.m. 
     The three blades, which are distributed at regular intervals around shaft 7 and rotate in a liquid, have a self-centring effect and do not introduce any bending moment or vibration in shaft 7, which is particularly important in the case where the shaft is mounted in cantilever manner with two upper bearings 43 and 44, as in the example shown. 
     In a variant, the bottom end 38 of the truncated cone 9 has a cylindrical or conical prolongation closed by an end member, the wall of the prolongation having radial or tangential inlets for liquid, and the end member may if required comprise a bearing for shaft 7. 
     The resulting pump can be used not only in the installation in FIGS. 1-4 but also for other applications. Its flexibility is a noteworthy feature. If the rotation conditions are fixed as previously described and the characteristic curve is drawn, showing the lifting height in dependence on flow rate as in FIG. 7, it can be seen that the curve is relatively flat. In practice, there is frequently an imposed flow which does not overflow into the tank 2 at the lower level 5 but which has to be raised to the upper level 40. In this case it is clear from the curve in FIG. 7 that if the flow rate abruptly decreases e.g. by half, the lifting height varies only by approx. 10%, i.e. the bottom level 5 is reduced by only a small amount. This property is particularly useful in a complete installation according to the invention, comprising a number of similar pumps transferring liquids between a number of tubes. Owing to the great flexibility, there is no adjustment problem during variations in conditions. 
     In addition to the last-mentioned property, the pump according to the invention is clearly of an extremely simple, rugged construction and of reduced size, in view of the very high flow rate which can occur over the entire periphery of threshold 47. On the other hand, the pump operates at low speed and uses little energy and is highly efficient, in view of the low turbulence of the liquid stream. The flow is almost laminar from the base to the top of the cone. In addition, the almost laminar motion is subjected to an appreciable, constant centrifugal force, which may be useful in certain cases, inter alia when the conveyed liquid contains solid particles in suspension or a dispersion of a second liquid which is not miscible in the first and which has to be separated therefrom.