[0001] This invention relates to a jet-based mixer settler system.

[0002] Various mixer settler arrangements have been proposed in the prior art.

[0003] US3126258 discloses the simultaneous use of separate mixer and settler compartments for each stage in a plurality of stages in a complex system. Jet mixing occurs as liquids are withdrawn from a mixing compartment and are injected back into the mixing compartment through several jets. Although internal moving parts are not required the jet mixing does not provide functionality which is meaningfully different from that achieved through mechanical mixing. A settler section operates continuously at a rate which is determined by the requirement of settling in each stage at the full rate of each counter-current stream. The throughput rate is therefore not controllable over a wide range and can be subjected to a condition known as "flooding", when settling is incomplete and mixed liquids are transferred instead of separated liquids. Transfers of liquids take place simultaneously through interconnecting pipes or ducts and depend on gravity action in combination with fluid density differences and the relative elevation of the successive stages. As these elevations are fixed the possibility of inducing widely different flow ratios is limited.

[0004] US2646346; US2754179; US2980514; US3089756 and US320688 disclose systems which have separate mixer and settler compartments with mechanical mixing and which generally exhibit characteristics similar to those identified herein for US3126258. [0005] US3549332 discloses three distinct timed sub-operations for mixing and for the transfers of light and heavy fluids in opposite directions. Inter stage ducts connect the bottom of one stage to the top of another stage so as to facilitate different transfers forwards and backwards. The ducts are not used for mixing. There is no suggestion in this patent of the possibility of using multiple physical stages per operational stage. Liquid transfers always involve one liquid entering another liquid. Thus a transfer of a liquid volume greater than the normal operating volume of that liquid, in a stage, requires the liquid to settle or rise quickly through the other liquid. This can possibly cause adverse mixing and entrapment. This is an important factor in large scale systems for adverse mixing can be problematic as flow cross-sectional areas in the stages scale up only in proportion to flow rates raised to 2/3, for similar residence times. In this citation mixing is done by mechanical stirring and there is no suggestion of eliminating the mechanical means of mixing, by the use of transfer flows instead. The technique described in this citation is not applicable to liquid/solid operations because the transfer of any fluid entails a transfer from deep within a settled bed of solids where blockages can easily occur.

[0006] In this specification the word "light" is used, for the sake of convenience, to designate a fluid which is less dense than another fluid which, in turn, is referred to as "a dense fluid".

[0007] Figure 1 of the accompanying drawings illustrates apparatus, 10 referred to as a mixer settler column in the specification of South African Patent Application No. 2012/03142, which allows for mixing, settling, and counter-current transfers of fluid to be done cyclically in a plurality of distinct modes of operation. These operations are carried out in different time periods in the same physical regions instead of using different physical regions for the various operations. [0008] The apparatus includes a column which is vertically orientated. A single column, with standard plates, can be used for any of a number of processes and can be used over a wide range of operating conditions. Modes of usage include liquid-to- liquid contacting operation, counter-current decantation, counter-current ion exchange, counter-current leaching and thickening.

[0009] An object of the present invention is to provide apparatus for multi-unit counter-current mixer settler operations in which the units are arranged horizontally. Such operations also include liquid-to-liquid contacting operation, counter-current decantation, counter-current ion exchange, counter-current leaching and thickening.

[0010] The horizontal arrangement of units can provide several advantages depending on practical needs and constraints. In some instances, headroom for support structure for a horizontal set of units (or a horizontally oriented apparatus) can usually be less complex and costly than for a vertical column. A horizontally arranged set of units can often be more easily maintained, by work at ground level, and by the draining, cleaning and repairing of individual units. In instances where many units are needed in a process, a high or a long series of units can be avoided by using a vertical column that has vertically stacked layers, each with a number of horizontally arranged units.

SUMMARY OF THE INVENTION

[0011] The invention provides a mixer settler which includes, at least, first and second units which are horizontally disposed relative to each other, each unit including a respective enclosed vessel, and a transfer duct which includes a first end which is exposed to an upper region of the first unit and a second end which is exposed to an upper region of the second unit, which connects the upper region of the first unit to the upper region of the second unit and which is shaped so that fluid passing from the first unit to the second unit increases in velocity.

[0012] The mixer settler may be connected by fluid ports to an external source which applies pressure to the fluids in the units.

[0013] The respective vessel in each unit may be cylindrical. The units may be positioned with a common horizontal axis. The units may abut each other and have vertical partitions between them. Thus adjacent units may be arranged with common or closely spaced walls. Alternatively the units may be horizontally spaced apart.

[0014] Each unit has a fluid-containing volume below the height of the ducts which tends to trap dense fluid and which tends to prevent the trapped dense fluid from flowing towards an adjacent unit.

[0015] The transfer duct may be tapered i.e. it may reduce in cross-sectional area in a direction from the first unit to the second unit.

[0016] When fluid flows at a relatively high flow rate from the first end to the second end of the duct, the second end may perform the function of a jet nozzle to eject fluid into the second unit, preferably in a direction towards a location inside an interior of the second unit which is displaced by a maximum distance from the nozzle (the second end of the duct). In this case, the jet of ejected fluid causes turbulent mixing of the fluid in the second unit.

[0017] When fluid flows at a low to moderate flow rate from the second end to the first end of the duct, the first end may perform the function of ejecting fluid into the first unit at a relatively low velocity in a way that tends not to disturb any dense fluid within a containing volume of the first unit. In this case, light fluid from above the bed of dense fluid in the second unit is transferred to mix with, or displace, light fluid above the bed of dense fluid in the first unit.

[0018] The transfer duct may be located externally to the first and second units. Alternatively it may be located within the first and second units. In each case the duct may protrude some distance into one or both of the units.

[0019] According to requirement a third unit may be connected to the second unit in a manner which is similar to that which has been described with respect to the second and the first units, a fourth unit may be connected to the third unit in a similar manner, and so on.

[0020] Other arrangements are possible. For example the units may be disposed in a column configuration which includes a number of layers of units wherein the layers are vertically stacked.

[0021] The individual units may be of varying lengths and shapes but their volumes and duct cross-sectional areas are typically substantially the same in size. According to requirement each unit may include a drainage port or valve for inspection and maintenance, as required.

[0022] The term "mixing feed end" is used to designate an end of the series of units of the apparatus such that if a fluid is introduced at that end, it tends to cause fluid to flow within the inter-connecting ducts of the apparatus from the first ends to the second ends of the respective ducts. Conversely, the term "transfer feed end" is used to designate an end of the series of units such that if a fluid is introduced at that end, it tends to cause fluid to flow within the inter-connecting ducts of the apparatus from the second ends to the first ends of the respective ducts. [0023] Mixing may be carried out for a predetermined period of an operational cycle ("a mixing sub-cycle") by introducing a fluid with a suitably high flow rate into the apparatus at or near its mixing feed end thereby causing a flow of fluid towards the transfer feed end and causing the mixing of dense and light fluids in each unit within the apparatus. The volume of dense fluid transferred in such a period may be the volume of dense fluid required to be processed by the apparatus for a complete operational cycle.

[0024] Settling may be carried out for a predetermined period of the operational cycle after mixing. During this period, fluid flows into and out of the ends of the apparatus are stopped.

[0025] The transfer of light fluid may be carried out for a predetermined period of the operational cycle, after settling, by introducing a fluid with a suitably low to moderate flow rate into the apparatus at or near its transfer feed end thereby causing a flow of fluid towards the mixing feed end and the transfer of light fluid only from unit to unit within the apparatus. The volume of fluid transferred in such a period may comprise a volume sufficient to return a similar volume of light fluid as was displaced during the preceding mixing sub-cycle and a further volume of light fluid to produce an overall light fluid movement as might be required to be processed by the apparatus for a complete operational cycle.

[0026] The counter-current flow of light and dense fluids requires an arrangement at the transfer feed end of the apparatus to separate a dense fluid product from the light fluid in the associated unit. This separation may be done outside the units of the apparatus or in an end unit of the apparatus. In the latter case, the transfer duct of the end unit at the transfer feed end should preferably not be tapered into a jet, and there may be two fluid ports at that end, with an upper two-way port for the light liquid, at a height similar to that of the transfer ducts, and a lower outlet port for the dense liquid, at a height below the top of the contained volume of the unit.

[0027] In the case of the mixing feed end, only light fluid exits from the mixing feed port, during the transfer period, and a feed of dense fluid and some returned light fluid is induced by external means to enter the same port during the mixing period.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention is further described by way of examples with reference to the accompanying drawings in which:

Figure 1 schematically illustrates from one side a jet-based mixer settler which includes a number of horizontally arranged units with external inter-unit ducts;

Figure 2 shows a system which is similar to that shown in Figure 1 but with internal inter-unit ducts;

Figure 3 shows an arrangement which is substantially the same as the Figure 2 system but with an end unit at the transfer feed end having two fluid ports and an inter-unit duct that is not tapered to a jet nozzle;

Figure 4 is a side view of an apparatus which includes stacked layers of horizontally arranged units; and

Figure 5 is a plan view in section of the Figure 4 mixer settler apparatus. DESCRIPTION OF PREFERRED EMBODIMENTS

[0029] Figure 1 shows in cross-section and from one side a mixer settler apparatus 10 according to the invention. The apparatus includes an array of horizontally arranged units 12, 14 and 16. Three units are shown. This is not limiting and the number of units in the array can be varied. Each unit comprises a respective closed vessel. The vessels are arranged on a common horizontal base or axis 20. In this example the vessels are close to each other, essentially abutting one another, or are integrally formed and have vertical partitions 22 between adjacent vessels.

[0030] Each vessel has a respective drainage valve 26. Ports, not shown, for inspection and maintenance can be installed at the top of each unit if necessary.

[0031] The first unit 12 has at least one fluid port 30 and the last unit in the array has at least one fluid port 32. Respective transfer ducts 40 and 42 are positioned between adjacent vessels. The duct 40 has an inlet 40A which is exposed to an upper region 12A of the unit 12. The duct has a tapered shape and reduces in cross- sectional area towards its outlet 12B which is in an upper region 14A of the unit 14. The duct outlet is not aimed vertically downwards but, instead, is angled so that fluid emitted by the duct tends to flow along a maximum length inside the unit 14 - this is indicated by a dotted line 44 which extends from the outlet 12B towards a diagonally positioned lower end 14B of the unit. The intention in this respect is to induce maximum turbulence in the fluid inside the vessel. The duct 42 is similarly orientated.

[0032] The apparatus 10 has a mixing feed end 46 and a transfer feed end 48. The fluid port 32, in the last unit 16, is a two-way port i.e. a port for the light fluid and an exit port for the dense fluid.

[0033] The transfer ducts 40 and 42 are external to the units.

[0034] A controlled pressure source 50 is connected to the port 30.

[0035] Figure 2 shows a mixer settler system 0A according to the invention which is substantially the same as what has been shown in Figure 1 with an array of horizontally arranged units 52, 54 and 56. However transfer ducts 60 and 62 between adjacent units are positioned inside the respective units. Similarly, a fluid port 64 has a portion located inside the unit 52 and a bidirectional fluid port 66 has a portion located inside the unit 56.

[0036] In both Figures adjacent units are arranged with common or closely spaced walls. Thus the units can be integral with one another or distinct vessels. The units may, however, be spaced apart and may be interconnected by means of ducts of the required tapering form. The units need not be on a straight line but instead could be arranged, viewed in plan, along a curve or in combinations of interconnected straight lines and curves.

[0037] Figure 3 shows an arrangement which is substantially the same as what is shown in Figure 2 and for this reason the Figure 3 construction is not described in detail. Like components bear like reference numerals. The main differences between the Figure 2 and Figure 3 arrangements are that, in the latter case, a duct 62A, between adjacent units 54 and 56, is not tapered to a jet nozzle, and there is an exit port 78 for the dense fluid which is positioned close to a base of the unit 56. Instead of requiring an external means for the separation of light and dense fluids, the arrangement of Figure 3 facilitates the separation of light and dense fluids within the unit at the transfer feed end of the apparatus, through the absence of turbulent mixing by a jet nozzle in the end unit,.

[0038] Figures 4 and 5 are side and plan views respectively of a mixer settler apparatus 80 which includes a cylindrical housing 82. Inside the housing are six stacked layers 84 to 94 of units and, within each layer, four respective horizontally arranged units 84A, 84B, 84C and 84D, etc. [0039] The individual units are of varying lengths and shapes but their volumes and duct cross-sectional areas are typically of the same order of magnitude. The end units of each horizontal layer are connected to units above or below by means of a respective duct P, Q, R etc. that can include a portion of the wall of the apparatus as part of the duct wall.

[0040] The ducts P, Q, R etc. are internal. Similarly, transfer ducts between adjacent units, marked, by way of example only, X, Y, Z for the layer 90 are internal and are configured in the manner which has been described hereinbefore in connection with Figures 1 and 2.

[0041] The mixer settler apparatus 80 has a fluid port 100 connected to the unit 94A in the uppermost layer 94 and a fluid port 102 connected to the unit 84A in the lowermost layer 84.

[0042] In a mixer settler apparatus according to the invention each unit, as noted, may comprise a sealed vessel. Thus the movement of fluids between adjacent units can be made to depend solely on an externally applied pressure and is not necessarily dependent on gravity.

[0043] The apparatus is operated cyclically, with periods of mixing, settling and transfer, in each operational cycle.

[0044] Mixing within each unit is induced by turbulent eddies created by a high flow rate of fluid, entering the mixing feed end of the apparatus, and produced by the pressure source 50 and a simultaneous high flow rate of fluid from unit to unit towards the transfer feed end. Each duct directs a downward jet of fluid through the corresponding downstream adjacent unit and creates turbulence in the fluid in that unit. During the mixing phase, dense and light fluids are transferred, from unit to unit, in the direction from the mixing feed end to the transfer feed end of the apparatus. The dense fluid does not flow between units at any time other than during the mixing phase.

[0045] Settling of the dense fluid in each unit occurs during a settling period of the operational cycle, when the flows to and from the apparatus are all made to be zero.

[0046] Transfer of light fluid, from unit to unit, in the direction from the transfer feed end to the mixing feed end of the apparatus occurs during a transfer period of the operational cycle, such that the movement of light fluid during mixing is substantially reversed and such that a further volume of light fluid is transferred to result in a net flow of light fluid over the operational cycle towards the mixing feed end.

[0047] The mixer settler system of the invention thus has a single compartment per stage in which mixing and settling are made to occur, at different times in an operational cycle. Mixing is effected by means of a fluid jet and by flow in one direction from one stage to another. Thus mixing requires only one driving pressure that is applied externally at one end of the apparatus.

[0048] In the system of the invention settling does not occur continuously but is a batch process which is part of a full operational cycle. Provision is thus made for operational stages to comprise multiple physical stages (i.e. not necessarily one stage only) thereby allowing adjustment of the settling area per operational stage to be effected dynamically in a manner which takes into account the number of operational stages over the column. Additionally, transfer flows can be effected by externally applied fluid pressures over different time periods to allow for significant flexibility that is not dependent on gravity. [0049] The ducts which connect the upper ends of adjacent stages facilitate mixing and fluid transfer. The ducts connect only light liquids in adjacent stages and transfer can be done by relatively high flows and in large volumes without mixing. This is important in large scale systems. Mixing is effected, as noted, by means of jets which are induced within the process by a single external pressure source. Thus the transfer of fluid between at least one pair of adjacent units is used to induce mixing and in this way the need for a separate mixing method is eliminated. The system of the invention is, furthermore, applicable to liquid/solid operations and allows for the effective handling of settled solid beds.