FIELD OF THE INVENTION

The present invention relates to the field of mineral engineering and metallurgy in general and to extraction of metal compounds from ores or concentrates by wet processes, and more particularly to a method and an arrangement for determining of the water content of organic phase in a solvent extraction process.

BACKGROUND OF THE INVENTION

Hydrometallurgical technologies are used for obtaining or extracting metal compounds from their ores. Hydrometallurgical processes involve the use of aqueous chemistry for the recovery of metals from ores, concentrates, and recycled or residual materials. Hydrometallurgy is typically divided into three general areas: leaching, solution purification and recovery technologies.

Leaching involves the use of aqueous solutions, which contain a lix- iviant brought into contact with a material containing a valuable metal. There are a number of leaching process options available for the hydrometallurgical treatment of ores and concentrates. In the leaching process, oxidation potential, temperature, and pH of the solution are important parameters. There are several versatile leaching methods ranging from chloride to sulfate leaching and from atmospheric to pressure leaching. Leaching technologies include the leaching of e.g. zinc, copper, nickel, cobalt, gold, arsenic, silver, molybdenum, manganese and synthetic rutile.

After the leaching process, in the solution purification, which can be a solvent extraction process, the pregnant leach solution is first mixed into emulsion with the stripped organic stream and after mixing allowed to separate in a settler. The valuable metal will be exchanged from the pregnant leach solution to the organic phase stream. The resulting streams will be a loaded organic phase stream and a raffinate stream.

After the solvent extraction loading process, in the solvent extraction stripping process, the loaded organic phase is then mixed into emulsion with stripping liquor and allowed to separate in a settler. In stripping the metal will be exchanged from the organic phase to the stripping liquor. The resulting streams will be a stripped organic phase stream and a rich stripping liquor stream. In the following, the prior art will be described with reference to the accompanying figures, of which:

Figure 1 shows a flow diagram of a hydrometallurgical process according to the prior art;

Figure 2 shows a top view of a solvent extraction settler of a hydro- metallurgical solvent extraction process according to the prior art;

Figure 3 shows a partial cross-sectional view of a solvent extraction settler of a hydrometallurgical solvent extraction process according to the prior art.

Figure 1 shows a flow diagram of a hydrometallurgical process according to the prior art. A hydrometallurgical process according to the prior art comprises the process blocks for leaching process 1 , solvent extraction process 2, and recovery process 3.

In a hydrometallurgical process according to the prior art the leach- ing process 1 is carried out first. The leaching process provides a pregnant leach solution for the solvent extraction process 2. In the loading stage of the solvent extraction process 2 the pregnant leach solution is first mixed into emulsion with an organic stream in a mixer tank. The resulting mixed emulsion is taken from the mixer tank to a solvent extraction settler of the solvent extrac- tion process 2 for separation. The loading stage of the solvent extraction process provides a loaded organic phase stream and a raffinate stream as output of the solvent extraction loading process.

The loaded organic phase stream from the loading stage of the solvent extraction process 2 is provided as an input for the stripping stage of the solvent extraction process 2. In the stripping stage the loaded organic phase is then mixed into emulsion emulsification with e.g. a lean electrolyte in a mixer tank. The resulting mixed emulsion is taken to a stripping settler for separation. The stripping stage of the solvent extraction process provides a stripped organic phase stream and a rich electrolyte stream as output of the solvent ex- traction stripping process.

Figure 2 shows a top view of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the prior art. A solvent extraction loading settler 5 according to the prior art comprises several fence structures 6-9, said fence structures 6-9 enabling the proper and con- trolled flow of the hydrometallurgical solvent extraction process. The fence structures 6-9 of the a solvent extraction loading settler 5 according to the prior art may be regular prior art fence structures 6-9, or even DDG-type fence structures 6-9 (DDG, Dispersion Depletor Gate). The rectangular cuboid volumes between the fence structures 6-9 are typically called settler cells.

Figure 3 shows a partial cross-sectional view of a solvent extraction settler of a hydrometallurgical solvent extraction process according to the prior art. A solvent extraction settler 10 according to the prior art comprises several double gate type fence structures 1 1 -12, and one such said double gate type fence structure 1 1 -12 is shown in Figure 3. In Figure 3 the flow direction is from left to right and the separated liquid organic phase 13 flows on the top from left to right and from one settler cell before the fence structure 1 1 -12 to another settler cell after the fence structure 1 1 -12. Likewise the separated liquid aqueous phase 15 flows on the bottom from left to right and from one settler cell before the fence structure 1 1 -12 to another settler cell after the fence structure 1 1 -12.

The flow of the dispersion 14 is restricted by the top part of the left fence 1 1 of the double gate type fence structure 1 1 -12 and the bottom part of the right fence 12 of the double gate type fence structure 1 1 -12. As shown in Figure 3 the thickness of the dispersion layer 14 of one left side settler cell before the fence structure 1 1 -12 is noticeably larger than the thickness of the dis- persion layer 14 of the other right side settler cell after the fence structure 1 1 - 12 as the dispersion breaks up along the path through the settler.

In the solvent extraction process the settler 10 part of a mixer-settler separates the phases after the liquid-liquid contact, in which the two immiscible solutions are mixed to dispersion. The phases exiting the settler 10, i.e. the separated liquid organic phase 13 and the separated liquid aqueous phase 15, should be clean of the other liquid phase but in practice some residue of the other phase, commonly called entrainment, will remain in the solutions.

Entrainment consists of isolated droplets of the other liquid phase that settle slowly by gravity due to the very small size of the droplets. The en- trained aqueous liquid in the separated liquid organic phase 13 typically contains impurities which can impair the purity of the product, cause degradation of the organic phase and lower the current efficiency of the electrowinning process following the solvent extraction.

The separated liquid organic phase 13 layer lies on top of the settler 10 and when this phase is to be the pure one of the two exiting phases, as in the case where the separated liquid organic phase 13 is loaded with the target element, it should contain a minimum amount of entrained aqueous.

Under normal operating conditions the amount of entrainment is quite low but in the event of process disturbances, which can take place for several possible reasons, the phase disengagement rate in the settler may decrease and result in an increase in entrainment. In solvent extraction processes there is currently no automated online measurement used for determining water content in the organic layer. Particularly, the determination of the water content of organic phase in a solvent extraction process, in which the aqueous phase and the organic phase are both liquid phases, is especially troublesome.

A typical practice is that the plant personnel take samples manually from the process and use a centrifuge in the laboratory to measure the water content. This method however, is time consuming, prone to human errors and, as being based on a single sample taken from a single point will only give an instantaneous indication of the water content.

In general, there are several problems with the prior art solutions for determining of the water content of the organic phase in a solvent extraction settler separating a liquid organic phase from a liquid aqueous phase, which solvent extraction settler comprises several fence structures. So far, the measuring solutions are relatively troublesome and difficult to process. Also the measurement reliability and sensitivity with the prior art measuring solutions has not been adequate enough.

The problem therefore is to find a solution for determining of the wa- ter content of organic phase in a solvent extraction process which can provide continuously reliable measurement data for the determination of the water content of organic phase with better measurement sensitivity.

There is a demand in the market for a method for determining of the water content of organic phase in a solvent extraction process which method would be continuous, more reliable and have a better measurement sensitivity when compared to the prior art solutions. Likewise, there is a demand in the market for an arrangement for determining of the water content of organic phase in a solvent extraction process which arrangement would be more reliable and have a better measurement sensitivity when compared to the prior art solutions.

BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is thus to provide a method and an apparatus for implementing the method so as to overcome the above problems and to alleviate the above disadvantages.

The objects of the invention are achieved by a method for determin- ing of the water content of organic phase in a solvent extraction settler separating a liquid organic phase from a liquid aqueous phase, said solvent extraction settler comprising several fence structures, which method comprises the steps of:

- measuring of the electrical conductivity of the liquid organic phase at different heights in an at least one settler cell with an at least one electrical impedance tomography unit, each of said at least one electrical impedance tomography unit comprising an electrical impedance tomography measurement probe arranged in said at least one settler cell after a fence structure and a data processing arrangement connected to said electrical impedance tomog- raphy measurement probe; and

- determining the water content of the liquid organic phase at different heights in said at least one settler cell with said data processing arrangement and/or a measurement server connected to said at least one electrical impedance tomography unit.

Preferably in the method, said solvent extraction settler is a solvent extraction loading settler. Alternatively in the method, said solvent extraction settler is a solvent extraction stripping settler.

Furthermore, the objects of the invention are achieved by an arrangement for determining of the water content of organic phase in a solvent extraction settler separating a liquid organic phase from a liquid aqueous phase, said solvent extraction settler comprising several fence structures, which arrangement comprises:

- at least one electrical impedance tomography unit for measuring of the electrical conductivity of the liquid organic phase at different heights in an at least one settler cell, each of said at least one electrical impedance tomography unit comprising an electrical impedance tomography measurement probe being arranged in said at least one settler cell after a fence structure and a data processing arrangement connected to said electrical impedance tomography measurement probe;

so that said data processing arrangement and/or a measurement server connected to said at least one electrical impedance tomography unit is used for determining the water content of the liquid organic phase at different heights in said at least one settler cell.

Preferably, said at least one electrical impedance tomography unit is arranged in said at least one settler cell so that it also is used to measure the thickness of the separated liquid organic phase layer. Alternatively, said at least one electrical impedance tomography unit is arranged in said at least one settler cell so that it also is used to measure the thickness of the separated liquid organic phase layer and the thickness of the dispersion layer.

Preferably, said at least one electrical impedance tomography units are several electrical impedance tomography units for measuring of the electrical conductivity of the liquid organic phase at different heights in several settler cells; and that said data processing arrangement and/or said measurement server are connected to said several electrical impedance tomography units.

Preferably, said solvent extraction settler comprises several electri- cal impedance tomography units in one settler cell. Preferably, said data processing arrangement comprises one or more data collecting devices, data coordinating devices, data gateway devices, data servers and data networks with fixed and/or wireless connections.

Preferably, said electrical impedance tomography measurement probe comprises several electrodes arranged in several electrode pairs; said several electrodes being arranged in a vertical row so that two vertically adjacent electrodes are coupled to form an electrode pair. Alternatively, said electrical impedance tomography measurement probe comprises several electrodes arranged in several electrode pairs; said several electrodes being ar- ranged in two vertical rows so that two horizontally adjacent electrodes are coupled to form an electrode pair. Alternatively, said electrical impedance tomography measurement probe comprises several electrodes arranged in several electrode pairs; some of the said several electrodes being arranged so that two electrodes being arranged at a distance from one another in a vertical row are coupled to form an electrode pair.

Further alternatively, said electrical impedance tomography measurement probe comprises several electrodes arranged in several electrode pairs; said several electrodes being arranged in at least two vertical rows so that some electrodes of said several electrodes being two vertically adjacent electrodes or two vertically distant electrodes in a same row are coupled to form an electrode pair. Further alternatively, said electrical impedance tomog- raphy measurement probe comprises several electrodes arranged in several electrode pairs; said several electrodes being arranged in at least two vertical rows so that some electrodes of said several electrodes being two vertically adjacent electrodes or two vertically distant electrodes in a different row are coupled to form an electrode pair.

Preferably in the arrangement, said solvent extraction settler is a solvent extraction loading settler. Alternatively in the arrangement, said solvent extraction settler is a solvent extraction stripping settler.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a flow diagram of a hydrometallurgical process according to the prior art;

Figure 2 shows a top view of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the prior art;

Figure 3 shows a partial cross-sectional view of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the prior art;

Figure 4 shows a partial cross-sectional view of one embodiment of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the present invention;

Figure 5 shows a partial cross-sectional view of another embodiment of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the present invention;

Figure 6 shows a partial cross-sectional view of a third embodiment of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the present invention;

Figure 7 shows a simplified view of one embodiment of a solvent extraction settler cell EIT measurement probe according to the present invention;

Figure 8 shows a simplified view of another embodiment of a solvent extraction settler cell EIT measurement probe according to the present invention;

Figure 9 shows a simplified view of a third embodiment of a solvent extraction settler cell EIT measurement probe according to the present invention; Figure 10 shows a partial cross-sectional view of one embodiment of a solvent extraction stripping settler of a hydrometallurgical stripping process according to the present invention.

The prior art drawings of Figures 1 to 3 have been presented earlier. In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings of Figures 4 to 10.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method and an arrangement for determining of the water content of organic phase in a solvent extraction settler.

Figure 4 shows a partial cross-sectional view of one embodiment of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the present invention. A solvent extraction loading settler 16 according to the present invention separates a liquid organic phase 13 from a liquid aqueous phase 15 and comprises several double gate type fence structures 1 1 -12, and one such said double gate type fence structure 1 1 -12 is shown in Figure 4. The flow direction is from left to right and the separated liquid organic phase 13 flows on the top from left to right and from one settler cell before the fence structure 1 1 -12 to another settler cell after the fence structure 1 1 -12. Likewise the separated liquid aqueous phase 15 flows on the bottom from left to right and from one settler cell before the fence structure 1 1 -12 to another settler cell after the fence structure 1 1 -12.

The flow of the dispersion 14 is restricted by the top part of the left fence 1 1 of the double gate type fence structure 1 1 -12 and the bottom part of the right fence 12 of the double gate type fence structure 1 1 -12. As shown in Figure 4 the thickness of the dispersion layer 14 of one left side settler cell before the fence structure 1 1 -12 is noticeably thicker than the thickness of the dispersion layer 14 of the other right side settler cell after the fence structure 1 1 -12.

A solvent extraction loading settler 16 according to the present invention comprises at least one electrical impedance tomography unit for measuring of the electrical conductivity of the liquid organic phase at different heights. Each of said at least one electrical impedance tomography unit com- prises an electrical impedance tomography measurement probe 17 arranged in at least one settler cell after the fence structure 1 1 -12. Each of said at least one electrical impedance tomography unit also comprises a data processing unit 18 for determining the water content of the liquid organic phase at different heights in said at least one settler cell, said data processing unit 18 connected to said at least one electrical impedance tomography measurement probe 17.

The at least one electrical impedance tomography unit measures the electrical conductivity of the liquid organic phase at different heights in said at least one settler cell. The data processing unit 18 determines the water content of the liquid organic phase at different heights in said at least one settler cell on the basis of the received electrical conductivity measurement data. The data processing unit 18 according to the present invention may comprise one or more data collecting devices, data coordinating devices, data gateway devices, data servers and data networks with fixed and/or wireless connections.

Measuring the water content of the organic layer online gives an opportunity to follow the process behaviour, to detect abnormal situations and to make corrective actions in time. Online measurement will also give a long time average measurement result instead of an instantaneous indication. The electrical impedance tomography measurement probe 17 can be arranged in said at least one settler cell so that it also can be used to measure the thick- ness of the separated liquid organic phase layer 13.

EIT technology (EIT, Electrical impedance tomography) can be used for reliable on-line measurement of a multiphase environment. In an EIT an estimate of electrical conductivity of the target as a function of location is calculated based on the measured voltages and the known injected currents. The calculations are based on a mathematical model determining the relations between the injected currents, the electrical conductivity distribution of the target and the voltages on the electrodes. An advantage of the EIT is that it is based on a mathematical model which also takes into account electrode impedances. The model can also be modified to take into account voltage losses due to contamination on the electrode surfaces.

The present invention introduces techniques based on a computational electrical resistance tomography approach which is applied to be used with a probe arrangement for determining the water content of the liquid organic phase at different heights in real time in a solvent extraction settler compris- ing a multiphase system. In this approach, metal electrodes can be attached on a surface of an electrical impedance tomography measurement probe, through which sinusoidal currents are injected to the target substance and resulting voltages are measured through at least two electrodes. Alternatively, voltages can be supplied between any two of the electrodes, and the resulting currents may be measured through the electrodes. The electronics in the system hardware (i.e. data processing unit) handles the injection and the actual measurements. The analysis based on the measurement results is performed either on a data processing unit or a measurement server.

In the method according to the present invention an injection signal, which can be either injected voltage or current, is applied to the electrodes. An estimate of the electrical conductivity of the target substance as a function of location is calculated based on the measured voltages and the known injected currents. Measured data is used to determine the actual situation, i.e. the con- ductivity distribution which caused the observations - this approach is known as an inverse problem. The calculation is based on a mathematical model determining the relation between the injected currents, the electrical conductivity distribution of the target substance and the voltages of the electrodes. Said technique is called electrical impedance tomography (EIT), and is applied in e.g. medical imaging or soil characterization. One advantage of EIT is that it is based on a mathematical model which also takes into account the electrode impedances; the model can be also modified to take into account the voltage losses due to contamination on electrode surfaces.

The objects of the invention are achieved by a method for determin- ing of the water content of organic phase in a hydrometallurgical process settler comprising a multiphase system using at least one electrical impedance tomography measurement probe comprising together a plurality of electrodes capable of being in contact with the medium, and the method comprises the steps of injecting currents or voltages through at least two electrodes; measur- ing voltages or currents, respectively, through the at least two electrodes. The method is characterized in that the electrical conductivity distribution in the multiphase system is calculated on the basis of the measurement results and the known injected currents, which calculation comprises reconstruction of a vertical conductivity profile as a function of location in the multiphase system; the water content of the liquid organic phase at different heights is concluded on the basis of the electrical conductivity distribution and electrode location information.

The system is further characterized in that a data processing unit or a measurement server is further configured to calculate the electrical conduc- tivity distribution in the multiphase system on the basis of the measurement results and the known injected currents, which calculation comprises reconstruction of a vertical conductivity profile as a function of location in the multiphase system, conclude, on the basis of the electrical conductivity distribution and electrode location information, the water content of the liquid organic phase at different heights.

Figure 5 shows a partial cross-sectional view of another embodiment of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the present invention. Another embodiment of a solvent extraction loading settler 19 according to the present invention sepa- rates a liquid organic phase 13 from a liquid aqueous phase 15 and comprises several double gate type fence structures 1 1 -12, and one such said double gate type fence structure 1 1 -12 is shown in Figure 5. The flow direction is from left to right and the separated liquid organic phase 13 flows on the top from left to right and from one settler cell before the fence structure 1 1 -12 to another settler cell after the fence structure 1 1 -12. The flow of the dispersion 14 is restricted by the top part of the left fence 1 1 of the double gate type fence structure 1 1 -12 and the bottom part of the right fence 12 of the double gate type fence structure 1 1 -12.

A solvent extraction loading settler 19 according to the present in- vention comprises at least one electrical impedance tomography unit for measuring of the electrical conductivity of the liquid organic phase at different heights. Each of said at least one electrical impedance tomography unit comprises an electrical impedance tomography measurement probe 20 arranged in at least one settler cell after the fence structure 1 1 -12. Each of said at least one electrical impedance tomography unit also comprises a data processing unit 21 for collecting the data from the electrical impedance tomography measurement probe 20 and/or determining the water content of liquid organic phase at different heights in said at least one settler cell, said data processing unit 21 connected to said at least one electrical impedance tomography measurement probe 20. The at least one electrical impedance tomography unit measures the electrical conductivity of the liquid organic phase at different heights in said at least one settler cell. The data processing unit 21 determines the water content of liquid organic phase at different heights in said at least one settler cell on the basis of the received electrical conductivity measurement data. The data processing unit 21 according to the present invention may comprise one or more data collecting devices, data coordinating devices, data gateway devices, data servers and data networks with fixed and/or wireless connections.

The electrical impedance tomography unit can be arranged in said at least one settler cell so that it also can be used to measure the thickness of the separated liquid organic phase layer 13 and also to measure the thickness of the dispersion layer 14.

Figure 6 shows a partial cross-sectional view of a third embodiment of a solvent extraction loading settler of a hydrometallurgical solvent extraction process according to the present invention. A third embodiment of a solvent extraction loading settler 22 according to the present invention separates a liquid organic phase from a liquid aqueous phase and comprises several double gate type fence structures and several electrical impedance tomography units for measuring of the electrical conductivity of the liquid organic phase at different heights. Each of said several electrical impedance tomography units comprise several electrical impedance tomography measurement probes 23, 25, 27, 29, 31 arranged in several settler cells after the fence structures. Each of said several electrical impedance tomography units also comprise a data processing units 24, 26, 28, 30, 32 for collecting the data from the electrical impedance tomography measurement probes 23, 25, 27, 29, 31 and/or determining the water content of the liquid organic phase at different heights in said several settler cells. A solvent extraction loading settler 22 according to the present invention may also comprise a measurement server 33 connected to said electrical impedance tomography units. Furthermore, a solvent extraction loading settler 22 according to the present invention may also comprise a routing switch 34 between said electrical impedance tomography units and said measurement server 33. The calculations and the determining of the water content of the liquid organic phase may be carried out in the measurement server 33 based on the measurement results from the said electrical imped- ance tomography units. Alternatively some or all of the calculations and the determining of the water content of the liquid organic phase may be carried out in the electrical impedance tomography measurement units and forwarded to the said measurement server 33.

The at least one electrical impedance tomography measurement units measure the electrical conductivity of the liquid organic phase at different heights in said several settler cells. The data processing units 24, 26, 28, 30, 32 determine the water content of the liquid organic phase at different heights in said several settler cells on the basis of the received electrical conductivity measurement data. The data processing units 24, 26, 28, 30, 32 according to the present invention may comprise one or more data collecting devices, data coordinating devices, data gateway devices, data servers and data networks with fixed and/or wireless connections.

Measuring the water content of the organic layer online gives an opportunity to follow the process behaviour in several settler cells, to detect abnormal situations and to make corrective actions in time. Online measure- ment will also give a long time average measurement result instead of an instantaneous indication. The electrical impedance tomography units can be arranged in said several settler cells so that they also can be used to measure the thicknesses of the separated liquid organic phase layer 13.

A solvent extraction loading settler according to the present inven- tion may also comprise at least one electrical impedance tomography measurement probes in the first settler cell before the first fence structure. A solvent extraction loading settler according to the present invention may also comprise several electrical impedance tomography measurement probes in one settler cell.

Figure 7 shows a simplified view of one embodiment of a solvent extraction settler cell EIT measurement probe according to the present invention. A solvent extraction settler cell EIT measurement probe 35 according to the present invention comprises several electrodes 36-39 arranged in several electrode pairs 40, 41 . The electrodes 36-39 of the solvent extraction settler cell EIT measurement probe 35 may be arranged in a vertical row so that two vertically adjacent electrodes 36, 37; 38, 39 are coupled to form an electrode pair 40, 41 . In an electrode pair 40, 41 of the said EIT measurement probe 35 the voltage difference between one electrode 36, 38 and another electrode 37, 39 coupled in the same electrode pair 40, 41 is measured. Said voltage meas- urement of the electrode pair 40, 41 gives the conductivity in the settler cell at the height of the said vertically arranged electrode pair 40, 41 . The solvent extraction settler cell EIT measurement probe 35 according to the present invention may comprise for example 10 to 64 electrode pairs 40, 41 .

Figure 8 shows a simplified view of another embodiment of a sol- vent extraction settler cell EIT measurement probe according to the present invention. Another embodiment of a solvent extraction settler cell EIT measurement probe 42 according to the present invention comprises several electrodes arranged in several electrode pairs. Some of the electrodes 43-46 of the solvent extraction settler cell EIT measurement probe 42 may be arranged so that two electrodes 43, 44; 45, 46 arranged at a distance from one another in a vertical row are coupled to form an electrode pair 47, 48. In an electrode pair 47, 48 of the said EIT measurement probe 42 the voltage difference between one electrode 43, 45 and another electrode 44, 46 coupled in the same electrode pair 47, 48 is measured. Said voltage measurement of the electrode pair 47, 48 gives the overall conductivity in the settler cell at the height of the said electrode pair 47, 48.

Figure 9 shows a simplified view of a third embodiment of a solvent extraction settler cell EIT measurement probe according to the present invention. A third embodiment of a solvent extraction settler cell EIT measurement probe 49 according to the present invention comprises several electrodes 50, 51 , 53, 54, 56, 57, 59, 60, 62, 63 arranged in several electrode pairs 52, 55, 58, 61 , 64. The electrodes 50, 51 , 53, 54, 56, 57, 59, 60, 62, 63 of the solvent extraction settler cell EIT measurement probe 49 may be arranged in two vertical rows.

In a solvent extraction settler cell EIT measurement probe 49 according to the present invention two horizontally adjacent electrodes 50, 51 may be coupled to form an electrode pair 52. Also two vertically adjacent electrodes 53, 54 in a same row may be coupled to form an electrode pair 55. Furthermore, two vertically distant electrodes 56, 57 in a same row may be cou- pled to form an electrode pair 58. Further, also two vertically adjacent electrodes 59, 60 in a different row may be coupled to form an electrode pair 61 . Further, also two vertically distant electrodes 62, 63 in a different row may be coupled to form an electrode pair 64.

In an electrode pair 52, 55, 58, 61 , 64 of the said EIT measurement probe 49 the voltage difference between one electrode 50, 53, 56, 59, 62 and another electrode 51 , 54, 57, 60, 63 coupled in the same electrode pair 52, 55, 58, 61 , 64 is measured. Said voltage measurement of the electrode pair 52, 55, 58, 61 , 64 gives the conductivity in the settler cell at the height of the said horizontally arranged electrode pair 52, 55, 58, 61 , 64.

Figure 10 shows a partial cross-sectional view of one embodiment of a solvent extraction stripping settler of a hydrometallurgical stripping process according to the present invention. A solvent extraction stripping settler 65 according to the present invention separates a liquid organic phase 68 from a liquid aqueous phase 70, i.e. from electrolyte 70, and comprises several double gate type fence structures 66-67 and one such said double gate type fence structure 66-67 is shown in Figure 10. In the solvent extraction stripping settler 65 the metal will be exchanged from the liquid organic phase 68 to the electrolyte 70.

The flow direction is from left to right and the stripped liquid organic phase 68 flows on the top from left to right and from one settler cell before the fence structure 66-67 to another settler cell after the fence structure 66-67. Likewise the rich electrolyte stream 70 flows on the bottom from left to right and from one settler cell before the fence structure 66-67 to another settler cell after the fence structure 66-67.

The flow of the dispersion 69 is restricted by the top part of the left fence 66 of the double gate type fence structure 66-67and the bottom part of the right fence 67 of the double gate type fence structure 66-67. As shown in Figure 10 the thickness of the dispersion layer 69 of one left side settler cell before the fence structure 66-67 is noticeably thicker than the thickness of the dispersion layer 69 of the other right side settler cell after the fence structure 66-67.

A solvent extraction stripping settler 65 according to the present invention comprises at least one electrical impedance tomography unit for measuring of the electrical conductivity of the liquid organic phase at different heights. Each of said at least one electrical impedance tomography unit com- prises an electrical impedance tomography measurement probe 71 arranged in at least one settler cell after the fence structure 66-67. Each of said at least one electrical impedance tomography unit also comprises a data processing arrangement 72 for determining the water content of the liquid organic phase at different heights in said at least one settler cell, said data processing ar- rangement 72 connected to said at least one electrical impedance tomography measurement probe 71 . The at least one electrical impedance tomography unit measures the electrical conductivity of the liquid organic phase at different heights in said at least one solvent extraction stripping settler cell. The data processing arrangement 72 determines the water content of the liquid organic phase at dif- ferent heights in said at least one settler cell on the basis of the received electrical conductivity measurement data. The data processing arrangement 72 according to the present invention may comprise one or more data collecting devices, data coordinating devices, data gateway devices, data servers and data networks with fixed and/or wireless connections.

Measuring the water content of the organic layer online gives an opportunity to follow the process behaviour, to detect abnormal situations and to make corrective actions in time. Online measurement will also give a long time average measurement result instead of an instantaneous indication. The electrical impedance tomography measurement probe 71 can be arranged in said at least one settler cell so that it also can be used to measure the thickness of the stripped liquid organic phase layer 68.

In any of the embodiments of the invention, the effect of contamination or dirtying of at least one electrode in the probe arrangement is taken into account. As widely known, the contamination around the electrode(s) leads into a non-ideal connection between the metallic electrode and target substance, which further causes additional electric resistance. The non-ideal connection can be seen as an additional voltage drop and it can be expressed by a quantity called contact impedance. The voltage (or current) measured through a pair of electrodes is generally a function of the injected current (or voltage), the conductivity distribution in the path of the electrical current and the contact impedances between the electrodes and the surrounding medium to be measured. The contact impedances may be used to compensate for the dirtying of the electrodes by inserting them to the calculation model as additional voltage loss parameters.

The solution for determining of the water content of organic phase in a solvent extraction according to the present invention provides reliable, online measurement data for the determination of the water content of a liquid organic phase with better measurement sensitivity.

The water content of organic phase can therefore be determined online from several heights of the said liquid organic phase. With the help of the arrangement according to the present invention the measurement reliability and sensitivity is substantially improved. When using several measurement probes according to the present invention it is possible to provide a three dimensional understanding of the conditions in the said hydrometallurgical process.

With the help of the solution according to the present invention the manufacturers and owners of solvent extraction settlers will be able to provide solvent extraction settler with a measurement arrangement producing more reliable measurement data for the determination of the water content of organic phase in a solvent extraction with said measurement arrangement having better measurement sensitivity. The solution according to the present invention may be utilised in any kind of a solvent extraction settler.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.