HK1016913B - Multi-stage extraction process - Google Patents

Description

Multi-stage extraction process

The invention relates to a process for the heterogeneous extraction carried out in an extraction plant comprising a plurality of separation stages connected in series, wherein each stage comprises an extraction chamber and a reextraction chamber, wherein the raffinate phase is contacted with an extractant in the extraction chamber and the extractant is contacted with the extract phase in the reextraction chamber for the reextraction.

Such extraction processes are used in the chemical, hydrometallurgical and microbiological industries as well as in other industrial sectors for the separation, concentration and purification of substances.

Single-stage plants are known which carry out a three-phase liquid extraction process in a two-chamber system, in which the two chambers communicate with each other at their upper part or comprise a porous partition. The two chambers are filled with the continuous phase through which two dispersed phases, insoluble in the continuous phase, pass in the form of droplets. Whereby the transfer of material from one dispersed phase (raffinate phase) to the other dispersed phase (extract phase) is effected via the continuous phase constituting the extractant. This method has been described, for example, in "the treatise che Grundling der Chemischen technology" journal 1984, Part 18, No.6 PP 736-738.

Further, the russian patent application No.94-015776/26 describes a three-phase extractor in which the first and second compartments are filled with a continuous phase. These chambers each have means for dispersing the raffinate and extract phases and are in communication with each other by means of an overflow for circulating the continuous phase. The overflow is of a tubular structure, interconnecting the upper and lower portions of each chamber. The extractor is also provided with a connecting pipe for feeding and discharging raffinate phase and extract phase.

Both the raffinate phase and the extract phase are dispersed in the respective chambers by means of a dispersing device into droplets. The droplets move through the continuous phase in the form of a population of droplets. With the proper selection and adjustment of the difference in specific weight between the raffinate phase, the extract phase and the continuous phase, a continuous circulation is achieved by upper and lower overflows only by the difference in specific weight between the emulsions in the first and second chambers, which causes the transfer of the substance to be extracted from one chamber to the other and thus from the dispersed raffinate phase to the dispersed extract phase, while the continuous phase acts as a transfer medium (carrier).

It is also known that in the extraction plants described above, a plurality of separation stages can be connected in series to increase the separation effect. In principle, the manner in which the phases are transferred through the separation stages and the separation stages are connected to one another can be chosen differently.

The basic object of the present invention is to improve the efficiency of a multistage extraction process by an optimal combination of phase transfer and separation stages. Particularly in systems where the distribution of the substance to be extracted in the various phases is very low, an economical and efficient separation is achieved. When using conventional multistage extraction processes, the separation problem to be solved is only with the aid of a very large number of separation stages, which makes the process uneconomical.

Starting from the above-described process, for the purposes of the present invention, the extractant always flows in a cross-flow with the extract phase and the raffinate phase, while the raffinate phase and the extract phase are passed through some or all of the separation stages in countercurrent.

In the present process, the extractant is always in cross-circulation with the extract and raffinate phases in the same separation stage. Alternatively, however, the extractant may flow counter-currently to the raffinate phase and co-currently to the extract phase for all stages, or alternatively co-currently to the raffinate phase and counter-currently to the extract phase.

The process of the invention is preferably carried out in such a way that the raffinate phase in one stage is dispersed in the extractant constituting the continuous phase in the dispersion zone of the extraction chamber and the extract phase is dispersed in the extractant constituting the continuous phase in the dispersion zone of the re-extraction chamber, so that the continuous liquid phase enriched in the extracted substances in the same stage passes from the extraction chamber into the dispersion zone of the re-extraction chamber and the continuous liquid phase depleted in the re-extraction chamber passes into the dispersion zone of the extraction chamber of the same or a subsequent stage. This process is comparable to single stage cross-flow operation when using a mixer settler for extraction and re-extraction.

The process of the invention is advantageously carried out as a three-phase extraction process, in which

a) A liquid used as a raffinate phase having a specific gravity greater than or less than that of the extractant, and

b) a liquid is used as the extractant phase, the liquid having a specific gravity less than that of the extractant if the raffinate phase has a specific gravity greater than that of the extractant and greater than that of the extractant if the raffinate phase has a specific gravity less than that of the extractant, such that the continuous phase is maintained in a circulating flow between the extraction and re-extraction chambers by only the difference in the specific gravities of the two dispersed phases relative to the continuous phase.

It was surprisingly found that the separation effect, i.e. the separation efficiency, of the process according to the invention increases more steeply as a function of the number of separation stages than in the known multistage extraction processes. This means that the problem of difficulty in separation caused by unfavorable partition coefficients can be solved with equipment having fewer separation stages than in the conventional extraction method.

The invention will be explained in more detail with the following specific examples and figures.

FIG. 1 is a flow diagram of a multi-stage extraction process according to the prior art;

FIG. 2 is a flow diagram of a series of separation stages with extractant circulation within each stage;

FIG. 3 is a flow diagram of a series of separation stages with internal and external recycle of extractant, wherein part of the liquid stream flows co-currently with the raffinate phase and the extract phase flows counter-currently;

FIG. 4 is a flow diagram of a series of separation stages with total external recycle of extractant, wherein the extractant flows counter-currently to the raffinate phase and co-currently with the extract phase;

FIG. 5 is a flow diagram of a series of separation stages with total external recycle of extractant, in which extractant flows co-currently with raffinate phase and counter-currently with extract phase;

FIG. 6 is a layout of the separation stage of the three-phase extractor;

fig. 7-10 are illustrations of embodiments of multi-stage three-phase extractor connections.

In the multi-stage extraction plant shown in fig. 1-5, each separation stage consists of an extraction cell 1 and a re-extraction cell 2. In the extraction chamber 1, the extractant is extracted from the raffinate phase into the substance to be extracted. In the stripping chamber 2, the extractant releases the extracted material into the extraction phase.

Fig. 1 shows a conventional counter-current connection system, with the extractant flowing from top to bottom and counter-current to the raffinate phase through each of the series-connected chambers 1 and then from bottom to top through the stripping chamber 2 and counter-current to the extract phase. The individual separation stages are connected directly in series, and the extractant is circulated via an external circuit or pumped.

Fig. 2 and 3 show a phase flow system and a connection system of separation stages according to the invention, in contrast to the same stage, in which the extractant is always circulated in cross-flow to the extract and raffinate phases, while the raffinate and extract phases are passed through the entire stage in countercurrent flow. In this way, the extractant is circulated with local internal circulation and with or without external circulation.

The improved phase flow system of the present invention is shown in fig. 4 and 5, where the extractant flows in the same stage in a similar fashion as the raffinate and extract phases are in cross-flow, but flows through all stages in a countercurrent fashion to the raffinate and in the same flow to the extract (see fig. 4), or vice versa, i.e., in the same flow to the raffinate and in the countercurrent flow to the extract (see fig. 5).

In both process flow systems, the raffinate phase is dispersed and finely distributed in the extractant constituting the continuous liquid phase in the dispersion zone of the extraction chamber 1 and the extract phase is dispersed and finely distributed in the extractant constituting the continuous liquid phase in the dispersion zone of the re-extraction chamber 2. As the continuous phase flows through the extraction chamber, the continuous phase is enriched with the substance to be extracted from the dispersed raffinate phase, and the enriched continuous phase then flows in the same separation stage to the dispersion zone of re-extraction chamber 2, where contact with the dispersed extract phase occurs, depletion of the continuous phase occurs, and enrichment of the extract phase correspondingly occurs. The depleted continuous liquid phase is then re-introduced into the extraction chamber 1 of the same or a subsequent separation stage, so that an internal or sequential circulation of the continuous phase takes place in each separation stage.

The three-phase extraction stage shown in fig. 6 can be used to practice the process of the present invention, where the raffinate phase is preferably a liquid having a specific gravity greater or less than that of the extractant. As for the liquid used as the extraction phase, if the specific gravity of the raffinate phase is greater than that of the continuous phase, a liquid having a specific gravity less than that of the continuous phase or the extractant is selected as the extraction phase, and if the specific gravity of the raffinate phase is less than that of the continuous phase extractant, a liquid having a specific gravity greater than that of the liquid phase is selected as the extraction phase, so that the circulating flow of the continuous phase extractant between the extraction chamber and the re-extraction chamber can be maintained only by the difference in specific gravity of the two dispersed phases with respect to the continuous phase.

The three-phase extraction stage essentially consists of an extraction cell 1 and a re-extraction cell 2, both of which are equipped with a dispersion device 3. The upper and lower parts of the chambers 1 and 2 are connected by a connecting channel or overflow 4. Depending on the specific gravity of the dispersed phase, the separation stage should be operated in such a way that the phase interface of the contacting phases is above or below the connecting channel of chambers 1 and 2. The three-phase extraction stage is provided with connecting pipes 5 and 6 as a feed port and a discharge port for raffinate phase, and connecting pipes 7 and 8 as a feed port and a discharge port for extract phase.

Fig. 7 and 8 depict a multistage three-phase extractor in which the chambers 1 and 2 are connected in series in a system of separation stages and are connected to each other to achieve the flow conditions of the flow diagrams shown in fig. 2 and 3. Both the chamber 1 and the chamber 2 are equipped with a dispersing device 3. In its upper and lower part, the chambers 1 and 2 communicate via a connecting channel or overflow 4. Depending on the specific gravity and/or the proportion of the respective dispersed phases, the phase interface of the contacting phases should be above or below the connecting channel between the chambers 1 and 2. Each stage is provided with connecting pipes 5 and 6 as the feeding ports of the dispersed extract phase and raffinate phase, and 7 and 8 as the discharging ports of the dispersed extract phase and raffinate phase. In the embodiment of fig. 8, a connection pipe 9 is provided as a continuous phase inlet and a connection pipe 10 is provided as a continuous phase outlet. For the flow of the dispersed phase, the chambers 1 of different stages are in communication with each other by means of a connecting line 12, while the chambers 2 of different stages are in communication by means of a connecting line 13. In addition, in the embodiment shown in FIG. 8, a connecting line 14 is provided for flowing the continuous phase between stages.

The multistage three-phase extractor shown in fig. 7 and 8 operates on the following principle:

the extraction cell 1 and reextraction cell 2 are filled with an extractant as the continuous phase. The raffinate phase and extract phase enter the chamber through connecting pipes 5 and 6 and the dispersing device 3. The droplets of the dispersed phase flow upward or downward in chambers 1 and 2, depending on the specific gravity of the contacting liquids, and coalesce at phase interface 11. This dispersion and coalescence process is repeated at each stage. The two dispersion phases are discharged through the connection pipes 8 and 7 in the first and last stages of the plant. As the droplet population flows through chamber 1 and chamber 2, dispersions of different specific gravities are formed. The result is a continuous upward movement of the phases and on the other hand a continuous downward movement of the phases. This is achieved by circulating the continuous phase between the chambers 1 and 2 by gravity alone, through the chambers 1 and 2 and through the connecting channel 4, and in the embodiment shown in fig. 8 from one stage to the other via the connecting line 14.

Fig. 9 and 10 depict two variants of a multistage three-phase extraction plant for carrying out the process according to the invention according to the flow diagrams of fig. 4 and 5.

In both embodiments, the multistage three-phase extractor still consists of a separation stage comprising an extraction chamber 1 and a re-extraction chamber 2, both chambers being equipped with a dispersing device 3. In the apparatus, the stages are arranged one below the other, and are connected to each other by a dispersed raffinate phase connecting line 12 and a dispersed extract phase connecting line 13, and a connecting line 14 flows the continuous phase from one stage to the other. The extractor is further provided with connection pipes 5 and 6 (as shown in FIG. 9) or 6 and 7 (as shown in FIG. 10) and connection pipes 7 and 6 (as shown in FIG. 9) as raffinate and extract phase outlets, or with connection pipes 7 and 6 (as shown in FIG. 9) and connection pipes 5 and 8 (as shown in FIG. 10) as raffinate and extract phase inlets, and with connection pipes 9 and 10 as continuous phase inlets and outlets, and with a return line 15 for external circulation of the continuous phase.

The multistage three-phase extractor of fig. 9 and 10 operates on the following principle:

the chambers 1 and 2 of each stage are filled with an extractant as the continuous phase. The two phases to be dispersed enter the chambers 1 and 2 of each stage via the connecting pipes 5 and 6 and the dispersing device 3. Depending on the specific gravity of the dispersed phase, the droplet population flows up or down in chambers 1 and 2 and coalesces at phase interface 11. This dispersion and coalescence process is repeated at each stage. The continuous phase flows through the stages in series via the connecting channel 14. In the lowermost stage of the embodiment of fig. 9, the raffinate phase enters the lowermost stage via connecting line 7 and the extract phase enters the lowermost stage via connecting line 6. The continuous phase flows from the bottom to the top, from the chamber 2 of the lowest stage through the chamber 1 of the previous stage, then flows into the chamber 2 of the same stage and from there through the chamber 1 of the further previous stage, thus continuing to the uppermost stage, from which it is recirculated to the chamber 1 of the lowest stage via the return line 15. In general, the flow regime is characterized by the continuous phase entering all the chambers 1 in cocurrent with the dispersed raffinate phase and passing through all the chambers 2 in countercurrent with the dispersed extract phase.

In the embodiment of fig. 10, the raffinate phase enters the uppermost stage through connecting line 5 and the extract phase enters the lowermost stage through connecting line 8. The continuous phase flows from top to bottom and cross-flows between chambers of adjacent stages; thus, in all chambers 1, the continuous phase counter-current to the raffinate and in all chambers 2, the continuous phase counter-current to the extract.

As the continuous phase flows through chambers 1 and 2, the continuous phase is successively contacted with the first and second dispersed phases. In this process, mass transfer occurs from one dispersed phase (raffinate phase) through the continuous phase (extractant) to the other dispersed phase.

The dispersion exits the apparatus via the first stage connecting pipes 5 and 8 (fig. 9) or via the last stage connecting pipe 7 and the first stage connecting pipe 6. Instead, the continuous phase circulates through the extractor via the connecting pipes 9 and 10, the connecting channel 14 and the return line 15 forming a closed circuit.

By suitably selecting the difference in specific weight between the phases and/or adjusting the mass flow of the dispersed phase components, the circulation flow can be generated by gravity alone. However, the use of a pump, for example a pump for circulating the continuous phase in the return line 15, is not prevented.

Claims (6)

1. A process for carrying out a multistage extraction in an apparatus comprising a plurality of stages connected in series, wherein each stage comprises an extraction chamber and a re-extraction chamber, and wherein a raffinate phase is contacted with an extractant in the extraction chamber and an extractant in the re-extraction chamber with an extract phase, characterized in that the extractant is always in cross-flow with respect to the raffinate and extract phases in the same separation stage, and the raffinate and extract phases are passed through some or all of the separation stages in countercurrent.

2. The process according to claim 1, characterized in that the extractant is always circulated in the same stage in a cross-flow of the extraction and raffinate phases.

3. The process of claim 1, wherein the extractant is passed through all stages in countercurrent flow to the raffinate phase and in cocurrent flow to the extract phase.

4. The process of claim 1, wherein the extractant is passed through all stages in a cocurrent flow with the raffinate phase and in a countercurrent flow with the extract phase.

5. The method of any one of claims 1 to 4, characterized in that:

a) in the first stage, the raffinate phase is dispersed in the extractant constituting the continuous phase in the dispersion zone of the extraction chamber, while the extract phase is dispersed in the extractant constituting the continuous phase in the dispersion zone of the re-extraction chamber,

b) the continuous liquid phase enriched in the extracted material in the same stage passes from the extraction chamber into the dispersion zone of the re-extraction chamber and the depleted continuous liquid phase in the re-extraction chamber passes into the dispersion zone of the extraction chamber of the same or a subsequent stage.

6. The method of claim 5, characterized in that,

a) a liquid used as a raffinate phase having a specific gravity greater than or less than that of the extractant, and

b) a liquid is used as the extractant phase, the liquid having a specific gravity less than that of the extractant if the raffinate phase has a specific gravity greater than that of the extractant and greater than that of the extractant if the raffinate phase has a specific gravity less than that of the extractant, such that the continuous phase is maintained in a circulating flow between the extraction and re-extraction chambers by only the difference in the specific gravity of the two dispersed phases relative to the continuous phase.