GB2321898A - Selective removal of metal ions from aqueous solution - Google Patents

Abstract

A method of selectively extracting one or more species of metal ion from an aqueous feed stream 104 containing two or more species of metal ion comprises passing the stream through an extraction cell 100 containing an immiscible carrier having a ligand which complexes with the desired species, ensuring that the ligand is saturated with the desired species, reacting the complex with a stripping solution to remove the desired species, releasing the ligand back to extract more of the desired species and repeating these steps in a series of connected extraction cells 102,104 until the concentration of the desired species produced 116 is no longer capable of saturating the ligand. At this stage, the product becomes contaminated with another species 120. Preferably the extraction cells are electrostatic pseudo liquid membrane-type cells having a feed side 106 and a stripping side 108 separated by a baffle 110 with the ligand/ligand complex migrating across the baffle.

Description

SEPARATION METHOD AND APPARATUS The present invention relates to improvements to the socalled "electrostatic pseudo liquid membrane" (ESPLIM) method of separation of metal ions from aqueous solutions.

Chinese patent application number CN 86101730A describes a separation technique which enables the purification of aqueous solutions and concentration of solutes in aqueous solutions.

The technique includes the steps of passing droplets of an aqueous feed solution which it is desired to purify and/or from which it is desired to extract metal ions for example, under the influence of gravity, through a first region of a non-polar carrier liquid in which is dissolved a chemical, or extractant ligand, having high affinity for the metal ion or ions to be removed whilst simultaneously subjecting the droplets to a high voltage electrostatic field so as to break up the droplets into a multiplicity of much smaller droplets in order to increase their surface area to volume ratio. The metal ions are complexed by the dissolved ligand chemical into the carrier liquid and are driven, principally by the concentration gradient so formed, to a second region in the non-polar carrier liquid through which is passing under the influence of gravity a stream of droplets of an aqueous "stripping" solution which has a chemically higher affinity for the metal ion than the complexing chemical in the carrier liquid. The stripping solution droplets are also simultaneously subjected to a high voltage electrostatic field so as to break them up into a multiplicity of much smaller droplets and thus to increase their surface area to volume ratio. The metal ions are thus concentrated into the stripping solution and the aqueous feed solution is largely purified of the metal ions. As the very small droplets of the purified feed solution and the stripping solution, the former now having a lower concentration of the metal ions and the latter now having a high concentration of the required metal ions, pass out of the high voltage electrostatic field, they coalesce and fall under gravity into mutually separated first and second collecting vessels, respectively, and from which they can be removed.

The first and second regions of the carrier liquid are separated by a substantially vertical barrier or baffle which is intended to allow substantially uninterrupted flow and passage of the carrier liquid to and from the first and second regions but, is also intended to impede or prevent the passage of the aqueous feed solution from the first region into the second region and, the passage of the stripping solution from the second region to the first region.

The ESPLIM system of the prior art is operated such that the ligand in the carrier liquid is never saturated, i.e.

the system is always operated with an excess of ligand present with regard to the species which it is desired to extract. Thus, since there is an excess of the ligand, all of the metal ions may be removed from the aqueous feed solution. This is not a problem when there is only one species of metal in the feed solution to be extracted or if it is desired to remove all species of extractable ions simultaneously. However, problems arise where there are two or more species in the feed solution which are capable of complexing with the ligand but, only selected ions from those present are required to be removed. Since there is an excess of ligand present, the two or more species will be selectively complexed with the ligand according to their distribution ratios which are dependent upon their rates of extraction from the feed and the thermodynamic equilibrium amount extracted. In published information available on the ESPLIM process, the selectivities achieved are similar to, or somewhat less than the ratio of the distribution coefficients of the individual species in isolation.

Furthermore, the situation where a high feed flow rate or high concentration of the species in the feed leads to saturation of the ligand in the carrier liquid is referred to as "overload" and is stated to be a counter-productive condition to be avoided. Thus, in the case where two or more species are present in the feed, the stripping solution will always be contaminated with a proportion of the other species present in the feed solution.

In the more general art of solvent extraction, it is known that improved selectivity between two or more extractable species in the feed solution may be achieved by means of the technique known as counter-current extraction. In this technique, two immiscible phases flow in opposite directions such that the most heavily loaded extracting phase is in contact with the most heavily loaded feed phase. The species present in the feed phase are thus in competition for a limited amount of the ligand in the extracting phase. Initially, all extractable species will complex with the ligand but as the process flow progresses, the species with the highest affinity for the ligand will tend to displace those species with a lower affinity back into the feed phase, even though the latter would extract in the absence of the former. When all of the ligand is occupied by one of the extractable species, the ligand is said to be saturated. However, in the case where there is insufficient of the desired species in the feed phase to enable the ligand to operate in the saturated condition, the only way of concentrating the desired species is to burn-off the bulk of the solvent such as by distillation for example which is undesirable from the economic and safety points of view.

It is an object of the present invention to provide an improved method of removing one or more desired species from a feed solution containing two or more extractable species without the need to remove large quantities of solvent as in the prior art counter-current solvent extraction process.

According to the present invention there is provided a method of improving the selectivity of at least one desired species from an aqueous first feed solution containing at least two potentially extractable species, the method comprising the steps of: a) passing a stream of said aqueous feed solution through a first extraction cell containing a first immiscible carrier phase having therein an extractant ligand to combine with and extract said at least one desired species from said feed solution; b) ensuring that operating conditions in said first extraction cell are such that said ligand is saturated with said at least one desired species; c) allowing said saturated ligand to react with an aqueous stripping solution stream also passing through said first immiscible carrier phase so as to remove said at least one desired species therefrom to form a product stream having a higher concentration of said at least one desired species than said feed solution, and to release said ligand back into said first immiscible carrier phase to complex with further at least one desired species; d) repeating steps a) to c) with subsequent extraction cells, a feed solution to each subsequent cell being constituted by a raffinate solution from an immediately prior extraction cell until a concentration of said at least one desired species in said raffinate feed solution falls below a level where said ligand is able to be saturated by said at least one desired species and a product contaminated with said other species is formed; and e) passing a raffinate from the last feed solution to at least one further extraction cell wherein operating conditions are such that a ligand in said further extraction cell is unsaturated.

Preferably, the contaminated product from the at least one further extraction cell is at least partially recycled to said first feed solution for step a).

In the present invention, the term "excess" has the meaning that there is more of the chemical in question, e.g. the at least one desired species, than is needed to combine or complex with all of another chemical, e.g. the extractant ligand, under the operating conditions of the process. In the method of the present invention when considered in the context of the invention being carried out in an ESPLIMtype extraction cell, this means that in the vicinity of the feed stream passing through the organic carrier liquid, at least all of the ligand in and adjacent the feed stream is complexed with the at least one desired species such that there is at least some of the at least one desired species passing through the cell into the raffinate and uncomplexed with the ligand.

The immiscible carrier phase may be a non-polar organic liquid in an ESPLIM-type cell as described above. However, the ESPLIM-type cell is operated with the extractant ligand in the saturated condition contrary to the teachings of the prior art. In the prior art, the ESPLIM cell is operated with an excess of ligand compared to the concentration of available species which it is desired to remove. In this way the ligand removes all of the species which it is required to remove from the feed and, the raffinate from the feed which separates out from the carrier phase is substantially purified of the particular species. Thus, the ESPLIM process may be used to remove a particular element, e.g. cobalt, from an aqueous solution so that the element may be recovered and/or the aqueous solution can be either disposed of or treated as a less toxic or harmful waste accordingly.

However, where the aqueous feed solution contains two or more species which are capable of combining with the ligand, the resulting product from the stripping solution stream will be contaminated with the other species present in the feed. This is a problem where it is required to remove one species selectively from the feed since when operated as in the known manner, the stripping solution will always be so contaminated.

The method of the present invention solves this problem by operating two sets of ESPLIM-type cells "in series". In the first set of cells conditions, including the concentration of the desired species and the rate at which the solution is passed through the first set of cells are such that the ligand in the carrier phase is saturated with the desired species, contrary to the teachings of the prior art. This ensures that the desired species is removed selectively by the ligand from the feed solution and transfers through the carrier phase to be removed by the stripping solution but, the raffinate from the feed solution is still contaminated with both the at least one desired species and the other species which are present. This contaminated raffinate then becomes the feed solution for a second ESPLIM-type cell in the first set where the process is repeated.

It may be that the remaining concentration of desired species in the raffinate feed solution contains insufficient desired species to ensure that only this species is selectively removed by the ligand. When this situation is reached, operation of the ESPLIM-type cells in the second set of cells may revert to that as taught in the prior art where the raffinate feed solution passes through the carrier liquid where there is an excess of the ligand present such that all of the remaining desired species present is removed to leave the emerging raffinate substantially purified of the desired species. In this case, the product from the stripping stream in this cell may be recycled back to the feed supplying the first set of ESPLIM-type cells.

One of the great advantages of the ESPLIM process is that an initially dilute aqueous feed solution can be greatly concentrated in the product produced by the stripping solution stream; concentration factors of 30 being common.

Therefore, once the stripping solution product becomes contaminated with the other species, the operation of the ESPLIM process may revert to the conventional, unsaturated ligand regime, for those ESPLIM-type cells in the second set downstream of the first set of cells where only the desired species is being removed. The ability of the ESPLIM system to concentrate dilute solutions is exploited to ensure that the recycle stream contains at least as high a concentration of the desired species as the original feed. An important advantage of the method of the present invention lies in this step, in that the high concentration of the desired species in this stream maintains the highest possible concentration of the desired product species in the feed to the first set of ESPLIM devices and hence allows said first set of ESPLIM devices to be more easily operated under the saturation regime.

Desirably, the concentration of the desired species in the product from the ESPLIM-type cell which is contaminated with the other species should be at least equal and preferably more concentrated than in the original feed solution to which it is being recycled. Thus the feed to the first ESPLIM extraction cell in the first set of cells is not diluted by a stream of lower concentration of the desired compound which allows saturation conditions in the first cell or cells to be more readily maintained.

Each ESPLIM-type cell in the process chain may be operated under chemical and/or process conditions which are most beneficial to the process objective of that particular cell.

In order for the ESPLIM process to function, it is necessary for there to be at least one different property between the feed and stripping streams. The most common property difference is that of pH where the stripping stream may be operated at a more or less acidic level than the feed stream. Therefore, when recycling the contaminated product containing other species back to the original feed, the pH may need to be adjusted to match that of the feed solution.

Different extractant ligand materials may be used in each ESPLIM-type cell as required.

In order that the present invention may be more fully understood, an example will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows a schematic arrangement of apparatus showing the basic operation and method of the prior art ESPLIM method; and Figure 2 which shows a schematic arrangement of a plurality of ESPLIM-type cells to carry out the method of the present invention.

Referring now to the drawings and where Figure 1 shows a schematic cross section through an apparatus 10 for carrying out the ESPLIM method of separation according to the prior art. The apparatus 10 comprises a reaction tank or vessel 12 which is divided at its upper portion by a wall 14 into an extraction cell 16 and a stripping cell 18.

At the lower end of the tank 12 there is a wall 20 which divides the tank into two receiving vessels or settling tanks 22, 24 for the purified feed solution or raffinate and, for the concentrated extractant in the stripping solution, respectively. Situated between the upper wall 14 and the lower wall 20 is a baffle 26 which allows an organic carrier liquid 28, in this case kerosene, to move freely throughout the tank 12. Electrodes 30, 32 are situated in the extraction cell side 16, between which a first high voltage AC electrostatic field may be applied.

Electrodes 34, 36 are situated in the stripping cell side 18, between which a second high voltage AC electrostatic field may be applied. In each of the cells 16, 18 at least one of the electrodes is insulated with, for example, a coating of polytetrafluoroethylene (PTFE) to prevent short circuiting within each cell. A controllable high tension supply (not shown) is provided for the electrodes so as to establish a desired potential therebetween. A conduit 40 is provided above the extraction cell 16 to supply a stream of feed solution 42 which is to be purified, into the carrier liquid 28. The conduit 40 has connected thereto pump means (not shown) and a reservoir tank (not shown) to provide a continuous supply of aqueous feed solution at a controlled rate. Another conduit 44 is provided above the stripping cell 18 to supply a stream 46 of aqueous stripping solution into the carrier liquid 28. The conduit 44 also has connected thereto pump means (not shown) and a reservoir tank (not shown) to provide a continuous supply of stripping solution at a controlled rate. Each of the receiving vessels 22, 24 have conduits 50, 52 to enable the raffinate 54 and the concentrate 56 to be drawn off as the level in each vessel rises or as required. The raffinate and concentrate are pumped to collection vessels (not shown) for disposal or further processing as required.

In operation, the apparatus 10 functions as follows and using as an example the extraction of cobalt metal ions from the feed solution 42 in which the Co ions are present at a concentration of lOOOppm in a 0.lM aqueous sodium acetate solution, the feed solution being supplied at a flow rate of 200 ml/hr into the carrier liquid. The stripping solution comprises a 1.OM solution of sulphuric acid which is supplied at a flow rate of 10 ml/hr into the carrier liquid. The diluent kerosene carrier liquid 28 has dissolved therein 10 volume% of di-(2-ethylhexyl)phosphoric acid (D2EHPA) extractant which is present in an excess quantity compared with the concentration of cobalt ions passing through the kerosene in the feed stream under steady conditions. An AC electrostatic field of 3KV supplied via a transformer from the mains supply is applied between the electrodes 30, 32 and 34, 36 to establish the first and second electrostatic fields. As the relatively large droplets of the feed solution 42 and stripping solution 46 fall into the extraction cell 16, they are subjected to the electrostatic fields between the electrodes 30, 32 and 34, 36 which have the effect of causing the relatively large droplets to break up into a multiplicity of microdroplets 60, 62 thereby greatly increasing the surface area to volume ratio of the two aqueous phases. In the extraction cell 16, the Co ions are extracted from the aqueous solution droplets due to the affinity of the D2EHPA thus causing the concentration of the Co-complex to rise in the extraction cell in the kerosene phase. Due to the concentration gradient so formed, the Co-complex diffuses through the kerosene through the baffle 26 towards the stripping cell 18 where the Co-complex reacts with the microdroplets 62 of the stripping solution where the Co-complex reacts with the sulphuric acid to free the D2EHPA, the Co ions reacting with the sulphuric acid and being concentrated therein. The D2EHPA then migrates back through the baffles 26 to the extraction cell 16 to establish a continuous chemical process. As the reacted droplets 60, 62 pass through the electrostatic fields under the influence of gravity, they eventually pass out of the electrostatic fields and begin to coalesce into larger droplets 70, 72 which fall into the receiving vessels 22, 24 as appropriate.

In experiments under the conditions described above, an initial feed solution of a Co concentration of 1000 ppm was purified to a concentration of 10 ppm in the raffinate 54, whilst the concentrate 56 had a concentration of 19,750 ppm of Co ions.

Therefore, it will be seen that the ESPLIM apparatus and method makes it possible to concentrate metal ions to a level where it is both practicable and economic to extract the concentrated metal ions so as to recover and reuse the metal per se. An example of this may be uranium. It is also clear that the feed solution may be so purified as to make disposal easier and/or less hazardous.

Referring now to Figure 2 where two ESPLIM-type cells 100, 102 are shown schematically, greatly simplified to aid clarity and arranged in series, i.e. the raffinate of one cell providing the feed solution to the next. Cell 100 is supplied with a feed solution 104 containing equal concentrations of three species A, B and C of which pure A is the desired product. All of the three species are capable of being extracted to some extent by the ligand in the organic carrier liquid (not shown) within the cells 100, 102. For illustrative purposes the concentration of A, B and C in the aqueous feed solution 104 is 1M.

In the first cell 100, the concentration of A in the feed stream is such that the ligand phase is saturated substantially only with the desired product A due to the higher selectivity attainable when the ligand is saturated and as explained hereinabove with respect to countercurrent solvent extraction. Under standard, non-saturated operating conditions, the ligand would also complex with a proportion of B and C depending upon their distribution ratios. By virtue of the concentration gradient, between the feed side 106 and strip side 108, of the ligand in the carrier liquid, the ligand complexed with species A migrates across the baffle 110 (in this diagram shown only as a diagonal line in each cell) towards the stripping solution stream (not shown so as to aid clarity) . The stripping solution then reacts with the saturated ligand to remove species A therefrom to produce a product 112 which is removed from cell 100 and which contains substantially pure A with substantially no B and/or C contaminants. This is only possible due to operating cell 100 with a saturated ligand contrary to the teachings of the prior art. The raffinate stream 114 then becomes the feed stream for the second ESPLIM cell 102 and still contains a relatively high content of A and all of the B and C contents of the original feed stream 104 contrary to the teachings of prior art ESPLIM systems. Example concentrations in the feed raffinate may be 0.4M A, 1M B and 1M C. The second cell 102 operates with the ligand in the conventional, non-saturated condition and efficiently extracts the remaining A but, with a certain proportion of B and C, in a ratio depending upon the distribution coefficients of A, B and C. In cell 102, advantage is taken of the ESPLIM cell's ability to concentrate dilute feeds into higher levels in the product stream by means of a property difference between the feed and strip streams. For example, an acidic extractant may be used in cell 102 and the strip phase (not shown) adjusted to a higher acidity than the feed 114. By operating cell 102 in the conventional unsaturated-ligand mode, the concentrations of species in the product 116 from cell 102 may, for example, be 1M A, 0.2M B and 0.1M C.

The product 116 from cell 102 may be recycled either to the original feed solution 104 or as an additive 118 to the strip solution for cell 102 in the process known as 'reflux', or to both. The raffinate stream 120 from cell 102 contains only the unwanted species B and C and may be disposed of as appropriate. The product stream 116 contains at least a similar concentration of species A as the original feed solution 104 and may therefore be recycled to the feed 104. Since the pH of product stream 116 will have a different value to that required for the feed 104, it may be readjusted in a suitable process step indicated at 122.

Claims (3)

CLAAS

1. A method of improving the selectivity of at least one desired species from an aqueous first feed solution containing at least two potentially extractable species, the method comprising the steps of: a) passing a stream of said aqueous feed solution through a first extraction cell containing a first immiscible carrier phase having therein an extractant ligand to combine with and extract said at least one desired species from said feed solution; b) ensuring that operating conditions in said first extraction system are such that said ligand is saturated with said at least one desired species; c) allowing said saturated ligand to react with an aqueous stripping solution stream also passing through said first immiscible carrier phase so as to remove said at least one desired species therefrom to form a product stream having a higher concentration of said at least one desired species than said feed solution, and to release said ligand back into said first immiscible carrier phase to complex with further at least one desired species; d) repeating steps a) to c) with subsequent extraction cells, a feed solution to each subsequent cell being constituted by a raffinate solution from an immediately prior extraction cell, until a concentration of said at least one desired species in said raffinate feed solution falls below a level where said ligand is able to be saturated by said at least one desired species and a product contaminated with said other species is formed; and e) passing a raffinate from the last feed solution to at least one further extraction cell wherein operating conditions are such that a ligand in said further extraction cell is unsaturated.

2. A method according to claim 1 wherein the contaminated product from the at least one further extraction cell is at least partially recycled to said first feed solution.

3. A method substantially as hereinbefore described with reference to the accompanying description and Figure 2 of the drawings.