An ion-exchange process is characterized by passing an aqueous solution countercurrent to an ion exchange material between at least two stages, the solution and material passing co-current through each stage and then being separated from each other. Regenerated anion-exchange resin and liquid from a regenerating tank 25, Fig. 1, are pumped (35) as a slurry to a vibrating screen 33 for separation, whence the liquid is pumped to a storage tank 37 and the resin is discharged into the third treating tank C of a series A, B, C. Resin and liquid are pumped in turn from tank C to a vibrating screen 30 above tank B, the resin finally reaching tank A. It is returned to the regenerating tank via pump 21, vibrating (separating) screen 23 and washing screen 24. Meanwhile, the liquid to be treated is pumped from a storage tank 11 to tank A where it is intimately contacted with the resin by stirrers 7 and baffles 8 and emerges as a slurry with the resin, to be passed on to tank B via pump 21, vibrating screen 23 and pipe 26. Thence it is passed on to tank C via pump 19, vibrating screen 17 and <PICT:0768050/IV(b)/1> <PICT:0768050/IV(b)/2> pipe 32. The liquid pumped together with the resin from tank C at 29 and separated from the resin at screen 30 is the original liquid freed from an ion preferentially adsorbed by the resin. It is centrifuged from any solid impurity at 39 and stored at 42. The liquid withdrawn from the regenerated resin at 33 is exhausted regenerant solution containing the above-identified ion desorbed from the resin by the regenerant, and is stored at 37, e.g. for subsequent recovery of that ion. In an example, the liquid to be treated is raw blackstrap molasses containing free aconitic acid and its calcium and potassium salts, and sulphate and chloride anions. Accordingly, the liquid received at 42 is molasses free from aconitate ions and the liquid received at 37 is a solution containing aconitic acid. The regenerant for the resin is sulphuric acid, added at 31 to the tank 25 at a rate to maintain a pH of 0.9. The adjustment of pH to from 4 to 6 (preferably 5) for the treatment process is achieved by addition of lime or the like to tank C at 2. Itaconic, citric, tartaric, malic, or oxalic acid may likewise be recovered from fruit or vegetable waste. In Fig. 2, liquid to be treated (e.g. molasses) is introduced to a tank 121 from outside the system and passed therefrom in turn through treating tanks 100 to 102 having associated therewith vibrating screens 108 to 106 (as in Fig. 1) countercurrent to anion-exchange resin (shown by a double line), the liquid emerging from screen 106 free from, e.g., aconitate ions. The resin is passed from the above (stripping) section to a rectification section 103 to 105 countercurrent to a second solution (e.g. ammonium hydroxide) fed to tank 105 and containing an anion (the OH ion) which is preferentially adsorbed by the resin so that as much as possible of undesired ions is displaced from the resin. The resin is regenerated in a tank 134 whith a mineral acid to produce a solution of aconitic acid. The "second solution" may contain, as effective anion, instead of the OH ion, the desired anion itself (i.e., aconitate ion in the above example) or an anion which is tolerable as an impurity in the aconitric acid solution eventually obtained. The rectification section ensures that the solution of recovered ion contains still less of the original impurities. Fig. 3 (not shown), applies the technique of Fig. 2 to the separation of two ions A and B, e.g. potassium and sodium, for which the ion-exchange material exhibits different affinities, present as their chlorides. In this case, if A is the ion preferentially adsorbed, the solution introduced at the end of the series (i.e., to tank 105, Fig. 2) preferably contains ion A. In the process of Figs. 1 and 2, the pH range may be maintained by mixing a cation-exchange resin in the H-form with the anion-exchange resin. In the process of Fig. 3, a mixture of cation-and anion-exchange material may be used to separate, on the former, a cation and, on the latter, an anion, simultaneously, the second solution then containing a salt having that cation or that anion.ALSO:An ion-exchange process is characterized by passing an aqueous solution counter-current to an ion-exchange material between <PICT:0768050/III/1> <PICT:0768050/III/2> at least two stages, the solution and material passing co-current through each stage and then being separated from each other. Regenerated anion-exchange resin and liquid from a regenerating tank 25, Fig. 1, are pumped (35) as a slurry to a vibrating screen 33 for separation, whence the liquid is pumped to a storage tank 37 and the resin is discharged into the third treating tank C of a series A, B, C. Resin and liquid are pumped in turn from tank C to a vibrating screen 30 above tank B, the resin finally reaching tank A. It is returned to the regenerating tank via pump 21, vibrating (separating) screen 23 and washing screen 24. Meanwhile the liquid to be treated is pumped from a storage tank 11 to tank A where it is intimately contacted with the resin by stirrers 7 and baffles 8 and emerges as a slurry with the resin, to be passed on to tank B via pump 21, vibrating screen 23 and pipe 26. Thence it is passed on to tank C via pump 19, vibrating screen 17 and pipe 32. The liquid pumped together with the resin from tank C at 29 and separated from the resin at screen 30 is the original liquid freed from an ion preferentially adsorbed by the resin. It is centrifuged from any solid impurity at 39 and stored at 42. The liquid withdrawn from the regenerated resin at 33 is exhausted regenerant solution containing the above-identified ion desorbed from the resin by the regenerant, and is stored at 37, e.g. for subsequent recovery of that ion. In an example the liquid to be treated is raw blackstrap molasses containing free aconitic acid and its calcium and potassium salts, and sulphate and chloride anions. Accordingly, the liquid received at 42 is molasses free from aconitate ions and the liquid received at 37 is a solution containing aconitic acid. The regenerant for the resin is sulphuric acid, added at 31 to the tank 25 at a rate to maintain a pH of 0.9. The adjustment of pH to from 4 to 6 (preferably 5) for the treatment process is achieved by addition of lime or the like to tank C at 2. Itaconic, citric, tartaric, malic or oxalic acid may likewise be recovered from fruit or vegetable waste. In Fig. 2 liquid to be treated (e.g. molasses) is introduced to a tank 121 from outside the system and passed therefrom in turn through treating tanks 100 to 102 having associated therewith vibrating screens 108 to 106 (as in Fig. 1) countercurrent to anion-exchange resin (shown by a double line) the liquid emerging from screen 106 free from, e.g., aconitate ions. The resin is passed from the above (stripping) section to a rectification section 103 to 105 counter-current to a second solution (e.g. ammonium hydroxide) fed to tank 105 and containing an anion (the OH ion) which is preferentially adsorbed by the resin so that as much as possible of undesired ions is displaced from the resin. The resin is regenerated in a tank 134 with a mineral acid to produce a solution of aconitic acid. The "second solution" may contain, as effective anion, instead of the OH ion, the desired anion itself (i.e., aconitate ion in the above example) or an anion which is tolerable as an impurity in the aconitic acid solution eventually obtained. The rectification section ensures that the solution of recovered ion contains still less of the original impurities. Fig. 3 (not shown) applies the technique of Fig. 2 to the separation of two ions A and B, e.g., potassium and sodium, for which the ion-exchange material exhibits different affinities, present as their chlorides. In this case, if A is the ion preferentially adsorbed, the solution introduced at the end of the series (i.e. to tank 105, Fig. 2) preferably contains ion A. In the process of Figs. 1 and 2 the pH range may be maintained by mixing a cation-exchange resin in the H-form with the anion-exchange resin. In the process of Fig. 3 a mixture of cation- and anion-exchange material may be used to separate, on the former, a cation and, on the latter, an anion, simultaneously, the second solution then containing a salt having that cation on that anion.