The process to be presented makes use of a commercial mixture of highly-branched monocarboxylic acids with the trade name of “Versatic 10” (Shell). The formula may be presented as:
where the total number of carbon atoms in the molecule is about 10. In short the Versatic acid will be termed RH.
The organic acid RH will act as a liquid cation exchanger and may generally extract metal ions from an aqueous solution by forming an organic metal salt solution:
As indicated by equation (1) the extraction will be dependent of the pH of the aqueous phase. The distribution curves for different metals in carboxylic acids have been investigated and reported at length over the years. The extraction curves for some common metals with the Versatic 10 system are shown in Figure 1. As can be seen, the curves for zinc and ferric iron are far apart, and a very efficient separation of the two metals is thus to be expected.
Although the use of carboxylic acids as extracting agents for metals has been known for many years, they have not been applied much in commercial processes so far. The reason for this is mainly to be found in the extracting reaction itself. It will be seen that the extraction of the metal ions liberates an equivalent amount of protons from the organic acid. To obtain an efficient extraction of metal ions from the aqueous phase, an equivalent amount of alkali has thus to be added to neutralize the acid proton formed.
In most cases this addition of alkali has been considered prohibitive for any commercial operation. Thus Spitzer (Shell Research) has pointed to this fact, discussing the use of Versatic 10:
“However, this type of extraction has one drawback in common with precipitation in requiring the addition of base (ammonia or caustic for example) in an amount sufficient to neutralize any free acid present initially, and to permit extraction of the metal ion of interest, and all metal ions preceding it in the extraction series. This means that in order to extract for instance copper, every other metal ion preceding copper in the extraction series has to be removed – either by precipitation, which however, is undesirable because of co-precipitation losses, or by extraction. In any case one molecule of caustic is consumed per valency of the iron removed. In the case of trivalent iron – which is invariably present in copper leach liquors – this means a considerable caustic consumption, which has been shown by calculation to make carboxylic acid extraction often economically unattractive for this type of separation.”
In the process presented in this paper, it will be shown, in contrast with the above quotation, that there will be no need for the addition of any external alkali, in order to extract ferric iron into the organic phase. The process makes use of the alkaline property of the calcine material itself by starting the process with organic leaching, i.e. leaching zinc oxide with the organic carboxylic acid RH.
It has been found that Versatic acid is highly reactive in a direct reaction with the zinc oxide of the calcine:
Having formed the Versatic zinc salt of the organic acid, it will be seen that according to the distribution curves in Figure 1, the extraction of ferric iron may take place by the ion exchange reaction:
Thus by reactions (2) and (3) zinc will be brought from the calcine material into an aqueous sulphate solution via the dissolution of zinc oxide in the organic phase. By the same overall operation iron has simultaneously been removed from the aqueous sulphate solution and extracted into the organic phase as a ferric organic salt. The crucial point and net result of this combined operation of leaching and solvent extraction is that no addition and thus no cost of alkali, has been necessary to have the iron extracted into the organic phase.
The use of the exchange reaction, Equation (3), for removing ferric iron from zinc sulphate liquors was demonstrated already by Fletcher and Fleet. Their conclusion was, however, along the same lines as the above quotation from Spitzer. Thus it is concluded by Fletcher and Flett that “A major factor in the economics of the use of carboxylic acids for the recovery of metals by solvent extraction is not the solvent loss but the amount of alkali required. Thus for divalent metals two moles of alkali are required for every mole of metal extracted.” Indeed, the presented calculations showed that the alkali cost was more than forty times greater than the cost for solvent make-up.
Further results on the use of the zinc salt of Versatic acid for the removal of iron zinc calcine leach solutions has recently been published by Van der Zeeuw. The results obtained confirm the excellent properties of Versatic acid for the extraction of iron. The fate of some of the other contaminants such as cobalt, copper, arsenic, antimony and germanium is also discussed with the conclusion that these elements are not likely to cause any serious problem. The last quantities if iron are suggested to be precipitated as hydroxide in order to remove the undesirable contaminants by co-precipitation.
Solvent Extraction of Iron in a Zinc Plant
The conceptual flowsheet in Figure 2 shows how the process for extracting iron by a carboxylic acid is integrated into the hydrometallurgical processing of zinc. The neutral leach and the hot acid leach will basically be the same as in the conventional Jarosite and Goethite processes. By replacing the precipitation steps for jarosite or goethite, the iron extraction will be brought about by the following operations: the organic leach, solvent extraction of iron, and the stripping of iron from the organic phase.
In the organic leach step enough zinc salt of the organic acid has to be produced in order to 1) neutralize the free sulphuric acid in the hot acid leach and 2) extract iron by the exchange reaction (3) at pH 2-3. Before the exchange reaction between zinc and ferric iron is taking place, the free sulphuric acid will be neutralized by the organic zinc salt:
The pH of the aqueous phase will thus increase until the exchange reaction takes place between pH 2-3.
The iron concentration in the hot acid leach solution will be reduced from some 10-25 g/l Fe to 0.5-1 g/l Fe. The iron may be removed quantitatively if necessary, but it is most likely of advantage to leave a small amount for precipitation as ferric hydroxide due to its scavenging effect on trace metals impurities.
Stripping of iron from the organic phase may be done by a mineral acid or by hydrolytic stripping which will be explained in detail later. The iron product, which may be completely free from zinc and other impurities, will be an iron salt or iron oxide, depending on the stripping procedure.
In the following, each of the three operations, the organic leach, the solvent extraction, and the stripping will be discussed and described in more detail.