Coprecipitation of Selenium and Tellurium with Collectors

I. P. Onufrienok, V. M. Aksenenko, E. S. Novikova

(Presented by the scientific seminar of the Department of Analytical Chemistry)

At low contents of selenium and tellurium, their quantitative precipitation from solutions is possible only with the use of collectors. For this purpose, the use of $\mathrm{Fe}^{3+}$ and $\mathrm{Al}^{3+}$ is known under conditions in which their hydroxides are precipitated [1, 2]. It is assumed that selenites and tellurites thereby form the corresponding salts of iron and aluminum. It is also known that tellurium is then separated quantitatively, while selenium is not; the incomplete precipitation of selenium is especially pronounced if the precipitation is carried out in an alkaline medium from ammonia. As for the coprecipitation of selenium and tellurium in elemental form with collectors, we have not encountered in the literature any works indicating such a possibility.

In studying the coprecipitation of selenium and tellurium, we used as collectors for selenites and tellurites $\mathrm{Fe}^{3+}$, $\mathrm{Al}^{3+}$, $\mathrm{Bi}^{3+}$, and $\mathrm{Ce}^{3+}$, and for the coprecipitation of elemental selenium and tellurium—arsenic. Intending subsequently to use the coprecipitation of selenites and tellurites with collectors for separating the latter from platinoids and copper, we decided to find collectors that would quantitatively coprecipitate selenites and tellurites in a weakly ammoniacal medium.

The coprecipitation of selenites and tellurites and the subsequent determination of selenium and tellurium were carried out according to the following procedure.

To aliquot portions of standard solutions of tellurites or selenites, the corresponding quantities of collector solutions indicated in Tables 1–5 were added. The resulting solutions were acidified with 1 ml of HCl (1:1), 10 drops of a 0.1% phenolphthalein solution were added to them, and precipitation was carried out with ammonia until a faint pink coloration appeared. If the coprecipitation was conducted in the presence of copper, ammonia was added until the solution above the precipitate became faintly blue. The precipitates obtained were filtered off and washed with a 2% ammonium chloride solution, to which ammonia had been added until a faint coloration of phenolphthalein appeared. If copper was present in the solution, the washed precipitate was dissolved in HCl (1:1) and reprecipitated again until the copper was completely removed.

After washing, the precipitates were dissolved in HCl (1:1); the solutions were collected in 50-ml volumetric flasks, where selenium or tellurium was reduced with an excess of $\mathrm{SnCl}_2$; then 2 ml of a 1% gelatin solution was added, the solutions were made up to the mark with HCl (1:1) and mixed, after which selenium or tellurium was determined colorimetrically. In colo-

rimetry, for comparison the same aliquot portions of standard solutions were taken into 50-ml volumetric flasks, where selenium or tellurium was reduced with an excess of SnCl₂, and further preparation for colorimetry was carried out in the same way as for the samples subjected to coprecipitation. The results obtained are shown in Tables 1–5.

Analyzing the data of Tables 1 and 2, we can see that tellurium is coprecipitated practically completely with Fe³⁺ and with Bi³⁺, but not completely with Ce³⁺; selenium, however, is coprecipitated practically completely with Ce³⁺ and Bi³⁺, but not completely with Fe³⁺.

As a collector for selenium and tellurium, Bi³⁺ would be very suitable for our purposes, but when we tested it for the coprecipitation of selenium in the presence of copper, it turned out that in this case it precipitates selenium non-quantitatively, even if introduced in a tenfold excess relative to what is required for coprecipitation in the absence of copper. It was evident that part of the selenite does not have time to form bismuth selenite before the pH at which its hydroxide precipitates, and, upon further increase of the pH, selenium(IV), not encountering trivalent cations, forms copper selenite, which subsequently passes into solution in an ammoniacal medium. For the same reason, Al³⁺, which we tested only qualitatively, was obviously unsuitable as a collector. It was clear that as a collector it was necessary to choose a trivalent cation that is not amphoteric, does not form ammine complexes, and at the same time has a hydroxide precipitation pH higher than that of Bi³⁺; therefore, we chose Ce³⁺ as the collector for selenium. In addition, we came to the conclusion that, for simultaneous coprecipitation of selenite and tellurite, a combined collector is necessary. Such a collector proved to be a mixture of Fe³⁺ and Ce³⁺ (Table 4).

Table 1

Table 2

Coprecipitation of selenium in the presence of copper by means of a combined collector is shown in Table 5.

Coprecipitation of elemental Se and Te with As.

It was known that NaH₂PO₂ and Ca(H₂PO₂)₂ precipitate elemental Se, Te, As, and Hg from an acidic medium [3]. It is also known that Se, Hg, and As can be precipitated by these reducing agents in a strongly acidic medium, whereas tellurium requires a more moderate acidity for its precipitation.

Table 3

Table 4

Coprecipitation of selenium with 5 ml of 0.4 N FeCl₃ and 2 ml of 25-percent CeCl₃ solution

Table 5

Anticipating that subsequently the determination of selenium and tellurium would be completed polarographically, we chose arsenic as the collector for the latter. It was therefore necessary to select the acidity limits of solutions from which selenium and tellurium could be quantitatively precipitated together with arsenic.

First of all, the precipitation of arsenic by calcium hypophosphite from solutions 1, 2, 3, 4, 5, 6, 7, and 8 N in hydrochloric acid was tested; it was found that, in the cold and on standing for 24 hours, calcium hypophosphite does not precipitate elemental arsenic from any of the solutions. Heating on a water bath for 10–15 minutes causes reduction of arsenic in solutions 8, 7, and 6 N in hydrochloric acid; later, after 20–25 minutes, reduction begins from a solution 5 N in hydrochloric acid. As for the reduction of arsenic from 4, 3, and 2 N hydrochloric-acid solutions, the onset of reduction is observed only after the solutions have been boiled for several minutes.

It was further established that selenium and tellurium from selenites and tellurites are quantitatively precipitated by hypophosphite from solutions 4, 6, and 8 N in HCl when these solutions are brought to boiling. Thus, the joint coprecipitation of selenium and tellurium with arsenic may be recommended according to the following procedure.

To 20 ml of a solution 8 N in hydrochloric acid, containing 1 mg each of Se and Te in the form of selenious and tellurous acids, add 1 ml of a solution of sodium arsenite containing 2 mg of arsenic and 4 ml of a 10-percent solution of \(\mathrm{Ca(H_2PO_2)_2}\). The solution is heated to boiling and then kept on a boiling water bath for 20 minutes, after which it is allowed to stand for 8 hours.

Conclusions

1. The optimum conditions have been found for the quantitative coprecipitation of selenites and tellurites in a weakly ammoniacal medium with a combined collector consisting of \(\mathrm{Fe^3}\) and \(\mathrm{Ce^3}\).

2. The optimum conditions have been found for the quantitative coprecipitation of elemental selenium and tellurium with arsenic as the collector.

REFERENCES

1. V. F. Gillebrand, G. E. Lendell, G. A. Bright, D. I. Gofman. Practical Manual of Inorganic Analysis. GONTI, Moscow, p. 355, 1957.

2. M. F. Proshkovich, P. V. Faleev. Analytical determination of selenium and tellurium in ores and products of their processing. Ore Beneficiation, No. 2 (8), Leningrad, pp. 21–24, 1957.

3. M. M. Stukalov. Arsenic and its ores. In: Analysis of Mineral Raw Materials, edited by Yu. N. Knipovich and Yu. V. Morachevskii. GONTI, Leningrad, p. 495, 1955.