DETERMINATION OF SELENIUM AND TELLURIUM IN COPPER ELECTROLYSIS SLUDGES

I. P. Onufrienok, V. M. Aksenenko

(Presented by Professor, Doctor of Chemical Sciences A. G. Stromberg)

The determination of selenium and tellurium in copper electrolysis sludges is a difficult task because the sludge contains large amounts of copper along with silver, gold, platinum, and platinoids. The latter are often precipitated by the same reagents as selenium and tellurium. Therefore, in analysis it is necessary to create conditions for the complete separation of copper and noble metals from selenium and tellurium, and only after this to determine the latter.

From the work of M. F. Proshkovich and P. V. Faleev [1] it is evident that tellurium, and also a large part of selenium, can be separated from copper and platinoids in the form of $\mathrm{Fe}_2(\mathrm{TeO}_3)_3$ and $\mathrm{Fe}_2(\mathrm{SeO}_3)_3$ either in a weakly acidic medium in the presence of sodium nitrate, or by hydrolysis of ferric acetate, or by precipitation with ammonia to a weakly alkaline medium. In all cases they found that tellurium is carried into the precipitate quantitatively, while selenium is not. From their work we concluded that $\mathrm{Fe}^{3+}$ is a good collector for tellurium and insufficient for selenium.

In our investigation we set ourselves the goal of finding suitable collectors which, upon precipitation with ammonia, would coprecipitate selenium and tellurium quantitatively, with simultaneous liberation from large amounts of copper and from most of the platinum and platinoids.

As collectors for selenium and tellurium we tested $\mathrm{Fe}^{3+}$, $\mathrm{Bi}^{3+}$, $\mathrm{Al}^{3+}$, $\mathrm{Ce}^{3+}$¹); it turned out that a combined collector consisting of $\mathrm{Fe}^{3+}$ and $\mathrm{Ce}^{3+}$ quantitatively coprecipitates selenium and tellurium in a weakly ammoniacal medium, and that during reprecipitations separation occurs from large amounts of copper and from most of the platinoids.

Subsequently selenium and tellurium are separated from iron and cerium by reducing them with stannous chloride from a hydrochloric-acid solution in elemental form; in this precipitate of selenium and tellurium small amounts of copper and platinoids were observed, from which selenium and tellurium are subsequently separated from alkaline solutions.

In one of our works [2] we showed the possibility of separating copper and selenium from tellurium in an alkaline medium if tellurium is precipitated with milk sugar from a weakly alkaline medium in the presence of $\mathrm{K}_4\mathrm{Fe}(\mathrm{CN})_6$, but for sludges, bearing in mind the subsequent determination of selenium, this pri-

¹) See the paper in this volume, “Coprecipitation of Selenium and Tellurium with Collectors.”

copper cannot be changed. Therefore we propose separating copper from selenium and tellurium with the aid of \(\mathrm{K_2CO_3}\) in the form of \(\mathrm{CuO}\). The residues of the platinoids must also be reduced and separated from selenium and tellurium in an alkaline medium, which we achieved by boiling an alkaline \(\mathrm{K_2CO_3}\) solution with magnitom; the latter, under these conditions, reduces neither selenium nor tellurium. Thus, for the determination of selenium and tellurium in copper-electrolysis sludges, it became possible to propose the following procedure.

A weighed portion of sludge of 1 g is taken in a 250-ml conical flask with a glass stopper, treated with 5 ml of \(\mathrm{H_2SO_4}\) (1:1) and 10 ml of \(\mathrm{HNO_3}\) (sp. gr. 1.40), with heating to boiling and disappearance of nitrogen oxides. After this, 20 ml of \(\mathrm{HCl}\) (sp. gr. 1.19) is added and the sample is heated on a water bath for 2 hours with periodic stirring; then 50 ml of water is added to the flask, heated to boiling, and allowed to stand for 1 hour. The insoluble residue is filtered off, washed with 1-percent \(\mathrm{HNO_3}\), and discarded.

To the filtrate are added 5 ml of 0.25 N \(\mathrm{FeCl_3}\) solution and 2 ml of a 25% \(\mathrm{CeCl_3}\) solution; precipitation is carried out with ammonia until the supernatant solution is faintly blue. The precipitate is filtered off and washed with a 2-percent solution of \(\mathrm{NH_4Cl}\) to which ammonia has been added, until phenolphthalein gives a pink coloration.

The precipitate is dissolved in \(\mathrm{HCl}\) (1:1) and reprecipitated once or twice to free it from copper.

After the separation from copper, the precipitate is washed from the filter, with the smallest possible amount of water, back into the flask; the solution from dissolving the residues of the precipitate on the filter with concentrated \(\mathrm{HCl}\) is added to the same flask. The solution in the flask is diluted with concentrated \(\mathrm{HCl}\), added in such an amount that the solution in the flask becomes \(\mathrm{HCl}\) (1:1). Then, disregarding the fact that not all of the \(\mathrm{Ce(OH)_4}\) precipitate has dissolved, the mixture is heated and an excess of 20-percent \(\mathrm{SnCl_2}\) solution is added; the latter promotes rapid dissolution of \(\mathrm{Ce(OH)_4}\) and reduces selenium, tellurium, gold, and platinum. The solution with the precipitate is boiled for 10 minutes for better coagulation of the precipitate, and then cooled.

The precipitate is filtered through a dense filter, washed first with cold 1-percent \(\mathrm{HCl}\), then two or three times with hot 1 N \(\mathrm{HCl}\), to dissolve \(\mathrm{Cu_2Cl_2}\). The washed precipitate together with the filter is placed in the flask in which the precipitation was carried out; 10 ml of \(\mathrm{HCl}\) and 5–6 drops of \(\mathrm{HNO_3}\) (sp. gr. 1.40) are added to the same flask, and the flask is heated gently on a water bath until the precipitate dissolves. The solution is then filtered through a small filter of medium density, and the filter is washed with 1-percent \(\mathrm{HCl}\).

The filtrate is collected in a 200-ml beaker, where it is neutralized with solid potash in the presence of phenolphthalein until the solution becomes pink; then the solution is heated to boiling, during which very small amounts of copper that have passed through precipitate as \(\mathrm{CuO}\). Without filtering off the precipitate, an additional 0.5 g of \(\mathrm{K_2CO_3}\) and 0.25 g of magnitom are introduced into the beaker, and boiling is continued to reduce platinum and gold; this requires boiling for 10–15 minutes. The precipitate is then filtered through a filter with the addition of filter-paper pulp, washed with hot 1-percent \(\mathrm{K_2CO_3}\) solution, and the filtrate is collected in a 250-ml conical flask.

To the filtrate are added 1 g of \(\mathrm{KNaC_4H_4O_6}\), 5 ml of a 20-percent freshly prepared solution of milk sugar, and 3 g of \(\mathrm{Na_2SO_3}\); the mixture is heated and then boiled for 25 minutes, after which it is allowed to stand until cooled. The tellurium precipitate is filtered through a porous crucible No. 3, washed first with a 2-percent hot solution of \(\mathrm{Na_2SO_4}\), then 5–6 times with freshly boiled hot water, and then twice with alcohol. The precipitate is dried

at 105° and weighed as tellurium. A variant of the titrimetric end-point determination of tellurium can also be used successfully [2].

The filtrate after separation of tellurium is used for the determination of selenium. To isolate selenium, the filtrate is acidified with hydrochloric acid until the evolution of CO₂ ceases; then 5 ml of a saturated solution of hydrazine sulfate, 2 g of Na₂SO₃, and an equal volume of HCl (sp. gr. 1.19) are added to it. The solution is mixed by rotary motion for 2–3 minutes and then left to stand for 8 hours. The separated selenium is filtered through a porous crucible No. 3, washed thoroughly with a 1-percent HCl solution, then twice with alcohol, dried at 100°, and weighed as selenium.

A sludge sample containing 1.72% tellurium and 8.04% selenium, when analyzed according to the procedure described above, gave 1.70, 1.71, and 1.74% tellurium and, respectively, 8.02, 8.00, and 8.03% selenium. Thus, the proposed method has sufficient accuracy and good reproducibility.

Conclusions

1. A method has been developed for determining selenium and tellurium in copper electrolysis sludges with sufficient accuracy and good reproducibility, and it is simple to carry out.

2. The use of a combined collector consisting of Fe³⁺ and Ce³⁺ for the coprecipitation of selenium and tellurium in the analysis of copper electrolysis sludge ensures the obtaining of correct results for the determination not only of tellurium, but also of selenium.

Literature

1. M. F. Proshkovich and P. V. Faleev, Obogashchenie rud, No. 2 (8), Leningrad, pp. 21–24, 1957.

2. I. P. Onufrienok and V. M. Aksenenko, Zhurnal analiticheskoi khimii, XIII, No. 1, pp. 119–122, 1958.