Results of the Study of Nickel Migration Forms in Sulfate Waters of the Norilsk Copper-Nickel Deposit

S. L. Shvartsev, E. P. Baburova

(Presented by Professor P. A. Udodov)

A more complete study of the microcomponent composition of groundwaters, carried out in recent years in connection with the introduction into practice of the hydrogeochemical method of prospecting, has shown that many microcomponents in groundwaters migrate in the form of complex compounds, which raise the pH of precipitation of the hydroxides of these elements, thereby promoting the stability of their aqueous solutions. In this connection, experimental verification of the forms in which many heavy metals occur in waters is of great interest.

One of the elements of great importance in hydrogeochemical prospecting for copper-nickel and other sulfide ores is nickel. Studies of recent years indicate that nickel in groundwaters migrates not only in the cationic form $\mathrm{Ni}^{2+}$, as is generally believed, but also forms complex complexes with organic matter [1]. In such complex multicomponent systems as the natural waters under study, the direction of reactions forming particular nickel complexes is regulated not only by the values of the constants of their stability, but also by the ratio of the concentrations of addends present in the solution. Elementary thermodynamic calculations show that in sulfate waters there should exist complex compounds of the type $\mathrm{NiSO_4^{0}}$, $\mathrm{Ni(SO_4)_2^{2-}}$, etc. The question also remains unstudied of the possibilities of nickel migration in the form of fine colloids. All this requires the formulation of appropriate experimental work.

In the present work we set ourselves the task of experimentally verifying the existence of complex nickel compounds in acidic and neutral sulfate waters. For the studies, groundwaters from the oxidation zone of the Norilsk deposit were taken. The pH value and composition of these waters are given in Table 1.

In addition, Cu, Co, Cr, V, Ag, Ti, and Mn were detected spectrally in significant quantities in these waters. A more detailed characterization of the composition of these waters is given in a work by one of the authors [2].

To determine nickel in natural waters, the reaction with dimethylglyoxime was used, making it possible to determine nickel by the colorimetric method with high sensitivity ($0.1\ \mu\mathrm{g}/\mathrm{l}$). Owing to the solubility of the complex compound of nickel with dimethylglyoxime in chloroform, nickel is separated from the components interfering with its determination. This method is applicable to the analysis of fresh and saline waters with mineralization up to $100$–$150\ \mathrm{g}/\mathrm{l}$. At the beginning of the investigations it was necessary to esta-

Table 1.

Chemical composition of groundwaters
of the Norilsk deposit, mg/l

to determine what portion of nickel in these waters is in the colloidal state. For this purpose two methods were used: filtration of the water through a ceramic filter and dialysis, which make it possible to separate the colloidal form from the ionic form in solutions. For filtration, a Buron bacteriological filter (France) was used, intended for purifying drinking water from microbes and having a pore size of 0.1–0.2 µm. In order to clarify the possibility of nickel sorption by the filter pores, a standard solution containing 50 µg/l of nickel was passed through it. Analysis of this solution at the outlet from the filter showed that sorption of nickel by the pore walls, if it exists at all, is only within the limits of the error of the analytical method used and cannot substantially affect the results of the investigations. Then samples of natural water were passed through the filter. The results of this experiment are given in Table 2.

Table 2

Results of water filtration through
a ceramic filter

From the data in this table it may be concluded that nickel is not retained by the ceramic filter and, consequently, in the waters studied it may occur in the form of simple ions, complex compounds, or colloidal particles whose size does not exceed 0.2 µm. To test this last assumption, experiments were set up on the dialysis of nickel solutions through cellophane films.

Dialysis was carried out in a dialyzer of the simplest design. The water under study was poured into a beaker whose bottom was replaced by a stretched cellophane film with a pore size of \(2 \cdot 10^{-3}\) µm. This beaker was then placed in a vessel with distilled water. The water level in both vessels was kept the same. Twice a day, a fresh portion of distilled water was poured into the lower beaker, and its nickel content was analyzed until its content in the solution became equal to zero.

Then, in the waters studied, the amount of nickel that remained in the natural solution was determined, i.e., that portion of nickel which did not

passed through the cellophane film and, consequently, was present in the form of compounds ranging in size from \(2 \cdot 10^{-3}\) to \(0.2\) microns. As can be seen from the results obtained (Table 3), the overwhelming part of the nickel diffused through the cellophane film, whereas a considerably smaller part remained in the upper beaker. Most probably, this latter part of the nickel is present in the solution in the form of the finest colloidal formations. This conclusion is in good agreement with the results of field observations.

Table 3

Results of dialysis of groundwaters through a cellophane film

The fact is that sample No. 2, with a low pH value, was taken in the Zapolyarny underground mine from an abandoned adit, where the water is practically in a motionless state, without any mixing. Prolonged interaction with ore minerals contributed to a sharp decrease in pH and, consequently, to the destruction of colloidal formations. The latter must either be destroyed in an acidic medium or, over a long period, must precipitate out of solution. Therefore it is logical that only 2.3% of the nickel in this sample is present in solution in the form of the finest colloids.

A different picture is observed in water sample No. 1, taken from a spring with a discharge of 0.2 l/sec, associated with the face of one of the adits of the same mine. The almost neutral water medium and its continuous movement through the pores of the rocks are a more favorable environment for the migration of nickel, along with the truly dissolved form, also in the form of fine colloidal particles or complex compounds of considerable size formed with organic matter. The possibility of the existence of the latter in groundwaters was shown by Yu. Yu. Bugel’skii in the work mentioned earlier. According to the data we obtained (Table 3), almost 30% of the nickel under these conditions migrates in the form of fine colloidal particles.

The next stage of the work was determination of the sign of the charge of nickel ions in the samples studied by the method of electrodialysis. For the work, a three-chamber electrodialyzer with two membranes (based on a cation exchanger and an anion exchanger) with a platinum cathode and anode was used. Into the cathode and anode chambers, 40 ml of distilled water was poured, and several crystals of KCl were added to increase the ionic strength of the solution. Into the middle chamber, the studied water sample was poured in a volume of 140 ml. Throughout the entire period of the experiment, it was necessary to maintain a constant pH value in the middle chamber. For this purpose, a constant flow of water is usually created in the cathode and anode chambers. The design of the instrument we used did not make it possible to maintain a constant flow. Therefore, once a day a fresh portion of distilled water was poured into the cathode and anode chambers; this water was then analyzed for nickel content. The results obtained are summarized in Table 4. Electrodialysis was carried out at a voltage of 200–250 V and a current not exceeding 100 mA.

Table 4

Results of electrodialysis of groundwater

The results of electrodialysis show that from acidic waters (sample No. 2) the greater part of the nickel migrated into the cathode chamber, whereas from neutral waters it migrated into the anode chamber. This circumstance, not entirely clear at first glance, is explained by the fact that in neutral waters the greater part of the nickel is apparently bound with organic matter in a complex compound carrying a negative charge. This fact is in good agreement with the dialysis results discussed above. As for acidic waters, in them the greater part of the nickel, as follows from the instability constant of a complex compound of the type \(\mathrm{NiSO_4^\circ}\), equal to \(4.0 \cdot 10^{-3}\) [3], should exist in solution in the form of the indicated ion pair. However, during electrodialysis this ionic compound decomposes, and as a result we observe that more than 95% of the nickel passes into the catholyte.

Thus, the results of the experiments carried out allow us to conclude that in groundwater nickel migrates in the form of various compounds. In acidic waters, ionic nickel compounds of the type \(\mathrm{NiSO_4^\circ}\) and \(\mathrm{Ni^{2+}}\) predominate, whereas in neutral waters the greater part of the nickel is bound in complex compounds carrying a negative surface charge. It is most probable that these compounds are organomineral. In individual cases the size of these compounds is so large that they approach subcolloids. The size of the latter, apparently, may reach 3 µm. Along with this, some part of the nickel migrates in the form of ordinary positively charged ions of the type \(\mathrm{Ni^{2+}}\), \(\mathrm{NiOH^+}\), etc. The ratios between the indicated forms depend on the specific geochemical and geological conditions.

LITERATURE

1. Yu. Yu. Bugel’skii. On the possibility of nickel migration in complex compounds with low-molecular-weight organic acids. Weathering Crust. Issue 10, 1968.

2. S. L. Shvartsev. Some features of the formation of the chemical composition of groundwater under conditions of the development of perennial frozen rocks. Proceedings of the Scientific-Technical Conference on Hydrogeology and Engineering Geology, issue III. Geochemistry of Groundwater. “Nedra,” 1970.

3. K. B. Yatsimirskii, V. P. Vasil’ev. Instability Constants of Complex Compounds. Publishing House of the Academy of Sciences of the USSR, 1959.