ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 7, pp. 1070–1075. © Pleiades Publishing, Inc., 2009.
Original Russian Text © A.I. Nikolaev, N.V. Kirichenko, M.P. Rys’kina, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 7, pp. 1131–1136.
COORDINATION
COMPOUNDS
Isolation of Niobium, Tantalum, and Titanium Complex Fluoride
Salts with Alkali Metal Cations
A. I. Nikolaev, N. V. Kirichenko, and M. P. Rys’kina
Tananaev Institute of Rare Element and Mineral Chemistry and Technology, Kola Research Center,
Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk oblast, 184200 Russia
Received February 14, 2008
Abstract—Fluoride and oxofluoride salts of niobium, tantalum, and titanium were isolated. They precipitated
from aqueous solutions and upon washing of organic extracts with aqueous solutions of ammonium, potassium,
and sodium salts. The compositions of the isolated compounds were studied. Different compositions were
established for the niobium salts that precipitated upon the dissolution of unwashed niobium hydroxide in
hydrofluoric acid under the atmospheric pressure, in an autoclave, and upon addition of sodium, potassium, and
ammonium salts to purely fluoride solutions of niobium, as well as for the tantalum ammonium and sodium
salts isolated from aqueous and organic solutions. The data obtained can be used for the synthesis of niobium,
tantalum, and titanium complex fluoride salts with various compositions.
DOI: 10.1134/S0036023609070134
Fluoride solutions of niobium, tantalum, and tita-
nium have found wide use in hydrometallurgical
schemes of extraction, concentration, and separation of
these allied elements. Niobium, tantalum, and titanium
concentrations in fluoride solutions achieve 8 mol/L,
which is considerably higher than in solutions of other
acids. Unlike other solutions, fluoride aqueous solu-
tions are less prone to form polymeric species [1],
which allows liquid–liquid extraction methods to be
used efficiently for the preparation of high-purity com-
pounds of these elements [2–5].
We have previously studied the dissolution of nio-
bium, tantalum, and titanium oxides and hydroxides in
hydrofluoric acid and the stability of the resulting solu-
tions [9]. The substantial differences in the sizes of
ammonium, sodium, potassium, and calcium cations
can induce the precipitation of complex fluoride salts
with different compositions. Therefore, it was of inter-
est to study the composition of solid phases precipitat-
ing from fluoride solutions of niobium, tantalum, and
titanium containing ammonium, sodium, potassium,
and calcium ions. The data presented in this work
develop investigations into the stability of solutions of
niobium, tantalum, and titanium fluorides and the com-
position of the isolated compounds in the presence of
ammonium, sodium, potassium, and calcium ions.
At the same time, the solubility of the elements in
fluoride solutions in the presence of alkali and alkaline-
earth metal or ammonium cations decreases substan-
tially due to the formation of less soluble complex flu-
oride salts with alkali or alkaline-earth metals [6].
Rare-earth titanoniobates, namely, loparite and perovs-
kite, being widely abundant raw materials in Russia,
contain alkali elements and calcium [7, 8]. In chemical
technology, potassium salts, namely, potassium hep-
tafluorotantalate and niobate, found practical use in
electrolytic tantalum production and manufacture of
materials for acoustical electronics. An ammonium ion
enters solutions together with ammonia and ammonium
salts used at the stages of washing of extracts, back-
extraction, or precipitation of niobium and tantalum
hydroxides. Precipitation at any extraction stage, which
is not specially associated with precipitation, violates
the technological process and decreases the parameters
of extraction of valuable components. Data on the sta-
bility of fluoride solutions of niobium, tantalum, and
titanium are scarce and often concern pure solutions,
whereas in practice we deal with solutions of more
composition from which they are separated [1, 8].
EXPERIMENTAL
To isolate niobium, tantalum, and titanium complex
salts, we used solutions of their oxides in hydrofluoric
acid and solutions of fluoride or non-fluoride salts of
ammonium, sodium, potassium, and calcium. The com-
positions of stock fluoride solutions of niobium, tanta-
lum, and titanium and the ammonium, potassium, and
sodium salts are given in Table 1. The starting reactants,
reagents, and products for the preparation of solutions
were characterized earlier [9]. Working solutions were
prepared by mixing the stock solutions with solutions
of salts and/or by dilution with water to required con-
centrations.
Organic solutions of tantalum were prepared by its
extraction with tributyl phosphate from aqueous fluo-
ride–sulfuric acid solutions. The tantalum concentra-
tion in the organic phase was 0.8 mol/L. The extracts
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