Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 11, pp. 1638−1642.
Pleiades Publishing, Ltd., 2013.
Original Russian Text © E.P. Lokshin, O.A. Tareeva, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 11, pp. 1686−1690.
AND INDUSTRIAL INORGANIC CHEMISTRY
Activation of Leaching of Rare Earth Elements
E. P. Lokshin and O. A. Tareeva
Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Resources, Kola Scientiﬁ c Center,
Russian Academy of Sciences, Apatity, Murmansk oblast, Russia
Received February 12, 2013
Abstract—Treatment of fresh phosphohemihydrate obtained in processing the Khibiny apatite concentrate by
hemihydrate sulfuric acid method was suggested by solutions containing alkali metal ﬂ uorides. It was shown that
such treatment reduces the transit time of low-hydrated rare earth elements phosphates contained therein, which
are poorly soluble in mineral acids, into the acid form from 2 months to 4–8 h.
In phosphogypsum obtained in the processing Khibiny
apatite concentrate to fertilizers by hemihydrate sulfuric
acid method (phosphohemihydrate PHH), rare earth
elements (REE ) are present mainly in the form of an in-
dependent phase: hydrated REE phosphates TrPO
. The value of x in TrPO
O depends on temperature
of proceeding of hemihydrate process, which is some-
what different in various plants formally using the same
technology. In fresh PHH removed from the rotary ﬁ lter
of “Ammofos”manufacture and JSC “Balakovo Mineral
Fertilizers” the value of x is less than unity. Such REE
phosphates at room temperature are difﬁ cult to dissolve
in solutions of mineral acids . By contrast REE from
the fresh PHH of JSC “Voskresenskie mineralnye udo-
breniya” is easily leached by mineral acids.
For ensuring a high extraction of REE the wet PHH
should be kept approximately 2 months in sulfuric acid
leaching solutions  thereby creating signiﬁ cant chal-
lenges associated with additional handling operations,
transportation, and milling, since at the keeping PHHs
are transformed into phosphate dihydrate and partially
clumped with the formation of enough coarse pieces.
The aim of this work was to develop a method of
acceleration of the transition of low-hydrated REE phos-
phates, which are poorly soluble in mineral acids, into a
readily soluble form.
Increasing the efﬁ ciency of leaching of REE from
PHH after its long aging in a humid environment is
explained by the transition of hydrated REE phosphates
O with x < 1 into ﬂ uorides .
Solubility of REE ﬂ uorides in sulfuric acid solutions
is large enough [4, 5] and in optimal ratios of l : s does
not limit the leaching of REE from PHH.
A transition into the ﬂ
uorides is determined by the
sequential occurrence of hydrolysis of anion SiF
the following reactions :
+ (2 + n)H
O + 4HF + 2NaF (1)
+ (2 + n)H
O = ↓SiO
O + 4HF + 2КF (2)
with the formation of free ﬂ uoride ion, which then reacts
with the hydrated REE phosphates:
O + 3F
In PHH a content of ΣTr
does not exceed 0.6 wt %,
therefore, 2.06 g of ﬂ uoride ion per 1 kg of PHH is re-
quired for their complete association in ﬂ uorides.
The concentration of ﬂ uoride ions in PHH is de-
termined by hydrolysis of the ion SiF
. At moisture