1070-4272/03/7601-0156$25.00C2003 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 76, No. 1, 2003, pp. 156 !157. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 1,
2003, pp. 162!163.
Original Russian Text Copyright + 2003 by Vershkova, Tareeva, Ivlev, Lokshin.
Solubility of Calcium Sulfate Dihydrate in Nitric Acid at 20oC
Yu. A. Vershkova, O. A. Tareeva, K. G. Ivlev, and E. P. Lokshin
Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials,
Kola Scientific Center, Russian Academy of Sciences, Apatity, Murmansk oblast, Russia
Received June 18, 2002
Abstract-The solubility of gypsum in 0330 wt % nitric acid solutions at 20oC was studied. The data were
compared with the results obtained at 60oC.
Methods involving preliminary leaching of calcium
ions with nitric acid show promise for recovery of
calcium-containing rare-metal raw materials (perov-
skite, sphene) . The forming solutions contain
free nitric acid, nitrates of calcium and rare-earth ele-
ments, and, to a small extent, nitrates of other im-
purity elements. Most of calcium ions can be precipi-
tated from the leaching solution by addition of sul-
furic acid, and the nitric acid solution after concentrat-
ing can be reused to break down the concentrates.
Reuse of nitric acid solutions is also of interest due to
the fact that under certain conditions rare-earth ele-
ments do not precipitate jointly with calcium sulfate
 and can be accumulated in the solution and sub-
sequently recovered by extraction.
Methods are being developed for extracting lan-
thanides by nitric acid leaching of phosphogypsum,
O, and phosphohemihydrate, 2CaSO
O, which are by-products in production of wet-
process phosphoric acid from Khibiny apatite concen-
trate . In the course of leaching, phosphohemi-
hydrate recrystallizes into phosphogypsum, and cal-
cium sulfate dissolves in the nitric acid solutions.
To estimate the efficiency of the calcium ion pre-
cipitation from nitric acid leaching solutions and the
amount of calcium sulfate dissolving in nitric acid
during leaching of phosphogypsum, we determined
experimentally the dependence of the solubility of
calcium sulfate dihydrate on the nitric acid concentra-
tion at 20oC.
The solubility of calcium sulfate dihydrate in nitric
acid was studied by the isothermal method. Analyti-
cally pure grade chemicals were used. The initial solu-
tions with a prescribed HNO
concentration and ex-
O were kept in hermetically sealed
temperature-controlled flasks at 20+ 0.5oC with stir-
ring until the equilibrium was attained. The process
was monitored by analysis of the liquid phase at
regular intervals for the content of Ca
ions by standard photometric and gravimetric proce-
dures. The presence of two endothermic peaks in the
ranges 1243150 and 1783192oC in the thermograms
and the X-ray phase analysis data for the samples of
the bottom phase confirm the existence of CaSO
O under the examined conditions.
As seen from the results obtained (see figure), the
dependence of the gypsum solubility on the nitric acid
concentration at 20oC passes through a maximum in
the range 22325 wt % HNO
. Data on the CaSO
O solubility at 60oC  are presented for com-
parison. The observed direct temperature dependence
of the CaSO
O solubility in nitric acid confirms
the previous results .
The results obtained are useful for engineering cal-
culations and physicochemical substantiation of flow-
sheets for reprocessing of calcium-containing rare-
metal raw materials. In particular, we can state that
Solubility S of CaSO
O in nitric acid at (1) 20 and