Reaction of corundum with a molten mixture of potassium and sodium tetrafluoroborates

Article abstract of DOI:10.1007/s11167-005-0447-0

Dissolution of synthetic corundum in KBF₄-NaBF₄ melts was studied as influenced by the composition of the salt mixture and temperature. The optimal parameters of the process for removal of synthetic corundum rods from internal channels of castings were determined.

Full text of DOI:10.1007/s11167-005-0447-0

Russian Journal of Applied Chemistry, Vol. 78, No. 7, 2005, pp. 1048 1051. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 7,
005, pp. 1070 1073.
Original Russian Text Copyright
2
2005 by Zarubitskii, Dmitruk.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Reaction of Corundum with a Molten Mixture of Potassium
and Sodium Tetrafluoroborates
O. G. Zarubitskii and B. F. Dmitruk
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of the Ukraine,
Kiev, Ukraine
Received December 22, 2004; in final form, April 2005
Abstract Dissolution of synthetic corundum in KBF NaBF melts was studied as influenced by the com-
4
4
position of the salt mixture and temperature. The optimal parameters of the process for removal of synthetic
corundum rods from internal channels of castings were determined.
A severe difficulty encountered in fabrication of
gas-turbine engines is removal of ceramic rods com-
posed of the chemically inert aluminum oxide from
internal cavities in blades, which are important ele-
ments of these devices [1].
The experiments were performed using the proce-
dure described in [3, 4]. The reactivity of the melts
was studied on planar rectangular samples of a virtu-
ally pore-free synthetic corundum. After being treated
in the melts, the samples were washed to remove reac-
tion products by prolonged boiling in distilled water
with a short-time intermediate treatment with a 10%
HCl solution. The samples were dried to complete
removal of moisture and then weighed. The change
in the sample mass was used to calculate the specific
reaction rate. The dissolution experiments were car-
ried out using corundum rods taken from internal
cavities in turbine blades quantity-produced at an avi-
ation plant. The porosity of the rods was 15 18%
at a grain size of 10 14 m. KBF and NaBF of
At present, industry uses for this purpose the meth-
od of dissolution of these rods in molten KHF [2],
2
which has a number of disadvantages: The process is
performed at a temperature at which KHF rapidly
2
decomposes, the melt aggressively acts upon the alloy
of castings, the quality of cleaning of channels in pre-
cision castings does not conform to the requirements
imposed, and the expenditure of KHF is too high
2
(
0.5 0.7 kg for removal of a rod with a mass of 12
1
5 g).
4
4
chemically pure grade were used. Thermogravimetric,
X-ray phase, chemical, and microscopic analyses were
made.
EXPERIMENTAL
Molten alkali metal tetrafluoroborates (MBF4)
As possible additives for improving the removal
were used in the process under consideration. It was
found in experiments on removal of ceramic rods
from internal cavities in turbine blades that the rod
dissolution rate in this method is comparable with that
in the existing technique [2]. Therefore, it was nec-
of rods in a melt based on KHF were tested KF
2
and NaF, KCl, KNO , KHF , CaF and BaF , and
3
2
2
2
Na SiF6 and NaB O . The experiments were per-
2
4 7
formed at 600 700 C.
It was found that the most promising among the
compositions studied, as regards their reactivity and
essary to find an additive to KBF that would raise
4
the etching rate of the ceramic by a factor of no less
than 2 3. In addition, the advantageous effect of the
additive might consist in that it would raise the flow-
ability of the molten mixture and make higher the
limiting solubility of aluminum oxide, i.e., improve
the reactivity of the fluoroborate melt or diminish the
specific expenditure of salts for blade cleaning and
simplify removal of the residue of the fluoroborate
fusion cake by washing from the internal cavities in
castings.
other properties, are molten mixtures of KBF and
4
NaBF . For example, the dissolution time (h) of
4
a synthetic corundum ceramics (rod) from a blade in
KBF NaBF mixtures with a 1 : 1 mass ratio of the
4
4
components is approximately three times that in pure
molten KBF (Table 1). It was found that the dissolu-
4
tion rate of synthetic corundum in KBF NaBF4
4
mixtures increases with the NaBF4 content up to
65 wt %, and then falls abruptly (see figure). Other
1
070-4272/05/7807-1048 2005 Pleiades Publishing, Inc.

REACTION OF CORUNDUM WITH A MOLTEN MIXTURE OF POTASSIUM
1049
advantages of the KBF NaBF melt were also re-
vealed: It has an enhanced reactivity, i.e., provides
dissolution of greater amounts of the ceramics, com-
Table 1. Cleaning of castings of gas-turbine engines to
remove the synthetic corundum ceramics in KBF4 and
KBF4 NaBF4 melts
4
4
pared to the case when individual salts KBF and
4
Melt composition, wt %
NaBF are used; after dissolution of a certain amount
4
T,
C
, h
of Al O (about 1%), the melt is covered with a crust
2
3
KBF4
NaBF4
which hinders volatility of the salts; and the viscosity
of the mixtures is lower than those of individual
KBF and NaBF .
1
1
00
00
600
4.0
3.0
2.4
1.4
1.5
0.8
1.0
0.8
4
4
650
680
600
650
650
650
680
Attempts were made to find why the rate of reac-
tion between aluminum oxide and molten KBF4
100
50
80
50
20
65
50
50
NaBF mixtures at a given temperature shows an
4
35
50
50
abnormal dependence on the mixture composition
(
see figure). The peculiar shape of the dependence at
NaBF content below and above 65 wt % is due to
the change of the reaction mechanism in the system.
4
Below this NaBF content, the process rate is limited
4
rosion rate. In particular, it decreases twofold at
00 C upon dissolution of 5 wt % Al O3 in the
by the chemical stage, with the main reaction product,
potassium sodium cryolite K NaAlF , being liquid
7
2
2
6
KBF NaBF melt.
4
4
at the temperature of the experiments. As the NaBF
4
content of the melt increases further, the process rate
falls dramatically because of the formation on the
Al O surface of a layer (film) of a solid product,
The optimal temperature of rod removal in the
KBF NaBF melt was chosen taking into account
4
4
the corrosion behavior of the material of the castings
and the intensity of rod dissolution. The rate of rod
removal was determined on castings produced by
oriented crystallization (rod length 86 mm). Below is
given the duration of rod dissolution as influenced by
temperature. In each experiment, 650 g of a freshly
2
3
which is mostly a sparingly soluble glassy phase
MOBF . It should be noted that reactions with Al O
2
2 3
in individual melts of KBF and NaBF proceed via
4
4
different intermediates. Potassium tetrafluoroalumi-
nate KAlF , which is in the liquid state at the temper-
4
prepared molten KBF NaBF mixture was used; the
ature of the experiment (mp 574 C), is formed in a
KBF Al O mixture, and a solid chiolite Na Al F
4
4
rod mass was about 15 g. Thus, thickening of the melt
with the ceramics was about 2% by the end of the
experiment:
4
2
3
5
3 14
(
mp 737 C), in a NaBF Al O mixture.
4 2 3
The following problems were solved in order to
optimize the technology: the optimal process tempera-
ture was chosen; the duration of rod removal as a
function of the amount of the ceramics introduced into
the tetrafluoroborate melt and its limiting reactivity
were determined; an efficient procedure was devel-
oped for washing of castings to remove the residue of
the tetrafluoroborate fusion cake; the corrosion of the
material of the castings was studied and its nature was
determined; and the composition of the crust formed
on the surface of the working melt was found by
X-ray phase analysis.
T,
, min
C
600 5
150
630 5
90 10
655 5
60 5
695 5
50 5
It follows from the data obtained that the tempera-
ture of 600 630 C cannot be recommended for use
v, mg cm 2 min 1
The corrosion of samples of a heat-resistant alloy
ZhS32 (material of castings) in the KBF NaBF melt
4
4
of an equimass composition was determined at 600
50 C. It was found that the corrosion rate of the
7
alloy, which is insignificant at 600 700 C, increases
dramatically at 750 C. A microscopic study of the
metal surface demonstrated that the corrosion is
mostly uniform. Thickening of the melt with alumi-
num oxide leads to a considerable decrease in the cor-
wt %
Etching rate v of aluminum oxide in molten KBF NaBF
mixtures at 650 C.
4
4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 7 2005

1050
ZARUBITSKII, DMITRUK
Table 2. Melting points and crystallization temperatures of
corresponding to the endothermic minimum in the
DTA curve was taken as the melting point in heating
the mixture, and that corresponding to the onset of
the exothermic effect of crystallization, as the crystal-
lization temperature in cooling the sample. It was
found that the melting point of the KBF NaBF mix-
KBF NaBF4 salt mixtures
4
Mixture composition, wt %
mp, Crystallization onset
C
temperature,
C
KBF4
NaBF4
4
4
tures recommended as the optical composition is 400
10 C.
1
00
565
508
482
463
427
400
380
360
372
406
4
9
8
7
6
5
4
2
1
0
0
0
0
0
0
0
0
10
20
30
40
50
60
80
90
505
455
445
403
385
350
322
340
The reactivity of the KBF NaBF melt is largely
4
4
determined by the composition of the interaction prod-
ucts and their solubility in a borofluoride mixture. It
was found by X-ray phase analysis that mostly mixed
cryolite is formed under the process conditions:
4
KBF4 + 2NaBF4 + Al O3 = 2K NaAlF6 + 3BF3
2
2
1
00
+
(BOF)3.
in the process because of the insufficient intensity of
rod etching. In the course of casting treatment, the
melt is continuously thickened by dissolution products
and its reactivity decreases. Consequently, the work-
ing temperature should not be lower than 650 C.
The upper limit to the working temperature was
chosen taking into account the fact that, above 700 C,
the volatility of the melt and its corrosivity toward
the material of the castings substantially increase.
Simultaneously occur some other reactions to give
potassium and sodium cryolites K AlF and Na AlF ,
3
6
3
6
oxofluoroborates NaBOF and KBOF , and sodium
2
2
chiolite Na Al F . It is noteworthy that the solubility
5
3 14
of cryolites in the KBF NaBF mixture markedly
4
4
exceeds that in KBF , with the reactivity of the melt
4
being, accordingly, higher, too.
The intensity of rod removal in the working melt
depends on both the temperature and the thickening of
the melt by interaction products. It was established
that the rate of rod dissolution markedly decreases
until approximately 3 wt % Al O is accumulated in
To choose the optimal composition and working
mode of the fluoroborate melt, it is important to know
the melting point and crystallization temperature of
KBF NaBF mixtures. The parameters of the phase
2
3
the melt at 650 C, and until 4 5 wt % Al O is ac-
4
4
2 3
diagram of the KF NaF BF system were reported
cumulated at 700 C. These values are apparently close
to the solubility of Al O in the form of the above-
3
in [5]. With account of the specific features of the
behavior of fluoroborates (e.g., their decomposition
can change the initial composition) and the conditions
of the process being developed, the fusibility diagram
of the system KBF NaBF was studied again by
2
3
mentioned compounds. When these limits are reached,
the rate of Al O dissolution virtually stabilizes. The
2
3
duration of rod removal strongly depends on the mass
ratio between the molten KBF NaBF mixture and
4
4
4
4
integrated thermal and X-ray phase analyses. The data
obtained somewhat differ from those reported in [2].
Table 2 lists only the liquidus and solidus tempera-
tures for a number of compositions. The temperature
Al O , as well as on the amount of a ceramics intro-
2 3
duced into the melt. Table 3 lists the results obtained
in an experiment in which the minimum expenditure
of the KBF NaBF melt is reached at a virtually
4
4
Table 3. Dissolution of a corundum rod in a KBF NaBF4 melt at 650 C
4
Casting mass, g
Mass of removed
ceramics, g
Thickening of melt by
ceramics, wt %
Rod removal
before experiment
after experiment
2
2
2
2
2
53.7
54.5
51.5
51.4
57.5
240.4
241.2
237.2
237.3
242.5
13.3
13.3
14.3
14.1
15.0
1.9
3.8
5.8
7.9
10.0
Complete
Residue present
Complete
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 7 2005

REACTION OF CORUNDUM WITH A MOLTEN MIXTURE OF POTASSIUM
1051
constant treatment duration of 1 h. Initial mass of the
melt 700 g, treatment temperature 650 C.
The reaction of tetrafluoroborate with an alkali is
strongly exothermic, and, therefore, it is recom-
mended to process castings at a temperature only
slightly exceeding the melting point of KOH (400
In this experiment, the expenditure of the melt per
blade was 140 g. Then, 15 more blades were proc-
essed and it was found that the minimum expenditure
of the melt for cleaning of one casting (with a rod of
mass 14 1 g) is 115 g.
450 C) in the course of 10 15 min. After the rods are
removed in the KBF NaBF melt, the castings are
4
4
cooled to the crystallization temperature of the tetra-
fluoroborate melt in order to preclude any interaction
between the liquid melt of the salts and the alkali.
The most important among the secondary proce-
dures of the process is their washing to remove the
residue of the tetrafluoroborate melt. The difficulties
encountered in washing are mostly due to the low
CONCLUSIONS
solubility of KBF and products formed in interaction
4
between the ceramics and the melt (cryolites etc.).
(1) The rate of the reaction between molten KBF4
KBF and NaBF strongly differ in the solubility in
NaBF mixtures and corundum at 600 700 C grows
4
4
4
water. For example, the solubilities of these salts
in 100 g of water at different temperatures are as
follows (g):
as the content of NaBF in the melt increases to
4
6
5 wt %, and then strongly decreases at still higher
concentrations of this salt.
T,
KBF4
NaBF4
C
20
0.45
25
0.55
107.4
30
0.98
40
1.4
100
6.3
210.0
(2) A method for removing synthetic corundum
rods from internal channels of high-precision castings
in melts was developed. The method includes treat-
ment in a molten mixture of KBF (50 55 wt %) and
4
Consequently, the solubility of the KBF NaBF4
4
NaBF (45 50 wt %) at 650 700 C for 1 2 h as the
4
mixture in water is sufficiently high owing to the dis-
main process, as well as some additional procedures.
solution of NaBF . As shown by the experiments, the
4
residue of the fusion cake of the fluoroborate mixture
can be completely removed both from the surface and
from the internal channels of the casting by washing
with warm flowing water and by boiling in distilled
water. If, however, the amount of the ceramics dis-
solved in the melt is considerable, cleaning to remove
the fusion cake of KBF NaBF is hindered: in some
(3) The advantages of the new process are as fol-
lows: the intensity of rod removal increases by a
factor of 2 3, the expenditure of the melt for removal
of a rod is three times lower, the molten mixture of
tetrafluoroborates is less corrosive toward the material
of castings, and the contamination of the atmosphere
and environment by noxious waste is diminished.
4
4
cases, high-quality washing cannot be achieved even
on repeating the procedures many times.
REFERENCES
In view of the fact that the given procedure, which
uses only water, is not efficient enough, other meth-
ods for removing the reaction products from internal
cavities in a casting were sought for. As a dissolving
medium were used acid and alkaline solutions (of
sulfuric and oxalic acids) and low-melting substances
1
. Chumakov, V.A., Stepanov, V.M., Ivanov, B.G., et al.,
Litein. Proizv., 1978, no. 1, pp. 23 24.
2
3
. USSR Inventor’s Certificate no. 1401071.
. Dmitruk, B.F., Zarubitskii, O.G., and Orel, V.P., Zh.
Prikl. Khim., 1981, vol. 54, no. 12, pp. 2659 2662.
(
KOH NaOH, KHF , NH KF , KOH, NaOH). It was
2 4 2
found that complete removal of the residue of the
fusion cake from the channels of castings is only
guaranteed in melts of potassium hydroxide or hydro-
fluoride.
4. Dmitruk, B.F., Marchenko, V.A., Dotsenko, V.K., et al.,
Zashch. Met., 1982, vol. 18, no. 3, pp. 376 380.
5. Barton, C.J., Gilpatcrick, L.O., Borman, I.A., et al.,
J. Inorg. Nucl. Chem., 1971, vol. 33, pp. 345 350.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 7 2005