Interaction of graphite with hydroxide-salt melts

Article abstract of DOI:10.1134/S1070427206040033

The mechanism and kinetics of graphite dissolution in melts based on sodium hydroxide were studied. The effect of various salt additives on the intensity of the occurring reactions is considered. A method recommended for removal of graphite in the form of remainders of molds and mold cores from titanium casts is described. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070427206040033

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 4, pp. 525 528.
Pleiades Publishing, Inc., 2006.
Original Russian Text
pp. 537 540.
O. G. Zarubitskii, B. F. Dmitruk, N. F. Zakharchenko, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 4,
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Interaction of Graphite with Hydroxide Salt Melts
O. G. Zarubitskii, B. F. Dmitruk, and N. F. Zakharchenko
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences
of the Ukraine, Kiev, Ukraine
Received December 8, 2005
Abstract The mechanism and kinetics of graphite dissolution in melts based on sodium hydroxide were
studied. The effect of various salt additives on the intensity of the occurring reactions is considered. A meth-
od recommended for removal of graphite in the form of remainders of molds and mold cores from titanium
casts is described.
DOI: 10.1134/S1070427206040033
The existing industrial methods for cleaning the sur-
face of machine and equipment parts to remove opera-
tional contaminations are imperfect: they are, as a rule,
low-intensive, fail to always provide a satisfactory
quality of cleaning, involve manual labor, and cannot
rule out health hazard and occurrence of dangerous
situations, e.g., inflammation [1]. This, in particular,
refers to cleaning of titanium alloy casts to remove
remainders of graphite molds and mold rods. Com-
monly, graphite is removed from casts of this kind
manually, with auxiliary cutting tools, with these tools
frequently broken; this procedure fails to always pro-
vide the due quality of surface cleaning and may cause
mechanical damage to articles so that they go to scrap.
on an OD-102 derivatograph at a heating rate of
5.1 deg min . The weighed portions of the starting
substances were in the range from 0.2 to 1.8 g.
1
An interaction between the components in mix-
tures of graphite and sodium and potassium hydrox-
ides is observed even at low temperatures. For ex-
ample, release of a certain amount of heat begins
in the C 2NaOH mixture at approximately 160 C.
The process is enhanced as the alkali melts. The quan-
titative characteristics of the effects observed strongly
depend on the ratio of the components. It is note-
worthy that oxidation of powdered graphite by at-
mospheric oxygen begins only at temperatures ex-
ceeding 400 C. The processes occurring in C NaOH
mixtures are accompanied by slight changes in the
sample weight: the weight loss is due to moisture
removal, and the weight gain upon melting of the al-
kali is associated with oxygen absorption. The thermal
and gravimetric effects are considerably more pro-
nounced for mixtures of graphite with potassium
hydroxide.
The aim of this study was to develop a high-tem-
perature procedure for cleaning the surface of titanium
alloy casts to remove graphite rods.
There is no published evidence about systematic
studies of the interaction between graphite and hy-
droxide salt melts. There have only been reports de-
scribing the behavior of carbon electrodes and struc-
tural elements in electrochemical processes [2].
In the C NaNO mixtures, the components inter-
3
act at a temperature markedly exceeding the melting
point of the nitrate. Depending on the component ra-
tio, the beginning of a vigorous reaction is observed
in derivatograms at 380 420 C. In mixtures with
component ratios of 2 : 1 to 10 : 1, the interaction
is markedly enhanced on reaching a temperature of
EXPERIMENTAL
A derivatographic study of processes occurring in
binary C NaOH, C KOH, C NaNO systems and
3
ternary mixtures C NaOH NaNO was carried out.
3
450 C. At large nitrate contents, the reaction proceeds
The molar ratio of the components was varied from
in a moderate mode.
5
: 1 to 1 : 4 for the first two systems and from 2 : 1
to 15 : 1 for the third. The molar ratios for the ternary
system were 5 : 1 : (0.7, 2.5), 1 : (2 4) : 2, 1 : 2 : 1,
and 1 : 1 : 0.5. The measurements were performed
As follows from the experimental data obtained,
the interaction of graphite with sodium and potassium
hydroxides is considerably weaker than that in mix-
525

526
ZARUBITSKII et al.
2
1
NaNO3 (initial weighed portion of the mixture
.867 g), correspond to the occurrence of the reaction
C + 2NaOH + 2NaNO3
=
Na CO3 + 2NaNO2 + H O.
(2)
2
2
It is noteworthy that the rate of reaction (2) strong-
ly depends on the component ratio. For example, its
intensity in the C 4NaOH 2NaNO mixture is con-
3
siderably higher, which can be judged from the abrupt
weight loss and a more pronounced rise in temperature.
In addition to carrying out a derivatographic study,
the kinetics of graphite etching in molten sodium hy-
Fig. 1. Derivatogram of the 2NaOH 2NaNO C mixture.
3
(
m) Weight loss, (T) temperature, and ( ) time.
droxide, nitrate, and nitrite and NaOH NaNO , NaOH
3
NaNO , and KOH KNO mixtures was examined.
2
3
The method of isothermal gravimetry of a sample
under free-convection conditions was used. Graphite
2
samples (99.9% C) in the form of rods (6 8 cm )
2
or plates (10 18 cm ) were treated in melts at 450
and 500 C. The mass of the melt was 120 140 g.
The samples were preliminarily polished to obtain
the same surface quality in each case. After treatment
in the melt, samples were cooled to room temperature,
washed in boiling distilled water for 15 20 min, and
calcined at 200 C. In determining the rate of graphite
etching in molten NaOH NaNO and NaOH NaNO
3
2
mixtures, the time of experiment was limited to 20 min
and the average value was calculated.
The kinetic curves of graphite etching in the indi-
vidual melts under study have a shape typical of auto-
catalytic processes (Fig. 2), which is caused by an in-
crease in the reaction area with time. The kinetic de-
pendences are described by a power equation whose
constants are listed in Table 1.
Fig. 2. Amount W of graphite etched away in melts of
(
(
1) NaNO , (2, 3) NaOH, and (4, 5) NaNO . Temperature
C): (1, 2, 4) 450 and (3, 5) 500. ( ) Time.
2 3
tures of graphite and sodium nitrate. Apparently,
atmospheric oxygen is involved in reactions after
the melting of the alkali:
The rate of graphite etching in molten NaOH
NaNO3 and NaOH NaNO2 mixtures depends to
a greater extent on the component ratio. The iso-
therms of etching rate in these mixtures pass through
a maximum (Fig. 3) at a nitrate content of approx-
imately 35% (500 C) and 45% (450 C) in the first
system and a nitrite content of 20% at both temper-
atures in the second system. It is noteworthy that
C + 2MOH + O2 = M CO + H O.
(1)
2
3
2
In the C NaOH NaNO mixtures, the components
3
intensively interact with carbon at temperatures 80
00 C lower than those in the mixture with sodium
nitrate (Fig. 1) and this interaction is accompanied by
a considerable weight loss. The values of the weight
loss, experimentally determined for the C 2NaOH
1
the data for the NaOH NaNO system are in agree-
3
ment with the results of a derivatographic study of
processes in the ternary mixture C NaOH NaNO in
relation to the ratio of the components of the hy-
droxide salt phase.
Table 1. Constants of the power equation d0 n, g cm 2 h 1
3
Melt
NaOH
T,
C
d0
n
A number of alkaline formulations used to process
titanium alloys, namely, to remove scale, are known
[3, 4]. However, preliminary experiments demon-
strated that formulations of this kind are ineffective
or at all unacceptable for removal of graphite present
500
1.08
2.3
3.3
0.9
0 4
0 1
0 4
NaNO3
NaNO2
500
450
99.1
2.02
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 4 2006

INTERACTION OF GRAPHITE WITH HYDROXIDE SALT MELTS
527
Table 2. Results of industrial tests of the method for cleaning of titanium casts to remove remainders of
the graphite-based mold mixture in a melt based on alkali metal hydroxide
Rod characteristics
Number of
simultaneously
processed
Time of
complete rate of rod
rod re-
moval, h
Average
Kind of cast
T, C
shape in
cross-section, mm
length,
mm
removal,
shape
1
casts
mm h
Turbine blade
Pipe
connection
Adapter
Flange (6 rods)
Base (12 rods)
Straight
90 bend
The same
5
20
120
55
40
70
10
27
1
5
5
3
3
1
560
540
560
520
500
520
7.5
3.0
2.0
4.2
0.5
1.75
8.0
9.2
10.0
8.3
10.0
7.7
d = 8
d = 10
d = 10, 6
d = 5
Straight
90 bend Oval: a = 13, b = 35
in the form of remainders of molds and mold rods
from titanium casts. It was found experimentally that
the intensity of graphite removal in such melts at tem-
peratures lower than 500 C is insufficient, in partic-
ular, for long rods. At the same time, treatment at
above 500 C may lead to inflammation of articles.
For comparison, a melt patented in a number of coun-
tries was used [5 7]. The melt contained (wt %): 50
and orthotitanate and potassium chloride has a higher
(by a factor of 2.5 10) reactivity toward graphite, and
no inflammation is observed at 550 C and a treatment
duration providing complete removal of a rod.
The experiments performed suggested as the op-
timal melt for removal of graphite in the form of re-
mainders of molds and mold rods from titanium casts
the following formulations (wt %): sodium nitrate
7
1
8 sodium hydroxide, 8 20 potassium hydroxide, 9
5 sodium nitrate, and 5 15 sodium chloride. The in-
4
2
0 55, potassium chloride 4 10, sodium orthonitrate,
6 and sodium hydroxide the rest.
tensities of graphite (rod) removal and types of cor-
rosion behavior of the titanium alloy were compared
for a particular alloy containing the above compo-
nents in close to the middles of the indicated ranges.
The optimal parameters of the process of cast cleaning
were found for the VT5 alloy.
Industrial tests of the technique suggested were car-
ried out at a titanium casting shop of a machine-build-
ing plant in an industrial thermal furnace. The melt
prepared in a rectangular steel bath had the following
composition (wt %): sodium nitrate 48, potassium
chloride 5.5, sodium orthotitanate 2.05, and sodium
hydroxide the rest.
The results obtained in the study served as a basis
for suggesting a melt containing four components.
The melt is based on sodium hydroxide and nitrate.
Potassium chloride contained in the melt improves
its technological properties and, in addition, serves to
prevent inflammation and explosion of potassium ni-
trate; KCl was introduced in an amount of 4 10 wt %.
Making its content higher is undesirable because of
a pronounced decrease in the dissolution rate of graph-
ite. New properties are imparted to the melt by sodi-
um orthotitanate. This additive to the alkaline melt
exerts the strongest inhibiting influence on the corro-
sion of titanium. Also, the thermal stability of tita-
nium and its alloys substantially increases. The solu-
bility of sodium orthotitanate in molten sodium hy-
droxide is limited. For example, it is somewhat higher
than 6 wt % at 550 C. The advantage of this salt is
that Na TiO can be obtained from a titanium waste,
4
4
and, provided that certain conditions are satisfied, this
can be done directly in the alkaline melt.
Fig. 3. Isotherms of etching rates v of graphite in molten
Comparative experiments demonstrated that the
composition containing sodium hydroxide, nitrate,
(
1, 2) NaOH NaNO and (3, 4) NaOH NaNO . Temper-
3 2
ature ( C): (1, 3) 450 and (2, 4) 500.
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528
ZARUBITSKII et al.
On melting the mixture and removing moisture at
00 500 C in the course of 1 h, the temperature was
gested. The optimal melt composition is as follows
4
(wt %): sodium nitrate 40 55, potassium chloride
4 10, sodium orthotitanate 2 6, and sodium hydrox-
ide the rest. The working temperatures of the method
are in the range 500 550 C.
elevated to the working point. Casts of a VT5L alloy
with remainder of molding sand and graphite rods of
variously intricate shapes were cleaned. The removal
of rods was periodically monitored in the course of
processing. The results of the tests are listed in Ta-
(2) The new technique was successfully tested in
1
industry. The tests demonstrated the advantages of
the chemical method for removal of graphite from
inner hollows of titanium alloy casts.
ble 2. A high rate of rod removal (8 10 mm h )
was achieved in the tests. A metallographic analysis
demonstrated that the surface of the casts remains un-
changed, with no corrosion etching occurring, and
the mechanical properties of the alloy satisfy the stan-
dard requirements.
REFERENCES
1
. Morshina, A.P., Tekhnol. Legk. Splavov: Nauch.-
Tekh. Byul. VILS, Moscow, 1973, no. 2, pp. 110 111.
The advantage of the chemical method for removal
of graphite from titanium alloy casts consists in that
there is no need for manual labor, the output capacity
significantly increases, cutting tools are saved, and
the cleaning quality is improved.
2. Zarubitskii, O.G., Ochistka metallov v rasplavakh
shchelochei (Cleaning of Metals in Alkali Melts),
Moscow: Metallurgiya, 1981.
3. Usova, V.V., Plotnikova, T.P., and Kushakevich, S.A.,
Travlenie titana i ego splavov (Etching of Titanium
and Its Alloys), Moscow: Metallurgiya, 1984.
CONCLUSIONS
4
. Aleksandrov, V.K., Anoshkin, N.F., Bochvar, G.A.,
et al., Polufabrikaty iz titanovykh splavov (Half-Prod-
ucts Made of Titanium Alloys), Moscow: Metallurgiya,
(1) The mechanism and kinetics of graphite dis-
solution in hydroxide salt melts and the effect of var-
ious salts (NaNO , NaNO , KCl, NaCl, Na TiO ,
1
979.
3
2
4
4
5
. French Patent 1477220.
KNO ) were studied. As a result, an efficient method
3
for removal of graphite in the form of remainder of
molds and mold rods from titanium casts was sug-
6. FRG Patent 1621556.
7. Swiss Patent 470491.
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