1070-4272/04/7701-0001C2004 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 77, No. 1, 2004, pp. 1!4. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 1,
2004, pp. 3! 6.
Original Russian Text Copyright + 2004 by Lapshin, Germanskii, Bogdanov.
AND INDUSTRIAL INORGANIC CHEMISTRY
Kinetics of the Reaction of Cubic Boron Nitride with a Melt
of Sodium Hydroxide
A. V. Lapshin, A. M. Germanskii, and S. P. Bogdanov
St. Petersburg State Technological Institute, St. Petersburg, Russia
Received October 9, 2003
Abstract-Kinetics of the reaction of cubic boron nitride with a melt of sodium hydroxide in the temperature
range 3803 480oC was studied. Kinetic parameters of the reaction were determined.
Intermediate products of the synthesis of cubic
boron nitride (CBN), so-called cakes, are polydisperse
mixtures of two polymorphic modifications of hexag-
onal graphite-like boron nitride (HBN) and CBN .
Crystals of cubic boron nitride are extracted from
intermediate products in two stages. In the first stage,
a cake is treated with an acid to remove compounds of
the metal initiating the synthesis, after which the resi-
due of nonrecrystallized HBN is removed by treat-
ment with a melt of alkali . The CBN extraction in
the second stage is based on the difference in rates of
reactions of CBN and HBN with a melt of alkali. The
degree of CBN decomposition in this reaction, yield-
ing ammonia and the corresponding metal borate, in-
creases as the sample grain size decreases. For exam-
ple, at 400oC and process duration of 30 min, a CBN
micropowder with the particle size from 5 to 3 mm
(5/3) decomposes to 55%, and that with the particle
size of 250/200, only to 6%. At 700oC, a 5/3 micro-
powder and also samples with the main fraction par-
ticle size from 50 to 40 mm decompose completely,
whereas CBN with the particle size of 100/80 and
250/200 decompose to 90 and 88%, respectively.
Thus, the treatment with a melt of alkali, which is
used to remove HBN during enrichment of the primary
product of the synthesis with CBN, even at low tem-
peratures results in considerable losses of this expen-
sive material, especially of its small-size fractions .
Furthermore, the following data on the reaction of
CBN with alkali melts are available in the literature.
According to , edges of CBN crystals with nitrogen
atoms on the surface have a higher activity in reac-
tions with a KOH melt than edges with boron atoms.
However, quantitative characteristics of CBN chemi-
cal stability were not given in that work. Mishina and
Filonenko  have correlated the observed cracking of
CBN bicrystals on etching by a KOH melt with the
presence of a mass defect on grain boundaries; how-
ever, it seems more probable that this phenomenon
results from the chemical decomposition of impurities
segregating on interfaces. Thus, according to , the
degradation of CBN crystals on their etching by an
alkali results from the chemical decomposition of
HBN inclusions contained in the crystals and of im-
purities captured during their growth. At the same
time, it is known  that the content of impurities
(in particular, of carbon and oxygen) on the surface of
LO-grade CBN crystals is higher than in LKV-grade
crystals having a more perfect structure and also
higher strength and crack resistance. Therefore, we can
conclude that the chemical stability of CBN toward
an alkali melt depends on both the shape of crystals
and the state of their surface.
Such conclusion seems to be important, because
wearing of tools equipped with CBN crystals depends
on their chemical interaction with a material under
treatment and with the environment . Klimenko
et al.  have found that chemical compounds con-
sisting of elements of contacting materials and en-
vironment arise in the region of the contact between
CBN and the material under treatment. It is probable
that the parameter of CBN chemical inertness to oxi-
dants can serve as one of criteria in selection of a type
of CBN to be used as a tool material.
We failed to find in the literature a quantitative
description of the kinetics of CBN reactions with
melts of alkalis. At the same time, the knowledge of
the kinetic relationships of the process is required to
improve the process for the enrichment of primary
products of the CBN synthesis.
The aim of this work was to determine the rate
constants of the CBN reaction with an NaOH melt.