FEATURES OF TEMPORARY BINDER-PRESERVATIVE REMOVAL
FROM MOULDED LIME REFRACTORIES DURING FIRING
A. V. Gropyanov
Translated from Novye Ogneupory, No. 5 pp. 38 – 40, May 2009.
Original article submitted January 28, 2009.
The effect of a moisture-containing atmosphere of furnace gases on the hydration rate of refractory objects
containing free CaO is considered. Results for studying the effect of binder thermal expansion on the condi
tion of lime objects and also analysis of comparing methods for preventing scrap that arises during heating of
objects during firing are provided.
Keywords: lime, CaO, binder, preservative, tallol, paraffin.
As already repeatedly noted [1, 2], one of the main rea-
son for scrap of refractories based on CaO is hydration. The
uniform reaction of CaO hydration is shifted to the right up
to 560°C, and this reaction proceeds actively in the warm-up
stage in any furnace unit, apart from a vacuum furnace.
The mechanism of hydration (more precisely hydroxy-
lation) of lime consists of the fact that initially at a CaO sur-
face there is very rapid formation of physically absorbed
moisture, that gradually changes into chemically bonded wa-
ter, forming with CaO at first CaO·H
O, and then portlandite
, whose specific volume is greater by a factor of 1.5
than for CaO . Under pressure of crystallization portlan
dite starts to crack and break objects during storage or on
heating during firing to 900 – 1000 K. Use even of high-
strength binders based on phenol formaldehyde resins does
not guarantee integrity of an object after moulding and firing,
since physically absorbed water remain s within it; here the
binder components, containing groups (–OH, –COOH), form
portlandite reacting with CaO.
Comparison of calculated data with experimental data
shows that currently for compacting lime and/or lime-
periclase refractories the optimum is use of a binder-preser
vative, containing 10 – 30 wt.% tallol unpurified oil and
90 – 70 wt.% paraffin, and also a preservative for finished
objects (impregnating composition) containing 10 wt.%
tallol and 90% paraffin.
The moisture content of exhaust gases in furnaces heated
by natural gas and solar oil (or fuel oil) is shown in Fig. 1. At
100°C the moisture content in a tunnel furnace is about five
times greater than in a chamber furnace, and at 500°C is
about seven times greater. Consequently, the rate of CaO
hydroxylation, proportional to water vapor pressure, should
be many times greater in a tunnel furnace than in a chamber
furnace. Therefore, one of the main reasons for possible
scrap during firing of objects made of CaO in a tunnel fur-
nace, heated by gas, is hydroxylation of CaO by moist ex-
haust gases. The hydrophobizing salt of calcium and organic
acids of the binder protects the raw material of an object
from possible hydration scrap.
The second reason for scarp is nonuniform thermal ex
pansion of the solid phase of raw material and binder. Calcu
lation is given below for the temperature range of safe be
havior of the solid phase – binder – pore system that expands
Refractories and Industrial Ceramics Vol. 50, No. 3, 2009
1083-4877/09/5003-0198 © 2009 Springer Science+Business Media, Inc.
St. Petersburg State Technological University of Vegetable Poly
mers (SPbGTURP), St. Petersburg, Russia.
Fig. 1. Moisture content in relation to tunnel furnace atmosphere
temperature (1 ) operating on gas, and the hearth (2 ) operating on