heat balance

1. HEAT ENGINEERING
ANALYSIS OF THE MOVEMENT OF MAGNESITE
IN A ROTARY KILN
K. V. Simonov, A. G. Luzin,
V. P. Mitin, V. V. Klimov,
D. A. Plotnikov, and S. D. Kozhevnikov
UDC 666.76:68.041.45.001.4
The processes developing in rotary kilns, notablywhen roasting cement and other construction mate-
rials [1-5], are being studied with the aid of radiosotopes in the Soviet Union and elsewhere. Itwas deemed
of interest to test this method in an investigation of the kiln processes when calcining indigenous magne-
site.
This article represents a report on an investigation of the magnesite movement and the ratio of dust
formation in the individual zones of a 90 meter long rotary kiln carried out with the help of radioisotopes.
The outside diameter of the kiln is 3.5 m, the inside diameter 3.04 m and the slope of the kiln 4.08~. The
kiln rotates at 1.11 rpm, the peripheral speed of the outer lining surface is 0.175 m/see, and the produc-
tivity:of the kiln is 10.5-12.5 tons calcined powder per hour. The charge rate is 39-40 tons raw magnesite
finer than 40 mm per hour. The length of the cooler is 36 m, the outside diameter 2.8 m, and the slope
3.5%. It rotates at 2.31 rpm.
The radioactive tracer was in the form of raw magnesite irradiated in IVV-2 apparatus with respect
to the elements Ca 47 and Fe 59. An analysis prompted a decision to the effect that account should be taken
solely of those components which contribute significantly to the activation of magnesite, viz., the stable
isotopes Ca4~ and Fe 58, which under irradiation with thermal neutrons are converted to the radioisotopes
Car and Fe 59.
Characteristics of the Isotopes
Indices Ca47 Fe59
Half-life Ti/2, days 4.7 45
Radiation energy ETmax, MeV 1.31 1.29
Gamma constant KT, r/h 5.14 6.25
The remaining elements (Si3~ Mg2G, and A127) were left out of account owing to their short half-life
of 2.62 h, 9.5 min and 2.3 min, respectively and small activation sections, viz., 3.4-10 -27, 3.0.10 -26, and
2.1.10 -25 cm z,respectively.
Calculations carried out in conformity with "Health Regulations No. 333-60" showed that the initial
activity Q0 of the magnesite for the experiment must not exceed 25mCi. The activity Q of the magnesite
varied exponentially Q = Q0 e-At during the period (seven days) between irradiation and charging into the
kiln and was 22.4 mCi.
The calculation* of the activation time of a weighted portion of magnesite to produce an activity of
25 mCi was carried out from the following equation:
~ractF ,VOmFe --Lt
*Report of the Volgograd Branch of the Specialized Administration of Installation and Alignment in Radia-
tion Engineering, the "MaKnezit"Plant, and the Eastern Institute of Refractories [in Russian] , Volgograd
(!968).
"Magnezit" Plant. Vo!gograd Branch of the Specialized Administration of installationand Alignment
in Radiation Engineering. Eastern Instituteof Refractories. Translated from Ogneupory, .No. ll, pp. 18-22,
November, 1974.
9 19 75 Plenum Publishing Corporation, 22 7 West 1 7th Street, New York, N. Y. 10011. No part of this publica~on may be reproduced,
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recording or otherwise, without written permission of the publisher. A cop), of this article is availab& from the publisher for $15.06<
679

2. /oZi~
8O0O
700
9~ 8oo
.~ 500
4oo
~. ~0o
200
" I00
O/
4
x 5 7
I I
f J 4
Fig. 1
rE0
8
80
E
9~ ~0
, • Z~
9 X
J
/0 ~ 50 7O 9o
Kiln length, m
Fig. 2
Fig. 1. Activity of the irradiated material recorded by detectors
Nos. 1-8 (noted against the curves) during the movement of
magnesite through a 90 m long rotary kiln and cooler.
Fig. 2. The travel time of the magnesite through the 90 m rotary
kiln: 1)-7) are the numbers of the detectors.
where Q0 is the activity (25 mCi); mFe is the weight of the isotope concerned (1.74.10 -3 g), ~act is the acti-
vation cross section (1.01 910-24 cm2), AFe is the atomic weight of the isotope (59), No is the Avogadro
number (6.0247-1023 mole-i), X is the decay constant (0.693/T1/2 )(h-l), F is the neutron flow for a given
power of the apparatus, viz. 1.0.1014 neutrons/(cm 2. see), t is the activation period, days.
The activation period for isotope Fe 5s of a 100 g portion of magnesite was found to be 46 days.
To determine the travelling time and average speed of the magnesite in the individual kiln sections
radiation detectors were set up at seven points along the kiln, one of them (No. 8 in Fig. 1) at the end of
the cooler cylinder. The passage of the tagged portion of magnesite along the kiln was monitored with a
measuring arrangement mounted on a ZIL-130 truck. The main element of the apparatus was a 26-chan-
nel technological device of the USIT-1-2A type. The gamma radiation was monitored with geiger counters.
The results recorded for several experiments in the form of diagrams on the strip of self-recording
instruments were in good agreement and were evaluated as follows. First, the time-dependences of the
activity of the materialwere reeordedin a single reference frame (Fig. 1) where the peak of each curve
denotes the passage of the maximum quantity of active material past a given detector at a given instant.
In the course of the calcining process the active portion of magnesite, like the charge as a whole,
undergoes considerable changes. The shape of the curves of its passage along the kiln is influenced by
the distribution of the isotope through the continuously increasing bulk of the material, by the intermixing
of the magnesite in the heat exchanger zone, by the distribution of the radioactive source as a result of
various factors, by the formation of a coating in the sintering zone, etc., so that the speed of the material
calculated from the meantravelling time of its active portion and the speed determined from the medians
of the intensity curves recorded by the detectors do not accurately express the rate of travel of the mate-
rial in the kiln. The 'peak' speed of the largest (across the kiln) amount of active material calculated for
the instants of peak counting rates is a more reliable indicator [2, 5].
An important point to note is that in the method of determining the passage time of the tagged mate-
rial from the peak activity the errors arising from the decrease in the absolute intensity due to the partial
decay of the isotope and from the absorption by the kiln lining do not affect the results to a significant
extent, i.e. it is merely the scale of the curves relative to the vertical axis which changes while the shape
of the curves and the position of the peaks relative to the horizontal axis remain the same provided the
experiments are conducted under identical conditions.
The travel rate of the magnesite in the individual kiln sections depends on the changes in the pro-
perties of the magnesite during the calcining process. Investigations [6] of the physicochemical pro-
cesses developing in the magnesite and of the changes in its properties have yielded the length of the tech-
nological zones in the kiln, viz.,heating (15 m), decarbonizing (50 m), sintering (16 m) and cooling (9 m).
The changes in the travel rate of the material in the individual sections must be considered in relation to
the changes in the properties of the magnesite and to the calcination-induced physico-ehemica[ processes.
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3. TABLE 1. Zonewise Parameters of the Travel of Magnesite in a
90 m Kiln
zone
. decarbonizing sintering cooling
Parameters heating a [ b I c a i b
KiLnsection length, m 0--15 16--38 56--65 66--74 2--90
Average travel r:ateof the
magnesite, rn rain 0,57 0,64 2,2 0,99 ] 1,34 0,23
Friability factor 1,3 1,5 0,94 2,2 I 3,1 0,230,5 0,5
In the kiln section between the 15th and 38th meter the travel rate of the magnesite varies only mar-
ginaHy (,Table 1), the explanation being the relatively slow rate of the decarbonizing process in which (tak-
ing dust entrainment by the flue gases into account) the space factor of the kiln decreases only slowly and
the travel rate of the maga~esite increases from 0.57 to only 0.64 m/rain. The decarbonization rate de-
termined from the volume of CO2 eliminated from the magnesite in this kiln section is only 0.7% CO2 per
minute.
In the kiln section between the 38th and 55th meter the mean travel rate increases sharply (see Table
1) as a result of the vigorous decomposition reaction of the carbonates which causes the magnesite layer
to 'bubble. ' In this kiln section the magnesite is hot enough and at a temperature above 640~ decarboniza-
lion develops throughout the bulk of the material together with peak carbon dioxide elimination (2.3,,~ CO2
per minute). Evidence that decarbonization proceeds at a peak rate is provided by the fact that the porosity
of the magnesite reaches 45-%.
The decarbonization of the magnesite lumps is accompanied by vigorous CO2 elimination and by a
consequent intermixing of the magnesite particles so that the material becomes highly mobile. The 'bub-
bling' effect was observed when extracting magnesite samples in this section of the working kiln with a
special sampler. The mobility of the material results in a higher travel rate in this section of the decar-
bonizing zone.
In the next kiln section (meters 55 to 65), which is relatively short compared with the preceding one,
the travel rate of the magnesite remains nearly the same as before. The bulk of the CO2 has been elimin-
ated and carried off with the flue gases but the magnesite lumps have not yet been broken up and the pro-
portion of fine fractions has increased only slightly. The minor changes in the phase composition and grain
size distribution of the magnesite in this section do not influence the travel rate to a sig-nificant extent.
In the section between the 65th and 75 meter the magnesite sintersbut the porosity is still fairly high.
The magnesite contains open mierocracks due to shrinkage, the material is completely decarbonized and
opened, and as a result of further attrition contains a large proportion (about 60%) fractions finer than
0.5 mm so that it is highly 'friable' and therefore travels faster in this section of the kiln.
The section between meters 75 and 90 includes the calcining and cooling zones. The travel rate of
the magnesite between the 75th and 81st meters fails sharply owing to the change in the ductility of the
material resulting from the development of a liquid phase and the coarsening of the grains due to sinter-
ing and the agglomeration of the fine particles into granules. The travel rate of the magnesite is affected,
moreover, also by the formation of a coating on the lining of this kiln section.
Towards the end of the kiln the powder being formed does not undergo significant changes and its
rate of travel remains constant at 0.23 m/min. In the cooler the powder moves at 1.125 m/rain.
The radioisotope-aided experiment thus showed that the travel rate of the material in the kiln varies
with the astute of the physico-chemical processes developing during the calcination of the magnesite.
The travel time of the magnesite through the 90 m rotary kiln is 166 rain (Fig. 2). The average rate
of travel is 0.542 m/rain.
The generalization of the experimental results yielded the following equation* for determining the
rate of travel of the magnesite in a 90 m rotary kiln:
v = KaDinntg ~,
*K. V. Simono% Technology of Magnesia Refractories from the Dolomites and Magnesites of the Sagkin
Deposits for Oxygen Converters. _Author's Abstract of Thesis [inRussian], Atma-Ata (1970).
68I

4. where Din is the inside kiln diameter, m; n is the rate of rotation of the kiln, rpm; r is the slope of the
kiln, degrees; K is the friability factor determined experimentally with account taken of the physical state
and the friability of the magnesite in its passage through the individual zones of the kiln.
The mean friability factor is 1.25. The values of K for determining the travel rate of the magnesite
in the individual kiln zones from the equation are given in Table 1.
CONCLUSIONS
A radioisotope-assisted investigation of the rate of travel of magnesite in a rotary kiln yielded the
boundaries of the technological zones of the kiln and the passage time of the material through these zones
in the calcination process. The friability and hence the travel rate in the individual kiln sections vary con-
siderably with the physico-chemical properties and grain size distribution of the magnesite.
Experimental data yielded the correction factor of friability which takes into account the changes in
the physical state and friability of the magnesite in the course of calcining and can be used for a suffi-
ciently precise determination of the travel rate and stay time of the magnesite in given kiln sections and
the stay time of the material in the kiln as a whole from the equation v = K~Dinntan r
1.
2o
3.
4.
5.
6.
LITERATURE CITED
F. G. Banit, in: "Research in the Chemistry and Technology of Silicates, " All-Union Scientific Re-
search Institute for Cement [in Russian], Promstroiizdat, Moscow (1957), pp. 170-180.
E. S. Kichkina, I. G. Abramson, F. I. Bednyakov, et al., Tsement, No. 4, 6-8; No. 6, 4-5 (1967).
K. Hogreba and W. S. Lehman, Zement-Kalk-Gips, No. 5, 210-215 (1955).
H. Costa and K. Peterman, Silikattechnik, No. 4, 209-210; No. 5, 253-259; No. 7, 345-350 (1959).
G. S. Valberg, A. Z. Kuleshenko, A. I. Vygodskii, et al., Tsement, No. 7, 10-11 (1972).
V. A. Perepelitsyn,K.V. Simonov, V. S. Korshunov, et al., Ogneupory, No. 9, 51-55 (1968).
682