INDUSTRIAL A N D ENGINEERING CHEMISTRY
July, 1926
727
Calcination Rates of Limestone' By Wallace A. Gilkey STANFORD UNIVERSITY, CALIF.
L
I M E BURNING was one of the first industries to be carried on by man wherein raw materials are converted into useful substances by chemical reactions. Lime mortars were used by the Romans in the construction of some of their buildings. Although the lime-burning industry had such an early beginning, it still presents a promising field for scientific research. The calcination of limestone plays an important part in the Portland cement industry. The powdered limestone contained in the raw mix is all burned to lime in the upper part of the rotary kilns a t a comparatively low temperature. It is then necessary to raise the mixture to a much higher temperature (about 1400" C.) until a state of incipient fusion is obtained. The material partly fuses together to form cement clinker. 300
2g 260
-B 0
.3
0
u
220
e" 180
.I
s 'c1 .-ea 140
B
;100 Y
a
'i
Per cent Si02 FezOa AlzOa CaO MgO SOI: Ka0, NazO Ignltlon loss
1.26
0.48 54.50 0.21
Traces 43.37
The crushed and ground sample was carefully separated into sizes from 4 to 6 mesh to through 200 mesh by screening. The sample was thoroughly dried, although only a trace of moisture was present. The procedure finally adopted was to heat the samples a t different constant temperatures for different intervals of time in small porcelain crucibles in an upright, cylindrical, 4 X 4 inch Hoskins electric furnace. The per cent of calcination during any time interval was determined by the loss of weight of the samples. Five samples could be heated at the same time. The temperature of the furnace could be controlled by a rheostat connected in series with the furnace to a lighting circuit. Very good temperature control was obtained by this method and also by slightly raising or lowering the lid of the furnace for a few seconds. When making runs a t the higher temperatures (above 800" C.), where the rates of calcination were very high and the time of heating the samples was short, the temperature of the furnace was raised to a hundred degrees or more above that a t which the rate of calcination was to be determined before the samples were placed in the furnace. The furnace cover was then removed and the crucibles were quickly placed in the furnace and the furnace cover placed in position. In this way the required tempera-
60
.3
u
E
F
20 700
750
800
850
900
950
1000
Temperatures in degrees Centigrade
Because of the high temperature required for the clinkering process and the large volume of air which rushes through the kiln, very high stack temperatures are produced and an enormous amount of heat is wasted. When it is considered that the average stack temperatures in a cement kiln are from 700" to 850" C., the large amount of waste heat is realized. A new process for burning Portland cement was under consideration in which the clinkering process would be carried out in a smaller furnace and the limestone content of the raw mix burned to lime by the waste heat in another furnace arranged in series with the first. The important question arose as to how fast the material could be passed through this calcining furnace-in other words, what are the rates of calcination of limestone a t different temperatures? No answer to this important question is found in the scientific literature. Procedure
A sample of selected limestone was obtained from the quarry of the Santa Cruz Portland Cement Company. The sample was of crystalline structure, varied in color from pure white to a light gray, and had the following analysis according to the cement company: 1
Received March 1, 1926.
700
750
800
850
900
950
1000
Temperatures in degrees Centigrade T h e Rates of Calcination of S a n t a Cruz L i m e stone a t Different Temperatures
ture could be reached in a very short time compared with the time required for the run. At the highest temperatures (900" to 1000" C.) it was necessary to work very rapidly. By exercising great care the crucibles could be brought to the required temperature in a small fraction of a minute. At the highest temperatures the full current was turned on and the temperature of the furnace was controlled by raising or lowering the furnace cover. A current of air was drawn through the furnace in order to obtain as nearly as possible the conditions that would prevail in a cement kiln. Temperatures were determined by means of a Brown nichrome
INDUSTRIAL AND ENGINEERI,VG CHEMITSTR Y
728
thermocouple. The instrument was found to be accurate to within 3" C. for any temperature on the scale of the galvanometer, which read up to 1000" C. Results
As might have been expected, the rates of calcination were greatly influenced by the concentration of carbon dioxide in the furnace atmosphere. Samples in covered and in uncovered crucibles were heated a t the same time a t the same temperature and for the same time interval. The rate of calcination under atmospheric pressure of carbon dioxide was only about 40 per cent of that obtained in uncovered crucibles in a current of air. There is need for more research to determine the effect of the concentration of carbon dioxide upon the rates of calcination. It was observed that the coarser particles of limestone would begin to decrepitate when heated to about 570" C. and that some of the material would thus be thrown out of the crucibles and lost. The crucibles could not be covered during the calcination because the reaction would be greatly retarded owing to the increased concentration of carbon dioxide.
Vol. 18, No. 7
Experiments were performed which showed that practically all of the decrepitation could be accomplished by heating the samples in covered crucibles for about 10 minutes a t 650" C. The very small amount of calcination a t this temperature for the short time interval could be neglected. Every sample was heated in a covered crucible at 650" C. for 10 minutes before being placed in the furnace. The rates of calcination were determined a t 25-degree intervals between 700" and 1000" C. It would have been practically impossible to have determined the rates of calcination of the limestone with any degree of accuracy a t temperatures much above 1000" C. because of the rapidity at which the reaction proceeds. The accompanying graphs represent the results of determinations made on over four hundred and fifty samples and were obtained from the average results plotted on a number of other graphs. Acknowledgment
The research was carried out a t the suggestion of Prof. S. W. Young, of Stanford University.
Measurement of Surface Temperatures' I-A Portable Thermocouple Device Compensated for Heat Losses By M. W. Boyer and J . Buss MASSACHUSETTS INSTITUTE OF TECHNOLOGY, SCHOOLOF
I
CHEMICAL
N MANY problems of heat transfer, investigators have been confronted with the difficulty of obtaining accurate surface temperatures. An important example is the problem of heat flow in pulp and paper driers, in which the quantitative consideration of the heat transfer coefficients necessitates the determination of the surface temperatures not only of the rotating drier shells, but of the moving sheet as well. Probably the most important methods in use a t the present time are the imbedded thermocouple, thermocouple and thermometer pads, and the radiation method. The imbedded thermocouple is the most accurate, but is limited in its application. The thermocouple pad usually consists of one or more couples backed by an insulating pad. The errors connected with this method are more or less evident. I n the first place, when applied to the surface of a wall through which there is a small temperature drop, the poor heat transfer to the pad along with the thermal losses from the pad may result in low values. On the other hand, when applied to a wall through which there is a large temperature drop, there may be a building up of the surface temperature beneath the pad. Although these errors tend to compensate each other, their relative magnitude is unknown and there is no assurance of obtaining the true surface temperature. The radiation method is based upon the Stefan-Boltzmann radiation law and depends upon the black body coefficients of the radiating surface. The lack of accurate information concerning these black body coefficients, or the alternative necessity of calibrating the instrument for each surface, renders this method of little practical value. Since none of these methods were considered suitable for obtaining true surface temperatures under various conditions, it was decided to develop if possible a device for this purpose. Although the development of such a device was prompted 1
Received March 12, 1926.
ENOIKEERINS PBACTICE,
EASTERN
MFG. CO.,BANGOR, ME.
by its possible use with moving surfaces, it was borne in mind that many kinds of surfaces are encountered in practice, such as metallic, lagged and fibrous, either moving or stationary. As the primary instrument for the measurement of the temperatures of these various surfaces, the thermocouple offered the most promise because of its ruggedness, accuracy, and ease of application. The use of the bare thermocouple, however, results in such large heat losses from the couple that it never reaches the temperature of the surface to which it is applied. The use of an insulating pad diminishes the error somewhat, but even with this, except in the case noted above, the heat losses are usually sufficient to prevent the couple from attaining the true temperature of the surface. It seemed possible, however, that these losses could be prevented by applying to the exposed side of the thermocouple sufficient heat to prevent any heat flow through the thermocouple. Under these conditions there should be no temperature drop from the surface to the thermocouple and the couple should indicate the true temperature of the surface. This is the principle of the so-called compensated thermocouple for surface temperature measurement. In order to apply this idea to a practical device, it was necessary to develop a means of determining the absence of heat flow between the surface and the couple. It was believed that this could best be accomplished by the use of an additional thermocouple, both couples being made in the form of disks, placed with their surfaces adjacent. Thus, if the disk in contact with the surface should show a higher temperature than the inner, or regulating, disk, it would indicate a heat flow from the surface to the disks, in which case heat mould be supplied by a heating element until the two couples attained the same temperature. With this condition there would be no heat flow and the temperature of either couple should be the true temperature of the surface.
