INDUSTRIAL AiVD ENGINEERING CHEMISTRY
January, 1927
fromequation (1). If the values for L and m are taken as previously given and the values for d are taken as obtained by method B, the instrumental constants CI and C2will be as given in Table VI, which contains all constants necessary when employing the consistometer, either as a visoometer for ordinary liquids or for testing other soft materials. Table VI-Instrumental
Constants of Two Consisrometers No. 1
CONSTANT
d (cm.)
0.05071 5.4 1.78 0.00002946 0.01312
L (cm.) m c1 c2
No $5 0.1116 2.0 1.07 0,001,363 0.02123
To exemplify the use of a buret consistometer in determining the consistency of a soft material, the results of tests made with instrument No. 5 on a n oil containing soap are shown in Figure 7 . As the lines in this graph are practically straight, the consistency of the material may be expressed, approximately a t least, by the yield shear value, f, and stiffness, S, as defined by the equations
where p is the pressure obtained as the intercept of the upper straight portion of the graph, prolonged, on the axis of abscissas.
'
- is also found graphically. 4
S and f are constants of the material.
Both ordinarily increase as the resistance to flow increases. Taking values for d, L and C1from Table VI, and reading p = 5.5 arid p-
9
=
363, from Figure 7 , it was found that f = 75 dynes per sq. em. and S = 0.68 dyne-second per sq. cm. I n the absence of published data on the consistency of oils containing soap, comparison may be made with the results
139
of Porter and Gruse15 on worked cup greases. These investigators do not give values for f, but for their grease of 1 highest mobility, 0.217'6, the stiffness would be 0.217~ = 4.60, or about 7 times as great as that for the oil containing soap. It is not to be inferred from the foregoing example that equations (13) and (14) are applicable to all materials which do not obey equation (l), and reference should be made to our paper presented before the June, 1926, meeting of the American Society for Testing Materials for a method that is preferred for rubber-benzene solutions. However, whatever equations may be necessary to describe the consistency of a material after a flow-pressure graph has been determined, i t is believed that the buret consistometer is of general application, provided only that the material is not opaque nor too stiff to flow readily under hydrostatic pressure. Summary
The buret consistometer may be used for rapidly and accurately determining the flow-pressure graph of a viscous or other soft material. The value of the coefficient of the kinetic energy correction should be determined for each instrument. It may be necessary to check the graduations of commercial burets. A satisfactory correction for surface tension effects was made by subtracting from the average head half the capillary rise, calculated from the usual formula, with a diameter equal to the outside diameter of the capillary tube. The inside diameter of capillary was found by flow tests for two buret consistometers. The difference between the maximum value and the minimum value of d4 for consistometer No. 1, as determined by five calibrating liquids by the recommended method, was 0.4 per cent. The ordinary formula for stream-line flow may be used in the calibration, without resorting to the laborious calculations proposed by Auerbach. 15
THISJ O U R N A L17, , 9.53 (1925), Green, Proc. A m , Soc.
kfeleutuis,
ao
Testzng
( z ) , 467 (1920).
Rubber as Lining for Grinding Mills' Solution of a Long-standing Problem of Chemical Grinding By B. W. Rogers? THE B. F. GOODRICH R U B B E RCo., AKRON,OHIO
HE engineers responsible for the trial of rubber as a grinding mill lining in the mining industry had in mind the abrasion-resisting quality only, to OT ercome the severe wear and frequent costly renewal of steel lining. The early tests showed that rubber could, if installed under proper conditions, effect a very desirable saving in maintenance cost. Contrary to expectations, the rubber lining bettered the performance of mills by reducing consumption of power and grinding mediums and by increasing capacity and fineness of the product being ground. These indications later suggested tests which formed the basis of research to determine the reasons for the difference in performance between metal and rubber lining, when operating under parallel conditions. It was found that the light weight of the rubber lining as compared with steel or stone reduced power consumption and permitted loading the mill with 20 per cent increased
T
1
2
Received July 26, 1926. Sales Engineer.
weight of grinding charge. Moreover, the rubber lining retarded the slippage of the grinding mediums against the shell, which was reflected in a more efficient agitation of the charge and a resulting increase in capacity and fineness. One series of tests included a photographic study of the operation of a laboratory mill with a glass front, which permitted visualization of the grinding action within the mill under varying conditions of pulp volume, density, lining material, and liner design. The photographs showed conclusively that slippage is detrimental to high efficiency. Slippage was not so pronounced in dry grinding operation as in wet, and it was observed that the nature of the material being ground had a strong influence on the action of the grinding charge. It was also observed that a corrugated lining helped to prevent slippage, and that a smooth rubber lining prevented slippage to a greater extent than smooth steel. This is accountable by the fact that the soft rubber allowed the individual pebbles or balls to indent the surface, forming a greater contact than the point contact of round grinding bodies resting on a hard lining.
Critical Speed
In the course of the study of grinding mills it was found that the revolving speed is a factor that has never been given sufiicient consideration, and many mills in service are not being operated at the critical speed conducive to the highest grinding efficiency. A Sormula has been evolved by which tho critical speeds of various diameters and varyiiig hall or pebble charges can be calculated, not taking into consideratioil, however, the factor of slippage. The theoretical paths of balls or pebbles can be plotted iuto a circle as parabolic lines, which almost coincide with the paths of balls in actual operation shown by photographs of a niill with a glass front. The portion of the mill occupied by the grinding charge has a hearing on the crit.ica1 speed, and it has been observed that undesirable slippage is considerably minimized as the volume of the grinding charge approaches 50 per cent of the total volume of the mill. The accompanying table of critical speeds is based on the best theoretical efficiency. Table of Critical Speeds
R . 9. m. R . 9.m. 50.19 6 0 . 4 5 41.8s 42.76 34.19 84.90 2 9 . 6 1 311.22 213.4s 27.04 21.18 24.09
n.9 . ,n. n.D.m. 61.82 41.72
36.l3'1 ao. 91
27.64 25.24
R . 0.m
653.80
03.10 46.04 37.m 3 1 , ti7 8 2 . 5 3 28.34 29.11 2 5 ~ 8 8 26.58 14.81 ;l(i.BX
~__
Speed at which renlrifusal force hcgiirs to ovemerne the gravity of tlir first row 01 balls in direct cezitarl with t h e shell.
A mill opcrating at critical speed, and having a given volume of grinding charge, eliniinatiug the factor of slippage, will produce 1.14 cyelcs of the grinding charge per revolution; and in this cycle the balls spcrid 5 i per cent of the t.itne rising
30 X 30-Inch Batch Mill with 3/4*1nch Linerite Applled by Vulcalock Process The lining in shown in i t s uncuied slate, arid belore curins the halves w i l l be bolted ioyelher and the central j o i r ~ tciorcd
in the circular path concentric to the surface of the shell lining and 43 per cent of the time followiiig the parabolic pat.h of the downward movement. A visual study of the motion and path of the grinding charge made possible with a glass-front mill will demonstrate t,hat a large percentage of the grinding takes place within the mass on its dou;nward path, which is in a much more violent state of agitation than the balls on the rising path. Slippage of the grinding charge against the shell lining consumes ineffective power, and ha.s the same effect on the action of the grinding charge as reducing the speed below the Critical point.
For fine grinding, where attrition axid the rolling action of the large surface of small grinding mediums accounts for a larger percentage of the grinding than the impact of balls, it is desirable to keep the halls in the downward path in a compact mass. I n such cases the critical speed for the best efficiency is the speed where the maximum cycles of the grinding charge are attained before centrifugal force becomes a sufficient factor to throw individual balls out of the mass BS they enter into the parabolic downward path. Where porcelaiii balls and flint pebbles are used for the grinding charge, it is common practice to reduce the revolving speed of mill much below the critical speed in order to minimize bredcage of the grinding medium. Pebble breakage is practically eliminated in rubber-lined mills, which results in a substantial reduction in the consumption of grinding medium per unit of material ground. Attachment of Rubber Lining
When the commercial possibilities OS rubber liming became evident, the problems of staudardieation and attachment came nuder consideration. h detachable lining with all metal grinding contact eliminated was developed with means OS positive anchorage in the mill by means of bolts. The detachable nature of the design requires joints which make the lining suitable only for abrasion-resisting service. This covers nearly e\.ery industrial wet-grinding condition, but does not include certain classes of grinding in the chemical industry which require the lining to resist both corrosion and abrasion. The advent of the Vulcalock process, which permits an integral bond to he made between soft rubber and metal, opened a new field for the rubber lining, with advantages which are lacking in the detachable type. Mills of both the batch type and the continuous feed a i d discharge type have been successfully lined by the Vnlcalock process using rubber ranging from 0.5 to 1 inch thick, depending on the sizc OS the mill and the conditions of operation. Mills to be lined by the Vulcalock process must he sent to the rubber fact.ory, and in the smaller sizcs, not sufficienbly large for a man to enter through the manhole, the construction must permit opening in the middle or the complete removal of one end head. After the inside oS the mill is processed, a rubber of specified thickness is applied in uncured condition and all joints are closed before the curing operation. The finished lining is water- or acid-tight, and protects the mill from corrosion as well as abrasion. The impervious surface permits the inill to be quickly and thoroughly washed out, in case one mill is required to grind products of various colors and qnalities. When all-metal contact must be eliminated, the rubber lining can be exiended around the frame of the opening, and the manhole covers can be lined on the wearing face and rubber-covered on the edges to prevent corrosion and contamination of the product that must be protected from iron contact. Rubber is primarily adapted for fine wet-grinding mills. In dry-grinding operations the application of rubber is limited because of the heat factor. Tests have shown that friction heat generated in the mill will build up in the mill, oiving to the slow radiation through the rubber lining, to a sufficient temperature to cause premature failure, unless a large volume of air is forced through the mill to dissipate the heat. Oil and grease have a deleterious effect on rubber, which precludes its use in grinding pigments in oil. I n the ceramic industry, where white materials are ground that must be kept entirely free of stains or any color contamination, the rubber lining is proving very satisfactory. Although t.he rubber compouiid used is black, the wear is so slight that contamination cannot be traced in white china
IXDUXTRIAL AND ENGINEERING CHEMlSTR Y
January, 1927
slip after fourteen days of continuous grinding. One of the first linings attached by the Vulcalock process was installed in a 3 x 3 feet batch mill used for grinding white glaze.
141
After eighteen months of continuous service, inspection showed that the wear from the original 3/4-inch thickness was too slight to be measurable.
Reactions in the Fuel Bed of a Gas Producer' 111-Effect of Steam-Coal Ratio By R. T. Haslam, J. T. Ward, and R. F. Mackie DEPARTMENT OF CHEMICAL ENGINEE:RING, MASSACHUSETTS INSTITCTE OF TECHNOLOGY, CAMBRIDGE, MASS.
With the same experimental technic as in the previous work of this series, a study was made of the effect of increasing the amount of steam admitted with the air blast (pounds of steam per pound of coal) on the reactions in the fuel bed of a gas producer. Runs were made using from 0.366 t o 1.03 pounds of steam per pound of coal with constant depth of fuel bed (3.0 feet) and constant rate of firing (40 pounds of coal per square foot of grate area per hour). I t was found t h a t the heating value of the final gas and the cold gas efficiency increased to a maximum and then decreased. The optimum steam-coal ratio for efficient producer operation lay between 0.7 and 0.8 pound of steam per pound of coal. This value is higher t h a n any found by other workers and is explained by the high rate
of firing employed. Apparently, the most desirable steam-
coal ratio increases as the ratio of firing increases. The percentage steam decomposition decreased with higher values of steam-coal ratio, as a result of the lower temperature in the primary reduction zone (where steam is decomposed by carbon) lowering the rate of reaction constant for steam decomposition. The thickness of the primary reduction zone and the thickness of the oxidation zone remained constant. As in the previous work, steam went through the oxidation zone undecomposed. This work indicates the value of operating gas producers a t a high rate of firing with a steam-coal ratio t h a t increases as the rate of Bring increases.
.. .... . .
I
N GAS-PRODUCER operation it is possible to vary three conditions-viz., depth of fuel bed, rate of firing, and amount of steam admitted with the air blast. Each of these primary variables has a definite effect on the results obtained from the producer. The effects of the first two are described in the previous article of this group.2 The work described herein is a continuation of the previous work and shows the effect of the steam-coal ratio (pounds of steam per pound of coal) on the composition of the gases progressively up through the fuel bed. T o tie in with results reported previously, a run was made a t Constant depth of fuel bed (3.0 feet) and constant rate of firing (40 pounds of coal per square foot of grate area per hour), these being the conditions of run 4 of the previous series. Run 4 was repeated and checked by run 201. I n all these runs the same apparatus was used and the same experimental procedure followed as reported p r e v i ~ u s l y . ~A carefully screened anthracite pea coal was used (Table I). T a b l e I-Analysis of Coals as Fired (Coal C was used for runs 4 and 103; coal D for run 105; coal E for runs 201, 202, 203, and 204) COALC COALD COALE Per cent Per cent Per cent Moisture 2.35 3.64 0.92 Volatile combustible matter 7.70 7.95 10.16 Fixed carbon 76.55 75,24 74.34 Ash 13.40 13,17 14,58 Total carbon
78.84
Higher heating value (dry basis), B. t. u. per Ib.
77.18
76.17
k', are plotted against the steam-coal ratio in Figures 1 to 4. Figures 5 to 11 show the changes in gas composition and temperature through the fuel bed, and Figures 12 to 18 show the change in the gas ratios through the fuel bed. Discussion of Results The accuracy of the results is controlled by the nearness to which the snap gas samples represent average conditions of operation under the external conditions imposed. The external conditions were held constant for a period of 15 minutes before the run was started. Nevertheless, variations in the coal and its combustion caused the final results to vary to an unavoidable extent.
.
..
.. LSS. STfAM PER LB. C O A L
Figure 1
-z
0
I
f E
2 4
BO
e
0
$
0
60
; z
w
b
I I
13,690
12,630
13,220
as
40 0.I
0,
0.5
The results of this series of experiments are tabulated against pounds of steam per pound of coal in Table 11. Some of the more important results, such as heating values of the final gas, cold gas efficiency, per cent of entering steam decomposed in the bed, and reaction rate constant, Received August 17, 1926.
* Haslam, Mackie, and Reed, page 119, this issue.
* Haslam, Entwistle, and Gladding, THIS J O U R N A L17,,
a1
586 (1925).
0,s
LB C ~ A L
Figure 3
Results
Figure 2
- 0
100
l B 5 . S T E A M PER
1
.
LB5. STEAM PER LB. COAL
1.1
9 0 0.1
0.3 0.5 0.7 0.0 L8S. S 1 f A W P f R L 0 . C O A L
1.1
Figure 4
(B. t. u. in Figure 1, gross, 30 in. Hg, 60° F., H20 saturated)
A glance a t Figure 1 shows that the cold gas efficiency increases to a maximum and then decreases as the pounds of steam per pound of coal is increased from 0.366 to 1.03. The maximum efficiency which can be obtained with this rate of firing (40 pounds of coal per square foot per hour) is found to lie a t a steam-coal ratio of from 0.7 to 0.8 pound of steam per pound of coal. This value is higher than any
