I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2234
have used neoprene, silicone, and Butyl rubber 0 rings. They can be supplied by Precision Rubber Products Corp., Dayton, Ohio. A constant speed motor oi' 1 r.p.m. turns the driving screw through suitable gears. In the present design, a flexo-actio11 motor manufactured by Merkle Korff Gear Co., Chicago, Ill., has been used. The clutch, supporting the plunger connecting rod, is equipped with threaded dogs to engage the driving screw. It is kept in proper alignment by two guide rods which are bolted
100
200
300
400
500
600
GRAMS DEL I VERED
Figure 2. Calibration of Feeder to the lower and upper plates of the driving mechanism. The plates also serve t o hold the bearings for the driving screw. The clutch can be disengaged from the driving screw by simply turning the knurled engaging knob. A small pointer attached t o the clutch passes over an accurately graduated metric scale fastened to the mounting board holding the feeder. It is particularly handy for calculating the rate of travel of the plunger during feeding. Four sets of gears with common centers have becn used with the present design. These will allow seven feed rates, from 0.8 to 12.8 ml. per minute. A wider range of feed rates can be attained by substituting a motor of different revolutions per minute or a oylirider of altered internal diameter. The accuracy of the feeder is well demonstrated by the rcsults
Engk;
rin g
Vol. 44, No. 9
of the following experiments shown graphically in Figure 2. One calibrat,ion was made with a low melting organic compound held at 100" C. by supplying steam t o the remrvoir jacket. Gear ratios were selected t o give a rate of 1.5 r.p.1~1.t o the driving screw. The amount delivered over each 10-minute period was weighed on a beam balance. Over 490 grams were deliverod at an average rate of 4.81 grams per minute. The maximum devitttion from this average was 0.7'3,. In a sccond experiment, a. second calibration was madc with a liquid organic compound at room t,emperature. Gear ratios were used to give a rate of 0 . 5 r.p.m. to Lhe driving screw. Weighings were made L ~ Rin the first case. Total delivery was 536.7 grams with an average feed rate of 1.73 grams per minut,c. The maximum deviation from this average was 1.1%. If 20 mm. from each end of the cylinder arc not included in the calibration, the maximum deviation from the average is 0.60/,. Slight distortion near the ling seals between the cylinder and jacket i s responsible for this variation in rate. A stainless steel cylinder was substituted for the glass cyliutlcr for pumping high melting point coal t,ar pitch requiring temperatures of from 150" to 200' C. The sanie accuracy in pumping rate was attained by machining and polishing the steel cylinder to comparable tolerances. The cylinder was heated electrically with an air space between the heat,er and cylindeT to prevent localized overheating. Acknowledgment
The author8 wish to acknowledge the help of George Driesen of the machine shop in building the feeder and designing the driving clutch. literature Cited (1) Lundsted, L. G . , Ash, A. B., and Koslin, ti.L., Anal. Chem., 22, 626 (1950). (2) Miohaeli, I., Chembtiu and Industry, 1951, No. 8 , 123. RECBIVW for review September 27, 1951.
ACCBPTEDApril 18, 1932.
Improving the Density and Strength I Briquets
Process development I
T H E O D O R E BREITMAYER'
AND
F R A N K B. W E S T 2
University of Washington, Seuftle, Wash.
ANY investigators have attempted t o briquet charcoal t o produce a satisfactory substitute for domestic or industrial fuel, metallurgical coke, or carbon electrodes. Such studies have been of particular interest t o regions having abundant wood wastes but little coal, gas, or petroleum. The principal advantages of charcoal over coal are its low ash, sulfur, and phow phoivs contents. Its principal disadvantages are its low density a s d crushing strength which make it difficult to store, t o transport, to handle, and to use. To be suitable for mctallurgicitl coke or carbon electrodes, charcoal moldings require high strength and density and low volatile matter content. The density and strength can be improved by briquetting with binders such as 1 Present
* 1'1 esont
addrese, Crown Zellerbaoh Corp., Camas, Wash. address, Shell Development Co., Emeryville, Calif.
starch, and some such briquets have been uscd in this country &E( domestic fuel (8). The use of by-product wood tar from the production of the charcoal as the binder for briquetting the charcoal haR been reported by numerous investigators (W, 5-7, 9, IO). The tar htm often been diluted with a solvent for better application. The resulting briquets have generally been baked a t temperatures above 300' C . to produce strong briquets. Beuschlein and coworlcers in this laboratory (2, 6) have also investigated the a h r p t i o n of the heavier tar components directly from the retort gtm onto the partially cooled charcoal, thus eliminating the steps of tar recovery, preparation, and mixing with the charcoal. They have also studied the manufacture of briquets by extrusion. The purpose of the present study was t o investigate two methods for further increasing the strength and density of char-
September 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
coal moldings. One proposal was t o grind the charcoal prior t o briquetting t o such a size as t o destroy the cellular structure inherited from the original wood. Another was to mold the briquets under a vacuum t o prevent any air trapped in the capillary structure of the charcoal from expanding the briquet on releaseof the briquettingpressure. The effect of briquetting pressure was t o be determined for each of the above, and the binder was t o be wood t a r of approximately the same viscosity and quantity as used by Meyers (7) in a previous investigation.
Table 1.
2235
Characteristics of Charcoal Mixes Mix, %
Tyler Mesh Size Distribution -10 f 14 -14 4- 20 -20 28
+4-35
-28
-35 f 48 -48 f 65 -200
Loose-pscked densities, gram/ml. Asprox. net kw.-hr./ton
A 2.6
20.8 29.7 25.2
11.6 10 2 0.0 0 246
... .
B
C
D
.... . ... .., . .. . .
. . . ..
.....
.*..
91.0 0.556 4.5
98.8 0.718 105
99.6
,..
... . . . .... . . ...
0.810
179
Effect of Particle Size on loose-Packed Densities
T o determine whether sufficient size reduction would destroy the cellular structure of the charcoal and thereby increase its density, preliminary tests were made t o find the loose-packed density of various size fractions of charcoal. Charcoal from mixed Douglas fir and western hemlock pulp chips waa ground by a disk crusher, and ortions were ground still finer by a Bantam Mikro-Pulverizer. 8 z e fractions down to 43 microns were separated by screening with a Ro-Tap shaker. Smaller sizes were separated by elutriation and were checked for size range with a calibrated field microscope. Loose-packed densities for the various fractions were determined by placing weighed samples in a 10-ml. graduate and lapping it gently on wood until no further settling could be detected. Very consistent results were obtained with the smaller particles, but i t was necessary to increase the size of sample and to use. a 250-ml. graduate for the coarser sizes t o achieve equal precision. Figure 1 shows the effect of particle size range on loose-packed density. The fractions separated by screening (43 microns or larger) increased moderately in density from about 0.25 for the larger to 0.35 for the smaller sizes, but all were lower than for solid chunks of unground charcoal which averaged 0.40 gram per ml. This aeems reasonably in accord with the findings of previous investigators (6, 7) t h a t there waa no advantage in using the finer sizes. However, below 43 microns there was a very sharp increase in loose-packed density to 0.69 for a 10- t o 25-micron fraction and t o 0.77 for a below-20-micron fraction with quite a range of particle sizes. Microphotographs of softwood cross sections indicate the diameter of the major tracheid cells t o be 15 to 40 microns (4). Thus i t appears t h a t the cellular structure of the charcoal had indeed been destroyed. The resultant loose-packed densities ranged up to three times those for the coarser fractions and up t o nearly twice thedensityof %olid”chunksof charcoal. This clearly warranted study of the properties of briquets formed from such small particle sizes. Preparation of Charcoal for Molding
The charcoal and wood tar used for molding were produced by heating western hemlock pulp chips t o redness in a retort until gas evolution ceased. The crude t a r condensed from the retort gas was further refined by boiling off the lighter fractions to yield a residue equal in weight t o half the charcoal. The final binder tar had a normal boiling point of 158’ C. and a viscosity of 950 centipoisee & 40’ C. which was similar to that used by Meyers (7). Batches of charcoal were ground t o various loose-packed densities under conditions permitting estimation of the power consumption with the results shown in Table I. A preliminary crushing and screening permitted synthesizing a Mix A t o approximate Meyer’s optimum size distribution. Mixes B, C, and D were ground by repeatedly passing the entire charge through the MikroPulverizer a t such a rate as t o load the motor t o a constant amperage. When the charge reached a loose-packed density of 0.556, a portion was set aside as Mix B. The rest of the charge was ground further in the same manner t o a loose-packed density of 0.718, at which point Mix C was set aside. A final grinding t o a densit of 0.81 produced Mix D. A rough indication of relative p a r t i d sizes in the four mixes is given by the percentage of each mix which is larger than 200 mesh. Ths decreased from 1 0 0 ~ o
for Mix A, t o 9.0% for Mix B, to 1.2% for Mix C, t o 0.4% for Mix D. The power consumption for each mix was approximated from its weight, the time of grinding, the voltage, and the motor current corrected for the current drawn at full speed without charcoal being fed. A power factor of 80% was assumed arbitrarily. The power consumptions so calculated should be considered as approximations only, but their relative values should be reliable. Table I indicates that the loose-packed density of Mix A can be doubled by approximately 5 kw.-hr. per ton, and that densities as high as 0.81 can be attained by expenditures of the order of 180 kw.-hr. per ton. These energy consumptions presumably could be reduced materially by use of closed circuit grinding. J
0.8
t
c
i0.6 0
a 04
p
In
02
s 0.0 2
4
10
20 40 100 e00 400 PARTICLE SIZE RANGE MICRONS
.
IO00
Figure 1. Effect of Particle Size Range On loose-wcked density d charcoal
The binder tar was applied t o samples in the proportion of 4 parts of tar t o 10 parts of charcoal b y weight, with the tar being dissolved in from 5 t o 16 parts of acetone t o ensure uniform distribution over the particles. Meyers (7) had used the same proportion of t a r t o charcoal but had kneaded the undiluted tar into the charcoal by hand. It was believed that application from an acetone solution would be more reproducible and would approximate the distribution attained by direct adsorption of the tar on the charcoal from the retort gases. After thorough stirring of the mixture, the acetone was boiled off on a steam bath under vacuum until no acetone odor could be detected in the hot mix. Production and Testing of Moldings
For the sake of simplicity in testing, the moldings were made in the form of 1-inch diameter cylindrical briquets, ranging in length from 0.7 to 1.8 inches. Figure 2 shows the mold which was constructed from a section of 1.5-inch extra-heavy iron pipe. The inside bore waa ground smooth t o a diameter of 1.04 inches, and a cylindrical plunger with a diameter of 0.98 inch was rovided for compressing the briquet. Vacuum ports were intro&ced through the pipe wall t o the narrow annulus between the plunger and the wall. The ends were closed with 1.5-inch pi e caps, one of which was equipped with a stuffing box for the pyunger. The briquettin pressure was applied t o the mold by means of a hand-operated fydraulic press in a steel frame. The oil pressure in the press was meagured by a gage and was multiplied by the ratio of the area of the lifting chamber t o the area of the
2236
INDUSTRIAL AND ENGINEERING CHEMISTRY
plunger in the mold in order to calculate the briquetting pressure. The briquetting was accomplished by charging the mold with the charcoal-tar mix, applying the desired pressure for 30 seconds, releasing the pressure, and removing the briquet. When using vwuum, the vacuum was applied to one port for 60 seconds before the mechanical pressure was applied and was maintained throughout the briquetting period. The va,cuum w a read ~ by m e a n s of a g a g e attached t o the o p posite port. T h e standard pressure used for briquetting was 5000 pounds per square inch, but some runs were made at 10,000 and 20,000 pounds per square inch. In two runs, measurements were made of plunger disp l a c e m e n t us. briquetting pressure during application of the pressure t o permit calculation of the energy consumed. Normally t w o briquets myere formed Figure 2. Defails of Briquetting Mold for every new condition used. The pressed briquets were hardened by baking in a covered crucible in a muffle furnace a t 725' F. for 40 minutes. Their densities were then determined from their weights and external dimensions. It should be noted that practically all of the tar seemed t o be driven off during the baking operation since the final briquets teighed practically the same as the original tar-free charcoal. Thus, the tar itself did not increase the density of the final briquets appreciably. A larger yield of much stronger and denser briquets should result if most of the t a r could be coked in situ rather than vaporized. The crushing strengths of the briquets were determined with a Southwark Tate-Emery testing machine belonging to the civil engineering department of the University of Washington. Rough ends on the briquet,s mere first squared with plaster of Paris. Pressure was then applied gradually t o the ends until failure occurred. Since the length-to-diameter ratios of the briquets varied from about 0.7 to 1.7, standard corrections ( 1 ) were applied t o correct the crushing strengths to a length-todiameter rat,io of 2.0. The corrections d e c r m e d the observed strengths by from 3 t o 35%.
Results of Briqwetting Experiments The briquets formed by pressing Mix A at 5000 pounds per square inch assure t h a t the basic procedures used are equivalent t o those used by Meyers (7). Mix A was made up t o duplicate Meyers' optimum mixture and gave briquet densities of 0.63 compared t o Bhe 0.62 found by Meyers. A crushing strength of 440 pounds per square inch was obtained, only slightly greater than for Meyers' briquets. Figure 3 shows the effect of the amount of grinding energy on the densities of the loose-packed mixes and on the densities of the briquets made from them using various briquetting pressures. The briquet densities roughly parallel but are higher than the loose-packed densities. The increase in density n i t h energy input is most significant between Mix A and Mix B. Mix D, with approximately 180 kw.-hr. grinding energy per ton, has the highest briquet densities, 0.92 and 0.94 a t 5000 and 10,000 pounds per square inch, respectively. The R l i x D (5000 pounds per square inch) briquets are 46% inore dense than those made from Mix A corresponding t o Meyers' optimum. Table I1 shows the effect of the amount of grinding energy on the corrected crushing strengths of the briquets made from the various mixes. The results must be accepted cautiously, since the strengths of duplicate briquets varied by 15 to 35y0 from their
Vol. 44. No. 9
average in about half the cases. K i t h this reservation, it may then be noted that among the briquets formed at 5000 pounds per square inch, the Mix I3 briquets average 30% lower than the reference Mix A briquets despite their higher density. This substantiates the observations of previous investigators that briquet strength is decreased by decreasing particle size within the sieve-size range. The still denser Mix C briquets formed a t 5000 pounds per square inch are back on a par with Mix A, while the corresponding Mix D briquets made from very fine particles average 1 0 0 ~ stronger. o Thus, briquet strength appears to pass through a minimum, as particle size is decreased, t o be followed by a rapid increase in both strength and density for very fine particles. The effect of doubling the briquetting pressure on crushing strength for Mixes C and D is somewhat contradictory in Table 11, perhaps because random variations between briquets are of the same order of magnitude as the improvement. The trend is clear for the fourfold range of pressures used with Mix B, the average increase in strength being 240%. The Mix B strengths and densities have been plotted against the briquetting pressure in Figure 4 and appear consistent, the strength increasing almost directly with pressure.
Table II. Crushing Strengths of Briquets from Various Charcoal Mixes B B Mix Grinding energy, kw.-hr. per ton 4.5 Correrted Crushing Strength_sLLb /Sq. Inch For briquets formed at, lb./sq. inch
. ~.
6,000 5,000 10,000 10,000 20,000 20,000
C
D
105
179
262
417
586
_.. ... io38
392
924
440 445
3.58 613 1202 551 383 1088
... ,
..
1066
... .... ...
Figure 4-also s h o m the approxiniate energy consumed in briquetting Mix B a t various pressures as computed from pressure-volume measurements from piston displacement as a function of the pressure. The energy consumptions were almost negligible, amounting to only 4 kw.-hr. per ton a t 5000 pounds per square inch. Significantly, increasing the pressure t o 20,000 pounds per square inch with a resulting increase in strength of 240% increased the energy consumption only 50%. I t is concluded that the optimum briquetting pressure will be determined by practical design limitations and costs, rather than by the power consumption. Figure 5 shows that briquetting the coarse particles of Mix A under various vacuums, measured in inches of mercury, increased
10
08
a
5 06
E04 5 0 2 zd
50 NET GRINDING
i
100 ENERGY
I 150
30
K.W.H./TON
Figure 3. Effect of Grinding Energy On densily of loose-packed mixes and of briquets formed st various pressures
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1952
2237
700
1
t
-1
600
E sI v)
t 600
I
I
0,ov 0
so00
Figure 4.
-
I
10000
BRlPUETTlNa
PRESSURE
lo 20000
11000
LE./SO. IN.
Effect of Briquetting Pressure
IO
O n net briquettine energy, briquet density, and corrected crushing strength br M i x B
IS VAWUM
-
20 IKHR
U
400 30
’
Figure 5. Effect of Briquetting under Vacuum On denslb and corrected crushing strength of M i x A briquets
the density and strength of the briquets. With 28 t o 29 inches of mercury, the density was increased 13% t o 0.71 gram per ml. A simultaneous 20% increase in strength is indicated although this is within the range of possible scattering of results. The mixes composed of finer particles also benefit slightly although erratically by vacuum operation. Whether vacuum was used or not, the plunger rebounded roughly 10% of the length of the briquet on release of the pressure, for all mixes. Various blends of Mixes A and B were also tested with and without high vacuum (28 to 29 inches of mercury), the resulting densities and strengths being reported in Table 111. Although Mix B appears definitely inferior in strength, at least without vacuum operation, blending as little as 25% of B into A increases the density by 15% and the strength b y 70% for nonvacuum operation. The densities and strengths of the blends exceed those of either mix alone. A similar but somewhat less striking effect is observed for vacuum operation. A few blends of Mixes A and D were tested similarly and are also reported in Table 111. No synergistic effect can be clearly observed with these blends, the results generally being intermediate. By and large, for blending with Mix A up t o 50% concentration, Mix B which requireeabout 5 kw.-hr. per ton is as effective as Mix D which requires about 180 kw.-hr. per ton. An outstanding exception consists of a single briquet formed under vacuum and containing 75% D which attained a density of 0.83 and a strength of 1705 pounds per square inch.
times that of coarser fractions and t o nearly twice the density of “solid” chunks of charcoal. The critical range is of the order of 25 t o 40 microns, approximating the diameters of the major cells in the original wood, A charcoal loose-packed density of 0.55 gram per ml. can be produced with a net energy consumption of the order of 5 kw.-hr. per ton by open circuit grinding. Densities as high as 0.81 can be attained for energy consumptions of the order of 180 kw.-hr. per ton. Briquet densities roughly parallel the original loose-packed densities and are increased by higher briquetting pressure. Briquet compressive strengths decrease at first with decreasing particle size but ultimately rise t o approximately twice those made from coarse charcoal. However, the strength frequently varies widely for supposedly identical briquets. Briquetting pressures of 20,000 pounds per square inch require a negligible amount of additional power over 5000 pounds per square inch for one mix and give an average improvement in strength of 240y0. Briquetting under vwuum improves t h e density and strength slightly. Blending 25 t o 5oy0 of moderately fine charcoal into coarse charcoal gives briquet densities and strengths which far exceed those from either starting material.
Conclusion
References
There is a critical size range for charcoal particles below which the loose-packed density increases rapidly t o as much atj three
(1) “A.S.T.M. Standards.” Vol. 2, p. 150, Designation C41-27, Philadelphia, American Society for Testing Materials, 1930. (2) Beuschlein, W. L., Univ. Wash., Eng. Expt. Sta.. Bull. 117 (1950). (3) Breitmayer, Theodore, M.S. thesis, University of Washington, 1948. Table 111. Densities and Crushing Strengths of Briquets Formed at (4) Brown, H. P., Bull. N . Y . State Coll. Forestry 5000 Pounds per Square Inch from Blends of Various Mixes Swacuse Univ., 1, No. 4, 11-14 (1928). (6) Ghormley, E. L., and Mock, E. T.,Jr., B.S. __ Mixes A B Mixes A D thesis, University of Washington, 1944. N o vacuum High VHCUUIII No vacuum High vacuum (6) Joseph,A. F., and Whitfield, B. W., J. SOC. Lb./sq. Lb.,‘sq. Lb./sq. Lb./sq. G./ml. inch Ainblend, % G./ml. inch G./ml. inch G./ml. inch C h m . I d . (London), 40, 190-2T (1921)
+
+
I
loo 75
G.63 0.62 0.72 0.72
63
O.fiO
60 25 0
0.74 0.71 0.66 0.66 0.68
440 445 758 740 460 821 822 840 262 358
0.70 0.72 0.73 0.72 0.70 0.73 0.72
0.67 0.71 0.71
535 527 696 696 514 814 639 780 488 626
0.63 0.62 0.69
440 445 490
0.70 0.72 0.71
535 527 683
............ ............
.... .... ....
0.76 0.81 0 92 0.92
884 1706
............ 741 1215 586 1202
0.77 0.83 0.92 0.96
.... ....
(7) Meyers, D. W., M.S. thesis, University of Washington. 1942. ( 8 ) Nelson, W. G., IND.~ ENG. CHEM., 22, 312
(1930). ....... (9) Pekshibaev, M. I., Russ. Patent 46,235 (March 31, 1936). (IO) Umlauf, Eduard, Austrian Patent 143,498 (Nov. 11,1935). REICEWEID for review May 9, 1951.
ACCI~PTED April 14, 1962.
