ABSORBENTS[27]
The absorbents used in both the British and American gas mask canister,
which afforded a degree of protection far superior to that of any
other allied or enemy nation except Germany, consisted of a mixture
of charcoal and soda-lime, as described in the preceding chapter. In
general, a gas mask absorbent must have certain requirements. These
are: absorptive activity, absorptive capacity, versatility, mechanical
strength, chemical stability, low breathing resistance, ease of
manufacture and availability of raw materials.
[Footnote 27: The basis of this chapter is the series of articles by
Lamb and co-workers which appeared in the _J. Ind. Eng. Chem._ for
1919.]
_Absorptive activity_, or a very high rate of absorption, is one of
the more important properties of a satisfactory absorbent. A normal
man when exercising violently breathes about 60 liters of air per
minute, and since inhalation occupies but slightly more than half of
the breathing cycle, the actual rate at which gas passes through the
canister during inhalation is about 100 liters per minute. Calculated
on the basis of the regular army canister, this corresponds to an
average linear air velocity of about 80 cm. per second. On the average,
therefore, a given small portion of the air remains in contact with
the gas absorbent for only about 0.1 second. Besides this, the removal
of the toxic material must be surprisingly complete. Though the
concentration entering the canister may occasionally be as high as one
half per cent, even the momentary leakage of 0.001 per cent (ten parts
per million) would cause serious discomfort and the prolonged leakage
of smaller amounts would have serious results in the case of some
gases. The activity of the present gas mask charcoal is shown by the
fact that it will reduce a concentration of 7000 parts per million of
chloropicrin to less than 0.5 part per million in less than 0.03 second.
Of equal importance is the _absorptive capacity_. That is, the
absorbent must be able to absorb and hold large amounts of gas per
unit weight of absorbent. Its life must be measured in days against
ordinary concentrations of gas. It is further necessary that the gas
be held firmly and not in any loose combination which might give up
minute traces of gas when air is, for long periods of time, breathed in
through a canister which has previously been exposed to gas.
The absorbents used must be of a type which can be relied upon to
give adequate protection against practically any kind of toxic gas
(_versatility_). The need of this is apparent when the difficulty of
having separate canisters for various gases is considered, as well
as the difficulty in rapidly and accurately identifying the gases
and the possible introduction of new and unknown gases. Fortunately,
practically all of the toxic gases are very reactive chemically or have
relatively high boiling points and can therefore be absorbed in large
amounts by charcoal.
Absorbents must be _mechanically strong_ in order to retain their
structure and porosity under conditions of transport and field use.
Further, they must not be subject to abrasion for the production of a
relatively small amount of fines would tend to plug the canister or to
cause channels through which the gas would pass without being absorbed.
Since the canister is filled several months before it is first used
in the trenches, and since the canister may be used over a period of
months before it is discarded, it is obviously the ultimate activity
and capacity (not the initial efficiency) which determines the value
of an absorbent. It must therefore have a very considerable degree of
_chemical stability_. By this is meant that the absorbent itself is
not subject to chemical deterioration, that it does not react with
carbon dioxide, that it does not disintegrate or become deliquescent
even after being used and that it has no corrosive action on the metal
container.
In a good general absorbent there must be a proper balance between its
various essential qualities, and hence the most suitable mixture will
probably always be a compromise.
CHARCOAL
The fact that charcoal would condense in its pores or adsorb certain
gases, holding them firmly, had been known for a long time.[28] In
general, it was known that so-called animal charcoal was the best
for decolorizing sugar solutions, that wood charcoal was the best
for adsorbing gases and that coke had very little adsorbing or
decolorizing power. No one knew the reason for these facts and no one
could write a specification for charcoal. The ordinary charcoal used
in the scientific laboratory was cocoanut charcoal, since Hunter had
discovered more than fifty years ago that this was the best charcoal
for adsorbing gases.
RAW MATERIALS[29]
The first charcoal designed to offer protection against chlorine and
phosgene was made by carbonizing red cedar. Since this had little value
against chloropicrin, attention was turned to cocoanut shell as the
source of raw material. This charcoal fulfilled the above conditions
for a satisfactory absorbent better than any other form tested. It must
not be supposed, however, that investigation of carbon stopped with
these experiments. In the search for the ideal carbon, practically
almost every hard vegetable substance known was tested. Next to
cocoanut shells, the fruit pits, several common varieties of nuts
abundant in the United States, and several tropical nuts (especially
cohune nuts), were found to make the best carbon. Pecan nuts, and all
woods ranging in hardness from iron wood down to ordinary pine and
fir, were found to be in the second class of efficiency. Among other
substances tested were almonds, Arabian acorns, grape seeds, Brazil
nut husks, balsa, osage orange, Chinese velvet bean, synthetic carbons
(from coal, lamp-black, etc.), cocoa bean shell, coffee grounds, flint
corn, corn cobs, cotton seed husks, peanut shells and oil shale. While
many of these substances might have been used in an emergency, none of
them would produce carbon as efficient, volume for volume, as that of
the cocoanut shell and other hard nuts.
[Footnote 28: Bancroft (_J. Phys. Chem._ =24=, 127, 201, 342 [1920])
gives a comprehensive review of “Charcoal before the War.”]
[Footnote 29: Part of this section is quoted from “Armies of Industry,”
by Crowell and Wilson, Yale Univ. Press.]
Some idea of the scale of charcoal production may be seen from the
requirement for cocoanut shells. When we first began to build masks our
demands for carboniferous materials ranged from 40 to 50 tons a day
of raw material; by the end of the war, we were in need of a supply
of 400 tons of cocoanut shells per day. This demand would absorb the
entire cocoanut production of tropical America five times over. (The
total production of cocoanuts in Central America, the West Indies and
the Caribbean Coast of South America amounted to 131,000,000 nuts
annually, equal to a supply of 75 tons of shells daily.) It was equal
to one-tenth of the total production of the Orient, which amounted to
7,450,200,000 nuts annually. This large demand always made a reserve
supply of charcoal material practically impossible. The “Eat More
Cocoanut” campaign started by the Gas Defense more than doubled the
American consumption of cocoanut in a brief space of time and in
October, 1918, with the help of importation of shell, we averaged about
150 tons of shells per day, exclusive of the Orient.
The first heating of cocoanut shells to make charcoal reduces
their weight 75 per cent. It was evident, therefore, that we could
more economically ship our oriental supply in the form of charcoal
produced on the other side of the Pacific Ocean. A charcoal plant was
established in the Philippine Islands and agents were sent to all parts
of the Oriental countries to purchase enormous supplies of shells.
While the work was only gaining momentum when the Armistice was signed,
the plant actually shipped 300 tons of cocoanut shell carbon to the
United States and had over 1000 tons on hand November 11, 1918.
In the search for other tropical nuts, it was found that the cohune
or corozo nut was the best. These nuts are the fruit of the manaca
palm tree. They grow in clusters, like bananas or dates, one to four
clusters to a tree, each cluster yielding from 60 to 75 pounds of
nuts. They grow principally on the west coast of Central America in
low, swampy regions from Mexico to Panama but are also found along the
Caribbean coast. The chief virtue of the cohune nut from the charcoal
point of view was its extreme thickness of shell; this nut is 3 inches
or more in length and nearly 2 inches in diameter but the kernel is
very small. Four thousand tons per month were being imported at the
time of the Armistice. A disadvantage in the use of cohune nuts was
that their husks contained a considerable amount of acid which rotted
the jute bags and also caused the heaps of nuts to heat in storage.
A third source of tropical material was in the ivory nuts used in
considerable quantities in this country by the makers of buttons.
There is a waste of 400-500 tons per month of this material, which was
used after screening out the dust. This material is rather expensive,
because it is normally used in the manufacture of lactic acid.
Another great branch of activity in securing carbon supplies was
concerned with the apricot, peach and cherry pits and walnut shells of
the Pacific Coast. A nation-wide campaign on the part of the American
Red Cross was started on September 13, 1918. Between this time and the
Armistice some 4,000 tons of material were collected. Thus the slogan
“Help us to give him the best gas mask” made its appeal to every person
in the United States.
A THEORY OF CHARCOAL ACTION
It has been pointed out that the first charcoal was made from red
cedar. While this was very satisfactory when tested against chlorine,
it was of no value against chloropicrin. In order to improve the
charcoal still further it was desirable to have some theory as to
the way charcoal acted. It was generally agreed that fine pores were
essential. The functioning of charcoal depends upon its adsorptive
power and this in turn upon its porosity. The greater the ratio of
its surface to its mass, that is, the more highly developed and
fine grained its porosity, the greater its value. Another factor,
however, seemed to play a rôle. As a pure hypothesis, at first, Chaney
assumed that an active charcoal could only be secured by removing the
hydrocarbon which he assumed to be present after carbonization. Being
difficultly volatile, these hydrocarbons prevent the adsorption of
other gases or vapors on the active material. To prove this, red cedar
charcoal was heated in a bomb connected with a pump which drew air
through the bomb. Although the charcoal had been carbonized at 800°,
various gases and vapor began to come off at 300°, and when cooled,
condensed to crystalline plates.
This experiment not only proved the existence of components containing
hydrogen in the charcoal, but also showed that one way of removing the
hydrocarbon film on the active carbon was to treat with an oxidizing
agent.
In the light of the later experimental work Chaney feels that there are
two forms of elementary carbon—“active” and “inactive”; the active form
is characterized by a high specific adsorptive capacity for gas while
the inactive form lacks this property. In general the temperature of
formation of the active form is below 500-600° C. The form is easily
attacked by oxidizing agents—while the latter is relatively stable.
The combination of active carbon with an adsorbed layer or layers of
hydrocarbon is known as “primary” carbon. Anthracite and bituminous
coal are native primary carbons, while coke contains a considerable
amount of inactive carbon, resulting from the decomposition of
hydrocarbon during its preparation.
PREPARATION OF ACTIVE CHARCOAL
“On the basis of the above discussion, the preparation of active
charcoal will evidently involve two steps:
“First.—The formation of a porous, amorphous base carbon at a
relatively low temperature.
“Second.—The removal of the adsorbed hydrocarbons from the primary
carbon, and the increase of its porosity.
“The first step presents no very serious difficulties. It involves,
in the case of woods and similar materials, a process of destructive
distillation at relatively low temperatures. The deposition of
inactive carbon, resulting from the cracking of hydrocarbons at high
temperatures, must be avoided. The material is therefore charged into
the retorts in thin layers, so that the contact of the hydrocarbon
vapors with hot charcoal is avoided as much as possible. Furthermore,
most of the hydrocarbon is removed before dangerous temperatures are
reached. A slight suction is maintained to prevent outward leaks,
but no activation by oxidation is attempted, as this can be carried
on under better control and with less loss of material in a separate
treatment.
[Illustration: FIG. 67. Dorsey Reactor for Activating Cocoanut Charcoal
with Steam.]
“The second step, that is, the removal of the absorbed hydrocarbons
from the primary carbon, is a much more difficult matter. Prolonged
heating, at sufficiently high temperatures, is required to remove or
break up the hydrocarbon residues. On the other hand, volatilization
and cracking of the hydrocarbons at high temperatures is certain to
produce an inactive form of carbon more or less like graphite in its
visible characteristics, which is not only inert and non-adsorbent,
but is also highly resistant to oxidation. The general method of
procedure which has yielded the best results, is to remove the
adsorbed hydrocarbons by various processes of combined oxidation
and distillation, whereby the hydrocarbons of high boiling points
are broken down into more volatile substances and removed at lower
temperatures, or under conditions less likely to result in subsequent
deposition of inactive carbon. Thin layers of charcoal and rapid gas
currents are used so that contact between the volatilized hydrocarbons
and the hot active charcoal may be as brief as possible. In this way
cracking of the hydrocarbons at high temperature, with consequent
deposition of inactive carbon, is largely avoided.
“While the removal of the hydrocarbons by oxidation and distillation
is the main object of the activation process, another important
action goes on at the same time, namely, the oxidation of the primary
carbon itself. This oxidation is doubtless advantageous, up to a
certain point, for it probably at first enlarges, at the expense of
the walls of solid carbon, cavities already present in the charcoal,
thus increasing the total surface exposed. Moreover, the outer ends
of the capillary pores and fissures must be somewhat enlarged by this
action and a readier access thus provided to the inner portions of the
charcoal. However, as soon as the eating away of the carbon wall begins
to unite cavities, it decreases, rather than increases, the surface of
the charcoal, and a consequent drop in volume activity, that is in the
service time, of the charcoal, is found to result.
“It is obvious, therefore, that conditions of activation must be so
chosen and regulated as to oxidize the hydrocarbons rapidly and the
primary carbon slowly. Such a differential oxidation is not easy
to secure since the hydrocarbons involved have a very low hydrogen
content, and are not much more easily oxidized than the primary
carbon itself. Furthermore, most of the hydrocarbons to be removed
are shut up in the interior of the granule. On the one hand, a high
enough temperature must be maintained to oxidize the hydrocarbons
with reasonable speed; on the other hand, too high a temperature must
not be employed, else the primary carbon will be unduly consumed. The
permissible range is a relatively narrow one, only about 50 to 75°.
The location of the optimum activating temperature depends upon the
oxidizing agent employed and upon other variables as well; for air, it
has been found to lie somewhere between 350 and 450°, and for steam
between 800 and 1000°.
“The air activation process has the advantage of operating at a
conveniently low temperature. It has the disadvantage, that local
heating and an excessive consumption of primary carbon occur, so that a
drop in volume activity results from that cause before the hydrocarbons
have been completely eliminated. As a consequence, charcoal of the
highest activity cannot be obtained by the air activation process.”
The steam activation process has the disadvantage that it operates
at so high a temperature that the regulation of temperature becomes
difficult and other technical difficulties are introduced. It has the
advantage that local heating is eliminated. The hydrocarbons can,
therefore, be largely removed without a disproportionate consumption of
primary carbon. This permits the production of a very active charcoal.
It has the further advantage that it worked well with all kinds of
charcoal. Inferior material, when treated with steam, gave charcoal
nearly as good as the best steam-treated cocoanut charcoal. Because of
the shortage of cocoanut, this was a very important consideration.
[Illustration: FIG. 68.—Section of Raw Cocoanut Shell. Magnified 146½
diameters.]
The air, steam and also carbon dioxide-steam activation processes have
all been employed on a large scale by the Chemical Warfare Service for
the manufacture of gas mask carbon.
[Illustration: FIG. 69.—Section of Carbonized Cocoanut Charcoal.
Magnified 146½ Diameters.]
[Illustration: FIG. 70.—Two-Minute Charcoal not Activated. Magnified
732 Diameters.]
“The above considerations are illustrated fairly well
by the photo-micrographs shown in Figs. 68 to 71. Fig.
68 shows a section of the original untreated cocoanut
shell crosswise to the long axis of the shell. In it
can be seen the closely packed, thick-walled so-called
‘stone-cells’ characteristic of all hard and dense nut
shells. Fig. 69 is a photograph of a similar section
through the same cocoanut shell after it has been
carbonized. As these photographs are all taken with
vertical illumination against a dark background, the
cavities, or voids, and depressions all appear black,
while the charcoal itself appears white. It is clear
from this photograph that much of the original grosser
structure of the shell persists in the carbonized
products. Figs. 70 and 71 are more highly magnified
photographs of a carbonized charcoal before and after
activation, respectively. As before, all the dark
areas represent voids of little or no importance in
the adsorptive activity of the charcoal, while the
white areas represent the charcoal itself. In Fig. 70
(unactivated) the charcoal itself between the voids
it seen to be relatively compact, while in Fig. 71
(activated) it is decidedly granular. This granular
structure, just visible at this high magnification
(1000 diameters), probably represents the grosser
porous structure on which the adsorption really
depends. These photographs, therefore, show how the
porosity is increased by activation.”
[Illustration: FIG. 71.—31-Minute Steam Activated Charcoal. Magnified
732 Diameters.]
The great demand for charcoal, and the need for activating other than
cocoanut charcoal led to the development of the Dressler tunnel kiln,
which seemed to offer many advantages over the Dorsey type of treater.
[Illustration: FIG. 72.—Sectional View of Dressler Tunnel Kiln, Adapted
to Activation of Charcoal.]
“The Dressler tunnel kiln is a type used in general
ceramic work. The furnace consists essentially of a
brick kiln about 190 ft. long, 12 ft. broad, and 9
ft. high, lined with fire brick. Charcoal is loaded
in shallow, refractory trays in small tram cars,
about 120 trays to the car. The cars enter the kiln
through a double door and the charcoal remains in the
hot zone at a temperature of about 850° C. for about
4 hrs., depending upon the nature of the material
charged. Water is atomized into this kiln, and a
positive pressure maintained in order to exclude
entrance of air. The kiln is gas-fired and the charcoal
is activated by the steam in the presence of the
combustion gases.
“Under such treatment the charcoal is given a high
degree of activation without the usual accompanying
high losses. Seemingly the oxidizing medium used,
together with the operating conditions, produce a
deep penetration of the charcoal particles without
increasing the extensive surface combustion experienced
in the steam activators. The capacity of such a type
furnace is limited only by the size of the installation.
“The advantages of this type furnace may be tabulated as follows:
1—High quality of product.
2—Small weight and volume losses.
3—Large capacity per unit.
4—Minimum initial cost and maintenance of installation.
5—Simplicity and cheapness of operation.
6—Adaptability to activation of all carbon materials.
7—Availability of furnaces of this general type already constructed.”
SUBSTITUTES FOR NUT CHARCOAL
The first experiments were made with a special anthracite coal
(non-laminated and having conchoidal fracture). This had a life of 560
minutes as against 360 minutes for air treated cocoanut charcoal and
800-900 minutes for steam-treated charcoal.
When the Gas Defense Service tried to activate anthracite on a large
scale in vertical gas retorts at Derby, Connecticut, the attempt was
a failure. They carbonized at 900° and then turned on the steam with
the result that the steam-treated coal had a slightly greater density
than the untreated, which was wrong, and had a shiny appearance in
parts with roughened deposits in other parts. When the hydrocarbons
are decomposed at high temperatures, the resulting carbon is somewhat
graphitic, is itself inactive, is not readily oxidized, and impairs
or prevents the activation of the normal carbon upon which it is
deposited. This discovery made it possible to treat anthracite
successfully. The conditions must be such as to minimize high
temperature cracking, to carry off or oxidize the hydrocarbons as fast
as formed, and especially to prevent the gases from cooler portions of
the treater coming in contact with carbon at a much higher temperature.
With these facts in mind, a plant was built at Springfield which
produced 10 tons a day of 150-300 minute charcoal from raw anthracite.
This was one-third of the total production at that time and was mixed
with the nut charcoal made at Astoria, thereby preventing an absolute
shortage of canister-filling material in October, 1918.
It was next shown that the cocoanut charcoal fines resulting from
grinding and screening losses and amounting to 50 per cent of the
product, could be very finely ground, mixed with a binder, and baked
like ordinary carbon products. By avoiding gas-treating in the bake,
the resulting charcoal is nearly as good as that from the original
shell. A recovery plant for treating the cocoanut fines was built at
Astoria. The product was called “Coalite.”
The great advantage of cocoanut shell as a source of charcoal is that
it is very dense and consequently it is possible to convert it into a
mass having a large number of fine pores, whereas a less dense wood,
like cedar, will necessarily give more larger pores, which are of
relatively little value. The cocoanut charcoal is also pretty resistant
to oxidation which seems to make selective oxidation a more simple
matter. By briquetting different woods, it is possible to make charcoal
from them which is nearly equal to that from cocoanut shell.
By heating lamp-black with sulfur and briquetting, it was possible to
make a charcoal having approximately the same service time as cocoanut
charcoal. A charcoal was made by emulsifying carbon black with soft
pitch, which gave the equivalent of 400 minutes against chloropicrin
before it had been steam-treated. This looked so good that the plans
were drawn for making a thousand pounds or more of this product at
Washington to give it a thorough test. This was not done on account
of the cessation of all research work. The possible advantage of this
product was the more uniform distribution of binder.
Instead of steam-treating anthracite coal direct, it was also
pulverized, mixed with a binder, and baked into rods which were then
ground and activated with steam. The resulting material, which was
known as Carbonite, had somewhat less activity than the lamp-black
mixes but was very much cheaper. A plant was built to bake 40 tons a
day of this material, which would yield 10 tons a day of active carbon
after allowing for grinding losses and steam treatment. The plant was
guaranteed to furnish an absorbent having a life of 600 minutes against
chloropicrin.
GERMAN CHARCOAL
After the Armistice was signed, Chaney took up the question of how
the Germans made their charcoal. The German charcoal was made from
coniferous wood and was reported to be as good as ours, in spite of
the fact that they were using inferior materials. Inside of a month
Chaney had found out how the German charcoal was made, had duplicated
their material, and had shown that it was nothing like as good as
our charcoal. The Germans impregnated the wood with zinc chloride,
carbonized at red heat, and washed out most of the zinc chloride. When
this zinc chloride was found in the German charcoal, it was assumed
that it had been added after the charcoal had been made. It was
therefore dissolved out with hydrochloric acid, thereby improving the
charcoal against chloropicrin. The German charcoal was then tested as
it stood, including the fines, against American charcoal, 8 to 14 mesh.
The most serious error, however, was in testing only against a high
concentration of chloropicrin. The German charcoal contains relatively
coarse pores which condense gases at high concentrations very well but
which do not absorb gases strongly at low concentrations. The result
was that the German charcoal was rated as being four or five times as
good as it really was.
[Illustration: German Charcoal. ×200.]
[Illustration: FIG. 73.—Charcoal from Spruce Wood.]
COMPARISON OF CHARCOAL
The following table shows a comparison of charcoals from different
sources. The method of activation was identical and the times of
treatment were those approximately giving the highest service time. The
results against chloropicrin, therefore, represent roughly the relative
excellence of the charcoal obtainable from various raw materials, using
this method of activation:
COMPARISON OF VARIOUS ACTIVE CHARCOALS ACTIVATED IN LABORATORY
-----------------+-----------------+---------------+----------------
| | | Accelerated
| Apparent |Steam Treatment| Chloropicrin
| Density | at 900° | Test Results
Base Material +-------+---------+----+----------+--------+-------
|Primary|Activated|Time| Weight | Weight |Service
|Carbon | Carbon |Min.| Loss |Absorbed| Time
| | | | Per Cent |Per Cent| Min.
-----------------+-------+---------+----+----------+--------+-------
Sycamore | 0.158 | 0.080 | 18| 53 | 41 | 7.3
Cedar | 0.223 | 0.097 | 60| 88 | 78 | 16.0
Mountain mahogany| 0.420 | 0.236 | 60| 44 | 32 | 16.3
Ironwood | 0.465 | 0.331 | 60| 44 | 31 | 20.8
Brazil nut | 0.520 | 0.316 | 120| 71 | 46 | 32.2
Ivory nut | 0.700 | 0.460 | 120| 70 | 48 | 47.0
Cohune nut | 0.659 | 0.502 | 120| 48 | 51 | 53.4
Babassu nut | 0.540 | 0.322 | 210| 68 | 85 | 58.7
Cocoanut | 0.710 | 0.445 | 120| 60 | 61 | 58.4
Cocoanut | 0.710 | 0.417 | 180| 75 | 72 | 64.4
-----------------+-------+---------+----+----------+--------+-------
BRIQUETTED MATERIALS
-----------------+-------+---------+----+----------+--------+-------
Sawdust | 0.542 | 0.365 | 120| 66 | 53 | 40.0
Carbon black | 0.769 | 0.444 | 240| 64.3 | 53 | 50.5
Bituminous coal | 0.789 | 0.430 | 165| 61 | 58.3 | 46.8
Anthracite coal | 0.830 | 0.371 | 480| 81 | 53 | 40.7
-----------------+-------+---------+----+----------+--------+-------
“In conclusion, it will be of interest to compare
the charcoals manufactured and used by the principal
belligerent nations, both with one another and with the
above mentioned laboratory preparations. Data on these
charcoals are given in the following table:
COMPARISON OF TYPICAL PRODUCTION CHARCOALS OF THE PRINCIPAL
BELLIGERENT NATIONS
--------+----------+-------------+--------+-------+-----------------
| | |Apparent|Service|
Country | Date |Raw Material |Density | Time | Remarks
| | | | Corr. |
| | | |to 8-14|
| | | | Mesh |
--------+----------+-------------+--------+-------+-----------------
U. S. A.|Nov. 1917 |Cocoanut | 0.60 | 10 |Air activated
U. S. A.|June, 1918|Mixed nuts, | 0.58 | 18 |Steam activated
| | etc. | | |
U. S. A.|Nov. 1918 |Cocoanut | 0.51 | 34 |Steam activated
England | 1917 |Wood | 0.27 | 6 |Long distillation
England |Aug. 1918 |Peach stones,| 0.54 | 16 |
| | etc. | | |
France | 1917-18 |Wood | 0.23 | 2 |
Germany | Early |Wood | ? | 3 |Chemical and
| | | | | steam treatment
Germany |June, 1917|Wood | 0.25 | 33 |Chemical and
| | | | | steam treatment
Germany |June, 1918|Wood | 0.24 | 42 |Chemical and
| | | | | steam treatment
--------+----------+-------------+--------+-------+-----------------
“It is at once evident that the service time of most
of these charcoals is very much less than was obtained
with the laboratory samples. However, in the emergency
production of this material on a large scale, quantity
and speed were far more important than the absolute
excellence of the product. It will be noted, for
instance, that the cocoanut charcoal manufactured by
the United States, even in November, 1918, was still
very much inferior to the laboratory samples made from
the same raw material. This was not because a very
active charcoal could not be produced on a large scale,
for even in May, 1918, the possibility of manufacturing
a 50-min. charcoal on a large scale had been
conclusively demonstrated, but this activation would
have required two or three times as much raw material
and five times as much apparatus as was then available,
due to the much longer time of heating, and the greater
losses of carbon occasioned thereby.
“It should furthermore be pointed out that the increase
in the chloropicrin service time of charcoal from
18 to 50 min. does not represent anything like a
proportionate increase in its value under field service
conditions. This is partly due to the fact that the
increased absorption on the high concentration tests
is in reality due to condensation in the capillaries,
which, as has been pointed out, is not of much real
value. More important than this, however, is the fact
that most of the important gases used in warfare are
not held by adsorption only, but by combined adsorption
and chemical reaction, for which purpose an 18-min.
charcoal is, in general, almost as good as a 50-min.
charcoal.”
TYPICAL ABSORPTIVE VALUES OF DIFFERENT CHARCOALS AGAINST VARIOUS GASES
LEGEND:
(A) = H₂O Content, (%)
(B) = Accel. Chloropicrin Service Time, (Min.)
(C) = Chloropicrin
(D) = Phosgene
(E) = Hydrocyanic Acid
(F) = Arsine
(G) = Cyanogen Chloride
(H) = Trichloromethylchloroformate
(I) = Chlorine
---+-----------------+--------+---+-----+---------------------------
| | | | | Service Time, Minutes
| | | | | Standard Conditions
No.| Charcoal | Nation | | +---+---+---+---+---+---+---
| | |(A)| (B) |(C)|(D)|(E)|(F)|(G)|(H)|(I)
---+-----------------+--------+---+-----+---+---+---+---+---+---+---
1 |Poor cocoanut |U. S. A.| 0 | 10 |120|175| 20| 18| 55| 50|270
2 |Medium cocoanut |U. S. A.| 0 | 30 |350|260| 25| 25| 65| 65|370
3 |Good cocoanut |U. S. A.| 0 | 60 |620|310| 27| 30| 75| 70|420
4 |Same as No. 2 | | | | | | | | | |
| but wet |U. S. A.|12 | 18 |320|330| 35| 16| 35| 95|
5 |No. 2 impregnated|U. S. A.| 0 | 35 |400|700| 70|400| 70|190|510
6 |Wood |French | 0 | 2.5| 25| 75| 9| 0| 1| 20|
7 |Wood |British | 0 | 6 | 70| 90| 18| 4| 5| 30|
8 |Peach stone |British | 0 | 16 |190|135| 30| 25| 65| 60|
9 |Treated wood |German | 0 | 42 |230|105| 20| 20| 22| 25|
10 |No. 9 impregnated|German |30 | 9 | 90|320| 16| 1|110|120|
---+-----------------+--------+---+-----+---+---+---+---+---+---+---
STANDARD CONDITIONS OF TESTS
Mesh of absorbent 8-14
Depth of absorbent layer 10 cm.
Rate of flow per sq. cm. per min. 500 cc.
Concentration of toxic gas 0.1 per cent
Relative humidity 50 per cent
Temperature 20°
Results expressed in minutes to the 99 per cent efficiency points.
Results corrected to uniform concentrations and size of particles.
SODA-LIME
Charcoal is not a satisfactory all-round absorbent because it has
too little capacity for certain highly volatile acid gases, such
as phosgene and hydrocyanic acid, and because oxidizing agents are
needed for certain gases. To overcome these deficiencies the use of
an alkali oxidizing agent in combination with the charcoal has been
found advisable. The material actually used for this purpose has been
granules of soda-lime containing sodium permanganate. Its principal
function may be said to be to act as a reservoir of large capacity for
the permanent fixation of the more volatile acid and oxidizable gases.
The development of a satisfactory soda-lime was a difficult problem.
The principal requirements follow: Its _activity_ is not of vital
importance, as the charcoal is able to take up gas with extreme
rapidity and then later give it off more slowly to the soda-lime.
_Absorptive capacity_ is of the greatest importance, since the
soda-lime is relied upon to hold in chemical combination a very
large amount of toxic gas. Both _chemical stability_ and _mechanical
strength_ are difficult to attain. The latter had never been solved
until the war made some solution absolutely imperative.
COMPOSITION OF REGULAR ARMY SODA-LIME
The exact composition of the army soda-lime has undergone considerable
modification from time to time as it has been found desirable to change
the raw materials or the method of manufacture. A rough average formula
which will serve to bring out the interrelation between the different
constituents is as follows:
COMPOSITION OF WET MIX
Per Cent
Hydrated lime 45
Cement 14
Kieselguhr 6
Sodium hydroxide 1
Water 33 (approx.)
AFTER DRYING
Moisture content 8 (approx.)
AFTER SPRAYING
Moisture content 13 (approx.)
Sodium permanganate content 3 (approx.)
Within limits, the method of manufacture is more important than the
composition or other variables, and has been the subject of a great
deal of research work even on apparently minor details. The process
finally adopted consists essentially in making a plastic mass of lime,
cement, kieselguhr, caustic soda, and water, spreading in slabs on
wire-bottomed trays, allowing to set for 2 or 3 days under carefully
controlled conditions, drying, grinding, and screening to 8-14 mesh,
and finally spraying with a strong solution of sodium permanganate with
a specially designed spray nozzle. The spraying process is a recent
development, most of the soda-lime having been made by putting the
sodium permanganate into the original wet mix. Many difficulties had to
be overcome in developing the spraying process, but it eventually gave
a better final product, and resulted in a large saving of permanganate
which was formerly lost during drying, in fines, etc.
FUNCTION OF DIFFERENT COMPONENTS
=Lime.= The hydrated lime furnishes the backbone of the absorptive
properties of the soda-lime. It constitutes over 50 per cent of the
finished dry granule and is responsible in a chemical sense for
practically all the gas absorption.
=Cement.= Cement furnishes a degree of hardness adequate to withstand
service conditions. It interferes somewhat with the absorptive
properties of the soda-lime and it is an open question whether the gain
in hardness produced by its use is valuable enough to compensate for
the decreased absorption which results.
=Kieselguhr.= The loss in absorptive capacity due to the presence of
cement is in part counterbalanced by the simultaneous introduction of
a relatively small weight though considerable bulk, of kieselguhr.
In some cases, there seems to be a reaction between the lime and the
kieselguhr, which results in some increase in hardness.
=Sodium Hydroxide.= Sodium hydroxide has two primary functions in
the soda-lime granule. In the first place, a small amount serves to
give the granule considerable more activity. The second function is
to maintain roughly the proper moisture content. This water content
(roughly 13-14 per cent after spraying) is very important, in order
that the maximum gas absorption may be secured.
=Sodium Permanganate.= The function of the sodium permanganate is to
oxidize certain gases, such as arsine,[30] and to act as an assurance
of protection against possible new gases. The purity of the sodium
permanganate solution used was found to be one of the most important
factors in making stable soda-lime. It was, therefore, necessary to
work out special methods for its manufacture. Two such methods were
developed, and successfully put into operation.
[Footnote 30: Which, however, was never used on the battlefield.]
Careful selection of other material is also necessary, and this phase
of the work contributed greatly to the final development of the form of
soda-lime.