CHAPTER XIII

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.

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