June, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
599
Recovery of Nitrogen Oxides from Gas Mixtures by Adsorption on Silica Gel’ By J. A. Almquist, V. L. Gaddy, and J. M. Braham FIXEDNITROGESRESEARCH LABORATORY, U’ASHINGTON, D.
c.
~
The adsorption efficiency and capacity of two samples of silica gel for dry nitrogen peroxide have been determined for various conditions of temperature, concentration, and rate of gas flow. The results indicate the desirability of low temperature adsorption from the standpoint of both capacity and adsorption efficiency. The rate of flow has no effect on the ultimate capacity of the gel, but is a n important factor in determining the amount of gel required for a given duty. Dry nitrogen peroxide is readily recovered from the gel by distillation a t 100’ C., and i t is probable that somewhat lower temperatures could be used to advantage. The comparatively large temperature range between efficient adsorption and recovery would present difficulties in commercial operation using granular gel, since a large mass of gel would have to be continually subjected to this temperature cycle. For this reason, the use of
powdered silica gel appears more promising, since in this form it can be carried continuously by the gas stream from the adsorption tower to the recovery chamber. In recovering nitrogen oxides by means of silica gel for a mixture containing water vapor, the difficulties of regeneration are greatly increased by the formation of nitric acid in the gel pores. The ratio of nitrogen oxides to water in the recoverable product will depend on the concentration of each in the gas phase and also on the length of time the gel has been exposed to the gas mixture. If the concentration of water vapor is 1 per cent or less, the gel will continue to adsorb water long after it has begun to pass practically all of the nitrogen oxides. For this reason i t would seem desirable in practice, in case granular gel were employed, to use the 6rst tower as a water adsorber, which would pass dry N204 on to the main body of gel.
HE recovery of nitrogen oxides from gas mixtures is a problem of great importance in connection with the catalytic oxidation of ammonia, the fixation of nitrogen
feasibility of using silica gel for nitrogen oxides recovery must be reserved until additional information is obtained.
T
by the arc process, and many industrial processes which yield oxides of nitrogen as by-products. The present practice of absorbing nitrogen oxides in water to form nitric acid is costly, in that it involves the use of a very large and expensive absorption system and furthermore the product does not usually exceed 50 per cent nitric acid, thus necessitating for many purposes the concentration of the acid. The removal of nitrogen oxides from gas mixtures by adsorption on suitable solid materials has been proposed as a step in producing liquid nitrogen peroxide. The latter is a very desirable product since it can be converted to nitric acid by treatment with water and oxygen under pressure and when dry is noncorrosive, so that it can be shipped in steel cylinders. Preliminary experiments2 with silica gel have indicated its applicability to nitrogen oxides recovery both as regards its adsorptive capacity and ts ability to retain its adsorptive properties after repeated use.3 The experiments described in this paper represent a study of the adsorption of nitrogen peroxide from dry and wet mixtures of this gas in air by silica gel, and the recovery of the peroxide from the gel. Judgment as to the commercial Received February 6, 1925.. Unpublished data obtained by N. W. Krase and V. L. Gaddy, Fixed Nitrogen Research Laboratory. a Sbortly after this article was prepared for publication, a paper by R. C. Ray on the “Adsorption of Nitrogen Peroxide by Silica Gel” appeared in J. Phys. Chem., 29, 74 (1926). The present investigation was undertaken from a more technical point of view than that of Ray, who made his adsorption experiments by the static method. 1
2
Adsorption of Dry N@4 The general method of experimentation consisted in passing a mixture of air and nitrogen peroxide of known composition a t a given rate of flow through a tube containing a known weight of silica gel maintained at a given temperature. The adsorption was followed by weighing a t intervals the tube containing the gel. PREPARATION OF GAS MIXTURES-In the presence of oxygen and at temperatures below 150’ C., nitric oxide (NO), which is originally produced in the arc and by the oxidation of ammonia, is completely converted to nitrogen peroxide when sufficient time is allowed for the reaction to take place. Accordingly, when the gas has been cooled to ordinary temperatures, there is present only the peroxide, which really consists of an equilibrium mixture of NO2 and its polymer, NzOa. Mixtures of air and nitrogen peroxide of any desired composition were prepared by the control of two metered streams of dry air, one of which bubbled through liquid nitrogen peroxide a t a rate sufficiently slow to permit saturation. The path of the air stream is shown in Figure 1. The flow through both flowmeters was accurately controlled by regulating the depth of submergence of the overflow tubes, A and B, in oil. The nitrogen peroxide saturator was kept at 0’ C. for all the experiments. At this temperature the vapor pressure of the peroxide is 264 mm.4 and the saturated vapor consists of Figure 1-Apparatus for Prepar a t i o n of Gas Mixtures a n d Adsorption
4 Scheffer and Treube, 2. phrsik. Chem., 81, 308 (1913).
600
4
I-VDUSTRIAL d S D E S G I S E E R I S G CHE.WISTRY
an equilibrium mixture of -\io2 and X204. The composition of this mixture was calculated from the equilibrium measurements of Schreber6 and Bodenstein6 to be 80.4 per cent N20a and 19.6 per cent NO2 by volume. The proportion of NzO, to NO2 changes with the temperature and partial pressure of the vapor. In this paper the concentration of nitrogen peroxide will be expressed as per cent of N204 equivalent. Thus, if it is desired to prepare a gas mixture containing the equivalent of 5 per cent NzOd (10 per cent NOZ) by volume,
$
j 2
0
-
Figure 2-Efficiency o f Adsorption a t 10' C for Gas Flows of 5 10. 20, 50, a n d 100 Cc. per M i n u t e per G r a m of 'Gel for Gel A with Gas Containing 9 Per c e n t b y Volume of N204 Equivalent
the gas mixture is made up to contain 5.00/0.902 or 5 . 3 per cent by volume of the saturated vapor. All gas volumes refer to 0" C. and 760 mm. PREPARATION OF NzOa--The liquid NzOa used in these experiments was obtained by oxidizing ammonia and adsorbing the nitrogen oxides on silica gel after the excess water had been condensed. The Nz04 resulting from the heating of the saturated gel was further purified by distilling through a column of activated gel maintained a t room temperature. THESILICAGEL-Silica gel is made by the careful dehydration of a specially prepared silicic acid gel and in the activated state consists of SiOs and H20 in indefinite proportions. It is evident, therefore, that the adsorptive properties of a gel will vary considerably with the activation treatment to which it is submitted, and it must be recognized that the results of adsorption measurements on a given sample are valid only for that particular gel in a certain state of activation. Samples of two commercial gels were used in these experiments and the results illustrate the above points. The combined results probably give a fair indication of the range of adsorptive capacities that may be expected from gels now available. The gel samples referred to as A and B were in the form of 10-14 and 8-14 mesh granules, respectively. They contained 2.8 and 3.4 per cent HzO, respectively, as determined by loss on ignition a t 1000" C., the initial weight being on the gels as received. Samples of 10 grams each were used; except for rates of flow of 50 cc. per minute per gram of gel and above, in which case 1 gram was used. The gel was contained in a tube like that of Figure 1. The gel bed was about 2.7 sq. cm. in cross section and 7.0 cm. in depth; since the average apparent density of the gel is about 0.5 gram per cubic centimeter the volume used was approximately 20 cc. Before being used, both samples were given a similar activation treatment. They were heated at 200" C. for half an hour with dry air passing at 20 cc. per minute after which they were saturated with dry nitrogen peroxide. This was distilled off and the gel was then activated for half an hour a t 100' C. with dry air passing at 20 cc. per minute. This was chosen as the initial state. After each adsorption experiment the nitrogen peroxide was distilled off and the gel activated a t 100' C. with air passing a t 20 cc. per min8
2. Phrsik. Chem., ar, 651 (1897).
l l b r d , 100, 121 (19'22)
5'01. 17, No. 6
ute until the weight of the gel was the same (within 1 per cent) as in the initial state. The duration of the activation treatment varied somewhat, but was on the average about 40 minutes for Gel A and about 2 hours for Gel B. PROCEDURE-In making an adsorption measurement, the flowmeters were adjusted for the desired composition and rate of flow, while the gas was by-passed a t the stopcock F. The weighed gel tube was then placed in position in the thermostat and the gas admitted to it. (Measurements were made a t -loo, O", and 25" C. A well-stirred ice bath was used for the runs a t 0" C. and a mixture of ice and salt was controlled to within 0.5" for -10" measurements. A water bath controlled to within 0.5' was used a t 25" C.) The gas leaving the gel tube was passed through an absorption bulb containing a slightly alkaline solution (1 drop 0.1 N NaOH) and a drop of indicator in order to determine the time when oxides of nitrogen begin to pass the gel. The gel tube was weighed a t intervals until it ceased to gain in weight. REsuLTs-Tahle 1 gives the results of a typical experiment to illustrate the method. Table I-Adsorption of Dry NaOc 10 grams Gel A ; temperature 0' C . ; total flow, 200 cc. per minute; 5 per cent NzO' equivalent; initial weight tube plus gel, 50.0850 grams; final weight, 51.6770 grams Gain in weight NZOP entering per interval Time interval per interval Gram Per cent adsorbed Minutes Gram 12 0.5042 0.492 100.0 0.656 91.8 16 0.6023 0.614 51.4 15 0.3163 0.614 17.8 15 0,1097 0.614 5,4 15 0.0335 0.614 4.2 15 0.0260 0.614 15 0.0000 0 Total adsorbed 1 , 5 9 2 0
The weight of nitrogen oxides entering the gel in a given interval was calculatea from the volume-percentage of %*04 equivalent, assuming a perfect gas. The figures in the last column represent the efficiency of the adsorption during each time interval. I n this experiment the time when nitrogen oxides began to pass the gel, referred to as the "break point," occurred after 12 minutes. The adsorption efficiencies of the two gel samples under various conditions of temperature, gas flow, and concentration are given by the efficiency-time curve8 in Figures 2 to 8, inclusive. The percentage efficiency over any given range of time is given by the ratio of the area under the curve to the total rectangular area under the line of 100 per cent efficiency.
T,me (mmute$
Figure 3-Efficiency of Adsorption a t O o C. for Gas Flows of 10, 20, 30, a n d 50 Cc. per M i n u t e per G r a m of Gel for Gel A with Gas Containing 9 Per c e n t by Volume of Na01Equivalent
The capacity of the two gels a t saturation, using a flow of 20 cc. per minute per gram of gel, is given in Table 11. Essentially the same saturation capacity was found for flows ranging from 10 to 50 cc. per minute per gram of gel. The marked influence of the temperature, as illustrated by these results, emphasizes the desirability of using some refrigeration in adsorbing the oxides.
lSDC;oTRI,4L A.VD ESGIiVEERISG CHEMISTRY
June, 1925
Capacities of Gels Capacity of gel at saturation as Temperature per cent of original gel weight
Table 11-Saturation Per cent by volume of N?O4eauivalent 9 0 5 0
0
c.
- 10 0 25
- 10 0
25
1.0
- 10
B
A
41.5 23.7 6.6 22.2 15.9 4 9
0
25
34.3 21.9 6.5 11.8 6.9 1.4
Although the rate of gas flow has no effect on the final capacity of the gel, it is seen from the efficiency curves, particularly in Figure 2, that it is one of the factors to be considered in' determining the quantity of gel required.
601
Adsorption of NzO, in Presence of Water Vapor
In considering the application of silica gel to the problem of recovering oxides of nitrogen from the products of ammonia oxidation, it is necessary to take into account the effect of water which is produced by the oxidation. Most of this water is removed by condensation as dilute nitric acid when the gases are cooled, and the concentration of water vapor present a t the time of adsorption will, of course, depend upon the temperature of the gas. The method of preparing the gas mixtures was the same as that described for the dry gas, except that the main air stream
Recovery of Liquid N2O4 from Saturated Gel
Recovery experiments were made on a number of gel samples saturated under various conditions. The tube containing the saturated gel was heated a t 100' C., and the N204 distilled into a refrigerated condenser, having a condensing surface of about 65 sq. cm. The distillation was carried out in a closed system except that a momentary release of pressure was necessary when the gel was first heated in order to permit 0 the escape of the air contained in the system. This period T/me (mmuie3) -Gel A x-Gel B of distillation was followed by the sweeping of the system with 5-Efficiency of Adsorption a t - 10' C. for Gas Flows of 20 air a t a flow of 20 cc. per minute. The amount of Nz04 a nFigure d 40 Cc. per M i n u t e per Gram of Gel with Gas Containing 5 Per recovered was determined by weighing the condenser. I n c e n t b y Volume of NzO4 Equivalent some cases weighings were made before sweeping with air (2 of Figure 1) was saturated with water vapor at the desired and the recovery by the direct distillation determined. temperature. When a stream of air containing Nz04 is mixed with air saturated with water vapor, the concentration of Table 111-Recovery of Liquid NZOIfrom Saturated Gel water vapor in the gas mixture will be reduced by condenPer cent Per cent adsorbed sation of nitric acid until it is in equilibrium at the given NzOd readsorbed Nz04 ficovered as Time ,of temperature with the solution of nitric acid formed. sweeping nally reTime of Temp. of liquid by The experiments to be described on the adsorption of E204 with air covered N2Or adsorbed distillation condenser direct distillation Minutes as liauid Minutes C. Gsams from wet gas mixtures prepared in this way were of two kinds. 2.427 45 - 10 91 4 5 91 0 In the one the gases were mixed in a preliminary condensing 2.368 1.5 - 10 92 2 1 95 6 2 330 5 -10 88 0 2 98 8 coil in which the condensation of nitric acid occurred, thus 0 2 95 5 4 145 28 3 278 30 0 5 97 0 decreasing the quantities of water vapor and N204 actually 0 695 30 - 10 5 81 6 reaching the gel. I n the other, the gas streams were mixed 2 309 25 (I 5 90 6 1 17s 25 0 5 84 6 just before entering the gel tube, so that the condensation of nitric acid occurred in the gel bed. In both cases, the gases It is seen from these experiments that efficient recovery of leaving the gel were passed through a weighed calcium dry N204 can be effected in a comparatively short time by chloride tube in order to determine the time when water distillation at 100' C. Distillation a t lower temperatures vapor began to pass the gel. An increase in weight of dry may be practical if a slower rate of recovery is satisfactory. calcium chloride is also a fairly accurate measure of the amount of water vapor passing the gel. After several hours' exposure to the moist gas, however, it was evident that some NOs was absorbed also. sc
9
Table IV-Adsorption
3
by Gel A with Preliminary Condensation of
"Os
N:O, entering coil per hour,2 460 grams, Hz0 entering coil per bour,O 254 gram Per cent by weight of affluGnt Ne04 Hz0 adNzOd i H20 Time interval Total time Grams adsorbed sorbed per Grams H20 Hours Hours per interval interval passing gel
't-
b
c
+
Tme (mnutrs)
Figure 4-Efficiency of Adsorption a t 25' C for Gas Flows of 5 10, 20, a n d 50 Cc. per M i n u t e per Gram of GG for Gel A with Ga; Containing 9 Per c e n t b y Volume of N?Ol Equivalent
The temperature to be used for the condenser depends largely on the type of system. If recovery is made by distillation in a closed system, higher temperatures may be used. If it is desired, however, t,o speed up the latter part of the distillation by passing air, the temperature of the condenser must be sufficiently low to prevent excessive evaporation of the liquid r C 1 2 0 4 .
ADSORPTION BY GEL A WITH PRELIMINARY CONDENSANITRICACID-A 10-gram sample of Gel A was used with a total flow of 200 cc. per minute. A stream of air containing 5 per cent N2O4 and one containing 3.13 per cent water vapor were mixed in a condensing coil a t 25' C. before passing into the gel a t the same temperature. The acid which condensed was retained in a trap a t the bottom of TION OF
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
602
the coil and its weight and concentration were determined, the latter by titration with alkali. The results are given in Table IV. I n this run 6 per cent of the entering NzOa and 65 per cent of the water were removed by condensation as nitric acid. The concentration of water vapor actually entering the gel was 0.92 per cent by volume, which corresponds to 7 mm.
rune
'(mmuteJ)
-Gel A x-Gel B Figure 6-Efflciency of Adsor t i o n a t O 0 C. a t Gas Flows of 10 20. a n d 40 Cc. per M i n u t e per & a m of Gel w i t h Gas Containind 5 Per c e n t b y V o l u m e of NlOc Equivalent
Vol. 17. No. 6
gram Nz04 per hour, or 7.6 per cent of the total entering the system, and the removal of 0.222 gram HzO per hour, or 87.5 per cent of the total entering. This reduced the amount of water vapor actually reaching the gel to 0.032 gram per hour which corresponds to a volume per cent of 0.33 and to a pressure of 2.5 mm. After the 28-hour run described above, the gel was heated at 100" C. for 25 minutes with the condenser a t 0" C., and 1.400 grams N ~ 0 4were recovered. It had a slight blue tinge, indicating the presence of a trace of water. The gel was then submitted to distillation at 200' C. for 30 minutes, followed by the passing of air for 60 minutes. This treatment resulted in the recovery of 1.449 gram acid, which was by titration with alkali. found to contain 82 per cent "03 It is probable that the figure for the acid concentration, reported in each case as " 0 3 , is somewhat high, owing to the presence of oxides of nitrogen dissolved in the acid condensate. ADSORPTION BY GEL B WITHOUT PRELIMINARY CONDENSATION OF NITRICACID-The rate Of flow was 200 CC. per minute with 10 grams gel. A stream of air containing 5 per cent Nz04and one containing 3.13 per cent water vapor were mixed just before entering the gel and condensation ahead of the gel was prevented by warming the connecting tubing. The temperature of the gel was 25" C. The results are given in Table V.
pressure. The concentration of nitric acid in the condenser was found to be 60 per cent "03, which should have a partial pressure of water vapor equal to 7.3 111111.7 Table V-Adsorption of Gel B without Preliminary Condensation of After the adsorption experiment, the gel was heated a t HNOJ 100" C. for 10 minutes. Some N204 vapor was driven out NzOa entering gel per hour, 2.460grams; HzO entering gel per hour, 0 254 gram Per cent by but not in sufficient quantity to produce a weighable condenweight of affluent NzOI+ Time NnO4 4-Hz0 sate. It was then submitted to distillation a t 200" C. for H20 adsorbed Gram H t 0 Total time (>rams adsorbed interval 10 minutes with the condenser a t 0" C., followed by the passper interval passing gel per interval Hours Hours None 0.852 62.5 0.5 0.5 ing of air for 30 minutes a t 20 cc. per minute in order to None 0.310 22.8 1 0.5 remove the nitric acid that had condensed in the connecting None 0.503 18.0 2 1 None 17.4 0.473 1 3 tubes. The amount recovered in this way was 3.553 grams None 16.1 1.322 6 3 None 16.3 1.345 9 3 which contained 63.0 per cent "03; and was 98.5 per cent None 16.2 1.768 13 4 of the gain in weight of the gel during the adsorption. 0.074 15.0 1.227 16 3 0.131 1.440 15.1 19.5 3.5 ADSORPTION BY GELB WITH PRELIMINARY CONDENSATION 3.0 0,580 1.060 13.0 22.5 0.703 0.896 11.0 25.5 3.0 OF NITRICAcm-The temperature of the coil and gel was 0" C., and the rate of flow was 200 cc. per minute with a 10It was observed that Nz04 began to pass the gel almost gram sample. The concentrations of Nz04 and water vapor were 5.0 and 2.63 per cent by volume, respectively. The ad- immediately. In the period of almost constant adsorption, sorption of Nz04 and water vapor fell off to less than 15 per between the third and sixteenth hours, a small amount cent in the first 4 hours, and after 8 hours the gain in weight (0.15 to 0.20 gram per hour) of Nz04 was being adsorbed, per hour was constant at 0.03 gram corresponding to the ad- together with all of the water. After this adsorption experiment the gel was heated a t sorption of all the water reaching the gel. The run lasted
T/me (minutes)
0-Gel A l eG-% B Figure ?-Efficiency of Adsor t i o n a t 25" C. for Gas Flows of 10, 20 a n d 40 Cc. per M i n u t e per & a m of Gel with Gas Containing 5 +er c e n t b y Volume of NaO4 Equivalent
28 hours and no water passed the gel during the time. The acid removed by condensation amounted to 12.357 To grams and was found to contain 58.1 per cent "03. make this acid during the run required the removal of 0.186 7
Interpolated from measurements of Burdick and Freed, J. Am. Chem.
Soc., 48, 518 (lQ2l).
Figure 8-Efflciency of Adsor tion a t -IOo, O', a n d 25' C. at Flow of 40 Cc. per M i n u t e per &am of Gel for Gel E w i t h Gas Containing 1 Per c e n t b y V o l u m e of NiOc Equivalent
150" C. for 2 hours and the distillate collected a t 0" C. It was found to consist of 6.790 grams of nitric acid containing 54.2 per cent HNOI by weight. A subsequent distillation under the same conditions for 3 hours resulted in the recovery of The total 4.049 grams of acid containing 72.2 per cent "0,. acid recovered was 10.839 grams, or 96.8 per cent of the total gain in weight of the gel during adsorption.
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June, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
It is evident from this experiment that as the adsorption proceeds there is a gradual conversion of the initially adsorbed Nz04to nitric acid by water for which silica gel is known to have a pronounced selective action.8 The ratio of nitrogen oxides to water in the product recovered from the gel will, therefore, depend on the length of time it has been exposed to the gas containing Nz04 and water vapor. An experiment was therefore carried out in which recovery tests were made a t definite time intervals. A total flow of 200 cc. per minute of gas containing 5 per cent NZO4and 2 per cent water vapor was admitted directly to a 10-gram sample of Gel B at 0" C. This corresponds to a flow of 2.460 grams N204 and 0.193 gram H20per hour. Adsorption experiments of 4, 8, 12, 16, 20, and 25 hours' duration were made. No water vapor passed the gel during any of these tests. After each adsorption run the gel was given the following treatment: It was submitted to distillation a t 100' C. with condenser at 0" C. for 30 minutes and the cona A recent observation of the selective adsorption of water by silica gel is that of Williams, J Soc Chem I n d , 45, 97T (1924) He found that benzene initially adsorbed on silica gel from a benzene water vapor mixture is gradually displaced by water on continued passage of the mixture through the gel.
603
densate (liquid -hJzo4)was weighed. It was then heated a t 200" C. for 30 minutes, followed by the passage of air for 5 minutes at 20 cc. per minute. The condensate of nitric acid was weighed and its concentration determined. The gel was then brought back to its initial condition-that is, to its original weight-by heating a t 200" C. for 1 hour with air passing at 20 cc. per minute. of T i m e of Adsorption on Relative A m o u n t s of NzOd a n d Water in Adsorbed Product Concentra- Per cent of Duration of NzOi Hn0 NnOi "03 tion of adsorbed " 0 8 NzOd +H2 adsorption adsorbed recovered recovered Hours Grams Grams Grams Per cent recovered 4 3.612 1.723 1.653 74.1 93.3 8 4.936 1.313 3.331 72 6 94.1 12 6.122 1.107 4.858 68.7 97.3 16 7.662 0.61ga 6.895 67.8 98.0 20 8.882 0.360a 8.296 66.1 97.4 24 10 181 ...b 9.994 67.1 98.0 a These condensates were blue in color, due to the presence of a small amount of water. b Some condensate here, but it was believed to contain but little NzOd. This was carried on with the condensate from the next distillation. Table VI-Effect
+
The results in Table VI show how the relative amounts of X204and water in the adsorbed product depend on the time of adsorption.
The Versatility of Ferrous Hydroxide' By Peter Fireman MAGNETIC PIGMENT Co., TRENTON, N . J.
UPPOSE you have before you a tub holding, say, 6000 gallons and that you fill it to three-fifths of its
.
S
- * capacity with a solution of ferrous chloride and then add a ton of slaked lime while passing air through the whole massq, Having done this, you may begin to make up your mind whether the product shall be black or yellow. If you decide that you want a black pigment, you turn on steam full blast and continue the oxidation a t a boiling heat until you get the right color. If you desire a yellow you operate for a while a t a low temperature but finally increase the heat to boiling. I n either case you obtain a valuable, readily marketable color. I n using soda ash as the precipitating agent the conditions are analogous. With an equal proportion of sodium carbonate you will obtain either a black or a yellow according as you use a high or low temperature while blowing in the air. Only when using lime you operate with an excess of ferrous chloride, but when using soda ash you operate with a considerable excess of alkali carbonate. If you diminish the excess of soda ash and carry on the oxidation a t an elevated temperature you produce a brown. A further decrease of the excess of soda enables you to obtain a red. I n all the reactions enumerated the first step is the precipitation of ferrous hydroxide, whose eagerness to take up oxygen is the cause of the formation of the various pigments. The remarkable feature of the reactions under consideration, however, is not so much the avidity of ferrous hydroxide for pxygen as its tendency to form a large number of substances varying in shade of color. This tendency to produce strikingly-different colors in response to slight changes in the conditions of the mother liquor may fittingly be characterized as the versatility of ferrous hydroxide. To restrain this tendency to react in many different ways and bring it under 1 Received April 2, 1925 Presented before the Division of Physical and Inorganic Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, .Md , April 6 to 10, 1925.
control has been the problem in the utilization of the reaction in question. The task is an attractive one. You start out with very common, cheap raw materials. You obtain pigments of fine structure, desirable shades, of great tinctorial power, and permanent in color. A gratifying feature in the production of these pigments is that you may utilize a waste liquor-namely, the spent acid obtained in cleaning iron wire or plates. It may be noted that during the last two decades there has been developed in this country a new industry based on the manufacture of precipitated iron oxides. A brief discussion of some of these pigments follows. Precipitated Ferro-Ferric Oxide
This black oxide of iron is produced by precipitation from either a solution of ferrous chloride by slaked lime or a solution of ferrous sulfate by soda ash. I n both cases a lively current of air and a rapid stream of steam are introduced. The mass soon becomes a greenish blue, but gradually loses its bluishness and becomes lighter, greener. As the boiling temperature is being approached the color darkens to a grayish brown. During the remaining few hours of the operation the black color becomes more and more pronounced. The oxidation is kept up until the ratio of the ferric to ferrous iron becomes 1.2:l to 1.5:l. The washed ferro-ferric oxide is best dried with the exclusion of air. Two typical analyses will show the composition of the black pigment. From FeClz and Ca(OH)z FeO 23.09 Fez03 76.52 coz 13 99.74
-
From FeSOi and NalC03 25.90 72.40 1.14 99.44
Considerable oxidation takes place during the drying. With suitable precautions this can be held in check, but it is of no particular moment whether the ratio of ferric to ferrous iron is a little greater or bss as long as it does not fall much
