Dec.. 1912
T H E J O U R A - A L OF I - Y D G S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y .
but merely t h a t they would become trustees responsible for the election of Directors who would give the Corporation a business administration, thoroughly practical but conforming t o the ideals implied by its objects and associations. The terms under which each new patent shall be acquired by the Corporation are entirely in the hands of the Board of Directors, but a t least for some time t o come it is probable t h a t only such patent rights will be primarily considered as are offered freely, without restrictions as t o mode of administration or obligation of any financial return from the Corporation, as present indications are t h a t the latter will find itself well occupied even by these offers alone. The Board has authority t o purchase patents where this may appear as good business policy, which may quite conceivably occur from time to time in rounding out fields in which it has already embarked. Contracts with owners of patents for administrating the same on a profit-sharing basis will probably not be considered, chiefly owing t o the unforeseen complications which it is easily possible, not to say certain, t h a t such agreements would eventually lead into as the further developments of different interests began t o overlap. A much simpler, safer and more expedient procedure appears t o be for the patentee to retain if he so chooses complete title and control of his patent in certain geographical territory, while assigning the same in other territory entirely unencumbered t o the Corporation. Any development which the latter can give it will then automatically enhance the value to both. This procedure is well illustrated in the case of the first patents to come into the possession of the Corporation, v i z . , those referred to above as initially offered t o the Smithsonian Institution itself. The owners of these a t the time of their original offer had already spent considerable time and money in their development, but from the outright sale of their foreign
1
867
rights and the rights of six western states (California, Oregon, Washington, Idaho, Nevada, and Arizona), together with a license for the one industry of Portland Cement manufacture throughout the whole United States, they felt adequately remunerated for their work and financial risks, and were willing to turn over all remaining United States patent rights as a nucleus for the experiment in economics which the Research Corporation represents. Together with this there came t o the Corporation a I O per cent. interest in the net profits of the parties who purchased the rights for the Western States and for the cement industry, while incidentally growing out of the negotiations on the foreign rights, another set of valuable patents has come t o the Corporation from Mr. Erwin Moeller, of Germany, which emphasizes in a most practical way the fact t h a t academic organizations, and particularly the Smithsonian Institution, are international in spirit, and so recognized by scientific men the world over, presenting a t once a nucleus from which may well be developed many activities leading toward world consciousness, cooperation and peace. The present movement, as stated, had its inception on the far western edge of this continent in very unpretentious beginnings, but has already overrun national borders both in the character of its work and the personnel of its supporters. I t is a question which should peculiarly interest this Congress as to how far and in what way international cooperation can best be assured in such activities which, from their very nature and aims, should from the outset transcend political boundaries and national pride and be treated by one and all from a standpoint as broad as humanity itself. I t was with this in mind t h a t the present paper has been presented, not so much as a record of present achievement, as t o stimulate discussion and cooperative effort toward ever wider and more effective activities in this most promising field.
F. G. C O T T R E L L . ~
ORIGINAL PAPERS
A STUDY O F THE VARIATIONS O F THE PHYSICAL AND CHEMICAL PROPERTIES O F R E D LEAD. B y 0 . b’. BROWNAND A. R . NEES. Received Aug. 2, 1912.
Red lead is a n oxide having the empirical formula Pb,O,. The chemical composition and physical properties vary within wide limits, depending upon the method of manufacture and the nature of the material from which it is made. Two distinct modifications of this oxide are known in commerce: ordinary red lead or minium, and orange mineral. The exact difference between these two modifications is not clearly understood. It is not our purpose, however, t o enter into a discussion of this. What we do want t o do, is to point out some of the variations in chemical and physical properties of ordinary red lead and the factors which control these variations. I n order t o make the subsequent discussion clear,
a brief rCsum6 of the various processes employed for the manufacture of red lead will be given. Companies which make white lead burn their “off color” white lead o minium. I n a second process molten lead is rabbled in a reverberatory furnace. The greenish yellow oxide, or massicot as it is called, thus obtained is ground and separated from the particles of metal by air or by levigation. This oxide is then burned t o red lead. I n some processes the levigation is dispensed with and the massicot is alternately ground and heated until conversion to red lead is complete.2 I n another methods litharge is first made from the metal, and tapped from the furnace in a fused condition. After being allowed t o cool i t is ground and 1 The substance of this article was presented in a n address before the Eighth International Congress of Applied Chemistry, New York, September, 1912. W.Eckford, Brit. P a t . 25.256, Dec. 4. 1908, and 15,423, J u l ~1 , 1909. J S.C. I.,29, 166. 3 Hoffman’s“ Metallurgy of Lead,” 4th Ed., pp, 15 and 16,
868
T H E J O U R N A L O F I N D U S T R I A L A-VD E L V G I N E E R I N G CHEiZIISTRE‘.
burned to red lead. Two modifications of litharge may be obtained by this process. If the fused litharge is allowed t o cool slowly, a so-called “ r e d ” litharge is obtained; but if it is cooled rapidly by allowing it t o flow onto water-cooled iron plates a “yellow” litharge is obtained. Both varieties are used for making red lead. Processes are also in use in which atomized lead, after previous treatment with water etc., is converted into red lead. The “Nitrate Method” in which sodium nitrate is reduced to nitrite by means of metallic lead is also in use. The oxygen from the sodium nitrate oxidizes the lead t o litharge or even to red lead. Several other methods have been patented and are in use.* The red lead obtained by these various processes differs widely in physical and chemical properties. I n most cases it is easy to determine the method of manufacture of a commercial red lead b y means of a microscopic examination. A review of the literature shows that little work has been done toward determining the conditions best suited for the production of a red lead having given chemical and physical characteristics. J . hlilbauer has made a study of the factors which affect the oxidation of litharge to red lead. The result of his work is given in a series of papers on “The Physical Chemical and Technical Studies of Minium.”z Milbauer carried out his experiments in a rotatable tube through which air or oxygen was passed. He found that the rate of oxidation was independent of the rate of rotation of the tube up to a certain limit. When the rate of rotation exceeded thirty revolutions per minute, small pellets were formed, thus reducing the surface expoced, and hence the rate of oxidation. The presence of moisture or preheating of the air or oxygen had no effect on the rate of oxidation. The surface being sufficiently great, the only things which affect the rate of oxidation are the temperature and partial pressure of the oxygen. For example, in one of Milbauer’s experiments lead oxide heated a t twelve atmospheres pressure for one hour contained 60 per cent. red lead, while in pure oxygen a t the same temperature and pressure only a few minutes were required for the oxidation to take place. At atmospheric pressure ab’out fifteen hours are required to effect the same degree of oxidation.3 He has pointed out t h a t the source of the litharge has a greater influence upon the rate of oxidation than the size of the particles. 4 Milbauer concluded from his experiments that under 450° C. no red lead is obtained from litharge, the product always being brown in color. From 4 j o o t o 5 5 0 ° C. the product was red. The best results were obtained a t about 470’ C. He also states that red lead made from lead carbonate is reduced a t this temperature. These statement are not all in accordance with our own experimental results, as will be shown later. He says that small amounts of impurities in the litharge have a very marked influence upon the rate of oxidation; that nitric acid and am1
E. L. Burton, Fr. P a t . 351,812, Feb. 2 8 , 1905. J . S. C. I . , 24, 892.
2
J. Milbauer, Chem. Z f g , 33, 522. J. Milbauer, Eng. P a t . 25,757, Xov. 18, 1911. J. Milbauer, Chem. Zto.. 34, 1341-42.
3
Dec., 1 9 1 2
monia vapors reduce the rate of oxidation to about one-third and one-seventh, respectively, of its original value; that small amounts (about 0 , I per cent.) of lead nitrate and of sodium nitrate slightly accelerate the rate of oxidation; that the presence of 0 . I per cent. of lead nitrate gives the product a fine pink color and that a similar amount of NaOH gives an intense red.I He found that an increase of ten degrees C. in the reaction temperature multiplied the rate of reaction by ,only about I .oj, as compared with the usual doubling or trebling the speed of an ordinary chemical reaction, for a n increase of ten degrees. From this fact he concluded that the change is a physical rather than a chemical one, and is to be explained by the theory of solid solutions advanced by Horstmann.2 I n another paper Milbauers states that decomposition is less rapid in oxygen than in air, more rapid in carbon dioxide than in either air or oxygen, and most rapid in a vacuum. I t should be borne in mind that in all of these studies of the rate of oxidation, Milbauer used litharge as a starting material. Litharge is much more difficult t o oxidize to red lead than normal or basic lead carbonate or lead hydroxide. I n the papers referred t o above, Milbauer gives 470’ C. as the temperature best suited for the oxidation of litharge t o red lead. He also points out t h a t a t a temperature only five degrees above this, red lead made from lead carbonate is rapidly reduced. We find that reduction does not take place a t this temperature and that red lead containing more than 30 per cent. lead peroxide (about 87 per cent. Pb,O,) can be obtained from lead carbonate a t 500’ C. I n the experiments which we have carried out, not only the effect of temperature upon the rate of oxidation of different starting materials but also the physical and chemical properties of the red lead formed from the different substances under different conditions of temperature have been studied. We have studied red lead made from litharge, lead hydroxide, white lead, lead sponge, metallic lead and lead tartrate, A microscopic examination of each sample was made. The real and apparent density and content of lead peroxide were determined. The relative plasticity of the different samples when made into putty-like paste with water was also noted. Besides the samples prepared in the laboratory we have examined a large number of samples of commercial red lead and litharge manufactured by different companies. I n the study of the rates of oxidation the “ R o t a r y Electric Laboratory Furnace,” t o be fully described by us in another paper, was used. A slow current of air was forced through the furnace; the rate of flow of the air through the furnace seemed to have little effect upon the rate of oxidation. Three porcelain balls about a n inch in diameter were kept in the rotating jar, for the purpose of keeping the charge well stirred. The rate of rotation was about nineteen to twenty revolutions per minute. J. Milbauer, Chem. Ztg., 33, 950. Hortsmann, 2. Physzk. Chem., 6 , 7. 3 J. Milbauer, Chem. Z t g . , 3 4 , 1 3 8 4 0 .
Dec.,
1912
T H E JOGR.YAL OF IL\-DC7STRIL4L AAYD E-YGISEERI-YG C H E I I I I S T R Y .
869
The plan of the furnace is shown in Fig. A. D and E are heating coils, F is the rotating jar, and B is the outer jacket. The air follows the course indicated by the arrows. I n this furnace the method of heat-
tion of the white lead. Oxidation can proceed only after the CO, is driven off. The steep part of the curve represents the real rate of oxidation before a condition of equilibrium is approached. The straight part of the curve a t the end, especially noticeable a t the s IO" higher temperature, represents the state of equilibrium between the red lead and air at that temperature. The rate of oxidation of white lead at different temperatures is illustrated by the curves on Plate I . Oxidation takes place most rapidly at 425' to 450' C. At 450' C. a state of equilibrium between air and red lead is reached a t about 32.7 per cent. PbO,, while a t 425' C. the state of equilibrium is above 34 per cent PbO,. Above 450' the rate of oxidation decreases. This is illustrated in Curve VI on Plate I , which shows the progress of oxidation at 4 7 j 0 C. At j o o C. oxidation is much slower, eight hours being required to obtain a red lead containing 3 1 . 5 0 per cent. of PbO,. It was not possible to obtain a smooth curve a t this temperature since the rate of oxidation isnot uniform. I n Plate IV the percentage of PbO, is plotted against the temperature, the time being the constant factor. FIJ. A. The percentage of PbO, in the product a t the end of ing insures that the rotating jar and its contents three hours and two hours at the various temperawill be heated to the same temperature and that a tures is shown in Curves I and 11, respectively. These pyrometer inserted in the jar will record the true curves clearly indicate that the rate of oxidation of temperature of the charge. white lead is a maximum a t a temperature of 4 2 j 0 I n the case of white lead the rate of oxidation >\-as c. t o 430' c. determined a t temperatures ranging from 350' C. to The rate of oxidation of sublimed litharge' at 5 0 0 0 C. 425' C. is shown by the curve on Plate 111. I n this During each run small samples were taken from case equilibrium was reached a t 33.72 per cent. PbO,. time to time and analyzed for lead peroxide. (The This composition was maintained during five hours amount of lead peroxide is a measure of the degree of heating. oxidation. Red lead of theoretical composition corI n Curve I on Plate I1 the rate of oxidation of a red responding t o the formula Pb,O, contains 34.89 per litharge at 450' is shown, while that of a litharge of
cent. lead peroxide.) The effect of temperature is very strikingly illustrated by the curves on Plate I , in which the percentage of lead peroxide is plotted as ordinate against the time as abscissa. In each case it will be noticed that the curve rises slowly at first, then more rapidly, and again very slowly toward the end. thus dividing it into three distinct parts. The slow rise a t the beginning is due t o the decomposi-
the yellow modification is shown in Curve 11. It will be noticed that the yellow litharge oxidized much more slowly than the red variety. If litharge is treated with water, dilute tannic acid, or glue solution, the rate of oxidation is materially decreased. Water and tannic acid solution have about the same effect, while glue solution has less retarding influence. Sample furnished by Picher Lead Company.
5
870
T H E J O U R N A L OF I X D U S T R I A L AA-D E S G I S E E R I A - G C H E i M I S T R Y .
Dec., 1912
The untreated red litharge was converted t o red lead containing 28.01 per cent. lead peroxide in eight hours, while sixteen hours were required t o obtain a red lead of like composition after the litharge had been treated with water or tannic acid solution. Eleven
pig lead, atomized lead, and lead tartrate were also studied. The lead hydroxide was prepared by the electrolysis of NaNO, solution, using lead electrodes. The hydroxide was thoroughly washed, dried and ground
hours were required in the case of litharge treated with glue solution. These facts are brought out in Curves 111, IV, and V, respectively. A like treatment affected the rate of oxidation of yellow litharge t o about the same extent. We did not make a n extended study of the effect this class of substances has upon the rate of oxidation
in a pebble mill. It was found impossible to remove all of the nitrate even by the most thorough washing. J. Milbauer has pointed- out that the presence of nitrates is not objectionable. The presence of the nitrates in this instance seemed t o reduce the rate of oxidation. Oxidation did not begin a t all until all of the nitrates were broken down. Hydroxide which has been exposed t o the air for several weeks is more easily oxidized than freshly prepared hydroxide, due to the fact t h a t it is partially converted t o carbonate. The old hydroxide can be oxidized t o red lead of almost theoretical composition a t 450' C. in about five hours, while the fresh material requires twice as long as the same temperature. The red lead prepared from lead hydroxide is amorphous as shown in Fig. 19 and has a slight pinkish tint due t o the nitrates which were present in the hydroxide. The specific gravity is low, varying from 8.46 t o 8.93, depending upon the temperature a t which it was made.
and the results stated above were noted in connection with a series of experiments carried out for a different purpose. However, it is of interest to know t h a t water and certain colloidal solutions have a very marked effect upon the rate of oxidation. The specific gravity and apparent density are also slightly influenced by these additional agents and by the single treatment with water, as will be shown later.
It is t o be noted t h a t other litharges which do not appear t o have been fused during the process of. manufacture require a much longer time for oxidation t o red lead. Red leads made from lead hydroxide, lead sponge,
The lead sponge used in these experiments was obtained by two methods: (I) by electro?yzing a hot, saturated solution of litharge in caustic soda, using lead electrodes, and ( 2 ) b y deposition from a n aqueous lead acetate solution acidified with acetic acid, using a large lead anode and a small lead cathode. The sponge thus obtained was carefully washed and dried a t 50' t o 60' C. It was then repeatedly moistened with water and dried. This exposure to air and water partially converts the sponge t o hydrated oxide and carbonate. The crystals of the acetate sponge disintegrate much more readily upon exposure t o air and moisture than do those from the alkali bath, due probably t o the acetic acid present. Both varieties of sponge yield a n excellent quality of oxide of a fine deep red color. A microscopic examination shows the product to be made up largely of crystals mixed with some fine amorphous particles. I n general, the specific gravity is about that of a red lead made from ordinary litharge. The rate of oxidation of both the acetate and alkali lead sponge is lower than that of fused litharge and more rapid than that of a litharge which has not been fused;
however, the alkali sponge oxidizes more rapidly than that from the acetate bath. It is interesting to note that a red lead of lower specific gravity than any so far obtained was produced from sponge taken from the alkali bath. The sponge was almost free from crystals and was rapidly oxidized t o nearly theoretical composition. I t had a specific gravity of 8.32. All other samples prepared from sponge had a much higher specific gravity; however, the samples of sponge from which they were made were more crystalline and not amorphous. Samples of red lead were also prepared from ordinary metallic lead and from atomized lead.1 It was not possible in any experiment to oxidize the metal directly to red lead. Various temperatures from just above the melting point (326'-327') up to 500' C. were tried. The metal first fuses, then becomes granular, due to the rotation of the furnace. The finer particles are entirely converted to red lead u-hile the larger ones merely become coated over with the oxide, making further oxidation very difficult. When the product reached this stage it was removed from the furnace and ground. The finer particles were separated by levigation, the residue was roasted again, and the grinding and levigation repeated The second treatment left only a small residue of unoxidized lead which was discarded. The finer portion, which was a reddish brown color, was then put into the furnace and further oxidized. Only a short time (three to four hours at 430' to 450' C.) mas required to convert the material into a brilliant, amorphous red lead. The atomized lead was treated in the same manner as ordinary metal and seemed t o possess no advantages over i t . The atomized lead, of course, did not fuse since the particles become quickly coated with oxide. By means of the microscope one is able to distinguish different modifications of red lead. I n general, there are two distinct modifications, amorphous and crystalline. Some commercial red leads are entirely amorphous, some are entirely crystalline, while others are a mixture of the amorphms and crystalline modification. So far as we have been able t o determine, red lead has no definite crystalline form of its own but retains the form of the substance from which i t is made. The fact will be made clearer by an examination of the photo-micrographs. All of the oxides shown in the photo-micrographs are magnified t o 2 1 5 diameters. A study of the effect of temperature, time of heating, chemical composition, and physical modifications upon the specific gravity was also made. Considerable difficulty was encountered in obtaining accurate determinations o the real specific gravity. The method finally adopted was to use I j cc. and 2 0 cc. graduated, glass-stoppered flasks, for pyknometers. Alcohol was used in the pyknometers instead of water. Water is not suitable because i t does not wet the par1 The atomized lead was procured from The Chemical Process c o , of New York City.
ticles of red lead thoroughly and because a portion of the sample always floats, due t o the high surface tension of the water. Ordinary g j per cent. alcohol was treated with silver oxide and caustic soda for several days and then distilled. This treatment removes the aldehydes and other impurities. Before distillation, a small amount of water was added and only that part of the distillate reserved for use which had the composition of the constant boiling mixture of alcohol and water. This treatment gives a pure alcohol, which does not appreciably change in density upon exposure to air. The alcohol thus prepared was put into a vacuum desiccator and the pressure reduced to about 3 cm. of mercury for two hours. The density of the alcohol was then carefully determined by means of the pyknometers mentioned above -the exact capacity of the pyknometers having been determined by means of distilled water a t a known temperature. Since a long series of determinations was to be made and since alcohol has a relatively high coefficient of expansion it was necessary to work under conditions of constant temperature. Twenty-five degrees C. was chosen as being the most convenient. For this purpose an ordinary electrically operated thermostat was used. The temperature remained constant to within less than 0 . I O C. The determinations were carried out in the following manner: Ten to fifteen grams of the sample are introduced into the weighed pyknometer and the total weight carefully made. The flask is filled about half full of the standard alcohol and thoroughly shaken. It is then placed in a vacuum desiccator and the pressure reduced to about 4 cm. of mercury for two to three hours. This treatment is necessary in order t o remove the small air bubbles which adhere, to the particles of red lead. The flask is then filled accurately to the mark with more alcohol (while the temperature is maintained at 2 j o C.) and weighed. The specific gravity is calculated according to the following formula, dw DEw1 - ( W -W,) ' where d = density of the alcohol, %L' = weight of the sample, w, = weight of alcohol required to fill flask, W = total weight of bottle sample alcohol, W, = weight of bottle sample. Close attention to details is necessary in order to obtain accuracy in these determinations. After each determination the pyknometers should be rinsed with.alcoho1 and then with nitric acid, which in presence of the alcohol quickly dissolves all of the red lead remaining. They are successively washed with water, alcohol, and finally with ether, the latter being removed by a current of air. The specific gravity of red lead is found to vary from 8.32 to 9.16. The lower value was obtained for a red lead made from lead sponge. The sample was slightly crystalline, as is shown by Fig. 7. The higher value was found for a commercial red lead (see Fig. 8) which appears from the photomicrograph to be a mixture of the crystalline and
+
+
+
Fro. I.
P16. 5 .
Lead Tartrate Crystals.
Red Lead made from the Lead Snonge shown ,"Fir. 6 .
FIG. 2. ~ e Lead d made frnni Lead Tartrate
~
Pic. 6 . ' Lend Sponge lrom m ACetrtc Bath.
Pic. 9. commercial Red Lcrd
b.
I ? m 3. A commercial " K d ' Litharge.
P i G . 7. ~ e Lend d made from Alkali Snonee.
PIC. 11.
Sublimed Litharge.
PIC.in. h mlrargc m i d r k o m t h c Red Lead shown In Fig. 9.
FiD.
is.
Red Lead nindr from \V!>ite Lead.
P I G . 14.
Red Lead made i m m TVhite Lesd.
amorphous modifications In general. amorphous samples prepared in the laboratory and found in comrnercc have a specific gravity of from about 8 . 6 6 to 8 . 9 , while the crystalline varieties have a higher real density. This generalization is not strictly true in all cases. In the following table thc specific gravity of various samples prepared in the laboratory
Pic. 16. Red l e a d made from Atomized Lend.
Fie. 19. Red Lead made froin Lead Hydroxide.
15.
constant, thc higher the spccific gravity. The more nearly thc sample approachcs the formula l'bp,, the lower is the spccific gravity. The specific gravity also increascs slightly with time of heating. For example, sample 13-C-4. heated a t qooo C. for four hours, contained 33.41 per cent. PbO, and had a specific gravity of X.X?. while sample
P I C . 17.
Red L e r d made irom Metal.
as well as a number of commercial samples is given. The data in Table I shows that the specific gravity of red lead made from white lead varies from 8 . 6 6 to 9.12. This variation seems to be a function of the temperature, the time of heating, and the chemical composition. In general, the higher the temperature a t which oxidation takes place, other factors being
Fro.
Red Lard made, from \Vhitc Lend t h a t h n i been reduced to Lithaizr and then re-oxidized.
Fm. 18. A n uuomhous Red Lead (See Pig. 9) #round
..,.
f,,? _ 5" "h,...rr
B-D-io heated at the same temperature for eight hours and containing 3 3 - 7 3 per cent. PbO, had a specific gravity of 8.90. The effect o' temperature on the specific gravity is shown very strikingly in the case of samples B-V and B-V-IC. Sample n-V was made from white lead, a t about 42s0 C. I t contained 3 2 . 5 7 per cent. lead peroxide and had a specific
w c . 20. Lead Carbonale from Sodium Methyl Salphnte 33sth.
Fie. 21. Ked Lend mrde frorn!the C.zrbonrtc :in Fix. 2 0 .
874
T H E J O U R N A L OF I N D U S T R I A L A N D ElVGINEERING C H E B T I S T R Y .
gravity of 8.66. I t was then completely reduced t o litharge by heating to a temperature a little above j o o o C., after which i t was reoxidized to red lead a t 42 j O C. The rate of oxidation in this case was very slow. About forty hours were required t o bring the per cent. of peroxide up to its former value. The specific gravity was now found t o be 9 . 1 2 instead of
Dec., 1912
made from litharge. Any amorphous red lead will, if heated t o j 2 5 O , be reduced to litharge. The litharge thus obtained will not be amorphous but crystalline. Fig. I O shows a litharge made by heating the amorphous red lead shown in Fig. 9. A comparison of the two figures will bring out the striking physical change which has taken place. The
TABLEI.
SAMPLE. B-E-8 B-E4 B-A-9 B-0-10 B-C-2 15-C-4 B-B-6 B-B-10 B-F4 B-F-8 B-F-6 B-G-11 B-1-11 B-K-3 B-V B-V-K F-.4-6 F-VI11 F-IV
STARTING MATERIAL. White lead
Lead hydroxide
F-I11 F-V H-A-9 H-I H-111-14 H-V-6 H-VI-6 K-A-7 K-B-13 M-A-17 M-E-15 35-F-17
Lead sponge
Sublimed litharge Litharge
11-G-11 M-H-12
, M-1-11
c-c-c
Atomized lead
R-PP S-R-L-50
Metal
S-R-L P-s-L A-R4-Y s-L R-R-L R-I-R R-I-L R-I-L-W R-I-L-T R-I-L-G R-D-L R-D-R L-S-R
Temp. C. 425' 425 350'
Hours heating.
Per cent. PbOn.
Specific gravity.
3% 2 6% 8 2%
34.42 22.72 25.28 33.73 28.73 33.41 22.39 33.06 24.33 32.70 32.27 33.06 31 .50 30.52
8.69 8.73 8.78 8.90 9.02 8.82 8.91 8.74 9.07 8.91 8.89 9.05 9.04 8.81
400' 400' 400'
4
3750 3750 450' 450' 450' 4740 5000 425
3% 8% 1 3 2 5% 8 2%
425O 425' 425 ' 445 400' 450' 400'
32.97 32.49 6 5% 15
8.46 8.92 8.58 8.93 8.55 8.96 8.32 8.78 9.04 9.10 9.04 9.02 9.24 8.78 8.87
Apparent density.
17.11
19.82
19.03 27.30 10.34'
1
23.31
400 ' 400' 400' 440' 3750 425 400' 450' 450'
6% 9 22 9 9 7 11 % 15 163/4 16
450°
11
28.24
8.87
20.62'
28.01 22.80 30,99
8.80 8.94 8.89
19.22' 25.88 29.2
32.41
9.02 8.83
24.41
450' 450' 425'-450'
8% 7
42j04500
32.41 26.93
27 .05 25.18
23.58
25.04 E-R s-L-50 1 Determined with Scott apparatus, a s modified by Dr. Schaffer.
8.66. The same experiment was tried in other cases and the same effect was always observed; in most instances the effect was much more marked than in the above example. From Fig. I j it is seen that partial crystallization takes place when the sample is heated t o j o o o or above. Indeed we are no longer dealing with a red lead made from white lead but one
Made from lead carbonate prepared electrolytically in a lead methyl sulphate bath. Sample B-V reduced to litharge a t 525' and reoxidized t o form B-V-K.
83.53 29.24 34.45 28.10 34.15 28.36 26.04 23.25 25.30 32.72 14.20 28.59 28.30
450'
REMARKS.
34.05
*
17.16 28.06 24.411 21.20
8.81 9.26 9.16 9.36 9.33 9.06 8.87 9.19 8.73 8.95 8.95
18.83 7.32 30.87 30.83 36.09 22.43 19.11' 26,831
9.32 8.92 9.45
31.541 22. i91 32.76
8.96 29.81
Alkali sponge. Acetate sponge Alkali sponge.
Red modification treated with water. Red modification treated with tannic acid solution. Red modification treated with glue solution. Red modification (untreated). Yellow modification (untreated).
Amorphous commercial red lead, ground fifty hours. Same as above. Not ground. Sublimed litharge. A commercial red lead. A commercial litharge. A commercial litharge. A commercial red lead. A commercial red lead. A commercial red litharge. Same as R-I-L treated with water. Same as R-I-L treated with tannic acid. Same as R-I-L treated with glue solution. Yellow litharge from fused litharge. Red lead made from R-D-L. Litharge made from amorphous red lead (S-R-L). A commercial red lead. Same as S-L ground fifty hours.
objection may be raised that the particles of litharge do not seem t o conform t o any of the crystallographic forms, and that, strictly speaking, they should not be called crystals. It is true that only in very rare cases do perfect crystals appear, as, for example, in Fig. I j . However, the shape of the particles indicate t h a t they are fragments of crystals.
Dec., 1912
THE JOL-R+\-AL OF I.\-DL-STRIAL
The marked change in specific gravity noted in the above experiments clearly indicates that a permanent physical change has taken place and that after a n amorphous red lead has been heated above the reduction temperature and again oxidized to its original peroxide content, i t exists as a different modification, having different physical properties. An amorphous red lead formed a t temperatures just below the’ reduction temperature (see B-1-1 I and B-G-I I , Table I) also has a high specific gravity, which goes t o show that the transformation begins below the reduction temperature. However, the fact that a red lead is crystalline does not necessarily mean t h a t it has a high specific gravity. On the other hand, a red lead made from lead sponge from a sodium hydroxide bath and having a slightly crystalline character, had a specific gravity of 8 . 3 2 , which is the lowest value we have observed in any case. (See H-I in Table I, Fig. 7 . ) Certain commercial red leads, for example R-I-R, Fig. 4 , made from a fused litharge, also have a specific gravity which is comparable to that of the amorphous red lead found on the market or made in the laboratory. I t would appear from these facts that some change other than of mere physical form is responsible, in part a t least, for the variations in specific gravity. This phenomenon may be explained on the basis of Wades” theory concerning the polymerization of lead compounds. He assumes that litharge, for example, does not have the simple formula PbO, but a complex formula, Pb,O,. He also assumes that when PbO is converted to red lead i t is not converted molecule by molecule, but that the oxidation takes place gradually and uniformly through the whole mass, whenever the conditions of oxidation are favorable. For example, if we assume the formula of litharge t o be Pb,,O,, the oxidation, according t o the theory, takes place in the following manner: Pb,,O,, = IzPbO; Pb,,O,, = I I P ~ O PbO,; Pb,,O,, = IoPbO 2Pb0,; Pb,,O,, = 8 P b 0 4Pb0, = 4Pb,O,; Pb,,O,, = 6 P b 0 +‘6PbO, = 6Pb,0,. The molecule of litharge is not converted directly t o Pb,O, but passes through successive stages of oxidation and may even, in some cases, pass as far as the sesquioxide Pb,O,. The application of this theory to the observed variations in specific gravity is a t once obvious. Let us assume that the formula of a red lead made under this certain conditions of temperature is Pb,,O,,, oxide will have a certain specific gravity. Now if the temperature is increased, the formula might become Pb,,O,,, for example, and the oxide will have a different specific gravity. Oxides formed a t different temperatures have different specific gravities, a n d i t may be assumed therefore that they. have different degrees of polymerization. The acceptance of this theory does not exclude the influence of the crystalline and amorphous modifications upon the specific gravity. The relative degree of polymeriza-
+ + +
Lyndon, “Storage Battery Engineering,” pp. 25-26.
‘4-YD E*YGI-YEERI.YG C H E M I S T R Y .
875
tion of the two modifications may be such that their specific gravities are about the same. The amorphous and crystalline modifications of red lead show a marked difference in character when mixed t o stiff putty-like paste with water. The paste made from amorphous red lead is tough and plastic, while that made from a crystalline oxide has much less plasticity and shows a tendency to harden and crumble. A commercial red lead made from fused litharge showed lack of plasticity t o a marked degree. We attempted to overcome this lack of plasticity by treating the litharge, before oxidation, with colloids such as tannic acid, glue, etc. One kilogram of the litharge was made to a thin paste with water in which was dissolved one gram of glue or tannic acid. I n the treatment of one sample, water without any addition agent was used. These samples were allowed to stand, with frequent stirring and occasional additions of water, for about sixty days, after which they were dried and ground in a mortar t o crush the lumps. Portions of about 300 grams of the treated and untreated’litharge were then oxidized to red lead. There was no noticeable difference in the plasticity of any of the samples. The only effect that the treatment seemed t o have was to cut down the rate of oxidation as noted above, and t o make a n oxide of a slightly lower specific gravity in the case of the sample treated with water and one of a slightly higher specific gravity where tannic acid and glue were used. I n each case the specific gravity of the litharge itself was considerably reduced by the treatment due probably t o the formation of lead hydrate. All of the amorphous samples prepared in the laboratory from white lead showed a high degree of plasticity as did also certain commercial samples which appear under the microscope to be a mixture of amorphous and crystalline oxides. Samples of amorphous oxide, which showed a very high plasticity, were also prepared in the laboratory furnace from metallic lead. We had no means a t hand for making a quantitative study of the plasticity, but the qualitative tests showed such striking differences that we have thought the results worth recording. The apparent densities which are given in Table I were determined with a Scott volumeter. The values are in grams per cubic inch. The apparent densities of the various samples have no definite relation t o the real specific gravities, but vary with the size of the particles. I n general, the finer the oxide the lower the apparent density. I n the case of an amorphous red lead, however, which was ground for fifty hours in a pebble mill, the apparent density as shown by the Scott apparatus was increased from 18.40 to 2 4 . 4 1 . By grinding a crystalline litharge for twenty hours the apparent density was decreased from 3 6 . 0 9 t o 2 8 . 5 8 , while a t the end of fifty hours the apparent density was 29.81. A full discussion of the effect of grinding upon the apparent density will be brought out in a later paper. The sample prepared from atomized lead had the highest apparent density ( 2 9 . 2 0 ) of any sample prepared in the laboratory.
876
T H E J O C R S A L OF I - V D G S T R I A L A S D EIYGIIVEERIn’G C H E M I . C T R Y .
Sample B-K-3 was prepared from lead carbonate. The carbonate was made b y electrolysis with lead electrodes in a sodium methyl sulphate bath in the presence of sodium carbonate and carbon dioxide. This sample of red lead had a n apparent density of I O . 3 4 , which is much lower than that of any other oxide which we have examined. The real density was 8 . 8I , Photomicrographs of the carbonate and red lead are shown in Figs. 20 and z I , respectively. The lead peroxide in all of the samples was determined by distillation with hydrochloric acid, according to the method described by the authors in the Proceedings of the Indiana Academy of Science, 191 I . co~cLusIo~s. From these experiments we may draw the following conclusiofis : I . Red lead may be either crystalline or amorphous. 2 . The crystalline modification has no definite form, but the crystals retain the form of the material from which the red lead is made. 3. The specific gravity varies between comparatively wide limits, from 8.32 to 9 . 16. 4. The variation in specific gravity depends upon: ( a ) the temperature a t which the oxide is formed; ( b ) the time of heating; (c) the chemical composition; a n d ( d ) the physical nature of the starting material. I t is supposed that these conditions influence the degree of polymerization of the oxide, and that the degree of polymerization is directly responsible for the variation in specific gravity. j . The apparent density depends mainly upon fineness of the particles of the oxide, decreasing as the fineness increases up to a certain point. 6 . The temperature best suited for the formation of red lead varies with the starting material used. About 425 O C. to 430’ C. is best for white lead, 450’ C. t o 470° C. for litharge and lead sponge, and about 4 j o o C. for converting lead hydrate and metallic lead to red lead. I n fact, 450’ C. may be taken as the temperature a t which red lead can be economically formed from any suitable starting material. 7 . Any red lead is rapidly and completely reduced t o litharge a t j 2 j o C. t o 5 3 0 ’ C. 8. Both “yellow” and “ r e d ” litharge are roasted t o red lead much more slowly after being treated with water, showing that “air separation” of th: litharge before roasting is to be preferred t o “floating” in water. ELECTROCHEMICAL LABORATORY, INDIANA UNIVERSITY, BLOOMIXGTON,
THE CONTINUOUS PURIFICATION O F COAL GAS WITH WEAK AMMONIA LIQUOR.’ B y J. G. O’NEILL.
I n the manufacture of coal gas, the crude gas from the retorts is pulled by a n exhauster through a condenser to free i t from tar and considerable ammoniacal liquor, and is then forced by the exhauster into another condenser to rid i t of excess heat, then through scrubbers which wash out the ammonia, and finally 1 P a p e r presented at the Eighth International Congress of Applied Chemistry, New York, September, 1912.
Dec., 1 9 1 2
through the purifying boxes where i t is purified from hydrogen sulphide and cyanogen, on into the holder. I n washing the ammonia from the gas, two scrubbers are used: the first scrubber, taking the gas, washes it with gas liquor, which absorbs a small amount of ammonia and hydrogen sulphide from the gas ; the second scrubber washes the gas with fresh water, which completely absorbs the ammonia from the gas. The present practice of purifying coal gas from hydrogen sulphide and cyanogen consists of passing the gas through a mixture of iron oxide and shavings in air-tight boxes, whereby the hydrogen sulphide and cyanogen in the gas combine with iron oxide to form iron sulphide and ferrocyanides. When the iron oxide no longer takes up any hydrogen sulphide, i t is taken from the boxes and exposed t o the air, whence by oxidation and throwing off of free sulphur, i t again becomes active or “ revivified.” This operation is repeated four or five times until the percentage of free sulphur in the purifying material increases to about 30 per cent., when the iron oxide will no longer revivify. I n earlier times the oxide would revivify and could be used until i t contained 60 per cent. free sulphur, but with the modern practice of making gas a t much higher temperatures the cyanogen in the gas has greatly increased, and a large percentage of the iron goes t o form inactive ferrocyanides. By introducing about z per cent. of air into the gas before the purifiers, a certain amount of revivification takes place in the boxes, and reduces the number of times necessary t o expose the oxide to the air t o again make i t active, There are many disadvantages which we encounter in the iron oxide method of purification, some of which are as follows: the process is non-elastic, consequently the normal increase of gas consumption from year t o year; the very much greater consumption during certain.periods of the year; the difficulty of obtaining coal low in sulphur-all these conditions are forcing us to rapidly increase our puri ying capacity a t a very large expense, especially considering the facts that the purifying boxes are extremely bulky and that many coal gas plants are already cramped for space; the danger of a gas explosion every time a box is opened, and the close attention necessary to prevent firing of the oxide when exposed to the air for revivification; the cost of oxide and the labor cost in filling and emptying boxes, also the accumulation of spent oxide, which gradually becomes a nuisance. All these disadvantages of oxide purification have been recognized in the past ; several investigators have devised other purification processes, none of which have met with general approval. Of these attempts, two are of interest to us, the first by F. C. Hills (British Patent No. 1369, of 1868, and 934, of 1874), whereby he heats ammonia liquor in a special apparatus t o 180O F. and obtains a liquor having caustic properties. He washes the unpurified coal gas with this liquor and absorbs the hydrogen sulphide from it. His process failed because of the loss of ammonia in preparing the caustic liquor and because of his failure t o determine the correct temperature of the liquor and
