SeDtember. 1923
INDUSTRIAL A N D ENGINEERING k H E M I S T R Y
951
The Quantitative Determination of Sulfur Forms in Coke17' By Alfred R. Powell PITTSBURGH EXPERIMENT STATION,
I
N another paper the
u. S .
BUREAUOF MINES,PITTSBURGH, PA.
Methods have been developed for the quantitative determination
method of attack must in-
writer3has described an of the four forms of s u ~ u r exist in coke. These methods valve chemical combination of the sulfur. investigation in which have proved to be accurate, and can be applied without the use of As stated before, the prethe nature of the sulfur elaborate vious investigation of -the present in coke was deterforms of sulfur in coke had mined. As the coke existed produced a method by means in the oven, two forms of sulfur were present-namely, fer- of which the quantity of the various sulfur forms present could be The examination of the coke involved a phase rule rous sulfide and sulfur held in the carbon apparently in the determined. study of the coke sulfur. With the coke a t a constant temperature, form of a solid solution. During the quenching of the coke, the pressure of free-sulfur vapor over the coke was measured. The however, oxidation takes place, so that a portion of the sulfur was then removed from the coke in small successive amounts ferrous sulfide is oxidized to iron oxide and free sulfur, a and the vapor pressure of sulfur was redetermined after each Thus, a curve was obtained which expressed the remuch smaller part of the ferrous sulfide is oxidized to sulfate, removal. lationship between the amount of sulfur in the coke and the presand another part of the ferrous sulfide remains unchanged. sure of free-sulfur vapor over the coke, the temperature being Therefore, the finished coke will contain sulfur in four constant. Phase rule deductions, then, give us a method for forms: (1) ferrous sulfide, ( 2 ) sulfates, (3) free sulfur ad- determining how much of the sulfur in the coke is ferrous sulfide and how much is in the'solid-solution form. The other two forms sorbed on the surface of the coke, and (4) sulfur held in of coke sulfur were not present under these conditions, since the solid solution in the carbon. method of measuring the vapor pressure of the sulfur depended Coke which has been made by fast heating, such as that on the passage of hydrogen over the coke, and the conditions made in small quantities in the laboratory or made in small were therefore strongly reducing. However, this previous had shown that the free sulfur and the sulfates in gas retorts, does not quite conform to these conditions, which investigation coke had resulted from the oxidation of some of the ferrous apply to coke made by the ordinary process in the by-product sulfide, and that the sum of the ferrous sulfide, the free sulfur, oven or in the beehive oven. Coke made by fast heating con- and the sulfates in the coke which had been subjected to oxidatains sulfur in three forms before removal from the retort, tion, was equal to the original content of ferrous sulfide in the under reducing conditions. Therefore, since the ferrous instead of the two mentioned for the ordinary oven coke. coke sulfide and sulfates could be determined in the coke by known The third forni is adsorbed free sulfur, which seems to be methods, and since the sum of the ferrous sulfide, the sulfates, driven to the surface of the coke by the fast heating, rather and the free sulfur had been determined by the phase rule study, than entering the carbon in solution. When this coke is the free-sulfur content of the coke was known. SOLID-SOLUTION SuLFuR-Since this was the only form of quenched, as a result of the consequent oxidation it contains sulfur in the coke for which an analytical method had not been the same four forms as the oven coke. worked out, it was very readily determined by difference from the The methods by means of which the nature of the coke total sulfur. sulfur was investigated, and which have been described in the The testing of samples for a quick and accurate method of paper referred to, gave a method for the quantitative determianalysis could therefore be carried on with the use of coke nation of the sulfur forms in coke, but it involved the use of a complicated apparatus, accurate temperature control, a samples of known free-sulfur content. Several of these samlong series of titrations, and many other disadvantages. It ples were available, and were used in the present investigation. was entirely unsuitable for the analysis of the sulfur forms in . ROASTING METHOD coke, although it proved to be invaluable in determining what The first method investigated was the roasting of the coke was the character of those sulfur forms. A quick, accurate, and convenient method was therefore sought, by means of a t such a temperature that the ferrous sulfide and adsorbed which quantitative analyses could be made of the four sulfur free sulfur would evolve their sulfur as sulfur dioxide, and be determined as such. Provided that this treatment were forms in coke samples. carried on under such conditions that the coke itself would EXPERIMESTAL not be oxidized, the sulfur held in solid solution by the carbon FERROUS SULFIDE-with the exception of very small quanti- would not be affected, since it is extremely stable. The sulties of calcium and magnesium sulfides, ferrous sulfide is the only fates, of course, would remain in the coke. Since the ferrous metallic sulfide occurring in coke. The determination of this sulfide content of the coke would have been determined alsubstance, therefore, amounts to a determination of the metallic ready, it would be possible to calculate the adsorbed free sulfide content, which is very easily performed. The method sulfur by difference. which the writer has used for the determination of ferrous sulTrials at several different temperatures showed that 500" C. fide in coke has been described in another paper,4 so t h a t no experimental work was necessary. was the best temperature for the oxidation of the sulfur withSULFATE-The determination of the sulfate content of coke out causing any measurable loss of coke. The powdered has been described in the paper just referred to. coke was placed in a boat, and this was put in a tube kept in FREE SULFUR-The determination of the free sulfur held on the surfaces of the coke in an adsorbed condition required consider- a furnace a t 500" C. Airwas passed over slowly for about a able experimental work in its development. At the outset it was half hour. For the purpose of absorbing the sulfur dioxide, realized that no direct method could be used, since solvents will the air passing out to the tube was bubbled through sodium not extract the adsorbed free sulfur from the coke quantitatively, heat will not drive out this form of sulfur to any extent, and any hydroxide solution. This solution was later acidified and titrated with iodine. 1 Received March 15, 1923. The results from this roasting method were very unsatisPresented before the Section of Gas and Fuel Chemistry a t the 65th Meeting of the American Chemical Society, factory. Analysis of the residual coke showed that the ferNew Haven, Conn., April 2 to 7, 1923. rous sulfide had completely disappeared. However, the re2 Published by permission of.the Director, U.S. Bureau of Mines. sults were invariably low, showing that the free sulfur and 3 J . A m . Chem. Soc., 45, 1 (1923). 4 THISJOURNAL, 12, 1069 (1920). the ferrous sulfide had not oxidized quantitatively to sulfur
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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dioxide. Indications pointed to the fact that some of the sulfur was oxidized to sulfur trioxide, as shown by the presence of a faint mist in the outlet air and an increase in the sulfate content of the coke. The low-temperature roasting method for the free-sulfur content of coke was therefore abandoned.
REDUCTION METHOD The next method tried was a reduction method. Since the free sulfur and the sulfates in the coke are formed by oxidation during the quenching of the coke, it was thought that the reduction of the coke would re-convert the free sulfur and the sulfates into ferrous sulfide. The ferrous sulfide could then be determined, and by subtracting the ferrous sulfide and sulfates in the original coke the free-sulfur content of the original coke would be known. The reduction was carried out by placing a weighed amount of the powdered coke in a boat, inserting this boat in a tube combustion furnace, and finally reducing the coke a t a red heat with hydrogen. The tube was fist rinsed out with hydrogen to eliminate all air, .then the outlet was closed and a very slight pressure of hydrogen was maintained in the tube to insure a thoroughly reducing atmosphere a t all times. After the tube had reached a red heat, the heating current was turned off and the tube was allowed to cool, the atmosphere of hydrogen being maintained, of course, until the tube was entirely cool. The ferrous sulfide was then determined in the coke in the usual manner. The following table gives the results from a variety of cokes: ---PERCENTAGE OF SULFIIR IN COKE---
CLASS OF COKE Gas-house Illinois by-product Pennsylvania by-product Pennsylvania by-product Beehive Vertical retort
FeS (After FeS SO4 Free S Reduc- (Orig- (Ong- (Origtion inal inal inal by Hz) Coke) Coke) Coke) 1.02 1 . 0 0 0.64 0.00 0.36 0.45 0.47 0.26 0.02 0 . 1 9 0.40 0.41 0.21 0 . 0 0 0.20 0.25 0.27 0.02 0.00 0.25 0.26 0 . 2 6 0.11 0.00 0 . 1 5 .. 1.30 0.58 0.10 0.62
FeS (Phase Rule Total Curve)
2.04 1.31 1.10 0.95
0.82 3.20
. .
Comparison of the figures in the second and third columns gives an indication of the accuracy of the method. The second column shows the percent'age of sulfur as ferrous sulfide in the coke under reducing conditions, obtained by measuring the ferrous sulfide line of the phase rule curve, which has been referred to earlier. This is an absolute measure of the ferrous sulfide content under the given conditions, and serves as the standard by which the accuracy of the method may be judged. The third-column gives the ferrous sulfide content obtained by chemical analysis after the coke had been reduced; the next two columns give the ferrous sulfide and sulfates in the original coke, as determined by analysis; while the final column gives the Eree-sulfur content of the original coke, calculated by subtracting the ferrous sulfide and the sulfate sulfur of the original coke from the ferrous sulfide sulfur after reduction of the coke. These results show that the method is reliable and can be depended upon for the determination of the free-sulfur content of coke. This method has been used in several investigations since its development, and the whole trend of the results has shown that it is a convenient and accurate analytical process. It has been mentioned that coke made by fast heating contains free adsorbed sulfur even in the reduced state. Cokes of this type are seldom encountered in industrial practice, but are confined mainly to cokes made in small batches in the laboratory. I n one of our investigations, however, a gas-retort coke was found which gave very irregular results when analyzed by the foregoihg method for free sulfur. Investigation showed that this coke contained free sulfur in the reduced state-in other words, more free sulfur than the
Vol. 15, No. 9
iron present in the coke could combine with to form ferrous sulfide. Special treatment of this coke during the reduction with hydrogen was therefore necessary in order to convert all the free sulfur into sulfide sulfur. This was very readily accomplished by mixing a small amount of calcium oxide with the coke before introduction into the reducing tube, thus furnishing the necessary material to convert all the free sulfur into sulfide sulfur. For the purpose of the analysis it did not matter into what sulfide the free sulfur was converted, because the subsequent treatment with hydrochloric acid released the sulfur as hydrogen sulfide. The results obtained with this modification were very consistent and satisfactory.
COMPLETE ANALYSIS OF SULFUR FORMS I N COKE After the total sulfur has been determined by any one of several well-known methods, two samples of the coke, which has been crushed to a t least 35-mesh size, are weighed out. These should preferably be 2-gram samples, although in the case of high-sulfur coke 1 gram may be sufficient. One of these samples is placed in a 200-cc. Erlenmeyer flask, and 25 cc. of water are added. This flask is connected with a test tube containing 10 cc. of ammoniacal cadmium chloride solution, in such a manner that any gas coming out of the flask will bubble through the cadmium chloride solution. Hydrogen is then passed through the flask in order to sweep out any air, and then 25 cc. of concentrated hydrochloric acid are added to the flask, which is gently heated until all the hydrogen sulfide has passed over into the adsorption tube and is then disconnected. The solution with the yellow precipitate in the tube is washed out, diluted with SO0 cc. of water, acidified, and then titrated with standard iodine solution. The sulfur so determined represents the ferrous sulfide content of the original coke. The contents of the Erlenmeyer flask are filtered, the filtrate is made slightly ammoniacal, and then brought back to a slightly 'acid condition with a few drops of hydrochloric acid. The sulfate is precipitated by barium chloride in the usual manner, and the subsequent analysis indicates the sulfate content of the original coke. This is always very small and in a great many cases is a mere trace. Separation of the iron from the solution before the precipitation of the barium sulfate is not necessary. The other sample of coke is placed in a small porcelain boat, and this is inserted into a silica tube in a heating furnace. While the furnace is still cold, the tube is thoroughly swept with hydrogen and then closed a t the outlet end. A slight pressure of hydrogen is kept in the tube, most conveniently by a T connection, which releases the excess hydrogen from the cylinder through a tube leading down through a f e w inches of water. The furnace is heated to a full red heat, and then the tube is allowed to cool to not more than 100' C. The boat is removed from the tube, and the contents are placed in an Erlenmeyer flask and treated in exactly the same manner as was the sample for the ferrous sulfide determination. The sulfur so determined represents the sum of the ferrous sulfide, sulfate, and free sulfur. The free-sulfur content of the original coke can be calculated by subtracting from this figure the ferrous sulfide and sulfate sulfur, which have been previously determined. I n the operation of the reduction furnace used for this determination due care should be taken to prevent any hydrogen explosion. I n case the coke under examination is suspected of containing free sulfur above the amount with which the iron c%n combine, a duplicate run should be made for free sulfur. I n this case about 0.2 gram of pure calcium oxide should be mixed with the powdered coke before it is placed in the combustion boat. Any difference between this determina-
September, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
tion and the one made in the normal manner would show .the amount of free sulfur in the coke above the amount necessary to combine with the iron. Such cases will arise only where the coke has been made by fast heating, such as coke made in the laboratory or in small retorts with very hot malls. Of course, if no differentiation is desired between the free sulfur which will combine with the iron of the coke and any excess free sulfur which may be present, the lime may be added in all cases. However, it is desirable to determine these separately, since free sulfur above that which will combine with the iron of the coke will have a much greater contaminating influence on the iron in the blast furnace or cupola.
953
After the foregoing determinations have been made, the ferrous sulfide, sulfate, and free-sulfur content of the coke will be known. By subtracting the sum of these, in terms of per cent of sulfur, from the total sulfur in the coke, the percentage of solid-solution sulfur will be known, and the analysis will be complete. ACKNOWLEDGMENT The writer wishes to extend his thanks to A. C. Fieldner, supervising fuels chemist of the Pittsburgh Experiment Station of the Bureau of Mines, under whose general supervision the present investigation’ was conducted.
Determination of Total Sulfur in Rubber Goods’” By S. Collier, Michael Levin, and R. T. Mease BUREAUOF STANDARDS, WASHINGTON, D. C .
and Adams, and Waterss HE method for the The Bureau of Standards method, by Waters and Tuttle, f o r clearly showed that the determination of the determination of sulfur in rubber goods has been modified as amount of salts occluded by total sulfur in rubfollows: Fifteen cubic centimeters of nitric acid-bromine mixture the barium sulfate precipiber goods, which was deadded to the sample and evaporated to dryness. Three cubic centitate should be taken into vised by Waters and Tutmeters of nitric acid and 5 grams of sodium carbonate are added consideration, if accurate tle,3 has been used in the and the resulting mixture is dried and fused. The fused mass is results are to be obtained. laboratory of the Bureau of lixiviated with water, filtered, the filtrate a c i d ~ e d with hydroSubsequently, in the use Standards for a number of chloric acid, and diluted to 300 cc. and the sulfur determined in of this shortened method, years. the usual manner. it was found that some I n this method 0.5 gram The amounts of occlusion, when the methods of the Bureau of samples were so violently of rubber is treated with Standards and of Kratz, Flower. and Coolidge are used, were deattacked by the nitric 20 cc. of concentrated nitric termined by precipitating barium sulfate from solutions containing acid-bromine mixture that acid saturated with brothe equioalent amount of salts usually present. and oarying amounts inaccurate results were mine. After digesting for of pure sodium sulfate. The results show that the amount of ocobtained. The amount of one hour in the cold and cluded salts must be corrected for in accurate work. one hour on the steam bath, acid left to be neutralized The sulfur in seoeral compounds, containing litharge and barytes, the excess of acid is evapovaried also, so that the rewas determined by both methods. After correction for occlusion, rated off. To the residue sulting fusion mixture was the results obtained by either method are practically the same and not always of the same are then added 5 grams of a suficiently accurate. composition. mixture of equal parts of It was deanhydrous sodium carbocided that more work should nate and potassium nitrate and enough water to make a be done to determine how little nitric acid-bromine mixture pasty mass, which is spread on the sides of the crucible will suffice for all types of compounds. Shortly after this work was started, the Committee on and dried on the steam bath. The crucible and contents are then heated over a sulfur-free flame until all car- Methods of Analysis of the Rubber Division of the AMERICAN bonaceous matter is burned. The melt is cooled, lixiviated CHEMICAL SOCIETY~ tentatively accepted as standard the with water, the solution filtered, acidified with hydrochloric methods of the Bureau of Standards and of Krats, Flower, acid, and the sulfuric acid precipitated by the addition of and Coolidge. It was decided to obtain some data on these barium chloride solution. methods and the amounts of occlusion, before k a l l y adoptRecently, some work4 was done to revise this method in ing them, in order to eliminate doubt as to their value. The work was divided into the following parts: the study order to save time without diminishing its accuracy. This work was based on the determination of sulfur in oils by of the effect of varying the amounts of nitric acid-bromine Waters.s I n the method as finally adopted 5 cc. of nitric mixture; the determination of the degree of occlusion to be acid-bromine mixture and 5 drops of bromine were used. expected in the two methods; and the comparison of results After heating on the steam bath the excess acid was neutral- obtained by both methods. ized with sodium carbonate, thus avoiding the evaporation EFFECT O F DIFFERENT AMOUNT^ O F NITRICACTD-BROMINE of the nitric acid and the use of a fusion mixture of potassium MIXTURE nitrate and sodium carbonate. Also the time for lixiviation The following method was used in this work, except that was shortened and the final volume of the solution decreased. the amount of nitric acid-bromine mixture was varied: I n addition, some work was done on the amount of occluded Place 0.5 gram of rubber in a porcelain crucible of about salts, because the work by Allen and Johnston,G Johnston 75-cc. capacity, add 20 cc. of the nitric acid-bromine mixture, cover the crucible with a watch glms, and let it stand for 1 Received February 23, 1923. hour in the cold. Heat for an hour on the steam bath, * Published by permission of the Director, U. S. Bureau of Standards. one remove the cover, rinse it with a little distilled water, and 8 Bur. Slandards, Tech. Paper 174; THISJOURNAL, 8 , 734 (1911). evaporate the solution to dryness. Add 3 CC. of nitric acid, 4 Levin and Collier, Rubbev Age and Tire News, 9 (2). 47 (1921).
T
Bur. Slandards, Tech. Paper 177; TEISJOURNAL, 12, 482, 812 (1920). J . Am. Chenz. SOL.,32, 588 (1910).
J . A m . Chcm. Soc., 98, 829 (1911). BTHXSJOURNAL, 14, 560 (1922). 7
