Oxidation of Pyritic Sulfur in Bituminous Coal H. W. NELSON,R. D. SNOW,].AND D. B. KEYES,University of Illinois, Urbana, Ill. of the cylinder through which holes I n experiments here described a n increase in were d r i l l e d to admit a theronly of c o m p o u n d s conmometer, level indicator, watertemperature increases the rate of the oxidation of taining carbon, hydrogen, cooled condenser tube, heating pyritic sulfur to soluble sulfate sulfur. B y element, and thermoregulator. A and o x y g e n , their combustion mercury relay was used in connecscreening a sample of the pulz$erized coal to several would be a relatively s i m p l e tion with the heater and the thersized fractions and running a series of oxidation matter. However, during the moregulator in order to maintain centuries of their gradual formaa desired constant temperature experiments on each fraction, the pyritic sulfur within the limits of * 1 C. The tion from decaying vegetable in the finer sizes is oxidized at a faster rate than level indicator was found necesand animal r e m a i n s , foreign sary while making runs at 70" C. the larger sizes. The amount of pyritic sulfur matter has been incorporated or higher, since the loss by evaporaoxidized at a gicen temperature and definite period and i m p r e g n a t e d in the coal tion and the passage of air containing water vapor made it difficult to of time is proportional to the inverse diameter substance. One of these impuricondense all of the water vapor ties, sulfur, has caused trouble. of the average size particles in each screened fracwithout having the condenser tube Owing to the steady depletion of (filled with short pieces of glass tion. The volume of air passed through ihe apthe low-sulfur coal in the IJnited tubing) fill up and create a back paratus per unit time and the quality of the air pressure. The level indicator conStates, increased a t t e n t i o n is sisted merely of a piece of stiff wire with respect to its oxygen content are comparabeing given t o the utilization of affixed into a cork at one end while tively unimportant factors in the oxidation process. coals that are suitable for varithe other end moved in a piece of ous industrial uses except that glass tubing stoppered into the Experiments made with ferric sulfate added io Bakelite plate. they are too highin sulfur. Rethe coal-water mixture show that this compound The air was obtained from the search of a fundamental characcompressed air line in the laboraassists materially in the oxidation of the pyritic ter is necessary for the solution tory, and the rate of flow was sulfur in fhe suspended coal. The effect is enof many of the problems. measured by means of a calibrated flowmeter, the rate being obtained hanced by a rise in temperature. A large The sulfur in coal is of two at atmospheric pressure. kinds, o r g a n i c and inorganic. amount of the pyritic sulfur is removed f r o m the A gas inlet (Figure 1) was used Classed as i n o r g a n i c are the for the entrance of chlorine as will coal during experiments in which a small amount sulfides of w h i c h iron pyrites be shown later. The vacuum line of chlorine gas is added io the air stream passing balanced the pressure when chlo(FeS2, isometric) and marcasite rine was used in order to operate through the apparatus. The organic sulfur is (FeS2, orthorhombic) a r e t h e the flowmeter at a t m o s p h e r i c unaffected by a n y of !he processes menfioned. most important. Sulfates are pressure, at which pressure it had also present, the most common been calibrated. ones being gypsum and iron sulfates. Previous investigators I n making a run, water was placed in the cylinder with air ( 1 , 8)have found that a t low temperatures the pyritic sulfur may be oxidized t o the sulfate form by air in the presence of being blown through the porous plate a t a given volume per moisture. The organic sulfur is little, if any, affected by this minute. The amount of water contained in the cylinder process. Winchell ( 7 ) found that pyrite was slightly oxidized while air was being passed through the apparatus was norby passing a stream of aerated water over a sample held on mally approximately 4.5 liters. To save time the water was a n aluminum screen. Stokes (6) oxidized samples of pyrite previously heated to about the desired temperature before and marcasite by means of ferric sulfate. A sample of the being placed in the cylinder. The heater was turned on, and, pyrite was boiled in a solution of ferric ammonium sulfate, when the desired temperature was maintained, the measured and the amount of oxidation taking place determined by a n amount of pulverized coal was added, and the level indicator, analysis of the iron. Ochi (S) treated pulverized coal with condenser, and thermometer were placed in position in the saturated chlorine water and found upon analyzing the washed Bakelite plate. After the appropriate time interval had elapsed, the top of residual sample that about two-thirds of the total sulfur had the cylinder was removed and the contents of the cylinder been removed, including some of the organic sulfur. I n the present investigation a study has been made of the poured out, and all particles of coal were washed from the oxidation by air of pulverized bituminous coal suspended in sides and bottom by a spray of water. The mixture was then filtered in a Buchner funnel, dried, and washed free from water. sulfates. I n most cases all of the sulfate sulfur was removed EXPERIMENTAL PROCEDURE by extraction in dilute hydrochloric acid. The coal residue was then sampled and dried in the vacuum The apparatus used for the oxidation experiments is shown oven for 4 hours, the temperature being 104" C. Before openin Figure 1. ing the oven, nitrogen was passed into the vacuum chamber, The body of the apparatus was a cylinder made of Allegheny after which the sample was cooled in a desiccator. The sample metal 5 inches (12.7 cm.) in diameter and 20 inches (50.8 cm.) was then analyzed for its total and pyritic sulfur content and long. A Filtros porous plate was cemented into the bottom of the cylinder by means of a mixture of litharge and glycerol. the percentage oxidation of the pyritic sulfur calculated. The bottom piece, which contained the connection tube for the The methods of Parr and Powell (4) were used for all entrance of air, was then screwed in place. A circular piece of determinations of the various forms of sulfur in the coal. half-inch (1.27-cm.) Bakelite was fitted into the other (top) end The total sulfur in the samples was found by means of the 1 Present address, Phillips Petroleum Company, Bartlesville, Okla. sodium peroxide fusion method.
F COALS w e r e composed
O
INDUSTRIAL AND ENGINEERING CHEMISTRY
1356
After removing the soluble sulfates from the sample, the pyritic sulfur was determined by extracting 1 gram of finely pulverized coal with a solution of 20 cc. of nitric acid (specific gravity, 1.42) and 60 cc. of distilled water. The extraction was carried on for 2 days a t room temperature. While Parr and Powell recommend that the time of extraction be a t least 4 days, it was found that the oxidation of the pyrite was completed within analytical limits in the 2-day period. At
AP
-
FIGURE1. APPARATUS FOR OXIDATION EXPERIMENTS
the end of the extraction period the coal was filtered off and washed, and the filtrate evaporated to dryness. The residue was then moistened with dilute hydrochloric acid and again evaporated to dryness. The residue was taken up in 5 cc. of 1:l hydrochloric acid and diluted to 100 cc. with hot water, The amount of sulfur in the sample was determined in the usual manner. Upon the completion of a run and after the coal residue had been filtered, the sample was washed with hot water in order to remove all of the soluble iron and sulfates formed during the oxidation process. It was found that as many as a hundred separate washings over a Buchner funnel were required to remove all of the sulfates, the final washings showing no precipitate upon the addition of a solution of barium chloride. Since this method of freeing the coal residue from sulfates could not be depended upon, it was decided to extract all residues with a 3 per cent solution of hydrochloric acid, which would remove all of the soluble iron and sulfates. When this was done, all of the coal samples would be analyzed on a common basis-i. e., free from soluble sulfates-and the amount of oxidation could be determined by the analysis of the residue for pyritic sulfur. OXIDATIONOF PYRITIC SULFURIN BITUMINOUS COAL The coal used was from Kincaid, Christian County, Ill,, and was a pulverized sample taken from the feed line of a power plant of the Commonwealth Edison Company in Chicago. The analysis, with special reference to the sulfur content, was as follows (dry basis):
Vol. 25. No. 12
gr,
%
I-
Ash Total sulfur Sulfate sulfur
Pyritic sulfur Organic sulfur
17.6 6.03 0.94
2.b8 2.51
I n all runs in which this coal was used the coal sample to be oxidized was of the following composition as regards the size of the particles: SCREEN MESH On 60 Through 60 on 100 Through 100 on 120
%
SCREEN.MESH Through 120 on 200 Through 200
5.6 13.5 8.7
% 29:9 42.3
Samples of 25 grams of pulverized coal were used for the experiments. Using Kincaid coal, a number of runs was made a t temperatures of 22", 50", 70", and 90" C. for one week. The rate of flow of air through the apparatus was kept the same for all runs-namely, 4 cubic feet (0.113 cubic meter) per minute. Each day a t the same time several samples were taken from the apparatus by means of a 100-cc. pipet. The samples were then washed, mixed thoroughly, and analyzed for the various forms of sulfur. The results of the oxidation of this coal for one week are shown in Table I. In all cases the basis of analysis has been that of samples dried in vacuo. Table I shows that an increase in temperature favors the oxidation of the pyrite in the coal, the greatest increase being from room temperature (22' C.) to 50". The oxidation is more rapid during the first day of the oxidation process, the rate decreasing during the following days until the fifth day when it is very low. This may be due to the fact that the particles of pyrite become coated with a layer of sulfate as the reaction proceeds and the solvent action of the water is insufficient to remove the layer fast enough to keep a fresh surface constantly exposed. It may also be difficult for the solution to be removed from the many small crevices and cracks in the particles which would slow down the oxidation reaction. Table I also shows that the percentage of organic sulfur in the sample of oxidized coal is in most cases approximately the same as the percentage of organic sulfur in the original coal. The difference may be assumed to be due to analytical errors and variations in samples, since the analyses of the residues of both runs a t 70" and 90" C. a t the end of 7 days show little difference in the amount of organic sulfur from that of the original coal, The organic sulfur is probably not affected by the oxidation process. ON PERCENTTABLEI. EFFECTOF TIMEAND TEMPERATURE AGE PYRITIC SULFUROXIDIZED
(Difference between total sulfur and sum of organic and pyritic sulfur in original coal is sulfate sulfur.) TOTAL SULFUR PYRITIC SULFURORGANIC SULFUR In In. In. In, I?. I? origi- 0x1orlgl0x1orlgi- 0x1nal dized Re- nal dized Renal dized TEMP. TIME coal sample moved coal sample moved coal sample a
C.
Day8
22 22 50 50 50 70 70 70 70 90
1 7
18 hours 3 5 1 4 5 7
s o % % % % .
5.97
..
..
2.58
2.39
%
%
%
6.7 13 7 27:4 49.5 54.4 35.1 63.3 67.3 70 9.1 0
2:51 2.51 2.51 2.51 2.51 2.51 2.51
2:48 2.33 2.28 2.50 2.47 2.31 2.45 4
..
..
7
Since the rate of oxidation is rapid during the first day of the oxidation process, a number of runs was made a t various temperatures for periods ranging from 2 to 24 hours. The weight of the samples and the rate of flow of the air were the same as for the longer runs. The results are shown in Table 11. The rate of oxidation during the first few hours is rapid as compared with the rate thereafter. Again, this is probably due to the rapid oxidation of the fresh surfaces of the pyrite.
December, 1933
INDUSTRIAL AND ENGINEERING
CHEMISTRY
1357
TABLE 11. OXIDATION OF PYRITIC SULFURIN PULVERIZED The results given in Table I11 show that the finer sizes KINCAIDCOAL oxidize a t a faster rate than the larger sizes. I n this series an (Time and temperature varied) attempt to find a correlation between the total surface area of PYRITIC SULFUR the particles (per unit volume) and the amount of oxidation I n original In oxidized TEMP. TIME coal sample Oxidized met with some success. If we assume that the areas of two a C. Hours % % % powders of given unit volume are inversely proportional to the 22 2 3.5 2.57 2.48 22 24 2.57 2.39 7.0 average diameter of their grains (also assuming that the dis50 1 2.48 2.29 7.2 tribution of each portion is uniform), we may represent the 5 19.7 50 2.48 1.99 50 18 2.48 1.80 27.4 ratio of the surface areas of the five samples of coal as follows 2 24.8 70 2.48 1.90 2 20.0 70 2.00 2.48 (based on the openings in Tyler Standard screens) : 5 26.2 70 2.48 1.83 35.1 24 2.48 1.61 70
--
r
1
1
When the filtrate from the oxidation process was tested with an indicator such as methyl orange, it was shown to be distinctly acidic. Also, when a sample of the filtrate was made acidic and a few drops of a solution of potassium thiocyanate added with a little hydrogen peroxide, the solution turned blood-red, showing the presence of the ferric ion. No color was obtained without the addition of a n oxidizing agent, which indicates that ferrous sulfate is formed by the oxidation reaction according to the equation: 2FeSn
+ 702 + 2Hz0 = 2HzS04 + 2FeSOc
I n order to determine whether the volume of air per unit of time was an important factor in the oxidation of the pyritic sulfur, a few experiments were made a t a constant temperature and for the same length of time but varying the rate of flow of air. It was found, however, that, if the rate of flow of air was sufficient to cause a steady stream of air to bubble through the porous plate and keep the coal particles from clogging the holes in the plate, there was no apparent change in the amount of oxidation. Conversely, an increase in the rate of flow did not cause an increase in the amount of oxidation of the sulfur. Oxygen, when substituted for air, did not cause a change in the amount of pyritic sulfur oxidized. The results of the oxidation experiments indicated that the degree of fineness of division of the coal particles was of major importance in the oxidation of the pyritic sulfur. I n order to determine the effect of this condition, the following series of experiments was made. EFFECTOF SIZE OF COALPARTICLES
A sample of the pulverized Kincaid coal was screened to five different sizes and the percentage by weight of each size determined. A number of experiments was then made in order to determine the effect of the degree of fineness of the coal particles on the rate and amount of pyritic sulfur oxidized. Since the fine particles present a much larger surface per unit volume on which the oxidation reaction may take place, the finer sized particles should show the greater amount of oxidation. I n making the oxidation experiments on the sized coal, about 10 grams of the various samples were used for each experiment and the temperature was maintained a t 50" C. After making a number of runs in the manner previously described, the samples were analyzed for pyritic sulfur; the data are shown in Table 111. TABLE111. DATA TI^ Hours 6 24 6 24 6 24 6 24 6 24
MESHSIZE 20-60 20-60 60-100 60-100
100-140 100-140 140-200 140-200 200-finer 200-finer
ON
SIZEDCOAL
-PYRITIC SULFURI n original I n oxidized coal sample Oxidized % % 7" ._ 1.65 1:is 7:3 1.59 1.78 10.7 2.39 2.39
2.86 2.86 2.73 2.73 2.37 2.37
2.15 2.04 2.34
2.20 2.25
1.91 1.84 1.48
10.1 14.6 18.1 23.1 17.6 30.0 22.2 36.2
1
1
1
0.0212 : 0.0078 : o.0049 : 0.0035 : 0.0025 47 : 128 : 202 : 288 : 400
or
The ratios of the percentage oxidation of the coal samples for 24 hours a t 50" C. are: 107 : 146 : 231 : 300 : 362
While the ratios of the two are not in exact agreement, the agreement is quite satisfactory for such a heterogeneous system as pulverized coal and shows that the oxidation of the pyritic sulfur in pulverized Kincaid bituminous coal is nearly inversely proportional to the average diameter of the coal particles. OXIDATIONOF PYRITICSULFURIN BITUMINOUS COALBY FERRIC SULFATE Stokes (6) has shown that pyrite or marcasite can be oxidized by boiling in a solution containing an excess of ferric ammonium sulfate. During this investigation it was decided to add a known amount of ferric sulfate to the mixture of pulverized Kincaid coal and water and determine the effect of the ferric sulfate upon the oxidation of the pyritic sulfur. Iron pyrites react with ferric sulfate according to the following equations: FeSz
2s
+ FeZ(S0J3= 3FeS01 + 25
+ 6Fez(SOa)s+ 8Hz0 = l2FeSOa + 8H2S04
For the experiments a quantity of ferric sulfate representing 0.05 per cent ferric ion was added to the water in the cylinder. After the solution had reached the desired temperature, the coal sample was added and the time of the oxidation noted. After the desired period of time the run was stopped, and the coal sample was filtered off, washed thoroughly, and dried in the vacuum oven. Because of the great amount of sulfates in the solution, particular attention was given the washing of the coal sample so that all traces of sulfate sulfur were removed before analyzing. A number of runs was made a t 22' and 70" C. and the percentage oxidation of the pyritic sulfur calculated from the analytical data. The results of a series of these experiments are shown in Table IV. It was found that the addition of the ferric sulfate hastened the oxidation process to some extent but not as much as might be expected. TABLE IV. EFFECTOF TIME, TEMPERATURE, AND FERRIC SULFATE ON PERCENTAGE OF PYRITICSULFUR OXIDIZED -PYRITIC SULFURIn origi- In oxinal dized TEMP. TIME coal sample Oxidized C. Hours % % % .22 2 2 :57 2 :is 3.5 22 22 22 70
70 70
717 ._
70 70 70 70 70
6 24 2
2.57 2.57 2.57 2.48 2.48
?
2.48 2.48 2.48 2.48
24
2 5 24
24 24 24 24
2.57
2.57
2.48
2.39 2.44 2.32 1.90 2.00 1.83 1.61 1.75 1.55
7.0 5.06 9.8 24.8 20.0 26.3 35.1 29.4
1.38 1.30
46.3 47.5
1.42
37.5 44.8
CATALYST None
None 0.0570 F e + + +as 0.05y0 F e + + +as None None None
1338
INDUSTRIAL AND ENGINEERING
The filtrate in each run was tested for the presence of the ferric ion by means of a solution of potassium thiocyanate. The filtrate from the experiments at 22" C. gave a strong test for the ferric ion, which indicates that all of the ferric sulfate had not reacted with the pyritic sulfur in the coal. However, using 0.05 per cent ferric ion as ferric sulfate in runs a t 70" C., the filtrate gave no color test of ferric thiocyanate when potassium thiocyanate was added t o the solution, This would indicate that all of the ferric sulfate had been used u p in the oxidation reaction or that part of it may have been changed to a basic sulfate. Several experiments were then made at 70" C. using 0.1 per cent ferric ion as ferric sulfate and the samples were analyzed as before. The results of these experiments indicated a further increase in the amount of pyritic sulfur oxidized. The filtrate in this case gave a clear color indication of the ferric ion; thus all of the ferric sulfate had not been used up during the oxidation reaction. Blowing air through the solution of ferrous sulfate which is formed by the oxidation of pyritic sulfur oxidizes but a trace of the ferrous sulfate t o the ferric form. Reedy and Machin (.5) have shown that ferrous sulfate is very slowly oxidized to the ferric form by blowing with air, so that the amount oxidized is exceedingly small. The ferric sulfate can be regenerated, however, by passing a small amount of sulfur dioxide through the solution according t o the equation: SO2
+ 2FeS04 + O2 = Fel(S04)a
No experiments were made on the regeneration of the ferric sulfate during the present investigation.
CHEMISTRY
Vol. 25, No. 12
more pyritic sulfur has been oxidized and removed a t 22" than a t 70" C. While these results are just the reverse of the other data collected during this study, they are in agreement with the results obtained by Ochi in his study of the effect of chlorine water on the removal of sulfur from various Japanese coals. TABLE V. EFFECTOF ADDINGCHLORINE TO THE AIR STREAM o s THE OXIDATION OF PYRITIC SULFUR IN KINCAID COAL (Sulfur in original coal:
total, 5.97 per cent; pyritic, 2.57; organic. 2.33: sulfate, 1.07)
PYRITIC SCLFURORGANIC S In oxiI N 0x1TOTAL S diaed ReDIZED TEMP. SAMPLEREMOVED sample moved SAMPLE
s I N 0x1-
TIME Min. 15 25 120 80 120
DIZED
c.
70 70 70 22 22
%
%
%
%
%
4.25 3.95 3.44 3.59 3.33
28.8 33.8 42.4 39.9 44.2
1.70 1.61 1.13 1.20 1.09
33.8 37.3 56.0 53.3 57.6
2.55 2.34 2.31 2.39 2.24
Even though the chlorine gas in the air was bubbled through the coal-water mixture in very small amounts, it is evident that the amount of chlorine going into solution in the water increases until a n equilibrium is reached for the solubility of chlorine gas in water a t that particular temperature. Thus the greater solubility of chlorine gas in water a t 22" C. results in the larger amount of pyritic oxidation when compared to that a t 70" C. The chlorine probably reacts with the water to form hypochlorous and hydrochloric acids and the former, which is a powerful oxidizing agent, may assist in the oxidation of the pyritic sulfur in the coal. The hydrochloric acid is probably not strong enough in this solution t o affect materially the pyritic sulfur. The results of these experiments do not agree with those obtained by Ochi inasmuch as there was no organic (or none within experimental limits) sulfur removed from the coal. The last column in Table V shows the amount of organic sulfur in each sample of the oxidized coal (the organic sulfur obtained by difference); the amount of organic sulfur is very nearly the same in each experiment, regardless of the temperature or length of time.
EFFECTOF CHLORINEON OXIDATIONREACTION The effect of adding a small amount of chlorine gas to the air stream bubbling through the coal-water mixture was investigated. About 0.05 cubic foot (1400 cc.) per minute of chlorine gas was mixed with the air stream while the experiment was being carried out. The temperature of the coalwater mixture was maintained at the desired constant temperature. For each experiment, after the water in the cylinder had reached the correct temperature, 20 grams of pulverized Kincaid coal were added, the apparatus was sealed except for the air inlet in the bottom and the outlet at the top, and the chlorine was admitted a t the desired rate. The space between the Bakelite top piece and the walls of the cylinder was covered with rubber electrician's tape over which was painted a seal of pyroxylin lacquer. The seal was effective in stopping any leaks from the apparatus and was easily applied and removed for each experiment. After the allotted time had elapsed for the run, the chlorine was shut off and the coal-water mixture washed from the cylinder. After filtering and washing the sample free of sulfates, it was dried and analyzed for total, pyritic, and organic sulfur. The length of time for the runs varied from 15 minutes to 2 hours. The results of the analyses of these experimental samples are shown in Table V. The experiments were made a t temperatures of 70" and 22" C., and Table V s h o m that
RECEIVED July 19, 1933. Presented before t h e Division of Industrial a n d Engineering Chemistry a t t h e 85th Meeting of t h e American Chemical Society, Chicago, Ill., September 10 t o 15, 1933.
The quarterly report of GERMAN CHEMICAL DEVELOPMEXTS. the I. G. Farbenindustrle, leading chemical producer in Germany, affords a general picture of conditions in the leading branches of the German chemical industry. There has been a continued improvement in chemical production and trade as a result of the government work-providing campalgn. The improvement is limited t o the domestic market, where the Reich measures intended to provide employment for the idle and inject new life into industry have quickened commercial turnover and raised the volume of industrial production. This stimulating influence is expected to be a factor in the business development for the future. Conditions in export business remained unchanged.
The dyestuffs trade showed no distinct change from the second quarter of 1933. In industrial chemicals the volume of orders received places this quarter on a par with the second quarter of 1933, which means a substantial improvement over the third quarter of 1932. Production of fertilizer nitrogen was maintained on the level of the preceding quarter, the rate of production being based on sales estimates for the fertilizer year 1933-34. The actual sale of nitrogen fertilizers during the third quarter showed a slight increase over the corresponding period of 1932. Production of motor fuel was increased further, with the part produced directly from lignite showing greater relative importance. Domestic sales of pharmaceuticals and insecticides showed much improvement.
ACKNOWLEDGMENT The late S. W. Parr was responsible for inaugurating this problem, The authors wish t o acknowledge also the assistance of H. F. Johnstone and the financial support of the project by the Utilities Research Commission in Chicago. LITERATURECITED (1) Drakely, J. Chern. SOC.,109, 723 (1916). (2) Li and Parr, IND.ENG.CHEY.,18, 1299 (1926). (3) Ochi, S . , J . Fuel SOC.Japan, 9, 246 (1930).
(4) (5)
Parr and Powell, Univ. Ill. Eng. Expt. Sta., Bull. 111 (1919). Reedy, J . H., and Machin, J. S., IND.ENQ.CHEM., 15, 1271
(1923). (6) Stokes, U. S. Geol. Survey, Bull. 186 (1901). (7) Winchell, Econ. Geol., 2, 290 (1907).
