AmiL 1922
THE JOURNAL OF ILVDUSTRIAL A N D ENGI:INEERING CHEMISTRY
295
Preparation and Evaluation of a Decolorizing Char from Bagasse' By C. LOUISIANASTATE
E. Coates2
UNIVERSITY,
There is need of a more accurate method of evaluation of decolorizing chars. In the present work, a commercial char of high eficiency was selected empirically as a standard and used in measuring the relative eficiency of a series of experimental chars made from bagasse. B y comparing the filtrates obtained from the standard char and the experimental chars, a more accurate comparison can be made than is possible between the dark initial solution and its decolorized product. The experiments demonstrate the importance of uniform acidity in comparing decolorizing power. Bagasse char, heated without treatment to 900", had a decolorizing power of 66 (standard- loo), which was increased to 86 by boiling with sodium hydroxide, and to 170 by subsequcnt boiling with hydrochloric acid. Chars prepared by this method had a decolorizing power of from two to two and a h a 8 times that of the standard char. I n addition to removing organic coloring matter, the chars showed a marked power of absorbing iron salts. Yellow commercial hydrochloric acid, which was used in washing the char, came through absolutely colorless. This suggests a possible technical application of highly activated chars in the remooal of iron from commercial hydrochlovic acid.
URING the last six years a great deal of work has
D
been done on activated chars, stimulated no doubt by their very great importance in gas warfare. It has become evident that for some reason there is little relationship between the gas absorbing power and the color adsorbing power of chars, though no satisfactory explanation of this lack of relationship has yet appeared. It i? perhaps too soon, therefore, to do more than add a few facts to those already at our disposal, in the hope that me may soon be able t o reach a point where we can generalize rationally and not be driven i o empirical conclusions. In 1914 William Clacher3 published an account of v, orlc on some bagasse chars, with the idea that the results might be applied technically. The process outlined by Clacher differs from one mentioned by Weinrich B number of years ago in certain patent specifications, in that Weinrich used the charred bagasse without subsequent chemical treatment, mainly as a filter aid, while Clacher activated his char by boiling with strong caustic soda, for the purpose of removing the siliceous ash, followed by washing with acid. So far as the present writer knows, neither process has ever been carried out technically. Dilute sodium hydroxide has been used for many years t o revivify spent bone chars. The purpose of the sodium hydroxide is to change the conditions of adsorption at the free surface of the carbon so that adsorbed substances may pass again into solution, and to dissolve certain alkali-soluble compounds. These substances include coloring matters, gums, colloids of one type or another, and mineral salts. In revivifying boneblack, the soda process was usually considered more expensive than burning. The use of stronger solutions of soda to activate inactive wood chars seems to the writer a different thing in many ways from its use in revivifying spent bone chars. It would
'
Presented before the Division of Sugar Chemistry and Technology a t the 62nd Meeting of the American Chemical Society, New York, N. Y . , September 6 t o 10, 1921. 2 Dean, Audubon Sugar School. a Intern Sugav J . , 1914, 64.
BATONR O U G E , t 0 U M A N A
naturally occur t o any chemist, v h o had to deal with a char with a siliceous skeleton, to extract this silica. Boneblack can be made much more active by extracting it with hydrochloric acid, thus removing the insoluble tricalcium phosphate, increasing the porosity, and leaving a greater amount of free carbon surlace available for adsorption. This is an ordinary laboratory procedure. In a similar way a siliceous char boiled with $odium hydroxide would lose its silica in the form of soluble sodium silicate, becoming more porous and more active. This procedure, however, would be without purpose in the case of bone char, which has a calcium phosphate base, and was probably not tried out on wood chars until the latter were developed into decolorizing chars some twelve to fifteen years ago. When high powcr chars of vegetable origin were developed from wood giving an ash of low silica content, the supporting skeleton was supplied by impregnating with lime or some other mineral substance, which was subsequently extracted by hydrochloric acid. Chars made of vegetable material high in silica, on the other hand, were extracted with sodium hydroxide, with the same general purpose in view. The growing importance of vegetable chars, which was stimulated by the successful work of Sauer and Kijnberg, has caused a good many chemists to seek different sources of decolorizing chars during the last few years. Bagasse, being a by-product of the cane-sugar industry, has naturally received considerable attention, and a number of experiments have been made with it by numerous workers, including the writer. The reports of these experiments vary considerably. It has therefore been thought best to repeat them for the purpose of ascertaining, if possible, the cause of these differences. In order to rule out the personal equation three lots of bagassc were treated separately by different men, first by the writer, then under his direction by N r . 8.D. Lipscomb and by Mr. P. M. Horton. The results in all cases were practically identical. It soon became evident that in most published accounts of the evaluation of chars the conditions of the tests mere rarely stated with any detail and definiteness, nor was any one method used generally, nor were the results expressed in tlhe same mathematical terms. It was determined, therefore, to test the effect upon the results of slight changes in conditions and to establish a standard method for work, which, while not altogether satisfactory, would at least have the merit of being definite. CALCULATION OF RESULTS In sugar work, decolorizing chars are probably of value in accordance with the amount of color they leave in the solution, rather than the amount which they take out. The usual Stammer and Duboscq figures give us roughly the percentage of color removal. On this basis a carbon which would remove 45 per cent would be one-half as good as a carbon which would remove 90 per cent total color, a statement which is incorrect technically. Similarly, a carbon which would remove 90 per cent of color would be callecl almost as good as one which would remove 95 per cent of color. This statement is also incorrect. On the other hand, it is, of course, not quite correct to call one which leaves 5 per cent twice as
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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
goodas the one which leaves 10 per cent. None the less, it is the writer's opinion that the second is the less inaccurate method of the two, and it was adopted for his work. These points were raised by Zerban some time since. The writer agrees with his point of view, in the main; but undoubtedly we need a more accurate system of evaluation. In order to get a rough idea as to the initial intensity of color, the sugar solutions were read against a Stammer double plate. While the tint of the solution generally differed materially from that of the plate, it was none the less found possible to get scale readings which were surprisingly uniform. The same solutions were then read before and after decolorizing, in the Duboscq colorimeter, and the comparison was made directly between the lengths of columns having the same tint, no allowance being made for the fact that, mathematically, color removal is not quite proportionate to length of column. The solutions were also read with a Hess-Ives tint photometer. For the sake of comparison, a good grade of commercial decolorizing char of high efficiency was taken as a standard, and its decolorizing power was called 100. This standard is, of course, purely empirical, and might be omitted if desired, but it. was found to be a convenient way of testing the relative decolorizing power of a series of experimental chars. Moreover, by comparing the filtrates obtained under standard conditions from the standard char and the experimental char, respectively, it was found that a more accurate comparison could be made between the light filtrates than between the dark initial solution and its decolorized product. Table I illustrates these points. TABLE I (Stammer reading on original solution, 3.5) ---Duboscq ColorimeterEquivalent Lengths of Column Color Color Mm. Removed Left 100 83 17 Filtrate from standard char . . .. . . . . . . Initial solution before decolorizing 17 Filtrate from experimental char.. 100 92 8 Initial solution before decolorizing. , , . . , , 8 Filtrate from standard char.. . 47 .. Filtrate from experimental char 100 17 Efficiency experimental char relative to standard 8 = 213
.
..
.
.
... .
-
.. .. .. ..
.. ..
..
100 = 213 47
Efficiency experimental char relative to standard-
It will be noticed that by direct comparison of the standard char filtrate with the experimental char filtrate the same figures are obtained as by the indirect comparison with the molasses solution. As the tints of the filtered solutions are more easily matched, this method was used in calculating the following data. (See also Tables 11,111,and V.)
TESTSOLUTION As a standard test solution to be decolorized, a 3 per cent solution of blackstrap or third molasses was chosen, because a solution of this type had been used by Zerban and others in char work. Different samples of molasses were found to differ materially in their color content, but the comparative figures on decolorizing were practically the same on all molasses samples employed. For this reason it may be inferred that the general nature of the coloring matters in third molasses is practically the same, and hence it may safely be used in obtaining comparative figures. It may be noted, however, that it is not possible t o compare results obtained on molasses with those obtained on other types of colored organic material, as, for instance, crude glycerol. The stock molasses solution was boiled and filtered with kieselguhr with practically no loss in color. Inasmuch as fermentation brought about color changes a fresh solution was made up every 2 days. METHODOF WORK To 200 cc. of the solution in a 500-cc. flask, 6 g. of char were added, the contents brought to a boil, then stoppered and
Vol. 14, No. 4
heated in a boiling water bath for 10 min. with occasional agitat,ion. The flask was then cooled and the contents filtered, discarding the first few cc. which were usually cloudy. In several experiments the flasks were boiled 5, 15, and 20 min. There was hardly an appreciable difference in the decolorizing effect after 5 min. and none a t all after 10 min.; the latter wag. taken as the standard time for boiling. The filtrates should be read after they are thoroughly cold. Though there is little difference in color in the same filtrate at 20" and 30",it generally becomes lighter as the temperature rises. This phenomenon is particularly noticeable with sirups, but different sugar solutions show it to different degrees. ACIDITY The importance of uniform acidity in comparing decolorizing power cannot be overestimated, as will be seen from Table 11,recording the results of tests made with definite degrees of acidity. Acidities were obtained with phosphoric acid and alkalinities with sodium hydroxide, with phenolphthalein as an indicator. pH values were obtained on a different series of solutions. TABLEI1 3 per cent molasses solution, made acid with phosphoric acid and alkali with NaOH Acidity Apparent Efficiency N Color before Decolorizing Standard Char 0.03 Light yellow 200 Light yellow 150 0.015 Brownish yellow 100 0.007 Dark yellow-brown 99 0.0035 80 Dark reddish brown 0.0015 0.007 Lighter, but redder 75 Alkalinity, N 0.07 Still lighter, but red 60
I n this case 200 cc. of the molasses solution were brought to the acidities indicated and decolorized with 6 g. of char. The acidulated kolutions were not of comparable colors because of the difference in tint, but there was no difficulty in obtaining comparable readings after decolorizing. I t will be noted that by simply changing the acidulation there was an apparent increase in efficiency of the char of 100 per cent and a corresponding decrease when the solutions were made alkaline. The figures were calculated on color remaining in solution. From the foregoing one might reasonably conclude that comparisons of decolorizing chars where no mention is made of acidities are of somewhat doubtful value. It might, be added that although these apparent decolorizing effects are only apparent, there seems nevertheless to be a certain relationship in actual color removed, as the filtered solutions when brought to the same acidity retained the same order of decolorization, though not to the same extent. The following colorimetric readings a t varying acidities on a molasses solution of different origins were made at our request by Mr. W. G. Raines, Jr., with the Hess-Ives tint TABLE 111 Solutions Used Acidity, N 0.02
0.015
0.01 0,005 0.003 Neutral Alkalinity, N 0.003 0.005
0.01 0.02
Red 46 50 50 48 55
52.5 65
69 69 71 71 71 64 61 62 52 50
Green 76 77 78 77 80 79 83 83 83.5 83.5 84 84
Blue 85 84 85 84.5 85 85 85
85 85.5 85
Total 207 211 213 209.5 220 216.5 233 238 238 239.5 240.5 240
84 84 83.5 83 82
85.5 85.5 85 84.5 84
233.5 230.5 230 5 219.5 216
S5 85.5
Total Color Remaining 49.5 53
55 51.5 61 68
79.5 79.5 81.5 83 86 85 76 72.5 72.5 60.5 57
April, 1922
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
photometer. The chart used in the color determinations is that published by Zerban.4 The readings are in duplicate series. It will be noted that there was no great difference in the amount of blue remaining, that the green was apparently removed to a slightly greater extent in acid solutions, and that the red was apparently much more affected. I n the following tests the acidity of the solutions to be decolorized was kept a t a constant acidity of 0.007 N . This is not far from the optimum clear juice acidity in raw sugar making, nhich in good practice ranges from 0.004 to 0.007. In melting raw sugars the optimum acidity for first liquors seems to be from 0.0015 N to 0.0025 N . In the direct manufacture of plantation granulated the best clear juice acidities, as determined by experience, range from 0.009 N to 0.011 N .
grade first molasses. In all cases the solutions were acidulated to 0.007 N , with phosphoric acid. CHARS-Bagasse chars were made as follows: CHARB-By heating Char A to 600' with a battery of Meker burners, washing water-soluble ash out with hot water, and drying. CHARC-By heating B to 800' in an electric furnace for 1 hr. CHARD-By boiling C for 10 min. with 15 per cent sodium hydroxide on weight of char plus enough water to make a cream, the final percentage being about 4 per cent sodium hydroxide solution. CHARE-Boiling D 5 min. with 1:l hydrochloric acid.
In Table IV the Stammer reading gives the approximate color, and the decolorizing powers are given in terms of standard char = 100.
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TABLB IV Stammer SOLUTIONReading
r
B
Decolorizing Power of Chars C D E Standard
..
PREPARATION OF CHARS About 8 lbs. of bagasse were charred in each of three series of experiments. The bagasse was heated in a large iron sand bath, which was heated red hot by a battery of M6ker burners and covered with a similar sand bath, the two being separated by a heavy triangle of iron wire and thus held about one-eight inch apart. This gave a fairly satisfactory retort, from which gases could escape readily without any increase in pressure. At the end of 15 or 20 min. most of the volatile matter had been driven off and the char rendered so brittle that one could powder it for subsequent treatment. The temperature of the retort ranged from 500" to 600" C., as recorded by a Northrup pyrometer. The percentage of recovery was about 50 per cent on the basis of dry matter. The char still contained a large quantity of substances volatile a t higher temperature, as was evidenced by the fact that when heated to 800" i t gave off gas with such rapidity as to blow the char in part from the clay crucible if the temperature were raised too rapidly. The char was ground to 70 to 90 mesh, and called primary Char A, and used in the subsequent preparation of activated chars with the electric furnace. Charring in two stages was found more convenient than charring in one &age on account of the bulky nature of the raw material. The results obtained in the two cases were identical.
297
0
..
..
116
1 1u
1UU
133 120
100
100
This bagasse char was not as active as batches subsequently made, probably because the sodium hydroxide solution was too weak. Tests made using the same amounts of molasses and omitting the sucrose gave similar results. The results show that: 1-The presence of sucrose did not materially affect the decolorizing power. 2-Bagasse char, when properly made, has a distinct decolorizing power before it is treated with sodium hydroxide (Columns B and C). 3-This decolorizing power is greatly increased by boiling with sodium hydroxide (Column D). 4-It is still further increased by boiling the char extracted with sodium hydroxide with a 1:1 solution of hydrochloric acid (Column E).
FURTHER EXPERIMENTS ON BAGASSE CHARS
The work was repeated with more attention to detail, it having become evident that the quality of the char was materially affected by several slight changes in process. For instance 20 g. of Char C were treated with 3 g. of sodium hydroxide, or 15 per cent by weight. To this was added enough water to enable the solution to boil readily. At the end of 15 mins.' boiling the ash content had not been greatly reduced. A rough estimate showed that about 100 cc. of SIZEOF CHARGRAINS water had been audded, giving a 3 per cent solution of sodium The chars were cooled., ground. and sifted. and separate . hydroidde, which attacked the silica slowly. If, however, tests were made of the various sieve fractions. *Chars this solution was allowed to boil down to a thick porridge, between 70 and 90 mesh were the most effective, and did not thus increasing the concentration of the sodium hydroxide differ appreciably in decolorizing power. These portions, solution, the silica content was reduced and the activity of the therefore, were selected for the experiments. A small char materially increased. It was also noted that for difquantity of char of 100 mesh upward was also obtained. ferent samples of molasses a given char showed different perThis was tested separately and showed about the same centages of color removed compared wit,h the standard, and qualities as a 70 to 90 char, to which it was subsequently the ratios of color removed by different chars on these different added. The high mesh chars gave somewhat slower filtra- molasses samples were not constant. tion, with no appreciable increase in decolorizing power.
PRELIMINARY EXPERIMENTS SOLUTIONS-A solution containing 400 g. of granulated sugar and 200 g. of first molasses was diluted to 2000 cc. This gave Solution 1, of 27.8 Brix and a reading against the Stammer double plate of 3.5. It approached in color and composition a rather dark sirup in ordinary sugarhouse practice. To 160 cc. of this solution (containing 50 g. of total solids) in a 250-cc. flask were added 5 g. of char, or 10 per cent on total solids. The stoppered flask was heated to 95' to 100' C. for 10 min., filtered, and cooled. Color comparisons were made on a Duboscq type colorimeter. Four solutions corresponding to Solution 1 were made up from different samples of molasses; 2, blackstrap; 3, blackstrap; 4, high4
Louisiana Planter. 1018.
CHARA-Ash, 11 per cent; SiOz, 7.06 per cent; decolorizing power, 0.0. CHARF-This was made by boiling Char A with sodium hydroxide, 20 per cent on weight of char, for 15 min., washing with water, then with dilute acid, and drying. Ash, 3.3 per cent; SiOz, 0.9 per cent; decolorizing power, 60. CHAR&Inasmuch as chars made at higher temperatures had shown more decolorizing power, Char A was boiled with sodium hydroxide as for Char F and then heated to 850' C. for 1 hr. A quantity of gas was given off during this stage. Ash of Char G, 8 per cent; SiOz, 3.2 per cent; decolorizing power, 225. CHARH-From the foregoing i t was evident t h a t the temperature of charring was of prime importance. Char A, therefore, was heated without treatment with sodium hydroxide to 900' for 2 hrs., giving Char H, with a decolorizing power of 66. CHARI-Char I was prepared by boiling Char H 30 min. with 20 per cent weight of char of sodium hydroxide, which gave a decolorizing power equal to 86. CHARJ-Char I was boiled with hydrochloric acid, and the resulting Char J showed a decolorizing power of 170.
