Activated Carbon for Sugar Decolorization E. S. HERTZOG AND S. J. BRODERICK Southern Experirnmt Stnlion, U. S. Bureau of Mines, Tuscaloosa, MH. This paper deals with the preparation of activated carbons from coal and coal refuse for the decolorizatioii of raw sugar solutions. The elementar? theory concerning the activation of carbon arid the adsorption of color from raw sugar solutions is given. -4standard color solution and simple decolorizing test used i n the experimental preparation of activated carbons are described. A method for the activation of carbon by differential burning in an atmosphere of steam is explained. The apparatus and technique of operation are given. Acid treatment of the steam-activated car-
T
bons increases their decolorizing power further. Data on four different raw materials are included. The difference between water-purification and sugar-decolorizing carbons is show-n to be due to greater degree of burning, and is attributed to the production of capillaries of larger diameter i n the sugar-decolorizing carbon. Possible yields from the raw materials are calculated. Good sugar-decolorizing carbons can be made from coal or coal refuse, but we should emphasize that the lower the ash of the original raw material, the better the results to be expected.
HE use of coal-mine waste as a raw material to make an activated carbon for water purification was the subject of a previous paper (b) on the utilization of this wahte. I n the earlier report large samples of coal waste froin two different mines were selected, and each of these samples was fractionated by tabling. Each fraction then varied in composition from the high-coal to the high-ash end. An ash analysis of the inorganic constituents mas made for each fraction. The high-coal fractions made the most acti7-e carbon product for water purification, and as the ash of the raw material increased, t,he activated carbon product decreased in quality. As activated carbons for water purification are not necessarily efficient removers of color from sugar solutions, a study mas made of the relation between a water-treatment carbon and a decolorizing carbon, as coal-mine waste might be advantageously used as a raw material to make sugar-decolorizing carbons. Consequently this paper deals with making activated carbons for sugar decolorization from coal wa3te and coal. Decolorizing Carbon in the Sugar lnduskry Ordinary wood charcoal has some decolorizing pover, and this fact has been known for centuries. According to Mantell (5) wood charcoal was used in England in the sugar-refining process with considerable success u p to the beginning of the nineteenth century when, owing t,o the advent of the bone black process, the charcoal and vegetable chars were virtually forgotten. Bone black could be revivified and used repeatedly, and was so effective and cheap that vegetable chars were unable to compete with it. During the first World War the urgent need for highly adsorbent charcoals for use in gas masks stimulated research on carbons. As a result, carbons were produced that had many times the power of adsorption of ordinary aharcoal. Such activated caibons are riov coinpeting successfully with bone black.
A brief description of the sugar decolorizing process may be of interest a t this point. The raw sugar as received is deep brown. The color is partly due to a thin film of residual mother liquor adhering to the surface of the crystals. T o remove the dark film the raw sugar is mixed with a saturated sugar solution and centrifuged. The sugar is made into a hot concentrated solutioii, and activated carbon is added and stirred into the sirup a t about 200" F. (93.3' C,), After a short reaction period the solution is filter-pressed. The treatment is repeated with fresh carbon which removes all but a trace of the remaining color. A one-stage process is effective if a large enough carbon dosage is used, but the two-stage treatment has been found to reduce the total carbon required. One refinery using the two-stage process reported that only 0.3 pound of activated carbon was needed for each 100 pounds of dry sugar produced.
Activation an.d Adsorption of Color Wood, coal, lignite, rice hulls, bagasse, and similar carbonaceous materials may be ignited or burned in retorts to form chars. The ignition decomposes the complex carbon compounds, drives off the rolatile hydrocarbon, and leaves a residue or char consisting largely of carbon. The carbon in chars deposited below 600" C. is capable of activation, but a gray modification formed a t higher temperatures is not activable ( 1 ) . The carbon deposited below 600" C. n-as considered very active a t the time of its formation, but since i t was surrounded by volatile decomposition products, it instantly adsorbed a surface coating of these materials and became so saturated that little adsorptive power remained. The carbon surfaces must therefore be cleaned to restore the original activity. This can be done by heating to 900" C. in a nioving atmosphere of stea,m. Activation with steam is a sort of differential oxidation in which the adsorbed constit,tients burn readily while the carbon burns with difficulty and leaves a clean carbon surface. The carbon is then found t o be itc-
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September, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
tive. The activity can be increased stii further by increasing its total surface area. Further burning in steam slowly oxidizes some of the carbon, enlarges the capillary pores, and thus increases the total surface area, The product is then found to be more active. However, burning must not be carried too far because the larger capillaries eventually will coalesce and reduce the surface area. With chars containing ash-forming material, burning increases the proportion of inert ash and reduces the amount of active carbon per unit of finished material. According to the above explanation, the optimum degree of burning comes when the combined adverse effects of the coalescing of the larger capillaries and the increase in inert ash nullify the increase in activation owing to further enlarging of the smaller capillaries and more complete cleaning of the surfaces. Activation with steam should be performed as a two-stage process. I n the first stage the carbonaceous material is charred below 600" C. with expulsion of the volatile matter and production of a porous carbon. The second stage requires heating the char at a much higher temperature, because oxidation with steam is very slow below 700' C. Temperatures of 900-950" C. were found satisfactory for the laboratory process. Activated carbons are specific in their behavior and depend on the mode of activation. Carbons especially activated t o remove tastes and odors from municipal water may be rated according t o their ability to adsorb phenol. On the other hand, sugar-decolorizing carbons are ranked according t o their ability t o remove color from a sugar solution. Early in this investigation a series of carbons was made in which the amount of burning during steam activation was correlated with the adsorption, and a certain degree of burning was found a t which maximum adsorption was obtained. Then the carbon having the maximum phenol adsorption value was tested for sugar decolorization and found t o be a poor adsorber of color. However, after this carbon had been reactivated in steam, it showed improved color adsorption. Results in other series of carbons illustrated t h a t sugar-decolorizing carbons must be burned t o a greater degree than those activated for phenol adsorption. The differences may be explained on the basis of molecular size and capillary diameter. According to Zerban (T), the color in raw sugar solutions is due to autocyanin and the oxidation products of certain polyphenols. The color bodies undoubtedly have much larger molecules than phenol, and it seems reasonable t o suppose that when the capillaries in the carbon are large enough t o permit free entrance of phenol molecules, a large proportion of the capillaries may still be too small to admit the color bodies. Longer burning of the carbon enlarges the capillaries and permits the color bodies t o enter, with an attendant rise in color adsorption. Moreover, the steam-activated carbons could be converted into better decolorizers by giving them an acid treatment. This effect was noted by McKee and Horton (4) who treated alkaline chars, such as black residue (from the soda pulp process for paper pulp), with acid and found the power t o decolorize was greatly increased. They attributed this improvement mainly t o neutralization of the negatively charged particles of carbon. The effect of acid on the steam-activated carbons has been studied, and the authors believe t h a t the improvement in decolorizing power must be attributed t o a combination of causes. The presence of a large amount of ash introduces some effects that may not be present in chars composed of fairly pure carbon. For example, decolorizing carbons appear t o be more efficient in acid than in alkaline solutions; consequently any free alkali in the ash would lower t h a t efficiency. I n t h a t case a n acid treatment would neutralize the alkali and allow the decolorizing carbon t o act more effectively.
1193
On the other hand, the acid may dissolve certain films of an inorganic nature and thus expose fresh carbon surfaces; however, it was found t h a t the increase in decolorizing power was almost the same, regardless of treatment with weak or strong acid. The explanation for the dissolving of surface coatings is therefore questionable. The data obtained in this investigation show that the acid removes a portion of the ash. Decreasing the ash content increases the relative amount of activated carbon and raises the decolorizing value. Early investigations showed that iron reacts with some of the organic compounds in sugar solutions to produce a darker color. Qualitative tests of the acid extracts from the carbons invariably showed the presence of considerable quantities of dissolved iron. All water-soluble iron is removed by the acid treatment to prevent possible contamination of the sugar solution by iron. Whereas the dissolving of surface films and the removal of excess alkali, inert ash, and iron are all part of the explanation, the largest single factor is probably a change in the electrical charge on the particles of carbon. Rideal (6) states t h a t "for substances which do not yield ions in solution, such as charcoal, the charge is always produced by the preferential adsorption of an ion from the solution, e. g., the hydrogen ion". The char adsorbs ions during the acid treatment and retains a portion of them throughout the washing and drying operations. The char then has a more positive charge, owing to adsorbed hydrogen, attracts the negatively charged color bodies, and produces better decolorization. Finally, when the pH is reduced, the apparent adsorption of color increases, owing to an indicatorlike color change of the sugar solution. Sugar solutions become lighter in color with increasing acidity.
Decolorizing Power of Carbons The sugar industry does not have a standard color solution, so that the decolorizing power of activated carbon is usually measured on the particular solution to be decolorized. Variations in testing procedure, depending on the local conditions, are found in different laboratories and refineries. The test used here is as follows: ' To 150 ml. of a yellow sugar solution in a 250-ml. Erlenmeyer flask add 1gram of Filter Cel. Heat in a flask to 90' C. in a shallow pan of water. Then add the decolorizing carbon and shake vigorously. Keep at approximately 90" for 30 minutes, shaking frequently. Then filter through ordinary filter paper, returning the first part of the liquid to the filter if cloudy. In filtering transfer the larger part of the Filter Cel t o the filter to form a retentive nonclogging layer on the paper. Filter directly into a 100ml. colorimetric tube, and adjust the level to exactly 100 ml. Then compare with a set of color standards. The color standards were made by heating several flasks of sugar solution with Filter Cel exactly as in the test, but with no carbon added. All were filtered and the filtrates combined. The combined filtrates were used to make up a set of twelve color standards by adding to the colorimetric tubes the following amounts of clear sugar filtrate: 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 ml. Distilled water was added t o each tube t o bring t o the 100-ml. mark. The tubes represented 0, 5, 10, 20 100 per cent of color remaining. The procedure was outlined by the Darco Corporation ( 3 ) . I n this work the color standards were preserved from fermentation and mold by the addition of 0.2 ml. formaldehyde to each tube. The color standards were stoppered and kept in the dark when not in use and would not change color for several weeks. The Darco Corporation recommends testing the carbon on the particular sugar solution available, allowing the operator t o use his own discretion as to the concentration of
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the sugar solution. For this investigation 15 per cent b y weight of Valentine's turbinado sugar in distilled water was adopted as the standard test solution; turbinado is raw sugar that has been washed and dried and has good keeping qualities. Formaldehyde was added as preservative in the preparation of a solution containing 2 ml. per liter. Since different lots vary in color, all decolorizing tests were made on turbinado sugar from a selected sack. The color of the standard sugar solution mas matched against Lovibond color glasses. A 100-ml. colorimetric tube was filled to the mark with standard sugar solution, which had a depth of 5'/~inches (14.9 em.). This tube was placed in a colorimeter and matched against a similar tube of distilled water and Lovibond color glasses. The color was approximately equal to a 30 yellow (N. T. 510) combined lvith a 4.5 red (N. T. 200). A dosage of 0.35 gram of a high-grade commercial sugardecolorizing carbon was required to remove 90 per cent of the color from 150 ml. of the standard sugar solution. Therefore, 0.35 gram was adopted as a standard dosage for all experimental carbons tested as detailed above. A carbon removing 90 per cent of color from the 15 per cent sugar solution using the standard dosage was considered acceptable No advantage was gained in determining exact dosages to remove 90 per cent of the color because each sugar solution has its own characteristics; what is applicable in one refinery is not in another.
Raw Materials
8 8
'
'
2 4
3
6 3 A
Fractions from two samples of coal-mine refuse and one sample of high-grade coal were used. The first refuse sample was from the Colta mine on the Mary Lee bed, Walker County, Ala. It was crushed to pass a 6/16-inch squareopening screen and then treated on a coal-washing table. The original sample contained 60.5 per cent ash. It was fractionated into five zones on the table, and the fractions from zone 1 with 21.1 per cent ash and from zone 2 with 41.8 per cent were used in the activation tests. The second sample of refuse was from the Corona mine on the Corona bed, Walker County, Ala. It had an ash content of 61.0 per cent and was separated on a coal table in a manner similar to the Colta refuse. It was divided into nine zones for better examination; then the first five zones were combined into one large sample called "Corona (zone 1-5)" with an ash content of 10.1 per cent. This material mas used for the activated carbon tests. Complete descriptions of the two refuse samples and detailed analyses of the ash of the various fractions were reported in the previous paper (d). A sample of a high-grade low-ash coal was tested to show what may be expected from that type of material. It was mashed coal from the Docena mine on the Pratt bed, Jefferson County, Ala., and had an ash content of 5.5 per cent.
Activation by Steam and Acid APPARATUS.The complete apparatus for activating carbon on a laboratory scale is shown in Figure 1. The large fuqedquarts tube in the middle of the chain is continuously rotated at about 2 r. p. m. by a motor-reduction drive. The furnace surrounding the quartz tube is heated electrically and maintained at any temperature up to 1000" C. by suitable resistances. One end of the quartz tube is connected directly through a stuffing box to the superheated steam supply; the other end is connected to the condenser and meter shown in Figure 1. The special stuffing boxes and fittings on each end of the quartz tube are virtually gastight, so that none of the oxidation.gases are lost before passing through the meter. The quarts reactlon tube has an overall length of 30 inches (76.2 em.) with an elongated bulb in the center 8 inches (20.3 cm.) long and 2 inches (5.1 cm.) in diameter, but the quartz lead ends are only 1 inch (2.5 cm.) in diameter. The bulb retains the sample in the central hot zone of the furnace and prevents spreading over the entire length of the quarts tube during rotation. It also tends to reduce the velocity of the hot
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1941
.
1195
slightly better and was adopted as standard. Hot acid treatment gases while passin over the h e l y powdered carbon and thus is about the same in effect as cold, but the operation is a little cuts down losses ofdust. faster with hot solutions. The technique of the acid treatment The steam is generated in the lower large horizontal tube of was as follows: A &gram sample of the steam-activated carbon the boiler assembly (Figure 1) by an electric immersion heater of was placed in a 400-ml. beaker, and then 200 ml. of 1N sulfuric 2000 watts capacity. The boiler is equipped with a water glass, acid was added. The mixture was stirred until every article of steam-pressure gage, and safety valve set to o en at 10 pounds the sample appeared to be wetted. The beaker anfcontents per square inch pressure. The steam is delivereito the apparatus were laced on the hot plate, brought to incipient boiling, and through small copper tubing thickly insulated with sheet askept i o t for 30 minutes. The suspension was stirred frequently bestos. However, as the flow of steam is not large, considerable during the 30-minute period, filtered, and washed with distilled condensation takes place in spite of the insulation. A T-fittin shown in the drawing serves as a drain to remove all condense8 water until no acid could be detected in the washings. The activated carbon was.dried at leaat 3 hours a t 110' C.; then it water collecting in the copper delivery tubing and prevent its was ready for decolorizing and other tests. entry into the quartz tube. The drain also serves as a pressure relief through the water seal, should any unexpected rush of steam occur. The globeshaped flask acts as a safety trap to prevent PULLEY suction of water from the water seal into the \ quartz tube in case of a sudden negative pressure. Some condensation did occur in the ends of the quartz tube protrudin out of the SPRING furnace, but the condensate was &ssi ated by I I the frequent use of a small Bunsen Eame on n n n n n those parts. The quartz tube has permanent brass fittings sealed on each end with a cement made of praster of Paris and 3 per cent acetic acid. The brass ends rest in split bearings, which may be raised or lowered by adjusting the slipsleeve bearing supports. A removable CEMENT brass fitting containing a carbon-ring stuffing box is attached to the permanent or fixed end PLUG by screws. Figure 2 is a detailed drawing of the drive-end fitting. 9' I . The steam from the boiler (Figure 1) passes into the quartz tube and over the finely powdered FIWRE2. FITTINGS OW THE DRIVEEND carbon in the reaction chamber where selective oxidation takes place. The exhaust gases are then passed through a water-cooled condenser to remove any excess steam and bring the gases to room temperature. As raw sugar solutions become lighter in color, when the acidity The permanent gases in the exhaust are measured by a precisionis increased, it was thought that the increased decolorizing power t y e wet test meter. Figure 3 is a photograph of the apparatus. of acid-treated carbons might be due entirely to the color change EOW-TEMPERATURN CARBONIZATION. A supply of the raw produced by an extremely small amount of residual acid adsorbed materials, either coal or coal refuse, was crushed to 10 mesh. by the carbons. This question was answered by running three Low-tem erature chars were made by heating 60-gram portions in standard decolorizing tests, one a blank in which no carbon was covered ikidmore iron crucibles of 180 ml. capacity. These added, another using a steam-activating carbon, and a third crucibles allow free escape of the volatile matter and minimize using the same carbon after acid treatment. The amount of oxidation of the contents. The charged retorts were placed in a color remainin and the pH of the filtrates were then determined. cold muffle furnace, and the temperature was slowly raised to The acidity o f the filtrate from the untreated carbon was in500' C.; then the heating was continued 30 minutes a t constant creased to equal that of the filtrate from the acid-treated carbon. If the increased decolorization from the acid treatment was due temperature. The low-temperature carbonization drove off large quantities of smoke, tars, and oils, which otherwise would entirely to residual acid, this should have decolorized the solution have tended to foul the discharge end of the activating apparatus. to the same point as the filtrate from the acid-treated carbon. All of the chars stopped smoking at least 10 minutes before the A slight bleaching of the color was noted, but it was small comend of the carbonization period. A cokelike residue was left in pared with the total difference in color between the two solutions, the retort, varying in physical ap earance with the coking charConversely, the acidity of the filtrate from the acid-treated acteristics of the coal. The cokearesidues were crushed to pass carbon was then reduced by adding a small amount of base until 10 mesh. Batches of 200-300 rams of the 10-mesh. material it equaled the pH of the filtrate from the untreated carbon. A were ground in a steel ball mill ofthe same size and design as the slight deepening of the color resulted. However, the following ordinary 8-inch porcelain jar mill, loaded with two-hundred 1-inch tabulation shows that the increased decolorization due to acid steel balls and rotated 45 minutes a t 40 r. p. m. Much of the treatment of the carbons cannot be explained by the change in final product passed a 325-mesh sieve. This was the material color due to change in acidity of the raw sugar solution: used to charge the activating awwaratus. .ACTIVATION WITH STEAM. A weighed charge of low-temperaStandard Test After Reversing p H ture char was placed in the quartz reaction tube and the tube Test % oplor P H of % oplpr pH of hT connected in the chain. The furnace was heated to 650' C. as ---mining filtrate remaining filtrate rapidly as possible, and then steam was turned into the reaction 1 none 100 6.2 100 6.2 chamber. Virtually no evolution of gas was noted with any of 2 Untreated 82 6.4 80 5.5 Aoid-treated the material below 850" C., and the use of steam at lower tem3 68 5.6 70 6.4 peratures would have served no useful purposes. After the steam was turned on, the temperature was raised to 900" C. and kept as Sugar refiners prefer to decolorize at a p H of 6.9-7.2 because constant as possible for the duration of the test. The quartz reaction tube was rotated at intervals to ensure even burning of higher acidity causes inversion of sugar. However, any exthe char. After a predetermined volume of gas had been process acidity due to the carbon can be removed b y the addition duced, the steam was shut off and the reaction tube closed tightly of a small amount of lime. All decolorizing tests could have while the furnace was cooled. The activated char was removed been adjusted to the same pH b y the addition of varying and dried several hours at 110" C. to ensure removal of any moisture taken up while the apparatus was cooling. The act;amounts of lime, but i t seemed preferable t o allow the changes vated carbon was then tested for ash content and for adsorption in pH due to each individual carbon to appear in the results. of color from sugar solutions. ACID ACTIVAGION.The decolorizing power of the steamactivated carbons can be increased by treating with acid. ExResults periments with sulfuric, h drochloric, and phosphoric acids showed all to be effective. dbfuric acid seemed best and is also The data of Table 1 are fisted in the order of increasing ash the cheapest, so it was used in treating the samples reported content of the steam-activated and untreated carbon, this behere, Various concentrations were tried, but exact strength of ing the order of increased amount of burning. Each series was acid is relatively unimportant, sulfuric acid of 0.25 normality is strong enough to give good results. Normal sulfuric acid was so planned that i t would show approximately the degree of
*
-
L°CKTNG
j
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U
TABLE I. DATAON 80-GRAM CHARGES O F STEAM-ACTIVATED DECOLORIZING CARBONS FROM LOW-TEMPERATURE CHARS Test No.
Total GasI Evolved, Cc.
1 16,226 2 20,671 3 25,117 4 27,326 5 30,158 6 32,989 7 35,170 8 39,021 9 45,307 10 49,130 11 53,802 High-grade commercial C Blank decolorizing test
7-Steam-Activated Carbons% color % ' phenol Mois- adsorbed p H of adsorbed ture-free by 0.35 augar by 30 filtrate p.p.m. C g. C ash, % A . From Colta Zone 1 Char with 30.4 0 6.4 56 33.2 0 6.5 65 34.7 0 6.5 68 37.1 6.6 74 5 39.2 70 5 6.7 40.9 10 6.7 70 42.8 10 6.6 60 45.6 13 6.7 55 22 53.8 6.6 50 61.1 40 35 6.7 74.1 30 2s 6.3
21.9
..
Acid Treatment% after color % Mois- adsorbed p H of adeorbed ture-free by 0.35 sugar by 30 g. C filtrate D . D . m.C ash, % 24,1y0Ash 28.5 5 5.7 55 10 66 31.0 5.6 13 32.5 5.6 68 33.7 18 50 5.6 36.8 20 6.6 72 38.4 25 5.7 65 40.1 5.7 63 28 41.8 62 30 6.0 48.4 55 38 6.0 57.6 48 5.9 50 71.0 38 5.9 3%
8.2
0
B.
14,130 19,822 3 22,654 4 28,317 25,485 25,372 7 33,980 8 34,745 High-grade commercial C
90
-Carbons
6.1 From Colta Zone 2 Char with 44.3% Ash
1 2
2.4 5.4
5.4 5.5 5.5 5.5 5.6 5.7
21.9 C.
20,077 26,!33 29,091 32,961 5 34,858 6 42,476 7 50,206 8 53,802 9 57,172 High-grade commercial C 1 2 3 4
28,317 33,980 39,644 4 42,476 5 45,307 6 50,971 7 59,466 8 65,129 9 67,961 10 72,945 11 79.288 High-grade commercial C 1 2
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Vol. 33, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
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21.1 23.4 24.6 25.2 26.7 33.1 39.4 42.7 53.4 21.9
90 6.0 From Corona Zones 1-5 Char with 157" Ash 0 6.8 55 16.4 8 72 18.2 6.8 12 6.8 18.3 75 19.2 77 15 6.9 17 19.3 7.0 SO 55 72 24.8 6.8 29.1 73 65 7.0 72 7.4 68 34.8 64 46.0 52 7.2
18 27 35 45 50
75 90 88 82
6.2 D. From Docena Washed-Coal Char with 6.9'33 Ash 9.8 5 6.5 .. 6.6 30 11.3 6.7 10 40 *. 8.0 12.0 25 48 6.8 7.4 14.0 6.7 40 58 9.0 45 15.2 6.8 ., 10.4 65 64 16.3 6.6 .. 11.6 68 19.6 13.3 76 60 6.8 .. 25.4 7.0 75 .. 17.8 86 34.1 6.5 82 25.3 .. 88 45.3 95 35.4 6.7 94 75.7 6.7 66 .. ..
5.9 5.8 5.7 5.7 6.2 6.0
50
52 66
43 35 25
50
58 70 75 86 84 80 75 50
90
.. ..
..
21.9
6.0 6.0 6.0
46 55
90
..
5.9 5.5 5.9 5.6 5.6 5.8 5.0 5.2 5.3 5.3
...
.. .. .. *. ..
..
.. .. ..
....
sugar solutions the carbon would have t o be burned much longer. The acid treatment had little effect on phenol adsorption. Series B gives data on carbons made from Colt& Zone 2 IOw-temDeratUre char. This material was t o i h i g h in ash to make a good activated carbon, but it shows what can be done when a large amount of ash-forming material is present. The effects of the ash are seen more clearly with a material of this kind. Theremoval of some of the inert ash by acid extraction increases the relative proportion of activated carbon in the sample and consequently should increase its decolorizing power. However, series B shows that the increase in decolorizing power due t o acid treatment is greater than can be explained on the basis of the amount of inert material extracted. For instance, in test 5 the carbon had an ash content of 64.2 per cent and removed 12 per cent color; this same carbon after acid treatment had an ash content of 62.3 per cent and removed twice as much color. Series C contains data on carbons prepared from Corona zones 1-5 refuse. The low-temperature char had an ash content of 15.0 per cent. The acid-treated carbon from test 7 had a decolorizing power equal to the commercial sugar carbon, although the ash was somewhat higher. This means that the pure carbon present must have been slightly more active than that in the commercial sample,
6.0
TABLE 11. COMBUSTION CALCULATIONS ON COLTA SUGARDECOLORIZING CARBONS (IN PERCENT) burning necessary t o prepare the most active carbon from that particular low-temperature char. For comparison the results obtained from a high-grade commercial sugar carbon are given for each series. This particular carbon has been widely used by sugar refineries for a number of years. The most active carbon from Colta zone 1 char was that from test 10 (Table IA) with an ash content of 61.1 per cent. It adsorbed 35 per cent of the color from the standard sugar solution. After being treated with acid, the same carbon removed 48 per cent of the color. From the total amount of gas evolved during the activation of each carbon, it is evident that the ash content increased consistently with each increase in amount of gas produced. Since the pH of the sugar filtrate from a blank decolorizing test was around 6.1, the results indicate that the untreated carbons made the solutions slightly more alkaline. This was probably due to some basic materials in the ash. On the other hand, the acid-treated carbons increased the acidity slightly. The data on phenol adsorbed give some idea of the value of each carbon for water purification. Test 4, series A, with a n ash content of 37.1, showed the maximum phenol adsorption of 74 per cent. This would be the best carbon for deodorizing water supplies, but for successful decolorization of
Ash in Carbon
Yie!da of Activated Material
Weight Lossb on Activating
Original Carbon BurnedC
Colta Zone 1
24.ld 30.4 33.2 34.7 37.1 39.2 40.9 42.8 45.6 53.8 61.1 74.1
76:3 72.6 69.5 65.0
61.5 58.9 56.3 52.9 44.8 39.4 32.5
2d:7 27.4 30.6
35.0 38.5 41.1 43.7 47.1 55.2 60.6 67.5
2?:3 36.1 40.2 46.1
69.0
7i:6 74.2 71.0 69.5 69.0
25.'4 25.8 29.0 30.5 31.0 35.8 42.3 48.2
64.2 57.7 76.8 51.8 85.5 % ash of low-temperature char poo* % ash of activated carbon b 100 minus yield. c Weight loss X 100 100 minus % ash of low-temperature char' d Low-temperature char before activation.
..0 0 0
5 5
50.7 54.2 57.6 62.1 72.7 79.8 88.9
10 10 13 22 35 28
4K6
..1
86.5
10 15 12 15 22 15
Colta Zone 2
44.31 59.4 59.7 62.4 63.7 64.2
Color Adsorbed
46.3 52.1 54.8 55.7 64.3 75.9
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The data in Table I1 on original carbon burned brings out an interesting relation between the Colta samples. When either char is fully activated, about the same amount of original carbon has been burned, the percentages being 75.9 and 79.8. The wide difference in yield, ash content, and de-
Series D gives results obtained by activating a low-temperature char from Docena washed coal. The series of tests shows results t o be expected when a high-grade coal is used as the raw material. The ash content of the low-temperature char from this coal was only 6.9 per cent. Test 10 gave the
-
FIGURE3. PHOTOGRAPH OF APPARATUS
most highly activated product, capable of adsorbing 95 per cent of the raw sugar color. The activity was considerably greater than that of the high-grade carbon selected as a basis for comparison. After this particular char was activated t o the maximum, the acid treatment did not increase the decolorizing power further. The demand for good decolorizing carbon is steadily increasing, and new uses are constantly being discovered. Many sugar refiners, a t present using bone black exclusively, believe that in the near future they will use activated vegetable carbon as a supplementary aid in the process. For instance, whenever an unusually dark batch of raw sugar is received, the excess color may be removed by adding an appropriate amount of activated carbon which throws a more constant load on the bone-char filters. The carbon would be used once and discarded; therefore, an inexpensive carbon would be desirable. However, decolorizing carbons cannot be produced so cheaply as might be supposed because of the extremely low yields. The low yield is an unavoidable result of quantitative physical relations involved in the activation process and not merely a question of needed improvements in the technique. Table I1 lists the yield calculations for both grades of Colta material. Since the ash was not consumed in the process and remained constant, and since there was practically no volatile matter in either the char or activated carbons, the yield of activated carbon and percentages of carbon burned can be calculated on the basis of change in ash content. Table I1 shows that, when the material from Colta zone 2 is activated to the maximum, the yield will be 57.7 per cent. The higher yield of Colta zone 2 is due to the large amount of ash (virtually inert) which makes the material considerably inferior to t h a t from zone 1. The carbon in both samples was activated t o about the same extent becpuse, on the basis of pure carbon present, the decolorizing powers are about equal: Ash, Per Cent 61.1 76.8
Carbon, Per Cent
Deoolorining Power
38.9 23.2
35 22
colorizing power of the two series can be attributed almost entirely to the difference in ash content of the low-temperature chars. Yield and combustion data for the carbon of maximum activity from each of the four low-temperature chars follow. These calculations are on the steam-activated untreated carbons. The yield percentages are on the basis of the low-temperature char used as the charge:
Material Colta 2 Colta 1 Coronal-5 Dooena
.
Low-Temp. ActiChar Ash, vated C % Ash, % 44.3 24.1 15.0 6.9
76.8 61.1 42.7 45.3
Yield,
%
57.7 39.4 35.1 15.2
OrigiColor nal C Cc. Gas/ Adsorbed, Burned, Gram of % % Char 22 35 68 95
75.9 79.8 76.4 91.1
1699 2457 2690 3647
These data indicate that the most active carbons were obtained from raw materials of low ash content. Moreover, the lower the ash of the raw material, the lower the yield at the point of maximum activation. However, the high yields are merely the result of accumulated ash which does not add t o the value of the product. Gases and volatile products given off would be greatest in amount from the lowest ash sample and, if collected and utilized, would help t o offset the low yield of carbon. The above data were recast to give carbon yields on the basis of the original coal: Material Colta 2 Colta 1 Corona 1-5 Docena
Original Coal Ash,
Yield
%
41.8 21.1 10.1 5.5
Ly;yemg. '
94.4 87.6
67.3 79.7
Yield Activated
c. %
Cc. Gas/ Gram of Coal
54.4 34.5 23.7 12.1
1604 2154 1811 2907
Only t h a t gas evolved during the activation process is given; the amount expelled during the low-temperature charring process was not measured. The gases evolved in these tests were not analyzed; but a typical analysis would be about 50 per cent hydrogen, 20 carbon monoxide, and 20 carbon dioxide.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1198
Vol. 33, No. 9
Acknowledgment
Literature Cited
This study was conducted a t the Southern Experiment Station of the Bureau of Mines in cooperation with the University of Alabama. The work was under the general supervision of 0. C. h l s t o n and W. H. Coghill, who made many helpful suggestions throughout the course of investigation. Specific acknowledgment is due M. A. McCalip of the field laboratory, of the U. S. Department of Agriculture a t Louisiana State University, and to A. G. Keller, in charge of the sugarhouse a t Louisiana State University, for their advice on the various testing methods used in the sugar industry. The authors wish to thank G. T. Adams, U. S. Bureau of Mines, for his assistance in planning and constructing the gastight rotary activating furnace used in this investigation.
(1) Bone, W. A., and Coward, H. F., J . Ciwm. SOC.,93, 1197-1265 (1908). (2) Broderick, S. J., and Hertzog, E. S., U. S. Bur. Mines, R s p t . Investigation 3548 (1941). (3) Darco Gorp., “Laboratory Manual for Use in Analyzing and Testing Decolorizing Carbon”, 1936, and “The Darcograph”,
1937. (4) McKee, R. H., and Horton, P. M., Chem. & M e t . Eng., 32, 56-9 (1925). (5) Mantell, C. L., “Industrial Carbon”, p. 192, New York, D. Van Nostrand Co., 1928. (6) Rideal, E. K., “Introduction to Surface Chemistry”, 2nd ed., p. 382, London, Cambridge University Press, 1930. (7) Zerban, F. W., J. IND.ENC.CHEW.,10,81617 (1918). Published by permission of the Director, U. S. Bureau of Mines. subject t o copyright.)
(Kot
Oxidation of Unvulcanized Rubber in Light J. T. BLAKE AND P. L. BRUCE Simplex Wire & Cable Company, Boston, Mass.
‘HEN unvulcanized rubber is exposed to light in the presence of air, it develops a tackiness which may pro-
W
ceed far enough to cause partial liquefaction of the rubber. The nature and intensity of the light is unimportant, although the rate is affected. The action is oxidation and is accompanied in many cases by peroxide formation and a Russell effect. This production of tackiness under the influence of light is susceptible to the action of organic catalysts, both positive and negative ( I ) . Many materials used regularly as antioxidants in vulcanized rubber are active agents in the development of tackiness. A number of other materials will effectively retard the development of tackiness. I n many cases they will suppress completely the positive action of these commercial antioxidants. Antioxidants appear, therefore, capable of playing a dual role-that of assisting rubber oxidation under some circumstances and that of retarding oxidation under others. This paper describes a method and apparatus for the quantitative study of the rate of oxidation of raw rubber. I n the previous paper ( I ) the tackiness of the raw rubber was of necessity judged wholely by appearance and feel. Although it increased with time of exposure t o light, in many cases it decreased again after having reached a maximum. We assumed that this subsequent decrease in tackiness was due to additional oxidation, but there was no proof. There was also no evidence that the development of the same degree of tackiness in rubber containing different materials represented the same degree of oxidation. The oxidation of vulcanized rubber is usually studied by exposure to air or oxygen a t elevated temperatures and frequently a t increased pressures. The decrease in tensile strength or elongation a t break is taken as a measure of the extent of oxidation, There have been attempts (3, 4,12) to follow the oxidation of rubber by determining the amount of
material soluble in acetone. It is assumed that unoxidized rubber is insoluble and that some of the oxidized rubber is soluble in this solvent, Stevens and Gaunt (IO) evaluated the deterioration of latex-impregnated fabric after exposure to light by determining the acetone extract. They found that phenolic materials and primary aryl amines have a protective action. Dufraisse (6) studied directly the rate of oxidation of vulcanized rubber. He contended that the rate of absorption of oxygen by a sample under given conditions is a truer measure of its probable service life than results obtained by the more common methods. He believed that the method offers practical advantages since the rate of oxidation may be ascertained in 2 or 3 hours instead of the many days required by some of the conventional accelerated aging tests. Milligan and Shaw (8) and Morgan and Naunton (9) studied the absorption of oxygen by vulcanized rubber in a manometric tube.
Apparatus The Dufraisse, Milligan and Shaw, or Morgan and Naunton types of apparatus were not believed entirely desirable. Oxidation in the former occurs a t a varying pressure. The rate of the development of tackiness is approximately the same whether a glass or a fused quartz container is used. On exposure to diffused daylight, the tackiness has the same characteristics but is developed more slowly, owing to the decrease in intensity of the light. It was concluded, therefore, that ultraviolet light was not essential and that a suitable apparatus could be constructed from glass. The apparatus (Figure 1) consists of a Pyrex glass tube of 20mm. bore, closed at one end with a ground-glass stopper. A horizontal tube of 3-mm. bore, about 75 cm. long, was attached t o the op osite end. The open end of the small tube was bent at right angTes and dipped into a mercury reservoir. The sample
