INDUSTRIAL AND ENGINEERING CHEMISTRY Lewis, W. X., and Luke, C . D., IND.E m . CHEM.,25, 725-7 (1933). Marsh, J. L., and Dodge, B. F., unpublished experimental data. Newton, R. H., IND. ENG.CREM.,27,302-6 (1935). Saddington, A. W., and Krase, N. W., J . Am. Chem. Soc., 56, 353-61 (1934). Sage, B. H., Lacey, W. N., and Schaafsma, J. G., IND.ENG. CHEM.,26, 214-17 (1934). Sisakind, B.,and Kasarnowsky, I., 2.unorg. allgem. Chem., 200, 279-86 (1931). Souders, M., Selheimer, C. W., and Brown, G. G., IND.ENO. CHEM.,24, 517-19 (1932). Van der Waals, J. D., and Kohnstamm, P., “Lehrbuch der Thermodynamik,” Leipzig, Johann Barth, 1912.
(21) (22) (23) (24) (25) (26) (27) (28) (29)
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Wan, S. W., Ph.D. dissertation, Yale University, 1935. Wiebe, R., and Gaddy, V. L., S. Am. Chem. SOC.,56, 76-9 (1934). Ibid., 57, 847-51 (1935). Wiebe, R., Gaddy, V. L., and Heins, C., Jr., Ibid., 55, 947-53 (1933). Wiebe, R., Gaddy, V. L., and Heins, C., Jr., IXD.ENO.CHEM., 24, 823-5 (1932). Ibid., 24, 927 (1932). Wiebe, R., and Tremearne, T. H., J. Am. Chem. SOC..55. 975-8 (1933). Ibid., 56, 2357-60 (1934). Wilson, T. A,, Univ. Ill., Eng. Expt. Sta., Bull. 146 (1925).
RECEIVED December 4, 1936.
Briquetting Coal with Sodium Silicate FOSTER DEE SNELL AND CYRIL S. KIMBALL Foster D.Snell, Inc., Brooklyn, N. Y.
T
HE use of sodium silicate as a binder for the manu-
facture of coal briquets has long been an interesting possibility. The material has good wetting properties against solids but has the obvious disadvantage of water solubility. The problem as developed through the literature has therefore pertained largely to that property. As would be expected in an art which has advanced much farther commercially in Europe than in the United States, the publications are largely from European sources, even though in many cases United States patents have been issued. The numerous ratios of sodium oxide to silicon dioxide in commercial grades of sodium silicate confuse the subject, since the ratio used is seldom quoted.
Literature Methods of briquetting coal may be classified as those without binder, those with organic binders, and those with inorganic binders. By far the most common are tar, pitch, or other forms of organic by-products. Inorganic binders include clay, lime, magnesia, and magnesia cement, and Portland, natural, and slag cements, as well as sodium silicate. The subject is further complicated by the use of sodium silicate with other binding materials. The outstanding advantage to be expected from an inorganic binder is that it will not alter the properties of the coal so far as smoking is concerned. When anthracite coal is used, a smokeless fuel should be obtained. Sodium silicate alone was patented for many uses, including the production of coal briquets (12), as early as 1866. Subsequently it has been used for lignite briquets (13),for sawdust briquets using 8 to 30 per cent of silicate (16), and in a 1 to 3 per cent solution of a 1:3.2 ratio for carbonized vegetable matter (14). The latter patent makes no claim of water resistance. Less than 5 per cent of a 1:3.2 silicate ratio is required, according to Hubbard (16). When a 1 :2 or 1:3 silicate ratio is used with alkaline nitrate, the briquet is reported to become insoluble on exposure (1, 20). Various other expedients have been used to render the sodium silicate insoluble, such as the incorporation of a major amount of calcium sulfate (5), or the addition of alkaline
A smokeless binder for briquets made by solution of silicic acid gel in sodium silicate appears to give properties quite different from those of a sodium silicate with the same ratio of sodium oxide to silicon dioxide. The briquets so made at a cost of $1.50 per ton for binder are moderately resistant when exposed to water. They show an increase of ash content under 4 per cent and no reduction in the fusion point of the ash. When made with anthracite culm, they burn completely, leaving an ash skeleton of the same shape as the briquet. Additional resistance to water can be obtained by pretreatment of the culm with a complex dispersion of aluminum palmitate or, to a lesser degree, by surface treatment of the briquets with a diluted form of the same dispersion. Such additional treatments increase the cost to a degree which is probably prohibitive commercially. earth hydrates or kaolin (31),lime and alkaline earth hydroxide and carbonate (24), or lime alone (9, 25). Calcium and magnesium compounds have been added to the coal before mixing with the silicate (3) or to the silicate before addition to the coal ( 2 ) . Others add ferric oxide and borax (26) or add acid to the mix before compressing ( 7 ) . Other additions to sodium silicate include dextrin @ I ) , lime and boiled starch or flour ( 8 ) , glutinous materials (4), crushed roots ( I @ , or coal tar (27). Another method of treatment has been by surface applications. Briquets bound with sulfite liquor have been surfacecoated with 15 per cent silicate solution (11). Others are waterproofed with silicate and iron oxide or manganese oxide (6). Spraying with potassium silicate has been practiced on briquets set with calcium sulfate ( I O ) . A combined binder of sulfite waste and silicate is treated with lime water (17).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Fixing silicate binders by a surface treatment with calcium or magnesium chloride (22) has been recommended. Briquets made from 1: 3.6 silicate ratio have been surface-treated with glue tanned with chromate (29). An attempt to use silicic acid is indicated by boiling it with wood moss (93). Another uses silicic acid and glucose obtained by mixing corn sirup, salt, and sodium silicate and acidifying with sulfuric acid (SO). The principal experimental work has been directed to the production of a smokeless briquet using anthracite culm, thus avoiding the smokiness resulting from a tar binder or the heating necessary to distill the volatile matter from the tar binder. The same binder has also been applied to other forms of fuel. Some of the data presented relate to uneconomical methods of manufacture. They were a necessary step in arriving a t an economical process experimentally. They are therefore presented for their technical rather than commercial value. The final remlts, believed to be economically feasible, are also given.
Specifications The following specifications were drawn up for the properties desired in anthracite briquets. They refer to pillowshaped briquets measuring 50 mm. (1.97 inches) on a side. They will require modification for application to other briquets: 1. Give a dry crushing strength not under 300 pounds. 2. Give a crushing strength after 24-hour direct exposure to continuous shower or soaking in water over 70 pounds. 3. Give a clean fracture when broken and not crumble when crushed either wet or dry. 4. Regain to within 50 pounds of the original crushing strength after 24 hours of continuous exposure to shower or soaking in water, followed by drying. 5. Give a maximum ash not over 18 per cent, of which not more than 4 per cent is from the binder. 6. Give a fusion tem erature of the ash from briquets not under that of original coa?
The Binder Preliminary work on various modifications of sodium silicate for the purpose will not be given in detail. Surface treatments after briquetting with silicate have the apparent disadvantage of failing to protect when the briquets are broken or even if a small part of the surface is injured. Work on surface treatment with calcium chloride also showed defects due to blooming. Sodium silicate can apparently be rendered insoluble by incorporation of a salt such as borax (26) which is essentially acid in nature. Some efforts in another laboratory to standardize the procedure to the degree requisite for large-scale use have been unsuccessful. The use of silicic acid as a binder proved unproductive because of lack of adhesion to the surface of the coal and lack of cohesion. Attempts to combine silicic acid with sodium silicate led to rather surprising phenomena as related to properties of the finished briquet. The binder (38) found most satisfactory is a solution of silicic acid in sodium silicate. This has a ratio of sodium oxide to silicon dioxide falling within the range of commercial silicates, but the properties of the binder are different from those of the commercial silicates. To prepare the silicic acid gel, dilute a sodium silicate (ratio, 1 : 3.25) to about 20' BB. This is a dilution of the usual commercial grade with an equal volume of water. Acidify it with dilute mineral acid such as 10 to 15 per cent sulfuric acid. Add the acid slowly with vigorous agitation to avoid the formation of lumps of silicic acid gel with free acid in the center. A thick pasty mass of silicic acid is formed by the time half the theoretical amount of acid has been added. At that stage discontinue the agitation and allow the gel to set. Break up this partially neutralized gel by agitation and add the balance of the acid necessary to complete the neutralization. Wash the gel with water until
725
the removal of free sulfate is substantially complete. The wet gel which separates contains about 16 per cent solid matter and is suitable for preparing the binder composition. Doubtless modifications of this technic would have produced more concentrated gels but there was no necessity for proceeding farther along those lines. To prepare the binder, mix approximately 85 parts by volume of 1:3.25sodium silicate with approximately 15 parts by volume of the wet gel. Heat in an autoclave to avoid loss of water and formation of an insoluble crust on the surface. Agitate during this heating which will usually take 30 minutes to an hour. The silicic acid dissolves or disperses in the solution, and the reaction is complete when the solution appears to be clear.
Preparation of Briquets For briquetting anthracite culm, mix 1part by weight of binder with 6 to 10 parts of culm and up to 3 parts of water. The water is added only to give the mixture a workable consistency. More than the maximum amount of binder specified will result in sticking to the molds; lesser amounts give bri uets which are porous and structurally weak. For briquetting lignite, oil-coke breeze, etc., the amount of binder may be increased. Bituminous coal requires about the same amount. Compress the mix into briquets at about ZOO0 pounds per s uare inch pressure. This has been satisfactorily carried out in a la%oratory press and also in a small rotary type of commercial machine. More than 7000 pounds pressure per s uare inch gives lowered ~ are crushed. strength, presumably because the C O particles Lower pressures give somewhat less strength, probably because the coal particles are more loosely packed. The briquet molds used were pillow-shaped with 50-mm. sides. Place the wet briquets in an oven a t 120' C . (248' F.) and raise the temperature over a period of 15 minutes to 220' C. (428' F.). Hold the briquets at that temperature for 15 to 20 minutes and remove them from the oven.
The results obtainable with this binder are shown in Table
I in contrast to those with commercial sodium silicates. The differences in water resistance are striking. When the briquets are removed from the oven, much of the air has been driven from their interstices. They must be allowed to cool before testing; otherwise water is soaked into them as into a sponge. A storage period of 24 hours was allowed before making these tests. The water immersions were made at room temperature for the requisite time. The shower test consisted of placing a single layer of briquets on a rigid wire screen and directing a needle shower against them from a height of about 600 mm. (24 inches). This was sufficient to avoid serious mechanical damage and to give greater effect than would be expected from rain. The crushing strength of the dry briquet is, in general, greater than that of coal itself, which is highly variable because of irregularities and cleavage planes. TABLEI. COMPARATIVE PROPERTIES OF COAL BRIQUETS MADEWITH SODIUMSILICATES AND WITH A SPECIAL BINDER Av. Crushing Binder Strength NazO: 2Si02 550 550 NanO: 3Sioz NazO: 3.25Sioz 450 NaeO: 3.86SiOz 450 Silicic acid gel 25 Exptl. without heating, 425 NazO :3.57SiOz Exptl. with heating, 450 NazO: 3.57SiOz
Disintegration after Water Immersion Within 1 hr. Within 1 hr. Within 24 hr. Within 24 hr. Within 24 hr. Within 24 hr.
None
Effect of Water Spray for 16 Hr. Washed away Washed away Washed away Soft and mushy Fragile Soft and mushy
No
appreciable erosion in 24 hr.
Representative data on the briquets made at 5500 pounds pressure per square inch show crushing strength of 400 to 500 pounds per briquet. Since they are pillow-shaped and about 50 mm. on a side, only a small area is in contact with the
726
INDUSTRIAL AND ENGINEERING CHEMISTRY
platens of the press used for crushing. The usual wet strength after 24 hours is about 75 pounds per briquet. This strength is such that they will stand a considerable amount of pounding before breaking and then break with a clean fracture rather than by crumbling. As a practical test a crushing strength of 60 pounds correlates with a strength such that a reasonably strong man can just break the briquet into halves. After soaking or exposure to the shower, the briquets dried at 150' C. (302' F.) gave a strength rather uniformly greater than that of the original briquet. The temperature of drying for measurement of regain was somewhat high but had been set by a tentative specification. By 24hour immersion in water the absorption, including surface moisture which could not be readily removed with a towel, was about 13 per cent. The corresponding figure when exposed to the shower for 24 hours was about 0.5 per cent higher. The absorption of moisture does not correlate particularly well with the wet crushing strength. In spite of the use of sulfuric acid in precipitation of the silicic acid gel, no noticeable increase in sulfur content over that of the culm was found. This indicates that it is washed out of the gel with surprising ease. Using a culm with an initial ash softening point of 1290' C. (2354" Fa),this was raised 10' C. (18' F.) by use of the binder. The same effect is obtained with commercial silicate of the same sodium oxidesilicon dioxide ratio. The final ash softening point of 1355' C. (2471' F.) was not, appreciably affected.
Burning Properties The following are typical of results obtained in burning of the briquets. The comparison quoted is of buckwheat coal and briquets because that form of coal is the nearest to the culm that is used commercially and because buckwheat coal had been in use with the boiler under test. When firing with a hot water supply boiler having an 800gallon rating and a 16 X 20 inch firebox, the briquets burned freely. After 3 hours there was no ring of dead ashes around the fuel box as would often be the case with buckwheat coal. It was not necessary to use a slice bar to provide sufficient draft for free burning; in fact, the draft was not noticeably reduced during burning. No clinkers formed. The ash which fell through the grate was soft and fluffy and shaking was necessary only a t infrequent intervals. Even with the firebox loaded to capacity, the draft was satisfactory. Similar use of buckwheat coal would so choke the draft that combustion would be unsatisfactory. In normal operation with buckwheat coal, the fire requires shaking and refueling every 2 hours. A charge of briquets was estimated to be good for 4 hours between fuelings. The fire was allowed to burn nearly out and was drawn. The ash showed no tendency to fuse and adhere to the grate. The particles of ash which had not been broken in withdrawal could usually be crushed by hand. A few small clinkers were found in the ash. This behavior contrasts sharply with the usual ash from buckwheat coal in the same firebox which is normally hard and somewhat clinkered. Such briquets burn in open fires with no more smoke than normally comes from anthracite coal. If not disturbed they burn completely to leave a gray skeleton of ash somewhat smaller in size but the same shape as the original briquet, without unburned core. This skeleton breaks down readily to give a finely divided and compact ash.
Pretreatment of Culm The admixture of materials with the binder to obtain an additional degree of waterproofness was not successful. Positive results were obtained by a roundabout route, applying the material to the culm and drying before treatment with
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the binder. A suitable agent for this purpose (999)was essentially a dispersion of 3 per cent aluminum palmitate and 2.5 per cent glue in water. Similar results can be obtained from a corresponding aluminum stearate dispersion produced by modified methods. When applied to the surface of the coal and dried before mixing with the binder, the waterproofing agent seems to dissolve slowly and disperse in the binder. By this pretreatment of the culm the wet crushing strength can be increased to an average value of over 200 pounds. The regain on drying is not increased.
Surface Waterproofing Relatively little experimental work was carried out on surface treatment of the briquets for improving their water resistance. The use of various dilutions of the waterproofing agent reduced water absorption about 2 to 3 per cent. The optimum results were not obtained with the most concentrated solution but at 10 to 15 per cent of the concentration of the original solution. Since the waterproofing agent is effective by surface repulsion rather than by mechanically filling the pores, this is not surprising. Data were reproducible but are not of sufficient interest to justify publication in detail.
Costs The cost of 12 per cent binder is about $1.60 per long ton of briquets. This is not an unreasonable cost, although greater than for tar briquets. The cost of preliminary treatment of the culm with the waterproofing agent discussed varies, according to experimental amounts used, from 50 cents to $1.60 per ton, in addition to the cost of the application and extra drying operation. The cost of surface treatment of the briquets with the waterproofing agent is only a few cents per ton in addition to the cost of the extra drying operation.
Literature Cited (1) Baker, H. M., U. S. Patent 446,505 (1891). (2) Boschan, Gyula, Hungarian Patent 103,142 (1930). (3) Ibid., 105,084 (1933). , Burnes, C. A., British Patent 328,082 (1929). I Chaills. Francois. U. S. Patent 697,234 (1902). Cornmelin, E., and Vian, R., French Patent 453,921 (1912). Cory, W. H., U. S. Patent 332,497 (1885). Crondace, G. L., Ibid., 828,999 (1902). Daddow, S. H., Ibid., 150,537 (1874). Drawbaugh, Daniel, and Gamble, B. E., U. S. Patent 867,918 (1907). Fekete, Deysti, Hungarian Patent 106,725 (1933). Fleury, A. L., U. S. Patent 61,931 (1866). Ganssen, Robert, German Patent 412,556 (1922). Hart, A. M., U. S. Patent 1,668,643 (1928). Hertig, Hans, Swiss Patent 142,960 (1930). Hubbard, Walter, British Patent 11,610 (1889). Hyogoya, Kisomatu, Japanese Patent 91,804 (1931). Klint, T. E. v. G., Swedish Patent 80,800 (1934). Kulhmann, French Patent 776,364 (1935). Lake, H. H., British Patent 2900 (1891). Leadbetter, J. W., U. S. Patent 909,626 (1909). Mitohell, A. M., Ibid., 1,029,022 (1912). Moerath, J. N., Ibid., 495,679 (1893). Mukoyama, Mikio, Japanese Patent 96,316 (1932), Roettgerr, W. C. A., U. S. Patent 190,724 (1877). Rouse, Thomas, Ibid., 1,109,704 (1914) and 1,381,748 (1921). Sewell, C. U., Ibid., 1,920,327 (1933). Snell, F. D., U. S. Patent 1,995,366 (1935); British Patent 394,572 (1933); French Patent 760,774 (1932) ; Belgian Patent 393,159 (1932); German Patent 619,244 (1935). Snyder, G. W., U. S. Patent 1,109,799 (1914). Taggart, W. P., Ibid.,1,396,603 (1921). Voinchet, A. L. J., and Lercin, A. A., French Patent 644,226 (1927). REOXJIVED February 8, 1937. Presented before the Division of Gas and Fuel Chemistry a t the 93rd Meeting of the American Chemical Society, Chapd Hill, N. C., April 12 t o 15, 1937.
