OPTICAL CRYSTALS O F SODIUM N I T R A T E POT.4SSIUXI
BROMIDE,SODIUal
CHLORIDE, AND LITHIUM
FLUORIDE
SYNTHETIC OPTICAL CRYSTALS 13. C. KREMERS The Harshaw Chemical Company, Cleveland, Ohio
NTIL the last decade several gaps existed in the total
nurked out with existing materials. Many desirable natural optical crystals are limited in size and quality. It is obvious gamut of electrical vibration. Cosmic rays had not that if a new type crystal, such as lithium fluoride, becomes been measured. Ultrashort x-rays were impossible. available in adequate size and quality, a new set of optical Even the comparatively long Hertzian waves had not all been combinations a t once becomes possible. measured. Today, however, modern tools in the hands of For the past two decades or more, efforts have been made to physicists and chemists have closed all prel-iously existing grow various optical crystals in the attempt either to duplicate gaps, and it is probable that the cosmic ray region will be exor augment natural crystals. The various alkali halides have ploited into even shorter wave lengths. Of the total seventy been produced in sizable crystals by a number of investigators octaves now known (Figure l), only one is in the form of (6, 9, 13, 18). I n 1927 Ramsperger and Melvin (13) grew visible light. Likewise, by a strange coincidence noncrystalseveral halide crystals, -and line transparent materials, determined many of their opsuch as the glasses, transmit tical and physical properties. rays only slightly beyond this Present-day advances in ultraviolet and inThey used cylindrical plativisible range. Penetration frared spectroscopy have brought about the num crucibles, placed in a into either the ultraviolet or need for more and larger optical crystalline rather close-fitting electric infrared ranges requires crysfurnace. Temperatures and talline o p t i c a l m a t e r i a l s . materials than are found in nature. Lithgradients were controlled by Furthermore, as transmission ium fluoride crystals up to 8 pounds in a set of multipIe heating elelimits of glasses are apweight are grown. Optically similar to m e n t s i n d i v i d u a l l y conproached, dispersion becomes fluorite, lithium fluoride can effectively trolled. poor. Crystalline materials replace this limited natural supply. SoCrystals of sufficient size such as quartz, fluorite, rock were grown so that most of salt, sylvite, etc., hare good dium chloride crystals, up to 25 pounds in the important optical properdispersion as transmission weight and superior to natural rock salt, are ties could be determined. limits are approached. grown. Potassium bromide and potassium Lithium fluoride was found The status of optical science iodide crystals for far-in frared transmission to possess properties unique today is limited by available enough to stimulate further are also grown. optical materials. The introinvestigation by many duction of special optical Natural calcite of optical quality is scarce, workers. Schneider (17),emglasses has been a great help. and the increasing demand for polarizing ploying the well known BridgRemarkable strides have been optics has stimulated the development of man method ( 2 ) ,grew several made in producing glasses producing large single crystals of sodium lithium fluoride crystals of more transparent to the near high quality and made a nitrate. ultraviolet. However, for careful study of the optical spectroscopic use in either of All of these optical crystals are grown properties in the extreme the invisible ranges, crystalfrom their molten salts under carefully ultraviolet. The Bridgman line optics only will suffice. controlled gradient and temperature. A method consists of meclianiIt is likewise obvious that 25-pound crystal requires some 10 days to cally lowering a platinum most of the optical combinac r u c i b l e c o n t a i n i n g thr grow and at least that long to anneal. tions possible have been
U
1478
NOVEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
molten salt through the core of an annular tube furnace. Crystals u p to 4 cm. in diameter were grown by this method. The present commercial method employed is based upon the technique developed by Stockbarger (20). The Bridgman technique was modified by using two annular tube furnaces separated by a close-fitting metal diaphragm. The cylindrical platinum crucible containing the molten salt is lowered through the diaphragm and crystallization takes place a t the high gradient maintained at this point. This procedure brings about several controllable factors not obtained by previous investigators. The upper furnace is held above the melting point of the salt but not high enough to cause excessive vaporization. The lower furnace temperature is adjusted sufficiently below the melting point of the salt so that when the crucible is lowered through the diaphragm, freezing or crystallization takes place in the region of high gradient between the two furnaces. The method thus constitutes a so-called continuous crystallization process, with the rate determined by the speed of lowering the crucible. A considerable refining of the salt is effected, and crystallization takes place in tlie region of high gradient. The upper and lower furnace5 thus also serve as reservoirs for the molten and solid salt. Figure 2 shows a furnace and controls for growing 8-inch (20.3-cm.) crystals. The crucibles used are constructed of the purest grade of platinum. Side walls are 0.003 to 0.006 inch (0.0762 to 0.1524 mm.) thick. The bottom cone-shaped section is slightly thicker. After crystallization is complete, the temperature is again brought up to n ithin 50'' C. of the melting point of the salt, and the crystal is annealed for a week to 10 days. I n the earlier stages of this development, crystals were annealed in their crucibles, and when cold, the tightly adhering platinum had to be ripped off (Figure 3 ) . Later a method was developed (19) wherebv the crystal is melted out of the. crucible aftet cryatallizaticm is complete. I n this method the crucible with the crystal is quickly transferred to a special electric furnace heavily ti our id with resistance wire to provide a large input of heat. The crystal and crucible are placed in an inverted position, and the furnace temperature is held slightly below the melting point of the salt for several hours. The maximum input of current is turned on, and shortly the crystal is released from the crucible. The method is similar to the removal of ice cubes from a refrigerator tray. T i t h a pair of preheated tongs the crystal is then transferred to the annealing furnace. By this meltingout method, the platinum crucible is saved and, after reshaping, is again put into service (Figure 4). Crystals u p to any diameter can theoretically Le grown by this method. ilctually &inch (20.3-em.) crystals are being produced, and it is probable that 10 or 12 inches (25.4 or 30.5 cm.) in diameter constitutes the practical limit. Depending upon size, up to 10 days are required to grow a crystal, and during this entire period all mechanical operations, such as lowering of the crystal, temperature control, variation in
1479
graclient. etc., must be under absolute inaiiual and automatic control. Temperatures of the crystallizing furnace.: varv by not m w e tlian 0.1" C.
Li thiuni F1 itoridt. L-dike the other alkali fluoricles, lithium fluoride is practically i n d u h l e , and adyantage is taken of this fact in the purification of the salt. Since ultraviolet transmission is a fuiiction of Ion- molecular m-eight, all heavy metal impurities mrist be carefully remoretl. This may he done by several methods. Schncider (17) recrystallized the nitrate several times and then converted the salt to fluoride, using platinum vessels t'liroughout. Stockbarger (270) took advantage of the fact t'hat with carbon dioxide under pressure, lithium carbonate can be converted to a bicarbonate or even a sesquicarbonate; most of the heavy metals are left in the form of insoluble carbonates. The latter method has been successfully used with an additional step i n n-hich last traces of heavy met'al are removed as sulfide. Single crystals of lithium fluoride up to 4000 grams in nright are growi. Contrary to many previous reports, single
1480
VOL. 32, NO. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
in the ultraviolet were worked out by Melvin (10) and later verified b y S c h n e i d e r (16). Hohls (6) e x t e n d e d these measurements to the limit of transmission in the infrared. The refractive index data of these various workers is in excellent agreement, and the complete index curve f o r l i t h i u m fluoride over its entire transmission range is shown in Figure 5. The absorption coeff i c i e n t s for lithium fluoride in the ultraviolet was determined b y S c h n e i d e r (17). Hohls (6) and Barnes (1) d e t e r m i n e d t h e infrared characteristics of l i t h i u m f l u o r i d e (Figure 5, lower graph). The transmission, T , is represented by the usual expression,
T = (1 - r)2e--Qd where T = reflection coefficient at normal incidence a = absorption coefficient d = thickness
FIGURE 2. FURNACES AXD COXTROLS FOR GROWISG OPTICAL CRYSTALS
This curve shows that for low ultraviolet work lithium fluoride must not only show transmission approaching the theoretical value, but also it is desirable that the size of
characteristics over the range from 2300 to 14,000 A.
Infrared Spectroscopy The importance of the infrared spectrograph as a tool in the study of molecular structure has already been demonstrated in both the inorganic and organic fields. The petro-
FIGIZRE 4. A 2 5 - P O U X D ROCKS.4LT CRYsT.4L AND PLATINUM FIGURE 3. PLATINUM CRUCIBLE A S D LITHIUM FLUORIDE CRUCIBLEI X WHICHIT W a s GROWK CRYSTAL WITH PLATINUM CRUCIBLE PARTIALLY REMOVED
NOVEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
1481
The availability of these exceptionally large prisms has stimulated the designing and construction of infrared spectrographs, previously considered impossible.
Potassium Bromide
Wave/engfth
-u
FIQURE5. REFRACTIVE INDICESAND ABSORPTION COEFFICIENTS OF LITHIUM FLUORIDE, SODIUM CHLORIDE, AND POTASSIUM BROMIDE
leum, gas, rubber, plastic, and allied industries are finding the infrared spectrograph the most convenient tool for research as well as control. Applications in the field of astronomical research are already giving us data on the composition of the planetary atmospheres. In biophysics we might mention the study of carbon dioxide respiration in both animal and plant life as being most conveniently measured by the infrared spectrograph. Depending upon the availability of proper refracting materials, this science may assume an importance comparable to modern x-ray analysis. Up to the limits of their transmission, prisms are usually preferred to gratings in infrared spectroscopy. Gratings produce a multiplicity of spectra both to the right and left of the undeviated image, with a consequent loss of energy. The problem of multiple image formation has in part been overcome by the echellete grating. Prisms will, however, conserve all of the energy of a given frequency in a narrow strip of a single spectrum. With larger prisms made available, the input of energy can be multiplied several times with consequent sharper definition of recorded energies.
Potassium bromide has optical characteristics similar to rock salt. Having a higher molecular weight, it transmits farther into the infrared. Crystals up to 25 pounds are grown. Potassium bromide crystals are considerably more difficult to grow than either rock salt or lithium fluoride. The refractive indices of both rock salt and potassium bromide have been accurately determined by a number of workers. The values of Melvin (IO) in the range 0.2-0.6 micron and those of Hohls (6),Rubens (IC), and Paschen (1.9)for the range 0.4-15 microns were selected for the points from which the index curve for sodium chloride is drawn (Figure 5 ) . For potassium bromide the indices as determined by Melvin (IO) for the range 0.21-0.6 micron and those of Gundelach (4) for the range 0.8-28 microns were used in drawing the index curve in Figure 5 . The absorption coefficients for both sodium chloride and potassium bromide in the infrared were determined by Mentzel(11). The values for the curves in Figure 5 were calculated from these data, using the foregoing transmission formula. The absorption curves show that a rock salt prism is effective to about 16 microns and a potassium bromide prism to about 25 microns. Several workers in infrared spectroscopy have found i t advantageous to have several interchangeable prisms available so that each spectral region can be covered by a prism material of the desired dispersion and transmission.
Potassium Iodide Because of its high molecular weight potassium iodide will transmit to about 40 microns and can be grown under conditions similar to potassium bromide. Owing to incompleteness of data, neither index curves nor absorption data are given a t this time.
Rock Salt Rock salt has long been afavorite for infrared spectroscopy. It shows excellent dispersion over its entire transmission range. It has been difficult, however, to obtain natural rock salt crystals of sufficient size and quality to provide the size of optics desired. For the last few years the Russian supply has been entirely cut off. Synthetic rock salt or sodium chloride crystals are grown up to 25 pounds (11.3 kg.) in weight (Figure 6). Asaresult, 60" prisms, 4 to 5 inches (10.2 to 12.7 cm.) tall with 6-inch (15.2-em.) faces, are now available. They are probably the largest rock salt prisms ever constructed. The quality of this synthetically grown material is superior to natural rock salt.
FIGURE 6. A 25-pOU;YD SINGLE CRYSTAL O F ROCKSALT FROM WHICHTHE LARGEST PRISM BLANK EVERMADEWAS CUT
The problem of purifying sodium chloride, potassium bromide, potassium iodide, and similar soluble alkali halide salts for crystal growing does not present serious difficulty. I n many cases commercial reagent-grade salts are satisfactory for use without further purification. Aqueous recrystallization will frequently assist in raising the quality of the salt used, but by far the most efficient method yet tried is recrystallization from the molten state in platinum vessels.
INDUSTRIAL AND ENGINEERISG CHEMISTRY
1482
Sodium Nitrate The acute shortage of calcite in the larger sizes has for several years stimulated the search for a proper substitute. Sodium nitrate, crystallizing in the same crystallographic system as calcite, has for years been a potential substitute. Single crystals up to 25 pounds in weight are grown in this furnace setup. These cry5talh are also grown from their molten salts.
VOL. 32, NO. 11
crystal is embedded in plaster of Paris, and the mouiited crystal is then clamped in proper position in the vise of a small power hack s a x (Figure 7). A plain copper blade and 320-mesh emery in water as abrasive are used. The total weight on the saw while cutting should not be over 1 kg. After the crystal has been mounted, the exposed surface is also covered with plaster of Paris BO as to cut down all vibration. Polishing lithium fluoride presellts no unusual difficulties, once the characteristics of the material are understood. Toward the end of the operation a soft abrasire must be used. Both tin oxide and titanium dioxide have been found effective. Too rapid a polishing operation tends to pull out fragments of crystal and leave pits. This is unlike natural crystals such as calcite, fluorite, etc. ' Water-soluble crystals are cut with a wet endless string (Figure 8) ; the most satisfactory type is a soft cotton warp. The ends can be joined with ordinary waterproof belting cement. Sodium chloride, potassium bromide, potassium iodide, etc., present no difficulties. The machine illustrated will cut optical blanks well within the tolerances usually demanded by lens grinders. I n the case of sodium nitrat'e and other water-soluble crystals possessing negative heats of solution, the cutting string is heated with a small torch just before entering the crystal. This procedure il; usually necessary to prevent such types of crystals f ~ o mrrarking.
FIGURE 7. POWER SAWFOR C U T ~ XLITHICM G FLUORIDE CRYSTALS
Sodium nitrate is one of the most difficult crystals to grow. Because of its low melting point (308' C.) high gradients cannot be maintained. Likewise, any crystalline material other than isometric tends to grow faster in certain directions. I n the case of sodium nitrate (trigonal system) crystallization is more rapid in the direction of the optic axis. The purification of sodium nitrate presents unusual problems. Aqueous recrystallization effects very little purification because many of the impurities have practically the same range of solubility. Recrystallization from the molten state is satisfactory. Oxidizable material, such as iron salts, is best removed as sludges by maintaining the salt in the molten state for several days. I n a comparison of refractive indices of sodium nitrate and calcite, the former shows greater dichromatism. The indices of these two materials are compared for characteristic wave lengths. Wave Length,v 434 501 589 668
---Calcite ( 7 ) y s o d i u r n K i t r a t e (8)Extraordmary Ordinary Extraordinary Ordinary ray. W ray, E ray. W rw, E 1.6126 1.340 1.67352 1.49428 1.5968 1.337 1.66604 1.48992 1.5848 1.336 1.65836 1.48641 1.48436 1.5783 1.334 1,63381
Cutting arid Polishing The cutting and polishing of lithium fluoride for optical purposes presents no unusual problems. For cutting, the
WET STRISGSAWFOR CUTTING WATERFIGUIIE 8. ENDLESS SOLUBLE CRYSTALS
As the uses of these crystals in special fields of optics become better known, it is expected that many other types will be developed.
Acknowledgment The author wishes to express his appreciation for the interest and cooperation of executives of The Harshaw Chemical Company. Many tedious and time-consuming tasks were materially lightened by the able assistance of Ralph McNabney.
NOVEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited Barnes, R . B., J . Optical SOC.Am., 28, 140 (1938). Bridgman, P. W., Proc. Am. Acad. Arts Sci., 60, 305 (1925). Cartwright, C. H., J. Optical SOC.Am., 29, 350 (1939). Gundelach, E., Z. P h y s i k , 66, 775 (1930). Gyulai, Ibid., 46, 80 (1927). Hohls, H. W., Ann. Physik, [5] 29, 433 (1937). International Critical Tables, Vol. VII, p . 24, New York, MoGraw-Hill Book Co., 1930. Ibid., Vol. VII, p. 26. Kyropolous, 2. anorg. albem. Chem., 154, 308 (1926). Melvin, E. H., P h y s . Rev., 37, 1230 (1931). Mentzel, A,. Z.P h y s i k , 88, 178 (1934).
(12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
1483
Paschen, Ann. P h y s i k , [1]26, 120 (1908). Ramsperger and Melvin, J. Optical SOC.A m . , 15, 359 (1927). Rubens, Ann. Physik, [N.S.] 54, 476 (1895); 60, 724 (1897). Schneider, E. G., J . Optical SOC.Am., 27, 72 (1937) Schneider, E. G., Phgs. Rev., 45, 152 (1934). Sohneider, E. G., Ibid., 49, 341 (1936). Slater, Proc. Am. Acad. A r t s Sci., 61, 136 (1925). Stockbarger, D. C., J . Optical SOC.Am., 27, 416 (1937). Stockbarger, D. C., Rev. Sci. Instruments, 7, 133 (1936). Stockbarger and Cartwright J. Optical SOC.Am. 29, 29 (1939).
PRESENTED before t h e Division of Physical a n d Inorganic Chemistry a t t h e 99th Meeting of t h e American Chemical Society, Cincinnati, Ohio.
Wet Milling of Corn Recovery and Utilization of Process Losses In the early days of the wet milling of corn, one of the largest plants in the industry lost dry-substance materials in its various process waters which were sewered, and caused a pollution load of the factory effluent equivalent to about 400,000 persons (in population equivalents) per 24 hours. This has been reduced to a n average of below 40,000 persons during 1938 w-itha lowest figure achieved during the last half of the year 1936 of 22,000 persons. Reviewing the work of the several investigators during this period, the greatest reduction and consequent recovery of product losses was accomplished through the recirculation and countercurrent flow of starch and gluten wash waters. Instituting a department to control losses continuously, devising a scheme for recovering volatile losses, study and subsequent improvement of processes and equipment, and a general survey of all openings to thc sewers from process departments contributed to this reduction with a corresponding saving of materials and products for a financial gain in all instances. Present work involving the recovery of ethyl alcohol and other volatile substances and the treatment of other process waters indicates that it may be possible to reduce the pollution load below 20,OOO persons and perhaps to less than 10,000, should these recovery processes work out as well in practice as they have in the pilot-plant stage.
I
N 1910thisindustry became conscious of the material losses in its process (5) because of the objections of residents in the vicinity of a plant located a t Waukegan, Ill., which disposed of its untreated process waters directly into Lake Michigan. The condition was improved by treating the waters with lime and iron sulfate and filtering to remove the solids, but the economic importance of these trade wastes did not receive significant attention until a study was started in 1920 by the Chicago Sanitary District in cooperation with the Corn Products Refining Company at the factory located a t Argo, 111. (3) Laboratory facilities were established on the factory premises, and an extensive investigation was car1
Ind
Present address, Yational Starch Prodrirts C o m p a n y , I n c
, Indianapolis,
-4. L. PULFREY’, RALPH W. KEKH, &NDH. R . REINTJES Corn Products Refining Company. Argo, 111.
ried out between 1920 and 1928. Before reviewing the results of their work, a brief description of this 24-hour continuous process is essential. Air-cleaned shelled corn is steeped in a weak solution of sulfurous acid. The corn is then milled, and the material is separated into the following fractions: starch, germ, gluten, fibrous material, and soluble solids. Dextrins, gums, food starches, corn sirups, dextrose sugar, and many other products are manufactured from the starch. Though zein and other products are derived from the gluten, the greater part is either combined with the soluble solids and fibrous materials or prepared directly as an animal feeding material. The entire process of steeping, grinding, and separation of the above components is a wet-milling process and all materials are handled as slurries to the last manufacturing steps. Deep wells furnish process waters. Before 1920 only the countercurrent, water used in the steeping operation was evaporated for the recovery of the solids, and even this practice was not followed a t all plants in the industry. Other process waters were sewered, which caused the total effluent losses reported by Mohlman and Beck (4) as amounting to over 2 per cent of the total dry substance ground. The pollution load of the entire factory effluent ranged from 250,000 to about 400,000 population equivalents per day ( 3 ) . “Bottling up” those process waters originating from the starch washing and the gluten settling processes, by recirculating and eventually concentrating them with the steep waters in the vacuum evaporators, contributed the first substantial reduction in the pollution load. Closer control of the escaping solids by automatic sampling, regular reports of excessive entrainment in the evaporation processes, and the recirculation of starch and of gluten wash waters reduced the pollution load to a range of 50,000 to 100,000 population equivalents (4) by 1928 and continiied in this range through 1935 (Table I). To concentrate all wet-starch process solubles eventually as steep water, however, involves two problems, one related to the other. First, efficient evaporation of the water in gallons of water evaporated per pound of exhaust steam must be
