MAY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
used to reduce the heat required for the evaporation process. In such a combination the relatively simple submerged combustion melting tank produces 46.1 per cent of the sodium sulfate whereas the multiple-effect system produces the remaining 53.9 per cent. Calculation shows that the heat requirement of a triple-effect evaporator working on this saturated solution is about the same as the submerged combustion melting tank, per gram of sodium sulfate produced.
Submerged Combustion for Heating When submerged combustion heating is used, evaporation is always taking place as the products of combustion leave saturated a t the temperature of the liquid. At lower temperatures very little water is carried out with the gas; in fact, water may be given up to the solution by the water of combustion. This fact allows submerged,combustion heating to utilize the gross heating value of the gas or permits an efficiency of over 100 per cent based on the net heating value of the gas, a value which astounded early workers and cast some discredit on their calculations. The boiling point of the liquid is that at which the heat of combustion is equal to the heat of evaporation of the water carried out a t that temperature, plus any other thermal effects. If the combustion reaction in which methane is burned with the theoretical amount of air in water is considered, the fraction of the heat available for heating purposes may be calculated. The results are shown graphically in Figure 7; below 50” C. more than the net heating value of the methane is available for heating purposes. As the temperature of the water rises, more of the heat is used for evaporation and less is available for heating. The amount of water evaporated by this combustion reaction increases rapidly with temperature. For sodium sulfate recovery, the optimum temperature of operation is the lowest possible, for here the greatest proportion of the heat available is used for heating the solution. The heat used for evaporation is not wasted, since sodium sulfate is deposited from the saturated solution, but the heat is used in a less efficient manner than in the heating cycle. Discussion of Results The submerged combustion cycle shown in Figure 6 was designed primarily to operate on natural Glauber’s salt where the cost of the raw material is small, ample outside storage space is available, and large crystallizing ponds may be constructed to care for the liquor from the evaporators. Evaporation either by submerged combustion or tube evaporators may be used on this liquor so that recrystallization is avoided. However, it has been definitely shown that the heat requirement per unit of sodium sulfate precipitated is lowest when the decahydrate crystal is melted, so that, whenever these crystals are available, a melting process should be used rather than a solution process to produce a saturated solution. Submerged combustion offers a ready solution to the technical difficulty of melting these crystals. Previous runs made for the submerged combustion of water and sulfite waste liquor (6) showed thermal recoveries of 92 to 96 per cent. The runs on sodium sulfate solutions gave thermal equilibrium between the liquid and the gases leaving, so that efficiencies should remain equally high if the evaporator body is insulated. Gaseous fuel was used in this work since the necessary equipment was already available; however, fuel oil can also be adapted to submerged combustion. Literature Cited (1) Badger, W. L., and Caldwell, H . B . , Trans. Am. Inst. Chem. Engrs., 16, 11, 131 (1924). (2) Cole, L. H., Canada Dept. of Mines, BUZZ. 646 (1926). (3) International Critical Tables, Vol. V, p. 100, New York, MoGraw-Hill Book Co., 1929.
593
(4) Kobe, K . A., Chem. & Met. Eng., 41, 300 (19343. (5) Kobe, K . A., and Anderson, C. H., J . Phys. Chem., t o appear. (6) Kobe, K. A., Conrad, F. H . , and Jackson, E. W., IND. ENQ. CHEY.,25, 984, 987 (1933). (7) Kobe, K . A , , and Hauge, C. W., Can. Chem. Met., 18,177 (1934). (8) Kobe, K . A., and Hauge, C. W., Power, 77,402, 460 (1933). (9) Landolt-Bbrnstein, Physikaliach-Chemische Tabellen, 5th ed., Vol. 11, p. 1262, Berlin, Julius Springer, 1923. (10) Palmer, L. A., Chem. & M e t . Eng., 32, 632 (1925). (11) Rich, P. C., Ibid., 40, 394 (1933). (12) Walker, W. H . , Lewis, W. K., and McAdams, W. H . , “Principles of Chemical Engineering,” p. 432, New York, McGrawHill Book Co., 1927. (13) Wells, R. C . , U. S.Geol. Survey, Bull. 717 (1923).
RECEIVED Marob 4, 1936. Part of the material of this paper was presented before the Division of Industrial and Engineering Chemistry at the 86th Meeting of the American Chemical Society, Chicago, I l l , September 10 to 1 5 , 1933.
+**
IV. Potassium Chloride from Sylvinite KENNETH A. KOBE
AND
ROBERT P. GRAHAM
HE discovery of large deposits of potassium salts in the Permian basin of Texas and New Mexico has founded a new industry in these states. Potassium chloride is produced by refining sylvinite, the approximate composition of which is 42.7 per cent potassium chloride, 56.6 per cent sodium chloride, and 0.7 per cent insoluble matter. The refining of sylvinite was discussed by Ward (4) and a flow sheet was given ( 2 ) . The method is essentially that of Blasdale (1). A solution saturated with sodium chloride and potassium chloride a t 20” C. is placed in contact with the sylvinite and the temperature raised to the boiling point (108” C.) by admitting live steam to the digesters. The numerous advantages of submerged combustion heating for many chemical processes (3) show that it might be Dossible to use this efficient form of heating in place of steam in the digesters. The solubility curves of sodium and potassium chlorides are typical of salts havSubmerged ing, respectively, flat and rapidly c o mbustion rising solubilities, and their beheat may be havior may then b e c o m p a r e d successfully with sodium sulfate and its inverted solubility curves (Figure 1, applied to the Part 111). recovery of potassium chloEvaporation of Salt ride from sylSolutions vinite. Conditions of opThe previous work with sodium sulfate showed the difficulty eneration for countered with a salt having an salts with flat inverted s o l u b i l i t y curve. In and rising solorder to determine the evaporaubility curves tion characteristics of salts with are given and other types of solubility curves, s a t u r a t e d solutions of sodium shown to be chloride and potassium chloride different from were e v a p o r a t e d . It was a t those with infirst believed that sodium chloverted soluride with its flat solubility curve bilities. might behave similarly to sodium ~
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INDUSTRIAL AND ENGINEERING CHEMISTRY
sulfate. A burner with a high-velocity nozzle formed by a '/r-inch (0.64-cm.) close nipple (upper part of Figure 4, Part 111) soon plugged with a hard scale about. 1/8 inch (0.32 cm.) inside the burner nozzle. The scale was not more than l/82 inch (0.8 mm.) thick and had built out from the circular opening. This type of nozzle was replaced by a cap, H , drilled to give a sharp-edged orifice, I (Figure 4, lower part). This burner was found to give continuous operation; however, the pressure in the mixing chamber fluctuated continually, indicating that scale was forming a t the orifice and being blown Off.
Potassium chloride has a rising solubility curve. Conditions for evaporation were found to be identical with those of sodium chloride. Pressure fluctuations covered the same range but were a little more rapid, undoubtedly because of the more concentrated solution.
Potassium Chloride from Sylvinite A saturated solution of sylvinite a t room temperature was made as the original leaching liquor. This was placed in a cone-bottom evaporator (Figure 3, Part 111), the lighted burner placed in the solution, and the calculated amount of sylvinite added to saturate the solution with potassium chloride. The agitation of the hot gases from the burner bubbling through the solution was sufficient to keep the sylvinite crystals suspended in the solution. The solution came to a boiling point of 90" C. ; because of the partial pressure of the products of combustion, the boiling point was below 100" C. After a short time a t the boiling point the burner was removed, the sodium chloride crystals were allowed to settle, and the solution was removed to a crystallizing tank. A pale pink mass of crystals of potassium chloride separated out as the solution cooled. Two modifications can be made in the process ( 2 ) when submerged combustion heating is used, The submerged combustion of natural gas replaces steam for the heating. The steam digester has a liquor circulating pump, which will be unnecessary with submerged combustion heating. The burners can be placed directly in the digester and thus give excellent agitation of liquor, or they can be placed in a side tube connecting the bottom and top of the digester. The release of the products of combustion will give the same effect as an air lift and circulate the liquor in the digester. The yield of potassium chloride per unit of solution will not be as great with submerged combustion heating as with steam heat. With the latter the boiling point of the solution is 108' C.; with the former it is reduced to 90' C. Reference to the solubility isotherms of Blasdale (1) shows that this will decrease the amount of potassium ahloride in solution from 37 pounds potassium chloride per 100 pounds water a t 108" C. to 32.5 pounds potassium chloride per 100 pounds water a t 90" C. If the solution is cooled to 20' C., the recovery in the latter case is only 80 per cent of the former. This indicates that the capacity of the plant is theoretically reduced by 20 per cent when submerged combustion is used. Actual reduction in capacity cannot be predicted, since certain features of submerged combustion might offset the lowered solubility by giving greater rate of solution.
Acknowledgment The writers wish to thank the U. S. Potash Company for supplying the sylvinite used in this investigation.
Literature Cited (1) Blasdale, W. C., J. IND.ENG.CHEX.,10,347-8 (1918). (2) Chem. & Met. Eng., 41, Supplement t o May issue, No.33 (1934). (3) Kobe, K.A.,Zbid., 41,300-2 (1934). (4) Ward, C.A., Ibid., 40,172-6 (1933). R E C ~ I V EMarch D 4, 1936. Presented before the Division of Industrial and Engineering Chemistry a t the 90th Meeting of the American Chemical Society, San Francisco, Calif., August 19 to 23, 1935.
VOL. 28, NO. 5
V. Sodium Carbonate Decahydrate KENNETH A. KOBE
AND
ROBERT P. GRAHAM
Q T
HE natural deposits of sodium carbonate in the western states differ, as do the deposits of sodium sulfate, from those in the southern and northern parts. The California deposits are trona (NanCOgNaHC03,2Hz0) which may readily be calcined to give sodium carbonate. The Oregon and Washington deposits are sodium carbonate decahydrate. This hydrate contains 63 per cent water and must be dehydrated before shipping. The methods are almost the same as those used for sodium sulfate, since both salts are so similar in their physical properties. Both have transition temperatures a t 32" to 35" C., and both have inverted solubility curves above the transition temperature, although the stable phase for sodium carbonate is the monohydrate whereas the sodium sulfate is anhydrous.
Submerged Combustion Cycle In order to study the dehydration of sodium carbonate decahydrate by the cycle of operations proposed for sodium sulfate decahydrate (Part 111), similar calculations were made. When sodium carbonate decahydrate is melted, sodium carbonate monohydrate and saturated solution are formed. When the NazC03.10Hz0 is introduced into a saturated solution a t 40", 18.6 per cent of the sodium carbonate precipitates as Na%C03.H20,and if the temperature is increased to 90" C., 25 per cent of the sodium carbonate precipitates. This is much smaller than the corresponding values for sodium sulfate from Na2SO4.10HzO. If one gram mole NazC03.10HzO is introduced a t A (Figure 6, Part 111) into a saturated solution a t 90" C., 25 per cent of the sodium carbonate will precipitate as the monohydrate (31 grams) leaving 75 per cent of the sodium carbonate in solution (79.5 grams sodium carbonate and 175.5 grams water). When this is pumped to the outdoor crystallizing pond a t an assumed temperature of 20" C., 61.3 per cent of the sodium carbonate crystallizes as the decahydrate (175 grams), leaving 13.7 per cent to be discarded (14.5 grams sodium carbonate and 65 grams water). Compared to sodium sulfate decahydrate, the sodium carbonate decahydrate gives a smaller recovery in the cycle, but less salt is discarded from the crystallizing pond so that more decahydrate is recycled. The data in the literature are insufficient to calculate a thermal comparison with the evaporation method, although the same general conclusions for sodium sulfate decahydrate hold for sodium carbonate decahydrate. The type of burner used for sodium sulfate solutions (Figure 4,upper part) was found tJooperate most satisfactorily. The salt that precipitated on the burner formed a chalklike mass and did not cake hard as did the sodium sulfate. Cone formation was very small and crumbly, and a large-size cone could not be formed with this type of nozzle. The formation was not even of sufficient strength to cause fluctuations in the gas-air pressure gage. The explanation is that the monohydrate first precipitated is dehydrated by higher temperature of the flame and leaves a crumbly mass easily blown off. The sodium carbonate does not fuse as does the sodium sulfate, for sodium carbonate taken off the burner combustion chamber dissolved readily, which is not the case for the sodium sulfate burner scale. RECEIVED hlarch
4, 1936.
