JULY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
801
RERUN OF TREATED ASD DEWAXEDLOKGRESIDUUM ucts consist of stable pressure distillate, stable light gasoline, and residue gas, S. A. E. MOTOROIL GRADES. A vacuum operation is The complete installation consists of: a debutanizer in dictated by the fact that lube fractions of high boiling point which the raw pressure distillate is stabilized; a stabilizer are taken overhead. for fractionating the overhead condensate from the debutaStabilization and Gas Recover) nizer and the condensate recovered in the absorption and stripping towers; an absorber in which the cracking still gas, the The gas recovery system is a necessary part of the modern refinery gas, and the uncondensed vapors from the stripping refinery. Wet gas as collected from the various operations is column are treated; and a steam stripping and rectifying processed for the recovery of stable light gasoline for blendtower for stripping the circulating absorption oil. ing. The gas recovery operation is conveniently combined Details of the debutanizing, absorbing, and stabilizing with the stabilization of cracked distillate. A combination operation are shown in Table VI. unit of this type which has been recently installed is shown in Figure 6. RECEIVED March 29, 1935. Presented as part of the Symposium on DistilThis plant receives cracking still gas and raw pressure dislation held under the auspices of the Division of Industrial and Engineering tillate directly from the cracking unit receiver. I n addition, Chemistry of the Smerican Chemical Society at the Massachusetts Institute refinery gas from the collecting system is processed. Prodof Technology, Cambridge, Mass., December 28 and 29, 1934.
FOR
Production of Potassium Sulfate from Poly halite and Svlvinite J
XPLORATIOSS by government and private agencies have disclosed the presence of large quantities of sylvinite, a mixture of potassium chloride and sodium chloride, and of polyhalite, a complex sulfate of potassium, magnesium, and calcium, occurring in bedded saline deposits in the Permian basin of Kern; Mexico and Texas (12, 16). Exploitation of sylvinite near Carlsbad, K.Mex., has already been undertaken actively by the U. S. Potash Company and the Potash Company of America, the former having produced refined potassium chloride since 1932 (16,dO). To date no attempt has been made either in this country or elsewhere to recover potassium salts from polyhalite on a commercial scale, but as a result of the prevalence of this mineral in the Texas-New Mexico deposits, this station of the U. S. Bureau of Mines has been investigating methods for its treatment. One of the possibilities considered has been the use of polyhalite and sylvinite together to produce potassium sulfate, which has always commanded an appreciably higher price than potassium chloride. As indicated by its formula, K2S04.MgSOc2CaS04.2H20,pure polyhalite might be made to supply three sulfate radicals in excess of the one equivalent to its potassium content. This paper presents a method of utilizing the sulfate content of polyhalite to convert refined potassium chloride into potassium sulfate by employing ammonia and carbon dioxide in a cyclic process. Process Outline The complete process, which will be designated as Bureau of Mines process 8, may be divided arbitrarily into four parts:
I. Raw polyhalite is washed to remove sodium chloride and
is calcined. The material is then decomposed by water at 25' C.
to roduce a solid consisting of a mixture of syngenite (KzS04Ca804.HZO) and gypsum (CaS04.2HzO)and a liquor containing essentially all of the magnesium sulfate and approximately 8 per cent of the potassium sulfate from the solid. 11. The syngenite-gypsum mixture is decomposed at 25' C. by ammonia and carbon dioxide in aqueous solution. This treatment precipitates calcium carbonate and yields a liquor containing potassium and ammonium sulfates.
J
ALTON GABRIEL
AND
EVERETT P. PARTRIDGE
Nonmetallic Minerals Experiment Station, U. S. Bureau of Mines, Rutgers University,
New Brunswick, N. J.
111. Solid refined potassium chloride is added to the otassium sulfate-ammonium sulfate liquor and agitated at 25" causing deposition of part of the potassium sulfate. The potassium sulfate remaining in solution is preci itated with ammonia. IV. The mother liquor, whick contains essentially ammonia and ammonium chloride with small amounts of potassium chloride and ammonium carbonate, is digested with lime and distilled t o recover ammonia, which is recirculated.
&,
The magnesium sulfate liquor derived from I, the calcium carbonate residue from 11, and the calcium chloride liquor from IV may be processed further as discussed subsequently. TABLEI. PERCENTAGE ANALYSISOF AVERAGESAMPLEFROM CARLOAD OF NEWMEXICO POLYHALITE" Polyhalite: 22.15 %SO4 15.25 MgSOa 34.64 Cas04 4.6 H20 Anhydrite Halite Magnesite RzOa Si02 Analysis by F. Fraas of this station.
+
5
76.64
8.17
12.81 0.73 2.29
Figure 1, which presents the process outline in greater detail, is based on the following considerations: (1) Crude polyhalite of the composition indicated in Table I, based on the analysis of an average sample from a carload shipment. (2) A loss of 10 pounds of potassium sulfate and 7 pounds of magnesium sulfate per 100 pounds of sodium chloride removed during washing of crude polyhalite to a sodium chloride content of 1.5 per cent (4). (3) A concentration of 32 pounds of magnesium sulfate and not more than 4 pounds of potassium sulfate per 100 pounds of
802
INDUSTRIAL AND ENGIXEERING CHEMISTRY
VOL, 27, NO. 7
quired to decompose the ammonium chloride liquor and to react with the magnesium sulfate and carbon dioxide present in it (8). (16) .Recovery of 98 per cent of the total ammonia content of the ammonium chloride liquor for re-use in the process. (17) Stripped ammonia containing 5 pounds of n-ater vapor per 100 pounds of ammonia. $n over-all recovery of 86 per cent of the potassium content, of the crude polyhalite and the refined potassium chloride is
indicated.
Fundamental Operations
1 1
iieih
I
raah
'Filter
1
filtrate
water in the mother liquor from the formation of the syngenitegypsum niixt,ure (I?', 18). (4) , A moisture content of 50 per cent on the wet basis in the syngenite-gypsum cake. ( 5 ) The use of ammonia and carbon dioxide equivalent' to the total calcium sulfate (combined in syngenit,e and present as both gypsum and anhydrite) in t,he syngenite-gypsum mixture. (6) The use of 10 per cent excess of water above the amount theoretically necessary to dissolve the potassium sulfate and ammonium sulfate resultsingfrom the decomposition of the syngenite-gypsum mixture (21). 171 of npr cent ..... of .. the total calcium .~ sulfate \., Decomnnsition r---"---- - g2 - - =.. (complete decomposition of syngenite and gypsum and partial decomposition of anhydrite) Tithin a period of 1 hour. (8) Practically complet,e removal of dissolved s a h from the calcium carbonate residue by washing with the water required in t,he decomposition step. (9) A moisture content of 40 per cent on the wet basis in the calcium carbonate residue. (10) Addition of potassium chloride equivalent t o the amnonium sulfate in the ammonium sulfate-potassium sulfate liquor. (11) Introduction of ammonia up to a concentration of 32 pounds per 100 pounds of water to precipitate potassium sulfate. (12) A moist,ure content of 30 per cent' on the dry basis in the potassium sulfate crop. ~~~~
(1 R I R e r o v w v nf \--, -_---' ~
1-
-
-
~
~
~
~
Thn
-
~
_
_
_
_
_
.
_
_
_
I
+ 2SH3 -+ C 0 2 4-H20 e CaC03 + ( N H d d O l
He calculated from the solubility products of calcium sulfate and calcium carbonate that the equilibrium constant for the reaction is 4.82 X l o 3 and that the theoretical yield a t 35" C. is 99.95 per cent. The rate of reaction iz very rapid a t first, but ultimate equilibrium is approached .lowly and 15 to 20 hours are estimated to be necessary for complete conversion. Both Keumann ( 1 2 ) and Matignon and Frejacques (10)point out that the rate of reaction iq influenced by the concentration of ammonium carbonate, by thP particle
T.4RLE
11. DECOMPOSITIOX OF SYNGEN~TE-GYPSU~~-ASHYDR~TE MIXTURE BY AMMONIUM C l i R B O S a l E SoLUTIoX Excess of (NHdaC03 Per cent
&-
nf fi .nov oont of l i m n in o v o n u ~nf tho n ~ n m i n f rp.
~IELI
CaSOl
RR npr r m t nf t h____-....-...e r o m h i n d notasfiium con-
"
tent of the ammonium sulfate-potassium sulfate liquor and the refined potassium chloride as potassium sulfate. (14) A potassium sulfate product containing approximately 9 per cent of ammonium sulfate and negligible amounts of other im urities. K\
I. PRODUCTIOX OF SYNGEKITE-GYPSUM ~ I I X T G RThe E. production of the syngenite-gypsum mixture to be used in the process involves a preliminary washing of crude polyhalite to reduce the sodium chloride content to about 1.5 per cent. The washing step (4, the calcination step (s),and the syngenite-gypsum formation step (17, 1 8 ) ) have been investigated previously by this station. 11. DECOMPOSITIOX OF SYNGESITE-GYPSUM MIXTURE. Hill and Adams (7) have investigated and Berliner (2) has patented a process which utilizes an aqueous wlution of carbon dioxide and ammonia or their salt equivalent to decompose uncalcined polyhalite. The former investigation indicated that magnesium ion was incompletely precipitated from solution even in the presence of a large exces of ammonium carbonate and that substantially complete precipitation was obtained only upon the further addition of appreciable quantities of ammonia. On the other hand, dtorch and Fragen (17, 18) showed that 98 to 99 per cent of the magnesium ion may be removed from solutions of varying concentrations of magnesium sulfate when a mole ratio of 3 or 4 of ammonium carbonate to 1 of magnesium sulfate is uqed. I n the present process, magnesium sulfate is separated from calcium sulfate and potassium sulfate prior to the ammonia-carbon dioxide treatment, and the function of the latter is t o precipitate calcium ion only. Experiments show that very little excess over the theoretical amount of ainmonium carbonate i p necessary for the essentially complete elimination of the calcium. Apropos of the decomposition of syngenite-gypsum mixture by ammonia and carbon dioxide is the work by Seumann (11) on the reaction:
100 100
Time of Decompn. Minutes
50
Total CaSOs Decomposed Per cant
96.1 l00b
0 Mixture allowed t o stand overnight after 120 minutes' agitatton. b Petrographic examination ahowed t h e conlplete absence ot syngenite, gypsum, and anhydrite.
JULY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
size of the gypsum, and by the rate of agitation. Baud (1) showed that the rate of reaction is increased by the introduction of clay. The results of experiments on the rate of decomposition of syngenite-gypsum mixture by ammonia and carbon dioxide are shown in Table I1 and are represented graphically in Figure 2. The syngenite-gypsum mixture, prepared from washed polyhalite and partly dehydrated during drying, had the following composition: Constituent cas04
Per Cent 61.07 30.74 0.6
&SO4
RIgSO4
Constituent RzOa .tSi02 MgCOs (magnesite)
Per Cent 2.5
Trace
The analysis for calcium sulfate includes that which occurred as gypsum, that which was combined with potassium sulfate as syngenite, and that which existed as anhydrite, an original impurity in the polyhalite. The syngenitegypsum mixture consisted of fibrous and bladed crystals, not over 50 t o 75 microns in length and 3 to 6 microns in width. The anhydrite occurred as broken tablets and irregular fragments ranging in size from 3 to 75 microns in diameter. I n the decomposition experiments, 12.5 grams of syngenitegypsum mixture were treated with 41.5 cc. of a solution 2.94 normal with respect to ammonia and 2.90 normal with respect t o carbon dioxide, prepared from solid “ammonium cazbonate” and aqua ammonia. Mixtures of the composition described were agitated at about 600 r. p. m. for varying intervals of time, were then filtered rapidly, and washed repeatedly with 50 per cent alcohol-water mixture, followed by 95 per cent alcohol, and dried overnight at 130” C. A representative sample of solid was then titrated with standard hydrochloric acid to determine the conversion of calcium sulfate to calcium carbonate. Petrographic examination of the solids after decomposition showed that the break in the curve of Figure 2 at about 90 per cent decomposition corresponded to complete conversion of syngenite and gypsum to calcium carbonate but incomplete decomposition of the larger particles of anhydrite, which continued to react slowly.
: I
I
I
I
I
!
Y
!
I
20
40
60
€n
TIME -MINUTES
100
I
I
120
140
! I
‘I
180
FIGURE 2. DECOMPOSITION OF SYNGENITE-GYPSUM MIXTURE BY AMMONIA .4ND CARBON DIOXIDE AT ROOM TEMPERITURE
The calcium carbonate residue was finely granular and could be filtered readily by suction. 111. PRODUCTION OF POTASSIUM SULFATE.The ammonium sulfate-potassium sulfate solutions resulting from I1 would produce on simple evaporation a solid containing about 66 per cent of ammonium sulfate and 34 per cent of potassium sulfate. Since these two substances form isomorphous mixtures in all proportions, a simple crystallization procedure will not yield a clean-cut separation of the constituents (7, 21). The effect of the addition of solid potassium chloride to the solution was therefore studied in an attempt to recover a greater part of the sulfate content as potassium sulfate. The system ammonium sulfate-potassium chloride-water
803
has not been extensively investigated, though in ammoniacal solutions it forms the basis for at least four patents on the preparation of potassium sulfate ( I S , 14, 16, 19). The low solubility of potassium sulfate in high concentrations of ammonia hydroxide was noted by Giraud (6) in 1885. When solid potassium chloride is added to the aqueous solution from 11, some potassium sulfate crystallizes out, but the concentration of potassium and sulfate ions in solution a t equilibrium is too high to be discarded. If ammonia gas is passed into the solution, the solubility of potassium sulfate is decreased greatly. The efficiency with which potassium ion is removed from solution depends in part upon the extent of coprecipitation of ammonium sulfate with potassium sulfate. If the amount of solid potassium chloride added is
I
I
1
I
I
INDUSTRIAL AND ESGINEERING CHE3IISTRY
804
The potassium sulfate product was uncolored and of uniform crystal grain size, the average particle size being about 0.16 mm. length and 0.075 mm. width. The chief impurity was 9 per cent of ammonium sulfate, which might be eliminated by calcination if a product of higher purity were required. This ammonium sulfate would scarcely be an objectionable constituent in material to be used for fertilizer. IV. RECOVERY OF B~molvIa. According to the estimates represented in Figure 1 the filtrate from 111will contain 32.5 pounds of ammonia, 15.7 pounds of ammonium chloride, and TABLE 111. EFFECT OF AMiVfOKI.4 CONCENTRATION T 3 O X EFFICIENCY OF CONVERSION OB AMMOSIUM SULFATE TO POT.4SSIUM SULFATEAND EXTENT OF COPRECIPITATIOS OB AMMONIUM SULFATE WITH P0TA4SSIUM SULFkTE AT R O O M TEAlPER.4TURE
Run S o .
Ammonia (SH32SOa in Concn. ICzSOa G . N H a / 1 0 0 g. H?O P e r cent
Recovery of I C as XzSO4 P e r cent
a Results of runs starting with syngenlre-gypsum mixture and following through Parts I1 and 111 a8 outlined.
4.2 pounds of potassium chloride per 100 pounds of ivater. The operation of recovering ammonia by digesting vith lime and distilling would be similar to that follom-ed in the ammonia-soda process (8). If syngenite were used in 11, ALTERNATIVE PROCEDURE. the mole ratio of potassium sulfate to ammonium sulfate would be 1 to 1, instead of approximately 1 to 2 for the syngenite-gypsum mixture derived from polyhalite, and 1 to 3 for polyhalite itself. The effect of increabe in potassium sulfate in the ratio is t o increase the amount of water necessary t o dissolve the salts. Table IV shows the approximate compositions of the solutions that would result from deconiposition of the three starting materials by ammonia and carbon dioxide, and the extent of coprecipitation of ammonium sulfate with potassium sulfate determined experimentally using these solutions. The amounts of water used in the starting solutions were calculated from the data of Weston (21)for the system potassium sulfate-ammonium sulfate-water a t 25 O C. The necessity for employing a relatively large amount of water for solution of the salts formed by the decomposition of syngenite led to the investigation of a procedure in which syngenite is decomposed by an ammonium carbonate solution in the presence of an amount of potassium chloride equivalent to the ammonium sulfate produced. After the precipitated solid has been removed, the ammonium chloride mother liquor is returned to a similar second decomposition cycle. The final liquor, now containing about 30 grams of ammonium chloride per 100 grams of water, is treated with ammonia to complete the precipitation of potassium sulfate. TABLEIv.
!vA4TER REQCIREMEKTG FOR V.4RIOUS STdRTING SULFATE CONTENT O F CORRESPONDMATERIALS AND AMMONICM I S G POTASSIUM SULF.4TE PRODFCTS Water (h-Hi)zSOa Mole Ratio Required in Content of CaSOd/KSOa XI aterial in Solid Decompn. &SO4 Product
G. Hn0/200 0. XdOa producedn Svneenite Syngenite-gypsum mixtureb
R a w polyhalite
1.11
465
Per cent 7 6
2.s2
280
9.0
2.4id 245 11.7 Calculated on basis of 92 per cent decomposition of CaSOa, saturation of (XH+SOA-K&O~hquor ?t 25' C., and 88 per cent recovery of KzS04 froin this liquor on trea!ment mth KCI and NHa. b Composition Indicated in Figure 1. e Composition given in ,Table I. d ("+SO4 ie also derived f r o m hlgSOa in polyhalite; complete decomposition of MgSOr is assumed. Q
VOL. 27, NO 7
Ilnder these conditions of decompobltion, the final solltl consists of a buff mixture containiiig about 68 per cent of potassium sulfate, 9 per cent of coprecipitated ammonium sulfate, 21 per cent of calcium carbonate, and varying sinall amounts of impurities originally present in the raw polyhalite, Although this solid probably ~ ~ o u be l d useful as a fertilizer material per se, a product richer in potassium sulfate would be desirable. Because of the considerably smaller size of the calcium carbonate particles, elutriation in an aqueous solution saturated with potaseium sulfate and containing 30 grams of ammonium chloride per 100 grams of water yielded two fractions-an overflow high in calcium carbonate and an underflow high in potassium sulfate. The results of three runs are recorded in Table V. This method for the treatment of byngenite may also be used for syngenite-gypsum mixture. It permits the use of less water than is necessary for the complete solution of potassium sulfate and ammonium sulfate in the treatment of syngenite-gypsum mixture. The decrease is appreciable, 100 grams of potassium sulfate product requiring approximately 135 cc. of water (cf. Table IV). It follows that less ammonia is required for the final precipitation of potassium sulfate, thus reducing the quantity to be stripped. rurthermore, a higher concentration of amnionium chloride in the end liquors is obtained, which would be an advantage in arnrnonia recovery. TABLE v.
SEPARATION OF POTASSILX SULFATE FROM CALCICW CARBOSATE BY ELUTRIATION
Per Cent of Total Per Cent Total &SO4 CaCOa Removed Remaining In Run from in CaCO3 KzSOa CaCO, KO. CaCO3 fraction fraction fraction 7 14.5 85 5 1 93 2 04.7 5.3 12.8 07 5 4.8 92.2 3 97.5 2.5 0 Including coprecipitated (NH4)nOa
-
Percentage Coiiipn Of KzSoa I 4 pounds of potassium sulfate and 35 pounds of ammonium sulfate per 500
JULY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
pounds of water. This liquor might be combined with the ammonium sulfate-potassium sulfate liquor in the main process, or it might be treated separately with potassium chloride and ammonia to produce potassium sulfate, Experiments have shown that the product thus obtained will contain 8 to 11 per cent of coprecipitated ammonium sulfate. TllBLE VI. EFFECTO F SODIUM CHLORIDE IN EXCESSUPON ~ O U N TAND PURITY OF SODIUM SULFATE PRECIPITATED BY
+
+
REACTION 2NaCl (NH&S04 F? Na,S04 2NH4C1 PERCENTAMMONIUM HYDROXIDE
G./100 0. HzO
14.0 14.0 14.0 14.0 21.9
Per cent
Per cent
Per cent
0.85 25 50 100 105
57.4 71.8 83.2 93.1 96.7
0.12 0.10 0.13 0.05 0.10
IN
28
Another possible outlet for this ammonium sulfate liquor would be in the production of sodium sulfate by the use of sodium chloride and ammonia. The data of Table VI indicate that by employing double the theoretical equivalent of sodium chloride and a concentration of approximately 39 grams of ammonia per 100 grams of water, a high recovery of anhydrous sodium sulfate practically uncontaminated by ammonium sulfate may be obtained. CALCIUMCARBONATE SLUDGE. According to Hou (8) rotary kilns have been employed in ammonia-soda plants for the calcination of precipitated calcium carbonate from causticizing operations. If the sludgcl of calcium carbonate obtained from the decomposition of the syngenite-gypsum mixture can be satisfactorily calcined in the same manner, it may be feasible to recover a large part of the lime and carbon dioxide required in the process. The choice between this recovery operation and the calcination of fresh limestone would depend upon the particular technical and economic factors affecting a given plant. CALCIUMCHLORIDELIQUOR.The liquor from the ammonia stills is estimated in Figure 1 to contain approximately 16 pounds of calcium chloride and 4 pounds of potassium chloride per 100 pounds of water. Separation and recovery of these constituents by direct evaporation and crystallization are probably impractical if not impossible. Somewhat less than half of the calcium chloride liquor might be combined with the magnesium sulfate liquor obtained earlier in the process to precipitate calcium sulfate and leave a liquor containing approximately 9 pounds of magnesium chloride, 7 pounds of potassium chloride, and 2 pounds of sodium chloride per 100 pounds of water. Evaporation mould allow the recovery of most of the potassium chloride and the ultimate production of magnesium chloride, if this were justified from the economic standpoint.
805
rich in potnssiuni compounds, usually being treated in an associated carnallite plant. In the case of the Texas-Kew Mexico deposits, which apparently do not contain industrially significant quantities of carnallite, the application of the German process to potassium sulfate-magnesium sulfate liquors derived from polyhalite would therefore involve either low recoveries or an elaborate plant layout. On the other hand, as pointed out by Fox and Turrentine (ij), the acid process for converting potassium chloride into potassium sulfate appears attractive where the hydrochloric acid simultaneously obtained may be utilized directly in the manufacture of dicalcium phosphate. I n place of hydrochloric acid, the process described in this paper yields magnesium sulfate in the form of a concentrated liquor as its chief by-product of industrial importance. The potential significance of this material should not be neglected in any comparison of the acid and ammonia-carbon dioxide processes. No adequate comparison of the two processes can be made without further information concerning the difficulties in operation and the heat requirements in each case, together with market analyses for the various products.
Literature Cited Baud, P., Con@ rend., 185, 1138-41 (1927). Berliner, J. F. (to E. I. du Pont de Nemours & C o . ) , U. S. Patent 1,909,606 (May 16, 1933). Conley, J. E.! Fraas, F., and Davidson, J . M., Bur. Mines, Repts. Investigations 3167 (1932). Davidson, J. M., and Fraas, F., Ibid., 3237 (1934). Fox, F. J., and Turrentine, J. W., IND.EWG.CHEM.,26, 493-6 (1934).
Giraud, H., B d l . soe. chim., 43, 552-6 (1885). Hill, J. R., and Adams, J. R., IND.ENG.CHEM., 23, 658-61 (1931).
Hou, T. P., “Manufacture of Soda,” A. C. S. Monograph 65, New York, Chemical Catalog Co., 1933. Majtl, H., Chem. Listy, 27, 230-3 (1933). Matignon, C., and Frejaoques, Compf. rend., 175, 33-5 (1922). Neumann, B., 2. angew. Chem., 34, 441-2, 445-7, 457-9 (1931). Partridge, E. P., IND.ESG.CHEM.,24, 895-901 (1932). Rusberg, F. (to Kali-Chemie -4.-G.), U. S. Patent 1,774,040 (Aug. 26, 1930). Rusberg, F., and Uebler, B. (to Kali-Chemi A.-G.), German Patent 590,633 (Jan. 6, 1934). Smith, H. I., Am. Inst. Mining M e t . Engrs. Contribution 52 (1933).
SociBt6 Btudes fabrication emploi engrais chimiques, French Patent 739,568 (Nov. 3, 1932). Storch, H. H., and Fragen, N., Bur. Mines, Repts. Znvesfigations3116 (1931).
Storch, H . H., and Fragen, N., IND.ESG. CHmr., 23, 991-5 (1931).
Thorssel, C. T., U. S.Patent 1,878,733 (Sept. 20, 1932). Ward, C. A , , Chem. 6c Met. Eng., 40, 172-6 (1933). Weston, A,, J . Chem. Soc., 121, 1223-37 (1922). RECEXVED February 9, 1935. Presented before the Division of Fertilizer Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 t o 14, 1934. Published by permission of t h e Director, U. 6. Bureau of Mines. (Not subject t o copyright.)
Comparison with Other Processes
Correction
The process described in this paper offers a means of utilizing simultaneously the sylvinite and polyhalite of the TexasNew Mexico potash deposits in the manufacture of potassium sulfate. As a means of producing this important fertilizer constituent, the ammonia-carbon dioxide process must stand comparison with existing methods. These are the process used for many years in the German potash industry, employing refined potassium chloride and magnesium sulfate, and the sulfuric acid process used since 1921 by the Fabrique de Produits Chimiques de Thann et de Mulhouse (5, 9). The recovery of potassium sulfate in the German process is limited t o not more than 65 per cent by the solution equilibria involved, the magnesium chloride mother liquors, still
SIR: In our article on “Mass Transfer (8bsorption) Coefficients,” which appeared on pages 1183-7 of the Kovember, 1934, issue, the data points on Figure 6 were calculated incorrectly. The coordinates of these points should be: DU/P
3
DUlP
135 214 310 440 830
0.026 0.019 0.014 0.018 0.012
1100 1500 2200 3500
j 0 010 0 0095 0.0084 0.0091
T. H. CHILTON AXD A. P. COLBURN E. 1. DU PONTDE NEMOTJRS &
COMPANY, INC.,W I L m x G r o s , DEL. June 5, 1933
KELLYPRESSURE FILTERFOR EXTREXELY SLOW-FILTERING MATERI 4 ~ s ; C4u BE STEAWJ~CKETED WRE\ NECESS4RY
Studies in Filtr W. Nature of Fluid Flow through Filter epta and Its Importance in
the Filtration Equation
QT
HE attempts of early workers to treat filtration with parabolic equations based solely upon Poiseuille’s law were unsatisfactory for three reasons: (1) The resistance of the septum was either neglected or inadequately accounted for; ( 2 ) most of the analytical methods employed were not capable of determining accurately the constants of a parabola when the origin of the experimental coordinates failed to coincide with the vertex of the theoretical curve; and (3) many of the experimental data were imperfect parabolas because of experimental error. As no basis existed for discarding imperfect parabolic data on the grounds that they were in error, two explanations for the failure of time-volume discharge curves to be parabolic were advanced: (1) I n the initial and early stages of filtration flow was not uniform or conditions were not established constant. and hence the early stage might be disregarded; ( 2 ) flow through the septum and thin layers of cake might be proportional to other powers of the pressure than unity.
B. F. RUTH University of Minnesota, Minneapolis, Minm
sidered as established until the l a m governing florv through filter septa have been in\-estigated. It has been shoan (11, 12) that the ordinarily observed stages of constant-pressure filtrations yield time-volume discharge curves which are portions of perfect parabolas, irrespective of the homogeneity or compressibility of the material being filtered. Examination of a large number of such curves has led to the formulation of a fundamental maxim of filtration, an ewential postulate of which is that the missing portion of the parabola near the vertex map be regarded as the course of a hypothetical filtration in which the resistance, already existing when the test began, v a s generated, thereby alloving the expression of such pretest resistance as a volume of filtrate. Upon the basis of this generalization and Poiseuille’s law, equations were derived which describe the course of constant-pressure filtrations in termr of the physical properties of the prefilt, the dimensions oi the test apparatus, and the pressure employed (10). The justification for presenting still another and perhapb more detailed treatment of the filtration unit operation is
Previous Work The first explanation was discussed in a previous paper (12). The second explanation is apparently supported by a variety of experimental evidence and has formed the basis for a comprehensive mathematical treatment of filtration, Although it has been shown that an equation based solely upon Poiseuille’s law is capable of treating the data of all properly performed filtrations, the fundamental basis of the equations developed in a previous paper (10) cannot be con806
