June, 1923
IATDUSTRIALA N D EXGINEERING CHEMISTRY
617
The Electrolytic Production of Acid and Alkali from Sodium Sulfate Solutions”’ By Harold V. Atwell and Tyler Fuwa MASSACHUSETTS INSTITUTE OF
T h i s paper summarizes the results of a thorough inoestigation carried out by this laboratory on the electrolysis of waste sodium sulfate solutions, with a oiew to the commercial production of caustic soda and sulfuric acid f o r local consumption. After ooercoming m a n y dificulties, a n electrolytic method has been developed f o r this purpose, which f r o m a n operating and technical standpoint appears to be quite satisfactory. While the precise o p t i m u m conditions have not been f u l l y determined a s yet, the recommended process in essence m a y be summarized a s follows: Cell: Horizontal, 2 diaphragms, both cathode and anode unsubmerged. Cathode: Perforated sheet iron, as in Allen-Moore cells. Anode: Perforated 100 per cent sheet lead. Distance between electrodes: 0.7 in. Diaphragms: Ordinary asbestos paper at cathode: JohnsManoille “High Grade” at anode. Liquid head on cell: 4 to 6 in. Electrolyte: 22 per cent sodium sulfate at 80 ’to 90 a C. Current density: I00 amps. per sq. f t .
T
HIS investigation was undertaken a t the request of the Viscose Company of America to determine the prac1 icability, from a commercial standpoint, of recoveriiig caustic soda and sulfuric acid of satisfactory concentration and purity by the electrolysis of sodium sulfate solutions, occurring as a by-product in the viscose process. It was required that the caustic soda be concentrated and nearly free from sodium sulfate, while the sulfuric acid should be not less than 8 per cent in concentration, and contain not more than 15 to 18 per cent of sodium sulfate.
SCOPEOF PROBLEM AND METHODOF ATTACK
MASS
T E C H N O L O G Y , CAMBRIDGE,
Operating under these conditions, it is possible to obtain a current eficiency of 85 per cent and operating voltage of 4.3 oolts, and a yield of 0.65 lb. of caustic and 0.79 Ib. of sulfuric acid per kilowatthour. T h e caustic soda is in the f o r m of 6.5 per cent solution, containing I5 per cent sodium sulfate which can apparently be concentrated to practically pure sodium hydroxide in an ordinary caustic eoaporator equipped with salt catchers. T h e sulfuric acid is in the f o r m of a n 1 I per cent solution containing about I3 per cent sodium sulfate which cannot be readily remooed. T h e total cost of production per pound of caustic soda compares oery faoorably with that f r o m sodium chloride, the only important di$crence being the saving in the cost of raw material if the sodium sulfate is considered a s a waste product. I n general, howeoer, the value of the two anode products, sulfuric acid and oxygen, is considerably less than that of the chlorine f r o m the sodium chloride process, and the sodium sulfate process can scarcely compete unless the oxygen can be sold or used locally without compressing into cylinders.
determination of brine concentrations, were made with halflength hydrometers, no corrections being made for the ex‘ pansion of the hydrometer bulb with increasing temperature. On this account the specific gravities as given are slightly higher than the true values, but since the same hydrometers were used throughout the investigation, no errors were introduced by the use of the uncorrected curves. Both the density and resistivity determinations were accurate to about 0.5 per cent. It will be noted that there was little gain in conductivity by working at concentrations of sodium sulfate above 22 per cent, and since higher concentrations introduce corre-
Since this problem had to be attacked without the benefit of previous experiments, nearly all the essential features of both cell design and operation had to be developed as the work progressed. No attempt was made to secure masses of data which could be tabulated and plotted to show the influence of all operating variables for a single type of cell. Instead, the fundamental principles of the process were established, a wide variety of cells and operating schedules were tried out, and enough general inlormation secured so that a dependable and reasonably efficient cell could be constructed for purposes of later detailed study. .FUXDAMENTAL DATAAPPLYING TO PROBLEM Owing to the rather surprising lack of data in the literature o n the specific gravity and electrical conductivity of fairly strong sodium sulfate solutions a t elevated temperatures, these measurements were made in this laboratory, the results being shown in Figs. 1 and 2. At seven different concentrations from 6 to 30 per cent sodium sulfate, the temperature was slowly raised by an external water bath, the conductivity being mpasured every three or four degrees in a carefully calibrated cell, and the density measured every ten degrees. The deiisity measurements, which were needed for the Received August 18, 1922. as Contribution No. 61 from the Research Laboratory of Applied Chemistry, M. I. T. 1
* Published
spondingly more sodium sulfate in the finished product, most of the runs were carried out with electrolyte of approximately this concentration. The sybtem NazS04-H2SOrH20 has been thoroughly studied by Saxton,s and a study of the system Na2S04-
* THISJOURNAL,
10 (1918), 897.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
618
NaOH-H20 has been begun by D'ans and S~hr'einer.~Data from these sources, supplemented by a few determinations made in this laboratory, indicate to what extent the cell effluents can be freed from sodium sulfate by concentration and ao crystallization. This method of purification cannot be ap/s p l i e d to the a c i d efRuent, since up to a certain concentration of sulfuric acid the 6 /2 solubility of sodium sulfate increases, while $I /o beyond this point con$6 siderable sulfuric acid h is lost by the crystalli3 zation of acid sulfates. C 6 This fact was of no con4 sequence in connection with this p a r t i c u l a r moblem. since the com1 I I 1 1 position'of the anolyte prAcrrir ' -.a'' N+S&" '' as it came from the cell satisfactorily met the requirements of the Viscose Company. The equilibrium curve for the system Na2SOd-NaOH-HZO a t 25" C., as determined by D'ans and Schreiner, is'shown as the solid line in Fig. 3. The separate points indicated along the lower part of the curve were determined in this laboratory a t 90" to 100" C. At 10" C. and above 20 per cent sodium hydroxide, the solution is so viscous and the precipitated sodium sulfate is so gelatinous that separation cannot be effected either by settling or filtration. At a temperature of 90" to 100" C. the precipitated sodium sulfate is distinctly crystalline up to 25 per cent sodium hydroxide and, although tending toward the gelatinous state, can be separated by a half-hour's settling a t caustic soda concentrations considerably above 25 per cent. Since the solubility of anhydrous sodium sulfate is probably lower a t 100" C than a t 25" C., the solubility of sodium hydroxide very much higher than a t 25" C., and the viscosity of the solution much lower, it is plain that crystallization should be allowed to take place from the boiling solution.
Vol. 15, No. 6
z
1 1
0
1
1
PRELIMINARY EXPERIMENTS In order to obtain a comparatively satisfactory anode material, a number of corrosion tests were made under conditions approximating that of the actual cell operation, an electrolyte of the following composition being used : Anhydrous NazSOa.. . . . . . . HBOd (sp. gr., 1.84). . . . . . Water,.
.................
Per cent 25.0
3.5 71.5
A current density of 0.8 ampere per sq. in. of anode surface was used, and a solution temperature of 65" to 75" C. Prohibitive corrosion was observed in the case of materials designated by (+), Silicon and a copper-tin alloy were only slightly corroded, but formed a nonconducting surface film of oxide.
} . , .. , ., .
I
.., . ,
4
.
Z. anovg. Chem., 67 ( l o l o ) , 437.
Badly disintegrated Rapidly dissolved Forms nonconducting surface film Badly disintegrated 0.03 g. per sq. in. per hr. 0.007 g . per sq. in. per hr. 0,007 g. per sq. in. per hr. initially (drops off with time) Copper dissolved readily leaving tin oxide film
I 1
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SYJTCN
Na&'+Na,SG
+a0
JGLID CUmC- DRTR OF DRNS RnDJC,5RE/NlER AT 2S.C RCCG88CD A?wTJ WTCRmfNLo lN
25
4 0
B
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1 5
20
35
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95
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P&RCZNT N a O n
Cell 1 (Fig. 4) was 2.5 in. between electrodes, had a thick duriron anode, and was open a t the top. Five 2-hr. runs made with saturated sodium sulfate solutions, and at varying current densities brought out the following points: 1-Such a thick cell has a high operating voltage and corresponding low energy efficiency. 2-Unsubmerged electrodes must be covered with hoods t o prevent loss by misting. 3-The iron content of the acid is too high when duriron is used as the anode. 4-Ordinary asbestos diaphragm paper weakens rapidly when used a t the anode.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1923
The second cell (Fig. 5 ) was designed to overcome the faults of the first cell and resulted in greatly improved performance. The decrease in distance between the electrodes from 2.5 in. to 0.75 in. lowered the operating voltage from 8.1 to 5.1 volts, a t 110 amps. per sq. ft. current density. The perforated sheet-lead anode was only slightly corroded, the acid solution giving no precipitate with hydrogen sulfide. The use of Johns-Manville “High Grade” asbestos paper a t the anode corrected the trouble due to disintegration of diaphragms. By closing the top of the cell and feeding through a “riser” whereby the diaphragms mere put under a greater hydrostatic head, the discharge of the electrolyte through the upper and lower portions of the diaphragm was more nearly equalized. Losses from misting were stopped by the use of spray hoods attached to each side of the cell. The next step was the construction of Cell 3 (Fig. 6), in which i,he distance between electrodes was only 0.6 in. The only other important modification over the plan of Cell 2 was one of proportion, which made Cell 3 relatively greater
619
in width than in height, in order to still further decrease the undesirable variation in the flow from the top and bottom of the cells. Now that the principles of cell design had become fairly well established, attention was focused on the control of the major operating conditions, it being especially desirable to fix a current density and rate of flow such that acid and alkali of a maximum concentration could be produced with the highest current efficiency. The results of nine carefully controlled runs on Cell 3 are given in Table I. Run 1 was made with a high rate of flow and with a current density of 144 amps. per sq. ft., but the concentrations of acid and alkali produced were low, although a current efficiency of 92 per cent was reached. Runs 2, 3, 4,and 5 were all made a t a current density of 115 amps. per sq. ft., and with successively lower rates of flow, this being controlled by varying the head of brine on the cell, and by increasing the thickness of the diaphragms. In Runs 2 and 3 single diaphragms were used, with a fast feed in Run 2 and a slower feed in Run 3, and, as would be expected, the current
TABLE I-OPERATION OF CELL3 IN BOTHVERTICALAND HORIZONTAL POSITIONS CONSTANTS O F OPERATION:
,
(1) Electrode area 100 sq. in. ( 2 ) Anode-cathode) distance, 0.6 in. (3)Anode, sheet lead (4) Diaphragm, Johns-Manville “High Grade” paper
1 2 3 4 6 6
7
8
9
75 210 45 195 120
144 115 115 115 115
4.85 4.65 4.57 4.95 5.20
0.61 1.30 0.27 1.29 0.83
19.2 19.8 20.0
60 60 195 300
115 115 115 115
5.05 4.70 4.95 4.65
0.40 0.38 1.30 1.86
17.7 22.8 20.8 18.4
20.8 18.9
60 65 60 75 70
5990 6500 5650 3950 2540
65 78
2390 1600 1550 1580
80
76
4.12 3.15 3.48 5.02 7.22 7.94 10.34
10.57
11.42
Vertical OPeration 215 5340 501 - e . 8710 103 5480 450 3940 256 2190 HorQontal OPeration 132 3130 115 . , . 2320 373 15.0 1840 630 13.2 2330
... ... ... ... ...
3.61 1.93 2.92 4.16 6.78
168 410 84 372 207
5.06 6.59 7.60 6.34
111 107 317 516
... ... .... .. ... ... ...
12.0 14.4
92.0 97.9 93.6 95.2 87.2
Slowfeedandhighcurrentdensity Fastfeednormalcurrentdensity Slower feed Diaphragm thickness doubled Diaphragm thickness tripled
!::;] Cathodeuppermost Anodeuppermost
80.0 86.3
Anodeuppermost
620
INDUSTRIAL A N D ELVGl*VEERING CHEMISTRY
efficiency dropped somewhat with the rate of flow; at the same time the acid and alkali concentration increased, although i t did not reach a satisfactory value. I n Runs 4 and 5 the same low-feed head was maintained, but in Run 4 two sheets of diaphragm paper were used at each electrode, and in Run 5 three sheets were used, thus lowering the rate of flow still further. It is a striking fact that Run 4, although producing stronger acid and alkali than Run 3, still showed a higher current efficiency. This was probably due to the fact that double diaphragms tend to prevent the back diffusion of the products of electrolysis. In Run 5 still higher concentrations of acid and alkali were obtained, but here, as in Run 4, the increase in voltage caused by the thicker diaphragm more than outweighs this advantage. The indications are that when single diaphragms are used in such a thin cell, losses due to back diffusion and neutralization must be very great. The final stage of this investigation was given over t o a consideration of methods of obtaining a uniform flow over the entire area of the diaphragm. Cell 4 (Fig. 7), operating in a vertical position but having two outer compartments to submerge the electrodes, was built for this purpose. Several runs were made with this cell at a constant current density of 111 amps. per sq. ft., and with decreasing rates of flow. With this decrease in rate of flow the acid and alkali concentrations increased roughly from 4 to 6.5 per cent, while the current efficiency dropped from 95 to 82 per cent. These comparisons indicate, as was expected, that there is a n abnormal facility for back diffusion in the submerged electrode type of cell. On the other hand, the operating voltage of 4.25 was considered very Satisfactory, and on the whole, the results were certainly promising enough to warrant further investigation on this type of cell. The final method of obtaining uniform flow through all parts of the diaphragm was the operation of Cell 3 in a nearly horizontal position, a hood being placed over the upper electrode and a tray under the lower one to catch the effluent. In Runs 6 and 7 comparatively satisfactory results were obtained, but i t was found that long-continued operation was impossible, due t o the trapping of gas in the middle compartment of the cell. In the final runs, 8 and 9, a gas outlet tube was provided a t the highest corner, and by this means the interior of the cell was kept entirely free from gas pockets. The electrolyte was fed from the lower edge as before and under a constant head of a few inches. Run 8, although productive of acid and alkali of high concentrations, showed a low current efficiency and an increase in voltage, both of which were probably due to the failure of the gases to escape freely through the electrodes. In Run 9 the anode was placed uppermost, and the cell tilted a little more, but in other respects the operating conditions were similar to those of the previous run. A 5-hr. continuous run was made with this cell, and i t amply fulfilled the expectations of horizontal cell operation. The acid concentration of 11.4 per cent was the highest ever reached, and the current efficiency, considering the combined concentration of acid and alkali, had never been equaled. The yields per kilowatt hour were also exceptional for such high concentrations.
It should be borne in mind that the cell used in the latter experimental work was so thin as to have high back diffusion losses, and for this reason it is safe to assume a somewhat improved current efficiency with a slightly thicker cell. With this single modification it is believed that this type of cell and the horizontal arrangement will give quite satisfactory results in commercial operation. ESTIX4TE O F COMMERCIAL PERFORMAKCE O F SODIUM SULFATE CELLSCOMPARED WITH THE ELECTROLYSIS OF SALT
While it is impossible to base final conclusions as to commercial cell performance on the results of a few short runs, an estimate can be made which in all probability can be approximated in practice. At a current density of 115 amps. per sq. ft., the voltage range was 4.2 to 5.0 volts, while the voltage in the caustic soda-chlorine cells is 3.5 to 4 a t a current density of 75 amps. per sq. ft. On a basis of 85 per cent current efficiency and 4.3 volts, which may be obtained by lowering the current density to 100 amps. per sq. ft., the yield would be about 0.65 Ib. of sodium hydroxide, and 0.79 lb. of sulfuric acid per kilowatt hour, which compares very favorably with the yield of 0.70 to 0.72 lb. of sodium hydroxide per kilowatt hour which is claimed for caustic soda-chlorine cells.
Vol. 15, KO.6
Instead of making cost estimates based on complete plant design, it seemed desirable to compare the cost and performance step by step with an Allen-Moore caustic soda-chlorine cell on which actual cost data were available. The important items of cost-namely, interest, depreciation and overhead charges, direct labor, and electric powerare substantially the same per pound of caustic produced for both processes, and the other minor items-such as steam, supervision and control, diaphragms and anodes, and incidental costs-practically offset each other. There is practically no anode cost in the sodium sulfate process, but on the other hand, it is certain that the life of the anode diaphragms will be much shorter than in the caustic soda-chlorine cells. For an installation of horizontal cells, it would be necessary to place several cells one above the other, in order to have the cell capacity per unit of floor space equal to that of Allen-Moore plants. The one item in favor of the sodium sulfate process for the assumed conditions is in saving approximately 0.6 cent per lb. in raw material costs, the sodium sulfate being considered of no value beyond the cost of crystallization. This slight advantage is more than offset, under most conditions, by the difference in the value of the by-products produced by the two processes. Neglecting the products common to both, the sodium sulfate process gives oxygen and dilute sulfuric acid containing sodium sulfate, as against chlorine for the salt electrolysis. Under favorable conditions, the sulfuric acid may be worth slightly more than 1 cent per Ib. of caustic soda produced, while the chlorine is worth between 3 and 4 cents for conversion into bleach or liquid chlorine. Unless the oxygen has an unusual local value, or can be sold or used without being compressed into tanks, it can scarcely counterbalance the net difference, assuming no value for the oxygen, of about 2 cents per lb. caustic soda in favor of the sodium chloride process. ACKNOWLEDGMENT The writers desire to express their appreciation of the courtesy of the Viscose Company of America in permitting publication of this paper. Acknowledgments are also due R. E. Wilson, who directed the course of the investigations, and Max Knobel for his aid in part of the experimental work.
Committee on Photosynthesis of t h e American Association for the Advancement of Science The following Committee on Photosynthesis of the American Association for the Advancement of Science has recently been appointed: C. G. ABBOT,Assistant Secretary, Smithsonian Institution, Washington, D. C. F. G. COTTRELL,Director, Fixed Nitrogen Research Laboratory, Washington, D. C. Moses GOMBERG, Department of Chemistry, University of Michigan, Ann Arbor, Mich. W. J. HUMPHREYS, United States Weather Bureau, Washington, D. C. D. T. MACDOLTGAS., Department of Botanical Research, Carnegie Institution of Washington, Tuscon, A r k , Chairman S. E. SHEPPARD, Research Laboratory, Eastman Kodak Co., Rochester, N. Y.,Secretary E. E. SLOSSON, Director, Science Service, Washington, D. C. H. A. SPOEHR,Department of Botanical Research, Curnegie Institution of Washington, Washington, D. C.
The first work of the committee will be to bring together information on current researches being conducted in various laboratories. Arrangements are being made for some special lectures, and the matter of obtaining funds for the furtherance of research on various phases of this subject will receive early attention.
