April, 1922
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEIMISTRY
281 -
Recrystallization of Niter Cake at 120' By Blair Saxton2 DEPARTMENT OF CHEMISTRY, YALEUNIVERSITY, NEW HAVEN,CONN.
In the following paper equations and curves have been deueloped by means of which the crystallization at 12' of the various solid phases of the system NazS04-HzS04-H~O can be followed. Calculations have been made showing the extent of separation into solution and solid, especially as NazS04.1OH20, which can be eflected at 12".
I
N a previous a r t i ~ l e the , ~ possibility was considered of separating niter cake into its components by recrystallization a t 25". Since that time, solubility results have been obtained by Foote4 at 12" for the system NazSOJH2SO4-H20, so that the possible separations at the latter temperature can now be determined. This system has also been studied recently by Pascal and EroSand by Dawson6 a t a number of temperatures. Dawson has also given results showing the amounts of Glauber's salt which can be crystallized a t various temperatures. The results, as given by Foote, expressed in per cent by weight, are given in Table I. TABLEI SOLIDPHASES
HzS04 NazSOclOHzO ............................. 0.00 6.47 12.90 hTazS04.10Hz0~NazS04.NaHS0~. . . . . . . . . . . . 16.52 NazS0~NaHS04........................... 21.95 NazSOa.NaHSO4~NaHS04.HzO.. . . . . . . . . . . . 27.96 NaHSO4.HzO .............................. 33.81, 36.69 58.79 COMPOSITION OF SALTS NazSOclOHzO............................. 0.00 ATazSOa.NaHSO4.. ......................... 18.70 NaHSOd.Hs0 .............................. 35.50 1 By difference.
SOLUTION NazSO4 9.49 12.87 18.02 32.93 29.59 26.42 15.16 11.82 4.33
Hz01 90.61 80.66 69.08 50.55 48.46 46.62 50.96 51.49 36.88
44.10 81.30 51.46
65.90 0.00 13.04
The results are plotted in Fig. 1, with per cents of acid in solution as abscissae, and per cents of sodium sulfate as ordinates. A diagonal is drawn across the diagram. This diagonal is then the hypothenuse of a right-angled triangle, each leg of which represents 70 per cent of acid and sodium sulfate, respectively. The per cent of water in solution is then represented by either the vertical or the horizontal distance of any point from the hypothenuse plus 30. The lines radiating from the origin represent the compositions of niter cakes of 20, 25, 30, and 35 per cent acid, together with a line for h'a2SO4.NaHS04 and a line tangent to the curve AB. This latter line, as will be explained later, gives a graphical method of determining from what concentration of solution the maximum amount of Glauber's salt will crystallize at 12". The intersection of any of the other lines from the origin with the solubility curve gives the composition of solution which first becomes saturated with the solid represented by that branch of the solubility curve intersected. Starting from the solubility of pure Glauber's salt there are two univariant points, at each of which two solids are in equilibrium. The curves correspond to divariant systems with one solid phase in each. The phase NazS04 which is present a t 25' drops out a t 12". The branches of the curve are not straight lines, except in the case of BC. Received August 8, 1921. Assistant Professor of Chemistry. *THISJOURNAL,10 (1918), 897. 4 I b i d , 11 (Isle), 629. 5 Bull. SOC. chim., 25 (IQlQ), 35. 8 Brit. Patent 127,677 (1919); C A , 13 ~1919),2421 1
2
In our former paperJ3because of lack of data, the lines were assumed to be straight between the univariant points. Previous calculations of this kind have, in general, been made on similar assumptions. Dawson6 has used the actual curves. He has recognized that a t a certain acidity, which he has called "the optimum acidity," a maximum yield of Glauber's salt is obtained. Our *identical conclusion (see below) was reached entirely independently. When the actual curves are considered some interesting results a t once appear, especially in the case of the branch of the curve representing the solubility of Glauber's salt.
FIG.1
THE CRYSTALLIZATION OF NazSOa.lOH2O Thc solubility of this solid is represented by the curve AB in Fig. 1. If we designate the concentrations of sodium sulfate and sulfuric acid in a water solution, saturated with Glauber's salt by 2 and y, respectively, we obtain the following empirical equation for the curve:' x = 5.58
86.6
4- wy
The average deviation of values of 2 calculated by means of this equation from the values determined is *0.40. If we represent the weights of hTaaSO4 and H2S04 in solution before crystallization by n and s and the weight of NazS04.10HzO separating from solution by z, then a t any point along AB the following relat,ion must exist: %-0.4410~ = S
2 -
5'58
d-
86.6 19.69 - y
Y
Y
and, solving for z, 7
The curve is very nearly a section of an equilateral hyperbola, the
equation of which can be written
5:
P
n
b + 6. -Y
c were evaluated by the method of least squares.
The values of a, b, and
The equation so derived represents the experimental facts better than any other simple equation, whether quadratic, logarithmic or exponential, which the author has tried.
THE JOURNAL OF INDUs'TRIAL AND ENGINEERING CHEMISTRY
282 z =2.268
(%
-
y)
=
2.268
5.58
+ *y)].86 6
Examination of this equation shows that as
X decreases,
2J
the value of z increases for fixed values of n and s. In other words, the amount of NazSOd.lOHzO separating reaches a maximum when the concentration of Nazi304 divided by the concentration of H2S04in the saturated solution is a minimum. This condition is represented graphically as the point at which a line from the origin becomes tangent X and no other line from
to AB. The slope of this line is
Y
the origin can intersect or touch AB and have a lesser slope. This point of maximum crystallization may be calculated by solving 5.58
2 -=
+86.6 19.69-y Y
Y X 21,
for minimum value of
=
+ my)] 86.6
2.268%-3.181~,
or, for 100 parts of niter cake, In this case s represents the per cent of acid in the niter cake. The weight, w, of water in the solution at crystallization is obviously given by the equation
loo-@ Y
S
+ Y).
Substituting the value of X, in the terms of y: this becomes =
(
-
1773 94.43' (19.69-y)y -l>"
For the maximum value of z, when y is 11.83, we find w = 6.052s.
The weight, w', of water with which the niter cake must be leached is w' = w
+ 0.559s = 1.268%+ 4.274s.
The total weight, W, at crystallization is W
= w
+ 0.5592 + + s = 2.268% + 5.274s. ~t
For 100 parts of niter cake w' = 126.8 3.006s and W = 226.8 3.006s. The maximum point may better be recognized by the titer of the solution, which should show 11.83 per cent acid, or by its density. In speaking of crystallizing Glauber's salt we refer to crystallizing the hydrate to this maximum point. A niter cake containing more than 41.6 Per cent of acid will not deposit Glauber's salt from solution a t 12", but rather the double salt Na2S04.NaHS04. This can be calculated from the values of 2 and Y at the Point of maximum crystallization. The percentage acidity of the solute is obviously
+
+
100. Since y at this point is 11.83, X, calculated from the
X+Y
x = 43.79-0.6576~.
Starting with this and the equation %-0.81302; S-O11870z
x
E
y'
equations similar to those given for Wa804.10H20can be developed. For any point on the line BC and for any weight of niter cake we have: (43.79-0.6576~)~ - ny 8.19-0.936~ w=(s-O.l870z) 56.21 - 0.3424) Y z =
-2 is Y
5
a t its lowest value. That x increases as - decreases can
Y
be seen by solving the original equation for,z and arranging in the following form: z = 5.35,
+
4 35s-12 0.187x- 0.813 Y
For all niter cakes which will saturate water solution directly with Na2S0*.NaHSO4,the numerator of this fraction is positive, while at any point on the line BC the denomination is negative. The fraction, therefore, becomes smaller and z becomes larger
z = 226.8-5.449s.
-W=
The solubility of this phase is represented by the line BC, the equation of which is
The maximum value of z is reached at C, for here
for maximum value of z. Either of these methods gives y = 11.83, which is checked by the graphical method. This value of y is independent of the values of n and s; hence it is the same for any niter cake. Obviously it is the point on AB at which to stop crystallization. If we substitute this value of y in the general equation for z, we obtain z
equation of the line AB, becomes 16.60. These values of 2 and y give 41.6 as the per cent of acid in the solute or dissolved niter cake. THECRYSTALLIZATION OF Na2S04.NaH804
(-
or the equat,ion 5.58
Vol. 14, No. 4
a t which 2~ = 27.96,
RS
z
- decreases. 2/
At the point C,
2; = 1.555%-1.414s w = 2.110~-0.4852%.
Or, if 100 parts of niter cake are considered, z = 155.5-2 969s w = W' = 2.595s - 48.51 W=w 100 = 2.595s 51.49.
+
+
At this point, C, the solution should show 27.96 per cent of acid. This is the point to which we have calculated all crystallizations3 of this double salt. Only a niter cake containing a t least 33.4 per cent of acid can directly saturate water at 12" with this double salt. This can be calculated from t,he values of 2 and y for point B in Fig. 1. If the water solution of a niter cake containing less than this per cent of acid were crystallized a t 12" so that the solution would contain more than 16.52 per cent of acid, corresponding to point B in Fig. 1, Glauber's salt would first crystallize until point B had beefi reached, then the double salt would separate. If equilibrium were reached, the solid phase would be double salt only until the concentration had been carried so far that the acidity of the solution in equilibrium with the solid had reached that corresponding to point C in Fig. 1, namely, 27.96 per cent. THE CRYSTALLIZATION OF NaHS04.HZO The solubility of this phase is represented by the curve CD, which, as far as it has been carried, follows the equation x = 35.15- 4 ~ ( 1 1 5 . 5 - y ) -2360.
This is the equation for a circle the center of which is only slightly t o the right of D and a little above C in Fig. 1. For points beyond D the equation is useless, but for the curve as determined it serves very well. The point D,
April, 1.922
THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEiMISTRY
furthermore, cannot be far from the univariant point at which NaHS04 and its monohydrate coexist, so in considering CD as a regular curve we cannot be f k wrong. From this equation and the relation n-0.51462 = E, S-0.3550~
283
if we consider 100 parts of the original cake, containing s parts of acid, the weight of Na~S04.NaHS04 which will separate after Glauber’s salt has been removed is 0.767s. This linear relation between the amount of acid sulfate and the acid in the original cake is plotted in Fig. 2. The amount
y
Values for x and w as follom are obtained s [ 35.15- dy(115.5-y) -23601 -%Y = 0.3550 [ 35.15- dy(115.5-y) -23661 - 0 . 5 1 4 6 ~ M =
(s-0.35502) (64.85
+ d~!115.5-~)-2360 Y
These calculations are not so laborious as they appear, since the value of the radical is easily simplified for a given value of y. This solid phase contains 35.50 per cent of acid and only a niter cake containing at least 5 2 . 4 per cent of acid can directly saturate water with this solid at 12’. If, however, Na2SO4.NaHSO4has been crystallized from the solution of a cake which originally contained less than 52.4 per cent of acid, and the per cent of acid in the solute has thereby been increased, KaHS04.HZ0 may be separated from solution, Any desired acid concentration along CD may then be reached. PROCESSES PROCESS A-Remove NazS04.10Hz0 from solution by using the proper amount of water to reach but not exceed the point of maximum yield; or completely leach with the correct amount of water. A method of calculating this amount has been given. The solid if properly washed will be fairly pure Glauber’s salt. Per 100 parts of niter cake the maximum amount, x , of Glauber’s salt which can be obtained in this way is (226.8- 5,449s). This expression is linear with respect to s; in Fig. 2 z has been plotted against s. From this plot one can read at once the maximum yield of the decahydrate obtainable from 100 parts of a cake which is s per cent acid. The weight of water, w’,with which the cake should be leached is also a linear function of s, as can be seen from Fig. 3. No advantage would be gained by adding fresh niter cake and water to the residual solution from this process. One would accomplish nothing except the adding of the solution residues as each crystallization or leaching is carried out. PROCESS c*-Remove NazS04.NaHS0~ from solution by concentrating to the point C in Fig. 1. The solution will contain 2 8 . 0 per cent of acid. The water required for complete leaching has been discussed. The solid has 18.7 per cent of acid, and, unlike Glauber’s salt, has little apparent value. It is simply an added batch of 18.7 per cent niter cake. There are two reasons, however, for its removal from solution. First, it permits concentration of acid in the residual solution up to 28 per cent. Second, it may be put through Process A either alone or with a new lot of niter cake and more Glauber’s salt removed. Both z and w’ are linear functions of s, and are plotted in Figs. 2 and 3. The equations relating to these values have been given. PROCESS Ac-Process C may follow A; that is, NazS04.lOHzO may be first removed and then Na2S04.NaHS04 may be separated from the filtrate. One hundred grams of solute, after Glauber’s salt has been removed from the solution of any niter cake, will contain 58.37 g. of NazS04 and 41.63 g. of HzS04. On concentrating to 160 g. or t o the appropriate titer, 31.9 g. of NaZSOb.NaHS04, composed of 25.9 g. of NazSO4 and 6 . 0 g. of acid, will separate. Or, 8 Process B was used in our previous paper to designate the separation of NaaSOi which is here abse:t.
% ac/d in oriqinal sake FIG.2
FIQ.3
of water in solution after the acid sulfate has been removed is also a linear function of s and is plotted in Fig. 3. PROCESS D-Recrystallize the Na2S04.NaHS04 from C. If Glauber’s salt only is desired, the above acid sulfate can be put through Process A. One hundred grams of Na~S04.NaHS04so treated will give 124.9 g. of the decahydrate, of which 55.1 g. are NaZR04, by evaporating to 283.0 g., or by leaching with 183.0 g. of water. The titer of the solution by which this operation may be followed has been given. In this way 67.8 per cent of the Na2S04 in the acid sulfate may be recovered as Glauber’s salt. PROCESS D’-If it is desired to concentrate the solution in sulfuric acid, NazS04. NaHSOd may be crystallized from the solution from D by using Process C. The weight of solute after 124.9 g. of Glauber’s salt have been removed is 44.92 g., of which 26.22 g. are sodium sulfate and 18.70 g. are sulfuric acid. From the solution 14.33 g of NazSO4.NaHS04, containing 11.65 g. of Na2S04and 2.68 g. of HzS04, can be removed. By these two operations 8 2 . 1 per cent of the sodium sulfate in the original acid sulfate as Na2SO4.NaHSOa, 67.8 per cent as NazS04, and 1 4 . 3 per cent as NaZS04. NaHS04, will have been removed from solution and 85.7 per cent of the acid will be left in solution. Naturally this cycle could be repeated as many times as desired, each time recrystallizing the NazS04. NaHS04. In this way practically all of the acid sulfate could be worked into Glauber’s salt, but the number of operations necessary makes such a cycle seem impracticable. However, these operations could be run
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T H E JOURNAL OF INDUXTRIAL A N D ENGINEERING CHEMISTRY
continuously, new niter cake being added and the sodium nearly entirely separated as Glauber’s salt. PROCESS Am-Glauber’s salt has been removed from solution, then Na2S04. NaHS04, and this has been recrystallized by Method D. This combined process gives nothing but decahydrate, but leaves two solutions, one after C, containing 28 per cent of acid, the other after D, containing only 11 8 per cent of acid. Again considering 100 parts of niter cake, the. total amount of decahydrate obtained by this method is 226.8-4.491s and hence is a linear function of s. This and the water in the fiolution after the last step are plotted in Figs. 2 and 3. PROCESS cn-Na2S04. NaHS04 has been crystallized from the solution of the original niter cake and then the acid sulfate has been recrystallized by Process D. The amount of Glauber’s salt obtainable in this way from 100 parts of niter cake containing s per cent of acid is given by the equ& tion z = 194.2-3.708s. This and the quantity of water necessary for the recrystallization is a linear function of 6. The lines showing these relations are in Figs. 2 and 3, respectively. PROCESS ACD’-If this process were used, from 100 parts of niter cake which was s per cent acid, the total amount of Glauber’s salt obtained would be 226.8-4.491s and the Na2S04.NaHS04 crystallized would be 0,1099s. PROCESS E-Remove NaHS04.HzO from solution. As has been stated, this cannot be done directly except in the case of a niter cake or solute high in acid. This process may, however, follow C, although the number of operations is such that it is practically excluded. If a solution high in acid were desired even at the expense of a complicated process and the loss of some acid in the solute, E might be resorted to. Solutions from 28 per cent to a t least 59 per cent acid could be obtained. One hundred grams of solute from C will contain 47.6 g. of Na2S04 and 52.4 g. of HzS04. The results which can be obtained from such a solute can be calculated by means of the general equation for this acid sulfate.
RESULTS The results which can be obtained by the use of these various processes are given in Table I1 and plotted in Fig. 2. TABLE I1 His04 H?SO4 Recov- Na?SO4 in Wt.of Acidered in Removed Niter No of Water Wt.of Solid, a = ity of Cake Treat- Opera- to NaiSOa.lOH~0.c = Solution Solution as Solid Per cent ment tlons Leach NaiSOdiaHSO4 Per cent Per cent Per cent 100.0 64.9 11.8 A 1 187.0 a = 1 1 7 . 8 97.7 10.2 28.0 1 3 . 4 C = 96.1 C 85.7 80.5 2 28.6 a=117.8;c=15.3 28.0 AC 75.5 1 0 0 . 0 28.0;ll.g 28.0 a=137.0 ACD 3 66.2 2 8 . 0 ; 1 1 . 8 100.0 2 4 . 4 a=120.0 CD 53.3 100.0 11.8 1 202.0 a = 9 0 . 6 A 39.2 88.1 28.0 1 16.4 C = 81.3 C 74.0 85.7 2 3 5 . 8 a= 90.6;c=19.2 28.0 AC 67.3 2 8 , O ; l l .8 100.0 35.1 a=114.5 ACD 3 59.7 2 8 . 0 : 1 1 . 8 100.0 2 21.2 a= 101.5 CD 39.9 100.0 11.8 1 217.0 a = 6 3 . 3 A 77.1 58.6 1 29.3 c = 66.4 28.0 C 66.6 85.7 2 4 2 . 9 a= 6 3 . 3 ; c = 2 3 . 0 2 8 . 0 AC 58.0 28.0;11. 8 100.0 42.1 a= 92.1 ACD 3 52.2 2 8 . 0 ; l l . S 100.0 CD 2 3 8 . 0 a=: 8 3 . 0 35 A 1 232.0 a = 3 6 . 1 11.8 100.0 24.4 28.0 72.4 64.5 C 1 4 2 . 3 C = 51.6 AC 2 50.0 a= 36.1;c=26.8 28.0 85.7 58.0 ACD 3 4 9 . 1 a = 6 9 . 6 28.0.11 8 100.0 47.1 CD 2 54.8 a= 64.4 28.0111:8 100.0 43.7
An examination of Table I1 and Fig. 2 shows that for the Processes A, C, ACD, and CD the yield of crystals decreases as the per cent of’acid in the niter cake increases. Only in one case, that of AC, is the reverse true for one of the solids, Na2SO4.NaHS04. Since Process A has preceded C in this combined process and the yield of Glauber’s salt decreases in A much faster than that of the acid sulfate increases in C, the result is that the total weight of sodium
Vol. 14, No. 4
sulfate, calculated as NazS04, falls off as the per cent of acid in the original niter cake increases. A comparison of the methods for producing Na2SOd. lOHtO shows that, of those here outlined, ACD is the most effective for a niter cake of less than 41.6 per cent acid, while CD, involving one less operation, is most effective when the original cake has more than that amount of acid. The difference in the yield of Glauber’s salt is probably not great enough to warrant the extra operation required by Process ACD even with niter cakes low in acid. The other method, A, is the simplest. The yield of Glauber’s salt, however, is the poorest. If the niter cake is comparatively low in acid, say, between 20 and 30 per cent, this process compares quite favorably with CD and is less expensive. We would recommend it for such niter cakes. The relations between the yields obtained by these three processes can be seen by a study of Table 11, or by a glance a t Fig. 2. If it is desired to crystallize ?Ja2S04.NaHS04, the amount which can be separated can be found in the table or read from the figure. In order to test these calculations, crystallization experiments were carried out with synthetic 30 per cent niter cake so that in the one case Glauber’s salt was separated near the point of maximum yield, and in the other case l\a2S04.NaHSO4 was crystallized. I n the first experiment 70.03 g. of anhydrous sodium sulfate were treated with 31.35 g. of 95.6 per cent pure acid. The cake, therefore, weighed 100.00 g. and contained 29.97 per cent of acid. To this were added 217.74 g. of water which, with that in the acid, made the total water 219.12 g. On warming slightly all sodium sulfate dissolved and Glauber’s salt only crystallized at 12’ after the solution was inoculated. This solution, in a stoppered Erlenmeyer flask, was left in a thermostat at 12 O for 4 days during which it was frequently shaken. At the end of that time the Glauber’s salt was filtered through a suction filter and quickly pressed between filter paper until it was dry. The yield was 60.2 g. The solution, on analysis, proved to contain 11.62 per cent of acid. The yield of Glauber’s salt calculated is 63.4 g. The agreement is satisfactory, considering the roughness of the experiment. In the second experiment 100.34 g. of synthetic niter cake, containing 30.01 per cent of acid, were dissolved in 56.73 g. of water. After warming until the sodium sulfate had dissolved, the solution was treated as in the previous experiment. The crystals were quickly filtered, washed twice with small portion of D’Ans’ solution (5 volumes of water, 1 volume of concentrated sulfuric acid, and 7 . 5 volumes of ethyl alcohol), then with a little absolute alcohol, and finally with ether. After being air-dried, it weighed 4 0 . 4 g. Its analysis gave 18.58 per cent of acid, while the calculated per cent for NanSOd.NaHS04 is 18.70. From the total weight a t crystallization and equations previously given the calculated per cent of acid in the solution was 19.32. That found was 19.44, For this acidity and for the weight of niter cake taken the estimated yield of acid salt is 43.2 g., which is in satisfactory agreement with the weight found. Butler and Dunnicliffg have apparently questioned the conclusion in the previous papera that we obtained Na~S04. NaHS04 in confirmation of calculations there developed. Their criticism is directed at the method of washing the acid sulfate as recommended by D’Ans. This consists in washing first with a mixture of alcohol and dilute sulfuric acid, then with alcohol, and finally with ether. They have shown that an acid sulfate such as NaHS04.HzO on prolonged extraction with moist ethyl alcohol will lose water and acid,
-
8
J . Chem. Soc., 117 (1920), 649,
