420
INDUSTRIAL AND ENGINEERING CHEMISTRY
tween 35” and 25‘ C. If the second fraction is dissolved in water and the solutisn evaporated to the point of saturation with K&O,.%HzO, some potassium chloride will precipitate. The remaining solution may then be recycled to the crystallization step. Thus the process is continuous and cyclic; pure K2C03.iHzO comes out a t one point and pure potassium chloride at another. The balanced flow diagram for the process is shown in Figure 6. This is based on a brine whose chloride content is 0.4% of the potassium carbonate content. Smaller or larger chloride contents would simply lower or raise the temperature at which the two fractions are separated. The flow diagram was based on the solubility data of de Ropp ( 7 ) and of Blasdale (1). The procedure was checked on a laboratory scale and found to be satisfactory. Potassium carbonate was produced containing only 0.06% chloride.
Vol. 36, No. 5
ACKNOWLEDGMENT
Acknowledgment is made to Robert D. Pike, who initiated the work, and to John E. Conley for valuable suggestions and criticisms throughout the investigation. LITERATURE CITED
(1) Blasdale, W.C., J. Am. Chem. Soc., 45,2935 (1923).
(2) Bur. of Mines, College Park, Md., unpublished work. (3) Findlay, A., “The Phase Rule”, 8th ed., London, Longmans, Green and Co., 1938. Hill, A. E., and Miller, F. M., J. Am. Chem. Soc., 49, 669 (1927). Hou, T.P., “Manufacture of Soda”, 2nd ed., A.C.S. Monograph 65, New York, Reinhold Publishing Corp., 1942. 25,256 (1933). Pike, R. D., IND.ENG.CHEM., Teeple, J. E., “Industrial Development of Searles Lake Brines”, A.C.S. Monograph 49, New York, Chemical Catalog Co., 1929. ILISHED
by permission of the Director, U. S. Bureau of Mines.
Silica from ALKALI C A R B O N A T E BRINES
Removal of Dissolved
b The removal of silica from alkaline brines by a two-step process involving the use of a basic magnesium carbonate, hydromagnesite, has been studied. Either compound formation or adsorption or a combination of both was considered possible as the desilication mechanism. Compound formation i s ruled out because of the inconstancy of the mole ratio of magnesia t o silica. Adsorption i s indicated as the correct mechanism by calculating the data i n the form of Freundlich adsorption equations. A logarithmic plot of the Freundlich adsorption isotherms gives straight lines whose slopes are constant. The intercepts of these lines are proportional to the N a 2 C o 3 - K 2 c o 3 molar concentration of the solutions from which silica i s adsorbed. Thus there i s a relation between the Freundlich coefficient and carbonate concentration of the solutions. The basis for this relation i s expiained. A final equation is established for the desilicating efficiency of basic magnesium carbonate as a function of the concentrations of silica and alkali carbonates i n solution. b b
E.
G e r a l d M e y e r and Charles U. S. BUREAU
I
1
I
E.
McCarthy
OF MlbES, COLLEGE PARK,
I
MD.
/
S A PROCESS for extracting K2COs.:Hz0 from Wyomingite, a leucite-bearing rock ( I ) , extraction is effected by a base-
I
exchange reaction with sodium carbonate. During the reaction a considerable amount of silica is leached into solution, along with potassium carbonate. This silica must be removed to make the Wyomingite process practical. The silica is removed by a combination of thermal and chemical means. For thermal desilication, the variation of silica solubility with temperature is pronounced enough to permit crystallization of 8Oy0of the silica present when the solution is cooled from 100” to 25’ C. However, the resulting brine still contains enough silica t o contaminate the products badly unless an additional desilication process is employed. For this additional process the following technique was devised. Its operation was originated in the laboratory and confirmed on a pilot-plant scale. Obviously some chemical desilicating agent must be employed. This desilicating agent must be relatively cheap, completely effective, and easy to handle, and must not introduce any additional impurities into the solution. A slurry of ba3ic magnesium carbonate was found to approach the requirements most nearly, This basic magnesium carbonate was prepared by mixing solu$ions of Epsom salts with solution5 of soda ash. The compound
CONCENTRATION OF AESlDUAL SOe, GRAMS/LITER
Figure 1.
Adsorption Isotherms for Various Carbonate Concentrations
INDUSTRIAL A N D ENGINEERING CHEMISTRY
May, 1944
Table 1.
D a b on Dcrilication Experiments 5iOa, Crame/L. Before After desil. de& 0.90 9.0 0.75 9.0 0.65 9.0 0.60 9.0
Source of Data Pilot plant
Basic MgCO: Moled. 0.0128 0.0193 0.0225 0.0257
Lab.
0.0106 0.0224 0.0316 0.0425 0.0532 0.0708 0.0887 0.1064
7.90 7.90 7.90 7.90 7.90 7.90 7.90 7.90
1.15 0.85 0.60 0.55 0.45 0.40 0.35 0.33
145.5 146.6 145.5 145.6 145.5 145.5 145.5 145.6
119.6 119.6 119.6 119.6 119.6 119.6 119.6 119.6
Lab.
0.0106 0.0183 0.0262
6.40 6.40 6.40
1.25 0.95 0.80
200.4 200.4 200.4
92.6 92.6 92.6
Pilot plant
0.0191 0.0191 0.0284 0.0286 0.0382 0.0478
7.00 7.62 9.00 8.20 9.33 9.32
1.05 0.85 0.80 0.72 0.65
153.0 153.0 153.0 153.0 153.0 153.0
139.0 139.0 139 0 139.0 139.0 139 0
Pilot plant
0.0251 0.0453 0.0932
7.00 7.00 6.80
1.00 0.75 0.55
208.0 208.0 208.0
122 5 122.5 122.6
Lab.
0.0306 0.0461 0.0615 0.0790 0.1024
1.70 1.40 1.20 1.10 0.88
325.0 325.0 325 0 325.0 325.0
15.0 15.0 15.0 15.0 15.0
11.4 11.4 11.4 11.4 11.4
1.00
NazC01, KzCOa, Grama/L.* QramdL. 61.2 168.8 61.2 168.8 61.2 168.8 61.2 168.8
I
Lab.
0.0123 0.0193 0.0257 0.0322
5.50 5.50 6.50 5.50
1.75 1.50 1.40 1.20
257.0 257.0 257.0 257.0
141.0 141.0 141.0 141.0
Lab.
0.0082 0.0163 0.0220 0.0295 0.0333
4.96 4.96 4.96 4.96 4.96
1.23 0.88 0.78 0.63 0.57
200.0 200.0 200.0 200.0 200.0
118.6 118.6 118.6 118.6 118.6
Lab.
0.0154 0.0233 0 0311 0 0493
4.00 4.00 4.00 4.00
2.00 1.85 1.55 1.30
285.0 285.0 285.0 285.0
276.0 276.0 276.0 276.0
thus precipitated was filtered and washed free of sulfate, and then repulped t o facilitate handling. Experiments designed t o detect any variation in the basic magnesium carbonate precipitate, due t o varying the mole ratio of magnesium sulfate t o sodium carbonate, indicated that only one compound was formed. Chemical analysis of this compound showed it t o have the empirical
Table II. Basic M COa S i 0 1 ReasftlgO, moved, Alole/L. i\lole/L. 0.0420 0.0858 0.0732 0.0908 1.048 0.0924
Ratios of Hypothetical Compounds
Mole Ratio MgOI Si01 0.4895 0.806 1.134
Change in Mole Ratio 0.4895 0.4165 0.338
0.1224 0.1844 0.2460 0 3160 0.4096
0.1615 0.1665 0.1698 0.1715 0.1766
0.7578 1.107 1.448 1.842 2.333
0.758 0.349 0.341 0.394 0.491
0.0512 0.0772 0.1028 0.1288
0.0633 0.0660 0.0683 0.0699
0.809 1.158 1.506 1.844
0.809 0.349 0.348 0.338
0.0512 0.0772 0.0900 0.1028
0.1348 0,1373 0.1390 0.1398
0.380 0.5625 0.6475 0.7355
0.380 0.1825 0.085 0.088
0.424 0.0896 0.1264 0.1700 0.2128 0.2832 0.3548 0.4256
0.1135 0.1168 0.1210 0.1222 0.1235 0.1244 0.1262 0.1256
0.3735 0.767 1.045 1.392 1.722 2.278 2.832 3.386
0.3735 0.3935 0.278 0.347 0.330 0.656 0.554 0.554
Basic Mole XIgCOt Si02 Re- Ratio a8 AI 0 moved, MgO! hIole?L.' Mole/L. SiOs 0.0764 0.116 0.659 0.0764 0.110 0,6945 0.1136 0.135 0.842 0.1144 0.123 0.931 0.1528 0.143 1.089 0.1012 0.144 1.327
Change in Mole Ratio 0.659 0.356 0.1475 0.089 0.138 0.258
0.1004 0.0999 1.005 0.1812 0.1041 1.740 0.3728 0.1037 3 1594
1.006 0.735 1.854
0.0330 0.0652 0.0880 0.1180 0.1332
0.0603 0.0678 0.0693 0.0719 0.0730
0.5475 0.962 1.266 1.641 1.826
0.5476 0.4145 0.303 0.376 0.185
0.0616 0.0932 0.1244 0.1972
0.0333 0.0358 0.0408 0.0450
1.849 2.803 3.048 4.385
1.849 0.754 0,445 1.337
42 1
formula 4Mg0.3COZ.4Hz0. An x-ray diffraction study of t h e dried substance indicated that its crystallographic structure corresponds t o t h a t of hydromagnesite, 4Mg0.3C02.4H20. Thus it is concluded t h a t the compound involved in this study is a hydrated double salt .of magnesium carbonate and hydroxide, which may be written 3MgCOa.Mg(OH)z.3H~0. The solutions desilicated with the basic magnesium carbonate varied in composition from 14% Na&Os-5% K2COs to 20% NazCOs-ZO% KzCOa t o 6% NazCOa-48% K&03. I n all cases the starting concentration of silica was not varied appreciably, but the final concentration was recorded as a function of amount of desilicating agent used. The reaction temperature was held at 27" C., and the time allowed for equilibrium t o be attained was one hour. Table I summarizes the experiments conducted under the conditions described. Some of the data were obtained in the laboratory, where the batch consisted of 100 ml. of solution; other data were collected in the pilot plant, where the batch consisted of 90100 gallons. The fact that d a t a from these two sources can be correlated verifies the adaptability of the process t o large-scale operat ion. M E C H A N I S M OF D E S l L l C A T l O N R E A C T I O N
An investigation indicated t h a t the reaction can take three possible courses: (a)A definite compound can form between the basic magnesium carbonate and the silica; ( b ) the silica can be adsorbed on the surface of the reagent; or (c) a combination of compound formation and adsorption can occur. Possibility (a) can be ruled out by the following evidence. If a compound is formed, it should show a constant mole ratio of magnesia to silica. This mole ratio can be calculated from the values of the silica concentration before and after desilioation and t h e values of basic magnesium carbonate used and Mg+ concentration in the desilicated solution. An attempt t o ascertain the latter by standard chemical analysis resulted in figures which bordered on the limit of quantitative detection. Spectrographic determinntion of the solids obtained on evaporating the solution t o dryness further indicated the low concentration of Mg++. Coneequently, since the concentration of magnesium in the solution wab of the order of 0.1 millimole per liter, no error is introduced in assuming t h a t all of the magnesia added as basic mdgnesium carbonate is withdrawn from the solution with the d i c a after desilication is complete. Thus the ratio of magnesia t o silica can be calculated for the solid obtained by desilication (Table 11). The inconstancy of these calculations rules out the possibility of compound formation, In addition, x-ray data substantiate the contention t h a t no compound is formed. Examination of the basic magnesium carbonate after it had been used for desilicating revealed a pattern similar t o t h a t of amorphous silica. That is, only a few broad diffraction bands were discernible. This pattern was compared with that of a prepared magnesium silicate and found t o difler, despite the fact t h a t the prepared silicate was nearly amorphous itself. The inference is t h a t a coating of amorphous silica is adsorbed on and surrounds each particle of basic magnesium carbonate. Since the reaction does not yield a compound, the possibilities of an adsorption phenomenon or of compound formation plus adsorption must be examined. The standard procedure for testing possible adsorption is t o calculate the data in the form of the Freundlioh adsorption isotherm equation, +
z/m = kcl/n where
2 = Si02 removed, moles m = MgO required, moles
c = residual Si02 concn., pram k,n
-
liter constantu
INDUSTRIAL AND ENGINEERING CHEMISTRY
422
Results for all experiments are listed in Table 111 in order of increasing concentration of sodium carbonate plus potassium earbonate. A plot of these data on logarithmic paper produces H. series of straight lines. This plot (Figure 1) indicates that the removal of eilica from the alkaline solution proceeds along the lines of an adsorption. Furthermore, since there is no tendency of the lines toward curvature, compound formation followed by adsorption is improbable (3)
Table 111.
Calculated Silica Adsorption Data
Baaic MgCOt. Mole/L. 0.0128 0.0193 0.0225 0.0257
Si01 Removed, Moia/L. 0.1348 0.1373 0.1390 0.1398
Mole8 SiOt Removed/ Mole Basic MgCOr 10.53 7.12 6.18 5.44
0.0106 0.0224 0.0316 0.0425 0.0532 0.0708 0.0887 0.1064
0.1135 0.1168 0.1210 0.1222 0.1235 0.1244 0.1252 0.1256
10.71 5.21 3.83 2.88 2.32 1.76 1.41 1.18
1.15 0.85 0.60 0.55 0.45 0.40 0.35 0.33
2.24 2.24 2.24 2.24 2.24 2.24 2.24 2.24
0.0105 0.0183 0.0262
0,0858 0.0908 0.0924
8.17 4.96 3.53
1.25 0.95 0.80
2.56 2.56 2.56
0.0191 0,0191 0.0284 0.0286 0.0382 0.0478
0.116 0.110 0.135 0.123 0.143 0.144
6.10 5.76 4.75 4.30 3.74 3.01
1.05 1.00 0.85 0.72 0.65
2.45 2.45 2.45 2.45 2.45 2.45
0.0082 0.0163 0.0220 0.0295 0.0333
0.0603 0.0678 0.0693 0.0719 0.0730
7.35 4.16 3.15 2.43 2.16
1.23 0.88 0.78 0.63 0.57
2.74 2.74 2.74 2.74 2,74
0.0251 0.0453 0.0932
0.0999 0.1041 0.1037
3.98 2.30 1.11
1.00
0.75 0.55
2.85 2.85 2.85
0.0306 0.0461 0.0615 0.0790 0.1024
0.1618 0.1665 0,1698 0.1715 0.1755
5.28 3.62 2.76 2.17 1.71
1.70 1.40 1.20 1.10 0.88
3.18 3.18 3.18 3.18 3.18
0.0123 0.0193 0.0257 0.0322
0.0633 0.0660 0.0683 0.0699
6.15 3.42 2.66 2.17
1.75 1.50 1.40 1.20
3.50 3.50 3.50 3.50
0.0154 0.0233 0.0311 0.0483
0.0333 0.0358 0.0408 0.0450
2.16 1.53 1.31 0.091
2.00 1.85 1.55 1.30
4.70 4.70 4.70 4.70
Si02 Concn. Dissolved after DeAlkali silication, Carbonates, Grams/L. Moles/L. 0.90 2.03 0.75 2.03 0.65 2.03 2.03 0.60
0.80
The lines of Figure 1 are parallel as wcll as straight. This signifies that the exponent of the residual silica concentration is constant for the various experiments. The only factor varied in the experiments was the concentration of sodium carbonate and potassium carbonate in the solution; also the only factor varied in the plot is the value of the intercept. The intercepts are equal to k ; therefore an attempt was made to correlate these values with the alkali carbonate concentrations of the solutions. The result is shown in Figure 2. D E R I V A T I O N OF STRAIGHT-LINE R E L A T I O N
A straight-line relation can be derived between the Freundlich
k and the concentration of alkali carbonates when these variables are plotted on a logarithmic scale. The following equation, obtained from Figure 2, holds for concentration between 2.0 and 4.7 moles per liter:
k = 131.3 wherc C
=
C 3 e i 4
cmoncn. of alkali carbonates, moles/liter
If an attempt is made to give k theoretical significance by comparing the Freundlich equation with the Langmuir adsorption
Vol. 36, No. 5
isotherm (2), it becomes apparent that k is a measure of the fraction of silica molecules condensing on the basic magnesium carbonate surface as compared with the fraction evaporating into the solution phase. Because the attractive force between silica and solid and the attractive force between silica and solution governs this fraction, the concentration of the major constituents of the solution will be a controlling factor. That is, the attractive force between silica and solid is unchanged (the same adsorbent, basic magnesium carbonate, is used throughCONCENTRATION OFALKALI out) ; therefore it CARBONATES.MOLS/LITER may be concluded that the attractive Figure 2. Correlation of Frcundlich K force between silica Values with Solution Concentration, and solution is changing. This force would undoubtedly be some function of the Concentration of the major solution constituents. However, the above picture is probably oversimplified, and the empirically derived reiation between k and the concentration would be expected t o hold for limited concentration ranges only. No particular significance can be attached t o the value obtained for n. Although l / n usually varies between 0 and 1, these limits are by no means fixed, especially for solution adsorptions. That l/n should remain constant seems in line with the fact that, throughout the experiments, the chief variable was the nature of the solution from which silica was adsorbed rather than the adsorbent or the adsorbate. Numerically the value for l/n as obtained from Figure 1 is 1.55. Hence, for carbonate concentrations between 2.0 end 4.7 moles per liter we obtain the following complete expression for adsorption of silica with prepared hydromagnesite:
= Narc02 and KzCOR,moles(1iter x / m = Si02 removed per mole basic MgCOa, moles
where C
ACKNOWLEDGMENT
Acknowledgment is made to Robert D. Pike, who initiated the work, and t o John E. Conley for valuable suggestions and criticisms throughout the investigation. LITERATURE CITED
(1) Bur. of Mines, College Park, Md., unpublished work. (2) Glasstone, S., Text Book of Physical Chemistry, 1st ed. pp. 1176-8,New York. D. Van Nostrand Co., 1940. (3) Roller, P. S., and Ervin, Guy, Jr., J . Am. Chcm. SOC.,62, 46! (1940). PUBLICWED by permission of the Director, U. 6. Bureau 01 Mine%.
