I"
LEO GARWIN' and JAMES M. WINTERBOTTOM* Oklahoma A. and M. College, Stillwater, Okla.
Liquid-Liquid Extraction of Zinc Chloride from Sulfate with Furfural A commercial process which can yield zinc chloride directly is proposed for separating aqueous mixtures of these salts 1
UNIT
r.
OPERATION of liquid-liquid extraction was formerly employed solely for separating organic compounds; recently, however, it has been extended to inorganic salt systems such as nickelom and cobaltous chlorides ( 5 ) , ferric and aluminum chlorides ( I O ) , and sodium dichromate and vanadic acid (23). Spray tower capacity coefficients for inorganic salt extractions have been reported by Kylander and Garwin (75) and Geankoplis and Hixson (6). A perforated plate pulse column has been used to separate rare earth metal nitrates in nitric acid, using tributyl phosphate as the solvent (7). Mass transfer rates for uranyl nitrate between water and two organic solvents in a wetted wall column have been determined by Murdoch and Pratt (18). For successful operation, metal chloride systems generally have one common requirement-high concentration of chloride ion from a n outside source. Mostly, this has been hydrochloric acid, but foreign metal chlorides have been used. Concentration of the salt mixture to be separated is also beneficial. Many mechanisms have been proposed for such extractions, the most common being the hydration, complex ion, and activ-
Present address, Kerr-McGee Oil Industries, Inc., Oklahoma City, Okla. * Present address;'Dow Chemical Co., Midland, Mkh.
ity coefficient hypotheses. For extraction under any circumstance, the solkent must have high dissolving power for the extractable salt in anhydrou8 form ( 5 ) . Extraction separation of salt mixtures having a common anion is fairly well established, and it is surprising that attempts have been made only rarely to separate mixtures of salts having the same metal cation ( 7 1 , 12). Even here, the purpose was primarily analytical. Certain common cation separations are commercially important-eg., preparing analytical grade reagents. Also, in manufacturirg lithopone and viscose rayon, zinc sulfate used must be low in chloride. Lithopone containing chloride tends to darken with age (7, 19). A good solvent must have not only solvent power for the anhydrous form of the salt to be extracted while being ineffective toward the second component, but it should have also mutual immiscibility with water, a specific gravity far from unity, cheapness, availability, nontoxicity, stability, noncorrosiveness, and ease of recovery. Literature on solubility of anhydrous zinc sulfate and chloride in organic solvents uoints .to furfural as a uossible solvent. Trimble (22) reports zinc chloride as about 2094 soluble in furfural, but the sulfate as practically insoluble-less than O.?l%. T h e SUIfate heptahydrate is even less soluble
than the anhydrous salt. Therefore furfural was selected as the solvent. I t was necessary to establish first whether zinc chloride could be extracted at any concentration from a simple aqueous solution and from mixtures with the sulfate, and, if not, whether introducing extraneous electrolytes would make extraction possible. Foreign electrolyte loading of practically any type was undesirable. Foreign metal cations could not be used because of product contamination, and strong acids in high concentrations would cause corrosion problems. Sulfuric acid causes furfural to polymerize. As a result, the experimental program resolved into a sequence of four steps. The first and second were extractC.) ing at room temperature (25' aqueous solutions of zinc chloride alone and then with the sulfate. T h e third was extracting combined aqueous solutions of the two salts at higher temperatures of 50' and 7 5 O C., and, lastly, if necessary, extracting solutions of the sulfate, chloride, plus extraneous electrolytes.
Experimental Zinc salts of ACS purity were used. The chloride had a sulfate content less than 0.01% and the sulfate heptahydrate had a maximum chloride content of 0.001%. The water was distilled, and VOL. 49, NO. 9
SEPTEMBER 1957
1355
Stock Solutions with Which Duplicate Runs Were Made (Second step)
ZnClz
Concn., Wt. % 2 5 10 20
ripprox. ZnSOc Concn.,
Wt. % 5, 9, 21, 28, 37a 5,7, 9,21,28,37a 5, 9,21, 28,37" 5, 9,21, 28U
Saturated solution.
the furfural, commercial grade, was vacuum distilled and then saturated with distilled water. ,4 large constant temperature bath was kept a t 25' & 1" C., with provisions for end-over-end equilibration of liquid mixtures in 50-ml. Erlenmeyer flasks. A smaller constant temperature bath was controllable to 1 0 . 1 ' C. and had no provisions for mechanical equilibration. .4 Westphal balance was used for determining density a t 25" C. of stock solutions and conjugate liquid phases. For the first step, anhydrous zinc chloride was dissolved in water to make approximately 5, 9, 17, 24, 30, and 35yo solutions by weight and a few drops of concentrated hydrochloric acid were added barely to dissolve the basic salts which had precipitated. More acid was required for the dilute solutions. The amount of extraneous chloride introduced varied from about 1 to 270 a t the lowest zinc chloride concentration to essentially OyGat the highest. Each solution was analyzed for chloride content. For the second step, solutions containing approximately 5, 7, 9, 21,28, and 377, by weight of zinc sulfate and 2, 5, 10: and 20% by weight of zinc chloride were used. The zinc chloride was dissolved in a measured volume of zinc sulfate stock solution of known density, and a few drops of concentrated hydrochloric acid were added to dissolve the basic salts of zinc chloride; a solid salt phase, however, remained in several of the most concentrated mixtures. Those which contained no solid were analyzed for zinc and chloride. For the third step, solutions of 270 of zinc chloride by weight in approximately 21, 28, and 377, of zinc sulfate by weight, prepared earlier, were used. Extraneous electrolytes were unecessary for the extraction, and no work was done with such systems. Procedure. For the first step, 25 ml. of both stock solution and water-saturated furfural (dz5 = 1.147) were equilibrated for a t least 2 hours in the constant temperature bath a t 25 ' C. Preliminary experiments showed equilibrium to be reached in ' / z hour or less.
1 356
After settling, the phases were separated and set aside for analysis. T h e stock solution containing 357c of zinc chloride showed complete miscibility with furfural under these conditions and could not be run. A411runs were made in duplicate. For the second step, equilibration procedure was the same as for step 1. Saturated stock solutions, when used, were heated to dissolve the salt, and then cooled to give a supersaturated solution, 25 mI. of which was used for the run. I n most runs using saturated stock solutions, a solid phase reappeared during equilibration; in some, however, enough zinc chloride was transferred to give an unsaturated solution and in one run, No. 49, the aqueous phase remained supersaturated to give significantly more extraction than was obtained in the companion run 50, which showed solid phase reprecipitation during equilibration. Where salt was present after equilibration, only the supernatant liquid was used. For the third step, single runs were made a t two elevated temperatures49.6" and 75.0" C. Equilibration flasks were suspended in the constant temperature bath and hand shaken periodically for about a day. After settling, the phases were rapidly separated and reserved for analysis.
(27) had advantages. Zinc could not be determined in the sol.vent layer; presence of furfural in the water extraci affected the diphenylamine indicator. Sulfate present in the solvent layer was estimated by comparing barium sulfate turbidity against that developed by a standard solution containing 0.0001 gram of zinc sulfate per ml. Saturating the standard solution with furfural did not affect its barium sulfate turbidity characteristics. No turbidity was obtained except in three samples which showed much lower turbidity than did the standard, corresponding to a solvent phase zinc sulfate content considerably less than 0.0005 gram per ml. Turbidity in these samples might have arisen from contamination by a fraction of a drop of aqueous phase during the separation ; because small samples were used, maximum recovery of each phase was attempted. T h a t samples provided by duplicate runs showed no turbidity, supports this view. Zinc chloride content of each phase or stock solution was calculated from its chloride analysis. Zinc sulfate in the aqueous phase or stock solution was calculated from the excess zinc not associated with chloride; that in the solvent phase was estimated by barium sulfate turbidity comparison. Res u Its
Analytical Samples for the aqueous phase were prepared by diluting with water a measured volume and analyzing an aliquot for zinc and chloride. For the solvent phase, a measured volume was washed two or three times with equal volumes of water. Periodic tests on third washings showed them to be free of chloride. Occasionally, a n emulsion formed which was broken by adding a few drops of concentrated nitric acid. The combined washings were diluted with water in a volumetric flask and a n aliquot was analyzed for chloride, and for sulfate when present. For chloride analysis, the method of Volhard was used (74) and for zinc that of Cone and Cady ( 2 ) as described by Kolthoff and Furman (73). The amount of zinc titrated was small, and the modification of Sutton and Mitchell
Degree of extraction is expressed by the distribution coefficient: ET, defined as the ratio of equilibrium concentrations of zinc chloride in the solvent and aqueous phases, expressed in grams per milliliter. Tables I? 11: and 111 give data for steps 1: 2, and 3! respectively. I n Tables I1 and 111, densities for several stock solutions and aqueous phases are missing; they could not be determined because ol the presence or slow appearance of a solid phase. The average of duplicate runs is reported, except for the single runs, Nos. 49 and 50. These differed in that the aqueous phase of run 49 )vas supersaturat.ed. I n Table IT.', smoothed density data at 25' C. for aqueous solutions of pure zinc sulfate are taken from the literature ( 9 ) either directly or by interpolation. Those for pure zinc chloride are esperi-
Table I. Zinc Chloride-Furfural-Water System at 25' Aqueous P h z Solvent Phase Stock Solution ZnCL
Run
INDUSTRIAL A N D ENGINEERING CHEMISTRY
So.
1-2 3-4 5-6 7-8
9-10
concn , wt, % 4.9 9.0 16.9 23.6 29.7
ZnClz
d?
1.042 1.081 1.154 1.225 1.294
C.
ZnCl1:
concn., g./ml.
d:"
conen., g /nil.
0.0472 0.0845 0.1596 0.225 0.274
1.052 1.085 1.145 1.198 1.242
0.00228 0.00577 0.0220 0.0510 0.1053
d?
1.149 1.151 1.160 1.178 1.206
I\
0.048 0.068 0.138
0.228 0.384
LIQUID-LIQUID E X T R A C T I O N Table II.
g./ml.
d:"
g./ml.
1.072 1.118 1.271 1.366 1.517
0.0189 0.0196 0.0224 0.0230 0.0246
0.0539 0.1021 0.264 0.367 0.503
1.079 1.122 1.268 1.360 1.474
0.0002 1 0.00028 0.00092 0.00192 0.00615
0
5.4 6.7 9.4 21.4 27.9 -37 -37
1.097 1.114 1.146 1.296 1.389
0.0472 0.0478 0.0490 0.0541 0.0537 0.0636 0.0556
0.0528 0.0702 0.107 0.263 0.37p 0.428 0.464
1.102 1.116 1.156 1.290 1.382 1.440
0.00162 0.00176 0.00234 0.00590 0.0114 0.0236 0.0232
0
5.4 9.4 21.4 27.9 -37
1.144 1.192 1.342 1.435
0.0953 0.0992 0.1016 0.0959 0.0886
0.0569 0.104 0.266 0.373 0.455
1.142 1.187 1.332 1.420 1.483
0.0083 0.0104 0.0238 0.0389 0.0531
0.0611 0.1072 0.281 0.328 0.321
1.212 1.254 1.400
0.0364 0.0443 0.0912 0.1270 0.1207
Stock Solution ZnSOc concn., wt. %
23-24 25-26 27-28 29-30 31-32
2.0 2.0 2.0 2.0 2.0
5.4 9.4 21.4 27.9 -37
11-12 13-14 13A-14A 15-16 17-18 19-20 21-22
5.0 5.0 5.0 5.0 5.0 5.0 5.0 10.0 10.0 10.0 10.0 10.0
33-34 35-36 37-3 8 39-40 41-42
Solvent Phase ZnSOc concn., g./ml.
Aqueous Phase enSon concn., g./ml.
ZnClz concn., wt. %
Run No.
Zinc Chloride-Zinc Sulfate-Furfural-Water System at 25' C. ZnClz concn.,
di
*..
...
...
ZnClz concn.,
...
0 0
0 0
0 0 0
0 0
0
