EXTRACTION BY PHASE SEPARATION WITH MIXED IONIC SOLVENTS Recovery of Magnesium Chloride from Sea Water Concentrates ROBERT
R .
G R I N S T E A D A N D
J A M E S
C .
D A V I S
Research Laboratories, T h e DOLLChemical Co., W a l n u t Creek, Calif. 94598 The application of mixed ionic solvent extraction systems to the recovery of magnesium chloride from sea water concentrates is described. Magnesium chloride is reversibly extracted from the aqueous feed brine by the organic phase, which is subsequently stripped with water to produce a magnesium chloride brine. Both quaternary ammonium (Aliquat 336) and primary ammonium (Primene JM-T) naphthenates in toluene were examined as the extractant, while sea water concentrates equivalent to threefold and tenfold concentrated sea water, and bittern (about 40 X), were studied as the aqueous feeds. For a given number of stages and specified recovery, the highest product concentration was predicted for the case of the 10 x sea water as a feed. The primary alkylammonium naphthenate should give a product containing about 14% magnesium chloride in a system recovering 90% of the magnesium chloride in a threestage extraction, five-stage strip process. The major cationic impurity, sodium chloride, should be largely rejected. Soluble losses of the extractant were determined.
MIXED ionic extraction systems, utilizing
organic salts as the active extractant, have been described by Grinstead et al. (1969). One of the more promising applications of these systems appeared to be in the separation of alkaline earth cations from sodium chloride brines-e.g., sea water concentrates-and as part of the over-all study we examined this possibility in greater detail. We report the experimental data leading to a definition of a solvent extraction process for separation of magnesium chloride from sea water concentrates. We believe the general conclusions are applicable also to calcium separation. At the outset of this work, two major potential applications appeared to exist. One was the “softening” of, or divalent ion removal from, sea water to be fed to a desalination process. Both McIlhenny (1963) and Klein et al. (1968) have done considerable work in the development of cation exchange processes for this purpose. Chemical precipitation processes have been developed by a group a t the W. R. Grace Co. (1966), and a demonstration plant utilizing them is planned. A related possibility is the softening of the blowdown or discard brine from the desalination process. The ion exchange softening process is based on the use of this brine to regenerate the resin beds, and the presence of some calcium and magnesium in this brine lowers the regeneration efficiency. Elimination of the divalent ions in the latter brine would, therefore, be of potential interest also. The second application is in the processing of sea water concentrates to recover magnesium chloride and other compounds. For both types of applications, the chemical 66
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
separation required is the same-removal of the divalent ions from an aqueous solution containing considerable quantities of sodium chloride. The purpose of this study was, therefore, to obtain the laboratory data necessary to evaluate these two possible applications of the mixed ionic system. Three possible sea water concei.trates were selected for study in order to provide information relevant to a determination of an optimum point of application in a sea water concentration flowsheet: 3 x (threefold) concentrated sea water, 1.0 x sea water, and bittern (about 40 x). The first level is roughly the concentration of the discard brine of a desalination plant, and would be the feed most readily available to any subsequent processing scheme. The 10 x sea water represents approximately the point a t which saturation with respect to sodium chloride is reached, and is a convenient intermediate point of application. I n addition, such a feed could be obtained through a solar evaporation step without the necessity of handling solid sodium chloride. Bittern represents the final stage in the processing of sea water to obtain salt, and as such is a commercially existent stream in some locations. The extraction of magnesium chloride by a mixed ionic system occurs by the equation:
+ Mg” + 2 C1- S 2RNHj- C1- + Mg2’ (R’C0O)r
2 R NH?’ R’COO
~
(1)
In this equation, and subsequently in the designation of concentrations, bars placed over a species denote constituents of the organic phase. As described earlier, this reaction is reversible, and a subsequent contact with water removes the extracted salt. The simple amine-carboxylate systems also extract the alkaline earths in preference to alkali metals, which makes them chemically well suited for removing magnesium and calcium from sea water concentrates. Furthermore, in systems such as sea water and other brines containing considerable sodium chloride, the extraction of a divalent metal chloride is assisted by the common chloride ion, as Equation 1 predicts, so that a product solution is obtained which is more concentrated in that species than the feed. The extraction system is rather simple, consisting only of a number of countercurrent extraction and strip stages, as shown in Figure 1. The major tasks of this stage of the process development work consisted of selection of the optimum extraction system and brine concentration, and determination of the extraction equilibria of the inorganic salt species and the extractants themselves. Experimental Methods
Materials. Aliquat 336, a product of the General Mills Corp., is a mixture of trialkylmethylammonium chlorides, the alkyl groups being primarily octyl and decyl. Equivalent weights of this material, as determined by chloride titration, run about 500 to 510. Primene J M - T is a product of the IRohm & Haas Corp., and is said to be a primary amine containing a highly branched alkyl chain, the amino group being attached to a tertiary carbon. The equivalent weight of this material, determined by titration with acid, runs about 340. The primary amines l-amino-2,2-dihexyloctane and 1,ldihexylheptylamine were prepared as described by Grinstead and Davis (3968). The compound l-ethyl-lmethylnonadecylamine was prepared in a similar manner, using a Grignard reaction to prepare, as the intermediate, the corresponding alcohol from methyl ethyl ketone and octadecyl bromide. Trioctylmethyl ammonium systems were prepared from the iodide salt, and shaken several times with 1M sodium chloride to remove the iodide ion. The original iodide salt was prepared by reaction of methyl iodide with trioctylamine in pentane. The precipitated product was analyzed as C = 60.4‘3, N = 2.7%, H = 10.9%; equivalent weight by silver titration = 492, theoretical = 495. Trioctylpropylammonium systems were prepared from the chloride
‘-4-
Feed brine
Rollinate
EXTR-
Loaded organic
Stripped organic
salt, obtainable from the Eastman Chemical Corp. Both 2-ethylundecanoic acid and 2-ethyltetradecanoic acids were prepared as described by Grinstead et al. (1969). Naphthenic acid E is a mixture of naphthenic acids of petroleum origin, obtained from the Chevron Chemical Corp. Equivalent weights of this material, by titration with base, run about 250. These compounds are cycloalkyl carboxylic acids, presumably containing alkyl substituents sufficient to give the observed equivalent weight. A reagent grade of toluene was used. Aqueous solutions were prepared with standard reagent grade chemicals. The synthetic sea water concentrates included mainly the major components (Table I ) . Designations 3 x and 10 x refer to the ratio of molar ionic concentrations in the brine to those in sea water, which are, respectively, 3 and 10. Calcium, although a significant component of concentrated, and in both 10 x and bittern is only a very minor component. Its omission from the 3 x sea water is discussed below. Procedures. The mixed ionic extraction systems were prepared by mixing weighed quantities of amine and acid, and diluting to the proper volume with toluene. I n the case of quaternary ammonium systems, the organic phase was shaken with a slight excess of dilute sodium hydroxide to convert the acid to the ionized form, then with 2 or 3 portions of water to strip the sodium chloride from the organic phase. Liquid-liquid contacts were made in screw-capped vials with Teflon lid inserts, and were shaken for one-half hour or longer a t ambient temperature. Earlier work with this system has shown that equilibrium is reached within a few minutes under these conditions. Phases were separated and centrifuged to obtain clear phases for analysis. Analytical Methods. Metal ions were determined by atomic absorption spectrometry. Organic phases were sometimes analyzed directly, and sometimes stripped with dilute aqueous HCl, the analysis being carried out on the resulting aqueous phase. Organic species in the aqueous phases were determined by two methods. The alkylammonium ion was determined by extraction as a picrate salt. Five milliliters each of the sample, chloroform, and the picrate reagent were shaken together. The absorbance of the chloroform phase was determined a t 410 mp against a reagent blank. The picrate reagent was prepared to contain 0.002M picric acid, 0.079M citric acid, and 0.041M disodium phosphate, and had a pH of 3. Standard curves were prepared by substituting a standard solution of the appropriate amine or amine salt in chloroform for the straight chloroform in the procedure, and replacing the sample volume with water. The use of saturated sodium chloride instead of water a t this point did not affect the standard curve. Concentrations as low as 1 x 10-6M amine are detectable. Neither the carboxylic acid nor the chloride ion interferes.
Table 1. Composition of Synthetic Sea Water Concentrates
