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Extraction of magnesium chloride from brines using mixed ionic extractants
J. inorg, nucl. Chem.. 1975, Vol. 37, pp. 191-198. Pergamon Press. Printed in Great Britain.
EXTRACTION OF M A G N E S I U M CHLORIDE FROM BRINES U S I N G MIXED IONIC EXTRACTANTS C. HANSON* and M. A. HUGHES Schools of Chemical Engineering, University of Bradford, Bradford BD7 1DP and S. L. N. MURTHY Department of Chemical Engineering, Indian Institute of Technology, Bombay-76, India
(Received 14 February 1974) Abstract--Data are presented on the distribution of magnesium chloride betwee0 aqueous phases and solvents comprising various mixed ionic extractants involving equimolar mixtures of amines and carboxylic acids. Aliquat-336/acid-810 is shown to have promise for the extraction of magnesium chloride from bitterns. The co-extraction of sodium, calcium, sulphate and bromide ions is considered. The system is shown to be insensitive to changes in pH and temperature but the choice of diluent can have an appreciable effect. INTRODUCTION DURING the last decade, considerable interest has been expressed in the possibility of recovering minerals from the concentrated brines resulting from sea water desalination plants or the bitterns produced during the solar evaporation of sea water for sodium chloride manufacture. Despite the immense quantities of many cations present in the oceans, the concentrations of most are low and an economic evaluation[1] has shown that only a few are likely to be recoverable even from concentrates at costs competitive with other sources. Amongst these is magnesium and the work described in this paper is concerned with the development of a solvent extraction process for the recovery of magnesium chloride from bitterns. Magnesium is already produced in large quantities from sea water by precipitation as the hydroxide, usually by the addition of some form of calcium hydroxide. Magnesium chloride is also recovered from some bitterns by crystallization routes via carnallite, a hydrated double salt with potassium chloride. In such cases it is essentially a by-product in recovery of a pure potassium salt. Some consideration has already been given to the possible use of solvent extraction for magnesium salts. Carboxylic acids are an obvious possible group of extractants and magnesium was included, for example, in an evaluation by Biumberg and Melzer[2] of •-halogen substituted carboxylic acids as possible extractants for metals. Such studies have usually been too general to permit any definite conclusions to be drawn on possible processes for the economic recovery *To whom all correspondence should be addressed.
of magnesium from brines. Carboxylic acids have the general disadvantage for magnesium that forward extraction requires a high pH, while magnesium hydroxide begins to precipitate when the pH of the aqueous phase exceeds about 8.0. This can limit the degree of extraction. Some preliminary experiments with naphthenic acid and versatic acid (1.0 M solutions in toluene) showed distribution coefficients for magnesium in the aqueous concentration rango of interest (about 2 M) at pH 8.0 to be only in the region of 10- 3. They have the added disadvantage that stripping and regeneration of the carboxylic acid requires use of a strong acid, with consequent reagent cost. There appears to be much more promise for the recovery of magnesium from brines with mixed anionic-cationic exchange extractants in the solvent. These have been developed by Grinstead[3, 4] and might typically comprise a stoichiometric mixture of an amine and a carboxylic acid. Extraction may be represented by : Mg 2+ + 2C1- + 2R4N. R'COO (aq.) (aq.) (org.) 2R4NCI + Mg(R'COOh. (org.) (org.)
(1)
Such systems offer several advantages. Firstly, forward extraction will be enhanced by the common ion effect due to the large excess of chloride ions present in typical feed brines. Secondly, both ions in magnesium chloride are co-extracted and the salt can be recovered from the loaded solvent simply by stripping with water. The use of the existing chloride ion concentration to provide the chemical potential for forward extraction, coupled with the possibility of stripping with water, 191
192
C. HANSON,M. A. HUGHESand S. L. N. MURTHY
eliminates the need for conditioning reagents a n d the associated cost. Finally, t h e r e is scope for optimizing the extraction power and selectivity of the solvent by choice of b o t h the anionic a n d cationic components. Grinstead et al.[3, 4] concluded that the most promising combinations of extractants for divalent metals were carboxylic acids with either quaternary or primary alkyl a m m o n i u m compounds. They showed the effectiveness of the amine c o m p o n e n t to be in the order quaternary > primary > secondary > t e r t i a r y ; a n d of acids to be carboxylic > alkylphosphoric > arylsulphonic. They subsequently[5, 6] studied the use of mixed ionic extractant systems specifically for the recovery of magnesium chloride from sea water concentrates. W o r k was confined to the systems primene J M - T / naphthenic acid E, and aliquat-336/naphthenic acid E. On the basis of a limited a m o u n t of distribution and selectivity data, together with measurement of solvent losses by solution, the former was considered the most promising and further development work was concentrated on this system. Recovery was evaluated from concentrates resulting from the evaporation of sea water by factors of 3, 10 a n d 30. The former corresponds to a desalination plant effluent, the second represents the saturation point with respect to sodium chloride, while the latter would be typical of a bittern resulting from solar evaporation of sea water to produce sail It was concluded that 10x concentration is the optimum for recovery of magnesium chloride, that solvent losses by solution do not make a significant contribution to the cost of the process except with dilute feeds, and that economic recovery of by-product magnesium chloride by solvent extraction is only possible in favourable market situations. While the work of Grinstead et al. on mixed extractants was very valuable, it was limited in the range of extractants studied a n d the consideration given to the presence of other ions. It was decided to undertake a more extensive survey of possible extractants with a view to developing a fl owsheet for economic evaluation.
The carboxylic acids were: (i) Acid-810, essentially a mixture ofiso-octanoic, iso-nonanoic and iso-decanoic acids. The average equivalent weight was measured as 157. Marketed by Novadel Chemicals Ltd. (ii) Naphthenic acid (180SP), a mixture of bi-cyclic acids (CnH2n_3COOH), 5-membered ring acids (CnH2n_ICOOH) and aliphatic acids (CnH2n+ICOOH). Manufactured by Shell Chemical Co. Ltd. (iii) Versatic acid 911, a mixture of secondary and tertiary aliphatic acids, present in the ratio 1:9, with hydrocarbon chain length range 9-11. Manufactured by Shell Chemical Co. Ltd. All other materials were of "Analar" grade. Amine-carboxylate solvents were prepared by mixing calculated amounts of amine and carboxylic acid, followed by addition of the diluent to give the required concentrations. In systems involving Aliquat-336, the solvent as prepared above was treated with a stoichiometric excess of 2 N sodium hydroxide. The organic phase was then washed thoroughly with distilled water to remove any sodium chloride or excess sodium hydroxide. Toluene was used as diluent for most of the work except when actually evaluating the effect of variation in this component of the solvent. It was not found necessary to employ a modifier with any of the systems studied. For extraction experiments involving bitterns, a synthetic bittern was prepared from "Analar" reagents, the concentrations of the principal constituents being: NaCI 1.60M, KC1 0.35 M, MgCI 2 1.70 M, MgSO,, 0.60 M. The minor constituents calcium, lithium, boron and bromine were added in the forms of calcium chloride, lithium sulphate, boric acid and potassium bromide but other trace constituents were neglected.
Procedure Most of the work was based on determination of distribution and selectivity data. For this, measured volumes of the two phases were equilibrated by shaking in a thermostat bath at the required temperature for 30 min. Experiments showed that both thermal and extraction equilibria were established in under 10min. The conjugate phases were then separated and analysed. The aqueous phase was analysed direct. The organic phase was first stripped with water in four contacts and the combined aqueous strip liquor analysed.
Analytical methods EXPERIMENTAL
Materials Consideration was given to the possible combination of four commercially available amines with three carboxylic acids. The amines were: (i) Alamine-336, a mixture of tertiary amines approximating to the molecular formula N[(CH2)nCH313, where n = 8-10. Manufactured by General Mills Inc. (ii) Aliquat-336, a mixture of quaternary ammonium chlorides approximating to the molecular formula {CH3-N[(CH2)nCH3]}CI, where n varies between 8 and 10. Manufactured by General Mills Inc. (iii) Amberlite LA-2, a secondary amine with N-lauryl(trialkyl methyl) amine and its isomers as the major components. Marketed by Rohm and Hass Co. (iv) Primene JM-T, essentially a mixture of isomeric trialkyl methyl primary amines in the Czs-C22 chain length range. Marketed by Rohm and Hass Co.
The project demanded analysis for magnesium, sodium, potassium and calcium in mixtures containing from one to all of these cations. Atomic absorption techniques were maialy used, although care had to be taken to avoid errors caused by interference. While it is generally claimed that the absorption of magnesium in an air-acetylene flame is not affected by the presence of sodium, potassium and calcium, preliminary investigation did not show such an assumption to be justified. While the interference was small, it could not be neglected, particularly at low concentration levels. The difficulty was overcome by adjusting the concentration levels of the interfering cations in the calibration standards to correspond to their concentrations in the samples. Sodium and potassium were determined by atomic absorption without difficulty. Flame emission was employed for calcium as being more sensitive in view of its low concentration relative to the other cations. The effect of interference again had to be eliminated
Magnesium chloride extraction
193
I
by the use of calibration standards containing the other cations at approximately the same concentration levelsas in the samples. Sulphate concentrations were not required with great accuracy and so were determined by the rapid volumetric method using sodium rhodizonate as indicator. Further details of the experimental work are available elsewhere[7].
RESULTS AND DISCUSSION
Comparison of amine components of mixed extractant Versatic acid 911 was used as the common acidic component for a comparison of the various amines. It was chosen as having a fairly constant composition (all work was conducted, of course, using material from the same batch). The results for extraction of magnesium chloride from pure aqueous solution are shown in Fig. 1. In agreement with the conclusions of Grinstead, the order of effectiveness of the amines is seen to be quaternary > primary > secondary > tertiary. While the distribution coefficients are not high, their magnitudes should be greater with a bittern due to the common ion effect of the excess chloride ions.
On the basis of forward extraction, aliquat-336 would appear the most promising basic component of the extractant. Amberlite LA-2 and alamine-336 are less effective than primene JM-T at high magnesium levels but show higher distribution coefficients at low concentrations. They would therefore have the rather unusual characteristic of being inferior to primene JM-T for both extraction and stripping. As a result, it appears that only aliquat-336 and primene JM-T merit further consideration as the basic component. While the former will be the superior for extraction and give higher loadings, its stripping characteristics will not be as good. It is of interest to note that the solvent loadings for amberlite LA-2 and alamine-336 are several times greater than those reported by Grinstead[6] for typical pure secondary and tertiary amines. This could be attributable not only to structural differences but also to the presence of small but significant amounts of active impurities (e.g. primary amines) in the commercial products.
Comparison of carboxylic acid components of mixed extractant With aliquat-336 and primene JM-T in turn as the common amine component, distribution data were 0.5
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Fig. 2. Comparison of carboxylic acid components for extraction of magnesium chloride.
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Component Acid-810 Naphthenic acid Versatic acid
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194
C. HANSON,
M. A. HUOHESand S. L. N. MURTHY
obtained for magnesium chloride between water and the organic solvent using the three carboxylic acids. The results for aliquat-336 are given in Fig. 2, from which it will be seen that no significant difference was found between the three acids. The behaviour with primene JM-T was similar. This comparable performance of the three acids is not really surprising, although the cyclic structure of naphthenic acid might have been expected to produce some difference. It is clear that a choice between the three acids cannot be made simply on the basis of the extraction isotherms. However, they do have substantially different physical properties and these could be significant for an industrial process. The available data are summarized in Table 1. Naphthenic acid is clearly at a disadvantage in comparison with the other two in having both a higher density and a higher viscosity. These would be expected to give slower phase disengagement and more difficult phase interdispersion. Furthermore, while naphthenic acid has the lowest cost per unit weight, its equivalent weight is greater than those of the other acids and the cost per equivalent weight is of the same order for all three. Overall, it would appear less attractive then either of the other acids considered. A logical choice between acid-810 and versatic acid is more difficult. However, the lower viscosity of the former would give a somewhat lower viscosity solvent phase with consequent reduction in power requirement and better phase disengagement. Acid-810 was observed to give the sharpest phase separation with aliquat-336. However, it should be noted that with all three acids difficulty was experienced in obtaining rapid phase separation at low magnesium loadings, i.e. corresponding to the later stages of stripping. In the light of the results obtained to this stage, it was decided to concentrate further work on the system aliquat-336/acid-810. This decision also reflected the extensive data already available on the system primene JM-T/naphthenic acid[6]. A 1-0M stoichiometric mixture of the two extractants in the solvent phase was used throughout, the saturation limit for magnesium chloride assuming a stoichiometric reaction then being 0.5 M. This should be a reasonable choice even for extraction from a bittern (-~ 30 x sea water) in which the magnesium concentration would be about 1.7 M.
Effect of chloride ion concentration It was important to establish that excess chloride ion in the aqueous phase would enhance extraction of magnesium chloride by the common ion effect. Extraction isotherms for magnesium chloride in the presence of sodium chloride are presented in Fig. 3 and show clearly that forward extraction is enhanced appreciably.
Selectivity for magnesium over sodium Previous work[6] has shown that sodium chloride
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presence of magnesium chloride between water and solvent phase comprising 1.0 M aliquat-336/acid-8 l0 in toluene.
Magnesium chloride extraction is co-extracted with magnesium chloride by mixed ionic solvents and it is important to establish the extent to which this takes place. The data obtained in studying the effect of chloride ion concentration provided this information and are cross plotted in Fig. 4. Values of selectivity vary from just under 10 to over 50. While high, it is clear that a loaded extract from a bittern would contain an appreciable concentration of sodium ions and scrubbing would be necessary to remove these.
Effects of potassium and sulphate ions In addition to sodium, magnesium and chloride ions, a bittern will contain appreciable concentrations of calcium, potassium and sulphate ions. N a + - K + Mg2+-CI - complexes and N a + - K + - M g 2 + - S O 2complexes are known to exist in aqueous solutions, the latter generally being the more numerous and stable. Hence the presence of these other ions might well lead to appreciable differences in the extraction behaviour of magnesium as against that reported above for systems in which they are absent. The magnesium chloride extraction isotherm was therefore determined using the synthetic bittern and is shown in Fig. 5. To illustrate the effect of the other ions, the
[] NoCI MgCI 2 Solutions I ' All, ~' 0 . 4 5 [ 0 SO~- free b i t t e r n / 351 X Bittern ~ Aci ® B i t t e r n - P r i m e n e JMT [6] A Bittern-0.5 M 0 . 4 0 -- clliquot ocid 810
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Fig. 5. Extraction isotherms for magnesium chloride from various feed materials.
195
Table 2. Distribution of magnesium sulphate between water and a solvent phase comprising l-0 M aliquat-336/acid-810 in toluene at 25°C Equilibrium concentrations of magnesium in the conjugate phases (M) Aqueous Organic 0.432 0.613 1.106 1.413
0.033 0.061 0.084 0.095
isotherm is also given for a pure magnesium chloridesodium chloride solution of the same strength. To ascertain the contribution of sulphate ions, a sulphatefree bittern was prepared (i.e. all metals in chloride form). The extraction isotherm is included in Fig. 5. It will be seen that the distribution coefficients for magnesium chloride are substantially less from a bittern than from the pure solution. At least half this effect appears due to the sulphate ions. However, its magnitude would not seem sufficient to justify prior desulphation as part of a commercial process since this will not only introduce an extra operating cost but will also dilute the feed. In addition to the impact of sulphate ions on the behaviour of the magnesium, there is also the problem of their being co-extracted with the chloride, thus introducing an impurity into the product. Previous work[6, 8] suggested that amines are poor extractants for sulphate. However, a study on the extraction of pure magnesium sulphate from aqueous solution by aliquat-336/acid-810 showed appreciable quantities to transfer. The results are given in Table 2. Nevertheless, comparison with Fig. 3 indicates that chloride/sulphate selectivities must be high. Preliminary experiments confirmed that, for solvent phase magnesium loadings of over if4 M, the sulphate concentration in that phase would be under 0.002 M. Thus the impurity level would only be in the region of 0.5 M per cent at most. The bulk of this should be removed during the scrubbing operation necessary to remove co-extracted sodium chloride. The difference in the extraction isotherm for a sulphate-free bittern and that for the equivalent mixture of just magnesium and sodium chlorides shows the effect of potassium. This leads to a small but significant reduction in the distribution coefficient for magnesium over almost the whole concentration range, presumably due to the formation of complexes in the aqueous phase. However, potassium is not co-extracted as readily as sodium and so should present no problems as an actual impurity (potassium concentrations in the organic phase in equilibrium with a bittern are <0.005 M).
Effects of calcium and bromide ions The presence of appreciable concentrations of sulphate ions in a bittern automatically limits the
196
C. HANSON,M. A. HuGhEs and S. L. N. MURTHY
possible concentration of calcium to a low level. No precise figures are available but it is probably less than 200 ppm. However, even at these levels calcium cannot be ignored as Grinstead [6] has shown amine-carboxylic acid extractant systems to be selective for calcium over magnesium. Thus, while these low levels of calcium should not directly affect the extraction of magnesium, they could influence the stripping process since this metal will tend to be preferentially held in the solvent and could build-up with recycle if stripping is not complete. There is also the question of calcium contamination of the product. Assuming virtually total extraction, the calcium chloride content of the product magnesium chloride could be in the region of 5 M per cent. One solution would be a two section stripping process with pure magnesium chloride being produced in the first by use of a relatively low aqueous to organic phase flow ratio, with clean-up of the solvent in the second by use of a much higher aqueous flow rate. A similar problem could arise on the anion side since aliquat-336/acid-810 is selective for bromide over chloride[7]. With a combined stripping section, the bromide content of the product could be around 1.5 M per cent. This could be greatly reduced by use of the two section stripping process suggested above for elimination of calcium. More detailed data on the effect and distribution of impurities are available elsewhere[7].
Effect of extractant concentration The work with a solvent phase containing a 1 M mixture of the extractants showed maximum loadings of magnesium chloride close to stoichiometric. The extraction isotherm was also determined for a bittern into a 0.5 M solution of the extractants. The results are incorporated in Fig. 5 and confirm near stoichiometric behaviour. Concentrations greater than 1 M were not considered in view of their high viscosities. Lower concentrations were considered since workers with other systems [9, 10] have reported lower solvent losses at lower extractant concentrations. However, these observations were probably dominated by entrainment losses since loss by solution would not be expected to vary greatly. It is considered that entrainment losses are better minimized in the design of contacting equipment rather than in choice of egtractant concentration since decrease in the latter would cause substantial increase in equipment size.
Effect of diluent The choice of diluent can have an important bearing on parameters governing plant performance such as phase disengagement and rate of mass transfer. In addition, it can influence the actual distribution through 0.45
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Comparison of aliquat-336 with primene JM-T A choice between aliquat-336 and primene JM-T as basic component of the extractant depends on their relative performance with a bittern rather than with pure components, as shown in Fig. 1, and also their stripping qualities. The extraction isotherm obtained by Grinstead [6] for extraction of magnesium from a bittern into primene JM-T/naphthenic acid is included in Fig. 5. It is clear from this that distribution coefficients are considerably higher with aliquat-336/acid-810, which would lead to a more compact unit for a continuous extraction plant. The advantage will be in the other direction, however, for stripping. In addition, the primene JM-T system appears considerably more selective for magnesium over sodium. However, these advantages for the primene JM-T system are only marginal in the context of a commerical process. A scrubbing section will be necessary in either case for removal of sodium but only a couple of stages would be perfectly effective even with aliquat-336. Similarly, any saving on the stripping section from primene JM-T would be less than the additional cost for the extraction section caused by using this extractant. It was concluded at this stage that aliquat-336! acid-810 was the most promising combination of the extractants considered.
0.55 5; .
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0.25
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0.10
0"05
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I
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Fig. 6. Effect of diluent on extraction isotherms.
Magnesium chloride extraction stabilization by solvation of the extracted species and by promotion or otherwise of aggregation. Distribution isotherms were obtained for five diluents and are shown in Fig, 6. The investigation was not exhaustive and was restricted to diluents obtainable in a pure form, thus excluding most of the commerical products now on the market. Nevertheless, the results do illustrate the influence of diluent. The chlorinated hydrocarbons are seen to give appreciably lower distribution coefficients than the others and third phase formation was observed with these above 1.75 M magnesium in the aqueous phase, n-Hexane and toluene are preferable to nitrobenzene in having a lower density. Of these, toluene gave marginally the better distribution coefficients and had adequate solvency so was considered the best.
Effect of pH While pH is an important parameter when considering extractions by amines or carboxylic acids alone, its influence on extraction with mixed ionic extractants should be small. This is one of their prime attractions for commercial use since the cost of repeated pH adjustments can be avoided. The effect of pH on extraction from a fixed feed solution was examined, the results being given in Table 3. It will be seen that it has comparatively little influence and so there would appear to be no advantage in making pH adjustment to the various streams.
Effect of temperature If t here is an appreciable ent halpy change accompanying the reaction represented by Eqn (1), the equilibrium constant would be dependent on temperature and so distribution coefficients would also be influenced by temperature. While it is difficult to conceive making any temperature adjustments to a feed bittern, it might be possible to exploit any such phenomenon at the stripping stage. Attention was therefore concentrated on the distribution of magnesium chloride alone and isotherms determined at both 25°C and 50°C. Very little effect was observed. At most the distribution coefficient decreased about 7 per cent with the 25°C rise in temperature. It is clear that enthalpy changes must be small and temperature will not be an important operating parameter for an industrial process. Table 3. Effect of pH on distribution of magnesium chloride between water and 0.5 M aliquat-336/acid-810 in toluene at 25°C
197
Solvent losses Solvent losses in a commercial process will result from both entrainment and solution of the solvent in the aqueous streams. The former is a function of plant design and operation and is diffficult to estimate in advance. However, with a relatively cheap product such as magnesium chloride, losses of expensive extractants could be a major economic parameter. Grinstead et al.[6] have reported the solubilities of various long-chain amines and quaternary ammonium chlorides in concentrated brines, while Fletcher and Flett[11] have determined the solubility of naphthenic acid in the raffinates from copper and nickel extraction. No data have been reported for acid-810. Some preliminary experimental results are given in Table 4. These are average values based on three determinations in each case (results varied by + 5 per cent). They are significantly higher than the reported soluble losses of naphthenic acid in copper extraction processes (-~ 100 mg. 1-1 at pH 4.7). Additionally, the soluble losses ofaliquat-336 would be in the region of 20 mg-1.- 1.
Mechanism of extraction The primary objective of the current investigation was the collection of useful data for design of a possible commercial process. However, it is of interest to consider the mechanism of extraction in the light of the data obtained. The mechanism has been studied by Davies and Grinstead[12], who concluded that (i) the magnesium carboxylate and aikyl ammonium chloride are not associated in the organic phase and exist as separate species, (ii) below an organic phase concentration of 0.02 M magnesium chloride, the magnesium carboxylate and alkyl ammonium chloride exist as unaggregated monomers, (iii) the extracted species are associated with several unused extractant molecules, and (iv) there is negligible hydrolysis of the extracted species. While these conclusions may have been valid for the conditions studied, the results of the present investigation suggest that they would not all apply under more concentrated conditions. Thus, while maximum loadings were approximately in agreement with the stoichiometry of Eqn (1), as discussed in Section 3.8, nevertheless many cases were observed of magnesium concentrations in the solvent somewhat greater than stoichiometric, e.g. 0.54 M in equilibrium with 3.0 M in the aqueous phase for pure magnesium chloride and 1~56 M in equilibrium with 2.77 M in an aqueous phase also 1 M Table 4. Solubility of acid-810 at 25°C
pH 2 4 6 8.5
Equilibrium concentrations (M) Aqueous phase Organic phase 1-66 1-66 1.67 1.66
0.137 0.132 0.129 0.137
Medium
Solubility Img. 1-tl
Water Synthetic bittern (pH -~ 4.5) Synthetic bittern saturated with aliquat-336
1,040 270 340
198
C. HANSON,M. A. HUGHESand S. L. N. MURTHY
in sodium chloride. The stoichiometric limit would be 0.50 M since the concentration ofextractants was 1.0 M. Many other limits of similar magnitude were found. Under these conditions, with magnesium concentrations just greater than apparent stoichiometric, it appears inconceivable that the extracted species could be associated with any free extractant molecules. It is thought probable that solvent phase concentrations in excess of the stoichiometric value could be due to the participation of hydrolysed species in the extracted complex and to the effect of water dissolved in the solvent phase. Such an explanation has been given by Fletcher and Flett E11] for similar observations during the extraction of cobalt and nickel by naphthenic acid. In the present investigations, it was noticed that significant amounts of water were extracted by the aliquat-336/acid-810 solvent. Thus dilution of a 1-0 M solution after equilibriation with water to 0.5 M by addition of toluene resulted in the separation of water. No quantitativemeasurements were made. Grinstead [6] has claimed that aliquat-336 will extract some 8 moles water per mole of extractant when contacted with pure water, although less when contacted with magnesium chloride solutions. It seems clear that the nonstoichiometric solvent loadings observed are related to the co-extraction of water.
CONCLUSIONS A mixed ionic extractant comprising an equimolar mixture of aliquat-336 and acid-810 is promising for the recovery of magnesium chloride from bitterns. It gives quite favourable distribution coefficients and loadings with good selectivity from the other ions present except for calcium. Forward extraction is enhanced by the high chloride ion concentration in a bittern, while stripping can be achieved with water. Thus there would be no need for conditioning steps, with their attendant cost. It would be necessary to incorporate a scrubbing section in a flowsheet and provision would also have to be made for solvent purification during recycle to prevent build-up of calcium and possibly bromide. Temperature and pH do not have significant effects on the process.
Solvent loadings in excess of stoichiometric were observed, probably due to co-extraction of water. The data presented show that a solvent extraction process based on this system could operate for the recovery of magnesium chloride. Whether it would be competitive can only be decided from an economic evaluation and this will be the subject of a future communicationE13]. Acknowledgement--One of the authors (S.L.N.M.) would like to record his indebtedness to the Association of Commonwealth Universities for providing a Commonwealth Scholarship which allowed him to undertake the work described.
REFERENCES
1. C. Hanson and S. L. N. Murthy, The Chemical Engineer (264), 295 (1972). 2. R. Blumberg and N. Melzer, International Mineral Processing Congress, New York (1964). 3. U.S. Office of Saline Water, Res. Dev. Prog. Rep. 320 (1968). 4. R. R. Grinstead, J, C. Davis, S. Lynn and R, K. Charlesworth, Ind. Eng. Chem., Prod. Res. Dev. 8(3), 218 (1969). 5. R.R. Grinstead, J. C. Davis, Ind. Eng. Chem., Prod. Res. Dev. 9(1), 66 (1970). 6. U.S. Office of Saline Water, Res. Dev. Prog. Rep. 406 (1969). 7. S. L. N. Murthy, PhD Thesis, University of Bradford (1971). 8. N. M. Rice, UniversityofLeeds. Private communication (1971). 9. C. G. Robinson and J. C. Paynter, Proc. Int. Solvent Extraction Conf., 214, The Hague (1971). 10. G. M. Ritcey, Mines Branch, Ottawa. Private communication (1971). 11. H. A. C. McKay, et al. (Ed.), Solvent Extraction Chemistry. Macmillan, London, 1966. 12. J. C. Davis and R. R. Grinstead, J. phys. Chem. 74(1), 147 (1970). 13. C. Hanson and S. L. N. Murthy, Paoer to be presented at Int. Solvent Extraction Conf. Lyon (1974).
EXTRACTION OF M A G N E S I U M CHLORIDE FROM BRINES U S I N G MIXED IONIC EXTRACTANTS C. HANSON* and M. A. HUGHES Schools of Chemical Engineering, University of Bradford, Bradford BD7 1DP and S. L. N. MURTHY Department of Chemical Engineering, Indian Institute of Technology, Bombay-76, India
(Received 14 February 1974) Abstract--Data are presented on the distribution of magnesium chloride betwee0 aqueous phases and solvents comprising various mixed ionic extractants involving equimolar mixtures of amines and carboxylic acids. Aliquat-336/acid-810 is shown to have promise for the extraction of magnesium chloride from bitterns. The co-extraction of sodium, calcium, sulphate and bromide ions is considered. The system is shown to be insensitive to changes in pH and temperature but the choice of diluent can have an appreciable effect. INTRODUCTION DURING the last decade, considerable interest has been expressed in the possibility of recovering minerals from the concentrated brines resulting from sea water desalination plants or the bitterns produced during the solar evaporation of sea water for sodium chloride manufacture. Despite the immense quantities of many cations present in the oceans, the concentrations of most are low and an economic evaluation[1] has shown that only a few are likely to be recoverable even from concentrates at costs competitive with other sources. Amongst these is magnesium and the work described in this paper is concerned with the development of a solvent extraction process for the recovery of magnesium chloride from bitterns. Magnesium is already produced in large quantities from sea water by precipitation as the hydroxide, usually by the addition of some form of calcium hydroxide. Magnesium chloride is also recovered from some bitterns by crystallization routes via carnallite, a hydrated double salt with potassium chloride. In such cases it is essentially a by-product in recovery of a pure potassium salt. Some consideration has already been given to the possible use of solvent extraction for magnesium salts. Carboxylic acids are an obvious possible group of extractants and magnesium was included, for example, in an evaluation by Biumberg and Melzer[2] of •-halogen substituted carboxylic acids as possible extractants for metals. Such studies have usually been too general to permit any definite conclusions to be drawn on possible processes for the economic recovery *To whom all correspondence should be addressed.
of magnesium from brines. Carboxylic acids have the general disadvantage for magnesium that forward extraction requires a high pH, while magnesium hydroxide begins to precipitate when the pH of the aqueous phase exceeds about 8.0. This can limit the degree of extraction. Some preliminary experiments with naphthenic acid and versatic acid (1.0 M solutions in toluene) showed distribution coefficients for magnesium in the aqueous concentration rango of interest (about 2 M) at pH 8.0 to be only in the region of 10- 3. They have the added disadvantage that stripping and regeneration of the carboxylic acid requires use of a strong acid, with consequent reagent cost. There appears to be much more promise for the recovery of magnesium from brines with mixed anionic-cationic exchange extractants in the solvent. These have been developed by Grinstead[3, 4] and might typically comprise a stoichiometric mixture of an amine and a carboxylic acid. Extraction may be represented by : Mg 2+ + 2C1- + 2R4N. R'COO (aq.) (aq.) (org.) 2R4NCI + Mg(R'COOh. (org.) (org.)
(1)
Such systems offer several advantages. Firstly, forward extraction will be enhanced by the common ion effect due to the large excess of chloride ions present in typical feed brines. Secondly, both ions in magnesium chloride are co-extracted and the salt can be recovered from the loaded solvent simply by stripping with water. The use of the existing chloride ion concentration to provide the chemical potential for forward extraction, coupled with the possibility of stripping with water, 191
192
C. HANSON,M. A. HUGHESand S. L. N. MURTHY
eliminates the need for conditioning reagents a n d the associated cost. Finally, t h e r e is scope for optimizing the extraction power and selectivity of the solvent by choice of b o t h the anionic a n d cationic components. Grinstead et al.[3, 4] concluded that the most promising combinations of extractants for divalent metals were carboxylic acids with either quaternary or primary alkyl a m m o n i u m compounds. They showed the effectiveness of the amine c o m p o n e n t to be in the order quaternary > primary > secondary > t e r t i a r y ; a n d of acids to be carboxylic > alkylphosphoric > arylsulphonic. They subsequently[5, 6] studied the use of mixed ionic extractant systems specifically for the recovery of magnesium chloride from sea water concentrates. W o r k was confined to the systems primene J M - T / naphthenic acid E, and aliquat-336/naphthenic acid E. On the basis of a limited a m o u n t of distribution and selectivity data, together with measurement of solvent losses by solution, the former was considered the most promising and further development work was concentrated on this system. Recovery was evaluated from concentrates resulting from the evaporation of sea water by factors of 3, 10 a n d 30. The former corresponds to a desalination plant effluent, the second represents the saturation point with respect to sodium chloride, while the latter would be typical of a bittern resulting from solar evaporation of sea water to produce sail It was concluded that 10x concentration is the optimum for recovery of magnesium chloride, that solvent losses by solution do not make a significant contribution to the cost of the process except with dilute feeds, and that economic recovery of by-product magnesium chloride by solvent extraction is only possible in favourable market situations. While the work of Grinstead et al. on mixed extractants was very valuable, it was limited in the range of extractants studied a n d the consideration given to the presence of other ions. It was decided to undertake a more extensive survey of possible extractants with a view to developing a fl owsheet for economic evaluation.
The carboxylic acids were: (i) Acid-810, essentially a mixture ofiso-octanoic, iso-nonanoic and iso-decanoic acids. The average equivalent weight was measured as 157. Marketed by Novadel Chemicals Ltd. (ii) Naphthenic acid (180SP), a mixture of bi-cyclic acids (CnH2n_3COOH), 5-membered ring acids (CnH2n_ICOOH) and aliphatic acids (CnH2n+ICOOH). Manufactured by Shell Chemical Co. Ltd. (iii) Versatic acid 911, a mixture of secondary and tertiary aliphatic acids, present in the ratio 1:9, with hydrocarbon chain length range 9-11. Manufactured by Shell Chemical Co. Ltd. All other materials were of "Analar" grade. Amine-carboxylate solvents were prepared by mixing calculated amounts of amine and carboxylic acid, followed by addition of the diluent to give the required concentrations. In systems involving Aliquat-336, the solvent as prepared above was treated with a stoichiometric excess of 2 N sodium hydroxide. The organic phase was then washed thoroughly with distilled water to remove any sodium chloride or excess sodium hydroxide. Toluene was used as diluent for most of the work except when actually evaluating the effect of variation in this component of the solvent. It was not found necessary to employ a modifier with any of the systems studied. For extraction experiments involving bitterns, a synthetic bittern was prepared from "Analar" reagents, the concentrations of the principal constituents being: NaCI 1.60M, KC1 0.35 M, MgCI 2 1.70 M, MgSO,, 0.60 M. The minor constituents calcium, lithium, boron and bromine were added in the forms of calcium chloride, lithium sulphate, boric acid and potassium bromide but other trace constituents were neglected.
Procedure Most of the work was based on determination of distribution and selectivity data. For this, measured volumes of the two phases were equilibrated by shaking in a thermostat bath at the required temperature for 30 min. Experiments showed that both thermal and extraction equilibria were established in under 10min. The conjugate phases were then separated and analysed. The aqueous phase was analysed direct. The organic phase was first stripped with water in four contacts and the combined aqueous strip liquor analysed.
Analytical methods EXPERIMENTAL
Materials Consideration was given to the possible combination of four commercially available amines with three carboxylic acids. The amines were: (i) Alamine-336, a mixture of tertiary amines approximating to the molecular formula N[(CH2)nCH313, where n = 8-10. Manufactured by General Mills Inc. (ii) Aliquat-336, a mixture of quaternary ammonium chlorides approximating to the molecular formula {CH3-N[(CH2)nCH3]}CI, where n varies between 8 and 10. Manufactured by General Mills Inc. (iii) Amberlite LA-2, a secondary amine with N-lauryl(trialkyl methyl) amine and its isomers as the major components. Marketed by Rohm and Hass Co. (iv) Primene JM-T, essentially a mixture of isomeric trialkyl methyl primary amines in the Czs-C22 chain length range. Marketed by Rohm and Hass Co.
The project demanded analysis for magnesium, sodium, potassium and calcium in mixtures containing from one to all of these cations. Atomic absorption techniques were maialy used, although care had to be taken to avoid errors caused by interference. While it is generally claimed that the absorption of magnesium in an air-acetylene flame is not affected by the presence of sodium, potassium and calcium, preliminary investigation did not show such an assumption to be justified. While the interference was small, it could not be neglected, particularly at low concentration levels. The difficulty was overcome by adjusting the concentration levels of the interfering cations in the calibration standards to correspond to their concentrations in the samples. Sodium and potassium were determined by atomic absorption without difficulty. Flame emission was employed for calcium as being more sensitive in view of its low concentration relative to the other cations. The effect of interference again had to be eliminated
Magnesium chloride extraction
193
I
by the use of calibration standards containing the other cations at approximately the same concentration levelsas in the samples. Sulphate concentrations were not required with great accuracy and so were determined by the rapid volumetric method using sodium rhodizonate as indicator. Further details of the experimental work are available elsewhere[7].
RESULTS AND DISCUSSION
Comparison of amine components of mixed extractant Versatic acid 911 was used as the common acidic component for a comparison of the various amines. It was chosen as having a fairly constant composition (all work was conducted, of course, using material from the same batch). The results for extraction of magnesium chloride from pure aqueous solution are shown in Fig. 1. In agreement with the conclusions of Grinstead, the order of effectiveness of the amines is seen to be quaternary > primary > secondary > tertiary. While the distribution coefficients are not high, their magnitudes should be greater with a bittern due to the common ion effect of the excess chloride ions.
On the basis of forward extraction, aliquat-336 would appear the most promising basic component of the extractant. Amberlite LA-2 and alamine-336 are less effective than primene JM-T at high magnesium levels but show higher distribution coefficients at low concentrations. They would therefore have the rather unusual characteristic of being inferior to primene JM-T for both extraction and stripping. As a result, it appears that only aliquat-336 and primene JM-T merit further consideration as the basic component. While the former will be the superior for extraction and give higher loadings, its stripping characteristics will not be as good. It is of interest to note that the solvent loadings for amberlite LA-2 and alamine-336 are several times greater than those reported by Grinstead[6] for typical pure secondary and tertiary amines. This could be attributable not only to structural differences but also to the presence of small but significant amounts of active impurities (e.g. primary amines) in the commercial products.
Comparison of carboxylic acid components of mixed extractant With aliquat-336 and primene JM-T in turn as the common amine component, distribution data were 0.5
--
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aliquat 336-versatic acid primene dM-T-versatic acid amberlite LA-2-versatic acid }< alarnine 3 3 6 versatic acid
ali~luat 556-acid 810 aliquat 336-napthenic acid aliquat 336-versatic acid
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Fig. 2. Comparison of carboxylic acid components for extraction of magnesium chloride.
5 g 0'10
Table 1. Physical properties (compiled from manufacturers' technical bulletins) 0,05
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Fig. l. Comparison of amine components for extraction of
magnesium chloride.
Component Acid-810 Naphthenic acid Versatic acid
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lif0 55-3 33.6
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194
C. HANSON,
M. A. HUOHESand S. L. N. MURTHY
obtained for magnesium chloride between water and the organic solvent using the three carboxylic acids. The results for aliquat-336 are given in Fig. 2, from which it will be seen that no significant difference was found between the three acids. The behaviour with primene JM-T was similar. This comparable performance of the three acids is not really surprising, although the cyclic structure of naphthenic acid might have been expected to produce some difference. It is clear that a choice between the three acids cannot be made simply on the basis of the extraction isotherms. However, they do have substantially different physical properties and these could be significant for an industrial process. The available data are summarized in Table 1. Naphthenic acid is clearly at a disadvantage in comparison with the other two in having both a higher density and a higher viscosity. These would be expected to give slower phase disengagement and more difficult phase interdispersion. Furthermore, while naphthenic acid has the lowest cost per unit weight, its equivalent weight is greater than those of the other acids and the cost per equivalent weight is of the same order for all three. Overall, it would appear less attractive then either of the other acids considered. A logical choice between acid-810 and versatic acid is more difficult. However, the lower viscosity of the former would give a somewhat lower viscosity solvent phase with consequent reduction in power requirement and better phase disengagement. Acid-810 was observed to give the sharpest phase separation with aliquat-336. However, it should be noted that with all three acids difficulty was experienced in obtaining rapid phase separation at low magnesium loadings, i.e. corresponding to the later stages of stripping. In the light of the results obtained to this stage, it was decided to concentrate further work on the system aliquat-336/acid-810. This decision also reflected the extensive data already available on the system primene JM-T/naphthenic acid[6]. A 1-0M stoichiometric mixture of the two extractants in the solvent phase was used throughout, the saturation limit for magnesium chloride assuming a stoichiometric reaction then being 0.5 M. This should be a reasonable choice even for extraction from a bittern (-~ 30 x sea water) in which the magnesium concentration would be about 1.7 M.
Effect of chloride ion concentration It was important to establish that excess chloride ion in the aqueous phase would enhance extraction of magnesium chloride by the common ion effect. Extraction isotherms for magnesium chloride in the presence of sodium chloride are presented in Fig. 3 and show clearly that forward extraction is enhanced appreciably.
Selectivity for magnesium over sodium Previous work[6] has shown that sodium chloride
0.6
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presence of magnesium chloride between water and solvent phase comprising 1.0 M aliquat-336/acid-8 l0 in toluene.
Magnesium chloride extraction is co-extracted with magnesium chloride by mixed ionic solvents and it is important to establish the extent to which this takes place. The data obtained in studying the effect of chloride ion concentration provided this information and are cross plotted in Fig. 4. Values of selectivity vary from just under 10 to over 50. While high, it is clear that a loaded extract from a bittern would contain an appreciable concentration of sodium ions and scrubbing would be necessary to remove these.
Effects of potassium and sulphate ions In addition to sodium, magnesium and chloride ions, a bittern will contain appreciable concentrations of calcium, potassium and sulphate ions. N a + - K + Mg2+-CI - complexes and N a + - K + - M g 2 + - S O 2complexes are known to exist in aqueous solutions, the latter generally being the more numerous and stable. Hence the presence of these other ions might well lead to appreciable differences in the extraction behaviour of magnesium as against that reported above for systems in which they are absent. The magnesium chloride extraction isotherm was therefore determined using the synthetic bittern and is shown in Fig. 5. To illustrate the effect of the other ions, the
[] NoCI MgCI 2 Solutions I ' All, ~' 0 . 4 5 [ 0 SO~- free b i t t e r n / 351 X Bittern ~ Aci ® B i t t e r n - P r i m e n e JMT [6] A Bittern-0.5 M 0 . 4 0 -- clliquot ocid 810
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Fig. 5. Extraction isotherms for magnesium chloride from various feed materials.
195
Table 2. Distribution of magnesium sulphate between water and a solvent phase comprising l-0 M aliquat-336/acid-810 in toluene at 25°C Equilibrium concentrations of magnesium in the conjugate phases (M) Aqueous Organic 0.432 0.613 1.106 1.413
0.033 0.061 0.084 0.095
isotherm is also given for a pure magnesium chloridesodium chloride solution of the same strength. To ascertain the contribution of sulphate ions, a sulphatefree bittern was prepared (i.e. all metals in chloride form). The extraction isotherm is included in Fig. 5. It will be seen that the distribution coefficients for magnesium chloride are substantially less from a bittern than from the pure solution. At least half this effect appears due to the sulphate ions. However, its magnitude would not seem sufficient to justify prior desulphation as part of a commercial process since this will not only introduce an extra operating cost but will also dilute the feed. In addition to the impact of sulphate ions on the behaviour of the magnesium, there is also the problem of their being co-extracted with the chloride, thus introducing an impurity into the product. Previous work[6, 8] suggested that amines are poor extractants for sulphate. However, a study on the extraction of pure magnesium sulphate from aqueous solution by aliquat-336/acid-810 showed appreciable quantities to transfer. The results are given in Table 2. Nevertheless, comparison with Fig. 3 indicates that chloride/sulphate selectivities must be high. Preliminary experiments confirmed that, for solvent phase magnesium loadings of over if4 M, the sulphate concentration in that phase would be under 0.002 M. Thus the impurity level would only be in the region of 0.5 M per cent at most. The bulk of this should be removed during the scrubbing operation necessary to remove co-extracted sodium chloride. The difference in the extraction isotherm for a sulphate-free bittern and that for the equivalent mixture of just magnesium and sodium chlorides shows the effect of potassium. This leads to a small but significant reduction in the distribution coefficient for magnesium over almost the whole concentration range, presumably due to the formation of complexes in the aqueous phase. However, potassium is not co-extracted as readily as sodium and so should present no problems as an actual impurity (potassium concentrations in the organic phase in equilibrium with a bittern are <0.005 M).
Effects of calcium and bromide ions The presence of appreciable concentrations of sulphate ions in a bittern automatically limits the
196
C. HANSON,M. A. HuGhEs and S. L. N. MURTHY
possible concentration of calcium to a low level. No precise figures are available but it is probably less than 200 ppm. However, even at these levels calcium cannot be ignored as Grinstead [6] has shown amine-carboxylic acid extractant systems to be selective for calcium over magnesium. Thus, while these low levels of calcium should not directly affect the extraction of magnesium, they could influence the stripping process since this metal will tend to be preferentially held in the solvent and could build-up with recycle if stripping is not complete. There is also the question of calcium contamination of the product. Assuming virtually total extraction, the calcium chloride content of the product magnesium chloride could be in the region of 5 M per cent. One solution would be a two section stripping process with pure magnesium chloride being produced in the first by use of a relatively low aqueous to organic phase flow ratio, with clean-up of the solvent in the second by use of a much higher aqueous flow rate. A similar problem could arise on the anion side since aliquat-336/acid-810 is selective for bromide over chloride[7]. With a combined stripping section, the bromide content of the product could be around 1.5 M per cent. This could be greatly reduced by use of the two section stripping process suggested above for elimination of calcium. More detailed data on the effect and distribution of impurities are available elsewhere[7].
Effect of extractant concentration The work with a solvent phase containing a 1 M mixture of the extractants showed maximum loadings of magnesium chloride close to stoichiometric. The extraction isotherm was also determined for a bittern into a 0.5 M solution of the extractants. The results are incorporated in Fig. 5 and confirm near stoichiometric behaviour. Concentrations greater than 1 M were not considered in view of their high viscosities. Lower concentrations were considered since workers with other systems [9, 10] have reported lower solvent losses at lower extractant concentrations. However, these observations were probably dominated by entrainment losses since loss by solution would not be expected to vary greatly. It is considered that entrainment losses are better minimized in the design of contacting equipment rather than in choice of egtractant concentration since decrease in the latter would cause substantial increase in equipment size.
Effect of diluent The choice of diluent can have an important bearing on parameters governing plant performance such as phase disengagement and rate of mass transfer. In addition, it can influence the actual distribution through 0.45
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Comparison of aliquat-336 with primene JM-T A choice between aliquat-336 and primene JM-T as basic component of the extractant depends on their relative performance with a bittern rather than with pure components, as shown in Fig. 1, and also their stripping qualities. The extraction isotherm obtained by Grinstead [6] for extraction of magnesium from a bittern into primene JM-T/naphthenic acid is included in Fig. 5. It is clear from this that distribution coefficients are considerably higher with aliquat-336/acid-810, which would lead to a more compact unit for a continuous extraction plant. The advantage will be in the other direction, however, for stripping. In addition, the primene JM-T system appears considerably more selective for magnesium over sodium. However, these advantages for the primene JM-T system are only marginal in the context of a commerical process. A scrubbing section will be necessary in either case for removal of sodium but only a couple of stages would be perfectly effective even with aliquat-336. Similarly, any saving on the stripping section from primene JM-T would be less than the additional cost for the extraction section caused by using this extractant. It was concluded at this stage that aliquat-336! acid-810 was the most promising combination of the extractants considered.
0.55 5; .
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0.25
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0"05
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Fig. 6. Effect of diluent on extraction isotherms.
Magnesium chloride extraction stabilization by solvation of the extracted species and by promotion or otherwise of aggregation. Distribution isotherms were obtained for five diluents and are shown in Fig, 6. The investigation was not exhaustive and was restricted to diluents obtainable in a pure form, thus excluding most of the commerical products now on the market. Nevertheless, the results do illustrate the influence of diluent. The chlorinated hydrocarbons are seen to give appreciably lower distribution coefficients than the others and third phase formation was observed with these above 1.75 M magnesium in the aqueous phase, n-Hexane and toluene are preferable to nitrobenzene in having a lower density. Of these, toluene gave marginally the better distribution coefficients and had adequate solvency so was considered the best.
Effect of pH While pH is an important parameter when considering extractions by amines or carboxylic acids alone, its influence on extraction with mixed ionic extractants should be small. This is one of their prime attractions for commercial use since the cost of repeated pH adjustments can be avoided. The effect of pH on extraction from a fixed feed solution was examined, the results being given in Table 3. It will be seen that it has comparatively little influence and so there would appear to be no advantage in making pH adjustment to the various streams.
Effect of temperature If t here is an appreciable ent halpy change accompanying the reaction represented by Eqn (1), the equilibrium constant would be dependent on temperature and so distribution coefficients would also be influenced by temperature. While it is difficult to conceive making any temperature adjustments to a feed bittern, it might be possible to exploit any such phenomenon at the stripping stage. Attention was therefore concentrated on the distribution of magnesium chloride alone and isotherms determined at both 25°C and 50°C. Very little effect was observed. At most the distribution coefficient decreased about 7 per cent with the 25°C rise in temperature. It is clear that enthalpy changes must be small and temperature will not be an important operating parameter for an industrial process. Table 3. Effect of pH on distribution of magnesium chloride between water and 0.5 M aliquat-336/acid-810 in toluene at 25°C
197
Solvent losses Solvent losses in a commercial process will result from both entrainment and solution of the solvent in the aqueous streams. The former is a function of plant design and operation and is diffficult to estimate in advance. However, with a relatively cheap product such as magnesium chloride, losses of expensive extractants could be a major economic parameter. Grinstead et al.[6] have reported the solubilities of various long-chain amines and quaternary ammonium chlorides in concentrated brines, while Fletcher and Flett[11] have determined the solubility of naphthenic acid in the raffinates from copper and nickel extraction. No data have been reported for acid-810. Some preliminary experimental results are given in Table 4. These are average values based on three determinations in each case (results varied by + 5 per cent). They are significantly higher than the reported soluble losses of naphthenic acid in copper extraction processes (-~ 100 mg. 1-1 at pH 4.7). Additionally, the soluble losses ofaliquat-336 would be in the region of 20 mg-1.- 1.
Mechanism of extraction The primary objective of the current investigation was the collection of useful data for design of a possible commercial process. However, it is of interest to consider the mechanism of extraction in the light of the data obtained. The mechanism has been studied by Davies and Grinstead[12], who concluded that (i) the magnesium carboxylate and aikyl ammonium chloride are not associated in the organic phase and exist as separate species, (ii) below an organic phase concentration of 0.02 M magnesium chloride, the magnesium carboxylate and alkyl ammonium chloride exist as unaggregated monomers, (iii) the extracted species are associated with several unused extractant molecules, and (iv) there is negligible hydrolysis of the extracted species. While these conclusions may have been valid for the conditions studied, the results of the present investigation suggest that they would not all apply under more concentrated conditions. Thus, while maximum loadings were approximately in agreement with the stoichiometry of Eqn (1), as discussed in Section 3.8, nevertheless many cases were observed of magnesium concentrations in the solvent somewhat greater than stoichiometric, e.g. 0.54 M in equilibrium with 3.0 M in the aqueous phase for pure magnesium chloride and 1~56 M in equilibrium with 2.77 M in an aqueous phase also 1 M Table 4. Solubility of acid-810 at 25°C
pH 2 4 6 8.5
Equilibrium concentrations (M) Aqueous phase Organic phase 1-66 1-66 1.67 1.66
0.137 0.132 0.129 0.137
Medium
Solubility Img. 1-tl
Water Synthetic bittern (pH -~ 4.5) Synthetic bittern saturated with aliquat-336
1,040 270 340
198
C. HANSON,M. A. HUGHESand S. L. N. MURTHY
in sodium chloride. The stoichiometric limit would be 0.50 M since the concentration ofextractants was 1.0 M. Many other limits of similar magnitude were found. Under these conditions, with magnesium concentrations just greater than apparent stoichiometric, it appears inconceivable that the extracted species could be associated with any free extractant molecules. It is thought probable that solvent phase concentrations in excess of the stoichiometric value could be due to the participation of hydrolysed species in the extracted complex and to the effect of water dissolved in the solvent phase. Such an explanation has been given by Fletcher and Flett E11] for similar observations during the extraction of cobalt and nickel by naphthenic acid. In the present investigations, it was noticed that significant amounts of water were extracted by the aliquat-336/acid-810 solvent. Thus dilution of a 1-0 M solution after equilibriation with water to 0.5 M by addition of toluene resulted in the separation of water. No quantitativemeasurements were made. Grinstead [6] has claimed that aliquat-336 will extract some 8 moles water per mole of extractant when contacted with pure water, although less when contacted with magnesium chloride solutions. It seems clear that the nonstoichiometric solvent loadings observed are related to the co-extraction of water.
CONCLUSIONS A mixed ionic extractant comprising an equimolar mixture of aliquat-336 and acid-810 is promising for the recovery of magnesium chloride from bitterns. It gives quite favourable distribution coefficients and loadings with good selectivity from the other ions present except for calcium. Forward extraction is enhanced by the high chloride ion concentration in a bittern, while stripping can be achieved with water. Thus there would be no need for conditioning steps, with their attendant cost. It would be necessary to incorporate a scrubbing section in a flowsheet and provision would also have to be made for solvent purification during recycle to prevent build-up of calcium and possibly bromide. Temperature and pH do not have significant effects on the process.
Solvent loadings in excess of stoichiometric were observed, probably due to co-extraction of water. The data presented show that a solvent extraction process based on this system could operate for the recovery of magnesium chloride. Whether it would be competitive can only be decided from an economic evaluation and this will be the subject of a future communicationE13]. Acknowledgement--One of the authors (S.L.N.M.) would like to record his indebtedness to the Association of Commonwealth Universities for providing a Commonwealth Scholarship which allowed him to undertake the work described.
REFERENCES
1. C. Hanson and S. L. N. Murthy, The Chemical Engineer (264), 295 (1972). 2. R. Blumberg and N. Melzer, International Mineral Processing Congress, New York (1964). 3. U.S. Office of Saline Water, Res. Dev. Prog. Rep. 320 (1968). 4. R. R. Grinstead, J, C. Davis, S. Lynn and R, K. Charlesworth, Ind. Eng. Chem., Prod. Res. Dev. 8(3), 218 (1969). 5. R.R. Grinstead, J. C. Davis, Ind. Eng. Chem., Prod. Res. Dev. 9(1), 66 (1970). 6. U.S. Office of Saline Water, Res. Dev. Prog. Rep. 406 (1969). 7. S. L. N. Murthy, PhD Thesis, University of Bradford (1971). 8. N. M. Rice, UniversityofLeeds. Private communication (1971). 9. C. G. Robinson and J. C. Paynter, Proc. Int. Solvent Extraction Conf., 214, The Hague (1971). 10. G. M. Ritcey, Mines Branch, Ottawa. Private communication (1971). 11. H. A. C. McKay, et al. (Ed.), Solvent Extraction Chemistry. Macmillan, London, 1966. 12. J. C. Davis and R. R. Grinstead, J. phys. Chem. 74(1), 147 (1970). 13. C. Hanson and S. L. N. Murthy, Paoer to be presented at Int. Solvent Extraction Conf. Lyon (1974).