Ind. Eng. Chem. Res. 1994,33, 329-335
329
Extraction of Cyanamide and Dicyandiamide from Ammoniacal Solution with Aliquat 336 Herbert Mizelli and Hans-Jorg Bart* Institut fur Thermische Verfahrenstechnik und Umwelttechnik, Technische Universitat Graz, Znffeldgasse 25, A-8010 Graz, Austria Solvent extraction of cyanamide and dicyandiamide (1-cyanoguanidine) from aqueous ammoniacal solution with Aliquat 336, a mixture of trialkylmethylammonium chlorides (trialkyl = Ca-C10, mainly capryl) dissolved in Shellsol T, has been studied. Extraction isotherms with different ionic forms of the anion exchanger are reported for separation of the compounds of interest, as well as for mixtures of the solutes. The search for an appropriate modifier to solve the apparent phase disintegration of the formed complexes results in a mixture of 10% (v/v) 2- ethylhexanol and 30% Aliquat in Shellsol T. With this phase the influence of temperature and extractant concentration and the effect of changing ammonia concentration in the feed solution have been investigated. A stoichiometric model was proposed and a temperature and concentration invariant 2:l (solute: amine) complex was found for dicyandiamide. A temperature variant stoichiometric complex with cyanamide allows prediction of optimal separation conditions.
Introduction Since the development of the high molecular weight amines in the early 1960's many applications have been described for the removal of anions from aqueous solutions. Applications using amines as extractants are reported in the field of hydrometallurgy and nuclear fuel reprocessing (Hanson et al., 1983), in the biochemical industry, and in the pharmaceutical industry. Most of these works deal with extraction from acidic aqueous solution (Ritcey and Ashbrook, 1984). To extract anions from alkaline solution only quaternary amine compounds like Aliquat 336 (Henkel) or Adogen 464 (Sherex) which are stable in this medium can be taken into consideration. This study is aimed at the selective separation of cyanamide and dicyandiamide from aqueous ammoniacal solution. Cyanamide, "2-CN, has many applications in organic synthesis (Weiss, 1978). The crucial property of this compound is its low stability. In acidic media with pH values lower than 4, cyanamide shows hydrolysis to urea, and in media with pH values higher than 5, it dimerizes to dicyandiamide (Buchanan and Barsky, 1930). That is why only stabilized solutions of cyanamide are stable. Dicyandiamide, (HzN)z--C=N-CN, which has its broadest application in the production of melamine, is also a very useful reagent in organic synthesis, as the following reactions are possible: (1) reaction of the guanidine group, (2) reaction of the cyano group, (3) reaction of both groups, and (4) splitting reactions. In the present paper the description of the extraction behavior of both components is given, with respect to temperature, concentration, and ionic form of the ion exchanger. Additionally the dependence on the type and concentration of the modifier used, the selectivities between both components, and the influence of ammonia concentration of the aqueous feed solution are part of the investigations. A mechanistic model for extraction is built, and equilibrium constants as well as stoichiometric coefficients of the extraction reaction and equilibrium concentrations are evaluated. Calculation of the Equilibrium Composition Liquid-liquid equilibrium distributions of the reactive solutes are calculated by means of the program QLEQUI
* Author to whom correspondence should be addressed. E-mail:
[email protected].
(Grigorianand Bart, 1993). The program uses the Brinkley method (Brinkley, 1947)to solve the mass balances which relate the feed and equilibrium concentrations in one equilibrium stage according to the proposed stoichiometric model. The procedure consists of two levels: (1) variation of parameters and (2) calculation of concentrations. For level 1 the Nelder-Mead method is used to evaluate the step size and the direction in space for the search of parameters. For level 2 the Brinkley method is applied to calculate equilibrium concentrations for each set of parameters, which-according to the proposed stoichiometry-yields a definite solution with squared convergence velocity. The calculated equilibrium concentrations are then compared with experimental data: the sum of squares deviation is the target function of the Nelder-Mead method. This sum is compared with the one of the previous iteration. The break condition is fulfilled if the difference of the square deviations lies within the determined accuracy. During the iteration process, graphics, showing experimental and calculated values, allow one to follow the procedure and give information about the accuracy and validity of the formulated model. The obtained stoichiometry and equilibrium constants allow the description of the system behavior under different conditions (concentration changes).
Experimental Section Materials. Cyanamide and dicyandiamide (1-cyanoguanidine) were obtained from Merck as analytical grade products of 98% purity. Ammonia was used as a 25 % aqueous solution (technical product) and diluted to the desired concentration. The mixture of trialkylmethylammonium chlorides (trialkyl = Ca-CIo, mainly capryl), known as Aliquat 336, was received from Henkel Co. The other components of the organic phase were Shellsol T by Shell Chemicals and 2-ethylhexanol (for synthesis), used as a modifier, by Merck. The various ionic forms of Aliquat 336 used in this work were converted from the commercially available chloride form according to the following procedure: (1)dilution of Aliquat 336 to a 50 % solution with Shellsol T, (2) contact of the organic solution with an equal volume of a 1.5 M solution of HzS04 (or 1.5 M Na2C03 or 1.5 M NaOH) in a shaker for 2 min, and (3) settling of the aqueous phase. This procedure was repeated four times. After phase separation the organic solution
ogss-5ga5/94/2633-Q329~04.50/0 0 1994 American Chemical Society
330 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 1. Extraction Degrees (%) for Cyanamide Extraction from 10% Ammoniwl Solution at 40 OC Using the Carbonate Form of Aliquat 336 and Different phase Ratios (A/O) as Well as Different Amounts (10%and 30%) of Various Modifiers AI0 = 0.1 AI0 = 10 10% 30% 10% 30 % 31.1 43.0 amyl acetate 85.5 86.6 84.6 86.3 10.1 14.2 diisobutyl ketone 24.4 49.8 2-ethylhexan01 91.0 96.6 heptan-3-one 96.4 96.8 26.9 17.3 92.4 88.6 32.7 27.3 isobutyl acetate 49.6 32.6 1-nonanol 90.8 92.4 83.8 29.4 octan-3-one 19.0 28.1 TOP0 89.1 89.1
was further diluted and modifier was added to obtain the desired composition of the solvent. Analytical Methods. Karl-Fischer titrations were performed on a Radiometer Copenhagen Titroprocessor for water determination. Cyanamide and dicyandiamide concentrations were determined by high performance liquid chromatography (HPLC): NH&N from the aqueous ammoniacal solution was analyzed by a modified method described in the literature (Prununosa et al., 1986). To avoid interference of ammonia the samples were extracted with ether after neutralization; then the ether was evaporated, and the residue was dissolved with water and then derivatized as described. The quantitative determination was made with external calibration and fluorescence detection similar to the described method. Dicyandiamide solutions were diluted to a detectable concentration and analyzed by HPLC using external calibration with ultraviolet detection at 218 nm. The measurements were carried out with a Perkin Elmer binary gradient pump, an RP-18 analytical column from Millipore Waters, a UV-VIS detector from Perkin Elmer, and a fluorescence detector from Shimadzu (Mizelli, 1993). Extraction Experiments. These experiments were carried out in thermostated glass shakers. Equal volumes of organic and aqueous phases were contacted with an automatic shaking device until reaching equilibrium. The concentrations of the solutions in the aqueous phase were
determined as described above; the concentration in the organic phase was calculated with mass balances.
Experimental Results and Discussion Composition of the Solvent Phase. Preliminary experiments have shown that extraction of cyanamide and dicyandiamide with Aliquat 336 in various ionic forms results-without the use of a modifier-in the formation of emulsions or third phase formation. To avoid this phenomenon, a systematical search for an adequate modifier was carried out a t various concentrations and phase ratios. Esters, ketones, alcohols, and organophosphorus compounds were investigated. Table 1shows that 2-ethylhexanol,1-nonanol,heptan-&one, and amylacetate yielded the best extraction degrees for cyanamide separation. Because of the low solubility in water and the small uptake of water and the shorter phase disengagement time, 2-ethylhexanolwas chosen further on. Figure 1shows that at modifier concentrations higher than 15% of volume the degree of extraction can only be improved very slightly. The minimum amount of modifier, Le., that amount where no phase disintegration appeared, was determined by titration of the loaded phase with alcohol. The sulfuric form of the ion exchanger needs about 4% modifier and the carbonate form only about 1% . The optimum ion exchanger concentration is 30 % (v/v) Aliquat 336. Higher concentrations will improve extraction results, but hydrodynamic constraints (i.e., viscosity and phase disengagement time) set a limit. Extraction of Dicyandiamide The experiments, having been carried out with different ionic forms of Aliquat 336 a t 20 "C, are summarized in Figure 2. Equilibrium isotherms have been used to formulate a model for the extraction procedure. The evaluation of stoichiometryand constants is done by means of the computer program QLEQUI. The extraction mechanism can be described by the following steps: (a) dissociation of ion exchanger
20 30 40 modifier content in solvent phase [vol-%] Figure 1. Dependence of cyanamide extraction on amount of added 2-ethylhexanol at 40 OC. Aqueous phase: 0.1 % cyanamide in 25% "3. Organic phase: 0.6 M Aliquat 336 (carbonate form) in Shellsol T With a variable amount of modifier. Phase ratio = 1. 10
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 331
30
g25
l
Y
2o
r5
*
10 5
" n
5
0
10
15
20
25
Caqu. [mmolL] chloride-fonn carbonate-form sulfate-form hydroxideform calc. values 0
*
A
0
Figure 2. Extraction of dicyandiamide with different ionic forms of Aliquat 336 at 20 OC.Aqueous phase: 0.1% dicyandiamide in 10% Organic phase: 0.6 M Aliquat 336 and 10% 2-ethylhexanol diluted in Shellsol T. Phase ratio = 1. Table 2. pK Values from Model 1 for Dicyapdiamide Extraction with Aliquat 336 ionic form
(b) deprotonation of dicyandiamide
+ OH- H 2 0 + (H,N),-C=N-CN (HNH,N-C=N-CN)-log
K2
= 0.4 (2)
+ OH- F? H 2 0 + (HNH,N--C=NH-CN)-log K = 0.8 (3) (HNHN-C=NH-CN)2(these -log K values contain the ionic product of water) F!
NH4++ OH-
-log K4 = 4.75
(4)
(d) association reaction xR4N++ y(HNH(2,N-C=N-CN)'2'-
F?
(R4N),(HNH(2,N-C=N-CN),
-log Kb (5)
For reaction a, a first approximation of the equilibrium constant from the literature (Zolotov et al., 1983) can be used. Definitely unknown parameters are the equilibrium constant of the association reaction (5) and ita stoichiometric coefficients ( x and y) in eq 5. Concerning that point, measurement and calculations (Bagreevet al., 1988) have shown that only a few molecules of anion exchanger can be placed in the vicinity of anions, because of the large size and the resulting steric hindrance. Two chemical models can be formulated for the description of extraction mechanistics: model 1assumes an overall 2:l stoichiometry in eq 5, as:
-
+
2R4N+ (HNHN-C=N-CN)"
-log K1 -log Ke
+
(6) The results of the mathematical computation and optirnizatipn procedure of the equilibrium parameters are listed in Table 2. As can be seen, the different ionic forms of the exchanger (eq 1)give a slight variation of K 1 . Model 2 takes into consideration that the pK values for deprotonation reactions of dicyandiamide are very similar and that two different complexes are formed during extraction: (R4N),(HNHN-C-N-CN)
OH-
SO,"
cos"
3.25 8.824
3.41 8.824
3.514 8.824
+
R4N+ (HNH,N-C=N-CN)-
-
(c) dissociation of ammonia ",OH
"3.
F?
(R4N)(HNH2N-C=N-CN)
+
2R4N+ (HNHN-C=N-CN)Z
c13.665 8.824
(7)
e
(R4N),(HNHN--C=N-CN) (8) Both models can be used to describe the equilibrium isotherm. From the chemical point of view the mixed model is more realistic, because of the small difference between the deprotonation constants of dicyandiamide (eqs 2 and 3). It is also important that dicyandiamide can easily form tautomeric forms (Schneider, 1950; Hughes, 19401, see Figure 3. NsC-N.
-N=c=N.
-N=c=N.
Figure 3. Tautomeric forma of dicyandiamide.
Using the latter model, it was concluded that the 1:l complex (eq 7) dominates over the 2:l complex (eq 8,Table 3). The total concentration of dicyandiamide in the aqueous phase can be understood as the sum of the concentrations of the various (de)protonated forms: caqu,equi = CHZDCA + CHDCA+ CDCA. Similiarly, the concentration of dicyandiamide in the organic phase is C,,q, = Cmmplexl:l + Ccomplefil. The high concordance between calculated and experimental data confirms the assumption of the mixed model. Computations also showed that the amount of the 1:lcomplex formed depends on the ionic form of the ion exchanger: this is due to the differences in the dissociation constants of the forms used. Calculations with other models, which used other integer as well as miscellaneously defined stoichiometric complexes could not solve the problem in the same accurate
332 Ind. Eng. Chem. Res.,Vol. 33, No. 2, 1994 Table 3. Equilibrium Concentrations of Involved Species for Dicyandiamide Extraction with Aliquat 336 Calculated and Experimental Values. Concentrations in mol/L. Organic Phase: 30% Aliquat, 336, Hydroxide Form, 10% 2-Ethylhexan01in Shellsol T, 20 OC experiment 2 3 4 5 Calculated Values for Concentrations of Species Involved in the Extraction 5.855 NH4OH 5.855 5.855 5.855 5.855 5.445 5.707 5.64 5.574 5.509 R4NOH complex 1:l 0.005825 0.008751 0.01169 0.01462 0.01757 complex 2:l 0.003965 0.005887 0.007771 0.009609 0.01141 0.002429 0.003692 0.00499 0.006317 0.007682 HaDCA O.ooOo20 O.ooOo30 O.ooOo41 O.ooOo51 0.000062 HDCA6.513-08 9.803-08 1.313-07 1.653-07 1.983-07 DCA” 0.005053 0.005076 0.0051 0.005124 0.005148 NH4+ RdN+ 0.001557 0.01546 0.01535 0.01525 0.01514 0.02061 0.02051 0.02041 0.02032 0.02022 OHCalculated Values for Total Dicyandiamide Concentration Cqu,q,,i 0.002429 0.003722 0.005032 0.006370 0.007746 Com,q,,i 0.009790 0.014638 0,019461 0.024229 0.02898 Experimental Values for Total Dicyandiamide Concentration c.gu,w,,i 0.002319 0.003758 0.005186 0.006399 0.007659 Com,,i 0.009921 0.0146 0.0193 0.0242 0.02907
Table 5. Extraction of Cyanamide at 40 OC from 26% Ammoniacal Feed Solution with Aliquat 336, Model Parameters ionic form -loa Ki -loa Kii ElrN+:HN-CNOH- form 3.15 3.79 1:l 3.45 3.85 1.251 cos” form
1
Table 4. pK Values from Model 2 for the Extraction of Dicyandiamide with Various Ionic Forms of Aliquat 336. Organic Phase: 30% Ion Exchanger, 10% 2-ethylhexanol in Shellsol T. Aqueous phase: Dicyandiamide in 10% Ammoniacal Solution, 20 OC ionic form
OH-
SO,”
-log K1 -log Ks -log K1
3.25 8.4 4.28
3.495 8.4 4.28
COS” 3.651 8.4 4.28
c13.89 8.4 4.28
1:l complex
60%
61.8%
73%
84%
way and have shown worse results. Table 4 summarizes the calculated results for all ionic forms of Aliquat 336 used. The extraction constants for 1:l (eq 7) and 2:l (eq 8) complexes have the same values for all forms used. The distinguishingpoints in the parameters are the dissociation constant (eq 1)of the ion exchanger and the amount of the 1:lcomplex in the organic phase which is formed during extraction.
Extraction of Cyanamide Using similar systems for the extraction of cyanamide as those described above, the results are distinguished by the markedly different pK values of cyanamide for the deprotonation reaction (Wayne, 1957): H,N-CN
+ OH- e H,O + HN-CN-log K, = -3.734 (9)
HN-CN- + OH- e H,O
+ N-CN2-
-log Klo 0.6 (10) Model calculations have shown that the stoichiometry is different from that of the dicyandiamide extraction and noninteger coefficients appear (Table 5). As pointed out for the dicyandiamide extraction (eq 5) the reaction mechanism for cyanamide can be stated zR4N++ y(H)NHN2- e (R,N),(H)NHN, -log&, (11) The higher probability for formation of a 1:l complex or for stoichiometries which are very close to a 1:l ratio can be explained by the different deprotonation constants of cyanamide (eqs 9 and 10): the second deprotonation reaction is much more unrealistic,since the CN+-ion exista only at very high pH values (Finkelshtein et al., 1963).
Temperature Dependence of Extraction According to van’t Hoff s equation dlnK AHa d(l/T) R changes in temperature effect changes of equilibrium constants. Experiments with constant modifier and extractant concentration at various temperatures have -I--
70 SO
50
1
40
Y
$30
u 20 10
0
2
4
20°C 0
10
S 8 C aqu. [mmoVL]
40°C
60fC
12
14
calc.values
Figure 4. Extraction of cyanamide a t various temperatures with Aliquat 336. Organic phase: 0.6 M Aliquat 336 (carbonate form), 10% Phase ratio = 1. 2-ethylhexanol in Shellsol T. Aqueous phase: 0.1% cyanamide in 25% “3.
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 333
2
0
4
6
10
8
12
C aqu. [mmoVL] A
dicya diamide
6
0
cyanamide
25 %
lo% ammonia
1%
Figure 5. Extraction of cyanamide (40 "C) and dicyandiamide (20 "C)at various ammonia concentrations. Organic phase: 0.6 M Aliquat 336 (hydroxide form), 10% 2-ethylhexanol in Shellsol T. Aqueous phase: 0.1% cyanamide (dicyandiamide)in ammoniacal solution. Phase ratio = 1. Table 6. Extraction of Cyanamide with the Carbonate Form of Aliauat 336 at Various Temperatures
-log K B -1- Kii
1.41: at 20 "C -3.606 4.00
1.241 at 40 "C -4.068 3.76
1.631 at 60 "C -4.381 4.10
shown that with increasing temperature the equilibrium constants for dicyandiamide in the sequence 20 > 40 > 60 "C decrease. For cyanamide extraction a different behavior was obtained. The results are shown in Figure 4. Model calculations, summarized in Table 6, have shown that-with changing temperature -the stoichiometry of cyanamide extraction changes, due to aggregation phenomena. For the dicyandiamide extraction a constant stoichiometry as described above was obtained.
Influence of Ammonia Experiments with different ammoniacal solutions have been examined, as ammonia has affected the deprotonation of the compounds under investigation. Care was taken to avoid ammonia loss due to evaporation which will shift the pH values of the solutions. Figure 5 shows the experimental results and the calculated values: as expected, for cyanamide and dicyandiamide extraction the tendency for increasing extraction rate with increasing ammoniaamount has been obtained. It has to be pointed out that the decrease of efficiency of the cyanamide separation is not so distinct as for dicyandiamide separation. At optimum temperatures cyanamide separation rates of 99% (97.1% ) were reached. For dicyandiamide 89.6%, 76.4%, and 69.6% could be achieved in a single step in the sequence of falling ammonia concentration (25% > 10% > 1%). Extraction mixtures of cyanamide having a 1% ammoniacal solution as the aqueous phase showed the tendency to form (unstable) emulsions, which is due to aggregation and a low interfacial tension of the organic phase. Nevertheless, the calculated values and parameters showed conformity with the previous evaluated extraction mechanisms and pK values.
Selectivity between Cyanamide and Dicyandiamide In alkaline solutions cyanamide tends to dimerize (Ploquin and Vergneau-Souvray, 1952)with the maximum speed dependent on the temperature and pH value. Because of the possible presence of dicyandiamide in alkaline solution, the selectivity between the monomer cyanamide and its dimer dicyandiamide must be studied. Coextraction experiments have been conducted to get information about separation, selectivity, and enrichment. Experiments have been carried out at 40 "C, which can be regarded as the optimum temperature for cyanamide extraction. The results are shown in Figure 6. At an ammonia concentration of 10% in the aqueous feed solution the degree of coextraction of dicyandiamide decreases linearly with increased loading of the solvent with cyanamide. At an ammonia concentration of 25 % , the degree of extraction of cyanamide is nearly constant. In contrast, the distribution coefficient of dicyandiamide rises until a certain loading of the solvent is reached. Then selectivity changes, and extraction of dicyandiamide is suppressed. At 60 "C enrichment factors for both components are lowered and ammonia affects extraction rates as described above. Experiments at 20 OC from 1 % ammoniacal feed solutions have resulted again in emulsions. Higher ammonia concentrations have shifted selectivity toward unselective extraction. Coextraction of Water The concentrations of the extracted species in the organic phase have been calculated from mass balances, which requires constant phase ratios. To control the water uptake, Karl-Fischer titrations have been carried out. Hence, it follows that only approximately 1% of the water has been coextracted and the amount decreased with increasedloadingof the organicphase. Thus water transfer has been neglected. Conclusions Extraction of cyanamide and dicyandiamide from aqueous ammoniacal solution with Shellsol T/2-ethyl-
334 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
loo 90
-
EEI
$ B 3
80
70
so
0
*
til
50 40
30 0
10
20
30
40
50
60
70
80
loading of org. phase with cyanamide[mmol/L] cyanamide dicyandlamide 25 % ammonia 10 % ammonia - ---- - - e 0
Figure 6. Coextraction of cyanamide and dicyandiamide (40 "C)at various ammonia concentrations in dependence of loading of organic phase with cyanamide. Aqueous phase: 0.15% cyanamide and dicyandiamide in ammoniacal solution. Organic phase: 0.6 M Aliquat 336 (OH- form), 10% 2-ethylhexanol in Shellsol T; phase ratio = 1.
hexanol solutions of trialkylammonium compounds (Aliquat 336) increases with rising concentration of amine and modifier. The optimum ion-exchangerconcentration is about 30 5% of volume; higher concentrations deteriorate the hydrodynamic properties of the organic solutions and do not improve the degree of extraction significantly. The modifier screening has shown that alcohols yield the best results in separation degrees. Ketones, esters, and phosphororganic compounds have been less suitable. %-Ethylhexan01has been used because of the small uptake of water, the low solubility in water, and a shorter phase disengagement time than 1-nonanol. The optimum amount of modifier (2-ethylhexanol) is between 10and 15% of volume, larger amounts of modifier do not affect the degree of extraction significantly. However, at very low ammonia concentrations (1%) emulsion formation during cyanamide extraction cannot be prevented. The size of the dissociation constants of the used ionic forms of Aliquat 336 are in the sequence OH- > S02-> C032-> C1-. Equilibrium isotherms for the extraction of dicyandiamide follow the same order. Cyanamide extraction gives better results when the hydroxide form is used then does application of the carbonate form. Temperature affects the extraction of dicyandiamide negatively, that means that the equilibrium constants are in the order 20 > 40 > 60 "C. Cyanamide extraction follows the sequence 40 > 20 > 60 "C. The stoichiometry of dicyandiamide extraction is obtained by a mixed model, which assumes 1:l and 2:l complexes (ion exchanger:solute) with a predominance of the 1:l complex. The nature of the formed complex depends on the ionic form of Aliquat 336 used and hence on its dissociation constant. For cyanamide extraction no integer defined stoichiometric reactions can be calculated. Stoichiometric coefficients depend on temperature, ionic form, and ammonia concentration of the aqueous feed solution. Ammonia concentration influences deprotonation reactions of the solutions and consequently the equilibrium
of the partition between the phases. A rising amount of ammonia increases the distribution coefficients. Selectivity between cyanamide and dicyandiamide depends on the loading of the organic phase, the ammonia concentration of the feed solution, and the temperature. Increasing the extraction of cyanamide diminishes the extraction of dicyandiamide. Optimum conditions for selective separation of cyanamide are at 40 "C and 10% ammonia concentration. Coextraction of water is in a range smaller than 1%.It falls with increased loading of the organic phase and can therefore be neglected. The formulated model allowsone to predict temperature and concentration changes in the mixture and thus to find an optimal separation regime for cyanamideldicyandiamide separation.
Acknowledgment We wish to thank Dr. M. Miillner and Dr. G. Stern from CHEMIE LINZ Ges.m.b.H and the Austrian Fonds zur Forderung der Gewerblichen Wirtschaft (Project FFF 5/607) for supporting this work. Thanks also to Dr. S. Grigorian for his assistance whilst calculating equilibrium parameters. Nomenclature HzDCA = dicyandiamide HDCA- = lstdeprotonation step of dicyandiamide DCA2- = 2nd deprotonation step of dicyandiamide R4NOH = hydroxide form of Aliquat 336 R4N+ = organic part of ion exchanger aqu = aqueous phase org = organic phase equi = equilibrium TOP0 = tri-n-octylphosphine oxide Literature Cited Bagreev, V. V.; Zolotov, Yu. A.; Fischer, C.; Kardivarenko, L. M.; Popov, S. 0. IR, NMR and conductivity study of extracts and the character of mutual influence of metals in their extraction with alkylammonium salts. International Solvent Extraction Con-
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 335 ference,ZSEC 88 USSR Acad. Sci., Eds.; Nauka: Moscow, 1988; 1, pp 187-190. Brinklev. S. R. Calculation of the eauilibrium composition of systems of m k y constituents. J. Chem: Phys. 1947,15 (2),107-110. Buchanan, G. H.; Barsky, G. The hydrolysis and polymerization of cyanamide in alkaline solution. J.Am. Chem. SOC.1930,52,195206. Finkelshtein, A. I.; Sukhorukov, B. I.; Mushkin, Yu. I. Russ. J.Phys. Chem. 1963,37,151-153. Grigorian,S.;Bart, H.-J. Calculationof reactive extraction flowsheets. In Solvent Extraction in the Process Industries;Logsdail, D. H., Slater, M. J., Eds.; Elsevier Applied Science: London, 1993;Vol. 2,pp 1260-1267. Hanson,C.;Lo,T.C.;Baird,M.H.1.HandbookojSoluentExtraction; John Wiley and Sons Inc.: New York, 1983. Hughes, E. W. The crystal structure of dicyandiamide. J.Am. Chem. SOC.1940,62,1258-1267. Mizelli, H. Cyanamid- und Dicyandiamid-Recyclingbeim Melaminprozess. Dissertation, Technische Universiat Graz, 1993. Ploquin, J.; Vergneau-Souvray, C. Dimerisation de la cyanamide. Bull. SOC.Chim. Fr. 1952,592-598. Prununosa, J.; Obach, R.; Valles, J. M. Determination of cyanamide in plasma by high-performance liquid chromatography. J. Chromatogr. 1986,377,253-260. ~
Ritcey, G. M.; Ashbrook, A. W. Solvent Extraction; Principles and applications to process metallurgy, Part Z & ZI; Elsevier Science Publishers: New York, 1984. Schneider, W. C. The structures of cyanamide and carbodiimide. J. Am. Chem. SOC.1950,72,761-763. Wayne, N. J. Cyanamide;Cyanamide International General Chemicals Department, brochure; CYANAMID Corp.: 1957,p 15. Weiss, S. Cyanamid: neuere Beispiele fiir die Synthese reaktiver Zwischenprodukte und biologisch aktiver Substanzen. Firmenschrift SKW Trostberg, 1978. Zolotov, Yu. A.; Bagreev, V. V.; Popov, S. 0. On the mechanism of the mutual influence of metals in their extraction with gas nitrates; International Solvent Extraction Conference,ZSEC 83, Denver: AIChE: New York, 1983;1, pp 428-429.
Received for review June 10, 1993 Revised manuscript received October 12,1993 Accepted November 7, 19930
* Abstract published in Advance ACS Abstracts, January 1, 1994.
