Reactive extraction for separation of KCl and NH4Cl from aqueous solution using tributylamine

Separation and Purification Technology 118 (2013) 58–63

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Reactive extraction for separation of KCl and NH4Cl from aqueous solution using tributylamine Jianxin Chen a,b,⇑, Quan Sun a, Aidang Lu a, Qing Wu c a

Engineering Research Center of Seawater Utilization Technology, Ministry of Education, Tianjin 300130, China School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China c School of Computer Science and Engineering, Hebei University of Technology, Tianjin 300130, China b

a r t i c l e

i n f o

Article history: Received 3 April 2013 Received in revised form 13 June 2013 Accepted 17 June 2013 Available online 28 June 2013 Keywords: Reactive extraction Ammonium chloride Potassium chloride Separation

a b s t r a c t The reactive extraction process with amine extractants for the separation of KCl and NH4Cl was studied. Tributylamine (TBA) with 1-butanol has been proven to be an effective extractant. A series of influencing factors of the NH4Cl conversion rate were systematically investigated (such as reaction temperature, pressure, reaction time, stirring speed and dilution ratio). The conversion rate of NH4Cl can reach up to 98.7% under the optimum conditions. The recovery of the extractant is mainly determined by the type of the diluents on the distribution result of the R3NHCl in the organic phase. The TBA can be regenerated by NH3 producing via self-reaction and the recovery efficiency of TBA could reach 91.0%. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction KCl is not only an important chemical fertilizer in agriculture but also the basic material for potash production in industry [1,2]. High purity KCl is also applied in clinical departments for treating some diseases [3]. Basically two sources for preparing KCl are reported [4] from the salt lake and the ocean. Previous works related to the extraction of KCl from salt lakes have been predominantly in the development of conventional processes of ion exchange, adsorption and electrodialysis [5–7]. With gradual decline of saline lake minerals, many countries throughout the world are forced to process seawater containing a high amount of KCl. In recent years, some widespread and abundant natural zeolites with selective specialties on ion exchange [8,9] are widely used to separate complicated-solution systems or to dispose sewage [10–13]. Some zeolites with excellent potassium adsorption capacity have been utilized for cost-effective adsorption of potassium from seawater [14]. Then, the processing of swapping KCl should be considered for recycle of zeolites, in which NH4Cl can be used as an eluting agent [8]. How to separate the KCl and NH4Cl from the potassium rich solution is a great challenge because of their similar chemical properties [15,16]. Recent studies have reported the possibility of using evaporation crystallization, ion ⇑ Corresponding author at: Engineering Research Center of Seawater Utilization Technology, Ministry of Education, Tianjin 300130, China. Tel.: +86 2260202241. E-mail address: [email protected] (J. Chen). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.026

exchange and solvent extraction to separate KCl and other inorganic salts. However, the drawbacks of these separation methods are either low versatility or high cost [17,18]. Therefore, there is a definite need for new more effective separation methods. Reactive extraction has been an important technology [19–25], to remove hydrophilic compounds from aqueous solutions by altering them by a reversible chemical reaction to improve the solubility in an organic solvent. Bagreev had studied trioctylmethylammonium chloride as a carrier which was first used to remove metallic ions from aqueous solutions [26], Some authors [27–29] have studied the recovery of carboxylic acids by liquid–liquid extraction with aliphatic tertiary amines dissolved in organic diluents. Grinstead had investigated the behavior and base strength of various amine types and classes in the reactive extraction of hydrochloric acid in toluene diluents [30]. Some important studies on the influence of diluents on amine extraction of carboxylic acids and acetic acid were carried out by Tamada, King and Ricker [31–33]. There are few reports on the reactive extraction for separating KCl from NH4Cl aqueous solution. This work was, therefore, undertaken to focus on separation of KCl and NH4Cl by reacting NH4Cl with extractant TBA in the presence of dilute 1-butanol. TBA is a strong Lewis base, the lon-pair electron of the nitrogen atom in the molecule can combine with H+ to become a stable ammonium ion through the formation of a coordinate bond, then the ammonium ion bonds with the chloride ion to form hydrochloride salt (TBAHCl) and complete the process of extracting HCl [34]. The remainder solution comprises mainly KCl, which could be

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concentrated and crystallized for the aimed product. Significantly, the bonds are still weak enough to be broken by NH3 producing via self-reaction, with NH3 the TBA can be recovered and the NH4Cl aqueous solution can be got back. Eq. (1) could be proposed the reaction process [35,36]. reversible

NH4 Cl þ R3 N )

* R3 N  HCl þ NH3

ð1Þ

The aim of this work is to separate KCl from NH4Cl aqueous solution by reactive extraction technology. It need to investigate some factors influencing the NH4Cl conversion rate, such as reaction temperature, pressure, reaction time, stirring speed and dilution ratio, extractant recycling also need to be studied.

regenerative bottle 500 mL, which was laid over a stirrer, was linked between the vacuum system and the outlet of the straight-type reflux condenser tube. Then the loaded organic extractant and water were fed to regenerative bottle, an additional reactive extraction happened again in the three-necked round-bottom flask and NH3 produced enter the regenerative bottle consecutively. As mentioned in the introduction, the NH3 can react with TBAHCl and the products are TBA as well as NH4Cl. NH4Cl can be obtained by concentration and crystallization from aqueous phase and TBA with 1-butanol is in the organic phase. TBA with 1-butanol can be recovered and used circularly. A flow chart for the separation processes is schematically shown in Fig. 1. 2.3. Analytical methods

2. Experimental 2.1. Materials TBA (>99%), tripropylamine (>98%), triethlamine (>97%), isooctyl alcohol (>99.7%), octanol (>99%), isoamyl alcohol (>98.5%) and 1-butanol (>99.5%) were purchased from Tianjin BoDi Chemical Reagent Factory. NH4Cl (>99.5%) and KCl (99.5%) were obtained from Tianjin Damao Chemical Reagent Factory. All other chemicals used in the experiments were of A.R. grade. The water was distilled before use.

The concentration of NHþ 4 was determined by the formaldehyde method [38]:

4NHþ4 þ 6HCHO ! ðCH2 Þ6 N4 þ 4Hþ þ 6H2 O

ð2Þ

+

The H formed was titrated by 0.1 N NaOH and phenolphthalein as an indicator. The concentration of the TBAHCl in the organic phase was determined by titration with 0.1 N aqueous sodium hydroxide and phenolphthalein as an indicator [39].

R3 N  HCl þ NaOH ! R3 N þ NaCl þ H2 O

ð3Þ

2.2. Experimental set-up and procedure The reaction of amine with NH4Cl which is in the aqueous solution was carried out in a three-necked round-bottom flask (500 mL). In the first neck of the flask a heat indicator was inserted to monitor the reaction temperature. The second neck of the flask was equipped with a straight-type reflux condenser tube, which ensured the steam of 1-butanol and water be condensed into liquid and returned into the flask to be condensed into solvent liquid and returned into the flask. The third neck was connected to a vacuum meter to monitor the reaction pressure. Thus, experiment apparatus was additionally equipped with a thermostatic bath and a vacuum system. The vacuum system was linked to the outlet of the straight-type reflux condenser tube. In a typical experimental procedure, the known amounts of KCl and NH4Cl were dissolved in water to prepare the aqueous solutions with initial concentrations of KCl 2 mol/L and NH4Cl 2 mol/ L, as the ingredients and their concentrations were nearly equal to potassium-rich mother solution of the technology for extracting KCl from seawater. 40 mL of aqueous, 0.1 mol of TBA, 150 mL of diluent were added to the three-necked round-bottom flask which was laid in the thermostatic bath at 105 °C for 4 h which was found to be the sufficient time for consuming NH4Cl during the preliminary tests. Vacuum system and agitator should be started during the whole experiment. Then the mixture was kept in a separating funnel for another 0.5 h to reach full separation of the phases. Each volume of organic phase and aqueous phase should be taken into account, in extraction of salts from aqueous solutions cannot be assumed constant volume of phases because of not small molar of NH4Cl [37]. In fact, the start total volume of the organic phase and inorganic phase is almost the same as that after the extraction process. Therefore, the amount of solvent loss in the process of experiment can be ignored. The lower phase was almost KCl solution which contains a little TBAHCl and the upper phase was the hybrid system of hydrochloride-amine complex as well as diluent. Efficient recovery and recycle of the TBA extractant is of pivotal importance for an economically sustainable process. For this purpose, it is essential that the used complexation reaction as shown by Eq. (1) is easily reversed by a moderate measure in process conditions. In the part of recovery of the extractant, a

3. Results and discussion 3.1. The screening of the extractant and the diluent With different cheap and commonly available tertiary amines in hand, screening of their activities and conversion rate was carried out with model reaction (Table 1, entries 1–3). Results, listed in Table 1, clearly indicated that their activities and conversion rates are highly dependent on their degree of the basicity and solubility. Moreover, the solubility in aqueous and organic phase of their salts also has considerable influence on NH4Cl conversion rate. High conversion of NH4Cl with the loss of the NHþ 4 in the aqueous phase value of (gA: 96.2%) was achieved when triethylamine was employed as extractant (Table 1, entry 1). However, high triethylamine hydrogen chloride salt in organic phase did not accompany with the high conversion of NH4Cl. Most of triethylamine hydrogen chloride salt was dissolved in the water because of the better sol-

TBA,n-butanol KCl NH4Cl

mix

H2O

react ammonia

layered solution

organic layered

amine

crystallize

solution KCl crystallize

NH4Cl Fig. 1. A schematic flow chart for KCl and NH4Cl separation.

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Table 1 Extractant and dilutent screening.a Entry

Amines

Alcohols

gAb (%)

gBc (%)

1 2 3 4 5 6

Triethylamine Tripropylamine Tributylamine Tributylamine Tributylamine Tributylamine

1-Butanol 1-Butanol 1-Butanol Isoamyl alcohol Octanol Isooctyl alcohol

96.2 89.4 56.6 36.0 17.5 15.9

10.3 24.0 57.3 35.3 18.0 16.5

a All reactions were carried out using NH4Cl (20 mL, 0.10 mol, 5.00 mol/L, 1.0 equiv.) and tertiary amine (0.10 mol, 1 equiv.) with dilute (40 mL) at 49.1 °C (bath temperature is 50 °C) for 3 h. b gA: NH4Cl conversion rate based on loss of the NHþ4 in the aqueous, calculated by Eq. (4). c gB: NH4Cl conversion rate based on generation of the amine hydrochloride in the organic phase, calculated by Eq. (5).

ubility than in organic phase. The use of tripropylamine resulted in a conversion of NH4Cl with (gA: 89.40%) (Table 1, entry 2), while (gB: 24%) of tripropylamine hydrogen chloride salt was extracted to 1-butanol. In contrast, tributylamine afforded the moderate conversion of NH4Cl with (gA: 56.6%) (Table 1, entry 3), and NH4Cl conversion rate based on generation of the amine hydrochloride in the organic phase (gB: 57.3%) was agreed well with the loss of the NHþ 4 in the aqueous phase. The results summarized in Table 1 demonstrate that tributylamine was highly efficient for separating NH4Cl from water. A survey of dilutents revealed that tributylamine as extractant demonstrated satisfactory allocation of tributylamine hydrochloride salt between water and alcohol. In contrast, NH4Cl conversion rates base on different factors with isoamyl alcohol in stead of 1-butanol are (gA: 36.0%) and (gB: 35.3%) (Table 1, entry 4), respectively. As illustrated in Table 1 (entries 3–6), long-chain aliphatic alcohol can lower the activity of tributylamine. With respect to both activity and conversion rate of NH4Cl, tributylamine with 1-butanol is optimal reaction extractant.

Fig. 2. Effect of dilution ratio on the conversion rate of NH4Cl. Stirring speed = 300 rpm; vacuum degree = 0.02 MPa; bath temperature = 105 °C; reaction time = 4 h.

3.2. Effect of various parameters on the reactive extraction Encouraged by the results of Table 1, the effects of reaction pressure, dilution ratio, stirring speed, reaction time and temperature on the reactive extraction have been thoroughly investigated as shown in Figs. 2–6. All reactions were carried out using NH4Cl and KCl (40 mL, each 2.00 mol/L.) and 0.10 mol tributylamine with 1-butanol at the certain temperature and pressure. The conversion rate of NH4Cl was calculated by Eqs. (4) and (5). The conversion rate of NH4Cl in the aqueous phase was calculated from the difference between the NHþ 4 concentration in the aqueous phase before and after the reactive extraction. Another calculation method is to analyze how much amine hydrochloride exists in the organic phase, which can confirm whether amine hydrochloride is fully transferred to the organic phase. Here, the corresponding conversion rates for NH4Cl are calculated using Eqs. (4) and (5):

Conversion rate ¼

CoV o  Cf V f  100% CoV o

ð4Þ

CV  100% CoV o

ð5Þ

and

Conversion rate ¼

where Co and Cf are the start and end concentrations of NHþ 4 in the solution (mol/L), respectively. Vo and Vf is the start and end volume of the aqueous phase (mL), respectively. C is the concentration of

Fig. 3. Effect of stirring speed on the conversion rate of NH4Cl. Vacuum degree = 0.02 MPa; reaction time = 2 h.

amine hydrochloride in the organic phase; V is the end volume of the organic phase. The NHþ 4 concentration in the aqueous phase and the concentration of amine hydrochloride in the organic phase need to be analyzed.

3.2.1. Effect of pressure The reactive extraction process can be greatly influenced by the pressure in the reactor and the experiment will not be conduct thoroughly if the pressure was out of consideration. Under the same conditions, the experiments without a negative pressure displayed a conversion rate of about 17% the NH4Cl. while it could rise to 40.2% after introducing a negative pressure (0.02 MPa) on the reactor at room temperature. Considering the response Eq. (1), It is evident that the reaction process will shifted toward the right with the NH3 be pumped out from the system constantly. The results indicated that negative pressure is helpful to remove the NH3 from the reaction system so that the reaction can be carried through to end smoothly.

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105 °C) for 4 h. The dilution ratio (V1  butanol: VTBA) was varied between 3/1 and 7/1 by altering the volume of 1-butanol. The results are shown in Fig. 2. The variation of the dilution ratio from 3/1 to 6/ 1 leads to an increase of the conversion rate from 77.1% to 98.7%. At ratios higher than 6:1, the dilution ratio had almost no positive effect on the conversion rate of NH4Cl.

Fig. 4. Effects of reaction time and temperature on the conversion rate of NH4Cl. Stirring speed = 300 rpm; vacuum degree = 0.02 MPa.

Fig. 5. Arrhenius plot of temperature dependent reaction rates at the various temperatures.

3.2.3. Effects of stirring speed The operating conditions used were the vacuum degree = 0.02 MPa at certain bath temperature for 2 h. The effect of speed of stirring on the conversion of NH4Cl was studied in the range 300–900 rev/min. The results are shown in Fig. 3. As it is evident from the figure, the variation of conversion of NH4Cl with speed of agitation is so small that the mass transfer factors become unimportant and the reaction is controlled by the kinetics only. Therefore, all other experiments were performed at a stirring speed of 300 rpm in order to ensure that the system is free from mass transfer resistance. 3.2.4. Effects of reaction time and temperature Five different bath temperatures 65 °C, 75 °C, 85 °C, 95 °C, 105 °C were investigated. The actual reaction temperatures in the reactor are 61.9 °C, 71.2 °C, 80.5 °C, 85 °C and 91 °C, respectively. The other working conditions were: stirring speed 300 rpm, and the vacuum degree = 0.02 MPa. The results are shown in Fig. 4. As the NH4Cl conversion rate calculated with Eq. (3) in the organic phase is almost the same as that calculated with Eq. (2) in the aqueous phase, the increase of TBAHCl concentration in organic phase also corresponds to the increase of NH4Cl conversion rate. Therefore, the concentration of TBAHCl is selected to indicate the effect of reaction temperature and reaction time on NH4Cl conversion rate. It is shown from Fig. 4 that the increase of reaction time improves the concentration of TBAHCl till it reaches a plateau region. Obviously, the highest concentration is 0.40 mol/L, the corresponding conversion rate is 98.7%. Results showed that reaction times to reach the plateau region at five different temperatures were 180 min, 360 min, 510 min, 700 min and 940 min, respectively. It is clear that reaction time decreases with the increase of temperature. It can also be indicated that reaction rate is proportional to temperature. Rate constant of reaction is calculated based on different temperatures and an Arrhenius plot of Ln (rate constant) versus 1/T is shown in Fig. 5. The apparent activation energy for the reaction of NH4Cl is calculated from the slope of the straight line as 38.16 kJ/mol. The effects of reaction time and temperature on the reactive extraction process of NH4Cl with TBA in 1-butanol suggested that it is an endothermic reaction. In order to explain the phenomenon, a better interpretation for the mechanism is to assume that five discrete equations in the reactive extraction process play an important role in determining the process is an endothermic process. They are expressed as in Eqs. (6)–(10). ionization

* NHþ4 Cl

NH4 ClðaqÞ ) NHþ4 NHþ4 

hydrolysis

þ H2 O )

*

NHþ4

ð6Þ 

 OH þ H

þ

DHðhydrolysisÞ > 0

 OH NH3  H2 O NH3 þ H2 O

Cl þ Hþ HCl

ð9Þ

coordination

HCl þ R3 N )

* R3 N  HCl DHðcoordinationÞ < 0

NH4 Cl þ R3 N R3 N  HCl þ NH3 DH ¼ DHðhydrolysisÞ þ DHðcoordinationÞ > 0 Fig. 6. Effect of volume of water on the recovery efficiency of the TBA extractant. All other conditions are same.

3.2.2. Effect of dilution ratio The operating conditions used were stirring speed 300 rpm and the vacuum degree = 0.02 MPa at 91 °C (bath temperature is

ð7Þ ð8Þ

ð10Þ ð11Þ

NH4Cl in water solution will be ionization and the products are þ  NHþ 4 and Cl , then the hydrolysis reaction between NH4 and H2O are almost put on stage simultaneously with the product of NH4OH as well as H+. The hydrolysis reaction of salt (NH4Cl) in water is an endothermic process [40]. The strong coordination interaction between the proton of the hydrogen chloride and the basic

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Acknowledgments Many thanks for the financial supports from National Natural Science Fund of China (21276063), PCSIRT (IRT1059), Hundred Excellent Innovation Talents from the Universities of Hebei Province (CPRC013), Tianjin Natural Science Fund (10JCYBJC04300), Natural Science Fund of Hebei Province (B2010000042), and the SRF for ROCS and SEM, China.

References

Fig. 7. Relationship between recovery efficiency of the TBA extractant and volume of water.

nitrogen of TBA will appear with the heat release so as to allow for the hydrogen bond formation along with the product TBAHCl [41]. The conversion rate of NH4Cl in the experiment is about 9% at room temperature without negative pressure. This is confirmed as the reactive extraction itself may have a direct coupling of energy for the endothermic reactions and the energy release by the exothermic reactions and may be spontaneous but weak. A series of experimental studies have confirmed that high temperature favor the reactive extraction process. So, it was necessary to improve the reaction temperature during the experiment process in order to speed up the reaction rate. Meanwhile, reducing ammonia solubility in water under the conditions of heating it also facilitates the reaction toward the right side. 3.3. Recovery of the extractant As presented in Fig. 6, exploratory experiments in batch mode were performed to determine how much volume of water added into the regenerative bottle is propitious to the recovery of the TBA, it is clear that at the same other conditions, the concentration of TBAHCl in the organic phase decreases with increasing water volume, which means that the amount of TBA recovered rises. This may be assumed because large amounts of water prolong the residence time of NH3 in water so as to ensure the reaction between the NH3 and TBAHCl in the organic phase. It is seen from Fig. 7 that the recovery efficiency of TBA increases from about 56.1% to 91.0% for change in the volume of water from 0 to 80 mL after 400 min of reaction under identical experimental conditions. Moreover, it also indicates that the reaction extraction technology is reasonable, which further supports the reaction between the NH4Cl and TBA is reversible. These studies have demonstrated the chemical and mechanical feasibility of the process.

4. Conclusions A reactive extraction using extractant and diluent to separate KCl and NH4Cl from aqueous solution has been investigated. TBA and 1-butanol are found to be the preferred extractant and diluent, respectively. Under optimum conditions, KCl and NH4Cl could be effectively separated from each other by the reactive extraction. Moreover, the separation conditions were very simply and mild, so it can be suitable for a scale production. In addition based on the application mentioned above, this reactive extraction may have some potential applications in the separation of other mixed salt.

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