J. Cent. South Univ. (2013) 20: 1797−1804 DOI: 10.1007/s11771-013-1675-8

Extraction kinetics of zinc by new extractant in ammoniacal system HUANG Ling(黄玲)1, 2, HE Jing(何静)1, CHEN Yong-ming(陈永明)1, YANG Sheng-hai(杨声海)1, JIN Sheng-ming(金胜明)1, TANG Mo-tang(唐谟堂)1 1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China; 2. Guangzhou Research Institute of Non-ferrous Metals, Guangzhou 510000, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2013 Abstract: The controlling step and the extraction reaction rate equation of zinc extraction from Zn(II)-NH3 solution by using a newly synthesized organic compound, 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester as the zinc extractant, were clarified. The effects of agitation speed, specific interfacial area, temperature, extractant concentration and Zn ion concentration on the extraction rate are studied in constant interfacial area cell. The results show that the extraction rate depends on interfacial chemical reaction and diffusion by using this new extractant to extract zinc, and the apparent activation energy of this extraction reaction is measured as 28.2 kJ/mol, which demonstrates that the extraction reaction is controlled by the mixed-controlled reaction rate. The apparent reaction orders a and b are measured as 1 and 0.38, and the constant k0 is 138.70. So, when extraction conditions are controlled as [HR]=20%−50%, T=0−30 °C, N=120−177 r/min and S=72.6−127.5 m−1, the solvent extraction reaction rate can be depicted as 28 206 v /( mol  m  2  s 1 )  138 .7  exp(  )  [ Zn 2  ] T  [ HR ] 0o.38 . 8 .314 T

Key words: solvent extraction; zinc; 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester; kinetics

1 Introduction Low-grade zinc oxide ore plays a more and more important role in China with the rapid development of zinc metallurgy. At present, “ammoniacal leaching− purification−electrodeposition” process is one of the most promising technologies [1−4]. Extraction of zinc from ammoniacal leaching solution is hard to carry out because of the low concentration of zinc ion in solution. Few reports were found in relative literatures about extraction of metal ions from ammoniacal solution. These extractants include Lix54, LIx-84I, Lix622N, Lix984N, DBHQ, cyanex272, TOPO and Hostarex DK-16, but there are still some difficulties to use them in industry, because of low extraction rate, or hard stripping, or high viscosity, etc [5−16]. Among these extractants, only a series of Lix extractants and Hostarex DK-16 are used to extract zinc from ammoniacal solution, and both of them have β-diketone structure which can interact with zinc during extraction process. These extractants can effectively extract zinc from ammoniacal solution under strict extraction conditions. For example, if extraction rate reaches 90%, the pH of solution should be controlled at 8−9, which makes them hard to meet

industrial requirement of zinc extraction [15−19]. According to the function characteristics of β-diketone structure and zinc [20−21], in this work, a new efficient extractant, 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester, was reported which could be used in efficient extraction and enrichment of zinc from Zn(II)-NH3 solution. Zinc extraction by 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester from Zn(II)-NH3 solution has the following key features: zinc single-stage extraction rate is over 97% and equilibrium distribution ratio is over 112.6 when the temperature ranges from 5 to 35 °C and the pH of ammonia solution ranges from 8 to11; no third phase is generated during extraction process because of small viscosity of 2-acetyl-3-oxo-2-thiobutyric acid14-alkyl ester. Loaded organic phase can be easy to strip by sulphuric acid, and the stripping rate is over 98%. Also, extractant can be regenerated during stripping process [22−23]. Based on these results, the kinetics of zinc extraction by 2-acetyl-3-oxo-dithiobutyric acidmyristyl ester is further studied, and its chemical kinetics is described.

2 Experimental 2.1 Materials The new extractant 2-acetyl-3-oxo-dithiobutyric

Foundation item: Project(51174240) supported by the National Natural Science Foundation of China; Project(2006BA02B04-4-2) supported by the National Eleventh Five-Year Research Program of China; Project(20100908) supported by Scientific and Industrial Research Organisation of Guangdong Province, China Received date: 2012−04−11; Accepted date: 2012−05−28 Corresponding author: HE Jing, Associate Professor; Tel: +86−731−88830470; E-mail: [email protected]

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acid-myristyl ester was synthesized with acetylacetone, carbon disulfide and bromotetradecane. Its molecular structure is shown in Fig. 1, and its properties are listed in Table 1. By constant temperature and crossing current experiment, the saturated loading capacity of zinc obtained was 76.4 mmol/dm3. Zn(II)-NH3 coordination compound aqueous solution was composed of analytically pure zinc oxide, ammonia, ammonium sulfate, and zinc concentration is varied from 0.07 to 0.44 mol/L.

3 Principle of experiment 3.1 Principle of extraction The β-diketone structure of the extractant 2-acetyl3-oxo-dithiobutyric acid-myristyl ester easily occur tautomeric change to enolform. The ion H+ of hydroxy can exchange with metal ion Men+, and then metal ion Men+ and extractant form metal chelate compound. The extraction of zinc by 2-acetyl-3-oxo-dithiobutyric acidmyristyl ester from Zn(II)-NH3 coordination system can be represented by [ Zn 2  ]aq  2[ HR ]o  [ ZnR 2 ]o  2[ H  ]aq

(1)

where HR denotes the extractant and subscripts “aq” and “o” denote the aqueous and organic phases, respectively. When the reaction reaches equilibrium, the equilibrium constant Kex can be represented as K ex 

Fig. 1 Molecular structure of new extractant 2-acetyl-3-oxodithiobutyric acid-myristyl ester Table 1 Properties of new extractant 2-acetyl-3-oxodithiobutyric acid-myristyl ester Relative Phase state Density/ Stability Note molecular (Room −3 (kg·m ) mass temperate) Easily Water content decomposited 1 200− Orange controlled by 345−429 under light 1 300 liquid dehydration or high process temperature

2 [ ZnR 2 ]o  [H  ]aq

(2)

[ Zn 2 ]aq  [HR ]o2

Zn ion can react with NH3 or hydroxy by coordination effect and react with water by hydration. According to the law of conservation of mass, zinc concentration in aqueous solution can be expressed as 4

4

i 1

j 1

[ Zn ]T  [ Zn 2 ]   [Zn(NH3 ) i2 ]   [Zn(OH)2j  j ]  [HZnO 21 ]  [ ZnO 22 ]  [Zn 2 ]  K1

(3)

Dex can be expressed as Dex 

[ZnR 2 ]o [Zn]T

(4)

According to Eq. (3) and (4), Dex can be expressed 2.2 Experimental method The kinetics of zinc extraction with 2-acetyl-3oxo-dithiobutyric acid-myristyl ester was studied in a constant interfacial area cell [24]. In this experiment, 20 mL of organic phase (10%−70% 2-acetyl-3-oxodithiobutyric acid-myristyl ester) and 20 mL of aqueous phase was placed in an empty beaker, and its specific interfacial area was noted as S; organic phase of the upper strata was stirred by electronic constant speed mixer, and the aqueous phase was stirred at the same stirring speed with the reversed direction by magnetic heated stirrer. The 1 mL of organic phase and 1 mL of aqueous phase were pipetted by automatic pipetting device respectively when the stirring time was 0, 1, 2, 3, 4 and 5 min. Zinc concentration in aqueous phase was analyzed by EDTA titration or WFX-210 type atomic absorption spectrophotometer.

as Dex 

K ex  [RH] o2 [H  ] 2  K1

(5)

where [HR]o and [ZnR2]o denote the concentrations of 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester in organic phase and extracted species; [Zn2+]aq and [H+] denote the concentrations of free ions Zn2+ and H+ in aqueous phase; [Zn]T denotes zinc total concentration in aqueous phase; K1 is a coefficient associated with the composition of aqueous phase, and it is a constant when the composition of aqueous phase does not change. 3.2 Processing kinetic data According to the method of invariable interface, zinc initial extraction rate is measured through constant interface experimental facility. Rate equation of zinc

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extraction by 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester is set up by differential method and can be represented by dn dn a v [HR ]bo  log v  log( )   K [ Zn 2 ]aq dt dt

log K  a log[Zn 2 ]aq  b log[HR ]o

(6)

where a and b denote apparent reaction orders; K denotes extraction reaction constant. The logK is a constant, when it was supposed that the composition of aqueous phase and temperature were changeless. Apparent reaction orders a and b were solved by experiments. Apparent reaction order a could be solved by following method: when [HR]o was changeless as well as [HR]o concentration was greater than [Zn]T, blog [HR]o could be seen as a constant; [Zn]T concentrations at various times were measured, and the slope of a plot of [Zn]T against t was the zinc initial extraction rate at different time, or v

d[Zn]T dn  dt dt

Fig. 2 Zn concentration in aqueous solution at different time ([HR]=40%; A/O=1:1; N=150 r/min; T=298.15 K; S=72.6 m−1)

(7)

Therefore, logνi and log[Zn]T,i could be obtained, and one drawing was made with logνi as ordinate and outer log[Zn]T,i as abscissas; finally, the slope of logνi− log[Zn]T, i was apparent reaction order a. Similarly, it can be applied to solve b. According to irreversible reaction, the equation lnv= /RT+C −Ea 1 can be deduced by Arrhenius equation [25]. A relationship between extraction reaction constant K and activation energy Ea could be expressed as K= k0exp(−Ea/RT), where k0 was a constant. Finally, a, b, k0 and Ea could be solved by experiments, and the rate equation of zinc extraction by 2-acetyl-3-oxo-dithiobutyric acid-14-myristyl ester was set up as E dn a  k0  [ Zn 2  ]aq  [HR ]bo  exp( a ) v (8) RT dt

4 Results and discussion 4.1 Measuring method of zinc initial extraction rate Figure 2 shows zinc(II) ion concentration in aqueous solution in different time at an agitation speed of 150 r/min. The result shows that zinc(II) ion concentration in aqueous solution and time have positive linear relationship in 0−4 min. Figure 3 shows the [Zn]o concentration in organic phase in 0−4 min. Because phase ratio of aqueous and organic phase is 11, [Zn]T concentration decreasing in aqueous solution or {[Zn]T-t=0−[Zn]T-t=i} is [Zn]o concentration increasing in organic phase. Besides, the rate of [Zn]T concentration decreasing in aqueous solution is the rate of [Zn]T concentration increasing in organic phase.

Fig. 3 Zn concentration in organic phase at different time

Zinc(II) ion concentration in organic phase and time have positive linear relationship in 0−4 min, as shown in Fig. 3, so the slope of a plot of [Zn]o against t is the zinc initial extraction rate by 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester. The linear regression analysis on zinc(II) ion concentration in organic phase shows that linear correlation coefficient is 0.998 2, and the slope of straight line is 7.30610−4. As a result, the zinc initial extraction rate in organic phase is 1.67×10−4 mol·m−2·s−1. 4.2 Effect of agitation speed Agitation speed which may influence the initial extraction rate of zinc is investigated. The effect of agitation speed is shown in Fig. 4. From extraction result shown in Fig. 4, there is a positive linear relationship of change in zinc(II) ion concentration in aqueous solution with time. The higher the absolute value of the slope of straight line is, the faster the zinc(II) ion concentration in aqueous solution decreases, and the greater the zinc initial extraction rate is. Initial zinc extraction rate increases with the increase of agitation speed at beginning, but the rate decreases when the agitation speed is over 150 r/min. Linear regression analysis is used to evaluate the

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specific interfacial areas with N=150 r/min. Zinc(II) ion concentration and time have positive linear relationship. Zinc initial extraction rate by 2-acetyl-3-oxodithiobutyric acid-myristyl ester becomes larger and larger with specific interfacial area increasing.

Fig. 4 Effect of stirring speed on Zn concentration in aqueous solution at different time

initial zinc extraction rate, and the result is shown in Fig. 5. The initial extraction rate of zinc by 2-acetyl-3oxo-dithiobutyric acid-myristyl ester extraction agent increases with the increase of agitation speed; initial extraction rate of zinc holds a line having nothing to do with the agitation speed when N is increased from 120 to 177 r/min; as the agitation speed continues increasing, the initial extraction rate of zinc decreases and phase contact area changes, as well as interfacial state becomes instable, which is against the specialty of extraction kinetics by the method of invariable interface [26]. According to the specialty of invariable interface [27−28], when the agitation speed is in the range from 120 r/min to 177 r/min, the effect of surface mass transfer resistance of zinc extraction by 2-acetyl-3-oxodithiobutyric acid-myristyl ester is reduced. So, N=120− 177 r/min is the best operating condition, and follow-up tests are carried out at the agitation speed of 150 r/min.

Fig. 6 Effect of specific interfacial area on Zn concentration in aqueous solution at different time

Figure 7 shows the relationship between initial zinc extraction rate and specific interfacial area. According to the diffusion-controlled reaction, rate of extraction reaction has relationship with stirring intensity and specific interfacial area. Initial zinc extraction rate increases with the specific interfacial area increasing. The initial extraction rate of zinc is not zero when the specific interfacial area tends to zero. The inverse of specific interfacial area is associated with the initial extraction rate of zinc, but they have a linear relationship. This suggests that extraction reaction process is controlled both by interface and aqueous phase [24−25]. It is suggested that extraction reaction is controlled by mixed-controlled reaction [25, 28]. The linear regression analysis on initial extraction rate of zinc shows that linear correlation coefficient is 0.981 7, and the result indicates that positive initial extraction rate of zinc is in proportion to specific interfacial area, or v∝Si.

Fig. 5 Effect of stirring speed on extraction rate

4.3 Effect of specific interfacial area Figure 6 shows the relationship between zinc(II) ion concentration in aqueous solution and time at different

Fig. 7 Effect of specific interfacial area on extraction rate

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4.4 Effect of temperature Figure 8 shows the relationship between zinc(II) ion concentration in aqueous solution and time at different temperatures (7−35 °C). The results give a positive linear relationship of change zinc(II) ion concentration with time. The absolute value of the slope increases with the extraction temperature increasing at the beginning, but the absolute value of the slope decreases when the extraction temperature is over 30 °C. It is suggested that initial zinc extraction rate by 2-acetyl-3-oxodithiobutyric acid-myristyl ester increases at first, and then decreases with the extraction temperature increasing.

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solution. In order to show the relationship between zinc(II) ion concentration with time, every group of experimental data is vertically shifted by a distance of {0.044 12−[Zn]T, t=0}. Extraction results are shown in Fig. 10. Zinc(II) ion concentration and time have positive linear relationship, and the absolute value of the slope increases with the initial zinc(II) ion concentration in aqueous solution increasing.

Fig. 10 Effect of [Zn]T on Zn concentration in aqueous solution at different time

Fig. 8 Effect of temperature on Zn concentration in aqueous solution at different time

Figure 9 shows the relationship between initial zinc extraction rate and temperature. The result shows that initial zinc extraction rate increases at first, while it decreases with the extraction temperature higher than 30 °C.

Figure 11 shows the initial zinc extraction rate with initial zinc(II) ion in aqueous solution increasing, and data are processed by the method of linear regression analysis. The linear regression analysis on initial extraction rates of zinc shows that linear correlation coefficient is 0.999 8. It is indicated that positive initial extraction rate of zinc is in proportion to zinc concentration in aqueous solution, that is v∝ [Zn]T,i.

Fig. 11 Effect of Zn concentration in aqueous solution on extraction rate Fig. 9 Effect of temperature on extraction rate

4.5 Effect of Zn concentration in aqueous solution Figure 10 shows the relationship between zinc(II) ion concentration in aqueous solution and time at different initial zinc(II) ion concentrations in aqueous

4.6 Effect of extractant concentration Figure 12 shows the relationship between zinc(II) ion concentration in aqueous solution and time at different 2-acetyl-3-oxo-dithiobutyric acid-myristyl ester concentrations. Extraction results show that zinc(II) ion

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concentration and time have positive linear relationship. The absolute value of the slope increases as the concentration of extractant increases in the range of 0−50%, while it decreases with concentration of extractant higher than 50%.

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abscissas. Both of them are processed by linear regression analysis, and the results are shown in Fig. 15 and Fig. 16, respectively. According to Eq. (6), the lnvi is in proportion to ln[Zn]T when the extraction conditions are changeless except the initial zinc(II) ion concentration. So, the slope of logνi−log[Zn]T,i is apparent reaction order a. Similarly, the lnvi is in proportion to ln[HR] when the extraction conditions are changeless except the extractant concentration, and the slope of log νi−log[HR]o,i is apparent reaction order b. The slope of the straight line shown in Fig. 15 is

Fig. 12 Effect of [HR] on Zn concentration in aqueous solution at different time (A/O=1:1; T=298.15 K; N=150 r/min; [Zn]T= 0.31 mol/L ; S=72.6 m−1)

Figure 13 shows the relationship between initial zinc extraction rate and 2-acetyl-3-oxo-dithio-butyric acid-myristyl ester concentration. Experimental data are processed by the method of linear regression analysis, and the results show that initial zinc extraction rate firstly increases, while it decreases with the extractant concentration increasing. The linear regression analysis in Fig. 14 shows that linear correlation coefficient is 0.734 3. So, positive initial extraction rate of zinc is not in proportion to concentration of extractant in aqueous solution when concentration of extractant is in the range of 20%−50%.

Fig. 14 Effect of extractant concentration on extraction rate

Fig. 15 Relationship between ln v and ln[Zn]T

Fig. 13 Effect of [HR] on extraction rate

4.7 Apparent reaction orders a and b According to the results of zinc concentration test and extractant concentration test, plotting is made with lnνi as ordinate and outer ln[Zn]T,i or ln[HR]o,i as

Fig. 16 Relationship between ln v and ln[HR]

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1.006, and the linear correlation coefficient is 0.988 5. So, apparent reaction order a is about 1, or a=1, which is consistent with the finding in Section 4.5. And the slope of the straight line shown in Fig. 16 is 0.376, and the linear correlation coefficient is 0.992 2, or b=0.38, which is consistent with the finding in Section 4.6. 4.8 Apparent activation energy Apparent activation energy Ea is evaluated by relationship between the temperature and zinc extraction speed coefficient K, which can be used to judge the kinetic models of extraction. According to Eq. (6), zinc extraction speed coefficient K can be obtained at different temperatures when temperature T, apparent reaction orders a and b, and v are known. Therefore, lnKi and 1/Ti could be obtained. Plotting is made with lnKi as ordinate and outer 1/Ti as abscissas, and processed by linear regression analysis. The results are shown in Fig. 17.

Fig. 18 k0 solved by graphic method

As shown in Fig. 18, the slope of the line is 138.68, or k0=138.7, and the linear correlation coefficient is 0.997 9. 4.9.2 Rate equation Ea=28 206 J/mol, a=1, b=0.38 and k0=138.7 are substituted into Eq. (9) when the extraction conditions are [HR]=20%−50%, T=0−30 °C, N=120−177 r/min and S=72.6−127.5 m−1. Equation (11) is the rate of zinc extraction by 2-acetyl-3-oxo-dithiobutyric acid-myristyl: v /( mol  m 2  s  1 ) 

138.7  exp(

28 206 )  [ Zn 2 ]T  [HR ]0o.38 8.314  T

(11)

5 Conclusions

Fig. 17 Relationship between lnK and 1/T

The slope of the straight line shown in Fig. 17 is −3 392.55, and the linear regression analysis on lnK shows that linear correlation coefficient is 0.998 9. So, −Ea/8.314=−3 392.55, and extraction apparent activation energy Ea is 28.2 kJ/mol. This suggests that the extraction reaction is controlled both by interfacial chemical reaction and diffusion [26]. 4.9 Rate equation of zinc extraction 4.9.1 Determination of k0 According to Eq. (7), Ea=28 206 J/mol, a=1 and b= 0.38 are substituted into Eq. (7) as

v  k 0  exp( 

 i  exp( 

28 206 )  [ Zn 2  ]T  [ HR ]0o.38 8 .314 T

28 206 )  [ Zn 2  ]T  [ HR ]0o.38 8 .314 T

(9) (10)

The χi curve for initial extraction rate of zinc is shown in Fig. 18, and the slope of the line is k0.

1) Extraction reaction of zinc by 2-acetyl-3-oxodithiobutyric acid-myristyl is controlled both by interfacial chemical reaction and diffusion by the study on the effect of agitation speed and extractant specific interfacial area on the extraction rate. 2) According to zinc concentration test and extractant concentration test, apparent reaction orders a and b are measured as 1 and 0.38, respectively, and the constant k0 is 138.70 by graphic method. 3) Extraction apparent activation energy Ea is determined as 28.2 kJ/mol, which demonstrates that the extraction reaction is controlled by mixed-controlled reaction rate. 4) When extraction conditions are controlled as [HR]=20%−50%, T =0−30 °C, N =120−177 r/min and S = 72.6−127.5 m−1, rate equation of zinc extraction by 2-acetyl-3-oxo-dithiobutyric acid-myristyl in the range of this experiment is v /(mol  m 2  s 1 ) 

138.7  exp(

28 206 )  [ Zn 2 ]T  [HR ]0o.38 8.314T

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