Article pubs.acs.org/IECR
Separation of Tungsten from Mo-Rich Leach Liquor by Adsorption onto a Typical Fe−Mn Cake: Kinetics, Equilibrium, Mechanism, and Thermodynamics Studies Rajiv Ranjan Srivastava,†,‡ Min-seuk Kim,‡ and Jae-chun Lee*,†,‡ †
Resources Recycling, University of Science and Technology, Daejeon 305-350, Republic of Korea Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejaon 305-350, Republic of Korea
‡
ABSTRACT: A typical mixed hydrated oxide cake of iron and manganese (FMC) was used to adsorb and separate tungsten from Mo-rich leach liquor obtained from the ammonia leaching of oxidatively roasted, spent hydro-desulfurization catalysts. The FMC considered in this study is itself a byproduct of the ammoniacal leaching of reduced manganese nodules. To yield a maximum tungsten-to-molybdenum separation factor of 199.6, the following optimum conditions were determined: FMC/W = 2.1, temperature = 50 °C, and time = 120 min. The adsorption kinetics was investigated and correlated with the common isotherm equations of Langmuir and Freundlich; the data fitted well with the Freundlich model and exhibited first-order kinetic behavior. The plot of qt vs t0.5 suggests boundary layer diffusion as the rate-limiting step of the adsorption process. The calculated thermodynamic parameters, ΔG°, ΔH°, and ΔS°, indicate that the adsorption of tungsten onto FMC is a spontaneous, exothermic, and physical sorption process.
1. INTRODUCTION Tungsten and molybdenum often occur together in natural minerals. Similarities in the ionic radius of tungsten and molybdenum creates major difficulties in their selective separation from each other.1,2 Because the elements often occur together in natural minerals, their separation is not only required from the point of view of meeting the demand but also from an economic standpoint due to their limited occurrence. One of the major uses of molybdenum and tungsten is as hydro-desulfurization (HDS) catalysts, which remove sulfur from crude petroleum. Even after the exhaustion of the catalytic activity and completion of their life cycle, the spent catalysts may be viewed as a very important secondary resource.3 Spent catalysts usually consist of Mo sulfides with Co or Ni on an alumina carrier. Over the years, numerous studies have been conducted on the recycling of spent catalysts based on hydrometallurgical processes which included alkali/acid leaching and metal separation.4−10 For the separation and purification of leach liquors, various methods including sulfide precipitation, ammonium salt precipitation, carbon adsorption, ion exchange, and solvent extraction have been investigated.4,8−13 However, limited literature reports are available regarding the treatment of ammoniacal leach liquors of molybdenum containing tungsten as an impurity. In most reports, drawbacks and/or limitations in the process economy and separation efficiency have been noticed.14 Hence, it is desired to develop an efficient method of molybdenum− tungsten separation rather than to follow the conventional methods without achieving the intended selectivity and product purity. An analysis of the mineralogical occurrence of tungsten and molybdenum has revealed that tungsten can exist in nature with iron and manganese, such as in (Fe,Mn)WO4, FeWO4, and MnWO4. Molybdenum has not been found in wolframite-like © 2013 American Chemical Society
ores, indicating the possibility of separating tungsten and molybdenum under favorable mineralogical conditions. However, when the separation of molybdenum and tungsten in caustic media was attempted using iron and manganese compounds separately, the methods yielded insufficient separation or contamination of the streams due to in situ compound formation during an intermediate stage of the separation process.2,15,16 Using hydrous ferric oxyhydroxide, Srivastava et al.14 proposed a flow sheet exhibiting 91% tungsten removal from ammonium molybdate leach liquor, but with a loss of 10% molybdenum. In the present investigation, a unique adsorption approach was applied by using both iron and manganese compounds together to separate tungsten from Morich leach liquor while minimizing the coadsorption loss of molybdenum. It should be noted that a typical iron−manganese cake (FMC), used as an adsorbent in this study, is itself a byproduct of the reductive roasting, ammoniacal leaching process of manganese nodules.17 The conversion of this byproduct to a potentially useful substance is attractive not only because of its chemical composition with 49% Fe and 12.5% Mn but also because of the desired adsorption properties of hydrated iron and manganese oxides. Hydrated metal oxides are considered to be better adsorbents than hydroxides due to their larger surface areas.18 In the present work, a kinetic and equilibrium study on the separation of tungsten from an ammonium molybdate solution by adsorption onto the surface of a typical sample of FMC was investigated by varying the adsorbent dosages, reaction temperature, and contact time. The adsorption isotherms and Received: Revised: Accepted: Published: 17591
July 28, 2013 October 25, 2013 November 1, 2013 November 1, 2013 dx.doi.org/10.1021/ie402434a | Ind. Eng. Chem. Res. 2013, 52, 17591−17597
Industrial & Engineering Chemistry Research
Article
Figure 1. Pourbaix diagram for a W−Fe−Mn−H2O system in ammonia medium at temperature = 25 °C, pressure = 1 atm.
speciation of molybdenum and tungsten. Optimization experiments were conducted at a 200 mL scale in a 250 mL flatbottom flask fitted with a condenser. To attain the reaction temperature, heating was provided by a hot plate with a magnetic stirrer and controlled by an electronic temperature sensor. After reaching the desired reaction temperature, a precalculated amount of the adsorbent, FMC, was added in to the solution while stirring the mixture at a constant speed of ∼300 rpm. To determine the adsorption kinetics, time scale sampling was performed by pipetting 5 mL of solution periodically for analysis. After the filtration and proper dilution of samples, molybdenum and tungsten content was analyzed by using ICP-AES (iCAP6000 series, Thermo Scientific). To analyze the iron and manganese in the precipitates, conventional titration methods were applied using K2Cr2O7 and KMnO4, respectively, after dissolving the precipitates in HCl. All experiments were performed in triplicate, and the averaged results are presented. The IR-spectra for the fresh and treated FMC were recorded using a NICOLET-380 spectrometer. 2.3. Determination of Separation Factor. The molar distribution coefficient of molybdenum and tungsten was calculated as follows:2
solute transfer mechanism to predict the boundary layer or intraparticle diffusion process were also evaluated. Furthermore, thermodynamic parameters such as Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were calculated using the obtained experimental data and are discussed in detail. Using this approach, we attempted to establish an efficient process for separating the tungsten contaminant from a Mo-rich solution. The process is applicable not only to the leach liquor obtained by the ammonia leaching of oxidatively roasted spent HDS catalysts but also to molybdenite processing as well.
2. EXPERIMENTAL SECTION 2.1. Materials. Synthetic mixture solutions of 18.2 g/L Mo and 956 mg/L W were prepared by dissolving the ammoniacal salts of both metals in ammonia solutions (distilled water + NH4OH) of pH 9.5. The metal hydrous oxides of iron and manganese were freshly prepared in the laboratory by precipitating the sulfate salts of iron and manganese in ammoniacal media, separately. During precipitation, air was purged to the system to maintain the conditions required for the formation of metal hydrous oxides as per the procedure described elsewhere to precipitate Fe and Mn from cobalt-rich ammoniacal leach liquor.17 After drying each precipitate at 45− 50 °C for 4 h and analyzing its metal content, the precipitates were mixed together to yield the desired composition of FMC, 49 wt % Fe and 12.5 wt % Mn, in the final adsorbent. All chemicals used were of LR grade and used as such without further purification. 2.2. Methods. All synthetic feeds were prepared in the laboratory at room temperature (∼25 °C); agitation was provided by magnetic paddles and stirrers. To predict the favorable adsorption conditions of tungsten onto ferric iron and manganese in an ammonia medium, an Eh-pH diagram was plotted using HSC chemistry software (version 6.0). By analyzing the diagram (Figure 1), an increased possibility of ferric and manganese tungstate formation was observed at pH < 7.5; hence, all of the experiments were performed at a pH level of 7.0−7.5. To this end, the stock solution pH was adjusted by the addition of dil. H2SO4 (25%, v/v) under stirring conditions prior to starting each experiment to avoid any change in the
λ=
C0V0 − CSVS CSVS
(1)
where λ is the molar distribution coefficient; C0 and CS are the initial and final concentrations of metal in solution [M], respectively; and V0 and VS are the initial and final volumes of the solution [mL], respectively. The separation factor (SF) was also calculated as follows:
SF =
λW λMo
(2)
where λW and λMo are the molar distribution coefficients of tungsten and molybdenum, respectively.
3. RESULTS AND DISCUSSION 3.1. Effect of Adsorbent Dosage. The effect of adsorbent (FMC) dosage on the adsorption of tungsten was studied in 17592
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of tungsten (92.5%) and the molar separation factor of tungsten to molybdenum were the highest at this temperature. Interestingly, at a temperature of 80 °C, a slight decrease in the adsorption efficiency of tungsten (3%) as well as in the SF was observed. This decrease may be attributed to the more favorable conditions for molybdenum as well than tungsten at higher temperature.2 Thus, an increase in molybdenum adsorption at higher temperature caused a remarkable decrease in SF. 3.3. Effect of Contact Time. The process kinetics were studied in a series of experiments by varying the contact time from 30 to 180 min and keeping FMC/W molar ratio = 2.1 and temperature = 50 °C constant. Adsorption isotherms are usually determined under equilibrium conditions. The amount of tungsten adsorbed at the equilibrium time reflects the maximum adsorptive capacity of the adsorbent under the corresponding operating conditions. The results shown in Figure 4 indicate that tungsten adsorption (93.3%) reached
the range of 1.1−4.3 for FMC/W molar ratio dosages equivalent to 0.5−2.0 g/L of the pulp density. Figure 2
Figure 2. Effect of dosage on percent of adsorption of tungsten and molybdenum as a function of FMC/W molar ratio and the corresponding separation factor (at 50 °C, 180 min contact time).
shows that the percentage removal of tungsten increased with the adsorbent dosage up to FMC/W = 2.1. This increase may be due to the increased availability of surface active sites resulting from the increased dosage and conglomeration of the adsorbent. The removal efficiency of tungsten was observed to be >93% at a FMC/W molar ratio of 2.1 but was only 28% at FMC/W = 1.1, whereas the adsorption loss of molybdenum was 10.3% and 4.8% at these ratios, respectively. At a molar ratio of FMC/W = 2.1, the ratio of adsorbed tungsten to metal (Fe + Mn) of adsorbent was calculated to be 1.46 (0.889:0.61). This value is quite similar to the ratio of tungsten to the metals such as Fe + Mn in the wolframite of 1.66 (1:0.6026). This clearly reflects that adsorption of tungsten on FMC followed the same molar ratio (tungsten to iron and manganese or vice versa) as that of the naturally occurring mineral such as wolframite. Moreover, the amount of metal adsorbed supports the mineralogical findings of several individual studies regarding the interaction between adsorbed species such as Me−O−O− Me on Fe/Mn oxides (where Me denotes metal adsorbed).19−21 Regarding the separation factor for tungsten to molybdenum, the molar ratio of FMC/W = 2.1 was observed suitable as the λW/λMo value was maximum at this point. 3.2. Effect of Temperature. The role of temperature on the adsorption of tungsten was investigated by varying the reaction temperature in the range 25−80 °C while maintaining other parameters constant such as FMC/W = 2.1 and contact time = 180 min. The results shown in Figure 3 indicate that the reaction temperature 50 °C is suitable as the removal efficiency
Figure 4. Adsorption kinetics of tungsten and molybdenum as a function of time (in minutes) and corresponding separation factors (at 50 °C and FMC/W = 2.1).
equilibrium in 150 min, but as the contact time increased, the loss of molybdenum also increased (up to 10.1% in 180 min). This behavior may be explained by the concentration gradient effect. At the contact time when the tungsten concentration was ≤50 ppm, the higher molybdenum concentration dominated adsorption, thereby increasing the loss of molybdenum. To maximize tungsten removal and minimize molybdenum loss, the value of λW/λMo was calculated at each time interval. The SF presented in Figure 4 was highest at 120 min in the time range studied. Hence, 120 min of contact was determined to be suitable when the removal of tungsten was >92% with 6% loss of molybdenum due to adsorption. 3.4. Adsorption Isotherm. The adsorption isotherm indicates how the adsorption molecules are distributed between the liquid and solid phases when the process reaches equilibrium. The isotherm also describes how the solutes interact with the adsorbent and is critical in optimizing the use of adsorbents. Two well-known isotherm models, the Langmuir22 and Freundlich23 models, were used to represent adsorption data. The linear equation of the Langmuir isotherm model is expressed as follows: ⎛ 1 1⎞ 1 M ⎟ + =⎜ ⎝ ab C ⎠ Y b
(3)
where M and Y are the concentrations of the adsorbent and adsorbate (mg/L), respectively; C is the equilibrium concentration of the adsorbate (mg/L); and a and b are the constants
Figure 3. Effect of temperature on percent of adsorption of tungsten and molybdenum with their calculated separation factors (at FMC/W = 2.1, 180 min contact time). 17593
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relationship. Values of k1 can be calculated from the plot for different concentrations of tungsten in the leach liquor. The pseudo-second-order model was proposed by Blanchard et al.25 and can be expressed in the linearized form as follows:
for the adsorbate and adsorbent, respectively, at any temperature. The logarithmic form of the Freundlich model is expressed as follows: ⎛x⎞ 1 log⎜ ⎟ = log K f + ⎝m⎠ n log C
t 1 1 = − t qt qe k 2qe 2
(4)
where m and x are the masses of the adsorbent (mg/L) and adsorbate (g/L), respectively; C is the equilibrium concentration of the adsorbate in solution (mg/L), and Kf and n are the constants for the adsorbate and adsorbent, respectively, at any temperature. Using the adsorption data, graphs were plotted for both models (Figure 5). The plots indicate that the experimental
(6)
where k2 is the rate constant of the pseudo-second-order adsorption (g·mg−1·min−1). The plot of (t/qt) vs t should yield a linear relationship from which k2 and qe can be determined for different concentrations of tungsten in the leach liquor. The data regarding tungsten sorption onto the FMC surface was plotted according to the pseudo-first-order and pseudosecond-order kinetic models, as shown in Figure 6a and b, respectively. The first-order plot of log(qe − qt) vs t (Figure 6b) shows good agreement with the experimental data.
Figure 5. Plots of isotherms for (a) Langmuir model, M/Y vs 1/C; and (b) Freundlich model, log(x/m) vs log C. Figure 6. Plots of (a) pseudo-first-order model, log (qe − qt) vs t; and for (b) second-order model, t/qt vs t, at 50 °C.
data do not satisfy the Langmuir isotherm (R2 = 0.449) but fit the Freundlich isotherm (R2 = 0.983) well. The value of the Freundlich constants were calculated to be Kf = 8.68 mg/g and n = 0.438 g. 3.5. Adsorption Kinetics. Adsorption kinetics depends on the adsorbate−adsorbent interaction that occurs under the studied conditions. The solute uptake rate determines the contact time required to complete the adsorption reaction and can be obtained from kinetic analysis. To determine the kinetic equation based on the solid sorption capacity, solution concentrations were analyzed using pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order equation, formulated by Lagergren and Svenka,24 is log(qe − qt ) = log(qe) −
k1 t 2.303
3.5.1. Error Analysis. The kinetics of tungsten adsorption onto FMC was tested using the data obtained under the optimized conditions. In addition to calculating the R2 values, the validity of each model was determined by calculating the residual variance and the sum of squares error (SSE, %):14 SSE (%) =
∑(qe,exp − qe,cal)2 N
(7)
For higher values of R and lower values of SSE, a better fit for the kinetic model can be observed. Table 1 lists the calculated results and shows that the adsorption of tungsten onto the FMC surface can be best described by the first-order kinetic model. Further, the rate constant values for each time interval are presented in Table 2. The average value of k (0.01856 min−1) shows best fit of the experimental data. 3.6. Sorption Mechanism. Although kinetics and equilibrium studies are sufficient to identify the adsorption process, the ability to predict the mechanism is required for design 2
(5) −1
where qt and qe are the amounts of metal (mg·g ) on the adsorbent at time t (minute) and at equilibrium, respectively, and k1 is the Lagergren constant for a first-order reaction (min−1). The plot of log(qe − qt) vs t should yield a linear 17594
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Table 1. Investigated Applicability of Kinetic Models in Terms of Their Calculated SSE Values kinetic model first-order second-order
qe,exp (mg·g−1) 892.43 892.43
k value
qe,cal (mg·g−1)
R2
SSE (%)
858.49 13.13
0.989 0.664
15.17 396.16
−1
0.019 min 0.0005 g·mg−1·min−1
Table 2. Rate of Reaction at Different Times versus the Rate Constant and Concentration of Tungsten in Solution (initial W concentration = 956 mg/L) t (min) CA, mg/L k, min−1 −rA, mg·L−1 min−1
30
60
90
120
150
180
764.80 0.0172 13.954
467.40 0.0181 8.459
256.30 0.0191 4.895
70.26 0.0217 1.524
63.57 0.0186 1.182
70.17 0.0167 1.172
Determining Gibbs free energy is the fundamental criteria of expressing the spontaneity. The change in enthalpy and entropy for the adsorption process was calculated from the slope and intercept of the plot ln Kf vs 1/T using the Van’t Hoff equation:
purposes. For a solid−liquid adsorption process, the solute transfer is usually evaluated by external mass transfer (boundary layer diffusion) or intraparticle diffusion or both mechanisms.26 The process may take place stepwise as follows: (i) solute transport from bulk solution to the adsorbent exterior surface through a liquid film, (ii) solute diffusion onto the pore of sorbent except for a small quantity of sorption onto the external surface (parallel to this is the intraparticle transport mechanism of the surface diffusion), and (iii) solute adsorption onto the interior surfaces of the pores and capillary spaces of the adsorbent.27 A common technique for identifying the mechanism is to apply the intraparticle diffusion equation formulated by Weber and Morris:28 qt = k idt 0.5
ΔH ° ΔS° + (9) RT R −1 −1 where R is the universal gas constant (8.314 J mol K ) and T is the absolute temperature (K). Further, the change in standard Gibbs free energy (ΔG°) was determined by using the equation: ln K f = −
ΔG° = ΔH ° − T ΔS°
(10)
The values of ΔH° and ΔS° from the linear plot in Figure 8 were found to be −14.29 kJ mol−1 and −0.026 kJ mol−1 K−1,
(8)
where kid is the diffusion rate constant (mg·g−1 min0.5) that can be obtained from the slope of qt vs t0.5. In Figure 7, the sharp-slope portion from 3.8 to 10.9 min0.5 is attributed to instantaneous adsorption and can be attributed to
Figure 8. Plot for ln Kf vs 1/T.
respectively. Thus, the value of ΔG° was calculated to be −6.38 kJ at 298 K. The negative value for change in standard Gibbs free energy confirms the feasibility of a process and the spontaneous nature of adsorption. Also, the negative value of ΔH° indicates that the adsorption reaction is exothermic. The negative value of ΔS° suggests that some structural changes occur on the adsorbent and that the randomness at the solid− liquid interface in the adsorption system reduces adsorption. The lower ΔH° value (
