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Leaching Kinetics and Recovery of Vanadium from Egyptian Boiler Ash Under Alkaline Oxidizing Conditions ( 67 )

ABSTRACT

KEYWORDS

Leaching Kinetics and Recovery of Vanadium from Egyptian Boiler Ash Under Alkaline Oxidizing Conditions

Laila Guirguis1, Maha Sharaf1, Abu Rehab Mahmoud1, Magdy Zahran2

E.mail:[email protected]

Oxidative Leaching, Vanadium, Leaching of Vanadium, Kinetics, Recovery.

4th Int. Con. Rad. Res. Appl. Sci., Taba, Egypt (2014) PP. 67 : 73

1. Nuclear Materials Authority, P.O Box 540 El Maadi, Cairo, Egypt.2. Chemistry department, faculty of Science, Menoufia University, Egypt.

Processing of Egyptian boiler ash in potassium hydroxide solution using potassium perchlorate as an oxidant has been investigated. The results showed that leaching temperature, potassium perchlorate, potas-sium hydroxide concentration, leaching time, liquid-to-solid ratio and agitation speed, have a significant effect; optimum process operating parameters were established as follows: temperature: 95 °C; concentra-tion of KClO3: 2%; potassium hydroxide concentration: 2 M; time: 3 h; liquid-to-solid ratio: 5:1; agitation speed: 600 rpm. Under these experi-mental conditions, the extraction efficiencies of Vanadium attained 99% with no co-extraction of Nickel and Iron. Leaching kinetics of Vana-dium showed that, the reaction rate of leaching process is controlled by chemical reaction at the particle surface. The leaching process follows the kinetic model 1− (1−X)1/3=kt with an apparent activation energy of 32.393 kJ/mol. Direct precipitation of pure Vanadium pentoxide from the obtained leach liquor with no co-extraction of Nickel and Iron had been done also, no need to use solvent for extraction. In fact this method will be the best to be applied to the Industrial-scale.

4th InternationalConference on Radiation

Sciences and Applications,13-17/10/2014, Taba, Egypt

Laila Guirguis et al.( 68 ) 4th Int. Con. Rad. Res. Appl. Sci., Taba, Egypt (2014)

INTRODUCTION

Vanadium, as a very important mate-rial, is widely used in various fields (Liao, et al 1985).

Vanadium is an important rare metal so, it is exposed to a rapid depletion

of it’s based ores. However, renewed sources for Vanadium production have to be established to face the continuous need of Vanadium. For this reason, the recovery of Vanadium from fly and boiler ashes has received attention in recent years and high pri-ority. On the other hand, the disposal and accumu-lation of this industrial waste may lead to environ-mental problems. Several methods have been used to recover Vanadium from fly and boiler ashes using hydrometallurgical processes. The recovery may be carried out directly by acid (Tsai et al., 1998, Vito-lo et al., 2000, 2001; Navarro et al., 2007) alkaline (Akita et al., 1995) or water (Akaboshi et al., 1987) leaching; this may be followed by an oxidation of Vanadium using air (Akaboshi et al., 1987), NaC-lO3 (Whigham, 1965), NaClO (Schemel, 1985), or oxygen (Goddard, 1987) . Vanadium in the attained leach liquor may be separated using precipitation (Vitolo et al., 2000), ion exchange (Tokuyama et al., 2003), or solvent extraction (Lozano and Juan, 2001, Miura et al., 2001 and Guibal et al., 2003).

Kinetic study of any chemical reaction is an in-dispensable part in designing the chemical process at leaching stage. Most of the leaching processes are reactions occur between liquid and solid phases. The models that have been used to express kinetics of non-catalytic liquid–solid reactions are the homoge-neous, grain, uniform pore and random pore models, shrinking (including shrinking particle and shrinking core), (Gbor and Jia, 2004; Georgiou and Papangela-kis, 1998).

The aim of this work is to investigate a simple leaching process of the Egyptian boiler ash from El-Korimat thermal power station by KOH solution in the presence of KClO3 as oxidizing agent. The ef-fects of the main system variables on the leaching

rate are examined. Moreover, the kinetic aspects of leaching Vanadium which provided a theoretical basis for the industrial application in the future had been studied.

EXPERIMENTAL

1. Chemical analysis of the Egyptian boiler ash

Boiler ash used in this study was from El-Kori-mat thermal power station, Egypt; its chemical com-position is 6.77 % SiO2, 2.93% Al2O3, 12.49% Fe2O3, 5.81% CaO, 9.65% MgO, 3.51% K2O, 2.85% Na2O Beside the presence of 10.51% Vanadium and 8.01% Nickel. All the chemicals used in this study were of analytical grade.

2. Analytical procedures

The ore was analysed for its major and minor elements using the proper analytical methods (Marc-zenko. Et al., 1986; Welz et al., 1999).

3. Leaching procedures

All leaching experiments were carried out in a glass reactor equipped with a Teflon stirrer, condens-er, thermometer, glass funnel for adding the solid sample and a sampling device. This set-up provides stable hermetic conditions and allows heating at con-stant temperature. The calculated volumes of KOH solution and KClO3 solutions were added to the glass reactor and heated-up to the selected temperature. When the temperature was reached, the solid con-centrate was added and the reaction commenced. Af-ter selected time intervals, the solution samples were taken for chemical analysis.

RESULTS AND DISCUSSION

1. Leaching results

1. 1. Effect of stirring speed

Fig (1) presents the effect of stirring speed under the conditions of -74 µm particle size, 2.5 M KOH, 2% KClO3 and 1:5 solid/ liquid ratio at 95°C for 3 h. The results indicated that, the leaching rate of Vana-dium increases quickly below 400 rpm and remains almost constant after 600 rpm. Clearly, stirring speed


improves leaching efficiency of Vanadium; increase stirring speed usually increases the rate of leaching due to the suspension of the compound particles and decreases the thickness of the mass transfer bound-ary layer on the surface of the particles. Therefore, the stirring speed is kept constant at 600 rpm in the following tests.

1. 2. Effect of KOH concentration

1. 3. Effect of KClO3 concentration

The effect of KClO3 concentration was exam-ined in the absence and presence of different concen-trations of KClO3 varying from 1 to 3%. The other leaching conditions were fixed at -74 µm particle size, 2 M KOH, and 1:5 solid/ liquid ratio at 95°C for 3 h. From the obtained leaching efficiencies as plotted in Fig (3), it was observed that, KClO3 con-centration play a critical role in dissolution of Vana-dium. Hence the dissolution of Vanadium increases from 84% to 99%, when the KClO3 concentration is increased from 0 to 2%. With increase in KClO3 concentration to 3% the extraction efficiency of Va-nadium sharply decreased to 90.50%. Therefore, the optimum KClO3 concentration appears to be 2%.

Fig.(1): Effect of stirring speed upon the leaching efficien-cies of V(IV) (2.5 M KOH, 2% KClO3 and 1:5 solid/ liq-uid ratio at 95°C for 3 h).

Fig.(2): Effect of KOH concentration upon the leaching efficiencies of V(IV), Ni(II), and Fe(II) at (2% KClO3 and 1:5 solid/ liquid ratio at 95°C for 3 h).

Fig.(3): Effect of KClO3 concentration upon the leaching efficiencies of V(IV), at (2 M KOH, and 1:5 solid/ liquid ratio at 95°C for 3 h).

Extraction of Vanadium is affected by KOH con-centration. A concentration range from 0.25 to 2.5 M was tested under the conditions of -74 µm particle size, 2% KClO3 and 1:5 solid/ liquid ratio at 95°C for 3 h with stirring speed 600 rpm. From the ob-tained leaching efficiencies as plotted in Fig (2), it was observed that, Vanadium extraction efficiencies increases from 35.11% to 99% when the KOH con-centration is increased from 0.25 to 2 M. A further increase in KOH concentration to 2.5 M resulted in very little increase in the extraction efficiency of Va-nadium. Therefore, the optimum KOH concentration appears to be 2 M and all further experiments were carried out at this KOH concentration.

1. 4. Effect of time

In order to evaluate the effect of time, a series of leaching experiments was carried out at different times ranging from 30 to 180 min. In these experi-ments, the other leaching conditions were kept fixed; namely, 2 M KOH, 2% M KClO3, and 1:5 solid/ liquid ratio at 95°C. From the results obtained, it is evident that, at only 30 min leaching time, the leach-ing efficiencies of Vanadium has been decreased to 36.30%. With increasing leaching time, the extrac-tion efficiency of Vanadium sharply increased to reach 99%.

Therefore, 180 min leaching time would be ad-equate and could be considered as optimum for Va-nadium extraction.

3. 1. 5. Effect of temperature

Four leaching experiments were carried out at room temperature, 50, 75, and 85ºC under the same


conditions previously used at 95ºC. The resultant extraction efficiencies plotted in Fig. (4) Indicates that, the temperature plays a critical role in leaching Vanadium. For example, working at room tempera-ture, the obtained extraction efficiency for Vanadium was decreased to only 21.51. By increasing tempera-ture to 75ºC the extraction efficiency of vanadium will reach 80.01, a further increase in temperature to reach 95ºC the extraction efficiency of Vanadium become 99 %. Therefore, it can be concluded that 95ºC leaching temperature would be adequate and considered optimum.

2. Leaching kinetics of vanadium

2. 1. Kinetic analysis

Leaching reactions do not often obey simple first- and second-order kinetics. A kinetic analysis of leaching reactions are generally expressed by non-catalytic heterogeneous reaction models. A kinetic analysis of leaching reactions is essential for the ef-fective design of leaching reactors to be used in a hydrometallurgical plant.

The kinetic of leaching reactions is often de-scribed by the shrinking core model. According to the shrinking core model, it is thought that the re-action between solid and fluid reactants takes place on the outer surface of solid. (Ibrahim et al., 2013; Levenspiel, 1999).

The overall rate of leaching reaction is affected by both the rate of diffusion of fluid reactant and products through the porous product layer that forms on the unreacted core of solid and the rate of the re-action at the surface of unreacted core (Ibrahim et al., 2013; Levenspiel, 1999).

These rate equations can be expressed as follows:

If the leaching rate is controlled by the diffusion through the liquid film, then the integrated rate equa-tion is:

X = k1t (2)

If the reaction rate is controlled by the diffusion through the ash or product layer, then the integrated rate expression is:

1 – 2/3X – (1-X)1/3 = Kdt (3)

If the leaching rate is controlled by the surface chemical reaction, then the integrated rate equation is:

1 – (1-X)1/3 = Kct (4)

where X is the conversion fraction of solid par-ticle, kl is the apparent rate constant (min-1) for dif-fusion through the fluid film, kd is the apparent rate constant (min-1) for diffusion through the product layer, kc is the apparent rate constant (min-1) for the

Fig.(4): Effect of temperature upon the leaching efficien-cies of V(IV) at (2 M KOH and 2% KClO3 solution, 1:5 solid/ liquid ratio for 3 h).

Fig.(5): Effect of solid/liquid ratio upon the leach-ing efficiencies of V(IV) at (2 M KOH and 2% KClO3 solution,95°C for 3 h).

60

65

70

75

80

85

90

95

100

1:02 1:03 1:05 1:07

leac

hing

effe

cienc

y, %

Solid/liquid ratio (V/V)

1. 6. Effect of solid/liquid ratio

Working with the fixed concentration of 2 M KOH and 2% KClO3 solution, another four leaching experiments were performed at solid/liquid ratios 1:2, 1:3 and 1:7 under the same leaching conditions used for the solid/liquid ratio 1:5. From the obtained results schemed in Fig. (5), it was found that, the extraction efficiencies of Vanadium was decreased at 1:2 and 1:3 solid/liquid ratios. And no obvious increasing in Vanadium extraction using 1:7 ratio. Therefore, it can be considered solid/liquid ratio 1:5 is optimum.


surface chemical reaction, and t is the reaction time.

2. 2. Effect of temperature

The temperature is a factor of great importance for the leaching kinetics. The effect of reaction tem-perature was examined from 298 to 368 K under the conditions of 2 M KOH, 2% KClO3 and 1:5 solid/liquid ratio from 0 to 180 min.. The extraction effi-ciency curves obtained are shown in Fig. 6. It is clear that, the quantity of Vanadium dissolved increases with increasing reaction temperature. In order to de-termine the kinetic parameters and rate controlling step, the experimental data as shown in Fig. (6) was analyzed on the basis of Eq. (2), Eq. (3) and Eq. (4) (shrinking core model) and the experimental data va-lidity was tested by statistical and graphical methods and then the multiple regression coefficient obtained for the integral rate expression were calculated. The results showed that, the experimental results (in Fig. 6) fit the reaction model of Eq. (4), as shown in Fig. (7) which demonstrated that the reaction rate of Va-nadium leaching process is controlled by chemical reaction at the particle surface under the experimen-tal conditions.

The apparent activation energy was determined based on the Arrhenius equation:

k = A exp(-Ea/R-T) (5)

or lnk = lnA – Ea/RT (6)

Where k is a reaction rate constant, A is the fre-quency factor, Ea is the apparent activation energy and R is the gas constant. The data for the five tem-peratures are presented in Fig. (8), the regression analysis showed that the linear relationship is also significant. The apparent activation energy (Ea) was, hence, determined to be 32.393 kJ/mol.

Fig.(6): Effect of temperature on leaching rate of Vana-dium at (2 M KOH, 2% KClO3 and 1:5 solid/liquid ratio ).

Fig.(7): Relationship between [1-(1-X)1/3] and leaching time for Vanadium leaching at various temperature at (2 M KOH, 2% KClO3 and 1:5 solid/liquid ratio).

RECOVERY OF V2O5

Before precipitating vanadium, Ammonium chloride in a solid/liquid ratio of 5 g NH4C1/250 ml solution was added to the filtered leach liquor con-taining either 0.01 M oxidizing agents HC1O4 or HNO3.

Fig.(8): Arrhenius plot for Vanadium leaching at (2 M KOH, 2% KClO3 and 1:5 solid/liquid ratio).

Fig.(9): XRD analysis of the pure Vanadium pentoxide (V2O5) product.


The obtained precipitate (ammonium meta vana-date) is dried at 110°C for 2 h, the product obtained was undergoing thermal decomposition at 400°C for 4 h. A pure form of Vanadium pentoxide (V2O5) was obtained as indicated by XRD, EDAX analysis as shown in Fig (9), Fig (10).

Based on the experimental results of the present investigation a technological flow sheet, describing the alkaline leaching , Vanadium and silica separa-tion and the recovery of V2O5 by precipitation fol-lowed by thermal decomposition of the obtained crystals, was presented in Fig. 11.

CONCLUSION

It is clear from the experiments that Vanadium can be easily leached from Egyptian boiler ash by using KOH in presence of KClO3 as oxidant. The results shows that, working 2 M KOH; 2% KClO3 ; stirring speed 600, solid/liquid ratio 1:5 at 95°C for 3 h, the extraction efficiency of Vanadium attained about 99%, with no co-extraction of Nickel and Iron. Recovery of pure V2O5 from the resultant leach li-quor is done to establish a good separation method of Vanadium pentoxide from the Egyptian boiler ash.

The leaching kinetics of Vanadium shows that, the rate of Vanadium leaching using KOH in pres-ence of KClO3 as oxidant is chemically controlled and follows the shrinking core model 1− (1−X)1/3=kt with an apparent activation energy of 32.393 kJ/mole.

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Fig.(11): Flow sheet for the process of recovery of V2O5 from El-Korimat boiler ash leach liquor.

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