Kinetics and mechanism of dissolution of synthetic molybdenum disulphide MoS2 in bromate solutions Part: I: Kinetics of dissolution of molybdenum disulphide in the MoS2KBrO3H2O system

HydrometaUurgy, 20 (1988) 235-248

235

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Kinetics and Mechanism of Dissolution of Synthetic Molybdenum Disulphide MoS2 in Bromate Solutions Part: I: Kinetics of Dissolution of Molybdenum Disulphide in the MoS2-KBrO3-H20 System A. BARTECKI and L. RYCERZ

Institute o/Inorganic Chemistry and Metallurgy o/Rare Elements, Technical University in Wroctaw, Wroctaw(Poland) (Received November 5, 1986; accepted September 26, 1987)

ABSTRACT Bartecki, A. and Rycerz, L., 1988. Kinetics and mechanism of dissolution of synthetic molybdenum disulphide MoS2 in bromate solution. Part I: Kinetics of dissolution of molybdenum disulphide in the MoS2-KBrO3-H20 system. Hydrometallurgy, 20: 235-248. The dissolution reaction of synthetic molybdenum disulphide MoS2 in aqueous potassium bromate solutions was investigated. The effect of the agitation rate, temperature, KBr03 concentration, specific surface of MoS2 and MoO~, B r - and Ag+ additives on the dissolution process was determined. The dissolution curves of MoS2 against time have a sigrnoidal character with a distinctly marked induction period. This period becomes markedly shorter with increasing temperature, KBr03 concentration and specific surface of the sulphide. The agitation rate has no effect on the dissolution process of molybdenum disulphide. The dissolution reaction constants depend considerably on temperature, KBr03 concentration and specific surface of MoS2. The apparent activation energy determined as 82.0 kJ/mol indicates that the reaction takes place in the kinetic region. The bromide ions act as an initiator of the dissolution reaction of MoS2 in bromate solutions. Introduction of the B r - ions into the reaction system results in a marked shortening of the induction period and has no effect on the reaction rate. The results of kinetic studies were analysed using the "shrinking core model" equation kt = 1 - ( 1 - ~ ) 1/3 for a reaction controlled by a chemical process. The experimental data (except for those of the induction period of reaction) were found to be in very good agreement with the equation.

INTRODUCTION

From among molybdenum sulphides, molybdenum disulphide, MoS2 is the most important one. It occurs in nature as mineral molybdenite and is an important natural source of molybdenum. On a production scale molybdenum is processed into M o Q by the oxidizing roasting method [ 1,2 ]. 0304-386X/88/$03.50

© 1988 Elsevier Science Publishers B.V.

236 Molybdenum disulphide is, in principle, almost chemically inactive. It becomes solubilized in highly oxidizing reactants, such as aqua regia, hot concentrated sulphuric and nitric acids, yielding soluble molybdenum (VI) compounds [3-71. Another method for dissolution of molybdenum disulphide is its oxidation in alkaline aqueous solutions under oxygen pressure [4,8-11] and in aqueous alkali metal hypochlorite solutions [4,12-14]. A large number of papers deal with electro-oxidizing dissolution of molybdenite [ 15-17 ]. Molybdenum disulphide becomes solubilized under the influence of potassium permanganate in basic solutions [ 18 ]. MoSe is also oxidized to NaeMoO4 by copper oxide in NaOH solutions at 523-563 K [3]. Apart from those described above, there are also other methods for dissolution of MoS2 in basic and acid solutions [4 ]: - oxidation with oxygen in basic media in the presence of a copper-ammonia catalyst. oxidation in acid media (sulphuric or hydrochloric acid) using potassium/ sodium chlorate, potassium permanganate, nitric acid, mixed acid and calcium hypochlorite. Effect of various oxidizing agents are compared in the paper by Bhappu, Reynolds, et al. [19 ]. In spite of extensive studies dealing with the dissolution process of molybdenite, the problem of finding an effective agent is still a live issue. The solubilizing agents mentioned above are either too ineffective or require extreme conditions to be applied (temperature, concentration). The present paper provides the results of studies on the dissolution kinetics of synthetic MoS2 using a strong oxidizer potassium bromate. The kinetics of dissolution of MoS2 in the MoSz-KBr03-H2S04-H20 system and mechanism of these processes will be presented in following papers. -

EXPERIMENTAL Materials Synthetic molybdenum disulphide for study was prepared by synthesis from pure elements, as described by Zelikman [4]. The chemical composition of the sulphide prepared and theoretical values for MoS: are provided in Table 1. Xray studies showed that the molybdenum disulphide obtained has a hexagonal structure. Potassium bromate, potassium thiocyanate and stannous chloride were analytically pure.

237 TABLE 1 Chemical composition of molybdenum disulphide

Molybdenum Sulphur

Theoretical

Experimental

(%)

(%)

74.99 25.01

74.85 25.25

Fig. 1. Schematic of the measuring apparatus: (1) oil seal; (2} to pump; (3) pipette; (4) water cooler; (5)stirrer; (6)thermostat.

Apparatus and procedures Measurements of the dissolution rate were carried out in an apparatus shown in Fig. 1. A 1L reaction flask was placed in a thermostat maintaining its temperature to _+0.1 ° C. A controlled-speed stirrer connected to the water cooler and oil seal was installed in the middle neck, whereas the side necks were provided with a thermometer and sampling pipette t e r m i n a t e d with a ceramic sinter. A flask with four side necks was used for potentiometric measurements. Two additional necks were used to introduce the electrodes into the reaction system.

238 TABLE 2 Specific surface of MoS2 determined by the sorptiographic method for various size grades Size grade (mm)

Specific surface ( m 2/g)

-0.044 -0.06 + 0.06 -0.075 + 0.075-0.102 +0.102-0.12

3.6 2.0 1.0 0.6 0.5

For kinetic measurements of dissolution each time the aqueous solution of KBrOa was treated with 1.476 g of MoS2. Each time the volume of the solution was 800 mL. Measurements were carried out as follows: when the KBrO3 solution attained the required temperature, it was treated with the weighed portion of MoS2. During the measurements 3 mL solution samples were taken for analyses at specified time intervals. The Mo concentration in solution was determined colorimetrically by the thiocyanate method [ 20 ]. The bromide ion concentration in solution was determined from a standard curve on the basis of potential measurements in the system: selective bromide electrode-calomel electrode. Diffraction patterns of the MoS2 samples were performed in a DRON-2 powder diffractometer. The specific surface of MoS2 was determined experimentally by the sorptiographic method using an ICSO sorptiograph. The specific surface of MoS2 determined for various size grades are presented in Table 2. Mathematical interpretation of the results of kinetic studies was performed on the basis of SCM (Shrinking Core Model) equations using a MERA-60D microcomputer. RESULTS AND DISCUSSION

E[fect of the agitation rate During the preliminary studies on dissolution of MoS2 in aqueous KBrO3 solution the dependence of the dissolution rate on the stirrer speed was determined. The results obtained for speeds ranging from 300 to 1550 min-1 are presented in Fig. 2. The dissolution rate was found to be practically constant over the entire stirrer speed range. This is an important indicator that the reaction proceeds in the kinetic region. Further measurements were carried out at the agitation rate of 640 min - 1. Standard dissolution conditions of MoS2 applied for further studies are provided in Table 3.

239 1.0

--

0.8

0.6

0.4-'

o.~36

72

108

i

E

ll,&

180

I 215 xlO0 t[s]

Fig. 2. Effect of the agitation rate on the dissolution process of MoS2; to = D: 300; A: 640; O: 820; X: 1050; and ~7:1550 min -1. TABLE3 Standard dissolution conditions of MoS2 Temperature KBrO3 concentration Specific surface of MoS2 Solution volume Weight of MoS~

298 K 0.333 M 3.6 m2/g 800 mL 1.476 g

Effect of temperature Studies of the relationship between the dissolution rate and temperature were carried out at 288, 298, 303, 308 and 318 K. Each time the concentration of KBrO3 was 0.333 M. When calculating the percentage conversion a at time t the mass of molybdenum removed from the solution in samples was taken into account. For interpretation of the kinetic curves shown in Fig. 3 the SCM (Shrinking Core Model) equations [21,22] were applied. The experimental results were found to be very well described by the SCM equation for a reaction controlled by the chemical process:

1 - (1-o~)l/3=kt

(1)

The reaction rate constants k were determined on the basis of eqn. (1) and the results are shown in Fig. 4. Apparent activation energy EA was determined from the Arrhenius equation by the graphical method (Fig. 5). The apparent activation energy EA determined is 82.0 + 5.0 kJ/mol. Thus, the dissolution

240

[]

J

2~.

L

48

96

120

1,

I 216

I

I

x

00

tIs]

Fig. 3. Dependence of the dissolution process of MoS2 on temperature. ~7: 288 K; []: 298 K; ; A : 303 K; X: 308 K; O : 318 K. 1.0--

,

,

,

c~ I

0,8

x

I

06

D

[3

0.4

0.2 ~ ~1

2J4

48

~

n~.

72

~A

96

120

145,

168

192

216

2L,0

26~,

xl00

tls]

Fig. 4. Dependence of the MoS2 dissolution reaction constant k on temperature. ~7:T=288 K, k= (2.33+0.06) × 10-~; []: T=298 K, k= (1.00+_0.06) × 10-4; A: T=303 K, k=(1.32+_O.11)XlO-4; X: T=308 K, k = (2.21 +-0.37) ×10-4; O: T=318 K, k= (6.42±0.99)×10 -4.

log k -3

-4

-5

I

I

I

3.~0

3.20

3.~0

I

I

34o 3so 3.6o 1000 / T [ K -1 ]

Fig. 5. Determination of the reaction EA in the MoS2-KBrO:~-H20 system. EA = 82 +-5 kJ/mol.

241 reaction of MoS2 in the system under investigation proceeds in the kinetic region. As shown in Fig. 3 the kinetic curves for all five temperatures have a sigmoidal nature. By graphically differentiating the kinetic curves a = f ( t ) the reaction rate was calculated for the temperature range under investigation. A plot of the reaction rate W(mol" d m - 3 s-1) versus time is shown in Fig. 6. At first, the process rate increases slowly, goes through a maximum and then falls. Such processes are characterized by the induction period at the initial stage. According to Delmon [23] the course of the reactions described by the sigmoidal kinetic curve is controlled by two basic processes: formation of the boundary surface (induction period) and its shifting towards the unreacted reactant. A shift of the boundary surface is controlled by the processes occur-

.8

22

2o o

10

8

2

96

120

lt.~,

168 xlO0 tis]

Fig. 6. Dependence of the MoS2dissolution rate versus temperature. X: 298 K; O: 303 K; [Z: 308 K; and A: 318K.

242

ring on it. In other words, the sigmoidal character of such kinetic curves depends on the nature of the chemical process. Such processes have been dealt with by many researchers who showed that in the initial stage they are developed only at specific points of the solic1 reactant, the reaction being unobservable during a certain period of time. Only at the final stages do the microreaction areas mentioned above expand rapidly, forming the reaction interface. After the reaction interface is formed, the process is continued by the interface being shifted inside the unreacted solid matter. Such a model explains fairly well the nature of the curves depicting the relationship between the MoS2 percentage conversion and time. In the initial period the reactions leading to the formation of interface take place. If the reaction rate in this period is measured by conventional methods, the course of reaction proceeding via the surface processes is imperceptible. Formation of the reaction zone initiates rapid conversion of the solid reactant. The rate of the chemical reaction which is initially low, increases on developing the reaction interface. The induction period occurring in the dissolution reaction of MoS2 depends strongly on temperature. On increasing the temperature it becomes shorter and at 318 K it is almost imperceptible (Figs. 3 and 6). For the entire temperature range under investigation the maximum reaction rate Wmaxis attained for the conversion degree a -- 0.60 and then falls rapidly.

Effect of KBr03 concentration The effect of initial KBrO3 concentration on the dissolution rate of MoS2 was investigated by determining variations in the total molybdenum concentration with time for four different initial K B r Q concentrations varying from

, 1.0

f

.

.

.

.

.

.

.

.

.

.

~

]

0.8 i 0.6

OJ.

0.2

, 21/~

:8

72 =

t 96

120

1/*/*

168

192

216

2/+0 "26~.

288

xlO0

t[S]

Fig. 7. Dependence of the MoS2 dissolution process versus KBr03 concentration. CKBrO 30 ~-X: 1.00× 10--2; [3: 2.28× 10--2; A: 6.35 × 10-2; and O: 0.333 M.

243

0.01 to 0.333 M. Measurements were performed at 318 K because of a very short induction time. The results of measurements are shown in Fig. 7. After determination of the reaction constants k, the reaction order with respect to the initial KBrO~ concentration was determined graphically from the relationship l o g k = f ( l o g CKBrO3 o ). An increase in the KBrO3 concentration results in both an increase in the reaction rate and considerable shortening of the induction period. For the KBrO3 concentration of 0.333 M the induction period is practically imperceptible.

Effect of the specific surface of MoS2 For studies of the relationship between the reaction rate and specific surface MoS2 samples of specific surfaces equal to 3.6, 2.0, 1.0 and 0.6 m2/g were used. Other parameters were constant: T=298 K, CKBrO3 o = 0.333 M. The results of measurements are presented in Fig. 8. An increase in the specific surface of MoS2 was found to result in a considerable increase of the dissolution rate and in shorter induction period. The reaction order with respect to the specific surface of MoS2 was determined graphically using the relationship log k=f(log S), where S is the specific surface of MoS2. The reaction order n determined experimentally with respect to the specific surface of MoS2 is n ~ 1 (0.84 _+0.10).

Effect of the MoO3 additive The relationship between the solubilization rate of MoS2 and the amount of molybdenum trioxide introduced into the reaction system was measured. The ~.

1.0

08

0.6

O.k

02 i

/x

/

/

/

x

_ 96

122

1/*~,

I 168

I 192

I 216

I 240

xlO0 t[s]

Fig. 8. Effect of the specific surface S of MoS2 on the dissolution rate. S-- A: 0.6; []: 1.0; C): 2.0; and X: 3.6 m2/g.

244

weight of MoO3 added varied from zero to 1.200 g. Other parameters were constant: T = 298 K, C°ro~ = 0.333 M. The problem of investigating the above mentioned relationship was assumed to be important because of the sigmoidal nature of the kinetic dissolution curves of MoS2 in KBrO3 solutions. According to Delmon [23 ], as mentioned when discussing the effect of temperature on the dissolution process of MoS2, the sigmoidal nature of the curves is related to the formation of the reaction interface and its shift inside the unreacted reactant. This model was applied by Smirnov et al. [ 7 ] for explaining the dissolution mechanism of MoS2 in nitric acid solutions. According to these authors M o Q is an intermediate product in dissolution of MoS2. Only after the reaction interface is formed with participation of MoO~ is the rapid dissolution process of MoS2 initiated. It was assumed that Mo03 or Mo (VI) ions which are formed in the course of MoO3 dissolution can play the role of initiator of MoS2 dissolution. When calculating the MoS2 percentage conversion ~ at time t, the mass of molybdenum present in solution of M o Q dissolution was taken into account (these data were taken from MoO3 dissolution in the MoO3-KBrQ-H20 system in the same conditions). The results of measurements show that M o Q added to the reaction system has no effect on the reaction course. This enables us to conclude that both MoO3 and the molybdenum (VI) ions formed by dissolving MoO3 have no effect on the course of the dissolution process of MoS2 in aqueous KBrO3 solution. Thus, the characteristic course of the MoS2 dissolution process in the system investigated should be attributed to the properties of bromate solution. It is known that in reactions with K B r Q participation the chemical induction effect often occurs. The B r - ions play a significant role in this effect [24,25]. This was the reason for our further studies in which the effect of Br ion addition to the system was investigated.

Ef[ect o[ the B r - ions In order to investigate the effect of the B r - ions on the dissolution process, the reaction system was treated with KBr solution together with MoS2. Molybdenum concentration was determined together with that of the B r - ions. Measurements were carried out for the following initial B r - ion concentrations: 0.00, 0.013, 0.055 and 0087 M. The results of measurements are presented in Fig. 9. Using an SCM equation the reaction constants k for particular initial bromide ion concentrations were determined (Fig. 10). They do not depend on the B r - ion concentration. Thus, the reaction is zero order with respect to these ions. However, their addition results in a considerable shortening of the induction period of the MoS2 dissolution reaction. It leads to the conclusion

245 1.0

CY-

~,

i

1.0

i

I

I

I

2&

48

72

96

~0.8

0.8 0.6

0.6

0.~

0.~

0.2

0.2 i 120

9'6

24.

I 1~

xlO0

t [s]

xlO0

t [sI

Fig. 9. Effect of the B r - ions added on the dissolution process of MoS2"C~r -- []: 0.00; X: 0.013; A: 0.055; and O: 0.087 M. Fig. 10. Effect of the B r - ions added on the MoS2 dissolution constant k. C ° r - = ( ~ : 0 . 0 0 ; X: 0.013; A: 0.055; and [:]: 0.087 M. Value of k is respectively: (2.02+_0.12)>< 10-4; (2.04+_0.06)X 10-4; ( 1.89 +_0.07 ) × 10- 4; and ( 1.99 +_0.08 ) × 10- 4. ~80 , -

)

6o

E ,-~

i 20

I

80

12 i

36 i

60 F

B~ =

108 T

132 I

156 xlO0 I

12

36

60

84

108

132

156 xlO0

%

rn

~ 60 ~ ~0

~20 I

t [s] Fig. 11. Dependence of the reaction rate W and B r - ion concentration on time for the dissolution

reaction of MoS2 "C°mo~ = O: 0.333; and El: 0.105 M.

246 that in MoS2 dissolution, without the addition of B r - ions, formation of these ions is the first stage of reaction. To obtain a more detailed picture of the role of B r - ions, the variation in their concentration during dissolution of MoS2 in bromate solution without any B r - ions added was measured. It was found that during the induction period, in spite of the fact that there is no dissolution of MoS2, the bromide ion concentration in solution increases. When the dissolution reaction is initiated, the bromide ion concentration suddenly increases and then falls. In order to relate the variations in the bromide ion concentration to the course of the MoS2 dissolution reaction, the dissolution rate of MoS2 ( WMos2 ) was calculated and compared with variations in the bromide ion concentration in solution. The results for 298 K are presented in Fig. 11. The results obtained show that only after a specific B r - ion concentration is reached is the dissolution process initiated. This is accompanied by a further increase in the bromide ion concentration. This increase results in a higher dissolution rate of MoS2. The highest dissolution rate of MoS2 corresponds to the maximum bromide ion concentration in solution. On exceeding its maximum value, the rate of MoSz dissolution falls and the bromide ion concentration in solution decreases suddenly. Thus, the bromide ions act as an initiator of the dissolution reaction of MoS2 in bromate solution. A chemical induction effect occurs here [25] which consists in coupling of two reactions: a primary and secondary reaction. The primary reaction (spontaneous) induces through its course a secondary process (non-spontaneous). It is well known that in the acid aqueous bromate solution an equilibrium is established which may be described by the following basic reactions [26,27 ]: BrO:i- + B r - .

' BrO~ + BrO-

(2)

BROW-+ B r - .

' 2BrO-

(3)

BrO-+Br-+H

+.

--'Br2+H20

(4)

In the literature there are a number of papers dealing with the kinetics and mechanism of homogeneous reactions with the bromates [28-35]. Because of the specific course of these reactions they were named autocatalytic reactions showing a characteristic induction period. One should believe that the dissolution reaction of MoS2 is also an autocatalytic reaction. In the first stage during the induction period the processes occurring on the sulphide surface yield hydrogen bromide.

MoS2 + 3BROW- +3H20~HMoO~- +2HSO4 + 3 H + + 3 B r -

(5)

Under the influence of the B r - ions in the presence of the H + ions an equilibrium described by eqns. (2-4) is established and then the BrOy and BrO-

247 ~6.0

f

f

'2

r

t

I

I

x

oEs.0

E

~

t,,0

.'7 3.0

2.0

1.0

36

72

108

15,$,

~60

216

xl00

108

15,4

190

216

xl00 tls]

¢~ 1.0

0.8

//

0.6

0.~

0.2

36

72

Fig. 12. Effect of Ag÷ ions added on the dissolutionprocess of MoS~-C°g. = A: 9.37× 10-3; [~: 2.50× 10-3; X: 6.25× 10-3; and C): 0.000 M. ions are formed. These are the main MoS2 oxidizing agents. The reaction products of BrO~ or B r O - with molybdenum disulphide are the bromide ions which cause reproduction of BROW- and B r O - entities. Thus, the maximum dissolution rate of MoS2 is accompanied by the m a x i m u m bromide ion concentration. In the final stage of the process a decrease in the bromide ion concentration is a result of reaction (4).

Effect of Ag + ions In order to confirm the importance of B r - ions in the MoS2 dissolution reaction in bromate solution, measurements were performed in the presence ofAg + ions which remove the bromide ions from the reaction system. The Ag + ions were introduced together with MoS2 and their concentration varied from

248 6.25 × 10 -4 to 1.5 × 10 -3 M. T h e r e s u l t s o f m e a s u r e m e n t s are p r e s e n t e d in Fig. 12. T h e r e s u l t s o b t a i n e d c o n f i r m fully t h e i m p o r t a n c e of t h e b r o m i d e ions in t h e d i s s o l u t i o n p r o c e s s of MoS2 in b r o m a t e solutions. E v e n a s m a l l a d d i t i o n of Ag + ions (C°g+ = 6 . 2 5 × 1 0 -4 M ) l e n g t h e n s t h e i n d u c t i o n p e r i o d b y b i n d i n g t h e b r o m i d e ions f o r m e d in t h e s y s t e m . An i n c r e a s e in t h e Ag + ion c o n c e n t r a t i o n r e s u l t s in b o t h l e n g t h e n i n g of t h e i n d u c t i o n p e r i o d a n d a d e c r e a s e in t h e rea c t i o n r a t e u n t i l t h e d i s s o l u t i o n p r o c e s s b e c o m e s a l m o s t fully d i s c o n t i n u e d (for COg+ = 9 . 3 7 × 10 -3 M ) .

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