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togram of the stibnite ore is given in Figure 1. Thechemical composition of the ore was determined byvolumetric and gravimetric methods (Table 1). Thedissolution experiments were carried out in a glassreactor of 250 mL volume equipped with a mechan-ical stirrer having a digital controller unit and atimer. The temperature of the reaction mediumcould be controlled within 1 C by a temperatureregulation system. After adding 100 mL of HCl so-lution to the reaction vessel and setting the reactiontemperature at the desired value, the reactor contentwas stirred at 500 min1. The solution was saturatedwith Cl2 gas, and 0.5 g solid was then added to thereactor while continuing to stir and add Cl2 gas. Atthe end of the reaction period, the contents of the

vessel were filtered and the filtered solution wasthen analysed volumetrically for Sb13.

The effects of quantities on the dissolution processwere investigated using the values given in Table 2 foreach quantity. In the experiments, while the effect ofone quantity was studied, the values of other quantitiesshown with asterisk in Table 2 were kept constant.

Results and discussions

Dissolution reactions

When stibnite ore is added into the hydrochlo-ric acid solution saturated with Cl2 gas, the reac-tions taking place in the medium can be written asfollows6:

Sb2S3(s) + 6HCl(aq) 2SbCl3(aq) + 3H2S(aq) (1)

SbCl3(aq)

+ HCl(aq)

SbCl4

(aq)+ H+

(aq)(2)

SbCl4

(aq) + Cl2 SbCl5(aq) + Cl(aq) (3)

H2S(aq) + 4Cl2(aq) + 4H2O(l) 8HCl(aq) + H2SO4(aq) (4)

Effects of quantities

The data obtained were plotted in the time ver-sus conversion fraction, described as XSb = theamount of Sb in the solution / the amount of Sb inoriginal stibnite ore.

As seen in Figure 2, the dissolution rate in-

creases, as the particle diameter decreases. This sit-uation can be attributed to the increasing contactsurface of the samples as the particle diameter di-minishes. Figure 3 shows that increasing reactiontemperature has an increasing effect on the disso-lution rate, as expected due to the exponentialdependence of the rate constant in the Arrheniusequation. The experiments to observe the effectof hydrochloric acid concentration on the dissolu-tion process showed that the dissolution rate in-creases with increasing acid concentration (Figu-re 4).

26 M. OPUR et al., Solubility of Stibnite Ore in HCl Solutions Saturated With Cl2 Gas, Chem. Biochem. Eng. Q. 15 (1) 2528 (2001)

T a b l e 1 The chemical composition of stibnite ore used inthe study

Component Sb Ca Pb Co Fe Mn

% 60.56 2.21 0.029 0.013 0.019 0.008

Component Ag Cu Zn As S SiO2

% 0.005 0.014 0.022 0.010 2 3.118 13.920

Ta b l e 2 Parameters used in this study and their values

Quantities Values

Reaction temperature, C 12, 25*, 35, 44

Acid mass fraction, % 23.72, 27.20*, 30.58

Particle diameter, mm 605, 433, 308, 215*, 152.56

Reaction time, min2.5, 5, 7.5, 10, 15, 20, 30,40, 45, 50, 60,70, 75

Stirring speed, min1 500*

Solid to liquid quotient, g mL1 0.5/100*

*The values kept constant.F i g . 1 X-ray difractogram of stibnite ore

F i g . 2 Effect of particle size on the dissolution of stibniteore

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Kinetics analysis

In a fluid-particle heterogeneous reaction sys-tem, the reaction rate can be expressed by one ofthe following models:

a) shrinking core model for particle of un-changing size,

b) shrinking particle model,

c) pseudo-homogeneous models.

These models can be applicable in both casesof gas-solid reactions and fluid-solid reactions1422.

In the presented study, the statistical analysis ofthe experimental data showed that the results fittedto the first order pseudo homogeneous model

(dXSb / dt) = k(1 XSb) (5)

When this expression was integrated, the fol-lowing equations was obtained

ln(1 XSb) = k t (6)

As seen in Figure 5, the plots of t versus ln(1 Xsb) gave straight lines. On the other hand,the quantities such as temperature, particle diame-ter, HCl concentration, chosen as quantities in thepresent work, affect reaction kinetics14. To deter-mine this effect, rate constant, k, in Eq. 6 was ex-pressed in the following form:

ln(1 XSb) = k0 [(d)a (w)b exp(EA/RT)] t (7)

and, the values of the constants of ko, a and b weredetermined as 2.97 105, 0.74 and 1.52 , respec-tively, using an statistical program and a PC. ThusEq. 7 could be written as follows:

ln(1 XSb) = 2.97 105 (d)0.74

(w)1.52 exp(4800/T) t (8)

From EA/R = 4800, activation energy (EA) wascalculated as 39.87 kJ.mol1.

To test the fit between the experimental con-version values and the values calculated from theEq. 8, the plot of X (experimental) versus X(calcu-lated) was drawn, as seen in Figure 6. The fit be-tween the experimental and calculated values isvery good, with a relative mean square errors of0.1065, calculated by the Eq. 9

M. OPUR et al., Solubility of Stibnite Ore in HCl Solutions Saturated With Cl2 Gas, Chem. Biochem. Eng. Q. 15 (1) 2528 (2001) 27

F i g . 4 Effect of acid concentration on the dissolution ofstibnite ore

F i g . 5 Plot of ln(1 X) versus t for different reaction

temperature

F i g . 6 The agreement between experimental conversionvalues and those calculated from the empirical ex-pression Eq. 8

F i g . 3 Effect of reaction temperature on the dissolutionof stibnite ore

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ERcal exp

cal

=-

=

12

21

1

2

N

X X

Xi

N ( )

( )(9)

Where Xcal is the calculated value, Xexp the ex-perimental value, and Nthe number of experimentaldata, which is 63 for the present case.

Conclusions

The dissolution of stibnite mineral in hydro-chloric acid solutions saturated with Cl2 gas was in-vestigated in a batch reactor. It was seen that thedissolution process was fitted pseudo-first order ho-mogeneous reaction model and the activation en-ergy of the process was calculated to be 39.87

kJ mol1. This semi empirical mathematical modelrepresented the process very well.

N o m e n c l a t u r e

XSb Conversion

t Reaction time, min

a A constant in Eq. (7)

b A constant in Eq. (7)

k0 A constant in Eq. (7)

d Particle size, mm

EA Activation energy, kJ mol1T Temperature, K

R Universal gas constant, J K1 mol1

w Mass fraction, %

ms Mass of stibnite, g

VHCl Volume of HCL solution

R e f e r e n c e s

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28 M. OPUR et al., Solubility of Stibnite Ore in HCl Solutions Saturated With Cl2 Gas, Chem. Biochem. Eng. Q. 15 (1) 2528 (2001)