Ind. Eng. Chem. Res. 1995,34, 3995-4002
3995
Solubility of Stibnite Ore in HCl Solutions Mehmet Copur,"Sabri Colak, and Sinan Yapici Atatiirk tiniversitesi, Miihendislik Fakiiltesi, Kimya Miihendisligi Boliimii, 25240 Erzurum, Turkey
In this study, the dissolution of stibnite, Sb2S3, which is an antimony ore, in HCl solutions was investigated, taking into consideration the effect of particle size, acid concentration, stirring speed, solid-to-liquid ratio, and temperature. It was observed that the dissolution rate increased with increasing temperature and with decreasing particle size and solid-to-liquid ratio, while no pronounced effect of stirring speed was observed. Using the experimental data, we attempted to establish a kinetic model for the dissolution of the ore in HC1 solutions. Employing graphical and statistical methods, the following kinetic model including the effect of the above choosen parameters was established to represent the dissolution process -ln(l - X ) = 9.79 x 10-lo(D)-0.908(C)10~600(S/L)-0.321e-6244/T t , where X is the conversion fraction of the mineral, D is the particle size, C the acid concentration in weight percent, and S/L is the solid-to-liquid ratio in grams per milliliter. The activation energy for this dissolution process was calculated as 52 kJ mol-l . 1. Introduction
Antimony trioxide (Sb203) is widely used as a fireresistant material in plastic, cable, and latex production (Morizot et al., 1980; Robert and Weast, 1986). Mandatory use of fire-resistant materials in the automobile industry by law in the United States has substantially increased the consumption of Sb2O3. The use of antimony in the production of fire-resistant materials in the United States is about 57 % of the total antimony consumption (MTA Bulletin, 1985). Stibnite has an orthorhombic crystal system and is available in nature in the color of lead-grey, sometimes tarnished and iridescent; it is opaque. It melts readily, even in a match flame. Stibnite, the most common antimony mineral, is commonly found with quartz in hydrothermal veins, as replacement bodies in limestone, and in hot spring deposits (Hamilton et al., 1987). The production of SbzO3 in high purity requires the production of SbC13 in high purity (Morizot et al., 1980). SbCl3 is also used for the production of other antimony compounds, in antimony electroplating, for coloring iron and zinc metals, and as a catalyst in organic chlorination and polymerization processes. The most common method for the production of SbCl3 is a dissolution process of stibnite, Sb2S3, in HC1 solutions followed by separation by a distillation process (Kirk and Othmer, 1952). Carrying out a reactor design for a technological process requires the knowledge of the kinetic behavior of the occurring process. Some studies were found in the literature t o enlighten the kinetic process for the dissolutin of stibnite in different solutions. In a study carried out by Shestiko et al. (19751, it was found that the leaching kinetics of stibnite in Na2S was controlled by diffusion through the fluid layer, and the activation energy of the process was calculated as 16.72 kJ mol-l. In the dissolution process of stibnite in NaOH solutions, KOet al. (1981) determined that the dissolution process was controlled by diffusion for the solution of 0.5 M NaOH concentration and by chemical reaction for that of 0.05 M NaOH. For the dissolution process of stibnite in acidic FeCl3 solutions, KOet al. (1981) found that the process was controlled by diffusion of the reactant Fe3+ through the sulfur layer for particle sizes smaller than 270 mesh, while for particle sizes larger than 270 mesh, the process was controlled by chemical reaction in the beginning of the dissolution and then by chemical 0888-588519512634-3995$09.0010
reaction as the process progressed. No study for the leaching of stibnite with HC1 solutions could be detected in the literature. There are also some studies for the production of SbCl3. Khundar et al. (1967) produced SbC13 by the effect of CC4 on stibnite in the temperature range 200500 "C. In another study carried out by Grossman (19761, SbC13 was obtained by the contact of HC1 gas with stibnite at temperatures above 80 "C. SbC13 obtained by the contact of an antimony ore with Clz gas was separated from S or H2S by distillation (Stewart and McKinley, 1976). The aim of this study is to investigate the dissolution kinetics of stibnite in HC1 solutions in a stirred batch reactor, using some kinetic data. As mentioned above, although the process is used to produce SbCl3 industrially, no study was found in the literature on the kinetic investigation of the dissolution of stibnite in HC1 solutions. 2. Experimental Section The antimony ore used in the experiments was provided from the region of Nigde in Turkey. After crushing and grinding the ore, it was sieved by a wet method using ASTM standard Sieves and separated into fractions of 0.5958, 0.4213, 0.2979, 0.2121, 0.1500, and 0.0822 mm. It was determined that the ore contains mainly Sb& and Si02 by X-ray diffraction; and X-ray diffractogram is given in Figure 1. The chemical analysis of the ore by volumetric and gravimetric methods and by atomic absorption spectrophotometry for trace elements is given in Table 1. For the kinetic study, the parameters were chosen to be temperature, HC1 concentration, particle size, stirring speed, and solid-to-liquid ratio. The ranges of these parameters and their values are given in Table 2. The dissolution experiments were carried out in a 250 mL glass reactor equipped with a mechanical stirrer with a digital control unit and a timer, a thermostate, and a back cooler. The temperature of the reaction medium could be controlled within f0.5 "C. First, 200 mL of an HC1 solution at a given concentration was put into the reactor, and after getting to a desired temperature of the reactor contents, a certain amount of the ore was added into the solution while stirring the contents of the vessel at a certain stirring speed during the reaction. At the end of the reaction period, the
0 1995 American Chemical Society
3996 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
u
40
44
36
28
32
2L
to
16
12
8
Figure 1. X-ray diffractogram of stibnite mineral. Table 1. The Chemical Analysis of Stibnite Used in the Studs element % (by weight) element % (by weight) Sb 60.560 Ag 0.005 Ca 2.210 CU 0.014 Pb 0.029 Zn 0.022 co 0.013 As 0.010 Fe 0.091 S 23.118 Mn 0.008 Si02 13.920
Table 2. Parameters and Their Ranges and Values Employed in the Experiments
contents of the vessel were filtered for 15 s and Sb3+in the filtered solution was then analyzed volumetrically (Furman, 1963). Each experiment was repeated twice within a deviation of approximately 1.5%, and the average values were used in the kinetic analysis. The number of experiments within the parameter ranges is enough for a kinetic analysis of this type.
temperature, 500 rpm for stirring speed, 0.2121 mm for particle size, 30.58 % for acid concentration, and 11200 (g/mL)for solid-to-liquidratio. The data obtained from the experiments were plotted in time versus conversion, described as (percentage of dissolved antimony from the ore)/(percentage of antimony in original ore). The effect of the stirring speed on the dissolution was investigated for the range of 100-900 rpm. The results are exhibited in Figure 2. As seen from this figure, increasing the stirring speed increased the dissolution rate up to 300 rpm, and no effect of the stirring speed on the dissolution could be observed by further increasing the stirring speed. This behavior can be explained by the higher density of stibnite (4649 kg mW3for pure stibnite (Perry, 1984)); for a good distribution of solid particles in the solution, a certain value of the stirring speed is necessary. It was also observed that some part of the solid settled down in the reactor at lower stirring speeds. This settling of the particles decreased with increasing stirring speed, and a good distribution was obtained after 300 rpm. The fact that stirring speeds above 300 rpm have no effect on the dissolution rate can mean that the process is not diffusion controlled through the film layer around the solid particle. For other solid-to-liquidratios, it was observed that this limit value of the stirring speed for complete suspension of solid particles in the reaction medium remained almost approximately at the same value, 300 rpm, thus the stirring speed value was chosen as 500 rpm to make sure of a good suspension of the particles in the solution while the effect of other parameters was investigated. The effect of the particle size on the dissolution was investigated for the particle size range 0.0822-0.5958 mm. The results obtained from these experiments are
3. Results and Discussions
3.1. Dissolution Reactions. Stibnite gives the following equilibrium reaction in HC1 solutions (Gilreath, 1954)
+
Sb2S3(s) 6HCUaq)
2SbC13(aq)+ 3H2S(g) (1)
Depending upon the HC1 concentration, the following reactions take place in the system
+
Sb2S3(s) 8HCUaq) B[SbCl,l-(aq) Sb2S,(s)
+ 12HCKaq) = 2[SbC1J3(aq)
+ 3H2S(g)+ 2Ht(aq)
(2)
+ 3H2S(g)+ 6Hf(aq) (3)
The complexing reactions 2 and 3 result in a shift of the dissolution process to the right, hence increasing the dissolution process. 3.2. Effect of Parameters. The effect of the parameters on the dissolution process was investigated for each parameter using the values given in Table 2. In the experiments, while the effect of one parameter was studied, the value of other parameters was kept constant: the constant values were chosen as 25 "C for
parameter
range
temperature ("C) acid concentration (% wt) particle size (mm)
12, 18, 25, 32,40,60 23.72,27.20, 30.58,33.84 0.5958, 0.423, 0.2979,0.2121, 0.1500,0.0822 100,200,300,500,700,900 0.5/100, 1/100,2/100,4/100
stirring speed (rpm) solid-to-liquid (g/mL)
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3997 I . 00
0.80
0.60 n
v,
X
0,40
0.20 A A A A ~0
oe0.0
Stirring
IS
"
....I 2 1
"
speed / r p m
Figure 2. Effect of stirring speed on dissolution of stibnite. 1.
00
0 .BO
0.60 n ln X
0,10
0.20
0 00 0
10
20
30
LO
50
60
70
time/ min
Figure 3. Effect of particle size on dissolution of stibnite.
shown in Figure 3. This figure shows that the dissolution rate decreases as the particle size increases; a behavior which can be attributed to the increase of surface with the decrease of particle size per amount of the solid. The experiments to observe the effect of the HC1 concentration on the dissolution process, carried out in
the concentration range 20.11-33.84 %, showed that the dissolution rate increases with increasing acid concentration, as seen in Figure 4. This increase in the dissolution with acid concentration can be explained by (a) the increase of the number of H+ ions per volume with the increase of HC1 concentration and therefore the shift of equilibrium reactions 1-3 to the right, (b)
3998 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
0
EO
40
160
260
200
t i me / m'i n
Figure 4. Effect of HCl concentration (wt %) on dissolution of stibnite. I. 00
I-
/ 0.8 0
0.60 0
v,
X
0,h0
00000 12OC 00000 l8OC
0.20
0.00
'
0
I
1
10
1
I
1
1
20
30
40
50
60
70
t i m e / min
Figure 6. Effect of temperature on dissolution of stibnite.
the increase of the dissolution rate due to the removal of HzS because of decreasing solubility of HzS with increasing acidity, and (c) the increase of the dissolution due to complex formation throughout reactions 2 and 3. The effect of temperature on the dissolution process was investigated in the temperature range 12-60 "C.
The results plotted in Figure 5 shows that the increase in the temperature substantially increases the dissolution rate. This behavior can be explained by the increase of the reaction rate constant with temperature expressed by the Arrhenius Law. Moreover, since the solubility of gases decreases with temperature, the removal of HzS formed through the dissolution reactions
Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 3999
k
"0°
0.8 0
0.6 0
-
0,40
-
n
In
X
..... oaaao
oooou
0 20
AAAAa
0.51100 11 100 2 1 100 4 1 100
0 00 0
5
10
I5
20
25
t imelmin Figure 6. Effect of solid-to-liquid ratio in g/mL on dissolution process.
from the reaction medium will shift the occurrence of the reactions 1-3 t o the right side increasing the dissolution process. The investigation of solid-to-liquid ratio on the dissolution was carried out for the solid-to-liquid range 0.005-0.040. The results exhibited in Figure 6 show that decreasing solid-to-liquid ratios are in favor of the the dissolution process, which can be explained by the decrease of solid amount per amount of reagent in the suspension. 3.3. Kinetic Analysis. The experimental results were analyzed by computer with statistical and graphical methods, applying fluid-solid reaction models (Levenspiel, 1.9721, and it was determined that the dissolution kinetics cannot be expressed by any of the fluidsolid reaction models. Therefore, the experimental data were analyzed using pseudo-homogeneous reaction models and it was found that this dissolution process can be expressed by a first-order pseudo-homogeneous reaction model in the form of
dx
-=I41 -x) dt
(4)
where D = particle size, mm; C = acid concentration, w t %; S/L = solid-to-liquidratio, g mL-l; X = conversion fraction; EA = activation energy, J mol-l; t = time, min; and a , b, d = constants. Statistical calculations by simultaneous multiple regression gave the following results for the constants in eq 7
ho = 9.790 x 10-l'
a = -0.908
b = 10.600 d = -0.321
E, = 51919.766 J mol-' Inserting the estimated values into eq 7 gives the following kinetic model -ln(l - x)= 9,790 ~ ~ - 1 0 ~ ~ ~ - 0 . 9 0 8 ~ ~ ~ 1 0 . 6 0 0 ~ ~ ~ ~ - 0 . 3 2 1 e - 6 2 4 4 / ~ t (8)
Integrating this equation gives -ln(l - X ) = kt
Then, eq 5 can be written as
(5)
The plots of -ln(l - X) vs t for various temperature and acid concentration ranges given in Figures 7 and 8, respectively, show the agreement of the experimental results with the model. To express the effect of the parameters it can be assumed that the rate constant of the dissolution process depends on the parameters as follows
As seen from the kinetic model for the dissolution process, eq 8, the most effective parameter is the acid concentration, and then reaction temperature follows it. Since there is no effect of the stirring speed on the dissolution, it was not included in the kinetic model. The particle size is more effective than the solid-to-liquid ratio. The fact that the stirring speed has no effect on the process and that activation energy is higher than 28 000 J mol-l (Jackson, 1986) supports the idea that the process is not controlled by diffusion through the fluid layer around the solid particle. Since the purity
4000 Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 4. 5
4.0
3.5
3.0
-T --
2.5
c
c I
2. 0
1. 5
I. 0
0.5
0.0 10
U
50
30 40 time / m i n
20
60
70
Figure 7. Plot of -ln(l - X ) vs t for different temperatures. 3.0
2.5
2. 0
X
c
-c
I
1. 5
I
1 0
os
0.o 0
25
50
75
100 12 5 time /min
150
175
200
225
Figure 8. Plot of -ln(l - X ) vs t for different acid concentration values.
of the ore is very high, approximately 83 % by weight, and it was observed by microscope that silica particles separated from stibnite by grinding, the formation of
an ash layer around the particle is out of the question, so it is being controlled by diffusion through the ash layer. The statistical calculations showed that the
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4001 I. 0
0.8
0.6
a
L
E
.-
L
0. 4
P,
a 0,
x
0.2
0. 0 0.0
0.4
0.6
0.8
1.0
x p r e d i c t i on Figure 9. The agreement between experimental conversion values and those predicted from the empirical expression.
process data fits t o a first-order pseudo-homogeneous reaction model, not a fluid-solid reaction model controlled by surface reaction, with regression coefficients of 0.9977 vs 0.9874 for the two models, respectively. In some studies of fluid-solid systems on mineral leaching (Ekmekyapar et al., 1993; Kum et al., 1994, Yartagi et al., 1994) the kinetic model was also found to fit to the pseudo-homogeneous models. To test the agreement between the experimental conversion values and the values calculated from the empirical equation, the plot of X e x p vs X p r d was drawn as seen in Figure 9. The agreement between the experimental and calculated values is very good, with a relative mean square of errors of 0.0627 calculated by the equation
where X p r d is the calculated value, xexp the experimental value, and N the number of experimental data, which is 76 for the present case. This empirical model can be applicable for the experimental condition ranges and reactor geometry used in the present study. The study can be extended to a pilot scale work with higher solid-to-liquid ratios and to production of antimony components from the Sb3+containing solution.
4. Conclusions
A kinetic study of the dissolution of stibnite with HC1 solutions was performed in a stirred semibatch reactor. The parameters were chosen t o be temperature, acid concentration, solid-to-liquid ratio, particle size, and sitrring speed. The results showed that the most effective parameter is the acid concentration followed by the temperature. The particle size is more effective than the solid-to-liquid ratio while no effect of stirring speed on the dissolution process was observed, after complete suspension at a critical stirring speed of 300 rpm. The kinetic analysis using fluid-solid reaction models and homogeneous reaction models incorporating graphical and statistical methods proved that the kinetic model best representing the process was a firstorder pseudo-homogeneous reaction model. A n empirical kinetic expression including the parameters used in the study was developed, and this expression can estimate the conversion fraction with a relative mean square of errors 0.0627. The activation energy for the process 51919.77 J mol-I. Nomenclature a constant in eq 6 aq: aqueous b: a constant in eq 6 C: acid concentration, w t % D: particle size, mm d : a constant in eq 6 EA: activation energy, J/mol a:
4002 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 g: gas S L : solid-to-liquid ratio, g/mL s: solid ko: a constant in eq 6 T temperature, K t: time, m i n X: conversion fraction
Literature Cited Ekmekyapar, A,; Ergahan, H.; Donmez, B. Calcination of Magnesite and Leaching Kinetics of Magnesia in Aqueous Carbon Dioxide. Doga: Turk. J . Eng. Environ. Sci. 1993, 1 7 (3), 197204. Furman, N. H. Standard Method of Chemical Analysis, 6th ed.; D. Van Nostrand Comp, Inc.: New York, 1963; pp 92-93. General Directory of Mineral Research of Turkey, Antimon; M T A Bulletin, Ankara, 1985, No. 192, pp 1-22. Gilheart, E. S. Qualitative Analysis; McGraw-Hill Book Comp., Inc.: New York, 1954; pp 198-202. Grosmann, H. Antimony trichloride. Ger. Offen. Pat. 2.511.616,8, 1976. Hamilton, W. R.; Wooley, A. R.; Bishop, A. C. Minerals, Rocks and Fossils; Country Life Books: Twickenham, Middlesex, England, 1987; p 28. Jackson, E. Hydrometallurgical Extraction and Reclamation; Ellis Horwood Ltd.: Chichester, England, 1986; p 46. Khundkar, M. H.; Khorasani, S. S. M. A.; Ahmad, K. R. Action of Carbontetrachloride Vapor on Sulfides. I. Antimony Sulfide and Cadmium Sulfide. Pak. J . Sci. Ind. Res. 1967,lO (3), 15560. Kirk, R. E.; Othmer, D. F. Encyclopedia of Chemical Technology; Interscience Encyclopedia Inc.: New York, 1952; Vol. 1,p 61. KO, I. Y.; Choe, J. S.; Oh, J . H. Leaching Kinetics of Stibnite in Sodium Hydroxide Solution. Taehan Kumsok Hakhoe Chi 1981a, 19 (51, 418-23.
KO, I. Y.; Kim, D. J.; Oh, J. H. Leaching Kinetics of Stibnite in Ferric Chloride Solution. On the Leaching Behavior of Stibnite. 2.; Taehan Kumsak Hakhoe Chi. 1981b, 19 (51, 410-17. Kum, C.; Alkan, M.; Kocakerim, M. M.; Dissolution Kinetics of Calcined Colemanite in Ammonium Chloride Solutions. Hydrometallurgy 1994, 36, 259-268. Levenspiel, 0. Chemical Reaction Engineering, 2nd ed.; John Wiley and Sons: New York, 1972; pp 357-377. Morizot, G.; Winter, J. M.; Barbery, G. Complex Sulphide Ores; Proceedings of the Congress, Rome, Italy, 1980; pp 151-158. Perry, V. H. Chemical Engineers’ Handbook, 6th ed.; McGrawHill Inc.: New York, 1984; pp 3-8. Robert, C.; Weast. CRC Handbook of Chemistry and Physics, 66th ed.; CRC press, Inc.: Boca Raton, FL, 1986; p B-8. Shestitko, V. S.; Titova, A. S.;Sedova, A. M.; Levin, A. I. Kinetics of Antimony Trisulphide and Sodium Antimonate Dissolution in a Sodium Monosulphide Aqueous Solution. Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metull. 1975, No. 6, 58-62. Stewart, R. D.; Mckinley, J. R. High Purity Antimony Trichloride and Antimony Oxide from Stibnite. US. Patent 3,944,653, 1976. Yartagi, A.; Kocakerim, M. M.; Yapici, S.; Ozmetin, C. Dissolution Kinetics of Phosphate Ore in SOz-Saturated Water. Ind. Eng. Chem. Res. 1994,33,2220-2225. Received for review December 13, 1994 Revised manuscript received May 25, 1995 Accepted May 25, 1995@ I39407363
Abstract published in Advance A C S Abstracts, August 1, 1995. @
