Determination of the Optimum Conditions for the Dissolution of Stibnite in HCl Solutions M. C ¸ opur, † T. Pekdemir,* ,† C. C ¸ elik, ‡ and S. C ¸ olak † Departments of Chemical Engineering and Industrial Engineering, University of Atatu ¨ rk, 25240 Erzurum, Turkey The Taguchi method has been used to determine optimum conditions for the dissolution of stibnite in HCl solutions. Chosen experimental parameters and their ranges were (i) reaction temperature, 25-70 °C; (ii) solid-to-liquid ratio (in weight), 0.1-0.25; (iii) acid concentration (in weight), 23.71-37%; (vi) mean particle size; 0.1061-0.8426 mm; (v) stirring speed, 200- 800 rpm; (vi) reaction time, 5-60 min. The optimum conditions were found to be reaction temperature, 70 °C; solid-to-liquid ratio, 0.125; acid concentration, 37%; mean particle size, 0.1061 mm; stirring speed, 700 rpm; reaction time, 60 min. Under these optimum conditions the dissolution of stibnite was approximately 99%. Next, for the experimental conditions of a time value 15 min less than its optimum value, a value of solid-to-liquid ratio twice its optimum value, and the remaining parameters at their optimum values, it was found that a benefit of 0.023 $/g SbCl 3 could be gained from the cost of acid and electricity against a loss of 7.5 × 10 -4 $/g SbCl 3 as a result of the 1.5% decrease in the Sb recovery due to the changes in optimum conditions. Introduction Antimony trioxide (Sb 2 O 3 ) is largely used in the production of plastics, cable, latex, and flame resistant materials (Morizot and Winter, 1980; Weast, 1986). Sb 2 O 3 consumed in the production of flame resistant materials used in the U.S. car industry forms 57% of the total antimony consumption in the U.S. (MTA, 1985). Antimony is mainly obtained from stibnite (Sb 2 S 3 ), which is a sulfuric mineral, and in small quantities from oxidized ores such as cervantite (Sb 2 O 3 ‚Sb 2 O 5 ), valen- tinite (Sb 2 O 3 ), and kermesite (2Sb 2 S 3 ‚Sb 2 O 3 ) (Dennis, 1974; Kirk and Othmer, 1952). Stibnite has an orthorhombic crystal system and is available in status in lead-grey color, sometimes tar- nished and iridescent, and opaque. It melts readily even in a match flame. Stibnite, the most common antimony mineral, is commonly found with quartz in hydrother- mal veins, as replacement bodies in lime stone, and in hot spring deposits (Hamilton et al., 1981). The production of Sb 2 O 3 requires production of anti- mony trichloride (SbCl 3 ) with a certain purity (Morizot and Winter, 1980). SbCl 3 is used in the production of other antimony compounds, in the processes of organic chlorination and polymerization, in electroplating, and in the coloration of metals such as iron and zinc (Kirk and Othmer, 1952). The most common SbCl 3 production method is the reaction of chlorine with antimony ores followed by purification processes such as distillation and volatiliza- tion. Stibnite gives the following equilibrium reaction in HCl solutions (Gilreath, 1954) Depending upon the HCl concentration, the following reactions may also take place in the system Because H 2 S is dissolved very little in strong acidic solutions, H 2 S formed at the end of these reactions will totally leave the reaction medium. Thus it will not have a significant effect on the dissolution of Sb 2 S 3 . A wide range of technology exists which may be used to convert H 2 S into a readily usable product, such as Na 2 S, instead of releasing it into the atmosphere. C ¸ opur et al. (1995) proposed a semiempirical equation for the dissolution of stibnite in HCl solutions for small solid-to-liquid ratios (less than 4/100) where x is the transformation fraction, D the particle size, C the HCI concentration, S/L the solid-to-liquid ratio, e the exponential function coefficient, T the reaction temperature, and t the time. Various optimization studies for the dissolution of antimony ores have been found in the literature. In a study done on the extraction of antimony from stibnite, optimum working conditions have been found to be FeCI 3 /Sb 2 S 3 molar ratio, 9; HCI, 1.3 mol; NaCI, 2.5 mol; reaction temperature, 103 °C (Kim and Kim, 1975). In another study, optimum working conditions in the dissolution of concentrate antimony have been found to be 300 g Na 2 S‚9H 2 O/L, 80-90 °C reaction temperature, 1/5 solid-to-liquid ratio, and 2 h reaction time (Djurkovic and Ilic, 1979). In a different study investigating working conditions for the leaching of sulfur concen- trates in alkali glycerine solution for antimony produc- tion, optimum leaching conditions have been found as 150-200 g NaOH/L, 150-200 g C 3 H 8 O 3 /L, 1.10 solid- † Department of Chemical Engineering. ‡ Department of Industrial Engineering. Sb 2 S 3(s) + 6HCl (aq) T 2SbCl 3(aq) + 3H 2 S (g) (1) Sb 2 S 3(s) + 8HCl (aq) T 2[SbCl 4 ] - (aq) + 3H 2 S 9(g) + 2H + (aq) (2) Sb 2 S 3(s) + 12HCl (aq) T 2[SbCl 6 ] 3- (aq) + 3H 2 S (g) + 6H + (aq) (3) -ln(1 - x) ) 9.79 × 10 -10 (D) -0.908 (C) 10.6 (S/L) -0.321 e -6244/T t (4) 682 Ind. Eng. Chem. Res. 1997, 36, 682-687 S0888-5885(96)00258-8 CCC: $14.00 © 1997 American Chemical Society