Kinetic modeling and experimental design of the sodium arsenojarosite decomposition in alkaline media: Implications Francisco Patiño a , Iván A. Reyes a, ⁎, Mizraim U. Flores a , Thangarasu Pandiyan b , Antonio Roca c , Martín Reyes a , Juan Hernández a a Área Académica de Ciencias de la Tierra y Materiales, Universidad Autónoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo km 4.5, Pachuca, Hidalgo 42081, Mexico b Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, Distrito Federal 04510, Mexico c Facultat de Química, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain abstract article info Article history: Received 2 October 2012 Received in revised form 30 April 2013 Accepted 15 May 2013 Available online 25 May 2013 Keywords: Jarosite Arsenic Decomposition Kinetics Experimental design The alkaline decomposition of a synthetic sodium arsenojarosite sample with formula [Na 0.87 (H 3 O) 0.13 ]Fe 2.50 [(SO 4 ) 1.95 (AsO 4 ) 0.05 ][(OH) 4.45 (H 2 O) 1.55 ] was studied in NaOH and Ca(OH) 2 media. The experimental data on the progressive conversion period are consistent with the spherical particle model with decreasing core and chemical control. Partially decomposed particles observed by SEM confirm the presence of a non-reacting arsenojarosite core, a reaction front and an amorphous ash halo consisting of Fe(OH) 3 with adsorbed AsO 4 , which coincide with the mentioned model. An experimental design was also developed to determine the ef- fects of the variables and their interactions that are directly involved in the decomposition reaction. It was found that T,[OH − ], and [OH − ]∙T interactions are the factors with the greatest influence on the reaction. With the data obtained from the chemical kinetics and experimental design, we proposed a series of equa- tions that satisfactorily describe the decomposition process in function of conversion and time. © 2013 Published by Elsevier B.V. 1. Introduction Jarosite-type ores belong to the supergroup alunite, whose general formula is: MY 3 (ZO 4 ) 2 (OH) 6 , where M_Na + ,K + , Ag + , Rb + ,H 3 O + , Tl + , NH 4 + , Hg 2+ , Pb 2+ ; and Y_Fe 3+ , Al 3+ , Cr 3+ , Cu 2+ , Zn 2+ ; and Z_S(VI), As(V) or P(V). The supergroup alunite consists of three groups of ores (alunite, beudantite and crandallite) that in combina- tion can make more than 40 different compounds (Jambor, 1999). The jarosite-type compounds where site Y is occupied by Fe 3+ and site Z is occupied by S(VI), are of mineralogical interest, and especially of metallurgical interest. Although nine kinds of jarosite can be syn- thesized, only six of these compounds can be found in nature as ores; the most common are the sodium, potassium and hydronium jarosites (Dutrizac and Kaiman, 1975, 1976). Substitution of hydroni- um ions H 3 O + for potassium or sodium in M-site showed that most of the natural jarosites were solid solutions of hydronium jarosite. Fur- thermore, other substitutions have been reported in the trivalent Y-site in synthetic jarosites [Al(III), In(III), Ga(III) and Cr(III)], as well as a complete substitution of the SO 4 2− in Z-site for SeO 4 2− and CrO 4 2− sites (Brophy and Sheridan, 1965; Dutrizac and Kaiman, 1976). From a geological point of view, their origin is related to alteration processes of sulfides and hosting rocks, both supergene and hydrother- mal (Desborough et al., 2010). The formation processes of jarosite-type compounds with a human origin range from the deliberate precipitation in hydrometallurgical processes, mainly in the zinc as a medium to con- trol impurities such as Fe, S, As, Sb, P, Cu, Mn, Ni and Pb (Arregui et al., 1979), to those produced as residues in environments polluted by acid rock drainage (ARD) or acid mine drainage (AMD). It has been previously reported that the jarosite-type compounds can incorporate elements of environmental importance into their structure, such as Pb 2+ , As 5+ , Cr 3+ , Cd 2+ , Hg 2+ ,F − (Dutrizac, 1991; Dutrizac and Chen, 2005; Dutrizac and Jambor, 1987; Dutrizac et al., 1980, 1987, 1996; Gunneriusson et al., 2009). For instance, As 5+ is widely precipitated in jarosite-type compounds (natural and synthetic), and the way it incorporates can influence its mobility and bioavailability in natural or controlled environments. The As in- corporated in the structure might influence the solubility of the jarosite, stabilizing it in a wide range of conditions that are tolerated by pure jarosites. For this reason it is important to know the behavior of this kind of compounds under different environmental conditions, both for the residues produced by the hydrometallurgical industries and for the compounds that are naturally produced, because arsenic or other toxic species contained in these compounds may be released in a bioavailable form into the ecosystems after their dissolution or decomposition. Several studies have been carried out on the decom- position of jarosite-type compounds, whether for industrial interests, such as the recovery of metallic values as Ag and Zn (Patiño et al., 1994, 1998, 2003; Roca et al., 1993, 2006; Salinas et al., 2001), or to know the chemical composition, thermodynamical properties or the way in which they are solubilized (Das et al., 1995; Drouet and Navrotsky, 2003; Drouet et al., 2004; Frost et al., 2005; Majzlan et Hydrometallurgy 137 (2013) 115–125 ⁎ Corresponding author. Tel.: +52 7717172000x2282; fax: +52 7717172000x2109. E-mail address: ivanalejandro2001@hotmail.com (I.A. Reyes). 0304-386X/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.hydromet.2013.05.005 Contents lists available at SciVerse ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet