Eleazar Salinas

Characterization and alkaline decomposition–cyanidation kinetics of industrial ammonium jarosite in NaOH media

Abstract A complete characterization was carried out on a jarositic residue from the zinc industry. This residue consists of ammonium jarosite, with some contents of H3O+, Ag+, Pb2+, Na+ and K+ in the alkaline “sites” and, Cu2+ and Zn2+ as a partial substitution of iron. The formula is: [Ag0.001Na0.07K0.02Pb0.007(NH4)0.59(H3O)0.31]Fe3(SO4)2(OH)6. Some contents of franklinite (ZnO·Fe2O3), gunninguite (ZnSO4·H2O) and quartz were also detected. The jarosite is interconnected rhombohedral crystals of 1–2 μm, with a size distribution of particles of 2–100 μm, which could be described by the Rosin–Rammler model. The alkaline decomposition curves exhibit an induction period followed by a progressive conversion period; the experimental data are consistent with the spherical particle with shrinking core model for chemical control. The alkaline decomposition of the ammonium jarosite can be shown by the following stoichiometric formula: NH 4 Fe 3 ( SO 4 ) 2 ( OH ) 6( s ) +3 OH ( aq ) − →( NH ) 4( aq ) + +3 Fe(OH) 3( s ) +2 SO 4( aq ) 2− . The decomposition (NaOH) presents an order of reaction of 1.1 with respect to the [OH−] and an activation energy of 77 kJ mol−1. In NaOH/CN− media, the process is of 0.8 order with respect to the OH− and 0.15 with respect to the CN−. The activation energy was 46 kJ mol−1. Products obtained are amorphous. Franklinite was not affected during the decomposition process. The presence of this phase is indicative that the franklinite acted like a nucleus during the ammonium jarosite precipitation.

Cyanidation kinetics of argentian jarosite in alkaline media

Abstract The cyanidation of argentian potassium jarosite in alkaline media is characterized by an induction period, followed by a conversion period, as was observed in other synthetic jarosites. The induction period shows an apparent order of 0.7 (NaOH) and 0.2 [Ca(OH) 2 ] with respect to the [OH − ], and an apparent activation energy of 86 kJ mol −1 (NaOH) and 36 kJ mol −1 [Ca(OH) 2 ]. The conversion period is characterized by the presence of a reaction front with an unreacted jarosite core. The decomposition products are amorphous. The process is chemically controlled in both media. In NaOH–CN − , the order of reaction is 0.6 with respect to the [OH − ] and the activation energy is 43 kJ mol −1 . In Ca(OH) 2 –CN − , the order of reaction with respect to [OH − ] is 0.5 and the activation energy is 80 kJ mol −1 . The order of reaction with respect to the [CN − ] concentration is zero in both media. The process is faster than that observed in natural arsenical potassium jarosite from gossan ores (Rio Tinto, Spain). The addition of potassium chloride to the reaction system increases the reaction rate; the order of reaction with respect to the [Cl − ] was 0.13.

Kinetics of alkaline decomposition and cyanidation of argentian ammonium jarosite in lime medium

Abstract The alkaline decomposition of argentian ammonium jarosite in lime medium is characterized by an induction period and a conversion period in which the sulfate and ammonium ions pass to the solution whereas calcium is incorporated in the residue jointly with iron; this residue is amorphous in nature. The process is chemically controlled and the order of reaction with respect to the hydroxide concentration is 0.4; the activation energy is 70 kJ mol−1. Cyanidation of argentian ammonium jarosite in lime medium presents the same reaction rate in the range of 0–10.2 mol m−3 CN−; in this range of concentration, the cyanide process can be described, as in other jarosites, in a two-step process: a step of alkaline decomposition that controls the overall process followed by a fast step of silver complexation. For higher cyanide concentration, the order of reaction with respect to cyanide is 0.65, and kinetic models of control by chemical reaction and diffusion control through the products layer both fit well; the activation energy obtained is 29 kJ mol−1; this is indicative of a mixed control of the cyanidation process in the experimental conditions employed. The process is faster than was observed in ammonium jarosite generated in zinc hydrometallurgy (Industrial Minera Mexico, San Luis Potosi, Mexico); it seems that the reaction rate decreases when the substitution level in the jarosite lattice increases; this behavior is similar to that observed for synthetic potassium jarosite and arsenical potassium jarosite from gossan ores (Rio Tinto, Spain) presented in a previous paper.