Jarosites são compostos que podem sofrer substituições com vários elementos de importância ambiental (tais como As 5+) durante a precipitação. Arsênico integrado na estrutura poderia influenciar a solubilidade da jarosita, potencialmente estabilizando a estrutura em uma ampla gama de condições que são toleradas pela jarosita pura. A reatividade álcali é caracterizada pela remoção de íons sulfato e sódio da rede e a formação de um gel composto de hidróxido de ferro com arseniato adsorvido. As curvas de decomposição mostram um período de indução seguida por um período de conversão. O período de indução é independente do tamanho das partículas e decresce exponencialmente com o aumento da temperatura. O período de conversão é caracterizado pela criação de um halo de hidróxidos em torno de um núcleo de natrojarosita arsênica não reagido. Dados cinéticos são consistentes com o controle químico do processo. A expressão obtida em meio NaOH para as concentrações de [OH − ] que vão desde 3,84 × 10 −3 para 1,08 × 10 −1 mol L −1 é: r 0 /v [1 − (1 − x) 1/3 ] = 3,11 × 10 9 exp(−57,110/RT) [OH − ] 0.7 t. A expressão em meio Ca(OH) 2 para as concentrações de [OH − ] que vão desde 2,21 × 10 −2 para 6,98 × 10 −2 mol L −1 é: r 0 /v [1 − (1 − x) 1/3 ] = 9,22 × 10 11 exp(−48,610/RT) [OH − ] 1.51 t. Jarosites are compounds that can undergo substitutions with several elements of environmental importance (such as As 5+) during precipitation. Arsenic integrated in the structure could influence the solubility of the jarosite, potentially stabilizing the structure under a wide range of conditions that are tolerated by pure jarosite. Alkaline reactivity is characterized by the removal of sulfate and sodium ions from the lattice and by the formation of a gel consisting of iron hydroxides with adsorbed arsenate. The decomposition curves show an induction period, followed by a conversion period. The induction period is independent from the particle size and decreases exponentially as the temperature increases. The conversion period is characterized by the formation of a hydroxide halo around an unreacted arsenical natrojarosite core. The kinetic data are consistent with the chemical control of the process. The expression obtained in NaOH medium for [OH − ] concentrations ranging from 3.84 × 10 −3 to 1.08 × 10 −1 mol L −1 is the following: r 0 /v [1 − (1 − x) 1/3 ] = 3.11 × 10 9 exp(−57.110/RT) [OH − ] 0.7 t. The expression in Ca(OH) 2 medium for [OH − ] concentrations ranging from 2.21 × 10 −2 to 6.98 × 10 −2 mol L −1 is the following: r 0 /v [1 − (1 − x) 1/3 ] = 9.22 × 10 11 exp(−48.610/RT) [OH − ] 1.51 t.

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The widespread use of jarosite-type compounds to eliminate impurities in the hydrometallurgical industry is due to their capability to incorporate several elements into their structures. Some of these elements are of environmental importance (Pb2+, Cr6+, As5+, Cd2+, Hg2+). For the present paper, AsO43- was incorporated into the lattice of synthetic jarosite in order to carry out a reactivity study. Alkaline decomposition is characterized by removal of sulfate and potassium ions from the lattice and formation of a gel consisting of iron hydroxides with absorbed arsenate. Decomposition curves show an induction period followed by a conversion period. The induction period is independent of particle size and exponentially decreases with temperature. The conversion period is characterized by formation of a hydroxide halo that surrounds an unreacted jarosite core. During the conversion period in NaOH media for [OH-] > 8 × 10-3 mol L-1, the process showed a reaction order of 1.86, and an apparent activation energy of 60.3 kJ mol-1 was obtained. On the other hand, during the conversion period in Ca(OH)2 media for [OH-] > 1.90 × 10-2 mol L-1, the reaction order was 1.15, and an apparent activation energy of 74.4 kJ mol-1 was obtained. The results are consistent with the spherical particle model with decreasing core and chemical control.

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ResumenSe ha realizado un estudio sobre la cinética de lixiviación de plata en el sistema S 2 O 3 2--O 2 -Cu 2+ que se encuentra contenida en las escombreras de Dos Carlos del Estado de Hidalgo en México. La concentración de [S 2 O 3 2-] influye considerablemente sobre la velocidad de lixiviación obteniéndose un orden de reacción de 0.98 en el rango de 15.8 a 126.4 gL -1 , logrando recuperaciones de plata en solución del 73%. Se ha analizado también el efecto de la temperatura y la adición de cobre en la cinética. La temperatura tiene una marcada influencia sobre la velocidad global de la reacción en el rango de 15 a 60°C. Se logró disoluciones de plata del 97% a la más alta temperatura estudiada, obteniendo un valor de 43 kJ mol -1 para la energía de activación. La adición de cobre favoreció la velocidad de lixiviación de la plata, obteniendo un orden de reacción de 0.35 respecto a la [Cu 2+ ] y alcanzando un 95% de plata lixiviada.Palabras clave: lixiviación, cinética, orden de reacción, producción de plata, residuos minerales Abstract A study on the leaching kinetics of silver involved in the system S 2 O 3 2--O 2 -Cu 2+ that is found in the tailings of Dos Carlos in the state of Hidalgo in Mexico. The concentration of [S 2 O 3 2-] considerably affects the leaching rate obtaining a value of 0.o98 for the order of reaction in the range of concentrations from15.8 to 126.4 gL -1 of S 2 O 3 2-, achieving silver recovery of 73%. The effect of temperature and of the addition of copper has been also analyzed. Temperature has a remarkable effect over the overall reaction rate in the range from 15 to 60 °C. Solutions of silver of 97% were obtained at the highest temperature of the study obtaining a value of 43 kJ mol -1 for the activation energy. Copper addition favored the leaching rate of silver, obtaining a reaction order of 0.35 with respect to the concentration of [Cu 2+ ] and reaching 95% of leached silver.