Abstract In this research work, novel amine-based leaching systems monoethanolamine (MEA) and ethylamine (EA) were studied from an electrochemical approach to improve the understanding of the dissolution reaction mechanism of precious metals, i.e., gold and silver. The effect of MEA and EA concentration on the oxidation rate of precious metals is presented. The study is complemented with electrochemical techniques such as: open circuit potential technique (OCP), linear voltammetry (VL) and chronoamperometry. The results show that these amines have a great potential in dissolving Au and Ag since average current densities of 0.008 and 0.013 A/cm2 are reached using the MEA, and 0.023 and 0.029 A/cm2 using the EA, respectively. Furthermore, the results indicate that both MEA and EA could decrease the environmental impact involved in hydrometallurgical processes.

Resumen En este trabajo de investigación se estudiaron, desde un enfoque electroquímico, los sistemas de lixiviación novedosos a base de aminas; la monoetanolamina (MEA) y la etilamina (EA), para mejorar el entendimiento del mecanismo de reacción de la disolución de los metales preciosos, i.e., el oro y la plata. Se presenta el efecto de la concentración de la MEA y la EA, en la velocidad de oxidación de los metales preciosos. El estudio es complementado por medio de técnicas electroquímicas tales como: Técnica del potencial de circuito abierto (OCP), voltametría lineal (VL) y cronoamperometrías. Los resultados demuestran que estas aminas tienen un gran potencial al disolver el Au y la Ag, debido a que se alcanzan densidades de corriente promedio de 0.008 y 0.013 A/cm2 empleando la MEA y 0.023 y 0.029 A/cm2 utilizando la EA, respectivamente. Además, los resultados dictaminan que tanto la MEA como la EA podrían disminuir el impacto ambiental que conllevan los procesos hidrometalúrgicos.

ABSTRACT Leaching of silver from minerals has been studied for decades. Recently, research has been focused on the substitution of cyanide (because of its high toxicity) from hydrometallurgical operations. In this sense, the development of alternative-green leaching solutions for the recovery of metallic silver from minerals and urban mines is necessary. In this research, a thermodynamic-kinetic study of the alternative system “monoethanolamine (MEA)-ammonium sulfate-cupric sulfate” was performed for the dissolution of metallic silver, varying the pH and the cupric ion concentration at room temperature. Furthermore, this research aims to evaluate the silver dissolution behavior at a pH lower than 10.2, elucidating the rate determining step of the silver dissolution process at these conditions of pH and cupric ion concentration. Thermodynamic results confirm that the MEA and NH3 can complex silver generating the species: Ag(MEA)2+, AgMEA-, Ag(NH3)2+, AgNH3+ respectively. It was evaluated the effect of varying the pH (8.8, 9.9 and 9) and the cupric sulfate concentration (0.1, 0.15 and, 0.2 M at pH 9) on the kinetics of silver dissolution. The results revealed, the possibility to leach a 39% of silver using a leaching solution containing: 0.15 M cupric sulfate, 0.05 M MEA and 0.4 M ammonium sulfate at pH 9. The kinetic analysis showed that the silver dissolution is controlled by the diffusion of fluid species through a porous layer composed of copper oxides, which was ratified by SEM and EDS analysis. Finally the present paper, open the possibility to optimize the molar ratio MEA:NH3 to maximize the silver dissolution kinetics.

Abstract In this work, the phosphatizing process of steel is investigated using open circuit potential and Tafel curves as well as scanning electron microscopy and energy dispersive X-ray spectroscopy. The results reveal that a pH of 2.57 in the phosphatizing solution promotes the dissociation of phosphoric acid which assists the formation of the manganese tertiary salt (Mn3(PO4)2), which is deposited on the substrate. It was also observed that an increase in the temperature from 25 to 90°C and the presence of HNO3 as catalyst enhances the manganese phosphatizing kinetics. On the other hand, the generation of iron phosphates and oxides is predominant at a pH of 1 and 90°C. These observations are supported by species distribution and Pourbaix thermodynamic diagrams.

Resumen En este trabajo, el proceso de fosfatado de acero es investigado usando potencial de circuito abierto y curvas Tafel, así como microscopía electrónica de barrido y espectroscopia de energía dispersiva de rayos-X. Los resultados revelan que un pH de 2.57 en la solución de fosfatizado promueve la disociación del ácido fosfórico, que ayuda a la formación de la sal de fosfato de manganeso terciaria (Mn3(PO4)2), que se deposita sobre el sustrato. También se observó que un aumento en la temperatura desde 25 a 90°C y la presencia de HNO3 como catalizador mejora la cinética del fosfatizado de manganeso. Por otro parte, la generación de fosfatos de hierro y óxidos es predominante en un pH de 1 y 90°C. Estas observaciones son soportadas con diagramas termodinámicos de distribución de especies y diagramas de Pourbaix.