Mechanism and Kinetics of Malachite Dissolution in an NH₄OH System

• Main Authors: Alvaro Aracena, Javiera Pino, Oscar Jerez
• Format: Article
• Language: English
• Published: MDPI AG 2020-06-01
• Series: Metals
• Subjects: malachite; carbonate; leaching; ammonium hydroxide; heterogeneous model
• Online Access: https://www.mdpi.com/2075-4701/10/6/833

Copper oxide minerals composed of carbonates consume high quantities of leaching reagent. The present research proposes an alternative procedure for malachite leaching (Cu₂CO₃(OH)₂) through the use of only compound, ammonium hydroxide (NH₄OH). Preliminary studies were also carried out for the dissolution of malachite in an acid system. The variables evaluated were solution pH, stirring rate, temperature, NH₄OH concentration, particle size, solid/liquid ratio and different ammonium reagents. The experiments were carried out in a stirred batch system with controlled temperatures and stirring rates. For the acid dissolution system, sulfuric acid consumption reached excessive values (986 kg H₂SO₄/ton of malachite), invalidating the dissolution in these common systems. On the other hand, for the ammoniacal system, there was no acid consumption and the results show that copper recovery was very high, reaching values of 84.1% for a concentration of 0.2 mol/dm³ of NH₄OH and an experiment time of 7200 s. The theoretical/thermodynamic calculations indicate that the solution pH was a significant factor in maintaining the copper soluble as Cu(NH₃)₄²⁺. This was validated by the experimental results and solid analysis by X-ray diffraction (XRD), from which the reaction mechanisms were obtained. A heterogeneous kinetic model was obtained from the diffusion model in a porous layer for particles that begin the reaction as nonporous but which become porous during the reaction as the original solid splits and cracks to form a highly porous structure. The reaction order for the NH₄OH concentration was 3.2 and was inversely proportional to the square of the initial radius of the particle. The activation energy was calculated at 36.1 kJ/mol in the temperature range of 278 to 313 K.