**Alvaro Aracena 1,\*, Javiera Pino <sup>1</sup> and Oscar Jerez <sup>2</sup>**


Received: 22 May 2020; Accepted: 15 June 2020; Published: 24 June 2020

**Abstract:** 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)4 <sup>2</sup>+. 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.

**Keywords:** malachite; carbonate; leaching; ammonium hydroxide; heterogeneous model

#### **1. Introduction**

#### *1.1. Consumption of Sulfuric Acid Due to Impurities*

Copper oxide compounds are often treated using hydrometallurgy, specifically through the use of chemical leaching with acidic leaching (dissolution) solutions composed mainly of diluted sulfuric acid (H2SO4). However, when copper oxides contain a large quantity of carbonates (CaCO3, MgCO3) or hydroxides (Al(OH)3, Ca(OH)2), acid consumption increases enormously, to a level that makes metallurgical treatment economically inviable [1,2]. This consumption is mainly because carbonates as well as hydroxides are quicker to react with sulfuric acid than the copper oxides, because they are very soluble in acids [3,4], while in some cases the copper compounds can contain considerable amounts of carbonate and hydroxide in their crystalline system. This is the case with basic copper carbonates, such as azurite (Cu3(CO3)2(OH)2) and malachite (Cu2CO3(OH)2). This leads to the complication of compounds that excessively consume leaching reagent (H2SO4), making the leaching process

inefficient and hindering copper recovery. There have been different works that have shown excessive consumption of acid with malachite; Bingöl [5] worked with a malachite mineral and obtained high copper recoveries (90%), along with other impurities, generating a consumption of 450 kg of acid per ton of ore. The same author tried to analyze the dissolution kinetics of malachite ore, but unfortunately he could not use a heterogeneous kinetic model (because the chemical reaction was very fast), obtaining a complete dissolution in a very short time caused by the present impurities that excessively consumed sulfuric acid (a group of minerals including pyroxene, quartz, goethite and magnetite, among others). Instead, Nicol [6] worked on the dissolution kinetics of malachite with H2SO4 (0.033 to 0.15 mol/dm3), finding that the kinetics was governed by the chemical reaction on the surface. This could be achieved because he worked with malachite without impurities (i.e., acid consumers). In his work [6], he did not find the consumption of H2SO4 per ton of malachite.

An alternative option for the treatment of copper oxide minerals containing carbonates is to use leaching in an alkaline system, i.e., in an ammonium system. The main objective is to decrease acid consumption so that the process becomes more economically viable. In addition, the use of an ammoniacal system promotes dissolution selectivity as well as the reduction of corrosive attacks. Aracena [7,8] treated copper oxide minerals in an ammonium hydroxide system (NH4OH). The experimental work was conducted using a stirred system with controlled temperature. The oxidized copper compounds such as tenorite (CuO) and cuprite (Cu2O) were of high purity. The results obtained showed that copper can be extracted from tenorite (particle size of 5 μm; 0.45 mol/L (mol/dm3) NH4OH; pH = 10.5; temperature of 298 K; time of 300 min) and cuprite (particle size of 5 μm; 0.10 mol/L (mol/dm3) NH4OH; pH = 10.5; temperature of 318 K; time of 240 min), up to recovery values of 98% and 82%, respectively. The reaction mechanisms established in each study were the following:

$$\text{Ca} \cdot 2\text{CuO} + \text{NH}\_4\text{OH} + 3\text{NH}\_4^+ \rightarrow 2\text{Cu}^{2+} + 4\text{NH}\_3 + 2\text{H}\_2\text{O} + \text{OH}^- \tag{1}$$

$$2\text{Cu}\_2\text{O} + 8\text{NH}\_4\text{OH} + \text{O}\_2 + 8\text{NH}\_3 \to 4\text{Cu(NH}\_3\text{)}\_4^{2+} + 4\text{H}\_2\text{O} + 8\text{OH}^-\tag{2}$$

The kinetic model representing tenorite and cuprite leaching was a chemical reaction on the surface. The activation energies calculated for tenorite and cuprite were 59.0 and 44.36 kJ/mol, respectively.
