# Statistical Study for Leaching of Covellite in a Chloride Media

^{1}

^{2}

^{3}

^{4}

^{5}

^{6}

^{*}

## Abstract

**:**

^{2}= 0.99).

## 1. Introduction

^{2+}in solution. All copper sulfides require the presence of Fe

^{3+}and O

_{2}as oxidizing agents for leaching to occur. Copper sulfide is oxidized by the presence of Fe

^{3+}, and the resulting Fe

^{2+}is reoxidized to Fe

^{3+}by O

_{2}. The redox pair Fe

^{2+}/Fe

^{3+}act as a catalyst in these reactions. The following reactions occur with the main secondary copper mineral, chalcocite, when the temperature is high (Equation (1)), and the sulfur is in the form of sulfate and not of elemental sulfur, as in natural conditions (Equations (2) and (3)) [5]:

_{2}S

_{(s)}+ Fe

_{2}(SO

_{4})

_{3(aq)}= Cu

^{2+}

_{(aq)}+ SO

_{4}

^{2−}

_{(aq)}+ CuS

_{(s)}+ 2FeSO

_{4(aq)}

_{2}S

_{(s)}+ 2Fe

^{3+}

_{(aq)}= Cu

^{2+}

_{(aq)}+ 2Fe

^{2+}

_{(aq)}

_{(s)}+ 2Fe

^{3+}

_{(aq)}= Cu

^{2+}

_{(aq)}+ 2Fe

^{2+}

_{(aq)}+ S

^{0}

_{(s)}.

_{2}S

_{(s)}+ 2Fe

^{3+}

_{(aq)}= Cu

^{2+}

_{(aq)}+ CuS

_{(s)}+ 2Fe

^{2+}

_{(aq)}

_{(s)}+ 2Fe

^{3+}

_{(aq)}= Cu

^{2+}

_{(aq)}+ S

^{0}

_{(s)}+ 2Fe

^{2+}

_{(aq)}.

_{2}:

_{2}S

_{2(s)}= CuS

_{2(s)}+ Cu

^{2+}

_{(aq)}+ 2e

^{−}

_{2(s)}= Cu

^{2+}

_{(aq)}+ 2S

^{0}

_{(s)}+ 2e

^{−}.

_{2}is only achieved from media with high chloride concentrations or high potentials [25]. Copper leaching processes in a chloride media are especially adequate for leaching non-ferrous minerals such as chalcocite, djurleite, digenite, and covellite, since in these cases, the leaching solutions contain low levels of dissolved iron [3].

## 2. Materials and Methods

#### 2.1. Covellite

#### 2.2. Reagent and Leaching Tests

#### 2.3. Experimental Design

_{2}SO

_{4}concentrations variables on leaching covellite [37,38,39,40]. An experimental design was carried out considering three levels per factor, resulting in a total of 27 samples [41]. The fit of the multiple linear regression model was generated in the statistical software Minitab 18 (version 18, Pennsylvania State University, State College, PA, USA), studying the linear and quadratic effects and the interactions of the factors considered in the study [42], as shown in Equation (9).

_{1}, x

_{2}, and x

_{3}are time, chloride, and H

_{2}SO

_{4}concentration variables, respectively. Table 3 shows the parameters used in the experimental model, and Equation (10) shows the transformation between the real values (Z

_{i}) and coded values (X

_{i}) of the experimental design.

_{high}and Z

_{low}are respectively the highest and lowest levels of each variable [43].

^{2}, R

^{2}

_{adj}, and p-values statistics were used to indicate whether the model obtained is adequate to describe the dependent variable under the sampled domain. The R

^{2}statistics measures the proportion of total variability that is explained by the model, the predicted R

^{2}statistic determines the performance of the model predicting the response, and finally, the p-values indicate whether there is a statistically significant association between the dependent variable and a determined independent variable [43].

## 3. Results

#### 3.1. ANOVA

_{2}SO

_{4}} and {Chloride, H

_{2}SO

_{4}} and the curvature of time variable contribute to explain the variability of the model.

_{2}SO

_{4}.

_{2}SO

_{4}concentration and the interactions of time–H

_{2}SO

_{4}concentration and of chloride–H

_{2}SO

_{4}concentrations affected the Cu extraction rate.

^{2}(0.9945) and R

^{2}

_{adj}values (0.9925). The ANOVA indicates that all the factors influence the Cu extraction from CuS, as indicated in the F statistic, where F

_{reg}(371.42) > F

_{T,95%}confidence level F

_{7,19}(2.543). Additionally, the p-value was lower than the significance level, which indicates that the multiple regression is statistically significant.

^{2}and R

^{2}

_{pred}reduces the possibility that the model is over-adjusted, and the leaching, chloride, and H

_{2}SO

_{4}concentrations, and the interactions of time–H

_{2}SO

_{4}and chloride–H

_{2}SO

_{4}are the most critical factors in explaining the process.

#### 3.2. Effect of Chloride Concentration

_{2}is possible with any chloride concentration, but the oxidation of CuS

_{2}is only possible with very high potential or high chloride concentrations [25].

## 4. Conclusions

- The linear variables of time and chloride concentration have the greatest influence on the model.
- Under ambient conditions of pressure and temperature, H
_{2}SO_{4}concentration–time and chloride concentration–time have significant effects on copper extraction kinetics from covellite. - The ANOVA analysis indicates that the quadratic model adequately represents copper extraction, which was validated by the R
^{2}value (0.9945). - The highest copper extraction rate at ambient temperature of 71.2% was obtained with a low sulfuric acid concentration (0.5 M), high level of chloride (100 g/L), and extended leaching time (600 h).

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Experimental contour plot of Cu extraction versus time and chloride (

**a**), time and H

_{2}SO

_{4}concentration (

**b**), and chloride concentration and H

_{2}SO

_{4}concentration (

**c**).

**Figure 3.**Interactions of time–chloride (

**a**), time–H

_{2}SO

_{4}concentration (

**b**), and chloride–H

_{2}SO

_{4}(

**c**) on Cu extraction.

Research Title | Dissolution Agents | Parameters Evaluated | Ref. | Maximum Cu Extraction (%) | Type of Covellite |
---|---|---|---|---|---|

The kinetics of leaching covellite in acidic oxygenated sulfate—chloride solutions | HCl, HNO_{3}, NaCl, H_{2}SO_{4} | Temperature, oxygen partial pressure, particle size, stirring speed, and sulfuric acid concentration | [23] | 85% | Synthetic covellite |

The kinetics of dissolution of synthetic covellite, chalcocite, and digenite in dilute chloride solutions at ambient temperatures | HCl, Cu^{2+} and Fe^{3+} | Potential effect, chloride concentration, acid concentration, temperature, dissolved oxygen, and pyrite effect | [24] | >90% | Synthetic covellite |

Element | Cu | S | Ca | O | H |
---|---|---|---|---|---|

Mass (%) | 56.14 | 31.08 | 3.66 | 8.76 | 0.36 |

Experimental Variable | Low | Medium | High |
---|---|---|---|

Time (h) | 48 | 72 | 144 |

Chloride Concentration (g/L) | 20 | 50 | 100 |

H_{2}SO_{4} Concentration (M) | 0.5 | 1 | 2 |

Codifications | −1 | 0 | 1 |

**Table 4.**Experimental configuration and Cu extraction data (at T = 25 °C, Stirring rate = 600 rpm, P = 1 atm).

Exp. No. | Time (h) | Cl (g/L) | H_{2}SO_{4} (M) | Cu Extraction Rate (%) |
---|---|---|---|---|

1 | 48 | 20 | 0.5 | 2.50 |

2 | 48 | 50 | 0.5 | 3.50 |

3 | 48 | 100 | 0.5 | 6.00 |

4 | 48 | 20 | 1 | 3.00 |

5 | 48 | 50 | 1 | 3.63 |

6 | 48 | 100 | 1 | 9.13 |

7 | 48 | 20 | 2 | 3.25 |

8 | 48 | 50 | 2 | 5.50 |

9 | 48 | 100 | 2 | 11.38 |

10 | 72 | 20 | 0.5 | 5.13 |

11 | 72 | 50 | 0.5 | 8.75 |

12 | 72 | 100 | 0.5 | 11.25 |

13 | 72 | 20 | 1 | 5.88 |

14 | 72 | 50 | 1 | 9.25 |

15 | 72 | 100 | 1 | 13.88 |

16 | 72 | 20 | 2 | 6.38 |

17 | 72 | 50 | 2 | 11.63 |

18 | 72 | 100 | 2 | 18.75 |

19 | 144 | 20 | 0.5 | 24.63 |

20 | 144 | 50 | 0.5 | 24.88 |

21 | 144 | 100 | 0.5 | 28.75 |

22 | 144 | 20 | 1 | 26.25 |

23 | 144 | 50 | 1 | 29.75 |

24 | 144 | 100 | 1 | 35.00 |

25 | 144 | 20 | 2 | 28.75 |

26 | 144 | 50 | 2 | 31.25 |

27 | 144 | 100 | 2 | 38.75 |

Source | F-Value | p-Value |
---|---|---|

Regression | 371.42 | 0.000 |

Time | 2624.36 | 0.000 |

Cl | 257.04 | 0.000 |

H_{2}SO_{4} | 105.5 | 0.000 |

Time × Time | 9.7 | 0.006 |

Cl × Cl | 0.56 | 0.466 |

H_{2}SO_{4} × H_{2}SO_{4} | 3.39 | 0.083 |

Time × Cl | 0.81 | 0.379 |

Time × H_{2}SO_{4} | 11.22 | 0.004 |

Cl × H_{2}SO_{4} | 22.6 | 0.000 |

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## Share and Cite

**MDPI and ACS Style**

Pérez, K.; Toro, N.; Saldaña, M.; Salinas-Rodríguez, E.; Robles, P.; Torres, D.; Jeldres, R.I.
Statistical Study for Leaching of Covellite in a Chloride Media. *Metals* **2020**, *10*, 477.
https://doi.org/10.3390/met10040477

**AMA Style**

Pérez K, Toro N, Saldaña M, Salinas-Rodríguez E, Robles P, Torres D, Jeldres RI.
Statistical Study for Leaching of Covellite in a Chloride Media. *Metals*. 2020; 10(4):477.
https://doi.org/10.3390/met10040477

**Chicago/Turabian Style**

Pérez, Kevin, Norman Toro, Manuel Saldaña, Eleazar Salinas-Rodríguez, Pedro Robles, David Torres, and Ricardo I. Jeldres.
2020. "Statistical Study for Leaching of Covellite in a Chloride Media" *Metals* 10, no. 4: 477.
https://doi.org/10.3390/met10040477