# Statistical Modeling of NaCl and FeSO4 Pretreatment Effect on Refractory Copper Ore Leaching
## 1 Introduction
The leaching of manganese ores using organic reducing agents has also been tested. It has been determined that oxalic acid, when utilized at elevated temperatures within an acidic medium, can effectively dissolve a significant portion (up to 98%) of the manganese present in the minerals [17; 18]. [PERSON] et al. emphasized the importance of manganese-containing species. They noted that these species have a non-crystalline structure and a variable composition. This composition mainly includes copper, manganese, iron, aluminum, and silicon. These traits enhance their resistance to dissolution. Similar characteristics can be found in mineral samples, such as marine polymetallic nodules and manganese ores [19; 20; 21]. Concerning the wide variety of mineral species present in ores containing black oxides, [PERSON] proposed the following dissolution reactions [12]. Tenorite (CuO) demonstrates leachability in acidic solutions according to reaction (1), with an equilibrium pH of around 4.5. Dissolution can be accelerated by using hydrochloric acid or increasing the temperature. \[\mathrm{CuO_{(s)}+H_{2}SO_{4(aq)}\rightarrowCuSO_{4(aq)}+H_{2}O_{(l)}} \tag{1}\]  
Hematite (Fe2O3), a mineral, exhibits insolubility in water and virtual insolubility in acidified and cyanide solutions, necessitating a more energetic acid, such as hydrochloric acid, for dissolution. Reaction (2) demonstrated that the reaction involving hematite manifests negative free energy, representing its spontaneous nature. Nevertheless, practical application reveals that the kinetics of this reaction are notably slow. \[\mathrm{Fe_{2}O_{3(s)}+6H^{+}_{(aq)}\rightarrow}2\mathrm{Fe^{+3}_{(aq)}+3H_{2 }O_{(l)}} \tag{2}\]  
Manganic oxide (Mn2O3) is insoluble in water, so it requires a reducing agent for complete dissolution in diluted sulfuric acid. Adding a suitable reducing agent, such as Fe^+2 (reaction (3)), can significantly improve this process. \[ \begin{array}{c}\mathrm{Mn_{2}O_{3(s)}+2H^{+}_{(aq)}\rightarrow}\mathrm{Mn^{+2 }_{(aq)}+MnO_{2(s)}+H_{2}O_{(l)}}\\ \mathrm{MnO_{2}+2 Fe^{+2}_{(aq)}+4H^{+}_{(aq)}\rightarrow}\mathrm{Mn^{+2}_{(aq) }+2 Fe^{+3}_{(aq)}+2H_{2}O_{(l)}}\\ \mathrm{Mn_{2}O_{3(s)}+2 Fe^{+2}_{(aq)}+6H^{+}_{(aq)}\rightarrow}2\mathrm{Mn^{+2 }_{(aq)}+2 Fe^{+3}_{(aq)}+3H_{2}O_{(l)}}\end{array}  \tag{3}\]  
Manganous oxide (MnO) is a compound known for its easy solubility in most acids (reaction (4)). \[\mathrm{MnO_{(s)}+2H^{+}_{(aq)}\rightarrow}\mathrm{Mn^{+2}_{(aq)}+H_{2}O_{(l )}} \tag{4}\]  
Cupric ferrite (CuFe2O4) exhibits high resistance to acid solutions at room temperature under typical leaching conditions. However, enhanced dissolution of this compound can be achieved by elevating the temperature, increasing the acid concentration, or varying the type of acid used, such as by employing hydrochloric acid (HCI). The chemical reaction depicting the dissolution of cupric ferrite is provided below (reaction (5)). \[\mathrm{CuFe_{2}O_{4(s)}+8H^{+}_{(aq)}\rightarrow}\mathrm{Cu^{+2}_{(aq)}+2 Fe^ {+3}_{(aq)}+4H_{2}O_{(l)}} \tag{5}\]  
Copper manganite (CuMn2O4) requires an acidic medium for the leaching reaction. Furthermore, the facilitation of dissolution and kinetics requires the presence of a reducing agent, such as a ferrous ion (Fe^2+). \[\mathrm{CuMn_{2}O_{4(s)}+8H^{+}_{(aq)}+2 Fe^{+2}_{(aq)}\rightarrow}\mathrm{Cu^ {+2}_{(aq)}+2 Fe^{+3}_{(aq)}+2 Mn^{+2}_{(aq)}+4H_{2}O_{(l)}} \tag{6}\]  
The pretreatment of ores is crucial for enhancing the solubility of mineral species. This process primarily involves the addition of concentrated sulfuric acid, a method commonly called \"curing\" in an industrial context. Furthermore, sodium chloride or nitrate ions have been used to pretreat refractory copper ores such as chalcopyrite [22; 23]. [PERSON] et al. have demonstrated that curing is essential for enhancing the leaching of ores containing black copper. The manuscript suggests prolonged pretreatment that involves the use of sulfuric acid and the addition of sodium chloride.