# Statistical Modeling of NaCl and FeSO4 Pretreatment Effect on Refractory Copper Ore Leaching
## 1 Introduction  
In certain regions of the world, special copper deposits, known as exotic, exist. These deposits are distinct due to their formation and the composition of their mineralogical species. For instance, porphyry copper deposits undergo oxidation when exposed to atmospheric oxygen and water, forming an oxidation zone. This zone is characterized by the oxidation of hypogene sulfide minerals and the formation of secondary mineral assemblages, which can cause acid rock drainage. The formation of this zone is influenced by various factors, including the mineralogy of the hypogene sulfides, the neutralization potential of gangue minerals, and the presence of an unsaturated zone, which is affected by tectonics, climate, erosion, and the incision of the hydrographic network [1, 2]. During the supergene processes, the leachate solution contains elements such as Cu, Mn, and K in solution and can be transported over long distances. These elements then reprecipitatefurther down in the supergene profile, where the conditions are reducing. In the Atacama Desert, Cu is reprecipitated in the oxide zone through carbonates, chlorides, sulfates, oxides, and copper [3; 4]. Exotic copper deposits found in Arizona, New Mexico, and Sonora are metal deposits associated with porphyry Cu deposits. These deposits are usually zoned from ferricrete upgradient and extend into the bedrock leached cap. Moving downgradient, they are seen as black manganese oxides or Cu wad and are eventually found as chrysocolla in the Cu-rich systems [5; 6]. The exotic copper deposits exhibit a wide range of size, with some reaching up to 3.5 million tons of copper. Notable examples of these deposits in northern Chile include Radomiro Tomic, El Tesoro, Spence, Mina Sur, and Lomas Bayas [6]. Researchers have made numerous efforts to categorize black copper oxides, but these have yielded inconsistencies. Before early 1988, copper pitch and copper wad were the only recognized minerals of this type. [PERSON] et al. proposed the term \"black copper silicate\" to refer to both minerals, since they share a similar composition and lack a defined crystalline structure [7]. In the copper mining industry, exotic deposits containing minerals refractory to traditional leaching agents, such as sulfuric acid, are commonly referred to as black copper oxides or black coppers. These oxidized ores consist of copper minerals and mineraloids exhibiting green and black hues, which contain significant levels of manganese. The exotic ores include atacamite, chrysocolla copper wad, copper pitch, and neotocite [8; 9; 10]. It is crucial to emphasize the scarcity of information in the literature regarding the leaching of oxidized minerals containing black copper oxides, despite the significant industrial value of these deposits. This underscores the novelty and potential impact of the proposed investigation, based on the pioneering studies conducted by [PERSON] and [PERSON] [11; 12]. From [PERSON], three of the five leaching media used have been selected, i.e., acid, chloride, and acid-reducing, which have shown great potential for the extraction of copper in mixed Cu-Mn-Fe compounds. From the manuscript by [PERSON] et al., the methodology focused on ore pretreatment has been adopted to reduce the redox potential of the leaching solution. It is essential to recognize that not all manganese-containing species are challenging to leach. For manganese carbonate ores, a combination of sulfuric acid and chloride ions effectively dissolves all the manganese present in the ore [13]. Among the species that are most studied for the leaching of black copper are those that contain manganese. Literature reports frequently discuss the use of reducing agents to enhance the dissolution of manganese from the ores [14]. Several studies have been conducted to dissolve manganese-bearing ores, mainly through pyrometallurgical, hydrometallurgical, and biohydrometallurgical routes [14; 15]. For instance, ferrous sulfate, sulfur dioxide, and sulfite solutions can serve as effective reducing agents [16].