Study on Process Mineralogy of the Combined Copper Oxide Ore in Tibet and Acid Leaching Behavior with Calcium Fluoride

Abstract

The Yulong copper deposit in Tibet is a typical porphyry copper deposit, with about 30 million tons of copper oxide ore in the surface layer. However, more than 40% of the copper resources are in a combination state, resulting in an extraction efficiency of only 50% for copper via the hydrometallurgical process. In this study, the process mineralogy of the combined copper oxide ore was systematically investigated and a calcium fluoride-enhanced leaching process is proposed to increase the leaching efficiency of the combined copper ore. The process mineralogy of the combined copper oxide ore was analyzed using various testing techniques, including chemical analysis, X-ray diffraction, and a process mineralogy parameter testing system (Mineral Liberation Analysis). The results revealed that limonite accounted for 86.12% of the sample, and 63.51% of the copper resource existed in the form of combined copper oxide in limonite. However, it is difficult for the uniformly distributed combined copper oxide in limonite to sufficiently make contact with sulfuric acid, which is the leaching agent, resulting in low copper leaching efficiency. The enhanced leaching behavior of the combined copper oxide ores was also investigated, thereby determining effective and economical enhanced leaching conditions. Under optimal conditions, at a grinding fineness ratio of −0.074 mm (accounting for 85%), liquid-solid ratio of 4:1, sulfuric acid concentration of 50 g/L, temperature of 30 °C, CaF₂ dosage of 1% of the ore mass, and leaching time of 4 h, the copper leaching efficiency increased to 60.57%, which was 7.34% higher than that of atmospheric pressure leaching. Finally, the enhanced leaching slag was analyzed using an electron probe microanalyzer. It indicated that fluorine ions can erode the combined copper oxide ore and facilitate the diffusion of hydrogen ions inside the limonite, thereby achieving a strengthening effect.

1. Introduction

Copper is an important industrial metal with wide applications in various industries and it plays a vital role in the nation’s economy [1,2,3]. China has become the largest copper consumer globally, with steady growth in demand and an external dependence on copper of over 70% [4,5]. With the depletion of copper sulfide and high-quality copper oxide ore, it has become urgent to improve the recovery of copper resources from the refractory copper oxide ore [6,7,8]. The Yulong copper deposit in Tibet is a typical porphyry copper mine, with copper reserves ranking second in China. There are four main types of oxidized copper in the Yulong deposit: disseminated porphyry oxidized copper, clay-type oxidized copper, marble-type oxidized copper, and combined copper oxide ore. The total amount of combined copper oxide ore in Yulong, Tibet, stands at 30 million tons, constituting 77% of the total copper oxide ore resources in Yulong.

Yulong copper oxide ore is mainly processed using the wet “leaching-extraction-electrowinning” process. The acid leaching efficiency for combined copper oxide ore is approximately 40%, while the copper recovery rate using conventional sulfide flotation is only 13%. In contrast, the leaching efficiency for the other three copper oxides exceeds 80%. With the depletion of the other three copper oxide resources, enhancing the leaching efficiency of copper from combined copper oxide ore is essential for future wet processing.

At present, the primary treatment methods for copper oxide ore encompass flotation, leaching, roasting, and beneficiation-metallurgy combination process [9,10,11,12,13,14]. The separation and recovery of combined copper oxide remains a focal point of research and a challenging aspect in the flotation separation of copper minerals [15]. The two main flotation methods are sulfide flotation and direct flotation. However, due to the high cost, low recovery, low sulfide separation efficiency, and high reagent dosage, floatation has not been widely employed in the industrial treatment of complex copper oxide ores [16,17]. The leaching method is commonly utilized in the industrial production of complex copper oxide ore [18]. However, acid leaching and ammonia leaching at room temperature and atmospheric pressure have been proven ineffective in treating combined copper oxides [19]. Combined copper oxide is usually encapsulated by gangue minerals, which hinders effective contact with the leaching agent. Presently, some researchers use mechanical activation and ultrafine grinding to introduce numerous cracks and defects in the gangue, creating pathways for the leaching agent and combined copper oxide. Nevertheless, this technology is characterized by high energy consumption and poses challenges for industrial applications [20,21]. Heap leaching, with its cost advantages in treating complex combined copper oxide, has found industrial applications in numerous enterprises [22,23,24]. However, in Tibet, where the Yulong copper mine is situated at an altitude exceeding 4500 m above sea level, issues arise due to the region’s low temperature, with winter lasting up to seven months and the lowest temperatures dropping to −30 °C. This has led to challenges such as winter spraying and heat preservation difficulties, rendering heap leaching challenging for industrial applications. Gai-rong Wang et al. discovered that increasing temperature is beneficial for the leaching of combined copper oxide [25]. However, given the Yulong copper mine’s high altitude, issues such as high evaporation of the hot leaching solution, elevated energy consumption, and limited processing capacity make it difficult to realize the industrial application of high-temperature and high-pressure processes. Therefore, it is urgent to identify a new process suitable for the development of the Yulong combined copper oxide ore mine in Tibet. It has been reported that, under acidic conditions, because of the small ionic radius of the fluorine ion, it has a strong permeability and can digest minerals containing silicon and iron [19]. Some researchers have found that fluorine can enhance the destruction of alum-bearing mineral structures during the leaching process, and the leaching rate of Vanadium can be improved by adding fluorides (such as NaF and CaF₂) [26,27]. Inspired by this, our research group proposed to use fluoride as a leaching aid to enhance the leaching of copper in combined copper oxide ore under atmospheric pressure.

This paper presents a process mineralogy study on combined copper oxide ore in Yulong, Tibet, aiming to address the low leaching efficiency of oxidized copper ore under atmospheric pressure. A novel approach using calcium fluoride as a leaching aid to enhance sulfuric acid leaching of combined copper oxide ore is introduced. The study explores the penetration effect and strengthening mechanism of calcium fluoride on combined copper oxide ore in diluted sulfuric acid solution, while examining various factors influencing copper leaching efficiency. This enhanced leaching with calcium fluoride offers a new direction for advancing hydrometallurgical copper extraction technology, particularly in Tibet, and holds promising application prospects.

2. Experiment

2.1. Material and Reagent

The combined copper oxide ore used in the experiment was sourced from Yulong Copper Co., Ltd., Xizang, China. After undergoing processes of drying, crushing, and grinding, the ore was ground to a particle size of 0.074 mm. Its chemical composition and phase composition were characterized through X-ray fluorescence analysis (XRF) and X-ray diffraction (XRD). The results, detailed in

Table 1. Main chemical composition of combined copper oxide ore.

Element	O	Cu	Fe	Al	Si	Mn	Ca	Other
Content (%)	40.10	1.97	40.64	2.92	8.80	0.09	1.28	4.20

and Figure 1, show that the ore contained 40.64% iron and 1.97% copper, with limonite being the main phase, along with small amounts of calcite and quartz.

Sulfuric acid (98% purity, Sinopharm) was obtained from Hunan Huihong Reagent Co., Ltd., Changsha, China. Ethylene diamine tetraacetic acid (99.99% purity, Macklin), ethylenediaminedisuccinic acid (98% purity, Macklin), and diethylenetriamine pentaacetic acid (99% purity, Macklin) were purchased from Hunan Hongjie Chemical Technology Co., Ltd., Changsha, China and Guangzhou Keqi Chemical Technology Co., Ltd., Guangzhou, China, respectively. Sodium fluoride (98% purity, Boer), calcium fluoride (98% purity, Aladdin), and ammonium fluoride (96% purity, Aladdin) were obtained from Hunan Dingkan Chemical Reagent Management Co., Ltd. and Changsha Kerui Chemical Glass Instrument Co., Ltd., Changsha, China. Tap water was used in all experimental procedures.

2.2. Experimental Method

Under normal pressure conditions, 100 g of ore was crushed and added to a 1000 mL beaker, along with a certain mass of sulfuric acid solution and leaching reagents (the reagents used in the experimental are chemically pure reagents).The stirring speed and leaching temperature were controlled with a quadruple stirrer. At the end of the reaction, the slurry was quickly pumped out with a vacuum pump, solid-liquid separation was carried out, and the filtrate was sent for analysis and testing. The process flow diagram is shown in Figure 2. The leaching slag was washed with tap water to remove the residual metal ions in the slag. After washing, the leaching slag was dried by an electric blast drying oven (80 °C) and then sent for analysis and detection after sample preparation. The contents of copper and iron in the samples before and after leaching were determined by chemical titration analysis. According to the content of copper and iron in the filtered slag, the leaching efficiency ($\eta$) of copper and iron was calculated according to Formula (1):

$$\eta =\frac{RW-R_t{W}_t}{RW}\times 100\%$$

(1)

In the formula, R is the metal grade of the raw ore, %; W is the dry weight of the raw ore, kg; R_t is the metal grade of the leaching slag, %; and W_t is the dry weight of leaching slag, kg.

Figure 2. Leaching process.

Figure 2. Leaching process.

2.3. Mineral Liberation Analysis (MLA) and Scanning Electron Microscopy (SEM)

In this study, FEI Quanta 600 (FEI Company, Hillsboro, OR, USA) was used for scanning the powder samples. Mineral Liberation analysis (MLA) can, using automatic scanning and energy spectrum analysis, accurately identify mineral species and determine content, and has the advantages of a high degree of automation, accuracy, and reliable measurement data. Scanning Electron Microscopy (SEM) is widely used to observe the morphology and composition of the surface ultrastructure of various solid materials. It has the advantages of abundant types of measurable samples, almost no damage to or pollution of the original samples, and the simultaneous acquisition of morphology, structure, composition, and crystallographic information. In this study, MLA and SEM were used to obtain relevant information, such as the mineral composition, intercalation relationship, and occurrence state of copper.

3. Process Mineralogy Analysis

The process of using mineralogy parameter testing (Mineral Liberation Analysis) systems and technology can quickly and accurately determine the mineral composition and content, the dissociation degree of mineral monomers, and their hyphenated mineral symbiosis characteristics, and process product particle size analysis, etc., which can satisfy the need for rapid analysis of ore selectivity in production and the evaluation of problems of the production process, etc. Therefore, MLA was performed on the combined copper oxide ore to determine its mineral composition.

Table 2. Mineral composition and content of combined copper oxide ore.

Mineral	Content (%)
Limonite	86.12
Kaolinite	8.29
Chrysocolla	0.41
Calcite	1.85
Anorthose	0.10
Quartz	1.41
Potassium-feldspar	0.19
Jarosite	0.22
Malachite	0.14
Biotite	0.51
Pyroxene	0.04
Manganite	0.21
Fluorite	0.00
Amphibole	0.04
Diaspore	0.05
Chlorite	0.11
Other	0.31

displays the mineral composition and content of the combined copper oxide ore. It is evident from Table 2 that the primary minerals in the combined copper oxide ore are limonite (86.12%), followed by kaolinite (8.29%), calcite (1.85%), quartz (1.41%), and trace amounts of chrysocolla (0.41%), malachite (0.14%), and ferromanganese oxide (0.21%). The mineral composition is complex and the embedded particle size is fine, which are qualities of typical complex copper oxide ore.

Table 3. Occurrence state of Cu in combined copper oxide ore.

Mineral	Limonite	Kaolinite	Malachite	Biotite	Chrysocolla	Other
Cu content (%)	63.51	7.65	8.58	5.28	14.34	0.64

shows the occurrence state of Cu in the combined copper oxide ore. It can be seen from Table 3 that the combined copper oxide in the combined copper oxide ore is mainly copper-containing limonite, and its copper content accounts for more than 63% of the total copper content in the sample, with a small amount of chrysocolla, copper-containing kaolinite-clay, and so on.

Table 4. Results of phase analysis of copper in the combined copper oxide ore.

Copper Phase	Content (%)	Proportion (%)
Free oxidized copper	0.99	50.25
Combined oxidized copper	0.79	40.10
Primary copper sulfide	0.04	2.03
Secondary copper sulfide	0.15	7.62
Total copper	1.97	100

shows the phase distribution of copper in the combined copper oxide ore. It can be seen from Table 4 that the content of free copper oxide in the ore sample which is easily directly leached by sulfuric acid is 0.99%, accounting for 50.25% of the total copper content of the sample. The content of combined copper oxide, which is difficult to leach using sulfuric acid at room temperature and atmospheric pressure, is 0.79%, accounting for 40.10% of the total copper content of the sample.

Figure 3 displays a backscattered electron microscope image of the primary phases in the combined copper oxide ore. As observed in Figure 3a, the predominant phase is limonite, accompanied by small amounts of kaolinite, calcite, quartz, etc. It can be seen from Figure 3b that most of the samples are limonite with relatively uniform copper content, and the copper grade in the limonite is relatively low and evenly dispersed.

Figure 4 shows an elemental scanning image of the sample, which shows the distribution of copper, iron, aluminum, and silicon elements in the combined copper oxide ore. It can be seen from Figure 4 that limonite is closely embedded with kaolinite, and limonite has an irregular porous and honeycomb structure. Kaolinite is a dense clay-like structure, wrapped in the periphery of limonite. Copper is evenly distributed inside the limonite in a low abundance state, and there is no obvious enrichment phenomenon, which makes it difficult for copper to make contact with the leaching agent, sulfuric acid, and to be directly leached, which is also the root cause of the low leaching efficiency of combined copper oxide ore. On the other hand, because the surrounding copper is adsorbed by limonite and evenly distributed on the surface and inside of limonite in the form of fine particles, if the direct acid leaching needs to dissolve limonite first, it will consume a large amount of acid and increase the production cost, and the excessive content of iron in the solution will adversely affect the subsequent extraction, which will increase the cost of later purification and iron removal. Therefore, it is difficult to economically and effectively recover copper resources from combined copper oxide ore by acid leaching at normal temperatures and pressures. New processes and methods are needed to effectively leach copper from combined copper oxide ore.

4. Results and Discussion

4.1. Leaching Behavior of Acid Leaching at Normal Pressure

In the context of normal pressure acid leaching behavior, the influence of sulfuric acid concentration, grinding fineness, leaching time, leaching temperature, and liquid-solid ratio on the leaching efficiency of copper and iron in combined copper oxide ore was examined individually. The leaching efficiencies of copper and iron are presented in Figure 5.

Figure 5a illustrates that the copper leaching efficiency was not affected significantly by sulfuric acid concentration, showing a gradual increase with elevated sulfuric acid concentrations. At a sulfuric acid concentration of 60 g/L, the copper leaching efficiency reached 54.75%. Further increases in sulfuric acid concentration led to a stabilization of the copper leaching efficiency but with a gradual increase in the iron leaching efficiency. The copper leaching efficiency was 53.23% at a sulfuric acid concentration of 50 g/L. Consequently, considering a comprehensive perspective, a sulfuric acid concentration of 50 g/L was chosen for normal pressure acid leaching.

Figure 5b illustrates that the copper leaching efficiency was not affected greatly by the proportion of grinding fineness −0.074 mm ratio, and the copper leaching efficiency increased slowly and then decreased with the increase of the proportion of grinding fineness −0.074 mm ratio. When the grinding fineness of −0.074 mm ratio was 70%, the copper leaching efficiency was 50.23%. When the grinding fineness of −0.074 mm ratio was 85%, the copper leaching efficiency was 53.23% and the iron leaching efficiency was 2.24%. At this time, we continued to increase the grinding fineness, and when the proportion of grinding fineness of −0.074 mm ratio was 90%, the copper leaching efficiency appeared to decline to 52.54%, and the iron leaching efficiency was 4.65%, which may be due to the finer grinding fineness, resulting in the exposed iron reacting with sulfuric acid first. Therefore, the proportion of grinding fineness −0.074 mm ratio was selected as 85% in normal pressure acid leaching.

Figure 5c illustrates that the copper leaching efficiency was not affected significantly by time when the leaching time was less than 10 h, and the copper leaching efficiency increased slightly with the increase of leaching time. When the leaching time was 10 h, the copper leaching efficiency was 55.63%. When the leaching time was 4 h, the copper leaching efficiency was 53.23%. When the stirring time was increased by 6 h, the copper leaching efficiency only increased by 2.4%. Therefore, considering comprehensively, the leaching time was selected as 4 h in acid leaching under normal pressure.

Figure 5d illustrates that the copper leaching efficiency gradually increased with the increase of temperature within 60 °C, but this increase was not significant, and that the increase was relatively large at 80 °C. The copper leaching efficiency at 80 °C was 60.12%, and the iron leaching efficiency was 6.23%; and the copper leaching efficiency at 30 °C was 53.23%, and the iron leaching efficiency was 2.24%. However, considering the high cost of heating in Tibet, and that the excessive iron in the solution will adversely affect the subsequent extraction, the temperature of 30 °C was selected for the normal pressure acid leaching.

It can be seen from Figure 6 that the leaching rate of copper and iron increased slowly with the increase of the liquid-solid ratio. However, if the liquid-solid ratio is too large, the load of leaching and liquid-solid separation equipment or the loss of the leaching agent will increase. As such, and also considering the rapid evaporation of water in Tibet, the liquid-solid ratio was selected as 4:1.

4.2. Effect of Type of Leaching Aid Agent on Copper Leaching Efficiency

Under normal pressure acid leaching conditions, the reinforcing impact of incorporating different leaching agents on the copper leaching efficiency in combined copper oxide ore was investigated. A dosage of 1.0% of the ore mass was used for sodium fluoride, calcium fluoride, and ammonium fluoride, while a dosage of 0.05 mol/L was applied for EDDS, EDTA, and DTPA. Figure 7 displays the obtained copper leaching efficiency.

Figure 7 reveals that chelating agents EDDS and EDTA exhibited no notable impact on the copper leaching efficiency, while DTPA demonstrated a modest strengthening effect, enhancing the copper leaching efficiency from 53.23% to 55.29%—a 2.06% improvement, though not distinctly pronounced. The reinforcing effect of fluoride on the copper leaching efficiency was more evident. Notably, ammonium fluoride demonstrated the most significant strengthening effect, elevating the copper leaching efficiency from 53.23% to 61.96%—an increase of 8.73%. Sodium fluoride and calcium fluoride followed closely, with increases of 8.01% and 7.34%, respectively. Given the comparable impact of the three fluorides on the copper leaching efficiency and considering factors such as comprehensive industrial applicability and economic benefits, calcium fluoride was chosen as the leaching aid agent for this experiment.

4.3. Effect of Calcium Fluoride Dosage on the Leaching Efficiency of Copper and Iron

The strengthening effect of the dosage of calcium fluoride on the leaching efficiency of copper and iron in combined copper oxide ore was investigated. The leaching efficiency of copper and iron obtained is shown in Figure 8.

As can be seen from Figure 8, with the increase of calcium fluoride dosage, the copper leaching rate initially increased quickly, then slowly, and finally reached equilibrium. As the calcium fluoride dosage rose from 0% to 3.0%, the copper leaching efficiency elevated from 53.23% to 63.06%, marking a 9.83% increase. Specifically, at a calcium fluoride dosage of 1.0%, the copper leaching efficiency reached 60.57%, a significant 7.34% increase, with an associated iron leaching efficiency of 3.45%. If the amount of calcium fluoride continued to increase, the copper leaching efficiency increased slowly and the iron leaching efficiency gradually increased. The amount of calcium fluoride was selected as 1.0% in this experiment.

4.4. Leaching Behavior of Calcium Fluoride in Enhanced Acid Leaching

For the enhanced acid-leaching behavior with calcium fluoride, the influences of sulfuric acid concentration, grinding fineness, leaching time, and leaching temperature on the leaching efficiency of copper and iron in combined copper oxide ore after the addition of calcium fluoride were individually examined. The corresponding leaching efficiencies of copper and iron are illustrated in Figure 9.

Figure 9a illustrates that after adding the leaching aid agent calcium fluoride, the copper leaching efficiency did not change much with the concentration of sulfuric acid and the copper leaching efficiency rose slowly with the increase of sulfuric acid concentration. When the concentration of sulfuric acid was 30 g/L, the copper leaching efficiency was 57.46%; when the concentration of sulfuric acid was 50 g/L, the copper leaching efficiency was 60.7%; and when the concentration of sulfuric acid was 70 g/L, the copper leaching efficiency was 61.62%. When the dosage of sulfuric acid reached 50 g/L, the copper leaching efficiency was very little affected by the concentration of sulfuric acid, so the choice of acidity in the enhanced acid leaching was 50 g/L.

Figure 9b illustrates that after adding the leaching aid calcium fluoride, the leaching efficiency of copper did not change much with the −0.074 mm ratio, and the leaching efficiency of copper increased slowly with the −0.074 mm ratio. When the −0.074 mm ratio accounted for 70%, the leaching efficiency of copper was 57.54%. When the −0.074 mm ratio reached 85%, the copper leaching efficiency was 60.57%, and if the −0.074 mm ratio continued to increase to 90%, the copper leaching efficiency remained unchanged. Therefore, the −0.074 mm ratio was selected as 85% in the enhanced acid leaching process.

Figure 9c illustrates that after adding the leaching aid calcium fluoride, the copper leaching efficiency did not change much with the leaching time and the copper leaching efficiency rose slowly with the increase of the leaching time. When the leaching time was 2 h, the copper leaching efficiency was 58.35%; when the leaching time was 4 h, the copper leaching efficiency was 60.57%; and when the leaching time continued to be increased, the copper leaching efficiency rose slowly. Therefore, the leaching time of 4 h was selected for enhanced acid leaching.

Figure 9d illustrates that after adding calcium fluoride, the leaching efficiency of copper was less affected by the reaction temperature when it was between 20 °C and 60 °C. When the leaching temperature rose to 80 °C, the leaching efficiency of copper was greatly affected by the temperature. When the leaching temperature was 30 °C, the leaching efficiency of copper was 60.57%, and the leaching efficiency of iron was 3.45%; when the leaching temperature was 60 °C, the leaching efficiency of copper was 64.57%; and when the leaching temperature was 80 °C, the leaching efficiency of copper was 67.83%, and the leaching efficiency of iron was 7.24%. However, considering that the high heating cost in Tibet and the high concentration of iron ions in the solution would adversely affect the subsequent extraction, the leaching temperature was selected to be 30 °C for the enhanced acid leaching.

It can be seen from Figure 10 that with the increase of the liquid-solid ratio, the copper leaching rate increased from fast to slow. When the liquid-solid ratio was 3:1, the copper leaching rate was 58.68%; and when the liquid-solid ratio was 4:1, the copper leaching rate was 60.57%. At this point, the copper leaching rate was basically unchanged when the liquid-solid ratio continued to increase. Therefore, the liquid-solid ratio of calcium fluoride-enhanced acid leaching was selected as 4:1.

5. Mechanism Analysis of Enhanced Leaching

According to the test and analysis results of the process mineralogy parameters of the raw ore, it can be seen that the valuable metal copper in the combined copper oxide ore mainly occurs in limonite and is evenly distributed inside the limonite. The process of the copper oxide leaching reaction mainly relies on the dissolution of acid, and sulfuric acid dissolves the valuable metal copper in the mineral into the solution. The main reason for the low leaching efficiency in acid leaching of combined copper oxide ore is that copper is uniformly distributed in limonite and is difficult to contact with the leaching agent, sulfuric acid. In this paper, the authors investigated the mechanism of enhanced leaching of copper from combined copper oxide ore by a sulfuric acid-calcium fluoride system, and the results are shown in Figure 11 and Figure 12.

Figure 11 shows the electron scanning pictures of the calcium fluoride-enhanced leaching slag of the combined copper oxide ore. According to the electron scanning picture of the leaching slag in Figure 11b, it can be seen that the leaching slag after fluoride ion erosion forms a honeycomb structure, the honeycomb structure is more conducive to the contact between the leaching agent sulfuric acid and the copper wrapped in limonite, thereby improving the leaching efficiency. From the surface distribution of the fluorine in Figure 11c, it can be seen that the content of fluorine in the periphery of the particles is higher, indicating that the erosion of fluorine mainly occurs in the periphery, and also reflects a certain erosion migration. It can be seen from the distribution diagram of the copper in Figure 11d that copper is evenly distributed inside the particles, which is different from the distribution of fluorine, indicating that the erosion of fluorine strengthens the leaching of copper. Figure 11 shows the erosion mechanism of fluorine on the combined copper oxide ore, and reveals the strengthening effect of the calcium fluoride leaching aid on the combined copper oxide ore.

Figure 12 illustrates the mechanism for sulfuric acid leaching and the enhanced leaching mechanism with sulfuric acid and calcium fluoride. In Figure 12, the sulfuric acid leaching process involves the migration of hydrogen ions replacing copper ions. Firstly, due to their negatively charged nature and small ionic radius (0.133 nm), fluoride ions can easily penetrate the interior of limonite. This penetration reduces the potential difference caused by the migration of hydrogen ions, facilitating the diffusion of hydrogen ions within limonite. This process strengthens the replacement and desorption effect of hydrogen ions on the combined copper oxide in the limonite. Secondly, fluoride ions, in the presence of sulfuric acid, generate hydrofluoric acid. This acid, along with silicate minerals, forms fluorosilicates, exerting a potent corrosive effect on silicate minerals like kaolinite. This action releases copper ions adsorbed in kaolinite.

Table 5. Copper phase analysis results of raw ore, atmospheric acid leaching, and calcium fluoride leaching slag.

Copper Phase	Raw Ore		Atmospheric Acid Leaching Slag		Calcium Fluoride Leaching Slag
	Content (%)	Proportion (%)	Content (%)	Proportion (%)	Content (%)	Proportion (%)
Free oxidized copper	0.99	50.25	0.06	6.32	0.06	7.59
Combined oxidized copper	0.79	40.10	0.79	83.15	0.63	79.76
Primary copper sulfide	0.04	2.03	0.04	4.21	0.04	5.06
Secondary copper sulfide	0.15	7.62	0.06	6.32	0.06	7.59
Total copper	1.97	100	0.95	100	0.79	100.00

presents a comparative analysis of copper phases in combined copper oxide ore, normal pressure acid leaching slag, and calcium fluoride-enhanced leaching slag. The table reveals that the content of combined copper oxide in the raw ore was 0.79%. Notably, after sulfuric acid leaching, the content of combined copper oxide remained at 0.79%, indicating the ineffectiveness of sulfuric acid in leaching combined copper oxide under normal pressure. In contrast, the content of combined copper oxide in the calcium fluoride-enhanced leaching slag was reduced to 0.64%. This 0.16% decrease in combined copper oxide content, compared to the raw ore and atmospheric acid leaching, signifies that calcium fluoride can partially leach combined copper in combined copper oxide ore, thereby enhancing the leaching efficiency.

From

Table 6. Occurrence state of copper in calcium fluoride-enhanced leaching slag.

Mineral	Limonite	Kaolinite	Malachite	Biotite	Chrysocolla	Other
Cu content (%)	89.88	9.24	0.09	0.08	0.09	0.62

, it can be seen that 89.88% of the copper in the leaching slag after calcium fluoride strengthening was present in the limonite, which also indicates that there was a large amount of bound copper in the limonite. This bound copper cannot be in contact with the leaching agent, sulfuric acid, which is also the main reason for the low recovery rate of combined copper oxide in Tibet.

6. Conclusions

(1)

The Yulong combined copper oxide ore sample was composed of 86.12% limonite, and most copper was distributed evenly within the limonite. The mineral components were complex and embedded in a fine grain size. Direct acid leaching requires the dissolution of iron, resulting in substantial acid consumption. Therefore, the direct atmospheric pressure acid leaching method is not suitable for the sample.

(2)

The optimal conditions for leaching combined copper oxide ore in Yulong, Tibet were established as follows: sulfuric acid concentration of 50 g/L, temperature at 30 °C, CaF₂ dosage at 1% of the ore mass, leaching time of 4 h, liquid-solid ratio of 4:1, and rotation speed of 200 r/min. Under these conditions, the copper leaching efficiency reached 60.57%, representing a 7.34% improvement compared to acid leaching without calcium fluoride under normal pressure.

(3)

Fluorine ions can migrate and penetrate into limonite particles to erode limonite, which can improve the leaching of copper in combined copper oxide ore, and the erosion of fluorine is more intense in the periphery. In addition, the penetration of fluoride ions into limonite particles can also strengthen the replacement and desorption of hydrogen ions for combined copper oxide in limonite. Moreover, fluoride ions can strongly corrode silicate minerals such as kaolinite in the presence of sulfuric acid, promoting the release of adsorbed copper ions in kaolinite.