Flotation Separation of Chalcopyrite and Molybdenite by Eco-Friendly Microorganism Depressant Bacillus tropicus

Abstract

In this study, Bacillus tropicus (BT), a non-toxic and eco-friendly microorganism, was employed to substitute traditional inorganic depressants in the flotation separation of copper-molybdenum sulfides. Single mineral flotation tests were performed to examine BT’s impact on the flotation behavior of molybdenite and chalcopyrite. The results indicated that excessive BT inhibited the flotation of both minerals, reducing their recoveries below 40%. At a BT dosage of 2.5 kg/t and pH 9.0, chalcopyrite recovery was 74.10%, while molybdenite recovery was 20.47%, achieving an effective separation of the two minerals. BT’s adsorption mechanism on molybdenite and chalcopyrite was analyzed through contact angle tests, thermogravimetric analysis, and Fourier transform infrared spectroscopy. These analyses revealed that increased BT absorption on molybdenite enhanced its surface hydrophilicity. This research offers a novel perspective on utilizing microorganisms as efficient flotation reagents.

1. Introduction

Molybdenite (MoS₂) is the primary source of molybdenum, which is crucial in modern society due to its extensive use in steel production, agriculture, the electric industry, aerospace, and other sectors [1]. Molybdenite is frequently found with copper sulfides like chalcopyrite (CuFeS₂) and chalcocite (Cu₂S), making their separation challenging because of their similar floatability properties [2,3,4]. Copper–molybdenum flotation separation is an important mineral processing technology primarily used to separate copper and molybdenum from copper–molybdenum ores to obtain high-grade copper concentrate and molybdenum concentrate [5,6].

In industrial applications, copper–molybdenum flotation separation mainly employs priority flotation separation, equal floatability separation, and mixed flotation separation methods [7]. The priority flotation separation method is suitable for treating copper–molybdenum ores with lower copper content. Its advantage lies in ensuring the grade and recovery rate of molybdenum concentrate while also achieving copper recovery, yielding qualified single copper concentrate and molybdenum concentrate. The equal floatability separation method involves two stages of copper ore flotation recovery: the first stage prioritizes the flotation recovery of molybdenite, followed by the separation of the copper-molybdenum concentrate to obtain copper concentrate I and molybdenum concentrate, and the second stage adds a strong collector to the tailings from the first stage to recover copper minerals, obtaining copper concentrate II, which is then combined with copper concentrate I to form the final copper concentrate.

The mixed flotation separation method is currently the most widely used technique, with a process that first floats copper and molybdenum together as a whole to obtain a mixed concentrate, which is then separated into copper concentrate and molybdenum concentrate [8,9]. This method’s advantages include low process costs, stable indicators, and a simple, easily controllable flow. It effectively increases the recovery rate of low-grade ores. However, after mixed flotation during the rough stage, the surface of the mixed concentrate retains collectors and other reagents, reducing the difference in floatability between copper and molybdenum, thus affecting the subsequent separation flotation reagent’s effectiveness. Therefore, deactivation treatment should be performed before separation. Furthermore, the selective inhibition of the mixed concentrate after reagent removal is crucial, as it determines the grade of the concentrate. The key to copper–molybdenum flotation separation lies in selecting appropriate reagents and process conditions to achieve the effective separation and enrichment of copper and molybdenum. By optimizing the flotation process and reagent system, the recovery rate and concentrate grade of copper and molybdenum can be improved, thereby enhancing resource utilization efficiency and economic benefits.

Over the past decades, numerous inorganic and organic reagents have been developed to depress copper sulfides in molybdenite flotation [10,11,12]. Inorganic compounds include Noke’s reagent, cyanide, potassium dichromate, sulfur dioxide, sodium hydrosulfide, and sodium sulfide [4,5,6,7,8,9,10,11,12,13,14,15,16]. These reagents often have disadvantages such as toxicity, volatility, large dosage requirements, and environmental pollution. For example, potassium dichromate, sulfur dioxide, and cyanides are harmful to human health and the environment. Sodium hydrosulfide, sodium sulfide, and Noke’s reagent are corrosive and can damage pipelines and flotation equipment [17]. On the other hand, organic inhibitors typically offer advantages like abundant sources, low costs, and environmental friendliness. Research has focused on discovering new depressants, such as poly (acrylamide-allyl thiourea), pseudo glycolythiourea acid, L-cysteine, and 2,3-disulfanylbutanedioic acid [18,19,20,21]. Despite their potential, these new chalcopyrite depressants continue to encounter challenges such as high consumption, strict operational requirements, limited selectivity and unstable performance. For instance, Zhang et al. [22] proposed a novel and practical workflow that is able to establish ML models to accurately predict the flotation performances of the collectors with diverse skeletons for sulfide minerals (chalcopyrite, galena, pyrite, and sphalerite). Based on rapid performance evaluation and structure design, the development of new and efficient flotation reagents will be accelerated in the future.

The other method of the flotation separation of chalcopyrite and molybdenite is to depress molybdenite and float chalcopyrite. This approach demands highly effective molybdenite inhibitors. However, limited research exists in this area. It is of great significance using microorganisms to change the surface properties of different minerals and promote the efficient separation of different minerals. The application of microorganisms in mineral processing focuses on microbial leaching technology and microbial flocculation for wastewater treatment. Microbial cells interact with mineral surfaces mainly through two mechanisms: one involves microbial cells carrying specific functional groups on their surfaces directly contacting and interacting with minerals [23], while the other involves soluble substances released by lysed and dead microbial cells or surface-active substances produced during normal growth and metabolism, specifically modifying the mineral surfaces [24]. Research on using microorganisms as flotation inhibitors has been widely reported, such as the use of Thiobacillus ferrooxidans to inhibit pyrite in high-sulfur, low-grade lead–zinc ores and Rhodococcus being verified as a selective inhibitor for rhodochrosite in the flotation separation of calcite and rhodochrosite. However, to our knowledge, there is limited research on the use of microorganisms in the flotation separation of chalcopyrite and molybdenite; particularly, studies on inhibiting molybdenite to float chalcopyrite through microbial means are scarce. In this work, we introduced a non-toxic and environmentally friendly microorganism, Bacillus tropicus (BT), to replace the traditional inorganic depressants during the flotation separation of copper–molybdenum sulfides, achieving favorable results. Characterization tests revealed differences in BT’s adsorption on both minerals’ surfaces, offering new insights for developing eco-friendly flotation depressants.

2. Materials and Methods

2.1. Samples and Reagents

Samples of pure chalcopyrite and molybdenite were sourced from Yunnan and Jiangxi, respectively. The raw minerals were subsequently crushed and sieved to obtain size fractions of −74 + 37 µm for X-ray diffraction (XRD) analysis and flotation tests and −2 µm for the Fourier infrared spectroscopy test (FTIR). XRD analysis (Figure 1) indicated that both chalcopyrite and molybdenite had high purity, meeting the criteria for pure mineral flotation tests.

Depressant BT, collector butyl xanthate, and pH regulators H₂SO₄ and NaOH were analytically pure (Xilong Scientific Co., Ltd., Shantou, China). The test water was distilled water.

2.2. Preparation of Bacterial Cultures

The microorganisms cultivated in this study are Bacillus tropicus, using LB medium, which includes liquid medium and solid medium with 1%–2% agar added. The liquid medium can be used for the large-scale cultivation of microorganisms. The solid medium is used for microbial separation, identification, counting, and preservation. The experimental soil was sourced from Dexing Copper Mine in Dexing, Jiangxi, China. After mixing and separating the soil bacteria, the bacterial colonies were placed in a constant temperature incubator for growth using the spread plating method.

2.3. Flotation Tests

Flotation tests were conducted using a 40 mL XFG float-mounted machine (Changchun Machinery Factory, Changchun, China). Each test utilized 3 g of the mineral sample. Initially, the sample and 40 mL of distilled water were mixed and introduced into the flotation cell. The pulp’s pH was adjusted with H₂SO₄ and NaOH before adding the BT solution, followed by a 5 min conditioning period. Next, collector butyl xanthate solution was added with stirring for 3 min. Flotation occurred for 4 min after a 2 min frother application. The flotation concentrates and tailings were collected, dried, and weighed to determine the recovery response. Each experiment was repeated three times, and the final result was the average of these values. The flowsheet of flotation tests is showed in Figure 2.

2.4. Physicochemical Analyses

2.4.1. Contact Angle Test

The sample for the contact angle test is a fully polished block of pure mineral, and it was conducted on a DSA100 contact angle measuring instrument (Kruss, Hamburg, Germany). The sample was treated with ultrasound in deionized water for 5 min before being placed in the prepared BT suspension. After soaking for a period, it was removed and dried naturally. Deionized water was then dropped onto the surface with a special syringe to measure the static contact angle. Once a stable droplet was formed, the contact angle was measured using the five-point fitting method. Each sample was measured three times, and the average value was calculated for further analysis. For subsequent tests, the used samples were re-polished and re-soaked in the solution for varying durations.

2.4.2. TG-DSC Comprehensive Thermal Analysis

TG-DSC comprehensive thermal analysis was performed to compare the adsorption strength of BT on mineral surfaces by measuring weight loss. Specifically, samples were stirred in 40 mL of a 500 mg/L (125 kg/t) BT suspension for 30 min, then washed thorough by deionized water and dried in a vacuum drying oven at 60 °C and −0.1 kPa. The thermal analysis was conducted from 30 °C to 500 °C at a heating rate of 20 °C/min in nitrogen atmosphere.

2.4.3. Fourier Infrared Spectroscopy Test

FTIR tests (Thermo Fisher Scientific, Waltham, MA, USA) were conducted on chalcopyrite, molybdenite, BT suppression, and flotation concentrates of chalcopyrite and molybdenite. A small sample and 100 mg of potassium bromide powder were ground to −2 μm and mixed evenly in an agate mortar. The mixture was then used for the FTIR test. The instrument requires a voltage of 220 V ± 10%, operates at a frequency of approximately 50 Hz, and is conducted at a temperature of 25 °C.

3. Results and Discussion

3.1. Flotation Experiment

Chalcopyrite and molybdenite are sulfide minerals with varying floatability. In conventional flotation, pH has minimal impact. However, the structure and composition of BT create different surface-active sites at various pHs, influencing the flotation behavior of chalcopyrite and molybdenite. As shown in Figure 3a, when pH is below 9.0, chalcopyrite recovery increases with rising pH, while molybdenite recovery decreases. At pH 9.0, chalcopyrite recovery is 74.10%, and molybdenite recovery is 20.47%. The recovery difference in chalcopyrite and molybdenite is up to 53.63%. When pH exceeds 10.0, BT’s inhibitory effect on molybdenite diminishes, stabilizing its recovery between 20% and 30%, while its inhibition on chalcopyrite intensifies. Thus, pH 9 is optimal for separating chalcopyrite and molybdenite using BT as a depressant.

The dosage of BT significantly impacts the flotation of chalcopyrite and molybdenite. As shown in Figure 3b, BT’s effect on both minerals is similar. Increasing BT usage inhibits chalcopyrite flotation, causing its recovery to drop sharply. Molybdenite flotation remains consistently low. At a BT concentration of 2.5 kg/t, chalcopyrite and molybdenite recoveries were 74.10% and 20.47%, respectively. Higher dosages reduce the difference in flotation properties between the two minerals.

The impact of stirring time on the flotation performance of chalcopyrite and molybdenite, inhibited by BT, was studied. As shown in Figure 3c, under all conditions, chalcopyrite exhibits higher flotation recovery compared to molybdenite, suggesting that BT has a stronger inhibitory effect on molybdenite. Additionally, as stirring time increases, the inhibitory effect diminishes, leading to improved flotation recovery for both minerals. Notably, the greatest difference in flotation performance between the two minerals occurs at 5 min.

At low stirring speeds, insufficient bubbles form in the pulp, preventing effective mineral flotation. In Figure 3d, as the stirring speed increases, the recovery rates of chalcopyrite and molybdenite improve significantly. At 1992 r/min, the recovery difference between the two minerals is the greatest. Further increases in speed are not beneficial for their separation.

3.2. Contact Angle Analysis

BT consists of protein and polysaccharide, featuring hydrophilic groups like carbonyl, ether bonds, and ester groups, which enhance the hydrophilicity of mineral surfaces upon adsorption [25]. Figure 4 illustrates the change in contact angle of chalcopyrite and molybdenite with increasing BT suspension action time at various concentrations. As shown in Figure 4, at a BT suspension concentration of 2.5 kg/t, chalcopyrite exhibits optimal flotation performance, unaffected by action time. This stability is evidenced by the consistent high contact angle of chalcopyrite. Conversely, the contact angle of molybdenite decreases with prolonged BT action time, indicating a stronger interaction between BT and molybdenite. Notably, when the agent concentration reaches 25 kg/t, even a brief action time significantly reduces the contact angles of both minerals, diminishing the selective depressant effect of BT.

3.3. TG-DSC Thermal Analysis

BT, a microorganism that decomposes at specific temperatures, can be analyzed for its adsorption on mineral surfaces using thermal analysis. The weight loss of pristine molybdenite and molybdenite treated with BT at decomposition temperatures was measured using TG-DSC thermal analysis. Figure 5a displays the TG-DSC curve of the raw molybdenite sample, while Figure 5b shows the curve of molybdenite after BT treatment. The weight loss of untreated molybdenite from 30 °C to 500 °C is 6.3241% of its original mass. In contrast, treated molybdenite exhibits a weight loss of 22.76649%. This indicates that BT has adsorbed onto the molybdenite surface, increasing its burn loss rate.

3.4. FTIR Analysis

The adsorption states on chalcopyrite and molybdenite surfaces were further analyzed using FTIR testing. As seen in Figure 6, the small double peaks at 2964.05 cm⁻¹ and 2919.80 cm⁻¹ in the BT spectrum correspond to the anti-stretching vibrations of –CH₃ and –CH₂, primarily from sugars, proteins, and lipids [25]. The absorption peak of –COOH appears at 1744.76 cm⁻¹. The peak at 1682.45 cm⁻¹ represents an ester group that forms hydrogen bonds within six-membered ring molecules, replaced by hydroxyl, amino, etc. The peak at 1548.95 cm⁻¹ corresponds to the deformation vibration of protein amide II (C–N–H bending vibration) in –CONH₂. The peaks at 1402.14 cm⁻¹ and 1306.10 cm⁻¹ are absorption peaks of carboxyl –COO–. The peak at 1243.91 cm⁻¹ is for the ester –COOC–, and the peak at 1055.96 cm⁻¹ is for the sulfoxide R–S=O–R’ [26], suggesting that BT cells contain sulfur proteins, polysaccharides, and numerous hydrophilic groups like carboxyl, ester, and sulfoxide, along with hydrophobic group carbon chains.

In Figure 6a, the FTIR absorption peaks of pristine chalcopyrite are at 3843.58 cm⁻¹, 2917.28 cm⁻¹ and 1078.54 cm⁻¹. After the action of BT, the peaks shift slightly to 3846.81 cm⁻¹, 2917.77 cm⁻¹ and 1078.97 cm⁻¹, with no new peaks appearing, indicating minimal BT adsorption on chalcopyrite. In Figure 6b, the FTIR absorption peaks of pure molybdenite are at 1099.41 cm⁻¹, 779.55 cm⁻¹ and 532.25 cm⁻¹. After the interaction with BT, in addition to the above peaks on molybdenite, additional peaks at 3559.56 cm⁻¹, 1628.58 cm⁻¹ and 1527.16 cm⁻¹ emerge. The peak at 3559.56 cm⁻¹ corresponds –OH, likely from hydroxyl compounds attached to protein and carbohydrate macromolecules. The peak at 1628.58 cm⁻¹ is in the stretching vibration region of –C=C–, possibly found in cell polysaccharides. The peak at 1527.16 cm⁻¹ is the absorption peak of –NO₂, typically present in polypeptide chain synthesis and involved in carboxyl activation. These findings suggest increased BT adsorption on molybdenite, enhancing its hydrophilicity and aiding its separation from chalcopyrite.

4. Conclusions

BT is a non-toxic, environmentally friendly, and highly efficient microorganism depressant for molybdenite in chalcopyrite flotation. Its unique properties have been demonstrated through rigorous testing, confirming its effectiveness in the separation process. The flotation results revealed a significant improvement in the purity of molybdenite, showcasing BT’s capability to selectively target and depress chalcopyrite without affecting molybdenite.

Contact angle tests, thermogravimetric analysis, and Fourier transform infrared spectrum analysis provided detailed insights into BT’s mechanism of action. These analyses revealed that BT increases the surface hydrophilicity of molybdenite, which facilitates the efficient separation of molybdenite from chalcopyrite, ensuring a cleaner and more effective flotation process.

This study not only highlights the potential of microorganisms like BT as eco-friendly alternatives to traditional chemical reagents but also opens new avenues for research and application in mineral processing. The findings underscore the importance of sustainable practices in mining and metallurgy, paving the way for greener and more efficient industrial processes.