The Evaluation of Carrageenan as a Novel and Environmentally Friendly Molybdenite Depressant

Abstract

In order to achieve the effective separation of copper-molybdenum in the presence of xanthate and kerosene, carrageenan was explored as a novel environmentally friendly molybdenite depressant in this work. The flotation behavior of molybdenite was studied by micro-flotation tests, and the depression mechanism was investigated through zeta potential, Fourier Transform Infrared Spectroscopy (FTIR) and atomic force microscope (AFM) analysis. The flotation results showed that molybdenite was significantly depressed by carrageenan in the pH range of 6–12 even in the presence of xanthate and kerosene. Zeta potential, FTIR and AFM measurement demonstrated that carrageenan could adsorb strongly on the molybdenite surface and change the surface wettability of molybdenite, thus significantly reducing the floatability of molybdenite.

1. Introduction

Molybdenum is a strategic metal with excellent properties. It is widely used in metallurgy, aerospace, electrical, chemical and other fields due to its high strength, low density, chemical stability, good corrosion resistance and high thermal conductivity [1,2,3]. Molybdenite is the main raw material for molybdenum extraction [4]. Most molybdenite ore is associated with sulfide minerals, especially copper sulfide minerals [5]. According to statistics, nearly 75% of copper and 50% of molybdenum in the world come from copper-molybdenum ores [6,7,8]. Porphyry copper ore is the main type of copper-molybdenum ore, in which copper and molybdenum mainly exist in the form of chalcopyrite and molybdenite [9,10]. These two minerals present similar wettability, which makes it difficult to separate them efficiently without depressants [11,12]. At present, the main strategy for copper-molybdenum separation is to depress chalcopyrite and float molybdenite [13,14]. Cyanide and NaSH are the commonly used chalcopyrite depressants [15,16]. However, cyanide has been limited because of its high toxicity to human health and the environment [14]. NaSH is decomposed easily under acidic conditions to produce hydrogen sulfide gas, which causes serious environmental pollution. Moreover, water is generally recycled in the Cu-Mo flotation plan, which inevitably contains collectors such as xanthate and non-polar oil originating from Cu-Mo bulk flotation [17].

At the same time, some copper or lead metals report to the molybdenum concentrate for many flotation plants, therefore, the quality of the molybdenum concentrate is affected and these impurities need to be removed. Taking into account the drawbacks of traditional chalcopyrite depressants, the complexity of recycled water, and the removal of impurities from the molybdenum concentrate, the exploration of more environmentally-friendly novel molybdenite depressants may be a good strategy for flotation separation.

Carrageenan is a natural polysaccharide plant colloid extracted from red algae [18]. Because of its good physical and chemical properties, such as gelation, thickening and stability, it is often used in pharmaceutical and food industries as an emulsifier, thickener and stabilizer [19]. Since carrageenan molecules contain a large number of hydrophilic hydroxyl and sulfonic acid groups, carrageenan could be used as a good depressant for molybdenite. Thus, the aim of this test work was to explore the use of carrageenan as a novel molybdenite depressant. The depression effect of carrageenan on molybdenite was studied by micro-flotation tests in the presence and absence of the collector. Furthermore, the micro-flotation test results showed that, compared with lignosulfonate, carrageenan showed a better depression performance for molybdenite under the same dosage. Furthermore, the interaction mechanism was revealed through zeta potential, infrared spectrum (FTIR) and atomic force microscope (AFM) measurements.

2. Materials and Methods

2.1. Minerals and Reagents

Molybdenite was obtained from Guangdong Province, China. The molybdenite samples were crushed by hand, then the samples were ground and sieved. The samples of 38–74 µm were used for the microflotation tests. The particles less than 5 μm were utilized for zeta potential measurement and Fourier Transform Infrared Spectroscopy analysis.

Analytical-grade sodium hydroxide (Aladdin Chemical Reagent Co., Ltd., Shanghai, China) was applied to regulate the pH. Emulsified kerosene and sodium butyl xanthate (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used as collectors. Analytical grade carrageenan (the molecular structure is shown in Figure 1) and methyl isobutyl carbinol (Macklin Chemical Reagent Co., Ltd., Shanghai, China) were used as a depressant and frother, respectively. Distilled water (18.25 MΩ∙cm) was utilized for all flotation and analytical measurement tests.

2.2. Microflotation Tests

The Microflotation tests were performed in an Microflotation machine (XFG, Jilin Exploration Machinery Plant, Changchun, China) with a 70 mL flotation cell. A measurement of 2.0 g molybdenite with 20 mL distilled water was dispersed in an ultrasonic cleaner for 2 min. Then the pulp was transferred to the flotation cell, and the slurry pH was adjusted by NaOH and HCl. After that, SIBX (Isobutyl Xanthate) or kerosene was introduced into the pulp and conditioned for 2 min. Carrageenan and MIBC (Methyl Isobutyl Carbinol) (20 mg/L) were added successively into the slurry and conditioned for 5 min and 1 min, respectively. After flotation, the concentrates and tailings were collected, filtered and dried to calculate the floatability. The calculation formula is as follows:

$$F=\frac{m_f}{m_f+m_s}\times 100\%$$

(1)

where m_f and m_s are the weight of floated and non-floated fractions, respectively. Each experiment was conducted at least twice, and the average results are reported in this work.

2.3. Zeta Potential Experiments

The zeta potential was determined using the Zetasizer Nano-ZS90 (Malvern Panalytical Ltd., Malvern, UK). 0.1 g molybdenite was dispersed in 100 mL distilled water, and 10⁻³ mol/L sodium chloride was added to the pulp. Then 500 mg/L carrageenan was introduced into the slurry and the pH of the pulp was adjusted (pH = 4–12). Finally, 1 mL of the upper suspension was taken into the sample cell to measure zeta potential. Sodium chloride was added as an electrolyte to stabilize zeta potential. In addition, the zeta potential was measured as a function of pH at a certain concentration of carrageenan to reveal the adsorption processes as affected by pH.

2.4. FTIR Measurement

The spectra (400–4000 cm⁻¹) were recorded by Nicolet-6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at room temperature (25 ± 1 °C). A measurement of 1.0 g molybdenite was added into 100 mL carrageenan solution (500 mg/L) at pH 10 and stirred for 10 min. Then the sample was filtered and dried in a vacuum drying oven at 25 °C for 12 h.

2.5. AFM Measurement

The morphology of the molybdenite surface before and after carrageenan treatment was tested by a Multimode V atomic force nanoscope (Bruker, Billerica, MA, USA) with acoustic tap mode. Firstly, a fresh molybdenite surface was cleaved with Scotch tape. Then the molybdenite surface was treated with a drop of 500 mg/L carrageenan solution for 5 min. After that, the molybdenite surface was dried with ultrapure nitrogen for observation. The information about the AFM unit: A V-shaped cantilever (resonance frequency 70 kHz, spring constant k = 0.4 N/m, dimensions of 115 μm × 25 μm × 0.65 μm) was used under air conditions.

3. Results and Discussion

3.1. Microflotation Test

Figure 2 presents the effect of carrageenan concentration on the floatability of molybdenite. In the presence of kerosene or SIBX, the floatability of molybdenite decreased sharply from 95% to less than 16% as the carrageenan concentration increased from 0 to 500 mg/L. Moreover, the depression effect of carrageenan on molybdenite was much stronger in the presence of kerosene than that in the presence of SIBX. This result indicated that carrageenan might be an effective molybdenite depressant, even in the presence of a collector.

The floatability of molybdenite as a function of pH is shown in Figure 3. In the absence of carrageenan, the molybdenite floatability kept above 90% at pH 4–12. After the introduction of 500 mg/L of carrageenan, the floatability of molybdenite reduced rapidly to less than 20%. Moreover, the depression ability of carrageenan on molybdenite was much stronger in the presence of kerosene. This result reconfirmed that carrageenan could depress molybdenite significantly, even if the surface was contaminated by a collector at a board pH range.

Figure 4 displays the effect of kerosene concentration on the floatability of molybdenite. Without the addition of carrageenan, the floatability of molybdenite was maintained at around 90%, while the floatability of carrageenan remained below 10% with increasing kerosene dosage in the presence of carrageenan. These results proved that the pre-adsorption of kerosene did not attenuate the depression effect of carrageenan on molybdenite.

The effect of SIBX concentration on the floatability of molybdenite is exhibited in Figure 5. Without the addition of carrageenan, the molybdenite floatability was consistently maintained above 90% as the SIBX concentration increased. After the addition of carrageenan, the floatability of molybdenite increased slowly as the SIBX concentration increased. Only approximately 30% molybdenite was floated at a SIBX concentration of 100mg/L, which indicated that SIBX insignificantly affected the selective depression effect of carrageenan on molybdenite, which was consistent with the micro-flotation results in Figure 2.

Without the addition of carrageenan, the floatability of molybdenite maintained a recovery of around 90%, while the floatability of molybdenite remained below 10% as the kerosene dosage increased in the presence of carrageenan. Refer to the research results of other scholars [20], when calcium lignosulfonate was used as a depressant, the floatability of molybdenite increased significantly (above 20%), particularly when the kerosene dosage was greater than 20 g/t. These results proved that the depression of carrageenan on molybdenite is more effective than calcium lignosulfonate. In other work [19], carrageenan was investigated as a novel depressant for chalcopyrite flotation. Microflotation test results demonstrated that carrageenan could depress the floatability of talc and had no effect on the chalcopyrite flotation. Compared with CMC (Carboxymethylcellulose) and Guar gum, carrageenan exhibits less effect on chalcopyrite flotation.

3.2. Zeta Potential Measurement

Figure 6 shows the zeta potential of molybdenite with or without carrageenan. The molybdenite surface was negatively charged throughout the whole pH range and the net charge increased as pH increased, which was in line with a previous investigation [1,2]. After the addition of carrageenan, the zeta potential of molybdenite became more negative. These results showed that carrageenan was adsorbed on the molybdenite surface, thus resulting in a high depression effect on molybdenite.

3.3. FTIR Measurement

The FTIR spectra of molybdenite before and after carrageenan treatment are displayed in Figure 7 For carrageenan, the peaks at 3420, 2943 and 2911 cm⁻¹ were ascribed to the stretching vibration of -OH, -CH₃ and -CH groups. The peak at 1646 cm⁻¹ was the absorption peak of associated water [19]. The characteristic peaks at 1267, 1071, 927 and 846 cm⁻¹ were assigned to O=S=O, C-O-, C-O-C and C-O-S stretching vibration [21]. For molybdenite, after being treated by carrageenan, a new peak appeared at 1079 cm⁻¹, which originated from the C-O- peak of carrageenan. These results indicated that carrageenan was adsorbed on the molybdenite surface.

3.4. AFM Measurement

The changes in mineral surface morphology before and after reagent adsorption can be visually observed by AFM. Figure 8 shows the AFM images of molybdenite with and without carrageenan treatment. Without being treated with carrageenan, the surface of molybdenite was relatively smooth. After being treated with carrageenan, it could clearly be observed that many aggregates appeared randomly on the molybdenite surface, which indicated that carrageenan was strongly adsorbed on the molybdenite face. This result proved again that the depression of molybdenite was mainly due to the adsorption of carrageenan on the molybdenite face.

4. Conclusions

(1) Carrageenan had a powerful depression effect on molybdenite at pH 6–12, even in the presence of kerosene or SIBX, and the depression of carrageenan on molybdenite was better in the presence of kerosene than with SIBX. Compared with calcium lignosulfonate, another depressant, carrageenan showed better depression performance.

(2) The concentration of kerosene did not affect the depression ability of carrageenan, while the concentration of SIBX slightly attenuated the depression effect of carrageenan.

(3) Zeta potential, FTIR and AFM measurement results demonstrated that carrageenan adsorbed on a molybdenite surface to increase its wettability resulted in the strong depression of molybdenite.