The Interaction between Ca²⁺ and Molybdenite Edges and Its Effect on Molybdenum Flotation

Abstract

In this paper, the influence of Ca²⁺ on the flotation of a skarn type molybdenum ore and pure molybdenite mineral at pH 8 was studied using diesel as the collector. It was found that Ca²⁺ had little effect on molybdenum flotation at low concentrations. By further increasing Ca²⁺ concentration, the floatability of molybdenite—especially from the fine size fractions—was depressed even without the presence of fine gangue minerals. The mechanism responsible for the deleterious effect of Ca²⁺ on molybdenite flotation was studied by a range of techniques including zeta potential measurements, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectrometer (SEM-EDS) analyses and molybdenum phase analyses. It was found that Ca²⁺ interacted with molybdenite edges producing CaMoO₄ precipitates which were responsible for the depression of molybdenite flotation of Ca²⁺ by preventing the adsorption of diesel.

1. Introduction

Flotation exploits a difference in surface wettability on valuable and gangue minerals, and water quality has a significant impact on mineral flotation through the modification of surface wettability by ions in water [1,2,3,4]. Due to the scarcity of fresh water and increasingly stringent regulations on the quality of discharged water, groundwater or sea water with a high strength of ions and recycled water are used by most flotation plants. This practice has resulted in an increase in ion strength of process water as a result of evaporation and ongoing ion inputs from groundwater (or seawater), chemicals added in flotation and run off from spoil [5,6,7].

The negative impact of calcium ions in process water on the flotation of molybdenite from Cu-Mo ores has been reported in a number of studies [8,9,10]. The negative impact of calcium ions has been attributed to the bridging role of calcium ions which promote the coating of fine gangue (e.g., quartz) particles on molybdenite. It has been found that calcium ions can adsorb on molybdenite as a result of electrostatic interactions and then increase the zeta potential of molybdenite or even reverse the zeta potential from negative to positive [11].Adsorbed calcium ions are attractive to negatively charged fine quartz or other gangue particles, deteriorating molybdenite flotation. These studies were conducted on Cu–Mo ores with bulk flotation of Cu and Mo at high pH and molybdenite recovered as a by-product [12,13,14,15].

Limited studies have been conducted to understand the direct depression of calcium ions on molybdenite flotation. While contributing the slime coating induced by calcium and magnesium ions to the depression of molybdenite flotation, Zanin et al. [10] also measured contact angles on coarse molybdenite particles (+150 μm) in the proximity of their faces and edges after conditioning at different pH values and calcium ion concentrations. They found that pH had little effect on the contact angle on molybdenite faces but a great effect on the contact angle on molybdenite edges. The contact angle on molybdenite edges was lower at pH 11 than at pH 8. At both pH 11 and 8, the contact angle on molybdenite edges decreased with increasing Ca²⁺ concentration, especially after Ca²⁺ concentration reached 6 × 10⁻³ M. The study from Zanin et al. [10] suggests that Ca²⁺ may directly depress molybdenite flotation even in the absence of fine gangue particles.

Lucay et al. [16] studied the effect of gypsum on the floatability of molybdenite in saline solutions using a modified Hallimond tube. In their study, the flotation was conducted at a fixed concentration of Ca²⁺ (0.05 mol/L) by varying the concentration of SO₄²⁻. Under these conditions, a concentration of 0.14 mol/L SO₄²⁻ was required to form gypsum. They found that the formation of gypsum at pH 7–8 depressed the floatability of molybdenite, especially at a fine size. They also noted that before gypsum began to form, the recovery of molybdenite was lower compared to the natural molybdenite flotation recovery, which was attributed to the adsorption of Ca²⁺ on the edges of molybdenite particles and the micro-edges of the micro-crystals that composed the faces of molybdenite particles. The study of Lucay et al. [16] also indicated the depression role of Ca²⁺ in molybdenite flotation at slightly alkaline pH.

Hirajima et al. [17] studied the effect of Mg²⁺ and Ca²⁺ on the floatability of chalcopyrite and molybdenite. They found that MgCl₂ and CaCl₂ solutions depressed the floatability of chalcopyrite and molybdenite at pH values higher than 9. It is interesting that Hirajima et al. [17] did not observe a depression effect of Ca²⁺ on the floatability of molybdenite at pH 8 in both flotation tests and contact angle measurements, which disagrees with the contact angle measured by Zanin et al. [10]. It is noted that the molybdenite sample used by Hirajima et al. [17] was a fine powder product (<30 µm) treated with acid and acetone. This molybdenite sample may be different from the natural molybdenite sample processed in flotation plants. Further, Hirajima et al. [17] did not distinguish the effect of Ca²⁺ on the property of molybdenite edges and faces.

In China, the major molybdenum plants—including those located at Jinduicheng and Luanchuan use sequential flotation, instead of bulk flotation, to recover molybdenite. In this case, nonpolar oil such as kerosene or diesel oil is used as collector and the pH of the flotation ranges between 7 and 8. In this study, the interaction betweenCa²⁺ and molybdenite edges and its effect on molybdenite flotation was investigated at pH 8 in the context of sequential flotation of molybdenum.

2. Materials and Methods

2.1. Materials and Reagents

The actual Mo ore (AM) used in flotation tests was supplied by the tungsten–molybdenum Mine of Luanchuan in Henan Province, China. It is a skarn type of tungsten–molybdenum ore, in which molybdenite (MoS₂) occurs as a major phase and scheelite (CaWO₃) as a minor phase. This ore was crushed to below 2 mm and then ground in a porcelain ball mill to 65% particles below 76 μm. The elemental composition of the actual Mo ore analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Thermo Fisher Scientific Inc., Waltham, MA, USA) and X ray fluorescence (Camscan Electron Optics, Ltd., Cambridge, UK) is given in

Table 1. The elemental composition of the actual Mo ore (wt %).

Compositions	Mo	WO3	Cu	Pb	Zn	S	CaO
Content	0.12	0.07	0.007	0.005	0.016	1.81	21.98
Compositions	Al2O3	SiO2	MgO	Na2O	K2O	TFe 1
Content	7.89	48.50	2.86	1.08	0.82	10.12

¹ TFe means the total concentration of Fe in metallic and non-metallic minerals.

. The mineralogical composition of the actual Mo ore analyzed by X-ray diffraction (Rigaku Corporation, Tokyo, Japan) analysis is given in

Table 2. The mineralogical composition of the actual Mo ore (wt %).

Compositions	Molybdenite	Powethite	Scheelite	Chalcopyrite
Content	0.19	trace	0.09	0.02
Compositions	Sphalerite	Galena	Pyrite	Magnetite
Content	0.24	trace	1.10	1.00
Compositions	Garnet	Diopside	Wollastonite	Quartz
Content	25.70	18.70	16.30	14.60
Compositions	Biotite	Feldspar	Calcite	Loss
Content	10.10	9.10	2.00	0.85

.

Pure molybdenite crystals (PM) used in this study were obtained from the tungsten–molybdenum Mine of Luanchuan in Henan Province, China, as well. They were first selected by hand to obtain high-grade crystals, then crushed and ground in a porcelain ball mill prior to collectorless flotation to further enrich the molybdenite. The enriched molybdenite was screened to obtain the −38 µm fraction for analysis. This material assayed 58 wt % Mo. The XRD analysis indicates 99 wt % molybdenite as shown in Figure 1.

AM and PM prepared in this study represented the nature molybdenite entering the flotation process without any chemical contamination.

Diesel oil was used as molybdenite collector in this study without emulsification. The addition rate of diesel was 180 g/t in AM flotation and varied in PM flotation. Pine oil was used as frother. The addition rate of pine oil was 100 g/tin AM flotation and 10 mg/L in PM flotation. Sodium silicate (Na₂O·nSiO₂, n = 1) was used as dispersant in AM flotation with an addition rate of 200 g/t. Calcium chloride was used to introduce calcium ions. Sodium hydroxide was used to adjust pH. Diesel and pine oil were of industrial grade and sodium silicate, calcium chloride, and sodium hydroxide were of analytical grade. Deionized water was used throughout this study. The different collector and frother dosages were used in AM and PM flotation to achieve a similar baseline flotation with a high molybdenite recovery in the absence of Ca²⁺ in the two cases.

2.2. Flotation Tests

AM flotation was conducted in a RK/FGC 3 L hitch groove flotation cell at a rotating speed of 1700 r/min. In the flotation process, 1 kg AM was added into the flotation cell, followed by the addition of dispersant, collector, and frother with conditioning time for 1, 2 and 1 min, respectively. The total flotation time was 7 min.

PM flotation was conducted in a RK/FGC 40 mL hitch groove flotation cell at a rotating speed of 1700 r/min. In the flotation process, 1 g PM was added to the flotation cell, followed by the addition of collector and frother with conditioning time for 2 and 1 min, respectively. The total flotation time was also 7 min.

2.3. Zeta Potential Measurements

Zeta potential was measured with a zeta potential analyzer from Micromeritics Instrument Corp. (Atlanta, GA, USA). The suspension containing less than 5% PM was conditioned for 15 min in a solution of 0~2000 mg/L Ca²⁺. Then 20 mg/L diesel oil was added and conditioned for 3 min. The suspension was then transferred to a sample vessel for the zeta potential measurement at room temperature (20 °C). The measurement was repeated five times and the average zeta potential was reported.

2.4. Scanning Electron Microscopy and Energy Dispersive X-ray Spectrometer (SEM-EDS) Analyses

A measure of 5 g PM was mixed in a solution of 800 mg/L Ca²⁺ and conditioned for 10 min at pH 8. The solids were then washed by deionized water, air dried and coated with carbon, and then analyzed by a Scanning Electron Microscope (SEM) fitted with an Energy Dispersive X-ray Spectrometer (EDS) from Camscan Electron Optics, Ltd., Cambridge, UK. SEM was used to map the surface topography and identify molybdenite faces and edges while EDS was used to identify elements on molybdenite faces and edges.

2.5. The Analysis of Molybdenum Phases

The presence of powellite [CaMoO₄] in PM before and after conditioning in the absence and presence of 800 mg/L Ca²⁺ was quantified by the phase analysis. The sample with or without the conditioning in Ca²⁺ solution was first washed by aqueous ammonia which can remove MoO₄²⁻ of molybdenite surface. The insoluble residue was heated with tartaric acid solution to dissolve powellite, but not molybdenite which would remain in the residue. Then the suspension was filtered and the filtrate was assayed for molybdenum by ICP-MS. This phase analysis has been detailed elsewhere [18,19]. MoO₄²⁻ may occur on the molybdenite surface, but this analysis only detects CaMoO₄. In this study, three repeat tests were conducted to determine the amount of powellite for each condition and the average was reported.

3. Results and Discussion

3.1. Flotation Tests

3.1.1. AM Flotation

The influence of Ca²⁺ concentration on AM flotation at pH 8 was studied and the results are shown in Figure 2. As can been seen, molybdenum recovery remained about 91.7% in the presence of Ca²⁺ below 60 mg/L. Above 60 mg/L, molybdenum recovery decreased with increasing the Ca²⁺ concentration and reached a minimum of about 87.5% at 600 mg/L Ca²⁺. Figure 2 also shows that Ca²⁺ only had a slight effect on molybdenum grade ranging from 3.5% to 4%. Table 1 shows that AM contains calcium minerals. Obviously, these calcium minerals did not affect the floatability of molybdenite since a high molybdenite recovery was obtained without the addition of Ca²⁺.

Figure 3 shows molybdenite recovery in AM flotation from different size fractions in the absence and presence of 800 mg/L Ca²⁺ at pH 8. It is clear that Ca²⁺ had a greater depression effect on the flotation of molybdenite from a fine size fraction than from an intermediate size fraction. For example, from −38 µm size fraction, molybdenite recovery decreased from about 90.1% in the absence of Ca²⁺ to about 79.1% in the presence of 800 mg/L Ca²⁺, while from −97 + 74 µm size fraction, molybdenite recovery only decreased from about 96.0% in the absence of Ca²⁺ to about 94.0% in the presence of 800 mg/L Ca²⁺. Obviously, the depression of Ca²⁺ on molybdenite flotation shown in Figure 2 is mainly attributed to depression from fine size fractions. As reported by Zanin et al. [10], Ca²⁺ did not affect the contact angle on molybdenite faces but reduced the contact angle on molybdenite edges at pH 8 [10]. This explains the more deleterious effect of Ca²⁺ on the flotation of fine molybdenite with a high proportion of edges in Figure 3.

3.1.2. PM Flotation

To further confirm the deleterious effect of Ca²⁺ on the flotation of fine molybdenite at pH 8, flotation of PM below −38 µm was conducted as a function of Ca²⁺ concentration and the results are shown in Figure 4.

At 20 mg/L diesel, molybdenum recovery remained high at about 94% when Ca²⁺ concentration was below 40 mg/L. Then, with Ca²⁺ concentration increasing, molybdenum recovery decreased significantly. At 600 mg/L, molybdenite recovery was about 45%. The decrease of molybdenite recovery slowed down when Ca²⁺ concentration was above 600 mg/L.

The single mineral flotation indicated in Figure 4 is consistent with the flotation results presented in Figure 3 in terms of a higher depression of Ca²⁺ on fine molybdenite flotation. It is clear that Ca²⁺ can depress molybdenite flotation especially from fine size fractions at pH 8 once Ca²⁺ concentration is high enough.

Figure 4a also shows that in the presence of 800 mg/L Ca²⁺, with increasing diesel concentration, the molybdenite recovery increased in PM flotation. At 400 mg/L diesel, molybdenite recovery was about 60%. Then, the increase of molybdenite recovery slowed down when diesel concentration was above 400 mg/L. This indicates that a high diesel concentration could mitigate the negative effect of Ca²⁺ on molybdenite flotation to some extent. Hirajima et al. [17] found that the recovery of fine molybdenite particles increased with increasing kerosene concentration in MgCl₂ solution. Song et al. [20] also reported that the flocflotation of fine molybdenite particles increased with increasing kerosene concentration.

3.2. Zeta Potential Measurements

The zeta potential of molybdenite below 38 µmin the absence and presence of 20 mg/L diesel as a function of Ca²⁺ concentration was measured at pH 8. Results are shown in Figure 5.

Figure 5 indicates that the zeta potential of molybdenite was about −30 mV in the absence and presence of 20 mg/L diesel without the addition of Ca²⁺. Obviously, the adsorption of diesel on molybdenite had little effect on the zeta potential of molybdenite. It has been documented that molybdenite edges are hydrophilic and composed of Mo–S bonds, while molybdenite faces are hydrophobic and composed of S–S bonds [10,15,21]. Oily collectors such as kerosene, diesel oil, transformer oil, and solar oil only adsorb on molybdenite faces through hydrophobic interactions and van der Waals forces [11,21,22,23]. Apparently, the zeta potential in Figure 5 was mainly controlled by molybdenite edges on which diesel were not adsorbed. Given the fine size (−38 µm) of molybdenite particles in the zeta potential measurements, molybdenite edges are predominant over molybdenite faces.

Figure 5 also shows that the addition of Ca²⁺ increased the zeta potential of molybdenite and the zeta potential of molybdenite reversed from negative to positive at 200 mg/L Ca²⁺. This indicates the adsorption of Ca²⁺ on molybdenite. However, the addition of diesel did not affect the zeta potential of molybdenite in the presence of Ca²⁺. It seems that diesel did not cover the sites on which Ca²⁺ was adsorbed. Since diesel adsorbs on molybdenite faces, it is highly likely that Ca²⁺ adsorbs on molybdenite edges, which was further discussed in the subsequent study.

3.3. Scanning Electron Microscopy and Energy Dispersive X-ray Spectrometer (SEM-EDS) Analyses

Figure 6a,b show SEM images together with EDS of molybdenite faces and edges after being treated with 800 mg/L Ca²⁺ and 20 mg/L diesel at pH 8. It can be seen that the element composition was different on molybdenite faces and edges. In Figure 6a, only Mo, S, and O existed on molybdenite faces. This indicates that Ca²⁺ did not adsorb on molybdenite faces. However, Figure 6b clearly shows the presence of Ca besides Mo, S, and O on molybdenite edges. Clearly, Ca²⁺ did absorb on molybdenite edges.

Some studies reported heterogeneous molybdenite surfaces which are composed of micro-crystal bands with micro-faces and micro-edges [16,19].Although the micro-edges are on molybdenite faces, they behave similarly as molybdenite edges [19]. For this case, Ca²⁺ may adsorb on molybdenite faces through micro-edges. If the faces are heterogeneous with micro-edges, Ca should be detected on molybdenite faces. In this study, Ca was not detected on molybdenite faces suggesting the homogeneous surface without micro-edges. This is also supported by the SEM image.

3.4. The Phase Analysis of Molybdenum

The amount of CaMoO₄ formed in the PM sample before and after conditioning in the absence and presence of 800 mg/L Ca²⁺ at pH 8 was determined by the phase analysis. The results are shown in

Table 3. The amount of CaMoO₄ formed in the PM sample before and after conditioning in the absence and presence of 800 mg/L Ca²⁺ at pH 8.

Title	Before Reacting	After Reacting
Concentration of calcium in pulp (mg/L)	-	0	800
CaMoO4/MoS2(mg/g)	0.00	0.00	1.25

. It is clear that the CaMoO₄ did not form on PM before conditioning and after conditioning in the absence of Ca²⁺. However, after the sample was conditioned in the presence of 800 mg/L Ca²⁺, 1.25 mg CaMoO₄ appeared on 1 g PM, which indicates that the interaction of Ca²⁺ with molybdenum edges formed this amount of CaMoO₄. It was reported that the edge/face ratio of molybdenite was about 0.51~0.67 for −38 µm fraction, but increased to 1.02~1.34 once CaMoO₄ precipitates formed on molybdenite edges [24]. In fact, the area of molybdenite edges is determined by the Mo–S bond length (0.154 nm) [25]. After oxidation, due to the formation of MoO₄²⁻ on molybdenite edges, the area of molybdenite edges is determined by the Mo–O bond length (0.177 nm) [26]. After Ca²⁺ reacts with MoO₄²⁻ to form CaMoO₄ precipitates, the area of molybdenite edges is determined by the Ca–O bond length (0.246 nm) instead of the Mo–O bond length, because Ca²⁺ is at the outer most layers [26]. As a result, the formation of CaMoO₄ precipitates on molybdenite edges increases the area of molybdenite edges but has little effect on the area of molybdenite faces and therefore increases the edge/face ratio of molybdenite. The results from this study together with the possibility of increasing molybdenite edge/face ratio by CaMoO₄ suggest that this amount of CaMoO₄ precipitates formed on molybdenite edges as shown in Table 3 is sufficient to depress molybdenite flotation. It is well known that a small coverage of hydrophilic species on a mineral surface can significantly depress the mineral flotation [27]. This study indicates that the formation of CaMoO₄ on molybdenum edges prevented the adsorption of diesel leading to the depression of molybdenum floatability.

It has been documented that molybdenite edges are easily oxidized but molybdenite faces are difficult to be oxidized in water at room temperature [8,9]. Therefore, the oxidation product (MoO₄²⁻ and HMoO₄⁻) will preferentially occur on molybdenite edges and make molybdenite negatively charged, enhancing the adsorption of calcium ions by electrostatic interactions. The Eh-pH diagram of molybdenite [28] (Figure 7) shows that when the solution pH is above 2, MoS₂ will be oxidized to form HMoO₄⁻. HMoO₄⁻ can exist at pHs ranging from 2 to 6. Then, HMoO₄⁻ starts to translate into MoO₄²⁻ at pH greater than 6 with the following reaction

$$2{\mathrm{MoS}}_2+9{\mathrm{O}}_2+10{\mathrm{OH}}^{-}\to 2{\mathrm{HMoO}}_4^{-}+4{\mathrm{SO}}_4^{2-}+4{\mathrm{H}}_2\mathrm{O}$$

(1)

$$\mathrm{HMo}{\mathrm{O}}_4^{-}+{\mathrm{OH}}^{-}\to {\mathrm{MoO}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(2)

In molybdenum flotation, the pH is generally over 7. Therefore, MoO₄²⁻ will be the main oxidation product on molybdenite edges. Table 3 suggests that Ca²⁺ may react with MoO₄²⁻ and form CaMoO₄. In fact, the reaction between Ca²⁺ and MoO₄²⁻ is a spontaneous process with the standard free energy of formation (ΔGf₀) of CaMoO₄ being −1405.19 ± 1.60 kJ∙mol⁻¹ [29]. In addition, Raghavan and Hsu [9] reported the solubility of CaMoO₄ which is 10^−7.38 g/100 g water. The concentration of CaMoO₄ presented in Table 3 is 3.12 × 10⁻³ g/100 g water far above the solubility of CaMoO₄ (10^−7.38 g/100 g), favoring the reaction between Ca²⁺ and MoO₄²⁻ as

$${\mathrm{MoO}}_4^{2-}+{\mathrm{Ca}}^{2+}\to \mathrm{CaMo}{\mathrm{O}}_4\downarrow$$

(3)

The combination of Equations (1)–(3) generates the following overall reaction which is responsible for the interaction of Ca²⁺ on molybdenite edges and its deleterious effect on fine molybdenite flotation [9,11]

$$2{\mathrm{MoS}}_2+9{\mathrm{O}}_2+12{\mathrm{OH}}^{-}+2{\mathrm{Ca}}^{2+}\to 2\mathrm{CaMo}{\mathrm{O}}_4\downarrow +4{\mathrm{SO}}_4^{2-}+6{\mathrm{H}}_2\mathrm{O}$$

(4)

Qiu et al. [30] reported that CaCO₃ precipitated on molybdenite surfaces in the presence of 10.33 mMCa²⁺ and 2.33 m MHCO₃²⁻ at pH above 9.5, inhibiting molybdenite flotation. While it is not clear whether CaCO₃ could have a negative effect on molybdenite flotation in this study, it is clear that CaMoO₄ had a deleterious effect.

4. Conclusions

A low concentration of Ca²⁺ in the water had little effect on molybdenite flotation with diesel as the collector at pH 8. However, a higher concentration of Ca²⁺ could depress molybdenite flotation even in the absence of fine gangue minerals. This depression was more pronounced on fine molybdenite particles. In this study, the deleterious effects of Ca²⁺ on molybdenite flotation resulted from the interaction between Ca²⁺ and molybdenite edges. The preferential oxidation of molybdenite edges produced MoO₄²⁻ which made molybdenite edges negatively charged and attractive to Ca²⁺, leading to the formation of CaMoO₄ precipitates responsible for depressing molybdenite flotation.