Use of Sodium Hexametaphosphate and Citric Acid Mixture as Depressant in the Flotation Separation of Scheelite from Calcite

The floatability of scheelite and calcite in the presence of single depressant (SHMP or H3Cit) and mixed depressant (SHMP/H3Cit) was studied by microflotation experiments and artificial mixed mineral experiments. Solution chemical calculation, zeta potential tests, thermodynamic analysis and XPS analysis were used to explain the relevant depressive mechanism. Mixed depressant (SHMP/H3Cit) exhibited excellent selective depressive effect on calcite. The optimal molar ratio of SHMP to H3Cit was 1:4. The depressant SHMP and H3Cit can be chemically bonded with Ca2+ to form CaHPO4 and Ca3(Cit)2 at pH 8. The CaHPO4 was more easily formed than Ca3(Cit)2 on the mineral surface, which indicated that the depressive effect of SHMP was stronger than H3Cit. The SHMP and H3Cit of the mixed depressant were co-adsorbed on the calcite surface, while the H3Cit of the mixed depressant was weakly adsorbed on the scheelite surface. The mixed depressant can significantly improve the separation efficiency of scheelite from calcite.


Introduction
Scheelite and wolframite are the two most important tungsten resources in nature. Since the wolframite ((Fe, Mn)WO 4 ) is more easily enriched by gravity methods and is easier to exploit and utilize, the wolframite resources have been exhausted [1][2][3]. Therefore, the exploitation and high standards utilization of scheelite (CaWO 4 ) are receiving more and more attention [4,5]. The extraction of scheelite from corresponding gangue minerals has been a problem due to its unique embedding characteristics and symbiotic relationship [6]. Foam flotation is an efficient method to recover minerals with low grade and fine grain size [7][8][9]. During the flotation process, the chemical properties of the mineral surface (such as wettability and surface electrical properties) are changed by adding flotation reagents [10][11][12][13]. Similarly, scheelite and calcite have similar chemical compositions and chemical interactions with corresponding flotation reagents, which makes the separation of scheelite and calcium-containing minerals different [14]. Therefore, the flotation of scheelite is also facing enormous challenges in the process of utilization of tungsten resources [15]. Flotation reagents are the key to improving flotation indicators. The most important flotation reagents are mainly collectors [16,17], depressants [18,19] and regulators [20]. The role of the depressants in flotation is mainly to selectively be adsorbed on certain mineral surfaces to reduce their floatability [21,22]. Therefore, increasing the selectivity and the depressive ability of depressants is the focus of scheelite flotation research. As such, the reasonable choice of scheelite flotation depressants is the key to recovering scheelite.

Material
The mineral samples with high purity of scheelite and calcite were from Guangxi and Hunan, China. Blocky pure scheelite and calcite are shown in Figure 1. At the beginning, the samples were hand-selected, and then the large particle size was hammered into a uniform small particle size. Then, it continued to be ground and the particle size of −74 + 37 µm was sieved by dry sieves for microflotation experiments. Particle size less than 37 µm was then further ground to below 5 µm for the mechanism experiments, chemical analysis and X-ray diffraction analysis (XRD). Chemical analysis showed that the scheelite sample had a purity of 96% and additionally contained 2.12% SiO 2 and 1.75% MgO, and the calcite sample had a purity of 98.91% and additionally contained 1.01% SiO 2 . The XRD also represented extremely high purity ( Figure 2).
The depressants SHMP (the content of P 2 O 5 is 65.0-70.0%) and H 3 Cit (purity greater than 99.5%) with analytical grade used in this experiment were from Tianjin Komiou Chemical Reagent Co., Ltd., China. and Hunan Huihong Reagent Co., Ltd., Changsha, China. respectively. Analytically pure NaOL was also obtained from Hunan Huihong Reagent Co., Ltd., China. Ultrapure water was used in all experiments (resistance over 18 MΩ × cm).

Microflotation
The floatability of the two minerals in the presence of different reagents was investigated using an XFGC II flotation machine (capacity 40 mL) with a spindle speed 1500 r/min ( Figure 3). 35 mL of ultrapure water and 2.0 g of pure mineral were mixed and stirred for 2 min. The pH of the slurry was then adjusted and recorded. The flotation reagents were then added in sequence and the conditioning time was as shown in Figure 4. The flotation time was also 3 min. The flotation recovery was calculated by the following formula: In the formula, m 1 and m 2 represent the floated products weight and sinked products weight, respectively. Microflotation experiments under the same conditions were repeated three times, and the average value was reported in the flotation results. The standard deviation for each test was calculated and presented as the error bars.
For the artificial mixed mineral tests, 1 g of scheelite and 1 g of calcite were mixed into mixed minerals. After the mixed minerals were stirred for 1 min, the flotation reagents were added. The foam product was collected as the concentrate product, and the sinked product was the tailing. The concentrate product and tailing product were weighed separately and the yields were calculated. The flotation recovery was calculated based on the yields and the scheelite grade of the two products.

Zeta Potential Analysis
Zeta potential analysis was performed using a zeta potential analyzer. The device model is ZetaPlus and the device manufacturer is Bruker, Karlsruhe, Germany. First, 30 mg of samples (−5 µm) and 35 mL of KNO 3 electrolyte solution were added to a beaker and stirred with a magnetic stirrer to bring the slurry in suspension. Then, corresponding flotation reagents were added, and the dosages of the reagents and the conditioning time were consistent with the microflotation experiment. The conditioning time of each flotation reagent was 3 min. After the conditioning, the slurry suspension was allowed to stand for 5 min, and the supernatant was withdrawn by a syringe for testing. Under each condition, the value was tested three times to obtain an average value. The standard deviation for each test was calculated and presented as the error bars.

XPS Analysis
In this experiment, XPS analysis was performed using an X-ray photoelectron spectrometer. The device model is ESCALAB 250Xi and the device manufacturer is Thermo Fisher, Waltham, MA, USA. X-ray source is monochromatic Al Ka X-ray source operating at 150 W. The fitting of the peaks was performed using the XPS PEAK fitting software (Version 4.1). The preparation process of the samples used for XPS analysis was consistent with the microflotation experiments. The samples were contaminated with hydrocarbons in an open system, so carbon can be detected on the surface of pure scheelite. The chemical state of the elements (such as binding energy, etc.) was referred to National Institute of Standards and Technology (NIST) XPS Databases and related literature.

Microflotation
The effect of the concentration of single depressant SHMP and H 3 Cit on the recovery of the minerals at a certain collector NaOL concentration (4 × 10 −5 mol/L) is depicted in Figure 5. When the concentration of SHMP reached 2 × 10 −3 mol/L, although SHMP greatly depressed the flotation of calcite, it also greatly depressed the floatability of scheelite, and the recovery was greatly reduced from 85.7% to 33.6%. This was not conducive to the flotation separation. As shown in Figure 5b, when single H 3 Cit was used as the depressant of the scheelite and calcite, the flotation recovery decreased to 40.3% and 54.1% when the H 3 Cit concentration reached 2 × 10 −3 mol/L. The depression effect of H 3 Cit on scheelite was even greater than calcite, which was also not conducive to the flotation separation between scheelite and calcite. Therefore, we considered the combination of two depressants to reveal the depression of the mixed depressant on the minerals. The effect of the total dosage of mixed depressant of SHMP/H 3 Cit on the floatability of minerals is depicted in Figure 6. When the total dosage of the mixed depressant SHMP/H 3 Cit was 2 × 10 −3 mol/L, the floatability of calcite was greatly reduced to 19.95%, but the scheelite still had a good floatability (61.05%). The large floatability difference between scheelite and calcite under mixed depressant conditions favored the flotation separation between the two minerals. Therefore, from the perspective of flotation selectivity, mixed depressant SHMP/H 3 Cit were more advantageous than single depressant SHMP or H 3 Cit. The effect of the molar ratio of SHMP to H 3 Cit of the mixed depressant on mineral floatability is depicted in Figure 7. The total concentration of mixed depressant was 2 × 10 −3 mol/L. As shown in Figure 7, When the molar ratio of SHMP to H 3 Cit was 1:4, the floatability of scheelite and calcite were 58.2% and 2.8%, respectively. At this ratio, the difference in floatability between scheelite and calcite was the largest, which was most beneficial to the flotation separation of the two minerals. Therefore, the ratio of 1:4 was the optimum ratio. Microflotation results have shown that mixed depressant can achieve good separation effect. To further verify the separation effect of the mixed depressant, the artificial mixed minerals experiments were conducted and the results are shown in Table 1. 2 g of mixed minerals used for this experiment was consisted of 1 g scheelite and 1 g calcite. The dosages of single and mixed depressant were all 1 × 10 −3 mol/L. The ratio of SHMP to H 3 Cit was 1:4. In the presence of single depressant H 3 Cit, a concentrate with scheelite grade of 52.33% was achieved at a recovery of 68.40%. When mixed depressant SHMP/ H 3 Cit was used, although the yield of concentrates decreased (from 65.35% to 62.15%), the grade and recovery of scheelite in concentrates increased to 62.19% and 77.3%, respectively. This indicated that the mixed depressant had better selectively, which was consistent with the microflotation results.

Solution Chemical Calculation of Dissolved Components of Minerals
The dominant components in the mineral slurry are different under different pH conditions. The components in the slurry solution follow the reaction equilibrium, and all reactions follow the proton transfer equilibrium, mass balance and charge balance. Based on these equilibrium reaction relationships, the dominant components in the slurry at different pH conditions can be calculated. The following reactions exist in the saturated scheelite slurry, and the corresponding reaction equilibrium constants are also listed in Table 2. Table 2. Reactions and reaction constants in saturated scheelite slurry.

Reactions
Reaction Constants For scheelite (CaWO 4 ), the pH at which Ca(OH) 2 precipitation begins to be formed is obtained by the following formula: The pH at which H 2 WO 4 precipitation begins to be formed is obtained by the following formula: After calculation, the relationship between the concentration (C) of each component of saturated scheelite and pH is shown in Table 3.
In Table 3, α WO 2− 4 and α Ca 2+ are the side reaction coefficients of WO 2− 4 and Ca 2+ , and the calculation formulas are: From the formulas in Table 3 and Formulas (10) and (11), the logarithmic of the concentration of each component in the saturated solution of scheelite can be calculated, and the figure is shown in Figure 8a.
In addition to Formulas (5) to (7), the equilibrium reactions and equilibrium constants in the saturated solution of calcite are shown in Table 4.  (17) In the atmosphere, take P CO2 = 10 −3.5 atm (standard atmospheric pressure), so H 2 CO 3 (aq.) = P CO 2 K 3 = 10 −4.97 . According to the reaction Formulas (5) to (7) and the reaction Formulas (12) to (17)  As depicted in Figure 8a, When the pH was 8, the positioning ions on the surface of the saturated scheelite solution were WO 2− 4 and Ca 2+ . This was consistent with previous related literature [14]. As shown in Figure 8b, the positioning ion on the surface of the saturated calcite solution was mainly Ca 2+ at pH 8, which made the surface of the calcite positively charged. This also agreed with related literature [6].

Solution Chemical Calculation of Components of SHMP and H 3 Cit Solution
The concentration of each component as a percentage of the total concentration in the SHMP and H 3 Cit solution is the distribution coefficient of each component. The dominant components of the dissolved reagents in the slurry determine the adsorption between the reagents and the minerals.
The equilibrium reaction formulas and corresponding reaction coefficients in SHMP and H 3 Cit solution are shown in Table 5.
The distribution coefficient diagram of every component of SHMP and H 3 Cit solution can be calculated from the above series of formulas, as shown in Figure 9. As depicted in Figure 9a, the dominant component in the SHMP solution was HPO 4 2− at pH 8.
Under this condition, Chelation of HPO 4 2− and Ca 2+ formed CaHPO 4 when SHMP was adsorbed on the mineral surfaces. As shown in Figure 9b, the main species in H 3 Cit solution at pH 8 was Cit 3− . When H 3 Cit was interacted with the minerals, the dominant component Cit 3− and Ca 2+ bonded to form Ca 3 (Cit) 2 on the mineral surfaces. The adsorption strength of SHMP and H 3 Cit was revealed with a further zeta potential experiment and thermodynamic analyses.

Zeta Potential Analysis
The zeta potential of minerals under different depressants is shown in Figure 10, respectively. The concentration of the single depressant (SHMP or H 3 Cit) and the mixed depressant SHMP/H 3 Cit were both 2 × 10 −3 mol/L. As shown in Figure 10a, bare scheelite had a negative zeta potential in ultrapure water over the entire pH range (6.5-10.2), which was consistent with previous study [6]. According to the solution chemistry calculation of scheelite, the positioning ions on the surface of scheelite were mainly WO 2− 4 and Ca 2+ at pH 8. Considering that the surface zeta potential was negative under this condition, the dominant component of the scheelite surface was mainly WO 2− 4 . Under the conditioning of depressant SHMP or H 3 Cit, the zeta potential of scheelite was decreased, indicating that the negatively charged HPO 4 2− and Cit 3− in the SHMP and H 3 Cit solutions specific adsorbed on the scheelite surface with the negative charge. After the conditioning of the mixed depressant SHMP/H 3 Cit, the magnitude of the decrease in the zeta potential of the scheelite was small, indicating that the adsorption of the mixed depressant on the surface of the scheelite was weak.
The zeta potential of calcite under different reagent conditions is shown in Figure 10b. The isoelectric point (IEP) of pure calcite appeared at around 9.3, which was consistent with the relevant literature [44,45]. Combined with the solution chemical calculation results of the calcite, the dominant component of the calcite surface was Ca 2+ . Depressant SHMP or H 3 Cit was adsorbed on the calcite surface, and the zeta potential was negatively shifted, indicating that the negatively charged components HPO 4 2− and Cit 3− were chemically adsorbed on the surface of calcite. The mixed depressant SHMP/H 3 Cit reduced the zeta potential of calcite to a greater extent, indicating that the mixed depressant had co-adsorbed on the calcite surface, and the adsorption on the calcite surface was more intense than scheelite.

Thermodynamic Analyses
In order to better explain the flotation phenomenon, the Gibbs free energy change of the reaction between the depressants (SHMP and H 3 Cit) and the minerals (scheelite and calcite) was calculated. From the above solution chemistry results, SHMP and H 3 Cit reacted with calcium ions on the mineral surface to form CaHPO 4 and Ca 3 (Cit) 2 precipitates at pH 8, respectively.
In the scheelite and calcite solution, the equilibrium reactions and equilibrium constants between the corresponding components of SHMP and calcium ions are depicted in Table 6. The Formulas (4) to (6) and Formulas (18) to (20) are shown in Tables 2 and 5, respectively, and are not listed in Table 6.  (37) The formula of Gibbs free energy change can be obtained from the equilibrium reaction (34), as follows: For the reactions between scheelite and SHMP, the calcium ion concentration is calculated as follows: For the reactions between calcite and SHMP, the calcium ion concentration is calculated as follows: [Ca 2+ ] = 10 13.31−2pH (43) As such, according to the reaction Formulas (4) to (6), (18) to (20), (24), and the reaction formulas (34) to (43), the Gibbs free energy change of the reactions between scheelite and calcite with SHMP can be calculated, as shown in Figure 11.
In the scheelite and calcite solution, the equilibrium reactions and equilibrium constants between the corresponding components of H 3 Cit and calcium ions are depicted in Table 7. The formulas (4) to (6) and formulas (21) to (23) are shown in Tables 2 and 5, respectively, and are not listed in Table 7. Table 7. Reactions and corresponding reaction constants.
According to the reaction Formulas (4) to (6), (21) to (23), (25), and the reaction Formulas (42) to (50), the ∆G of the reactions between scheelite and calcite with H 3 Cit can be calculated, as shown in Figure 11. The ∆G of the reaction when CaHPO 4 precipitate was formed on calcite surface was smaller than that on scheelite surface at pH 8. Therefore, the precipitation of CaHPO 4 was easier to be formed on the calcite surface, so the depressant effect of SHMP on the calcite was stronger than that of the scheelite. The ∆G of the reaction when Ca 3 (Cit) 2 precipitate was formed was bigger than that when CaHPO 4 precipitate was formed, illustrating that SHMP had a stronger depressive effect on minerals than H 3 Cit. Figure 11. The ∆G as a function of pH for the reaction of depressants with minerals.

XPS Analysis
The relative concentrations and the shifts of mineral surface elements (before and after the conditioning of corresponding depressants) tested by XPS are shown in Table 8. The relative concentration was the relative concentration percentage between the elements being characterized.
After the conditioning of depressant H 3 Cit with the minerals, the relative content of the C and O elements on the mineral surfaces became larger due to the adsorption of H 3 Cit, but the relative content of the Ca element became smaller. The decrease in the relative content of Ca was mainly due to the increase in the relative content of the corresponding C and O elements. When SHMP was used as a depressant, the relative contents of P element on the scheelite and calcite surfaces increased by 2.76% and 3.00%. This indicated that SHMP had similar depressive effect on scheelite and calcite. When scheelite interacted by the mixed depressant SHMP/H 3 Cit, the relative content of the P element increased by 2.16%, less than 2.76%, indicating that the adsorption amount of SHMP in the mixed depressant on scheelite surface was smaller than that of the single depressant SHMP. Because the relative contents of the C and O elements were reduced (−0.72% and −0.53%), the adsorption of H 3 Cit in the mixed depressant SHMP/H 3 Cit on the surface of the scheelite was also very weak. In contrast, the relative content of P element on the surface of calcite treated by the mixed depressant was increased by 4.12%, much larger than the surface of scheelite (2.16%) and the surface of the calcite after the reaction of single SHMP (3.00%). This indicated that the SHMP in the mixed depressant was more strongly adsorbed on the calcite surface than that of scheelite. In addition, the adsorption strength of SHMP in the mixed depressant on the surface of calcite was also larger than that of the single depressant SHMP. In contrast to scheelite, the relative contents of C and O elements on the surface of the calcite increased after adding of SHMP/H 3 Cit, indicating that the H 3 Cit in the mixed depressant was also adsorbed on calcite surface. This demonstrated the co-adsorption of SHMP and H 3 Cit occurred on the calcite surface. It was indicated that the mixed depressant had a strong depressive effect on the calcite. The separated and fitting of the peaks of O 1s on calcite surface after the conditioning of the mixed depressant are shown in Figure 12. According to the NIST XPS Databases and related literatures [42,46,47], the peak at 531.22 eV was contributed by the oxygen in the CO 3 2− of the calcite.
The peak at 532.55 eV was probably the binding energy of the oxygen in the Ca-COOR, indicating that the depressant H 3 Cit of the mixed depressant was absorbed on the calcite surface. Moreover, the peak at 533.42 eV was assigned to PO 4 3− of the SHMP, indicating that the depressant SHMP of the mixed depressant was also absorbed on the calcite surface. This indicated that the SHMP and H 3 Cit in the mixed depressant were co-adsorbed on calcite surface. Therefore, the depressive effect of mixed depressant on calcite was strong.

Adsorption Model
According to the experimental mechanism that has been studied, a possible adsorption model of the mixed depressant SHMP/H 3 Cit and collector NaOL on the mineral surfaces can be obtained. The advantage of the mixed depressant is mainly reflected in the stronger flotation selectivity on the two minerals. This is reflected in the difference in the adsorption behavior of the mixed depressant on the scheelite and calcite surfaces. As depicted in Figure 13, after the mixed depressant interacted with the minerals, SHMP and H 3 Cit in the mixed depressant are co-adsorbed on calcite surface, and this hindered further adsorption of the collector NaOL. This made the floatability of calcite very strongly depressed. However, the adsorption of mixed depressant on scheelite surface was very weak, particularly, the H 3 Cit of the mixed depressant had weak adsorption on the surface of scheelite. This made the floatability of scheelite less affected.

Conclusions
The single depressant SHMP or H 3 Cit cannot achieve the flotation separation between scheelite and calcite because of their poor selectivity. When the mixed depressant SHMP/ H 3 Cit was used, the flotation recovery of scheelite and calcite reached 58.2% and 2.8% at an optimum molar ratio of 1:4, respectively, and the flotation separation could be achieved. Artificial mixed mineral experiment indicated that the mixed depressant can significantly improve the separation efficiency of scheelite and calcite. The depressants SHMP and H 3 Cit were chemically bonded with the calcium ions on the mineral surface to form CaHPO 4 and Ca 3 (Cit) 2 , respectively. Thermodynamic analysis indicated that the CaHPO 4 was easier to form than Ca 3 (Cit) 2 on the mineral surfaces, indicating that the depressive effect of SHMP was stronger than H 3 Cit. XPS indicated that SHMP and H 3 Cit of the mixed depressant were co-adsorbed on calcite surface, while H 3 Cit of mixed depressant was weakly adsorbed on scheelite surface. The mixed depressant SHMP/ H 3 Cit had a stronger depressive effect on calcite than scheelite, and the flotation separation efficiency of scheelite from calcite can be improved.

Conflicts of Interest:
The authors declare no conflict of interest.