Effects of Sodium Silicate on Flotation Separation of Sphalerite and Dolomite and Its Mechanism

Abstract

Sphalerite often co-exists with dolomite, a carbonate mineral containing calcium and magnesium. In the flotation process of sphalerite, dolomite entering into the concentrate will have a considerable negative impact on the subsequent smelting. Therefore, the effects of sodium silicate on the flotation separation of sphalerite and dolomite and its mechanism were investigated in this study. It was found that alkaline conditions and the addition of sodium silicate were conducive to the flotation separation of sphalerite and dolomite. Under alkaline conditions, sodium silicate improved the hydrophobicity of sphalerite and the slurry turbidity. The yield stress and apparent viscosity were significantly reduced when dolomite was present in slurry. In addition, the surface electrical properties of dolomite shifted from positive to negative with an increase in the dosage of sodium silicate at pH 11, leading to electrostatic repulsion between sphalerite and dolomite. EDLVO results indicated that the total interaction energy between dolomite and sphalerite particles was repulsive when sodium silicate was present. This study provided a theoretical basis for the flotation separation of sphalerite and dolomite.

1. Introduction

Among non-ferrous metals, the global consumption of zinc ranks third after aluminum and copper. With the continuous development of the social economy and the continuous improvement of consumption levels, the demand for zinc products in the world is increasing year by year. Therefore, the efficient development and utilization of zinc ore resources are of great significance to the development of the national economy [1]. Sphalerite is the main zinc-bearing mineral and often coexists with carbonate minerals containing calcium and magnesium. Due to the poor interaction between sphalerite and xanthate, the sphalerite usually needs to be activated by adding copper ions. Carbonate minerals containing calcium and magnesium are often found in the form of dolomite in sphalerite ores [2]. Dolomite is mainly gelatinous and metasomatic with sphalerite, and some particles are fine-grained and wrap inside sphalerite, which leads to difficulty in ore grinding and dissociation, serious gangue inclusion, and entering flotation concentrate with sphalerite [3]. When zinc concentrate is smelted by pyrometallurgy, the magnesium oxide content in the furnace material is required to be strictly controlled within 5% [4]. This is due to the fact that most magnesium-containing minerals are substances with high melting points, increasing the smelting energy consumption, and causing the slag to stick to the furnace wall. When hydrometallurgical is used, the content of magnesium-containing silicate minerals is too high, resulting in an increase in the amount of sulfuric acid, and the accumulation of dissolved magnesium ions in the acid solution eventually clogs the filter cloth due to the precipitation of magnesium sulfate crystals. Therefore, there is an urgent need to develop a method to reduce the magnesium content during sphalerite flotation.

Currently, the method of separating sulfide minerals from calcium–magnesium minerals is mainly flotation. The gangue containing magnesium in sulfide ore mainly includes serpentine, talc, and dolomite. It is often necessary to add depressants or dispersants to the slurry to reduce the floatability of gangue minerals, thus preventing them from entering the concentrate [5,6,7,8]. Magnesium depressants can be divided into two categories: organic depressants and inorganic depressants. The depressant types of gangue containing magnesium are shown in

Table 1. Depressants of gangue containing magnesium.

Categories	Depressants	Other Reagents	Research Technique	Application	Ref.
Organic reagents	Starch	Acetic acid	Zeta potential	Lab	[21]
		HCl/NaOH	XPS
		Dodecyl amine	AFM
	Dextrin	HCl/NaOH	Zeta potential	Lab	[22]
		NaBX	Infrared spectroscopic
		Terpenic oil	Adsorption experiments
	Locust bean gum	PBX	Zeta potential	Lab	[23]
		SHMP	Turbidity tests
		Pine oil
Inorganic reagents	Sodium hexametaphosphate	HCl/NaOH	Zeta potential	Lab/Plant	[24]
		CMC	FTIR
		MIBC	Absorption and ion release
	Sodium silicate	Zinc sulfate	FTIR	Lab/Plant	[25]
		Kerosene	Zeta potential
		Terpinol oil	ToF-SIMS

. Organic depressants include polysaccharides, tannins, humic acids, and organic synthetic polymers [9]. Inorganic depressants include phosphates and silicates [10]. Polysaccharide organic depressants mainly include starch, dextrin, guar gum, xanthan gum, sweet potato gum, lignin, pectin, carrageenan, chitosan, and locust bean gum [11,12,13,14]. In addition, in order to improve the depressor effect of polysaccharide organic depressants, the predecessors have been modified, and the common ones are modified starch, carboxymethyl cellulose, and carboxylated chitosan [15]. The most commonly used dispersants are sodium hexametaphosphate, sodium tripolyphosphate, and sodium silicate [16,17]. Sodium hexametaphosphate can alleviate the heterogeneous condensation of magnesium-containing gangue and useful minerals and change the surface potential of magnesium-bearing gangue, enhancing the electrostatic repulsion between particles. The reaction mechanism of sodium hexametaphosphate is mainly through the formation of hydrophilic complexes, adsorption on the surface of minerals, changing their surface electrical properties, and dispersing and depressing them [18,19,20]. The dispersion mechanism of sodium silicate is mainly due to the reduction in mineral surface potential and the change in electrostatic force between mineral particles through SiO(OH)₃⁻, so as to improve the dispersion effect between particles.

Sodium silicate is an inorganic colloid, which is a commonly used regulator in flotation, and it has a good depressor effect on gangue minerals such as quartz and silicate. When fluorite, calcite, and scheelite float with fatty acids as collectors, sodium silicate can be used as a depressant of gangue minerals. When the amount of sodium silicate is large, it also has a depressor effect on sulfide ore [26]. The depressing of sodium silicate is caused by HSiO₃⁻ and H₂SiO₃, which adsorb on the surface of the minerals, making them highly hydrophilic. In addition, the adsorption degree of HSiO₃⁻ and H₂SiO₃ on the surface of minerals is different, and minerals with strong adsorption capacity will be depressed, while conversely, minerals with poor adsorption capacity will not be depressed [27,28]. Another possible depressing mechanism is the repulsion between hydrophilic negatively charged mineral particles and hydrophobic negatively charged bubbles caused by the addition of sodium silicate [29,30,31]. In addition, sodium silicate is also often used as a dispersant in flotation. It is employed to improve the froth stability and increase the grade of concentrate. Sodium silicate is very useful for the flotation of minerals with a large sludge content. However, adding too much sodium silicate can make concentrate filtration difficult. The mechanism of sodium silicate as a sludge dispersant is that the silicate colloids are negatively charged and have a hydration layer on their surface, and the silicate colloids are stably dispersed and suspended in the slurry without agglomeration and sedimentation with each other [32]. When this colloidal particle and HSiO₃ are adsorbed on the surface of the sludge, the sludge is in a dispersed state, which can play an important role in improving the froth performance in the beneficiation and desliming operation [33]. Although a large number of studies have been carried out on the role and mechanism of sodium silicate in flotation, it is unclear whether sodium silicate can improve the flotation effect of dolomite and sphalerite.

In the study, sphalerite and dolomite were taken as the research objects, and the effects of sodium silicate on the flotation behavior of sphalerite and dolomite were investigated by microbubble flotation tests. Turbidity tests were used to determine the dispersion of single minerals and artificially mixed minerals. The effects of sodium silicate on the surface wettability of dolomite and sphalerite were studied by contact angle measurement. The effects of sodium silicate on the rheological properties of slurry were clarified by rheological properties. The dynamic potential measurement was used to reveal the change rules of mineral electrical properties before and after reaction with sodium silicate. Through the theoretical calculation of EDLVO, the interaction force between particles before and after the reaction with the reagent was revealed. This study lays a theoretical foundation for the flotation separation of sphalerite and dolomite.

2. Materials and Methods

2.1. Samples and Reagents

Sphalerite and dolomite samples used in this study were sourced from Jiangxi Province and Yunnan Province, China. The samples were first crushed using crushers, followed by grinding with a three-head agate grinder. The milled sphalerite and dolomite were sieved with standard sieves to obtain sphalerite and dolomite particles with a size of −37 μm. Sphalerite and dolomite samples were analyzed by X-ray diffraction (XRD), and the results of the XRD analysis are charted in Figure 1. There were no characteristic peaks of other minerals in the samples of sphalerite and dolomite. Chemical element analysis indicated that the Zn grade in sphalerite was 64.23% and the purity of sphalerite reached about 95.85%. Dolomite contained 20.93% MgO and its purity was approximately 96.28%.

CaO and HCl (Aladdin Biochemical Co., Ltd., Shanghai, China) were used to adjust the pH of the slurry. Copper sulfate (CuSO₄, 99%, Maclin Biochemical Technology Co., Ltd., Shanghai, China) and sodium silicate (Na₂SiO₃, 97%, Shanghai Maclin Biochemical Technology Co., Ltd.) were used as the regulator. Butyl xanthate and terpineol oil (Tiefeng Mineral Processing Pharmaceutical Co., Ltd., Kunming, China) were employed as collectors and the frother, respectively. The reagents used were analytically pure, and deionized water was used for all the experiments.

2.2. Flotation Tests

The flotation tests were conducted using a flotation machine (XFG-II, Exploration Machinery Plant, Jilin China), with a 40 mL effective-volume cell. First, the sphalerite and dolomite samples were mixed according to the weight ratio of 1:1, and 4 g of mixed samples was poured into the flotation tank with 35 mL of deionized water. The impeller speed of the flotation machine was set to 1993 r/min, and an airflow rate of 6 L/min was used. The pH of the slurry was adjusted with CaO and HCl, and then copper sulfate with a concentration of 1 × 10⁻⁴ mol/L, sodium silicate with a concentration in the range of 0–100 mg/L, and butyl xanthate with a concentration of 1 × 10⁻⁴ mol/L were added sequentially for 3 min. Then, pineol oil with a concentration of 5 mg/L was conditioned for 1 min, and the flotation time was fixed at 3 min. Finally, the obtained concentrate and tailings were filtered, dried, and weighed for tests. The average value was recorded by repeating tests three times.

2.3. Contact Angle Measurements

The contact angle is an important indicator of the wettability of the mineral surface [34]. The contact angle measurement was conducted by a contact-angle-measuring instrument (JA-200, IFEisi Precision Instrument Co., Ltd., Shanghai, China). Samples of sphalerite and dolomite with a volume of 1 × 1 × 0.3 cm³ were obtained by slicing. The sliced samples were polished with 1000-mesh sandpaper, after which the sanded samples were cleaned with deionized water. The contact angles were measured after reaction with the flotation reagents, and the average values were taken by repeating measurements 3 times.

2.4. Turbidity Experiments

Turbidity is used to characterize the dispersed state of mixed minerals [35]. A 0.5 g amount of dolomite was added to 40 mL of deionized water and stirred with a magnetic stirrer for 3 min. During the stirring, the slurry was adjusted to the required pH with CaO solution, 0.5 g of sphalerite was added, and after stirring for 3min, it was transferred to the settling tube. Deionized water in a 100 mL volume was added to the settling tube, and settled for 20 min after stirring. After that, 10 mL of supernatant was transferred to an LH-WGZ-1B turbidity meter. When the turbidity is large, it means that the number of mineral particles in the upper suspension of the sedimentation system is large, and the slurry is in a dispersed state. On the contrary, when the turbidity is small, it means that the number of mineral particles in the upper suspension is small, and the slurry is in an aggregated state [36].

2.5. Slurry Rheology Measurements

Rheology measurement analyzes the relationship between stress and strain in the flow of fluids [37]. The rheology tests were performed using a rheometer (MCR702, Anton Paar GmbH, Graz, Austria). The pure mineral sample was stirred after adding reagents according to the requirements, then 40 mL of slurry was added to the test sample cup. The shear yield stress and apparent viscosity of the slurry were measured. Shear stress data and rheological characteristic data were obtained, and the average values were taken.

2.6. Zeta Potential Measurements

The potential was determined using sphalerite and dolomite samples with a particle size of −5 μm. A 20 mg sample was transferred to a beaker with 40 mL of deionized water. After stirring with a magnetic stirrer, the pH of the solution was adjusted with CaO solution for 3min. Then, the mixture was left to stand for 10 min. A 2.5 mL amount of supernatant was transferred to an electrophoresis tank, and measured by a Malvern Zetasizer Nano-ZS900pro instrument (Sibaiji Instrument System Co., Ltd., Shanghai, China) three times. The average value was taken.

2.7. EDLVO Theory Analysis

EDLVO theory [38,39] is an extension of the classical DLVO theory, which is often used to explain the interaction forces between particles. The formula of EDLVO theory is shown in Equation (1). The equations used to calculate the interaction between sphalerite and dolomite particles are listed in

Table 2. Formula for EDLVO calculation between sphalerite and dolomite particles [29,30].

Van der Waals interaction energy	$A_{132}=\left(\sqrt{A_{11}}-\sqrt{A_{33}}\right)\left(\sqrt{A_{22}}-\sqrt{A_{33}}\right)$ $V_W=-{\displaystyle \frac{A_{132}}{6H}}{\displaystyle \frac{R_s{R}_d}{R_s+R_d}}$	A11 = 5.45 × 10−20; A22 = 4.87 × 10−20; A33 = 3.7 × 10−20
Electrostatic interaction energy	${\mathsf{\kappa }}^{-1}=0.304/\sqrt{\mathrm{C}}$ $V_E={\displaystyle \frac{\pi {\epsilon }_{\alpha }{R}_s{R}_d}{R_s+R_d}}\left({\phi }_{01}^2+{\phi }_{02}^2\right)\left\{{\displaystyle \frac{2{\phi }_{01}{\phi }_{02}}{{\phi }_{01}^2+{\phi }_{02}^2}}\ln \left[{\displaystyle \frac{1+\exp \left(-\mathsf{\kappa }\mathrm{H}\right)}{1-\exp \left(-\mathsf{\kappa }\mathrm{H}\right)}}\right]+\ln \left[1-\exp \left(-2\mathsf{\kappa }\mathrm{H}\right)\right]\right\}$.	C = 0.01 mol/L; ${\epsilon }_a$= 6.95 × 10−10 C2J−1m−1 ${\phi }_{01}$ = −44.14 × 10−3 ${\phi }_{02}$ = −39.45 × 10−3
Hydrophobic interaction energy	$\left(1+\cos \theta \right){\gamma }_L=2\left(\sqrt{{\gamma }_s^d{\gamma }_L^d}+\sqrt{{\gamma }_s^{-}{\gamma }_L^{+}}\right)$ $V_H^0=2\left[\sqrt{r_3^{+}}\left(\sqrt{r_1^{-}}+\sqrt{r_2^{-}}-\sqrt{r_3^{-}}\right)+\sqrt{r_3^{-}}\left(\sqrt{r_1^{+}}+\sqrt{r_2^{+}}-\sqrt{r_3^{+}}\right)-\sqrt{r_1^{+}{r}_2^{-}}-\sqrt{r_1^{-}{r}_2^{+}}\right]$. $V_H=2\pi {\displaystyle \frac{R_s{R}_d}{R_s+R_d}}{h}_0{V}_H^0\exp \left(-{\displaystyle \frac{H}{h_0}}\right)$	${\mathrm{r}}_3^{+}={\mathrm{r}}_3^{-}=25.5$; ${\mathsf{\gamma }}_1^{+}={\mathsf{\gamma }}_2^{+}={\mathsf{\gamma }}_3^{+}=0$ ${\mathrm{h}}_0=1$

.

$${\mathrm{V}}_T^{ED}=V_W+V_E+V_H$$

(1)

where $V_T^{ED}$ is the total interaction energy; V_W is the Van der Waals interaction energy; V_E is the electrostatic interaction energy; and V_H is the hydrophobic interaction energy. When $V_T^{ED}$ is positive, there is a repulsive force between the particles. When ${\mathrm{V}}_T^{ED}$ is negative, there is an attraction between the two particles.

3. Results and Discussion

3.1. Micro-Flotation Experiments

The effects of slurry pH and dosage of sodium silicate on the flotation separation of sphalerite and dolomite were investigated, and the results are shown in Figure 2 and Figure 3, respectively.

Figure 2 exhibits the effects of slurry pH on the flotation separation of sphalerite and dolomite in mixed minerals under the condition of a sodium silicate dosage of 20 mg/L. As can be seen from the figure, the slurry pH demonstrated a great influence on the flotation separation of sphalerite and dolomite. Under acidic conditions, the flotation recovery of Zn was inferior. When the pH rose from 3 to 11, the flotation recovery of Zn increased from 71.52% to 84.20%, and the grade of Zn increased from 53.51% to 56.96%. This result showed that the increase in slurry pH was conducive to the improvement of Zn grade and recovery. In the flotation process, the flotation recovery of MgO showed a downward trend with an increase in pH, from 4.48% to 3.54%. Simultaneously, the MgO grade in the flotation concentrate was reduced from 1.12% to 0.80%. The flotation results indicated that the slurry pH exhibited a significant effect on the flotation separation of sphalerite and dolomite, and it was conducive to the flotation separation of sphalerite and dolomite under alkaline conditions.

When the pH value of the slurry was fixed at 11, the effects of the dosage of sodium silicate on the flotation separation of sphalerite and dolomite in mixed minerals were studied, and the results are presented in Figure 3. As can be seen from the figure, the flotation recovery and Zn grade increased with an increase in sodium silicate dosage. At a sodium silicate dosage of 80 mg/L, the maximum recovery of Zn reached 86.61%, and the grade of Zn reached 57.75%. However, the flotation recovery and grade of MgO gradually decreased with the increase in the dosage of sodium silicate. When the sodium silicate dosage was in the range of 0 to 40 mg/L, the flotation recovery and grade of MgO decreased significantly. When sodium silicate was absent, the flotation recovery of MgO was 7.30%, and the MgO grade was 1.60%. When the sodium silicate concentration reached 100 mg/L, the flotation recovery and grade of MgO were minimized at 3.04% and 0.68%, respectively. The results implied that the addition of sodium silicate effectively reduced the flotation recovery of dolomite, and the floatability of sphalerite was improved, which promoted the flotation separation of sphalerite and dolomite.

3.2. Wettability Analysis

During the flotation, mineral wettability is usually measured by the contact angle. The larger the contact angle on the surface of minerals, the more hydrophobic and floatable the mineral [40]. Figure 4 illustrates the contact angles of sphalerite and dolomite before and after the addition of the sodium silicate at pH 11. This study indicated that in the absence of sodium silicate, the contact angle of the dolomite surface was 28.95° and that of the sphalerite surface was 42.3°. The results were consistent with previous studies [41,42]. When sodium silicate was present, the contact angles of dolomite and sphalerite were 20.08°and 55.38°, respectively. Compared with the absence of sodium silicate, the surface contact angle of dolomite was reduced, indicating that sodium silicate enhanced the hydrophilicity of dolomite. However, the surface contact angle of sphalerite was significantly increased, which indicated that the sodium silicate improved the hydrophobicity of sphalerite. The contact angle results were consistent with the previous flotation results.

3.3. Turbidity Tests

The slurry turbidities of sphalerite, dolomite, and mixed minerals under different concentrations of sodium silicate were measured by a turbidity meter, and the results are demonstrated in Figure 5. In the absence of sodium silicate and at pH 11, the slurry turbidity of sphalerite was lower than that of dolomite, which indicated that the dispersion of dolomite in the slurry was better than that of sphalerite in the slurry. With the increase in the sodium silicate concentration, the turbidity of dolomite slurry showed a significant upward trend, while the turbidity value of sphalerite slurry increased slowly, indicating that the concentration of sodium silicate exhibited a significant influence on the turbidity of dolomite slurry, and sodium silicate achieved an excellent dispersion effect for dolomite. When sphalerite and dolomite existed at the same time, the turbidity value showed an upward trend with an increase in sodium silicate concentration, indicating that sodium silicate can improve the dispersion of the slurry system and reduce the covering degree of dolomite on the sphalerite.

3.4. Rheological Properties of Slurry

Rheological analysis can reveal the level of interparticle interaction and aggregation in the slurry, which is closely related to flotation performance. Shear yield stress and viscosity are the main parameters for evaluating the rheological properties of flotation pulp [43].

In order to study the effect of sodium silicate concentration on the rheology of slurry of sphalerite and dolomite, the yield stress and apparent viscosity of sphalerite and dolomite slurry under different sodium silicate concentrations were investigated at pH 11, and the results are shown in Figure 5. As can be seen from Figure 6a, the yield stress of dolomite slurry was higher than that of sphalerite slurry over the entire sodium silicate concentration range. With an increase in sodium silicate concentration, the yield stress of dolomite slurry decreased from 0.098 Pa to 0.024 Pa. Nevertheless, the yield stress of sphalerite slurry remained almost constant around 0.004 Pa. In addition, when both sphalerite and dolomite existed in the slurry, the yield stress of the slurry decreased with an increase in sodium silicate concentration, indicating that the addition of sodium silicate changed the rheological properties of the mixed slurry. Figure 6b shows the apparent viscosity of the slurry as a function of sodium silicate concentration. It was found that the viscosity of sphalerite slurry was almost not affected by sodium silicate concentration, and was lower than 11 mPa·s. However, with an increase in sodium silicate concentration, the apparent viscosity of dolomite decreased significantly, from 15.21 mPa·s to 10.21 mPa·s. In addition, the apparent viscosity of the slurry in the coexistence system of sphalerite and dolomite showed a downward trend, from 13.78 mPa·s to 10.12 mPa·s. The results indicated that the addition of sodium silicate reduced the yield stress and apparent viscosity of the mixed slurry, thereby increasing the flotation rate and selectivity of the flotation reagent during the flotation process [44,45]. Thus, the flotation separation effect between sphalerite and dolomite was improved.

3.5. Zeta Potential Analysis

The adsorption of flotation reagents often alters the electrostatic properties of the mineral surface. Figure 7 depicts the zeta potential of dolomite and sphalerite as a function of sodium silicate concentration at pH 11. In the absence of sodium silicate, the potential of dolomite was positive, while the potential of sphalerite was negative, leading to an electrostatic attraction between them. As the concentration of sodium silicate increased, the zeta potential on the surface of sphalerite and dolomite gradually decreased. When the concentration of sodium silicate was higher than 11 mg/L, the potential of dolomite and sphalerite were negative, resulting in them repelling each other and reducing the cover phenomenon of dolomite on the surface of sphalerite. These were consistent with the turbidity and flotation results.

3.6. EDLVO Theory

Figure 8 shows the interaction energies between dolomite and sphalerite particles before and after reaction with sodium silicate at pH 11. In the absence of sodium silicate, the Van der Waals energy, electrostatic energy, and hydrophobic energy between sphalerite and dolomite particles were negative, indicating that the forces between the particles were attractive. When sodium silicate was present, the electrostatic energy, hydrophobic energy, and total energy of dolomite and sphalerite became positive, indicating that a strong mutual repulsion force between the two particles appeared, and the hydrophobic energy was the main force [46].

3.7. Discussion

In the absence of sodium silicate, the zeta potentials on the surface of sphalerite and dolomite were negative and positive, respectively, resulting in electrostatic adsorption and inferior flotation separation. After treatment with sodium silicate, both sphalerite and dolomite exhibited negative zeta potentials, resulting in potential repulsion. In addition, sodium silicate enhanced the hydrophilicity of dolomite and the hydrophobicity of sphalerite, and the dispersion effect and rheological properties of pulp were improved. In the presence of sodium silicate, a strong mutual repulsion force between the two particles appeared. Therefore, sodium silicate effectively strengthened the flotation separation of sphalerite and dolomite. A schematic diagram is shown in Figure 9.

4. Conclusions

The effects of dolomite on sphalerite flotation and the enhanced separation mechanism of dolomite and sphalerite were investigated in this study. The conclusions are as follows:

(1)

Sodium silicate improved the grade and recovery of sphalerite, while it reduced the floatability of dolomite, enhancing the flotation separation of sphalerite and dolomite. The surface contact angle of dolomite decreased, while that of sphalerite increased after reaction with sodium silicate. With an increase in sodium silicate dosage, the dispersion effect of slurry was improved.

(2)

Sodium silicate improved the dispersion of the slurry system and reduced the covering degree of dolomite on the sphalerite. In addition, it reduced the yield stress and apparent viscosity of the mixed slurry.

(3)

In the absence of sodium silicate, the zeta potentials on the sphalerite and dolomite surfaces were negative and positive, respectively, resulting in heterogeneous agglomeration. In both sphalerite and dolomite treated with sodium silicate, the zeta potentials turned negative, resulting in electrostatic repulsion.

(4)

EDLVO theoretical calculations indicated that the cover between sphalerite and dolomite particles mainly depended on electrostatic interaction energy when sodium silicate was absent. In the presence of sodium silicate, a strong mutual repulsion force between the two particles appeared.