New Technology of Zinc Oxide Concentrate Flotation + Mechanical Stirring Defoaming Zinc Leaching

Abstract

During the acidic leaching of flotation zinc oxide concentrates, CO₂ released from carbonate decomposition generates viscous foams that disrupt process stability. This study introduces an innovative synergistic defoaming process combining air flotation and mechanical methods. Fine air bubbles destabilize the foam, while mechanical defoaming enhances the removal of residual bubbles. The results indicate that the defoaming process combining air flotation with mechanical stirring effectively reduces foam generation during the acid leaching of zinc oxide concentrates, enhances leaching efficiency, and improves process stability. This method provides an effective solution for foam control and offers a new approach for the treatment of zinc oxide concentrates.

1. Introduction

Currently, the main raw material for zinc smelting is sphalerite, which undergoes a process of roasting (with fluidized bed), zinc calcine leaching, purification, and electrorefining, or direct oxidative pressure leaching of sphalerite, followed by purification and electrorefining to produce zinc [1,2]. However, with the rapid development of the zinc smelting industry, the available sphalerite resources are becoming increasingly scarce. To meet the growing demand for zinc-containing materials in China’s economic development [3,4], it is essential to conduct research and development on the utilization of low-grade, high-clay, and difficult-to-select zinc oxide ores. Through extensive exploration by mineral-processing workers, significant progress has been made in the development of flotation technology for zinc oxide ores [5,6]. Under appropriate beneficiation conditions, zinc oxide concentrates containing up to 44% Zn can be produced [7,8,9]. Alongside the advancements in zinc oxide ore beneficiation technology, there are still numerous deficiencies in the recovery and utilization processes of zinc oxide concentrates [10]. For example, the leaching process of zinc oxide concentrates generates a large number of bubbles that lead to overflowing, which can severely impact the entire zinc smelting leaching process [11,12,13]. Residual flotation reagents may also pose a potential risk of causing short-circuiting during the zinc electrowinning process [14]. The common treatment processes for zinc oxide concentrates currently include pre-treatment with a certain amount of oxidants before leaching, low-temperature roasting to remove flotation reagents followed by leaching, blending zinc oxide concentrates with zinc calcine for combined leaching, and adding specific defoamers to eliminate the impact of excessive bubbles generated during the leaching process.

The flotation of zinc oxide ore has a low grade, and the combined selection of oxygen and sulfur is not thorough, containing a large amount of organic matter, resulting in the raw material being unsuitable for traditional zinc smelting processes or having high production costs. The direct acid leaching process can cause serious overflow phenomena (Figure 1), making acid leaching impossible. The pyrometallurgical volatilization process has high production costs and is not environmentally friendly. The ammonia leaching process requires the construction of new production lines, involves significant equipment investment, has poor working conditions, and results in high production costs. The flotation zinc oxide concentrate from a certain Chinese enterprise contains a large amount of carbonates (calcite, smithsonite, dolomite, siderite) and amine organic matter. During acidic leaching, the escape of CO₂ causes the amine organic matter to generate a large amount of sticky foam which carries the pulp overflow, making acid leaching difficult. The project aligns with the actual production situation of the enterprise, focusing on the research of direct acid leaching processes to enable the acidic leaching of flotation zinc oxide concentrate. The project tackles key technologies such as the flow rate and pressure of air stirring, the rotational speed of mechanical defoaming devices, the installation method of blades, and the selection of defoaming areas, thereby achieving the industrialization of direct acid leaching of flotation zinc oxide concentrate.

2. Preparation Methods

2.1. Experimental Materials and Equipment

The experimental materials used were zinc oxide concentrate (dry sample), 98% concentrated H₂SO₄, and tap water. The selected raw material was flotation zinc oxide concentrate, and its composition is shown in

Table 1. Chemical composition analysis table of zinc oxide concentrate (%, Zn_d: dissolubility).

Zn	Znd	Fe	SiO2	Cd	Co	Na2S	MgO	CaO	C	Moisture	Pb	Oxidation Rate
14.75	10.63	13.88	27.12	0.18	0.004	0.2	0.28	7.18	6.8	15.6	0.87	72.07

and

Table 2. Zinc chemical phase analysis of zinc oxide concentrate (%).

ZnSO4	ZnO	ZnS	Zinc Iron Spinel and Other	Total Zn
1.54	9.09	4.08	0.044	14.75

. The total zinc content is approximately 14.75%, which includes various forms of zinc compounds such as zinc sulfate (1.54%), zinc oxide (9.09%), zinc sulfide (4.08%), as well as zinc ferrite and other impurities (0.044%).

2.2. Outcome Representation

The leaching rate is calculated according to the following formula:

$$P= (1 -{\displaystyle \frac{m_0-m_1}{m_0}}) \times 100\%$$

Among them, P represents the leaching rate (%), m₀ is the zinc content of the raw material (g/kg/t), and m₁ is the mass of slag zinc (g/kg/t) excluding water solubility.

3. Results and Discussion

3.1. Leaching Results

The results of the air defoaming exploration test are shown in the table, and the experiment is shown in Figure 2. In the experiment, when compressed air at different pressures (0.02 and 0.05 MPa) was introduced, intense foam formation appeared on the slurry surface, especially when the gas flow rate reached 7.5 L/min and 15 L/min, at which point foam generation became more pronounced. This indicates that the flotation process significantly promotes foam generation and stability, and the variations in gas flow rate and pressure directly affect the intensity of foam formation.

In terms of leaching efficiency, the experimental results in

Table 3. Defoaming leaching data of ‘air flotation + mechanical stirring’ in the leaching process of zinc oxide concentrate.

No.	Oxidation Rate (%)	Quantity of Slag (g)	Leaching Liquid (g/L)	Leaching Rate of ZnO (%)	Slag Rate (%)	Barometric Pressure (Mpa)	Amount of Wind (L/min)	Leaching Method
1	76.59%	603.83	2240	93.14	53.52	0.02	7.5	air flotation
2		596.54	4500	96.56	59.65	0.05	15	air flotation
3		702.89	4610	93.33	62.30	0.08	22.5	air flotation
						Stirring rate r/min	Problem
4		557.85	4610	91.93	65.73	100	A large amount of foam is generated during the acid leaching process, and the foam is viscous and difficult to break down.	mechanical stirring
5		579.44	4770	94.21	61.95	300		mechanical stirring
6		610.21	4900	93.05	67.02	500		mechanical stirring
7		607.89	4900	98.15	61.02	0.05	15	air flotation+ mechanical stirring (300 r/min)

show that using a flotation combined with mechanical stirring defoaming process, the ZnO leaching rate ranged from 93% to 99%, while the slag rate was between 53% and 63%. When only mechanical stirring was used, a large amount of foam was generated immediately after acid addition before slowly dissipating. Even after subsequent acid additions, foam continued to form, resembling the phenomenon observed during the initial acid addition. When only flotation defoaming was applied, the liquid-to-solid ratio in the slurry was seriously imbalanced.

Flotation technology works by introducing fine bubbles, which interact with the flotation reagents and surfactants in the slurry [15,16,17]. The buoyancy of the bubbles helps bring the foam to the surface of the slurry, forming a froth layer, which effectively reduces the concentration of flotation reagents, weakens foam stability, and reduces foam generation. When the flotation is combined with mechanical stirring, the foam that rises to the surface of the slurry after the flotation is subjected to shear forces under mechanical stirring, which destroys its structure, promotes foam rupture, and accelerates bubble dissipation.

Moreover, mechanical stirring not only effectively reduces bubble volume but also improves the slurry’s flowability and uniformity, thereby enhancing the stability and efficiency of the acid leaching reaction. By introducing stirring forces, bubbles can react more easily with other substances in the slurry, improving leaching efficiency. The combination of flotation and mechanical stirring not only provides a stirring effect but also aids in bubble breakup. Compared to single leaching methods, this combined approach can significantly improve the ZnO leaching rate and reduce zinc loss.

3.2. H₂S Dilution Experiment

During the sulfuric acid leaching of zinc oxide concentrate, H₂S gas is produced. According to China’s H₂S emission standards, the permissible volume concentration in the air is 0.002%, which corresponds to 20 ppm or 28.83 mg/m³ [18].

①

Theoretical Calculation

The content of Na₂S in zinc oxide concentrate is 0.3%, which means there is 3 kg of Na₂S in 1 ton of zinc oxide concentrate. The chemical reaction occurring during the leaching process is: Na₂S + H₂SO₄ = Na₂SO₄ + H₂S. Therefore, 0.871 kg of H₂S gas is produced when leaching 1 ton of zinc oxide concentrate. To meet the emission standard of 28.83 mg/m³, assuming the entire neutral leaching period is 1 h, an air volume of 871,000/60/28.83 = 503.53 m³/min is required.

②

Experimental Measurement

Samples from groups 2, 5, and 7 were analyzed in the experiment (Table 3), and the acid leachate concentration was varied. The optimal leaching conditions for group 7 had a leachate pH of 3–4, while the new experiment (#8) reduced the pH to <1.5. The total sampling time was 30 min. The sampling process is shown in Figure 3a, and the measurement results are presented in

Table 4. Sample analysis of H₂S gas content.

No.	H2S (mg/m3)
2	10.30
5	5.21
7	1.16
8	44.15

.

The sampling and analysis results (Table 4) show that the leaching process generates H₂S gas, and as the acidity increases, the concentration reaches 44.15 mg/m³. Despite the ZnO leaching rate being as high as 98.83%, the amount of H₂S produced is significantly higher than the emission standard. A comparison between the H₂S emissions from the synergistic and single leaching processes clearly indicates that the flotation + mechanical stirring, combined with the defoaming leaching process, leads to a more uniform distribution of sulfuric acid in the slurry, promoting the leaching reaction of zinc oxide concentrate. The effective utilization of acid allows for a more complete leaching reaction, thereby reducing the possibility of excess acid reacting with the minerals to generate hydrogen sulfide. At the same time, this process is effectively applied to the zinc oxide concentrate leaching production, addressing the issues of severe foaming (Figure 3b,c).

4. Conclusions

In addressing the issue of CO₂ gas generation from carbonates during the acid leaching process of a domestic flotation oxidized zinc concentrate, which leads to the production of large amounts of sticky foam by flotation reagents and causes foam overflow, severely affecting the stability of hydrometallurgical processes and even making the leaching process infeasible, this experiment innovatively introduces an “air flotation + mechanical” synergistic defoaming technology. This technology introduces fine bubbles through compressed air, causing the bubbles to attach to the foam liquid film, making it unstable and thereby disrupting the foam. The bubbles rise and carry the foam away from the liquid surface, achieving rapid defoaming. The flotation defoaming process, combined with mechanical defoaming, enhances the removal of residual bubbles, significantly improving the stability and leaching efficiency (98.15%) of the zinc oxide concentrate leaching process. Furthermore, the H₂S gas emissions meet the required standards. This process not only effectively solves the problem of foam interference in the leaching of oxidized zinc concentrate but also achieves efficient synergy between the smelting and beneficiation processes, providing a novel solution for the disposal of oxidized zinc concentrates in the industry.