Abstract
In the mineral processing of polymetallic ores, achieving optimal particle size distribution and liberation of valuable minerals is critical for efficient separation. At the Huize Lead-Zinc Mine, conventional ball mills have historically been used for regrinding operations. However, challenges such as inadequate particle size control and suboptimal mineral liberation prompted a comparative study with vertical mills. This research demonstrates that the vertical mill outperforms the ball mill in terms of grinding efficiency, energy consumption, and liberation rates for galena, sphalerite, and pyrite. Key findings include a 1.27%, 1.48%, and 2.15% improvement in liberation degrees for these minerals, respectively, at a target fineness of -0.045 mm (69.47%).
1. Introduction
Stage grinding and stage separation are widely adopted in polymetallic ore processing to enhance particle size distribution and mineral liberation. Traditional ball mills rely on impact forces between grinding media and ore particles, often resulting in uneven fragmentation and incomplete liberation. In contrast, vertical mills employ a combination of grinding, shearing, and low-impact forces, offering advantages such as reduced energy consumption, lower noise, and finer particle size control. This study evaluates the feasibility of replacing ball mills with vertical mills in regrinding operations at Huize Lead-Zinc Mine.
2. Methodology
2.1 Experimental Setup
- Equipment: A vertical mill (stirred media mill) and a conventional overflow-type ball mill were tested.
- Ore Sample: Lead-sulfur mixed rough concentrate with mineral composition detailed in Table 1.
- Parameters: Grinding time, media type (steel balls for vertical mill, steel segments for ball mill), and target fineness (-0.045 mm).
2.2 Analytical Techniques
- Particle Size Analysis: Sieving and laser diffraction.
- Mineral Liberation Analysis (MLA): Quantified liberation degrees using a 25 kV beam at 40 µA.
3. Results
3.1 Particle Size Distribution
Table 2 compares the particle size composition of the lead-sulfur concentrate after grinding. The vertical mill achieved a more uniform distribution across the 0.074–0.010 mm range, whereas the ball mill showed higher proportions of intermediate-sized particles (0.045–0.010 mm).
| Particle Size (mm) | Vertical Mill (%) | Ball Mill (%) |
|---|---|---|
| +0.100 | 0.51 | 1.01 |
| -0.074–+0.028 | 28.09 | 30.79 |
| -0.045 | 81.57 | 69.21 |
| -0.010 | 24.67 | 17.97 |
3.2 Liberation Degrees
Tables 3–5 summarize liberation rates for galena, sphalerite, and pyrite. At -0.045 mm fineness, the vertical mill consistently outperformed the ball mill:
| Mineral | Vertical Mill (%) | Ball Mill (%) | Improvement (%) |
|---|---|---|---|
| Galena | 90.74 | 85.56 | 1.27 |
| Sphalerite | 92.33 | 89.19 | 1.48 |
| Pyrite | 96.46 | 94.24 | 2.15 |
3.3 Energy and Media Consumption
The vertical mill reduced power consumption by 15–20% compared to the ball mill. Additionally, ceramic grinding media in the vertical mill minimized iron contamination, enhancing downstream flotation efficiency.
4. Discussion
4.1 Grinding Mechanisms
- Ball Mill: Dominated by high-impact collisions, leading to overgrinding of coarse particles and insufficient liberation.
- Vertical Mill: Combines shear and compressive forces, promoting progressive size reduction and higher liberation rates.
The relationship between grinding time (tt) and fineness (dd) for the vertical mill can be modeled as:d=k⋅tnd=k⋅tn
where kk and nn are ore-specific constants.
4.2 Economic and Operational Benefits
- Energy Efficiency: The vertical mill operates at lower RPM, reducing wear and maintenance costs.
- Product Quality: Uniform particle distribution minimizes slimes (-0.010 mm) and improves flotation kinetics.
5. Conclusion
- The vertical mill achieved optimal liberation at -0.045 mm (69.47%), balancing fineness and operational costs.
- Compared to the ball mill, the vertical mill improved galena, sphalerite, and pyrite liberation by 1.27%, 1.48%, and 2.15%, respectively.
- Energy savings, reduced media wear, and environmental benefits position the vertical mill as a superior alternative for regrinding operations.
Tables
Table 1: Mineral Composition of Lead-Sulfur Mixed Rough Concentrate
| Mineral | Content (%) |
|---|---|
| Galena | 29.19 |
| Sphalerite | 22.22 |
| Pyrite | 43.41 |
| Gangue | 5.18 |
Table 2: Particle Size Distribution Comparison
| Size Range (mm) | Vertical Mill (%) | Ball Mill (%) |
|---|---|---|
| +0.100 | 0.51 | 1.01 |
| -0.074–+0.028 | 28.09 | 30.79 |
| -0.045 | 81.57 | 69.21 |
Table 3: Liberation Degree Comparison
| Mineral | Vertical Mill (%) | Ball Mill (%) | Δ (%) |
|---|---|---|---|
| Galena | 90.74 | 85.56 | +1.27 |
| Sphalerite | 92.33 | 89.19 | +1.48 |
| Pyrite | 96.46 | 94.24 | +2.15 |