In polymetallic mineral processing, uneven dissemination of valuable minerals necessitates staged grinding and separation circuits to optimize particle size distribution and mineral liberation. Conventional overflow ball mills dominate regrinding operations but exhibit limitations. Ball mills primarily rely on impact breakage via point-contact collisions between media and ore. Excessive impact forces often cause preferential fracturing along mineral grain boundaries, yielding poor liberation of target minerals:
$$
\text{Impact Efficiency} \propto \frac{E_k \cdot \rho_{\text{media}}}{d_{\text{grain}}}
$$
where $E_k$ is kinetic energy, $\rho_{\text{media}}$ is media density, and $d_{\text{grain}}$ is grain size. Vertical stirred mills (VSMs) offer an alternative with dominant abrasion mechanisms, reducing overgrinding. This study compares a VSM against a conventional ball mill using steel segments in regrinding Huize lead-sulfur bulk concentrate.
1. Feed Characteristics
The lead-sulfur bulk concentrate assayed 29.19% sphalerite, 22.22% galena, and 43.41% pyrite. Initial particle size analysis revealed suboptimal liberation:
| Size Fraction (mm) | Yield (%) | Cumulative Oversize (%) | Cumulative Undersize (%) |
|---|---|---|---|
| +0.100 | 5.42 | 100.00 | 94.58 |
| 0.100–0.074 | 16.56 | 94.58 | 78.02 |
| 0.074–0.045 | 21.08 | 78.02 | 56.94 |
| 0.045–0.038 | 8.77 | 56.94 | 48.17 |
| -0.010 | 14.11 | 14.11 | 0.00 |
2. Experimental Methodology
Vertical Mill (JM-260): Operated at 50% pulp density with φ15 mm steel balls (35% filling). Regrinding durations targeted -0.045 mm yields of 69.47%, 76.14%, and 81.57%.
Ball Mill (Ø450×450 mm): Used steel segments (20% filling) to match production conditions, achieving -0.045 mm ≈70%. Liberation was quantified using Mineral Liberation Analysis (MLA).
3. Grinding Performance & Liberation Analysis
VSM Product Size vs. Grinding Time:
| Parameter | 0 min | 1 min 40 s | 2 min 20 s | 3 min |
|---|---|---|---|---|
| -0.045 mm (%) | 56.94 | 69.47 | 76.14 | 81.57 |
| +0.100 mm (%) | 5.42 | 0.41 | 0.40 | 0.51 |
| -0.010 mm (%) | 14.11 | 19.31 | 23.48 | 24.67 |
Mineral Liberation at Different Fineness (VSM):
| Mineral | Liberation Degree (≥3/4) at -0.045 mm 69.47% | Liberation Degree (≥3/4) at -0.045 mm 76.14% | Liberation Degree (≥3/4) at -0.045 mm 81.57% |
|---|---|---|---|
| Galena | 86.83% | 90.74% | 92.48% |
| Sphalerite | 90.67% | 92.33% | 93.39% |
| Pyrite | 96.39% | 96.46% | 96.90% |
Liberation gains diminished beyond 70% fineness:
$$
\Delta \text{Lib}_{70→76} > \Delta \text{Lib}_{76→81}
$$
4. VSM vs. Ball Mill at Equivalent Fineness (-0.045 mm ≈70%)
| Mill Type | -0.045+0.010 mm (%) | -0.010 mm (%) | Galena Lib. ≥3/4 (%) | Sphalerite Lib. ≥3/4 (%) | Pyrite Lib. ≥3/4 (%) |
|---|---|---|---|---|---|
| Vertical Mill | 50.16 | 19.31 | 86.83 | 90.67 | 96.39 |
| Ball Mill | 51.24 | 17.97 | 85.56 | 89.19 | 94.24 |
| Difference (Δ) | -1.08 | +1.34 | +1.27 | +1.48 | +2.15 |
The VSM produced a more uniform particle size distribution across the 0.074–0.010 mm range. Its superior liberation stems from dominant shear/abrasion breakage:
$$
\text{Abrasion Efficiency} = k \cdot \int_{t_0}^{t} \tau \cdot \dot{\gamma} dt
$$
where $\tau$ is shear stress and $\dot{\gamma}$ is shear rate.
5. Conclusion
At optimal VSM regrinding fineness (-0.045 mm 69.47%), target mineral liberation exceeded 90%. The VSM outperformed the ball mill by 1.27–2.15 percentage points in ≥3/4 liberation at equivalent product size. Combined with inherent advantages like lower energy consumption and reduced overgrinding, VSMs present a viable replacement for conventional ball mills in Huize’s regrinding circuit. Future work will optimize VSM media type (e.g., ceramic balls) to eliminate iron contamination.