Performance Comparison of Overflow and Grid-Type Ball Mills: How Discharge Methods Shape Grinding Efficiency

For metal concentrators, grinding efficiency directly impacts the entire mineral processing chain—and the performance of ball mills, the core grinding equipment, is largely determined by their discharge methods. Overflow ball mills (with free overflow discharge) and grid-type ball mills (with low-liquid-level forced discharge) exhibit distinct performance characteristics, making them suitable for different grinding scenarios.​

The overflow ball mill’s performance is closely tied to its "free overflow" discharge. During operation, the pulp level in the cylinder is relatively high—this high level ensures that ore particles are fully in contact with steel balls, laying the foundation for fine grinding. However, the high pulp level also leads to two notable traits: first, ore discharge relies on self-flow, meaning that ore particles that have already reached the required fineness may remain in the cylinder and be ground repeatedly, causing "over-grinding." Over-grinding not only wastes energy but also produces excessive fine particles that are difficult to recover in subsequent processes (such as flotation), reducing overall mineral recovery. Second, the high pulp level increases the resistance encountered by steel balls when they fall, weakening the impact and grinding effect on ore particles, resulting in slightly lower productivity compared to grid-type mills.​

Despite these limitations, the overflow ball mill has irreplaceable advantages for fine grinding. Its simple structure (no complex grid components) makes operation and maintenance extremely convenient—workers only need to regularly check the wear of the cylinder lining and the lubrication of main bearings, reducing downtime. More importantly, it can stably produce ore particles with a fineness of less than 0.2mm, which is critical for subsequent processes like flotation. Flotation requires ore particles to be fine enough to expose mineral surfaces, and the overflow mill’s ability to control fineness precisely ensures the efficiency of the flotation process.​

In contrast, the grid-type ball mill’s "forced discharge" design revolutionizes its performance. The discharge grid plate at the end cover creates a height difference in pulp level from the feed end to the discharge end, allowing ground ore particles to be quickly discharged through the grid. This rapid discharge fundamentally solves the over-grinding problem—ore particles that meet the coarse grinding requirement (>0.2mm) are promptly removed, avoiding repeated grinding. At the same time, the grid plate blocks steel balls, enabling the mill to load more steel balls (including small balls). When these steel balls fall, the low pulp level reduces resistance, maximizing their impact force on coarse ore particles. As a result, the grid-type mill’s productivity is 10%-25% higher than that of the overflow type, and its specific productivity (output per unit power consumption) is also superior—an important advantage for concentrators pursuing high throughput.​

However, the forced discharge structure comes with trade-offs. The addition of radial ribs, dustpan-shaped lining plates, and grid plates makes the grid-type mill’s structure more complex, increasing its weight by 15%-20% compared to the overflow type of the same specification. This not only raises manufacturing costs but also increases power consumption during operation—since more energy is needed to drive the heavier equipment and overcome the resistance of the grid structure.​

In summary, the overflow ball mill excels in fine grinding with stable fineness and easy maintenance, while the grid-type mill leads in coarse grinding with high productivity and low over-grinding. Choosing between them requires aligning their performance traits with the concentrator’s specific grinding goals.