Overflow Ball Mills VS. Grid-Type Ball Mills

In the grinding process of metal concentrators, ball mills are the most widely used core equipment, which can be divided into two main types: overflow ball mills and grid-type ball mills according to the ore discharge method. The two types have distinct characteristics in structural design and working performance, adapting to different grinding needs, and are key factors determining grinding efficiency and mineral processing effects.

1. Overflow Ball Mill: The Preferred Equipment for Fine Grinding Scenarios

The overflow ball mill is characterized by "free overflow discharge", and its structure consists of a cylinder, end covers, main bearings, hollow journals, transmission gears, and ore feeders. Its cylinder is welded with 15-36mm thick steel plates, with flanges at both ends precisely matched with the end covers. The hollow journals are welded to the end covers and supported on the main bearings — among them, the journal close to the transmission gear ring is fixed with a shoulder, and the other journal can expand and contract freely to adapt to the deformation of the cylinder caused by heat or load. The inner diameter of the hollow journal at the discharge end is slightly larger than that at the feed end, and some are equipped with reverse spiral blades inside to prevent the discharge of coarse-grained materials and the loss of small balls.

The transmission method is flexibly selected according to the specifications: large-scale equipment usually uses a synchronous motor for direct drive, which has high transmission efficiency and improves the power factor of the power grid; medium and large-scale equipment mostly uses an asynchronous motor with a gear reducer; small-scale equipment is driven by an asynchronous motor through a V-belt. Ore feeders are divided into three types: drum-type, volute-type, and combined-type. The drum-type is suitable for open-circuit grinding, the volute-type can scoop ore from the lower ore tank, and the combined-type ore feeder handles both coarse-grained feeding and backfill processing, adapting to the closed-circuit grinding process.

In terms of performance, the pulp level in the cylinder of the overflow ball mill is relatively high, and ore discharge relies on self-flow. Although it may easily cause "over-grinding" of some ore particles that have met the standard and has a slightly lower productivity, it has a simple structure, convenient operation and maintenance, and can produce finer ore particles (grinding fineness < 0.2mm). Therefore, it is often used for fine grinding operations or the second stage of two-stage continuous grinding.

2. Grid-Type Ball Mill: The Efficient Choice for Coarse Grinding Links

The grid-type ball mill is basically the same in structure as the overflow type, with the core difference being the addition of a discharge grid plate at the discharge end — the end cover is divided into eight fan-shaped chambers by radial ribs, each chamber is equipped with a dustpan-shaped lining plate, and a grid lining plate (divided into two groups, pressed by wedge iron) is fixed on it, forming a forced discharge structure.

Its discharge method is "low-liquid-level forced discharge". The pulp in the cylinder forms a height difference from the feed end to the discharge end, and the ground ore particles are quickly discharged through the grid plate, effectively reducing the "over-grinding" phenomenon. At the same time, the pulp storage capacity is small and the grid plate blocks the steel balls, allowing more steel balls (including small balls) to be loaded. When the steel balls fall, the impact of pulp resistance on the grinding effect is small. Its productivity is 10%-25% higher than that of the overflow ball mill, and the specific productivity under unit power consumption is better. However, due to the grid structure design, its structure is more complex, the weight is larger, and the power consumption is higher.

With the advantage of efficient forced discharge, the grid-type ball mill is suitable for coarse grinding operations with a grinding fineness > 0.2mm, or as the first stage of two-stage continuous grinding, and is widely used in the coarse grinding link of concentrators.

3. Core of Selection: Matching Grinding Needs and Production Goals

The choice between overflow ball mills and grid-type ball mills depends on the requirements for grinding fineness and production efficiency: if fine-grained products are needed (such as fine grinding before the subsequent flotation process), the overflow ball mill is preferred to balance product fineness and operational convenience; if it is for the coarse grinding link and high productivity and low "over-grinding" rate are pursued, the grid-type ball mill is a better choice. Reasonable selection can maximize grinding efficiency and lay a good foundation for subsequent mineral processing processes.