Introduction to the Sag Mill Design

Sag mill design, as an advanced form of autogenous milling technology, cleverly integrates the milled ore and a modest amount of added steel balls as dual media. These media collide, slide, and compress against each other inside the drum, forming an efficient pulverizing mechanism that finely grinds the ore. Compared to wet grinding and conventional crushing methods, the semi-autogenous milling process significantly reduces dust generation, optimizes the operational environment, lowers energy consumption, and greatly facilitates material transportation and management. Its broad applicability is attributed to its powerful crushing capability, which can reduce the feed size (200mm—350mm) to below a few millimeters in one go, achieving a high crushing ratio. This effectively shortens the process flow, saves space and infrastructure investment, and reduces staffing and maintenance costs.

Key Role of Steel Balls in Sag Mill Design

In sag mill design, the precise addition of a small number of steel balls is crucial for eliminating the accumulation of hard rocks, enhancing mill efficiency, and reducing liner wear. Typically, the steel ball filling rate is controlled between 2% and 8%, while the rotation speed is maintained in the high-efficiency range of 70%—80%, ensuring an impressive crushing ratio of 100—150. For instance, a gold mine's sag mill design specifies a ball load of 6 tons, with an accurate ball filling rate of 6%. The grate hole width is meticulously set at 12mm, with a stable rotational speed of 17rpm, achieving a crushing ratio of about 145, while maintaining the grinding concentration in the ideal range of 75%—85%.

Application Range and Ore Types of Semi-autogenous Milling Design

However, it is worth noting that the application of semi-autogenous milling technology is not universal; it is strictly limited by the type of ore. To fully leverage its advantages, one must closely integrate the ore characteristics and production conditions of the mine in the design and practice, continuously optimizing solutions to potential issues. Specifically, the design of sag mill excels in handling ores with high mud content, high viscosity, difficult-to-crush and screen materials, and severely weathered ores, as well as ores with altered surrounding rocks, coarse grained mineral intergrowth, well-developed dissociation, high relative density, or strong corrosiveness.

Analysis of the Internal Crushing Mechanism of Semi-autogenous Mills

Inside the semi-autogenous mill, the ore crushing process is intricate and complex, relying mainly on the impact force during the free fall of the ore and steel balls, the mutual grinding and stripping force between the steel balls, and the instantaneous stress generated by the transition between pressure and tension on the ore. This process not only achieves natural grading of ore blocks and particles but also promotes efficient autogenous grinding, media grinding, and mutual milling effects. The unique design of semi-autogenous mills, such as the gravitational potential energy generated by a large diameter drum, the anti-segregation discharge advantage of a short drum, and a reasonable length-to-diameter ratio (0.30—0.35), collectively form the cornerstone of its efficient operation.

Operational Optimization and Performance Improvement of Sag Mill Design

Additionally, the operational optimization of semi-autogenous mills should not be overlooked. Under fixed specifications and rotational speed, precisely controlling the input amount of material and the ratio of large to small ore blocks is key to ensuring a stable and efficient grinding process. It is generally recommended to maintain the material filling rate between 30% and 40%, striving to keep the feed quantity and size ratio relatively stable, with an optimal ratio of 50% for both large and small blocks. However, when dealing with hard ores, one must remain vigilant against the accumulation of hard rocks, timely adjusting strategies to tackle the challenge of "difficult-to-grind particles."