The largest Premier™ mill yet

44’ SAG mill

Pursuing more sustainable comminution circuits is a top priority for mining companies. Especially as today’s projects face increasing environmental, social, and governance pressures, rising energy costs and decreasing ore grades. From a grinding perspective, the most efficient circuits and high-capacity plants often use the largest grinding equipment, which in turn minimizes the number of equipment lines. Metso continues to stretch the limits and has designed a SAG mill with increased diameter and power beyond 42 ft and 28 MW respectively.

Innovative design

Horizontal grinding mills are often seen as mature technology. With the first mills delivered in the early 1900s and trunnion-supported mills appearing unchanged for 30 years, it's easy to assume little technological advancement. However, engineering tools and manufacturing have advanced significantly. New mill sizes can now be evaluated with high accuracy.

Metso’s design experience and use of advanced technologies and simulation tools minimize manufacturing risks while delivering robust, safe solutions. Metso components provide best-in-class availability. Drive options include gearless and geared. When evaluating drive technology, the following parameters should be considered:

Required power
Efficiency
Reliability
Footprint
Sustainment

System performance

System analysis is essential for any mill delivery, as all structures are susceptible to vibration. This ensures the combined system meets vibration and motor rotor to stator clearance requirements, serviceability and limit state. Assessments of the combined mill system, consisting include:

Strata
Foundation
Stator (or pinon)
Rotor (or girth gear)
GMD air gap stiffnesses
Grinding mill
Mill main bearings
Main bearing oil film stiffness

Modern software and hardware have fueled the development and adoption of finite element analysis (FEA). This includes a completely detailed model of all geometrical features and the simulating rotation of more than 16 load cases. Metso extends the scope of normal analysis (system stiffness (air gap check) and modal analysis) to include harmonic response analysis. This allows the measurement of vibration levels resulting from known excitation source and response to all modes of concern.

Optimize process efficiency

Metso provides a holistic approach to risk mitigation to improve mill availability and utilization. There is a plethora of design, manufacturing, installation, and operational considerations to be detailed when pursuing a step-up beyond the current size limits. It is critical that the design is developed with consideration of manufacturing requirements and capabilities. Design iterations are often required to balance mill design with manufacturer feedback to ensure that the design intent can be translated to manufacturing, and any manufacturing constraints are captured in design.

This collaboration process has been conducted as part of the detailed design for the 44 ft SAG mill. The culmination of this work is that trunnion, head, and shell manufacturing feasibility for a 44 ft SAG mill have been confirmed to fall well within the current capabilities of the respective manufacturers. It is important to note that only proven suppliers were engaged to confirm feasibility. Specifically, only manufacturers who have previously produced components for 40 ft SAG mills.

For SAG mills, the risk of increasing the SAG mill size, and mill throughput, can be mitigated with the combined implementation of high-efficiency grate-discharge solutions and a highly accurate simulation tool. Overflow ball mills require inlet and outlet designs capable of supporting very high throughputs, considering both material flow and media retention. Emperical calculations and detailed simulation are now capable of predicting overflow conditions.