Background
A copper concentrator in Zambia operated three 5.5-meter diameter ball mills in its primary grinding circuit. The mills processed copper sulfide ore ground to 80% passing 75 microns, using 65mm steel grinding balls. The mills ran 24 hours per day with planned maintenance stops every 8 weeks.
The existing mill liners were manganese steel β a standard configuration. Each liner set lasted approximately 10β12 months. A liner change required a planned 36-hour shutdown per mill, with a crane crew and four maintenance staff to remove and replace the heavy steel liner sections.
The concentrator's maintenance manager wanted to explore rubber liners after seeing them used successfully at a neighboring operation.
Evaluation
Before committing to a full conversion, the concentrator management wanted to understand whether rubber liners were appropriate for their specific mill configuration. Key questions:
1. Are the grinding balls (65mm) within rubber liner capability? 65mm balls are within the standard range for rubber mill liners β typically rubber handles balls up to 80β100mm without excessive impact damage. This was confirmed acceptable.
2. Is the ore characteristics suitable? The copper sulfide ore was moderately hard (Bond Work Index approximately 14 kWh/t). No highly abrasive minerals (quartzite, corundum) were present in significant quantities. Suitable for rubber.
3. Will rubber liners affect grinding efficiency? Liner profile (lifter bar geometry) determines grinding efficiency, not liner material. The rubber liner set was designed with the same lifter bar geometry as the existing steel liners. No change in grinding performance was expected or observed.
Trial on One Mill
One of the three ball mills was converted to rubber liners. The other two continued with steel liners for direct comparison.
Rubber liner change installation observations: - Liner set weight: approximately 4.2 tonnes vs approximately 24 tonnes for steel - Installation time: 21 hours vs 36 hours for steel - Crane lifts required: 18 lifts vs 42 lifts for steel (rubber sections are smaller and lighter) - Staff required: 3 maintenance staff vs 4 for steel
12-month inspection of trial mill: Rubber liners showed approximately 30% wear β projected total life of 36β42 months
12-month inspection of steel liner mills: Steel liners showed approximately 55% wear β at this rate requiring change at 18β22 months
Rubber liner projected life: 3Γ steel liner life in this application.
Full Conversion
After 18 months of trial operation (which confirmed the extended liner life projection), the remaining two mills were converted to rubber liners.
Annual impact of full conversion (3 mills):
| Metric | Steel Liners | Rubber Liners |
|---|---|---|
| Liner changes per mill per year | ~1.0 | ~0.33 |
| Shutdown hours per change | 36 hours | 21 hours |
| Total shutdown hours per year (3 mills) | 108 hours | 21 hours |
| Reduction | β | 87 hours |
87 hours of recovered production time per year across 3 mills, plus reduced crane usage and maintenance labor.
Noise reduction was a secondary but significant benefit β the ball mill area became measurably quieter with rubber liners. The concentrator's control room team and maintenance staff working near the mills noted the improvement.
Key Points
Trial before full conversion. Running one mill on rubber for 12β18 months before converting all three gave the concentrator management data from their own specific ore and mill conditions. Published data suggests rubber liners work well in this configuration β site data confirmed it.
Weight reduction accelerates liner change. The 4.2 vs 24 tonne weight difference reduced installation time by 15 hours per change β more than offsetting the higher cost per rubber liner set.
Profile geometry matters. The rubber liner set was designed to replicate the existing steel liner profile. Changing to rubber while simultaneously changing lifter geometry would have made it impossible to isolate the effect of the liner material change.
Elephant Rubber supplied the rubber mill liner sets for this project.