With industrial development, the volume of solid waste like fly ash and slag continues to increase. Traditional storage methods pollute environments and consume land resources. Ultra-fine powder composites—primarily fly ash and mineral powder—serve as high-performance cement additives and concrete admixtures. These materials can fully replace S95 mineral powder or partially substitute cement, significantly reducing production costs while enhancing product durability. Dry ball mills remain essential equipment for superfine grinding in building materials, especially when targeting specific surface areas exceeding 6,500 cm²/g or even 7,000 cm²/g. Compared to bed-grinding equipment, ball mills produce ultra-fine particles with superior sphericity and specific surface area, improving particle packing density and slurry fluidity. However, their elongated design (high length-to-diameter ratio) intensifies friction between grinding media and materials, generating excessive heat. This heat transfers to sliding bearing bushes, causing overheating that triggers shutdowns or bush-burning accidents. This article explores structural optimizations for φ3.5 m×17.5 m dry ball mills to mitigate bush temperature issues.
Working Principles
Dry superfine ball mills employ dual sliding bearings supporting rotating shells through slide rings. Each sliding bearing contains 2–4 bushes that support and slide against the rings. Hydrostatic-hydrodynamic lubrication is standard: high-pressure pumps lift the shell before startup, creating a 0.2 mm oil film. After 1–2 minutes, low-pressure pumps maintain lubrication during operation. Material enters via a feed chute, undergoes progressive grinding, and exits through discharge grates. Heat generation follows:
$$Q_f = \mu \cdot F_n \cdot v$$
Where \(Q_f\) is frictional heat (W), \(\mu\) is the friction coefficient, \(F_n\) is normal load (N), and \(v\) is sliding speed (m/s).
Structural Optimizations
1. Feeding Device Modification
Uncontrolled material entry causes impact-induced dry friction in the initial 1 m of the ball mill, shortening effective grinding length and accelerating liner wear. Adding a return plate redirects material flow vertically, minimizing friction and heat generation.
| Parameter | Before Optimization | After Optimization |
|---|---|---|
| Material Impact | High-velocity frontal impact | Controlled vertical descent |
| Friction Zone | 1 m from inlet | Eliminated |
2. Cylinder Structural Redesign
Traditional conical transmission sleeves allowed hot material accumulation near slide rings, conducting heat to bushes. Replacing them with bolted straight sleeves isolates heat. Additional measures include:
- Applying thermal insulation paint on discharge-end internals
- Installing nano-thermal barriers between liners and shell
- Increasing discharge grate open area by 25%
- Adding material guide plates
Heat conduction reduction is quantified as:
$$Q_c = k \cdot A \cdot \Delta T / d$$
Where \(k\) is thermal conductivity (W/m·K), \(A\) is area (m²), \(\Delta T\) is temperature gradient (K), and \(d\) is insulation thickness (m).
3. Lubrication System Enhancement
Original lubrication parameters proved inadequate during high ambient temperatures (>38°C), causing bush temperatures >70°C. Optimized flows ensure stable oil films and cooling:
| Parameter | Original | Optimized |
|---|---|---|
| Low-Pressure Flow | 80 L/min | 125 L/min |
| High-Pressure Flow | 2×2.5 L/min | 2×5 L/min |
| Oil Film Thickness | 0.2 mm | 0.3 mm (+30%) |
Continuous high-pressure operation during heat waves improves heat isolation. Hydrodynamic film thickness follows:
$$h = \sqrt[3]{\frac{6 \mu U R^2}{W}}$$
Where \(h\) is film thickness (m), \(\mu\) is viscosity (Pa·s), \(U\) is speed (m/s), \(R\) is radius (m), and \(W\) is load (N).
4. Bush Surface Geometry
Conventional oil distribution relied on slide-ring rotation and wedge effects, limiting oil supply. Adding an axial groove and direct oil inlet ensures abundant lubrication:
| Feature | Original Bush | Optimized Bush |
|---|---|---|
| Oil Inlet | End-face groove | Axial groove + direct inlet |
| Oil Coverage | Partial | Full surface |
This maintains full-fluid friction, reducing friction coefficient by 18%.
5. Double Sealing Mechanism
Single rubber seals permitted oil leakage and dust ingress, increasing bush temperature. A dual-seal solution combines:
- Rotating oil-retaining rings on slide rings
- Stationary rubber-felt seals with adjustable compression bolts
Contaminant exclusion maintains oil purity, critical for thermal stability.
Performance Validation
Implementing these optimizations on a φ3.5 m×17.5 m ball mill reduced bush temperatures from >70°C to 60–65°C during summer operation. Over 18 months, no temperature-induced shutdowns occurred. Comparative results:
| Metric | Standard Ball Mill | Optimized Ball Mill |
|---|---|---|
| Avg. Bush Temp. | 75°C | 60°C |
| Temp. Reduction | Baseline | 10–15°C |
| Unscheduled Stops | 4–6/year | 0 |
Operational efficiency improved by 12%, and maintenance costs dropped by 30%.
Conclusion
Optimizing feeding, cylinder, lubrication, bush, and sealing structures in dry ball mills effectively reduces bush temperatures. Key achievements include:
- Heat Source Control: Return plates and thermal isolation minimize frictional and conducted heat.
- Enhanced Lubrication: Higher oil flows and grooved bushes stabilize oil films, validated by:
- Reliable Sealing: Dual-seals prevent oil contamination and leakage.
$$h \propto \sqrt[3]{\mu U}$$
These modifications enable continuous operation of superfine grinding ball mills under high-temperature conditions, boosting productivity while lowering operating costs.