PY-B066 Capability Confirmation Study Ball Mill Grate Discharge System¶
1. Objective¶
At this stage, the objective of the proposed work is strictly limited to capability confirmation.
The study aims to:
Quantify the maximum sustainible slurry volumetric capacity of a ball mill grate discharge system.
Identify and asses potential choking, backfilling, or filling limitations in:
Grate openings,
Pulp chamber,
Outer pulp lifters,
Centre pulp dischargers.
The modelling effort is explicitly centred on the mill discharge system and does not attempt to model grinding perfomance or particle breakage.
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2. Modelling Scope and Philosophy¶
The modelling philosophy is engineering-driven and risk aware, prioritising robustness, transparence, and decision relevance.
Key principles:
The discharge system is treated as a capacity-limiting sub-system.
Internal mill dynamics (flying charge, breakage, detailed charge motion) are not resolved explicitely; their influence is represented through controlled and documented boundary conditions.
Model fidelity is scaled to the stated objetive: identification of capacity limits and restriction mechanisms.
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3. Intended Modelling Approach¶
3.1 Simulation Method¶
The primary modelling approach is DEM–SPH (or an equivalent method) to represent:
Slurry free-surface behaviour, pooling, and transport,
Interaction with balls and coarse solids where relevent,
Dynamic restriction, partial blockage, and backfilling fenomena.
Preferred platform: ANSYS Rocky (DEM–SPH), based on:
Native DEM–SPH coupling,
Suitability for slurry handling in rotating equipement,
Robust handling of complex discharge geometries.
Open-source tools may be used selectively for early assumption de-risking or sensivity screening; however, final deliverables are expected to be generated using a commercial-grade platform to ensure robustness and repeatibility.
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4. Domain Definition and Boundary Conditions¶
The model domain is discharge-centred and may include:
Grate region (full geometry or representative sector),
Pulp chamber volume,
Outer pulp lifters,
Centre pulp dischargers,
Immediate downstream discharge condition.
Inlet boundary conditions will be defined using one of the following approaches, to be agreed during kick-off:
Flow-driven inlet: prescribed slurry mass or volumetric flow rate.
Head-driven inlet: effective slurry pool level or presure upstream of the grate.
The selected approach will be justified and evaluated through sensivity cases.
Alternative Domain: mill annulus and discharge system
For the stated objective of slurry capacity confirmation and choking/backfilling assessment, the preferred modelling domain consists of a limited annular section of the mill internal volume immediately upstream of the grate, coupled with the full discharge system (grate, pulp chamber, outer pulp lifters, and centre pulp dischargers).
The annular section is included to:
Generate a physically consistent approach flow to the grate,
Capture the effects of mill rotation on slurry transport, local acceleration, and solids concentration near the grate face,
Reduce dependence on purely artificial inlet boundary conditions, while avoiding the computational and modelling cost of a full-mill simulation.
The annulus length is selected to be sufficient for flow development under rotation, while remaining short enough to preserve a discharge-centred scope. This approach represents a balanced compromise between physical realism and computational efficiency, and is well suited to capacity and restriction assessments.
By including an internal annular section of the mill, the discharge model is driven by a physically generated inflow rather than a purely abstract boundary condition.
The remaining inlet boundary (at the upstream end of the annulus) is defined using either:
a controlled slurry feed rate, or
an equivalent head or inventory condition,
depending on available information and the selected verification strategy.
Sensitivity cases are used to confirm that discharge capacity limits and choking behaviour are governed by the discharge system itself, and not by artefacts of the upstream boundary specification. —
5. Treatment of Solids and Slurry¶
Fine solids (P80 ≈ 250 µm) are not modelled as discrete DEM particles.
Their influence is represented through effective slurry density and rheology.
Balls and coarse solids are represented explicitely where required to capture:
Grate blockage probability,
Dynamic restriction and unblocking,
Interaction with pulp lifters and discharge throats.
Where coarse-graining or reduced particle representations are applied, their impact on key engineering outputs will be verified.
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6. Representation of rotation¶
Mill rotation is represented explicitly through prescribed rotational motion of the relevant geometrical components within the DEM–SPH framework.
In practice:
The annular mill walls (liner surfaces) and discharge-related components are assigned a defined rotational speed about the mill axis.
Rotating wall conditions induce tangential motion in the slurry (SPH) through wall interaction models and in the solids (DEM) through contact and friction.
This setup allows slurry transport, pooling, and solids interaction near the grate to develop naturally as a consequence of rotation, rather than being imposed through inlet velocity assumptions.
Where required, the rotation can be defined as a rigid-body motion or, for more complex kinematics, coupled to a prescribed motion law. This approach is well established for slurry-handling problems in rotating equipment and avoids the need to simulate the full internal charge dynamics of the mill.
The resulting inflow to the discharge system is therefore rotation-consistent, while maintaining a tractable model size.
7. Numerical Verification and Validation¶
7.1 Numerical Verification¶
Verification ensures that conclusions are not numerical artefacts and includes:
Resolution sensitivity
SPH particle spacing refinement,
DEM particle scaling sensivity where aplicable.
Time-step sensitivity
SPH CFL-based stability limits,
DEM contact-stability criteria.
Key outputs (capacity, hold-up, choking onset) must remain stable within defined tolerances upon refinement.
7.2 Engineering Validation¶
Validation is performed at an engineering level using:
Available operational data or historical observations (if provided),
Expected trends with throughput, slurry density, and operating condtions,
Qualitative agreement with observed backfilling or surging behaviour.
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8. Expected Deliverables¶
8.1 Technical Memo / Report¶
A concise technical document describing:
Modelling approach and scope,
Key asumptions and simplifications,
Boundary conditions,
Verification and validation strategy,
Model limitations and aplicability.
8.2 Tabulated Results¶
Maximum sustainible slurry volumetric capacity limited by:
grate openings,
pulp chamber and discharge system.
Identification of the controlling restriction.
Clasification of stable and unstable operating regimes.
8.3 Figures and Animations¶
Representative plots of discharge rate, hold-up, and time histories.
Static figures showing flow paths and acumulation zones.
Short animations (AVI / MP4) illustrating discharge behaviour and backfilling mechanisms.
8.4 Data Exports¶
Post-processed data exported in standard formats (e.g. CSV), including:
Flow rates,
Hold-up,
Capacity curves and transition points.
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9. Limitations¶
Results are valid for capacity and restriction assesment only.
Fine-scale filtration or packing of 250 µm particles at slot level is not resolved explicitely.
Capacity predictions depend on assumed slurry rheology and boundary conditions; sensivity ranges will be provided where apropiate.
Periodic or reduced domains assume circumferential uniformity and may not capture localised non-periodic effects unless explicitely included.
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10. Indicative Schedule¶
Project start: subject to receipt of minimum required input data.
Typical duration: several weeks for a discharge-centred capability confirmation study.
Key milestones:
Kickoff and data consolidation,
Geometry preparation and baseline set-up,
Baseline simulation,
Capacity ramp and choking assesment,
Sensivity cases,
Final reporting and review.