Mineral processing plants become unstable when control loops, instruments, and operating strategies fall out of sync with the orebody and constraints. The fastest path to stability is targeted loop tuning supported by disciplined control strategies. This page explains why instability occurs, how tuning fixes it, and the steps to get started with audits, health checks and pragmatic retuning, setting the foundation for APC (advanced process control) and sustained performance.
Some definitions before we start ...
PID (proportional–integral–derivative) control: A standard feedback control method where proportional responds to present error, integral accumulates past error, and derivative anticipates future error. Good tuning delivers fast, stable responses without overshoot or hunting.
APC (Advanced Process Control): Multivariable control that coordinates several manipulated variables and constraints across circuits to reduce variability and optimise targets. APC relies on well‑tuned base PID loops.
Cascade control: A hierarchical control structure where the output of a master loop becomes the setpoint of a secondary loop to improve disturbance rejection.
Feed‑forward: The controller uses measured upstream variables to anticipate disturbances and mitigate them.
Integral‑windup: Situation where the controller is requesting more integral input than the control variable can provide; the error in the controller continues to accumulate, which needs to be “unwound, causing overshoot
Stiction: Valve/actuator behaviours of stick-slip motion causing jerky movement, which creates control noise and instability.
Hysteresis: Valve/actuator characteristic where the effect of the valve action on the process lags the controller signal to the actuator
Why plant stability matters in mineral processing
If your concentrator swings between good days and firefighting, you’re seeing process variability in action. Stability is the foundation of recovery, throughput, and predictable performance. When grinding and flotation circuits operate within their designed limits:
- operators can make confident decisions
- metallurgists can optimise’ and
- managers can gain clear visibility into plant behaviour.
The fastest, most defensible path to stability is targeted loop tuning, supported by sound control strategies and disciplined operating practices. Done well, it:
- turns oscillation into stability,
- improves concentrate grade and recovery, and
- reduces spillage and equipment wear.
Stable loops minimise variability, so grind size and froth conditions stay within target bands. That consistency enables repeatable decisions and credible optimisation.
Common causes of instability in concentrators
Instability rarely has a single cause. In concentrators, several patterns recur.
Outdated PID tuning and control strategy drift
PID (proportional–integral–derivative) loops set the pace for your plant. The controller uses the tuning parameters to return the process to setpoint. Over time, the process variables change;
- processing priorities change.
- parameters drift.
- maintenance rebuilds changes equipment performance. or
- grade changes erode stability.
Manual overrides and inconsistent operator actions
Operators step in to “steady the ship,” but frequent and changes from auto to manual mode can introduce inconsistent responses and mask underlying issues. The plant becomes reactive rather than controlled.
Variable feed and orebody changes
Fluctuations in hardness, moisture, and feed grade shift the process’s natural dynamics. Without adaptive strategies, loops tuned for last month’s ore fight today’s conditions.
Instrument issues and bad data
Sensor drift, noisy signals, non-calibrated transmitters, or poor valve performance (stiction, hysteresis) lead to false control actions. Bad data equals bad decisions. The control system can only react to the data it receives
Conflicting control objectives
Tight control of a hopper level can cause downstream disturbances. When upstream loops aggressively chase short-term setpoints the downstream loops try to damp disturbances, causing the circuits to work against each other. Clear control hierarchies and cascade structures matter.
Symptoms your operations teams might recognise
- Oscillating grinding mill load or cyclone pressure, frequent overload or high-level alarms.
- Fluctuating flotation air and cell level the froth racing or collapsing.
- Overdosing or underdosing of reagent and unstable froth behaviour.
- Rising alarm counts, frequent operator mode changes, and chronic setpoint nudging.
- Volatility in recovery, concentrate grade, and specific energy KPI.
Loop tuning explained: the fastest path to plant stability.
Loop tuning is the discipline of selecting PID parameters and control strategies so loops respond appropriately to disturbances and setpoint changes. In concentrators, it is the foundation for advanced control.
What good tuning looks like
- Controller response is prompt with minimal overshoot without setpoint hunting
- Controller input signals represent process conditions to provide a timely controller response
- Processing characteristics are modelled into the control response, and the reaction time between the process signal and process response is fitted to the model.
- Downstream processes are not impacted by controller response.
- Controller responds to setpoint changes and process disturbances.
Where it matters most in concentrators
- Grinding circuits:
Stable mill load, feed rate, and cyclone pressure maintain a consistent grind size, enabling downstream flotation efficiency. - Flotation circuits:
Smooth air flow, level control, and reagent dosing deliver repeatable froth stability and grade outcomes. - Thickeners:
consistent underflow density and flow rate, constant settling rate and bed mass.
How APC complements tuning
APC builds on well-tuned base loops, coordinating multiple variables to manage constraints and variability. Without tuned base layers, APC fights noise rather than controlling the process.
Technical examples, from variability to predictability
Keeping it conceptual, here are typical scenarios we see in the field and how loop tuning changes outcomes.
Grinding, mill load and cyclone pressure
Problem:
Mill load swings ±8–12 per cent, cyclone pressure oscillates, and grind size varies. Operators chase setpoints, throughput fluctuates.
When the mill overloads, the feed rate reduces to reduce the mill load (decreased production). The mill load decreases below the setpoint as the feed rate increases. Low mill load causes liner damage. The feed rate increases to increase the mill load. The mill load overshoots, and the recirculation load increases.
Causes:
Feeder deadtime, there is a delay between changing the feed rate and observing the change at the mill. The number of operating cyclones is not reacting to pressure changes, or the pressure parameters are not matched to the cyclone unit capacity. Cyclone feed pumps ramp too quickly.
Tuning approach:
- Retune load control with measured process response, and add signal filtering as necessary
- Check cyclone control strategy, retune cyclone circuit
- Check processing deadtimes
- Align setpoints with constraints and introduce gentle ramping
Result: Steady mill load, tighter cyclone control, consistent grind size, improved downstream recovery.
Flotation, air and level control
Problem: Airflow and level loops hunt; froth stability varies; grade and recovery jump between shifts.
Causes: Poor instrument calibration, derivative amplification of noise, and interacting loops within the same bank.
Tuning approach:
- Calibrate air meters and level transmitters, apply appropriate filtering
- Reduce derivative, adjust P/I for critically damped response
- Clarify control hierarchy, e.g., cascade air flow to blower pressure, decouple bank interactions
Result: Stable froth, repeatable pull, narrower grade band, easier optimisation.
Reagent dosing, pH and collector addition
Problem: pH swings lead to erratic reagent consumption and variable hydrophobicity.
Tuning approach: Tune pH loop with sensible integral action, add feed-forward for significant disturbances, and verify valve linearity.
Result: Less chemical variability, improved selectivity, reduced cost per tonne.
What changes when stability improves
For metallurgists, stability reduces variability in recovery and grade, so optimisation decisions stick. For operational managers, stability increases visibility, replacing reactive firefighting with predictable performance.
Performance outcomes
- More consistent recovery and concentrate grade
- Higher utilisation, fewer trips and alarms, lower energy per tonne
- Clearer KPI trends, enabling credible improvement programs
- Reduced spillage and operator firefighting
People and process outcomes
- Operators spend less time chasing setpoints, more time monitoring quality
- Shift-to-shift consistency improves, training is easier, and dashboards tell a coherent story
How to get started with loop tuning
A phased approach helps teams move quickly without disruption.
- Baseline the control layer
- Audit PID loops for responsiveness, overshoot, and integral windup
- Validate instrument health, signal quality, and valve performance
- Retune critical circuits first
- Prioritise grinding load, cyclone pressure, and key flotation loops
- Align setpoints and constraints, remove conflicting objectives
- Strengthen control strategies
- Implement cascades, feed-forward for known disturbances, anti-windup, and filtering
- Once base layers are stable, deploy APC to coordinate multi-variable control
- Embed discipline
- Define change control for tuning parameters, instrument calibration schedules, and operator guidelines
- Use dashboards to monitor stability metrics and alarm rates
Take the next step
- Start with our Plant Stability Health Check (self-assessment) to check in on your plant’s current performance.
- Move to a Loop Tuning Audit to identify and fix your critical unstable loops. We’ll start with a discovery call to understand your plant and its current performance.
- Consider a comprehensive improvement program to address instruments, control strategies, and operating practices.
- Explore Mipac’s Process Optimisation, Loop Tuning and Control Strategy Optimisation services.
Book a discovery call
We’ll review your stability challenges together and outline a pragmatic tuning plan for your concentrator.
Some FAQ’s about loop tuning and plant stability
What is loop tuning in mineral processing?
Why is PID tuning critical for plant stability?
What are the first steps to improve stability?
How do APC and loop tuning work together?
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