Closed-circuit Grinding System Circulating Load: Optimization Guide

Circulating Load (CL) represents a critical performance metric in mineral processing. It is not merely an indicator of grinding efficiency but a primary control variable for system throughput. In a Closed-circuit Grinding System, the circulating load is the ratio of the mass of coarse material returned to the mill to the mass of new feed entering the circuit.

Optimization of this ratio offers a direct path to lowering energy consumption per ton. While standard engineering texts often cite 250% as a baseline, operational realities frequently require adjustments based on pump curves and classification efficiency. This technical guide outlines the calculation methods, adjustment strategies for Hydrocyclones, and operational protocols to prevent over-grinding and maximize Ball Mill Efficiency Improvement.

Definition and Mechanism: Why Circulating Load Matters

In a closed-circuit operation, the material passes through the mill rapidly. The short residence time means the mill does not finish grinding in a single pass. Instead, the mill functions to break down particles just enough to pass the classifier.

Mathematical Calculation: The Screen Analysis Method

The only accurate method to determine circulating load is through particle size analysis (Screen Test). Flowmeters are often inaccurate due to air entrainment in the slurry. The calculation uses the percentage of material passing a specific mesh size (typically the circuit target size, e.g., 200 mesh or 74 microns).

The Standard Formula

CL(%)=u−dd−o​×100

Where:

• d (Discharge): Cumulative % passing the reference mesh in the Mill Discharge.
• o (Overflow): Cumulative % passing the reference mesh in the Classifier Overflow.
• u (Underflow): Cumulative % passing the reference mesh in the Classifier Return/Underflow.

Calculation Example

• Mill Discharge (d): 45% passing 200 mesh.
• Cyclone Overflow (o): 90% passing 200 mesh.
• Cyclone Underflow (u): 15% passing 200 mesh.

CL=15−4545−90​×100=−30−45​×100=150%

In this scenario, a 150% circulating load is generally considered low for a ball mill, suggesting potential capacity for increased feed or a need to adjust the classifier to return more material.

Key Variables Influencing Circulating Load Stability

Several physical factors dictate the natural circulating load of a circuit. Understanding these helps in setting realistic targets.

1. Ore Hardness and Grindability

Harder ores (high Bond Work Index) naturally generate higher circulating loads. The mill breaks fewer particles per pass, resulting in more coarse material reporting to the underflow. Soft ores grind quickly, often resulting in lower circulating loads. Operators must increase the feed rate for soft ores to maintain an efficient bed of material in the mill.

2. New Feed Particle Size

Coarser feed (F80) requires more impact energy. If the Crushing Plant delivers oversized rock, the mill simply cannot break it fast enough. This builds up in the circuit, spiking the circulating load until the mill chokes. Stable operation requires a consistent feed size.

3. Classification Efficiency

The efficiency of the Hydrocyclone or Spiral Classifier is the gatekeeper. Ideally, the underflow should contain zero finished fines. In reality, water carries fines into the underflow (Bypass). If the classifier is inefficient, the calculated circulating load may appear high, but it is a “False Load” composed of fines rather than coarse particles requiring grinding.

4. Ball Charge Volume and Size

The grinding media must match the circulating load. A high circulating load implies a high volume of coarse particles. This requires a ball charge with sufficient impact force (larger balls) and sufficient surface area. Worn media reduces the breakage rate, causing the load to accumulate uncontrollably.

Optimization Strategies: Adjusting the Circuit

To manipulate the circulating load towards the optimal range (typically 250%-350%), specific mechanical and operational adjustments are necessary.

Hydrocyclone Parameter Adjustment

The cyclone determines the cut point and the split of mass.

• Apex (Spigot) Diameter: This is the primary physical control.
  • Decreasing Apex: Restricts the underflow. Forces more water and fines to the overflow. Increases underflow density. Risk of roping.
  • Increasing Apex: Allows more material to return to the mill. Reduces underflow density. Increases circulating load.
• Vortex Finder: Changing the vortex finder diameter affects the cut point size. A larger vortex finder creates a coarser overflow and reduces circulating load.

Pump Speed and Pressure

The slurry pump feeding the cyclone provides the energy for separation.

• Pressure Increase: Increases centrifugal force. Produces a finer separation and typically increases the circulating load by sending more marginal particles to the underflow.
• Pump Capacity: A common bottleneck. Optimizing for 400% load often requires upgrading the pump motor and pipeline diameter to handle the increased volumetric flow without settling.

Water Balance Control

Water addition is the fastest operational control.

• Sump Water: Dilutes the cyclone feed. Lower density (e.g., 50% solids) improves separation efficiency and reduces the bypass of fines to the underflow.
• Mill Inlet Water: Controls the density inside the mill. High circulating loads return dry solids (75-80% density). Sufficient water must be added at the mill inlet to maintain grinding density at 70-75% for effective rheology.ParameterActionEffect on Circulating LoadEffect on Product Size (P80)Feed RateIncreaseIncreasesCoarserCyclone ApexDecreaseDecreases (initially)Coarser (due to bypass)Feed DensityDecrease (Dilute)Decreases (improved efficiency)FinerPump SpeedIncreaseIncreasesFiner

Practical Guidelines for Process Control

• Monitor Underflow Pattern: The discharge from the cyclone apex must form a “spray” or “umbrella” shape (20-30 degrees).
• Avoid Roping: A “rope” or “sausage” discharge indicates the apex is overloaded. Separation efficiency drops near zero, and coarse material contaminates the overflow.
• Density Checks: Regular manual density checks on the cyclone overflow ensure the automated density gauge is calibrated.
• Screen Alternatives: For heavy mineral applications like Gold Processing, replacing cyclones with Vibrating Screens eliminates specific gravity issues and prevents the infinite recirculation of heavy, fine gold particles.

Troubleshooting: Detecting “Belly Full” (Mill Overload)

A grinding circuit has a maximum transport capacity. Exceeding this leads to “Belly Full.”

• Symptoms:
  • Sound level drops significantly (muffled).
  • Power draw fluctuates or drops (load center shifts).
  • Spillage at the mill feed inlet.
  • Cyclone feed sump overflows.
• Immediate Action:
  1. Cut the new feed supply to zero.
  2. Maintain mill rotation and pump operation.
  3. Increase water to the mill inlet to flush the load.
  4. Resume feed only when the mill sound becomes crisp and power draw stabilizes.

2026 Trends: Automation and AI in Grinding

Advanced circuits now employ real-time particle size analyzers (PSI) coupled with AI control systems.

• Dynamic Balancing: The system automatically adjusts pump speed and water addition to maintain a constant P80 product size, allowing the circulating load to float within safe limits.
• Micro-hydrocyclones: Use of smaller diameter, multi-cyclone clusters to handle higher pressures and achieve finer cuts, pushing circulating loads higher without losing efficiency.

Frequently Asked Questions

Q1: What is the ideal circulating load percentage for a ball mill?
While 250% is a standard textbook figure, modern high-efficiency circuits often target 300% to 400%. The ideal number is the point where the mill consumes maximum power (drawing peak amps) while producing the target grind size without overloading the pump or cyclone.
Q2: How does circulating load affect the specific energy consumption (kWh/t)?
Optimized high circulating load generally reduces specific energy consumption. By removing fines rapidly, the mill energy is focused on coarse particles, preventing energy waste on over-grinding. Although pump energy increases, the gain in mill grinding efficiency outweighs the pumping cost.
Q3: Why is my calculated circulating load extremely high (over 600%)?
A result over 600% usually indicates a “short circuit” or measurement error. It implies the classifier is sending almost everything back to the mill. Causes include a clogged cyclone overflow, an extremely large apex, or very low feed pressure preventing separation.
Q4: Can I increase circulating load without changing the pump?
Usually, no. Increasing the load means moving more volume. If the pump is already running at 50Hz/60Hz, it cannot move the extra mass. Optimizing for higher loads typically requires upsizing the pump and the cyclone feed lines.
Q5: What is the difference between “True” and “False” circulating load?
True circulating load consists of coarse particles that need regrinding. False circulating load consists of fine particles (already finished) that are carried back to the mill due to inefficient classification (bypass). Reducing False load improves circuit capacity.
Q6: How does ore variability impact the circulating load setting?
If the ore becomes harder, the circulating load will naturally rise as the mill breaks less material per minute. The operator must either reduce the new feed rate or increase the grinding media charge to compensate and prevent overload.

About ZONEDING

ZONEDING Machine is a premier manufacturer of mineral processing equipment. We provide complete comminution solutions, including Ball Mills, Rod Mills, and advanced classification systems. Our technical team assists mines globally in optimizing circuit designs to handle high circulating loads for improved recovery rates.
Contact ZONEDING today for a circuit audit and efficiency analysis.