3.Determining the Optimal Comminution Size – Balancing Adequate Liberation Against Over‑Grinding

In the previous articles, we clarified the three fundamental concepts and their interrelationships. Yet the question that clients most frequently ask is: "Exactly how fine should I crush or grind my ore?"

There is no universal numerical answer, but there is a well‑established engineering methodology for arriving at the right answer for a given ore. This article focuses on that decision‑making process.

What Constitutes a "Reasonable" Comminution Size?

A reasonable particle size must satisfy two conditions simultaneously:

• Achieve the degree of liberation required by the subsequent separation process – this is the minimum technical threshold;
• Avoid generating excessive and unnecessary fine particles – this is the economic constraint.

In essence: comminute just enough, but no more than necessary.

A Typical Decision‑Making Process

Consider an ore in which the target mineral is disseminated as coarse grains of 5–8 mm. Observing the liberation behaviour at progressively finer sizes:

In this scenario, 8 mm or 4 mm may both be viable candidates for further evaluation. The final selection should be based on a comprehensive assessment of liberation, concentrate quality, tailings losses, throughput capacity, energy consumption, and equipment compatibility.

Why Is Pursuing Ever‑Finer Sizes Not Advisable?

Both crushing and grinding are energy‑intensive operations, with grinding being particularly costly. Over‑reduction of particle size brings a cascade of negative consequences:

• Increased slimes generation → complicates both separation and solid‑liquid separation steps;
• Higher energy consumption → grinding costs escalate disproportionately;
• More difficult dewatering → settling and filtration performance deteriorate;
• Reduced equipment efficiency → many separators are not optimised for fine or ultra‑fine feeds.

In engineering terms, this is referred to as over‑crushing or over‑grinding. Sound process design seeks the optimal balance between sufficient liberation and the avoidance of unnecessary fines.

What Information Is Required for a Proper Ore Evaluation?

When assessing an ore to determine its optimal comminution size, knowing the head grade and maximum feed size alone is insufficient. The following data are essential:

• ✅ The dissemination size distribution of the target mineral;
• ✅ The intergrowth relationship between the target mineral and gangue;
• ✅ The liberation response – i.e., the degree of liberation achieved at various comminution sizes.

Such information is typically obtained through mineralogical characterisation and liberation analysis performed on size‑fractionated products from comminution tests. Only with these data can a technically sound and economically rational particle‑size decision be made.

Final Synthesis – Decision Implications of the Three Concepts

To conclude this series:

• Crushed particle size tells us what has been achieved;
• Dissemination size tells us what needs to be achieved;
• Degree of liberation tells us how effective the achieved size actually is.

These three pieces of information together determine:

• The required fineness of comminution for the ore in question;
• The appropriate downstream separation flowsheet;
• The likely scale of capital investment and operating costs for the project.