Stop Measuring Blasts by Cost Alone

What metrics do we actually need to measure? The first group is fragmentation. It is important not to rely on a single number. P80 is useful, but it does not tell the full story.

We need to look at the complete size distribution, including P10, P50, P80, top size, oversized fraction, and fines fraction. Two blast designs can have the same P80 and still behave very differently in the processing plant.

One design may create more fines, while another may generate more oversized material. Even when the P80 is identical, the size distribution curves can be completely different. The plant will feel these differences immediately. Excess fines can create screening losses and dust, while excess oversize can slow down crushing and require secondary breaking.

The real question is not only what the P80 is. The real question is what the entire size distribution looks like.

The next group of metrics relates to loading. Here we track:

• Average bucket fill factor
• Loading rate in tonnes per hour
• Specific excavation diesel consumption
• Loading cycles per hour

Fragmentation affects much more than the crusher. It also influences how easily an excavator can dig, fill the bucket, and load material into trucks.

For primary crushing, we need to track:

• Throughput
• Power consumption
• Downtime caused by oversized material

This is where poor fragmentation becomes highly visible. If oversize increases, the crusher may slow down, stop more often, or require additional secondary breaking.

For the mill, the key metrics are:

• Throughput in tonnes per hour
• Specific energy consumption in kWh per tonne
• Availability

Milling often feels the impact of blasting very strongly. Better fragmentation can increase throughput and reduce grinding energy. However, excessive fines or unstable feed can create new problems.

The entire value chain must be measured.

We also need to evaluate the surrounding environment. Key metrics include:

• Peak Particle Velocity (PPV)
• Flyrock incidents
• Backbreak distance

A blast design cannot be considered successful if it improves fragmentation but creates unacceptable vibration levels, flyrock risk, or wall damage.

One of the most common mistakes is evaluating only drill and blast costs.

The key metric is total mining cost, including:

• Drilling
• Blasting
• Loading
• Hauling
• Crushing
• Grinding

All of these costs should be combined into a single metric: dollars per tonne.

Only then can we understand the true economic impact of blast design decisions.

Let's examine a practical example involving copper-molybdenum ore with a rock strength of approximately 165 MPa.

• Hole diameter: 152 mm
• Burden: 5.0 m
• Spacing: 5.8 m
• Staggered pattern
• Heavy ANFO blend (50/50)
• Stemming: 4.5 m
• Non-electric initiation
• Delays: 25 ms and 65 ms
• Powder factor: 0.78

Results:

• P80: 320 mm
• Oversize: 4%
• Fines: 9%
• Crusher throughput: 1,850 t/h
• Mill throughput: 1,320 t/h
• Drill and blast cost: $0.38/t

Design B uses:

• Burden: 4.5 m
• Spacing: 5.2 m
• Pumped emulsion in wet zones
• Heavy ANFO blend (60/40) in dry zones
• Stemming: 4.0 m
• Electronic detonators
• Inter-hole delay: 60 ms
• Inter-row delay: 55 ms
• V-cut initiation pattern
• Powder factor: 0.92

Results:

• P80 reduced to 215 mm
• Oversize reduced below 2%
• Fines increased slightly to 11%
• Crusher throughput increased to 1,985 t/h
• Mill throughput increased to 1,410 t/h
• Drill and blast cost increased to $0.45/t

At first glance, Design B appears more expensive because drill and blast costs increased by 18%.

However, the entire value chain tells a different story.

Design B:

• Improves fragmentation
• Reduces oversize
• Increases crusher throughput
• Increases mill throughput
• Improves plant productivity

The additional blast cost is offset by gains in downstream performance.

The important question is not how much the blast cost. The important question is what the blast did to the total mining cost per tonne.

Finance departments often focus only on blast costs.

In this example:

• Drill and blast cost increased by $0.07/t
• Mill savings generated approximately $0.52/t
• Net benefit reached approximately $0.45/t

For a mine processing 12 million tonnes per year, this translates into roughly $5.5 million in annual net benefit.

This is why blast optimization must be communicated in financial terms.

One blast is not enough data.

Rock conditions, water conditions, weather, and drilling accuracy can all vary.

A meaningful comparison typically requires four to six blast rounds per design.

A blast may look cheaper on paper while creating:

• More oversize
• Lower loading efficiency
• Lower crusher throughput
• Higher grinding energy

This can increase total operating costs.

The human eye is unreliable.

Oversize content can easily be misjudged by 30–50%.

Without image analysis, discussions become subjective rather than data-driven.

Modern mining operations typically follow several principles:

1. Use a total mining cost framework covering drilling, blasting, loading, hauling, crushing, and grinding.
2. Implement image analysis systems throughout the mine-to-mill process.
3. Compare blast designs within the same geological domain or adjust results using a blastability index.
4. Conduct regular reconciliation reviews involving blasting, loading, and processing teams.

The blast engineer sees the design. The loading team sees the muckpile. The processing plant sees feed behavior.

Only by combining all of these perspectives can a mine determine which blast design creates the greatest value.