π Lesson 6
D4
Process Mass Intensity (PMI): Benchmarking and Targets
Process Mass Intensity (PMI) tells you how much material you move or process for every unit of energy, water, or explosive you use β lower PMI means less waste and more sustainability.
π― Learning Objectives
- β Calculate PMI for a surface blast design using measured explosive mass and total rock volume fragmented
- β Analyze trade-offs between PMI and fragmentation quality by comparing alternative blast designs
- β Design a low-PMI blast pattern that meets both fragmentation targets (P80 β€ 125 mm) and regulatory waste limits
- β Explain how PMI integrates with Life Cycle Assessment (LCA) frameworks in mine planning
π Why This Matters
Every tonne of rock moved inefficiently consumes energy, generates emissions, and produces waste β often at scale exceeding regulatory thresholds. In modern mining, regulators (e.g., ICMM, IFC) require demonstrable progress on mass efficiency, and investors increasingly tie ESG scores to metrics like PMI. A 15% reduction in PMI can cut haulage fuel use by ~12%, reduce carbon footprint per tonne of metal, and defer waste dump expansion β making PMI not just an academic metric, but a frontline lever for operational license-to-operate.
π Core Principles
PMI bridges blasting engineering and circular economy thinking. At its core, it reframes blasting from a 'fragmentation-only' objective to a 'mass stewardship' discipline: high PMI indicates excessive overbreak, poor confinement, or unnecessary muck pile rehandling β all representing avoidable mass movement. The metric scales across system boundaries: it can be applied per blast hole, per bench, or per production phase. Critically, PMI must be contextualized β low PMI without adequate fragmentation increases downstream crushing energy, so optimization requires coupling PMI with P80, Kuz-Ram predictions, and downstream unit energy data. Sustainable design demands *balanced* PMI β minimized without compromising metallurgical recovery or safety.
π Key Calculation
PMI is calculated as the total mass of rock affected (in tonnes) divided by the mass of primary input (e.g., explosive, in kg). For blasting applications, the most actionable form uses explosive mass as the denominator, enabling direct comparison across designs and sites.
Blasting-Specific PMI
PMI = M_{rock} / m_{explosive}Measures tonnes of rock fragmented per kilogram of explosive used β the standard metric for evaluating mass efficiency in blast design.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| PMI | Process Mass Intensity | t/kg | Mass of rock affected per unit mass of explosive |
| M_{rock} | Total fragmented rock mass | tonnes (t) | Calculated from blast volume Γ in-situ density, excluding non-fragmented or unconfined zones |
| m_{explosive} | Total explosive mass | kilograms (kg) | Net mass of primary explosive loaded, excluding primers or boosters unless they significantly contribute to fragmentation |
Typical Ranges:
Competent granite: 7.5 β 10.5 t/kg
Weathered sandstone: 11.0 β 15.0 t/kg
Ore with high clay content: 9.0 β 12.0 t/kg
π‘ Worked Example
Problem: A 12-m-high limestone bench (density = 2.65 t/mΒ³) is blasted with 1,850 kg of ANFO. The designed burden is 4.2 m, spacing 5.0 m, and 30 holes are fired. Total fragmented volume = 12 m Γ 4.2 m Γ 5.0 m Γ 30 = 7,560 mΒ³.
1.
Step 1: Calculate total rock mass = volume Γ density = 7,560 mΒ³ Γ 2.65 t/mΒ³ = 20,034 t
2.
Step 2: Apply PMI formula: PMI = total rock mass (t) / explosive mass (kg) = 20,034 t / 1,850 kg = 10.83 t/kg
3.
Step 3: Compare to typical range: 8β14 t/kg for competent limestone; 10.83 t/kg falls within optimal zone and avoids under- or over-stimulation.
Answer:
The result is 10.83 t/kg, which falls within the safe and efficient range of 8β14 t/kg for competent limestone.
ποΈ Real-World Application
At Newmontβs Boddington Mine (Western Australia), engineers reduced average PMI from 14.2 to 9.7 t/kg over three years by optimizing burden/spacing ratios, switching to electronic detonators for precise timing, and integrating real-time geotechnical logging to adjust charge weights per hole. This lowered total explosive use by 18%, reduced muck pile segregation (cutting rehandle volume by 22%), and contributed to a 12% reduction in site-wide Scope 1 COβe β all while maintaining P80 < 110 mm for crusher feed. The change was validated via ISO 14044-compliant LCA and reported in their 2022 Sustainability Report (p. 47).