🎓 Lesson 3
D2
Material Cost Flow: BOM, WIP, and Scrap Accounting
Material cost flow tracks how money moves through materials—from the bill of materials (BOM) listing what’s needed, to work-in-progress (WIP) as items are being made or blasted, to scrap accounting for unusable or wasted material.
🎯 Learning Objectives
- ✓ Calculate material cost per blast round using BOM-derived explosive and accessory quantities
- ✓ Analyze WIP inventory valuation for partially completed blast designs across multiple benches
- ✓ Apply scrap accounting rules to classify and cost non-fragmented boulders and oversize material
- ✓ Explain how BOM accuracy impacts variance analysis between planned and actual material costs
- ✓ Design a simplified material cost flow worksheet aligned with ISO 50001 energy-cost integration requirements
📖 Why This Matters
In open-pit mining, a single misestimated detonator cost or unrecorded drill steel scrap can cascade into $200k+ annual cost variances—and worse, mask systemic inefficiencies in fragmentation control. Material cost flow isn’t just accounting: it’s the financial spine of blast optimization. When engineers understand how BOMs drive procurement, how WIP reflects real-time blast readiness, and how scrap quantifies fragmentation failure, they gain leverage to improve both cost discipline and rock breakage performance.
📘 Core Principles
Material cost flow rests on three interlocking pillars: (1) The Bill of Materials (BOM) defines *planned* material inputs—explosives, primers, stemming, accessories—per blast round, derived from blast design software (e.g., DigiShot, BlastLogic). (2) Work-in-Progress (WIP) captures *actual* material status mid-cycle: loaded but unfired holes, staged explosives awaiting initiation, or partially drilled patterns—valued at standard cost until completion. (3) Scrap accounting classifies and costs *non-conforming outputs*: oversize boulders requiring secondary breaking, misfires, or damaged drill bits—categorized as either recoverable (e.g., salvageable casing) or non-recoverable (e.g., waterlogged ANFO). Together, these form a closed-loop cost model required by IFRS 2 and MSHA compliance frameworks.
📐 Blast Round Material Cost
This formula computes total direct material cost per designed blast round, integrating BOM quantities with unit costs and adjusting for expected scrap loss. It serves as the baseline for variance analysis and WIP accrual.
Blast Round Material Cost (BRMC)
BRMC = Σ(Q_i × C_i) × (1 + R_scrap)Total direct material cost per designed blast round, adjusted for expected scrap-related cost uplift (e.g., secondary breakage, rehandling).
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| Q_i | Quantity of material i | kg, units, m³ | Planned quantity from BOM for material type i (e.g., ANFO, detonators, stemming) |
| C_i | Unit cost of material i | $/kg, $/unit, $/m³ | Standard procurement cost, including freight and handling |
| R_scrap | Scrap cost uplift factor | decimal | Estimated additional cost ratio attributable to scrap (e.g., 0.08 for 8%) |
Typical Ranges:
Hard rock (granite, hematite): 0.05 – 0.12
Soft rock (shale, coal overburden): 0.01 – 0.04
💡 Worked Example
Problem: A bench blast design specifies: 42 mm diameter holes, 12 m depth, 5.2 m spacing, 4.0 m burden; ANFO density = 0.85 g/cm³; 1.2 kg/m ANFO loading; 1 electronic detonator per hole; 0.3 m stemming per hole (sand, $12/ton); ANFO = $650/ton; detonators = $8.50/unit; expected oversize scrap rate = 8% of fragmented volume.
1.
Step 1: Calculate total hole volume = π × (0.021 m)² × 12 m × (100 m² / (5.2 × 4.0)) ≈ 2.07 m³ per 100 m² pattern area
2.
Step 2: ANFO mass = 2.07 m³ × 850 kg/m³ = 1,759.5 kg → cost = (1,759.5 / 1000) × $650 = $1,143.70
3.
Step 3: Detonators = (100 / 20.8) ≈ 4.81 → round to 5 units × $8.50 = $42.50
4.
Step 4: Stemming = 5 holes × 0.3 m × π × (0.021)² × 1,600 kg/m³ ≈ 0.223 ton → $2.68
5.
Step 5: Total BRMC = $1,143.70 + $42.50 + $2.68 = $1,188.88; apply 8% scrap cost uplift (for secondary breakage labor/fuel): $1,188.88 × 1.08 = $1,284.00
Answer:
The blast round material cost is $1,284.00 per 100 m², falling within the typical range of $1,100–$1,500 for hard rock iron ore applications.
🏗️ Real-World Application
At Rio Tinto’s Pilbara operations (2023 Blast Performance Review), a 12% increase in non-recoverable scrap (oversize > 1.2 m) was traced to outdated BOMs that omitted moisture-compensation factors in ANFO density assumptions. Revised BOMs—integrating real-time weather station data and updated bulk density calibrations—reduced scrap-related rehandling costs by $4.2M/year. WIP tracking revealed 23% of ‘loaded’ holes were delayed >48 hrs due to logistics bottlenecks, triggering automatic cost reclassification from WIP to inventory obsolescence—enabling proactive rescheduling and $1.7M in avoided demurrage.