1.1 Surface Mining Unit Operations & Method Selection

Surface mining accounts for the majority of global mineral production, extracting metallic ores, industrial minerals, and coal located near the Earth's surface. Choosing surface mining over underground extraction depends fundamentally on deposit depth, geometry, spatial distribution of grade, geomechanical conditions, and economic stripping boundaries.

Introduction to Surface Mining & Unit Operations

Surface mining operations consist of sequential tasks called unit operations, categorized into primary unit operations (directly involved in rock extraction) and secondary or auxiliary unit operations (supporting production efficiency and safety).

Cyclic vs. Continuous Unit Operations

Primary unit operations follow a fundamental sequence:

1. Drilling: Rotary or percussion drill rigs bore blast holes according to engineered pattern geometries (burden, spacing, sub-drill, and hole diameter).
2. Blasting: Blast holes are loaded with commercial explosives (e.g., ANFO, heavy ANFO, or emulsion blends) and initiated using non-electric or electronic detonators to fragment the rock mass.
3. Loading (Excavation): Heavy loading units—such as hydraulic excavators, electric rope shovels, or front-end loaders—excavate fragmented ore and waste rock.
4. Hauling (Transport): Off-highway rigid-frame haul trucks, articulated dump trucks, or overland conveyor belts transport material from the pit face to destination points.
5. Dumping / Spoiling: Waste rock is placed in engineered waste dumps (external rock piles), while ore is dumped into primary crushers, stockpiles, or run-of-mine (ROM) pads.

Auxiliary operations include haul road grading, dust suppression, bench dewatering, pit wall scaling, and topsoil stockpiling. Operations are classified as cyclic (e.g., traditional drill-blast-truck-shovel systems) or continuous (e.g., bucket-wheel excavators paired with shiftable conveyor systems and spreaders).

Classification of Surface Mining Methods

Method selection depends on deposit thickness, dip, depth of cover, and rock strength.

1. Open-Pit Mining

Open-pit mining extracts steep or thick disseminated ore bodies (e.g., porphyry copper-gold deposits) via a series of horizontal steps or benches. As mining deepens, the pit expands laterally, requiring the removal of surrounding wall rock (waste) to maintain safe slope angles. Open-pit bench heights typically range from 5 to 15 meters, tailored to excavator reach and blast design limits.

2. Open-Cast / Strip Mining

Open-cast mining (strip mining) is applied to horizontal or flat-lying tabular deposits, such as coal seams or lateral oxide mineralizations, lying under shallow overburden. Overburden is stripped and cast directly into the adjacent mined-out void (backfilling) in a continuous progression, eliminating long waste haulage distances. Equipment includes high-wall draglines, continuous bucket-wheel excavators (BWE), or shovel-truck fleets.

3. Placer, Dredging & Hydraulic Mining

• Placer Mining: Exploits heavy mineral concentrations (e.g., alluvial gold, cassiterite, magnetite sands) deposited in unconsolidated river beds or beach sands.
• Dredging: Floating barges equipped with bucket ladders or cutter-suction heads excavate underwater sediments, pumping slurry to aboard or onshore gravity separation plants.
• Hydraulic Mining: High-pressure water jets (monitors) disintegrate unconsolidated alluvial banks, washing slurry into sluice boxes or gravel pumps.

Stripping Ratio Fundamentals & Break-Even Analysis

The stripping ratio ($SR$) quantifies the volume or mass of waste rock removed to extract a unit quantity of ore.

Volumetric and Mass Stripping Ratios

• Volumetric Stripping Ratio ($VSR$): $VSR = \frac{V_{\text{waste}}}{V_{\text{ore}}} \quad (\text{m}^3 / \text{m}^3 \text{ or } \text{m}^3 / \text{tonne})$

• Mass Stripping Ratio ($MSR$): $MSR = \frac{W_{\text{waste}}}{W_{\text{ore}}} \quad (\text{tonne / tonne})$

Break-Even Stripping Ratio (BESR)

On the simplified incremental basis used here, the Break-Even Stripping Ratio ($BESR$) is the maximum additional waste stripping ratio supported by recoverable ore value after the listed processing and G&A deductions:

$BESR = \frac{R - C_{\text{processing}} - C_{\text{G\&A}}}{C_{\text{stripping}}}$

Where:

• $R$ = Recoverable value per tonne of ore (Price $\times$ Grade $\times$ Recovery)
• $C_{\text{processing}}$ = Ore milling and processing cost per tonne
• $C_{\text{G&A}}$ = General and administrative cost per tonne of ore
• $C_{\text{stripping}}$ = Waste stripping cost per tonne of waste rock

If the actual stripping ratio of a pit increment is less than this simplified $BESR$, the increment clears the stated economic screen. The equation does not by itself establish that open-pit mining is preferable to an underground alternative; a full decision also includes ore-mining cost, selling and refining terms, time, capacity, tax, recovery, geotechnical constraints, and the cash flows of each real alternative.

Equipment Selection, Match Factor & Fleet Efficiency

Matching loading and hauling equipment helps control waiting and identifies nominal fleet bottlenecks. The Match Factor ($MF$) evaluates fleet balance:

$MF = \frac{N_{\text{trucks}} \times t_{\text{loading}}}{N_{\text{loaders}} \times t_{\text{cycle}}}$

Where:

• $N_{\text{trucks}}$ = Number of operating haul trucks
• $N_{\text{loaders}}$ = Number of operating loaders/shovels
• $t_{\text{loading}}$ = Total time required for shovel to load one truck (minutes)
• $t_{\text{cycle}}$ = Nominal truck cycle time, including loading, haul, dump, return, and spotting but excluding stochastic queue delays (minutes)

In the deterministic model, a Match Factor of $1.00$ indicates nominal balance, $MF < 1.00$ indicates nominal loader under-utilization, and $MF > 1.00$ indicates nominal truck queuing. Dispatch rules, equipment availability, and cycle-time variability can change the observed queues.

Philippine Surface Mining Operational Context

In the Philippines, surface mining operations face severe tropical weather, intense rainfall, and strict environmental regulations under the Philippine Mining Act of 1995 (RA 7942). Major applications include:

1. Nickel Laterite Strip Mining (Surigao del Norte, Dinagat Islands, Palawan, Zambales): Deposits consist of shallow, flat-lying weathered mantles (limonite and saprolite zones). Operators utilize contour strip mining with hydraulic excavators loading 20 to 30-tonne articulated or rigid dump trucks. Wet season operations require intensive bench grading (1–2% slope), sediment retention basins, and progressive rehabilitation.
2. Porphyry Copper-Gold Open-Pit Operations (e.g., Toledo/Carmen in Cebu and the open-pit phase at Didipio): Large-scale, deep open-pit operations requiring multi-bench slope stability control, bench dewatering pumping systems, and large-capacity truck-shovel fleets.

Comparative Summary of Surface Mining Methods