Heap leaching is a cost-effective and efficient method for extracting gold and silver from low-grade ores. This process involves stacking crushed ore on an impermeable pad and irrigating it with a leaching solution (typically cyanide for gold and silver). Over time, the precious metals dissolve and are collected for further processing.
Testing gold and silver-associated ores is crucial to determine the optimal heap leaching conditions, recoverability, and economic feasibility. This blog provides a detailed guide on:
- Introduction to Gold and Silver Associated Ore
- How to Beneficiate Gold and Silver Associated Ore
- How to Conduct Gold and Silver Associated Ore Testing
- Lab Equipment Needed for Gold and Silver Associated Ore Tests
Gold and Silver Associated Ore
What Are Gold and Silver Associated Ores?
Gold and silver often occur together in different types of deposits:
Free-milling ores liberate metals easily through crushing and grinding, making them ideal for heap leaching.
- Refractory Ores: Gold and silver are locked in sulfide minerals, requiring pre-treatment (e.g., roasting, pressure oxidation) before leaching.
- Oxidized Ores: Weathering exposes gold and silver, making them more amenable to leaching.
Common Host Minerals
Gold and silver are frequently associated with:
- Quartz veins (free gold deposits)
- Sulfide minerals (pyrite, arsenopyrite)
- Tellurides (gold and silver tellurium compounds)
- Oxidized zones (after weathering)
Understanding raw ore composition is critical for designing an effective heap leaching process.
How to Beneficiate Gold and Silver Associated Ore？
Beneficiation is a crucial step in maximizing gold and silver recoveries from associated ores before heap leaching. The goal is to liberate, concentrate, and prepare the precious metals for efficient extraction. Various techniques are applied depending on ore mineralogy, grain size, and economic feasibility. Below is a detailed exploration of these methods:
Crushing & Grinding (Size Reduction)
Heap leaching efficiency heavily depends on particle size distribution. Crushing the ore finely enough to expose gold and silver particles, but not so fine that it causes permeability issues in heaps.
- Primary crushing (jaw crushers) reduces ore from 500 mm to 100–150 mm.
- Secondary crushing (cone crushers) further breaks it down to 10–50 mm.
- Tertiary crushing/grinding (HPGR, ball mills) achieves target sizes (1–25 mm).
Optimal heap leach sizes:
- Coarse crushing (~25 mm) reduces energy costs but slows leaching kinetics.
- Fine crushing (~1–10 mm) speeds up dissolution but may lead to compaction or poor percolation.
- Crushing must strike a balance between leach recovery vs. processing costs.
Gravity Separation (Recovery of Free Gold/Silver)
If gold and silver occur as native or coarse grains, gravity separation efficiently recovers them before leaching, reducing cyanide consumption and increasing overall recovery.
Common Equipment:
- Gold centrifugal concentrators: Centrifugal force separates heavy gold/silver particles.
- Shaking Tables: Stratifies particles by density, capturing liberated metals.
- Jigs & Spirals: Used for coarse gold recovery in lower-grade ores.
Applications:
- Ideal for alluvial/placer deposits with liberated gold.
- Effective as a pre-concentration step before leaching sulfide ores.
Limitations:
- Less effective for ultra-fine or locked gold.
- Requires high-density differences between gold and gangue minerals.
Flotation (For Sulfide-Associated Gold/Silver)
When gold and silver are locked in sulfide minerals (e.g., pyrite, arsenopyrite), froth flotation helps concentrate them before leaching.
Process:
- Grind ore to ~75 µm (liberates sulfide-bound gold).
- Mix with collectors (xanthates) that attach to sulfides.
- Inject air to form froth—sulfide-gold particles float, while waste sinks.
Advantages:
- Improves Au/Ag recovery from refractory ores.
- Reduces heap leach volume (pre-concentrates pay metals).
Limitations:
- Requires fine grinding (energy-intensive).
- It requires additional dewatering before heap-stacking.
How to Conduct Gold and Silver Associated Ore Testing？
Laboratory testing helps determine: Optimal leach conditions (pH, cyanide concentration, particle size). Recovery rates & kinetics
Economic feasibility
Step 1: Sample Preparation
Collect representative ore samples (1–50 kg).
Crush and screen to the desired size (1–25 mm).
Homogenize to ensure uniformity.
Step 2: Bottle Roll Tests (Kinetic Leaching Test)
Purpose: Evaluate leachability under agitated conditions.
Process:
Add ore + leaching solution (NaCN, pH 10–11).
Roll for 24–72 hours.
Monitor gold/silver dissolution rates.
Step 3: Column Leach Tests (Simulate Heap Leaching)
Purpose: Mimic actual heap leaching conditions.
Process:
Pack ore into a column (1 m tall, 15 cm diameter).
Irrigate with leach solution.
Collect pregnant solution at intervals (1–60 days).
Analyze gold/silver recovery over time.
Step 4: Analysis & Optimization
Assay pregnant solution (AAS, ICP-OES, fire assay).
Adjust parameters:
Cyanide concentration (0.2–1.0 g/L NaCN)
pH (10–11, using lime)
Irrigation rate (5–20 L/hour/m²)
Lab Equipment for Gold and Silver Associated Ore Tests
Laboratory testing is essential for determining the best approach to heap leaching gold and silver ores. The right equipment setup ensures accurate data collection, allowing metallurgists to optimize extraction parameters before scaling up to commercial operations. Below is a comprehensive breakdown of the necessary lab equipment for gold and silver heap leaching tests, including their functions, specifications, and significance.
Sample Preparation Equipment
Before testing, ore samples must be properly crushed, ground, and classified to ensure results.
Jaw Crusher (Primary Crushing)
Function: Reduces large chunks (Run-of-Mine ore, 500mm+) to smaller fragments (100–150mm).
Key Features:
- Adjustable discharge gap (controls final particle size).
- High manganese steel jaws (wear-resistant).
Roll Crusher (Secondary Crushing)
Function: Further crushes ore to ~10–50mm.
Advantages Over Jaw Crushers:
- Better control over final particle size.
- Produces more uniform fragments (improves heap test consistency).
Ball Mill / Rod Mill (Fine Grinding for Flotation Tests)
Needed For: Refractory ores requiring ultra-fine grinding (P80 ≤75µm).
Specs:
Stainless steel/tungsten carbide lining to prevent contamination.
- Motor power: 1–10 kW (lab-scale).
- Use Case: Ball mill/rod mill is used for grinding sulfide ores before flotation tests.
Hammer Mill (Alternative for Brittle Ores)
Best for: Soft, oxidized ores.
Sieve Shaker & Test Sieves (Particle Size Analysis)
Purpose: Ensures the desired size fraction (critical for column leach tests).
Standard Mesh Sizes:
Coarse heap leaching: 6–25mm
Fine heap leaching: 1–6mm
Flotation prep: ≤75µm
Leaching Test Equipment
Once the sample is prepared, conduct actual leaching tests to simulate heap leaching conditions.
Bottle Roll Test Apparatus (Kinetic Leachability Studies)
Function: Evaluates how quickly gold/silver dissolves under agitated conditions.
Setup:
- Polypropylene bottles (500mL–2L capacity).
- Roller assembly (~20–50 RPM, adjustable speed).
- Cyanide solution (NaCN concentration: 0.2–1.0 g/L).
- Duration: 24–72 hours, with periodic sampling.
Column Leach Testing System (Heap Leach Simulation)
Purpose: Closely mimics real-world heap leaching on a lab scale.
Key Components:
- Acrylic/PVC columns (1m height, 10–25 cm diameter)
- Peristaltic pump (controlled irrigation rate: 5–20 L/h/m²)
- Solution collection system (separate pregnant/barren solution storage)
- Test Duration: 30–60 days, depending on ore type.
pH/ORP Meter (Monitoring Leach Chemistry)
Critical For: Maintaining optimal cyanide leaching conditions.
Recommended Range:
- pH 10–11 (using lime or NaOH for stabilization).
- ORP (Oxidation-Reduction Potential): +200 to +400mV (optimizes Au/Ag dissolution).
Cyanide Detoxification System (Environmental Safety)
Needed For: Treating effluent before disposal.
Methods: Hydrogen Peroxide (H₂O₂) oxidation.
Solution & Solid Analysis
Atomic Absorption Spectrometer (AAS) / Inductively Coupled Plasma (ICP-OES)
Fire assay furnace (high-precision gold/silver measurement)
Additional Tools
Filter press (separate leached solids)
Balance (weigh samples accurately)
Agitators & mixers (prepare leaching solutions)
Conclusion
Heap leaching is a cost-effective method for extracting gold and silver from low-grade ores, and testing is critical before full-scale implementation.
Key Takeaways:
- Ore characterization (free-milling vs. refractory) determines pre-treatment needs.
- Beneficiation (crushing, gravity separation, flotation) improves recovery rates.
- Bottle roll and column leach tests help optimize leaching parameters.
- Accurate lab equipment (crushers, AAS, fire assay) ensures reliable data.
By following structured testing protocols, miners and metallurgists can maximize metal recovery while minimizing operational costs.
JXSC lab mineral processing equipment manufacturer has more than 38 years of experience in mining processing. We provide various lab mining equipment including gravity-separating equipment for processing minerals such as gold, tin, tungsten, lead, zinc, tantalum, niobium, iron, manganese, silver, titanium-iron, etc. Lab machines include laboratory jaw crusher, hammer crusher, roller crusher, grinding equipment, lab gravity separator, screening, washing equipment, etc. Welcome to consult!
