Standard for Initial Gold Liberation in Gravity Concentration
The standard for initial liberation in gold recovery processes is derived from the paper titled “Some factors influencing gold recovery by gravity concentration,” published in the Journal of the South African Institute of Mining and Metallurgy (now the Journal of the Southern African Institute of Mining and Metallurgy), Volume 73, Number 11 (June 1973), pages 363–384. This document provides foundational insights into gravity recovery, including tests on Witwatersrand ores such as Vaal Reefs, where approximately 75% gravity recovery was achieved from material crushed to pass 1.5 mm, with only a slight increase to 79% at finer grinds around 93% minus 74 μm. The paper emphasizes the risks of overgrinding, such as flattening of gold particles, which reduces recoverability. The full PDF is accessible at: https://www.saimm.co.za/Journal/v073n11p363.pdf.
Key Terminology in Gold Processing
Several terms are essential for understanding gold liberation and recovery processes:
Head Feed (or Head Grade): The original, unprocessed material (ore or slurry) fed into a processing stage or equipment before any separation or concentration occurs. It represents the incoming material’s gold grade, used as a baseline for calculating recovery percentages by comparing to concentrates and tailings.
Bond Work Index (Wi or BWi): A measure of ore grindability, quantifying the energy (in kWh per short ton or metric tonne) required to reduce ore from a large particle size to a P80 of 100 μm. Lower values indicate easier-to-grind ores, while higher values signify harder materials. The Bond equation, ( W = 10 \times Wi \left( \frac{1}{\sqrt{P}} - \frac{1}{\sqrt{F}} \right) ), predicts power needs, where W is net power, F is the feed P80, and P is the product P80. Typical values for gold ores range from 14–17 kWh/t.
Gravity Recoverable Gold (GRG): The portion of gold in an ore sample that can be recovered by gravity concentration methods, expressed as a percentage of total gold. GRG tests assess liberation and recovery potential through staged grinding and centrifugal concentration.
P80: The particle size at which 80% of the material passes through a screen or sieve, commonly used to define grind targets (e.g., P80 of 75 μm for fine grinding).
P95: The particle size at which 95% of the material passes, sometimes referenced in liberation diameter calculations but ore-specific.
Liberation: The separation of gold particles from gangue minerals, defined not as a fixed size but as the particle size that maximizes profit for the downstream process, such as gravity concentration requiring near-complete separation.
Particle Size Distributions in Initial Grinding Stages
Even when material passes through a 1.5 mm or 1 mm screen, the resulting particle size distribution varies significantly. Typically, only a small fraction of particles remains coarse, while the majority becomes finer due to the milling action. This distribution arises because impact mills fracture rocks unevenly, producing a range from dust-like fines to larger fragments that are recirculated if they do not pass the screen. The purpose of using such screens is to adhere to guidelines for achieving near-optimum initial liberation of gold, as coarser primary grinds (e.g., passing 1.5 mm) liberate a substantial portion of coarse-to-medium gold while minimizing overgrinding risks like particle flattening. This represents the first stage in a staged recovery process. Subsequently, particles are recirculated at the next level to achieve a P80 or P96 of 200 mesh (approximately 74 μm) for further recovery using Cleangold technology. This intermediate step enhances capture of finer liberated gold before advancing to any leaching processes. The decision to proceed to leaching must be based on empirical measurements, such as assays of concentrates and tailings, to ensure economic viability and avoid unnecessary processing of refractory material.
Cleangold Technology in Gold Recovery
Cleangold is a patented magnetic riffle sluice system that utilizes magnetite to form a “corduroy” bed, designed specifically for fine and micron gold recovery in mercury-free operations. It excels in capturing free micron gold, with documented performance down to 5 μm and sometimes as low as 1–5 μm in optimized setups. Independent lab and field tests demonstrate that it outperforms mercury amalgamation for particles below 70 μm, making it particularly effective for fine gold in black sands or magnetite-heavy material common in alluvial or reef-derived deposits. The system operates non-mechanically with low water usage, ideal for artisanal and small-scale gold mining (ASGM) environments and is scalable for large scale operations.
In staged processing, Cleangold is applied after primary grinding to coarser sizes (e.g., passing 1.5 mm), where it efficiently recovers coarse-to-medium liberated gold, as well as any finer particles already freed. This approach leverages Cleangold’s ability to handle fines without requiring ultra-fine primary grinds, thus avoiding flattening of malleable gold particles during initial milling. For ores with coarse-to-medium gold associations, such as those in Witwatersrand-style or paleoplacer deposits, primary liberation at 1.5 mm aligns well with Cleangold’s strengths, capturing up to 75–80% of gravity-recoverable gold in the first pass. Tailings from Cleangold are then re-milled finer (e.g., to P80 75 μm) for additional recovery or leaching if assays indicate remaining values. Optimization involves adjusting water velocity, feed rate, and angle to trap particles in the 5–50 μm range without losses. This mercury-free method supports sustainable practices, especially in regions like East Africa, by maximizing early recovery while minimizing environmental impact.
Gravity Recoverable Gold (GRG) Test Protocols
Gravity Recoverable Gold (GRG) test protocols provide a standardized method to evaluate the potential for gravity concentration in gold ores. Developed by André Laplante in the 1990s, these tests use a laboratory-scale batch centrifugal concentrator (e.g., 3-inch Knelson KC-MD3 or Falcon L40) with sequential grinding stages to simulate progressive liberation.
The standard procedure involves:
- Preparing a 50–100 kg representative sample.
- Stage 1: Grinding to P80 ≈ 550–850 μm, followed by concentration to recover coarse gold.
- Stage 2: Re-grinding tailings to P80 ≈ 180–200 μm for medium gold recovery.
- Stage 3: Final re-grinding to P80 ≈ 75 μm for fine gold recovery.
Cumulative GRG percentages are calculated from assays of concentrates and tailings, with plots of recovery versus grind size identifying optimal liberation points. Typical GRG values range from 25–94%, averaging 60–70% in gold ores. Simplified versions use a single fine grind, while extended protocols include detailed size-by-size analysis. These tests inform flowsheet design, such as integrating Cleangold for early recovery in staged systems.
Implications for Grinding Design in Gold Ores
In gravity-focused recovery for ores like those in Witwatersrand or East African deposits, a 1.5 mm screen in primary impact mills serves as a reasonable starting point for liberation, particularly when targeting early capture of free gold. This size facilitates high throughput and sufficient liberation for gravity systems while reducing overgrinding risks. However, optimality varies by ore characteristics: coarser gold-dominant ores benefit from 1–2 mm targets, whereas finer or refractory gold may require 0.5–1 mm for adequate liberation.
Modern studies indicate that refractory gold locked in sulphides or silicates necessitates finer overall grinds (P80 ~75 μm) for total recoveries exceeding 90%, but gravity can secure 70–80% of liberated portions earlier. Testing liberation/recovery curves—by processing batches at varying grinds, applying gravity separation, and assaying products—mirrors the 1973 paper’s approach and identifies peaks in recovery relative to energy and cost.
Recommendations for Design and Testing
Designs should incorporate staged grinding: primary to 1.5 mm for early gravity recovery, followed by re-milling tailings to ~75 μm for final extraction. Start with 1.5 mm as a low-risk primary target, then conduct tests at multiple grinds (e.g., 3 mm, 1.5 mm, 0.8 mm) to plot recovery versus throughput. Optimize based on assays, ensuring decisions on leaching stem from measured data.
For ores with variable gold sizes, characterize distributions via mineralogy or size-by-size assays prior to scaling. This approach aligns with sustainable, mercury-free operations in ASGM.
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