Gold Gravity Separation Guide: Placer vs. Hard Rock Process Optimization

Gravity separation remains the primary method for gold recovery in both artisanal and industrial mining operations. It functions on the principle of Specific Gravity (SG), utilizing the density difference between gold (SG approx. 19.3) and typical gangue minerals like silica (SG approx. 2.65). This guide analyzes the technical requirements for processing placer and hard rock deposits, equipment selection based on particle size, and methods to improve operational efficiency.

Fundamental Principles of Gravity Concentration

Gravity separation relies on the differential settling velocities of particles in a fluid medium (usually water). Efficient separation requires two main conditions:

1. Liberation: The gold particle must be physically separate from the host rock.
2. Density Difference: The specific gravity difference between the valuable mineral and the waste rock must be significant.
   For gold, the Concentration Criterion (CC) is generally high, making gravity separation an effective initial stage. It offers low operating costs (OPEX) as it does not require chemical reagents such as cyanide or xanthate.

Processing Placer Gold Deposits: Washing and Screening

Involves extracting gold that has already been liberated by natural erosion. The primary operational challenge in these deposits is the presence of clay and the wide distribution of particle sizes.

1. Clay Disintegration (Scrubbing)

Many placer deposits contain cohesive clay. If this clay is not broken down, it forms agglomerates (“clay balls”) that trap gold particles and transport them into the tailings.

• Solution: Use a Trommel Scrubber or a Log Washer. These machines utilize mechanical agitation and water pressure to disintegrate clay into a slurry. This step releases the gold particles for subsequent recovery.
• Operational Note: Simple screening is insufficient for high-clay ores. Mechanical scrubbing is a mandatory pretreatment step.

2. Sizing and Screening

Gravity separators operate most efficiently on a narrow particle size range. Large rocks (oversize) cause turbulence that disrupts the settling of fine gold.

• Process: After scrubbing, material passes through a Vibrating Screen or the screen section of a trommel.
• Classification: Material is separated into oversize (waste gravel) and undersize (gold-bearing sand). Only the undersize material feeds the concentration equipment.

Processing Hard Rock Gold: Grinding and Liberation

Hard Rock Gold Mining requires crushing and grinding to liberate the gold from the quartz or sulfide matrix. Gravity separation in this context serves to recover coarse gold early in the comminution circuit.

1. The Grinding Circuit Integration

In a closed grinding circuit (consisting of a Ball Mill and Hydrocyclones), heavy minerals accumulate. Gold, being dense, sinks to the bottom of the cyclone and returns to the mill.

• Issue: Repeated grinding flattens gold particles and creates “smears,” making them difficult to recover by flotation or cyanidation.
• Solution: Install a gravity concentration unit to treat a portion of the circulating load.

2. The Split-Stream Approach

It is unnecessary to treat the entire mill discharge.

• Standard Practice: Direct 15% to 30% of the cyclone underflow or ball mill discharge to a Centrifugal Concentrator.
• Benefit: This recovers the “Gravity Recoverable Gold” (GRG) efficiently without requiring large equipment to treat the total plant throughput.

Equipment Selection Guide: Matching Machine to Particle Size

Choosing the right tool is critical. Here is how I categorize equipment based on the gold particle size you are targeting.

1. Coarse Gold (+1mm): The Realm of Sluices and Jigs

For nuggets and coarse flakes, simple tools work best.

• Sluice Box: Low cost, high volume. Best for alluvial roughing. However, they are prone to theft and require frequent cleanups.
• Jigging Separator Machine: The Mineral Jig is superior for continuous operation. It uses a pulsating water column to dilate the bed. The heavy gold penetrates the ragging (bedding material) and is drawn off the bottom. It handles feed fluctuations much better than a sluice.

2. Medium to Fine Gold (50 microns – 1mm): The Centrifugal Revolution

This is where modern mining has changed. Gravity force (1G) is often not enough to settle fine gold against the turbulence of water.

• Centrifugal Concentrator: These machines (like Knelson or Falcon types) spin to create 60G to 100G of force. This amplifies the weight difference between gold and sand.
  • Key Insight: The quality of your fluidization water is paramount. If you use dirty water, the fluidization holes clog, the bed packs, and recovery drops to zero. Always filter your process water.

3. Cleaning and Final Recovery: The Shaking Table

Once you have a high-grade concentrate from your jigs or centrifuges, you need to upgrade it to smeltable quality.

• Shaking Table: (e.g., Gemini or 6S tables). This offers the highest enrichment ratio. You can visibly see the separation bands. It allows you to separate free gold from sulfides and magnetite. It is low capacity but high precision.

Particle Size	Recommended Equipment	Primary Force	Water Consumption
> 2mm (Nuggets)	Jig / Sluice / Metal Detector	Gravity (1G)	High
100µm – 2mm	Jig / Spiral Chute	Gravity + Flow	Medium
20µm – 100µm	Centrifugal Concentrator	Centrifugal (60G+)	Low (but needs pressure)
< 20µm	Flotation / CIL (Chemical)	Surface Chemistry	High

Optimizing the Flowsheet: How to Link It All Together?

Designing a flowsheet is not just buying machines; it is about balancing loads.

Placer Flowsheet Recommendation

1. Feed: Hopper + Feeder.
2. Washing: Trommel Scrubber (removes clay).
3. Sizing: Vibrating Screen (removes +20mm waste).
4. Roughing: Jigging Separator Machine (recovers 90% of gold).
5. Scavenging: Sluice Box (catches surges from Jig tails).
6. Cleaning: Shaking Table (upgrades Jig concentrate to >50% gold).

Hard Rock Flowsheet Recommendation

1. Crushing: Jaw Crusher -> Cone Crusher.
2. Grinding: Ball Mill closed with Hydrocyclones.
3. Gravity (Kidney Loop): Screen cyclone underflow (2mm) -> Centrifugal Concentrator -> Tails return to Mill.
4. Intensive Leach: Centrifuge concentrate goes to Acacia Reactor or Shaking Table.
5. Main Circuit: Cyclone overflow goes to Gold CIL Plant or Flotation.

Frequently Asked Questions

 Q1: Can gravity separation operate without chemicals?
Yes. Gravity separation utilizes only physical forces (gravity, centrifugal force) and water. It does not require cyanide, mercury, or flotation reagents, making it an environmentally compliant method.
Q2: Why is classification important before gravity separation?
Gravity separation efficiency decreases when the size range of particles is too wide. Large particles create turbulence that prevents fine heavy particles from settling. Screening or hydraulic classification improves the performance of Jigs and Tables.
Q3: How does a centrifugal concentrator recover fine gold?
A centrifugal concentrator spins at high speeds to generate G-force (up to 60-100 times gravity). This force amplifies the weight difference between fine gold and waste rock, allowing the gold to settle against the bowl wall even when particles are very small (micron size).
Q4: What is the purpose of the shaking table?
The shaking table is used as the final cleaning step. It takes the “concentrate” from larger machines (like sluices or centrifuges) and separates it into a final high-grade product, often producing a visible line of clean gold ready for melting.