Hard Rock Gold Recovery

High-Recovery Concentration for Milled Hard-Rock Ores

Hard-rock gold recovery begins with liberation. Gold contained within quartz, sulfides, or other host rock must first be crushed and milled finely enough to separate the valuable particles from the surrounding material. Once the gold is liberated, the recovery system must separate a relatively small amount of dense material from a much larger volume of ground rock.

Yuba Mining and Machine focuses primarily on the gravity recovery and concentration side of the process. Our systems are designed to receive properly milled and classified material, recover free gold and other heavy minerals, and progressively upgrade the concentrate through primary, secondary, and final recovery stages.

For many hard-rock applications, we recommend the Low-G Horizontal Centrifuge for primary recovery, followed by the Reverse Multi-Helix Spiral Cleaner and Finisher for concentrate upgrading. However, the best recovery circuit depends heavily on the mineral composition of the ore, particularly the amount of magnetite, black sand, sulfides, and other heavy minerals present.

The Challenges of Hard-Rock Gold Recovery

Hard-rock processing is not simply a matter of crushing rock and passing it through a concentrator. The ore must be milled to the proper size, classified consistently, and matched with a recovery circuit that can handle both the gold and the surrounding mineral matrix.

Common challenges include:

Proper Gold Liberation

Gold that remains locked inside larger rock particles cannot be recovered effectively by gravity concentration.

The ore must be crushed and milled until the gold is liberated from the surrounding host rock. The required grind size varies from one deposit to another. Some ores release gold at a relatively coarse size, while others must be reduced to 100, 200, or even finer mesh before meaningful recovery is possible.

Milling too coarsely may leave recoverable gold locked in the rock. Milling unnecessarily fine can increase power consumption, reduce throughput, and make gravity recovery more difficult.

Material testing is therefore important when determining the correct grind size and recovery circuit.

Maintaining a Consistent Feed

Gravity recovery equipment performs best when the feed is:

Properly milled
Consistently classified
Delivered at a controlled rate
Mixed into a stable slurry
Free from oversized material

Large variations in particle size, slurry density, or feed rate can reduce separation efficiency and make the concentrator more difficult to tune.

Fine Free-Gold Recovery

As gold becomes finer, it becomes increasingly difficult to recover using conventional gravity equipment.

The Low-G Horizontal Centrifuge is designed specifically for production-scale recovery of fine free gold. Its centrifugal separation system is engineered to maximize recovery across a wide range of fine particle sizes, helping operators capture as much recoverable gold as possible under suitable operating conditions.

At extremely fine sizes, gold may remain suspended in process water and begin to behave almost colloidally. Recovery at these sizes becomes increasingly dependent on the ore, water conditions, classification, and whether the gold is truly liberated.

Heavy-Mineral Loading

Many hard-rock ores contain magnetite, iron minerals, sulfides, and other dense materials commonly referred to as black sand or heavies.

These minerals respond to gravity concentration along with the gold. In some ores, the amount of heavy material is relatively low, allowing a centrifuge to produce a compact, high-grade primary concentrate. In other ores, large quantities of magnetite or dense sulfides can rapidly fill a centrifuge concentrate bed and reduce its effective gold-holding capacity.

The correct primary recovery equipment should therefore be selected according to both the gold size and the heavy-mineral content of the ore.

Choosing the Correct Hard-Rock Recovery Circuit

There is no single recovery circuit that is ideal for every hard-rock deposit. Two ores may contain similar gold grades but behave very differently during concentration.

The most important considerations include:

Gold liberation size
Gold particle-size distribution
Magnetite content
Sulfide content
Amount of black sand and other heavies
Required production rate
Target concentrate grade
Water availability
Desired level of automation

Low to Moderate Heavy-Mineral Content

For milled ores containing relatively low amounts of magnetite, black sand, and other competing heavy minerals, we generally recommend:

Milled and Classified Ore
↪ Low-G Horizontal Centrifuge
↪ 16-inch Spiral Cleaner

↪ 16-inch Spiral Finisher

In this arrangement, the Low-G Centrifuge serves as the primary concentrator. It processes the full classified slurry and captures the free gold in its internal concentrate chambers.

Because the centrifuge operates as a batch-type concentrator, it can accumulate a high concentration of valuable material during production while continuously discharging the lighter ground rock.

At the end of the production period, the centrifuge concentrate is discharged and processed through the Reverse Multi-Helix Spiral equipment for secondary concentration, cleaning, and finishing.

Best suited for:

Free-milling gold ores
Fine liberated gold
Low to moderate magnetite content
Low volumes of competing heavy minerals
Ores requiring high primary concentration ratios
Operations seeking a small primary concentrate volume

High Magnetite, Black Sand, or Heavy-Mineral Content

When the milled ore contains large amounts of magnetite, black sand, dense sulfides, or other heavy minerals, we may recommend using Reverse Multi-Helix Spirals as a larger part of the primary and secondary recovery circuit.

A centrifuge does not distinguish between gold and every other dense mineral. If the feed contains an unusually high percentage of heavies, these minerals can occupy the concentrate chambers and increase the frequency of concentrate discharge.

For high-heavy-mineral ores, an adjustable spiral circuit may provide a more practical way to continuously remove lighter material while progressively concentrating the gold and valuable heavy fraction.

A typical arrangement may include:

Milled and Classified Ore
↪ 24-inch Spiral Rougher
↪ 16-inch Spiral Cleaner
↪ 16-inch Spiral Finisher

Depending on throughput requirements, multiple spirals may be installed in parallel. While we readily manufacture our standard Spiral Bank configuration, we have also designed and built a variety of custom spiral arrangements to meet specific processing and recovery requirements.

Best suited for:

Magnetite-rich ore
High black-sand content
Dense sulfide mineralization
Mixed heavy-mineral concentrates
Circuits requiring continuous concentrate discharge
Applications where concentrate volume must be reduced progressively

The exact arrangement should be based on material testing. In some cases, the centrifuge may be omitted from the primary circuit and the spirals used as the main gravity concentration system.

Crushing and Milling Equipment

Yuba Mining and Machine primarily specializes in fine gold recovery, gravity concentration, and final concentrate cleaning, rather than conventional crushing and milling circuits.

Our equipment is generally designed to receive ore after it has been crushed, milled, and classified to the appropriate liberation size.

However, we can work with customers who need a more complete system. Depending on the project, we can:

Incorporate customer-supplied crushers and mills
Integrate third-party crushing equipment
Design conveyors, hoppers, and structural supports
Connect the milling circuit to the recovery system
Fabricate a complete plant around the required process
Assist with overall flowsheet layout and equipment selection

The crushing and milling equipment must be selected according to the hardness of the rock, feed size, target grind, moisture content, and required production rate.

Build the Recovery Circuit Around the Ore

No two hard-rock ores liberate or concentrate in exactly the same way.

A recovery circuit should be selected according to:

Head grade
Liberation size
Free-gold content
Gold particle size
Magnetite and black-sand content
Sulfide content
Required throughput
Water conditions
Concentrate volume
Final product requirements

For ores with low amounts of competing heavy minerals, the Low-G Horizontal Centrifuge followed by Reverse Multi-Helix Spiral Cleaner and Finisher is generally the preferred arrangement.

For ores with large amounts of magnetite, black sand, or dense heavy minerals, a spiral-based recovery circuit may provide better concentrate management and more consistent continuous operation.

Material testing is the best way to determine the correct equipment arrangement, operating settings, and expected concentrate characteristics.

Yuba Mining and Machine can help evaluate your ore and develop a gravity recovery circuit built around the actual behavior of your material, not simply a standard equipment package.