What Are The 4 Core Gravity Separation Methods in Gold Mining?

Gold mining profitability hinges on efficient ore processing. Gravity separation, using techniques like sluicing and centrifuging, is a key step in recovering significant gold before more complex processes.

Four main gravity separation methods dominate gold recovery: sluicing, spiral concentration, centrifugal concentration, and jigging. Each technique uses different principles to separate gold based on density, offering unique advantages for various ore types and gold particle sizes. Choosing the right method significantly impacts overall recovery rates and operational costs. 

Let’s delve into each of these methods, exploring their strengths and weaknesses.

How Does Sluice Box Gold Recovery Work?

Sluice boxes, a simple yet effective technology, have been used for centuries to recover gold. Their low cost and simplicity make them attractive for small-scale operations.

Sluice boxes utilize a gently sloping channel lined with riffles to trap heavier gold particles. Water carries the ore mixture through the channel, with gold particles settling in the riffles while lighter materials are washed away. While simple, sluices are best for recovering larger gold particles from readily liberated ores.

Sluice boxes are inexpensive to build and operate, making them ideal for small-scale or artisanal miners. They’re particularly effective in recovering larger gold nuggets and flakes. However, their recovery efficiency decreases significantly with finer gold particles. The angle of the sluice, riffle design, and water flow rate all impact efficiency. Many modern sluices incorporate enhanced riffle designs and even incorporate classifiers to improve performance on finer material. Their effectiveness is largely dependent on the ore characteristics. Ores with high clay content or very fine gold will require additional processing steps after sluicing.

The gold grade of the original ore of Zijinshan Gold Mine is only 0.8g/t. The concentrator adopts a heap leaching-chute gravity separation-carbon leaching process to recover the gold in the ore and obtains relatively satisfactory technical and economic indicators. The gold recovery rate exceeds 80%, and the economic and social benefits of the concentrator are very significant. The production process of the concentrator is shown in Figure 1.

The circulating load of the classifier circuit of the Les Mines Camchib gold and copper concentrator in Canada is as high as 500%~700%. The ball mill discharge is processed by the constriction chute for rough separation. The gold recovery rate of the coarse concentrate is 10%~20%, the gold enrichment ratio is 2, and the tailings of the constriction chute enter the classifier. The coarse concentrate is finely concentrated by the Nelson centrifugal gold concentrator in two stages to obtain a concentrate containing 5%~10% gold, and then the shaker is finely concentrated to a grade of about 65% for smelting. The gravity separation process is shown in Figure 2.

How Effective is Spiral Concentration for Gold?

Spiral concentrators offer a more advanced gravity separation method compared to sluice boxes, improving recovery rates for finer gold.

Spiral concentrators use a helical channel to separate materials based on density differences. Ore is fed into the top of the spiral, and as it moves down, heavier particles (like gold) migrate towards the inner side of the spiral, while lighter particles are carried outward and discharged. This allows for higher throughput and better recovery of smaller gold particles than sluice boxes. 

Spiral concentrators handle larger volumes of ore than sluice boxes and are more suitable for medium-scale operations. The spiral geometry, flow rate, and feed characteristics significantly affect their efficiency. They are particularly effective in processing finer materials than sluice boxes can handle. However, their performance is still affected by the presence of clays or other fine particles that can interfere with gold separation. Many modern designs include features like adjustable water sprays and different spiral configurations to optimize performance for specific ore characteristics. They’re often used in combination with other gravity separation methods.

The Boliden Ore Processing Plant in Sweden sets up a gravity separation circuit to recover coarse gold before the second stage of grinding to prevent over-crushing of gold, which increases the gold recovery rate by 5%. The main function of gravity separation is to recover monomer gold before flotation. The gravity separation process consists of DSS and DS cone concentrators, two three-head LGT Reichert cone concentrators, a spiral concentrator, and a Sala Deister shaker. The ground ore is separated by a sieve to separate materials larger than 3mm before being fed to the Reichert cone concentrator and then returned for re-grinding to avoid over-coarse materials causing pulp turbulence to interfere with the separation effect of the cone concentrator. The gravity separation process is shown in Figure 3.

What is Centrifugal Concentration and When is it Used?

Centrifugal concentrators use high-speed rotation to separate gold, providing high efficiency and handling fine particles effectively.

Centrifugal concentrators, such as Falcon concentrators, use centrifugal force to separate materials based on density. Ore is fed into a rapidly rotating bowl, with heavier gold particles being forced outwards against the bowl’s wall, while lighter materials are carried away. This method achieves high separation efficiency, particularly for fine gold particles. 

Centrifugal concentrators offer significantly higher throughput and recovery rates compared to sluice boxes or spirals, particularly when dealing with fine-grained gold. They are often used as a secondary or tertiary concentration method after coarser separation techniques. Their capital cost is significantly higher than sluice boxes or spirals. However, their high throughput and efficiency can offset this cost for large-scale operations. Operating parameters, such as rotational speed and feed rate, must be carefully controlled to maximize efficiency.

 
More than 51% of the gold ore processed by the concentrator of Henan Jinyuan Gold Mining Co., Ltd. exists in the form of natural gold with a particle size greater than 0.06mm. In order to recover this part of the gold economically, reasonably, and efficiently, the concentrator set up a re-selection operation in the grinding circuit and used two KC-XD40 Nelson concentrators to recover the coarse gold. The production process is shown in Figure 4. When the ore product particle size is -0.075mm, the content accounts for 55%, and the feed concentration (solid mass fraction) is 36%, the gold concentrate yield of the Nelson concentrator is 0.03%, the gold grade of the concentrate is 2800~3500g/t, and the gold recovery rate in the concentrate is 38%.

How Does Jigging Separate Gold from Ore?

Jigging, a pulsating bed separation technique, is another effective method for recovering gold particles, especially when dealing with a wide range of particle sizes.

Jigging uses a pulsating bed of water to separate materials based on size and density. Ore is fed into a container with a perforated bottom, and a pulsating upward flow of water lifts lighter particles while heavier particles (like gold) settle. This process repeats continuously, concentrating the gold in the bottom layer. Jigging handles a wider range of particle sizes than sluicing or spiral concentration.

Jigging is particularly effective when dealing with a mixed-size distribution of gold particles. It’s robust and can handle ore with a significant amount of clay or fine materials. Different jig designs exist, each optimized for particular particle sizes and ore characteristics. The pulsation frequency, water flow rate, and bed depth all influence the separation efficiency. The capital cost of jigs is comparable to spiral concentrators. They are frequently used as a primary concentration step, especially in operations that need to handle a wide range of particle sizes.

The Pamour Porcupine gold mine in Canada uses two-stage jigging for pre-separation, and the concentrate is floated and cyanided respectively. The first stage uses a 1.5m×4.9m ball mill, and the ball mill discharge port is connected to a Pan American jig. The coarse concentrate is fed into a Denver double-chamber jig for jigging gold concentrate smelting. The first stage jigging tailings are regrinded, and copper and sulfur are separated by flotation. The selected jigging tailings and sulfur concentrate are regrinded and then decyanidized. The process is shown in Figure 5.

Conclusion

Gravity separation plays a critical role in gold recovery, significantly reducing the load for downstream processing. The choice between sluice boxes, spirals, centrifugal concentrators, and jigs depends on factors such as ore characteristics, gold particle size, and operational scale. Each method offers unique advantages and disadvantages, and often a combination of techniques is employed to maximize gold recovery.