What are the most effective methods for pre-concentration and sorting of low-grade ores?

In my experience, Dense Media Separation (DMS) stands out as a highly effective method for pre-concentration and sorting low-grade ores. Using fluids like water or ferrosilicon suspension, DMS leverages density differences to separate particles. Those denser than the fluid sink, while lighter particles float, dividing the ore into fractions with distinct grades. Remarkably versatile, DMS finds application in various ores like coal, iron, copper, nickel, and diamonds, making it a go-to solution for enhancing ore quality and extraction efficiency.

Dense media separation is done with "dense media" (it's in the name), the description above falsely says it may be done with a gas or plain water. Ordinary density separation may be done in other fluids, but note that often it is a rate of sedimentation that is exploited rather than bouyancy. Thus particle size and shape also play a role (often dominant).

Dense media consumption is an important consideration in operations. Losses in the circuit may not be obvious. It is important to understand the dense media PSD, size fraction of the fines, and the setup of the DMS circuit.

Presentation to the sorter is critical….. it’s not a “one size fits all” piece of equipment - it is worth the to properly screen and even wash material being fed to the sorter!

Ore sorting needs to be thought of as any other type of concentrator. There is a fixed and marginal cost to concentrate using the method. It should not be thought of as improvement to an ore body but as part of the process system. Important items to know, detection limits, recovery, concentration, and processing cost and capex, and throughput limitations. Be aware that as commodity appreciates the level of which the sensor needs to detect has to improve. Because it comes down to identifying particles above a marginal value, the detection limits may need to improve in increasing commodity environments.

In my experience, Gravity Concentration is a robust method that uses gravity to separate particles based on their specific gravity or density. Heavier particles settle faster in water or air. Equipment like jigs, spirals, shaking tables, centrifugal concentrators, or dense media cyclones can perform gravity concentration. This method is especially effective for ores with heavy minerals like gold, tin, tungsten, or chromite, making it a reliable choice for sorting low-grade ores efficiently.

Don’t be afraid to add redundant gravity separators in a gravity circuit - the payback of having a second piece of equipment is short.

When doing gravity concentrates it is important multiple operating parameters are adjusted as well the test work is performed with various mass pulls. This allows to begin to establish what the relationship are between mass pulls, recovery, flow rates, grindsize ect. The testing should be systematic otherwise the testwork will risk coming to conclusions that may have been premature.

The actual benefits to adding gravity concentrates are typically marginal by the literature. Typically additional recovery is in the 1% range. However with the popularization of both Falcons and Knelsons that meant they have become a standard circuit addition, specifically in Au circuits. This is more an operator philosophy rather than a recovery strategy. The benefit of the gravity concentrator typically is it provides a level of insurance that large nuggets or gold doesn't get by or trapped in unit operations along the circuit.

With magnetic separation, one important area which is not always noted is the importance of water clarity and quality of the feed. High oxidized ores or ores with lots of clays can be problematic as it can coat and insulate magnetic particles that can help prevent the magnetic particle from reporting to concentrate or middlings. It is good practice to conduct regular magnetic surveys as feed conditions change or to confirm that the drums are set optimally.

With all pre-concentration methods, the most important factor will be liberation size. The impact of crush size is usually evaluated by size-by-size assays and testing at specific sizes. Most of these methods can be operated in series to reject as much of the non valuable material from the high grade ore as possible.

Another common method is to exploit differences in grindability and natural crystallite sizes, which means that after crushing a ore below its liberation size, the fine and coarse fractions may have differing compositions allowing for concentration by simple size fractionation, which may be achieved economically with screens or a sedimentation technique.