Dense medium separation (DMS) is a process of mineral beneficiation based on specific gravity. There are two key components of DMS separation: the dense medium and the cyclone. The dense medium most commonly used in mineral processing is a slurry of ferrosilicon suspended in water. The ferrosilicon-water slurry is mixed with the mineral particles to be separated and injected tangentially into the cylindrical portion of a dense medium cyclone. The centripetal acceleration and tangential inertia of the slurry and mineral particles create a core of relatively low-pressure air in the centre of the cyclone. The denser “heavy” minerals pass through the slurry and move along the wall of the conical section of the cyclone and are ejected out the narrow apex of the spigot. The less dense mineral particles are carried towards the low pressure core in the cyclone and are ejected out of the vortex at the top of the cyclone. Because the net motive force acting on the mineral particles is centrifugal rather than gravitational the rate of mineral separation is greatly increased.
Many mining operations use DMS for pre-concentration prior to processing or milling to reject barren material and improve downstream recovery efficiency. Using DMS technology in mineral exploration, for example recovering indicator minerals, however, is still relatively new. DMS plants used in mineral processing are typically designed for large volumes (tonnes per hours) and coarse particle sizes (millimetres to centimetres). Attempts to scale down processing plants for recovering heavy minerals from sediment samples have thus far met with limited success. With this technology, the two major challenges that must be overcome when modifying a large-scale DMS plant into a micro-DMS are the small size of the target grains and the potential for cross-contamination between samples. Indicator minerals commonly sought after in exploring for new mineral deposits are small – typically 0.25 to 0.6 millimetres. For the smallest mineral grain sizes, the high slurry viscosity becomes a significant factor requiring longer cyclone residence times for efficient separation. Additionally, small grains can get caught on horizontal or near-horizontal surfaces in pumps and pipes which can cause cross-contamination if they are not completely cleaned out between samples. This can lead to false positives and can result in wasted time and money.
Over the last two year, research at the Saskatchewan Research Council (SRC) has focused on resolving these challenges and designing a new type of micro-DMS purpose-built for recovering microscopic indicator mineral grains from sediment samples. The prototype micro-DMS at SRC addresses the above-mentioned challenges by focusing on processing small samples (grams to kilograms) in batches. One necessary design limitation of the batch-fed micro-DMS method is that the cyclone must be fed gravitationally. The gravity-fed system eliminates the slurry feed pump used in most large-scale DMS plants which is the most significant source of potential cross-contamination. The design of most slurry pumps makes it nearly impossible to be sure that all sample material has been cleared before adding the next sample. The gravity-fed batch method involves mixing a single sample and quality control tracers to the dense ferrosilicon slurry which is fed into cyclone. After a sample has been processed the entire feed-cyclone plumbing system is cleaned and the heavy mineral concentrate is visually checked for the quality control tracers. Any grains trapped in the micro-DMS, regardless of specific gravity, will report to the recovery screen and thus no grains are lost and the possibility of cross-contamination is eliminated. The heavy indicator minerals and quality tracers are removed from the recovery screen for further processing that may include electrostatic, magnetic and heavy liquid (e.g. methylene iodide) separation followed by mineral observation and mineral chemical analysis.
Using micro-DMS for recovering heavy minerals from sediment samples offers several key advantages. The first is speed of processing. In the prototype micro-DMS currently in operation at SRC a sample up to five kilograms can be processed in five minutes. A new micro-DMS facility being built at SRC will have multiple cyclones to triple the throughput for even faster sample processing. The second advantage of micro-DMS is that, unlike Wilfley shaking tables and panning, obtaining reproducible and consistent mineral separation using DMS does not rely on the skill of the operator. Indeed, when the micro-DMS plant is properly tuned and balanced it will quickly return to a steady state following density perturbations. A third reason to use micro-DMS for indicator mineral recovery is that it can be used to target specific mineral deposits based on the density of their respective indicator minerals. Lithium deposits, for example, have indicator minerals that have lower specific gravity than diamond indicator minerals. Alternatively, micro-DMS can be used to recover indicator minerals for all deposit types simultaneously thereby maximizing the amount of information that can be extracted from each sample. Thus a program focused on exploring for diamond deposits can also be useful for discovering gold and platinum, base metal, rare earth element, and uranium deposits without any additional expense.
Editorial and photos provided by the Saskatchewan Research Council