Although there are a number of different diamond recovery technologies being utilized worldwide, the two principal types are Rotary Pan plants and Dense Media Separation (DMS). Both methods are in actual fact used for bulk material reduction and require a further process for the final diamond recovery. These systems are based on the property of diamonds having a much higher specific gravity (density) of ~3.52 g/cm3 compared to most of the other minerals that make up material in which diamonds are found.
The first stage of kimberlite processing is to crush and screen the mined ore as required in order to reduce its’ size. For alluvial type deposits crushing is rarely required and the material goes straight to screening. Depending on the processing method being employed, material under a certain chosen size fraction may go through a scrubbing system to clean the ore prior to it being fed into the processing plant.
Rotary Pan Plants
In a Rotary Pan plant, crushed ore, when mining kimberlite, or alluvial gravel and soil is mixed with water to create a liquid slurry called “puddle” which has a density in the 1.3 to 1.5 g/cm3 range. The mix is stirred in the pan by angled rotating “teeth”. The heavier minerals, or “concentrate”, settle to the bottom and are pushed toward an extraction point, while lighter waste remains suspended and overflows out of the centre of the pan as a separate stream of material. The concentrate, representing just a small percentage of the original kimberlite ore or alluvial gravels, is drawn off for final recovery of the diamonds.
Dense Media Separation (DMS) Plants
In a Dense Media Separation (DMS) plant, powdered ferrosilicon (an alloy of iron and silicone) is suspended in water to form a fluid near the density of diamond (3.52 g/cm3), to which the diamond bearing material is added to begin the separation process of the heavier minerals from the lighter material. Additional separation of the denser material occurs by centrifuge in “cyclones” that swirl the mixture at low and high speeds, forcing the diamonds and other dense minerals to the walls and then out the bottom of the cyclone. Waste water rises at the center of the cyclones and is sucked out and screened to remove waste particles. The DMS process results in a concentrate that generally weighs less than one percent of the original material fed into the plant at the beginning of the process.
Both recovery methods have their advantages and disadvantages. The capital required for an equivalent (high) throughput DMS plant is in the order of ten times higher than the capital required for a Rotary Pan plant, but can typically offer better recoveries. Water usage and operating costs for an equivalent DMS plant are also typically much higher than that of a Rotary Pan plant.
Rotary Pan plants are most often employed when mining alluvial deposits and DMS plants for kimberlite deposits. A primary kimberlite deposit generally has a much longer life-of-mine then an alluvial deposit and due to the nature of the deposit, allows for the development of stationary infrastructure. Thus the higher capital costs associated with a DMS plant are more easily justified. Alluvial deposits are, in most cases, spread out over a large geographical area and often require the processing plant to be moved in order to remain close to the area being mined so that material handling costs can be minimized. The requirement for a semi-portable processing plant that can handle large volumes of material is generally more conducive to the Rotary Pan plant applications.
The industry perception that Rotary Pan plants yield poorer diamond recoveries is only partly correct. The efficiency of any plant depends on the skill of the operator and the technology applied. Proper process design and automated control of the critical variables, especially puddle density, indicate that approximately the same efficiency can be obtained from Rotary Pan circuits as from DMS circuits.