The diamond extraction method is driven mostly by mine geology, topography and formation, economic considerations and any laws and regulations applying in each diamond producing country. However, it is important to note that, in most circumstances, miners do not just pull diamonds out of the ground. Extraction usually involves diamond-bearing ore and gravel. Once these materials are brought to the surface, metallurgists then extract the diamonds from the host ore.
Diamond recovery requires a sophisticated system that has evolved greatly over the years and which benefits from today’s advanced technology.
The grade of a diamond mine is often one of the most critical factors in determining the economic viability of a project. Many diamond mines have less than 1 carat of diamonds per metric ton of rock. This means that the diamond content in a ton of rock is typically less than 1 part per 5 million. There are many other minerals and rocks that are transported to the surface together with diamonds, which requires extracting this tiny volume of diamonds from the surrounding material.
Since kimberlite is formed underground, it is a relatively hard rock, similar to granite. The first step to finding the ‘needle in a haystack’ diamond, is crushing the kimberlite ore. This has to be done in a controlled way in order to avoid breaking or damaging the diamonds trapped inside. Some alluvial mines can completely skip the crushing stage if the diamonds are found in sand, earth or fine gravels.
Crushing is a multi-stage process of breaking the rocks into ever smaller pieces and re-circulating larger pieces until everything is the right size to enter the processing plant. It actually begins with the liberation of the rock, where explosive engineers must carefully design their blasts in a way that results in pieces that are small enough to be transported by truck to the primary crusher. Primary crushing often utilizes gravity and an inverted cone shaped machine that forces the larger rocks through a small opening, thereby breaking them against each other.
Secondary crushing stages often utilize jaw crushers, aptly named for their resemblance to a human jaw, as they chomp through the rocks and pass through an opening. Crushing is a dirty process that generates a lot of dust and debris and requires various stages of washing and screening. This is a water intensive operation, and mines often develop sophisticated water treatment facilities on-site in order to reduce the need for fresh groundwater.
There are two main methods of mineral processing used in diamond recovery. Both methods have the same objective, namely to greatly reduce the volume of mineral material that must be processed in order to find diamonds. Both techniques rely on the principal that diamond is a relatively heavy mineral with a specific gravity of 3.52 g/cm3 (compared to water, which has a specific gravity near 1 g/cm3). Diamonds are heavier than most of the minerals that surround them inside the kimberlite. This fact can be used to separate diamonds from other minerals and simplify the recovery process.
The first type of process, which is generally used in alluvial operations, is called a Rotary Pan Plant (RPP). In an RPP, the diamond-bearing gravel, sand and earth are mixed with water to create a slurry, often known as a ‘puddle,’ with a specific gravity in the range of 1.3 to 1.5 g/cm3. The puddle is then stirred in the pan by rotating angled ‘teeth’. The heavier minerals will settle at the bottom of the pan where they are forced down to an area where the concentrate can be extracted. Many of the lighter minerals overflow the pan and can be removed to waste.
The most common separation method is called dense media separation or DMS. A DMS plant also uses the principal that diamonds are heavier than most of the surrounding rocks and minerals. Most modern DMS plants utilize a hydrocyclone (or ‘cyclone’), which is essentially a large centrifuge. Once all the kimberlite has been crushed to an appropriate size, the resulting mix of minerals is combined with water and ferrosilicon, a fine-grained powder that increases the density of the resulting water solution. The cyclone spins at a high rate of speed and the lighter minerals will flow to the top of the cyclone while the heavier minerals, including diamonds, will sink to the bottom where they can be recovered.
Even after most of lighter rocks and minerals are removed, only a small percentage of the heavy mineral concentrate is actually diamond. This concentrate must still undergo several rounds of diamond extraction. Final diamond recovery harnesses two other unique physical properties – diamonds are hydrophobic and they fluoresce when exposed to x-rays.
Several different minerals fluoresce when exposed to ultraviolet light, X-rays or electron beams, among them diamonds. Sophisticated technology has been developed to take advantage of this fact. In an X-ray flow sorter, a thin stream of heavy mineral concentrate is fed into a unit where the diamonds and remaining ore are led on a conveyor belt. When exposed to X-rays, a diamond fluoresces and activates a photo-detector that triggers a jet of air that deflects the diamond into a collector box. This system is also completely enclosed or hands-off and provides significantly increased security as well as safety during the final recovery process.
X-ray sorting recovery has ~ 98% recovery factor on diamonds between 1mm and 25mm. However, it has proven to be less effective in recovering very large diamonds and some diamonds have surface coatings that prevent the diamonds from fluorescing in a way that the sorters can detect.
TOMRA, a Norwegian company that specializes in developing sensor-based solutions for optimal resource recovery, has developed a very promising technology known as X-Ray Transmission or XRT. This XRT technology is utilized at the Karowe Diamond Mine in Botswana and was in use for the recovery of the 1,111 carat Lesedi La Rona diamond last November. XRT uses the diamonds’ unique carbon signature to recover each stone. The company claims that the DMS stage can be bypassed altogether and that recovery factors exceed traditional X-ray sorting machines. At the moment, XRT is only in use for large diamond recovery (+6mm), however this may change as the technology develops.
Diamonds also have the physical property of being hydrophobic, meaning they resist water, but easily adhere to grease. This property was used extensively in diamond recovery in South Africa in the late 19th century, long before X-ray technology was harnessed. Miners wash the heavy mineral concentrate with water, as the wet stones are vibrated over a grease film. The wet minerals will repel the grease and continue on to waste, whereas diamonds will remain stuck in the grease. The grease mix can then be scrapped and boiled off to remove the traces of grease and leave mostly diamonds behind.
A few variations of grease recovery have developed over the years. The challenge with grease tables has been that they provide a very exposed process, which creates security concerns. It is also very messy and requires many trained personnel, further exacerbating the security challenge. Although grease is still in use in many world-class diamond operations, it is slowly being phased out as new technology replaces it. Many operations actually utilize both grease and X-ray recovery. Typically, X-ray would be employed first, to recover a large majority of diamonds from concentrate. Then grease can be used on a much smaller scale to attempt recovering anything missed by the X-rays.
Despite all of the effort and technology, the resulting concentrate feed will still have non-diamond material. Therefore, this concentrate is then fed into a secure sorting facility where specially trained diamond sorters separate diamonds from non-diamonds.
Even after multiple phases of recovery and screening, the resulting non-diamond concentrate will usually be fed back through the recovery circuit at least once before being designated as ‘waste’ and transported to the tailings dump.
Caustic Fusion Dissolution
Another method of diamond recovery is caustic dissolution. This method is a specialized process, which is only used for the recovery of micro-diamonds typically found in small kimberlite samples from grassroot exploration projects. Caustic dissolution is designed to completely dissolve all of the surrounding rock and minerals except diamonds, yielding 100% diamond recovery. This is done by subjecting the rock to a high basic solution of caustic soda at high temperatures. This renders almost all of the host minerals soluble, except for diamonds, which can be visibly detected and extracted.
Although effective in recovering diamonds, this process is labor intensive, expensive and typically only used with small samples of just a few kilograms. It would not be economically viable for a commercial mining operation.
I am often amazed at the staggering complexity of modern diamond mines. Many people in the industry, even those who have been around for a long time, often fail to grasp the lengths that companies must go to in order to extract diamonds from the earth. Perhaps even more wondrous is how these diamond deposits are found in the first place, a subject that I will discuss next week.
The views expressed here are solely those of the author in his private capacity. No one should act upon any opinion or information in this website without consulting a professional qualified adviser.