The grease table was the primary diamond recovery technology from the 1870s through most of the 20th century. It worked on a simple principle: diamonds are hydrophobic, they repel water and adhere to grease, while most other minerals are hydrophilic and wash away in a water flow. Crushed ore was washed over inclined tables coated with petroleum jelly or grease; diamonds stuck; everything else flowed off. It was effective but required enormous tables, constant maintenance, and could not recover very small or very large diamonds effectively. The introduction of X-ray luminescence sorting in the 1980s and 1990s transformed the economics of diamond recovery, smaller, faster, more precise, with the ability to detect every diamond above a minimum size regardless of shape or surface condition. : De Beers Group mining technology documentation, debeersgroup.com; historical diamond recovery technology documentation

Dense Media Separation (DMS)

Dense Media Separation is the first major concentration step in a diamond processing plant. Crushed, screened ore is fed into a drum or cone vessel containing a heavy medium, typically ferrosilicon powder suspended in water, adjusted to a specific density between that of the gangue minerals (typically 2.6–2.8 g/cm³) and that of diamonds (3.52 g/cm³). Heavy minerals including diamonds sink and are collected; lighter gangue minerals float and overflow. DMS can process large volumes of ore efficiently, dramatically concentrating diamond-bearing material before the more precise X-ray sorting step. Recovery efficiency is high for stones above approximately 2–3mm (De Beers Group mining documentation; industry processing descriptions).

X-ray luminescence sorting

X-ray luminescence (XRL) sorting is the primary diamond recovery technology at modern mines. Diamonds emit a distinctive fluorescence under X-ray irradiation, they glow brightly and characteristically, while most gangue minerals do not fluoresce or fluoresce differently. Material from DMS concentration passes on a conveyor belt under X-ray beams; arrays of sensors detect the fluorescence signal; pneumatic ejectors fire precisely timed air jets to redirect fluorescing particles (diamonds and any other XRL-active minerals) into a separate collection stream for subsequent hand-sorting confirmation (De Beers Group Diamond Recovery Technology documentation, debeersgroup.com).

Modern XRL sorters operate at high belt speeds and can process tonnes of material per hour. They are effective for stones from approximately 1–2mm up to any size. Their limitation is false positives, certain other minerals (some garnets, some carbonates) also fluoresce under X-ray and are ejected alongside diamonds, requiring a second separation step (De Beers technology documentation; industry processing descriptions).

Automated optical sorting

Automated optical sorting, using cameras, spectrometers, and machine learning algorithms to identify diamonds by their visual and optical characteristics, is being deployed at an increasing number of operations as the technology matures. Optical sorting can supplement XRL by detecting the specific optical properties of diamonds (high refractive index, characteristic reflection pattern) that distinguish them from false-positive minerals. The combination of XRL and optical sorting can reduce the volume requiring hand confirmation considerably (De Beers Group technology development documentation; industry technology analyses 2020–2025).

De Beers Diamond Recovery Technology (DRT)

De Beers developed its Diamond Recovery Technology programme as a proprietary suite of recovery technologies including XRL sorters, DMS systems, and optical sorters deployed across its operations. DRT improvements over the past decade have increased diamond recovery rates (the proportion of diamonds actually recovered relative to those present in the ore) by several percentage points at major operations, a significant financial impact when processing millions of tonnes of ore annually (De Beers Group Annual Reports and technology documentation, debeersgroup.com).

Primary sources

De Beers Group mining technology documentation and Annual Reports. debeersgroup.com. [Dense Media Separation process; X-ray luminescence sorting methodology; Diamond Recovery Technology programme; automated optical sorting development; recovery rate improvements.]

Debswana Annual Reports. debswana.com. [Recovery technology deployment at Jwaneng and Orapa; processing plant descriptions.]