Gravity Separation in Diamond Recovery: Grease Tables vs. XRT Sorters
Diamond recovery circuits must protect valuable stones while rejecting as much barren material as possible. The choice between grease tables and X-ray transmission (XRT) sorters affects recovery, plant footprint, water consumption, operating skill, and the treatment of coarse and fine material.
Grease-based concentration has a long history in diamond processing because diamonds are naturally hydrophobic and tend to adhere to a greasy surface. XRT sorting uses a different principle: it identifies differences in X-ray absorption between diamond-bearing particles and surrounding rock, then removes selected stones or waste with accurately timed air pulses.
Neither technology is universally superior. The right solution depends on ore mineralogy, diamond size distribution, liberation, feed preparation, throughput, and the position of the recovery stage within the processing plant. A well-designed circuit may use either method independently or combine both with crushing, screening, dense medium separation, and final recovery controls.
How each technology captures diamonds
A grease table or grease belt presents a prepared concentrate to a coated surface while water carries away lighter or less adherent minerals. Diamonds, along with certain heavy and hydrophobic minerals, attach to the grease. Operators periodically scrape or wash the concentrate from the surface for cleaning and final diamond recovery.
This approach is selective rather than purely gravitational. Particle density, surface characteristics, size, shape, and water flow all influence performance. Grease recovery is especially useful after upstream concentration has reduced the mass of material, although the circuit must control clay, organic contamination, oil, and fine slimes that can mask diamond surfaces.
XRT sorters work with a dry or carefully conditioned stream of individual particles. The machine measures how each particle absorbs X-rays as it passes through the inspection zone. A control system evaluates the signal and activates compressed-air ejectors to separate likely diamond-bearing particles from waste.
The technology is often used for coarse pre-concentration because it can process large rocks at high capacity without grinding them. Removing barren material early can reduce downstream energy use and improve the economic value of dense medium or final recovery stages.
Operating principles and circuit placement
Grease tables are usually placed after crushing, scrubbing, screening, and a concentration stage. Feed preparation is critical: excessive fines can coat diamonds, while oversize particles may shield smaller stones or prevent reliable contact with the grease. Water quality, grease formulation, table speed, slope, and wash-water distribution require regular adjustment.
A grease circuit can be compact and relatively simple, but it creates a concentrate that still requires secure handling and cleaning. The recovered material may include other hydrophobic minerals, so final hand sorting, X-ray fluorescence, Raman screening, or another diamond verification method can be needed depending on the operation and product specification.
XRT sorting requires accurate size classification and stable particle presentation. Overlapping particles, wet feed, excessive dust, and irregular trajectories can reduce detection accuracy. The sorter therefore depends on effective screening, drying or moisture control, controlled acceleration, and a clean separation zone.
In a plant design, XRT may be installed ahead of dense medium separation to reject barren coarse rock, or after a concentration stage to recover diamonds from a smaller mass. Its location should reflect the target size range and the balance between diamond exposure, throughput, and the cost of treating misdirected material.
Comparing recovery, cost, and operating demands
The following comparison summarizes the practical differences between the two technologies. Actual results should be confirmed through representative ore testing because diamond deposits can vary substantially across zones and mining depths.
| Criterion | Grease tables or grease belts | XRT sorters |
|---|---|---|
| Main separation basis | Surface hydrophobicity supported by water flow and density concentration | X-ray absorption and particle-by-particle optical or sensor control |
| Typical feed condition | Concentrated, washed, relatively fine to medium material | Classified, liberated, usually coarse and dry or moisture-controlled particles |
| Water requirement | Moderate to high, depending on washing and cleaning arrangements | Low during sorting; water may still be needed elsewhere in the plant |
| Best application | Final or near-final recovery of exposed diamonds | Coarse pre-concentration and rejection of barren rock |
| Treatment capacity | Suitable for concentrate streams and controlled feed rates | High capacity for coarse, well-presented material |
| Main sensitivity | Slimes, surface contamination, grease condition, wash-water balance | Moisture, particle overlap, size distribution, coating, and diamond exposure |
| Diamond protection | Gentle handling when correctly operated | Generally gentle, though conveying and impact points must be controlled |
| Operating skill | Requires frequent cleaning, inspection, and concentrate security | Requires sensor calibration, air-system maintenance, and process control |
| Capital profile | Often lower for small concentrate streams | Higher initial investment, especially with advanced sorting modules |
| Key risk | Loss of stones masked by fines or poor surface contact | Missed or misclassified diamonds in covered, wet, or composite particles |
Grease equipment can be attractive where water infrastructure already exists and the feed has been reduced to a manageable concentrate. It may also suit smaller operations that favor mechanical simplicity and hands-on control. However, its recovery response can change quickly as ore chemistry and slurry conditions change.
XRT typically offers stronger value when a large proportion of the run-of-mine feed is barren rock. Early waste rejection can lower transport, comminution, and separation loads. The financial case depends on stone value, feed grade, sorter recovery, electricity and compressed-air costs, and the consequences of losing even a small number of high-value diamonds.
Ore characteristics and recovery risks
The first question is whether diamonds are sufficiently liberated and exposed for the selected technology to detect them. A diamond locked inside a dense host rock may not present the expected surface response on a grease belt. XRT may also struggle if the surrounding rock creates a similar absorption signal or if the diamond is too small relative to the particle being inspected.
Particle size distribution is equally important. Grease tables generally require a narrow, well-prepared feed so that water flow and surface contact remain consistent. XRT systems also operate within defined size windows because detector resolution, air-jet timing, and particle spacing determine sorting accuracy.
Clay and weathered material create additional complications. Scrubbing and desliming can improve grease recovery by exposing particle surfaces, while drying and screening can improve XRT presentation. These preparation steps add capital and operating cost, but poor feed conditioning can cause greater losses than the equipment itself.
Representative bulk sampling and pilot testing should measure recovery by size fraction, diamond liberation, concentrate yield, false rejection, and the behavior of indicator minerals. Engineers can then compare a grease-based circuit, an XRT stage, or a hybrid arrangement using realistic mass balances rather than supplier specifications alone.
Integrating separation equipment into a plant
A complete diamond recovery plant normally combines several functions rather than relying on one separator. Crushing must preserve stones and limit breakage, screening must create suitable size fractions, and scrubbing must remove clay without damaging valuable material. Dense medium separation can concentrate heavy particles, while grease or XRT technology performs a targeted recovery or pre-concentration role.
The best circuit also includes secure concentrate transfer, tamper-resistant cleaning areas, redundant recovery paths, and sampling points before and after every major stage. Plant management teams should track diamond recovery, mass yield, water use, energy consumption, reject value, and equipment availability. These measures help reveal whether a sorter is improving total recovery or simply shifting losses between streams.
For operators evaluating a new mine or upgrading an existing facility, integrated mineral processing solutions can provide a broader framework for connecting ore testing, equipment selection, plant engineering, and commissioning. A technology decision is more reliable when it is assessed as part of the complete flowsheet instead of as an isolated purchase.
Practical selection priorities
Equipment selection should reflect the deposit and the commercial objective. A coarse, competent ore with a high barren-rock fraction may justify XRT pre-concentration, while a washed concentrate with exposed stones may favor grease recovery. Some operations can use both: XRT rejects waste early, and a downstream grease stage treats the enriched heavy fraction.
Use these priorities when comparing alternatives:
- Confirm diamond size distribution, liberation, and host-rock properties through representative bulk samples.
- Test grease recovery under realistic slime, clay, water-quality, and concentrate-loading conditions.
- Test XRT performance across moisture levels, particle sizes, stone colors, coatings, and composite particles.
- Calculate value recovery alongside mass reduction, energy savings, compressed-air demand, water use, and maintenance cost.
- Design secure concentrate handling and independent recovery checks before final equipment approval.
A pilot campaign should include deliberate searches of reject streams, because the economic risk of losing a large stone can exceed the average recovery percentage reported for the circuit. It should also examine seasonal feed changes and ore domains, since weathering and lithology can alter both sensor response and grease adhesion.
The decision should then be documented in a mass balance and financial model that includes installation, civil works, utilities, staffing, spares, commissioning, and long-term support. For site-specific engineering, project consultation can help connect metallurgical testing with plant configuration and implementation requirements.
Bring the selected technology into the project early, before layout, water systems, electrical design, and material handling are finalized. A properly tested grease circuit, XRT sorter, or combined recovery flowsheet can protect diamond value while reducing waste treatment and improving plant reliability.