Background to X-Ray Sorting Machines.

X-ray based sorting machines are used extensively in diamond mines to sort diamonds from gangue material. Using X-ray technology to recover diamonds would be ideal if only diamonds luminesced. However, this is not the case; there is a significant amount of gangue material that luminesces when irradiated by X-rays. The intensity of the luminescence signal for diamonds and gangue varies at different wavelengths, so do the rise and decay times of the luminescent signal differ. One also needs to differentiate surface luminescence from absorption coefficient properties of diamonds and associated minerals.

From analysing the luminescence, wavelength, rise time, decay time and absorption characteristics, one can discriminate between diamonds and minerals. In order to sort diamonds successfully from gangue material one needs to utilize as much of the luminescent characteristics as possible into the sorting criterion.

The efficiency and intensity of the luminescence response of a diamond or a gangue particle is also a function of the configuration of the X-ray and detection equipment. When assessing sorting machines using X-ray technology; the following parameters should also be considered:

1. 1.Detection wavelength(s) used;
2. 2.The length of time the particle is irradiated by X-rays;
3. 3.The flux density and flux intensity of the X-ray beam;
4. 4.Position of the detection module; i.e., same side as x-ray radiation or opposite to radiation side or both sides.
5. 5.In the detection section, note should be made of the presence and efficiency of any optical filters. Diamond luminescent wavelengths profile range between 400nm and 600nm, but so do other minerals. For example, calcite has a peak luminescence that occurs at wavelengths greater than 500 nm, while feldspar has a peak luminescence signal around 410 nm. Zircon on the other hand has a luminescence profile that does not drop off over the entire optical band (300 nm to 700 nm). Refer to figures at the end of this document.

The latest x-ray luminescent sorters introduced into the South-African market are the Bourevestnic technology from Russia. This technology poses the following differences compared to other:

1. Pulsed, high energy x-ray source (e.g., 33kV @ 320mA);
2. Detection of luminescent material on two sides;
3. Utilization of peak luminescence, rise and decay times of luminescent signal;
4. High material throughput rates;
5. Recovery of all diamond types, including type IIa (low luminescent rise time) due to high energy x-ray source used.

Application

Freefall, material feed application is Wet, double pass, single input, single detection four viewing channels per pass, single x-ray per pass; primary or secondary bulk recovery machine.

Material size range (-5mm + 1mm) classified in appropriate ratio and feed rates.

Features and Specifications

• v Double pass with single x-ray radiation, single detection per pass
• v Four photo multiplier detection per pass
• v Detect luminescence difference between diamonds and associated luminescent minerals
• v Robust air ejector system
• v Compact, robust, ergonomic design
• v Separate control and service rack
• v Single network interface for control and information
• v User friendly menu driven HMI

Benefits

• Mature Technology
• Maximum diamond recovery at high feedrates
• Minimal ejection of non-diamond luminescent material
• Spillage free; low yield at maximum throughput rate
• Self-testing and on-line calibration
• Operator and maintenance friendly
• Complete operator safety
• Low operating cost
• Comprehensive maintenance support