Optical Sorting Technology

Optical sorting technology use sophisticated detection, processing and separation technology to remove waste material from ores economically, at high speed and with close to 100% accuracy.

Optical sorting technology is relatively new to the African mining environment. This technology was still being experimented with only 4 years ago within the diamond field. The first industrialised machines were installed and commissioned only 2 ½ years ago at two diamond mines in South Africa.

The sorting principle is very simple. A light source is focussed on the material path; a line scan camera detects diffracted (and / or deflected) light from the material, a dedicated image processing system processes the information, and decides to eject a particle or to ignore the particle. The rate at which all of the above happens is one of the fundamental differences between the current products available on the market.

This type of technology can be seen as “intelligent” since the image processing system can be taught what to sort. Since colour is the fundamental principal used to discriminate against different particles, the applications are almost endless. Some machines work only with diffracted light which limits its applications since it will almost always use white light as an excitation force. Other machines use different colours and wavelengths of light which allows it to be a lot more flexibility. By using different wavelengths of light, certain properties of the material being sorted can be used to distinguish one particle from another.

A weighting system can be applied to the colour classification of the detected material. That means the detection system can analyse a specific particles’ colour, look at the make up of the different colours detected, and decide if there is enough of a specific colour make up present in that particle for it to be recovered. In the colour classification stage the unwanted components are coded in a hierarchical way. In this way pixels of these unwanted components of the mineral can be counted individually for the different reject types, and compared in relation to the total number of pixels of the complete mineral piece. The morphologic filtering of the detection images improves the quality of reject detection. Small misclassified spots can be removed, and missing detection areas can be filled in according to the context of the local neighbourhood.

An added advantage of this technology is that the particle dimensions can be used as a parameter for the sorting process. By looking at the length, width and total area of the particle, a more complex sorting algorithm can be created. The shape of the particle can also be added to this algorithm.

All of this happen at a rate of up to 30 000 particles a second!!

The standard OptoSort machine complies with the following specifications:

• MS Windows operating platform
• Dedicated image processing hardware and software
• Line Scan frequency of 10kHz
• 1 line consists of 2048 pixels, in red green and blue
• Any combination of 16 million colours can be detected
• One pixel is typically 0.3*0.3mm and can be reduced to 0.15*0.15mm
• 20 Mega pixels per second is processed and can be increased to 74 Mega pixels
• Image classification is done within a 3-dimentional colour room

Material sorting parameters

• Particles from 0.5 mm up to 300 mm can be sorted with various machine models
• Typical sizing ratio of about 4 of the material being sorted
• +1 -4mm material is sorted at about 4 tph (600 mm wide sorting area)
• +4 -10mm material is sorted at about 6 tph (600 mm wide sorting area)
• +10 -32mm material is sorted at about 30 tph (600 mm wide sorting area)
• +32 – 75mm material is sorted at about 80 tph (1800 mm wide sorting area)

Various models are available for specific applications.

Belt sorters

Material is transported into the detection plane of the camera system via a conveyor belt moving at a defined speed. The camera system covers the width of the conveyor belt with the number of pixels available on the CCD chip. With, for example, 2048 pixels and a working width of 1200 mm and a belt speed of 3.0 m/s we obtain a pixel area of approx. 0.6 mm² which the camera system detects.

If objects smaller than 4 mm are to be sorted - as is frequently required with precious stones - pixel areas of even 0.6 mm² are already too large to be a basis for optimum sorting decisions. In such cases the working width and belt speed are adjusted accordingly. This makes it possible for objects with dimensions in the range of just 0.5 mm to be detected via multiple (very small) pixel areas in a number which the sorting decision requires.

The following Belt Sorters are offered:

Gravity Sorters

Gravity sorters are used preferably for particle sizes larger than 40 mm. Depending on the design of the sorter nozzle bank, objects with a size up to 300 mm can be ejected by a compressed air pulse.
The standard working width of the OptoSort gravity sorters is 1200 mm.

Where so required in individual cases, we can also supply machines with working widths up
to 1800 mm.

References:

Fraunhofer Institute for Information and Data Processing.

Behind the development of the OptoSort sorting systems up to their present levels of capability lie many years of exclusive collaboration with the IITB in the application field of sorting mineral and metal bulk materials.

The IITB is the Fraunhofer Society's Institute for Information and Data Processing which is located in Karlsruhe, Germany. Following this tradition, we are committed to continuing with this exclusive collaboration in the future as well.

Please don't hesitate to contact us.