Sensor-based ore sorting
Sensor-based ore sorting is a non-disruptive technology in mining that responds to mega–trends such as the reduction in ore grades and increasing effort to reduce the specific resource input per tonne of product produced. It is a generic term for all physical separation processes in which material properties are detected by a sensor system and subsequently rejected through an amplified mechanical, hydraulic or pneumatic process.
Different sensors at various relevant positions in the process flow allow wide application in all segments of mineral production for a variety of mineral commodities, e.g. ferrous metals, non-ferrous metals, industrial minerals, coal and diamonds.
The use of sensor technology for sorting enables separation according to completely new criteria and opens up new possibilities for the mining industry. Sensor-based sorting processes are separation processes that do not require process chemicals or water and have significant energy-saving potential when used to separate barren by-products before energy- and water-intensive beneficiation processes.
There are three main sensor-based ore sorting processes:
- Particle ore sorting
- Bulk ore sorting
- Batch ore sorting
1. Particle Ore Sorting
Sensor-based particle sorting is an umbrella term for all physical separation processes where particles are singularly detected by a sensor technique to be rejected through an amplified mechanical, hydraulic or pneumatic process. It is automatization of hand-picking, the oldest form of physical separation, and expands this by adding separation criteria using detection of radiation outside the spectrum visible to the human eye.
The terms “ore sorting”, “automated sorting”, “electronic sorting” or “optical sorting” are also used in the industry to refer to this technology. The application of detection technologies enables new separation criteria. It has the unique property of detaching the separation criteria from the separating force. X-ray transmission based particle ore sorting is a single particle detection approach based on a planar projection of the differential attenuation caused by different minerals. Currently, there are two available sorter designs, namely chute-type and belt-type sorters. The sorting process can divided into four interactive sub-processes:
- Particle presentation
- Particle detection
- Data analysis
- Particle separation.
An alternative for subdividing the sub-processes of SBS according to their contribution to the overall process efficiency was introduced by Robben in 2013. [link to
There are two main types of equipment used in today’s industrial systems, the belt-type and chute-type sorter.
Chute-type machines have less moving parts and a smaller specific footprint per tonne of feed, leading to lower investment and operating costs. They also allow for double-sided detection which is made possible by a small free-fall part of the moving flux where surfaces are visible from two sides. Therefore, chute-type systems are mainly used for surface detection principles.
- When the material is fed into the detection zone, the particles are isolated, by spreading them evenly on the chute or belt with 15-35% area occupancy to ensure sufficient spacing between individual particles and to allow independent detection of particle properties. A vibration feeder transports the material to a chute or onto a conveyor belt
- Detection acquires information about the physical properties, and the location of individual particles using various detection principles and systems, which must be contact free, fast and cost-effective. A distinction is made between surface detection technologies, where the radiation is reflected from particle surfaces, and transmissive technologies, in which radiation penetrates the particles and the phase-specific attenuation forms the basis for ejection decisions.
- The signals provided by the detection system form the basis for classification, i.e. for decision-making on precise ejection from the material stream. The most commonly used detection technologies are presented here [link to different sorting technologies].
- Physical separation in state-of-the-art sensor-based particle sorters is usually achieved by pneumatic ejection. A combination of high speed air valves and an array of nozzles perpendicular to the accelerator belt or chute allows timed application of air pulses to change the direction of flight of individual identified particles. The nozzle pitch and diameter can be adjusted to suit for different particle sizes. The air pulse must be precise enough to change the direction of flight of a single particle over the mechanical splitter plate. There is limiting resolution issue with all SBS.
1.1 XRT-based particle ore sorting
XRT-based particle ore sorting is the predominant technology due to its flexibility for different ores and minerals, and the advantages offered by transmittive detection technology. It is based on a planar projection of X-ray attenuation of a particle stream. These particles are distributed either on a chute or conveyor belt, and are individually scanned and evaluated while passing. The particles to be sorted are penetrated by X-ray radiation at certain energy levels and the intensity of the remaining radiation after having passed the particle is measured by suitable detection systems.
For waste rejection applications, where recovery is is a priority, the ability to detect properties within a particle is of critical value and offers a distinct advantage over surface detection technologies, but also over density-based separation processes, such as dense-medium separation, or jigging.
XRT-based particle sorters are successfully commissioned in various mining industries dealing with industrial minerals, precious stones, ferrous and non-ferrous metals, coal and metal slags. Some of the most well-known XRT applications are: Recovery of large diamonds at the Lucara Karowe mine, the Mittersill tungsten mine, and the San Rafael tin mine and the Umm Wu’al Phosphate Mine in Saudi Arabia.
1.2 NIR-based particle sorting
The NIR-based particle sorting is based on a surface detection of the resonance vibration of molecular bonds, near-infrared spectrometry. NIR spectrometry detects absorption of NIR radiation which is a primary property of the minerals with diagnostic features in this wavelength region. NIR absorption is caused by the different movement regimes of molecular bonds and creates a spectrum which is as unique as a fingerprint for a mixture of heterogeneous materials.
The NIR fingerprint is in the form of a spectrum, intensity vs. wavelength. The diagnostic absorption features of minerals, the molecular bonds, can also be visible in inhomogeneous material. As long as spectral differences are detectable, the material can theoretically be sorted with an NIR sorter. NIR-based particle sorting has found various applications, e.g. sorting of industrial minerals.
1.3 Color-based particle sorting
Optical sorting, is a surface detection technique which forms the backbone of sensor based particle ore sorting technology. In this technique particles are detected and separated based on their optical properties, such as brightness or color. Color sorting is an optical sorting approach widely used in mineral processing, agriculture and in recycling industries.
Secondary properties of the particles such as; color, transparency, density, reflectance are the detected and then used as separation criteria. If there is a visible difference between the properties of the waste – and the ore particles, color sorting is applicable as in the mining industry, where it is used extensively in the sorting of industrial minerals such as talc, feldspar, calcite and quartz sorting, precious stones, e.g. diamond sorting, base metals and precious metals.
1.4 LASER -based particle sorting
Two LASER-based detection principles are available on the market. One principle measures reflection and quantifies diffraction, and is this a mehod to detect the presence of transparent structures, such as quartz veins or quartz coatings but also massive quartz particles. The source emits LASER light that hits a point on the surface of the particles for each consecutive measurement. Depending on the colour and transparency of the material, the light is either reflected and/or scattered and diffracted within the transparent structure. Reflection, is the reversal of a wave when it hits a surface or boundary that does not absorb all of the wave’s energy. Diffraction, is the propagation of waves when passing through or on objects.
Light hitting the surface of particles is partially “deflected” in form of scattering. In some scattering processes, a change in frequency is added to the change in direction. The following figure shows the three different types of particle reactions to the laser beam.
This enables the detection of quartz structures with a thickness of less than 1 mm, which are too small for conventional RGB line scan cameras under operating conditions.
The second LASER principle is the so called 3D laser
3D laser, is an advanced version of laser sorting. This is a contactless three-dimensional scanning method.
The function of the 3D laser sensor is to obtain information about the properties of the particle surface such as: shape, size, roughness and brightness. To obtain more detailed information about the position of the particles and to enable more accurate classification, 3D lasers can be used in conjunction with different sensors, such as colour or even X-ray transmission.
2. Bulk Ore Sorting
Bulk ore sorting (BOS) is another sensor-based physical separation technology. This technology relies on Process Analytical Technology (PAT) and material diversion. Here, material is diverted after a measurement on a loaded conveyor belt.
Most ore deposits are heterogeneous. Run-of mine ore on a conveyor belt therefore not only presents inter-particle heterogeneity that can be exploited by particle ore sorting, but also larger scale heterogeneity fluctuations. These fluctuations can be measured with a suitable PAT. Whenever the sensor measurement detects a string of bulk load on the belt which is below a defined separation cut-point, a separating mechanism is activated.
The main sensors used in BOS technology that fully penetrate the material are prompt gamma neutron activation analysis (PGNAA), pulsed fast and thermal neutron activation (PFTNA), magnetic resonance (MR), radiometry based on micro waves (RM) and electromagnetics (EM). Also X-ray fluorescence (XRF) and near-infrared (NIR) for surface detection are applied.
The main adventages of BOS technology are as follows:
- Scalable to all sizes of conveyor belts
- Relatively cost-effective
- Explotation of larger scale distributional heterogeneity characteristics
- Significant reduction in the amount of material fed to downstream processing, which not only reduces costs but also water, energy and reagent consumption.
- Upgrading of concentrator plant feed
- Reduction of specific amount of the tailings and improving resource utilization.