GOLD EXTRACTION
which could not be differentiated with existing
technologies, such as colour-sorting, X-ray
transmission or near-infrared sensors, can now
be identified and separated. The new ore sorting
technology (Lightsaber mineral processing)
enables the user to define and apply a greater
number of sorting criteria, and thus sort with
much greater precision.”
The laser identification technology consists of
a multi-channel laser scanning system with high-
resolution imaging, and cutting-edge colour and
textural selectivity. Multiple material
characteristics such as brightness, colour, size,
shape and surface texture are processed
simultaneously.
TOMRA’s patented multi-channel sorting
technology measures reflection, absorption and
fluorescence and uses the scattering effect of
multiple lasers inside a detector unit. Typically,
the focused laser beam, which is a monochromatic
ore sorting are not able to directly detect gold
occurring at the typical fine grain size of most
deposits, be they refractory or free milling.
Therefore, the sensors are used to detect
minerals associated with gold or alternatively
characteristics of gangue or waste material. For
example, gold is often associated with sulphides
such as pyrite or arsenopyrite, which can be
detected by XRT sensor technology. This sensor
type is most often used on belt type particle
sorters as shown below in Figure 1. Higher atomic
weight atoms have higher X-Ray absorption and
therefore can be identified by the system.
Gold is often also associated with quartz and
in particular, concentrated in the contact zone of
quartz veins. Quartz can be detected by the
recently introduced laser sensors, which are most
commonly used in the chute-type particle sorter.
This is a multi-channel technology capable of
measuring reflection, absorption, refraction,
Figure 1 - Diagram of a belt type XRT
sensor-based ore particle sorter
Copyright: TOMRA Sorting
light source, hits a rock or mineral and is
absorbed or reflected. When this laser beam hits
a larger, translucent crystal – for example quartz
– the laser beam enters the crystal and is
reflected, refracted and scattered inside the
crystal. The beam then returns as a blurry glow
which is of relatively low intensity. Large
translucent crystals will scatter more efficiently
than smaller ones. This differentiated physical
behaviour is captured in high-resolution by high-
sensitivity tools inside the new sorting system.
“Analysing results from tests and practical
trials at several installations, we have
demonstrated that we can achieve higher
recovery, better quality and more consistent
sorting of quartz material with laser sorting than
with other sensor technologies,” comments Ines
Hartwig, Product Manager at TOMRA Sorting
Mining. “But other segments could also benefit
from this technology: any application in which a
crystalline structure helps to separate the valued
material from the waste can also be targeted. So,
possible applications could, for instance, include
minerals such as gold, calcite, fluorite, rock salt,
or talc to name just a few.”
Looking generally at gold, TOMRA explains
that the available sensor technologies for particle
48 International Mining | AUGUST 2017
scattering and fluorescence in order to
characterise the measured material. Multiple
lasers are used on both sides of the particle
stream as shown in Figure 2. In the case of quartz
and other crystalline
materials, the lase