Precise separating Sensor-based sorting of quartz using multi-channel laser detection

Introduction

The multi-channel laser detection system patented by TOMRA measures the reflection, absorption and fluorescence of multiple lasers of differing wavelengths simultaneously and congruently at one and the same point. This makes it possible to analyse the textures of diverse minerals. One such textural analysis assists, for example, in the direct detection of high-purity quartz in alluvial deposits.

High-purity quartz is notable for its coarsely monocrystalline structure. The crystal structure of quartz is translucent, and the incident laser light therefore penetrates into the mineral, and is scattered in the process. This scatter appears to cause the mineral to luminesce. The light is only reflected from the finely crystalline and opaque minerals, on the other hand (Fig. 1). TOMRA multi-channel laser technology is capable of measuring this possibly only slight luminescence, which provides an indication of the textural difference mentioned above, and thus of achieving extremely good differentiability of the materials to be separated, irrespective of their colorations.

Laser sorting

Applications for optical lasers have increasingly been introduced in many technological fields and industries during the past sixty years. The TOMRA group has, for example, had laser technology in its range of food sorting systems since as long ago as 1997. This has become a globally established and acknowledged identification method, with more than 3000 laser sorters now installed. This TOMRA-patented sorting technology has now been specially adapted and further developed for the unique needs of the mining industry.

The basis for the new laser sorter is provided by the compact PRO series (Fig. 2), which has proven its capabilities in mining over many years. This free-fall sorting machine is equipped with two multi-channel laser systems arranged at 180° to each other and permitting identification of the material from two sides.

This so-called double-sided sensor arrangement makes it possible to scan the largest part of the surface of every stone and thus to obtain the largest possible amount of information on the quality of the material’s surface. This is necessary in the sorting of materials with contact zones, since these are generally apparent only on one side of the stone.

Application

One important application for laser sorting in the minerals sector is, for example, the recovery of high-quality quartz for the production of silicon. Coarse-particled quartz (SiO₂) is required for this purpose, and is reduced to silicon (Si) in industrial melting furnaces. In addition to a particle size of 20 to 120 mm, the quartz must also have low iron, aluminium and titanium contents. Up to now, the quality levels required have generally been attained by means of manual sorting, and also, in approximately the last fifteen years, increasingly with the aid of colour sorters. These instruments may be used if there is, for example, a correlation between iron content and the coloration of the quartz [1]. This correlation does not always exist, however. Quartz gravels from sedimentary deposits, for example, exhibit the most diverse range of colorations and cannot be differentiated from the gangue on the basis of colour (Figs. 3 and 4)

Additional identification methods which perform sorting tasks which could previously be accomplished either not at all or only inadequately by means of colour-based sorting have become established in recent years, in the form of X-ray transmission and near-infrared sorting [2]. Neither of these technologies is suitable for the sorting of quartz gravels, however, since they are not capable of detecting the only slight differences in iron contents. Here, the multi-channel laser sorters recently developed for the mining sector provide potentials for the solution of a whole series of previously impossible sorting tasks.

Capable of detecting textural differences, and thus achieving extremely good differentiability in the feed material, regardless of its coloration (Figs. 5 and 6).

Experience in the field

The first laser sorter for this sorting function went into operation at a plant in Spain in October 2016, following detailed customer testing at TOMRA’s Test Center at Wedel, near Hamburg, which specialises in mining tasks. The customer open-cast mines pit gravel, which is pre-screened in the mine and then conveyed to the preparation plant, for intensive washing, grading and sorting. Up to now, the material, with a particle size of 40 to 120 mm, has been manually sorted, with an average of twelve to sixteen employees being required for this work.

The laser sorter installed in October 2016 (Fig. 2) sorts the 20 to 70 mm particle-size fraction at a throughput rate of 70 t/h. The coarse +70 mm fraction continues to be manually sorted, but now needs fewer employees.

Two further benefits have resulted, in addition to reduced labour costs and improved safety:

1. The customer now additionally sorts the 20 to 40 mm fraction, thus increasing the plant’s overall yield.

2. He can now also process poorer grades of pit gravel, which could previously not be manually sorted to obtain the target quality levels. This boosts the mine’s yield, sales and productive life.

Table 1 shows analyses of sorting results. All quality levels needed for the production of silicon are achieved.

Prospects

Several other applications using multi-channel laser technology are currently in their development phase or are already installed in the field, in addition to the identification and sorting of high-quality quartz for the production of silicon. The following, inter alia, may be mentioned here:

Detection of gold-bearing minerals via the detection of the contact zone between quartz and the gangue

Sorting of rock salt and gangue and/or its intergrowths

Recovery of fluorite (fluorspar) from tailings

Separation of lithium-containing minerals from gangue

The double-sided sorting identification function permits precise evaluation of both pure and intergrown material. It is thus possible, depending on relative mineral values, to focus on high product purity or on high recovery rates.