The Economic Potential of Automatic Rock Sorting

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1 The Economic Potential of Automatic Rock Sorting Tako P.R. de Jong, The Netherlands The availability of high-throughput automatic sorters for coarse and lump sized material ( kg per piece) can improve the overall efficiency of both mining and milling. Using automatic rock sorting as pre-concentration it is possible to mine heterogeneous ore deposits of a low average grade, but with local sections, bands or veins of high grade. By pre-sorting the ore pieces before grinding overall milling costs decrease considerably. Figure 1 The conventional process requires processing of all rock leading to a huge finely ground tailings dam, sludge, loss of flotation reagents etc. Using automatic pre-concentration, various rock types can be sorted before milling. Apart from considerable energy and reagent savings, the preconcentration tailings have good mechanical properties (crushed rock) and there is no loss in reagents. The sorting can be carried out in-pit or underground for direct refill. TU Delft investigates separation efficiency of the most promising new sorting techniques and draw conclusions regarding the overall effect on mining operations (recovery, economy, mining and processing aspects). A number of cases will be investigated by obtaining and characterising samples (being from drill cores or sampled on-site), pre-treatment, sorting experiments using industrial sorting equipment, and finally working out the economic potential for the given cases. The general objectives are to assist the mining industry in increasing its resource base, minimise energy needs, waste and reduce costs. In this project TU Delft co-operates with world edge leading suppliers of sensor and sorting technology. Sorting Technology Today Automatic sorting is the separation of a particle mixture in two or more fractions based on measured properties. It relies on an automatic system with actuators responding differently as function of the material properties (Fig. 2). Depending on their properties and value, automatic sorting is suitable for particles ranging from 2 mm to 250 mm. In principle two different set-ups are applied (Fig. 2): on-belt and free-flight (or free fall). Width of a single unit can be up to 2 meters and depends on the required processing capacity. In the on-belt geometry the material is fed to a conveyor having at least several meters length between feed point and inspection zone. This is to allow the particles to come to rest after dropping them on the conveyor that has a speed of typically 3 m/s.

2 Figure 2 - Principle of an automatic particle sorter with on-belt sensor geometry for conductive and X-ray sensors (A) and free-flight geometry for colour cameras (B) While crossing the inspection zone, a measurement is done by an array of cells. On-belt inspection is common for X-ray transmission and electromagnetic sensors. Free-flight geometry is very suitable for optical inspection using line-scan cameras. The optical background against which the particles to be detected must be recognised can be freely chosen and the particles can be observed from both sides simultaneously. In principle X-ray transmission inspection can also be done in this way. For electromagnetic inspection, observation on a belt is often more favourable due to the smaller distance between sensor coils and material. The data-processing system has the tasks of successively recognising particles from the background, determine one or more distinctive features from each particle, classify the particles as belonging to one of both fractions, and of controlling the required pneumatic actuators. This process is carried out real time and for up to several thousands of particles per second. This is accomplished by an advanced combination of hard- and software. Material specific distinctive features, e.g. derived from known imaging algorithms, can be integrated, of which several are available in the system library. A better classification is possible at an increasing number of independent features. This can be achieved by extracting several features from a single sensor image, for example particle size, shape, and texture. A number of optical sorters in use for minerals processing applications is shown in Fig. 3. The design of these models is based on considerable experience in mining applications and is suitable for over 8000 h/y operation. The optical systems rely on the use of one or more colour line scan cameras and illumination from specially designed light sources. From the camera a variety of distinctive properties can be extracted that include shape, area, intensity, colour, homogeneity etc. For a new application the user describes the various fractions that are present in the feed by setting independent classification criteria. During sorting these are applied on all particles and based on these criteria a particle is sorted or not. Typical applications are various base metal and precious metal ore, industrial minerals and gem stones (Fig. 4). Figure 5 shows how automatic sorting is integrated in a 500 t/h plant for limestone concentration in Finland (NordKalk). The system design that is now successful in the colour sorting of rocks is also the basis for future improvements and introduction of new sensors.

3 Figure 3 Range of optical sorters for minerals and typical applications (Images courtesy Scan&Sort GmbH, Germany) Figure 4 Applications of automatic sorting (Images courtesy Scan&Sort GmbH, Germany)

4 Figure 5 Limestone colour sorting Dolomite / Calcite at NORDKALK, Finland (Courtesy Scan&Sort GmbH, Germany) X-ray sorting X-ray sorters are insensitive for dust, moisture and surface contamination and sort directly on the difference of average atomic number of the rock fragments. Even if there is no visible, electric or magnetic difference, many materials can still be concentrated with X-ray sorting. In recent years using fundamental principles of X-ray transmission and electromagnetism Delft University aims at developing and improving new solutions for automatic minerals and rock sorting. These developments are now being implemented and further developed for integration into existing industrial sorting systems that are available on the market. As a result, reliable sorting technology is available capable of achieving considerably better sorting results at higher capacities and against lower costs. The principle set-up of an X-ray sorter is given in Fig. 6. X-ray imaging appeared very suitable for automatic sorting of a variety of materials, including minerals and coal (Figure 7). It is particularly suitable for the high-speed identification that is typically required in mining applications. The detector, an array of scintillation crystals, needs a sufficient number of counts on each element of the array for an accurate reading. The modern electronic transmission imaging enables sufficiently fast and sharp reading with X-ray tubes that are common in X-ray imaging equipment. No permanent nuclear X-ray sources are necessary. Conveying speeds of typically 3 m/s are possible. Second advantage of transmission is that the particle volume is detected and not just a surface layer, as is the case with optical sorting. X-ray imaging enables a fast determination of an average atomic number range of the inspected material.

5 In combination with the high-resolution image this gives an estimation of the content and properties of the scanned materials. Figure 6 Principles of an X-ray sorter and industrial test system (Courtesy CommoDas GmbH, Germany) By X-ray imaging crushed and sized heterogeneous ore (coarse grained minerals in the 5 50 mm range, brecciated or veins of the mm to cm range) the pre-concentration potential can be made visible (Fig. 8). In this way the optimised crushing size is determined. A larger size gives lower sorting costs, but liberation may be insufficient. Smaller crushing sizes give better liberation, but sorting costs go up. In the example of Fig. 8 effective pre-concentration can already be done between 20 and 10 mm. The determination of the optimised liberation size in pre-concentration is a straightforward experimental procedure that is much easier than the conventional polished section and imaging analysis, since with X-ray the metal grade can be detected directly from the particles without the stereographic conversions that are required in image analysis of flat sections. The X-ray transmission sorter shown in Fig. 9 has been introduced in Apart from the necessary X-ray shielding to exclude radiation exposure, this system is very similar to an optical sorter and operates as fast and at similar resolutions and features. The shown model has 3 m/s belt speed, 1.2 m width and 0.8 mm geometric resolution. Depending on feed properties and specifications the approximate capacity is 5 45 t/h for mm sized material of density 2600 kg/m 3. The maximum width for this range of sorters is 1.8 m.

6 Figure 7 With X-ray imaging techniques the average atomic number of particles can be detected. An ore can be graded on metal content, or a coal on calorific value (ash content) or the presence of pyritic mineral inclusions (Sulphur content). Images TU Delft (left) and CommoDas GmbH (right). Figure 8 Liberation as function of size, directly determined with X-ray imaging, indicating the pre-concentration potential of this lead ore (Images TU Delft).

7 Figure 9 Industrial X-ray sorter introduced in 2004 (Courtesy CommoDas GmbH, Germany) Electromagnetic sorting of minerals Another development comprises the electromagnetic pre-concentration of minerals. It is based on differences in magnetic susceptibility or in electrical conductivity of the various minerals. Though this principle is well known, the latest generation sensor technology allows a much wider application range as known so far. It has the advantage of, compared to optical or X-ray sorting, providing the lowest cost solution in rock sorting. Recently TU Delft investigated the efficiency of electromagnetic sorting in coal cleaning (Fig. 10). Besides coal cleaning other applications are the sorting of industrial minerals and ore, wherever the detectable difference in magnetism or electric conductivity is sufficient. This can be easily established in laboratory tests. Benefits for mining operations Automatic sorting is hardly applied in the heart of a separation process. It is either applied for pre-concentration of the plant feed, for instance in the case of ore lump sorting preceding the mill, or for inspection of the products, in order to remove remaining contaminants that the mechanical process was unable to remove. The economics of automatic sorting are strongly proportional to particle size and mass: the larger and heavier the particles, the cheaper the costs per tonne. Particles that need to be removed from the feed are ejected by means of air blows, consuming considerable energy (pressured air). As a consequence, the smaller the percentage that needs to be removed, the lower the costs related to feed tonnage. Experience indicates that wear of automatic sorting is low and negligible compared to those of crushers, screens, mills etc. This is due to the fact that besides feed conveyor and catching bins the feed is not directly into contact with critical parts of the system. Besides the costs of the sorting itself, the avoidance of costs elsewhere in the process and added value of the products are determining. This is illustrated with the following examples.

8 Figure 10 Electronic coal cleaning using electromagnetic sorting technology is a low cost and compact alternative to conventional density separation, for instance in underground or temporary operations. Pre-concentration of Ore After primary crushing each ore piece below a certain cut-off grade can be individually removed. This leaves only ore of sufficient grade for the mill. Employing this method, ore sections of variable grade or composition can be economically mined that would, on average, have a grade that is at present too low for economic exploitation. The additional costs of the automatic removal of lowgrade rock before milling will be compensated by savings in the grinding process regarding energy and maintenance savings of the mill, in the mineral dressing circuit and its subsequent processing. Coal preparation Coal washery plants are usually designed for a maximum top feed size of mm. Oversize coal may be crushed, but especially when there is a strong host rock, coal percentage in the oversize can be as low as 20%. Instead of crushing the oversize and add to the washery feed, or consider oversize as waste and accept coal loss, sorting the oversize to recover the pure coal and middlings (bony or banded coal) is preferred. For this material conventional density concentration or handpicking are often too expensive in terms of investment or labour. A compact automatic coal lump sorter can be used instead. The sensor can be either electromagnetic or XRT depending on the required specifications.

9 Figure 11 The need for increased automation in mining puts the minimum feasible panel sizes and seam height under pressure. Low cost automatic lump sorting close to the mining front, e.g. using electromagnetic sensors, releases transport and washery capacity, especially in poor deposits (deep, banded, inclusions, bands with inferior quality, low seam height, splits etc.). Courtesy drawings: Thomas, Coal Geology, 2002, Photograph: DSK, Germany). A totally dry flow sheet for the preparation of coal can be particularly advantageous. One of the main problems of conventional dry processes, which rely on pneumatic mechanical cleaning, is the lower separation efficiency, sensitivity for moisture fluctuations in the feed and inability to concentrate fines. From earlier studies it was concluded that qualities and recoveries can be obtained that are competitive to conventional wet separation. Future Research The objective of the project is to investigate separation efficiency of electro-magnetic and X-ray transmission sensors and draw conclusions regarding the overall effect on mining operations (recovery, economy, mining and processing aspects). Specific cases in ore and/or coal sorting will be worked out in close co-operation with the mine operator and equipment supplier. The work itself consists of obtaining and characterising samples, pre-treatment, laboratory sorting experiments (in Delft and elsewhere), and finally working out several processing scenario s for at least one specific mining project (working out a flowsheet).

10 References Contact information: Bertin, E.P (1978). Introduction to X-ray spectrometric analysis. Plenum press, New York. CommoDas GmbH Dalmijn, W.L., T.P.R. de Jong, N. Fraunholcz, H.J. Glass (2000). Werkwijze en inrichting voor het scheiden en analyseren van materiaalstromen. Patent pending, Delft University of Technology. Harbeck, H.: Klassifizierung von Mineralien mit Hilfe des Röntgentransmissionsverfahrens. Colloquium Sensorgestuetzte sortierung, 6, 7 april 2004, Stolberg, Germany. ECSC Project 7220-PR-066. An evaluation of the economic benefits of pneumatic dry cleaning processes. Report to the European Coal and Steel Community, University of Nottingham, University Park, Nottingham, Jong, T.P.R. de, Mesina, M.B., Kuilman, W.: Electromagnetic deshaling of coal. Physical Separation in Science and Engineering V12, N3, P , 2003 Jong, T.P.R. de, Houwelingen, J.A. van, Kuilman, W.: Automatic sorting and control in solid fuel processing: opportunities in European perspective. Geologica Belgica Vol. 7, No 3-4, p , 2004 Jong, T.P.R. de; Dalmijn, W.L.; Kattentidt, H.U.R.: Dual energy X-ray transmission imaging for concentration and control of solids. Proc. IMPC-2003, XXII International Mineral Processing Congress, Cape Town, 28 September - 3 October 2003 Kattentidt, H.U.R., T.P.R. de Jong, and W.L. Dalmijn (2003). Multi sensor identification and sorting of bulk solids. Control Engineering Practice 11, 41-47, Kirchner, A.; Maude, Chr.: On-line analysis of coal symposium review. IEA Coal research, London, Salter,, J.D., Wyatt, N.P.G.: Sorting in the minerals industry: past present and future. Minerals Engineering, Vol. 4, Nos 7-11, pp , 1991 Thomas, L.: Coal Geology. Wiley, Chichester, 2002 Scan&Sort: