UTILIZATION OF OPTICAL SORTING TECHNIQUE IN THE PHOSPHATE INDUSTRY

Rachida Daoudi, Violeta Kukenska

JACOBS ENGINEERING Lakeland, Florida

Presented at: CENTRAL FLORIDA 2013 SYMPOSIUM

Sheraton Sand Key, Clearwater, Florida June 8 & 9, 2013

Abstract: One factor that increases operating costs in many mineral processing plants is the treatment of waste material. Mining methods, although greatly improved over the years, still take in varying amounts of waste material. This waste has the effect of increasing costs in transportation, crushing, milling and often chemical and other costs. Optical sorting technology is quite used in several mining industries other than phosphate to reduce the waste material. Only one application in the phosphates industry was used by one phosphate mine in the early 1980s in Western United States, the equipment at the time was expensive to maintain, but advances in detection equipment, instrumentation, and control have made this type of equipment much more reliable and user friendly over the next 30 years. Other phosphate mining companies have recently shown interest in optical sorting, and done some prefeasibility test on bench and pilot scale. This paper covers tests conducted by TOMRA Sorting Solution using X-ray transmission (XRT), to differentiate chert from phosphatic limestone. The results are promising and could improve the processing performance in phosphate mining and beneficiation.

Introduction

Automated optical ore sorting equipment is already used by some mining companies for industrial minerals projects. Ore sorting systems allow mining operations to reduce dilution, lower strip ratios and transport costs, remove contaminants and optimize process flows by separating ore from gangue on a rock by rock basis. To incorporate this technique in the treatment of phosphate, it is interesting at first to define the optical sorting regarding the benefit and the disadvantages and second how to integrate this technology to increase processing performance in the phosphate mining industries.

Over the last ten years, improved optical components and advances in computing technology have substantially reduced costs for sorting equipment. Normally, suppliers of optical sorters only offer equipment for small or intermediate size particles (10-30 mm), one new supplier offers high capacity sorting equipment for handling a wider size range sorting, which in the past was based on brightness levels of reflected light, can now employ other criteria such as surface texture and color spectra. Two sided viewing enhances efficiency in some applications.

Optical Sorting Technique

Sorting units or Optical Sorting is quite simple: this process relies on the detection and separation of singles particles in bulk stream. The first step in sorting is therefore to isolate each particle in the whole stream. This is achieved by accelerating the speed of the particles and increasing the width of the transport axis. The information of the sensors is picked up by a central processing unit which classifies each particle into a pre defined category and sends a signal to the separation unit. The adjacent separation unit removes positively detected particles from the stream by air valves which are powered with compressed air.

Depending on the industry and materials desired for separation, there are different detection technologies in optical sorting that can be used. Figure 1 below lists the technologies available today.

Figure1: Detection technologies in optical sorting

(Mineral Processing Technology G.V.Rao-Vibhuti N Misra)

Benefits of optical sorting

- Pre-concentration of mill feed by separating it into high-grade, low-grade fractions and waste fractions

1- Extend reserves into lower grade resources 2- Utilize lower cost mining methods – Accept more dilution up front in mining and get rid of it before it hits the mill

- Selective high grading of existing waste dumps

- Reduction of environmental risks and costs, such as segregation of waste rock to send to appropriately designed dumps

- Optimization of multiple process streams, for example, sending appropriate ore to the mill and leach heaps

- Pre-concentration of ore underground or at remote sites in order to reduce haulage and hoisting costs and effectively increase hoist capacity.

Disadvantages of the optical sorting

- Installation costs of the optical sorting machine

- The optical sorting system use a sophisticated equipment which can be difficult to maintain

- XRT applications are limited by the thickness of the fragments that intercept the beam and power of the X-rays tubes.

- Capability of optical or color sorting is limited by poor liberation of mineral constituents presence of dust and coatings on the surface of fragments and by the power of compressed air jets used to deviate the coarsest rejected fragments.

- Sometimes it is necessary to use water sprays to clean surface of fragments before they enter the sensing zone of sorter.

- The efficiency required depends on the average particle size and the quantity of particles to be rejected. ( for example on optical sorting of ceramic, a fraction of 3–5 mm may be sorted with capability of 5 t/h, if only a small quantity has to be rejected. An increase of the quantity to be rejected to 30 %, reduces the possible capability to below 2 t/h. The sorting of fractions between 100–250 mm achieves performances of 150–200 t/h).

Operating cost

- The dominating cost for sorting lies in operation. The possible exception would be small particle (-10mm, -3/8”) sorting, where the capital cost may dominate. Compressed air consumption comprises the dominating operating cost element. Blasting small rocks requires somewhat more air per ton of blasted material than large rocks, but the difference is insignificant. Quoted numbers are about 40m3/ton for large rock and 50 m3/ton for small rock. In the US with relatively low energy cost ($0.03-0.04/kWh), the compressed air cost may be approximately $0.35 to 0.70/ton feed rock. (The cost needs to be prorated according to the electric energy cost).

- Maintenance cost can vary from roughly $0.35 per ton feed for some large rock (100-140mm) M16 sorter plants to a few pennies per ton for less maintenance intensive, free-fall sorters processing intermediate size rocks (less than 50 mm).

Optical Sorting Bench Scale test using XRT by Tomra (Commodas

sensor)

As mentioned above the type of technique and corresponding vendor sensor recommendation will depend on the industry and materials processed. In our case Tomra Sorting solutions

recommends XRT (X-ray transmission) technology in evaluating atomic density for optical sorting application.

Bench scale testing was conducted by Commados Facilities for separating chert from phosphatic limestone. The initial objective for test work was to improve concentration performance of downstream upgrading process. Removal of coarse, sized, hard chert of high abrasion index value, upstream of impact crushing and rod milling, will contribute to reduced energy requirement, wear rate of comminution equipment and maintenance cost.

As the response to X-ray transmission mainly depends on atomic numbers of elements constituting the material intercepted by the incident beam, the XRT is not affected by presence of mineral coatings or dust onto the surface of fragments under sorting. This is a great advantage with regard to optical sorters. However, XRT applications are limited by the thickness of the fragments that intercept the beam and power of the X ray tubes. Coarse sized, high absorption materials composed of heavy elements cannot be effectively sorted since the incident beam would be totally attenuated.

Figure 2 illustrates a typical flow diagram for two stages XRT optical sorting used in beneficiation bench testing.

Figure 2: Flow sheet for sample preparation and XRT sorting test by Commados

Results:

High separation performance was achieved in two stages on both size fractions:

- 97.1% of the phosphate and 97.9% of the calcite fed to the sorter are recovered in the final phosphate products, (R1A+R2R), at a weight recovery of 87.3%, whereas 94.4% of the silica and silicates report to the final rejected fraction, (R2A), for the -60+40 mm feed fraction.

- 96.5% of the phosphate and 97.7% of the calcite, most of the later mineral constituting the matrix of the phosphate containing fragments, are recovered in phosphate products at a weight recovery of 88.4% for the -40+20 mm feed fraction. 87% of the silica and silicates are recovered in the final silica tailings. It should be noted that the fraction

(R1R) of the initial feed entering the second sorting stage is moderate, representing 15.5% by weight for the combined -60+20 mm size fraction

Promising efficiency is also obtained in one sorting stage since 92.9% of the phosphate contained in the -60+20 mm feed fraction are recovered in the phosphate product at a weight recovery of 84.5%, whereas about 90.9% of the silica and silicates report to the tailings R1R. Table 1 and Table 2 below show the results of two separate bench scale tests. Commodas data reveal that the materials as received were much finer than expected since 60mm oversize fractions were: 3.1% for Ore 1 Sample and 1.7% for Ore 2 Sample, corresponding to 2.3% for the blended samples. This unfortunately did not allow an evaluation of the sorting techniques on coarse sized fragments in the 150 to 60 mm size range where liberation of chert and phosphate could be less favorable when compared to the -60+40 mms fraction. In addition,

Table1: Overall material balance for sample separation and XRT sorting tests by Commados ( chemical compositions)

Table 2: Overall material balance for sample separation and XRT sorting tests by

Commados ( mineralogical compositions)