Density Tracer Testing

The following procedures focus on coal-washing dense medium cyclone circuits but are adaptable to other units. For a DMC circuit which has not previously been tested in this way, one must; select appropriate size(s) of density tracers ensure that they can be retrieved determine an appropriate density range to be spanned determine the appropriate number of tracers to be used at each density interval assemble the required personnel and conduct the test assess the data.

TRACER SIZE(S)

If tests are to be conducted with feed off, a range of sizes - say 8, 16 and 32mm may be used. However it is usually preferable to determine performance with feed on. To facilitate retrieval it is then usual to use only one size of tracer. For installations with a feed topsize in the 20-70mm range, 32mm tracers are most often used. For circuits with smaller feed it is sometimes possible to use 16mm tracers. For dense medium cyclones the relative behaviours of different size fractions can be inferred from the results of a test using coarse tracers (Wood, Davis and Lyman, 1987).

TRACER RETRIEVAL

Tracers are added to the circuit feed and are usually retrieved manually from the discharge lips of product and reject drain-and-rinse screens. Retrieval rate is typically greater than 95 percent per test. Many dense medium cyclone circuits incorporate two cyclones with separate product screens and a common rejects screen, and a person must be located at each of those screens. Prior to the first test of a circuit it is recommended that a small number of tracers be thrown on the feed ends of the product and rejects screens to ensure that they can be retrieved at the discharge ends. If the screens are heavily loaded it may be that some tracers become buried in the particle bed and are not seen before they fall into the screen discharge chute.

DENSITY RANGE

Density tracers are available at density intervals of 0.01 RD units (1.24, 1.25, 1.26 etc). In some cases adequate definition of the partition curve can be obtained using only every second density increment. Under common operating conditions, the separation density for a coal-washing dense medium cyclone is typically around 0.1 RD units higher than the feed medium density. One may conduct a "sighting" test with small numbers of tracers at densities from say 0.1 RD units below the feed medium density to 0.3 RD units above the estimated separation density. For example, if the feed medium density were 1.40, three tracers at each of the densities 1.30, 1.32, 1.34,....1.68, 1.70 may be used - a total of 63 tracers. The results of this test provide an improved estimate of separation density or definition of a density range of particle retention (refer following notes on interpretation of partition curves). If tracers larger than the feed topsize are employed, this test will also show whether they will pass freely through any unusual sieve-bend feed chute and skirt arrangements. If there is little or no retention, the full test should use tracers spanning a density range of approximately 0.3 RD units, centred on the estimated separation density. For example, if the separation density estimate from the "sighting" test is 1.52 RD units, tracers of densities 1.36, 1.38, 1.40 ...... 1.66, 1.68 may be used. After further experience with the circuit it may be possible to reduce the range to less than 0.2 RD units, but still have confidence that the entire partition curve (partition numbers from 0 to 100) will be defined. For circuits with separation densities higher than 1.40, if there is any evidence of a low-density "tail" (Figure 3) to the partition curve it is desirable to include tracers of a density close to that of clean coal - say 1.30.

NUMBERS OF TRACERS

To ensure adequate definition of the partition curve it is recommended that 40 tracers be used at each density - 440 to 640 per test. They should be mixed so that the various densities are added in random order. In cases where retention is expected over an RD range exceeding 0.06 RD units, there is no partition curve to be defined. In order to minimise tracer losses the numbers may be reduced to just three per RD increment over a density range which adequately spans the range of retention.

CONDUCT OF THE TEST

The tracers should be added at the rate of one tracer every two-three seconds - usually to desliming screen oversize. The test is normally concluded ten minutes after the addition of the last tracer. Any tracer retrieved after that period is designated as having been retained. In cases where products from individual cyclones in a module are drained on separate screens, it can be helpful to separately record the tracers which report to those screens. The data sheet on the back page has separate columns for this purpose.

REFERENCES

Davis, J.J., Wood, C.J. and Lyman, G.J., 1985a. Density tracers can improve coal preparation plant yield. Australian Coal Miner, July, pp9-1 1.

Davis, J.J., Wood, C.J. and Lyman, G.J., 1985b. The use of density tracers for the determination of dense medium cyclone partitioning characteristics. Int. J. of Coal Processing, 2(2) 107-126.

Davis, J.J., Wood, C.J. and Lyman, G.J., 1985c. The effects of operating variables on dense medium cyclone operation. Proc 3rd Aust. Coal Prep. Conf., Wollongong.

Wood C.J., Davis, J.J. and Lyman, G.J., 1987. "Towards a medium behaviour based model for coal-washing dense medium cyclones". Aus IMM Dense Medium Operators' Conference, Brisbane, 1987, pp247-256 and Coal Preparation, 1989, Vol 7, ppl83-197.

INTERPRETATION OF DMC PARTITION CURVES

The figures below illustrate the common forms of density tracer partition curves for dense medium cyclones. A module of one or more well-operated and well maintained dense medium cyclones should show an efficient separation (Figure 1). By contrast with conventional float/sink techniques, density tracers provide the resolution which shows that large particles can be partitioned with an Ep of less than 0.01 RD units.

Fig 1 Normal (efficient) partition curve.

Figure 2 shows a reasonably small RD range of particle retention. Separation is still quite efficient but there is a danger that a small change in operating conditions may increase the density range of retention. The cyclones rapidly become choked with "near-density" material and frequently clear themselves by ejecting surges of slurry, including low-density coal, to underflow.

Fig 2 Tracer Retention

The resulting partition curve is shown in Figure 3. The Ep is large; there is a low-density "tail" and a low (sometimes negative) offset between feed medium density and cutpoint. The performance shown in Figure 3 can also arise from vortex finder overload when the medium flow from the vortex finder is insufficient to carry out all the particles which should report to the low-density product. As with surging, the yield loss can be very significant.

Fig 3 Surging or Vortex Finder Overload cause yield loss

A curve with a plateau (Figure 4) is indicative of differing cutpoints between separators in the module. Examination of the data for individual product screens will suggest which units are separating at high, and which at low density.

Fig 4 Two DMCs with different cutpoints.

Means for the correction of these separating inefficiencies may be found in the references listed above or by contacting Partition Enterprises Pty Ltd.

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