Density Tracers are particles having precise densities to mimic the densities of particles in a plant feed stream:
- valuable minerals such as diamond (3.53 g/cc) or coal (1.25 g/cc).
- low-value gangue such as sandstone (2.3g/cc), shale (2.7g/cc), kimberlite (2.25g/cc).
- composite particles such as a piece of coal attached to some shale. The density of the particle is an indicator of the proportion of valuable mineral.
Density tracers with densities spanning the range of interest are added to the circuit feed and retrieved from the product and rejects streams, manually or with the assistance of magnets. For statistical confidence, a test may utilize some thousands of tracers in selected sizes and densities. Additional properties such as bright colour or ferromagnetism facilitate retrieval. After retrieval they are sorted into their various densities, and the resulting data are used to plot a partition curve.
The form of the curve can indicate whether the metallurgist should take actions such as adjust medium density, replace a worn circuit component, or correct an overload or medium instability situation.
Density Tracers recovered from Floats & Sinks.
immediately show the Partition Curve.
Spiking Tracers mimic the density of a valuable mineral and are used in situations where the mineral particles are well liberated. Their most common application is to test for recovery of free diamonds from an alluvial deposit. They can be inserted in the ore stream anywhere between the pit and the final sorting stage. If that final sorting stage utilizes X-ray sorters, the tracers should luminesce strongly under X-rays to ensure none will be rejected at that point. Thus, if 100 spiking tracers are added to a plant feed conveyor and 90 are recovered by the sorters, one may estimate that 10 percent of diamonds are being rejected by the processing units between those points. The metallurgist may then seek the cause of those rejections by conducting further spiking tests, with different addition points, or a density tracer test (as illustrated bottom left) to determine the partition curve.
Luminescence Index Tracers mimic diamonds in terms of density and are offered in seven luminescence levels to simulate the response to X-rays of high-luminescent and low-luminescent diamonds. Like diamonds, they should be translucent to light, so that the luminescence is a volume effect, not just a surface effect.
They are commonly passed through a sorter to provide a basis for adjusting the sensitivity of the photomultiplier tubes which detect the presence of diamonds. Those adjustments are usually made with no feed gravel passing through the sorter. It is important to also conduct tests with feed on to check for diamond losses which may result from overloading or other causes.
In summary, all these classes of tracers are tools to assist the metallurgist inachieving a financially rewarding compromise between:
- greater yield of valuable product; and
- less contamination of that product.