In these challenging times it makes sense to treat the more easily accessible and more easily treated ore resources. Units produced at a lower cost may well see the whole operation through the economic dip. Even in good times, a sweetener to make up grade and tonnage short-falls is most welcome.

The resource

Time has done a pretty good job of loosening up the upper zones of most ore bodies. In most cases it is still there as the oxidized over layer, but in others it has been transported a little way off and reworked. Where the target mineral is hard and has a high specific gravity, reworking usually results in a natural upgrade that we can take advantage of. One good example of this is chromite where the individual grains have been completely liberated out of the original host to make a high grade, closely sized product – an almost perfect target.

Figure 1 It is easier to mine the errosional deposition in the vlei (foreground) than the thin seam on the hillside (background)

Why is it attractive?

Gravity recovery of these resources is ideal for many reasons. Firstly, it is chemical free so permitting is simplified. The plant and infrastructure necessary to deal with them is simple, involves lower capital and operating cost and consumes less energy per tonne than conventional systems. All of these factors combined result in a far less onerous project definition, with the likelihood of a shorter lead time to production, set-up time of less than a week is common. These resources are generally easy to locate and since the resulting tailings are just the reworked feed material, thoughtful back placement can result in minimal disruption and quick rehabilitation. 

How is it done?

Whether re-worked or in-situ oxide material, the first break is that loose oxides are amenable to scrubbing. The matrix is disintegrated into its constituents which is, of course, a pre-requisite for efficient separation. In most cases the target minerals remain relatively coarse as the scrubbing is not so harsh as to cause grain breakage. Having completely disintegrated the host, the competent coarse gangue can be screened off and by doing so the undersize is upgraded inversely. The choice of screen size is important as the aim is to remove the vast majority of the gangue via this simple initial step. It is unlikely that there will be an absolute cut-off and some recovery loss to the coarse material is inevitable. If the oversize were found to contain viable values, then this could be crushed and returned into the system, possibly after a sorting step to identify the pay ore.

The disintegrated undersize slurry is then fed through a Knelson concentrator. If the target is precious metals (gold, silver or platinum), then the appropriate machine would be a batch Knelson concentrator. If the target is a bulk mineral, for example cassiterite, then the Knelson CVD continuous concentrator would be used to cope with the higher mass yield. The primary concentrates are then upgraded via an appropriate step, for example, a Gemini concentrator for precious metals, or a smaller Knelson CVD for bulk minerals. 

Figure 2 A 20 tph RG150/Knelson CD20 combination for recovering alluvial gold. Note no formal foundations necessary.

What scale are we talking about?

Scale is almost limitless, Knelson Concentrators are available from 6tph through to 650tph, and obviously multiple units could result in a very large instillation indeed.  However it is equally important that viability can be achieved at the lower scale, even down to 6tph. 

An ideal system for artisanal mining

This paves the way for entrants in the artisanal field who, through their primary success, can grow into larger processing modules. An elementary study will show that there is a lower tonnage limit for viability. If anything, the processing at smaller scale is also more difficult. Knelson and APT have therefore concentrated upon producing a small but serious entry lever unit, the 6tph RG60/MD12, utilizing the same, proven technology of the larger scale.   

The lower capacity units are also important because they provide the means for large scale field exploration.  This is particularly important because deposits of this nature are difficult to sample and evaluate and large sample lots are necessary. 

Figure 3 A 6tph RG60/KnelsonMD12 combination. Together with a cleanup table comprises a complete solution. All powered by 25kVA generator.

Complete processing plants

Knelson Gravity Solutions (KGS), through their association with Appropriate Process Technologies (APT), are able to supply fully comprehensive process modules of 6tph, 20tph and 80tph.  These packages are designed for extreme simplicity and are supplied absolutely comprehensively to allow immediate assembly and start-up.  The units are designed to be easily translocated as the deposit is depleted so that ore-trucking is kept to a minimum. Being of low energy consumption, typically around 3kwh/t in total, on-site power generation is feasible and indeed units can be supplied with generators.

What sort of costs are involved?

Capital costs obviously vary according to the mineral to be targeted, but for a simple gold module to treat 20tph, the all-up cost, inclusive of scrubber, Knelson MD Concentrator, clean-up table, water supply tanks and pumps and power generation, would cost approximately $250 000 at today’s rates.  A complete 80tph gold module would be approximately $800 000. 

Process operating costs are extremely low, with power generation fuel being the main consumable.  A typical gold module of 20tph would have an operating cost in the region of 10USc per tonne. 

How do you test for applicability?

Knelson have developed a test methodology which mimics the full-scale principles, in essence a mini pilot plant.  A sample of 60 - 250 kg is tumbled in an autogenous scrubber and the resulting slurry is screened to remove oversize.  The undersize product is then fed through the appropriate Knelson Concentrator and the resulting concentrates are upgraded on the appropriate device.  This relatively large sample size not only allows more representative sample, but also generates a reasonable amount of concentrate for any subsequent investigations.

An even smaller amenability test can be run prior to committing to the large sample, in this case, just five kilograms are necessary and this material is first scrubbed and screened prior to passing through a batch Knelson 3” concentrator fitted with a special cone.  Tails are re-passed several times generating recovery and grade vs mass pull information.  These tests can be performed at Knelson’s Laboratories in Vancouver and Southern Africa.