A coin found lying in the street is hardly worth the stoop to collect it, right, especially when it would take a fist full to buy even a slice of bread, let alone the loaf?
What if someone had been walking down the road with a hole in his pocket, replenished by an endless re-supply of coins? And what if every now and again a biggie dropped out, even the odd note!
Back to subject and the context here is recovering seemingly insignificant amounts of gold from cyanidation tailings. We will quantify these amounts later, but first let’s look at whether we are chasing reality or thin air. The fact is that in every cyanide tailing you wish to examine there will be some residual free gold that has survived the dissolution process, either because of insufficient residence time to dissolve a coarse particle, or perhaps some passivation coating, or maybe a breakdown of conditions. The presence of these peaks does not indicate a deficiency of the current process nor a failure of the process management, but simply an acknowledgement that conditions can and will wander beyond the design capabilities of a plant and tailing gold spikes are inevitable.
Never mind the reasons, the realization that even at steady state there will be something and with an imbalance there is likely to be more, is cause for excitement rather than concern. These are the spikes that give us the +/- after a correctly quoted tailings value, the bits that statisticians like to smooth away, and the stuff that we at KGS would rather add to your recovery. These are not numerical peaks, ladies and gentlemen, they are gold every bit as real as the rest of your recovery and worthy of consideration. This is ‘new gold’ as opposed to ‘alternative gold’, since its capture is over and above everything else being done.
Just how much GRG might be in tailings? The tabulation below gives results of Knelson MD3 tests upon various tailings that we have tested. Admittedly these figures represent the maximum that may be expected, in practice only a fraction of this may be recovered, depending upon the gravity effort applied. Nevertheless, the amount available is often rather a shock.
Table 1 Recovery from CIP plant tailings, South Africa
GRG RECOVERY FROM TAILINGS via Knelson MD3
Mine
Feed
Yield
Recovery
g/t Au
%
%
A
0.20
4.01
23.9
B
0.21
2.59
16.5
C
0.54
1.66
5.2
D
0.25
1.55
37.7
E
0.31
4.48
44.3
F
0.35
2.58
38.6
G
0.18
2.51
15.6
And just what is involved in scavenging values from tailings? Since the application is in open circuit we de-rate the tonnage capacity of our concentrator by 50% or more to optimize capture, but even so a surprisingly compact installation results. The idea is to locate the gravity module next to the final tailings sump so in effect we are simply interrupting the tailings line with a short side hop. The concentrates produced may either be returned to mill feed or upgraded via sub gravity.
The giant XD70 is particularly suited to this scavenging duty bringing not only economy of scale, but a simple unit installation requiring the absolute minimum of ancillary engineering. The XD70 gravity module shown below would be capable of treating a stream of about 350 tonnes per hour!
Figure 1 Knelson XD70 Tailings Module
And is it worth all the fuss? Let us explore that through an example of a single XD70 complete module on a tailings stream:
Stream tonnage: 350 tph solids as slurry at 33%
Grade: 0.2 grams per tonne
Gravity recovery: 10%
Gold price: US$28/gram
Recovery value: US$1.6 million/annum
Total capital and installation cost: US$1.1 million
And if the plant sees a few of those spikes that push the recovery to just 12%, the IRR jumps to over 137%!
Not mere crumbs, not just a few slices, but a loaf worth more than a million dollars a year that would otherwise probably be lost forever. Sobering thought.
A coin found lying in the street is hardly worth the stoop to collect it, right, especially when it would take a fist full to buy even a slice of bread, let alone the loaf? 
