Well actually, quite a lot for very little, but still a very attractive deal nevertheless.
Lately our team has been treading leather on base metal and bulk mineral sites investigating applications for Knelson’s newest version of the CVD continuous concentrator. This basic winning formula has been elevated to a higher level by the incorporation of new innovative features that sharpen the grade-recovery capability of the CVD considerably. This will all be rolled out in the near future, but what I want to address here is an application for this technology that was always there, but which we can now really take advantage of.
Unless they incorporate their own smelters and refineries all base metal producers are subject to a supply contract that typically specifies a minimum acceptance grade. Of course these criteria are biased in favour of the smelter and the often arbitrarily specified minimum grades introduce all sorts of trauma at the mine end. You see, grade and recovery are not separable, they are tied together in a particular relationship. So the production plant has little latitude: their flotation process, for instance, does what it does and in order to achieve that minimum grade some extra mineral has to go to the tailings.
Well, let’s take a closer look at that one. Actually, there are two parts to this grade/ recovery relationship: mineralogical and process. The mineralogical part has to do with mineral associations, and degree of liberation at a particular grind size. We could call this the intrinsic relationship since it defines the maximum theoretically possible, as exemplified by the curve in figure 1. The second part is how closely we manage to approach the intrinsic curve by the process method applied. It is the product of these two that the process people call their ‘recovery’ and it is by the manipulation of this resultant that we hope to steal a little back for a change.
FIGURE 1 The components of grade and recovery (hypothetical)
First let us get back to our hypothetical chosen example, a copper flotation plant producing a concentrate for shipment to a smelter. And let us put some numbers to that for ease of understanding:
- Daily feed rate: 24000 tpd
- Feed grade: 2%Cu
- Recovery: 83%
- Flotation concentrate grade: 34%Cu
- Minimum acceptance grade for concentrate: 26%Cu
The met balance for this scenario looks like this:
FIGURE 2 plant daily met balance
If we want to scavenge some extra recovery from this plant, then the place to do it is the tailings, so we apply gravity and a cleaner step to this stream to scalp a concentrate assaying about 2.5%Cu into about 0.9% of the tailings mass. If we blend this directly into the flotation concentrate then the mass balance on the blend looks like this:
FIGURE 3 blended concentrate
So we are still way above the minimum acceptance grade of 26%Cu, but we have added 184tpd x 2.53%Cu = 4.6 additional tones of copper per day. That’s around US$38000 worth!
It sounds pretty good so far, but it doesn’t stop there. Whether through daily operational glitches or fluctuations in ore feed grade, the flotation plant concentrate grade will also fluctuate. It will exhibit the peaks and troughs that all superintendants know only too well. The same concentration method that we applied to scavenge values from the tailings can also be applied to separate out a super high grade concentrate from the flotation concentrate, a cleaner-cleaner step if you like. The peaks can then be used to supplement the troughs in concentrate grade such that the blend with the concentrate scavenged from the tails always achieves the desired overall minimum grade. In times of high grade feed to the plant It may even be possible to relax the scavenger conditions further to add even more tonnes of daily copper.
So in summary, two relatively small and simple Knelson CVD concentrators, working quietly at either end of the flotation plant end up bringing huge potential benefits. It’s like a synergistic feedback loop to push that curve closer to the theoretical maximum.
Here is a prediction for you: a blending department is going to become a very important feature of process plants, it just makes too much sense to ignore. And yes, you may have guessed, those numbers are not that hypothetical after all, we are working on real plant trials at this very moment – I’ll keep you informed!
