There are plants out there that use no chemistry at all, but very few of them. Of course alluvial/placer plants fall into this category, but for this discussion let us focus upon the hard rock crushing and milling plants that literally suck carbon in like a black hole. For sake of consistency I am going to focus on gold plants specifically, but most of the principles will apply across the board, so please feel free to read your own theme into it.
Let us focus our attention here on changes we can make to the typical gold plant, and a good place to start would be to define the typical gold plant , shown in Figure1 below.
Basically we can break the circuit down into four unit blocks: Crushing, Milling, Dissolution (CIP/CIL), and Winning (Elution and electrowin). Obviously the energy consumption will vary according to scale, but I have considered a fairly small operation of 180tph.
Whether it is kilojoules of electrical energy or kilograms of reagents, all inputs into processing can ultimately be boiled down to a carbon equivalent. Even the water we use in processing comes at a high energy cost. Add it all up and the bottom line is no longer just financial, but we have an additional item: responsibility. Responsibility for the wastage, responsibility for the immediate environmental impact and responsibility for the long term global damage, and responsibility for the maximum possible gold recovery to make it all worth while.
Figure 1.
Heavy stuff! Yes, for those who have the conscience to appreciate the gravity of it, and fortunately most of our sterling industry fall into that category. And to those who don’t, heavy stuff too, because the cost of the excess is high, and becoming higher daily. And if that were not enough to stir awareness, then add in this cost: legislation and correction. Many countries are already preparing legislation and mechanisms to quantify, monitor and regulate wastage in mining under the general banner of carbon usage. To coin the buzz phrase, the carbon footprint is under scrutiny and our processing footprint stands out boldly because it includes all the nasties to boot, like emissions, toxic chemicals and long term oxidative seepage.
The burning question is, how to address this? The gravity-only option is an obvious solution, and in a previous article we looked at a plant with an all-up process energy consumption of just 12 kwh/tonne, and absolutely chemistry free at that. Certainly this would be the way to go where one has the luxury of a clean sheet of paper, and an amenable ore. But what of the multitude of existing gold plants like our typical example? Do the principles that brought such massive savings in the gravity-only plant also bring us savings sufficient to warrant implementing them in addition to the existing process? The savings we are interested in are obviously energy and consumables, but also the gold itself. Remember, the ratio that will be of interest will be carbon use/kg of gold produced, so attacking it at both ends brings a bigger benefit. Let us take a look at what is possible.
The first block of our typical circuit, crushing, seldom earns the processors attention, but it should! It is well known that fines are generally higher grade and we have seen grade ratios of more than 3 to 1 on -2mm crusher dust versus oversize. On the Eureka plant in Southern Africa, screening off the -2mm from a heap leach feed and processing it through Knelson batch concentrators generated 26% of the gold immediately, for an overall increase in recovery of 10%. The leach time was also shortened by 17%, with proportional savings of power and reagent consumption. Ok, that’s a heap leach, but pre-screening and taking the fines directly to the gravity circuit before it sees the mill in our typical circuit would again add the free gold right up front and avoid all the potential losses in milling.
Which brings us to our second block, milling. It is now very well established that gold losses on media in milling can be very significant. An audit by Mintek on a mine into which we had fitted Knelson batch concentrators on the cyclone underflow gave an increase in overall recovery of 5%. When the ball skats mags were collected at startup they assayed 673.8g/t Au. As the circulating free gold load was pulled down after four hours the mags assay had dropped to 429.4 g/tAu, and after 16 hours it was 201.1 g/t Au. At a kilogram per tonne of iron consumption, that reduction of the gold on iron equates to 0.47 g/t relative to mill feed, more than enough to account for the increased recovery observed. And here is the shocker, that was one of the lowest mill iron assays I have observed over the years, a few others being 800.7g/t Au (medium grade Wits mine), 2410 g/t Au (high grade copper/gold mine in Tanzania) and the highest was 9443g/t Au on mill iron on a high grade wits ore!
So certainly we can avoid wastage by putting gravity into milling circuits, and whilst there may not be an energy saving in the typical milling circuit per se, there is a very tangible benefit in the next block, dissolution: CIL/CIP. All that gold we took out in the milling circuit does not have to suffer the dissolution circuit. Residence times can be reduced dramatically and we have seen cases where a third of the CIP train was rendered redundant. The resulting energy saving alone was of the order of 0.4kwh/t, with significant cyanide savings too. Less cyanide added obviously means less leaving the plant too, whether it be as free cyanide or WAD, and therefore less to contain, or neutralize. We have talked about Knelson concentrators scavenging values on CIP/CIL tailings in a previous article. Our ongoing research shows that the action of the Knelson also reduces both free and WAD cyanide considerably, a very welcome added benefit whilst scavenging that last bit of gold value.
The last block in the typical plant is Elution and Electrowinning. The more gold we recover upstream, there is proportionately less to have to elute and electrowin. The number of electrowin cycles will also decrease as a consequence of the downsized cyanidation circuit. That means less energy, less cyanide usage, and less carbon usage - no, not the same stuff this article was about, but rather a neat place to end the journey don’t you think?
So there you have it. The humble little Knelson concentrator, so tiny in comparison with all those other huge components of our typical process plant, can, and does make a significant reduction in the carbon footprint. And, yes, on a good few of those plants it recovers more than the chemical plant too!
