There has been a lot of talk about Egina lately. Discussions about processing, gold distribution and the efficiency of ore sorters. A forum poster had the belief that the Egina lag gravels needed to be crushed and that the gold concentrate would need further milling and what not:
“Its all about size. They need to get it down to a certain size to put it through the sorter. There will be oversized boulders. They cant remove them one at a time so they will have to crush. Primary and then maybe cone crush to get smaller.
There is transport. Not far but they still need trucks. They are not going to move the plant every 500 feet. And then when they do finally get a concentrate that also needs to be sent to a mill where its crushed and reprocessed to get gold. Concentrate is not gold. Its mixed with waste. Its not pure.”
… This sounded far fetched to me so I sent the above comments to Novo’s CEO, and mining engineer, Rob Humphreyson in hopes that he could shed some light on what Novo currently envisions for Egina and what the Novo team is currently working on.
This was his response:
Constructive criticism is always welcome – that is in fact one of Novo’s agreed team behaviours owing to the unique style of deposits we have and the necessity for innovation. Good questions and very difficult for people to understand without the context of actually seeing it first hand.
Doing my best to answer the queries in order.
From our in-field observations so far, oversize at Egina will be minimal – per what you observed in the rockpiles at the mill and in the rockpiles ready for trucking at the trenches, plus what you can observe in the walls of the trenches. Not anticipating any crushing for Egina as mother nature has kindly provided this service free of charge over the millenia. Only possible need for any crushing is if the geology substantially changes away from Egina or the infinitesimally small amount of +50mm material we observe at Egina has commercial quantities of gold contained within and it’s economic to crush the oversize and extract the gold (unlikely). Current concept is that large nuggets above 50mm would be detected and scraped off tails conveyor belt scanners (twin to be ensure detection in case of individual unit failure – big nuggets too valuable to lose) with smaller material being processed through mechanical sorters or other. Comet Well on the other hand would definitely require crushing prior to screening and further processing, per the mechanical sorter trial run late 2018.
Transport is interesting – if we can design a plant that can trail behind a continuous miner, this opens up a world of opportunity in terms of reducing costs. A concept under consideration is to handle the material only once and return it to the ground via a tails conveyor in a single pass, then flatten it and rehabilitate it straight away. Intuitively seems feasible but there will no doubt be technical issues linking the mining, processing and rehabilitation that will require further work – hence a staged approach to field trials. Key is mechanical sorter thoughput rates to match continuous miner production rates and hence how big the trailing plant would need to be to keep up and whether or not this is technically feasible – hence field trials to test miner productivity, understand the rock size fractions that a continuous miner will generate, ore sorter productivity for these various size fractions, gold deportment (size fraction analysis, statistical distribution of gold throughout the gravel and basement horizons etc). If successful, this would result in very low operating costs. It should be said that we are not wedded to any particular concept and are actively conducting desk top assessments of all possible solutions and ranking them, though I must say the prospect of automated mining, low operating costs, water and chemical free processing and all with the gold reporting to secure concentrate bins without being touched by human hands is somewhat enticing. It’s then a matter of economics and matching this to the geology that we are learning about in real time – so two streams in parallel, geological understanding and economics. First and foremost, we need to continue to get a handle on geology / grades, bearing in mind the field and geological model has only had around 6 months of proper systematic exploration. As we gain confidence in the model, we’ll look at timely and prudent mining and processing technology testing in the field and in labs as required (test fast / test cheap) to understand how operational costs may play against the grades we are seeing and if we have a very large lowish grade system amenable to bulk scale mining or whether we have areas of higher grade that may require a selective and more conventional approach (or a bit of both ). We have some very smart employed to focus their attention on both potential outcomes, bearing in mind the dynamic nature of making plans against a background of continual geological learning. All knowledge we build up is important for submitting a mining operations plan, as such plans must outline in detail the mining, processing and rehabilitation methodology contemplated prior to gaining approval for mining to proceed.
Concentrate from a mobile plant consisting of mechanical sorters (and possible other technology) would largely be in the form of raw individual nuggets in the larger size fractions, with some ‘hot rocks’ captured also. Tails conveyor would capture any very large nuggets. The lower size fractions would rely on a ‘panel blow’ to ensure gold capture by blowing and area rather than a point, resulting in a ‘dirty con’. Given the total actual gold concentration in rock, even in high grade areas is only a fraction of total mass (it’d be really nice if it were larger !) an extremely high grade / low mass portion would be transported to another facility to be refined / smelted / sold as nugget samples etc.
That’s the current thinking, but tomorrow’s another day and Egina continues to throw up surprises every single day.
Hope this helps.”