If it is lepidolite the key then becomes grade - if for example it grades 1.5% then need 5 tonnes of ore to produce 1 tonne 6% grade concentrate at 80% recovery. It also means the higher the grade the less deleterious elements to be treated. At 1% grade you need 7.5 tonnes of ore. Given where concentrate prices are at, having high grade is a key if it is lepidolite as this reduces costs of removing deleterious elements per tonne of concentrate produced. If spodumene well all good even at 1% grade, because spodumene has less deleterious elements to treat than lepidolite when the ore grades the same.
Illustrative: 1. Deposit grades 1% Li20 and has 0.8% Fe203 - at 7.5 tonnes to produce 6% grade spodumene, you have 6% cumulatative Fe203 to deal with. 2. At 1.5% you have 4% cumulatative Fe20 to deal with, hence why grade matters if it is lepidolite, because removing deleterious elements from lepidolite is more difficult than from spodumene. 3. This is an example to repeat as there are other deleterious elements too (see below)
in essence, the issue is that lepidolite refining practices are less developed than those associated with spodumene - i.e. just have to look at patent applications to see that - but having someone onboard with experience is a key if it is lepidolite. Obviously spodumene is preferred, but the key is grade if it is lepidolite, noting just how many new mines one needs for the EV revolution itself (so suspect when there is a shortfall technology will improve).
Another way to put it, if your deposit is 1% Li20, you would want it to be spodumene, but if you had two deposits, one been 1% Li20 spodumene and the other one been 2% lepidolite, well what would you prefer, if the spodumene one had Fe203 of say 0.5%, and the lepidolite one was 1% Fe203. You might find little processing difference in cumulatative Fe203 - as they both have cumulatative Fe203 count of 3.75%. It is why grade is a key if it is lepidolite here, and obviously understanding the deleterious element count. Note - this is an example to repeat, as there are other types of deleterious elements to deal with, and they are higher in lepidolite generally.
When talking deleterious elements there are three main types you don't want - phosphorous (P), fluorine (F) and Fe (iron) especially. These are essentially your penalty elements in concentrate pricing. When one talks mica as well, the key thing you are really focussing on is fluorine and iron, because fluorine and iron is an element of mica. The other is magnesium (Mg), potassium (K) and calcium (Ca).
Most converters operate on the assumption of recovering 85% - 90% of the lithium concentrate in the 6% grade concentrate exported, when converting to say carbonate/hydroxide for example. Now, high deleterious impurities means that these impurities, if not removed in the 6% concentrate making process adequately to meet sales specs, stick at high enough concentrations in the 1050 degrees roasting process meaning they form clinkers and therefore in their removal you end up losing more lithium in dpwnstraem converters, and this is why downstraem converters (those producing lithium carbonate of lithium hydroxide monohydrate) would penalise sales not at right specs for example and/or pay a premium for concentrate products with very good deleterious specifications. This article explains the concept of clinkers in more detail: https://www.linkedin.com/pulse/all-chemical-grade-spodumene-concentrates-same-harman-grant/
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