my guess is what you are seeing is feed from the primary crusher with 2 conveyers under the 2 different sized screens on a single deck, one conveyer takes the fines to a ball mill one takes the medium sized rock to a secondary crusher and the oversized gravel vibrates off the screen falling in a pile for a FEL to return to the primary crusher. Robert or a poster familiar with the plant probably have a better understanding though and may explain better.
You are on the right track most mills I have seen would have a double or triple deck vibrating screen rather than a single deck screen with 2 different apperture sizes so this is quite novel.
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