st-bob wrote: Twigs, pieces of timber,
That's possible, you get some runs where there is some wood in it. Only takes one small log to get to get splintered into a million pieces.. It's not uncommon to find a piece of wood or two in any load, if you go to any breaker you'll find a big stack of small pieces of wood next to nut and pea hoppers where they load the delivery trucks.
old chisels, a belt-buckle - .
That shouldn't make it through. They seperate the coal and rock by adding magnetite to water which changes the gravity of the water, the coal floats and the rock sinks as should any other heavy object.