I would have to agree with SAU on skill of operators effecting coal quality. We remove overburden with a dragline, then the dozer does an initail clean up of the top of coal. If the dozer is sloppy, the scraper or 834 has a heck of a time doing the next stage of clean up. And if they don't do so well, the blade that does the final clean up has an even harder time. And if the blade doesn't get it down to hard, then the drill has problems which then effects the blasting of the coal. If every step of the process has been accomplished well, the driller can still drill too deep or too shallow so that when the coal is shot, either the innerburden gets shot up into the coal or the bottom of the coal doesn't get shot. And if the coal doesn't get shot, the loading equipment has a hard time digging. Sometimes the loading operator digs into the floor or the wall for a little extra "ash," or doesn't seperate out the parting or a sulphur band. And if the trucks were hauling for reclamation prior to hauling to the retail yard and the operator doesn't clean the bed of the truck, well, there's your mud ball. And of course, de-water the pits at every stage of the process can effect everything, too. And a poor engineering plan or failure to follow a good plan can effect the coal quality as well. Not every operator in this process is skilled and/or consciencious. One operator can effect the quality of thousands of tons of coal.
Also, our pits vary quite a bit in contents of sulfur, sodium, ash and BTUs.
Is the molted metal scoria?