Without more context, it sounds like the term you're looking for is '(ore) concentrate'.

In the mining process, ore is fed into a mill that crushes the ore into a powder ('fine-grained sand' according the 'slig' Wikipedia article that I've included). However, that powder/crushed ore is then processed and separated into two groups: the ore concentrate (valuable) and gangue (waste). The waste is sent to a tailings pond, which is usually very close by. Tailings are just useless residue from ore, and they don't normally get transported (hence why I think tailings wouldn't really work in this context).

That said, what does get transported is the ore concentrate that will then get transported to a smelter. You could also keep it vague and just use 'crushed/ground ore'.