the mine guild may be close to bank but tis slower then the mining site north of the dwarf cannon quest area. If you go there once you have full inventory of coal use it with the coal cart there and once the cart is full go to the ones by seers bank and there will be like 100 something coal in it and you just make a few trips to cart and bank and youve just gotten alot of coal. For getting there use duel ring to duel arena and gnome glider to grand tree, then walk to mine site. To get to bank use cammy teletab, i recommend wearing the duel ring