This is the quest to be able to smelt Dark Iron. More information can be found at https://vanillawowdb.com/?spell=14891 https://vanillawowdb.com/?spell=14891.

If I drop mining.
Can I do this quest again if I decide to pick up mining again?

The quest is only offered if you're a miner. I think you might also need to be 230, not positive but I dont remember being offered the quest til 230+

This is the quickest route but requires either BS made keys, or a Rogue. You enter the instance and open the door to your left. Go until you see a door to your right and open that as well. Go through and there is a door on your left. Open it and watch for the patrolling Fireguards. There are stairs to your right, you want to go to the left. You will see Shadowforge Lock and you need to click on it to unlock it. Be careful not to aggro any of the mobs. After unlocking it go back to the stairs and then go up them. Take the path to the left and continue until you find a door. Go through it and continue bypassing all mobs that you can. Watch out for Fireguard Destroyer's that patrol the area. You will come upon an open area, you will want to go right, down to the room with the lamps.

Go away down a bit and turn right to the last path before the dead end room. If you have gone right way you will see a passage with some Fireguard Destroyers on left and right. Go straight through and you will be in 'The Bar'. Go right up to the stairs. At the end of the room you will see Plugger Spazzring. Pick pocket him to get Grim Guzzler Key. After this he will yell and aggro the area to you!. Go back just after the stairs there is a door behind Phalanx. Open it with the key.

You will find yourself in a large circlular room. Bypass as many mobs as you can then on your left there is an entrance. Go down and you will see Ambassador Flamelash. Ignore him and go to the left. There is an entrance here. Take it and go down to a large room with Moltem War Golems. Go through them and keep going straight. You will see a stairs leading down, take them and go through the door. Congratulations you are in the 'Summoners Tomb'.

Talk to Gloom'Rel (2nd ghost dwarf on your right). Talk to him and a Spectral Chalice will appear. Click on it and turn in the required materials to complete the quest and talk to Gloom'Rel again who will now you know teach you how to smell dark iron!

This is found in the Summoner's Tomb region of Blackrock Depths. Talk to Gloom'rel before starting the event and he will create the chalice.

Make sure to have items for quest with you when you heading there, because its an ""instant-turn"" quest.

Ah now that's gratitude for you. ""teach me to smelt dark iron please""... ""Thanks, now I'll just kill you & your 6 buddies off""...

Ok here is a very detailed account of what to do to be able to learn the ability https://vanillawowdb.com/?spell=14891 Smelt Dark Iron. I am creating this one detailed as possible because it is annoying to get bits and pieces by reading through several posts to get all of the info needed.
1. Before heading to https://vanillawowdb.com/?maps=46:205328 Blackrock Mountain to go into https://vanillawowdb.com/?zone=1584 Blackrock Depths you should gather these items in these amounts for the quest https://vanillawowdb.com/?quest=4083 Spectral Chalice.
https://vanillawowdb.com/?item=3577 Gold Bar x20
https://vanillawowdb.com/?item=6037 Truesilver x 10
https://vanillawowdb.com/?item=7910 Star Ruby x 2
2. Next you head to https://vanillawowdb.com/?maps=46:205328 Blackrock Mountain to go into https://vanillawowdb.com/?zone=1584 Blackrock Depths.
3. Once inside the instance https://vanillawowdb.com/?zone=1584 Blackrock Depths, turn to your right and you will see a bullet like drilling machine(aka digger) sticking out of the ground. When you click on it choose the option ""take me just past The Grim Guzzler"". When you load back in from taking the digger there is a mob, after the mob you will turn right, go down a flight of stairs, turn right again and bypass https://vanillawowdb.com/?npc=9156 Ambassador Flamelash by turning left in his chamber. Follow the little hallway to the Mold Foundry, once in the Mold Foundry you will follow the room around to the right where you will enter the Summoner's Tomb.
4 . You will speak to https://vanillawowdb.com/?npc=9037 Gloom'rel which is the second dwarf on the right upon entering the Summoner's Tomb.
5. When you speak to him you will ask him to tell you all his secrets and he will summon the https://vanillawowdb.com/?quest=4083 Spectral Chalice to the right of him that will have an exclamation indicating a quest. Pick up the quest and if you have brought the above mentioned items you will be greeted with a lit up question mark signaling you are able to turn it in.
6. Be sure you speak to him again after you have turned in the quest and you will tell him ""I have paid the price now tell me your secrets"" and he will grant you the ability to smelt Dark Iron.
When you get or if you have already aquired the Dark Iron you wish to smelt, then you can either kill the seven and the door will open. Once you have gone through the door there is a door to your immediate right that leads you to the bridge that takes you to the Molten Core. To the left and just before the bridge is a set of train tracks leading to where you actually smelt the Dark Iron.

Hope this helped!