Cast iron tubs can weigh over 300lbs easily depending on the size, shape, and wall thickness, which makes them very difficult to remove. Depending on what your plans are for the tub, you have several different methods you can use to remove one. I've tried both methods in the past so I can help you prepare for what you're in store for.
Plumbing Preparation Prior to the Tub Removal
No matter what method you choose from below, you will need to makes sure that you disconnect any plumbing running to the tub. Usually this entails disconnecting the drain waste and overflow pipes from the tub itself. Sometimes these are the only things keeping the tub in place due to the tubs weight alone.
You may also need to cut caulking around the tub using a utility knife, if the tub is located next to one or more tiled walls.
Sometimes a special tool called the "Tub Drain Remover" is required to unscrew the bottom drain from the tub's plumbing below. I typically used a massive screwdriver and wedged it into the slot in the upper portion of the tub drain, then unscrewed that drain strainer counterclockwise if my memory serves me right. The tub drain remover tool just makes it easier, you can pick one up for $10.
The tub overflow piece that's located inside the tub can usually be unscrewed with two Phillips head screws.
Okay, now that you properly disconnected the plumbing from the tub itself, as a safety precaution, you can turn off your water supply, just in case you accidentally damage a water pipe during the actual bathtub removal process.
Skip to the method that pertains to your actual needs, one is for people who plan to keep the tub in one piece, and the other is for people who rather remove the tub in smaller chunks.
Remove a Cast Iron Tub in 1 Piece
You should have everything disconnected from the tub at this point, now you have to break loose the tub from the walls if that's how it's installed. You're going to have to do whatever it takes to remove it, this can entail removing wall tile, and other obstacles that can be holding the tub in place.
If you cut the silicone bead and still can't get the tub to budge, you may have to break the walls that hold the tub there. I would personally break as little of the all as possible if, you're plan is to retrofit a new tub in that area. If you're planning a major remodel, it may be a lot easier to remove the surrounding tile and cut the wall board off 6" above the tub, so that you can give yourself plenty of wiggle room to wedge the tub out of its cavity.
Sometimes people want to relocate or reuse an old cast iron tub and it is doable, but you're going to need some man power because of the sheer weight of the tub. I remember one of our first whole house remodels back in 2005, we removed a cast iron tub and had to carefully navigate it through the hallway and kitchen and then down the 2 flights of stairs. I remember that removal clearly because it was a bit of a nightmare. If you have to walk your tub down a flight of stairs, I recommend at least 2 strong guys at the bottom of the tub, because the weight is too much for one guy to bare all on his own. With this method you are going to risk damaging the stairs and any flooring you can possibly drop or drag the tub on. Also you may scrape or bump into walls, doors, and other obstacles on your way out of the house, so plan ahead and be careful.
Break the Cast Iron Tub Into Pieces
I've used this method 3 times, 2 on the job for previous customers and once for myself. First make sure you've done the work mentioned previous regarding the plumbing disconnect . You can stuff some crumpled up newspapers into the exposed tub drain to prevent debris from clogging your drain pipes, that's what I did.
For this method I recommend the following materials:
1. Old comforter
2. 10 lb sledgehammer
3. Safety Glasses
4. Thick work gloves
5. Ear protection, because it's going to be very loud.
I removed both cast iron tubs by placing a blanket or old comforter over the areas I was planning to strike with the sledgehammer. Why use the blanket? The porcelain coating is going to shatter into tiny sharp pieces and they can cause injury if they hit your body. The comforter will prevent them or pieces of cast iron from flying at you or somebody else. I wouldn't even think of using a sledgehammer on a cast iron tub or sink without taking the right safety precautions.
Once you have yourself protected and ready for work, simply strike covered areas of the tub with the sledgehammer, paying attention to how much force you actually need to break pieces off. My particular tub took a ton of force, watch the YouTube Clip below to see me in action, I even show footage of what my new bathroom looks like after I converted the old outdated bathrooms into one larger bathroom instead. I think you're going to like what I did with it.
One last thing I want to point out is that the corner walls of the tub might require more force to strike but they are also great starting points in my opinion. Once you break the top corner of the tub, you can continue whacking pieces off the tub wall. Remove all the pieces that area easy to handle out of your way and keep working at it until the whole tub has been obliterated and removed from your bathroom.
For cleanup, I suggest a good shop vac, because you'll have hundreds if not thousands of porcelain shards and small pieces of cast iron as well to clean out from the floor.
Good luck, and please be careful. If you don't trust yourself doing a job like this, hire somebody how has experience.