[Farmplantsandseeds (OP)]

Do you think if I tipped 20L bottle of hydrochloric acid into a tank of calcium deposits, that was at least 15 cm thick, it would dissolve it completely?

The tank is cement, it has had bore water running through it for years. Calcium is calcium carbonate or limescale, most likely with magnesium in it too. There is a lot in this ‘hard water’ here.

It says on the Hydrochloric Acid bottle “Corrosive 8” (Bondalle from Bunnings).

[evan_au]

The tank may be concrete, but are there any metal fittings or metal pipes that will be dissolved too?
How would you dispose of any acid left in the tank?

Hydrochloric acid is rather dangerous.
- Use gloves and goggles when handling it
- Be careful not to splash it

[chiralSPO]

Chemically speaking this should work, but from a safety standpoint there are many issues, see below:

Hydrochloric acid should work quite well for dissolving calcium/magnesium deposits (probably mostly carbonates and hydroxides). The protons of the acid will react with carbonate to make CO₂ and water, and with the hydroxide to make water. Both calcium chloride and magnesium chloride are very soluble in water.

You can do a rough calculation of how much acid you need based on the reaction:
CaCO₃ + 2 HCl → CaCl₂ + CO₂ + H₂O

Concentrated HCl is about 12M, so 20 L of 12M HCl can react with about 12 kg of calcium carbonate (based on a density of about 1.8 g/cm³ and a thickness of 15 cm, this would clean an area of about 440 cm². You can scale up or scale down the amount of acid based on the area required.

A few warnings:
• You mention that the tank is made of cement, which is largely composed of calcium salts, and will also be dissolved by the acid (though this is only an issue once the acid gets through the 15 cm of deposit...)

• If the acid is commercial strength it is probably concentrated (32% HCl). This is very nasty stuff , so as Evan mentioned, you will need eye protection (a drop in your eye can cause permanent blindness) and gloves (it will burn your skin within a second of contact). Based on the large volume of acid (20 L), I would go a few steps further and recommend using a face shield, elbow-length gloves (butyl or nitrile--latex won't help you) and a rubberized apron (or rain coat or poncho), because larger volumes mean larger splashes/spills are possible. You also need to be careful about the vapors from the acid--don't use it in a small room.

• Because the reaction releases carbon dioxide there is a danger that it will splash, spray, bubble over etc. The reaction is analogous to the classic vinegar and baking soda "volcano" demonstration, but with much, much stronger acid! You do not want to add the acid too quickly, or it will just come right back at you. Also, make sure that the tank is open to the air to allow the gases to escape easily (20 L of acid will liberate 6000 L of CO₂). Again, this should be done outside or in a very well ventilated room, as a sudden release of so much CO₂ can easily displace the air in a small room and suffocate you!

• You will probably want to dilute the acid with water (always add acid to water, not the other way around!) This will make the reaction less vigorous and reduce the risk of bubbling over or splashing back. I would recommend (without knowing the dimensions of this tank or how easy it is to access etc.) That for each liter of acid you want to use, first add a liter of water to the tank. Then add the acid slowly in portions. For instance, say you add 20 L of water to the tank (nothing should happen at this stage). Then add a 500 mL portion of acid. It should start fizzing immediately, but shouldn't be too violent. When the bubbling stops or dies down significantly add another portion (you can add smaller or larger portions based on how vigorous the reaction appears to be.) and continue adding acid until the deposit has dissolved or adding more acid doesn't result in more bubbles (this might never happen because the cement would likely bubble as well).

• HCl will also react with rust, iron, mild steel, galvanized steel (vigorously!), zinc (vigorously!), aluminum (vigorously!), and some types of plastic/rubber. Be sure that there is nothing else in there that you don't want to corrode.

• The solution of calcium and magnesium salts should not be particularly hazardous (if you don't use too much acid, it will still be close to neutral). Based on the volume involved, you might want to check local regulations for how to dispose of it, but I would imagine that it can go down the drain.

Long story short: yes HCl will dissolve calcium deposits, but at such a large scale you need to be extremely careful, because there is potential for serious harm or even death. I recommend finding someone who can assist you in this undertaking.

[Bored chemist]

It may be more practical to remove most of the deposits mechanically, rather than chemically.

[chris]

It may be more practical to remove most of the deposits mechanically, rather than chemically.

I agree. As well as being personally safer, it's also less likely to result in damage to the tank itself. The acid cannot discriminate between the limescale and the tank lining. And in any areas where there is a paucity of calcium carbonate deposition the tank surface will be exposed and will be attacked. The damage could be costly, and most likely produce a pitted surface more prone to further limescale formation.

I'd use a hammer and a chisel...

[chiralSPO]

ahhh yes, elbow grease, the universal solvent!