What happens if you put Eggshells in the wrong bin?

Never expect whole shells to disappear in a heap; they will not.

Where do Eggshells go?

Compost or food waste. Crush them first or they will still be recognisable next year.

Food waste collection

Eggshell is calcium carbonate and breaks down very slowly in a heap, which is why whole halves survive a full composting cycle intact. Crushed to a powder they break down faster and add calcium. The claim that scattered shell deters slugs does not hold up in trials, though it does no harm. Food waste collections take them without any preparation at all. Where there is no local food waste collection they go in general waste, and the council list decides that.

How to prepare it

Dry and crush them before adding to a compost heap.
Rinse if they are going to sit in a caddy in warm weather.

What not to do

Never expect whole shells to disappear in a heap; they will not.

Kerbside collection rules genuinely vary by council and county, and local rules always win over general guidance. The reasoning above is what transfers — if your service differs, it will usually be for a reason described here.

Other things that go to food waste collection

Food scraps and leftovers Food waste collection where you have one — it accepts cooked food, meat and dairy, unlike a home compost heap.
Teabags Food waste or compost — but many bags are sealed with polypropylene, and that plastic mesh stays behind.
Cooking fat, lard and dripping Let it solidify, scrape it into the food waste or general bin. Never down the drain, at any temperature.
Bones Food waste collection if you have one. Never a home compost heap.
Coffee grounds Compost heap or food waste. Good material in both, in moderation.

Common questions

Where do Eggshells go?: Compost or food waste. Crush them first or they will still be recognisable next year.
What happens if you put Eggshells in the wrong bin?: Never expect whole shells to disappear in a heap; they will not.

Where do Food scraps and leftovers go? Food waste collection where you have one — it accepts cooked food, meat and dairy, unlike a home compost heap.
Where do Teabags go? Food waste or compost — but many bags are sealed with polypropylene, and that plastic mesh stays behind.
Where do Cooking fat, lard and dripping go? Let it solidify, scrape it into the food waste or general bin. Never down the drain, at any temperature.
Where do Bones go? Food waste collection if you have one. Never a home compost heap.
Where do Coffee grounds go? Compost heap or food waste. Good material in both, in moderation.
Where do Household batteries (AA, AAA, C, D, 9V) go? Shop or manufacturer take-back
Where do Lithium-ion batteries go? Shop or manufacturer take-back
Where do Vapes and disposable e-cigarettes go? Shop or manufacturer take-back
Where do Medicines and tablets go? Pharmacy
Where do Pizza boxes go? Kerbside recycling bin
Where do Leftover wet paint go? Household waste recycling centre

Sources

US Environmental Protection Agency — EPA (Public domain (US Government))

How these figures are compiled and checked