What you have described is a simple situation of Super Saturation. Let me
explain, as you were using the hot Citric acid solution, it became
concentrated. In other words, it contained all the acid that the water
could hold. Hot water can hold more Citric acid than can cold water. As
the solution cooled down, it then became “super-saturated” , it contained
more acid than would stay in solution. The Citric acid then precipated out
in the form of crystals. If you had added a little water to this and again
heated it, the crystals would have redesolved and you could have disposed
of it. I’m not going to suggest any disposal solutions. Far too many tree
huggers out there to suit me. Good luck.
" Families are forever… Don’t start."