del statement does not delete an instance, it merely deletes a name.
When you do del i, you are deleting just the name i – but the instance is still bound to some other name, so it won’t be Garbage-Collected.
If you want to release memory, your dataframes has to be Garbage-Collected, i.e. delete all references to them.
If you created your dateframes dynamically to list, then removing that list will trigger Garbage Collection.
>>> lst = [pd.DataFrame(), pd.DataFrame(), pd.DataFrame()]
>>> del lst # memory is released
If you created some variables, you have to delete them all.
>>> a, b, c = pd.DataFrame(), pd.DataFrame(), pd.DataFrame()
>>> lst = [a, b, c]
>>> del a, b, c # dfs still in list
>>> del lst # memory release now