while you take out the acid portion by neutralizing, you didn't remove the waste products.

you wouldn't be doing too much damage just soaking iron in it. as it is an iron salt that would form.
but salt is going to be bad for your lawn.

however, would not dump oil in your yard, or grease? because that is the majority of caked on crap.
and honestly I don't know what compounds that would form when combining with the acid.

regardless, it isn't as simple as just neutralizing the acid.
it did react with whatever else you dumped in there forming other compounds.

heavy metals is not relavent here.
it is proper disposal of waste, whatever it is made out of.

we all know better now.
you can go back less than 100 years where it was fine to have x-ray machines in shoe shops.
the argument we have always done it and it never hurt anyone is a bad argument.
we know it can be bad.
so spend a little extra effort to do it right, or don't do it.