The answer is that it is true that the maintenance of evil does indeed involve gradation – in the sense that evil itself initially had a great root, having also been rooted in Keter, but afterwards it descended one level, and was rooted only in Chochmah and below. Then afterwards it descended even further and was rooted only in Binah and below. This shows how the world is on the way to repair, since evil has some affinity only with the lower levels but not with the higher ones.
Thus although in one aspect it appears as if the waste was “thrown back into the pile”, the truth is that the maintenance of evil passed immediately to Chochmah and all the more so to the levels below it. (That is to say, the task of maintaining evil fell to all that remained unpurified – in terms of the metaphor, it fell into the grainpile as a whole – except that it still relates to Chochmah and even more so to the levels below it.)
However, when Chochmah was purified, it reached only Binah, and so it was in all cases until it remained on that bottom level which I mentioned.