From briefly looking it up in a dictionary, it seems to me that 至少 is a direct equivalent of "at least" in English, and as far as I can tell its placement in a sentence follows the same set of rules. For instance:
At least I know how to cook.
I see at least 5 people.
But 至少 (and "at least" in English) doesn't seem to adhere to any rules pertaining to where in the sentence it's allowed to be? So what part of speech would you refer to this as? An adverb? A preposition?