What part of speech does 至少 belong to? From briefly looking it up in a dictionary, it seems to me that 至少 is a direct equivalent of &quot;at least&quot; in English, and as far as I can tell its placement in a sentence follows the same set of rules. For instance:
至少我会做饭。
At least I know how to cook.
我看至少5个人。 
I see at least 5 people.
But 至少 (and &quot;at least&quot; 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?
In the book 汉语国际教育用词语声调组合及轻重音格式实用手册, p.241 (which is rumored to be the post-reform HSK replacement), 至少 is classified as a 副词 (adverb).  Furthermore, it's considered a 程度副词 (adverb of degree) in the book 现代华语概论.
In English, we might describe it as an adverbial since it can affect a whole phrase.