Hello! <br /><br />Yes, "at" is correct and supposed to be in the sentence. "At" is used to describe production rates: "cell phones are produced at a rate of ten per second in the United States". In the sentence in question, the phrase "at a rate of" can simply be "at", to make "at over one million tons per year". If you'd like, however, you can say "...globally produced at a rate of over one million tons..." to make it more explicit. =)