In 1999, the Phosphoric Acid Production National Emission Standards for Hazardous Air Pollutants (NESHAP) set maximum achievable control technology (MACT) standards for phosphate rock calciners that regulated mercury emissions by means of standards for particulate matter. In 2015, EPA promulgated the residual risk and technology review for this source category, and further amended the standards in September 2017. The revised standards established mercury emission limits for calciners based on the one source that operates phosphate rock calciners, the Aurora, North Carolina, facility operated by PCS Phosphate Company, Inc. (PCS), now a subsidiary of Nutrien Ltd. (Nutrien). PCS/Nutrien has requested EPA review the Phosphoric Acid NESHAP mercury standards. This final rule would consider additional mercury test data that were not available at the time the rule was promulgated and that are relevant to the average performance of the source used to set the MACT floor mercury emissions for calciners.