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The Potash Corporation of Saskatchewan (PCS) Phosphate, now Nutrien, Aurora, has petitioned EPA for reconsideration of the NESHAP Mercury (Hg) limit. One source (PCS-Aurora, N.C.) operates calciners and, therefore, defines the maximum achievable control technology (MACT) floor for hazardous air pollutant emissions. In 1999, the Phosphoric Acid Production NESHAP set MACT standards for calciners that regulated total fluoride as a surrogate for hydrogen fluoride and established particulate matter limits as surrogates for Hg. In 2015, EPA promulgated the residual risk and technology review for this source category, and further amended the standards in September 2017. This proposed rule considers additional Hg data received from PCS/Nutrien concerning the MACT floor emission limits for calciners.