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This action will address the agency's residual risk and technology review (RTR) of the National Emission Standards for Hazardous Air Pollutants (NESHAP) for Primary Magnesium Refining. The Primary Magnesium Refining NESHAP, subpart TTTTT, was promulgated pursuant to section 112(d) of the Clean Air Act (CAA) on October 10, 2003. The NESHAP established emission limitations, work practice standards, and operation and maintenance requirements based on maximum achievable control technology (MACT) for controlling emissions of hazardous air pollutants (HAP) from spray dryers, magnesium chloride storage bins, melt/reactor systems, and launder off-gas systems. The HAP emitted from these sources include, but are not limited to, hydrochloric acid, chlorine, dioxins and metal HAP. This action will implement the residual risk review requirements of CAA section 112(f)(2) and the technology review requirements of CAA section 112(d)(6). The statute directs the EPA to promulgate emission standards under CAA 112(f)(2) if such standards are required to provide an ample margin of safety to protect public health or to prevent, taking relevant factors into account, an adverse environmental effect. Any such standards are to be promulgated within 8 years after promulgation of MACT standards under CAA section 112(d). CAA section 112(d)(6) requires the EPA to review and revise the MACT standards as necessary, taking into account developments in practices, processes and control technologies, no less often than every 8 years. Pursuant to a court order (Community In-Power and Development Association v. Pruitt, No. 16-cv-1074KBJ (D.D.C. 2018)), the EPA is obligated to complete this final action by October 1, 2021.