The provisions of this subpart are applicable to discharges resulting from the manufacture of sulfuric acid by sulfur burning, wet-process phosphoric acid, normal superphosphate, triple superphosphate and ammonium phosphate, except that the provisions of §§ 418.12, 418.13, and 418.17 shall not apply to wet-process phosphoric acid processes that were under construction either on or before April 8, 1974, at plants located in the State of Louisiana.