[Code of Federal Regulations] [Title 40, Volume 28] [Revised as of July 1, 2006] From the U.S. Government Printing Office via GPO Access [CITE: 40CFR421.20] [Page 460] TITLE 40--PROTECTION OF ENVIRONMENT CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) PART 421_NONFERROUS METALS MANUFACTURING POINT SOURCE CATEGORY--Table of Contents Subpart B_Primary Aluminum Smelting Subcategory Sec. 421.20 Applicability: description of the primary aluminum smelting subcategory. The provisions of this subpart are applicable to discharges resulting from the production of aluminum from alumina in the Hall- Heroult process.