This major group includes establishments primarily engaged in mining, developing mines, or exploring for metallic minerals (ores). These ores are valued chiefly for the metals contained, to be recovered for use as such or as constituents of alloys, chemicals, pigments or other products. This major group also includes all ore dressing and beneficiating operations, whether performed at mills operated in conjunction with the mines served or at mills, such as custom mills, operated separately. These include mills which crush, grind, wash, dry, sinter, calcine, or leach ore, or perform gravity separation or flotation operations. Magnesite and brucite operations are classified in Industry 1459, and crushed dolomite operations are classified in Industry 1422. Smelters and refineries are classified in Manufacturing, Major Group 33, and establishments engaged in producing primary magnesium metal are classified in Manufacturing, Industry 3339. The operation of brine wells or sea water plants for the production of magnesium is classified in Manufacturing, Major Group 28.