Ammonia Leaching at Kennecott

This document describes the development, operation, and advantages of the ammonia leaching process at the Kennecott Copper Corporation plant in Alaska. Built after successful laboratory and pilot-plant tests beginning in 1913–14, the commercial plant started in 1916 and ultimately reached a capacity of 800 tons per day, operating continuously until 1931. The ore treated is a high‑grade limestone replacement deposit with both sulfide and carbonate copper minerals; gravity concentration first recovers sulfides, while the carbonate-rich tailings are treated by percolation leaching with aqueous ammonium carbonate solutions. The process flow includes initial drainage, a “first leach” and “second leach” with circulating ammoniacal solutions, followed by an innovative hot steam wash that strips residual ammonia economically and without significant copper precipitation. Copper is finally recovered by boiling the pregnant solution with steam to drive off NH₃ and CO₂, precipitating copper oxide, while the regenerated ammonia is condensed and recycled. The paper emphasizes the process’s advantages—limited fine grinding, low water consumption, high-grade copper product, and minimal solution fouling—and outlines operating conditions, tank design, and a typical cycle for one leach charge.