Betterton–Kroll process

The Betterton-Kroll Process is a pyrometallurgical process for refining lead from lead bullion (lead that still contains significant amounts of impurities). Developed by William Justin Kroll in 1922, the Betterton–Kroll process is one of the final steps in conventional lead smelting. After gold, copper, and silver are removed from the lead, significant amounts of bismuth and antimony remain. The Betterton–Kroll process is used to remove these impurities. In the process, calcium and magnesium are added to the molten lead at temperatures around 380 °C. The calcium and magnesium react with the bismuth and antimony in the bullion to form alloys with a higher melting point, which then can be skimmed off of the surface. This process leaves behind lead with less than 0.01 percent bismuth by weight. The process is crucial to cheap industrial lead smelting and offers significant advantages over more expensive processes like the Betts Electrolytic process and fractional crystallization.