John Benjamin Zadra developed a carbon adsorption/desorption process to extract gold from pregnant cyanide solution while economically regenerating process carbon. This technological breakthrough greatly increased gold recoveries in processing plants and helped spur the world-wide resurgence of the gold industry during the second half of the 20th century.
In 1948, John Zadra was Chief of the Hydrometallurgical and Ore Dressing Branch at the U.S. Bureau of Mines’ Reno, Nevada Rare and Precious Metals Experiment Station. The Getchell Mine in Nevada had a particular problem. The mine was successfully loading carbon to 400 ounces of gold per ton, but smelter recovery totaled only 200 ounces per ton.
Carbon leaching of gold and silver had been practiced since the 1890s, with precious metals recovered either by precipitation in a zinc box or by the Merrill-Crowe method. Both of these methods commonly supersaturated the solution with zinc cyanide, precipitating cyanide and preventing cyanide recycling.
John Zadra set to work and in 15 months developed a carbon adsorption/desorption process using hot sodium hydroxide-cyanide solution to strip gold and silver values from carbon and an electrolytic mass transfer cell, the Zadra cell, to plate precious metals from the solution onto steel wool.
In a Zadra cell, a perforated, cylindrical inner container, the cathode, contains a central feed tube, a current distributor, and steel wool. A second cylindrical container, the anode, contains a stainless steel screen that surrounds the cathode. Pregnant strip solution enters through the central feed tube and flows upward and outward through the steel wool. Precious metals plate from the solution onto the steel wool. The system allows carbon to be repeatedly loaded, stripped, and re-loaded.
The first commercial Zadra cells were successfully installed at the Getchell Mine in February 1951. Additional installations at other gold plants soon followed. At the New Carlton Mill in Cripple Creek, Colorado, a new Zadra cell installation treated nearly 2,000 tons of ore per day and realized 98% gold recovery. The Zadra cell quickly became an industry standard and remains in common use at gold plants throughout the world.
The Zadra cell was only one of John Zadra’s notable contributions to metallurgical technology. Working as Chief Chemist and Metallurgist at the Nkana Mine in Northern Rhodesia (now Zambia) from 1929 to 1933, he developed a method for recovering cobalt from reverberatory furnace slag. As Mill Superintendent for Celo Mines, Burnsville, North Carolina, during 1939 and 1940, he received national recognition for converting a dry mica, kyanite, and garnet processing plant into the company's first non-metallic flotation concentrator.
John Benjamin Zadra’s innovative contributions to precious metals and electrowinning technology remain his most important legacy to the minerals industry. Precious metals producers throughout the world will long benefit from his practical process research and development.
