Lead miner Doe Run Co. hasn't turned lead into gold quite yet. But Dave Olkkonen, manager of research and technical development, has come close, helping to successfully adapt the electrowinning refining process to lead concentrates.
"The technology we're talking about is very similar to what microchips have meant to telecommunications and computers. It's that significant of a technological advance for the lead industry," Olkkonen said.
Traditional lead smelting can be a dirty business. Lead concentrates are mixed with coke and various other raw materials, like silica, limestone or soda ash, and fed into a red-hot furnace where lighter impurities like sulfur are burned off. The energy-intensive process is not fully efficient, as it doesn't draw 100 percent of the lead out of the concentrate mixture and allows lead particles to escape into the air.
More importantly, traditional lead refining is environmentally hostile. Sulfur dioxide is released in large quantities and must be converted to sulfuric acid, and lead emissions leak into the atmosphere—lead particles can escape even the most high-tech installations—pollute land and water. Then there is the slag, a glass-like by-product that's a mix of silica and heavy metal compounds that is hazardous to release into the environment, forcing lead refiners to simply store it.
With that in mind, St. Louis-based Doe Run has been looking for a more environmentally friendly alternative to its Herculaneum furnaces, and began work on lead electrowinning in 1997. The process, widely used in copper and zinc refining, involves electroplating pure metal directly from a concentrated acid solution. The technology was never successfully applied to lead, but in the late 1990s Engitec Technologies SpA, an Italian technology firm, cracked the puzzle. It found that fluorboric acid successfully dissolved lead from concentrates and then allowed the metal to be plated out of the solution.
"What Engitec gave us was the prospect of a commercial wet chemical process to extract lead from lead sulfides," Olkkonen said. But while the technology worked in a laboratory setting, Doe Run faced a variety of problems when it tried to scale it up. "When you're dealing with beakers and lab-sized equipment and scaling it up to using large tanks and large-scale electroplating cells, there are always, always issues and growing pains," Olkkonen said. The project required the team to design everything from the process and operational procedures to equipment like the electrowinning cells and tanks.
The blank slate provided by the scale-up efforts gave Doe Run the leeway to develop a variety of patented processes and technologies along the way. One of the proprietary features of Doe Run's technology is the reverse current leaching tanks, a three-step system that selectively absorbs the lead sulfates from concentrates into the fluorboric acid solution.
Lead concentrates arrive at the plant containing around 80-percent metal and enter the first leach tank to mix with the acid, which dissolves some of the lead out of the lead sulfur particles. The weaker concentrates are then transferred to a second tank where the process is repeated, and the now lead-poor solids and solution are then placed in the third tank to leach out the remaining lead.
What's left behind is basically lead-free sulfur, which has been separated from the lead using a wet chemical process instead of being incinerated in traditional fire smelting and can be used in sellable co-products, Olkkonen said. "We're looking at higher lead metal recoveries from our concentrate. What we've seen is that we can recover 99 percent of our lead from concentrates. I'm not sure what fire-based operations are doing on average, but we're getting anywhere from 2 to 4 percent better recovery."
The leaching solution starts off as lead-free anolyte and travels through the tanks in reverse order, meeting the most lead-poor material when it is strongest and absorbing lead to become catholyte. "We want our richest, highest-potential leaching solution in contact with the weakest lead-poor material," Olkkonen said.
When the lead-bearing catholyte is ready to leave the first leach tank, it is fed into the electrowinning cells where the real magic takes place. The electrowinning tank is divided into cells for lead cathode starter sheets and inert carbon anodes. The lead-rich catholyte is poured into the cathode compartments while the lead-free leach solution, or anolyte, surrounds the carbon anodes. "There's a permeable membrane between the cathodic and anodic compartments that allows the current to pass, and the solution, once it changes its form, to pass as well," Olkkonen said.
Once the cells are filled, the operator flips a switch and an electric current plates pure lead from the solution onto the cathode. The whole process takes around 24 hours, or "very similar to what commercial plating times would be," Olkkonen said. The result is a 150-pound lead cathode (including the 45-pound starter sheet).
Doe Run's demonstration facility in southeast Mo. runs two commercial-sized tanks, each producing 64 cathodes at a time. If operated continuously throughout the year, the facility could generate about 2,450 tons of lead annually, exclusive of starter sheets.
"The beauty of this advance is that we have a sustainable technology for our lead industry here in North America, and potentially a global game changer. It means a better, safer work environment, less exposures, cleaner communities," Olkkonen said.
The new technology comes at a timely moment for Doe Run. The U.S. Environmental Protection Agency is rolling out new national ambient air quality regulations for lead in 2017 that will force the company to stop traditional lead refining at its Herculaneum smelter in Missouri. "We have several options that we are exploring for the plant," said Jose Hansen, vice president of marketing and sales. "We know primary smelting using our current technology at this location will have to cease by 2017 due to sulfur dioxide emission standards. Our desire is to make the transition to the new technology even earlier than that."
Taking the project from demonstration plant scale to a commercial-sized operation is expected to require investments of more than $150 million. Doe Run officials said earlier this year that they were actively pursuing funding and loan guarantees to help bring the new technology to fruition.