Electrowinning

Electrowinning or Electroextraction is an electrolytic process used for recovering metals, like gold, silver and copper from a solution. In this process, electrons from an electric current chemically reduce metal ions, forming a solid metal compound on the cathode (the negatively charged electrode). The deposited metal takes a form that can be easily smelted into a more usable form. Typically, the metals are initially dissolved in a solution through a leaching process from ores. Electrorefining employs a similar process of electrolysis to eliminate impurities from a metal.

The electrowinning process dates back to 1747 when it was first demonstrated experimentally by von Leuchtenberg. In 1807, the English chemist Humphry Davy, recovered sodium in elemental form by the electrolysis of sodium hydroxide for the first time. The first commercial scale refinery Balbach and Sons Refining and Smelting Company using electrowinning technology was set up 66 years later, which then became the second largest metal processing company in the United States.

The Process

In the electrowinning process, the electrolyte contains dissolved metals that need to be recovered. It employs electroplating which is particularly effective for purifying non-ferrous metals like copper and silver.

A typical electrowinning unit comprises a tank, rectifier, and pump, with cathodes (negatively charged electrode) and anodes (positively charged electrode) positioned inside the tank. The pump fills the tank with an electrolytic solution, while the electrowinning rectifier supplies electrical current to the cathodes and anodes, causing cations to migrate towards the cathode. Over time, positively charged ions plate onto the cathodes. It's worth noting that as metal accumulates on the cathode, the metal deposition in the solution diminishes, slowing down the plating process. Once the rate of metal deposition decreases to a point where electroplating is no longer effective, the cathodes containing pure metal deposits are collected. In wastewater treatment applications, the solution (wastewater) undergoes cleaning or significant purification of non-ferrous metals and can then be further processed through chemical precipitation or reused in industrial processes.

Electrowinning for ironmaking

Electrowinning for ironmaking (Source: Energy.nl)

Electrowinning for recovering metallic iron from an alkaline media is considered a promising alternative technology to blast furnaces and direct reduced iron that uses coal as reductant. Though commercial scale electrolytic iron production has faced several challenges in the past, significant advances have been made in the past two decades. Over the past decade, numerous significant R&D initiatives have been sanctioned, aiming to explore alternative CO2-free technologies characterized by low energy consumption. Examples include projects like IERO, ULCOS, LowCarbonFuture, GREENSTEEL, and others. A 2022 project called the Siderwin research project, demonstrated various benefits of using electrowinning for refining iron ore including, around 87% reduction in direct CO₂ emissions, 31% reduction in energy use, the ability to involve iron oxides containing residues like red mud, and potential compatibility of the process with renewable energies. A startup called Electra has raised US $85 million to develop commercial scale electrowinning technology for iron production even from iron ores that are too impure to cost-effectively refine it at a commercially viable scale.