When Solvent Extraction came to Africa

My career in mineral processing began in October 1969 at the the Nchanga Copper Mine in Zambia. In 1969 Nchanga produced copper from a mixed ore containing about 3% copper by weight. Ore had been mined from underground since 1938, but in 1955 open-pit mining commenced, and the giant Nchanga open-pit and the smaller Chingola and Mimbula-Fitula pits began to contribute to a total milled tonnage of about 30,000 tonnes per day treated at the massive operation. Underground ore contains mainly suphide minerals, chalcocite, chalcopyrite, covellite, bornite, while the shallower open-pit ores contain oxidised minerals, mainly malachite and azurite, as well as cuprite and chrysocolla.

My first two years at Nchanga were on the concentrator, mainly involved with flotation. The milled ore from underground and the open pits was first fed to sulphide flotation; then the sulphide tailings were sulphidised (using sodium hydrosulphide) before recovering the oxidised minerals. High grade sulphide concentrates were shipped 30 miles to Rhokana in Kitwe for smelting and high grade oxide concentrates were pumped to the high-grade leach-electrowinning plant.

Oxidised copper flotation is always less inefficient than sulphide flotation, and at Nchanga the situation is exacerbated because of the presence- in addition to the recoverable malachite and azurite- of relatively large amounts of poorly floatable cuprite and almost totally non-floatable chrysocolla.  Consequently, with an overall recovery of about 85%, the final tailings from oxide flotation still contained around 0.4% acid-soluble copper, resulting in a loss to the final tailings dams of around 150 tonnes per day of copper. 

It was decided that these losses must be recovered and the metallurgical manager, Jack Holmes (posting of 11 November 2017) was taken out of the direct management stream to determine how to deal with the  losses. He looked at many schemes but homed in on leach-solvent extraction after having travelled to the USA with a small team* to evaluate its potential.

Jack Holmes with the MEI team in 2017

In the late 1960s (1968 at Ranchers Bluebird Mine and 1970 at Bagdad Mine), solvent extraction had been successfully introduced to copper processing in the USA; so, after much process evaluation, a decision was made to develop and build a huge tailings treatment plant at Nchanga, to recover copper from not only continuing tailings arisings, but also from past tailings stored in the dams, and in 1974 this, then (and for many years) the world's largest solvent extraction plant, came on-stream, treating about 900,000 tonnes of tailings per month.

The huge plant was built in two stages, to achieve copper production as soon as possible. In stage 1, thickeners and neutralisation vessels were erected, together with the leaching pachucas in which the tailings minerals would be intensively agitated in a dilute solution of sulphuric acid to produce a weak copper sulphate solution. This would be upgraded and purified in Stage 2 by the solvent extraction process, before precipitating the copper by electrolysis.

During construction of Stage 2 I was on the commissioning team which put into operation Stage 1, to treat a large stockpile of low grade oxide concentrate, which had hitherto been found difficult to treat economically . The copper in this stage was precipitated from solution by cementation onto scrap iron in temporary Kennecott Cone units.

Leach pachucas and Kennecott Cones

I left before the Tailings Leach Plant came into operation, but was amazed when I visited in 2012 to see the scale of this process, which now treats up to 50,000 tonnes per day of current sulphide flotation tailings and old tailings in the leach-SX plant, 4500 tonnes per month of final copper being produced in a giant electrowinning tankhouse. 

A section of the massive Tailings Leach Plant in 2012,
with the smelter in the background on the site of the old high-grade leach plant

At MEI's Hydrometallurgy '27 conference in Cape Town next year, Dr. Kathy Sole will present a keynote lecture and will show how this elegant copper processing technology conforms to the principles for a circular hydrometallurgical flowsheet, and can be considered to have been ahead of its time with respect to these ideas. Today, some 20% of world primary copper cathode is produced using this technology, with the dominant contribution coming from the African Copper Belt and the keynote will draw on examples of developments and advances during the past five decades, and continuing in the modern era.

Kathy Sole holds a PhD from the University of Arizona. She has been providing independent hydrometallurgical consulting services since 2011, with clients on six continents, and has worked across a wide range of commodities, including base metals, uranium, titanium, vanadium, and precious metals. Her main areas of expertise are in solvent extraction, ion exchange, and electrowinning. She also holds an adjunct professor position at the University of Pretoria. She received the 2019 Milton E. Wadsworth award from the SME and was twice awarded the Silver Medal of the SAIMM. Kathy is a Fellow of SAIMM and the South African Academy of Engineers.

* I would like to thank Ken Severs, who was part of the Stage 2 commissioning team, for checking details in this posting for accuracy.

#Hydrometallurgy26
#MEIBlog