Memories of the Nchanga Tailings Leach Plant

I have just been on my afternoon coastal path walk, and was caught in a torrential downpour, with some flash-flooding, which brought back memories of an evening almost 40 years ago in Zambia:

In the suite of copper-containing minerals mined at Nchanga were minerals which responded poorly to froth flotation, notably cuprite, chrysocolla and, to a lesser extent, malachite and azurite. In the USA at that time solvent extraction had been successfully introduced to copper processing, and a decision was made to develop and build a huge tailings treatment plant, to not only recover copper from existing tailings, but also from past tailings stored in the dams. In 1974 this, the world’s largest solvent extraction plant, commenced treatment of over 10 million tonnes per year of tailings to produce an extra 100,000 tonnes per year of finished copper.



The Kennecott Cone plant

The huge plant was built in two stages. In stage 1 thickeners and neutralisation vessels were erected, together with the leaching tanks, 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 in Stage 2 by the solvent extraction process, before precipitating the copper by electrolysis. During construction of Stage 2, Stage 1 was put into operation to treat a large stockpile of low grade oxide concentrate, which had hitherto been found difficult to economically treat. The copper in this stage was precipitated from solution by cementation onto scrap iron in temporary Kennecott Cone units.

I was assigned to the commissioning team for this project with three other metallurgists, Stuart Mellor, Peter Glass and Tom Whitehouse, and for almost 6 months we worked a three shift rota. The operating experience proved invaluable, but also gave me the opportunity of working with my own crew of African operators, and to share their great sense of mischief, which was exemplified by an episode on one particular night shift.


With my shift crew on the Kennecott Cone plant

The storm that hit the plant just after midnight was one of those rarely experienced outside the tropics. The rain was torrential, and a bolt of lightning plunged us into darkness, pumps tripped out, and very soon the whole area was covered in several inches of water. I immediately phoned for the call-out electrician, who arrived within minutes. A huge Afrikaner, he obviously commanded the respect and fear of the African crew, who followed him meekly as he strutted imperiously towards the electrical control panel, with his huge belly wobbling before him.

Slowly the expressions on the faces of the crew changed, to be replaced by sly grins. They knew exactly where the electrician was heading, and his next step took him plunging into a submerged pump sump, where in a mass of bubbles he disappeared from view. The following moments would have graced a Laurel and Hardy movie. A yellow hard hat bobbed to the surface, followed, amid another flurry of bubbles, by a large round face. Totally without expression, and in the best Oliver Hardy fashion, he blinked and then slowly wiped his eyes with the tips of his forefingers. The Africans dissolved into fits of hysterical laughter, and one totally lost control and lay like a stranded cockroach on his back in a pool of water, kicking his legs and holding his aching ribs.

This is just one fond memory that I have of my time on the tailings leach plant. Does any one else have stories of those days, and does anyone know what happened to the Kenecott Cones- were they eventually abandoned, as they were never particularly efficient? Also what became of my fellow metallurgists? Peter Glass I met up with a few years ago in Perth, WA. I last saw Stuart Mellor 29 years ago on the Harmony Gold Mine in South Africa, but I have totally lost track of the affable Tom Whitehouse.