Return to Nchanga, Zambia's greatest copper mine

During my visit to Chingola last week, I spent a whole day as a guest of the Konkola Copper Mines (KCM) management at Nchanga, the Copperbelt's largest mine, and Chingola's raison d'être.

There was a great feeling of nostalgia as I drove down Wattle Avenue to mine security, and then on to meet the concentrator manager, Barry Kalumba. I first set foot here in October 1969, my first job since leaving University, and as I then nervously found my way to the metallurgical offices, I tried to comprehend the sheer scale of the mine complex, the size of a small town.

It is now more like a small city. Since I left in 1973 an 'environmentally friendly' smelter (is there such a thing?) has been built, as well as a vast tailings leach plant, together with major concentrator expansions.

Although I did not appreciate it at the time, Nchanga was, in the late 1960s, one of the great copper mines of the world. The Copperbelt Province of Zambia and the neighbouring Katanga province in the Democratic Republic of the Congo host a massive orebody, formed over 500 million years ago by the collision of tectonic plates.

In 1969 Nchanga produced copper from an ore containing about 3% copper by weight (it is now less than 2%). This was high grade in world terms, ore grades in the USA averaging about 0.6% copper, and about 1.2% in the giant mines of South America. The mine produced only copper (it now also produces cobalt), which is unusual for a copper mine, which would commonly produce by-products such as lead, zinc, molybdenum and gold. Nchanga was therefore subject to the vagaries of the international copper price, which was high in 1969, but in 1973, the year that I left, the mine suffered due to a crash in the copper price.

Chingola Open Pit in 1969

Ore has 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 contributed to a total 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 ore contains oxidised minerals, mainly malachite and azurite, as well as cuprite and chrysocolla.



Ball mills, West Mill

 Comminution of the oxidised open pit ore is undertaken close to the pit at the East Mill, in rod and ball mills, the slurry then being pumped to join the underground ore, which is crushed at the West Mill and ground by 12 closed circuit ball mills. Little has changed since 1969 in the grinding circuits, except that the two mechanical classifiers in the ball mill circuit have been replaced by cyclones, but current commissioning of a 7m diam SAG mill - 5m diam ball mill-cyclones circuit will mean that the ancient parallel ball mill circuit will soon disappear.

Concentrator expansions

The new SAG-ball mill circuit

Much has changed in the flotation plant too. In my day the ground ore from underground and the open pits was first fed to sulphide flotation, the concentrate then being thickened, filtered in rotary drum filters and thermally dried before being transported 45 km to the Nkana smelter at Kitwe. Now Nchanga has its own Outotec Flash Smelter, producing around 12,000 tonnes of anode copper per month, and the drum filters and dryer have been replaced by a single large Larox Pressure Filter.

Sulphide rougher flotation and concentrate dewatering

The old sulphide flotation machines,
now lying derelict

Much of the old flotation plant now lies derelict, the small flotation cells being replaced by very large machines, with final cleaning in columns. In the past, sulphide tailings were sulphidised before floating the oxidised minerals, which were then pumped to the high grade leach-electrowinng plant. The 'oxide' flotation plant no longer exists, neither does the high grade leach-electrowinning plant, this site now being occupied by the smelter.

Oxidised copper flotation was always inefficient, as in the suite of copper-containing minerals mined at Nchanga are minerals which respond poorly to the froth flotation process, notably cuprite, chrysocolla and, to a lesser extent, malachite and azurite. In the late 1960s solvent extraction had been successfully introduced to copper processing in the USA, and a decision was made to develop and build a huge tailings treatment plant at Nchanga, to not only recover copper from existing tailings, but also from past tailings stored in the dams, and in 1974 this, then the world's largest solvent extraction plant, came on-stream.

The huge plant was built in two stages. 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 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 economically treat. The copper in this stage was precipitated from solution by cementation onto scrap iron in temporary Kennecott Cone units.

Stage 1 of TLP, 1972 Kennecott Cone plant

I left before the Tailings Leach Plant (TLP) came into operation, but was amazed last week to see the scale of this elegant process, which now treats up to 50,000 tonnes per day of current 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, with the smelter in the background

Leach pachuca with residual suphides

So efficient is the TLP that the major losses to final tailings are now due to suphide minerals, particularly the poorly floating chalcocite. This is evidenced by the black froth floating in the air-agitated leach pachucas, and a pilot plant has been built to find ways of succesfully floating these residual sulphides.





I thank Barry Kalumba and the management team at KCM Ltd for spending so much time with me on what was a truly memorable visit. I was also very impressed by the motivation, enthusiasm and knowledge of the Nchanga metallurgists, all from Zambian Universities, who showed me around the plants.

With Tailings Leach Plant metallurgists



With KCM Managers, Lodson Musukwa (Shift-Incharge Crushing),
Clemento Milimo (East Mill Plant Manager)
Barry Kalumba (Concentrator Manager)
Ravi Kumar (New East Mill Manager)
Luciano Mwango (West Mill Manager)