Monday, 15 June 2020

Memories of Nchanga's High-Grade Leach Plant

During my 4 years at Nchanga in the early 1970s my first two years were spent on the concentrator, before being transferred to the commisioning team for Stage I of what would eventually be the giant tailings leach-solvent extraction-electrowinning plant, the brainchild of the legendary Jack Holmes (MEI Blog 5th September 2018). Stage I was an interim plant designed to treat stockpiled low grade oxide copper concentrates by leaching and cementation in Kennecott Cones.
Stage I Tailings Leach Plant 1972. Kennecott Cones and Leach Pachucas
The cementation plant did not operate for very long as it was soon absorbed by Stage II and I did not see the final enormous tailings leach plant, which began operation in 1974, until my visit to Nchanga in 2012.
A section of the Tailings Leach Plant in 2012 with the smelter in the background
Copper is characterised by having a whole suite of economic ore minerals, and many are present in the Nchanga ore body. Underground ore contains mainly suphide minerals, chalcocite, chalcopyrite, covellite, bornite, while the shallower open-pit ore contains oxidised minerals, mainly the bright green malachite, blue azurite, as well as cuprite and chrysocolla. All these minerals responded to processing in various ways.
When I left Nchanga in 1973 these minerals were concentrated by flotation in two stages, first a relatively straightforward sulphide flotation circuit, and then treatment of the sulphide tailings by sulphidisation using sodium hydrosulphide, before floating the oxidised minerals. Not too successfully, however, as the final tailings contained substantial amounts of refractory minerals, notably chrysocolla and cuprite, and to a lesser extent malachite and azurite. The tailings leach plant was initially conceived to treat these tailings, but so successful was piloting that the 'oxide' flotation circuit was dispensed with and the giant plant treated the tailings from sulphide flotation as well as tailings impounded in tailings dams.
So when commisioning of Stage I had been completed I did not realise that my transfer, for my few remaining months in Zambia, would be to a plant which would also soon become obsolete, the high-grade leach plant. This section tank-leached the oxide flotation concentrates in dilute sulphuric acid, the resultant copper sulphate solution being transferred to the tank house to produce electrowon copper.
With fellow metallurgist Peter Glass, 1973
Electrowinning tankhouse
Copper cathode stockpile with Sandy Lambert
I found things had changed enormously when I returned to Nchanga 8 years ago. In the early 70s sulphide concentrates were shipped by rail to the smelter at Kitwe around 30 miles away. Now a giant flash smelter treats these concentrates at the rate of 12,000 tonnes of anode copper per month, and sits on the site of the old high-grade leach plant.
Although my few months on the leach plant provided a catalyst for our move from Zambia, I have fond memories of those few months, particularly of the people that I met, many of whom I still keep in touch with.
Leach plant personnel 1973, BW, Eric Plumridge, Phil Cudby, Ian Noble, Corrie Koen, Peter Glass and Paul Smithson
Times change but memories linger on.

10 comments:

  1. So THAT'S where the cover image from mineral processing technology came from...


    James Spangenberg, UCT

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    1. Well spotted James. Yes, the photo was on the 6th and 7th editions

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  2. It is indeed tricky; in India also we get oxide ore veins while the plant is designed for sulphides; good challenge for innovation.
    Didn't you have any values along with copper such as moly/gold?
    Lack of proper and accurate mineralogical/geochemical information at the exploration /borewell analyses leads to many issues if the plant is designed with limited information--- continuous characterisation even while the plant is operating gives so much information to minimise loss of values in regular operations.

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    1. The ore grade at Nchanga in the 1970s was around 3% Cu, and the mine produced only copper (it now also produces cobalt, I believe), which is unusual for a copper mine, which would commonly produce by-products such as lead, zinc, molybdenum and gold.

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  3. Days fondly remembered Barry. At the time you were at Nchanga I was based at the R & D Pilot Plant facility in Kitwe but spent much time on Nchanga projects.
    At the Concentrator I spent many happy days (Test Bank 614!)working with the bundle of energy that was Dr Filip Ser testing a new approach to floating oxide copper minerals using a selective sulphydric collector rather than the non-selective fatty acid Palm Kernal Oil collector used historically. I fondly remember characters like Jack Macdonald, Paul Piercy, Roger Kelley, Clifford Eales and others from those days.
    Then for quite a while I worked on shift on the small SX/EW Pilot Plant in Kitwe testing different feed materials and reagents (Lix and Kelex) for use in the Stage 2 Tailings Leach Plant. This brought me into contact with people like Dave Deuchar, Dave Parker, Les Stewart, Paul Smithson and other veterans of that era.
    Great days and thinking back on it, what an incredible way to start a career as a metallurgist!

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    1. Yes, wonderful days indeed Rod. I remember all those names bar one- Filip Ser- must have missed him. In my first few weeks at Nchanga, on the concentrator, the acting plant metallurgist was our late and dear friend Roger Thomas. Paul Piercy was assistant superintendent and Jack Mac I think was metallurgical superintendent.

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  4. Never knew you once worked at Nchanga Tailings Leach Plant
    Ulemu Chirwa, First Quantum Minerals, Zambia

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    1. Well before your time, Ulemu

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    2. It is a great plant and I learnt a lot, too bad it's deteriorating.
      Ulemu

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