Grinding mill supplier New Concept Projects (NCP) has commissioned the world’s first trunnion-supported, fully stainless steel mill at a copper plant in the Democratic Republic of Congo (DRC), which operates with the use of a highly acidic slurry solution as a grinding medium.

Having been commissioned in May, the trunnion-supported mill manufactured from complex duplex stainless steel has proven the benefits of its design, says NCP sales engineer Meveshan Moodley. The company hopes, therefore, to see increasing demand for this solution in Zambia and the DRC’s Copperbelt regions, or globally, where end-users are considering rafinate-based copper recovery.

Moodley explains that, instead of using neutralised water in the grinding circuit, the DRC plant uses rafinate, which is highly acidic, with a pH of between one and two. He notes that, traditionally, when process water is returned from the plant’s tailings to the mill, a neutralisation step, which involves adding lime or some other agent to neutralise the process water, has to be included, owing to the acidic leach associated with copper.

Moodley notes that significant operational cost savings can be achieved by eliminating the neutralisation step, while simultaneously improving the leach kinetics by using acidic slurry in the tumbling mill. It is reported that plants using rafinate on suitable ores can reduce operating costs by up to 40%, compared with traditional copper recovery processes. This operating cost advantage quickly pays for the additional capital expenditure required for the high-grade stainless steel capital equipment.

The plant needed a mill to be designed that could withstand extremely low pH conditions without corroding. Stainless steel trunnion-supported mills have been experimented with before; however, manufacturing technology was limited by the materials and manufacturing methods. Traditionally, shell flanges were manufactured in mild steel and then welded to stainless steel shell plate material. Further, heads had to be cast or fabri- cated from conventional mild steel and be bonded with a stainless steel welded cladding. This was not very successful. NCP has now pioneered manufacturing processes that allow the supply of the trunnion-supported mills made entirely from stainless steel. This will allow new plants that are considering the use of rafinate to run trunnion-supported mills of larger diameters, which had previously been a limiting factor. The DRC mill, which is operating well, and

also features a fully fabricated stainless steel feed chute, a stainless steel trommel screen, a variable-voltage variable-frequency drive and rubber liners with corrosion-resistant fasteners designed to handle variable run- of-mine feed.

NCP has been very active in 2016, and is currently commissioning a new semiautogenous grinding (SAG) mill and a matching ball mill for a uranium project in Namibia. Moodley notes that at 36' × 19.5', the 13.5 MW SAG mill is the largest- diameter mill that NCP has manufactured to date. The company believes that it will be the largest SAG iron gear-driven mill in Africa and among the largest gear-driven mills in the world.

“In the challenging economic climate, large gear-driven mills offer significant cost advantages over gearless drive mills. Incorporating latest technology, NCP can now offer gear-driven mills of up to 40' and 18 MW.”