Multotec has supplied two 52 ton self-driven trommel screens to a mineral sands project in West Africa, and will shortly be supervising erection and commissioning. The self-driven trommel screens have inside diameters of 3 365 mm and are 16 544mm in length. Each unit is capable of handling 4 375 tons per hour of solids feed, or 12 888 cubic metres per hour of pulp. The trommel screens are mounted on support pedestals, plumber blocks and steel tyres on the feed end, with a plumber block supporting the discharge end coupled to a hydraulic drive.

On track to commence production this year, the mineral sands project is set to become one of world’s largest producers of zircon and titanium dioxide products, coinciding with a period of unprecedented demand. It has a projected mine life of at least 20 years and is expected to produce on average about 85 kilotons of zircon and 575 kilotons of ilmenite per annum, as well as small amounts of rutile and leucoxene, when in full production, making it also the largest new mineral sands project currently under construction. Multotec was originally awarded a contract to implement only up-front design, finite element analysis and engineering for these units, but subsequently received an order to manufacture and supply the self-driven trommel screens from the EPCM contractor for the floating (wet) concentrator, mineral separation plant and associated site infrastructure.

“The client required a primary screening machine to remove vegetation and tramp material from the beach sands,” Anthony Yell, Multotec project manager - screening, says. “The high volumes made the self-cleaning capabilities of a self-driven trommel screen the most cost effective option for the application, as has already been proved in a number of existing applications. “Self-driven trommel screens of this size are unique in terms of the multi-disciplinary nature of their manufacture, involving large steel fabrication, rubber lining, stress relief, machining, casting of polyurethane panels and liners, assembly, hydraulic drives and electronics featuring Human Machine Interface. Test running is also a requirement, along with specialised logistics to transport these units to site — followed by supervision, erection and commissioning.

“Owing to the magnitude of the order, we deployed a dedicated project manager to serve as a single point of contact for the client.” Self-driven trommel screens have been designed in a number of different ways over the years, but Multotec’s design uses a direct hydraulic drive on the discharge end of the trommel, which simplifies the drive and support mechanism. The hydraulic drive caters for variable speed as standard, making it possible to optimise the screening process to suit the application. Discharge of the oversize is accomplished through peripheral discharge ports, an approach that also simplifies the chute arrangement. The undersize is fed to a dewatering cyclone and then onto heavy mineral spirals and magnetic separation.

All this equipment is supplied from Multotec’s minerals sands industry offering, comprising a spectrum of screening, gravity separation, magnetic separation, cyclone, sampling and wear resistant solutions. “Multotec has an excellent track record of designing and manufacturing large self-driven trommel screens, having supplied a number of these units over the years to projects such as Exxaro’s Hillendale mine and Rio Tinto’s Mandina and Richards Bay Minerals operations,” says Yell. “Hillendale mine has been operating successfully for more than ten years. “Our unique drive arrangement has proved to be reliable, with low maintenance costs. Multotec manufactures virtually the complete unit in-house, importing only the hydraulic drive. We’ve also supplied smaller self-driven trommels to India, Australia and Canada and we manufacture mill trommels for the local and international markets.

“We differentiate ourselves in this arena by being involved from inception to completion, including training of plant personnel and, in many instances, maintaining our trommel screens for customers through cent-per-ton maintenance contracts using our proven Hawkeye predictive maintenance programme.” In manufacturing the two massive self-driven trommel screens for this mineral sands project, the Multotec team worked from February 2012 to a non-negotiable deadline of November 2012, in order to transport the units by road to the Port of Durban before the annual holiday season roads embargo on abnormal loads was imposed. En route to Durban, the units required special routing and escorts.