Modern Mining June 2021

VIBRATING SCREENS

Kwatani uses research to double banana screen throughput By applying research findings on the performance of multi-slope – or ‘banana-type’ – vibrating screens, South African vibrating screen and feeder specialist Kwatani has improved the design to double the throughput on a customer’s dewatering machine.

T his success was documented in a peer- reviewed presentation by Kwatani chief operating officer Kenny Mayhew-Ridgers and director Derrick Alston, where they show how the company’s mechanical expertise in vibrating screen design – combined with material flow data – was key to finding the most efficient solution. “Banana screens are in general more complex than single-slope screens, so it is not uncommon to find installations where performance is not optimal,” says Mayhew-Ridgers. “It was therefore very valu- able to learn from these research findings, which indicated exactly where performance was falling short.” The research was conducted by PW Cleary, JW Fernandez, MD Sinott and RD Morrison from Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the University of Queensland. Using mainly numerical simulation methods such as discrete element modelling (DEM) and smoothed particle hydrodynamics (SPH), they modelled the granular behaviour of solid particles as well as the fluid-like behaviour of water and slurry mixtures. Mayhew-Ridgers explains that it was clear from the research results – whether the material was wet or dry – that the traditional feed-end slopes of around 34° were not ideal. “Material velocity in excess of three m/s is too high

Kenny Mayhew-Ridgers, COO at Kwatani. for efficient screening,” he says. “High veloci- ties also reduce the life of screening panels due to wear.” Slope changes A more suitable slope at the feed-end was consid- ered to be 25° – reducing by 5° for five or six deck slopes in the screen and ending with a discharge slope of just 5° or even 0°. The smaller angular incre- ments between the deck slopes allow the material a smoother directional transition, for longer wear life of the panels. The drive angle was also a significant factor in this project. “The drive angle of 45° used in the numerical simulations is too low for the first few slopes of the banana screen,” he says. “This low drive angle propels the material away from the deck; another reason why mass flow rates were low for the first few slopes in the simulations.” Another reason was that the material velocity was still about one m/s at the discharge end of the screen, when it should preferably have been less than 0,5 m/s. Kwatani’s proposed 5° or 0° slope at the discharge, together with an increased drive angle of 50° to 55° would reduce the velocity to better effect. “An interesting finding from the simulation research was that the mass flow rate of the fluid

A single deck banana screen being tested at Kwatani.

through the first few slopes was actually better when the screen was stationary than when it was excited,” he says. “This means that – for dewatering or drain- and-rinse applications – a lower stroke is actually preferable. One can also con- sider a design where the feed-end has a lower stroke, and the discharge has a higher stroke, by off-setting the centre of gravity slightly.” Problem solved Applying this analysis, Kwatani was able to solve a customer’s severe challenge of excessive carry-over of water from a competitor’s banana screen to the con- veyor belt feeding the dense medium separation (DMS) or pan plant sections. The two degrit screens were dewa- tering +0,8 to 5 mm material, but the

34  MODERN MINING  June 2021