Screen panel selection cannot be considered in isolation when identifying the effect that this process has on media recovery. Screen aperture size determines the cut point, whilst open area determines the achievable drain rate. However, the drain rate is also influenced by material type, aperture shape, media density, fines and viscosity characteristics.
Multotec Manufacturing provides recommendations on correct screen operation, discharge bed depth, density, aperture sizes, weir bar configuration and spray water when assisting customers with screen panel selection.
Basic principles of screen panel selection include:
- Speed vs stroke: most vibrating screens are linear motion units using twin vibrator motor drives or gearbox type excitors. Vibrator motors are usually four, six or eight pole operating 1440, 960 or 720 r.p.m. respectively, generating strokes of between 2 – 4 mm for four pole motors, 6 – 8 mm for six pole motors and 12 – 16 mm for eight pole motors.
- Gearboxes normally operate between 720 and 960 r.p.m. with strokes between 6 and 16 mm. Multotec Manufacturing has a table indicating the correct stroke required for different aperture sizes on inclined and horizontal screen
- Discharge bed depth: should ideally be no more than four times the aperture size. The depth is affected by feed rate (t/h), screen width (m), velocity (m/s) and bulk density of material (t/m3). Conversely, discharge bed depths below 3 to 4 times the aperture size do not form a bed and cause material to bounce down the screen deck. This also results in poor screening efficiency.
Medium losses make up a significant component of operating costs. A number of factors contribute to medium losses, including forces of attraction between ore and medium particles, ore porosity, inefficient washing, poor magnetic separation and classification inefficiencies. Corrosion and abrasion of the medium and excessive circuit loading during the addition of fresh medium and lastly, housekeeping are also factors.
A considerable quantity of medium is lost through adhesion, which is a function of ore porosity, media and screen operation. An increase in medium density leads to increased medium losses as a result of an increase in viscosity, which in turn results in poor drainage. Plants should optimise drainage to minimise medium losses. This is achieved by minimising operating density, diluting the medium prior to drainage and optimising the screen load and distribution.
Drain and rinse screens use spray nozzles for spraying clean water and flood nozzles for plant water. Flood boxes are used to evenly distribute large volumes of water, although these tend to silt up over time and have been replaced by super flood nozzles, which distribute the same volumes when rinsing without silting up. Dewatering screens are designed to have high bed depths by forming a cake where water can drain and fines be retained. The discharge moisture content depends on the average particle size. The smaller the average particle size, the higher the discharge moisture content due to the surface tension between the particles and the water
Degrit screens remove oversize particles from Magnetic Separator effluent. It is a de-sliming application, which requires low discharge bed depths for efficient stratification, fines and effluent removal. Spray water can be added to assist the de-sliming process. When selecting screen panels, there are a number of important factors that contribute to screening efficiency and media recovery. If all the factors are taken into consideration such as the physical properties of stroke, G-force, discharge bed depth as well as the metallurgical properties of adhesion, density and viscosity. Then the size, shape and open area of a screen panel can affect screening efficiency and media recovery.
Screen panels are designed to give the optimum cut point, while maximising open area and life. There is a direct relationship between open area and life – the higher the open area, the shorter the life; therefore panel design is always a compromise.