Widely used in fine wet screening applications, these high frequency screening machines comprise of up to five individual screen decks positioned one above the other operating in parallel. The “stacked” design allows for high-capacity units in a small footprint. The flow distributor splits the feed stream evenly to the individual polyurethane screen decks (openings down to 45 pm) where feeders distribute the stream across the entire width (up to 6 m) of each screen. Dual vibratory motors provide uniform linear motion to all screen decks. The undersize and oversize streams are individually combined and exit toward the bottom of the Stacked Sizer. Repulp sprays and trays arc an optional addition in between screen sections, which allow for increased screen efficiency.
By classifying by size-only, screens compared to hydrocyclones, give a sharper separation with multi-density feeds (for example, in Pb—Zn operations), and reduce overgrinding of the dense minerals. Operations that replaced hydrocyclones with stacked high frequency screening machines in closing ball mill circuits can result in a decrease in the circulating load from 260% to 100% and 10 to 20% increase in circuit throughput.
In the late 1990s. Began the development a new concept tor high-capacity fine wet screening: the Stacked Sizer Screen. The machine design is based on the following wet sizing principles:
Sufficient water coupled with high-frequency vibration is necessary to allow fine solids to pass through a screen surface. Sizing will stop when all available free liquid has passed through the screen surface.
Slurry must be introduced to the screen surfaces in uniform, thin layers.
Oversize material must be quickly conveyed off the screen surface to free up effective screening area.
Width is the most important factor in determining wet screening capacity and efficiency.
The high capacity Stacked Sizing/screening machine consists of up to five decks positioned one above the other and all operating in parallel. Its use together with urethane screen surfaces as fine as 75 microns (200 mesh) has made fine wet screening a practical reality in mineral processing operations worldwide. The application of this technology in closed circuit grinding is demonstrated with specific application examples.
Screening is the process of separating particles by size and fine screening typically refers to separations in the range of 10 mm (3/8″) to 38 microns (400 mesh). Fine screening, wet or dry, is normally accomplished with high frequency, low amplitude, vibrating screening machines with either elliptical or straight-line motion. Various types of wet and dry fine screening machines and the factors affecting their operation have been discussed previously. In fine particle wet screening, the undersize particles arc transported through the screen openings by the fluid and the fraction of fluid in the slurry will therefore affect the efficiency of the separation. From a practical standpoint, the feed slurry to a fine screen should be around 20% solids by volume to achieve reasonable separation efficiency. As most of the fluid passes through the screen openings rather quickly, the fine screening process can be completed in a short screen length. Therefore screen width, rather than screen area, is an important design consideration for fine wet screening.
Recognition of this concept led to the development of multiple feed point fine wet screening machines, or example, the Multifeed screen consists of three screen panels mounted within a rectangular vibrating frame and is actually three short screens operating in parallel. Each screen panel has its own feed box and the oversize from each panel flows into a common launder and then to the oversize chute. Similarly, the undersize from each of the three panels flows into the undersize hopper. The popular 1.2 m (4 ft) wide by 2.4 m (8 fl) long version has a total effective width of 3.0 m (10 fl) In general, multiple feed point machines have been shown to have 1.5 to 2 times more capacity than a single feed point machine of equivalent size and screen area.
Expanding further on this concept, the Stacked screening machine was introduced in 2001. With a capacity considerably greater than any other type of fine wet screening machine previously available, the Stack Sizer has up to five vibrating screen decks operating in parallel for a total effective width of 5.1 m (17 ft). The decks are positioned one above the other and each deck has its own feed box. A custom-engineered single or multiple-stage flow distribution system is normally included in the scope of supply to representatively split the feed slurry to each Stacked screen and then to the decks on each machine. Ample space is provided between each of the screen decks for clear observation during operation and easy access for maintenance and replacement of screen surfaces. Each screen deck, consisting of two screen panels in series, is equipped with an undersize collection pan which discharges into a common launder with a single outlet. Similarly, the oversize from each of the screen decks collects in a single hopper with a common outlet large vibrating motors rated at 1.9 kW (2 5 HP) each and rotating in opposite directions produce a uniform high frequency linear motion throughout the entire length and width of all screen decks for superior oversize conveyance.
As mentioned above, the fluid passing through the openings carries the undersize particles through the screen openings. The screening process is essentially complete when most of the fluid has passed through the openings. Any remaining undersize particles adhere to the coarse particles and are misdirected to the oversize product An optional ‘’repulping” system is available for the Stack Sizer in which spray water is directed into a rubber-lined trough located between the two panels on each deck With this feature, oversize from the first panel is reslurried and screened again on the second panel. This repulping action maximizes the correct placement of undersize particles and its use will depend upon the particular objective of the screening machine.
To date, 1000s of screening machines are in operation at mineral processing plants worldwide. Dry mass flow capacity typically ranges from 100 to 350 t/h. This is roughly equivalent to 3 or 4 of the older style Multi-feed screens discussed above Like all screening machines, capacity depends upon many factors such as screen panel opening, weight recovery to oversize, the amount of near-size particles, particle shape, and slurry viscosity.
Sizers are for high capacity in a short compact machine. Generally you can make good cuts or separations with high efficiency. If you need near absolute 99.9% precision cuts, then a sizer cannot do that, and most inclined screeners also cannot. So that is why it is very important to understand what the separation goal is before selecting a screener. You cannot have high capacity and high accuracy + 99.9 in the same machine! This machine does not exist! A sizer generally can accomplish a similar separation of a single inclined screen in 2 or 3 screens, and 1/3 the length. of course a lot depends on the PSD, and how close the remaining particles are on each side of the desired cut.