4.3 Chute Exit (Spoon)
The chute exit should be dimensioned to allow the unhindered passage of the material. The exit opening minimum dimension should be at least 2.5 times the maximum lump size of the material, and must have an area at least 2.5 times the area of the material, based on the design capacity of the conveyor and the belt speed, as indicated earlier. Where the chute feeds into skirts, the chute width must not be greater than the width of the skirts. Any necking or reduction in width of the chute body must comply with the chute wall slope and valley angle requirements.
The chute exit should be designed to impart to the material some velocity in the direction of flow, where the feed is onto another conveyor, wherever possible. A common specification is for the exit velocity to be within 10% of the receiving belt speed.
(Fig. 7: Typical chute outlet requirements wp_1294) For chutes exiting at right angles from screens onto conveyors, the chute should cover the full width of the screen discharge and ought to be equipped with adjustable, replaceable deflector plates, placed at approximately 70° to the horizontal, located above the conveyor belt surface. The deflector plates must be dimensioned to allow the full passage of material, without creating a cut-off area over the conveyor belt. Chute exiting screens may be provided with a cut-off or isolating mechanism, such as a clam-shell gate, or radial gate, in order to prevent flooding of the receiving conveyor under trip-out conditions, when the conveyor coasting time is less than the loaded screen run-down time. The gates may be programmed to automatically close rapidly (in less time than the conveyor coasting time), and to open smoothly and slowly, in order to deliver the screen run-off to the conveyor in a reasonably controlled manner when the system is restarted.
For chutes exiting hoppers, bins, silos and stockpiles, the chute design must accommodate the requirements of the feeding device, such as the vibrating, belt, apron or other type of feeder.
4.4 Chute Liners
Chute Hood: Under normal conditions, the chute hood side plates are not lined. If the material trajectory is such that the hood front plate or side plates will experience impact, then only the areas subject to impact should be lined, in order to minimise the mass (and cost) of the liners.
Chute Body: Only the areas where the material impacts or slides should be lined.
Chute Exit and Skirts: The exit of the chute is normally lined wherever the material impacts or slides. Skirts should be lined full length. The depth of liners should be at least equal to the depth of material contacting the skirts. In the impact and acceleration zones, the skirts should be lined full depth.
Types of Liners: The following types of liners may be considered for selection:
- VRN-500. This is preferred at locations where impact is high, or where the material lump size is greater than 100 mm – 150 mm.
- Ti-Hard, Rio-Carb or other harder grades of liner steel have high wearing properties and should be considered, based on the specific application.
- Solidur or equivalent UHMWPE. This may be specified in locations where the material is sliding. These liners should not be specified in areas of high impact, or material with large lumps and sharp edges. This material is especially useful for lining the back plates of chutes and for lining dribble chutes, in order to improve material flow.
- Ceramics. This is useful where the action of the material is largely sliding and there is a significant moisture content in the material. These liners should not be specified in areas of high impact, or where the material lump size is greater than 100 mm to 150 mm. Ceramics are best utilised where the body of the chute has a long sliding portion and where water is introduced to wash down fines collection areas.
- Rubber. These liners are best utilised in primary crushed material bins and hoppers, or where impact is likely to be high. The location of the liner with respect to the trajectory of the material must be carefully considered, in order to present the material flow normal to the surface of the liners as far as possible. Other locations where rubber may be used are in the body of the chute which may be subject to material splash, in order to reduce noise levels. Rubber liners should not be specified in areas where sliding takes place without the introduction of wash water
General
- The bare chute plate should be prepared in accordance with the requirements of the engineer. For replaceable metal liners, the chute surface should be clean and free from rust and scale. The surface to be lined may be coated with epoxy primer to 30 μm. For other methods of attachment, such as adhesives or riveting, the surface of the chute to be lined should be prepared in accordance with the requirements of the liner supplier.
- Liner plates of thickness 12 mm and above should be secured with M16 countersunk bolts.
- The minimum number of securing bolts per liner plate should be as follows:
- For triangular sections: 3 bolts
- For any other shape: 4 bolts - For other lining materials, the securing bolts or rivets should be in accordance with the requirements of the liner supplier.
- Nib head bolts may be used in areas that are not subject to flexing or heavy impact. Note that cracks in the harder steel liners originate at the notch for the bolt head nib. For this reason, liners in hardened steels that are subject to flexing ought to be secured with conventional countersunk bolts, with slot heads or hexagonal socket heads. The securing bolts may be grade 4.4.
- In areas where wash down water is used, the bolt joints should be made water tight.
- The liners ought to be so patterned that the gaps between the liners are staggered in the direction of flow, in order to prevent the material fines ‘channelling’, and creating ‘pagging’ areas (the rapid build-up of very fine material). In corners, the liners must be so arranged that the edges overlap and the corners of the bare chute are protected.
- The welding of liners is unacceptable.
- The recommended thickness of steel liners shall be as follows, subject to input from the liner supplier:
- 20 mm: on high wear, heavy impact areas and chutes handling material of average lump size greater than 100 mm to 150 mm
- 2 mm: on surfaces subject to light impact and material sliding only, and on skirts
- 10 mm: on fines chutes that are not subject to impact. - The thickness of other lining materials, such as ceramics and Solidur should be as determined by the liner supplier.
- All liner plates must be sized for ease of handling, with an average mass of 30 kg and a maximum mass not exceeding 35 kg. Keep in mind that liner plates are often difficult to manipulate within the confines of the chute body. A recommendation is that liner plate could be provided with removable ‘handles’ to facilitate handling.
- Metal liners should be secured with countersunk bolts. The maximum bolt diameter is usually determined by the thickness of the liner. The countersunk holes should allow a base thickness of about 3 mm between the back of the liner and the underside of the countersink. The maximum securing bolt diameter may then be determined as
d = 2 · (t – 3) (3)
where:
t = liner plate thickness [mm]
