Non Metallic Linings For Chutes
John Rozentals, Pr.E B.E. MSAIME MSAICE
Richard Holmes, Pr.E M.A.
Acknowledgements : Bionic Research Institute - Chute Design Conference 1992
NON METALLIC linings for chutes can effectively resist abrasive and impact wear, limit dust generation, and reduce product degradation.
This paper examines non metallic lining materials, and provides guidelines for their selection and use.
John Rozentals is a Partner of BRI Tec Consulting, and an associate of Hamilton Associates.
Richard Holmes is a Partner of BRI Tec Consulting and specialises in materials engineering.
NON METALLIC AND SOFT LININGS FOR CHUTES
The lining of chutes may be required for a variety of reasons - such as one or more of the following:
- To facilitate repair or replacement of the sliding surface
- To improve the performance of an inadequate chute
- To resist abrasive wear
- To resist impact wear, noise, dust generation, or product degradation
- To promote flow and avoid build-up - particularly with sticky materials
- To protect against corrosion or chemical reaction by the product being handled
- To protect the product from contamination or chemical reaction.
Often the lining material must fulfill a number of conflicting requirements - dictated by the overall design and layout of the chute. Hence a compromise may have to be accepted in selecting an optimum lining material.
WEAR AT CHUTE SURFACES
A great deal of research and investigation has been conducted on the general subject of abrasive wear. There is, however, little quantitative information available from field trials covering wear in chutes.
A number of authorities have established the relative resistance to wear of lining materials by laboratory testing.
The performance and comparisons established under laboratory conditions for one set of circumstances and materials have, however, often been reported as unrepresentative of the situations arising from in-plant operation.
The selection of materials must therefore be largely determined on a qualitative basis from experience in handling products in various industries.
At the initial point of impact - at say the discharge trajectory from a belt conveyor head pulley - the impact wear can be high.
At the impact surface wear will be a combination of two components:
- direct impact
- scouring along the surface
The resultant damage to the surface will depend largely upon the relative hardness (and other properties) of the particles and the surface material.
The wear resulting from impact can be many times greater than that resulting from abrasion. It is important, therefore, to reduce impact on the chute surface.
It may pay to use one type of lining material in the impact zone, and a less expensive material in the abrasive sliding area.
The rate of wear in abrasive or sliding wear depends on a number of factors:
- the relative hardness of the particles and the chute surface
- the size and shape of the particles
- the wall friction coefficient
- the product pressure on the chute surface
- the sliding velocity
Many authorities report abrasive wear being proportional to the square of the sliding velocity.
WALL FRICTION ANGLES
The wall friction angle of the lining material will vary with the product handled.
Test information shows that variation in friction angles may not follow a regular pattern. A lining surface exhibiting low friction when handling one product may not exhibit these same qualities to the same extent when handling a different product.
There is a scarcity of published information for values of wall friction angles of lining materials handling different products. Published values are usually limited to specific materials and products on an individual job basis.
Specific tests should be carried out to establish values in specific cases if accurate prediction of chute performance is critical.
Even less information is available on product flow exhibiting some measure of cohesion.
Some features of chute construction can affect wall friction:
- minor ledges
- transverse weld or joint lines
- bolt heads
- surface changes due to abrasion or corrosion and subsequent caking
- hold-up of product on surfaces
- indentation of soft materials such as rubber, plastics, or paints
HARD METAL LININGS
The selection of hard metal lining materials is discussed by Mike Hall in his contribution to this Seminar.
RUBBER OR SYNTHETIC RUBBER
Rubber is frequently used as a chute lining material - both as a means of handling impact, abrasive wear, and for noise abatement.
For impact absorption the thickness of rubber at the impact point, and the direction of impact, are important.
The method of fixing the rubber lining in the impact area must be very secure. Bolted fixings for plain rubber sheeting may fail in the impact area due to movement and stretching of the rubber. This results in a bulging away from the backing plate. The rubber lining should incorporate a bonded metal, perforated or woven metal or fabric stiffening layer to keep the lining stable.
The wall friction angle for rubber (cannot read?????) a lower slip value than other materials. But the resistance of rubber can prevent the build-up of fines -even with cohesive products.
Polyurethane elastomeric coatings and linings can be applied or rebuilt on site by airless spray or trowelling methods. But it is essential to employ qualified and experienced applicators to safely handle the sometimes toxic chemicals and to achieve adequate adhesion to the substrate surface.
Polyurethane in its cured state is generally considered as flammable. Special treatment is required to make the material fire retardant.
Ultra-High Molecular Weight Polyethylene (UHMWP) is not suited to handling very highly abrasive product - such as coke or sinter. It is used for the lining of chutes, hoppers, bins, vibrators, feeders, and bunkers.
In impact areas UHMWP will abrade rapidly.
UHMWP is one of the world's toughest known thermoplastics. Compared with other plastics it offers considerable advantages when suitably applied as a chute lining material.
The material is flexible and can distort under load unless adequately supported. It must be mechanically fixed to the chute backing. Due to its non-stick properties it cannot be satisfactorily bonded by adhesives unless sheets are backed with rubber. This enables sheets to be glued with contact adhesives.
Incorrect or insufficient number of fixings is the most common cause of failure.
UHMWP is flammable and gives off toxic fumes when burning. It is a potential fire hazard. The use of naked flames or welding equipment must be avoided.
UHMWP is suitable for temperatures up to 65 deg.C.
UHMWP is available in differing grades. Since it is hard to identify visually care must be exercised to ensure that the correct grade is employed for the duty.
Quarry tiles provide a low-cost lining material to resist abrasive wear. Appropriate cement or epoxy mortar fixing may be used.
Initially the tiles offer a smooth surface with a fairly low-slip value. Wear of the tiles and the mortar filling between joints can increase sliding friction.
Basalt tiles are particularly suited to withstanding sliding abrasion.
Under impact conditions the surface may be subject to erosion.
Cast basalt is generally used in wet material handling situations such as coal chutes and ash sluiceways.
Basalt can resist wear, corrosion, and temperatures up to about 400 deg.C.
Ceramic materials are used mainly where extreme heat is a problem.
Alumina ceramics are resistant to most acids, solvents, salt solutions and molten salts, and have good resistance to alkaline solutions in mild to medium concentrations.
Alumina ceramics are recommended in high wear applications where there is sliding abrasion or low angle impact.
High Alumina Ceramic Tiles are widely used to resist sliding abrasion in hoppers and chutes where a measure of impact resistance is an added requirement. The product is particularly suited to the coal industry for chutes handling coal and shales. The thickness of tiles must be made adequate to suit the degree of impact which they have to withstand.
Various bonding methods are available. The most common is epoxy resin which provides a high shear strength and good resistance to chemical or moisture attack and ageing. Epoxies can withstand temperatures exceeding 200 deg.C.
Chute linings are subject to wear. Sooner or later the liner material will have to be replaced. Access to the inside of many chutes and skirt areas is extremely difficult, if not impossible.
Chutes with belts 900 mm or wider should have ample room for maintenance men to enter the chute and replace liners. Even so, extra access doors should be provided in chutes 3 m or more in length. These doors should be large enough to pass liner plates through, and must be bolted and gasketted. The doors should be located in pairs, directly opposite one another, and be no smaller than 450 mm square. By locating two doors opposite each other the openings make a convenient place to insert planks or scaffolding.
For smaller chutes on narrow belts careful design and detailing is required to make adequate provision for liner replacement. Fabricating the chutes in short lengths for easy disassembly is one approach .The non wear side of them chute can be flange bolted to facilitate maintenance access.
Good access does not just happen. It takes careful thought during design. Such consideration at the design stage can be stimulated by appropriate specification by the conveyor end users.
1. H .J Taylor "Guide to the design of transfer chutes ." Mech. Handling Engineer's Assoc. 1989.
2. R.E Holmes 'Selection of material: for the lining of chutes" First International Chute Design Conference, South Africa 1991