 |
 |
Selection Of Materials For The Lining Of Chutes
R.E. Holmes
M.A. (Eng) Pr. Eng MSAIMEAcknowledgements : The Bionic Research Institute, Chute Design Conference 1991
A wide variety of lining materials are available for the diverse applications called upon in the bulk materials handling industry.
This paper reviews the range of lining materials available, and provides practical guidance for their use.
Richard E Holmes is a Consulting Engineer. with many years experience in the use and application of non-ferrous materials. He is a Director of The Bionic Research Institute.
CONTENTS
- GENERAL
- REASONS FOR LINING CHUTES
- CONSIDERATION OF ALTERNATIVES TO LINING
- PROTECTION OF THE CHUTE
- PRODUCT PROTECTION
- FRICTION MODIFICATION
- PROCESSING MATERIALS
- DESIGN CONSIDERATIONS
- MAINTENANCE OF CHUTES
- LINING MATERIALS FOR PROTECTION OF CHUTES
- LININGS FOR PRODUCT PROTECTION
- LININGS TO MODIFY FLOW
- RETARDING THE TRANSFER OF PRODUCT
- PROCESSING THE PRODUCT DURING TRANSIT ON CHUTE
- CHUTE DESIGN CONSIDERATIONS RESULTING FROM ADOPTION OF LININGS
- CONSIDERATION IN THE CHOICE OF LINING MATERIALS
SELECTION OF MATERIALS FOR THE LINING OF CHUTES
1. GENERAL
Lining of chutes is a relatively simple concept, but the study is vastly complicated by the number of variables involved in the application of chutes. Not the least of these is the nomenclature. Three items need to be defined to avoid confusion :-
1.1 Chute: This includes launders. deflectors breeches chutes, flaps and vibration assisted chutes.
1.2 Materials: This refers to materials of construction of the chute and of the lining. It does not refer to the items in transit in the chute .
1.3 Product: This refers to the substance or substances transported by the chute, including slurries, mixtures, primary products and waste materials .
2. REASONS FOR LINING CHUTES
2.1 To protect the chute structure from abrasion, erosion, temperature imposed by the product.
2.2 To protect the product from damage by the chute due to chemical reaction or mechanical damage by the chute.
2.3 To increase or reduce friction or otherwise change the velocity of the product.
2.4 To process the product during transit along the chute.
3. CONSIDERATION OF ALTERNATIVES TO LINING
Obvious alternatives to lining of chutes are:
3.1 Manufacture the whole chute in alternative materials.
3.2 In some circumstances discard chutes altogether.
These alternatives ore not now discussed, but choice of such a different approach must be guided by economic considerations.
4. PROTECTION OF THE CHUTE
This is the major reason for lining chutes because a long list of conveyor-handled products are corrosive, abrasive or at extreme temperature.
4.1 Corrosion Protection : Corrosion occurs through electro-chemical reactors reactions. Highly reactive products such as chemicals, wet products and contaminated material in damp situations react with metal chutes. Inert or low reactive sheet materials or coatings are used for this purpose.
4.2 Abrasion Protection : Soft resilient materials or exceptionally hard material materials are used for this application depending on the conditions .
5. PRODUCT PROTECTION
Linings are used to prevent pick-up of chute material or chute corrosion products. Diminution of products may also be prevented by using soft resilient linings, although change in chute geometry may be more effective.
6. FRICTION MODIFICATION
Low coefficient of friction material such as fluoropolymers, high density polyethyline and others may be used to speed the product and prevent hang ups. High friction linings are seldom used to slow products, but occasionally corrugated soft materials are used to even out the flow onto the next process.
7. PROCESSING MATERIALS
A variety of materials are used to line chutes that require a special geometry for mixing or separating or grading materials during transit down a chute. Special textured linings can be used on vibrating chutes for sorting mixed media.
8. DESIGN CONSIDERATIONS - WHEN SELECTING LININGS
Space and mass are often limiting factors in the choice of thick or heavy lining materials. These will be considered when discussing individual cases.
9. MAINTENANCE OF CHUTES
This can be a big problem and mention will be made of problems arising with various materials.
10. LINING MATERIALS FOR PROTECTION OF CHUTES
10.1 Rubber Lining : This is the most common and most successful solution. Rubber may be adhered in sheets to clean abraded chute surfaces, or it may be vulcanised on.
Choice of sheet thickness depends on the size of the particles being conveyed and the distance of drop onto the chute. This solution is often adopted in mining and quarrying applications. The lining must have sufficient thickness to absorb and redeliver the energy of the falling product. Figure 1.
|
|
Figure 1: Thickness of rubber for rebound
Table 1 indicates the necessary lining thickness for various sizes of product against the free fall height of the product. Here soft resilient rubber is specified as Shore Hardness 30 against the hard resilient rubber having a hardness of 60.
Such chute protection is used largely for heavy rock products. Fine granular dry product can be handled in unlined chutes
10.2 Hard Metal Lining of Chutes : This is a less popular solution to the abrasion problem. These are examples where engineers standardised on chute sizes and could thereby use standard lining plates. In this situation cast metal linings can prove economical.
Manganese steel is generally used where heavy impact is experienced and high chrome steel where sliding abrasion only is experienced. Chrome molybdenum steels are also used.
To prevent cracking, a plate thickness of at least 25mm is required. At R2,50 to R3,00 per kg for manganese steel, and R3,00 to R4,00 for chrome steels this can be a heavy and costly solution. However, it may well outlast rubber in certain applications. Figure 2.
Figure 2: Abrasion - Resisting plate liners
10. 3 Plastics and Synthetic Elastomers : Other than Neoprene, these have normally very little advantage as protection for chutes against abrasion. They are either susceptible to damage, difficult to adhere, or too costly.
Neoprene is useful where some solvent contamination is present.
10.4 Ceramics and Stone : These materials are now seldom used in chute protection. Quarried granite slabs were used in the lust century for some chute applications, and abrasion-resisting ceramics tend to be used currently only in ducts and pipe work. Loose stone is often used in a "drop-box" , a device in a rock chute to cushion the first falloff a conveyor prior to the product sliding thereafter on an unlined chute. Figure 3
Figure 3: Drop Box
10.5 Corrosion Protection Linings
10.5.1 General: Corrosion is a process by which a chemical solution reacts with a solid material resulting in damage or discoloration of the solid material. The usually best-known form of corrosion is the rusting of iron which is caused by air (oxygen, carbon dioxide and nitrogen principally) in water solution, reacting with iron to cause iron hydroxides, oxides, and carbonates. Essentially water is necessary to effect or promote the corrosion. The reaction normally produces an increased surface area of the solid resulting in increased reaction speed.
Corrosion in chutes will therefore mainly occur where damp products are handled or where reactive products are handled in a moist atmosphere.
The effect of abrasion by a moving product sometimes improves the situation by polishing the surface of the chute and thereby reducing the rate of corrosion.
10.5.2 Rubber lining is usually resorted to in protecting chutes against corrosion. This material has the advantage that it can be a protection against abrasion at the same time.
10.5.3 In the event that rubber would react with the product, epoxy, polyester or urethane linings may be used .Urethanes, although less corrosion-resistant than the others, are often adopted due to their better performance in resisting abrasion and impact,
10.5.4 Acidic cakes, sludge and precipitates, involving nitric and sulphuric acids, will attack most organic lining materials, consequently austenitic stainless steels have to be used. This material is fairly resistant to abrasion and is sometimes used as a lining material. More often nowadays, with the comparatively lower price of stainless steels the whole chute will be manufactured from austenitic stainless.
Be warned however, 13% chromium stainless will not resist these materials.
10.5.5 Products contaminated with acidic chlorides, fluorides and phosphates-plus-fluorides may be handled with specially formulated rubber compounds. Stainless cannot be used.
10.6 Protection against high temperature products :
Conveyor chutes rarely have to handle very hot products. Some form of cooling is usually achieved before the chute is reached. However, on occasions, hot products are handled. Each process has a different solution to the problem, usually because these are problems of abrasion and corrosion associated with the high temperature.10.6.1 Carbide conveyors handle the hot product using 3CR12 steel liners on chutes and conveyor pans.
10.6.2 In pyrite-burning sulphuric acid plants, the hot calcined oxide discharge chutes were lined with high alumina refractory tiles which resisted heat as well as abrasion. Silicon iron liners may also be used for this purpose, but the material is very brittle, and can only be cut with diamond grinding wheels.
11. LININGS FOR PRODUCT PROTECTION
11.1 Pick up of particles of steel or rust can be damaging to many products. A particular problem involves PVC or polythene granules, used for electrical insulation.
Chutes are generally coated heavily with an epoxy or polyurethane twin pack paint.
The small particle size and weight have little effect on such linings. Alternatively, the chute can be made of a compatible material, stainless steel, timber, etc.
11.2 Food products are protected from contamination by reaction with chute material by seamless polymeric screeds such as those used abattoirs. These replace the old jointed tiles chutes and floors, which are now considered internationally to be unhygienic.
12. LININGS TO MODIFY FLOW RATE
Deliquescent or damp and sticky materials need special low friction chutes. This is achieved by applying linings of fluorocarbons or ultra high density polyethylene. These materials are highly solvent-resistant ar1d their low coefficient of friction make adhesion virtually impossible. Liners are moulded to conform to the inside of the chute where they can be secured by metal fastenings. Figure 4.
Figure 4: Flexible polymer linings to assist flow of sticky product
13. RETARDING THE TRANSFER OF PRODUCT
Linings are not normally used directly on the chute to retard the product. However, a retarding flap is often used and high friction rubber lining is applied to the abrasion surfaces. Figure 5.
Figure 5: A retarding flap to regulate product flow
14. PROCESSING THE PRODUCT DURING TRANSIT ON CHUTE
Chute surfaces are modified when it is required to process the product as it proceeds along or down the chute. The principal processes thus achieved are (a) product mixing and (b) product separation.
However, these units are not strictly conveyor chutes, but rather separate processing units.
Such examples are sorting chutes for industrial diamonds and spiral media separators. Both these have chutes modified by additional of various roughnesses to suit the materials to be separated. Much experimentation is required for each new application or product.
15. CHUTE DESIGN CONSIDERATIONS RESULTING FROM ADOPTION OF LININGS
15 .1 Dimensions :
It is obvious that when a lining considered the theoretical cross-section of the chute necessary to handle the product must be carefully calculated. The construction will have to allow for the thickness of the lining, but it may have to be re-computed due to the possible retarding effect of the lining.15.2 Whatever kind of lining is selected for a chute, maintenance of the system can be expected to be a regular and often costly activity. Conveyors are usually continuous in operation and stoppage of any unit will usually require halting all the operations contributing to that unit. Hence chute lining repair or replacement has to be carefully planned. On new and untried processes considerable allowances should be incorporated in dimensions to accommodate unknown factors. Chute systems should be designed in sections which may be easily handled into position. Where heavy chutes or linings are involved, hoisting gear will be necessary to handle the components. Permanent gantries are recommended .
15.3 Obviously the impact area in a chute can be expected to wear most frequently. If continuous operation is necessary, such parts must be backed up by replacement spares.
Where direct high-impact cannot be avoided, for space or other considerations, the impact areas should be specially treated. This usually involves one of two options ;-
(a) Increasing the thickness of the lining at the impact point.
(b) Changing the material of lining at the impact point.
Either solution may require a re-design or recalculation of chute cross section requirements.
15. 4 Handling Lined Chutes:
Repairs to linings in situ is unsatisfactory and often impossible. It has been recommended above that chutes be designed in flanged sections to assist in removal and replacement when the lining has to be replaced. It is strongly recommended that lifting points be built into the conveyor structure to enable easy removal of the chute components. It is uneconomic to discount this recommendation and plan to rely on mobile cranes for maintenance. Processing plants very soon get cluttered up with additions and modifications. This will often obstruct access to conveyer chutes. Ultimately this may necessitate the use of long-reach heavy-lift cranes to move relatively light chute components.16. CONSIDERATIONS IN THE CHOICE OF LINING MATERIALS
It is the writer's experience that many industries started in a Small way by using what machinery and equipment was available. Scaled up laboratory or pilot plant equipment was adopted and modified as successes and failures were encountered. Eventually standard solutions were adopted and used thereafter without question. The same rather hit-and-miss practice is often adopted in the use of linings and the selection of materials of construction .
A material engineer should be involved in the evaluations linings. This position should not necessarily be given to a material science graduate. A better candidate would be a well qualified and experienced maintenance engineer. In many cases, a low priced lining material, well engineered will be much more successful than an expensive alternative. Sometimes an unlined alternative structural material may be used. It could be more expensive initially, but less of a maintenance problem. The best information available on linings is provided by the major suppliers .
In the field of rubber linings, there are large suppliers who have good reputations to maintain. They can quote many case-histories to back their information.
For hard chemical resistant materials, again the major suppliers of resins give a very good service. In this province, a very valuable book of reference is the Chemicals, Plastics and Rubber Yearbook for 1982, which provides a most extensive chemical resistance table for plastics and elastomers. The principle foundries which provide special ferrous alloy castings can advise on hard metal liners.
If any lining material supplier is unable to provide case histories and references, it is generally unwise to engage in extensive experimentation. In effect, use proven methods where possible.
Maximum
Particle Size
mmMinimum Recommended Thickness of Hard Resilient Rubber in mm
For Given Product Fall0.6m 0.9m 1.2m 1.5m 1.8m 2.4 25 USE 19 25 38 50 SOFT 32 38 44 100 RUBBER 38
44 50 64 150 38 44 50 64 75 200 50 50 64 76
CONSIDER 250 50 64 76 ALTERNATIVE 300 76 76 89 MATERIALS TABLE 1
