There are a number of different criteria to consider when selecting the right chambers for your crushing needs. There is one, however, that must always be considered – that you have a well-graded feed to the chamber. A well-graded feed is generally thought to be 90-100 per cent passing the closed-side feed opening, 40-60 per cent passing the midpoint and 0-10 per cent passing the closed-side setting (Figure 1).
Before selecting a crushing chamber, you should take a sieve analysis of the feed to find out if you have a well-graded feed. Look first at the feed opening of the chamber. Take the closed-side feed opening (CSO) plus the open-side feed opening (OSO), and divide by two to establish the maximum feed size to the chamber.
For example, assume a closed-side feed opening of 165mm and an open-side feed opening of 205mm:
Max Feed Size = (CSO + OSO)/2
Max Feed Size = (165 + 205)/2
Max Feed Size = 185mm.
The maximum feed size to this chamber would be 185 mm and not more than 10 per cent larger than the closed-side feed opening (CSO).
To determine the midpoint of the crushing chamber, take the closed-side feed opening (CSO) plus the closed-side setting (CSS), and divide by two. Assume, for example, a closed-side feed opening of 165mm and a closed-side setting of 25mm:
Midpoint = (CSO + CSS)/2
Midpoint = (165 + 25)/2
Midpoint = 95mm.
In this hypothetical case, between 40 and 60 per cent of the material should pass 95mm in feeding this chamber (Figure 2).
A feed that is not well graded will invariably cause one or more problems with the crushing operation. That is the first thing you should carefully examine.
If the feed is too coarse (Figure 3), one or more of the following problems can develop in the operation:
? There will be reduced tonnage through the crusher
? Low horsepower consumption
? The product from the crusher will be coarser than it should be
? The product will not be as cubical as it should be
? The upper part of the crushing chamber will wear out (Figure 4)
If the feed to the crusher is too fine, you will notice one or more of the following problems:
? There will be reduced tonnage per hour through the crusher
? High horsepower consumption
? Upper frame movement
? High oil temperature
? The difference in capacity from a new liner to a worn-out liner can be as much as 50 per cent less. (The reason for this reduction in capacity is that the lower portion wears faster than the upper portion, which causes the feed operation to close off).
The second thing to evaluate is the reduction ratio of the plant. An open-circuit Gyrasphere, cone crusher or the second-stage of crushing should have a reduction ratio of 6:1 to 8:1 in a single-pass operation. If this crusher is in a closed-circuit operation, the reduction ratio should not exceed 6:1. The reason for this limitation is that the recirculating load is filling in some of the void area within the chamber. Material needs room for expansion, therefore, you are unable to crush at the higher reduction ratio. Trying to do so will cause your crusher to pull higher amperage and may cause the upper frame to move.
A closed-circuit Gyrasphere crusher or third-stage of crushing should have a reduction ratio of 4:1 to 6:1, maximum. In most cases, 3:1 to 4:1 will work out to be the proper reduction ratio.
Beware – some suppliers offer liners with enlarged feed openings to ?improve? on factory-recommended reduction ratios for the crusher. Operating with a larger than recommended reduction ratio may result not only in premature liner wear, but catastrophic bearing or bushing failures, and even cracked or broken castings.
A mistake to avoid
One thing you should never do is to place a new concave liner in a crusher with a worn mantle – or a new mantle in a crusher with a concave liner. Why?
If you have properly selected the replacement component, you will change the complete profile of the crusher by mating new and worn components. The receiving opening will tend to close down, thereby restricting the feed from entering the chamber – and causing a reduction in tonnes per hour.
For example, assume that you have a crusher with the capacity of 272 tonnes per hour. And let us further assume that the feed opening is restricted by improper replacement components, reducing the capacity by 10 per cent – which is a 27 tonnes per day reduction. Over the next four weeks (say, 160 production hours), it will cost your company 4354 tonnes of production! How much is that in terms of lost revenue? $30,000?
This initial lost revenue does not take into consideration the fact that four weeks later you will probably have to shut down, once again, to replace the other liner component. For an investment of just two additional hours, the second liner could have been replaced. Changing both liners simultaneously would have saved your company at least four hours of complete shutdown time – this is additional lost revenue.
Timing is critical
When should you replace a liner in your crusher? It is a frequently asked question.
It is good practice to chart liner wear on a daily basis. By doing this, you will get a feel for how quickly the liners are wearing.
If the liner is wearing evenly throughout the chamber, you should consider changing the manganese when it has worn down to about 25mm thick at the bottom. At about 19 to 16mm thick, the manganese will crack, causing the backing material to begin to disintegrate. And this, in turn, will cause the liners to break loose. If this should happen, continued operation could destroy the seat on the support bowl or the head of the crusher. Repairing these items is very costly.
Liners require a period of easier production to work harden the manganese. Work hardening ?green? manganese cone liners can take days to weeks, depending on the feed material.
To maintain the maximum levels of capacity, gradation and cubical product, a crusher must be choke-fed at all times. The best way to keep a choke-feed to the crusher is with a surge bin (or hopper) and feeder that are located prior to the crusher.
Choke-feeding is almost impossible to achieve without a hopper and feeder.
When the crusher feed is reduced, the product tends to become coarse, with slivers and flats occurring. Customer and industry specifications tend to require more cubical products. The photos below show a product that has been choke-fed (Figure 5) and a product that has not been choke-fed (Figure 6).
In Figure 6, the product appears to be coarser, with a higher percentage of slivers and flats.
Abrasion of the manganese should also be considered. Work hardened material can be abraded away when the cone head spins before or during crushing, when the crusher is not being choke-fed. As a telltale sign, you may notice the operator throw in a bit of rock to stop the cone head from spinning. Cone breaks prevent this type of wear.
Well-graded, choke feed, with correct liners: these provides maximum tph through the crusher, maximum utilisation of horsepower, maximum cubical product, maintaining capacity within 10 per cent to 15 per cent from new-to-worn manganese, and maximum utilisation of mantle and concave liners.
Well-graded, not choke-fed, with correct liners: these provide reduced tph through crusher, reduced horsepower consumption, diminished cubical product, coarser product and increased liner wear.
Coarse choke feed, with correct liners: this provides low tph through the crusher, low horsepower consumption, non-cubical product, product gradation coarser than with graded feed, and irregular manganese liner wear.
While all cone and gyratory crushers differ somewhat in design, most liner changes are basic. A cone liner change for a remote-adjust cone ? where the assembly hydraulically lifts and the liner change begins ? can be completed in as few as four hours. A standard cone takes a little longer, requiring quick couplers to help raise the cone assembly, along with the use of shim stacks.
These guidelines might seem to be elementary to veterans in the industry. But there is one thing that you must remember: selecting the proper liner and maintaining a consistent feed pattern to your crusher are proven ways to put more rock on the pay-pile ? and more revenue in your company.
Dudley Schaal is a product engineer for Telsmith crushers, which are distributed in Australia through Metaltec.