Construction & Demolition Recycling,
Sept-Oct, 2004 by Jack Byrne
The following feature is based on a presentation prepared by Jacky Alix of Sandvik AB and edited by Sandvik’s Jack Byrne.
Real business don’t have money to waste on purchasing the wrong equipment for the job. The decision on what type of crusher to buy in order to meet recycling requirements needs to be correct the first time.
It can be argued that good crushing equipment carries a high price tag and, because of that, needs to last a long time while operating in one of the toughest possible applications.
Making the right decision involves a lot of different factors, ranging from the type of feed material and tons per hour of production expected through the desired product sizes, travel requirements, and even environmental considerations.
Often a good starting point is to consider the raw feed material. What is it? It could be pavement, bricks, concrete slabs or even asphalt. It could be concrete with, or without, steel reinforcement. The reinforcement can be small re-bar, heavy re-bar, wire mesh, or other materials.
Is the fines content high or low? If the feed material sometimes involves demolition rubble, it may contain contaminants like plastic, wood, plaster or a variety of other things.
What are your expectations? Keeping in mind local market needs, you may be looking for quality products for roadways, parking lots and sub-base. It could be for low-grade concrete for pavement, or fill material. What are the product sizes you desire, and what are required gradations within the product sizes?
Recycling involves size reduction of rocks or minerals already sized and bound together by cement, bitumen or other binding media. The rock hardness, while sometimes a factor, is not generally a major issue. Generally, low crushing forces are required to break the binder. On the other hand, rock abrasiveness is a major consideration. It can have a strong influence on operating costs, especially with impact crushing.
At the same time, feed preparation is always an asset to upgrade plant efficiency and reduce maintenance costs. Steel is going to go through the crusher in any recycling operation. However, it really doesn’t make a lot of sense to put steel through the crusher when you don’t need to. It takes away from crusher capacity, creates added wear and poses potential equipment damage possibilities. The same holds true for large lump sizes that should be broken up before being fed to the crusher.
STYLE AND SUBSTANCE
The crusher types most commonly used in recycling operations are jaw crushers and impact crushers. Cone crushers are also used in some parts off the country.
In terms of jaw and impact crushers, each has its advantages (and shortcomings), depending on the application requirements.
Jaw crusher benefits, for example, are:
* Versatility: The design allows for input of a wide variety of dissimilar materials.
* Much greater tolerance of abrasive materials.
* Dower maintenance costs, and less potential for damage to the crusher.
* Slower RPM range, another way to prevent potential damage.
There are restrictions, however, with jaw crushers as well:
* Produce a flat, elongated rather than cubical product.
* Provide a low reduction ratio–approximately 3:1.
* May not completely remove the wire mesh from concrete, and may leave a beaded necklace of concrete on re-bar.
* Yields a lower quantity of fines.
When looking into a jaw crusher for recycling, there are certain helpful features you should look for. First, you will need a clear, unobstructed, wide-feed opening. Secondly, because of the abrasiveness of recycled feed material, it is desirable to have an automatic, or controlled Closed Side Setting (CSS) System. (In order to assure continuous quality of product gradations, and because of the abrasive, high-wear qualities of the material being processed, it may be necessary to frequently adjust the CSS.)
Finally, it is not required, but helpful to have a Hydraulic toggle-retaining device. (The toggle is the safety device protecting a Jaw Crusher from serious damage when an un-crushable item passes through it.) A hydraulic toggle, while not inexpensive, can pay for itself quickly in tough recycling applications where, potentially, materials that can damage a crusher are processed daily.
Impact crushers offer entirely different benefits:
* High reduction ratio–8:1 to 12:1, even up to 20:1 in closed circuit systems.
* More cubical product shape.
* High quantity of saleable fines.
* Efficient separation of wire mesh and re-bar from concrete.
* Versatility: Impact crushers can process nearly all types of recycled material, including asphalt.
* Easy replacement of wear parts. The limitations are different also:
* Impact crushers are more sensitive to abrasive material.
* Maintenance costs may be high if too coarse steel contamination prevails in the feed, or insufficient feed preparation is done.
* Higher dust emissions.
When considering an impact crusher there are a number of important areas to explore. First, remember that the correct crusher feed opening versus feed size will keep production levels up. On primary impact crushers, especially in recycling applications, large feed openings should not automatically be considered the gauge for admission of even coarser lumps. Large feed openings are important to ease the inward flow of dissimilar feed materials, as is always the case in recycling.
Rotor inertia and mass are always important in impact crushers. Look for large rotor diameters, which can ensure smooth crusher operations even with uneven distributed feed sizes, as is often the case in concrete recycling.
Hammer design and fitting are important issues, as well. It is always important in impact crushing for the hammers to retain a sharp leading edge. This is what keeps the production high, gradation consistent and wear distributed to all areas of the crusher under control.
The hammers should have strong support inside the crusher, and should always have the same surface contacts in order to provide the proper strength. In some models, the hammers have a large backing bar that can provide strength to the rotor and will help to provide high rotor inertia. Hammers should have a high amount of metal to wear. For ease of servicing, or removal, the hammers should be vertically installed, and should utilize a simple, strong locking device.
Hammer lifetime is dependent upon metallurgy and abrasiveness of the feed material. Even in recycling, some feed material is far more abrasive than others and some material is much harder than other material. These items combine to dictate the metallurgy that can be used in the application. Different common metallurgies that are available manganese (M2), chrome steel, chrome iron and chrome steel with ceramic pads.
Also important in impact crushers is to focus on serviceability. Gravity-hung impact curtains can offer real advantages. They will fall back when an un-crushable item enters the chamber, and will also move back to help relieve bridging (clogging). Protected Impact curtains can allow for local repair of concentrated wear on the curtains. Cast curtain liners, with a single size fitting the entire range of locations, can be another significant benefit.
SETTLING THE DUST
Environmental issues continue to play an increasingly larger role in our everyday lives, especially, it seems, in the crushing business.
Permitting, if required, can take on a life of its own. Because of this, it is important to give some attention to the area of dust control when looking into crusher purchase considerations.
Different types of crushers have different emission levels. As a general rule, the following figures can be used when estimating airflow generated by crushers (in cubic feet per ton of material processed):
Jaw Crushers: 650-800 cu. ft./ton
Cone Crushers: 950-1,150 cu. ft./ton Impact Crushers: 1,300-1,450 cu. ft./ton
Hammermills: 1,300-1,600 cu. ft./ton.
Solutions commonly used when dealing with Dust Emissions include:
* Water spraying
* Dust enclosures at equipment and/or transfer points
* Airbag filters.
* A roofted building (for stationary plants) combined with dust enclosures and filters.
When using a mobile single-stage jaw crusher, there are important considerations on the subject of final product sizes.
The crushed fraction will, generally, be in the range of 4×0 inches to 6×0 inches. There will be low content of 1-1/2 x 0 inches.. The fraction 1-l/2×0 inches is unlikely to meet the specification for sub-base.
The inclusion of pre-screened fines is usually only acceptable when they are free of contaminants. Generally, you have no control of the proportion of pre-screened fines when including them in the crushed product.
When using a single-stage mobile impact crusher, the crushed fraction will be in the 2-1/2 x 0 inches to 3 1/2 x 0 inches range. 75-90 percent will be minus 1-1/2 inches. The fraction 1-1/2 x 0 inches generally meets the specs for sub-base. Once again, inclusion of pre-screened fines is only feasible when they are free from contaminants. Also, there is generally no control of the proportion included, but in this case it is of lesser influence.
“When using a two-stage mobile crushing plant (jaw/cone or jaw/impactor), the crushed fraction is generally 1-1/2 x 0 inches to 2 x 0 inches. The 1-1/2 x 0 inches fraction generally meets the specifications for sub-base. Inclusion of pre-screened fines may be needed to further improve fines content, but must he free of contamination.
All of the items listed above hold true for stationary plants just as for individual crushers and portable plants. This is true regardless of whether the issue is crushing fractions, plant emissions or the particular benefits of jaw crushers or impact crushers.
The right plant set-up improves production and decreases cost per ton. Building portable crushing plains involves working within certain height, width, and weight restrictions. Stationary plants are generally free of those restrictions.
This can allow for better plant efficiencies and flow improvements through the use of better chute and conveyor angles, properly sized screens, and better clearances under crushers and screens allowing for better flow and handling of the material. It can also allow for easier maintenance access. All of this comes back to improving or lowering the everyday cost to operate your crushing plant.
A lot to consider? Yes, there is. However, if you will keep these ideas in mind when considering a crusher purchase you will make a much better buying decision and you will be much happier for all of the years you own the crusher.
Common Impact Hammer Metallurgies
Metal Type Lifespan Applications
Manganese (M2) Baseline Index: 1.0 Primary and secondary
Chrome Steel Index: 1.5-1.7 Primary and secondary
Chrome Iron Index: 2.7-3.2 Secondary applications of <7 inches
Chrome Steel Index: 3.0-3.5 Primary and secondary ceramic pads applications
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