There are two main stages in a crushing plant, namely crushing and screening.
The first step of processing begins after the extraction from quarry or pit. Many of these steps also are common to recycled materials, clay, and other manufactured aggregates. The first stage in most operations is the reduction and sizing by crushing. Some operations, however, provide a step prior to crushing called scalping.
Scalping (Figure 1) most often is used to divert fines at a jaw primary crusher in order to improve crusher efficiency. In this way the very coarse portion is crushed and then recombined with the portion of crusher-run material before further processing. This first step may, however, be an excellent time to improve a deleterious problem. If a deleterious or fines problem exists in the finer fraction of crusher-run material (namely, clay, shale, finely weathered material, etc.) the fall-through of the scalping operation may be totally or partially diverted and wasted, or may be made into a product of lesser quality. In any case, only acceptable amounts, if any, should be returned back into the higher quality product. Consideration of process variables in this early stage may be very important.
2. Primary crushing
In stone quarries or in very “boney” gravel pits, large material usually is reduced in size by either a jaw (Figure 2) or a gyratory crusher. Both types are compression crushers. Although economical, they have the tendency to create thin, elongated particles. Particle shapes sometimes may be a problem for Producers of hot mix asphalt. In some operations impact crushers are used for primary crushing, but they may have a slightly higher cost per ton. Impact crushers may upgrade poor-quality aggregate and increase separation, such as removal of rebar from concrete in recycling operations.
After primary crushing/reduction the resulting aggregate generally is placed in a large “surge” pile where the aggregate may be fed into the secondary operation whenever convenient. Care is always taken when building up and loading out surge piles, as this step may be a major source of segregation of material going to the secondary plant. Variation at this point may affect both mineral quality and gradation.
Drawing from an inverted cone over a load-out tunnel works well after material has been deposited and left undisturbed to form the walls of the draw-down cone. If the need ever arises to consume the entire pile, care is taken to thoroughly mix the older material a little at a time with fresh product to make the surge as uniform as possible as the aggregate is being pushed into the tunnel. If the operation relies on end loaders to feed the secondary plant from the surge pile (Figure 3), the same care is taken to mix coarse with fine material from the outside to the inside of the pile.
3. Secondary and tertiary crushing
Secondary and tertiary crushing, if necessary, are the final steps in reducing the material to a desired product size. Historically, cone and roll crushers were the most common choice crushers, but in recent years impact crushers are more widely used. These crushers also are sometimes used as primary crushers for fine-grained gravel deposits.
The cone crusher (a compression type) simply crushes the aggregate between the oscillating cone and the crusher wall (Figure 4). Clearance settings on this equipment are required to be checked and maintained as part of standard operating procedure.
As with other compression crushers, the cone crusher yields a somewhat elongated and slivery particle shape. This may be minimized, however, by “choke” feeding the crusher. This technique will also make the shape and size more uniform. One way to choke feed is with a surge hopper and a controlled belt-feed to the cone crusher (Figure 5). Automatic level controls measure the head of the material over the top of the cone.
A roller crusher (Figure 6) is another compression type crusher that simply breaks the material by pinching the aggregates. These types of crushers are often found in gravel operations. Roller crushers have constant maintenance problems and are prone to excessive wear. The rollers are required to be checked frequently to insure proper clearance and uniformity across each roller. Rebuilding and re-milling the roller is a standard operating procedure.
4. Impact crushing
Impact crushers may be used as primary, secondary, or tertiary crushers. Despite having a somewhat higher operating cost than other crushers, they tend to produce a more uniform particle shape. Impact crushers usually will benefit the aggregate better than compression crushers, and they may generate more fines. Common types are the horizontal shaft (Figure 7), vertical shaft, and hammermill impactors.
The horizontal shaft single or double rotor may aggressively handle large and odd-shaped material. Large horizontal impactors sometimes are used as primary crushers. Fracturing occurs at the same time by rock against rotor, rock against breaker bar, and rock on rock.
The vertical shaft impactor (Figure 8) is operated in rock against anvil, or rock against rock (through the installation of a rock shelf) modes. The Producer is required to decide carefully the mode best suited to the raw material.
The hammermill impactor (Figure 9) provides excellent reduction and beneficiation through the impacting and shearing action of the hammers and grates; however, a large amount of fines is produced. This type of crusher is sometimes used in the manufacture of agricultural ground limestone.
Screening is another technique to control both quality and gradation of the aggregate product.
1. Product quality
If deleterious material exists at undesirable levels after crushing and may be identified as being predominantly in one size range that is not needed for product size, the material may be screened out (namely, fines or top size). This step may occur between crushing so that an opportunity exists to recreate the same size downstream, if needed, to create a product. The screened-out lower-quality material may be used for a lower quality product or wasted if no use exists.
The rinse screen (Figure 10) is also commonly used. By processing the material over a screen that retains all of the product, the clay and deleterious fines may be rinsed away to make the product acceptable.
2. Gradation control
The best technique for gradation control is screening (Figure 11). Screening may be done wet or dry, depending on the type of aggregate being processed and the degree of consistency required for each product.
Washing, for example, may be necessary to clean a concrete aggregate, but may not be needed for hot mix asphalt products, which may contain more fines. For gradation control alone, however, consistency sometimes may only be maintained by using wet screening. Gradation consistency is usually an overriding factor for a hot mix asphalt customer. Water volume and flow direction are critical in wet screening. Frequent checking of the gradation is a standard operating procedure.
Dry screening is a slight misnomer because the material passing over the screen decks is wet, ranging from slightly damp to very wet, depending on conditions such as rain or subsurface moisture. Non-washed screening is a more accurate description of this screening process. High moisture is a
concern because the wet aggregates may cause some material to become sticky and bind together, making the aggregate harder to separate. Furthermore, high-moisture conditions may cause binding of lower screen decks, causing override of the material rather than separation. If these conditions are encountered, the Producer may need to establish a balance between the moisture content of the incoming material and the feed rate through the screens. This balance is required to be made for each hour of operation. If reduced feed rates do not solve the problem or is too costly, washing or an additional screen area may be needed.
Sometimes screening variation is too great even under the most favorable of conditions. When this occurs the Producer is required to check that the equipment and the screen cloth are in good repair. The most common reason for high screening variability is the tendency to push too much material over
a screen. The only way to maintain a bed of material thin enough for optimum efficiency is to provide enough screening to allow the desired rate of production. Standard operating procedures should reflect the maximum feed rate for the design of the plant.
For well-graded products having many sieves, gradation control may not be done without first separating the material into fractions. Separating the material into numerous small fractions and then back-blending at a set rate for each fraction may be necessary to control the gradation. Frequent sampling, testing, and control charting are necessary for monitoring because aggregate gradation is subject to so many variables.