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Aggregate benefaction and product quality, washer, screening

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Aggregate benefaction and product quality, washer, screening

June 25, 2023 nflg 0 Comments

Other benefaction

Other forms of benefaction for quality are available to the Producer. These include the log washer, heavy media separator, and attrition mill. The log washer commonly is used in wet operations to agitate and scrub clay and other objectionable fines from coarse aggregate. A Producer may need to use a log washer when rinsing screens do not remove these objectionable fines.

Heavy media separation is somewhat costly, but may be the only practical way for a Producer to meet quality requirements. This method works only when the undesirable material has a different specific gravity than the desirable material. The deleterious material is discarded after the media is separated for recycling.

Attrition mills are seldom used but remain an option when the deleterious particles are uniformly softer than the non-deleterious particles. The attrition mill abrades the deleterious particles into fines that may be screened out of the system.


Screening is another technique to control both quality and gradation of the aggregate product.


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 crushings 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 is also commonly used. By running the material over a screen that retains all of the product, enough clay and deleterious fines to make the product acceptable may be rinsed away.


The best technique for gradation control is screening. Screening may be done wet or dry, depending on the kind 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 can contain more fines.

For gradation control alone, however, consistency sometimes may only be maintained by using wet screening, especially for hot mix asphalt products. 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 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, (namely, #53s), 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.

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