PHYSICAL SEPARATION PROCESSES (cont.)

ERDC TN-DOER-C13

July 2000

often are, coupled with chemical treatment and extraction processes. The principal properties used

in separation of sediment fractions are particle size and density. Surface chemistry and magnetic

properties may play a role in isolated cases, but the majority of separations are made on the basis

of size and density. Additionally, with the exception of screening processes, most separations are

not purely size or density separations. While one parameter may predominate, some density effects

will be felt in a size separator, and vice versa. There are five principal components of physical

separation treatment trains: prescreening, primary physical size separation, density separation,

solid/liquid separation, and dewatering.

"Prescreening refers to the removal, or reduction in size, of oversize materials from the bulk

sediment that would interfere with downstream processing operations. Oversize materials are

roughly 50 mm in size or larger." Oversized dredged material may consist of stones, tree limbs,

and soil clumps, but may also include such unwieldy objects as cable, refrigerators, tires, or car

bodies. As a result, the prescreening component may present some difficult challenges for dredged

material processing operations. "Prescreening equipment may involve one or more of the following:

feed hoppers, fixed bar screens (grizzlies), rotating trommel screens, comminutors, attritioners, log

washers and hand picking."

Size separation is the central unit operation of the physical separation treatment train. "Because

many contaminants associate chiefly with the finer soil fractions, separation of sand size particles

(>75 m) from silts and clays (<75 m) is typically the foundation on which the remainder of the

soil washing treatment train is established and refined. Size separation equipment may include one

or more of the following processes: screens (fixed or vibrating, wet or dry), hydrocyclones and sieve

bends."

Density separation is useful when there are significant amounts of either low-density organic

material or high-density metal fragments. Most mineral components of sediments do not differ

enough for density separations to be valuable, but sediments and dredged material may well contain

various amounts of organic material that can be successfully separated from the mineral fraction.

Because anthropogenic contaminants have a high affinity for organic material, removal of organic

material may be necessary to produce a clean fraction, even within the coarsest materials. "A density

separation circuit might include: spiral concentrators, mineral jigs, multi-gravity separators, dense

media, shaking tables or a pinched sluice. Spiral concentrators and jigs are most commonly utilized

in remediation."

With the possible exception of prescreening for oversized material, physical separation processes

require that the material be slurried with water for processing. The high volume of water introduced

is one of the chief disadvantages of physical separation as a management strategy. "Gross separation

of the solids and water is achieved with solid/liquid separation processes, typically utilizing

clarifiers, sedimentation basins, lamella clarifiers or flotation cells."

Dewatering is essentially a second-stage solid/liquid separation process, necessary to produce a

material dry enough to handle. "Solids concentrations of 45% to 80% are possible, depending upon

the size of the material and the dewatering processes used. Fine materials are the most difficult to

dewater and typically represent a significant portion of overall processing costs. A dewatering