Simplified soil washing processes for a variety of soils
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.Journal of Hazardous Materials 66 1999 3145
Simplified soil washing processes for a variety ofsoils
M.I. Kuhlman a,), T.M. Greenfield ba MK Solutions, 12843 Coey Lane, Houston, TX 77099, USA
b Greenfield Industries, 25306 Mill Pond Lane, Houston, TX 77373, USA
A soil washing process is described in which high shear mixing, sprays, hydrosizing, flotation,and screening are integrated to countercurrently clean sand, silt and clay. Chemical techniques aresummarized that can be used to remove selected contaminants from the soil and then clean thewash water for reuse. Descriptions of simple and complex processes, and a recent project are used
.to illustrate the points: 1 Soil washing is not a single process, but a collection of unit operations .assembled for each project; 2 all process must be coordinated for the project to be successful;
.and 3 combining techniques in a few pieces of equipment debottlenecks soil washing. q 1999Elsevier Science B.V. All rights reserved.
Keywords: Soil washing; Processes; Simplified
1. The soil washing process
w xSoil washing processes are described in numerous papers and a recent book 1 . Thew xevolution and use of this companys process is described in four papers 25 . The
.purposes of the present paper are to expand on these earlier publications by 1promoting the concept that soil washing is not one process, but a combination of
.techniques that clean the soil and 2 providing examples of treatments for a limitednumber of problems. Fig. 1 illustrates modules in soil washing process. Discrete groupsof equipment include the following.
.1 The soil preparation module for removal and cleaning of oversize material anddebris.
.2 A soil washing module to remove the contaminants. The washing module can besubdivided further into equipment for washing sand and fines generally -200-mesh,
) Corresponding author. Tel.: q1-281-564-8851; fax: q1-281-933-7441; e-mail: firstname.lastname@example.org
0304-3894r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .PII: S0304-3894 98 00212-X
( )M.I. Kuhlman, T.M. GreenfieldrJournal of Hazardous Materials 66 1999 314532
Fig. 1. Flow chart for generic soil washing process.
. w x74-mms . This division has developed 1 because lower surface area, larger sand grainsare easier to process than the fines fraction, which has one to three orders of magnitudelarger surface area than the )200 mesh sand. If the fines are contaminated, they can bemore difficult to clean.
.3 The wastewater treatment module in which the dispersed soil and contaminant areremoved from the water.
.4 A residual management module. Soil washing either removes the contaminantfrom the soil or concentrates contaminants in the fines. Thus, the contaminated materialmust either be recycled, desorbed, bioremediated or stabilized for landfilling.
.5 A volatile emissions control module. .6 The wash water storage and management module. Some processes recycle more
than a million gallons of water per day. The water can be heated and fresh makeup watermust be added, since the washed soil contains more water than the feed soil.
The overall soil washing process can be simple or sophisticated, but it must be wellthought out in order to combine high productivity with low processing and capital orequipment rental costs.
1.1. The soil washing module
The soil washing described here begins by first breaking up the soil in an attritionscrubber and separating the sand in sandscrews that combine countercurrent spraywashing of the soil with hydrosizing and flotation to remove fines and contaminants
( )M.I. Kuhlman, T.M. GreenfieldrJournal of Hazardous Materials 66 1999 3145 33
from a soil slurry. This minimizes the volume of water that must be managed. Then,countercurrent hydrosizing and flotation are used to clean the 20- to 70-mm fraction.Finally, countercurrent washing can sometimes be used to extract metals or lighthydrocarbons from the sub-20-mm clay fraction.
A drawing of such a process is presented in Fig. 2. This process differs from thoseoffered by many companies in that the equipment is modified sand washing equipmentrather than equipment designed for the mining industry. Because of its origins, thisprocess is very mobile, uses much less water and power, is more adaptable to differentconditions, and is more tolerant of debris found in surface soil than mining equipmentdesigned to process crushed ore. While mining equipment does an excellent job in itsindustry, each component is focused towards a specific task which it faithfully executesat fixed sites for years. However, soil washing processes are mobile and operate for twoto three months at a time with limited support facilities. Thus, the ideal soil washingprocess is one which minimizes capital, setup time, training of crews, and the number ofunit operations.
1.1.1. Sand washingThe practice of using sandscrews for removal of undesirable materials was developed
in the sandrgravel industry and in grinding loops in the mining industry. Thesesandscrews separate lighter material from a coarser or denser fraction that has a highersettling velocity. In this process, the soil matrix is initially torn apart in an attritionscrubber, reducing the viscosity of the slurry, so that the sand settles quickly.
Fig. 2. Generic TVIES soil washing process.
( )M.I. Kuhlman, T.M. GreenfieldrJournal of Hazardous Materials 66 1999 314534
Fig. 3. Hindered settling velocity for sand grains and creosote particles.
w xFig. 3 summarizes the hindered settling velocity calculated for 20% slurries 6 ofsand, silt and dried creosote. The figure shows that the settling velocity of 75-mm 200
. . .mesh sand point A is 0.79 ftrmin fpm , while the settling velocity of 75-mm .creosote particles point B is 0.042 fpm. The settling velocity of a 75-mm sand grain is
.the same as that of a 295-mm creosote particle point C . This means that equal sizecreosote and sand grains can be segregated by gravity in a pool of moving or risingwater. The lighter particle will be carried away by water while the heavier particle
.remains. Air bubbles increase the volume of larger creosote particles point C1 thatwould settle without air at the same rate as smaller sand grains, thereby facilitating theirremoval.
Thus, a sandscrew that includes a hydrosizingrflotation pool can separate )200-mesh sand from fine sand, silt, clay and creosote particles or contaminated woody debrisas long as enough water is used to maintain a dilute slurry. The water in the equipment
.described here for soil washing comes from three sources: 1 dilution water mixed with .the soil in an attrition scrubber preceding the sandscrew, 2 hydrosizing water added to
.the pool of the sandscrew, and 3 wash water from spray jets that countercurrently flushcontaminants from the soil. Combing high shear mixing, hydrosizing, flotation andsprays removes 80 to 90% of the contamination from the sand in a single step.
1.1.2. ScreeningSelective use of screens to remove trash or floatable debris is also important. For
instance, fine mesh screens in Fig. 2 are used to remove floating debris between thesandscrew and hydrosizer. In addition, profile wire screens are useful to remove 30- to
( )M.I. Kuhlman, T.M. GreenfieldrJournal of Hazardous Materials 66 1999 3145 35
50-mm debris from the overflow of the hydrosizerrflotation cells before it can beconcentrated in the fines. The most important design criteria for screens is that they bevery durable, since the cost of replacing screens can be one of the largest costs in aproject.1.1.3. Silt washing
Silt, clay, and debris flowing over the weirs of sandscrews are pumped to hydrosizingand flotation equipment after most of the floatable debris is removed with a 50- to150-mesh screen. Countercurrent extraction in settling tanks and hydrosizers are well-
w xknown techniques 7,8 . This process includes several flotationrhydrosizing cells inseries. As Fig. 3 shows, flotation and hydrosizing are complementary processes since a
.75-mm creosote particle that is attached to an air bubble point B1 can be separated .from 20-mm grains of silt point D in rising water.
1.1.4. Clay washingSoil particles finer than 20 mms can also be washed, but only if the contaminant can
be solubilized in the water, a suitable extractant, or separated by flotation. Metals likew xlead, mercury, cadmium, zinc or uranium are extractable 24,9 . Organic chemicals
.with high water solubilities i.e. benzene or those that partition into a surfactant micellecan also be extracted from clay. Clay wash equipment has been built that combines pulpremixing, flotation, and clarifier sections into a single apparatus. Several of these claytanks are used in series to wash the clay. The extracted pulp which settles in one claytank is countercurrently washed by being pumped to the next clay tank for remixing,extraction or flotation, and clarification.
1.1.5. Specialized equipmentProperties of contaminants such as density, magnetic susceptibility, and surface
chemical properties are valuable in remediation processes. Density, for instance, isuseful for removing contaminants like mercury, lead and the actinides. The equipmentused for this purpose are jigs or gravity tables and spirals. These are described in detail
w xelsewhere 8 . Jigs and tables work because dense particles, like bullet fragments andbattery chips, separate better from sand when rythmatically shaken in water than whenhydrosizing alone is used. Spirals work because centrifugal force separates lighterparticles from the denser contaminant.
Magnetic segregation can remove metals with appreciable magn