CA2091079A1 - Method and apparatus for washing soil - Google Patents

Abstract

Abstract of the Disclosure A soil washing apparatus utilizing a water-based surfactant to separate contaminants from soil particles. After removing the surfactant/contaminant/water mixture from the soil particles, the soil is returned to the remediation site, while the mixture is processed through an oil/water separator. Oil and contaminants are then disposed of, while the water and surfactant are recycled back into the apparatus for use with more contaminated soil. A method of soil washing is also provided.

Description

2 0 9 ~ ~ 7 9 5428/o7235 METHOD AND APPARATUS FOR WASHING SOIL

Field of the Invention This invention relates generally to devices for pollution remediation. More specifically, this invention relates to a device for extracting contaminants from soil.

~ackground of the Invention As government, industry, and society in general have become more aware of pollution and its effect on the environment, the vas~ extent of existing contamination has also become fully realized. Responding to the public interest, government has imposed restrictions on industry to reduce and prevent further pollution and to mandate the clean-up of past contamination and return the local environment to its previous natural state.
Of course, the incidents involving chemical industries that produce toxic waste are well documented. Less known, but affecting a possibly broader segment of the population is the contamination produced by the petroleum industry and its products. Tanker spills on the open seas can be enormous but cleaning these spills often may only require oil booms and water skimmers to reclaim some of the oil, while the remainder simply evaporates. The oil spills and leaks on land, however, present more difficult obstacles to cleaning.
This type of contamination can be caused by leaking or incorrectly connected pipes, leaking fuel tanks, and improperly used fuel dispensers to name a few. When these leaks are either above or below ground, the petrochemicals, i.e., volatile and 2~ 79 semi-volatile organic compounds, become absorbed into the soil and water table, making their removal difficult. Often, these areas of contamination are not discovered until they have become a large problem affecting many cubic yards of soil. From leaking underground tanks at gasoline stations alone, these contaminated areas are counted by the hundred thousands, throughout the nation. And in each case, without prompt action, the contaminants can reach local water supplies or agricultural areas.
Removing contaminants from underground water supplies can be accomplished through many known methods and devices.
Removing the same contaminants from soil, however, has to date been a crude process achieving limited success. Current state--of-the-art devices and methods make use of alcohol- or petroleum based solvents in connection with settling tanks to dissolve the contaminants from the larger particles, such as those larger than 10 microns in diameter and returning those cleaned particles to the previously contaminated site.
Smaller particles remain in the slurry with the solvents and the contaminants. This slurry is then containerized and shipped to a waste disposal site. Except for the now-cleaned large soil particles, the solvents, contaminants and smaller particles become contaminated waste for another, admittedly more controlled, site. To clean more soil, it is necessary to begin the process again with new solvents. In essence, these are batch processes in that a load of soil is treated with the device through to completion. Then, another load can be installed into the device for treatment.
Thus, in view of the above-mentioned deficiencies in the art, an object of the invention is to provide a device and method for removing contaminants from a wide range of particle sizes for return to the previously contaminated site.
It is another object of the invention to provide a continuous device and method for soil washing that allows continuous addition of contaminated soil.

, . . .

2 ~ 7 ~

It is yet another object of the invention to provide a closed-loop system that reuses the contamination removal agent.
It is a further object of the invention to provide a device and method for soil washing that does not add to the pollution waste stream.
It is a still further object to provide a device and method that is simple and cost-efficient to manufacture and operate.

Summary of the Invention In accordance with the above objects, a soil washing apparatus is provided utilizing a water-based surfactant to separate contaminants from soil particles. After removing the surfactant/contaminant/water mixture from the soil particles, the soil is returned to the remediation site, while the mixture is processed through an oil/water separator. Oil and contaminants are then disposed of, while the water and surfactant are recycled back into the apparatus for use with more contaminated soil. A
method of soil washing is also provided.
Brief Description of the Drawings These and other objects, advantages and embodiments will become apparent to those skilled in the art upon a reading of the detailed description of the preferred embodiments in conjunction with a review of the appended drawings, in which:
Fig. 1 is a schematic flow diagram of a soil washing apparatus according the invention;
Fig. 2 is a longitudinal cross-sectional view of a skimming tank;
Fig. 3 is a transverse cross-sectional view of a skimming tank;
Fig. 4 is a side cross-sectional view of a soil shaker;
Fig. 5 is a side view of a desilter.

209~7~

Detailed Description of the Prefqrred Embodiments Referring now to Fig. 1, a schematic diagram of a preferred soil washing apparatus 10 i9 shown. The general components, all of which will be described more fully below, are a crusher 12, a skimming tank 14, soil shakers 16, desilter units 18, pumps 20,22, and an oil water separator 24. The process begins by collecting contaminated soil 26 from the slte 28.
Various contaminants can be removed using the apparatus or method of the invention, although it is preferred that the soil 26 is contaminated with volatile and semi-volatile organic compounds, such as petroleum products. As will be seen below, an end result of the apparatus 10 is the replacement of a large majority of the same soil 26 to the same site 28, but without the contamination.
For ease of description, the specific components of the apparatus will be described in the general order they would be encountered by contaminated soil 26 travelling through the device, as follows:
Soil 26 is collecte~ and introduced into the apparatus.
The soil preferably first enters a crusher 12 at A. The crusher 12 can be any known device that mechanically applies compressive and/or shearing pressure to break the soil 26 into a small granular or powder size form. This operation can be particularly important in areas that have high clay or silt concentrations.
Clay and silt tend to retain contaminants and to form and remain in clumps, with themselves or other soil components, providing little surface area for the action of any treating agents. By breaking up the soil, the surface area is increased exponentially.
The crushed soil is then transported, such as by a conveyor belt (not shown), and deposited at B into the first chamber 3Oa of the skimming tank 14. Preferably before any soil is introduced into the apparatus, the skimming tank 14 is partially filled with a combination of water and a water-based surfactant, such as Fluid D-10, a proprietary non-ionic synthetic fatty acid surfactant available from Coastal Chemical Co. in 209~79 houisiana~ The exact surfactant wlthin that category and the proportion relative to the w~ter content ls determined based on the composicion of the 90il. Other water-ba~ed surfactants will work similarly.
As the soil 26 enters the first chamber 30a, it iY
~uickly stirred into the water/surfactant mixture at C by the action of flow from an inlet pipe 31 that form~ a jet near the bottom of the tank 14, with the pressure being produced by the first pump 20. As seen from Fig. 3, the sidewalls 34 of the tank 14 are sloped with the pipe 31 entering the tank preferably along one wall 34 and directed towards the opposite sloped wall 34.
This creates swirling motion of the fluid 36 within the chamber 30a, as indicated by the arrow D. The fluid 36 is thus sheared by the turbulent flow within the chamber 3Oa, increasing the interaction between the surfactant and the soil. The sidewalls 34 of the tank are also preferably sloped to form a v-bottom 38, which helps prevent soil particles from resting in the bottom.
The surfactant has been shown to act on the contaminants in two ways. First, the surfactant emulsifies the contaminants. Second, it encapsulates the small contaminant particles, aiding them in floating to the top layer of the fluid, ; which the contaminants are already predisposed to do, being less dense than water. Thus, one result of the mixing action within the first chamber 30a is the separation from the soil particles of some contaminants, which tend to float to the top 40 of the fluid 36.
The soil particles 42 within the fluid 36 have two routes for leaving the first chamber 30a. Larger particles, i.e., those having a diameter larger than about 10 microns will tend to settle near the bottom 38 of the tank 14 in spite of the churning action of the inlet pipe 31. Obviously, the larger the particles 42, the lower it will likely be found in the tank 14.
These larger particles 42 and some smaller ones caught in the fluid 36 near the bottom 38 are drawn, with fluid 36, out of the tank 14 at the exit pipe 32 at E which is opposite to the inlet 2~9~79 pipe 31. The fluid 36 and particles 42 are pulled through the first pump 20, which is a modified impeller pump. There are more blades on the impeller than in a conventional unit in order to move the solids through the pump 20 as well as the fluid 36.
This also provides further justification for the crusher 12, as the particles 42 must all be small enough to pass through the first pump 20. The fluid 36 and suspended solids 42 are then deposited at F onto the top surface of a series of soil shakers 16a-16e, described more fully below. The fluid 36 is also split off at G and fed to the inlet ports 32 to produce the motion D.
The other route for soil particle~ to leave the first chamber 30a is by being suspended in the fluid 36 and carried by the fluid flow over the first skimming wall 44 into the second chamber 30b, of the tank 14 at H. The height of the skimming wall 44 is preferably adjusted in conjunction with the flow rate of the pumps 20,22 to cause a certain depth of fluid to be continuously skimmed off the top of fluid in the first chamber 30a. This skimmed fluid will contain a majority of the contaminants from the fluid in the first chamber 30a due to the emulsifying and floating action of the surfactant.
The several shakers 16a-16e preferably all operate in a similar manner, except for the porosity of their respective screens 46. Each shaker, as shown in Fig. 4, has a rear chamber 48 into which a pipe 49 discharges fluid from the first pump 20.
The fluid fills the chamber 48 and spills over the forward edge 50 of the chamber onto the sifting screen 46. The screen 46, which is mounted in the bottom of the shaker 16 at an angle, allows the passage of all materials having a particle size smaller than the size of the holes 52 in the screen.
As is known in the industry, the entire shaker 16 is shaken with reciprocating linear movement along a horizontal axis, as shown by arrow I. This movement is created by rotating eccentrically mounted weights (not shown) with motors mounted to the shakers 16, as is known in the industry. The movement is designed to cause material resting on the screen 46 to slide 2~9~ ~7~

forward slightly with each cycle of motion perhaps falling through one of the holes 52 lf the particle 19 small enough. The size of the holes 52 is exaggerated for illustrative purposes.
Since the screen 46 is sloped upward in the forward direction, soil particles 42 will move against gravity and thus, more slowly up the screen 46, giving them more of a chance to fall through the holes 52. Eventually, if the particles 42 do not fall through the screen 46, they will be ejected off of the forward end 54 of the screen 46 into a collection bin (at J, Fig. 1) for immediate return to the site 28 or, if necessary, perform analytical tests on the 90il before its return.
All of the fluid and the smaller particles that fall through the shaker screen 46 will fall back into the skimming tank 14, but into a later chamber than the first chamber 30a. As can be seen in Fig. 1, the shakers 16a-16e are preferably positioned directly over the chambers 30b-30f, although the forward edge 54 of the screen 46 is preferably positioned past the outer edge of the tank 14 such that particles 42 ejected from the front and 54 of the screen will not fall back into the tank 14, but can be returned to the site 28. Of course, the number of chambers 30 and shakers 16 may be varied depending on soil composition and the level of contamination.
The subsequent chambers 30b-30f after the first are generally similar, except the last chamber 30g, which is described more fully below. All of the subsequent chambers 30b -30g have input pipes 32 to cause the swirling effect within the chambers 30. All of the chambers 30b - 30f except the last one 30g have exit ports and pipes that lead through the first pump 20 to the soil shakers 16a - 16e.
From the last chamber 30g, the exit port and pipe leads through a second pump 22 (at K), which is preferably a standard fluid pump, since the fluid is free, by this point, of larger particles 42. The pump 22 forces the fluid from this chamber 30g to the inlet ports of the last three chambers 30e-30g (at L), but also passes the fluid through desilter units 18a,18b mounted over -- 209:~7~

the last two shakers 16d,16e (at M).
By the time the fluid has reached the end chamber 30g, it preferably contains only tho~e 90il particles 42 that are smaller than 6 microns. Particles of this size are essentially in suspension in the fluid and become difficult to separate through mere sifting. To first remove the particles 42 from suspension, it is preferred to pass the fluid from pump 22 through the desilter units 18 which are known in the industry as "hydrocyclones." As seen in Fig. 5, these units include cones 58, at the top of which are fluid inlets 57 that receive fluid from pump 22 and direct it into the top of cones 58 along the inner sidewall of the cone 58. This sets up a high-speed swirling effect in the cones, as shown at Z. The centrifugal force generated by this swirling causes the particles 42 to separate out of the fluid and fall through the bottom of the cones 58 and onto the last two shakers 16d,16e. These shakers have the finest screens with holes 52 preferably having a 2 micron diameter.
Simultaneously, the same force causes the clean water to spiral upward along the inner slope of the cone 58 and exit through the tubes 56. The tubes 56 empty back into the fifth and sixth chamber 30e,30f of the tank 14.
Through the actions of the progressive skimming in the chambers 30, in combination with the surfactant in the tank 14, the soil shakers 16, and the desilting units 18, the fluid 36g in the final chamber 30g of the tank 14 is substantially free of any soil particles 42 having a diameter larger than 2 microns. Thus, this fluid 36g is chiefly composed of water, the surfactant and the contaminants that were separated from the soil 26. Despite the churning of the fluid 36g, the surfactant and contaminant~
will tend to float to the surface.
A floating skimmer unit 60 within the last chamber 30g of the tank 14 removes the fluid 36g from the top layer of the chamber 30g. This skimmed fluid is passed to any known oil/water separator 24 at N, such as the Verisep system, available from Monosep Corporation, Lafayette, Louisiana. In the separator 24, , 20~ln7s the oil and other organic contaminants are separated from the water and the water-based surfactant. The oil is then either containeriæed and shipped to a disposal site at 0 or, depending on the nature of the contaminants, processed through a scavenger that can reclaim the petroleum products for later refining. In either case, only the contaminants must be dispo~ed of. Soil particles less than 2 microns in diameter remain suspended in the water and are also disposed of, but this only accounts for a minute fraction of the total soil processed. The remainder of the soil 26, which i5 now free of contamination, is returned to the site 28 from which it was removed. ~ecause the device 10 may be installed at the contaminated site 28, minimal excavation and transportation costs are incurred for carrying the soil 26 to the device or returning the clean soil 26 to the site 28.
The water and surfactant that has been separated from the fluid 30g from the last chamber 30g is then piped back to the beginning of the process at P, preferably to the first chamber 30a of the skimming tank 14, completing the closed loop of the system. Essentially, the only materials that are ejected by the device are clean soil particles and contaminants, which are the only materials introduced to the device while it i9 running. The water and surfactant travels through the machine continuously.
While it is possible that trace amounts of the surfactant may remain on the cleaned soil 26, the preferred surfactant meets all government standards for environmental release and may be returned to the site 28 with the soil 26.
Since the surfactant contains fatty acid, the biodegradation process is enhanced. The removal of trace amounts of surfactant from the system, over time, will cause the concentration of the surfactant in the skimming tank to decrease slightly. Thus, it is preferable to monitor the concentration and adjust it accordingly. The monitoring may be done by manual samples, or preferably with an optical detection system (not shown) that automatically monitors the light transmissivity of the water/surfactant solution, as is known.

~ 20~7~

The amount of time a partlcular 90il sample spends in the device or in any particular sectlon of the device can preferably be adjusted through a number of valves 70, positioned along the various pipes of the device. For example, if valve 70a were closed, soil particles 42 would tend to remain in or be processed through the second and fourth chamber 3Ob-3Od for a longer period, since the only route for entering the last three chambers 30e-30g would be over the skimming wall 44 between the fourth and fifth chambers 30d,30e. Other valve positions and configurations are contemplated.
It is contemplated by the invention that other contaminants besides organic compounds could be separated from the soil. To remove some metals, the pH of the water may be altered and a conventional stripping tower added to the device, preferably at O in Fig. 1.
While the embodiments shown are fully capable of achieving the objects of the invention, it is to be understood that these embodiments are shown for the purpose of illustration and not for limitation.

, ::
' ~
.

..

Claims (12)

1. A method of washing soil having incorporated contaminants, comprising the steps of:
mixing said soil with water and a water-based surfactant to form a slurry;
sifting said slurry to remove soil particles substantially not containing contaminants therefrom;
removing a top layer of said slurry, said layer including a portion of said surfactant and a portion of said contaminants;
separating said layer into first and second components, said first component including said contaminants, said second component including said water and said surfactant.

2. A method as in claim 1, further comprising the step of crushing said soil.

3. A method as in claim 1, further comprising the step of desilting said slurry to remove soil particles therefrom.

4. A method as in claim 3 wherein said step of desilting includes the removal of said particles having a diameter greater than 2 microns.

5. A method as in claim 1 further comprising the step of mixing said separated water and surfactant with a further amount of soil.

6. An apparatus for washing soil having incorporated contaminants, said soil being mixed with water to form a slurry, comprising:
a tank for holding said slurry having first and second chambers with a skimming wall therebetween;
a water-based surfactant mixed into said slurry;
means for sifting said slurry to remove soil particles from said slurry, said means for sifting positioned above said tank such that said particles will be deposited remote from said tank, while the remaining portion of said slurry will be deposited into said tank;
a skimmer within said second chamber for removing a top layer of said slurry from said tank, said layer being substantially devoid of said particles; and an oil/water separator having an inlet connected to said skimmer and having two output flows, said first output flow including said contaminants, said second output flow including said water and said surfactant.

7. An apparatus as in claim 6, wherein said second outlet is directed to said tank.

8. An apparatus as in claim 6, further comprising a pump for drawing said slurry from said tank and directing it to said means for sifting.

9. An apparatus as in claim 8 wherein said pump further directs said slurry back into said tank for agitating said slurry.

10. An apparatus as in claim 6 further comprising means for desilting said slurry to remove soil particles therefrom.

11. An apparatus as in claim 10 wherein said means for desilting is comprised of a hydrocyclone and is capable of removing particles having a diameter greater than approximately 2 microns in diameter.

12. An apparatus as in claim 6 further comprising means for crushing said soil before said soil is mixed with water to form said slurry.