soil washing enhancement with solid sorbents

Soil Washing Enhancement with Solid Sorbents Y. M. El-ShOubav

Merck & Co., Manufacturing Division, Somerset, NJ 08889

and

D. E. Woodmansee GE Corporate Research & Development, Schenectady, NY 12309

Soil washing is a dynamic, physical process that reme diates contaminated soil through two mechanisms: particle size separation and transfer of the contaminant into the (mostly) liquid stream. I;heper$ormance of different sorbents and additives to remove motor oil from sea sand was tested. Hydrocyclone, attrition scrubber, and froth flotation equipment were used for the decontamination study. Sorbents and additives were mixed with soils in the attrition scrubber prior to flotation. Sor- bents used were granular activated carbon, powder activated carbon, and rubber tires. Chemical additives used were calcium hydroxide, sodium carbonate, Al- conox@, Triton@ X-100 and Triton@ X-114. When a froth flotation run was per$ormed using no additive, washed soils "tails' 'contained 4000ppm of total oil and grease (TOG). However, when carbon or rubber (6% by weight) was added to the contaminated soils the washed soils "tails" contained less than 1000 ppm of total oil and grease (TOG). m e addition of sodium carbonate or calcium hydroxide (6% by weight) had same effects as sorbents. In both cases washed soil "tails" contained total oil and grease of less than 1000 ppm. I;he use of these non-hazardous additives or sorbent can enhance the soil washing process and consequently saves on time (residence time in equipmend and number of stages (equipment design) required to achieve the tatget clean up levels.

INTRODUCTION

Soil washing is a promising ex situ decontamination treatment method. It is a physical process in which excavated soils undergo particle size separation. The separation of fine particles, usually below 35 microns, eliminates a large portion of the contaminant. This can be easily achieved using a hydrocyclone. In addition the large particles (above 4 mm) are directly removed using screening techniques since they contain very small concentrations of the contaminant. The other soil fractions undergo intimate contact with wash water in an attrition scrubber. This pro- motes contaminant transport from the soil phase to the liquid phase. This soil/water slurry is the feed for the flotation unit where the contaminant is floated and skimmed. The contaminant collects in the froth while the clean soil

collects as tails. Figure 1 gives a schematic diagram of the soil washing process.

The soil washing process, in and of itself, does not de- stroy or immobilize the contaminants, so it cannot be considered a complete remediation process. The principal value in a standard washing process with water is the pro- duction of a clean stream of sand from which the contaminant has been removed.

The soil washing decontamination performance could be enhanced by two methods. The two methods are mechanical and chemical methods. The mechanical method approach involves the use of several flotation stages or the use of longer flotation residence time. This procedure does not guarantee meeting the required clean up standards. Some contaminants remain trapped in the soil pores and are not liberated regardless of residence time or the number of unit operations. The chemical method approach involves the addition of a chemical additive or a physical sorbent material. The sorbent and/or the additive is added to the contaminated soil in the attrition scrubber stage prior to flotation. The mechanical mixing motion, in the pres-

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FIGURE 1 Schematic Diagram of the Soil Washing Process

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FIGURE 2 Schematic Diagram of the Equipment Used in This Study

Environmentd! Progress (Vol. 15, NO. 3) Fall, 1996 173

ence of the sorbent additive, accelerates and increases the contaminant release from the soil. The additive displaces or adsorbs the contaminant from the soil pores and pro- duces a clean soil. This work was performed to test the enhancement of the soil washing process by adding safe, non hazardous additives or sorbents, rather than the use of ex- tra unit operations (energy).

EQUIPMENT

Familiar unit operations in soil washing involve coarse soil screening, hydrocyclone separation, attrition washing, and flotation as shown schematically in Figure 2.

Screening

Separations of the coarse particle sizes are accomplished by screening, with a water rinsing action to promote removal of fine materials (usually contaminated) from the coarse material (usually uncontaminated). The screen used in this study was a 30 mesh screen supplied by W. S. Tyler Co. of Cleveland, Ohio. The oversize (1 mm diameter) from the screening processes was shown to be sufficiently clean that they can be set aside and not proceed further through the washing process. Only in very rare cases the coarse materials might be porous and contain considerable inter- nal contamination. In that case, the coarse material needs to be crushed and processed through the entire process. In other cases where the coarse material indeed contains a lot of contamination on their surface, the usual procedure would be to wash them in a rotary gravel washer. The tumbling action in such a washer causes the gravel surfaces to be abraded, thereby removing much of the contamination.

Hydrocyclone Separation

The hydrocyclone operates similarly to a gaseous cyclone, providing a centrifugal force to sling large and dense particles to the outside wall of the cyclone where they leave as underflow from the spigot at the conical cyclone base, while the fine materials stay in the center of the cyclone and rise through the vortex finder to discharge through the overflow. The motive force for the separation comes from the pressure drop across the hydroclone which is provided by a pump.

Each run, performed in this study, started by mixing commercially available sea sand with water (90% water-10% sea sand-by weight) and run in a 2" hydroclone. Braun et. a1 [ I ] and Savaovsky, et al. [ 21 showed that an increase in the feed concentration usually leads to a coarse cut size, reduced sharpness of separation, and raised the pressure drop. In addition, an extensive increase in the soil concentration causes spigot plugging and shutoff time for mainte- nance. Although there are several models to design [ 3, 41 Scale up [5, 61, and to predict [G, 71 the hydroclone performance, a commercially available unit was used. A one and two inch diameter hydroclones, with several spigot/vortex combinations, were purchased from Carpco Inc., Jacksonville, FL.

Attrition Wasbing

After over-screening coarse material and hydrocloning out the ultra fine materials, the remnant fraction is mainly

sand. These particles have sufficient momentum in solu- tion that a vigorous agitation of a sandy slurry causes them to abrade each other's surfaces. This phenomenon is accel- erated in an attrition washer in which an approximately 50% solids slurry is vigorously agitated. This is driven by plac- ing two impellers on a rotating shaft with opposite pitches so that the flows impact on each other. This action pro- vides excellent mixing and surface abrasion, giving rise to the name attrition washing. At the conclusion of such washing, the hydrocarbon materials have largely been scrubbed from the sand and form emulsions in the wash water. To two cell counter current propeller type scrubber obtained from Denver Equipment, Colorado Springs, Col- orado was used in this study.

Flotation Separation

Two types of flotation techniques exist in literature, column flotation [ 8, 91 and froth flotation. Mahne [ 101 compared the results of both units and concluded that the performance of both units was comparable. Most available flotation books deal with mineral flotation [ G, 11, 12, 131 rather than soil decontamination. However, the limited study on soil contamination using fine elimination [ 14, 151 agreed that major contamination is concentrated in the silt, humus and clay. The elimination of these materials by hydroclone and froth flotation yields cleaner soils. In order to recover the contaminant, silt, humus and clay to provide a clean sand tailing, the washed slurry is then directed to a mechanical air flotation system (4 cell flotation device) model #5-dr float. The unit was supplied by Denver Equipment, Colorado Springs, Colorado. In this operation, a stream of air released near the bottom of a tank of washed slurry is broken into fine bubbles by rotating mechanical impellers. Hydrophobic materials (contaminants and ultra fines), which would rather attach to an air/water interface than stay in suspension, attach to the bubbles. The bubbles rise to the surface by their own buoyancy where a froth is created. This froth is sufficiently stable to allow excess liquid to drain from it back into the flotation chamber while the remaining froth films are directed into a launder at the side of the flotation tank. Any hydrophobic material, such as hydrocarbon liquid emulsions, stays with the froth films and is moved into the launder. When the bubbles in the froth break, a concentrate stream of the hydrophobic materials is created. This stream is highly contaminated and re- quires contaminant destruction.

MATERIALS

Feed

The feed for the flotation runs was commercially avail-

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FIGURE 3 Screen Analysis Obtained for Sea Sand and Ground Soil Used in this Study

174 Fall, 1996 Environmental Progress (Vol. 15, No. 3)

able sea-sand obtained from King Packaging Company of Schenectady, NY. In addition, clean soil obtained from the ground was used. Figure 3 shows the screen analysis obtained for both soil matrices used in this study.

sorbents

Shredded rubber tires were obtained from North Amer- ica Tire Recycling Corp. of Fort Edward, NY. Fine and granular activated carbon was obtained from Calgon Car- bon Corporation, Pittsburgh, PA.

Frothers

Technical grade pine oil and 4-methyl-2-pentanone were obtained from Fisher Scientific, New Jersey.

Additives

The enhancement of soil washing using surfactant addition and the role of surfactant in flotation was discussed in detail by Furstenau et al. [IG]. In addition, the types, the structures, the effects and the chemistry of most available surfactants is given in Rosen’s “Surfactant and Interfacial Phenomena” [ 2 71. No study on the use of adsorbents and chemical additives was found in the literature. Alconox@ surfactant was the only surfactant used in this study. The Alconox@ was obtained from Alconox Inc., New York, NY. The other additives used were: sodium carbonate obtained from Fisher Scientific Co., Fairlawn, NJ. Triton X-114 and X-100 obtained from Rohm and Haas, Philadelphia, PA. Alum obtained from J. T. Baker Inc., Phillipsburg, NJ. Cal- cium hydroxide obtained from Aldrich Chemical Inc., Mil- waukee, WI.

EXPERIMENTAL PROCEDURE

Laboratory Sequences

Each flotation run starred by mixing commercially available sea sand with water (90%/ water-10% sand-by weight). The 2“ hydroclone was operated at 30 psi with vortex/spigot combination of 14 mm/5.4 mm. The overflow (fines) were collected for disposal and the under-flow was dried in an oven for 24 hours at 95°C. The amount of fines were below 2% (by weight) in all cases. The dried sand (under-flow) was thoroughly mixed with 10,000 ppm of motor oil. The contaminated soil was then mixed with water (1:l by weight) and fed to the attrition scrubber. The feed residence time was kept at 15 min. in the attrition scrubber followed by 20 min. in the flotation cell. This procedure was kept exactly the same for all runs. The variabil- ity between runs was in the type of frother, additive, adsorbent and collectors used.

Analytkal Method

EPA test method 413.1 (total oil and grease-TOG) gravimetric was used to determine the motor oil concentration in the soil. This method is a soxhlet extraction method with solvent to measure the total oil and grease in any sample. The extraction was performed using methylene chloride rather than freon. Both methylene chloride and freon were tested and same extraction effectiveness was observed. The methylene chloride extract is dried in a ro-

FIGURE 4 Motor Oil Concentration in Soil Tails Treated With Carbon

tary evaporator and the weight of the extracted materials is determined with respect to the weight of the soil extracted sample.

RESULTS AND DISCUSSION

Thirty-four runs were performed using the hydroclone, attrition scrubber and the flotation equipment. All runs were performed using sand or soils that were contaminated in the laboratory rather than environmentally aged samples. This action eliminated any inconsistency in the particle size distribution or the characteristics of the starting feed, but left open the issue of the role of environmental aging on cleanability results.

N o Additive-Base Line Run

One run was performed using only clean tap water, i.e., no additive or/and sorbent was used. Pine oil was used as a frother (less than 2 cc). The tails produced in this run were analyzed for TOG using EPA test method 413.1 gravimetric. The TOG concentration in the treated tails was found to be 3773 parts per million (ppm). This result was considered a base line value and any improvement in the process should yield a better final clean up result. The 3773 ppm is considered high in most states. Our experience in the field showed that for concentration above 1000 ppm, remedial action is required. The use of water does not produce clean tails which meet most of the state regulations.

Carbon as an Auitive

Three runs were performed using three different mesh sizes of carbon (large, medium and fine carbon). The carbon weight was kept at 6% with respect to the dry feed soil weight. These runs demonstrated the effect of carbon mesh size (surface area) on the decontamination of motor oil from sandy soils. Figure 4 illustrates the effect of the carbon mesh size on the amount of motor oil left in the cleaned soil “tails.” As the carbon surface area increased the amount of motor oil in the tails decreased. When pine oil was initially

FIGURE 5 Amount of Carbon Found in the Tails After Flotation Separation

Environmental Progress (Vol. 15, No. 3) Fall, 1996 175

FIGURE 6 Effect of Carbon Addition in Reducing Contam- ination of Clayey Soil

used as a frother, no froth was obtained, even when high amounts (55cc) was used, because the pine oil was adsorbed by the carbon. These runs were dropped and 4- methyl-2-pentanone (MIBK) was used instead. This frother was not effectively adsorbed by the carbon and consequently it can form a strong stable froth even at low concentration (1Occ). The froth behavior was very similar to the pine oil.

The carbon separation from the tails using the flotation technique was poor for the large carbon runs. However the carbon separation was excellent when powder activated carbon was used. Figure 5 shows the percent carbon left in the tails with respect to the carbon mesh size. In case of coarse carbon, the separation efficiency could be im- proved by using screen separation techniques.

Carbon Addition to Chyey Soil

Three runs were performed under exactly the same conditions mentioned above. The only difference was that the feed (sea sand) was not treated by the hydroclone. How- ever it was directly dried and mixed with 6% (by weight) Kaolinite clay. The soil matrix was then mixed with 10,000 ppm by weight motor oil. These runs illustrated the effect of fines (clay) on the decontamination process. The motor oil concentration in the tails, using the three different carbon sizes, in comparison with the run performed under the same conditions with no clay are illustrated in Figure 6. The motor oil concentration in the tails was higher than the same runs performed with no clay (see Figure 4). This could be blamed on the porous hydroxyl structure of clay and its ability to adsorb and hold contaminants.

Eff& of Fitze Carbon Concentration

The effect of the amount of carbon in the feed was stud- ied in these runs. The runs were performed using 1%, 3% and 12% powder activated carbon (with respect to the dry soil feed). The soil/carbon mixture was mixed with water (1:l by weight). The slurry was fed directly to the attrition scrubber with no hydrocloning. At the end of the flotation

FIGURE 7 Effect of Fine Carbon Amount in Reducing Contamination on Soil

FIGURE 8 Carbon Percentage Left in the Decontaminated Tails

run, a sample from the tails was collected and analyzed for total carbon content as well as total oil and grease.

Figure 7 shows the effect of the carbon/feed ratio on the soil decontamination process. As the carbon concentration increased the decontamination process efficiency increased, i.e. the tails contained lower amount of motor oil. However, the carbon separation from the tails decreased with the increased carbon percentage (see Figure 8). An increase in the residence time of the sand/carbon mixture in the flotation cell definitely improves the carbon separation efficiency. The reason for not increasing the flotation residence time was for consistency between runs. The soil tails, for all performed runs, were analyzed with no further carbon separation.

Rubber Tires as an Adsorbent

Michelin tire rubber was frozen with liquid nitrogen and crushed to small particle size ( 4 mm pieces). These pieces were mixed with the sea-sand slurry in the attrition scrubber. The slurry was then treated in the flotation cell. N o rubber particles were noticed to be separated by flotation. Accordingly, the tails were screened to recover the mixed tire rubber pieces. After the removal of the rubber, a soil sample was obtained for analysis. The tails contained 855 ppm TOG.

Two additional runs were performed using shredded tire rubbers (1 mm diameter). These runs were performed to investigate the surface area effects and the percent tire rubber on the decontamination process. The first run was performed with 6% shredded rubber and the second run was performed with 12% shredder rubber. The flotation of the rubber particles in the flotation cell was poor and screening was used for the separation. The tails of the 6% rubber particles run contained higher TOG concentration than the 12% rubber particles run (see Figure 9). The efficiency of screening was poor and fine rubber particles were noticed in the screened tails. Since these samples were extracted using methylene chloride, the TOG concentration pre- sented here should be higher than expected. Methylene chloride extracts oils from rubber particles causing the higher readings.

FIGURE 9 Effect of Rubber Particles Addition to the Con- taminated Sea Sand


FIGURE 10 Results Obtained Using Alconox@ as a Collec- tor and Frother

It should he mentioned here that additional analysis of the tails was performed using a modified EPA test method 413.1. The modified method uses silica gel to adsorb oil and

12 Effect of the Addition of Triton@ x-114 to Contaminated Soils

greases from nonpetroleum hydrocarbons origin. The results obtained by using both methods were identical.

Alconox isr as a Frother and Collector

Three runs were performed using Alconox@ (laboratory detergent). The Akonox@ was added to the attrition scrubber prior t o flotation. The Alconox@ amounts ranged from 0.2% t o 1% with respect to the dry soil weight. The contaminant concentration left in the tails was almost con- stant (around 2000 ppm) regardless of the amount of Al- conox@ mixtd with the feed. Figure 10 shows the results. When the modified EPA test method 413.1 was used to an- alyze the tails. the amount of total oil and grease was found to be 755 ppm in all cases. This result indicated that the amount o f surfxtant did not affect the efficiency of separation. In addition, the surfactant's (Alconox@) fatty acid could be the cause of the false high reading of TOG (2000 ppm) originally detected in the tails.

Ca(OH)Z, NaZCO3 and Alum (AIK(S04)Z. 12HZO) as Additives

To study the effects of these nontoxic, waste water treatment additivrs on the decontamination of motor oil from sea sand, each material was added to the feed in the attrition scrubber. The amount of the additive was kept con- stant at 6%) with respect to the dry soil weight in all runs. Both calcium hydroxide and sodium carbonate decontamination runs showed tails with less than the 1000 pprn of TOG (the target number). However, the froth flotation run performed with alum showed tails with TOG concentration of 4383 ppm ;host equal to the base run (3773 ppm). Pine oil was used 3s a frother for all runs. Figure 11 summarizes the results.

The temperature of the slurry prior to the addition of the additive was about 30°C. After the addition of the Calcium hydroxide this temperature rose to 48°C and the pH rose from 7 to 12 (caustic). The same observation was noticed with sodium carbonate where the pH was about 10 and the temperature rose from ambient to 38°C. In case of the Alum

run, the temperature slightly rose to 34°C and the pH went down to 4 (acidic). Although no detailed study was performed on these additives role on the decontamination process, these exothermic reactions may help in the contaminant liberation from the soil surfaces.

Triton" X-114 and X-100 as Additives

Two runs were performed using different amounts of Triton@ X-114 and Triton@ X-100. When 0.07% by weight of Triton@ X - 1 1 4 was added to the feed, the amount of TOG in the tails was 2351 compared to 6990 ppm when 1.5% of Triton@ X - 1 0 0 was used. The froth was very strong in both runs and some sand floated with the froth. When the tails were extracted, the smell of the Triton@ was noticed. Fig- ure 12 summarizes the results. The objective of this study is to float fines and contamination rather than sand and heavy particles. Accordingly, Triton@ was not pursued.

Conclusions:

The frother has the least effect on the flotation performance. It is recommended to use 4-methyl-2-pentanone as a frother whenever carbon adsorbent is to be used. 4- methyl-2 pentanone was not adsorbed by carbon. The carbon addition to the feed was very effective in producing clean tails. The amount of contamination left in the washed tails was a function of the carbon size (surface area). As the surface area increased, a lower contamination was detected in the tails. The carbon would end up in the concentrate. The addition of shredded tire rubber to the soil feed enhanced the removal of TOG. The removal efficiency increased as the rubber surface area increased. The rubber particles have to be treated as waste. The clean up of a soil matrix containing fine particles was less efficient than the same matrix with no fines. Sodium carbonate and calcium hydroxide are efficient additives in the soil washing process. Tails with less than 1000 pprn was produced in both cases and residues of both chemicals would be nonhaz- ardous. Alconox@ detergent amount did not affect the concentration of contaminants in the cleaned soil. Triton X - 1 1 4 as an additive was not only ineffective as a soil wash additive but also produced strong froth that floated too much sand.

RECOMMENDATION

Additional work is needed to study environmentally aged samples contaminated with heavy chlorinated hydrocarbons, specifically PCBs.

FIGURE 11 Effect of Different Additives on the Decontam- ination of Sea Sand

Environmental Progress (Vol. 15, No. 3) Fall. 1996 177

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3. Boysan, F., Ayers, W. H., and Swithenbank, J., “A Fundamental Mathematical Modelling Approach to Cy- clone Design,” Trans. Z Chem. Eng., Vol. 60, 1982.

4. w e , E. O., and Plitt, L. R, “The Cone Force Equa- tion and Hydrocyclone Design,” Material Technology -An Interamerican Approach, Proc. of the Interameri- can Conference on Material Technology, ASME 108- 118, San Antonio, TX (May, 1968).

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soil washing enhancement with solid sorbents

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