29 New Developments in Soil Washing Technology

Th. NeeBe Department of Environmental Process Engineering and Recycling, University of Erlangen-Nilrnberg, Paul-Gordan-StraBe 3, 91052 Erlangen, Germany

29.1 Introduction

In Germany we can now look back on more than a decade of experience in soil washing based on the state of the art of mineral processing techniques (NeeBe and Grohs 1990a; NeeBe and Grohs 1990b; NeeBe and Grohs 1991a; NeeBe and Grohs 1991b; Wilichowski and Werther 1996). In recent years washing technology has changed remarkably. The cost of soil cleaning 10 years ago is now no longer ac­ceptable. As a result of competition with biotechnology, in situ-technologies and natural attenuation, the tonnages sent for soil washing have been reduced dramati­cally. This is a challenge that can be met only by a new generation of soil washing plants, which is the subject of this paper.

29.2 Process Flow Sheet of a Soil Decontamination Plant

Industrial soil washing processes to meet the specific requirements of the con­tamination to be removed can be developed on the basis of laboratory tests (Feil et al. 1994; Feil1997; NeeBe et al. 1997). The process stages of a soil decontamina­tion unit are shown schematically in fig. 29.1. The configuration of individual process stages can vary greatly, depending on the material and the objective of decontamination.

The actual washing process is preceded by a dry soil preparation. This includes storage, feed regulation and pre-crushing of the material. Of decisive importance for the washing effect is the subsequent wet liberation, the aim of which is to disperse the agglomerated fme particles. In this process, soil is suspended in water by the application of mechanical energy. If required, liberation may be performed in two stages, when the sand particles are subjected to a second treatment in the previously thickened suspension. The relevant apparatus are encapsulated and equipped with air exhaust systems to allow highly volatile components to be re­moved. The exhaust air is passed through activated carbon filters and cleaned.

R. Stegmann et al. (eds.), Treatment of Contaminated Soil© Springer-Verlag Berlin Heidelberg 2001

462 Physical Treatment

.. " .2 :a ~

" II u

_.6._ gravel

sand

silt

contaminated soil

, ................................................ ~ ...... . destiming ~ .... :.:.!.~.~.:.?J..~~ .... .

! 10-IOOJ!m

residue

residue

additional modules core modules

Fig. 29.1. Process flow sheet of a soil decontamination plant

residue

fresh water

water repellend

The main principle of soil washing is a selective classification of fmes, which are highly contaminated due to the large specific surface. In stepwise fashion the soil is sized on sieves and hydrocyclones, where cleaned gravel and sand can be separated.

The remaining suspension with fines < 63 11m is subjected to a solid/liquid phase separation process resulting in a cohesive contaminated waste product. The process water is returned to the wash water circuit. These processes are basic to the core modules of every soil washing plant.

For special cases auxiliary modules can be added. In these, density sorting for the elimination of highly contaminated organic matter is often carried out.

Further, alternative processes for the cleaning of fines < 100 11m can be consid­ered: flotation (Wilichowski 1994), leaching processes (Kramer and Koch 1992), high gradient magnetic separation and microbiological processes.

New Developments in Soil Washing Technology 463

29.3 First Generation of Soil Washing Plants

The first generation plant in Germany was characteristically:

• Stationary centralised plants with a relatively high throughput of up to 30-70 t h-1.

• Expensive cleaning of fines and waste water treatment. • Negative water balance and consequently indirect discharge of a more or less

large amount of cleaned waste water. • Operational costs of up to 20Q-400 DM f 1, depending on the level/type of

contamination.

Advantages of the washing technologies are:

• Soft technology using natural principles of wet self-cleaning of the soil with a moderate energy requirement.

• High cleaning efficiency, as can be seen from table 29.1. • Low operational costs for the coarse soil fractions. • High level of technology based on the state of the art of modern mineral

processing.

Disadvantages are:

• No soil remediation, but production of a low-cost building material (gravel and sand).

• Highly contaminated residue of the fine fractions. • High operational costs for cleaning of fine fractions and waste water treatment.

Table 29.1. Data on soil washing plants

Process Parameters

Throughput [t h"1]

• low • moderate • high soil recovery [%] cleaning efficiency [%] recirculation water flow [m' f 1]

fresh water demand [m' h"1]

specific energy consumption [kWh f 1]

reagent demand [kg f 1] (flocculants, surfactants)

Value

< 10 10 -30 30 -70 75 -90 85 -95

3 - 4 0.1- 0.3 7 -35 0.3- 0.8

464 Physical Treatment

29.4 New Generation of Soil Washing Plants

The second-generation plants have been developed mainly because of the need to reduce operating costs. This can be accomplished by the following measures:

• Use of mobile or semimobile plants in modular form with low or moderate throughput of 10-20 t h-1•

• Treatment of contaminated soil and other mineral waste. • Reduction of the core cleaning modules to those needed for gravity processes,

classification and density sorting. • Application of costly processes for fme fraction cleaning, such as leaching, and

flotation only in special cases. • High intensity attrition as a special development for soil washing. • Use of more sophisticated waste water treatment if possible with a positive

water balance (no waste water discharge).

A typical flow sheet of a modem soil washing plant, offered by AKW Apparate + Verfahren GmbH & Co. KG in Hirschau/Oberpfalz (Germany), can be seen in fig. 29.2.

29.4.1 Wash Water Circuit

For a soil washing plant to function correctly, the logistics of water flow are of prime importance. Some variants of the flow circuit are presented in fig. 29.3. Fig. 29.3a, b demonstrate two traditional variants with negative water balance (waste water discharge). Fig. 3c shows the modem configuration with process water treatment integrated into solid/liquid separation. The feed to waste water treatment is a fine sludge with a solids content of approx. 5Q-100 g r 1• In solid/liquid separation, the objective pursued is to bind contaminations largely to the solid phase. One requirement for a positive water balance without waste water discharge is a suffcient proportion of fines in the soil. However, a fmes fraction > 20 % threatens the profitability of soil processing because the recovery of washed soil will be too low. When the proportion of fme sludge is too low, more sludge can be produced artificially by means of hydroxide precipitation or addi­tion ofbetonite.

The addition of Fe (III) or AI {III) leads to the formation of voluminous hy­droxide sludges which, for example, can bind heavy metal ions through adsorp­tion. For de-emulsification of hydrocarbon contaminants, organic emulsion split­ting agents are used. This is achieved with polymers having molar masses of between 50,000 and 500,000, which have a flocculation effect on the solid phase and a coagulating effect on the oil phase. By integration of flocculation and ad­sorption, a highly contaminated sludge mixture of organic and mineral substances is formed, which is dewatered in the solid/liquid separation process stage to obtain a solid product. The sludge mixture is either taken to a special disposal site or subjected to a special process (e.g. thermal, biological). In this process stage,

New Developments in Soil Washing Technology 465

process water in which the dissolved and emulsified contaminants are reduced is recirculated.

Cleaned Gravel +- Density >2mm

Sorting

ReLe

Residue +-

Cleaned Sand +-63 .... 200 Jlffi

Contaminated Soil

l Dry Soil Preparation

l Wet Liberation

l Sieving

l <2mm

Hydroclassification

l Density Sorting

l High Intensive Attrition

l Desliming

< 10 jlffil ~-

Solid/Liquid Separation Waste Water Treatment

! Residue

Fig. 29.2. New flow sheet of a soil washing plant

Freshwater

< 63 jlffi

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New Developments in Soil Washing Technology 467

29.4.2 High Intensity Attrition - a New Process for Fine Particle Cleaning

One result of research and development in soil washing is a new variant of me­chanical fme particle cleaning that takes account of the need to reduce costs - high intensity attrition (Tiefel et al. 1999; Schricker 1999; Information by AKW Appa­rate + Verfahren 2000).

With today's state-of-the-art processing technology, conventional attrition is performed relatively simply by agitation of the material in a thickened suspension. Practical experience with available attrition systems, however, has shown that they are often unable to maintain the necessary parameters and thus to guarantee maximum cleaning efficiency.

A common reason is that the processing conditions necessary for an intensive attrition effect, that is, intensive particle contact, vary during the course of the processing cycle. In this connection, the solids concentration in the suspension fed to the attrition unit must be regarded as a determining factor for efficient process­ing. An adequate attrition effect can only be ensured if the solids content in the suspension is so high that the free mobility of the individual particles can be largely restricted.

High intensity attrition uses solid concentrations > 40 vol. % and higher me­chanical energy input, for producing a higher percentage of attrited fmes < 1 0 !Jm of up to 1 Q-15 %. The production of fines by attrition of an oily sand can be seen in fig. 29.4.

In the industrial scale application of attrition units, high solids concentrations of 40% are hardly ever reached, vary widely, or cannot be maintained permanently. When the concentration falls below the pre-set level, which corresponds to a de­fined solids content, additional coarse material is fed to the first attrition cell. The coarse material consists of part of the > 0.5 mm fraction, which is separated from the abraded material and then recirculated to the feed. Both the recirculated coarse material fraction, and the material fed into the attrition cell, are monitored and controlled by a central SPC unit.

The flowsheet for this new processing stage is shown in fig. 29.5, as exempli­fied by cleaning contaminated sands.

The suspension is fed ( 1) directly from the upstream processing installations, e.g. via hydrocyclones, into the attrition unit (2), where additional reagents (8) can be added if required. A torque sensor mounted to the drive shaft of the agitator determines the actual suspension viscosity and passes the signal to the central control system. If the actual value falls markedly below the set value, coarse mate­rial is added from the storage (4). The coarse material is obtained by screening at 0.5 mm and dewatering (3) of the particle fraction after attrition. Depending on the filling level in the storage container, controlled diversion of the coarse material flow to the container takes place. The main part can be discharged directly as cleaned material fraction.

In a hydrocyclone (5), the fine fraction formed during attrition is separated from the < 0.5 mm fraction. Depending on the specified processing aims and the cleaning values required, a cut size between 20 and 80 !Jm may be necessary. The fine fraction ( 6), which contains the contaminants in concentrated form, is directed to the process water or sludge preparation, where flocculation and sedimentation

468 Physical Treatment

180

160 --+- Oily sand, feed -o- Oily sand, attrited,

140 20 min, n=1200min·'

120 before attrition

:::r:: 100

& 80 fines by attrition

60

~ 40

20 ~~OoooeP

0,1 10 100 1000

Particle Diameter [~m]

Fig. 29.4. Particle size distributions of an oily sand before and after high intensity attrition (In­formation of AKW Apparate + Verfahren 2000)

cleaned gravel

cleaned sand

Fig. 29.5. Flow sheet of the high-performance attrition (Tiefel eta!. 1999)

contaminated sludge

(lJ

take place to produce a product suitable for dumping. From the underflow of the hydro-cyclone (7), the cleaned sand fraction flows on to a dewatering screen and is then stockpiled by means of belt conveyors.

New Developments in Soil Washing Technology 469

The actual attrition process is carried out in conventional attrition units, no ad­ditional alterations are required for high-performance attrition (Mitteilung der AKW Apparate + Verfahren 2000). A particular advantage of this latter is that the entire plant equipment required - with the exception of the measurement and con­trol system- consists of conventional, commercially available units, so that exist­ing plants can be upgraded cost effectively.

29.5 Cleaning Example

In order to study the efficiency of high-performance attrition in comparison with conventional attrition for cleaning different feed materials, and to determine the achievable contaminant concentrations, tests were conducted in a special test rig.

High-performance attrition can be used, for example, for cleaning soils and soil-like materials contaminated with mineral oil hydrocarbons. A typical oily sand with particles ranging between 63 l!m and 2 mm and a pollutant concentra­tion of 1845 mg mineral oil (kg dry solids)"1 was cleaned by means of high­performance attrition to 300 mg kg" 1 in the> 63 l!m fraction. This corresponds to a cleaning efficiency of 84 %. In comparison, cleaning efficiency with conven­tional attrition treatment reached just 840 mg (kg dry solids)"1• The fine fraction < 63 l!m formed during high-performance attrition accounted for 9.5 mass.-%. For high-performance attrition, part of the cleaned coarse material > 1 mm was recir­culated to the material feed and added to the attrition process (mass ratio of fine to coarse material1:1). In this way a constant solids concentration of 1200 g r 1 (cor­responding to a solids content of 45 vol.-% for quartz) could be maintained in the attrition process.

Table 29.2. Process conditions and cleaning results for high-performance attrition of a 63 )!m-2 mm sand contaminated with mineral oil (Tiefel et al. 1999)

Process Parameters

Specific energy input [kWh f 1]

Attrition time [ s] Primary feedback ratio [-] in the closed circuit system Quantity of fine particles< 63 )!m after high-performance attrition[%] MHC concentration> 63 )!ill before high-performance attrition [mg kg-1]

MHC concentration> 63 )!ill- after conventional attrition [mg kg-1]

MHC concentration> 63 )!ill after high-performance attrition [mg kg-1]

Decontamination efficiency[%]

Value

20.0 280

1.0

9.5 1845 840 300 84

470 Physical Treatment

29.6 Summary

A new generation of soil washing plants has been developed mainly because of the need to reduce operating costs. This can be accomplished by the following measures:

• Reduction of the core cleaning modules to those needed for gravity processes, classification and density sorting, and attrition.

• Application of costly processes for fme fraction cleaning, such as leaching, and flotation only in special cases.

High intensity attrition uses solid concentrations > 40 vol.-% and higher me­chanical energy input for producing a higher percentage of attrited fmes < 1 0 f.!m up to 1Q-15 %.

The stabilisation of a high-solids concentration by controlled recirculation of endogenous coarse material ensured the cleaning of fractions that could otherwise not be cleaned sufficiently, using conventional attrition techniques.

References

Feil A, Wetzel H, NeeBe T (1994) Charakterisierung der Sanierbarkeit kontaminierter BOden. Aufbereitungs-Technik 3: 56--60

Feil A (1997) Untersuchungen zur Aufbereitbarkeit kontaminierter BMen. PhD-Thesis, Univer­sitlit Erlangen-Ntlmberg, Gennany

Kramer U, Koch P (1992) Zur Anwendung von Laugungstechnologien bei der Dekontamination von Boden. Neue Bergbautechn 22 1: 13-17

Mitteilung der AKW Apparate + Verfahren GmbH und Co KG, Hirschau (2000) Hochleistung­sattrition unter Produktionsbedingungen. Aufbereitungs-Technik 41 4: 195

NeeBe T, Grohs H (1990a) NaBmechanische Aufbereitung kontaminierter BOden. Aufbereitungs­Technik 31: 563-569

NeeBe T, Grohs H (1990b) Die Aufbereitungstechnik des Bodenwesens. Aufbereitungs-Technik 31:656--662

NeeBe T, Grohs H (1991a) Waschen und Klassieren kontaminierter BOden. Aufbereitungs­Technik 32:72-77

NeeBe T, Grohs H (1991b) Fest/fltlssig-Trennung filr die Entsorgung in Bodenwaschanlagen. Aufbereitungs-Technik 32: 294-302

NeeBe T, Feil AU, Schricker B (1997) Washability curves for contaminated soil. Proc XX. Int Mineral Processing Congr, Aachen, Gennany, 21-26 Sept

Schricker B ( 1999) Intensivierung der Attrition bei der physikalisch-chemischen Sanierung kontaminierter mineralischer Abfiille. PhD-Thesis, Universitlit Erlangen-Ntlmberg, Gennany

Tiefel H, Schricker B, NeeBe T (1999) Hochleistungsattrition zur mechanischen Reinigung von mineralischen Roh- und Reststoffen. Technische. Aufbereitungs-Technik 40 4: 160-164

Wilichowski M, Werther J (1996) Mathematische Modellierung von NaBaufschluB, Siebung und Hydrozyklontrennung bei der physikalischen Reinigung mineral61-kontaminierter BOden. Aufbereitungs-Technik 37: 87-96

Wilichowski M (1994) Aufbereitung mineral6lkontaminierter BOden durch Bodenwiische und Flotation. PhD-Thesis, TechnischeUniversitlit Hamburg-Harburg, Gennany