Treatment of Contaminated Soil || New Developments in Soil Washing Technology

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  • 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 ac-ceptable. As a result of competition with biotechnology, in situ-technologies and natural attenuation, the tonnages sent for soil washing have been reduced dramati-cally. 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 con-tamination 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 decontamina-tion 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 re-moved. 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

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    _.6._ gravel

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    contaminated soil

    , ................................................ ~ ...... .

    destiming ~ .... :.:.!.~.~.:.?J..~~ .... . ! 10-IOOJ!m

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    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 consid-ered: 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 addi-tion ofbetonite.

    The addition of Fe (III) or AI {III) leads to the formation of voluminous hy-droxide sludges which, for example, can bind heavy metal ions through adsorp-tion. For de-emulsification of hydrocarbon contaminants, organic emulsion split-ting 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 ad-sorption, 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

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    ' 63 l!m fraction. This corresponds to a cleaning efficiency of 84 %. In comparison, cleaning efficiency with conven-tional 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 recir-culated to the material feed and added to the attrition process (mass ratio of fine to coarse material1:1). In this way a constant solids

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