12    TRANSIENT RESPONSE OF A MULTI-STAGE COUNTER
CURRENT WASHING SYSTEM FOR REMOVAL OF
METALS FROM CONTAMINATED SOILS
Allen Chao, Associate Professor
Yi-tang Chang, Graduate Student
Department of Civil Engineering
North Carolina State University
Raleigh, North Carolina 27695-7908
Mark Bricka, Environmental Engineer
C. Nelson Neale, Environmental Engineer
USAE WES EER
Vicksburg, Mississippi 39180
INTRODUCTION
Mediation of heavy metal contaminated soils can be carried out in many different ways. The
process of chemical extraction using acids or EDTA solutions will be addressed in this paper. Results
of limited field and laboratory studies reported in literature provides sufficient evidences to show that
the chemical extraction method is promising for mediating heavy metal-contaminated soils. The
results presented in available literature are informative but do not provide an insight into system
design and operation of this method. For example, a continuous-flow reactor is likely to be used for
large field applications. The equilibrium and efficiency data reported on this process1,2,3,4 were
mostly obtained with batch reactors. Thus, the results reported cannot be directly used for field
applications. Further, the extraction chemical method for remediation of contaminated soils is site
and soil specific, there is no rule of thumb for predicting the optimum operational conditions and the
maximum treatment efficiency. Hence, treatability studies must be conducted to determine whether
the chemical extract method is effective for mediating a soil contaminated with certain types of heavy
metals. Additionally, the type of system to be used for conducting the treatability study should be
similar to that for full-scale field applications.
A counter-current extraction system with the solvent flowing in one direction and the soil slurry
flowing in the opposition direction will be used to conduct treatability studies. The concept of
counter-current washing is simple and well-known but has not been reported in literature for mediation of contaminated soils. A major advantage of using the counter-current washing is that tremendous savings of chemical reagents can be achieved. Since the consumption of chemical reagent
contributes to most of the treatment costs for the extraction method, the counter-current system is
more cost-effective than other types of reactors.
The proposed soil extraction system consists of a series of 4 stages (Figure 1). Each stage consists of
a complete mixing tank and a clarifier. Influents of soil slurry and extraction fluid are mixed in the
tank reactor where washing of the soil particles occurs. After a suitable detention time, the mixture is
passed on to the clarifier for separating the washed soil particles from the liquid. The supernatant
flows by gravitation from the clarifier of a stage to its immediate downstream stage reactor and the
underflow solid suspension is pumped back to its immediate upstream stage reactor.
TRANSIENT RESPONSE MODELS
Full-scale continuous counter-current systems have been widely used in chemical processing industries. Mathematical equations describing the transient behavior of the extraction processes mostly
concerns the initial start-up period of the process. The soil washing treatability studies need to be
carried out to reach a steady-state for evaluating its system control parameters and efficiency. However, the system dynamic transient response must be evaluated in order to detect when a steady-state is
reached. Once the system reaches the steady-state, all pertinent parameters are measured and the
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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