(Concentrated Chloride Extraction and Recovery of Lead)
This technology recovers lead from soils using an aqueous solvent extraction process that takes advantage of the high solubility of chlorocomplexes of lead. The extract solution contains greater than 4 molar sodium chloride and operates at a pH of -4. The figure below depicts the three-stage continuous countercurrent pilot plant used to study the process.
To operate the pilot plant, soil is sieved to remove particles greater than 1.12 millimeters in diameter, and is placed in the first chloride extraction tank (M1) for extraction with concentrated chloride solution. This soil and solvent slurry passes into a thickener (S1). The soil and solvent slurry has an average residence time of 1 hour in each extraction tank in the system.
The bottoms of the thickener flow by gravity to the second chloride extraction tank (M2). The solution exiting the second chloride extraction tank flows to the second thickener (S2). The bottoms of the second thickener feed the third stage.
The third stage is the last soil stage, but the first solvent stage; fresh solvent enters the system at stage three. The bottoms of the third thickener (S3) flow by gravity into the soil rinse system (VF1) to remove excess salt. The rinsed soil in VF1 is the clean product soil. The overflows from S3 pass to M2, the overflows from S2 pass to the M1, and the overflows from S1 pass to the lead precipitation system (M4/S4). In M4/S4, lead hydroxide [(Pb(OH)2] is recovered by simply raising the pH of the spent extraction solution to 10. After Pb(OH)2 removal, the spent chloride solution flows to the solvent makeup unit (T1) where it is acidified to pH 4 in preparation for reuse.
This technology produces treated soil, suitable for replacement on site, and Pb(OH)2 , possibly suitable for reprocessing to recover pure lead. The ease of solvent regeneration minimizes waste disposal. Solvent recycling is very successful, and pilot-plant tests have required little or no salt or water makeup.
The pilot plant has treated soil from two lead battery waste sites (LBWS). One LBWS soil contained high fines (about 50 percent clay and silt) and the other contained low fines (less than 20 percent clay and silt). The pilot plant's method of transferring soil by gravity eases much of the soil handling problems inherent in high clay soils. After treatment, both soils easily passed the toxicity characteristic leaching procedure test. The total lead concentration in the high fines and low fines soil was reduced from 7 percent to about 0.15 percent and 1.5 percent to 0.07 percent, respectively.
This technology removes high concentrations of lead from soil, particularly at LBWS, while producing a treated soil usable as backfill and a recyclable, concentrated lead salt.
This technology was accepted into the SITE Emerging Technology Program in September 1994. Batch extraction testing was completed in 1995. Treatability tests using the pilot plant on the high and low fines soils were completed in August 1996. The high fines soil came from a LBWS located in Houston, Texas, and the low fines soil came from the Sapp Battery National Priority List site in Florida. Future plans include expanding the applications of the technology by studying additional waste-site soils. The evaluation of the technology is expected to be completed by August 1997.
EPA PROJECT MANAGER:
Eugene Harris
U.S. EPA
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7862
Fax: 513-568-7676
TECHNOLOGY DEVELOPER CONTACTS:
Dennis Clifford or Tim Nedwed
Department of Civil and Environmental Engineering
University of Houston
4800 Calhoun Street
Houston, TX 77204-4791
713-743-4266 or 713-743-0751
Fax: 713-743-4260
E-mail: DAClifford@uh.edu or
w408@egr10.cive.uh.edu