Recovery of chromium from tannery sludge using hydrometallurgy
E. Pantazopouloua, O. Zebiliadoua, M. Mitrakasb, A. Zouboulisc*
a Department of Chemistry, Aristotle University, Campus University, Thessaloniki, GR 54124, Greece.
b Department of Chemical EngineeringAristotle University, Campus University, Thessaloniki, GR 54124, Greece.
c* Department of Chemistry, Aristotle University, Campus University, Thessaloniki, GR 54124, Greece (zoubouli@chem.auth.gr).
Keywords: tannery sludge, chromium, recovery
Solid waste management is of high environmental concern. European Union waste management policies aim to reduce the environmental and health impacts of waste and improve the resource efficiency. Major problem in Greece is the considerable amounts of solid wastes that are temporarily stored in areas next to the industries.
The process of leather tanning consists in the transformation of animal skin to leather. The predominant tanning method is based on chromium salts; specifically the trivalent chromium salts are the most widely used chemicals in tanneries (Kilic et al. 2011; Zouboulis et al. 2012).
From the total chromium amount applied during the tanning process, about 60% is consumed during the processing of raw materials (animal skin). The residual chromium remains in the exhaust tanning bath and is subsequently discharged into the wastewater. The dissolved chromium and other spent chemicals, which discharged in the wastewater, are mainly removed by chemical precipitation with inorganic coagulants, before the wastewater is allowed to enter the biological treatment process. The precipitated chromium along with other co-precipitated organic compounds is discharged as sludge (Swarnalatha et al. 2006).
Consequently, direct discharge of tannery sludge is not allowed, because chromium can cause serious environmental contamination of soils and water (Jing et al. 2006; Moon and Dermatas 2007). Thus, chromium recovery from this sludge is necessary for re-using it in tanning industry with additional environmental and economic benefits. In this study, the chromium recovery, based on hydrometallurgical treatment processes, directly from tannery sludge was investigated.
Tannery sludge is produced from the central wastewater treatment unit of tanneries cluster in the industrial area of Thessaloniki in Northern Greece, where is temporary stored in the plant backyard at large piles. This air-dried sludge was collected and is abbreviated as chromium- rich tannery waste (Cr-RTW). The chemical composition of Cr-RTW is presented in Table 1.
Table 1. Chemical composition of Cr-RTW.
% wt.
Moisture C N Al Ca Cr total Mg Na
10.8 22.6 1.7 0.3 9.1 8.6 1.3 0.7
mg/kg of dry mass
As Ba Cd Cu Mn Ni Pb Zn
62 100 nd* 61 120 110 11 373
*nd: not detected
Chromium recovery from Cr-RTW was carried out using 1g of Cr-RTW and 50 mL of deionized water, i.e. a liquid/solid (L/S) weight ratio of 50 mL/kg at different temperatures (25, 50, 60oC), mixing times (60, 90, 120, 180 min) and pH values of the mixture (2.0, 3.0), which was adjusted by adding 1 M H2SO4. Chromium total concentration was determined by atomic absorption spectrophotometer, Perkin Elmer Analyst 400. Chromium recovery was calculated based on the chromium content of Cr-RTW (8.6%), according to Table 1.
Table 2. Chromium recovery (%) from Cr-RTW.
pH Mixing time (min) Temperature (oC) Cr recovery (%)
3.0 60 25 1.5
3.0 90 25 1.8
2.0 60 25 4.9
2.0 90 25 11.1
2.0 120 25 11.7
2.0 180 25 12.3
2.0 180 50 62.7
2.0 180 60 72.3
Chromium recovery (%) from Cr-RTW is presented in Table 2, in which the effect of pH and mixing time, along with temperature is given. It is observed reduced chromium recovery for pH 3, as well as for short mixing times, below 90 min. Temperature of the process is another factor, which affects chromium recovery. Recovery of chromium increased from 12.3% to 62.7% for pH 2.0, mixing time 180 min and temperature 50oC, as the temperature was raised from 25 to 50oC. The maximum observed chromium recovery was 72.3% for pH 2, duration 180 min and temperature 60oC.
Acknowledgements
This research has been co-financed by the European Union (European Social Fund - ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES: Reinforcement of the interdisciplinary and/or inter-institutional research and innovation.
References
Kilic, E., Font, J., Puig, R., Colak, S., Celik, D., 2011. Chromium recovery from tannery sludge with saponin and oxidative remediation. J. Hazard. Mater. 185, 456–462.
Zouboulis, A., Samaras, P., Krestou, A., Tzoupanos, N., 2012. Leather production modification methods towards minimization of tanning pollution: “Green tanning”. Fresenius Environ. Bull. 21, 2406–2412.
Swarnalatha, S., Arasakumari, M., Gnanamani, A., Sekaran, G., 2006. Solidification/stabilization of thermally-treated toxic tannery sludge. J. Chem. Technol. Biotechnol. 81, 1307–1315.
Jing. C., Liu, S., Korfiatis, G., Meng, X., 2006. Leaching behavior of Cr(III) in stabilized/solidified soil. Chemosphere. 64, 379–385.
Moon, D.H., Dermatas, D., 2007. Arsenic and lead release from fly ash stabilised/solidified soils under modified semi- dynamic leaching conditions. J. Hazard. Mater. 141, 388–394.
