Environ. Sci. Technol. 2000, 34, 3802-3808
Zinc and Lead Removal from Blast Furnace Sludge with a Hydrometallurgical Process PETER VAN HERCK,† C A R L O V A N D E C A S T E E L E , * ,† RUDY SWENNEN,‡ AND RONALD MORTIER§ Department of Chemical Engineering, University of Leuven, W. de Croylaan 46, B-3001 Heverlee, Belgium, Fysico-Chemische Geologie, University of Leuven, Celestijnenlaan 200C, B-3001 Heverlee, Belgium, and SIDMAR N.V., J. Kennedylaan 51, B-9042 Gent, Belgium
During the production of pig iron in a blast furnace, a Znand Pb-containing sludge is generated in the air pollution control system. This toxic waste can be landfilled after dewatering and pretreatment, which is very costly. The sludge particles contain large amounts of Fe and C that could be recycled in the furnace. However, the Zn content of the sludge is high, and the Zn input to the blast furnace must be limited, so Zn has to be removed. This paper describes a hydrometallurgical process whereby the sludge is leached under both acid (HCl) and oxidizing conditions. After the separation of the solids, which mainly contain C and Fe, from the leaching solution, the latter is passed through an anion exchanger to remove Zn and Pb and is recirculated to the reactor. Investigation of the leaching behavior showed that a pH below 1.5 and a redox potential above 650 mV are required to obtain high leaching efficiencies for Zn and Pb. Sequential extraction showed that the largest part of Zn and Pb is extracted in acid medium, whereas an additional 16-18% is extracted in oxidizing acid medium. XRD analysis showed that after acid leaching, Zn partially occurs as sfalerite and as Franklinite. Comparison of the leaching behavior of the sludge with that of sfalerite and Franklinite geochemical standards confirmed this and showed that sfalerite dissolves in acid oxidizing conditions, whereas the Franklinite does not. Calculations with the geochemical speciation program MINTEQA2 were compared with experimental results, and the process developed was tested in a pilot plant.
Introduction Extraction of Fe from its ores and its conversion into alloys is the most important metallurgical process (1). Besides Fe and C other elements are also introduced into the blast furnace. Zn especially forms a problem: during the metallurgical process it evaporates because of the very high temperatures in the furnace and subsequently condenses on the walls of the furnace at lower temperature. The condensed Zn prevents the descending of the furnace load * Corresponding author telephone: 0032(0)16/32.27.27; fax: 0032(0)16/32.29.91; e-mail:
[email protected]. † Department of Chemical Engineering, University of Leuven. ‡ Fysico-Chemische Geologie, University of Leuven. § SIDMAR N.V. 3802
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000
(the mixture of the iron ores, additives, and cokes). This leads to a sudden falling of the load, which generates large amounts of dust and may even damage the installation and the fireproof coating of the furnace. To ensure proper working of the furnace, the input Zn concentration should not exceed an average value of 120 g/ton of pig iron. Consequently, the ores are carefully selected, and the quality of the additives is checked. Part of the evaporated Zn leaves the blast furnace with the effluent gas and condenses on the dust particles, the concentration of Zn being highest on the finest dust particles. Between 8 and 12 kg of dust is produced/ton of pig iron. This dust is normally removed in the air pollution control system. Large particles (>50 µm) are removed from the flue gas in a dust bag and a cyclone and can directly be recovered in the blast furnace after sintering because the Zn content is in general low (
