Environ. Sci. Technol. 2004, 38, 5977-5984
Chemical and Mineralogical Characterization of Blast-Furnace Sludge from an Abandoned Landfill T I M M A N S F E L D T * ,† A N D REINER DOHRMANN‡ Arbeitsgruppe Bodenkunde & Bodeno¨kologie, Fakulta¨t fu ¨ r Geowissenschaften, Ruhr-Universita¨t Bochum, D-44780 Bochum, Germany, and Bundesanstalt fu ¨r Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover, Germany
Blast-furnace sludge is generated during the production of pig iron and is disposed of in the environment in large surface landfills. We investigated blast-furnace sludge samples of an abandoned landfill in order to determine its chemical and mineralogical nature and to evaluate some environmental hazards that may arise from this industrial waste. The mineralogical inventory, which was quantified by Rietveld refinement of XRD analyses using the fundamentalparameter approach, revealed that blast-furnace sludge is dominated by X-ray amorphous substances (with a mean of 590 g kg-1) including coke and (hydr)oxides of Fe, Si, Al, Zn, and Pb. Calcite (CaCO3) (136 g kg-1), dolomite (Ca,Mg[CO3]2) (14 g kg-1), quartz (SiO2) (55 g kg-1), kaolinite (Al2[OH]4Si2O5) (40 g kg-1), graphite (C) (27 g kg-1), and chemically not specified layered double hydroxides (28 g kg-1) were identified in almost all samples. Iron is present as magnetite (Fe3O4) (34 g kg-1), hematite (Fe2O3) (38 g kg-1), wuestite (FeO) (20 g kg-1) and R-iron (Fe0) (6 g kg-1). Chemically, blast-furnace sludge is dominated by C (190 g kg-1) and Fe (158 g kg-1) reflecting the process of pigiron production. On the basis of total contents, environmentally problematic metals (including As) are Zn (32.6 g kg-1), Pb (10.3 g kg-1), Cd (81 mg kg-1), and As (129 mg kg-1). As the forested landfill is used by residents for leisure activities, the exposure assessment by pathway oral uptake of blastfurnace sludge particles by humans has to be critically evaluated, particularly as significant proportions of metals are acid-soluble. However, under the prevailing slightly alkaline pH values of the sludge (pH 7.6-9.2), the solubility of the metals is very low as indicated by low pore water concentrations. Currently, groundwater monitoring should be focused mainly on F- since the F- concentrations in the pore water of blast-furnace sludge are at high level (2.65-24.1 mg of F- L-1).
Introduction Before strict environmental laws were enacted, industrial wastes frequently had been disposed of in uncontrolled landfills. As these wastes often contain harmful substances, considerable hazards against the environment may arise from them. One example of such an industrial waste is blast* Corresponding author telephone: +49 234 3223439; fax: +49 234 3214469; e-mail address:
[email protected]. † Ruhr-Universita ¨ t Bochum. ‡ Bundesanstalt fu ¨ r Geowissenschaften und Rohstoffe. 10.1021/es040002+ CCC: $27.50 Published on Web 10/16/2004
2004 American Chemical Society
furnace sludge that is generated during the production of pig iron. Pig iron is produced commonly in large blast-furnaces. Preheated air is blown into the lower part of the blast-furnace. As a result, a dusty gas leaves the blast-furnace at the top during the operation. The gaseous phase of this top gas is composed of roughly 49% N2, 22% CO2, 23% CO, 3% H2, and 3% H2O (1). The calorific value of the gas is 3300-4000 kJ m-3. Hence, this gas is used for preheating the air in cowpers, in sinter plants, for running pumps. or for generating electricity. As the valuable top gas contains about 30 kg of dust (Mg of pig iron)-1, it was purified long before environmental laws were enacted. After a preliminary dry cleaning in dust catchers and centrifugal separators, a wet treatment with flue gas scrubbers follows. The washing water is pumped into clarification basins where the particles are separated from the water by sedimentation. The remaining muddy waste is referred to as blast-furnace sludge. It was a common practice to pump the blast-furnace sludge into surface landfills. These landfills are composed of a number of ponds, which were reciprocally pumped over decades with blastfurnace sludge. The generation of blast-furnace sludge in Germany amounted to 130 000 Mg yr-1in the 1980s (2). Today, the steel industry produces about 500 000 Mg of blast-furnace sludge yr-1 in Europe (3), which is all landfilled. The loading of the blast-furnace consists of iron ores, metallurgical coke, and flux. Sodium, K, and Zn enter the blast-furnace in both iron ores and ash-containing coke. They are partially reduced to elemental vapor near the bottom of the furnace at high temperature and subsequently condense on the walls of the furnace at lower temperature. This process becomes cyclic and leads to an accumulation of Na, K, and Zn compounds in the furnace. Since these compounds form solids on the walls, they cause damage in the blast-furnace operation. Hence, inputs of these elements into the blastfurnace should be minimized. Evaporated Na, K, and Zn leave the furnace partially with the top gas. Therefore, blastfurnace sludge is always enriched in these elements and cannot be recycled within the blast-furnace operation. Blastfurnace sludge was and continues to be an industrial waste. In 1936, the maximum number of 128 blast-furnaces had been in operation in Germany on 42 sites (4). Most of them were in the coal-mining area around the rivers Ruhr, Emscher, and Lippe in North-Rhine Westphalia. This area was the largest industrial region in Europe and is known as Ruhr area. Nowadays, in the Ruhr area almost all blast-furnaces are abandoned. However, blast-furnace sludge landfills still exist as industrial heritage. Such a landfill is in the City of Herne, North-Rhine Westphalia. Today, the sludge of this landfill is de-watered and vegetation has established. The landfill is used by the residents of the surrounding housing areas for leisure activities. Environmental research regarding blast-furnace sludge is limited since the sludge is a waste and not a valuable byproduct. Recently, a nontoxic crystalline cyanide compound of low solubility was detected in several blast-furnace sludge landfills (5). In other studies, the ability of blast-furnace sludge to adsorb heavy metals, cyanide, and iron-cyanide complexes was checked (6-11). Knowledge of the chemical and mineralogical composition of blast-furnace sludge as well as of solubility of harmful substances is rather narrow. For an improved risk assessment, it is essential to obtain precise information about the composition of landfilled blastfurnace sludge. In the present study, we investigated samples from an abandoned blast-furnace sludge landfill in the Ruhr area, Germany. The main objectives were (i) to identify the VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5977
FIGURE 1. Location of the experimental site.
FIGURE 2. Abandoned blast-furnace sludge landfill in the Ruhr area, Germany. With the exception of the youngest basin, all basins are forested (the air photo was taken in 1989). chemical and mineralogical composition of blast-furnace sludge and (ii) to study some aspects of the environmental/ human hazards originating from this industrial waste.
Experimental Section Site, Sampling, and Sample Preparation. The blast-furnace sludge landfill is in the Ruhr area, North-Rhine Westphalia, Germany (Figure 1). The landfill had been in operation from about 1930 to 1982. It consists of nine single ponds with a thickness of about 10 m. The sludge was not covered by any material but left exposed to the surface. At present, the ponds are forested (Figure 2). On the basis of a field survey, nine pits were excavated to depths of about 150 cm in the center of the ponds. In total, 32 samples with about 30 kg of wet material per sample were obtained. Main criterion for sampling was the color of blast-furnace sludge. Mostly, the material was black (high contents of coke), sometimes it was 5978
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 22, 2004
gray (high contents of carbonates). The water content, which was determined gravimetrically, ranged from 46 to 245% (wt/wt) with a mean of 147% (wt/wt). Fresh blast-furnace sludge was homogenized manually. Subsamples used for chemical and mineralogical analysis were dried at 60 °C. All material passed a 2-mm sieve. Subsamples were ground in an agate mortar. Samples used for centrifugation and pHstat experiments were not dried but stored at 4 °C after sieving to pass a 2-mm sieve in sealed, N2-flushed plastic containers. Analyses of Chemical Composition. Total C was determined by dry combustion of the material at 1200 °C (TR 3600 Deltronik). The CO2 was absorbed in an alkaline solution and detected by Coulomb electrochemical titration. Inorganic C was measured by adding HClO4 (15%) to the samples, which were preheated to 60 °C, using the same analyzer. The difference between total C and inorganic C is mainly cokebound C as explained later. Total S and total N were determined by dry combustion at 1200 °C, and the evolved SO2 and N2 were detected by thermal conductivity (vario EL, Elementar Analysensysteme). Most elements were analyzed by XRF (X-ray fluorescence) spectroscopy. For this, 1 g of sample was heated to 1030 °C for 10 min to determine loss on ignition. After mixing the residue with 5.0 g of LiBO2‚2H2O and 25 mg of LiBr, it was fused at 1200 °C for 20 min. Crucibles (Pt95-Au5) and an automatic fluxer (Herzog 12/1500, Engelhard-Clal) were used for the fusion. For quantitative analysis, a wavelengthdispersive XRF spectrometer was used. It was equipped with a Cr tube (PW 1480, Philips) for the analysis of Ti, Ca, K, Cl, Ba, Cs, Sb, and Sn and with a Rh tube (PW 2400, Philips) for the analysis of all other elements. In the case of the metals Zn, Pb, Cd and As (although As is a metalloid, the term “metal” is used in this paper to refer to the elements As, Zn, Pb, and Cd collectively), the samples were digested by Na2O2 at low temperature to avoid losses of these elements. For this, 50 mg of sample and 800 mg of Na2O2 were mixed in a Zr crucible and heated over an open flame. After fusion, water was added to the crucible, and the crucible was set into a water bath until the melting was dissolved. Concentrated HNO3 was added to the crucible, and the solution was transferred into a 100-mL flask, which was filled with water. Metal concentration were determined by inductive-coupled plasma (ICP) emission spectrometry (Spectro Ciros CCD, Spectro Analytical Instruments). The pH of the sludge was measured potentiometrically in a 0.01 M CaCl2 suspension (10 g of sludge, 25 mL of solution) using a glass electrode and pH-meter (pH 192, WTW, Weilheim, Germany). Total cyanide was extracted from the blast-furnace sludge by means of an alkaline extraction (12). Surface Area. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Five points were used representing an equilibrium between different amounts of available N2 and N2 adsorbed on the samples’ surface by a Gemini III surface area analyzer (Micromeritics). The samples were preconditioned by heating and evacuating. Analyses of Mineralogical Composition. Conventional X-ray diffraction (XRD) analysis yields qualitative mineralogical composition. To clarify the environmental significance of the blast-furnace sludge, accurate quantitative data are essential. This can be achieved by Rietveld refinement of XRD analyses (13). Rietveld refinement is only possible for crystalline phases with known crystal structures. However, qualitative overview analysis indicated large amounts of XRD amorphous material in blast-furnace sludge. The quantification of XRD amorphous phases is possible in the following way. An internal quantification standard has to be mixed with the sample. Subsequently, the crystalline phases are quantified by Rietveld refinement. This yields absolute portions of all crystalline phases calculated. On the basis of the known addition of the internal standard, absolute XRD
crystalline phase contents can be calculated. The amount of XRD amorphous phases is given by the difference between 100%, and the sum of all crystalline phases quantified above. In our study we used corundum, Al2O3 (SRM 676; National Institute of Standards and Technology NIST, Gaithersburg, MD), as the internal standard. Corundum was mixed with the samples to give a proportion of 20% wt/wt. Each mixture was homogenized thoroughly by hand in an agate mortar for about 15-20 min. Mineralogical composition of these mixtures was determined by XRD using a Philips X’pert diffractometer PW 3710 (Philips) with Cu KR radiation. Random powder samples were run with a step size of 0.01° and counting time of 10 s per step over a measuring range of 2-90° 2θ. The machine is equipped with a fixed divergence slit and a secondary graphite monochromator. Starting from the qualitative XRD analysis, quantification of blast-furnace sludge was performed using Rietveld refinement. The commercial software AutoQuan (Agfa NDT Pantak Seifert) was used; it is based on the fundamental-parameter approach. This approach involves modeling of the peak profiles through deconvolution of wavelength distribution and the instrumental profile by using a ray-tracing algorithm and real structure effects such as texture, microstrain broadening, and crystallite size. To start the refinement, the XRD profile and the starting models of all minerals or crystalline phases are selected. These models allow the assignment of fixed physically meaningful restrictions for the cell parameters. All parameters to be refined are unlocked in the starting models. Error calculations for the parameters are performed on completion of the refinement. The analysis of large deviations up to a limit defined in the starting model allows a judgment whether the starting model was sufficient or not (14). The least-squares refinement is performed without any operator influence. This last aspect differs markedly from conventional Rietveld programs where the refinement strategy is essential for the success. Finally, the difference line, the statistical factors Rwp (weighted profile), and Rexp (expected) allow an evaluation of the refinement success (13). The Rwp factor considers all differences between the experimental and calculated XRD pattern, whereas Rexp describes the minimal theoretical statistical error. The goodness-of-fit (GOF) is a suitable tool to evaluate the success of the whole refinement procedure with only one factor by
GOF ) Rwp/Rexp
(1)
Composition of Pore Water. To check the solubility of compounds in blast-furnace sludge, the pore water of the samples was obtained by centrifugation. This was possible since the water content of the samples was very high (see above). On the basis of water content, a total mass of 180 g dry material was weighted into six centrifugation tubes. Samples were centrifuged for 30 min at 15300g under N2 and vacuum-filtered through a 0.45-µm cellulose nitrate filter. Immediately after filtration, a 10-mL subsample was acidified with concentrated HNO3. The pore water was analyzed by ion chromatography (DX 300, Dionex) for anions, by ICP emission spectrometry for metals in the acidified samples and by potentiometric titration down to pH 4.5 with 0.1 M HCl (725 Dosimat and 691 pH-meter, Metrohm) for alkalinity. Electrical conductivity was determined by a conductometer (LF 521, WTW), and pH was determined by a glass electrode (pH 192, WTW). Pore water redox potential (EH) readings were performed in only few samples (n ) 10) since the water volume obtained by centrifugation was limited. For the EH measurements, 50 mL of the pore water was immediately transferred after centrifugation into a 50-mL four-neck glass jar, which allowed access for a combination pH electrode, a combination EH electrode, an O2-free N2 inlet tube, and a N2 outlet tube. Readings were taken after 30 min.
TABLE 1. Chemical Composition of Blast-Furnace Sludge element
mean
median
min.
max.
C Fe Ca Si Al Zn Mg Pb K S Mn N P Na
g kg-1
unit
190 158 83.0 77.3 35.8 32.6 20.6 10.3 6.39 5.78 5.75 3.52 1.53 1.10
149 159 81.8 80.1 34.0 30.4 19.2 9.83 4.28 3.94 4.47 2.88 1.60 0.59
69.0 57.9 35.2 45.1 22.7 15.7 6.30 1.42 1.36 2.38 1.11 1.47 0.48
