Journal of Environmental Quality 30:1927-1932 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Ecological Risk Assessment
Identification of a Crystalline Cyanide-Containing Compound in Blast Furnace Sludge Deposits
Tim Mansfeldt*,a and
Reiner Dohrmannb
a Arbeitsgruppe Bodenkunde und Bodenökologie, Fakultät für Geowissenschaften, Ruhr-Universität Bochum, D-44780 Bochum, Germany
b Institut für Mineralogie und Lagerstättenkunde, Rheinisch-Westfälische Technische Hochschule Aachen, D-52056 Aachen, Germany
* Corresponding author (tim.mansfeldt{at}ruhr-uni-bochum.de)
Received for publication December 27, 2000.
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ABSTRACT
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During blast furnace operation, a cyanide-containing muddy waste referred to as blast furnace sludge is generated in large amounts. In Germany it was and is still common practice to pump this sludge into surface deposits. Depending on species, cyanide has very different toxicity. To this day there is no information about the type of cyanide occurring in blast furnace sludge deposits. In order to identify the type of cyanide we investigated by means of wet chemical and powder X-ray diffraction analyses 37 samples of three blast furnace deposits. Wet chemical results indicate that both the extremely toxic free cyanide (HCN and CN-) and toxic weak metal–cyanide complexes, for example [Zn(CN)4]2-, are not present in the sludge. By powder X-ray diffraction we identified the crystalline cyanide-containing compound potassium zinc hexacyanoferrate(II) nonahydrate, K2Zn3[Fe(CN)6]2 x 9H2O, as the cyanide-bearing compound. Our study is the first that identifies potassium zinc hexacyanoferrate(II) nonahydrate in the environment. As the iron–cyanide complex [Fe(CN)6] is not acutely toxic, any direct hazard comes from cyanide occurring in the investigated wastes. Under the predominant pH milieu of the sludge (pH about 8) the solubility of potassium zinc hexacyanoferrate(II) nonahydrate is low, thus minimizing the mobility of cyanide.
Abbreviations: CN, cyanide • HCN, hydrogen cyanide • XRD, X-ray diffraction
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INTRODUCTION
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HYDROGEN cyanide (HCN) is one of the most rapidly acting toxic substances known (Egekeze and Oehme, 1980). In an aqueous solution the HCN molecule hydrolyzes:
 | [1] |
The distribution of total cyanide (CN) with respect to the HCN molecule and the CN ion depends on pH (pKa = 9.21; Beck, 1987). Both the molecular HCN and the CN ion are called free cyanides. From HCN several salts can be derived. The simple cyanides have the general formula A(CN)x, where A is (i) ammonium, (ii) an alkali metal, (iii) an earth-alkali metal, or (iv) a transition metal, and x is the valence of A. Some of these salts are readily soluble, for example KCN, and some are sparingly soluble, for example Zn(CN)2. In an aqueous solution, some of the simple cyanide will dissolve, thus liberating free cyanide, analogous to Reaction [1]. Because the CN group is a strong ligand, it forms with transition metal complexes of the form [M(CN)x]+m-x, where m is the valence of the metal, and x is the number of CN groups (Sharpe, 1976). Like simple cyanide these complexed cyanides can form both soluble and sparingly soluble salts. In general, cyanides complexed with alkali metals are readily soluble, for example K4[Fe(CN)6], and complexed cyanides with mixtures of alkali metals, earth-alkali metals, and transition metals are sparingly soluble, for example K2Ca[Fe(CN)6]. If all other relevant factors remain unaltered, the degree of dissociation of the metal–cyanide complex is a function of the complex stability constant. By dissociation, free cyanides are liberated:
 | [2] |
where m is the valence of the metal, x is the number of CN groups, and y is the number of water molecules. Weak metal–cyanide complexes are those with zinc, cadmium, and copper, medium with nickel, strong with iron and palladium, and extremely strong with cobalt and gold. By complexation the toxicity of cyanide is lowered.
During pig iron production, KCN and NaCN are produced in the blast furnace (Owen, 1983; Trömel and Zischkale, 1971; von der Dunk et al., 1964). Potassium- and sodium-containing compounds enter the blast furnace in both coke and iron ores. They are partially reduced to elemental K and Na vapor near the bottom of the blast furnace. With N, which originates from the preheated air blown into the blast furnace, and C, which originates from coke, they react to alkali cyanides (Reaction [3]), where M represents K or Na. Another possible reaction is the reduction of alkali carbonates, which are deposited on the walls of the blast furnace, by C (Reaction [4]), where M represents K or Na:
 | [3] |
 | [4] |
Some alkali cyanides are deposited on the walls of the blast furnace, some leave the blast furnace at the bottom with the slag, and some leave the blast furnace at the top with the top gas. The top gas can be used as a fuel elsewhere in pig iron or steel production. Prior to further use it has to be cleaned up by both dry and wet purification. A muddy blast furnace sludge is created in large amounts during the wet purification step. Due to high contents of Zn and alkali metals it is not possible to recycle the blast furnace sludge in the blast furnace operation. Thus, it was and it is still today a common practice to pump the worthless blast furnace sludge into surface deposits. The generation of blast furnace sludge in Germany amounted to 130000 Mg per year in the 1980s (Mertins, 1986). Owen (1983) stated that the minor proportion (less than 10%) of cyanide in the scrubber water, which is the first cleaning step in blast furnace operation, was in the form of iron–cyanide complexes. The rest was free cyanide. Pablo et al. (1997) reported that in the effluent water, which is the last cleaning step, the relative proportions are deferred in such a way that only about 50% of total cyanide was free cyanide. With increasing contact time of sludge and water, the proportion of dissolved iron–cyanide complexes on total cyanide increased. In contrast to the washing waters there are no studies dealing with the type of cyanide in blast furnace sludge deposits. Only Steuer (1986) postulated the occurrence of slightly soluble Zn(CN)2 in blast furnace sludge. He concluded this from the abundance of Zn occurring in blast furnace sludge, but he did not perform any further experiments. Thus, the type of cyanide occurring in blast furnace sludge remains speculative until now. Knowledge about the type of cyanide is essential in order to assess the environmental behavior and toxicity of cyanide-containing compounds.
It was the aim of this study to identify the type of cyanide in blast furnace sludge deposits. For this, 37 samples of two abandoned blast furnace sludge deposits and one operating blast furnace deposit in Germany were investigated by wet chemical methods and by powder X-ray diffraction (XRD).
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MATERIALS AND METHODS
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Samples
Samples were taken from two abandoned blast furnace sludge deposits. One deposit is in the Ruhr area, North-Rhine Westphalia, Germany. It covers an area of about 5 ha and had been in operation from about 1930 to 1983. It consists of nine single basins having a volume of about 250000 m3. The sludge is now dewatered. Today the blast furnace sludge deposit is covered by vegetation, and development of soils has taken place. Parts of the deposits are used by the population for leisure-time activities. Based on a field survey, 10 pits were excavated to depths of about 150 cm. In total, 32 samples were collected from the pits. The deposit in Lübeck, Schleswig-Holstein, Germany, had been in operation from 1971 to 1980. Prior to 1971 the sludge was pumped into the Baltic Sea. Four samples were taken during a remediation campaign. Finally, one blast furnace sludge sample of a still-operating blast furnace in Duisburg, North-Rhine Westphalia, Germany, was investigated.
Chemical Analyses
For a general characterization of blast furnace sludge, C, Si, and some metals were analyzed by elemental analyzer, X-ray fluorescence (XRF), and inductively coupled plasma–atomic emission spectroscopy (ICP–AES). Total C was determined by dry combustion of the material at 1200°C (Deltronik, Düsseldorf, Germany). The CO2 evolved 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 at 60°C, using the same analyzer. Organic C was calculated as the difference between total and inorganic C. For analysis of metals (Na, K, Ca, Mg, Fe, Mn, and Al) and Si, 1.5 g of pulverized samples were fused at 1200°C in a Pt-crucible with 6 g LiBO2–Li2B4O7 (Spectromelt A 12; Merck, Darmstadt, Germany) to obtain discs. X-ray fluorescence spectroscopy (Philips [Almelo, the Netherlands] PW 2404, Rh-tube) was used for the determination of these elements. In the case of Zn and Pb the sludge was digested by Na2O2 at low temperature in order to avoid losses of these elements. For this, 50 mg sample and 800 mg 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 in a water bath until the fused material was dissolved. Concentrated HNO3 was added to the crucible, and the solution was transferred into a 100-mL flask, which was filled up with water. Zinc and Pb were determined by ICP–AES using a Philips PU 7000 spectrometer. The pH of the sludge was measured potentiometrically in a 0.01 M CaCl2 suspension (10 g sludge, 25 mL solution).
Total cyanide was extracted from the blast furnace sludge by means of an alkaline extraction (Mansfeldt and Biernath, 2001a). For this, the sludges were extracted by 1 M NaOH repeated three times. The extracts were digested under acid conditions and boiled according to the German Standard Methods (1988) by means of a microdistillation technique (Mansfeldt and Biernath, 2000). Additionally, easily liberatable cyanides were determined according to the German Standard Methods (1988). This digestion was performed at pH 4 and room temperature for 4 h with a microdistillation technique and a modified digestion vessel (Mansfeldt and Biernath, 2001b). Easily liberatable cyanide includes free cyanide and weak metal–cyanide complexes. In all cases, the HCN evolved was absorbed in an alkaline solution and cyanide was determined spectrophotometrically at 600 nm using a barbituric acid–pyridine solution (Mansfeldt and Biernath, 2000). In order to check the solubility of cyanide occurring in blast furnace sludge, aqueous extracts were obtained for all samples by adding 500 mL demineralized water to 50 g dry sludge in a 1000-mL polyethylene flask. Samples were shaken end-over for 24 h at 5 rpm. After extraction, 250 mL of extract was centrifuged for 10 min at 15300 x g and vacuum-filtered through a 0.45-µm cellulose nitrate filter. The extract was analyzed for total cyanide according to Mansfeldt and Biernath (2000).
Mineralogical Analysis
Blast furnace sludge samples for random powder XRD analyses were dried at 40°C, ground in a mortar for 10 min and then analyzed using a Siemens D-500 diffractometer (Bruker, Karlsruhe, Germany) with CuK
radiation. In order to improve the signal to noise ratio long-term XRD scans were performed on all samples. Thereby the step size was 0.01° 2
and the counting time was 10 s within a range of 2 to 72° 2. Additionally, five blast furnace sludge samples were treated with a NaOH solution. This was done in order to dissolve the cyanide-containing compound detected in the sludge. For this, 1 g of blast furnace sludge was weighed into a 250-mL polyethylene flask, and 200 mL 1 M NaOH were added. The flasks were shaken end-over for 16 h. The samples were vacuum-filtered through a 0.45-µm membrane filter. The remaining solid was transferred into a new flask, and the whole procedure was repeated two times. Subsequently, the solid was washed free of NaOH using deionized water. Finally, the samples were dried at 40°C and analyzed by XRD.
Preparation of Potassium Zinc Hexacyanoferrate(II) Nonahydrate
In a preliminary XRD study, peaks at 0.541 nm (16.38° 2; d [113]), 0.450 nm (19.73° 2; d [024]), and 0.408 nm (21.79° 2; d [116, 211]) were observed for some sludge samples. These peaks can be related to the compound potassium zinc hexacyanoferrate(II) nonahydrate. For a detailed study, potassium zinc hexacyanoferrate(II) nonahydrate was prepared by drop-wise mixing 1 M K4[Fe(CN)6] with ZnSO4 under continuous stirring at room temperature (Vlasselaer et al., 1976). After aging for 7 d the white precipitate was centrifuged and the solid was dried at 70°C for 2 d. After pulverizing, the white precipitate was washed with deionized water, dried again at 70°C for 7 d, and sieved. An aliquot of the precipitate was dissolved in a 0.1 M NH4–EDTA solution. Zinc and K were analyzed by ICP–AES, cyanide was digested and distilled by a microdistillation technique (Mansfeldt and Biernath, 2000), and water content was determined by weight difference.
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RESULTS AND DISCUSSION
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The composition of the blast furnace sludge samples is dominated by C and Fe as shown in Table 1. Both elements are used in large amounts in the blast furnace operation in the form of coke and iron oxides. In the blast furnace, coke and iron oxide particles are carried over by the preheated air stream in the top gas. Besides these purely physical processes, many chemical reactions occur in the blast furnace. As a result, cyanide is generated, and alkali metals, Pb, and Zn are partially reduced to the elemental form in the vapor phase. These elements enter the blast furnace as by-elements in coke (ash), iron oxides, and fluxstones (limestone and dolomite). Zinc is especially enriched due to its low boiling point in the blast furnace dust. The pH of the sludge is slightly alkaline (Table 1). This can be explained by the presence of calcite and dolomite in the sludge (data not shown). Both minerals enter the blast furnace as additives. Additionally, some CaO is created in the blast furnace, which reacts later in the deposits with water to Ca(OH)2, thus raising pH into a slightly alkaline range.
Related to total cyanide content, the solubility of cyanide in the aqueous extracts ranged from 0.01 to 1.6% with a mean value of 0.31% (data not shown). This indicates that the largest proportion of cyanide is present as an insoluble compound in sludge. Free cyanide and weak metal–cyanide complexes were not detectable in the blast furnace sludge, which can be inferred from the absence of easily liberatable cyanide (Table 1). Thus, all cyanide must be in the form of strong metal–cyanide complexes, and the occurrence of Zn(CN)2 as postulated by Steuer (1986) can be excluded. From an ecotoxicological point of view this finding is important, since Zn(CN)2 can liberate the extremely toxic free cyanide. As we did not have the opportunity to analyze the alkaline extracts for various metal–cyanide complexes, we performed a simple qualitative experiment. The alkaline extracts obtained were acidified by HCl to a pH of about 2, and some dissolved Fe(Cl)3 was added. Immediately, a blue precipitate started to fall out according to Reaction [5]:
 | [5] |
The blue precipitate is known as Berlin (or Prussian) blue, and indicates the presence of ferrocyanide or hexacyanoferrate(II), [FeII(CN)6], in blast furnace sludge. Thus, we focused our XRD study on slightly soluble ferrocyanide containing compounds.
In Fig. 1
the XRD pattern of the synthetic potassium zinc hexacyanoferrate(II) nonahydrate is shown. The precipitate produced was identified as potassium zinc hexacyanoferrate(II) nonahydrate by comparing powder XRD patterns with those reported previously (Renaud et al., 1979). Wet chemical analysis yielded a composition with a molar ratio of 1.94:3.06:2 for K to Zn to Fe(CN)6, and of 8.71 for H2O to Fe(CN)6 normalizing hexacyanoferrate(II) equally to two.
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Fig. 1. Powder X-ray diffraction patterns of synthetic potassium zinc hexacyanoferrate(II) nonahydrate, K2Zn3[Fe(CN)6]2 x 9H2O, where CN is cyanide. The strongest peaks are marked with corresponding d spacings in nm.
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The XRD patterns of the synthetic cyanide compound together with those of the three sludge samples from different locations are presented in Fig. 2
. The occurrence of potassium zinc hexacyanoferrate(II) nonahydrate is clearly indicated in the two older blast furnace sludge samples (Herne and Lübeck). Five coincidence-free diffraction peaks corresponding to potassium zinc hexacyanoferrate(II) nonahydrate were identified with d spacings at 0.362 
, 0.408 
, 0.450 
, 0.541 
, and 0.646 
for the samples from Herne and Lübeck. For the recent sludge material from Duisburg the presence of the potassium zinc hexacyanoferrate(II) nonahydrate is suggested by a weak reflection at 0.541 nm. From chemical analysis, differences in total cyanide content of all three samples discussed are negligible with regard to XRD analysis. This indicates that precipitation of potassium zinc hexacyanoferrate(II) nonahydrate might be just in process in the youngest sample.
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Fig. 2. Powder X-ray diffraction patterns of synthetic potassium zinc hexacyanoferrate(II) nonahydrate, K2Zn3[Fe(CN)6]2 x 9H2O, where CN is cyanide, and three blast furnace sludge samples from different locations. Note that the intensity scaling factor is 0.66 for the synthetic potassium zinc hexacyanoferrate(II) nonahydrate whereas it is 4 for Samples 2076 and 2128 and 10 for Sample 2153. Relevant peaks are marked with corresponding d spacings in nm. Cyanide contents are 9280 mg kg-1 CN for Sample 2076; 3230 mg kg-1 CN for Sample 2128; and 6760 mg kg-1 CN for Sample 2153.
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Potassium zinc hexacyanoferrate(II) nonahydrate was clearly identified on the basis of at least three reflections in 20 of the 37 samples studied. For the remaining 17 samples potassium zinc hexacyanoferrate(II) nonahydrate concentration is apparently below the detection limit, or this compound is not present, or it does not form stoichiometrically. Our powder XRD patterns do not exclude the presence of other minor crystalline or amorphous iron–cyanide containing phases. Based on total cyanide content determined by wet chemical analyses and assuming all cyanide being present as potassium zinc hexacyanoferrate(II) nonahydrate, the maximum possible potassium zinc hexacyanoferrate(II) nonahydrate content is in the range of 0.1 to 5.7%, with a mean of 0.7%. This low quantity precludes quantification by XRD.
We assume that precipitation of potassium zinc hexacyanoferrate(II) nonahydrate immediately starts when blast furnace dust enters the process of wet purification. First, the CN ions react with ferrous ions forming soluble iron–cyanide complexes. Next, the iron–cyanide complexes form potassium zinc hexacyanoferrate(II) nonahydrate with Zn and K ions. The precipitation will proceed as long as soluble iron–cyanide complexes are present in the blast furnace sludge deposits.
The XRD results of the sludge samples alone were not an ultimate proof for the existence of potassium zinc hexacyanoferrate(II) nonahydrate. Therefore, an additional experiment was conducted to verify indirectly the occurrence of potassium zinc hexacyanoferrate(II) nonahydrate in blast furnace sludge by a dissolution treatment. Potassium zinc hexacyanoferrate(II) nonahydrate is soluble under strong alkaline conditions. The dissolution reaction is:
 | [6] |
This reaction suggests that blast furnace sludge treated with strongly alkaline solutions (e.g., a concentrated NaOH) should lose its characteristic peaks corresponding to potassium zinc hexacyanoferrate(II) nonahydrate. Therefore, five samples were treated with NaOH. The XRD patterns of these five blast furnace sludge samples before and after the alkaline treatment as well as the pattern of the pure potassium zinc hexacyanoferrate(II) nonahydrate are shown in Fig. 3
. The respective two patterns of each sample are grouped together. Only the strongest XRD peaks of potassium zinc hexacyanoferrate(II) nonahydrate at 0.408, 0.450, and 0.541 nm are presented. The characteristic reflections corresponding to potassium zinc hexacyanoferrate(II) nonahydrate disappeared for all samples after the alkaline treatment. This can be regarded as additional evidence for the existence of the crystalline phase potassium zinc hexacyanoferrate(II) nonahydrate in blast furnace sludge.
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Fig. 3. Powder X-ray diffraction patterns of synthetic potassium zinc hexacyanoferrate(II) nonahydrate, K2Zn3[Fe(CN)6]2 x 9H2O (the intensity scaling factor is 0.1 and CN is cyanide), and five blast furnace sludge samples before and after a treatment with a NaOH solution. Some peaks are marked with corresponding d spacings in nm.
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To avoid misinterpretation concerning the presence of potassium zinc hexacyanoferrate(II) nonahydrate, two pure coke samples were analyzed by XRD. This was necessary because coke contains mullite, Al[AlSiO5], and this mineral is a possible phase for the reflection position at about 0.54 nm. Chemical results indicate high amounts of C, presumably coke, in the sludges (Table 1). As expected mullite was identified in these two cokes giving a diffraction peak (0.539 nm) at nearly the same position as the potassium zinc hexacyanoferrate(II) nonahydrate (0.541 nm). Mullite has three significant diffraction peaks in the range between 15 to 28° 2 with d spacings at 0.539 
, 0.343 , and 0.339 nm (Joint Committee on Powder Diffraction Standards, 1994). However, by comparing the relative intensities of all other strong reflections of mullite, it can be concluded that mullite is not detectable in the blast furnace sludge samples. Regarding the problematic influence of texture on relative intensities in XRD analysis, an additional experiment was performed to ensure that the whole diffraction intensity at 0.541 nm derives from potassium zinc hexacyanoferrate(II) nonahydrate. One of the coke samples with significant mullite content was treated with an NaOH solution in exactly the same way as the blast furnace sludges. As can be concluded from the XRD pattern of this material, mullite was still present after the alkaline treatment. This is an ultimate evidence for the presence of potassium zinc hexacyanoferrate(II) nonahydrate in the sludges.
What are the implications of our results? The toxicity of cyanide strongly depends on the type of cyanide. While free cyanide has a high acute toxicity, the iron–cyanide complexes have a very little toxicity, even at relatively high exposure levels (Shifrin et al., 1996). Thus, there is no actual risk to human health and the environment at cyanide-contaminated blast furnace sludge sites assuming all cyanide is bound as potassium zinc hexacyanoferrate(II) nonahydrate. Leaching of cyanide from blast furnace sludge deposits may cause contamination of ground water. The solubility product of potassium zinc hexacyanoferrate(II) nonahydrate is comparatively low, Ksol = 5.5 x 10-39 (Bellomo, 1970). Its solubility depends on pH as shown in Reaction [6]. Only under extremely strong alkaline conditions is it soluble. Considering the slightly alkaline milieu of the investigated blast furnace sludge and assuming that all cyanide is present as potassium zinc hexacyanoferrate(II) nonahydrate, leaching of iron–cyanide complexes into the ground water is not to be expected to a significant extent at these sites. This can be inferred from the low water solubility of cyanide being present in deposited blast furnace sludge. However, detailed studies like ground water monitoring or pH-dependent solubility experiments should clarify this. If aging is required to form this less toxic, complexed form of cyanide as can be concluded from the fresh blast furnace sludge sample, it raises environmental regulations for disposal from operating blast furnaces: Younger sludges have to be evaluated more critically concerning environmental aspects of cyanides than older sludges.
The pig iron production process is relatively uniform due to energy aspects and materials used worldwide. Hence it is concluded that the potassium zinc hexacyanoferrate(II) nonahydrate detected might be a typical trace phase in blast furnace sludge at all locations. This is inferred by the overall presence of cyanide, Fe, K, Zn, and water in deposited blast furnace sludge.
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ACKNOWLEDGMENTS
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We are grateful to Gertrud Siebel (Rheinisch-Westfälische Technische Hochschule Aachen), Heidi Biernath, Kirsten Keppler, and Dr. Thomas Fockenberg (all Ruhr-Universität Bochum) for technical assistance. Financial support was provided by Ministerium für Umwelt, Raumordnung und Landwirtschaft, Nordrhein-Westfalen, Germany.
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NOTES
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R. Dohrmann, present address: Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), D-30655 Hannover, Germany.
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REFERENCES
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- Beck, M.T. 1987. Critical survey of stability constants of cyano complexes. Pure Appl. Chem. 59:1703–1720.
- Bellomo, A. 1970. Formation of copper(II), zinc(II), silver(I) and lead(II) ferrocyanides. Talanta 17:1109–1114.
- Egekeze, J.O., and F.W. Oehme. 1980. Cyanides and their toxicity: A literature review. Veterinary Quarterly 2:104–114.
- German Standard Methods. 1988. German Standards Methods for the investigation of water, wastes water, and waste. Anions (Group D). Determination of cyanides DIN 38405, Parts 13 and 14. VCH, Weinheim, Germany.
- Joint Committee on Powder Diffraction Standards. 1994. Powder diffraction file. JCPDS Card 15-776. International Center for Diffraction Data, Swarthmore, PA.
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- Trömel, G., and W. Zischkale. 1971. Gmelin-Durrer—Metallurgie des Eisens. Volume 3. 4th ed. Verlag Chemie, Weinheim, Germany.
- Vlasselaer, S., W. D'Olieslager, and M. D'Hont. 1976. Caesium ion exchange equilibrium on potassium zinc hexacyanoferrate(II) K2Zn3[Fe(CN)6]2. J. Inorg. Nucl. Chem. 38:327–330.
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TOP
ABSTRACT
INTRODUCTION
