Ind. Eng. Chem. Res. 1997, 36, 4381-4384
4381
Treatment of Chromic Tannery Wastes Using Coal Ashes from Fluidized Bed Combustion of Coal E. M. Bulewicz, A. Kozak, and Z. Kowalski* Institute of Inorganic Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Cracow, Poland
A new method of treatment for chromic tannery wastes containing chrome and large amounts organic substances has been investigated. It has been found that the addition of certain types of coal ash from fluid bed combustion technologies, at a suitable temperature and pH, results in effective removal of Cr(III) compounds present in the wastes. The wastes could then be subjected to further processing in conventional biological treatment units. The method is very simple, cheap, and effective and could be used for chromic tannery wastes of different compositions. Introduction Leather tanning processes using chromic agents produce large amounts of waste waters. These contain, among other pollutants, Cr(III) and organic substances. According to current estimates, Poland produces 0.50.7 million m3 of such wastes per annum. Chromic tannery wastes differ considerably from one another with respect to the concentration level of both chrome and organic pollutants. The former ranges from 0.5 to 2.0 g/dm3, and the latter depends mostly on the leather type and the parameters of the tanning process. This is why there are substantial differences in COD (chemical oxygen demand) and BOD5 (biological oxygen demand), which, respectively, range from 2000 to 5000 g/m3 and from 700 to 2000 g/m3. According to Kowalski (1990, 1994), the concentration of Fe(III) lies between 0.2 and 0.3 g/m3 and the usually high concentrations of chloride can lie between 1 and 5 g/m3. Earlier work by Kowalski (1990) has shown that Cr(III) in tannery wastes can be converted into chrome salts by treating the wastes with sodium chromate. Extraction of chrome compounds using an excess of sulfuric acid was used too. An older, conventional method of chromic waste treatment (Awierbuch and Pawlow, 1969) was based on ensuring suitable conditions for Cr(OH)3 precipitation and then filtering off the precipitate. The high concentration of colloidal organic compounds is the principal factor that makes the removal of Cr(III) from tannery wastes difficult. Simple precipitation of Cr(OH)3 particles is precluded, since during the neutralization of the waste with Ca(OH)2 or NaOH coagulation and sedimentation of chromium hydroxide does not take place readily, and the colloidal dimensions of the suspended particles make filtration difficult. This is why the treatment of waste waters with Ca(OH)2 usually employed in Polish tanneries can only be partially effective and only if the level of pollution by organic material is low. Nevertheless, it appears that the currently employed direct precipitation of Cr(OH)3 from waste waters is the simplest and cheapest method of treatment. In the course of previous studies on selecting precipitation conditions to bring about and improve the coagulation of chromium hydroxide and thus facilitate filtration, it was found that the addition of cement at a suitable temperature (363 K) and pH (∼9) resulted in rapid sedimentation of the slurry (Kowalski, 1994; * To whom correspondence should be addressed. E-mail:
[email protected]. S0888-5885(96)00764-6 CCC: $14.00
Kuczyn´ski et al., 1988). The role of the cement was to aid in the formation of large agglomerates, the sedimentation of which was faster than that of the original particles. In addition, the thickened slurry could be filtered relatively fast. After dilution with, e.g., municipal sewage, the filtrate obtained could be subjected to further cleaning in conventional biological treatment plants. Over the last 7 years, this very simple but effective method has been implemented in six Polish tanneries. Moreover, work on the utilization of the precipitate (Kowalski, 1990; Gollinger and Kowalski, 1994) resulted in developing methods of using it as a component of concrete and also as an additive to the raw materials for sodium chromate production. Rising prices of cement and energy encouraged us to undertake further research aimed at finding new and cheaper precipitation agents that could bring about sedimentation at lower temperatures. Different materials were examined, and promising results were obtained with some types of coal ash, especially from fluidized bed combustion (FBC) technologies. It would be a novel and attractive possibility of utilizing such ashes: employing a waste material to treat another waste stream. Ashes produced in conventional pulverized fuel and in FBC installations differ with respect to both chemical and phase composition, as recently documented in reviews by Mulder et al. (1995) and by Carr and Colclough (1995). These differences are due to lower combustion temperatures in FBC technologies (about 850 °C, both for bubbling, BFBC, and circulating, CFBC, beds) and the use of calcium-based sorbents (limestone or dolomite) to combine the sulfur present in the fuel. Because of lower combustion temperatures than in pulverized fuel furnaces, the mineral matter in the coal does not come near the ash fusion temperature, and because the sulfur dioxide evolved during fuel combustion is captured in the combustor itself, the solid wastes produced consist of coal ash mixed with the product of the desulfurization process, anhydrite, and contain a high proportion of free lime, derived from the calcination of an excess of limestone sorbent that must be used to achieve 80-90% flue gas desulfurization. From the point of view of the present work it is important that the waste can contain as much as 50% of CaSO4 and CaO, in comparable proportions, but most of the other phases present are not unlike those in Portland cement. In the presence of water, free CaO is turned into Ca(OH)2, and anhydrite is converted to gypsum, but in the highly alkaline medium there is also a reaction between Al-containing coal ash components and calcium sulfate, © 1997 American Chemical Society
4382 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 Table 1. Chemical Composition of the FBC Ash Used (CII), Mass % major components Bulewicz et al. (1992) SiO2 Al2O3 Fe2O3 CaO SO3 ignitation loss
13.0 9.5 9.4 41.8 14.5 14.2
minor components CANMET (1996) MgO Na2O K2O MnO TiO2 P2O5
0.89 0.21 0.51 0.24 0.25 0.05
resulting in the formation of a Ca sulfoaluminate (ettringite, 3CaO‚Al2O3‚3CaSO4‚32H2O). There is evidence (e.g., Murdiel, 1986) that the Al3+ ion in the ettringite structure can be replaced with Cr3+ ions, which have a similar ionic radius. According to Kozak (1995), the pure compound, isomorphous with ettringite, containing Cr in place of Al, can also be synthesized under conditions similar to those for ettringite precipitation. Physical and chemical characteristics of different types FBC ashes described in the literature were examined, and also the present methods of their utilization and disposal have been reviewed (e.g., by Mulder et al. (1995), Carr and Colclough (1995), and Weinberg et al. (1991)). Mulder et al. (1995), Weinberg et al. (1991), Solemtishmack and McCarthy (1995), and Anthony et al. (1995) paid particular attention to ettringite formation. It is worth noting that the tendency of FBC ashes to retain a number of elements has been considered: these elements include Se, B, Ba, Cu, Ni, and also Cr (e.g., Solemtishmack and McCarthy, 1995; Tawfic et al., 1995; Behrandres and Hutzler, 1994). Experimental Section The FBC wastes employed in tests on Cr3+ precipitation came from Canadian experimental CFBC installations and were made available to us for other work (Bulewicz et al., 1992; CANMET, 1996). The fly ash fraction, designated CII, was used. Its composition (in conventional oxide form) is given in Table 1. First, crude experiments were performed using aqueous Cr2(SO4)3 to assess how Cr3+ solutions would behave in contact with FBC ashes. Different quantities of CII ash were added to prepared solutions. After mixing and settling of the solids containing any precipitated Cr, the Cr content in the filtrate was determined using the usual (colorimetric, sensitivity 0.01 ppm) method and the pH of the filtrate was recorded. The phase composition of the drained solids was examined using the X-ray diffraction method. The results are presented in Table 2. From the point of view of removing Cr3+ ions from solution, these results were surprisingly good, suggesting that an effective method of chrome waste treatment could possibly be developed using FBC ashes. In further experiments, typical chromic tannery wastes from the “Niepolomice” tannery were used. They typically contained 0.9 g/dm3 of Cr(III), and the BOD was 1500 g/m3 and COD 4500 g/m3. In 1993, this tannery introduced the waste treatment method mentioned above, employing cement as the flocculating agent. About 1.5% of cement is usually added, and 1.25% and 2.0% addition of FBC ash CII was tried. The average mixing time after ash addition was 10 min, to allow any free CaO to be converted to Ca(OH)2. Sedimentation tests were made at temperatures of 293, 313, and 333 K (the optimum temperature for the cement method is 363 K).
The results of these tests are shown in Figures 1 (at low alkalinity) and 2 (at high alkalinity), in terms of the volume of the thickened suspension expressed as a percentage of the initial volume, denoted as nf, given as a function of time. At first glance, the addition of 2.0% of the FBC ash at 60 °C appears to be most advantageous, but after longer times even 1.25% of the ash at 20 °C brings about the same degree of sedimentation. This is practically the same as the effect obtained with cement addition. The fact that good sedimentation results were obtained in tests at pH ) 9 as well as those at pH ) 11 could indicate that the effect of the ashes might be different in the two cases. Microscopic examination showed that at pH 9 the presence of the ash brings about the aggregation of solid particles and the formation of large agglomerates with an apparently higher sedimentation rate than the initial particles. At pH ) 11, either some substitution of Cr3+ ions for Al3+ ones in the ettringite structure takes place or the Cr compound is precipitated alongside ettringite. Further work could help to elucidate this problem. The optimum conditions recommended for the treatment of the “Niepolomince” tannery waste would be as follows: temperature 333 K, filtrate pH ) 9-11, and addition of 12.5 of ash per 1 m3 of waste to be treated (ash/Cr mass ratio ∼24). X-ray phase analysis of five samples of solids after sedimentation at pH ) 8.8-9.1 showed that they contained gypsum, quartz, and calcite. At pH ) 1111.5, however, the solids also contained ettringite, and there wa evidence of the presence of CaSO4 hemihydrate. The X-ray diffractograms are given in Figure 3, but with size reduction only strongest lines can be seen. The Cr content in all samples of the thickened slurry was 1-1.5%, the H2O content 66-80%. Slurry samples were filtered at 323-333 K, under a pressure of 0.030.04 MPa, using a laboratory frame press with BT-17 filter cloth. The filtration rates achieved (350-400 dm3/ m2/h) were considered to be relatively high. The filtrate contained less than 0.01 ppm of Cr(III). After dilution with municipal sewage (typical proportions could be 1:5), the filtrate obtained could be subjected to further cleaning in a conventional biological treatment plant. Cakes from the filter press were stored, and changes in their consistency were examined. After 10-12 days, a solidification process took place, with the formation of a hard brittle material resembling plaster. After drying 2 h at 378 K, the cake contained 6.4% Cr2O3 30.5% CaO, 9.1% Fe2O3, 8.8% Al2O3, and 12.7% SiO2. The mean water content (for four samples) was 50%. Ignitation loss after calcination at 873 K for 3 h was 24.4%. The precipitate similarly, as after the cement treatment, could be used as a component in concrete. As an alternative, according to Kowalski and Walawska (1997), partial recovery of the chromium could be achieved if the waste was added to the mixture used for sodium chromate production. Conclusions On the basis of experimental tests it is suggested that fluidized bed coal combustion ashes could be used for the treatment of chromic wastes containing high concentrations of organic compounds, which is typical of tannery wastes. The addition of the ashes to the waste promotes the sedimentation of solids incorporating compounds con-
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4383 Table 2. Treatment of Aqueous Solutions Containing Cr3+ Ions Using FBC Ashes (CII) starting materials test no.
[Cr3+] solution (g/dm3)
FBC ash CII amt added (g/dm3)
1 2 3 4 5
0.0314 0.190 0.175 2.08 3.93
4.96 11.52 12.64 40 40
after treatment ash/Cr mass ratio
filtrate [Cr3+] (ppm)
filtrate pH
solid residue, main crystal phases (X-ray method)
158 60.6 72.2 19.2 10.2
