Abstract
Electric arc furnace dust (EAFD) is a toxic waste product made in the remelting of scrap steel. The results of a Toxicity Characteristic Leaching Procedure (TCLP) conducted on a sample of EAFD originating from the remelting of stainless steel scrap showed that the total Cr and Cr (VI) liquor concentrations (9.7 and 6.1 mg/L, respectively) exceeded the Toxicity Characteristic Regulatory Level (TCRL). The EAFD showed a complex heterogeneous mineralogy with spinel minerals group predominance. A sequential extractions method has permitted the determination of the amount of available metals (potentially mobile component) from the EAFD as follows: Cr (3%), Ni (6%), Pb (49%) and Zn (40%). Solubility controls on Cr, Pb, Zn and Ni were identified in the EAFD. This means that the Cr, Pb, Zn and Ni concentrations in solution were controlled by the solubility of some phases from EAFD. The concentrations of Ni and Zn, which are metals not regulated by TCRL were below 0.41 and 1.3 mg/L, respectively. The solubility control on Pb was sufficient to decrease its concentration (<0.24>
Extracts
One of the most important problems in the secondary steel mill industry throughout the world is the disposition of dusts produced from electric arc furnaces [1]. A large quantity (10-20 kg) of electric arc furnace dust (EAFD) is generated per tonne of steel produced and around 700,000 and 50,000 tonnes of EAFD are generated each year in the United States and Canada, respectively [2,3]. The cost of EAFD disposal is not negligible. For example, 200 million dollars per year are necessary to dispose EAFD in the United States [4]. Moreover, the EADF is listed hazardous waste under Resource Conservation and Recovery ACT (RCRA) in United States and under the Transportation of dangerous Goods Regulations (TDGR) in Canada. Also, high levels of several contaminants cause it to fail the Ontario Regulation 347 leachate extraction procedure [5]. Thus, the disposal of these industrial solid toxic wastes can cause environmental risk due to the mobility of toxic elements. Some heavy metals of EAFD like chromium are toxic and have high solubility. Chromium (VI) is particularly problematic because it must initially be reduced before being fixed in an insoluble phase. A solubility control by a material implies that the concentration of some constituent elements in solution will be controlled by the solubility of the secondary precipitates. Reardon et al. [6] recommended conducting leaching tests on a particular waste material at least at two different water/solid ratios. They point out that if an element's concentration does not double when the water/solid ratio is halved, there must be a solid phase control on its concentration in solution.
... The chemical composition of dusts varies according to the type of steel produced and the variation’s range of compositions is considerable. For example, the Fe, which is the major element, can vary from 15 to 62% and represents around 43% in the EAFD from the stainless steel industry [3]. ... The concentrations of heavy metals are given in Table 2 along with some comparisons with data from the literature. The particle size distribution of the EAFD established by a Sedigraph particle size analyser (SediGraph 5000ET by Micromeritics) at 36 °C in an adequate liquid (density: 0.829 g/cc, viscosity: 9.14 cp) is presented in Table 3. The EAFD contains two major size fractions: a very fine-grained portion and a coarser part. Particle sizes range from less than 2.8 µm to more than 176 µm. The majority (94%) of the particles are smaller than 5.5 µm in diameter. ...
5.3. Leaching test results
5.3.1. TCLP results
The TCLP results presented in Table 6 show that the EAFD is a toxic waste. The total Cr (9.7 mg/L) and the Cr (VI) (6.1 mg/L) liquor concentrations after leaching are over the value of Toxicity Characteristic Regulatory Level (Table 7). The Pb concentration is under the regulatory level. The Ni and Zn concentrations are not regulated by the TCLP but it is important to note the high Zn concentration reached (93.9 mg/L). The EAFD is a toxic waste according to its high Cr concentration and must be treated. Stabilization/solidification (S/S) processes could be used to treat EAFD. This will be the subject of further research. The following section tries to evaluate the long-term leaching behaviour of EAFD in a neutral initial environment. ...
Conclusions
The EAFD particles appear as elongated grains, spherical, fine-grained and irregular particles. The predominance of spinel group minerals was determined. Magnetite is the main phase present in the EAFD together with chromite. Most grains are franklinite–magnetite–jacobsite solid solutions. No phase containing Zn or Pb was observed, but the hydroxides, carbonates and iron release Pb and Zn according to the sequential extraction results. These results revealed evidence of Zn in iron oxides. The Ni was found in Fe–Ni oxides (magnetite). The chromium appears both in chromium carbonate and chromite (spinel mineral). Many EAFD particles present different textures (zonation, exsolution and degassing phenomena). The EAFD is a toxic waste. The total Cr and Cr (VI) concentrations (9.7 and 6.1 mg/L, respectively) exceeded the Toxicity Characteristic Regulatory Level. A solubility control on Cr, Pb, Zn and Ni was identified in the EAFD. However, this control was not sufficient to decrease the Cr concentration to below the regulatory level. Future works may include analysis of leachates from the different granular sizes-fraction of EAFD.
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