Leaching behavior of pollutants in ferrochrome arc furnace dust and its stabilization/solidification using ferrous sulphate and Portland cement

https://doi.org/10.1016/j.jhazmat.2008.05.114Get rights and content

Abstract

In this study, dissolution properties under different conditions and pollution potential by toxicity characteristic leaching procedure (TCLP) of arc furnace dust generated in the production of ferrochrome were examined and some stabilization/solidification (S/S) techniques were applied to the dust depending on contaminants determined. Dissolution properties and pollution potentials of all the materials stabilized/solidified were also studied under the similar conditions. It was determined that the metallic components concentrations dissolved from the ferrochrome arc furnace dust (FAFD) except for chromium and zinc were below the detection limits. The chromium concentration dissolved from the FAFD by TCLP was found to be 9.8 mg/l. Portland cement (PC), PC–FeSO4 and PC–sand–FeSO4 mixtures for S/S of the FAFD were tested. Although metal ions in the cationic form were stabilized when the PC was only used, Cr(VI) in the sample was not changed depending on PC amount and remained in the soluble chromate form. The stabilization efficiency of Cr(VI) increased by the increasing amounts of PC and FeSO4. The best S/S of the FAFD was accomplished when the 5 stoichiometric amounts of FeSO4, 30% PC and 16% sand mixture were used. TCLP leaching results of the samples obtained under the optimum conditions were below the EPA landfilling limits.

Introduction

Ferrochromium is an iron and chromium alloy, containing 45–80% Cr and various amounts of Fe, C and other elements. Ferrochromium is produced pyrometallurgically by carbothermic reduction of chromite ore (FeO·Cr2O3) [1]. Slag and arc furnace dusts are generated as waste material in enormous quantities during the ferrochrome production. While small percentage of the slag has been used as construction materials and aggregate [2], [3], [4], the majority of it is held in dumps. Slags can contain high levels of toxic elements [5], [6].

Fine solid particles (dusts) released into environment during the ferrochrome making process are also an important pollution source due to its high levels of extractable toxic elements content which could pose environmental pollution. It has been reported that the most abundant heavy metals in these dusts are Cr, Zn, Pb, Ni and Cd [7], [8], [9]. Particularly, it has been indicated that the filter dust or its sludge produced during the ferrochromium production contain high levels of soluble hexavalent chromium [10], [11], [12]. Arc furnace dusts have been categorized as hazardous waste by USEPA, EU and a lot of country due to their chemical, physical and leachability properties [13]. Therefore, it is not allowed to disposing of at landfills without treatment by regulations.

Stabilization/solidification (S/S) technologies are widely applied for treatment of hazardous wastes such as sludges, slags and ashes containing heavy metals. Main purposes in the S/S processes are to reduce the hazard of a waste by converting the contaminants into less soluble, mobile or toxic forms and to encapsulate the waste in a monolithic solid of high structural integrity by using some additives such as metal stabilization additives and binding materials [14]. Since electric arc furnace dusts are considered as hazardous pollutants, several researchers have focused on the stabilization of them [15], [16], [17], [18]. For this purpose, Salihoglu et al. have studied properties of steel foundry electric arc furnace dust solidified/stabilized with Portland cement. In the study, leachability of lead and zinc from the dust solidified/stabilized has been evaluated by TCLP and SPLP and it has been found that the concentrations of the lead and zinc released exceeded the EPA landfilling limits [13]. Solidification/stabilization of electric arc furnace dust using coal fly ash was investigated by Pereira et al. [19]. The solidification/stabilization mechanism and leaching behaviors of the Pb, Zn, Cd and total Cr were determined depending on pH, TCLP and DIN-38414 S4. They have reported that the final pH of the leachate must be within a range of values corresponding to the minimum solubility of the metals in the leaching medium, which is achieved in the pH 8.0–11.3 interval for TCLP leachates. Similar investigations by using cement-based additives for stabilization/solidification of wastes containing heavy metal have also been made by Andres et al. [20], Valls and Vàzquez [21], Janusa et al. [22], Diet et al. [23], Ruiz and Irabien [24], Islam et al. [25], Zain et al. [26], Viguri et al. [27].

Arc furnace dusts contain zinc, nickel, lead and cadmium and trivalent and hexavalent chromium. Oxidation stage of the chromium in the some researches related to stabilization/solidification and leaching behavior of chromium in these wastes has not been taken into account. Although S/S of the heavy metals having cationic form in the wastes could be achieved, it has been reported that Cr(VI) in the wastes stabilized/solidified released into leaching solution [28], [29], [30]. Cohen and Petrie were studied dissolution of chromium and zinc in ferrochromium flue dusts by cement-based solidification. They also taken into account chromium species together with other metal contaminants in their study and found that the alkaline nature of the cementitious product reduces the solubility of Cr(III) and zinc, but Cr(VI) in the solidified waste is mobile.

Chromium contamination is a significant problem since hexavalent form of chromium is highly mobile, toxic and carcinogenic to living organisms [31], [32]. The trivalent chromium is quite immobile in the water environment because of limited solubility of its hydroxide which is readily formed in the natural pH range of waters. Therefore, detoxification and immobilization processes of Cr(VI) is based on its reduction to Cr(III). For this purpose, it is needed to use some additives having reducing potential for Cr(VI). Kindness et al. used blast furnace slag in order to fix trivalent and hexavalent chromium ions in the solid waste. They have reported that Cr(VI) can reduced to Cr(III) in the presence of blast furnace slag [33].

Point of view the knowledge mentioned above, in this study, dissolution properties under different conditions and pollution potential by TCLP of arc furnace dust generated in the ferrochrome production were examined and some S/S techniques were applied to the dust depending on contaminants determined. Dissolution properties and pollution potentials of all the samples stabilized/solidified obtained were also studied under the similar conditions and results were compared.

Section snippets

Materials

Ferrochrome arc furnace dust (FAFD) used in the study was collected from Eti Krom AS located near Elazığ, Turkiye. The chemical composition of the sample is presented in Table 1. As seen in the table, sample contains 13.9% Cr, 17.18% Mg, 10.13% Si, 5.19% Fe, 2.83% Al and 1.5% Zn as main components. The FAFD sample mean particle size of which is about 105 μm was used in the experiments. Ordinary Portland cement (PC), sand and ferrous sulphate (Merck-1.03965) were used as S/S reagents. The working

Leaching behavior of FAFD

Wastes can be affected from the some environmental conditions in landfilling or stored area. For example; concentration of the contaminants released can change depending on pH, contact time and amount of rains (liquid/solid ratio) or water contacted. This situation has been taken into account in the standard test methods determining the pollution potentials of the wastes [35], [36], [37], [38]. Starting from this point, effects of pH, contact time and liquid/solid ratio on the dissolution of

Conclusions

In this study, dissolution properties under different conditions and pollution potential by TCLP of the FAFD were examined and some S/S techniques were applied to the dust depending on contaminants determined. Dissolution properties and pollution potentials of all the stabilized/solidified materials prepared were also studied under the similar conditions.

In the dissolution experiments, it was determined that except for chromium and zinc, metallic components concentrations dissolved from the

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