Zinc Cementation of Heavy Metals from Electric Arc Furnace Dust Recycling Process Extracted by Hydrochloric Acid Solution Chanwit Thititanagul1 , Porntip Lekpittaya2 and Sureerat Polsilapa3 Interdisciplinary Graduate Program in Advanced and Sustainable Environmental Engineering, Department of Chemical Engineering, Faculty of Engineering, Kasetsart University1 Department of Materials Engineering, Faculty of Engineering, Kasetsart University2,3 Email: [email protected], [email protected], [email protected] Abstract The benefits of hydrochloric acid for selective leaching electric arc furnace (EAF) dust has been reported by many authors. Low concentration hydrochloric acid can extract zinc which presence as the majority element contained in the EAF dust while leaving the iron in the solid residue. After extraction, there are also hazardous elements that were leached into an aqueous solution such as lead and chromium. These two elements could affect the efficient currency in the electrolytic cell on the electrowinning process for zinc separation from the solution. Thus, in this study, the cementation process was investigated using zinc powder in order to cement the solute heavy metals in the zinc-rich solution prior to the further process. Then optimal conditions were found to be; Zn/Pb 3.0 molar ratio, the temperature at 60oC, and rotation speed at 500 rpm for 10 minutes. The final solution was very high purity (Pb and Cr concentration < 2 ppm) which was suitable for zinc deposition to produce metallic zinc. Keywords: Zinc; Cementation; Electric Arc Furnace Dust; Hydrochloric Acid; Heavy Metals 24 วิศวกรรมสาร มก. 1. Introduction very stable structure of ZnFe2O4, and the emission of greenhouse gases are considered Steel has played an important role in as two main drawbacks of this method. world economics for ages. Many industrial sectors, Therefore, the combination of pyro-hydro- such as electronic devices, transportation, metallurgy was investigated [6,8,9]. A low mechanic, and construction, use steel as concentration of acid solution was reported a primary material. Most of the Thai steel to extract approximately 20 wt% of zinc from makers use a large amount of scrap material the EAF dust [5]. Other researchers have that is melted in an electric arc furnace at a o focused on ZnFe2O4 decomposition using very high temperature (>1,700 C) to produce reducing agents, such as carbon [6] and iron new steel alloys. The electric arc furnace [7], to lower the energy consumption in the (EAF) dust, a by-product is generated and process. It was reported that approximately typically collected by bag house filter and 70 wt% of zinc ferrite was transformed into accounts for approximately 1-2% of feedstock zinc oxide [6-9]. However, calcium oxide and [1]. It is reported that the main chemical eggshell, which was consists of calcium oxide composition of EAF dust consists of three as the main chemical composition, have been main compounds; zinc ferrite (ZnFe2O4), found to be an economical reducing agent zinc oxide (ZnO), and iron oxide (FexOy) [8,9]. Lime and eggshell were employed by [2]. X-ray fluorescence analysis reveals trace mixing with EAF dust in order to decrease elements, in the EAF dust, of lead (Pb), the temperature, in the pyrometallurgical chromium (Cr), cadmium (Cd), and process, for the decomposition of the very manganese (Mn) [3,4]. The zinc and other stable ZnFe2O4. The product of ZnFe2O4 hazardous elements in the EAF dust arise decomposition is ZnO, which is less stable from much of the scrap having been and is extractable by weak acidic solutions galvanized. Therefore, the EAF dust is [10,11]. Various acidic solutions have been classified as hazardous waste, and disposal studied, such as sulfuric acid (H2SO4), of which requires specific management. nitric acid (HNO3), and hydrochloric acid (HCl) However, the EAF dust has potential as a [5]. Low concentrations of HCl selectively secondary source of zinc and iron. Previous leaches zinc in solution leaving iron as solid research has reported on methods to extract residue [8]. Furthermore, using hydrochloric zinc from the EAF dust [4-9]. Pyrometallurgy acid as a leaching solution also gives a benefit has been widely studied to treat electric arc in further electrowinning processes. The furnace dust over the past years [3]. However, solution, after zinc extraction by HCl, high energy consumption a result of the contains ZnCl2, which could be used out- Zinc Cementation of Heavy Metals from Electric Arc Furnace Dust 25 Recycling Process Extracted by Hydrochloric Acid Solution right in zinc deposition for metallic zinc or zinc alloy production. After the extraction 2. Experimental process, it was reported that trace elements 2.1 Materials and Chemicals such as lead (Pb), copper (Cu), and cadmium Electric arc furnace dust, collected (Cd) are extracted into the solution as well by baghouse filter, was obtained from 13 [1,5]. These elements could reduce efficiency steel recycling factories in Thailand. Calcium in the electrowinning process in addition to oxide powder was obtained from DAEJUNG environmental issues. The cementation Ltd., South Korea, hydrochloric acid (37% process has been investigated for many w/v) from Merck KGaA Ltd., USA and applications, mainly for the removal of heavy zinc powder with a purity of 99.9% and metals from solution. A well-known cementa- an average size of approximately 1.27 μm tion process is the Merrill-Crowe process for from AJAX Finechem Ltd., Australia. HCl, removal of gold from the solution [11]. The zinc powder, and CaO were of analytical advantage of zinc cementation not only for grade (AR) and laboratory-grade (LR). removing heavy metals in the solution but 2.2 Analytical instruments recovery these valuable metals back in solid- The phases of the solid samples were state. Zinc was found to be a potential mate- analyzed by X-ray diffraction (XRD) (Philip rial for cementation due to its high galvanic series X’pert). The conditions for analysis θ o o series. It is good at reducing electrons from were 2 , 20 to 70 , the step size was 0.02, the present time 1 second, CuKα radiation other metals [10,11]. The investigation of fac- was used. Chemical composition of the EAF tors affecting zinc cementation of lead and dust, calcium oxide, and zinc powder was chromium in low concentration hydrochloride analyzed by X-ray fluorescence (XRF) (Horiba acid solution prior to the electrowinning pro- XGT-5200). Each sample was measured at 3 cess and the kinetics involved are the main points and the average value of wt% by ele- objectives of this research. Operating vari- ments was calculated. The conditions were; ables of the Zn/Pb stoichiometric ratio, tem- live time: 100 second, processing time: P2, perature, stirring speed and cementation time XGT Dia: 1.2 mm, X-ray tube vol: 50 kV, were investigated in order to maximize the X-ray filter: 5 Element, Cell: Nonexistence. Atomic absorption spectrophotometer (AAS) removal of heavy metals from the aqueous (Perkin Elmer with GBC Avanta Ver 2.01) solution. Further removal after cementation was used to analyze the chemical concentra- could be provided by an electrowinning pro- tion of the aqueous solution. Each sample cess to produce metallic zinc or zinc alloy for was measured three times and reported by use as a primary material for many industries. the average value with part per million (ppm). 26 วิศวกรรมสาร มก. 2.3 Methods stirrer with temperature monitored by ther- Three steps were used to extract zinc mometer. The optimum conditions for zinc in this study. The EAF dust was firstly extraction have been found to be 0.5M, HCl, calcined using CaO as a reducing agent, a solid/liquid ratio of 1:10, a solution temper- o in order to transform the ZnFe2O4 to ZnO. ature of 60 C, stirring speed of 900 rpm, and The resultant EAF dust was then leached extraction time of 20 minutes [5][8]. Atom- by a low concentration HCl solution. ic absorption spectrophotometer (AAS) was The heavy metals now leached out used to determine the concentration of lead underwent was cementation by zinc powder. and chromium leached into the aqueous so- 2.3.1 Calcining lution prior to the cementation process. After The phases present in the EAF leaching, solid residue was dried and dust samples determined by XRD. They characterized by XRD to confirm ZnO were then mixed with CaO: ZnFe2O4 molar extraction. ratio of 1:2 and hand pressed into a disk of 2.3.3 Cementation approximately 1 cm radius and 0.5 cm thick. Two liters of the leachant was The EAF dust mixed with CaO was calcined separated into 100 ml. aliquots for each at 800oC for 2 hours in Modultemp furnace experiment. After analysis of the leachant to at a 10oC/min heating rate. The resultant determine the concentration of the lead, material was analysed by XRD to confirm zinc powder was added in the molar ratios phase transformation and comparative (Zn/Pb) of 1.5, 3.0, and 4.5. The solution o intensities of ZnFe2O4 to ZnO peaks. temperatures were varied between 40 C, 2.3.2 Leaching 50oC, and 60oC. The effect of stirring The calcined EAF dust was leached speed was investigated at 100, 500, and by HCl solution. The leaching was carried out 900 rpm. The measurement of Pb and Cr based on the work of previous researchers. concentrations were taken initially, then after The solution was heated on hot plate- magnetic 10 minutes and 30 minutes cementation time. 700 absorption spectrophotometer (AAS) was used 1 – ZnFe2O4 , 2 – ZnO, 3 – Ca2Fe2O5 Zinc Cementation of Heavy Metals from Electric1 Arc Furnace Dust to determine the concentration of lead and 600 4 – CaO , 5 – Fe3O427 absorptionchromium spectrophotometerleached into the aqueous (AAS) wassolution used 700 Recycling Process Extracted500 by Hydrochloric Acid1 – ZnFe Solution2O4 , 2 – ZnO, 3 – Ca2Fe2O5 prior to the cementation process.