Article pubs.acs.org/est
Inertization of Heavy Metals Present in Galvanic Sludge by DC Thermal Plasma Anelise Leal Vieira Cubas,*,† Marília de Medeiros Machado,† Marina de Medeiros Machado,† Frederico Gross,† Rachel Faverzani Magnago,† Elisa Helena Siegel Moecke,† and Ivan Gonçalvez de Souza§ †
Environmental Engineering, Universidade do Sul de Santa Catarina (Unisul), Palhoça, Santa Catarina, Brasil Environmental Engineering, Universidade do Sul de Santa Catarina (Unisul), Palhoça, Santa Catarina, Brasil § Department of Chemistry, Universidade Federal de Santa Catarina (UFSC), Florianópolis, Santa Catarina, Brasil ‡
ABSTRACT: Galvanic sludge results from the treatment of effluents generated by the industrial metal surface treatment of industrial material, which consists in the deposition of a metal on a surface or a metal surface attack, for example, electrodeposition of conductors (metals) and non conductive, phosphate, anodizing, oxidation and/or printed circuit. The treatment proposed here is exposure of the galvanic sludge to the high temperatures provided by thermal plasma, a process which aims to vitrify the galvanic sludge and render metals (iron, zinc, and chromium) inert. Two different plasma reactors were assembled: with a DC transferred arc plasma torch and with a DC nontransferred arc plasma torch. In this way it was possible to verify which reactor was more efficient in the inertization of the metals and also to investigate whether the addition of quartzite sand to the sludge influences the vitrification of the material. Quantification of water content and density of the galvanic raw sludge were performed, as well as analyzes of total organic carbon (TOC) and identify the elements that make up the raw sludge through spectroscopy X-ray fluorescence (XRF). The chemical composition and the form of the pyrolyzed and vitrified sludge were analyzed by scanning electron microscopy with energy-dispersive X-ray spectrometer (SEM-EDS) analysis, which it is a analysis that shows the chemical of the sample surface. The inertization of the sludge was verified in leaching tests, where the leachate was analyzed by flame atomic absorption spectroscopy (FAAS). The results of water content and density were 64.35% and 2.994 g.cm−3, respectively. The TOC analysis determined 1.73% of C in the sample of galvanic raw sludge, and XRF analysis determined the most stable elements in the sample, and showed the highest peaks (higher stability) were Fe, Zn, and Cr. The efficiency of the sludge inertization was 100% for chromium, 99% for zinc, and 100% for iron. The results also showed that the most efficient reactor was that with the DC transferred arc plasma torch and quartzite sand positively influenced by the vitrification during the pyrolysis of the galvanic sludge.
■
INTRODUCTION
surfaces, and make the products more attractive appearance throught the deposition of a thin metal layer on the surface.3 Galvanizing occurs through the baths in which the structures are immersed in a electrolytic solution and occurs the coating by the two principles for the electroplating: principle of metal deposition with power source, wherein the galvanic deposition of metals is based on electrolytic phenomena by electric current, and the principle of metal deposition without external power source, wherein the electrons needed for depositing the metal are produced directly in the solution by a chemical reaction.3 Different procedures for the inertization of galvanic sludges have been proposed in the literature4−6 and notable examples
All industrial processes generate residues, which in many cases are toxic and hazardous and thus cannot be discarded directly into the environment without damaging the environment or human health. The solution for the problem of storage, collection, transport, treatment, and/or final disposal of hazardous wastes are strongly associated with the composition of the residue. Due to the presence of heavy metals in its composition, this type of residue that contains hazardous substances, which according to the Brazilian norm NBR 10004/ 2004 is of Class I − Hazardous, must undergo special treatment.1 Electroplating is a treatment in which certain materials, especially metal, suffer coat to acquire protection against weathering and handling, conferring beauty, durability, and improved surface properties to meet the needs and demands of the market.2 The eletrocplating objective is to prevent the corrosion, to increase the hardness and conductivity of the © 2014 American Chemical Society
Received: Revised: Accepted: Published: 2853
October 3, 2013 February 6, 2014 February 7, 2014 February 7, 2014 dx.doi.org/10.1021/es404296x | Environ. Sci. Technol. 2014, 48, 2853−2861
Environmental Science & Technology
Article
are stabilization and solidification with calcium oxide.4,7−11 However, these forms of treatment, although they can render the sludge inert, do not have the advantage of reducing the volume as in the case of inertization by thermal plasma, but instead high quantities of inertized sludge remain. In fact, in the case of treatment with calcium oxide a 40% increase in volume can occur in relation to the initial residue, and a suitable location for its disposal is required.12 Several authors have verified that heavy metals can be stabilized when submitted to thermal treatments,13−16 but few studies have demonstrated the successful application of thermal treatments with plasma to residues containing chromium.17 The term “thermal plasma” is employed to describe gases which are partially ionized when heated to high temperatures (between 5000 and 50 000 K) at pressures close to atmospheric pressure.18 The formation of plasma occurs when a gas, normally argon, flows across an electrical field, fed by a DC source. When the gas is submitted to a voltage difference formed by the two electrodes (electrical field) and a high current is provided it ionizes forming the plasma.18 The high plasma temperatures cause rapid and complete pyrolysis of organic substances and can also result in the fusion and vitrification of certain inorganic residues.19 Thermal plasma with a DC transferred arc plasma torches and a DC nontransferred arc plasma torches were used to carry out the experiments. The operational principle is the same in both cases and is based on forced convection of the gas through the column of an electric arc established between two electrodes (anode and cathode) applying a direct or alternating current.18 The basic difference lies in the fact that in the case of the DC nontransferred arc plasma torch the anode is positioned within the torch and the plasma jet does not conduct the current through it, in the other hand, in the DC transferred arc plasma torch the cathode lies within the torch and the anode outside the torch, at the place where the material to be pyrolyzed is deposited.20 The treatment of hazardous wastes using plasma is recognized worldwide as being highly efficient and it has been used for the elimination or transformation of different types of residues including municipal wastes,21−27 incinerator ash,28−30 and even oily sludges produced by refineries.31 In all of the cases the pyrolyzed material is inert and can be incorporated into concrete or transformed into other materials of high added value such as fiberglass, carbon black, and others materials,32,33 but the literature shows no studies on the application of plasma for inerting galvanic sludge. In this way, the aim of this study was to investigate the inertization of galvanic sludges applying thermal plasma technology, considering that this pyrolysis technique can be used to transform hazardous wastes into residues which are not harmful to the environment and which could be reused in other processes.
galvanic sludge studied come from the plating process. In this process, the pieces after going through the steps of washing, degreasing chemical, electrolytic degreasing, for washing metals with dry solvents, detergents hot alkaline cleaning and acid etching, pickling, where oxidation and removal of inorganic impurities occurs, and activation, where goes to a tank containing water and the metal to be deposited along with additives (eg, boric acid) which effectively occurs chrome electroplating. Then, the piece undergoes a further washing step for removal of waste. Some specific applications require a second round of plating to impart greater strength to the piece. The opening of the sample, in order to characterize it, the sample was carried out using 0.5 g of the residue sample dried in an oven at 100 °C, with the addition of 20 mL of heated water and 20 mL of concentrated nitric acid. The solution was filtered to determine the iron, zinc and chromium contents of the raw sample on a flame atomic absorption spectrometer (Cole Parmer, BUCK Scientific 200A) using an air/acetylene mixture and flow rates of 8 L min −1 and 4 L min −1. Analysis of X-ray fluorescence spectroscopy (XRF), total organic carbon and measured density and water content of the galvanic sludge sample were also performed. To determine the water content in the raw sludge, weighed 2 g of galvanic sludge, and to dry the sludge, the sludge was arranged in a drying cabinet at a temperature of 45 °C for 3 h. To determine the density the pycnometric method was used, which is disposed 100 g of sludge in a pycnometer at a temperature of 100 °C. Analyses were performed in duplicate. For the analysis of the total organic carbon (TOC) was used the method described by Allison (1965).36 The samples were dried at 45 °C and milled in a 2 mm sieve. Were performed in duplicate, where were weighed 0.5 g of galvanic sludge sample and added 15 mL of 1.25 mol L−1 potassium dichromate (K2Cr2O7) and 30 mL of concentrated sulfuric acid (H2SO4). The samples were heated in 150 °C. After the cooling, was added 100 mL of distilled water and three drops of ferroin indicatorto make possible the titration procedure. The samples were titrated with ferrous sulfate (FeSO4) to 0.50 mol L−1, until the color change, from green to red. The analysis of X-ray fluorescence spectroscopy (XRF) was performed by the Laboratory of Catalysis and Interfacial Phenomena (SXRF-LACFI) of the Federal University of Santa Catarina (UFSC), Brazil, and the fluorescence measurements were performed on a Varian spectrofluorometer, model Cary Eclipse, operating with xenon lamp and detector 80 points per second. Is also connected to a data acquisition system and data processing Cary Eclipse version 1.1 (Varian, 2002). Preparation of Samples for Vitrification. The preparation of samples was carried out weighing 2.5 g of sludge on a standard analytical balance (Bel Mark 210 A). The pyrolysis was carried out on the raw sludge and with the sludge mixed with quartzite sand with a size distribution of
