UTILIZATION OF ZINC IN POWDERS FROM ELECTRIC ARC FURNACES IN STEEL PRODUCTION

The processing of waste powders from steel production in electric arc furnaces is a world problem. The presence of Fe, Zn, Cd, Pb, etc. in the powders makes them a valuable source for these metals. Zinc is the metal that is most often utilized, which leads to reduction of environmental pollution. The greatest problem in this connection is the presence of chlorine and fluorine in the powders, which influences the electroextraction of zinc in a significantly negative way. The presented paper shows the results from the study of the possibilities for zinc leaching from powders obtained in the steel production in Bulgaria. A detailed characterization of the powders was made using chemical analysis, XRD, DTA and TGA and Mössbauer spectroscopy. The results from the powder leaching with different solvents give the reason to recommend a technological scheme for the complete and total processing of the waste powders from steel production.


Introduction
Each year about 30 million tons of waste materials from the steel production industry in the world are recycled.These waste products are powders from blast-furnace and electric arc furnaces (EAF), fine and coarse fractions from oxygen convertors, sediments from the water treatment of large ground fractions, etc. [1].
About 20 kg of powder is released from 1 t of produced steel.In this connection different methods are developed (hydrometallurgical and pyrometallurgical) to process these powders with the aim of leaching the valuable metals in them such as Zn, Cd, Pb, etc.One of the main problems connected with the processing of the EAF powder is the high halogen content.Halogens are especially harmful in the electroextraction of zinc from zinc-sulphate solutions [2].
At low concentrations of the useful components or the presence of radioactivity it is necessary for the powders to be stabilized and safely stored [12][13][14].R.Sekula [15] tests physical separation methods, and C.Solisio [16] discusses the solubility of zinc and aluminium using Thiobacillus ferrooxidans.Selective dissolution of zinc before iron is carried out using ultrasound.With their research G.Ye [2,17] present alternative treatments for leaching of halogens.In laboratory conditions [18][19][20] experiments were carried with EAF powders using a wealz process to produce zinc oxide.
Regardless of the numerous investigations done, currently there is no universal method to apply in order to utilize the zinc from steel production powders.Depending on the content and quantity of the powders, investments are made in new installations and technologies or existing ones are used.
The high Fe, Cl and F content in the powders leads to significant difficulties in leaching according to the classic hydrometallurgical scheme for zinc leaching, because it leads to an increase of Cl and F in the solution, and there are strict requirements concerning their content in the zinc sulphate solution for the electroextraction of zinc.
In this connection the aim of the present study is to carry out as complete as possible a characterization of EAF powders obtained in Bulgaria and to study the possibilities of recycling the zinc in them.

Experimental
The studies are carried out with a powderlike sample obtained when the outlet granules are ground, and sifting the whole quantity through a 0.2 mm sieve.A study of the influence of temperature and time upon the degree of removing of Cl and F from the powder is carried out in a tube furnace.A sample of 5 g is thermally treated at different temperatures and durations.
DTA and TGA (a Q Derivatograph, Hungary) were carried out under the following conditions: sensitivity of DTA, 0.5 mV; DTG, 1 mV; TG, 100 mg; heating rate, 10 o min -1 ; sample mass 100 mg.A ceramic crucible was used.All the studies were performed in air medium.
The X-ray phase analysis of the samples was carried out with an apparatus "TUR-M62" (Dresden, Germany) equipped with a computer-controlled HZG-4 goniometer with Co-K α radiation and Bragg-Brentano geometry.Data base (Powder Diffraction Files, Joint Committee on Powder Diffraction Standards, Philadelphia PA, USA, 1997) was used for the phase identification.
The Mössbauer spectra were taken with an electromechanical spectrometer (Wissenschaftliche Elektronik GMBN -Germany) functioning at a constant acceleration mode and at room temperature.
A 70 mCi 57 Co(Cr) source and an α-Fe standard were used.The experimentally obtained spectra were computer fitted to a series of Lorenzian lines by the least squares method.The parameters of hyperfine interactions such as isomer shift (IS), quadrupole splitting (QS) and effective internal magnetic field (Heff) were determined.The relative weights (G) of the partial components of the spectra were found from the intensities of the corresponding lines.

Results and Discussion
The powders obtained in electric arc steel production are granulated and transported to a zinc plant for processing.Their average chemical content is the following (in mass %): Zn-20.15;Fe-32.5;Pb-3.53;Cl-2.13;F-0.35; Sb-0.2; humidity-8.68.
Based on the chemical content the following conclusions can be made: 1.The zinc content in the separate batches is within 18.8-23.1 % and remains within this range during the study.The low zinc content renders the powder unsuitable for independent hydrometallurgical processing.
2. The high content of Cl and F is especially unfavorable in comparison with the requirements for initial raw materials (Cl<0.05% and F<0.02 %), used with the hydrometallurgical method for zinc production.The Sb content also exceeds the requirements for initial raw materials (Sb<0.005%).
3. In all samples the Fe content falls within the range of 28-35 %.
The steel production powder is studied with DTA and TGA analysis.The obtained DTA and TGA curves are presented in Fig. 1.
The DTA curve shows that in the temperature interval up to 110 °C a slight endothermic effect is observed, which is due to moisture released from the sample.This is confirmed by the TGA curve, according to which the reported mass loss is of the order of 8 %.The second endothermic effect of the DTA curve (340 ºС) is not accompanied with mass loss and is probably due to some phase transition.According to the TGA curve up to 805 ºС no mass change of the sample is observed, and after the indicated temperature there is a small mass decrease up to 1000 ºС.The obtained results show that the powder from steel production is stable, inert and is only slightly influenced by heating in the studied temperature interval.
The measured X-ray pattern (Fig. 2) is characterized with comparatively narrow and intensive lines, which is indicative of crystallized phases.On the other hand, the base line registers certain deviations from a straight line, which shows that the presence ZnFe 2 O 4 ).The ratio of the two components -paramagnetic and ferromagnetic is 57:43 (Table 1).
It can be presupposed that in a solution of H 2 SO 4 the whole quantity of ZnO will dissolve, while ZnFe 2 O 4 will remain in the cake.A part of the iron linked as Fe 3 O 4 will also pass into the solution together with the ions of Cl and F. This confirms once again that it is impossible to begin directly with powder leaching in the zinc production.
The EAF powder subjected to study contains 2.05 % chlorine that contaminates the electrolyte.During the electroextraction of zinc the chlorine ions corrode the lead anodes, lead lining of the baths, lead or aluminium coils for cooling water.The lead ions passed into the solution and after that in the cathodic zinc.The concentration of chlorine in the electrolyte sometimes reaches 500-600 mg/dm 3 at an admissible content of 100-150 mg/dm 3 .
The behavior of chlorine at the thermal treatment of EAF powder is studied at different temperatures and durations.The intervals in which these factors change are chosen on the basis of the data from the DTA and the operation practice of the wealz furnaces.
The obtained results show that the degree of chlorine removal is influenced by both the time for heating and the temperature (Fig. 4).More significant is the influence of the temperature factor.From the obtained results it is clear that the influence of the time is manifested more strongly up to 30 -40 min, after which there is hardly any change of the graphic dependencies.
It is indicative that at 1100-1200 ºС and duration of 30 min a degree of chlorine removal of over 95 % is achieved.These results show that at the processing of steel production powder in wealz furnaces a considerable part of the chlorine will pass into the wealz oxides, which will render them unusable for direct leaching with H 2 SO 4 .
It is imperative in such cases to take precautions for preliminary removal of the Cl and F from wealz oxides before zinc leaching.
The obtained results for the removal of fluorine at tempering of up to 1200 ºС in air medium are shown in Fig. 5.
The results unambiguously show the positive influence of the increase of temperature on the degree of fluorine removal.A maximum degree was obtained at 1200 ºС (59.0 %).
In comparison with the Cl removal in the same condition it is obvious that the degree of removal of F is considerably lower.This shows that a smaller part of F will pass in the wealz oxides compared to Cl, but yet its concentration will be high and will hinder the direct leaching of the wealz oxides.It is known from literature and the practical implementation of the processing of such powders that Cl can be leached with the help of H 2 O, and for the leaching of F a favorable influence is the addition of Na 2 CO 3 .
In this connection an investigation was carried out about the behavior of Cl leached at different temperatures with and without the utilization of Na 2 CO 3 (Fig. 6).
The presented results disclose that a considerable part of the chlorine passes into a solution even without the addition of Na 2 CO 3 (over 75 %).The addition of Na 2 CO 3 does not influence the degree of leaching of Cl.The influence of the temperature increase factor is of far greater significance.
Based on literature data, the practice of water-alkaline washing and the experiments carried out by us, it was established that the addition of Na 2 CO 3 influences mainly the passing of F into a solution, but the degree of leaching of F does not exceed 20 %.
In recent years work on zinc electrolysis from a chloride solution has been more and more active.This process has a few advantages, especially when leaching raw materials or semi-finished products that have significant quantities of admixtures.Some of the most widely used reagents for zinc leaching are NaOH and NH 4 Cl.In this connection an initial investigation was carried out on zinc leaching from EAF powder using these two reagents.
The factor temperature of leaching was mainly studied at all other parameters constant (Figs.7 and 8).The concentration of the NaOH and NH 4 Cl solutions used is chosen on the basis of literature data.
In both cases of leaching it is obvious that the factors temperature and time of leaching influence positively the passing of zinc into a solution.
However, we have to note that the obtained results for the degree of zinc leaching at these conditions are very low.This is probably due to the high content of the stable and hard to dissolve ZnFe 2 O 4 , contained in the EAF powder.
A main part of the iron in the researched powders is in the form of The results obtained from the magnetic separation showed that when the process is repeatedly carried out, a magnetic fraction with a very low zinc content, and a nonmagnetic fraction with a zinc content exceeding the initial one more than twice can be obtained.This gives the reason to propose a technological scheme for the processing of EAF powders (Fig. 9), including magnetic separation.4. The washing of the EFA powders with water and Na 2 CO 3 leads to the passing of a considerable part of the chlorine into the solution (over 75%), while the degree of leaching of F does not exceed 20 %. 5.A technological scheme was proposed for the processing of EAF powders using preliminary magnetic separation.

Fig. 5 .Fig. 6 .
Fig.5.Degree of removal of F at thermal treatment (τ = 30 min) Fe 3 O 4 .At the same time the zinc is almost equally distributed between ZnFe 2 O 4 and ZnO.Having in mind that the two zinc-containing phases are paramagnetic, and Fe 3 O 4 is highly magnetic, it is of a certain interest to check the possibility of magnetic separation of the powder.

Table 1 .
Results from the Mössbauer analysis of the powder B.S. Boyanov / JMM 45 (1) B (2009) 15 -22 Fig.4.Degree of chlorine removal depending on the temperature and the duration of thermal treatment