PROCESSING OF SOLUTION AFTER LEACHING OF STEELMAKING DUST BY CEMENTATION

Abstract The work includes a description of a hydrometallurgical processing of steelmaking dust a waste product of steel production from electric arc furnace (EAF). Most attention is paid to cementation. The aim is to study purification of solution by cementation of impurities that could cause problems in zinc electrowinning process. Most attention is focused on removal of cadmium and lead from the sulphate and chloride solutions. The highest efficiency of cadmium removal was achieved in the sulphate solution, at 25, 50, 80 °C and at pH = 4, 5, 6. The highest efficiency of lead removal from sulphate solution (87.61 %) was achieved at pH = 6 and temperature of 80 °C. In chloride solution, the highest efficiency of lead removal (99.94 %), at the same pH but at a lower temperature of 50 °C was achieved.


Introduction
Steelmaking production is accompanied by generation of different kinds of waste.Except a slag and emission, sludge and steelmaking dust, which consist of mainly iron oxides, are generated.When a scrap is melted, volatile elements like zinc, cadmium and lead, get into a steelmaking dust [1].Input material in EAF varies by its composition, what leads to variable composition of EAF dust.The contents of the main elements vary in a certain range.The example of composition of EAF dust is shown in Table 1.
Table 1 The example of composition of EAF dust [2] Element There are three possible ways of processing EAF dust: hydrometallurgical, pyrometallurgical and combined [1].Pyrometallurgical methods of processing steelmaking dust require high temperature and reduction agent, whereby product is crude zinc oxide of low commercial value.By hydrometallurgical methods, zinc can be recovered at lower energy consumption by various procedures [2].
The main part of hydrometallurgical treatment includes a purification of solution after leaching in order to decrease concentration of ineligible elements to the level of allowable limits (Tab.2), which do not cause problems in the further electrolytical recovery of zinc.Impurities present in the unpurified neutral solution coming from iron precipitation step, can lead to: lower current efficiency of zinc electrolysis, deposition of impurities on the cathode together with zinc, reverse reaction on the cathode and anode.Basically, following purification processes can be applied in order to remove ipurities from a solution: chemical precipitation, electrochemical deposition, ion exchange, cementation with zinc powder and solvent extraction.In industrial practice, mainly cementation with zinc powder in a continuous multi-step proces, is used for purification of solutions [3].The aim of this paper is to study purification of sulphate and chloride solutions coming from leaching of steelmaking dust.The attention was paid to removal of cadmium, lead and iron from the solution and comparison of conditions, at which the process reaches the highest efficiency.

Theoretical part
Cementation belongs to physico-chemical methods of extracting the metals from a solution, based on the electrochemical reaction between the cementing metal and the ion of the precipitated metal.Thermodynamic feasibility of cementation is determined from the ratio of the values of the electrode potential.The electrode potential of the displacing metal E M must be more negative than that of the displaced metal E Me (E M < E Me ).The cementation of the metal is accompanied obviously by a change of its concentration in the solution, and consequently, of its potential.
When the equilibrium values are reached (E M= E Me ), the process stops.On the basis of the difference in the electrode potentials, it can be determined the electrode pairs of the cementing and cemented metal [5].The cementation process can be described by the equation (1) [5]: n 1 , n 2 -ion charge; Me-cementated metal; Mcementing metal (element) Cementation is heterogenous process between the solid and the liquid phase.On the surface of the reducing metal is fomrmed a stationary layer of the solution-Nernst layer [6][7][8].Whereas, cementation is preferred environmental and economic refining operations, several results of metal cementation from the solutions have been published in the literature. of conditions, under which the cementation was carried out is included in the Table 3.Some of these works deal with the cementation from the synthetic solutions [7][8][9][10] and some deal with cementation from a solution after leaching of steelmaking dust [11][12].Concentrations of monitored metals in the solutions after cementation were compared with the concentrations of metals in the solutions before cementation and the results are shown in Fig. 2 -9.Efficiency of the cementation and purity of the solution were monitored parameters after cementation.From the results, it is clear that cadmium is removed from the sulphate solution at pH 4, 5 and 6 at all temperatures (Fig. 2a).According to the E -pH diagram of Cd-S-H 2 O system (Fig. 3a), cadmium is in the solution in the form of Cd 2+ at temperatures 25, 50 and 80 °C at pH 4, 5 and 6.Phase analysis (Table 6) confirmed the presence of cadmium in the solid residue after cementation at pH 4 and 5 and temperature 25, 50 and 80 °C.It follows from Fig. 2 that pH value about 1 leads to lower efficiency of removal cadmium comparing to higher value of pH (pH > 4).This was also confirmed by another literature where pH value higher than 1.5 is required for cementation of cadmium from a solution [15].At cementation of lead (Fig. 2b) the highest efficiency, 87.61 % at temperature 80 °C and pH = 6 was achieved.The lowest efficiency of lead cementation (39.6 %) was achieved at 25 °C and pH = 4.It is clean, from Fig. 2b that efficiency of lead cementation raises with increasing pH.filter cake after cementation (Table 6) confirmed the presence of iron in the form of Fe 2 O 3 and FeO(OH).At the experiment without adjusting pH at 50 °C, iron was only partially precipitated, i.e. there is still iron remaining in the solution after cementation.From Fig. 5b it can be seen, that zinc starts to precipitate from the solution at pH = 6.At pH lower than 6 zinc remains in the solution in the form of Zn 2+ .Precipitation of zinc was confirmed by phase analysis, where phases ZnO and Zn(OH) 2 was identified in solid residue coming from cementation carried out at pH = 6 and 80 °C (Table 6).In the case of experiments without pH adjusting, cemetation efficiencies of cadmium and lead were very low, 19.1 % and 4.28 % respectively.Thermodynamic study of cadmium and lead cementation from chloride solution at temperatures 25, 50 and 80 °C is listed in Table 7.It results from thermodynamic study that the reactions will proceed in the direction of product formation at all studied temperatures.As can be seen from achieved efficiencies, in chloride solution unlike sulphate solution, 100 % cementation of efficiencies were reached at all studied conditions.Achieving higher cementation efficiency in the sulphate solution was probably inhibited by formation of PbSO 4 .from combined E-pH diagram of the Fe-Cl-H 2 O system at temperatures 25, 50 and 80 °C (Fig. 8b), iron is present in the chloride solution in the form of FeCl 2 -to pH = 5.At pH > 5, iron starts to precipitate.Efficiency of iron cementation at pH = 4 was 96 % at all selected temperatures.Iron was precipitated from the solution by adjusting the pH solution to values 5 and 6 by adding a hydroxide sodium solution.XRD phase analysis confirmed the presence of iron in the form of FeO(OH) in the solid residue after filtration (Table 8).In the case of the experiment without adjusting pH (pH about 1) iron was cemented with lower efficiency (13.13 %) comparing to higher values of pH.increases its concentration in the solution at pH = 4 and 5 and at all studied temperatures.Similarly, as in the case of sulphate solution, zinc starts to precipitate at pH = 6.This fact is confirmed by E-pH diagram of the Zn-S-H 2 O system at 25, 50 and 80 °C, where zinc is in the soluble form till pH = 6, but over this value zinc starts to precipitate.Phase analysis of the solid residue after cementation confirmed the presence of phases ZnO and Zn(OH) 2 (Table 6).In the case of the experiment without adjusting pH, zinc concentration raised too, what could be casued by its dissolution due to the presence of free acid in the solution.Phase analysis of solid residues after cementation confirmed, that iron precipitates from sulphate solution by increasing pH above 4 as Fe 2 O 3 and FeO(OH).Adjusting pH to the value of 6 casued partial precipitation of zinc from suphate solution.Phase analysis of solid resiue confirmed its presence in the form of Zn, ZnO and Zn(OH) 2 .The same phases were identified also in solid residues coming from chloride solution.

Conditions of the experiment
It results from comparison of experiments with and without adjusting pH in both sulphate and chloride solutions, that increasing pH value has a positive effect on efficiency of cementation cadmium and lead from the solution.However, adjusting pH to the value of 6 causes precipitation of iron and partial precipitation of zinc, which stays in solid residues.

Fig. 2 Fig. 3
Fig.4ashow the efficiency of iron removal from sulphate solution.It can be seen from E -pH diagram of the Fe-S-H 2 O system, that when pH is adjusted by adding NaOH solution, iron starts to precipitate as iron hydroxide before cementation at all studied temperatures.Phase analysis of

Fig. 4 a
Fig.5agraphically shows a comparison of zinc content in the sulphate solution before and after cementation at selected conditions.Adding of zinc powder as cementing metal into the solution led to incresing its concentration at all selected temperatures and at pH = 4 and 5.However, at pH = 6 part of zinc, present in the solution, precipitated as it is evident in Fig.4bwhat caused lower concentration of zinc in the solution after cementation in comparison to zinc concentration before cementation.Fig.5bshows E -pH diagram of the Zn-S-H 2 O system at temperatures 25, 50 and 80 °C.From Fig.5bit can be seen, that zinc starts to precipitate from the solution at pH = 6.At pH lower than 6 zinc remains in the solution in the form of Zn 2+ .Precipitation of zinc was confirmed by phase analysis, where phases ZnO and Zn(OH) 2 was identified in solid residue coming from cementation carried out at pH = 6 and 80 °C (Table6).

4 . 2
a.) Content of zinc in the sulphate solution before and after cementation b.) E-pH diagrams of the Zn-S-H 2 O system at temperatures 25, 50 a 80°C Cementation from chloride solution Efficiencies of cadmium cementation in chloride solution are ilustrated in Fig.6a.These results show that cadmium cementation from chloride solution reached 100 %.It means, that cadmium in chloride solution (as in the case of sulphate solution) was completely cemented without any influence of temperature and pH.Fig.7ashows a combined E-pH diagram of the Cd-Cl-H 2 O system at temperatures 25, 50 and 80 °C.According to this diagram, cadmium is in the region of water stability in the form of CdCl 3 (-a) at studied conditions.
Fig. 7 shows a combined E-pH diagram of the Pb-Cl-H 2 O system at temperatures 25, 50 and 80 °C.It results from the E-pH diagram, that lead is in the region of water stability in the form of PbCl 4 (-a) at all studied temperatures.

Fig. 6 Fig. 7
Fig. 7 Combined E-pH diagrams of the systems a.) Cd-Cl-H 2 O , b.) Pb-Cl-H 2 O at temperature 25, 50 and 80 °C Fig.9agraphically shows comparison of zinc content in the chloride solution before and after cementation at selected conditions.It results from Fig.9a, that using zinc as cementing metal

Table 3
The review of works and conditions of Cd and Pb cementation The resulting filter cake was washed by distilled water to the neutral pH and dried at temperature 80 °C per 24 hours.After drying, the filter cake was weighted and analyzed by XRD diffraction phase analysis with diffractometer Panalytical X-Pert Pro.DOI10.12776/ams.v21i2.451p-ISSN 1335-1532 e-ISSN 1338-1156

Table 4
Concentration of metals and pH in solutions before cementation

Table 5
shows calculated ΔG°T of the cementation of cadmium and lead from sulphate solution at temperatures 25, 50 and 80 °C.Calculation have been performed by use of program HSC 6.1.The reactions (2 and 3) have tendency to proceed spontaneously in direction of products formation.DOI 10.12776/ams.v21i2.451p-ISSN 1335-1532 e-ISSN 1338-1156

Table 5
Calculated ΔG°T of the cementation of kadmium and lead from sulphate solution at temperatures 25, 50 and 80 °C.
Conditions of the experiment

Table 6
Phase composition of selected solid residues after cementation from sulphate solution

Table 7
Calculated ΔG°T of the cementation of cadmium and lead from the chloride solution at temperatures 25, 50 and 80 °C

Table 8
Phase composition of selected solid residues after cementation from chloride solution Experimetal results show the possibility of using cementation as way of removing metals (mainly Cd, Pb and Fe) from solutions to the acceptable limits.The most effective conditions of cementation found out in this work are listed in Table9.However, it is obvious that at pH 6 except of cementation occure also precipitation, what contribute to improvement overall refining process.

Table 9
The most effective conditions of cementation Cd, Pb and Fe From experimental study of cadmium and lead cementation from solutions coming from leaching of steelmaking dust, following conclusions can be stated:Cementation of cadmium was carried out under selected conditions (pH = 4, 5 and 6 and at temperatures 25, 50 and 80 °C) with 100 % efficiency in both sulphate and chloride solution.Cadmium was completely cemented at all selected conditions without any influence of temperature and pH.Phase analysis of solid residue after cementation confirmed the presence of cadmium as Cd and CdO, which probably formed by oxidation of Cd in solid residue.The highest efficiency of lead cementation in suplhate solution, 87.61 %, was reached at pH = 6 and temperature 80 °C.In chloride solution the efficiency of 99.94 % was achieved at pH 6 and 80 °C.Lead was present in solid residues in the form of Pb in both suplhate and chloride solution, what confirmes its cementation.