THE INFLUENCE OF HYDROCHLORIC ACID ON THE ZINC EXTRACTION FROM FLUX SKIMMING

This paper presents experimental results obtained by agitation leaching of hazardous waste sample – flux skimming. Such flux is formed by Zn(OH)Cl and NH4Cl phases. Distilled water and water solutions of hydrochloric acid were selected as a leaching media. The aim of this study was to determine optimal conditions of zinc extraction into the solution. The apparent order of reaction with regard to the initial HCl concentration in the leaching solution is n=0.18 in the concentration range from 0.01 mol.dm -3 to 0.25 mol.dm -3 . The obtained results also indicate that the temperature does not significantly affect zinc extraction. The value of apparent activation energy was estimated to Ea=3.985 kJ.mol -1 . Optimal liquid to solid ratio L:S=80 for both used leaching media was also determined experimentally as well as the stirring rate at 200 rpm.


Introduction
In the field of surface treatment the technology of zinc hot-dip galvanizing is wide-spread method of anticorrosion protection of steel products.It is a metallurgical process where the formation of regular zinc coating takes place at the temperature between 450 and 470°C by interaction between the melted zinc and steel part [1 -6].Zinc hot-dip galvanizing can be divided according to the used technology to continuous and batch.Batch zinc hot-dip galvanizing can be divided to a dry or wet depending on the way of fluxing agent application on the steel what is a part of its preliminary treatment.During the wet batch hot-dip galvanizing the steel parts proceed to zinc dipping still wet from the process of pickling.

Specific waste formation
During the wet zinc hot-dip galvanizing process a foam like fluxing agent occurs on the surface of separated part of the zinc bath.The main purpose of fluxing agent is to dissolve surface oxides forming on the surface of steel after the pickling and to activate the surface of steel for the reaction with melted zinc [7,8] takes place during the process of hot-dip galvanizing.Solid wastes such as bottom dross and zinc ash belong to a group of wastes with a significant amount of zinc [9 -12].Specific waste forming only during the wet hot-dip galvanization is flux skimming.It is a spent and inactive fluxing agent which forms on the surface of zinc bath in the separated part of the bath.NH 4 Cl is lost by evaporation as well as reaction between zinc, zinc oxide and iron during the hot-dip galvanizing process [13 -15].According to authors Kunhalmi [16], Krištofová [6], flux skimming is formed by ammonium chloride, oxides, sulfides, etc.Except original salts such as NH

Experimantal part 2.1 Material and methods of experiment
For experimental purposes a flux skimming sample was supplied by Slovak company.Supplied flux skimming sample had an irregular shape and heterogeneous composition, therefore it was necessary to homogenize it.The treatment of the sample consisted of drying-up, crushing and grinding.After the homogenization, the sample was quartationed with the aim of obtaining representative sample.Afterwards a representative sample was dissolved in 65% HNO 3 and analyzed by atomic absorption spectrometry (AAS).For the determination of chloride content in the samples, the titration method was used (0.1 M AgNO 3 with the addition of 5% K 2 CrO 4 ).Table 1 summarizes the chemical composition of the sample in weight percentage (wt.%). 3 Results and discussion

Material characterization
The qualitative phase analysis of the investigated sample of waste revealed that the sample contained two phases, i.e.NH 4 Cl and Zn(OH)Cl.Other zinc-containing phases were not identified.The x-ray diffraction pattern is plotted in Fig. 1.

Fig. 1 Results of x-ray diffraction analysis of flux skimming
The content of identified phases: 96.43 wt.% of Zn(OH)Cl and 3.57 wt.% of NH 4 Cl was estimated by quantitative x-ray diffraction phase analysis (Fig. 1).These concentrations of HCl (0.05, 0.01, 0.25M HCl) were chosen on the base of the previous experiments.Leaching at higher concentrations of hydrochloric acid (0.5, 1 and 2 M) were already published [21,22].After the 5 minutes of leaching at 295 K in 0.25 M HCl the observed extraction of Zn into a solution was at 92 % and 50 % for the 0.01 M HCl (Fig. 3).From Fig. 3 it follows that the 0.01 M HCl is not sufficient for the maximum extraction of zinc in solution.

Apparent activation energy E a
The value of apparent activation energy E a was calculated from the initial values of zinc extraction rates according to the author Zelikman [20] in the time interval of 0 -60 seconds.Table 2 shows the values of zinc extraction rates experimentally obtained at the following conditions: c(HCl) = 0.01 M, ratio L:S = 80:1, stirring rate = 200 rpm.The values of leaching rate, given in Table 2, shows that an increase of temperature above 313 K did not significantly affect zinc leaching rate.At temperature of 353 K the value of zinc leaching rate decreased for all time intervals comparing to the temperature of 333 K. where k is rate constant, A is frequency factor and R is gas constant (8.314J K -1 mol -1 ), the values of apparent activation energy in the temperature interval of 293 -333 K (E a = 3.985 KJ.mol -1 ) and frequency factor (A = 0.0431 s -1 ) were calculated using the method of linear regression.Calculated value of E a indicates that the rate of waste sample dissolving with the content of Zn(OH)Cl in the investigated temperature range is determined by diffusion speed, what is in accordance with the author Habashi [23], who states that if the E a lies in the interval of 4.18 -12.54 kJ.mol -1 then the leaching is controlled by diffusion.The behavior of ln k dependence on 1000/T illustrated in Fig. 4 also indicates that the mechanism of sample leaching is constant in the investigated temperature interval of 293 -333 K.

Determination of apparent order of reaction "n"
The apparent order of reaction, n was determined with regard to the initial concentration of HCl in the leaching solution based on the following equation: (2.) or as a logarithmic equation (3.) where n represents slope of a linear dependence ln v = f(lnc HCl ) which is given in Fig. 5 and reaches the value of n = 0.18; ln k = -3.944.The apparent order of reaction with regard to the initial HCl concentration in the leaching solution is n = 0.18 in the concentration range from 0.01 mol dm -3 to 0.25 mol dm -3 .

Conclusions
Experiments showed favorable effect of an increasing concentration of hydrochloric acid in the interval of 0.01 to 0.25 M HCl on the zinc extraction, where after 5 minutes of leaching in 0.01 M HCl the zinc extraction was at 51 % and in 0.25 M HCl 92%.It has been determined that leaching of zinc in HCl solution is more efficient than in distilled water.Obtained results confirmed that the temperature did not significantly affect zinc extraction into the solution.An example can be seen for raising the temperature from 295 to 313 K when leaching in 0.25 M HCl, where the zinc extraction changed from 92 % to 95 % after 5 minutes of leaching.The values of kinetic parameters such as apparent activation energy E a = 3.985 kJ/mol and apparent order of reaction with regard to initial concentration of HCl n = 0.18 were also experimentally determined.From the calculated value of the apparent activation energy E a it may be concluded that the rate of waste leaching in the HCl solution at the given temperature range is determined by the diffusion rate.

Fig. 2 3 .
Fig. 2 The influence of concentration of HCl leaching solution on the Zn extraction in the dependence on leaching time

Fig. 3
Fig. 3 Influence of the temperature on zinc extraction into a solution for 0.01 and 0.25MHCl

Fig. 4
Fig. 4 Arrhenius dependence of temperature influence on rate constant of leaching the Zn containing sample, correlation index r = -0.9976.

Table 1
Chemical analyses of the flux skimming sampleThe phase analysis of the investigated samples was carried out by X-ray diffraction method using SEIFERT XRD 3003/PTS (Germany) under following conditions: Co 35 kV, 40 mA radiation, scan step 0.02 2theta.The range of measurements: from 10 to 130° 2θ.The weights of the sample were 5 g, what represents L:S=80 ratio.The mixing of the sample in the leaching solution of 400 ml volume was supplied by glass stirrer with the constant stirring speed in the range of 200 rpm.Leaching experiments of zinc extraction into the solution were carried out in a solution of HCl with different concentrations (0.01M, 0.05M and 0.25M HCl).The leaching tests were carried out at the following temperatures 293, 313, 333, 353 K.The constant temperature was maintained in each experiment by water thermostat.At fixed time intervals(1, 5, 10, 15, 20 min.),liquid samples were taken in the volume of 5 ml.These samples were analyzed for zinc determination by AAS.The maximum duration of leaching test was 20 minutes.DOI 10.12776/ams.v21i2.502p-ISSN 1335-1532 e-ISSN 1338-1156

Table 2
Dependence of X Zn on the leaching time at temperatures of 293, 313 and 353 K