JournalJournal of Chemical of Chemical Technology Technology and Metallurgy, and Metallurgy, 52, 1, 52, 2017, 1, 2017 124 - 130

EVALUATION OF THE EFFECTIVENESS OF LIQUATION REFINING OF ZINC MELTS FROM IRON IMPURITY

Vladimir A. Kechin, Evgeny S. Prusov

Vladimir State University named after Alexander Received 06 June 2016 and Nikolay Stoletovs Accepted 15 September 2016 Vladimir, Gorky St., 87, 600000, Russian Federation E-mail: [email protected]

ABSTRACT

The methodology of selection of additive elements for purification of zinc melts from iron impurity by liquation refining is proposed. Calculations of the effectiveness assessing the possibility of application of the formula of the periodic process of extracting to a single stage of extraction are performed. The extracting ability of the additives chosen is established. The experimental results show that the highest efficiency of unalloyed zinc purification from iron impurity is achieved by using silicon as an extraction additive. Aluminum and manganese are recommended to be used as extracting additives in zinc alloys refining from iron. Keywords: zinc melts, liquation refining, effectiveness, additive elements, iron impurity.

INTRODUCTION tion [8-14]. The method of liquation refining draws the greatest interest towards production of zinc melts of a Zinc is widely used in various industries for produc- low content of iron and other metallic impurities as it is tion of casting and wrought alloys for structural and easily adapted to the conditions of the acting industrial functional purposes including antifriction, sacrificial, enterprises. This method is based on the use as refining high-damping and other materials [1 - 4]. additives of various metals, which form high-melting Modern requirements to the purity of primary zinc used phases at interaction with the impurities [15]. These for alloys production are very high. The main impurities in phases segregate due density difference of the newly zinc produced by electrolysis refer to iron, lead, cadmium, formed phases and the zinc melt. Subsequent separation copper, tin and arsenic [5]. Concentrations of these impu- of the phases can be carried out by standing, centrifuga- rities determine the grade of primary zinc in accord with tion or filtration of zinc melts. ASTM B6-13 or other standards. The production of zinc The main objective of the present work is to evaluate the of decreased iron content is an especially actual problem. effectiveness of liquation refining of zinc melts from metallic Due to its low solid solubility in zinc (  0.001 %) the iron impurities (iron for an example) by additives introduction. impurity forms brittle intermetallic compounds (FeZn,

FeZn3, etc.), which significantly decrease the corrosion THEORETICAL CONSIDERATIONS resistance and deteriorate the mechanical and casting properties of zinc and its alloys [5 - 7]. Selection of refining additives for purification of Many of the known physical and physicochemical zinc melts from iron impurity methods of zinc melts refining from metallic impurities The selection of additives in case of production of (Fe, Pb, Cu, etc.) are low productive and difficult from metal melts of low impurities content by the liquation the point of view of constructive-technological realiza- refining method is based on the assessment of the nature 124 Vladimir A. Kechin, Evgeny S. Prusov

of interaction of the elements of the periodic system with The analysis of the data in Table 2 shows that the the impurities present and the base metal. The possibility additive elements of group A, having low solubility in of application of various elements as extractive additives zinc, and elements of group B, dissolving in zinc, form to zinc melts purification is determined by their solubility on high-melting chemical compounds with iron whose in zinc and the efficiency of their interaction with the melting point is above 1000°C. impurities present. The analysis of the metal-chemical properties of the elements in the course of their interac- Theoretical evaluation of the effectiveness of zinc tion with the zinc melts and the iron impurity shows liquation refining from an iron impurity that various types of bonding are formed [5, 16]. The The extraction theory fundamentals [24] are used selection of the elements retained for further considera- to evaluate the effectiveness of zinc purification from tion as potential refining additives is carried out taking metallic impurities. In relation to the technological pro- into account the nature of their interaction with zinc and cesses of zinc melts purification from iron the formula of iron, and also their cost and toxicity. The elements of the periodic process is applied to a single stage extrac- the periodic system are screened on the ground of these tion on the ground of the analogy with the metallurgical requirements aiming to allocate the potential additives. processes of extracting [25]: The characteristics of their interaction with zinc and iron Km⋅ are presented in Table 1. XX= (1) 0 Km⋅+ L The analysis of the data in Table 1 shows a possi- bility to use C, Si, S (group A) as refining additives for where Х0 is the initial concentration of the impurity in purification of unalloyed zinc from iron impurity. These the metal (zinc) in mass % (in this case and hereinafter), elements practically do not interact with zinc or have Х is the concentration of the impurity after the extraction very small solubility in it, but form chemical compounds operation (mass %), К is the equilibrium coefficient of and solid solutions with iron. Elements Al, Mn, Ti (group distribution of the impurity between the phases of the B) may be used in zinc alloys production as they act as segregating system, m is the mass of the solvent (zinc) alloying elements and refining additives as well. in g, while L is the mass consumption of the extracting Table 2 presents the characteristics of the phases additive ( g). formed in system “zinc-iron-additive” for the chosen The impurity distribution between two phases at a potential extraction additives in liquation refining of temperature set in advance is determined by the value zinc melts [16, 23]. of the equilibrium coefficient of distribution, K, which

Table 1. Characteristics of additive elements interaction with zinc and iron impurity. System Zn-Fe-X Element Type of interaction Solubility in zinc Ref. (X) with iron * % mass. T / °C Ti CC 0.019 400 [17] Mn CC + PSS 0.62 416 [18] 0.017 200 [18] Al CC + PSS 0.42 382 [19] 0.25 275 [19] C CC + PSS none [20] Si CC + PSS none [21] S CC + PSS partial miscibility [22] in the melt * PSS – a partial solid solution; CC – a chemical compound.

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Table 2. Characteristics of the main phases in the Zn-Fe-X system. Element (X) Compound Melting point / °C Density / g·cm-3 Group A

C Fe3C 1252 7.69

Fe3Si 1240 7.08 Si FeSi2 1220 4.75 FeS 1194 4.84 S FeS2 1171 5.02 Group B FeAl 1255 5.58 Al FeAl3 1157 3.85 Mn CSS* 1138 ∼7.8 TiFe 1377 6.52 ….Ti TiFe2 1427 6.84 * CSS - a complete solid solution. is approximately equal to the ratio of iron solubility in ing must be somewhat greater than the calculated one. molten zinc and iron solubility in the solid solution or Calculations of the efficiency of extraction of iron its content in the new phase formed with the additive impurity from zinc melt are executed for system Zn- participation: Fe-X (where X = C, Si, S, Al, Mn, Ti). The following input data are accepted: iron initial concentration X = C Fe 0 = Zn (2) 0.015 %, base metal mass m = 100 g, temperature of K Fe Zn CCC impurity extraction - 500°C. Iron solubility in the zinc melt is 0.02 % [5] at this temperature. Averaged values Fe whereCZn is the solubility of iron in molten zinc at the of the concentrations of iron in the formed solid solutions Fe process temperature (%), while CCC is the iron content or compounds with the extracting additive are assumed in the solid solution or the chemical compound formed on the ground of the phase diagrams of iron and the ad- with the additive (%). ditives selected. The extracting additive consumption The consumption of the additive element is calcu- is accepted to 10 times higher than impurity element lated taking into account its solubility in solid zinc. It concentration, i.e. L1 = 0.15 % on the ground of the is necessary to consider two cases aiming this. In the base metal mass. first one the extracting additive is insoluble in the base For an example, consider the principle of calculating metal. Its consumption is designated by L. In the second the final iron content in molten zinc in case of Zn-Fe-Ti case the additive has a known solubility determined by system according to the formula of the periodic process the system phase diagram. Its consumption is presented of extracting (1). The equilibrium coefficient of iron dis- =Fe Fe = = ⋅ −4 by L = L1 – L0, where L1 is the additive element amount tribution will be KCZn/ C CC 0.02 / 50 4.0 10 . The required to obtain the final concentration of the impurity, solubility of titanium in molten zinc is accepted equal while L0 is the amount of the additive element dissolved to 0.04 % at 500°C. Thus it follows that the extracting in the base metal (determined by the additive element additive consumption per 100 g of base metal will be solubility at the temperature of extraction). LTi = 0.15 – 0.04 = 0.11 g, while the final concentration As the equilibrium state between the liquid and of the iron impurity will reach: solid phases is not attained under the real conditions Km⋅ 4.0⋅⋅ 10−4 100 XX==Fe Zn 0.015⋅=0.004% of crystallization and phase separation presence, the 0 − 4 Km⋅+Zn L Ti 4.0 ⋅ 10 ⋅+ 100 0.11 experimental value of the impurity content after refin- (3) 126 Vladimir A. Kechin, Evgeny S. Prusov

The calculations carried out in accord with the pro- iron content decrease from 0.015 % to 0.002 - 0.004 % cedure described above show that the selected 10-fold at their 10-fold excess, 30-fold excess of manganese and increase of the consumption of titanium as an extracting 50-fold excess of aluminum are required to obtain the additive leads to 3.75 times iron content decrease in the same purification effect. Obviously, the use of manga- zinc melt. nese and aluminum will be possible in production of zinc Similar calculations are performed for other ele- alloys of low iron content only in case they are present ments considered as potential extracting additives. Inter- as basic alloying elements. mediate data and calculations results are listed in Table 3. The efficiency assessment of zinc purification at The efficiency of zinc purification,η (%), from iron different initial content of iron impurity and 10-fold impurity is evaluated in correspondence with: excess of the extracting additive (Fig. 2) shows that it Fe− Fe grows up to 85 % - 90 % with iron content increase in η =01 ⋅100 % Fe0 (4) initial zinc (0.003 % - 0.030 %) excluding aluminum where Fe0 and Fe1 are the initial and final concentrations and manganese. Purification in the presence of the latter of iron impurity in the zinc melt, correspondingly. in fact does not proceed under the conditions pointed Table 3 shows that in case of initial iron content in above. It is also seen, that the liquation refining method zinc of 0.015 % and 10-fold increase of the consumption is not suitable for technologies of deep purification of of the extracting additive the additive elements line in unalloyed zinc with initial iron content up to 0.003 %. respect to their extracting ability decrease is as follows: Si → S → C → Ti → Al (Mn) . EXPERIMENTAL VERIFICATION The efficiency estimation of zinc purification from iron at the accepted 10-fold increase of the extracting Materials and methods additive consumption shows that η = 81.2 - 83.3 % if Melting of high grade (99.95 % Zn) zinc was per- elements of group A are used. Only titanium among the formed in an alundum crucible in a electric resistance elements of group B provides refining with η = 78.3 %. furnace. The iron content of the zinc melt prior the refin- Fig. 1 illustrates the results obtained in calculating ing was 0.014 % - 0.016 %. Carbonyl iron was added to the iron content change in a zinc melt in case of a various zinc at 500 °C, Al and S were introduced to the melt as consumption of the additive elements referred to the iron refining additives at 490 °C, while Ti, Si, Mn and C - at content in zinc. It is seen that while C, Si, S, Ti provide 600°C. The purity of all materials used was higher than

Table 3. Calculations results referring to the process of extraction of iron from zinc by introduction of various additive elements.

Averaged iron Solubility content of the Additive Final impurity Purification Additive in the solid phase additive element K · 104 concentration efficiency element formed element consumption Fe1 / % η / % by the additive in zinc L / % Fe CCC / % L0 / %

C 57 3.5 0 0.15 0.0028 81.2 Si 66 3.0 0 0.15 0.0025 83.3 S 63 3.2 0 0.15 0.0026 82.6 Al 36 5.6 0.5 0 0.015 0 Mn 100 2.0 0.3 0 0.015 0 Ti 50 4.0 0.04 0.11 0.004 78.3

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Fig. 1. Theoretical changes of iron content in zinc melt depending on consumption of extracting additive.

Fig. 2. Theoretical efficiency of purification of zinc with different iron content at 10-fold excess of extracting additive.

128 Vladimir A. Kechin, Evgeny S. Prusov

99.5 %. The melt was stirred with a graphite rod within from the melt, etc.), as well as with possible losses of 8-10 min during the additives introduction. Then it was the extractive additives and their incomplete recovery at cooled at a speed of 40°C/h-60°C/h down to the tem- entering the melt. Fig. 3 shows that only silicon should be perature of crystallization (  420°C). Then the mixture considered as an effective extracting additive in relation was quickly heated to 480°C and a sample for chemical to iron in case of unalloyed zinc production. As expected, analysis was taken from the crucible bulk using a quartz aluminum and magnesium are partially dissolved when tube. The chemical composition of the samples was introduced to the zinc melt and stay there after purifi- determined by X-ray fluorescence spectrometer ARL cation. In case of large consumption of additives their Advant’X (Thermo Scientific, USA). Refining additives content in zinc reaches high values as expected from the were introduced to the melt in various ratios in respect nature of their interaction with zinc in correspondence to the iron impurity ranging from 0.1 to 1.3 %. with the phase diagrams considered. Obviously, the process of purification of unalloyed zinc from iron using Experimental results on purification of zinc melt the additives studied will be difficult under conditions of from iron using additive elements large-scale production due to their high consumption. At The results of the experimental investigation of zinc the same time, the use of aluminum and magnesium as purification from iron impurity using additive elements alloying and extracting additive elements in production (Fig. 3) shows the different refining capacity of the ad- of zinc alloys provides neutralizing the negative effect ditives used. The discrepancy between the calculated of iron because of the formation of complex products and experimental data referring to the efficiency of of interaction. In this case the refining is simultaneously purification of zinc from iron impurity can be explained combined with alloying of the base metal with aluminum with some of the assumptions made (complete interac- or manganese. tion of the extracting additives with iron impurity used, Thus, it is recommended to use silicon as an extract- selected phases of only one stoichiometric composition ing additive in the production of unalloyed zinc of a low taken into consideration in equilibrium phase diagrams iron content, while aluminum and manganese - in case preparation, complete removal of the reaction products of zinc alloys production.

Fig. 3. Experimental data on efficiency of purification of zinc from iron by extracting additives.

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