Thermodynamic Modeling of Zinc Speciation in Electric Arc Furnace Dust

Chris A. Pickles1; 1 Introduction

In the secondary steelmaking process, steel scrap is 1 Robert M. Buchan Department of Mining, Queen’s Uni- remelted in an electric arc furnace (EAF) and the outputs versity, Kingston, Canada. are steel, slag, off-gas and a dust. On average, about ten to twenty kilograms of dust are generated per ton of steel and Abstract. The remelting of automobile scrap, contain- as a result about one million tons of dust is produced annu- ing galvanized steel, in an electric arc furnace (EAF) re- ally in North America. A portion of the automobile scrap is sults in the generation of a dust, which contains consid- galvanized steel and consequently, the dust has high levels erable amounts of zinc and other metals. Typically, the of zinc, which is of considerable economic value. In addi- amount of zinc is of significant commercial value, but tion to zinc, there are substantial amounts of iron and cal- the recovery of this metal can be hindered by the var- cium and numerous other metals and also non-metals in the ied speciation of zinc. The majority of the zinc ex- dust. The dust composition is variable since it depends upon ists as zincite (ZnO) and zinc ferrite (ZnFe2O4)orfer- the scrap composition and the operating parameters of the ritic spinels ((ZnxMny Fe1xy /Fe2O4), but other zinc- furnace. Numerous studies have been performed in order to containing species such as zinc chloride, zinc hydroxide characterize the dusts from various plants [1–7]. Since the chlorides, hydrated zinc sulphates and zinc silicates have dust contains leachable lead, cadmium, chromium and zinc also been identified. There is a scarcity of research lit- then it is considered as a hazardous waste in most countries. erature on the thermodynamic aspects of the formation of There are two major approaches for dealing with the these zinc-containing species, in particular, the minor zinc- dust [8–11]: (1) stabilization followed by disposal in a haz- containing species. Therefore, in this study, the equilibrium ardous landfill and (2) processing for the recovery of the module of HSC Chemistry® 6.1 was utilized to calculate the valuable metals. In the metal recovery processes, usually types and the amounts of the zinc-containing species. The zinc and lead are extracted, but in some cases, depending variables studied were: the gas composition, the tempera- on their concentrations and the economics, copper and iron ture and the dust composition. At high temperatures, zincite may also be recovered. Many processes have been pro- forms via the reaction of zinc vapour with oxygen gas and posed and evaluated and these are generally based on py- the zinc-manganese ferrites form as a result of the reaction rometallurgical (High Temperature) and/or hydrometallur- of iron-manganese particles with zinc vapour and oxygen. gical (Aqueous Chemistry) techniques. Only a few pro- At intermediate temperatures, zinc sulphates are produced cesses have been developed commercially and the major- through the reaction of zinc oxide and sulphur dioxide gas. ity of these are based on the well proven, Waelz rotary kiln As room temperature is approached, zinc chlorides and flu- technology [12–14]. orides form by the reaction of zinc oxide with hydrogen The dust contains numerous elements, many of these chloride and hydrogen fluoride gases, respectively. Zinc sil- can be considered as contaminants, and some are penalty el- icate likely forms via the high temperature reaction of zinc ements in the metal recovery process and this disadvantages vapour and oxygen with silica. In the presence of excess the economics of metal recovery from the dust, in compar- water and as room temperature is approached, the zinc sul- ison to zinc concentrates. Although present in only small phates, chlorides and fluorides can become hydrated. amounts, the halogens in the dust can have deleterious effects on the subsequent processes for the recovery and the Keywords. Automobile scrap, electric arc furnace, dust, recycling of the metals or for the disposal of the dust it- zinc, zinc ferrite. self. In pyrometallurgical processing, the presence of salt vapour inhibits the operation of the zinc metal condenser. Consequently, the non-ferrous metals are recovered in oxide form along with some metal halides as a crude zinc oxide. In the hydrometallurgical extraction of zinc from the Corresponding author: Chris A. Pickles, Robert M. Buchan EAF dust via the sulphate route, the halogens can contam- Department of Mining, Queen’s University, Kingston, Ontario, inate the electrolyte utilized in the electrowinning of zinc. K7L-3N6, Canada; E-mail: [email protected]. Also secondary wastes, containing halogens, can be gen- Received: June 29, 2010. Accepted: July 20, 2010. erated as a result of the processing of the dust. With re- 4 C. A. Pickles gards to stabilization and disposal, some of the minor zinc- als, physical ejection of metal and slag particulates into the containing species may be more leachable than both the furnace atmosphere by bursting of gas bubbles and the en- zinc oxide and the zinc-containing ferrites and thus pose a trainment of low density metallurgical reagents, such as potential contamination risk. Furthermore, if the EAF dust carbon or lime, in the off-gas [22–28]. The boiling point or any residues formed as a result of dust processing are of zinc (1180 K) is much lower than the typical operat- utilized in construction materials such as concrete, then the ing temperature of the steelmaking process (1873 K) and presence of the halogens can have harmful effects on the thus the zinc coating on the galvanized steel evaporates and properties of the concrete. As a result, either expensive py- enters the gas phase. Once in the gas phase, the zinc can rohydrolysis or aqueous dissolution is required to remove react with other gaseous species such as oxygen, chlorine the halogen contaminants prior to metal recovery or stabi- and sulphur. Additionally, zinc could react with both oxy- lization. Additionally, dioxins and furans may be present gen and other oxides, such as silica. The boiling point of in the dust or form during subsequent dust treatment pro- iron is 3135 K and therefore it would be expected that there cesses. Thus, it is important to understand both the source would be little evaporation of iron. However, a number of and the formation mechanisms of the halogen-containing oxygen-enhanced evaporation [29–31] and bubble-bursting compounds. mechanisms [32–34] have been proposed to explain the ori- With regards to the zinc-containing species, some pre- gin of the considerable amount of iron in the steelmaking vious research has been performed on the Zn-Fe-O [15– dust. Most commercial automotive steels contain a rela- 19], Zn-Fe-O-H-C [20] and Zn-Fe-Mn-O-H-C [21] sys- tively small percentage of manganese (0.3% to 0.8%). In tems concerning the formation of zincite (ZnO), zinc fer- general, the behaviour of manganese is similar to that of rites (ZnFe2O4/ and zinc-manganese ferrites or ZMFO iron. However, the boiling point of manganese (2335 K) (ZnxMny Fe1xy /Fe2O4/ in EAF dust. Zinc ferrite or is significantly lower than that of iron (3134 K), and as a ZMFO formation is mitigated by high zinc to iron ra- result, the manganese to iron ratio of the dust can be sig- tios, low oxygen potentials, high dust loadings, promot- nificantly higher than that of the steel, from which it origi- ing the formation of zinc hydroxide (Zn(OH)2/, zinc car- nated. Calcium and silicon are present in oxide form in the bonate (ZnCO3/ and hydrozincite (Zn5(OH)6(CO3/2/ and slag and particles of the slag can be ejected into the furnace also by high levels of calcium oxide. Additionally it atmosphere via the bursting of gas bubbles. Additionally, was proposed that the ZMFO is formed by the sequen- calcium oxide is added as a flux and because of its low den- tial oxidation of iron-manganese particles in the pres- sity it can be captured in the off-gas with the dust. Silica ence of zinc vapour as follows, with decreasing tempera- mainly originates from various glasses that are present in ture; Fe-Mn, (Fe,Mn)O, (Fe,Mn)O-Fe3O4, (Zn,Mn)Fe2O4- the scrap. Fe3O4,(ZnxMny Fe1xy /Fe2O4 [21]. However, there Automobile scrap can contain chlorine and fluorine in is very little information in the literature regarding the various forms. Organic compounds, such as paints, plas- formation of other zinc-containing species in the EAF tics and rubber, are present in the scrap and usually contain dust. chloride- and fluoride-containing species, such as polyvinyl In this research, the conditions for the formation of the chloride. In addition, road salt, which consists of mainly various zinc-containing species that are found in EAF dust sodium chloride but also smaller amounts of potassium and were investigated. Firstly, the origin of the various ele- calcium chloride and possibly calcium fluoride, can be en- ments of interest in the EAF dust is described. Secondly, trapped in the scrap. Some of the chlorine, resulting from the procedure utilized to calculate the equilibrium com- the decomposition of these materials in the furnace, can re- position is discussed. Thirdly, the method by which the act with hydrogen to form hydrogen chloride gas or with simulated gas composition was selected is illustrated. Fi- the various metals to form metal chlorides and metal oxy- nally, since the major gaseous species, which affect the chlorides. Electric arc furnace dusts are known to con- dust composition are oxygen, chlorine, fluorine and/or sul- tain chlorine [35]. The properties of zinc chloride are well phur, then equilibrium calculations were performed for known but it has only recently been shown that zinc oxy- the Zn-Fe-Mn-O-C-H-Cl, Zn-Fe-Mn-O-C-H-F and Zn-Fe- chloride (ZnOCl) can form under certain conditions in the Mn-O-C-H-S systems. Furthermore, EAF dusts also con- gas phase [36]. Furthermore, several zinc hydroxide chlo- tain some calcium and silicon and therefore calculations rides are known to exist and the two most commonly stud- were carried out for the Zn-Fe-Mn-O-C-H-Cl-F-S-Ca-Si- ied are; Zn5(OH)8Cl2H2OorZnCl24Zn(OH)2H2O and system. ˇ-Zn(OH)Cl. Li and Tsai [37] have reported the presence of ZnCl24Zn(OH)2H2O in dusts from Taiwanese steel plants. Unfortunately, there is very little information in the literature on the thermodynamic properties of zinc oxychlorides 2 Origin of Relevant Elements in EAF Dust and zinc hydroxide chlorides. The deportment of fluorine The dust forms as a result of three major processes: evapo- in the dust can be quite different from that of chlorine, ration and oxidation of relatively low vapour pressure met- with most of it being reported as CaF2,MgF2 and Ca-F- Thermodynamic Modeling of Zinc Speciation in Electric Arc Furnace Dust 5

Cl compounds [6]. The differing behaviour of fluorine, in contrast to chlorine, may be due to the higher stability Nitrogen Sulphur dioxide of calcium fluoride in comparison to the major chlorine- Oxygen Sulphur trioxide containing species such as sodium chloride and potassium Carbon monoxide Vaporized hydrocarbons chloride. The major sources of sulphur in the off-gas from the fur- Carbon dioxide Ozone, nitrous oxides nace are the various hydrocarbons that are in the scrap or Hydrogen Hydrogen sulfide deliberately added to the furnace. Turnings and borings Water vapour Hydrochloric acid can be present in the charge and automobile scrap contains Hydrofluoric acid some oil. Furthermore, fuels added to the furnace such as Silicon tetrafluoride coal or oil can be a major source of sulphur. Ultimately this can result in the production of sulphur dioxide at high Ammonia and acetlylene temperatures in the furnace atmosphere and subsequently Dioxins as the off-gas begins to cool, the sulphur dioxide can be converted into sulphur trioxide. On further cooling, metal sulfides and/or sulfates can be produced and EAF dust is Table 1. Typical constituents present in the electric arc known to contain these species [1]. If there is considerable furnace atmosphere during the remelting of carbon steel moisture in the off-gas, then hydrated zinc sulphates may (adapted from reference [39]). form.

chloride (ZnOCl) gas has been reported [36]. Additionally, the heat capacity of a compound can be approximated as 3 Equilibrium Calculations the sum of the known heat capacities of the elements. Thus, The equilibrium amounts of the various species were calcu- for zinc oxychloride, the elemental values of zinc and oxy- ® lated using the Equilibrium Module of HSC Chemistry 6.1 gen from gaseous ZnO and chlorine from gaseous ZnCl2 [38]. For any given reaction system, the various phases and were utilized in the calculations. There is even less infor- species that are known to be present are specified and the in- mation available on the thermodynamics of the hydroxide put consists of the amounts of the reacting species. Then the chlorides, but the thermodynamic properties of the simplest calculations are performed for isothermal and isobaric con- species, i.e. Zn(OH)Cl can be approximated. The standard ditions. The equilibrium composition is calculated using enthalpy and entropy were taken to be the sum of one-half the Gibbs free energy minimization method. Ideal mixing of the corresponding values for solid Zn(OH)2 and solid of species in a solution is assumed unless activity coeffi- ZnCl2. The heat capacity was determined in a manner sim- cients have been inputted. Kinetic effects can be taken into ilar to that for zinc oxychloride using the heat capacity val- account by deleting species, which would not be expected ues for solid Zn(OH)2 and solid ZnCl2. The estimated ther- to form. The major variables studied were; the composi- modynamic information for these species was then added to tion of the elements in the off-gas, the composition of the the HSC database. off-gas and the temperature of the off-gas during cooling. The typical components in the atmosphere of an elec- The furnace atmosphere was assumed to consist of mainly tric arc furnace are shown in Table 1 [39]. The major con- carbon monoxide, hydrogen, carbon dioxide, water vapour, stituents are seen to be carbon monoxide, carbon dioxide, oxygen and nitrogen. Other gases such as chlorine, fluorine hydrogen, water vapour, nitrogen and oxygen. Therefore, and sulphur could be added either individually or simulta- the gas composition can be described in terms of the N-O- neously. C-H system. Also of interest are the minor constituents: There is very little information in the literature regard- hydrogen chloride, hydrogen fluoride, silicon tetrafluoride, ing the thermodynamics of some of the species that can hydrogen sulphide, sulphur dioxide and sulphur trioxide. form in the dust such as ZMFO, zinc oxychloride and zinc There is very little information in the literature with re- hydroxide chlorides. Therefore, it was necessary to ap- gards to actual EAF off-gas compositions. In the absence proximate the thermochemical data for these species. The of any metallurgical additions, the conditions in the atmo- ZMFO ..ZnxMny Fe1xy /Fe2O4/ wasassumedtobea sphere of the freeboard of the furnace would be oxidizing. separate phase consisting of an ideal mixture of zinc ferrite However, if carbon is added to the slag and/or additional (ZnFe2O4/, manganese ferrite (MnFe2O4/ and magnetite hydrocarbon-based fuel sources are utilized this could pro- (Fe3O4/. The composition could be expressed in terms mote more reducing conditions. Furthermore, those studies of the amounts of zinc ferrite (ZnFe2O4/, manganese fer- that have been performed indicate that the gas composition rite (MnFe2O4/ and magnetite (Fe3O4/ or in terms of the can vary widely even within one heat. However, from the amounts of Zn .D x/,Mn.D y/ and Fe .D 1 x y/. literature, the following ranges of gas composition can be Recently, the standard free energy of formation of zinc oxy- considered as typical in the freeboard of the EAF in the 6 C. A. Pickles temperature range of 1250 to 1750 K: 5–30% CO, 10–30% ganese reaches a maximum at about 1500 K and this corre- H2O, 5–10% H2 and 10–30% CO2 [40]. The balance of the sponds to the peak in the amount of ZMFO. Additionally, as gas is mainly nitrogen and a small amount of oxygen. Us- the temperature drops, a large proportion of the zinc vapour ing 100 kmole of gas, an average gas composition of 17.5% is converted into condensed zinc oxide. Subsequently, the CO, 20% H2O, 7.5% H2, 20% CO2, 34% N2 and 1% O2 amount of manganese decreases while the amount of zinc was inputted into the Equilibrium Module and this gave the continues to increase and by about 1200 K the amounts of gas composition at 1600 K as follows: 17.28% CO, 21.83% manganese and iron have become insignificant. Therefore, 8 H2O, 5.95% H2, 20.59% CO2, 34.34% N2, 2:52 10 % at 298 K, the ZMFO is mainly zinc ferrite. However, ki- 8 CH4and 5:8810 %O2. In an actual EAF off-gas system, netic factors could play a role in the ZMFO composition at considerable air is introduced for post-combustion and/or room temperature, and if it was quenched from high tem- cooling purposes and therefore an extra 100 kmole of air peratures then it could contain more iron and/or manganese (21 kmole O2 and 79 kmole N2/ was added to give the stan- than the equilibrium values. As the temperature declines, dard gas composition. The standard amounts of zinc, iron the amount of hydrogen chloride gas remains relatively con- and manganese utilized in the calculations were 3 104, stant until room temperature is approached. Then the equi- 1 104 and 0:5 104 kmole, respectively. librium composition of the dust undergoes a dramatic shift, Another important factor is the dust loading, which is with the amount of hydrogen chloride gas falling rapidly. the mass of dust in a given volume of gas. During partic- The zincite and the ZMFO vanish and are replaced by zinc ularly turbulent portions of the heat such as oxygen injec- carbonate (ZnCO3/, hydrozincite Zn5(OH)6(CO3/2,zinc 3 tion, very high dust loading values of over 150 g/m have hydroxide (Zn(OH)2/, zinc chloride (ZnCl2/ and a small been reported [41]. As a result of government regulations, amount of zinc hydroxide chloride Zn(OH)Cl. The amount discharge values of less than 100 mg/m3 are typically re- of ZnOCl was found not to be significant at any tempera- quired [42]. For the standard conditions used in the equilib- ture. rium calculations, the dust loading was assumed to be about Typically, zinc carbonate, hydrozincite and zinc hydrox- 10 mg/m3. This ratio ensured that there was an oversupply ide are not usually present in EAF dusts. The formation of of gas to react with the dust particles and therefore the gas zinc carbonate, hydrozincite and zinc hydroxide would in- composition was independent of both the amount and the volve low temperature gas-solid reactions between gaseous composition of the dust. carbon dioxide and/or water vapour and solid zinc oxide and since these reactions are extremely slow then these species can be eliminated as possible reaction products. Figure 2 shows the equilibrium composition of the dust in the ab- 4 Results and Discussion sence of these three species. At high temperatures, the be- 4.1 The Zn-Fe-Mn-O-H-C-Cl System haviour is similar to that described previously in Figure 1. However, as room temperature is approached, again the Inputting the Zn-Fe-Mn-O-H-C-Cl system into the Equilib- amounts of zinc oxide and ZMFO decrease rapidly, but in rium Module of HSC Chemistry®6.1 generates a list of 246 this case the decline is limited. The reduction in the amount species. However, many of these would be unstable under of these oxides and also the amount of hydrogen chloride the present conditions and by eliminating these species, the gas corresponds to an increase in the amounts of zinc chlo- number was reduced to 58, as shown in Table 2. Seven sep- ride and zinc hydroxide chloride. Since these chlorine- arate phases were considered as follows: gases, chlorides, containing species form by the reaction of hydrogen chlo- oxides, metals, carbon, liquid water and spinel (ZMFO). ride gas with zinc oxide, then it would be expected that the Figure 1 shows the behaviour of the zinc-containing species kinetics of these reactions would be more favourable than and also hydrogen chloride gas from 298 to 2000 K for the formation of zinc hydroxide, zinc carbonate and hydroz- 4 a chlorine (Cl2/ gas addition of 1 10 kmole and for incite. Thus, zinc chloride and zinc hydroxide chlorides are the standard gas composition. Zinc and hydrogen chloride possible equilibrium reaction products as the dust compo- gases predominate at high temperatures with some ZMFO nents cool down to room temperature. In all subsequent and a small amount of gaseous zinc oxide. The spinel be- calculations, zinc carbonate, hydrozincite and zinc hydrox- gins to form at about 1900 K and it has been shown previ- ide have been excluded. ously [21], that initially it contains a large amount of iron It would be expected that the major variables affecting and some manganese and is therefore mainly magnetite. As the quantity of zinc chloride and zinc hydroxide chloride the temperature drops, the amount of iron decreases, while would be the amounts of chlorine, iron, zinc and the oxy- the amount of manganese increases. The amount of zinc in gen potential. Figures 3(a) to (c) show the effects of each of the ZMFO begins to increase at about 1700 K and this cor- these variables at room temperature (298 K) for the oth- responds to the decrease in the amount of zinc vapour. As erwise standard conditions and 1 104 kmole of chlo- the temperature drops even further, the amount of iron in rine gas. With regards to the effect of chlorine as shown the ZMFO continues to decrease, while the amount of man- in Figure 3(a), it can be seen that zinc hydroxide chlo- Thermodynamic Modeling of Zinc Speciation in Electric Arc Furnace Dust 7

GASES GASES CHLORIDES OXIDES OXIDES CARBON

N2 Mn FeCl2 FeCO3 Mn3O4 C CH4 MnCl FeCl3 FeO MnOFe2O3 WATER (l)

CO MnCl2 FeCl22H2O Fe2O3 Mn(OH)2 H2O.l/ CO2 MnO FeCl24H2O Fe(OH)2 MnOOH SPINEL Cl2 MnO2 FeCl36H2O Fe(OH)3 Zn(OH)Cl ZnFe2O4 Fe O2 MnCl2 Fe2O3H2O ZnO Fe3O4 FeCl2 Zn MnCl3 FeOOH METALS Fe2MnO4 FeO ZnCl MnCl2H2O MnCO3 Fe H2 ZnCl2 MnCl22H2O MnO Mn HCl ZnO MnCl24H2O MnO2 Zn H2O ZnOCl ZnCl2 Mn2O3

Table 2. Standard phases and species utilized in the equilibrium calculations.

Figure 1. Equilibrium amounts of the zinc-containing species and gaseous HCl as a function of temperature for the Zn-Fe-Mn-O-H-C-Cl system. Figure 2. Equilibrium amounts of the zinc-containing species and gaseous HCl as a function of temperature for the Zn-Fe-Mn-O-H-C-Cl system, in the absence of ZnCO3, ride begins to form at low chlorine levels, reaches a max- Zn(OH)2 and Zn5(OH)6(CO3/2. imum and then decreases as the amount of zinc chloride increases. The amount of zinc oxide continuously decreases with increasing chlorine and is mainly replaced by zinc and the amount of Zn(OH)Cl increases to some extent. chloride. Eventually, the zinc oxide completely disappears At high iron additions, the amounts of the zinc-containing at high chlorine additions, and subsequently zinc chloride species remain relatively constant but the amount of hy- coexists with an increasing amount of gaseous HCl. The drogen chloride gas begins to increase. As shown in Fig- amount of ZMFO initially decreases slowly with chlorine ure 3(c), higher zinc levels initially favour an increasing and reaches a slight minimum, which corresponds to the amount of zinc chloride, which levels off and then decreases maximum in the amount of zinc hydroxide chloride. There- as the amounts of Zn(OH)Cl and ZnO begin to increase. after, the amount of ZMFO increases, and reaches a max- Also, there is some gaseous HCl at low zinc amounts but imum, where both ZnO and Zn(OH)Cl have almost disap- it disappears with increasing zinc. Initially, the amount peared, and then the amount of ZMFO decreases rapidly. of ZMFO is very low but increases, reaches a maximum Figure 3(b) shows that initially, small iron additions re- and then decreases as the formation of ZnO becomes more sult in only small changes in the amounts of the zinc- favourable. At high zinc levels, only zinc hydroxide chlo- containing species with the amount of ZnCl2 increasing ride and zinc oxide coexist. slightly and the amounts of ZnO and Zn(OH)Cl decreas- The effect of the amount of oxygen in the system on the ing somewhat, while the amount of ZMFO remains low and zinc-containing species is shown in Figure 3(d). At low constant. Subsequently, the amounts of ZnO and ZnCl2 de- oxygen additions, the amounts of zinc oxide, zinc chloride, crease, while the amount of ZMFO increases signiﬁcantly zinc hydroxide chloride and ZMFO are relatively constant. 8 C. A. Pickles

Figure 3(a). Effect of the amount of chlorine on the Figure 3(c). Effect of the amount of zinc on the equilib- equilibrium amounts of the zinc-containing species and rium amounts of the zinc-containing species and gaseous gaseous HCl for the Zn-Fe-Mn-O-H-C-Cl system. HCl for the Zn-Fe-Mn-O-H-C-Cl system.

Figure 3(b). Effect of the amount of iron on the equilib- Figure 3(d). Effect of the amount of oxygen on the equilibrium amounts of the zinc-containing species and gaseous rium amounts of the zinc-containing species and gaseous HCl for the Zn-Fe-Mn-O-H-C-Cl system. HCl for the Zn-Fe-Mn-O-H-C-Cl system.

However, at an oxygen addition of about 12 kmole there fect of the amount of calcium oxide in the dust on the is a rapid decrease in the amount of zinc oxide and a cor- amounts of the condensed zinc-containing species and also responding increase in the amounts of zinc chloride, zinc hydrogen chloride gas at a temperature of 298 K. The stan- hydroxide chloride and ZMFO. This discontinuity corre- dard conditions were utilized and the amount of chlorine 4 sponds to the oxidation of the hydrated iron and manganese gas (Cl2/ was 1 10 kmole. It can be seen that the chlorides, and the released chlorine converts some of the amounts of ZnCl2, Zn(OH)Cl and also HCl(g) decrease zinc oxide into zinc chloride and zinc hydroxide chloride. rapidly with increasing calcium oxide addition, while con- The addition of calcium will give rise to additional versely there is increased formation of ZnO. It is noteworthy species in the dust such as calcium oxide (CaO), various cal- that some Zn(OH)Cl exists even at calcium oxide additions 4 cium ferrites (xCaOyFe2O3/ calcium carbonate (CaCO3/ above 1 10 kmole. Figure 4(b) shows the behaviour and calcium hydroxide (Ca(OH)2/. Furthermore, the pres- of the calcium-containing species. The amount of calcium ence of chlorine will result in the formation of calcium chloride hexahydrate (CaCl26H2O) initially increases but chloride (CaCl2/ and various hydrated calcium chlorides then levels off and at this point the amount of calcium car- (CaCl2 xH2O). Since all these species are relatively sta- bonate becomes signiﬁcant and continues to increase with ble then it would be expected that the amounts of the other increasing calcium oxide. This reduces the amount of chlo- metal chlorides would decrease. Figure 4(a) shows the ef- rine available to react with zinc. Thermodynamic Modeling of Zinc Speciation in Electric Arc Furnace Dust 9

Figure 4(a). Effect of the amount of calcium oxide on Figure 5(a). Effect of the amount of calcium oxide on the the equilibrium amounts of the zinc-containing species and equilibrium amounts of the zinc-containing species for the gaseous HCl for the Zn-Fe-Mn-O-H-C-Cl-Ca system. Zn-Fe-Mn-O-H-C-Cl-Ca system in the absence of CaCO3 and Ca(OH)2.

Figure 4(b). Effect of the amount of calcium oxide Figure 5(b). Effect of the amount of calcium oxide on on the equilibrium amounts of the calcium-containing the equilibrium amounts of the calcium-containing species species for the Zn-Fe-Mn-O-H-C-Cl-Ca system. for the Zn-Fe-Mn-O-H-C-Cl-Ca system in the absence of CaCO3and Ca(OH)2.

In a manner similar to zinc carbonate and zinc hydrox- ZMFO. At calcium oxide additions above 2 104 kmole, ide, as discussed previously, it would also be expected that the amount of free calcium oxide increases rapidly. the formation of calcium carbonate and calcium hydroxide would be limited by kinetic factors. Therefore, Fig- ures 5(a) and (b) show the results for the zinc-containing species and the calcium-containing species in the absence 4.2 The Zn-Fe-Mn-O-H-C-F System of calcium carbonate and calcium hydroxide. A comparison of Figure 4(a) with Figure 5(a) shows that, in the absence Figure 6 shows the behaviour of the zinc-containing species of calcium carbonate and calcium hydroxide, the conver- and also hydrogen fluoride gas as a function of temper- 4 sion of zinc chloride and zinc hydroxide chloride to zinc ature for a fluorine (F2/ gas addition of 1 10 kmole oxide occurs more readily. Additionally, the amount of and for the standard gas composition. The behavior of the ZMFO declines more quickly with increasing calcium ox- zinc-containing species is similar to that discussed previ- ide. As can be seen by comparing Figures 4(b) and 5(b) ously, however, it can be seen that some hydrated zinc flu- the more rapid decrease in the zinc-containing species is oride (ZnF2H2O) and also ZnF2 form as room tempera- due to the increased amounts of the hydrated calcium chlo- ture is approached and this corresponds to a decrease in the rides. The more rapid reduction in the amount of ZMFO, amount of hydrogen fluoride gas. These zinc fluorides form in the absence of zinc carbonate and zinc hydroxide, is due at the expense of the zinc oxide. Since the formation of the to the formation of 2CaOFe2O3,which is more stable than zinc fluorides involves a reaction between hydrogen fluo- 10 C. A. Pickles

Figure 7(b). Effect of the amount of iron on the equilib- Figure 6. Effect of temperature on the equilibrium amounts rium amounts of the zinc-containing species in the Zn-Fe- of the zinc-containing species in the Zn-Fe-Mn-O-H-C-F Mn-O-H-C-F system at room temperature. system.

Figure 7(c). Effect of the amount of zinc on the equilib- Figure 7(a). Effect of the amount of ﬂuorine on the equi- rium amounts of the zinc-containing species in the Zn-Fe- librium amounts of the zinc-containing species in the Zn- Mn-O-H-C-F system at room temperature. Fe-Mn-O-H-C-F system at room temperature.

zincite has been expended to form ZMFO, then some zinc ride gas and zinc oxide, then this reaction could be kinet- from the hydrated zinc fluoride is employed to form ad- ically favourable. In a manner similar to the Zn-Fe-Mn- ditional ZMFO. As shown in Figure 7(c), the amount of O-H-C-Cl system it would be expected that the amount of hydrated zinc fluoride increases with increasing zinc and zinc-containing fluorides at 298 K would be affected by the then becomes constant, as the amount of fluorine is limited. amounts of fluorine, iron, zinc, the oxygen potential and Thereafter, the amount of ZMFO increases and eventually the amount of calcium oxide. Figure 7(a) shows the ef- begins to level off, at which point the amount of zinc ox- fect of the fluorine content of the gas and it can be seen ide increases. The amount of zinc fluoride increases slowly that the amount of hydrated zinc fluoride increases with the with increasing zinc. In contrast, to the chlorine-containing fluorine content and this is first at the expense of the zinc system, the amount of oxygen had no effect on the amounts oxide and subsequently ZMFO. The amount of zinc fluo- of the zinc-containing fluorine species and this can be at- ride increases slowly with increasing fluorine. At high flu- tributed to the higher stability of these species. orine additions the amounts of zinc fluoride and hydrated The effect of calcium oxide is shown in Figure 8, where zinc fluoride are constant and the amount of gaseous HF it can be seen that the amounts of zinc fluoride and also increases. Increasing the amount of iron favours the for- hydrated zinc fluoride decreased relatively rapidly with in- mation of ZMFO and this consumes the zincite, as shown creasing calcium oxide. Correspondingly, the amounts of in Figure 7(b). At high iron additions and after all of the calcium fluoride and zinc oxide increased and then levelled Thermodynamic Modeling of Zinc Speciation in Electric Arc Furnace Dust 11

Figure 9. Effect of temperature on the zinc-containing species and sulphur dioxide and sulphur trioxide for the Zn- Figure 8. Effect of the amount of calcium oxide on the Fe-Mn-O-C-H-S system. equilibrium amounts of the zinc-containing species in the Zn-Fe-Mn-O-H-C-F system at room temperature. at high sulphur levels the ZMFO disappears. Similarly, the amount of zincite decreases and ultimately ceases to exist. off once all the available fluorine had been consumed. The On the other hand, the amounts of the hydrated zinc sul- amount of zinc oxide continued to increase slowly and the phates increase and this is more significant for the sulphates amount of ZMFO decreased as a result of the liberation of with higher water contents. zinc oxide from the ZMFO due to the higher stability of the As shown in Figure 10(b), the amounts of the hydrated calcium ferrites. For calcium oxide additions of over about zinc sulphates are essentially independent of the amount of 1 104 kmole there is an insignificant amount of the zinc iron. At an iron addition of about 1 104 kmole, ZMFO fluorides. This is due to the higher stability of calcium flu- begins to form and the amount of zincite decreases. How- oride. ever, at high iron additions, the amounts of zincite and ZMFO begin to level off as the standard amount of zinc is 3 104kmole and therefore, an increasing amount of 4.3 The Zn-Fe-Mn-O-C-H-S System iron favours the formation of ZMFO at the expense of the zincite. Figure 10(c) shows the effect of the zinc content of Figure 9 shows the effect of temperature on the zinc- the dust on the zinc-containing species. At low zinc levels containing species, sulphur dioxide and sulphur trioxide for the hydrated zinc sulphates predominate. However, at a zinc 4 the standard conditions and a sulphur (S2.g// addition of content of about 2 10 kmole, zinc oxide begins to form 1 104 kmole. At high temperatures, sulphur dioxide and beyond this point the amount of zinc oxide increases exists with gaseous zinc and some ZMFO. The amount of continuously, while the amounts of the hydrated zinc sul- sulphur dioxide begins to decrease below about 1400 K and phates level off. The amount of ZMFO remains very small. is converted into some sulphur trioxide and zinc sulphate. The effect of oxygen is shown in Figure 10(d), where it This reduces the amounts of zinc oxide and ZMFO. Also a can be seen that at low oxygen levels, zinc sulphide, zincite small amount of zinc oxysulphate forms. As the tempera- and ZMFO are stable. At higher oxygen levels, the zinc sul- ture drops towards room temperature, the zinc sulphate is phide disappears; the amount of ZMFO drops, the amount replaced by hydrated zinc sulphates with increasing water of zinc oxide increases and hydrated zinc sulphates form. content. Correspondingly, the amount of ZMFO exhibits Again, the hydrated zinc sulphates with a higher moisture an increase but then disappears as room temperature is ap- content are present in larger amounts. The discontinuities proached. On the other hand, the amount of zinc oxide in- at about 14 kmole of oxygen correspond to the availability creases slightly towards room temperature. However, since of sufficient oxygen to convert the zinc sulphide to the sul- the formation of zinc sulphate involves the reaction of zinc phates. Below and above this discontinuity, the amounts of oxide with gaseous sulphur dioxide then this reaction would the species are relatively constant. not be expected to be kinetically favourable. Figure 11 shows the effect of calcium oxide on the zinc- In a manner similar to chlorine, the significant variables containing species. Higher amounts of calcium oxide result affecting the zinc-containing species at room temperature in increasing amounts of zinc oxide and a decrease in the would be the amounts of sulphur, iron, zinc and the oxygen amounts of the hydrated zinc sulphates. This is mainly due potential. As shown in Figure 10(a), increasing the amount to the formation of the more stable calcium sulphates and of sulphur results in a decrease in the amount of ZMFO and hydrated calcium sulphates. 12 C. A. Pickles

Figure 10(a). Effect of the amount of sulphur on the Figure 10(d). Effect of amount of oxygen on the equilib- amounts of the zinc-containing species in the system at rium amounts of the zinc-containing species at 298 K for 298 K for the Zn-Fe-Mn-O-C-H-S system. the Zn-Fe-Mn-O-C-H-S system.

Figure 10(b). Effect of the amount of iron the equilibrium Figure 11. Effect of amount of calcium oxide on the equi- amounts of the zinc-containing species at 298 K for the librium amounts of the zinc-containing species at 298 K for Zn-Fe-Mn-O-C-H-S system. the Zn-Fe-Mn-O-C-H-S system.

4.4 The Zn-Fe-Mn-O-C-H-Cl-F-S-Ca-Si System

In order to provide a more realistic description of the dust composition, all of the elements that have been discussed previously were included and additionally silicon was added, since most dusts contain some silica. Here the amounts of chlorine (Cl2.g//, sulphur (S2.g// and ﬂuorine 5 (F2.g// were reduced to 1 10 kmole, again to more closely approximate the composition of actual dusts, and the amounts of calcium oxide and silica were each 1 104 kmole. For the remaining elements, the amounts utilized in the calculations were the same as the standard composition. The major zinc-containing species are shown as a function of temperature in Figure 12(a) and it can be seen Figure 10(c). Effect of amount of zinc on the equilibrium that the main species at room temperature are zinc oxide, amounts of the zinc-containing species at 298 K for the ZMFO and zinc orthosilicate (ZnSi2O4/. It can be seen Zn-Fe-Mn-O-C-H-S system. that the zinc orthosilicate forms in two stages. From about Thermodynamic Modeling of Zinc Speciation in Electric Arc Furnace Dust 13

5 Conclusions

(1) The formation of zinc-containing species in EAF dust was investigated using the equilibrium module of HSC Chemistry® 6.1. Thermodynamically, zinc hydroxide, zinc carbonate and hydrozincite should be the predominant species in EAF dusts at room temperature but they are not usually found to any great extent in EAF dusts because of kinetic limitations. The thermodynamic calculations demonstrate that zincite, zinc-manganese ferrites, zinc chlorides, zinc hydroxide chloride, zinc fluorides, hydrated zinc sulphates and zinc silicates are possible species that could be present in EAF dust. Zincite arises from the oxidation of gaseous zinc. The zinc-manganese ferrite forms as a result of the reaction of zinc vapour and oxygen with an Figure 12(a). Effect of temperature on the equilibrium iron-manganese particle. Zinc sulphates form at intermedi- amounts of the major zinc-containing species in the Zn- ate temperatures via the reaction of gaseous sulphur diox- Fe-Mn-O-C-H-Cl-F-S-Ca-Si system. ide and zinc oxide. As room temperature is approached, zinc chlorides and zinc fluorides are produced by the chlo- rination or fluorination of zinc oxide by hydrogen chloride and hydrogen fluoride gases, respectively. Zinc silicate is produced by the reaction of gaseous zinc and oxygen with silica. (2) Increasing levels of chlorine in the off-gas favour the formation of zinc chloride at the expense of zinc hydroxide chloride, zincite and ZMFO. Increasing amounts of iron promote the formation of ZMFO and this lowers the amount of zinc available to form either zinc chloride or zinc hydroxide chloride. Increasing the amount of zinc results in increases in the amounts of zinc oxide and zinc hydroxide chloride, but lowers the amounts of ZMFO and zinc chloride. Low oxygen potentials favour the formation of zinc oxide, while higher oxygen potentials favour zinc hydroxide chloride, zinc chloride and ZMFO. High calcium oxide Figure 12(b). Effect of temperature on the equilibrium levels in the dusts result in reduced amounts of zinc chloride amounts of the minor zinc-containing species in the Zn- and zinc hydroxide chloride. Fe-Mn-O-C-H-Cl-F-S-Ca-Si system. (3) The calculations show that zinc fluoride and hydrated zinc fluoride can form in EAF dust if there is sufficient fluorine in the off-gas. These form at the expense of zincite 1500 K to about 1300 K, zinc vapour reacts with oxygen and ZMFO. Hydrated zinc fluoride is more stable than zinc and silica to form Zn2SiO4, while from about 1000 K to fluoride. The amount of zincite decreases with increasing 700 K zinc oxide reacts with silica to form Zn2SiO4.Again iron as more zinc ferrite forms but the amounts of zinc since solid-solid reactions are relatively slow particularly fluoride and hydrated zinc fluoride are not significantly af- at low temperatures, then it would be expected that the fected. Both of these fluorides increase with increasing zinc majority of the zinc silicate would be formed as a result but their continued formation is limited by the amount of of the higher temperature reaction. The minor-zinc con- fluorine. The amount of these fluorides increases with in- taining species are shown in Figure 12(b), where it can be creasing zinc until all of the fluorine is consumed. There is seen that the amount of zinc metasilicate (ZnSiO3/ reaches no effect of oxygen but calcium oxide significantly reduces a maximum value at about 1450 K, but then decreases to the amounts of the fluorides because of the higher stabil- insignificant amounts at room temperature. As room tem- ity of calcium fluoride. Additionally, the amount of zincite perature is approached a number of minor zinc-containing increases, mainly because of the higher stability of the cal- species appear and these are zinc chloride, zinc hydroxide cium ferrites in comparison to zinc ferrite. As a result of chlorides and the hydrated zinc sulphates: Zn2SO47H2O, the high stability of calcium fluoride, the amounts of zinc Zn2SO46H2O, Zn2SO42H2O and Zn2SO4H2O. fluoride-containing species in the dust tend to be very low. 14 C. A. Pickles

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