Mineralogical and Chemical Characteristics of Slags from the Pyrometallurgical Extraction of Zinc and Lead

: The slags derived from the fire refining of lead bullion, differ distinctly in the mineral composition, which results from the fact that these slags are end products of a series of chemical reactions (of both reduction and oxidation). The most common phases included in the refining slags are sulphates and hydrated sulphates (anglesite, gypsum, ktenasite and namuvite), oxides and hydroxides (wustite and goethite), nitrates (gerhardtite) and silicates (kirschsteinite and willemite). The other phases are sulphides and hydrated sulphides (sphalerite and tochilinite), metals (metallic Pb) and glass. Among the mineral components of these slags can be distinguished—primary mineral constituents, phase constituents formed in the ISP process and lead refining, secondary mineral constituents, formed in the landfill. The slags contain, in chemical terms, mainly FeO, CuO and SO 3 , PbO, in smaller contents SiO 2 , Al 2 O 3 and CaO, TiO 2 , MnO, MgO, K 2 O, P 2 O 5 . The mineralogical and chemical composition indicate that slags may be a potential source of metals recovery and pyrometallurgical processing of these wastes seems to be highly rational.


Introduction
Pyrometallurgical extraction of zinc and lead using the Imperial Smelting Process (ISP) is based on the reduction of roasted Zn-Pb concentrate with coke at 1000 • C in a shaft furnace. The feedstock for the process is a mixture of zinc-lead concentrates; materials recycled from the process, that is, sludges, dusts, dross; secondary raw materials such as scrap zinc alloys, zinc dross, crude lead from other sources and waste, including-dusts from steel making, zinc dust and dross, zinc sludge, lead oxide, cable scrap. The diversity of feedstock implicates varying chemical and mineral composition of products and waste generated in the ISP process.
The products of the process include crude zinc and crude lead, which are then rectified (Zn) or refined (Pb). In the process of multi-stage lead fire refining, slag is produced, which is the only waste of the ISP process deposited in a landfill [1][2][3].
These slags contain a number of chemical constituents, of which the dominant ones are-ZnO, PbO, Fe 2 O 3 , CaO, SiO 2 and their content in the slag exceeds 10% [4].
The mineral composition of slags from the refining process is varied and the most common phase constituents are oxides and hydroxides (zincite ZnO, wustite FeO, hematite  x-average, s-standard deviation, V-coefficient of variation, LLOD-lower than limit of detection. The results are given with estimated uncertainties.
The content of the other chemical constituents is low. The average content of SiO 2 , Al 2 O 3 and CaO is a few wt %, whereas that of TiO 2 , MnO, MgO, K 2 O and P 2 O 5 hardly exceeds 0.60 wt %. Important information, arising from the chemical composition of the tested samples taken from the top layer of the landfill, is the total amount of metals that could be recovered. These constituents include PbO, CuO and ZnO, the total content of which ranges from 40.89 to 51.99 wt % and the value of the coefficient of variation V is 10%, which indicates a potential for their recovery.

Trace Elements
Among the trace elements, Ag, As, Ba, Bi, Cd, Co, Cr, In, Ni, Sb, Se and Tl were determined. The prevalence of As is evident, with its contents ranging from 0.94 to 1.33 wt %, hence in principle arsenic can be regarded as one of the major chemical constituents (Table 2).
Cadmium and antimony are the elements the contents of which are lower than that of arsenic but several times higher than that of the other. Their content ranges from 0.16 to 0.35 wt % for Cd and over 0.40 wt % for Sb. In the case of antimony, it should be pointed out that these are the values determined for two samples (WZI and WZII), as the content in the other samples was below the lower limit of detection.
The content of elements such as Ag, Ba, Co, Cr, In, Ni or Se does not exceed 0.05 wt % and in the case of Bi and Tl it is even less than 0.01 wt %. In addition, some of these elements are not present in all of the tested samples. This is especially true for bismuth and thallium, which were found only in sample WZI and for silver, which was found in samples WZI, WZII and WZVI.

Major Phase Constituents
The main phase constituents present in the charge mixture as shown in the studies [17,18] included galena PbS, sphalerite ZnS, iron sulphides, zincite ZnO, anglesite PbSO 4 , lead oxide PbO, FeO-ZnO oxides and glass. They were included in the feedstock for the process (zinc blend concentrate, galena concentrate) and other constituents, which formed the charge mixture, such as-semi-finished products (Zn-Pb sinters), waste (dust, dross, slag), products from the process (crude zinc, crude lead). Yet in the composition of the main phases of the tested samples taken from the top layer of the landfill, as indicated by the present studies, only anglesite (lead sulphate), sphalerite (zinc sulphide) and glass were present among the phases of the charge mixture. However, new phases formed during the process and resulting from hypergenic transformations of these phases, taking place in the landfill, were also found. Among the phases found in the tested samples of the top layer of the landfill, the following groups were distinguished: The presence of these phases is evidenced by the characteristic reflections in the diffractograms of the tested samples ( Figure 1). Worth noting is the varied intensity of some basic reflections in the diffractograms, assigned to individual phases, which indicates different content of these in the tested samples.
The study of the chemical composition in the micro-areas revealed that the grains of the main phases formed intergrowths of very small clusters of individual phases. Grains composed of one phase only occur very rarely. The determined chemical composition of the grains of the main phase in the tested samples indicates that in some cases admixtures of various elements, mainly metals, are present. It was found, for instance, that:  in the tested samples indicates that in some cases admixtures of various elements, mainly metals, are present. It was found, for instance, that: − wustite (Figure 2, Table3) may contain admixtures of Zn (up to 5.68 wt%), Cu (up to 5.59 wt%), Sn (up to 1.61 wt%), Sb (up to 2.09 wt%) and of Pb (2.95 wt%), − kirschsteinite ( Figure 3, Table 3) contained Zn (up to 6.17 wt%), Pb (up to 2 wt%), Cu (up to 1.12 wt%), Mn (up to 0.83 wt%).     There are two characteristic relationships in the chemical composition of wustite. With increasing iron content, the content of copper and zinc decrease (Figures 4 and 5). At the same time the R 2 value of the trend line is very high: 0.74 for Cu and 0.83 for Zn. Therefore, it can be concluded that both copper and zinc substitute iron in wustite, which is quite common in this type of iron oxides [19][20][21][22].  There are two characteristic relationships in the chemical composition of wustite. With increasing iron content, the content of copper and zinc decrease (Figures 4 and 5). At the same time the R 2 value of the trend line is very high: 0.74 for Cu and 0.83 for Zn. Therefore, it can be concluded that both copper and zinc substitute iron in wustite, which is quite common in this type of iron oxides [19][20][21][22].
There are a few characteristic relationships in the chemical composition of kirschsteinite:        The relationship between silicon and aluminum may indicate a diadochal substitution of silicon and aluminum, due to their crystallographic similarity (similar ionic radius), which is quite common in silicates [23]. In turn, the relation between Ca and Fe is also geochemically justified in the group of minerals to which kirschsteinite belongs [24][25][26][27].
However, the relations between copper and zinc and iron in kirschsteinite are interesting in the context of those previously determined in wustite. Although the zinc content in both phases decreases with increasing iron content, the copper content in kirschsteinite increases while in wustite it decreases. It is difficult to explain this phenomenon conclusively.
Thus, among the phase constituents of the examined waste of the top layer, three types can be distinguished, taking into account their origin: − mineral constituents, which are part of zinc and lead concentrates used as feedstock in the The relationship between silicon and aluminum may indicate a diadochal substitution of silicon and aluminum, due to their crystallographic similarity (similar ionic radius), which is quite common in silicates [23]. In turn, the relation between Ca and Fe is also geochemically justified in the group of minerals to which kirschsteinite belongs [24][25][26][27].
However, the relations between copper and zinc and iron in kirschsteinite are interesting in the context of those previously determined in wustite. Although the zinc content in both phases decreases with increasing iron content, the copper content in kirschsteinite increases while in wustite it decreases. It is difficult to explain this phenomenon conclusively.
Thus, among the phase constituents of the examined waste of the top layer, three types can be distinguished, taking into account their origin: -mineral constituents, which are part of zinc and lead concentrates used as feedstock in the pyrometallurgical process-this includes sphalerite only, -phase constituents formed in the technological process-these include mainly kirschsteinite and wustite, -secondary mineral constituents, formed in the landfill under the action of hypergenic factors-these include gypsum, ktenasite, namuvite, tochilinite, gerhardtite, goethite.
Anglesite, willemite, metallic lead and glass may be of problematic origin, as on the one hand they may belong to the type of mineral constituents contained in small quantities in zinc and lead concentrates (anglesite and willemite) and on the other hand to the type of phase constituents formed in the technological process which are returned to the process (anglesite, metallic lead, glass).

Trace Phases
The results of chemical composition analysis of the grains in micro-areas revealed the presence of phases which are present in much smaller quantities and which therefore did not exhibit their characteristic reflections in the X-ray diffractograms. As in the case of the main phases, the phases occurring in trace quantities do not form individual grains. Usually they form small size inclusions in the main phases or occur at the border of the main phases.
As in the case of the main phase constituents, among the phase constituents present in trace amounts in the examined waste of the top layer, three types can be distinguished, taking into account their origin: -mineral constituents, which are part of zinc and lead concentrates used as feedstock in the pyrometallurgical process-these probably include alamosite, quartz and cerussite, which are contaminants of the concentrates resulting from the mineralization of the Zn-Pb ore deposits from which the concentrates are derived, -phase constituents formed in the technological process-these probably include metal alloys, Pb oxides and metallic Ag, -secondary mineral constituents, formed in the landfill under the action of hypergenic factors-these probably include leiteite and paulmooreite.        The provenance of franklinite and bornite may be problematic. They may belong to the group of mineral constituents contained in small quantities in zinc and lead concentrates or to the group of phase constituents formed in the technological process which are returned to the process.
As indicated by numerous studies, both leiteite and paulmooreite are known to be present as secondary phases in many deposits and landfills of metallurgical plants (the Tsumebdeposite, Namibia; Bushveld Complex, South Africa) [33][34][35].

Discussion
The identification of the main phases by X-ray diffraction and the established chemical composition of the tested samples allowed to calculate the content of these phases and the results of these calculations are presented in Table 5.
The dominant phases in the samples are anglesite, gerhardtite and wustite, the total content of which varies from about 52% to about 68 wt%, with the exception of the WZIV sample, where the total of these three constituents is 27.50 wt%. In that sample, kirschsteinite, ktenasite and goethite are significant in quantitative terms.
It can therefore be concluded that the samples are quite diverse in terms of phases and in particular in terms of the proportion of individual phases. And although it seems that samples WZI, WZII and WZIII are very similar to each other as compared to WZIV, they still show quite strong qualitative differences between them. This is manifested, for instance, in the presence of:  The provenance of franklinite and bornite may be problematic. They may belong to the group of mineral constituents contained in small quantities in zinc and lead concentrates or to the group of phase constituents formed in the technological process which are returned to the process.
As indicated by numerous studies, both leiteite and paulmooreite are known to be present as secondary phases in many deposits and landfills of metallurgical plants (the Tsumebdeposite, Namibia; Bushveld Complex, South Africa) [33][34][35].

Discussion
The identification of the main phases by X-ray diffraction and the established chemical composition of the tested samples allowed to calculate the content of these phases and the results of these calculations are presented in Table 5.
The dominant phases in the samples are anglesite, gerhardtite and wustite, the total content of which varies from about 52% to about 68 wt %, with the exception of the WZIV sample, where the total of these three constituents is 27.50 wt %. In that sample, kirschsteinite, ktenasite and goethite are significant in quantitative terms.
It can therefore be concluded that the samples are quite diverse in terms of phases and in particular in terms of the proportion of individual phases. And although it seems that samples WZI, WZII and WZIII are very similar to each other as compared to WZIV, they still show quite strong qualitative differences between them. This is manifested, for instance, in the presence of:  x-average, s-standard deviation, V-coefficient of variation.
The three distinguished types of phases (primary constituents, which form the feedstock in the process; constituents formed in the course of the process; secondary constituents, formed in the landfill under the action of hypergenic factors) are set out in a diagram ( Figure 14). In the tested waste of the top layer, three samples (WZI, WZII and WZIII) are located in the middle of the diagram, which suggests similar content of these three types of phases, while the location of the projection point of the WZIV sample indicates a dominant content of secondary constituents. It will be noted, however, that in the waste under investigation there are still significant amounts of primary constituents, that is, feedstock in the process, which can be reused for metal recovery, like the other constituents. An important element of this recovery is certainly the content of metals in these constituents. diagram, which suggests similar content of these three types of phases, while the location of the projection point of the WZIV sample indicates a dominant content of secondary constituents. It will be noted, however, that in the waste under investigation there are still significant amounts of primary constituents, that is, feedstock in the process, which can be reused for metal recovery, like the other constituents. An important element of this recovery is certainly the content of metals in these constituents.  However, from the point of view of the recoverability of metals, it is more important to consider the quantitative aspects of individual phases, grouped in the form of chemical compounds or the classification of minerals used in mineralogy, which illustrates the forms of metal occurrence. For this reason, the phases occurring in the tested samples were divided into 7 groups-(i) silicates, (ii) sulphates and hydrated sulphates, (iii) nitrates, (iv) sulphides and hydrated sulphides, (v) oxides and hydroxides, (vi) metals and (vii) glass (Table 5).
Sulphates and hydrated sulphates (anglesite, gypsum, ktenasite and namuvite) show the highest content in the tested samples, at an average content of about 38 wt %. The second group in terms of quantity are oxides and hydroxides (wustite and goethite), with their average content close to 20 wt %. The content of nitrates (gerhardtite) and silicates (kirschsteinite and willemite) are comparable, the average figures being 13.56 and 11.57 wt %, respectively. Average content of the other phase groups is below 10 wt % and these include sulphides and hydrated sulphides (sphalerite and tochilinite, average content 9.93%), metals (metallic Pb, average content 1.74 wt %) and glass (average content 5.64%).
A very important factor that must be considered in the technological recovery of metals (Pb, Cu and Zn) is their content in the individual phases.

Conclusions
Refining slags deposited in the top layer of the Hazardous Waste Disposal Site are characterized by varied development of the phase constituents and hence-(i) the main phases are usually in the form of conglomerates or multiphase intergrowths, (ii) trace elements form small size inclusions in the main phases or occur at their border.
The predominant phase constituents in the tested slags include-sulphates and hydrated sulphates (anglesite, gypsum, ktenasite and namuvite) at an average content of ca. 38 wt %, oxides and hydroxides (wustite and goethite) at an average content close to ca. 20 wt %, nitrates (gerhardtite) and silicates (kirschsteinite and willemite) at an average content of 13.56 and 11.57 wt %, respectively. Average content of the other phase groups is below 10 wt % and these include sulphides and hydrated sulphides (sphalerite and tochilinite), metals (metallic Pb) and glass.
Among the main and trace phase constituents of refining slags, the following can be identified-(i) primary mineral constituents, which are part of Zn-Pb concentrates (sphalerite, alamosite, quartz and cerussite), (ii) phase constituents formed in the ISP process and during chemical transformations occurring in the course of lead refining (kirschsteinite, wustite, metal alloys, Pb oxides and metallic Ag), (iii) secondary mineral constituents, formed in the landfill under the action of hypergenic factors (gypsum, ktenasite, namuvite, tochilinite, gerhardtite, goethite, leiteite and paulmooreite).
The major chemical constituents of the tested samples taken from the top layer of the landfill include FeO, CuO and SO 3 , PbO, the total content of which is over 74 wt %. Constituents found at substantially lower concentrations include SiO 2 , Al 2 O 3 and CaO, TiO 2 , MnO, MgO, K 2 O, P 2 O 5 . The content of the main constituents shows little variation, as the coefficient of variation V is below 22%.
Both the high content of metals, Zn, Pb, with their low variability and the significant content thereof in the individual phases, as well as their presence in the primary constituents, make refining slags a potential source of these metals and pyrometallurgical processing of these wastes seems to be highly rational.