Effect of galena contained in dust from Zn–Pb metallurgical processes on environment

Based on the results of spectrometric tests, the basic constituents of electrostatic precipitator dust are Pb and Zn, which jointly constitute 88 wt% of the dust (Table 1). The other two prevailing elements in the composition of the dust are S and Cd, at concentrations of 4.50 wt% and 4.08 wt%, respectively (Adamczyk et al. 2010; Nowińska et al 2015; Adamczyk and Nowińska 2016).

Based on the identification of the mineral composition of the electrostatic precipitator dust, it was found that the main phase constituents of that dust were sulphides, PbS (galena) and ZnS (sphalerite), with lead oxide (PbO, minium) and zinc oxide (ZnO, zincite) present in lower amounts. Apart from these four constituents the dust from the electrostatic precipitator also contains an amorphous substance (glaze) (Adamczyk et al. 2010; Hongxu et al 2016; Cappuyns et al 2014; Kucha et al 1996; Kushnir 2014).

Interpretation of the results of phase composition examination of electrostatic precipitator dust in the context of the environmental impact thereof should mainly apply to one of the basic phases; that phase is lead sulphide. This constituent has admixtures of such elements as Fe, Mn, Zn, As, Se, Sb, Sn and In (Table 2).

If lead sulphide is released into the soil and water environment, the decomposition of this phase may mobilize these elements into the environment, depending on the conditions of that environment, primarily Eh and pH (Ettler and Johan 2014, 2015; Mizerna 2016; Mizerna and Król 2015; Piatak et al 2015; Songa et al 1999, Taylor and Lapa 1983).

The basic phase component of the dust, lead sulphide, will decompose in hypergenic processes occurring mainly under the influence of factors such as: seasonal temperature variability, atmospheric precipitation, insolation and Eh and pH conditions of soil–water environment. The Eh and pH conditions, in turn, depend on many other factors, principally on chemical interactions with precipitation water and on soil quality. The Eh–pH diagrams that show the stability regions of the main chemical constituents of lead sulphide (sulphur and lead) under adopted temperatures (− 3 and + 25 °C) and additionally in temperatures of + 5 and + 11 °C are presented in Fig. 1a, b. The Eh–pH diagrams plotted for the additional temperatures (+ 5 and + 11 °C), enable observation of the changes in the behaviour of the forms of occurrence, mainly of lead, with rising temperature. The final task required for plotting the diagrams was establishing the concentrations of Pb and SO₄²⁻ in an aqueous environment, which were assumed to be equal to permissible limits specified in the Ordinance of the Minister of Environment of 24 July 2006 on the conditions of discharging wastewater to waters and soil and on substances particularly harmful to the aqueous environment (Dz.U. 2006, No. 137, item 984): 0.5 mg Pb/L, 500 mg SO₄/L.

The water and soil environment under investigation (Miasteczko Śląskie) has a pH range of 4.5 to 7.5.

In the surface region lead sulphide is decomposed by acidic solution in the following reactions:

Reaction (1) represents the oxidative decomposition of PbS which produces Pb²⁺ and sulphur. Pb²⁺ may also be formed as a result of oxidation (Scheetz and Rimstidt 2009; Czaplicka and Buzek 2011):

Reaction (2) suggests that PbSO₄ forms when the redox potential of the solution is too high, and reaction (3) indicates that H₂S forms in the course of PbS decomposition in acidic solution when the redox potential is too low. Pb²⁺ formed in reaction (1) or (5) may undergo hydrolysis: Pb²⁺ → PbOH⁺.

Changes of Eh and pH that determine the stability region of lead sulphide in aqueous environment, may cause its decomposition. The products of this decomposition in the specified environment which include Pb, PbO₂, PbSO₄, Pb²⁺, PbOH⁺, whereas sulphur, depending on the conditions, adopts the following forms: HS₂O₈⁻, HS₂O₇⁻, S₅O₃²⁻, S₄O₃²⁻, SO₄²⁻. Among the forms listed, Pb, PbO₂ and PbSO₄ are solid, the remaining products will occur in the aqueous environment in ionic form (Pb²⁺, PbOH⁺, HS₂O₈⁻, HS₂O₇⁻, S₅O₃²⁻, S₄O₃²⁻, SO₄²⁻) (Fig. 1).

At − 3 °C, under oxidative conditions, Pb²⁺ prevails at a lower pH (below 6.20), whereas Pb(OH)⁺ starts to prevail at higher pH (above 6.20). At + 5 °C the boundary pH value between Pb²⁺ and Pb(OH)⁺ prevalence is 6.18, at + 11 °C it is 6.19, and at + 25 °C it is 6.21. The range of pH value variability is between 6.18 and 6.21; this corresponds to seasonal differentiation of temperature from − 3 to + 25 °C and may in fact be neglected when determining the boundary pH value between Pb²⁺ and Pb(OH)⁺ prevalence.

At − 3 °C, under alkaline conditions (pH > 8.05), for slightly reductive environments, (Eh below − 0.32) lead sulphide will decompose to Pb and S₄O₃²; this is in contrast with the increase of temperature to 25 °C, where the pH value shifts towards 7.26 and Eh remains virtually unchanged.

The environmental Eh and pH values, derived from the Eh–pH diagrams (Fig. 1), are listed in the table below. This list clearly shows that with increasing temperature, the stability region of lead sulphide is gradually reduced (decreasing absolute values of |ΔEh| and |ΔpH|) (Table 3).

The other phase that is important from the viewpoint of the soil and water environment is lead sulphate, which may form as a result of lead sulphide decomposition. The stability of this component under the conditions of the soil and water environment, as determined by the diagrams, is presented in Table 4. The values of these parameters indicate that with increasing temperature the stability region of lead sulphate is gradually reduced (decreasing absolute values of |ΔEh| and |ΔpH|), and at ca. 12 °C this component is completely decomposed in the environment.

The findings presented above show that:

The narrow range of Eh and pH values within which lead sulphide and lead sulphate are stable is unfavourable for the soil and water environment. This is mainly due to the possibility of changing this environmental parameter within a narrow range beyond the boundary values. The effect will be the decomposition of this phase into metallic Pb, Pb²⁺, PbOH⁺ and sulphate ions (Fig. 1), posing a hazard for surface and underground waters. In addition to lead, other elements present as admixtures in lead sulphide (Fe, Mn, Zn, As, Se, Sb, Sn and In) will also be released into the environment (His-Hsiung 2016; Morrison et al. 2016; Nang-Htay 2016; Lia 2010; Sobanska et al. 1999).

Similar analyses have been performed for fayalite, the main constituent of slag generated and landfilled by the Miasteczko Śląskie Smelting Plant (Atlas of Eh–pH diagrams 2005; Adamczyk and Nowińska 2013; Puziewicz 2007; Ettler et al. 2001; Ray 1993). The Eh and pH parameters of the stability region of this constituent, established under similar environmental assumptions using diagrams, indicate that maintaining the pH value of soils at above 7 is favourable for the stability of fayalite. In the case of lead sulphide, the Eh potential seems to be the parameter of the soil that should be controlled to ensure stability.