Recycling Technologies for Secondary Zn-Pb Resources

Current demand and future projections of metals lead to the fast depletion of scarce resources. Thus, lean ore beneficiation and secondary resource utilization become urgent. While the recycling of metallic scrap is nowadays state-of-the-art, metal-containing by-products from the metallurgical industry represent a new potential secondary resource. Huge amounts of such materials are available worldwide either in dumps or continuously produced. However, their complex composition renders a detailed characterization difficult, resulting in few and weakly developed recycling concepts available to date.

When addressing the sustainable use of metals, one must consider not only primary metals in the natural environment but also alternative resources, such as secondary metals found in society. For that purpose, elucidating the availability of secondary metals, that is secondary metal reserve, is very important. A classification framework of the secondary resources was applied to investigate their applicability to Zn and Pb and to assess the secondary Zn and Pb reserves and resources of major targeted countries. Estimates show that Japan and the United States have secondary Zn reserves of 14 Mt and 13 Mt, respectively, and showed the total estimated amount of secondary Zn reserves of the studied countries is equivalent to about 24% of the global primary Zn reserves. On a per-capita basis, France, Germany, and Japan have the largest secondary Zn reserves. The application of a classification framework showed that a considerable amount of secondary Zn resources is found in landfills, providing a future potential target for secondary Zn landfill mining. The framework provides details about the sizes and locations of secondary Zn and Pb resources. This information is useful for both industry and policymakers to maximize access to valuable secondary Zn and Pb sources. This study also highlights the necessity for the integrated management of primary and secondary resources.

This chapter is a brief review of the past and future of the Zn extraction process. Firstly, the history of Zn extraction is given. Then pyro- and hydrometallurgical Zn production methods are covered. Fe removal processes are briefly explained. Finally, the current Zn extraction situation is highlighted.

Steel and zinc (Zn) make a good coupling during their service life and are easily recyclable. Zn coatings lengthen the useful life of steel products and structures and keep the steel performing at its peak condition. When the Zn coated steel is scrapped and recycled, the steel and Zn have new lives. Therefore, the combination of steel and Zn is especially fortunate because it is possible to separate and recover both the original metals. This means that galvanized steel and electro-Zn coated steel sheets are recyclable materials which can be reused to make further contributions to the life of the society. The metals can be separated because Zn is naturally much more volatile than steel. When scrap Zn coated sheet is melted in the steelmaking process, the scrap steel is turned into new steel for reuse. The Zn enters dust which is recovered from the furnace. They form a secondary resource of Zn which we can use again, alongside other Zn raw materials. The recycled Zn is also used in Zn coated steel, with a new life in new products, such as cars, buildings and construction products. This cycle, in which Zn and steel are infinitely renewable, contributes to the virtuous circle of recycling for sustainability.

This chapter covers the most significant Zn secondary leaching using inorganic sulphuric acid (H₂SO₄) as lixiviant. The physical, chemical, and thermodynamic properties of Zn compounds and H₂SO₄ are introduced in detail. The effects of reagent concentration, temperature, time, particle size, solid/liquid ratio, and oxidant addition on Zn dissolution are covered. The effects of sulfating roasting on Zn-ferrite are explained. Previous studies on Zn leaching were summarized along with possible reactions.

Like other industrial processes, hot-dip galvanizing of steel generates some by-products and wastes. Secondary materials contain a relatively high percentage of zinc being valuable sources of the metal. Solid by-products like zinc ash, bottom dross, and top dross are currently sold to pyrometallurgical recycling plants, although zinc ash consisting of easy leachable components is suitable for hydrometallurgical treatment. In turn, flux skimming is deposited in landfills for hazardous wastes, but it could be leached for zinc recovery. Waste aqueous solutions from pretreatment steps, such as spent baths from pickling, stripping, and fluxing, or washing waters can be also regenerated by hydrometallurgical methods instead of going to landfills. This chapter presents the characteristics of main secondary raw materials originating from hot-dip galvanizing lines and reviews hydrometallurgical methods developed for their recycling or regeneration.

The growing of collected waste lead-acid battery quantity means the growing demand for secondary lead (Pb) material for car batteries, both needed for increased cars’ production and for replacing of waste batteries for the increased number of automobiles in service. Pb recycling is critical to keep pace with growing energy storage needs. In recent years, tightening emission regulations have forced many developed country smelters to close. This has driven battery manufacturers and distributors to increasingly rely upon unregulated smelting operations in developing nations, negatively impacting the environment and human health. Therefore, finding a cleaner and more cost-efficient Pb recovery and recycling method is critical to the Pb recycling community.

Different processing techniques are practiced to treat the inorganic (such as H₂SO₄) or organic acid (such as citric or malic acid) leached Pb–Zn oxide ores/wastes to recover the remaining Pb as well as Ag with a high Fe content in the first stage leach residues. These wastes may come from the Waelz process, mine/mineral processing tailings, etc. Brine solutions (NaCl and CaCl₂), alkaline solutions (NaOH), organic acids (malic, acetic, and oxalic acids), urea, and potassium sodium tartrate were tested alone or combined leaching (such as H₂SO₄ + NaCl; H₂SO₄ + FeCl₃.6H₂O), citric acid + NaNO₃; acetic acid + Na-citrate, and citric acid + Na-citrate) were also performed. Chloride leaching is the most recognized and widely used Pb recovery method. Chloride leaching processes have been employed using either NaCl, or MgCl₂ and CaCl₂, or FeCl₃ along with HCl. NaOH + potassium sodium tartrate was found the best lixiviant for Pb leaching from Turkish oxidized Pb–Zn oxidized flotation tailings.

The recycling of lead (Pb), which has a limited reserve in the world, has great importance in terms of sustainable and efficient use of resources. Currently, more than half of the lead, which is the softest of base heavy metals, is recovered by recycling. In addition to the insulation of the cables and its use as a radiation shield, lead is mostly used in the manufacture of lead-acid batteries (LABs). Generally, lead smelting flue dust, also known as lead smelting fly ashes, formed during the smelting stage in secondary Pb production is fed back into the smelter. However, the impurities contained in this dust and the other required specifications for feeding into the furnace prevent dust from being fed back into the furnaces. Therefore, it is essential to evaluate these by-products with an effective process and to obtain valuable content from them. In this chapter, firstly the characterization of lead smelting flue dust has been investigated. Afterwards, the processes that can be applied to obtain contents such as Pb, Sb, Zn, and As from these materials were compiled from the literature and a comprehensive review study was presented.

This chapter covers the impurity removal techniques before electrowinning. Alkaline hydraulic NaOH precipitation, Zn dust cementation, and Na₂S precipitation can be used to purify ZnSO₄ and PbSO₄ PLSs. Fe and As impurities can be removed by NaOH + H₂O₂ hydraulic precipitation at a pH of 3.5. Al, Cu, and Cd can be precipitated at pH: 5.0 using Zn dust. Two-stage separate precipitation was performed better impurity removal than combined one-stage precipitation. Na₂S precipitation of PbSO₄ and Pb brine solution was also covered in detail.

Solvent extraction (SX) of zinc (Zn) can be used to recover high-purity Zn from sulphide mineral leaching solutions, low-grade ores, and secondary resources. The method is quick, environmentally friendly, and can be adjusted to treat a variety of aqueous solutions.