Elsevier

Minerals Engineering

Volume 22, Issues 9–10, August–September 2009, Pages 742-751
Minerals Engineering

Top submerged lance direct zinc smelting

https://doi.org/10.1016/j.mineng.2008.12.014Get rights and content

Abstract

A long history of zinc processing starting with pilot plant studies in the 1980s has progressed to the treatment of close to 800,000 tonnes a year of zinc bearing feeds in Top Submerged Lance (TSL) furnaces in the form of residues and slags. Whereas these Ausmelt TSL applications are end-of-pipe applications treating residues from classical Roast-Leach-Electrowinning (RLE) process routes, this paper discusses Direct Zinc Smelting (DZS) Process. In Stage 1 of this novel two stage TSL application the sulphide sulphur from the zinc concentrate is the energy carrier used to smelt the concentrate and fume 60–65% of the zinc content. Subsequently in Stage 2 zinc is fumed from the already molten Stage 1 slag to create a final discard slag with a total zinc recovery of 99%. The use of sulphide sulphur as fuel to smelt has economic advantage over the present end-of-pipe TSL applications, that require two steps (for smelting and sulphur removal) and then fuming using primary fuels. The slag product of the DZS process overcomes inherent environmental liability and ever increasing economic burden associated with the traditional RLE processes by eliminating the creation of large volumes of iron bearing residues (mainly jarosite and goethite) while complementing high recoveries of Zn with those of the valuable elements e.g. Cu, Ag, Au, Pb, Ge, and In. High levels of Fe, Mn, SiO2, and MgO in some concentrates render them unsuitable for processing in RLE, however are elegantly directed by DZS to the benign slag. This paper will discuss Ausmelt’s applications in zinc processing with a focus on DZS to show how this process can extend the life of present RLE sites.

Introduction

With growing pressure on the margins for the processing of zinc feeds as a result of increasing costs of energy and labour, the ever increasing drive for sustainable use of zinc resources, interest in the economical processing of secondary feed material sources has surged. In addition the drivers for environmental conformity and stricter health regulations in an increasingly educated and environmentally aware and active society will affect the future of metal production. Intensifying the processes in metallurgy by maximizing mass and energy transfer in state-of-the-art furnace technology will be a key. Since its invention, Top Submerged Lancing (TSL) Technology has gained widespread commercial acceptance in non-ferrous metal production by doing exactly this.

The processing of among others copper, lead, tin, and zinc bearing primary, secondary (residue) or recycled materials in TSL is commercial reality. Close to 40% of Sn will be produced through TSL furnaces in the next years, while an ever increasing amount of copper is produced in TSLs while e-waste processes, lead battery recycling and copper recycling are already commercial reality (Matusewicz and Reuter, 2008). Producing these various metals in close to 50 commercial Ausmelt TSL furnaces is testimony to this technology’s flexibility to economically process both primary and/or secondary feeds, while using varying fuels, oxygen and good process control to produce high metal recoveries.

Being environmentally compliant in regions for example such as Europe, Japan, and South Korea evidences TSL’s excellent environmental performance by operating within all norms for offgas emissions and other environmental legislation.

This paper describes the technical and commercial aspects of Ausmelt’s progress in the field of zinc processing. The discussion briefly provides an overview of Korea Zinc’s commercial applications for the treatment of zinc leaching residues (and other materials) and subsequently focussing on the developments of Direct Zinc Smelting (DZS). The DZS process has the potential to make an important contribution to treating lower grade unclean concentrates in an innovative combination of the pyrometallurgical recovery of zinc as a ZnO fume and subsequent processing it in a simplified leach and electrowinning circuit while harnessing the sulphide sulphur as a fuel. This complements the success TSL already has in the processing of iron containing residues from the classical Roast-Leach-Electrowinning (RLE) primary zinc industry, fuming zinc from slags and treating secondary materials such as Electric Arc Furnace (EAF) dusts to recover both the iron and zinc units.

Traditional RLE processes are complex flow sheets that incorporate a number of fundamentally simple steps to produce zinc sulphate liquor that undergoes electrolysis to produce the final zinc metal product. The complexity in the flowsheet comes from the need to purify the leach liquor to remove the iron, copper, cobalt, nickel, and other elements such as arsenic and bismuth to prevent them affecting the final product or reducing the efficiency of the electrowinning processes. The leach residue (usually jarosite or goethite) is already a cost driver in terms of disposal and increasing dumping costs, as well as negative publicity around them is continuing to be a major concern as stricter environmental standards are enforced preventing the creation of ponds and other storage mechanisms for economically viable disposal. In addition, the RLE process is less efficient in the recovery of valuable minor elements such as Indium (In) or Germanium (Ge), which are commonly found with zinc ores.

TSL zinc processing technology has been implemented in various commercial applications for the efficient recovery of zinc from these intermediate industrial products (residues, slags) in South Korea, Japan, and Australia (Hughes et al., 2008). Therefore, retrofitted to existing zinc smelting installations, the TSL furnace can treat primary leach residues to recover the zinc units and minor valuable elements such as In and Ge to produce a discard saleable slag low in zinc (<2%) enabling maximum zinc recovery from a concentrate source. The zinc is returned in the form of zinc oxide fume can easily be treated in simplified existing zinc RLE works for which only a neutral leach step is required (see Fig. 1).

The Korea Zinc facility in Onsan (South Korea) is the most comprehensive evidence to date as to the success of TSL technology (Lee and Park, 2003, Kim and Lee, 2000, Lee et al., 2006). It comprises an integrated flow sheet of seven interdependent projects utilising twelve TSL furnaces (10 for the specific recovery of zinc) that allows Korea Zinc to maximize the recovery of values from these intermediate industrial residue products (e.g. goethite, QSL slag, old residue ponds, indium, and germanium, etc.). This demonstrates the flexibility of the TSL furnace and zinc technology in particular as depicted by Fig. 2. According to Korea Zinc’s 2008 annual report the Korea Zinc Group (Korea Zinc, its wholly owned subsidiary Sun Metals Corporation, and its sister company Young Poong) ranks second in the global zinc market and has refined zinc smelting capacity of 925 kt/a. It is notable that this processing recovers through its TSL technology (Fig. 2) 63.5 kt/a Zn, 39.0 kt/a Pb, 267 t/a Ag, 600 t/a Cd, and 75 t/a In (substantial amount on a global basis), adding considerably to the profit margin (Korea Zinc, 2008 Annual Report). These fuming activities make Korea Zinc the only zinc smelter free from environmental pollution issues. While fuming recovers all the valuable metals from waste it also turns the leftovers into environmentally stable slag to be used as construction materials.

These fully commercialized TSL applications are complemented by TSL flowsheets to process EAF dusts (Hughes et al., 2007, Lee et al., 2006) as well as direct processing of zinc concentrates, which is the topic of this paper.

Over the past 25 years, fundamental investigations have been conducted into the direct smelting of zinc sulphide concentrates with the objective of producing crude zinc metal directly by condensation of the zinc metal vapour from the SO2 bearing gas stream or technologically best as oxidized ZnO fume.

Thermodynamic calculations by Yazawa (1979) on the volatilization of zinc as metal vapour from pure ZnS material during oxidation of sulphide showed that 99% of the Zn as sulphide is converted to zinc metal vapour. However, this tends to convert to ZnO and ZnS during cooling, because of the presence of SO2 and CO2, i.e. the pO2 is high enough for this to occur. These experiments were extended to industrial concentrates by Davey and Turnbull (1980), producing similar results, claiming as well that the process is thermally autogenous. However, it has proven challenging and elusive to produce metal zinc by condensation from zinc vapour/gas mixtures other than for the Imperial Smelting Furnace. Test work has been conducted by Minproc (Foo et al., 1992) for producing ZnO from Bolivian sulphide ores in a submerged combustion smelting process with zinc recoveries exceeding 92%.

In the late 1990s Ausmelt carried out extensive R&D resulting in a two stage ZnS concentrate smelting–fuming process being proposed and developed in conjunction with BUKA Minerals Ltd., Australia. In this process finely grained Zn–Pb bulk concentrate from their existing operations were treated. The Buka Zinc Process (BZP) was based on a two stage smelting process in the TSL furnace (Buckett and Sinclair, 1998). It was shown that high levels of zinc (>50%), lead (>90%) and silver (>90%) are recovered as fume in the 1st Stage smelting process, with the majority of the balance of the zinc (>40%), lead (<5%) and silver (<10%) recovered in the 2nd Stage reduction/fuming step. It was observed that in excess of 5% of the sulphur input reported as sulphate to the Stage 1 fume, principally due to sulphatisation of the lead present in the Stage 1 fume in the gas handling system.

In the BZP process the treatment of the lead-rich ZnO fumes was conducted via a hydrometallurgical ammoniacal leaching process (ammonia–ammonium carbonate (AAC)-leach solution). The advantage of this process is its good separation of the lead from the zinc, as lead largely remains un-dissolved in the residue as lead carbonate, whereas zinc dissolves as a zinc–amine complex. After solution purification (by cementation using Zn-dust), the zinc is recovered from solution by steam distillation of the ammonia and precipitation of zinc in the form of a basic zinc carbonate. The distilled ammonia is absorbed and re-circulated.

In order not to contaminate the closed ammonia solution circuit with sulphate, the ZnO fume is pre-leached in an alkaline Na2CO3-solution in order to remove sulphate in the form of Na2SO4. While the selected treatment of the fume by the above method was ideal due to its small scale (i.e. pilot plant scale), it is disadvantageous when used in large scale due to its economics and intermediate residue streams etc. Due to renewed market interest coupled with the expected growth in the supply of concentrates that cannot be readily processed in the traditional RLE process, a process was developed in which zinc is recovered from ZnS concentrate by the direct ZnS concentrate smelting/volatilization in a Top Submerged Lance (TSL) furnace process. The recovered zinc in the form of a crude zinc oxide product is suitable to be treated by the conventional zinc RLE process. This will be discussed in more detail in the next section.

Section snippets

Features of the Direct Zinc Smelting (DZS) Process

In this section the various main operating steps of the DZS process are presented and discussed in some detail based on a 200,000 t/a feed rate of concentrate. The main steps of the DZS process are depicted by Fig. 3 noting that the furnace details are depicted in Fig. 4:

  • During Stage 1 the zinc concentrate is smelted and zinc fumed directly as zinc metal. The basic reaction is: ZnS+O2(g)=Zn(g)+SO2(g)(ΔG°1250°C=-168.1kJ/mol). Oxygen and air pass through the TSL lance, while the concentrate and

Conclusion

Since the early 1980s Ausmelt has been developing novel processes to recover zinc from both primary concentrates and secondary/recycle/residue resources. The success at Korea Zinc and the innovation of Ausmelt is a clear example as to the technological advantages that can be gained by using TSL technology in the processing of residues originating/slags from primary hydrometallurgical and pyrometallurgical processing of zinc and lead concentrates.

The DZS process as proposed and discussed has the

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