pyrite redox chemistry

Iron Pyrite For Lead Refining And Copper Removal

Iron pyrite already has myriad uses, from the production of sulfur compounds to an additive in several types of commercial glass. In the metals refining space, pyrite is prized for use in the refining of lead, where it is an essential component, and the removal of copper from that molten lead and associated slags.

Introduction

As with any metal production, an element of refining needs to take place to go from mined ore to useful, viable product. Lead occurs naturally as the galena ore, the natural ore of lead (ii) sulfide (PbS), but often contains impurities.

Lead is often mined for its content of other metals, particularly silver. Additionally, small amounts of copper compounds may be found in these ores. From both economical and practical viewpoints, it is imperative to remove the silver and any other metal from the lead -  oftentimes these metals are far more valuable than lead, has a different downstream/late stage refining process and removal at the earliest opportunity ensures a more efficient overall process. Lead ores often contain a plethora of other useful metals too.

The crude ore is fed into a sintering machine with a variety of fluxes, reductants and oxidants, including coke, iron and silica. Therefore, it can be stated that the treatment of lead ore begins with the oxidation of the sulfide to a sulfate, shown here with oxygen as oxidant:

2 PbS + 3 O2 → 2 PbSO3

Which undergoes thermal decomposition into lead (ii) oxide and sulfur dioxide gas, which is expelled.

PbSO3 → PbO + SO2

Lead smelting is the next process which employs heat and several reducing substances to reduce the oxidising compounds that have bound to the metal, and further reduction to the intermediate, elemental, metal from the lead (ii) oxide that has been generated in situ takes place. Smelting continues with the application of heat in a blast furnace, still under reducing conditions.

In the blast furnace, slag and molten lead sink to the bottom and other metal impurities rise to the top. The molten lead (which at this point still contains a moderate quantity of copper) and the slag are allowed to leave the bottom front face of the furnace, in separate channels. The other metal impurities often include antimony and arsenic and is referred to as a ‘speiss’, which is recovered and sold on for further purification.

Pyrite is a key material in the lead refining process(1). In addition to primary production, it finds important roles in secondary treatment of smelting wastes and other remediation-type processes.

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Pyrite In The Smelting Process

As part of the cocktail of fluxes, reductive and oxidative additives added at the early states of lead smelting, pyrite is a valuable inclusion. Without an effective oxidation process, the sulfide would not be converted to the sulfate, which can then not be converted to lead oxide and this is largely taken care of by components such as silica.

Pyrite is essential to the process as it acts as a reductant in the conversion of lead oxide to lead metal, in the equations(2):

2 FeS2 + 15 PbO → Fe2O3 + 4 SO3 + 15 Pb
FeS2 + 5 PbO → FeO + 2 SO2 + 5 Pb

These reactions are incredibly thermodynamically favourable, with Gibbs free energy values of -23,804 and -9.548 kcal mol-1 at 1,100 °C respectively(3). The iron species produced typically dissolve in the slag with the sulfur dioxide being lost to the atmosphere, effectively pushing the position of equilibrium further to the right. Sodium carbonate is typically present to sequester the sulfur trioxide (SO3), stabilising it to sodium sulfate - which enters the slag -  and carbon dioxide which is also lost to the atmosphere(4).

Pyrite is industrially proven, whereas elemental sulfur has a low melting and boiling point - which leads to low efficiency(5). Furthermore, pyrite is a much easier material to handle and work with compared to the direct addition, owing to the former’s non-toxicity.

metal casting process with red high temperature fire in metal part factory
copper wire used in making copper oxide

Purification Of The Molten Lead: Copper Drossing

Copper drossing is the next stage in lead purification, where sulfur is added directly to the molten lead. In many processes, the source of sulfur is pyrite(6,7), with scrap metallic iron also being added. The iron species produced end up in the matte layer.

Copper compounds and other non-lead metal sulfides rapidly rise to the surface forming a layer called a matte. This layer can be manually removed and taken or sold as a viable starting material for copper smelting. Further processing of the molten lead to remove more metals involves cooling to ca. 700 to 800 °C with agitation, forming dross that separates out from the molten lead. Dross contains residual lead oxides, more copper compounds and incredibly small amounts of residual antimony. The dross is skimmed off and, like the matte and speiss before it, is sold on for further purification. Finally, the lead is purified using a method such as the Betterton-Kroll process, which removes any residual bismuth from the lead, allowing it to be casted.

Continuous sulfur (pyrite) -assisted copper drossing is an identical process, except that it uses two or more pots which are continuously stirred, into which the molten lead and pyrite and iron are added.

Secondary Lead Removal

Secondary lead processing refers to the isolation of lead from primarily scrap sources, such as batteries, solder or the lead flashing from houses. The process is very similar to the initial lead production method, except that no oxidising agents are used, as oxidation of already primarily elemental lead is pointless. As such, pyrite has no role to play here.

Lead-Ingot-Block-Plate-Billet-Lead-Metal-Plumbum-Ingot

Pyrite And Acids

It has been shown that the lead ore galena can be oxidised with pyrite and nitric acid in the aqueous phase - compared to the traditional method above when lead is produced in the solid and subsequently molten phase. In the research(8), anglesite (PbSO4) is produced from galena through the heating with pyrite and nitric acid, causing the lead sulfide to leach. At temperature, the acid is able to oxidise the pyrite and subsequently aid in the formation of anglesite and a moderate amount of plumbojarosite. Anglesite is a secondary ore, exclusively formed as the oxidation product of galena.

This is an interesting approach as it suggests that a leaching method using the concentrated acid may be effective in lead production from its ore, when that lead is destined as a higher sulfate, which would negate the use of expensive and environmentally damaging blast furnaces by obtaining the sulfate directly. Lead (ii) sulfate has uses in battery technology.

Pyrite For Cleaning Up Speiss And Related Wastes

Because of its tendency to contain relatively large amounts of silver, speiss as a material is valuable and should be treated. Researchers have claimed that one tonne of lead smelter speiss can contain up to 0.901% silver by mass, in addition to being composed of approximately 54% copper, 19% arsenic and 9% antimony. Their research showed that by roasting the speiss with pyrite in an oxidising environment, antimony oxide was readily produced(9) in conversion rates in excess of 98% at 800 °C after only two hours. Initially, the authors had thought that pyrite would act as a source of sulfur, as in copper drossing.

Pyrite does, however, have a use in the copper production arena. Research has shown that metals can be readily recovered from copper smelting slags via the application of pyrite. Slags are also present in the production of copper, and here pyrite can be used to enhance yields and therefore make a process more competitive. Briefly, slags are roasted in the presence of pyrite and then leached with water. Using a 1:4 ratio of pyrite to slag, in excess of 95% of residual copper was removed from the copper rich slag, leaving other impurities behind, after only one hour at 550 °C(10).

Iron Pyrites nugget fools gold

Conversion Back To The Sulfide

There has been interest in the conversion of metal oxides to their equivalent sulfides, which may have applications in the energy storage field. Vulcanisation is a process more commonly associated with rubber production - and correctly refers to the treatment of natural rubber with sulfur for hardness -  but researchers have demonstrated that lead oxide can be vulcanised (i.e. treated with sulfur for conversion into the sulfide) using pyrite. When heated together at 900 °C, the oxide and pyrite convert to the sulfide at near complete conversion levels, affording PbS in highly pure crystal form(11).

Summary

  • Lead smelting is the process by which lead ore - galena - is converted to metallic lead
  • In the blast furnace, pyrite is an essential inclusion, converting lead oxide to molten lead metal in a highly thermodynamically favourable process
  • Pyrite is preferred over elemental sulfur owing to the poor performance of the latter and with the former being significantly easier to handle
  • Pyrite is used in the copper drossing process, as a source of sulfur, removing copper and other metal impurities from the post-smelter lead
  • Other applications using pyrite include slag purification and secondary oxide isolation from ores
  • Overall, pyrite is an essential component in the refining of lead and copper
Pyrites powder in a pot

References

1         J. R. Parga et al., JOM, 2001, 19, 53

2         Z. Zsczygiel et al., JOM, 1998, 4, 55

3         H. Wang and D. Shooter, Environ. Tech., 2000, 21, 561

4         E. R. Cole and A. Y. Lee, Hydrometallurgy, 1984, 12, 49

5         C. Zscheische et al., Challenges and Opportunities of a Lead Smelting Process for Complex Feed Mixture, in: B. Davis et al. (eds), Extraction 2018, Ottawa, 2018

6         US Patent US3694191, 1970

7         B. Xu et al.,  Removal of Sulfur from Copper Dross Generated by Refining Lead, in: J. Y. Hwang et al. (eds) 9th International Symposium on High-Temperature Metallurgical Processing, Phoenix, 2018

8         R. G. Zárate and G. T. Lapidus, Hydrometallurgy, 2012, 115, 57

9         M. Peterson and L. G. Twidwell, J. Haz. Mater., 1985, 12, 225

10       F. Tümen and N. T. Bailey, Hydrometallurgy, 1990, 25, 317

11       Y.-X. Zheng et al., Physiochem. Probl. Mineral Process., 2018, 54, 270