Pyrite And Lead Recovery: An Overview
Adding further to its myriad arsenal of uses, pyrite is an effective and essential part of the production and recovery of lead. Relying on either porosity profiles or pyrite’s redox chemistry, industrial inclusions of pyrite in stoichiometric and sub-stoichiometric quantities of pyrite (treated or untreated) provide comprehensive lead removal properties from aqueous solution and are solely responsible for pushing the equilibrium to the right in lead smelting. Additionally, pyrite finds uses in its calcined or cindered form for removal of metals (including lead) from aqueous solution such as in mining tailings.
Pyrite In The Lead Smelting Process
It is without a doubt that smelting, as simple as it may seem, is a complex process. Many components are added to the smelter alongside the ore and/or recycled metal. Shown below is the chemical equation of converting lead oxide (PbO, the primary ore) to the metal(1). Using heat and a combination of reductive and oxidative additives, the process is effective and efficient. The reduction and oxidation pathways below are crucial, and would not function without pyrite. Pyrite here acts as a reductant.
2 FeS2 + 15 PbO → Fe2O3 + 4 SO3 + 15 Pb ΔG = -23,804 kcal mol-1
FeS2 + 5 PbO → FeO + 2 SO2 + 5 Pb ΔG = -9,548 kcal mol-1
Despite the high temperatures at which these reactions are carried out, they are thermodynamically favourable at 1,100 °C(2), as can be seen from the Gibbs free energy values (ΔG) quoted above. Equilibrium pushing is very much in effect here, with the iron species being produced having the effect of pushing the equilibrium to the right as they dissolve in the slag. This effect is further modulated by the sulfur dioxide being readily and rapidly lost to the atmosphere. Not shown is the sodium carbonate that is added to sequester the sulfur trioxide and convert it to sodium sulfate and carbon dioxide (SO3 to Na2SO4 + CO2) which enters the slag and disappears to the atmosphere respectively(3). Elemental sulfur could theoretically be used, but it is harder to handle than pyrite and has a lower melting and boiling point, rendering it less efficient(4).
Secondary Lead Removal
Because of the lack of need of redox chemistry for the secondary removal of lead (i.e. lead that has been derived from scrap sources in the most part), pyrite has no part to play here unless the lead is melted down and added into a conventional (primary) lead isolation process. Lead, like many other metals, is infinitely recyclable providing impurities can be removed.
Chlorination roasting is a method to extract high value metals from tailings. By roasting tailings materials (from lead, gold and silver mining, for example) and then subjecting this material to a chlorine rich atmosphere, high value metals readily form their chloride salts in situ, which are then easily separable and dechlorinated later. Pyrite has been proposed as a material to enhance the conversion of the chloride source (calcium chloride) into elemental chlorine(5), as per the following formula:
CaCl2 + FeS2 + O2 → CaSO4 + Fe2O3 + Cl2
Chlorine gas may then react with any metal present (such as lead, gold, etc., that is in low concentration and widely dispersed through the tailings) to form the chloride of that metal:
M + Cl2 → MCl2 (or other chlorides of the metal, where ‘M’ is a metal)
The chlorine would eventually be produced by simple thermal decomposition of the calcium chloride, but pyrite enhances this. The use of pyrite also has broader applications in the clean up of various metal tailings, including cyanide(6). Overall, the advantage of being able to use easy to handle and relatively inexpensive pyrite to enhance the calcium chloride process pays dividends.
Pyrite And Acids
As mentioned in other discussions on this website, pyrite is broadly tolerant of acids and bases. This means that it can be used in an even wider spectrum of applications. One method for the extraction of lead ore galena is oxidising it with pyrite and concentrated nitric acid. This ‘wet chemistry’ method takes lead into the aqueous phase compared to using a furnace and melting the ore down. Therefore, it may have a lower energy requirement. In heating the galena ore with pyrite and nitric acid, lead sulfide leaches into the aqueous phase, eventually producing anglesite - PbSO4. Because of the temperatures involved, the oxidation is easily carried out by the acid, with a small amount of plumbojarosite also being formed(7). This method may be particularly attractive if the lead is eventually required as a higher sulfide/sulfate as opposed to elemental or molten lead. Any modern chemist would prefer to use a more direct route if it is the sulfide/sulfate that is required - completely negating the need for the blast furnace. Lead (ii) sulfate has promising uses in electrical storage and batteries.
Treated Pyrite: Further Applications
Whilst many of the prevailing applications for pyrite use it in its raw, untreated form, there are many others where a simple process such as calcination can open up more doors. Pyrite’s melting point of 1,177 °C means that calcination temperatures must be well below this. Formation of pyrite cinder - effectively a roasting process followed by desulfurisation - also has a modest array of uses.
As with other materials undergoing calcination, heat treated pyrite will become more porous and polycrystalline - affording it with properties especially well suited to contaminant or metal recovery. Calcined pyrite has been demonstrated as effective at removing mercury(8) and copper(9) from mine tailings and drainage. In agriculture, pyrite in its calcined form can remove moderately high concentrations of ammonium, phosphate and nitrite ions from standing and runoff water, preventing its egress into water courses and protecting against eutrophication(10).
Moving back to lead, and the recovery of it from aqueous solution, we are presented with a similar story: porous calcined pyrite is effective at removing lead. One study looked at how calcined pyrite could be supported in fixed bed columns and have aqueous solutions of single and mixed metal character passed over them. In addition to being able to remove copper, pyrite was able to remove 73.7 mg g-1 of lead (mg of lead per gram of calcined pyrite) from a single metal solution, representing a circa. 15% efficiency.. When this was applied to a multi-metal solution containing cadmium and zinc in addition to the copper and lead, efficiency for isolated lead removal dropped to 2.5%(11). Authors describe the mechanism of operation as that of precipitation and dissolution via the formation of covellite an galena in situ, as described earlier.
Pyrite cinder is a compound formed where pyrite has been roasted at incredibly high temperatures and thereafter had all residual sulfur removed whilst still in the furnace. The resultant cinder is composed mostly of hematite and magnetite. It may also arise as a waste product from sulfuric acid production. It is iron rich, with some hydroxides also present(12) - examples for the recovery of spent lead-acid battery materials are just one use case.
Iron, lead, zinc and copper recovery from smelter and furnace slags is enabled through the use of pyrite cinder. Pre-heat treated pyrite cinder followed by reduction allowed the removal rates of copper, lead and zinc to be 36.36%, 92.86% and 20.0% respectively(13). Authors claim that the use of compressed pyrite cinder can enhance the operation of the furnace. Other research in the area points towards the sulfur content in pre-heat treated pyrite cinders to be primarily elemental - meaning that there has been some decomposition of the pyrite into its component elements prior to the heat treatment required to form the cinder - and that the higher pyrite component in the cinder (i.e. higher purity) led to a greater surface area to volume ratio, meaning enhanced removal properties(14).
Overall, it has been reported that the identity of the post-heat treated pyrite cinder is predominantly iron oxides and hydroxides(15), with the crystal structure predominating being of the hematite type. Using pyrite cinders for metal removal and recovery needs to be undertaken at pH 6 or above (neutral to alkaline) so as to prevent leaching of metals off the cinder.
- Lead recovery is an essential industrial process - ensuring the prevention of losses to water courses and the environment in general is crucial
- As part of the production of lead smelting, pyrite is added to ensure that lead oxide is reduced to metallic lead
- Pyrite is able to enhance chlorination roasting, a process to remove metals and precious metals in small quantities from tailings, for example from lead production
- With acids, pyrite offers a ‘wet chemistry’ approach to lead extraction, avoiding the need for smelting
- In industrial clean-up and mining waste treatment operations, calcined pyrite may serve as a ‘sponge’ to remove lead from runoff water; cindered pyrite acts in a similar way to remove lead from aqueous solution
- Pyrite cinder is a form of pyrite that is effective at removal of metals - including lead - from solution, effective for a variety of metals at pH values above 6
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