Pyrite In The Manufacture Of Iron
Iron pyrite (iron(ii) disulfide, FeS2, herein referred to as pyrite) is a naturally occurring sulfide of iron, and is one of the most common sulfides. Pyrite is found in large amounts in quartz seams, in sedimentary and metamorphic rocks, and alongside coal deposits. Aside from its metallic lustre giving it a visually attractive appearance, it finds use in industry in the manufacture of brake pads and as an additive in glass production, amongst other areas, many of which rely on its high natural purity and impressive levels of hardness.
But an important contemporary role for pyrite is when used as a supplementary material in the production of cast iron. Often added in powdered form at approximately 100 mesh, pyrite can act as a source of sulfur, as an aid to machinability. As a source of sulfur, it is inexpensive and non-toxic when compared to other notable sources.
Pyrite is not regarded as a good source of iron itself, especially compared to magnetite and hematite - despite being approximately half iron by weight - but it is a good source of sulfur(1). The addition of materials such as pyrite to molten/cast iron is referred to as inoculation, and is a common liquid-state treatment of the iron(2).
As A Source Of Sulfur In Graphitic Cast Irons
Graphitic iron is an important cast iron material in its own right, the applications of this material are where material stiffness is more important than tensile strength, for example the engine blocks of automobiles and pump housings. Graphitic iron is, by its nature, iron that contains graphite. Carbonaceous materials are added to the molten iron, which precipitates on cooling to provide a dispersed graphite network through the iron. Such precipitation is enhanced by the presence of pyrite, which provides sulfur in the melt and therefore produces many points of nucleation for the precipitation of graphite(3).
The tensile properties of graphitic iron are modulated by the presence of sulfur - experiments adding between 0.023 and 0.080 weight% of sulfur (from the addition of a maximum 200 μm diameter pyrite powder) have caused a more uniform and clearly defined microstructure, but beyond a certain point, pyrite addition causing increased sulfur content contributes to a reduction in tensile strength(4). At the other end of graphitic iron manufacture is the formation of nodules - where the graphite formed is spherical in nature and affords a more ductile cast iron. From the same study, it was found that higher additions of sulfur from pyrite provide more points of nucleation and thus more compact- and flake-like forms of graphite, i.e. fewer nodules. Inoculation to give rise to more nucleation points is made more efficient by increasing sulfur content via pyrite(5).
Nodule counts tend to be greater when sulfur/pyrite grains are added to the molten iron at later stages in the process across a range of iron compositions(6). Up to 0.55% by weight of sulfur as an inoculant is common. Patents to produce automotive-grade cast iron suggest common sulfur contents in the region of 0.2 to 0.3% by weight sulfur(7).
Overall, it is said that the addition of pyrite to graphitic iron production is especially beneficial for highly compacted deployments of graphitic iron, in addition to iron where a moderate degree of ductility is required. Compacted graphite iron, produced with sulfur added from pyrite, has strength and toughness properties approaching those seen with ductile iron(8).
To Improve Machinability In Cast Grey Iron
Grey cast iron is a type of graphitic iron, so called because of its grey colouration, however machinability can be problematic due to its hardness. Machinability is of paramount concern in the production of any metal, but especially one as globally economically important as iron. Machinability can be any process from drilling, turning, milling and grinding.
One method of ensuring easier machining is by the addition of so-called ‘engineering inclusions’ which may cause reductions in wear to tooling and/or lesser machining forces required to achieve the same outcome(9). Excess sulfides (arising from pyrite) are transformed into engineering inclusions using manganese, forming manganese sulfides, which are regarded as optimal inclusions. Increased sulfur content from pyrite increases the size of the inclusions. Research suggests that machinability is affected not by morphology of the inclusions but the percentage area instead(10). Sulfur contents well in excess of 0.2 weight% are tolerated. Suitable and accurate determination of engineering inclusions is imperative as micro cracks can form at the inclusion-matrix interface resulting in reductions of cutting force and energy consumption(11). Manganese is a common inclusion in cast iron.
Other studies have shown that the addition of sulfur (via pyrite or other means) at a later stage, in quantities as low as 0.06 weight% results in improved machinability vastly(12), and that a target sulfur inclusion level should be in excess of 0.12 weight% for optimal machinability enhancement.
Impact Of Pyrite Addition On The Casting Process
Non-metallic inclusions such as graphite and sulfur can dramatically modulate the solidification, structure and global casting quality of iron. Examination of the patent literature reveals the production of iron via agglomeration is enhanced by the addition of metal sulfide powders (such as pyrite) alongside metal oxides - with no chemical binders - suitable for use in rolling(13). One study from Japan suggested that increased sulfur content in a melt can cause surface defects when employed in a green sand casting situation, but this can be negated by the use of elevated levels of carbonaceous material in the mold(14).
In cast iron, sulfur levels lower than 0.04% by weight are associated with higher levels of eutectic undercooling, which can lead to the formation of undercooled graphite and/or carbides(15); eutectic undercooling can be controlled and slowed by adding the sulfur source at a later stage in the overall liquid-stage treatment of molten iron(16). Excess carbide formation should be avoided as such materials reduce machinability, and moderate to high sulfur/pyrite concentrations can negate these effects. This notion is backed up by Chinese research which states that the amount of undercooled graphite in grey steel is essentially eliminated when sulfur inclusion is increased beyond 0.143% by weight(17) and optimal microstructure in the sulfur from pyrite range 0.078 to 0.121%.
As a source of sulfur, pyrite is one of the three major sources alongside iron sulfide (FeS) and elemental sulfur (S8). FeS is noted for its high sulfur content but high costs; elemental sulfur is arguably the best source of sulfur but it is easily and rapidly vaporised upon addition to a melt - such vaporised sulfur renders any recovery of that sulfur difficult and unstable. Not insurmountable problems relating to the addition of pyrite to molten iron are that pyrite can float into slag due to its low density - remediation is via slow addition of the iron pyrite in the first instance. It is noted that the propensity to form slag in the first place is as much related to the method of smelting and the smelting pot as it is the smelted materials themselves(18).
In grey cast iron, whilst there is a wide tolerance of sulfur levels, the presence of FeS causes hot shortness. Hot shortness is a process where at high working temperatures, cast metals exhibit particularly low mechanical strength and tendencies to crack rather than deform. The aforementioned engineering inclusions that arise from the presence of sulfur (from any source) and manganese negate these somewhat(19).
- Pyrite (iron(ii) disulfide, FeS2), is a widely available and highly pure source of sulfur used in the iron casting industry
- In the manufacture of graphitic cast iron, pyrite/sulfur is responsible for the provision of nucleation sites, allowing the formation of graphite
- Cast irons made with pyrite/sulfur additions are known for their clearly defined and highly regular microstructures
- Pyrite/sulfur inclusions inhibit the formation of nodules, which can lead to more ductile iron
- In grey iron manufacture, machinability is improved via the use of pyrite/sulfur due to the production of manganese sulfides as engineering inclusions - such inclusions make several machining processes easier
- In cast iron in general, the use of pyrite as a source of sulfur modulates the casting quality of the iron, in addition to minimising the effects of eutectic undercooling
- Compared to other sources of sulfur, pyrite is cheaper and has fewer vaporisation problems
1 C. R. A. Wright, J. R. Soc. Arts, 1873, 22, 536
2 I. Riposan and T. Skaland, Modification and inoculation of cast iron, in: D. M. Stefanescu (ed.) Cast Iron Science and Technology, ASM International, Novelty, Ohio, United States, 2017
3 I. Riposan et al., AFS Trans., 2003, 3, 93
4 H. R. Abbasi et al., J. Mater. Proc. Tech., 2009, 209, 1701
5 A. de A. Vicente et al., J. Alloys Compounds, 2018, 10.1016/j.jallcom.2018.10.136
6 O. M. Suárez et al., Int. J. Cast Metals Res., 2003, 16, 1
7 US Patent US2887421A, 1955; France patent FR2887421X, 1955
8 S. Dawson and T. Schroeder, AFS Trans., 2004, 2, 9
9 H. Opitz, Proc. Int. Prod. Eng. Res. Conf., 1963, 107
10 A.A. Pereira et al., J. Mater. Proc. Tech., 2006, 179, 165
11 B. Mills et al., Wear, 1997, 208, 61
12 R. Z. Wu, Ironmaking and Steelmaking, 2008, 35, 638
13 US Patent US6866696B1, 2002
14 Y. Awano et al., J. JFS, 2011, 83, 20 (in Japanese)
15 I. Riposan et al., Sulfur - a key element in graphite formation in irons, in: UgalMet 2016: The 7th Conference on Material Science and Engineering, Galati, Romania, 2016
16 I. Riposan et al., Sulfur in cast irons - friend or enemy?, in: AFS International Ferrous Melting Conference, Nashville, USA, 2015
17 W. Liu et al., Foundry, 2011, 1, 1
18 A. S. Zavertkin, Refract. Ind. Ceram., 2013, 54, 35
19 K. K. Saxena et al., Mater. Today: Proc., 2020, 10.1016/j.matpr.2020.02.577