Iron Pyrite

Iron Pyrite Applications in Resin Bonded Wheels and Brake Linings

What is Iron Pyrite?

Iron pyrite is a naturally occurring sulfide mineral, and is the most common example of the sulphide mineral. It is found naturally in quartz seams and alongside coal deposits. In its pure form, it has a metallic lustre that gives it a superficial appearance to gold - hence pyrite is often referred to as ‘fool’s gold’. Pyrite finds many uses, but due to its relatively inexpensive cost and moderate-to-high levels of hardness of 6-7 on the Mohs scale, compared to iron (4 Mohs) and human fingernail (2 Mohs).

Due to this hardness it can often be found being utilised in mechanical settings where high tolerance of resistive pressure is required, such as a component in brake pads and as an active filler in grinding wheels for the manufacturing sector, but where a super-hard material such as diamond is not required. In addition to high levels of hardness, pyrite has a moderate thermal decomposition temperature, decomposing between 540-700 °C into iron(ii) sulfide and elemental sulfur. It is also non-toxic and despite being composed of mostly iron by weight, pyrite is not considered a useful source of iron.

Iron Pyrite for Brake Lining

Brake pads are made up of frictional additives. Brake pad additives include, binders, fillers and reinforcing fibres which collectively produce a pad that is both easy to manufacture and reliable, whilst at the same time reducing undesired side effects such as excess noise and heat.

Iron Pyrite Powder is primarily used as a brake pad additive/ frictional additive and as a lubricant in the manufacture of brake pads, for a variety of applications. Iron pyrite application may include a brake lining filler. The addition of pyrite to the pad modulates the hardness - friction ratio, a crucial factor in the performance of a brake pad, as a solid that is too hard could also be brittle, which would then fail in the high pressure and heat scenario of friction braking. A failed brake pad means inefficient braking and potential risk to life. Braking functions via the conversion of kinetic energy to heat energy - thus it is crucial that any brake pad has a high overall ability to distribute heat evenly.

Brake pad and brake disc

How is it Used?

Iron Pyrite for brake lining is used as a lubricant in the brake pad design, as well as a brake lining filler. The term ‘lubricant’ seems counterintuitive, but in the application of braking, it is crucial that a uniform braking pattern is achieved every time. As such, brake pad manufacturers use frictional additives - both lubricants and abrasives - to modulate the braking pathway for uniformity.

The choice of lubricant is oftentimes to ensure operating temperatures are maintained below that of the metallic brake pad mounting. Semi-metallic pyrite’s decomposition temperature fits the requirements perfectly. Lubricants can make up between 5 and 30% of the composition of a brake pad, and typically pyrite is added to the brake pad manufacturing process as a finely ground powder. In addition to the lubricant pathway, pyrite aids in heat dissipation in the pad, where pyrite is acting in a filler role, in addition to the frictional additive role.

Iron pyrite for brake lining is found and used  in high-performance brake pads and shoes (for different modes of braking) as both a frictional additive and a brake lining filler, particularly in the automotive sector, by brands such as Ferodo, Road House and Bosch - alongside metallic iron, haematite and carbon as primary friction materials.

brake pads and brake disc manufactured using iron pyrite

Iron Pyrite in Resin Bonded Wheels

A resin bonded wheel is one type of grinding wheel employed in the industry, they are formed of an abrasive (such as silicon carbide or synthetic diamond, often referred to as ‘grit’), a filler (typically an inorganic compound) and a binder. The binders are organic in nature, typically phenolic resins. Iron pyrite for grinding wheels is an effective and economical choice as a grinding wheel filler. The addition of a filler to a resin improves the resin (and thus the overall tooling) by adding heat resistance, toughness and resisting breakages and is often responsible for increases in porosity.

Porosity is an important concept in industrial grinding applications. A greater level of porosity increases the supply of coolant to the grinding area, whilst simultaneously enabling the removal of debris and residues. A filler can also act as a secondary abrasive. Common abrasive wheels typically have a resin content of around 8 wt%, filler content around 3 wt%, with the balance being primary abrasive/grit.

How Iron Pyrite for grinding wheels is used?

During manufacturing of phenolic resin bonded grinding wheels, pyrite is added as a filler in the resin alongside the grit, with the addition of fillers partly responsible for the grade and hardness of the resin overall. Iron pyrite applications include being used as a grinding wheel filler, pyrite is described as an ‘active filler’, as the surface of the iron pyrite interacts with other compounds in the resin, accounting for an interconnected filler microstructure. Sulfure – containing additives such as pyrite prevent the formation of metal oxide layers through a high temperature redox reaction, which in turn causes the delay of oxidation to the phenolic resin bond itself, in turn prolonging the lifetime of the tool.

Grinding wheel with sparks
grinding machine that uses iron pyrite

Resin bonded grinding wheels are used in conventional, high precision and super abrasive grinding situations, for the latter two scenarios, the high heat capacity of pyrite comes in beneficial as a heat sink, reducing the temperature at the site of grinding activity and thus enhancing performance. The bond itself is known for its ability to withstand high levels of shock load.

Iron Pyrite for grinding wheels is added in the pure monocrystalline form, taking a splinter, or needle-type shape, this addition not only helps with the enhancement of thermal conductivity as mentioned before, but enhances tensile strength and porosity of the resin bonded abrasive. Overall, durability is increased, which results in more grinding uptime and fewer requirements to replace the grinding tool. Pyrite has been utilised in many different resin bonded abrasive grades, including applications with ceramic, carbide and alumina abrasives.

Where Else Has Pyrite Been Used?

Iron Pyrite uses have a vast range in other industrial situations as a source of sulfur, pyrite cinders have been found to be a good additive/filler for bitumen to be used in road construction. Performing as well as established fillers (Portland cement, limestone dust), the resulting pyrite-doped bitumen resulted in good data in testing for mixability, stability and flow. This addition of pyrite cinders can be seen as environmentally beneficial as it prevents pyrite cinders (a by-product of coal production) from otherwise going to waste. As a component of ‘Shungite’ deposits, iron pyrite has been found in small quantities and has been used overall as an enhancer to synthetic rubber production, alongside other minerals.

Aside from being a filler, pyrite has found other uses in recent years, notably as a cathode in new types of battery cells. A nanocrystalline pyrite cathode has been shown used alongside a magnesium anode in a dual sodium/magnesium salt electrolyte, affording energy densities of circa. 210-watt hours per kilogram, which is comparable to market-leading lithium-ion cells, and twice the capacity of magnesium-ion cells.

This performance is especially good considering the low cost of materials. Iron pyrite has been identified as a potentially useful material for the manufacture of flexible solar cells, displaying high solar absorbing character using much thinner layers than would be used for silicon. Researchers, however, have found that difficulties arising from surface defects (a lack of sulfur atoms in the crystal structure) are causing inefficiencies in use as solar cells. Despite this, work is ongoing into this seemingly promising material.

grinding wheels

Summary

  • Iron pyrite powder is a naturally occurring mineral of sulfur that has a good level of hardness and has a thermal stability profile that is useful in braking and grinding applications, it is also relatively inexpensive and non-toxic.
  • Iron pyrite uses include acting as a frictional additive in the manufacture of brake pads and shoes, as well as a lubricant, to ensure uniformity in the braking process.
  • It is used in resin bonded wheels as an active filler, enhancing the performance of the overall grinding wheel by increasing levels of porosity, modulating heat conduction and physical strength of the wheel.
  • It has been shown that pyrite is a useful filler in bitumen production, replacing traditional fillers such as limestone and cement and small quantities in rubber production
  • Iron Pyrite uses outside of fillers include being in mixed battery cells, and as a potential material for solar power generation.
brake pad and brake disc

References

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Pyrites powder in a pot