Iron Pyrite Applications in Resin Bonded Wheels and Brake Linings

High quality iron pyrite is used in a variety of settings including brake pad linings and resin bonded wheels in grinding settings. Africal Pegmatite is a leading miner, miller and supplier of the highest quality iron pyrite for these and a wealth of other applications

What is Iron Pyrite?

Iron pyrite (iron(ii) disulfide, FeS₂) is a naturally occurring sulfide mineral, and is the most common example. 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 many are due to its relatively inexpensive cost and moderate-to-high levels of hardness of 6 to 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 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, making it well suited as a component in a broad array of grinding and frictional processes. It is also non-toxic. Despite being composed of mostly iron by weight, pyrite is not considered a useful source of iron. One of the many advantages of pyrite addition to abrasive and frictional processes is that not only are only small amounts of pyrite typically required, but pyrite’s inexpensive nature means that its inclusion even in the largest scale processes is far from being economically prohibitive.

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.

Pyrite is primarily used as a - perhaps counterintuitively - 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.

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.

Performance of brake pads containing pyrite is generally excellent, affording a uniform braking capacity across a range of speeds and applied braking intensities. Longevity of the pad is related to how much of the brake pad compound is removed by the act of friction breaking - and some of the ‘dust’ that escapes contributes in part to localised air pollution alongside debris such as microplastics/rubber dust from the tyres on the road. Research has shown that brake pads containing pyrite are stable across varying braking types, with minimal pyrite lost to ‘dust’(7). The particulate dust that is emitted ranges in size from 10 nm through to 10 μm, and can be hazardous to human health(8). It stands to reason that lesser amounts of higher performing materials will be emitted as particulate dust. The only drawback to the prolonged use of a pyrite containing brake pad is that during decomposition of the pyrite - after long use at high temperatures - sulfur gas is released(9), which is considered harmful. Because of the amount of pyrite used, time taken to reach a decomposition pathway and the low overall amount of sulfur released in any case, this is not viewed as a major concern.

As a filler, pyrite provides additional benefits in addition to those available through the braking process. The pad’s overall mechanical strength is increased by improved adhesion of the abrasive grains to the binder; the abrasive grains are better protected from harmful gas release during the firing of the pad as part of its manufacturing process; the filler prevents thermal degradation of the binder itself.

Pyrite is rarely, however, used as the sole component in a brake pad. Heat dissipation and regulation is an important part of brake functionality. Research has shown that other metallic inclusions, which are already known for their superior heat dissipation properties outside of braking, may be used in a pyrite led braking system(10). Pyrite is highly tolerant of these inclusions and has been demonstrated as working synergistically with inclusions such as powdered zinc, copper fibres and brass fibres. Despite additions of these other materials, the new composite brake pads showed consistent frictional performance.

Other sulfide based binding materials have been used in the research environment. Solid lubricants/binders including bismuth trisulfide, tin disulfide and antimony disulfide were tested in an idealised braking scenario with graphite being the major component of the brake pad. Testing showed that these non-pyrite materials had slightly better performance in terms of frictional stability, but showed poorer performance in terms of wear(11).

As established previously, a great amount of heat and pressure is produced at the interface of the brake pad and the wheel. There is research which shows that under these immense conditions, pyrite itself can actually form in situ. Researchers employed Mössbauer spectroscopy to probe the condition of the brake surface and to see what compounds were present. It is hypothesised that a third surface, i.e. a friction film, is produced between brake pad and wheel and this can contain pyrite, amongst other compounds(12).

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 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 is an effective and economical choice for a grinding wheel filler(13). The addition of a filler to a resin improves the resin (and thus the overall tooling) by adding heat resistance, toughness and resisting breakages(14) - 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(15).

How it is used?

During manufacture 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(16,17). When used as a filler, pyrite is described as an ‘active filler’, as the surface of the iron pyrite interacts with other compounds in the resin, accounting for a strength providing interconnected filler microstructure. Sulfur-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(18).

In addition, the sulfur dioxides that can develop during pyrite decomposition can decelerate the thermal decomposition of the synthetic bonding resin itself, providing an additional longevity of tooling benefit(19). Fillers, such as pyrite, are typically used in the grinding and resin bonded wheel space in the loading range of up to 20% by weight, compared to the abrasive itself which may be found in 65 to 90%, and the phenolic/synthetic resin binder being employed anywhere between 5 and 20% by weight(20). Pyrite is considered a “favoured” filler for such applications - ostensibly due to its low price and high performance across a variety of grinding scenarios.

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(21). The bond itself is known for its ability to withstand high levels of shock load.

Pyrite 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. 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 abrasive grades of grinding wheels of the resinoid bond type, including applications with ceramic, carbide and alumina abrasives.

As a filler, pyrite is added to resin bonded grinding wheels to primarily improve strength, but other benefits include an ensured provision of space to allow for the addition of machining or cooling lubricant; provide better shock resistance through the bulk of the wheel; provide an environment to allow the gradual and controlled erosion of bonds to expose fresh cutting edges and to provide a degree of chemical resistance for the wheel to the cooling or machining lubricants(22). General overall stability of the resin is particularly enhanced by the presence of fillers, as is the overall flexural strength performance characteristics(23).

Grinding wheels using pyrite as a component alongside fibreglass and bakelite as a binder has been shown to be more durable over long-term grinding times than those containing just carbon elements(24), by some 30%, when as little as 7.8% pyrite by weight was used. Not only applicable to the conventional production of grinding wheels and/or media, pyrite can be used in novel new production methods for grinding media such as those produced using laser sintering and 3D printing(25).

Specialised Grinding Types

Modern grinding technologies can also take advantage of pyrite - and are especially well suited to modern generation engineering projects like the building of high speed rail infrastructure. High speed rail grinding is a highly efficient grinding method which relies on the relative motion between the grinding wheel and the rail - over typically much longer distances than conventional grinding. From this relative motion, efficiencies are gained and overall grinding speed is increased, especially important for large infrastructure projects. Pyrite - as in other grinding scenarios - is prized for its ability as an abrasive and a heat dissipator(26). It fares well in the harsh grinding settings found in the railway industry, with the pyrite absorbing the grinding heat, combining with oxygen to form ferric oxide and sulfur dioxide, bringing the overall grinding temperature down. Because of the accumulation of grinding heat, it is posited that pyrite based rail grinding wheels are best suited to grinding in short sharp bursts, as opposed to sustained and continuous action(27).

With an ever developing interest in uses for contemporary and/or fashionable materials such as carbon fibre, interest has turned to see whether these may be suitable for use in brake systems. As ever, using pyrite as a binder, carbon fibre based wheels have been developed and shown to be decently effective in the primary grinding role(28). The choice of such a strong binder is regarded as important, with excellent durability properties observed.

Where Else Has Pyrite Been Used?

Pyrite has been used 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. In addition, this addition of pyrite cinders can be seen as environmentally beneficial as it prevents pyrite cinders (a byproduct of coal production) from otherwise going to waste(29,30). 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(31).

Aside from as 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(32).

Due to its incredibly low electrical band gap of 0.95 eV and characterisation as an n-type semiconductor, 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(33). 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(34).

Summary

Iron pyrite 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.
It is used as a frictional additive in the manufacture of brake pads and shoes, as a lubricant, to ensure uniformity in the braking process. It may be used in concert with other solid lubricants or fillers
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. One highly promising use case is in the grinding of rails for the use of high speed railways
It has been shown that pyrite is a useful filler in bitumen production, replacing traditional fillers such as limestone and cement; and in small quantities in rubber production
Pyrite has found other uses outside of fillers, such as in mixed battery cells, and as a potential material for solar power generation.

African Pegmatite is the go-to industrial partner for the supply and processing of the highest quality iron pyrite for a whole host of applications including in resin wheel and brake disk lining scenarios. African Pegmatite has the experience, capacity and reach to be able to provide pyrite for any application to the exacting specification of any customer worldwide.

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Iron Pyrite Applications in Resin Bonded Wheels and Brake Linings

What is Iron Pyrite?