What is corundum?

Corundum is a mineral from the category of oxides and hydroxides and is the most frequently used industrial mineral worldwide. It also occurs naturally as a gemstone (sapphire, emerald, ruby). Chemically corundum is aluminium oxide with the chemical formula Al2O3.

How is fused alumina obtained?

Raw materials containing aluminium oxide are electrothermally melted at a reaction temperature of more than 2,000 °C to produce the various fused aluminas. Different starting materials are used depending on the desired type of fused alumina. For instance, bauxite is used for the manufacture of brown and semi-friable fused alumina, whereas pure aluminium oxide (alumina) is exclusively used for white fused alumina. This is obtained in a preceding process from bauxite in the Bayer process.

What is silicon carbide?

Silicon carbide is a chemical compound of silicon and carbon and belongs to the group of carbides. The chemical formula is SiC.

Silicon carbide is at the same time the hardest and lightest ceramic material and is also highly resistant against acids and alkaline solutions.

How is silicon carbide obtained?

Silicon carbide (SiC) is produced from coke and quartz sand applying the Acheson process in an electric resistance furnace. When petrol coke and quartz sand are grouped around a graphite electrode and heated to a temperature of 2,000°C, black and slightly higher-quality green SiC are obtained. Green SiC has a slightly higher purity and hardness, but is also less tough.

Why is corundum or silicon carbide used?

Corundum is the most frequently used abrasive. The purer the corundum, the harder it also is, which can be determined from the colour. The toughness can be increased by adding various metal oxides and by shortening the cooling process in production.
The following distinctions are made:

Brown fused alumina
with more than 94% Al2O3 (aluminium oxide) is e.g. used to treat steel castings and cast iron. Its toughness permits especially high pressing forces.

White fused alumina consists of more than 99.9% Al2O3
Due to its hardness and thermal resistance up to 2,000°C, white fused alumina is suitable for tough steel types (tool steel), for grinding and polishing glass as well as for all steel types requiring cool grinding.

A very important field of application for the ultrapure white fused aluminas are varnishes and coatings, particularly powder and UV coatings, in which they enhance the scratch-resistance of furniture surfaces, worktops and floors. Fused alumina is most effective in ramps and highly wear-resistant industrial floors.

Semi-friable fused alumina with approx. 97% Al2O3 (aluminium oxide) is classified between brown fused alumina and white fused alumina due to its technical properties. It is less tough than brown fused alumina, but has a higher cutting quality.

Pink fused alumina gets its characteristic colour when chromium oxide is added to melt. It has a slightly higher grain toughness. Its higher edge stability makes it possible for use in form and profile grinding; otherwise it is identical to white fused alumina. Due to its hardness, fused alumina is often added to surfaces to protect them against wear (e.g. in varnishes and laminates).

Ruby fused alumina has further metal oxides added to it, particularly Cr2O3 (chromium(III) oxide). This results in maximum toughness making ruby fused alumina suitable for the grinding of high-alloy steel types.

Silicon carbide is heat-resistant up to approx. 1,600°C and contains hard, sharp-edged crystals. One abrasive grain usually only consists of a single or a few crystals. Compared with fused alumina, SiC is harder and more brittle. There are many fields of application, ranging from conventional blasting agents to hard concrete aggregates which make industrial floors abrasion-resistant and conductive and vaults particularly durable. It is used in engineering ceramics due to its low weight and low thermal expansion. In addition, SiC is a semiconductor and is used in the manufacture of blue LEDs, for example. SiC also produces a very pronounced glitter effect in dark colours (e.g. façades, plaster).

Higher-quality green silicon carbide is primarily used to treat glass, china, marble, gemstone, artificial stone, for precision treatment of light and non-ferrous metals as well as leather.

What is FEPA and what does it do?

FEPA is the Federation of European Producers of Abrasives. Its mission is to develop and publish standards, documents and comprehensive safety instructions for the abrasive industry.
Abrasive grain sizes are produced and sold according to the FEPA standard, among other things. This is to ensure that a consistent quality for any abrasive can be achieved across different manufacturers.

For more information go to https://www.fepa-abrasives.com/

What grain sizes are available?

Macrograins       ________________ Micrograins      
FEPA F Abrasive Micron µm FEPA P Paper Micron µm   FEPA F Abrasive Micron µm Micron µm FEPA P Paper Micron µm""
F 5 4125           P 240  
F 6 3460       F 230   P 280  
F 7 2900           P 320  
F 8 2460       F240      
F 10 2085           P 360  
F 12 1765 P 12 1815   F 280   P 400  
F 14 1470              
F 16 1230 P 16 1324   F 320   P 500  
F 20 1040 P 20 1000       P 600  
F 22 885       F 360   P 800  
F 24 745 P 24 764       P 1000  
F 30 625 P 30 642   F 400      
F 36 525 P 36 538       P 1200  
F 40 438 P 40 425   F 500   P 1500  
F 46 370           P 2000  
F 54 310 P 50 336   F 600      
F 60 260 P 60 269       P 2500  
F 70 218           P 3000  
    P 80 201       P 5000  
F 80 185       F 800      
    P 100 162   F 1000      
F 90 154       F 1200      
F 100 129 P 120 125   F 1500      
F 120 109       F 2000      
    P 150 100          
F 150 82 P 180 82          
F 180 69 P 220 68          
F 220 58              

Which measurement methods are applied?

  • Dynamic image analysis (DIA)
  • Static light scattering (SLS, also called laser diffraction)
  • Screen analysis
  • Impedance method
  • Sedimentation (not used at Wester Mineralien GmbH)

What is the d50 value?

The d50 indicates the mean particle size.
d50 means that 50% of particles fall within the specified grain size.

What does the bulk weight indicate and what impacts the bulk weight?

According to the FEPA, the bulk density of an abrasive is the quotient of the mass and the volume a grain type poured out in a particular manner will occupy. The bulk density, also called bulk weight, is stated in g/cm3.

The rounder the particles, the higher the packing density, meaning the smaller the volume/grain size, the higher the bulk density.

What is the specific weight?

The specific weight is the ratio of the weight of a body to its volume. The specific weight of corundum under normal conditions is 3.94 g/cm3.

What does the form factor (grain shape) indicate?

The description of the grain shape can range from “angular” to “acicular” to “spherical”.

When the value of the dimensionless form factor is 1, the grain is spherical.

The smaller the form factor, the pointier the grain.

What is the specific surface?

The specific surface means the outer surface of a substance including all accessible pores in relation to the mass. It is determined using the Brunauer-Emmett-Teller (BET) method.

What is porosity?

Porosity is defined as the gas proportion (cavity volume) of a particle’s total volume. It can be determined using different measurement techniques, such as gas adsorption and mercury porosimetry.

With the same grain size and shape, a lower bulk density corresponds to a higher porosity of the grains.

What does the term “flow properties” mean?

The flow behaviour of powders depends on many factors. The following applies in general: Coarser, dry grain types flow easily, but the fine the grains get, the stronger is the impact of morphology and various adhesive forces on the flow behaviour:

  • Van der Waals forces
  • Liquid bridges
  • Electrostatic interaction

The 4 most common methods for determining flow properties are

  • Shear cell
  • Angle of repose
  • Compressibility index or Hausner ratio
  • Flow rate through a nozzle

A product is usually considered as having good flow properties if it flows out of a silo without interruption or does not solidify during storage or transport.

What is Mohs hardness?

Mohs hardness (according to Friedrich Mohs, 1773 - 1839) ascribes a relative hardness value to certain known minerals on a scale ranging from 1 (talc) to 10 (diamond). Accordingly, a mineral is considered harder than another if it can scratch the latter.

Please note that these values merely form an ordinal scale: A higher value means greater hardness, but the distances between the scale values are not always the same.
A Mohs hardness of 10 is assigned to diamond, the hardest material. A diamond therefore can only be cut using another diamond.
Diamonds are closely followed by the corundum group, which includes rubies and sapphires.

Mohs hardness – a scale ranging from 1 to 10

What is a silane coating used for?

When functionalising fused alumina, a very thin layer of a silane adsorption group is applied to the fused alumina particles. This organic surface coating has the effect of better embedding and increased scratch resistance in a melamine resin matrix, for example. Fused alumina has a high refractive index (n = 1.76). “Greying” would occur in transparent coatings, such as varnishes and melamine resin, without additional coating. The coating adjusts the refractive index to that of the varnish, thereby improving the latter’s transparency.

What is the refractive index of white fused alumina compared with that of glass?

The refractive index of fused alumina is approx. 1.76, whereas that of normal glass types is approx. 1.4. The closer the refractive indices are to each other, the more transparent the system (e.g. varnishes).

What is the difference between aluminium oxide and white fused alumina?

Al2O3 alumina is the starting material of aluminium production, both of metallic aluminium and aluminium oxide. It is porous an has a large reactive surface.