Survey of raw materials used by us for the manufacture of grinding wheels and dust that may be produced during the grinding operation of such products.

We are regularly asked questions on the above subjects, hence we composed this survey with the aim to inform our customers to the best of our abilities.

Both for vitrified and bakelite bond grinding wheels we are using as abrasive material:
aluminium oxide (Al₂O₃). It is fused at very high temparature(2000°) and produced in several types :

Normal	Al2O3	= Approx.	96%	Al2O3 and 4% Ti O2
White	Al2O3	= Approx.	99.5%	Al2O3
Pink	Al2O3	= Approx.	99.5%	Al2O3 and 0.25% Cr2O3

Next to Al₂O₃ a second basic product is used as abrasive material namely Silicon Carbide (SiC). SiC is a product crystallized at very high temperature (1600-2500°C.) and can be subdivided into 2 types:

• Green SiC = Approx. 99.6% SiC.
• Black SiC = Same.

None of both types Al₂O₃ and SiC are dangerous to health, neither their dust particles produced during the grinding operation. The volume of dust originating from the material to be ground (in general metal) during the working is many times higher than that of dust produced by abrasive grains.
Needless to say that inhaling dust, of whichever nature, is to be prevented. Grinding with dust extraction or grinding with a dust mask is always recommendable.

Moreover the applied bondings for vitrified (ceramic) wheels consist of glazes, which connect the abrasive grains together during a firing process at a maximum temperature of 1300°C. Basically the glazes contain: Na₂O, K₂O, CaO, MgO, B₂O₃, Al₂O₃ and SiO2, compositions which are more or less comparable with window- or bottle glass.

These glazes are also pulverized more or less during the grinding Operation, however the greater part melts together with the metal partides which are ground away. Anyway the dust from these glazes are not harmful.

The abrasive grains of resin bonded grinding wheels are combined by means of phenol-formaldehyde resins (bakelite resins). During grinding bakelite resins burn entirely, during which process carbon dioxide and watervapour arise as waste products, which are harmless to health.

The proportion of abrasive grain to bond are both for vitrified and bakelite bonded wheels between 95 to 85% and 5 to 15%. Consequently the abrasive grain always forms the major part of the compositions. As far as filiers are concerned we have fully abstained from the use of poisonous materials, such as derivates of lead, antimony or arsenic, which are sometimes applied at he manufacture of grinding products. Anyhow we are unknown of any risk caused by composed partides of a grinding wheel during Operation, apart from the risk dust partides of whichever nature can give when inhaling.

Besides we can render the follwing supplementary information about silicon carbide grinding wheels, since sometimes the impression is created, that there is danger to silicosis at the use of such wheels. Apart from stock removal there will arise wear of silicon carbide grains, when ground with wheels containing silicon carbide and this will partly happen by means of fine dust. In case of such a high grinding pressure that a considable rise of temperature will occur at the contact surface, the fine SiC-dust will oxidize, causing CO or CO₂ next to SiO₂.

However at this temperature there does not arise quartz but a SiO₂ modification, namely
alpha-tridymite.
There is a chance that alpha-crystobalite comes into existence in case of higher temperatures. Ifthe composed tridymite (or crystobalite) is cooled down, there does not appear beta-quartz, but the low temperature modification of tridymite and crystobalite, so the beta forms. Neither the beta-tridymite northe beta-crystobalite gives any risk to silicosis.

This problem was already put before us earlier by some large customers. We then collected dust samples at various places and from different grinding operations. This dust was examined by means of röntgendiffraction in the physical laboratory of Ferro (Holland) B.V., Rotterdam. In none of these samples quartz, crystobalite or tridymite was found. Amorphous SiO₂, cannot be determined in this way, but it is known that amorphous SiO₂ does not offering any risk of silicosis.

The results of an investigation made by the Dutch T.N.O. Institute (semigovernmental) about the risk and in particular about liberated fine quartz dust, if any, when grinding with SiC were entirely negative. No alpha- or beta quartz could be discovered as derivated products from SiC.

We, too, could never find any evidence of silicosis at people, who have regularly been working with grinding wheels containing silicon carbide. Neither we know about any literature about this subject. However this does not remove the fact that we strongly believe in good dust exhausting at any grinding activity.

In case it turns out at any investigations carried out by you, that the afore mentioned opinion needs rectification, we shall be very glad to be informed. Since, in connection with other customers, we, too, want to be up-to-date certainly when the optimal safety of the labourers is at stake.

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