25-04-2014, 12:37 PM
SILICA SAND: FOUNDRY REQUIREMENTS AND CLASSIFICATION
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ABSTRACT
Silica Sand is the most essential raw material and its importance is sometimes forgotten amongst Foundry
personnel. Silica Sand as used by Foundries is desired for its thermal resistance and availability. In Queensland
Australia it falls under the minerals act as it is processed and sold for its chemical properties. Whilst Silica Sand
is abundant throughout Australia the technical requirements of Foundries allow only a few deposits to be mined
and processed for their use. This paper will cover some of the processing routes along with grading and
physical/chemical attributes of Silica Sand.
Bulk Density and Compaction
Foundry Silica Sand is chosen for its ability to compact with a range of Chemical Binder systems. The
Surface Area of the Sand Grains must be low as the use of Resin has a major impact on a Foundries
competitiveness and profitability. With correct sizing and sphericity a maximum Bulk density can be
achieved, a high grade Silica Sand will have a dry bulk density of approximately 1.56 tonnes / cubic
metre with an AFS GFN of 50.
There are formulas and tables for the calculation of sphericity however if we imagine perfect spheres
and the way they would pack given the correct vibration then we can picture the following scene:
Perfect spherical grains of sand would in theory pack the closest and give a theoretical permeability of
zero. That is gas would not pass through it. It was from this theory that sand size distribution and
shape have been decided upon. Sand is rarely round and spherical and the terms: well rounded, sub-
angular to rounded and angular have been adopted. Fortunately a certain degree of permeability is
necessary and is why we can use Silica that is sub-angular to rounded, refer photo 2.
Chemical and Thermo-Physical Properties
Silica Sand is used predominantly due to its chemical purity and advantageous thermal properties. It
is resistant to molten Steel and Iron it has high hardness and is compatible with all types of Foundry
Binder systems. Silica has a high fusion point above 1690 degrees Celsius. This is dramatically
lowered however by Fluxing agents such as Calcium, Sodium, Potassium, and Iron. These elements
can drastically lower the sintering point of Silica the alkaline nature of the elements and their oxides
listed above can drop a 99.8% Silica Sand from 1700C to less than 1200C!
|Deleterious agents such as lime for example not only raise the pH of the sand but will render some
binder systems useless. Acid Catalysed Furan will not harden whilst the phenolic urethane systems
will react and harden instantaneously!
Silica does exhibit a rapid thermal expansion as it undergoes a phase change from Alpha Quartz to
Beta Quartz and at a temperature of approximately 570 Celsius. This expansion can lead casting
defects such as “Veining or Rat Tails” however this can be compensated for
by the use of complex
binder systems and additives to the sand mix dispensed via a controlled dosing system. The expansion
of Silica Sand is compared below against the popular “Special Refractory Sands” such as Chromite
and Zircon.
The Acid Demand Value test:
The acid demand value is important because it displays the amount of alkaline materials that should not be
present in washed and classified foundry silica sand. Minerals such as limestone and shell, (CaCO3),
dolomite Ca/Mg CO3, Lime CaO.
The importance of the Acid Demand in testing should not be overlooked as it reveals various carbonates
and salts that may be missed with a standard pH test. A high ADV will significantly shorten bench life in
the cold box system and lead to pinhole defects in castings due to the production of CO2 gas evolution.
Typical Values for Queensland silica Sands are: FSD 710M 0-4ml, Rouse Channel Sand 15-40ml.
The Best Value is 0 and a usable maximum value is 15.
The acid demand test can be obtained by the use of hydrochloric acid or Sulphuric Acid as per the BCIRA
Broadsheet 210-4 (1983)
Conclusion:
Whilst we have an abundance of high purity silica within Australia, without the correct understanding of
it’s properties, ideal sizing for a given application and the available resins systems at the disposal of today’s
Casting engineer the true benefits will only be partially realized.