SILICA SAND: FOUNDRY REQUIREMENTS AND CLASSIFICATION. M. DAWSON TECHNICAL MANAGER, FOUNDRY SAND SYSTEMS & CASTING METHODS CAST METAL SERVICES PTY. LTD. PH # 0732666266 FAX#073266366 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.

When referring to Silica Sand for Foundry use we define it as the mineral quartz; its sizing, chemical purity, shape and physical durability will be discussed along with the thermal properties.

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.

Figure 1: Theoretical Spheres Photo 2: Reclaimed Silica, AFS 50 To introduce the sizes most commonly used the Red Spheres denote 0.425 mm, Green 0.212mm, Yellow 0.3mm, Salmon 0.15mm and an undesirable black sphere at 0.075mm or 75 microns. Ideally the sand grains should be spread across four sieve sizes in a Bell type distribution. The Older Silica Sand deposits are subjected to a greater degree of weathering hence a rounder grain evolves. This explains photo 2 where this material was laid down a mere 12,000 Years ago!

Sand Size Distribution Sand particle distribution is controlled by selective mining from dune systems of known particle size via drilled surveying and/or by thorough sampling. The wet product can then be blended and literally cut off at a selected grain size effectively between 425 microns, (0.425mm) and 106 microns. Wet and dry screens, select silica Sand sizing hydraulic classification such as hydro cyclones, settling tanks and high velocity T cell classifiers.

Photo 4: 300,000 Tonne /annum Wet Plant Photo 5: Aerial View of an Open Pit Mine A Modern Wet Plant, Beachmere QLD photo’s courtesy of Southern Pacific Sands, QLD. The washing process can involve as many as three to four cycles to ensure that the sand has an exceptionally low turbidity, free from clay and fines, low in unwanted salts and an acceptable pH value. The Industry Standard in Australia is a hybrid system using Metric sized sieves that match the old Mesh Numbers of the US Standard (ASTM E11-61) and allow an AFS Grain Fineness Number to be calculated. A Table displays the way in which this is calculated and a simple spreadsheet can be devised and used. Sieves of known size are assembled and placed in a vibratory jig. The grains fall through square mesh sizes that are placed coarse on top usually 0.60mm or 30 mesh through to 300 mesh or 53 microns at the bottom above a collector tray. The sand that is retained on each sieve is weighed and tabulated, that weight is multiplied by a factor dependent upon the sieve size it was recorded against and an AFS GFN Number is calculated. The Table Below demonstrates a typical Sieve Analysis for the Mine above and the Bell Curve obtained is nearly ideal!

THE AFS GFN SPREADSHEET

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.

IMPURITIES CONTAMINATES AND TESTING PROCEDURES 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) Moisture: Moisture contents of 0.2% and above are detrimental to the overall quality of the Cold Box, Phenolic Urethane process, the water reacts with the hydroscopic poly-isocyanate part 2. The Bench life is considerably shortened, core strength is lowered and Flowability is reduced. Other key indicators are lower scratch hardness and core rigidity. In general terms there is a 30% loss of tensile strength for every 0.1% moisture increase!

Moisture versus Tensile Strength

0

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0 0.1 0.2 0.3 0.4 0.5

Percent Moisture

Tens

ile S

tren

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(PS

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CONTAMINATES AND TESTING PROCEDURES cont...

pH The pH test is a measure of hydrogen ion concentration, it reveals whether the chemically soluble ions in water have acid or alkaline properties. Lime for example causes the pH level to significantly rise; acid causes the pH to fall below 7. A pH of 7 is neutral and distilled water has a pH of 7, a strong acid would have pH of 2 and conversely a strong base or alkali would measure 13. To have a pH of 9 or higher on the phenolic urethane cold box system is to dramatically shorten the bench life of the sand. A very low pH such as 2 or 3 will inhibit the sand from setting at all especially in self set cold box systems. The pH is measured using buffer solutions of known pH and propriety “wet glass bulb” measuring equipment.

Fines and Clays, Turbidity or the Sedimentation Test. These are classified as all materials below 20 microns in size, the quickest tell tale sign is to take 100g of sand and place it into 200ml of water, shake the mixture well, note the amount of sedimentation after a settling time two minutes and the clarity of the water above the sedimentation line. Note this and compare this to previous batches of sand. Acceptance should be based on a test result that gives a satisfactory result.

Beachmere Sand (left) vs. NZ Foundry Sand (Right)

VEINING, RAT TAILS CAUSES AND CURES

The expansion of Silica as it undergoes various phase changes has been well discussed needless to say as the sand heats up it expands this can be overcome to a certain extent by using an angular sand rather than a well rounded sand, the downside to this is lower tensile strength. Usually Iron Oxide is added to the sand or specially engineered core sand additives. These additives sinter at low temperatures and offer the sand some form of thermal plasticity as they dissolve they allow the sand to expand into the voids left behind.

This impellor core made from the Cold Box process and has 6% of a proprietary CMS Anti- Veining additive. 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.