Laboratory tests - Standard test methods

Clay grade content

Many different methods are used to assess the amount of fine material present in a sand. The purposes are diverse; it may simply be to obtain trend information about unprocessed sands from the quarry face or to provide a customer with information about trace residues of fine material after processing.

The range of values provided from a Clay Grade evaluation can be large, depending on the nature of individual sands and because of this it is not possible to devise a single test which exhibits similar accuracy over the range encountered, and indeed, for some purposes, e.g. trend analysis, great accuracy is not required.

Because of the multiplicity of uses for this test and the general use of the word "clay" to describe any test, at any cut off size which represents the finer end of a natural sand distribution, it is not possible to define a single test that satisfies all of the variations encountered between sand suppliers. Three tests have been described, which may be employed in appropriate circumstances.

Note:

Although the name of the test suggests that the measurement is to determine fine clay particles, most of the material normally measured is fine quartz. There should not be any attempt to use the values obtained from this test as a measure of the true content of clay minerals.

Apparatus

• 25cm. diameter stainless steel or nickel basin
• Gas burner or electric hotplate
• Tripod
• 500 ml graduated beaker
• Elutriator
• Rapid sand washer-for clay silts removal
• Dish
• Balance

Reagents

1. Ammonia solution S.G. 0.88 (about 35% NH₃
2. Sodium Hexametaphosphate, (NaPO₃)₅ GPR grade
   Sodium Tripolyphosphate Na₅P₃0₇1OH₂O GPR grade.

Method

Weigh approximately 1OOg of a representative sample of the material under test to one decimal place (or 50g for naturally bonded sands).

Inside a fume hood or ventilated space, place approximately 300ml. of water and 5 ml. of 0.88 ammonia into a basin. The ammonia is used as a deflocculating agent to maintain the .fine particles in suspension during the test.

Heat the liquid to boiling point and then add 1OOg of sand carefully with constant stirring to avoid bumping. Allow to simmer with occasional stirring for 5-1 Ominutes depending on the amount of "clay" present.

Where extraction or local ventilation is not available, this process may be modified by use of a small quantity of one of the sodium salts in Reagents 2), as a floculating aid. Heating of the solutions is not normally required.

Clay Results

The clay residue by the test methods 3.1 (i) and (ii) are dried before weighing and calculated as a percentage of the original weight. In test methods 3.1 (iii) the weight loss from the original weight is calculated as the percentage clay content.

Example methods using a rapid clay washer (agitation mixer) or an elutriator are described below at 3.1.(i), (ii) and (iii) respectively.

3.1.(1) Clay content less than 5%

If the material has a clay content less than 5% then the suspended slurry may be removed by careful syphoning or decantation, through a 63 μm sieve. Any oversize particles are returned to the main body of the sample.

The residual mixture is transferred to a graduated 500ml beaker. Distilled or deionised water is added to the 300ml mark. This is stirred and left to stand for one minute for each 2.5cm depth of water in order to settle out coarse quartz grains. The mixture is siphoned/decanted again through a 63μm sieve. This procedure is repeated until the water above the sand is clear. The sand is then transferred to a dish, dried and cooled.

3.1 (ii) Rapid Sand Washer, Clay con· tent greater than 5o/o

A sample is placed in a marked beaker. Fill to about two-thirds with tap water and add a small quantity of one of the sodium salts in Reagent 2). Stir with the impeller for 1Ominutes and then fill to the mark with water. Allow to stand for 1O minutes and siphon off the supernatant liquor. Repeat once more then on the third change of water allow only

"·5 minutes prior to siphoning. Repeat this until the supernatant is clear. The water is decanted off.and the residue is dried.

3.1 (iii) Elutriator, Clay content greater than 5%

The sample is placed into the elutriator cone with a small quantity of one of the sodium salts in Reagent 2). The water is adjusted to give an upward flow·· consistent with the removal of particles of less than 20μm diameter. When the water flow becomes clear (after approx.one hour), turn off the water and allow the settled sand to drain out of the base of the elutriator into a dish for drying.

** FLOW RATE REQUIRED TO SUSPEND A 20μm
PARTICLE OF QUARTZ DENSITY AT DIFFERENT WATER TEMPERATURES.

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