Laboratory tests - Standard test methods

Loss on ignition (LOI)

LOI is the weight change percentage caused by heating the sample to high temperature and is used to give a measure of the extent of volatile substances. The original use of this test was to estimate the amount of coal dust in recycled foundry sand.

Although the content of carbon bearing materials may be related to the LOI, it is not a direct measure of carbon. Some recycled materials experience weight gains because they contain significant amounts of metallic particles. During heating these can be converted to high oxidation state metal oxides.

Other components may show weight losses due to phase changes associated with dehydroxylation of clay minerals or with the loss of carbon dioxide from carbonate minerals such as calcite and aragonite in shell fragments.This normally means that LOI is a balance of all of the competing weight changes that occur during ignition.

It is important to carry out this test on samples that have not been modified by grinding to reduce the grain size. Analyses for LOI on samples ground for chemical analyses in tungsten carbide mills, for example, show consistently higher values than their unground equivalents due to conversion of the contaminating carbide grinding medium to an oxide of tungsten.

Apparatus

• High temperature electric furnace-capable of maintaining 1000°C ±50° c
• Crucible- fused silica, alumina or platinum.
• Balance- accurate to at least 3 decimal places.

Method

The test sand should have been pre-dried at 110°c ±10°C and desiccated prior to use.

The empty crucible should also be stabilised prior to use by heating and desiccation, then weighed (W1). About 10g of sand is added and both are reweighed (W2).

The loaded crucible is placed in the furnace at 1000°C, ±50°C. After 90 minutes the crucible is withdrawn and placed in a desiccator. When cool the crucible is reweighed (W3).

The percentage loss on ignition is given by

{ (W2-W3) / (W2-W1) } x100

Normally the test should be carried out in duplicate and the average value quoted.

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