Why we get "Gain on ignition" in industrial byproduct metallurgical slag such as copper,lead zinc, LD and GGBFS , instead of "Loss on ignition"?
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The reason is that these materials contain significant amounts of Ferrous iron (Fe+2). During heating in air Fe+2 reacts with oxygen and is oxidised to Fe+3. e.g.
4FeO + O2 --> 2Fe2O3
The increase in weight (gain on ignition) is equal to the amount of O2 absorbed by the material during ignition minus the actual loss on ignition, if any.
Hope this helps,
Possibly, if the right conditions exist, for example if the sample is high in metal sulphides and the atmosphere of the furnace is well oxygenated, sulphates may be produced.
However most sulphides also oxidise easily (with the loss of SO2), and in my experience it is more likely that a small portion is lost during the heating process, adding to the LOI.