re SO3 & Alkalies at bottom stage & LOI of bottom stage
The effect of SO3 and alkalis is to slightly increase the value of the LOI test. This is because at the temperature of the standard LOI test (950°C) conducted over one hour, SO3 and particularly alkali chlorides in the hot meal can evaporate slightly. For a one gram sample, this slight increase can amount to 0.5-2% LOI. The actual increase will be dependent on the amount of SO3 and alkali chlorides typically in your hot meal.
To minimise this effect, the LOI test should be conducted at 950°C for the shortest time possible(ie to just calcine any remaining limestone). Some rapid LOI tests use a heating time of only 30mins instead of 1hr. You can determine this time experimentally by determining the time it takes for a 0.5g sample of raw meal (not hot meal) to achieve constant ignition weight. The sample is removed and weighed after 10,20,30,40,50 and 60 mins and the point of constant ignition weight is observed. This time then is used to analyse the LOI of hot meal samples @ 950°C.