The invented heat treatment process is characterized in that a crystalline ammonia alum is subjected to dehydration in two stages in such a manner first is instantaneously burned in a burner flame at a temperature ranging 1000 to 1500 degC and then it is brought into contact with combustion gases at a temperature ranging from 250 to 600 degC. So dehydrated product is the thermally decomposed by heating to a constant temperature in the range of 500 to 600 degC and the resulting product is subsequently immediately or additionally dissolved in water with temperature of 40 to 95 degC to aqueous solution of aluminium sulfate, from which optionally crystalline aluminium sulfate can be isolated.
