The Strong Black Liquor, from the Digesters was passed through the Evaporators and the Concentrators where most of the water was taken out. It was then passed through a Cascade Evaporator, which was located in the Flue Gas outlet of the Recovery Boiler. Here the hot flue gas evaporated more of the water as well as depositing some chemicals that would otherwise be exhausted to the atmosphere. The concentrated Black Liquor was then sprayed into the furnace and burned along with Soda Ash and other chemicals too, I suppose.
The big difference between burning oil and burning Black Liquor is that the liquor contains some non-combustible elements that, along with the chemicals that are added, melt and become fluid like molten metal. This was no surprise to the people who knew about such things, it is supposed to do that.
The molten material (Smelt) containing the chemicals that are to be recovered, collects in the bottom of the furnace and then runs out through spouts into the Smelt Tank that is located beneath the Furnace and Boiler. You may think that you could imagine what it would sound like to hear several streams of molten metal running into an enclosed tank of water. You couldn’t even come close. It is a steady succession of explosions, sometimes violent enough to damage the equipment.
After the smelt is dissolved, in the smelt tank, it is called Green Liquor, and is pumped to the Causticizing Building where it is treated with Lime and becomes White Liquor. This white liquor is then pumped to the Digester Building where it is put into the Digesters, along with wood chips and black liquor, that has been recovered from previous cooks, and the whole mess is cooked with steam at 150 PSI. This is where the entire process starts all over again.