Deepak Sharma.

One of the best examples that we use at our home is curdle the curd (dahi jamaana). In order to get the curd, you need small amount of curd (output becomes input).
 
Being in the chemical process industries, I have observed that many times, the output of the process is being used as the key input to the same process. The usage of the output as the main input is for the Crystallization or as Seeding material.  For instance, in Zinc Smelting (Hydrometallurgical route) process, one of the major outputs is Jarosite. But for the Jarosite production, Crystallization/ seeding of Jarosite is required, which is being done by the output (Jarosite) only via recirculation piping. Also, for the production of Sulphuric acid (i.e. output), Sulphuric acid (at low concentration) is used as input in the Absorption towers and Drying towers.
 
The main challenge of the Crystallization/ Seeding process is how to proceed from scratch, i.e. how to commission it first time.  Moreover, material balance poses a big hassle. Due to the complexity of the Mass flow diagram, it requires extensive training to train and get the team at the same platform.