relay control with network and sms with main controller
Industrial control systems of a safety/mission-critical nature are designed with the concept of "redundancy", which is probably what you refer to. In such safety systems, you have two 2 pole safety relays with forcibly guided contacts. One pole controls the application, the other pole is used to supervise the state of the relay. If it gets stuck, then you'll be able to tell.

On the highest integrity levels, such as ISO 13849-1 PL d or PL e, each relay is typically supervised by 2 microcontrollers, that checks that the relay is indeed in the expected position. That may or may not be overkill here, depending on how critical the application is. In my experience, everything in melting plants is critical - if not in terms of safety, then in terms of cost at production standstill.

All of this, PCB and software both, needs to be designed by professionals. So if you have no clue what any of the above means, you must consult someone who does.

I wouldn't recommend using hobbyist junk such as Raspberry Pi for professional use in general, but not inside a melting plant in particular. You'll have massive transients, voltage surges and magnetic fields. The hobbyist board is likely to run amok in such an environment, since it was not built to handle that. I'm guessing this is for some "IoT" where you intend to access the device over internet? It can perhaps work if you place the board outside the plant...

There are plenty of companies that specialize in various controllers for industrial use, so why not buy a pre-made system rather than re-inventing the wheel?