Gathering sulphite act 4

hello can anyone tell me how to gather sulphite ?
I’m stuck in braking wall in mine1 mission . I cannot continue exploring mine cos I don’t know where to get sulphite
All info what I found on web is how to gather sulphite over 4act
But I’m stucked in 3 rd niko mission to break the wall with sulphite . How I can farm/ gather it ? I have to go to mine lvl 2 to continue story missions without it I’ll not pass act 4 . So I need sulphite ! How to get it ? In act 4 ! In the beggining on mine quests

You dont need the mine to progress through the story, you can play without ever going into the mine.

You will encounter niko later on randomly and you get much more sulphite. So just move on for now, the niko quest chain is just in act 4, because this is where you can encounter him the first time but its by no means required.