Chlorine in No Man’s Sky is a great way to make units if you’re just getting started. What you need to do is simply loop it in the refiner (medium refiner is enough) and you do that by stocking up on oxygen (buy it from the NPCs) and having at least a little bit of chlorine initially.
If you can’t find chlorine on any NPC, go to a planet with water and farm some salt to refine it into chlorine. You only need a little bit to start, after that you will be multiplying it by adding oxygen to it in the refiner.
Keep inputting chlorine and oxygen into the refiner and sell off excess to NPCs (not the galactic trade market).
There’s not much else to be said, so if you want a practical example, just watch the video above I embedded.