As mentioned by others - print out a short letter with your flights, contact info, and short explanation (hi i am travelling for diving, this is used to scrub CO2 - keep it simple, any complicated terms might scare customs), MSDS (material safety data sheet), manufacturer air transport statement (manufacturer provides one for both 797 and 812) and technical data sheet. All can be downloaded from sorb manufacturer site.
I normally put them in a plastic pouch then tape it to the box of sorb (4.5kgs only, not taking 20kgs). Additionally I put a paper tape across the pouch opening - basically a little gimmick, so i know if somebody actually took it out, as most likely they will rip that tape
I would not take it as hand baggage, walking around with a box of white powder might get you unwanted attention...I do not mention it at check in neither, unlikely they will know what is it but sure they will make a fuzz.
Also, I put a sticker with my name/address/mobile right on those lovely corrosive labels on each keg
From recent travels (last 3 months or so):
Germany to Mexico via Canada, only way out:
2x 4.5kg in checked baggage, no issues whatsoever. Nobody checked anything.
Germany to Indonesia via Taiwan, then local Bali-Komodo, both return, as had some unused sorb:
1 x 4.5kg in checked baggage, on the Germany to Indonesia leg zero issues. My 2nd checked bag was inspected (found security note inside) but that one had o2 cylinder. Also came back to Germany with one box (used less than expected and still had weight allowance), zero issues.
2x 4.5kg plus another 2.2kg in the rebreather itself - on the way out Bali to Komodo ther was huge fuzz about o2 cylinder (Citilink are known for that) but zero efs given about sorb.
Way back from Komodo to Bali, still had some unused sorb, initially it was fine but after 20 mins heard my name being butchered on the loduspeaker system. Had to go back for a chat with security, they were very interested in the cases of white powder, but upon seeing all the paperwork (as above) with the official company letterheads/signatures etc they calmed down, though still opened and stuck finger inside then just took copies of the paperwork and sent me on my merry way (well had to do a selfie with a security girl xD)