Seeing as it started it's journey at Antonov's own airfield in Hostomel near Kiev, stopping in Almaty before flying to China, I'm assuming it's picking up something there.

If that is so, the F at the end of an ADB callsign could mean the flight is not carrying a load. It either unloaded where it came from or is going to load where it's going.

Maybe Mriya's destination gives us a clue as to what really big thing, if anything, it picked up in Tianjin.