Read more about this since and I think deicing can only do so much and can be overwhelmes by severe icing perhaps cause by supercooled water (liquid but below zero).

Usually at -47C it is too cold for this to happen but speculators have said this may be supercooled liquid coming up from the lower thunderstorm.

Happy for people to comment if any of this is wrong. I am not a pilot or aircraft engineer or fluid mechanic etc.

My layman's idea of how hard to get rid of ice: try cleaning your freezer of built up ice or chuck a massive block of ice in the sun and see how many hours it takes to melt. Snowmen can survive a sunny few days after the snow.