filling the whole sand pit with oil? Hehe
sounds like good plan. What could go wrong.
There are two primary ways of transferring heat:
Conductivity
Radiation
the question is - is this an issue? If you got low conductivity you increase surface - people figured out how to cool a space station in vacuum, which is close to zero conductivity, still works.
How? Radiation - when you get your heat exchange hot enough it radiates heat and transfers it that way. Same the other way around - just heat the sand up to multiple hundred degrees and it will radiate into the pipe - no conductivity needed.