liquid silicon 是第幾類危險品??

Not enough information to determine.

Dangerous goods are determined based on their chemical formula.

You can refer here for the sample of this list:

http://www.hkfsd.gov.hk/chi/source/notices/Fire_Protection_Notice_No_4.pdf

Also - please note that as you intend to ship overseas, you may have to check with the country's transportation department for further guidance.

Liquid silicon? Silicon melts only when heated strongly.
So you're transporting a hot molten liquid?

I think you're talking about some sort of silicone; but still you have to specify the identity.