LONDON, Nov 13 (Reuters) - Aluminium prices hit their highest in nearly 20 months on Friday as speculators pushed prices higher due to continued optimism over a COVID-19 vaccine and strong demand in China.

Three-month aluminium on the London Metal Exchange was up 0.1% at $1,931 a tonne by 1705 GMT after touching $1,941, the strongest since March 21, 2019.

LME aluminium has rebounded 32% since March despite forecasts of huge surpluses - analysts polled by Reuters last month pencilled in 2 million tonnes of excess supply this year.

Prices have been lifted not only by optimism over a vaccine boosting a recovery in the global economy but strong consumption of aluminium in China as the country unleashed heavy stimulus spending.

Analysts say some signals have been misleading, however, such as China flipping from a net exporter this year into importing 1.99 million tonnes, up 381% year-on-year.

"The market is taking China moving to be a net importer very positively, even though it is only driven by price arbitrage. The more metal that is sucked up by China, less the arbitrage will continue," said analyst Carsten Menke at Julius Baer in Zurich.

"This means that LME prices have to come down eventually, not a big crash, but I see about 10% downside."

* Another factor supporting aluminium prices is that large amounts of the surplus are hidden in off-exchange warehouses in financing deals.

* Chilean state-owned Codelco, the world's largest copper producer, said it would continue to dig another year at its century-old open cast Chuquicamata mine, which has had good yields.

* LME copper gained 0.8% to $6,992.50 a tonne.

The upside of copper has been capped by producer hedging, Alastair Munro at broker Marex Spectron said in a note. "This week a producer offer has certainly been apparent from the trades we have seen."

* LME nickel fell 0.2% to $15,900 a tonne, zinc added 0.5% to $2,639, lead shed 0.3% to $1,893 and tin rose 0.6% to $18,400 a tonne.

* For the top stories in metals and other news, click or (Reporting by Eric Onstad;Editing by Elaine Hardcastle)