Alcoa launches legal action against Alumina Ltd.

Alcoa has taken the surprise step of launching legal action in the US against Alumina to stop attempts by its Australian partner to derail a historic break-up of the aluminium giant.

The action comes amid threats by Melbourne-based Alumina as recently as this week to try to "interfere with the (Alcoa) seperation" to cast "a cloud over the seperation ... and disrupting" the operations of the two partners' Alcoa World and Alumina Chemicals (AWAC) venture, according to court documents filed in the US state of Delaware on Friday night.

AWAC, split 60-40 between venture operator Alcoa and Alumina, is one of the world's biggest bauxite and alumina producers and owns world-class mines and refineries in the South West.

Alcoa announced its historic split, to separate the upstream operations of bauxite, alumina and aluminium from the specialty metals division, in September.

It hopes to complete the demerger in the second half of this year, having argued the two resultant companies - new Alcoa and the specialty metals group Arconic - will perform better on a stand-alone basis.

Alumina has remained largely silent on the demerger.

But the court action reveals Alumina claims the Alcoa demerger triggers AWAC pre-emptive rights.

According to the court filing, Alumina has threatened to take its grievances public including asserting that it has secured marketing rights for AWAC.

Alcoa argues Alumina's pre-emption claims are invalid - none of AWAC's assets will be spun into Arconic - and points to its partner's own creation in 2002, when mining house WMC spun out its 40 per cent AWAC stake into a stand-alone ASX-listed company.

Alcoa has asked the court to declare that Alumina has no right nor ability to block the demerger, and no right to assume AWAC marketing rights.

It is unclear how long the court process will take.

Alcoa remained tight-lipped today while Alumina was asked for comment.

An Alcoa spokesman said: "Alcoa’s separation does not require Alumina Limited’s consent. "We look forward to putting this matter behind us and launching new Alcoa and Arconic in the second half of 2016."