UPDATE 1-Kazakhmys sells half of power unit, up from 25 pct

* Sells 50 pct stake instead of 25 pct, in Ekibastuz plant

* State welfare fund pays $681 mln, up from $339 mln

* Forms strategic partnership to run plant

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LONDON, Dec 10 (Reuters) - Kazakh copper producer Kazakhmys (KAZ.L) will double the stake it is selling in the country's biggest power plant to 50 percent for an increased price of $681 million, it said on Thursday.

On Oct. 13, the London-listed group said it had sold a 25 percent stake in Ekibastuz GRES -1, the largest power plant in Kazakhstan, to Kazakh state welfare fund Samruk-Kazyna for $339 million in cash. [ID:nL0565789]

The proceeds will be used to pay off debt, the company said in a statement.

"Kazakhmys retains its investment in a highly attractive business, with the security of owning sufficient power generation for all our future needs, whilst freeing up additional capital for the group," Chief Executive Oleg Novachuk said.

The revised deal is in line with an original memorandum signed with Samruk-Kazyna a year ago, which envisiged a strategic partnership to run the power plant.

The partners will now create a joint supervisory board and management positions will alternate between Kazakhmys and Samruk-Kazyna every five years.

"Partnership with Samruk-Kazyna will enhance the value of Ekibastuz by providing a closer relationship with the Bogatyr mine... which supplies approximately 80 percent of Ekibastuz's coal requirements," Kazakhmys said..

The coal mine is jointly owned by Samruk-Energy JSC and Russia's UC RUSAL, the world's top aluminium maker.

Full year earnings before tax, interest, depreciation and amortisation (EBITDA) attritutable to Ekibastuz were $86.7 million.

Separately, Eduard Ogai, head of the company's Kazakh operations, told reporters on Thursday it plans to invest $4 billion in its copper deposits and a further $1 billion in refining facilities within the next five to six years. [ID:nGEE5B907L] (Reporting by Eric Onstad; Editing by Hans Peters)