Rio Tinto has started to sell non-core assets, including a $750 million (£392.4 million) mine in Alaska, and is hoping to raise a further £200 million this year when it puts its headquarters on the market.

Rio, the world’s second-largest miner, said that it would sell $15 billion of assets to streamline the company after the $38 billion acquisition of Alcan, the aluminium producer, last year.

The company, which is the subject of a £75 billion takeover approach from BHP Billiton, said yesterday that it had sold the Greens Creek gold and silver mine in Alaska to Hecla Mining, of the United States.

Rio is also planning to sell its head office at 6 St James’s Square in Mayfair. The square is one of the most sought-after locations in London, with premises carrying some of the capital’s highest rents, and Rio is understood to have received a valuation of more than £200 million. The company has moved staff to a building in Paddington and senior executives to the City.

The sale of the Alaskan mine is the first significant disposal since the company decided to streamline. Greens Creek is already part-owned by Hecla and Rio will receive $700 million in cash and $50 million in Hecla stock. Guy Elliott, Rio’s chief financial officer, said: “The sale of our interests in the Greens Creek Mine is a very positive first step towards our target of realising asset sales.”

Rio will reveal its financial results for 2007 today and is hoping that a strong set of results, in combination with its disposal programme, will convince shareholders that it is more valuable as an independent company. BHP, its industry rival, has made an offer of 3.4 shares for one in a deal that would create the world’s third-largest company, with a market capitalisation of more than $350 billion.

In a letter to shareholders yesterday, Don Argus, BHP’s chairman, said: “Bringing these companies together will provide a unique opportunity to unlock value for both BHP Billiton and Rio Tinto shareholders.” He said that the process would take a long time as the deal must win regulatory approval before shareholders can accept the offer.

Analysts expect Rio to announce today a small drop in profits to $7.1 billion, from $7.3 billion last year. UBS is predicting earnings of $6.9 billion on revenue of $25.9 billion and said that it expected lower earnings because Rio paid more tax in 2007 and also booked a $314 million charge from higher costs at its Argyle diamond mines.

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