Piles of problems: Rio’s acquisition of Alcan, and assets such as this alumina refinery in Australia, are the reason the group’s debt soared to $39bn

Rio Tinto’s gains this year from asset disposals reached $6bn (£3.7bn) on Tuesday after the sale of Alcan Composites, a niche manufacturer of aluminium products, for $349m.

The disposal of the unit to Schweiter Technologies, a Swiss machinery maker, is the latest small step taken by the Anglo-Australian mining group to reduce its net debt, which stood at $39.1bn on June 30. A rapid series of asset disposals this year, coupled with a $15.2bn rights issue, is on track to more than halve that figure by the end of the year.

Alcan Composites is one unit of Alcan Engineered Products, itself a division of Alcan, the North American aluminium producer. Rio’s debt woes stem entirely from its 2007 acquisition of Alcan for $38bn.

In 2008, Rio planned to pay down its Alcan debt by immediately selling Alcan Engineered Products and a host of other Alcan assets that had little to do with Rio’s core mining business. But the financial crisis wrecked these plans, leaving Rio with debt levels that nearly led to it selling a strategic stake to Chinalco, the Chinese miner.

Guy Elliott, finance director, said: “In the last three months we have made significant inroads into divesting the downstream assets acquired with Alcan.”

Rio said last month it received a $2bn offer from Australia’s Amcor for a large part of Alcan Packaging, including its pharmaceuticals, tobacco, and European and Asian food businesses. That followed the sale of Alcan’s American food packaging business to Bemis for $1.2bn.

Most of Paris-based Alcan Engineered Products, including units that make aircraft wings, remains to be sold. Rio made a small gain last week by selling a majority stake in that division’s cable unit to a private equity company for an undisclosed, apparently marginal, sum.

The sale to Schweiter is expected to close by the end of the year.

With the completion on Monday of Rio’s $750m sale of Brazilian assets to Vale, Rio is capable of reducing its net debt to about $19bn by December 31. The figure could be lower if it completes a planned initial public offering of Cloud Peak Energy, a suite of US coal mines, which analysts say could fetch more than $1bn.