Mining group Xstrata plans to boost capital spending next year by 89 percent to $6.8 billion to expand output and is considering whether to close or sell any of its four profit-squeezed copper smelters.

The firm, which aims to boost overall production by 50 percent by 2013, estimated capex of $3.6 billion this year, Chief Financial Officer Trevor Reid told an investor seminar on Thursday.

Most of the increased capex would go toward the nickel, coal and copper divisions for new and expanded mines, he added.

The copper unit has six advanced projects to deliver 60 percent production growth by 2015, Charlie Sartain, chief executive of Xstrata's copper division, said.

We're analyzing in some detail the value benefits of the different elements of those smelters and each one of those assets needs to stand the scrutiny of whether or not it deserves to remain within the portfolio, he said.

The outcome of that I think we'll see progressively in the coming months where we need to take some decisions ... whether we have to do some further rationalization.

Xstrata, the world's fourth-biggest producer of mined copper and third largest in refined copper, has four smelters with a combined capacity of about 800,000 tons.

(Reporting by Eric Onstad, editing by Will Waterman)