(Adds data on worst loss, CFO comment)
RIO DE JANEIRO, Feb 25 (Reuters) - Brazilian miner Vale SA reported a fourth-quarter net loss of $8.57 billion, its worst ever as a private company, as weak commodity prices and hefty writedowns heaped further pressure on the world's largest producer of iron ore.
The loss, reported early Thursday, is Vale's fifth in the past six quarters, with the company hit by a collapse in the price of its principle profit driver, iron ore, just as it entered a period of heavy investment in large projects.
Falling far short of a loss of $56 million predicted by analysts in a Reuters poll, the result is Vale's worst since at least 1997 when it was privatized.
It comes despite record iron ore and nickel production, highlighting treacherous market conditions.
A difficult fourth quarter brought Vale's full-year loss to $12.1 billion, the biggest suffered by a listed Brazilian company since at least 1986, according to data compiled by market-analysis firm Economatica.
The annual loss was mainly due to non-cash factors such as impairments and currency moves, which "do not affect the day-to-day cash generation of the company," Chief Financial Officer Luciano Siani said in a video posted on the company website.
Impairments, almost entirely on coal and nickel assets, totaled $9.37 billion, while a 47 percent depreciation of the Brazilian real against the dollar in 2015 increased the cost of servicing foreign-currency debt.
Despite an iron ore price that fell as low as $37 per tonne in late 2015, Vale said it remained competitive, pointing to production costs of $11.9 per tonne excluding royalties.
Vale reported earnings before interest, tax, depreciation and amortization (EBITDA) of $1.39 billion, in line with forecasts but down 36 percent from a year earlier.
The company is betting on the completion of its new massive iron ore mine in Brazil's Amazon, known as S11D, to steady its finances.
The largest project in Vale's history costs $14.5 billion, a heavy burden, given the collapse in iron ore prices.
But the 90-million-tonnes-per-year mine should be up and running by the end of the year, cementing the company's position as the largest iron ore producer in the world and, according to Vale, pushing its costs to the lowest in the industry. (Reporting by Stephen Eisenhammer and Jeb Blount; Editing by Jason Neely and Bernadette Baum)