BHP Billiton has posted an 11% increase in iron-ore output for the quarter ended 31 March.

Iron ore production is BHP’s biggest revenue earner and for the quarter the company produced 31.16 million metric tons, up from 28.19 million a year earlier.

Coking coal output gained 7% on the year but dropped 8% from the previous quarter, while petroleum production gained 16% to 36.84 million barrels of oil equivalent in the March quarter from a year ago.

BHP said its uranium production from the Olympic Dam mine in South Australia, the world’s largest uranium deposit, fell 90% from a year ago following an incident where a transport shaft was damaged.

Last month BHP Billiton ended its 40-year-old annual iron ore contract by moving to a quarterly pricing system with Asian mills.