RIO DE JANEIRO—Brazilian miner Vale SA said on Thursday it had begun using self-driving trucks for the first time at its Carajas complex, its largest iron ore mining operation, as it continues to expand its use of the driverless technology. Vale expects to boost productivity and safety by using the trucks to haul iron ore, said Pedro Bemfica, the executive heading the autonomous technology program. The miner’s six self-driving vehicles at Carajas are nearly twice as tall and more than three times as wide as a conventional trucks and capable of holding 320 tonnes of iron ore. The behemoths will operate alongside its fleet of about 120 normal offroad vehicles at Carajas, which is located in the northern Amazon state of Para. The company plans to add four additional self-driving trucks by year’s end. The company forecasts that the trucks, which operate constantly and at a higher speed, will cut …