* Vale nickel plant at Goro closed early May after acid leak
* Spill sparked violent protests causing more than $20 mln in damage
* Goro nickel plant has troubled history with spills, protests
* World nickel prices up strongly this year on supply concerns (Adds quotes, detail on statement)
SYDNEY, June 2 New Caledonia authorities said on Monday they have authorised a conditional restart of Brazil-based Vale's nickel operations, which were suspended more than three weeks ago after acid-tainted effluent spilled into a river.
The leak sparked violent riots by young Melanesians that caused more than $20 million in damage to buildings, equipment and vehicles and left three policemen with gunshot wounds.
Police remained on alert as protesters maintained a presence near the plant.
"The situation is calm and some (protesters) are still camping near the plant," said a local government spokeswoman.
Some protesters, who have been frustrated by the lack of response from indigenous Kanak chiefs to the chemical spill, are seeking the permanent closure of the mine - something Vale said last week was not on the table.
Vale could not immediately be reached for comment on when production would restart.
The Southern Province of the French-administered Pacific island noted it was Vale's sixth major incident at the $6 billion Goro site and set hightened safety standards as a condition of the mine operations resuming.
Conditions also included a full audit of safety management and the establishment of a multi-party surveillance committee.
Vale, the world's second-largest nickel miner, employs high pressure technology and acids to leach nickel from abundant tropical laterite ores.
"Every time (there is a problem with Vale), it's the same response by the Province... Yet, it happens again," said Louis Kotra Uregei, president of New Caledonia's Labour Party.
Uregei is asking for a halt of the acid-based nickel processing.
"We need to focus on ore mining and let the processing get done somewhere else," he said.
Nickel mining is a key industry in New Caledonia, which holds as much as a quarter of the world's known reserves. Vale's plant is the second-largest employer in the southern province, with some 3,500 employees and contractors.
LME nickel prices finished May up 5 pct at $19,250, having soared 40 percent so far this year, largely on supply concerns from major producer Indonesia. (Additional reporting by Melanie Burton; Editing by Lincoln Feast and Richard Pullin)