* Seeks court certification to file class action suit
* Disease from inhaling silica dust from gold-bearing rocks
* S.Africa miners already hit by costs, labour militancy
JOHANNESBURG, Dec 28 (Reuters) - A South African lawyer has moved to file a class action suit against over 30 gold firms on behalf of 17,000 former miners who say they contracted silicosis, a debilitating lung disease, due to negligence in health and safety.
The companies include third-largest global bullion producer AngloGold Ashanti, fourth-largest Gold Fields and Harmony Gold. Also named is Anglo American's South African unit, which owed gold assets in the past but no longer produces it.
Attorney Richard Spoor said on Friday he had filed last week for class certification for an action for damages in the South Gauteng High Court in Johannesburg.
"We need to ask the court for permission to proceed on a class action basis. We filed the papers last week, and that matter will have to be argued in the court if it's opposed," Spoor said.
He expected the matter to be heard in April or May of next year.
The damages sought in what could be Africa's biggest ever class action suit have not been disclosed but could be huge at a time when South Africa's mining industry is battling with soaring power costs and wage costs as well as violent labour militancy.
Spoor has signed up 17,000 plaintiffs so far from South Africa, Botswana and Lesotho, the landlocked kingdom that has provided hundreds of thousands of migrant workers to South Africa's gold mines over the past century, he said.
Spoor said the number of plaintiffs was growing by around 500 a month.
The planned suit, which has little precedent in South African law, has its roots in a landmark ruling by the Constitutional Court that for the first time allowed lung-diseased miners to sue their employers for damages.
Silicosis is a chronic and progressive disease that cannot be cured. Miners contract it by inhaling silica dust from gold-bearing rocks.
African Rainbow Minerals, one of the companies named, said it was "too soon to discuss the outcome of the matter as none of the merits of the matter has yet been established, let alone tested in court".
An Anglo American spokesman said, "We are aware of the case, but Anglo American has not yet been served, so it would not be appropriate to comment any further."
Officials at AngloGold Ashanti, Gold Fields, Harmony and some other companies named in the filing could not be reached for comment.
The companies named in the filing owned or operated 78 different gold mines from 1965 to the present.