Campaigners against the mine first took their case to Europe in 2008, claiming the site flouted EU environment rules.
Their petition was heard again this week and the European Commission asked for more time to look into potential breaches.
The firm behind the scheme said it was constantly assessing its impact.
The petition is expected to return to the European Parliament later this year.
It says work is done just 36m from houses and would have a detrimental impact on residents' health.
Work to mine 10m tonnes of coal over 17 years from Ffos-y-Fran began in 2007.
Miller Argent (South Wales) Ltd initially received planning permission following a public inquiry, only for that to be overruled by the High Court.
But the Welsh government, which backed Ffos-y-Fran, won the right to appeal and a Court of Appeal judge allowed it to go ahead.
A spokesman for Miller Argent said: "We currently employ over 230 people at Ffos-y-fran providing high quality jobs.
"We work closely with Merthyr and Caerphilly councils, Natural Resources Wales to ensure we consistently meet their requirements as well as our own.
"We are constantly assessing the impact of our activities upon others and diligently implementing mitigation techniques across our operations.
"The scheme itself is saving the public purse many millions of pounds reclaiming over 1,000 acres of dangerous and derelict land, at the same time providing substantial benefits to the local community through the Ffos-y-fran benefit fund."
Plaid Cymru MEP Jill Evans has written to the parliament's petitions committee urging it to acknowledge "residents are continuing to suffer noise and dust from Ffos-y-fran".
She said: "Ffos-y-fran has had a devastating environmental impact, primarily on the residents but also on a much wider scale considering our commitment to reducing carbon emissions."
Miller Argent said it would be happy to discuss the scheme in detail with Ms Evans.
The European Commission is to seek more answers about Ffos-y-fran opencast mine, near Merthyr Tydfil, from both the UK and Welsh governments.