* Global trade in some mercury products to be banned by 2020
* NGO criticises "soft language" of convention (Recasts, adds detail throughout)
By Tom Miles and Emma Farge
GENEVA, Jan 19 (Reuters) - More than 140 countries have agreed on the first global treaty to cut mercury pollution through a blacklist of household items and new controls on power plants and small-scale mines, the United Nations said on Saturday.
The legally-binding agreement aims to phase out many products that use the toxic liquid metal such as batteries, thermometers and some fluorescent lamps, through banning global import and exports by 2020.
The treaty will require countries with coal-fired power plants such as India and China to install filters and scrubbers on new plants and to commit to reducing emissions from existing operations to prevent mercury from coal reaching the atmosphere.
"We have closed a chapter on a journey that has taken four years of often intense but ultimately successful negotiations and opened a new chapter towards a sustainable future," said Fernando Lugris, chair of the negotiations.
The deal also includes measures to reduce mercury use in small-scale gold mining, although stopped short of an all-out ban. Gold prices near $1,700 a tonne have spurred the use of mercury as a catalyst to separate gold from its ore.
Emissions of mercury from artisanal and small-scale gold mines, which are usually unofficial and often illegal, more than doubled to 727 tonnes in 2010 from 2005 levels, overtaking coal-fired power plants as the main source of pollution from the metal.
The Minamata Convention on Mercury - named after the Japanese city where people were poisoned in the mid-20th century from industrial discharges of mercury - needs ratification from 50 countries and is expected to be formalised later this year.
The treaty requires governments to draw up national rules to comply and could take between three to five years to take effect.
As mercury, also known as quicksilver, is released to the air or washed into rivers and oceans, it spreads worldwide, and builds up in humans mostly through consumption of fish. The brains of foetuses and infants are particularly vulnerable to damage from mercury.
Officials said the financing required to bring in cleaner technology for industry and help developing countries come up with local solutions was one of the major sticking points of the six-day negotiations.
"Financing was agreed very early this morning and it was one of the most difficult aspects," said Lugris.
Japan, Norway and Switzerland have made initial pledges totalling $3 million in financing and an interim financial arrangement will be discussed in April by the Global Environment Facility, said Tim Kasten, head of the chemicals branch of UNEP.
Countries failed to agree on including vaccines where mercury is sometimes used as a preservative.
While negotiators celebrated the deal reached after all-night talks in the fifth and final round of talks, the response from some non-governmental organisations (NGO) was more muted.
"The treaty will not bring immediate reductions of mercury emissions. It will need to be improved and strengthened, to make all fish safe to eat," said David Lennett from the Natural Resources Defense Council.
NGO IPEN, which aims to reduce the health risk of chemicals, described the language of the treaty as "soft" and "somewhat voluntary in nature" and said it was unlikely to result in a global reduction of mercury releases.
"Countries that do not want to do this can escape quite easily," said IPEN's Joe DiGangi.
In one notable climbdown, countries abandoned their goal of setting concrete targets for pollution levels from coal-fired power plants and cement factories, but negotiators said they would defer these discussions to a later meeting.
For mining, the treaty requires action from governments to reduce mercury use where artisanal and small-scale gold mining is "more than insignificant" but has no list of countries.
Alternatives to mercury in small mines are available, such as magnetic sluices, but developing countries have complained about the cost of implementation.
Many developing countries including Brazil and Mali strongly resisted attempts to limit imports of mercury, according to IPEN, because of the economic importance of small mines.
"The supply is still available, the practice of artisanal mining is still polluting and we are left with a mess at the end and there is no funding to clean it up," said DiGangi.
Artisanal and small gold mines now account for around 35 percent of global mercury pollution, according to a study by the U.N. Environment Programme published last week.
Other NGOs welcomed the number of products included in the treaty.
"The list of products was much longer than we expected," said Elena Lymberidi-Settimo, a coordinator at Zero Mercury Working Group. "The treaty sends the right market signal and will eventually lead to less exposure worldwide."
Many nations have already tightened laws - the United States barred exports of mercury from Jan. 1, 2013. The European Union, until 2008 the main global exporter, barred exports of the liquid metal in 2011. (Reporting by Emma Farge and Tom Miles; Editing by Sophie Hares)