The Lafarge cement plant between Route 9W and the Hudson River was the greatest source of mercury emissions in New York from 2004 to 2006. But a project announced Wednesday, designed to improve fuel efficiency and enhance environmental protection, would change that.

The work, which will cost in the hundreds of millions of dollars, will "have significant impact on reducing all emissions," said Joe Goss, president of the Lafarge United States East Cement Business Unit, which includes the Ravena plant.

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The project would replace the 46-year-old plant’s equipment with what Lafarge describes as state-of-the-art manufacturing technology.

The investment is intended to cut operating costs and ensure the plant’s long-term viability, plant officials said. As a result, they expect environmental performance to improve, although the decision was driven by a broad set of business considerations, not specific complaints.

Over the years, the Ravena plant has drawn the ire of environmental and consumer groups. Of this latest project, "if it’s effective, it’s not a moment too soon," said Susan Falzon of Friends of Hudson.

Added Keri Powell of Earthjustice, another environmental group, "It does seem like a good idea to replace this outdated technology with the dry kiln process."

With the new process, she said, it’s easier to install pollution controls, and Earthjustice will seek to "make sure the pollution controls they install will be sufficiently protective."

The company said it has submitted an environmental assessment form to state regulators, an initial step as it seeks permits to allow the project to move ahead. Lafarge employs 220 people at the plant, which has a total payroll topping $20 million.

The actual construction work would begin in 2013 and take three years to complete.

Powell raised concerns about the emissions from the plant "between now and when they get the new kiln complete." She said she’d like to see the company end its use of fly ash in the manufacturing process, which she said is believed to be a source of the mercury being emitted.

At the plant, which produces 1.8 million tons of cement annually, the new equipment would reduce fuel use by half. Electricity usage would edge slightly higher, although the company is looking at a cogeneration plant or other source of energy to cut costs.