For the last two nights, we've been telling you about the incredible $100-million mansion that secretive billionaire Ira Rennert is building in New York. His critics in Utah say he ought to spend some of that money to clean up his Magcorp plant in Tooele County. It turns up at the top of EPA's list every year as the nation's Number One Toxic air polluter. Tonight, Environment Specialist John Hollenhorst finishes his report.

The company says there is hope of cleanup at Magcorp. A major cleanup is tentatively planned over the next three years. The company consistently has been extremely tight-lipped. They didn't return our calls until Monday, after we started doing stories about the New York owner's extravagant mansion. Now Magcorp apparently hopes to do what critics have been demanding for a decade.

In 1989, the magnesium plant was called AMAX. And even then officials rarely spoke to reporters. But with confidential sources nine years ago, we gave KSL viewers an insider's look at the key reason so much chlorine gas goes into the air. Ancient, leaky electrolytic cells. Back then, we reported that most of the cells AMAX used were designed in the 1930's.

That same year, secretive businessman Ira Rennert bought the company and gave it a new name, Magcorp. But nine years later, the old cell technology is still spewing chlorine.

Dianne Nielson, Utah director of Environmental Quality, said, "They can certainly reduce chlorine emissions by implementing newer cell technologies."

Magcorp *has* made substantial progress since Rennert took over. Emissions have been cut a whopping 40 percent. But MagCorp emissions are still far higher than the nation's Number Two toxic polluter.

Chip Ward, of the group Citizens Against Chlorine Contamination, says, "I think the state should be tougher and could be tougher."

State regulators say their powers are limited because the federal government has never set a national standard for chlorine, and health studies don't indicate any real problem. According to Nielson, "There is not an impact to public health from those chlorine emissions.

" But critics say dioxin and other threats haven't been studied at all. Ward says, "The chlorine emissions may be an ozone precursor for the Wasatch Front. We don't know that, because we haven't looked at it."

Critics want total replacement of the old electrolytic cells. And they just might get it. In recent months, Magcorp has been testing an entirely new approach. Nielson explains, "I think they are sincere in looking at the technologies. I don't know what their plan is in terms of implementation. I am hopeful that they will move forward with those more efficient and less polluting technologies."

Just this week, Magcorp told us testing is nearly complete and looks very promising. If it pans out, they'll spend $55 million modernizing the plant, including $40 million to replace those pesky electrolytic cells.

That's only about half what the owner is spending on his fabulous New York mansion. Critics have jumped on that project as a symbol, to keep the pressure on the company. "It's the old idea of the squeaky wheel gets the oil. And until very recently that wheel hasn't squeaked," Ward said. "We're squeaky now," he adds.

The company still refuses an on-camera interview. But they say they've put their $55-million plan in writing and sent it to the federal government.

It's tentative. The testing isn't complete. But they're very optimistic it will work and substantially reduce chlorine emissions.