An offer by Kennecott to build a double-contact acid plant for better sulfur-dioxide control at the Magna smelter is welcome news to state pollution fighters, but the Sierra Club says Kennecott is only offering a serious threat to health and economic well-being.

"I think it's a good idea. I'm really pleased that it happened," said Kenneth L. Alkema, director of the Utah Division of Environmental health."We've got to make them put in reasonable control technology, and we said all along it's double-contact acid plants. They disagreed with us until this point."

Ivan Weber, chairman of the Environmental Health Committee of the Utah Chapter of the Sierra Club, denounced the Kennecott plan because the decrease in sulfur dioxide emissions would be coupled with "what amounts to approximately doubling the PM10 (fine particulate) output, or something like that."

The reason he is inexact is that it's hard to calculate the results of the Kennecott plan. But it definitely will reduce air quality, he charged.

Weber said Salt Lake area oil refineries should be upset about Kennecott's plan because it would shift more of the burden of controlling PM10 onto them. If Kennecott shut down, he said, that wouldn't be such a bad thing as it might open the opportunity for more jobs in cleaner industries.

During a meeting of the Utah Air Conservation Committee Wednesday, Kennecott offered a plan to reduce peak sulfur dioxide emissions from the smelter by 70 percent. It would build the new acid plant by 1994 and meanwhile change operations and install additional equipment to improve emission control.

The $200 million plan would result in increasing the percentage of sulfur captured during the smelter process from the present 92 percent to 96 percent.

However, the company asked that several rules in the state's air-quality plan be changed concerning fine particulates. The company wants emission limits for the smelter's main stack raised to what Kennecott calls "achievable levels."

Also, Kennecott officials said an expert consulting firm hired by the company, Meteorological Evaluation Services Co. of Amityville, N.Y., concluded that Kennecott's contribution to PM10 pollution in Salt Lake County was only 2 percent of the fine particulates in the air, not the 15 percent assessed by the state.

At the heart of the disagreement is the proposed State Implementation Plan on controlling PM10, particles so tiny they can lodge permanently in lungs and cause disease. Sulfur dioxide is an issue because once the gas is released into the atmosphere, it can convert into particles - PM10.

Alkema concedes that Kennecott, oil refineries and other companies made good points in their criticism of the implementation plan.

In fact, the proposed State Implementation Plan is so flawed, he said, that it will be rewritten. The present draft went to the state archivist two weeks ago in preparation for a round of hearings scheduled for Dec. 17 and 18.

After the hearings, the Air Conservation Committee will analyze the comments and rewrite the proposal. Then the corrected version will be released for an additional hearing, probably in February.

Originally, state officials proposed that Kennecott rebuild the existing acid plants at the smelter, converting them to double-contact plants, where the stream of gas bound for the tall stack goes through acid baths twice in order to remove sulfur impurities in the smoke. The cost estimate for this upgrading was $40 million.