JAY — Investigators began Thursday to try to figure out what caused an explosion at the Androscoggin Mill that left part of the complex a smoking ruin but caused no serious injuries.

Just before noon Wednesday, company officials said, one of the plant’s major pieces of equipment, called a digester, blew up, leaving a key section of the paper mill a charred and shattered mess.

It’s already clear, a company spokeswoman said, that without the digester the mill won’t be able to produce pulp “for a significant period.”

The mill’s paper machines, however, were not damaged so its owner, the Pennsylvania-based Pixelle Specialty Solutions, is “exploring options to resume paper machine operations as soon as possible to serve our customers,” said Roxie Lassetter, its human resources manager at the mill. How long it might take to restart is uncertain.

While the State Fire Marshal’s Office and others probe the wreckage, the company is trying to figure what, if anything, it can do to keep operating. With so much equipment ruined, the mill’s prospects, which seemed bright, suddenly look less certain.

Gov. Janet Mills said at a Wednesday news conference that “we don’t know yet what the future may bring for the Androscoggin Mill. That will come more fully into view in the coming days.”

Steve McCausland, spokesman for the Maine Department of Public Safety, said in a news release Thursday that “investigators began interviews with employees and witnesses, and got their first close look at the area where the explosion took place within the mill property.” They plan to return Friday to continue their work.

Town officials are overseeing a cleanup — warning people the debris blown into the sky and settling in the area could be irritating but isn’t unhealthy.

The town posted Thursday on social media that “as the mill begins assessing their next steps and what the future holds, we will stand strong with them as a community. This area has already faced so much and we will continue to do it — together.

“To all the workers at the Androscoggin Mill, we are thinking of you and keeping you in our prayers in the days ahead,” the town said.

Fire Chief Mike Booker said that “we are entering into the next stage of this where everyone is concerned about the financial implications, and I definitely share the same concerns.”

Booker said the air quality is safe despite “a nuisance smell.”

He said people who want to sweep up the debris can do so but should wear gloves and, if dust is present, a face mask.

“The chemical with the pulp is classified as a mild irritant,” Booker said in a Facebook post, and it “will dissolve over time, and is known as a good fertilizer.”

The mill survived a post-recession paring that wiped out five Maine mills. It was sold in January by the Ohio-based Verso Corp. to Pixelle Specialty Solutions, the largest specialty paper producer in North America. It employs about 500 people and is valued by the town at more than $325 million.

Not every paper mill has an attached pulp mill. Pixelle itself said it had four specialty paper mills, including the one in Jay, and only three of them also produced pulp. It is a material that can be hauled in, though whether it is worth doing financially is part of what the company is figuring out.

The Jay mill had two continuous Kamyr digesters, one for hardwood and one for softwood, according to Adriaan R. P. van Heiningen, a professor of chemical engineering at the University of Maine, who has a research specialty in pulp production and spent time at the mill a decade ago as part of an environmental project.

He described the digester as “one of the most important processes in the entire production of pulp and paper.”

After logs arrive at a mill, they’re chipped, screened for size and steamed. From there, “they make a slurry of chips into white liquor, which is the water solution with sodium hydroxide and sodium sulfide, and they transport that as a slurry in a pipe to the top of the digester,” van Heiningen said.

Once piped in, “basically what you have is a pressure cooker,” he said. “The pressure is about 150-pounds per square inch.”

Experts from the Ernest Orlando Lawrence Berkeley National Laboratory in California said in a report on the digester process that the ingredients are cooked at about 330 degrees for several hours, allowing the liquid “to permeate the wood chips and dissolve most of the non-fibrous constituents in the wood.”

While van Heiningen said he has no direct knowledge of the cause of Wednesday explosion, one theory would be a rupture due to alkaline corrosion cracking.

“The steel inside or the iron inside (the digester), in alkaline conditions, at high temperatures, it has a tendency to slowly develop little cracks,” he said.

Those cracks would be looked for during the annual maintenance shutdown.

“Maybe they missed it at the time or maybe it’s not a problem,” he said.

He couldn’t tell from photos if it was the hardwood or softwood digester involved in Wednesday’s explosion. It is possible to run just one, if undamaged, to supply the plant, but “production goes down about half,” van Heiningen said.

“It’s an expensive piece of equipment,” he added. “This will lead to quite a long outage, I think. It’s unfortunate, for all the people.”

The two companies in the world making digesters, based in Austria and Finland, have subsidiaries in U.S., he said.

Craig Aderman, a retired stationary steam engineer from Freeport, said he worked for years for Sappi Global, which has paper mills in Skowhegan and Westbrook.

He described his job there, at root, as trying “to keep things from blowing up and killing people.”

Aderman said that operating continuously under high pressure, the digester circulates the chips in cooking liquor until they float down to the bottom, where a huge scraper removes the brown pulp for the next step in the papermaking process. Hundreds or thousands of tons of chips can move through daily.

Typically, the pulp is then bleached white, Aderman said.

The risk is that stress fractures can arise on the pressurized vessel, he said, especially at transition points where the shape changes. He recalled finding one once near the top of a vessel in a Sappi plant that could have posed a serious risk.

Aderman said it was fortunate nobody was around when the digester at the Pixelle Specialty Solutions-owned mill exploded. But it would not have been unusual that nobody was nearby, he said.

He said in his experience, people checked digesters every couple of hours to eyeball them and see if everything appeared to be running smoothly. They are actually operated from a control room, he said, that doesn’t need to be close by.

Blasts like the one in Jay are rare, Aderman said, and not necessarily catastrophic.

“This is probably one of the worst explosions I’ve heard of,” he said.

When then-owner International Paper proudly cut the ribbon on a $54 million investment in Jay for a new mill in 1964, it proclaimed it had built it “around a gigantic continuous digester, the newest development in the pulp and paper industry.”

“Towering over 210 feet in the air,” the company said, “the digester will be able to manufacture 500 tons of high-quality kraft pulp every day.”

What changes were made in the more than half century since are unclear. Mentions of investments in the mill online do not note anything more about its digesters except for a temporary shutdown of one between 2016 and 2018.

At a new conference late Wednesday afternoon, Mills hailed the fact that nobody was seriously hurt in the Jay explosion.

“There’s a common saying that ‘God will not give you more than you can carry.’ And without question the burden for us now is heavy,” she said during her news conference. “But Maine people can carry it and we will carry it. I just want to say, if ever there was a day when we should believe in miracles, today is it.”

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