Multiple people have been treated for chemical burns caused by the toxic substance inside the collapsed tank.

Nine employees of an American pulp and paper mill are missing and one person is confirmed dead after a chemical tank implosion.

A firefighter and eight more employees have also been hospitalised with injuries suffered as the tank collapsed at Nippon Dynawave Packaging Company in Longview, in Washington state, about 7.15am local time on Tuesday.

The tank held a chemical brew known as “white liquor,” a highly corrosive substance that consists mainly of sodium hydroxide (caustic soda) and sodium sulfide.

The chemical was used in the paper mill to break down wood to make a durable paper for packaging products — but upon contact with human skin, the toxic mixture causes severe chemical burns and poisoning.

The tank contained nearly three-and-a-half million litres of white liquor, according to the latest reports from the Longview Fire Department.

The department said that only 10 per cent of the chemical material remains in the damaged tank.

Recovery efforts “remain complex” due to ongoing safety concerns at the site, firefighters said.

“Responders are continuing efforts to structurally reinforce and stabilise the site before additional recovery operations can safely proceed.

“These conditions continue to impact portions of the facility and limit access to some areas of the site.”

Authorities said some victims had suffered burns or inhalation injuries, and that the severity of the injuries ranged from minor to critical.

The facility which dates back to 1953, and employs about 1000 people, makes material for tissues, printing paper, cups, plates, cartons and other goods.

Longview, a city of about 38,000, has had a relationship with the paper and lumber industries since its founding by a Kansas City timber baron in the 1920s.

Cowlitz Fire and Rescue Chief Scott Goldstein is is too early to determine the cause of the implosion.

“The people who are responders here have friends and relatives that work on site,” he said.

“It is something that is impactful, and we have support networks to support the workers as well as the emergency responders.”

‘Absolute tragedy’

US Senator Patty Murray of Washington called the implosion an “absolute tragedy”.

“My heart is with everyone who lost a loved one today — as well as the injured workers,” she said in a written statement.

Emotional family members of employees gathered at the company’s visitor entrance, with three women seen sharing a tearful embrace before heading inside a nearby union hall serving as a family assistance centre.

Mike Gorsuch, battalion chief with the fire department in Longview, Washington, described it as a “mass casualty scene”.

First responders decontaminated patients and took them to hospitals in Longview and Vancouver, Washington.

About 40 firefighters and paramedics had responded, along with a regional hazmat team, Gorsuch said.

Recovery efforts have now paused until Thursday morning due to ongoing safety concerns.

“State officials are planning a visit to the site. The site will continue to be monitored overnight,” Longview Fire Department said.

The tragic incident comes just days after an overheated chemical tank in Southern Californian aerospace plant prompted evacuation orders for 50,000 people in the Orange County city of Garden Grove.

A crack that formed by chance on the tank relieved pressure and helped avert a catastrophic explosion.

But the risk of a smaller explosion or potential spill kept orders in place for about a third of the evacuees. Many are still living out of hotel rooms, tents, emergency shelters at schools or staying with family or friends.