A massive storage tank containing tens of thousands of gallons of a highly corrosive substance collapsed inward on Tuesday at the facility in Longview, Washington, prompting a large-scale search and rescue operation.

"We have recovered the ninth and final missing employee of this incident," Brad Hannig, Chief of the Longview Fire Department, told a news conference.

Rescue officials had announced on Wednesday that two workers had died, while nine others were still unaccounted for.

The incident at Nippon Dynawave Packaging occurred during an early morning shift change when a 900,000-gallon (3.4-million-litre) tank carrying a substantial volume of white liquor ruptured.

White liquor is a highly alkaline chemical solution composed of sodium hydroxide and sodium sulfide, which is used to break down wood chips and produce the pulp used in paper manufacturing.

Nippon Dynawave Packaging, a subsidiary of Nippon Paper Group, states on its website that it produces eight billion single-serve containers annually, supplying customers across North America, Asia and other global markets.