The site, in the port city of Skikda, was devastated, according to officials. Three liquefied natural gas (LNG) plants were destroyed by the blast, which was believed to have been caused by an accident at a gas boiler.
Most hospitalized workers had suffered facial injuries.
Angry workers complained to government officials that they had warned an accident would happen and that the boiler was faulty but no one listened, according to Reuters.
The head of security at the gas plants told state radio he heard vibrations and irregular noise at a boiler just before the blast. Specialists had been told a year ago of the defective boiler but it had only been superficially repaired, he said.
Algeria is the world's second largest LNG exporter after Indonesia. A quarter of its shipments leave from the port of Skikda. The refinery, which employs 12,000 workers, was shut down after the explosion as a preventive measure.
Officials told Reuters it was too early to assess damage because it was so extensive, with metal, glass and concrete debris spread across the 92-hectare site.
It was the worst LNG incident since 1975 when about 40 people died in an explosion in Staten Island, U.S., according to Andrew Flower, an independent gas consultant.