A soybean processing plant in Sergeant Bluff was closed down for maintenance when an explosion there this morning injured several people. The plant south of Sioux City’s uses flammable hexane gas to extract oil from soybeans and managers said some of the gas exploded, though they don’t yet know the cause. Doctor Larry Sellers is medical Director at Mercy Hospital in Sioux City. Sellers says three workers were evaluated and were able to leave. Five patients had to stay, as he explained four were critical and one serious, and they remained in the intensive care unit. Omaha-based Ag Processing Incorporated owns the plant, as well as five others in Iowa, and plants in Nebraska, Missouri and Minnesota. The soybean plant’s in the Port Neal Industrial Complex where an explosion in 1994 at the Terra Industries plant killed four people, injured more than fifteen and released a toxic cloud of ammonia.