Abstract: At approximately 10:00 a.m. on October 31, 1997, Employees #1 through #5 were near the north heavy black liquor storage tank. Black liquor is a highly corrosive combination of sodium hydroxide, sodium sulfide, wood fibers, and other components of negligible quantities, with a pH of between 12 and 14; in this tank, the mixture was 45 percent solids. The 40 ft diameter by 60 ft tall tank was on the east side of the facility and was receiving black liquor from the power plant during a normal boil-out of the concentrators. The liquor entered the tank from the side, at a height of approximately 8 ft, under 60 psi pressure, and at 295 degrees F, as opposed to the contents of the tank, which was at 235 degrees F. Steam at 60 psi and weak liquor also entered the tank through the same location during boil-out. On the day of the accident, the boil-out process started at approximately 7:45 a.m. and the liquid level in the tank rose dramatically over a short period of time. At about 8:00 a.m., the level started to rise from 36 to 45 ft; just prior to 10:00 a.m., the level spiked to 55 ft. It was normal for the liquor level to rise rapidly during the boil-out of the concentrators, which was done approximately every three weeks. During normal operations, black liquor circulated through the north heavy black liquor storage tank from the evaporators, with the level running at approx 37 ft. On this day, the tank had filled to approximately the 43 ft level, as indicated from control room readings, when it failed and ruptured at the 20 ft level. Approximately 300,000 gallons of the hot, heavy, black liquor was released. Employee #1 was coated with the material and was killed. Employees #2 and #5 suffered burns and were admitted to the LSU/Shreveport burn unit. Employees #3 and #4 were treated locally and released. The storage tank was an atmospheric tank constructed to API 650 standards and inspected using API 653. It was made of A36 carbon steel plate in 6 ft to 10 ft rings of various thickness ranging from 0.5 in. for the top course (#6) to 0.645 in. for the bottom course (#1), with a 08125 in. thick base. The tank was placed in service in 1992 and had been experiencing a high rate of corrosion (between 0.03 and 0.07 in. per year) on the interior surface. From 1992 until 1996, the company had used it only for black liquor, with flashing as the only major problem occurring. Since the liquor entered the tank from the top, flashing would occur near the outlet. The company solved this problem by installing a bell-shaped covering at the end of the inlet pipe. The company did not consider black liquor to be a covered process under the PSM standard.