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CSB releases report on likely causes of fatal explosion at Pennsylvania zinc refinery

20 March 2015

The US Chemical Safety and Hazard Investigation Board (CSB) has released the results of its investigation into the explosion and fire on July 22, 2010, at the Horsehead Corporation zinc refinery in Monaca, Pennsylvania, that killed two of its employees. The blast likely resulted from a build-up of superheated liquid zinc inside a ceramic zinc distillation column, which then "explosively decompressed" and ignited.

A sump at the bottom of the column had not been replaced when the column was rebuilt a month earlier, and the problematic sump became partially blocked, allowing the buildup, according to CSB, which said there were multiple zinc distillation columns operating in the refinery when the explosion occurred and released a large amount of zinc vapour, which at high temperatures combusts spontaneously in the presence of air. The two victims had been performing unrelated maintenance work on a nearby column when the explosion and fire occurred; a third operator was seriously injured and could not return to work.

The incident was investigated by many agencies, including the CSB and the Occupational Safety and Health Administration (OSHA), but its underlying cause had remained unexplained.
CSB then asked UK-based zinc distillation expert William Hunter to review the evidence, including witness interviews, company documents, site photographs, surveillance videos, lab test results, and data from the facility's distributed control system. CSB released Hunter's 57-page report after sharing draft versions of it with Horsehead and the United Steelworkers union that represented Horsehead workers in Monaca.

The refinery was closed and dismantled after the 2010 incident, according to CSB, which added that the "New Jersey" distillation-based zinc refining method used there was first developed in the 1920s and used for decades in Monaca, but is no longer practiced anywhere in the United States, although it is overseas, notably in China. "Although this particular zinc technology has ceased being used in the US, we felt it was important to finally determine why this tragedy occurred," said CSB Chairperson Dr. Rafael Moure-Eraso. "Our hope is that this will at last provide a measure of closure to family members, as well as inform the safety efforts of overseas companies using similar production methods."

The explosion occurred only 12 days after the construction and startup of Column B, which operated at more than 1,600° F and normally has only small amounts of liquid metals in its various trays. Hunter's analysis said flooding of the column creates a very hazardous condition, which is likely what occurred on July 22, 2010. "Under extreme pressure the tray wall(s) eventually failed, releasing a large volume of zinc vapour and superheated zinc that would flash to vapour, and this pressure pushed out the combustion chamber blast panels," Hunter's report stated. "The zinc spray and vapour now had access to large amounts of workplace air and this created a massive zinc flame across the workplace."

The previous column that used the same sump had to be shut down prematurely due to sump drainage problems, the analysis found, and problems with the sump were observed during the July 2010 startup of the new Column B. Over at least an hour preceding the explosion, DCS data indicate a gradual warming at the base of Column B, as liquid zinc likely built up and flooded the lower trays, while vapour flow to the overhead condenser ceased. "Ten minutes before the explosion, an alarm sounded in the control room due to a high rate of temperature change in the column waste gases, as zinc likely began leaking out of the column into the combustion chamber, but by then it was probably too late to avert an explosion, according to the analysis. Control room operators responded to the alarm by cutting the flow of fuel gas to Column B but did not reduce the flow of zinc into the column. The unsafe condition of Column B was not understood, and operators inside the building were not warned of the imminent danger," the CSB reported.

The report noted New Jersey-type zinc distillation columns have been involved in serious incidents around the world. In 1993 and 1994, two column explosions at a former French zinc factory killed a total of 11 workers, and an international committee of experts who investigated them identified up to 10 other major incidents at other sites attributable to sump drainage problems.

The Monaca facility had five documented column explosions prior to 2010, but none of them involved fatalities, according to the report.

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