The July 2010 explosion and fire at the former Horsehead zinc refinery in Monaca, Pennsylvania, likely resulted from a buildup of superheated liquid zinc inside a ceramic zinc distillation column, which then “explosively decompressed” and ignited, according to a technical analysis released by the U.S. Chemical Safety Board (CSB).

Two fatalities

Two Horsehead operators, James Taylor and Corey Keller, were killed when the column violently ruptured inside the facility’s refinery building, where multiple zinc distillation columns were operating. The rupture released a large amount of zinc vapor, which at high temperatures combusts spontaneously in the presence of air. The two men had been performing unrelated maintenance work on another 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 multiple agencies including the CSB and the U.S. Occupational Safety and Health Administration, but its underlying cause had remained unexplained. In the fall of 2014, CSB contracted with an internationally known zinc distillation expert to conduct a comprehensive review of the evidence file, including witness interviews, company documents, site photographs, surveillance videos, laboratory test results, and data from the facility’s distributed control system (DCS). The 57-page report of this analysis, prepared by Mr. William Hunter of the United Kingdom, was released today by the CSB. Draft versions of the report were reviewed by Horsehead and by the United Steelworkers local that represented Horsehead workers in Monaca; their comments are included in the final report as appendices.

Method discontinued in U.S.

In the years following the 2010 incident, the Horsehead facility in Monaca was shut down and dismantled. The “New Jersey” zinc process, a distillation-based method that was first developed in the 1920’s and was used for decades in Monaca, is no longer practiced anywhere in the United States, although a number of overseas companies, especially in China, continue to use it.

“Although this particular zinc technology has ceased being used in the U.S., 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 Hunter report was based on expert professional opinion, and did not involve any onsite examination of the evidence. CSB investigators made several short deployments to the Horsehead site in 2010 following the incident, interviewing a number of witnesses and documenting conditions at the site.

500 days

The explosion involved an indoor distillation column several stories tall. The column consisted of a vertical stack of 48 silicon carbide trays, topped by a reflux tower, and assembled by bricklayers using a specialized mortar. The bottom half of the column was surrounded by a masonry combustion chamber fueled by natural gas and carbon monoxide waste gas. Horsehead typically operated columns of this type for up to 500 days, at which time the columns were dismantled and rebuilt using new trays.

The explosion on July 22, 2010, occurred just 12 days after the construction and startup of “Column B.” Column B was used to separate zinc – which flowed as a liquid from the bottom of the column – from lower-boiling impurities such as cadmium, which exited as a vapor from the overhead line. The column, which operated at more than 1600 °F, normally has only small amounts of liquid metals in the various trays, but flooding of the column creates a very hazardous condition, the analysis noted. Such flooding likely occurred on July 22, 2010.

A massive zinc flame

“Under extreme pressure the tray wall(s) eventually failed, releasing a large volume of zinc vapor and superheated zinc that would flash to vapor, and this pressure pushed out the combustion chamber blast panels,” Mr. Hunter’s report concluded. “The zinc spray and vapor now had access to large amounts of workplace air and this created a massive zinc flame across the workplace.”

After examining all the data, the report determined that the explosion likely occurred because of a partial obstruction of the column sump, a drain-like masonry structure at the base of the column that had not been replaced when the column was rebuilt in June 2010. The previous column that used this sump had to be shut down prematurely due to sump drainage problems, the analysis found. These problems were never adequately corrected, and various 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 vapor 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.

A dangerous pressure  build-up

The technical analysis determined that there was likely an underlying design flaw in the Column B sump involving a structure known as an “underflow” – similar to the liquid U-trap under a domestic sink. The small clearance in the underflow – just 65 millimeters or the height on one brick – had been implicated in other zinc column explosions around the world, and likely allowed dross and other solids to partially obstruct the sump and cause a gradual accumulation of liquid zinc in the column. Liquid zinc in the column causes a dangerous pressure build-up at the bottom and impairs the normal evaporation of vapor, which would otherwise cool the liquid zinc. Instead the liquid zinc becomes superheated by the heat from the combustion chamber, with the pressure eventually rupturing the column and allowing the “explosive decompression.”

The report noted that the Column B sump had previously been used with a different type of column that had a much lower rate of liquid run-off through the sump, so the problem with the sump was only exacerbated when Column B was constructed to separate zinc from cadmium, increasing the liquid flow rate into the sump by a factor of four to five.

Missed opportunities

The report concluded that Horsehead may have missed several opportunities to avoid the accident, overlooking symptoms of a blocked column sump that were evident days before the accident. “Missing these critical points indicates that, in large measure, hazardous conditions at Monaca had been ‘normalized’ and that process management had become desensitized to what was going on. This raises the question whether sufficient technical support was provided to the plant on a regular basis,” according to Mr. Hunter.

The report noted that New Jersey-type zinc distillation columns have been involved in numerous serious incidents around the world. In 1993 and 1994, two column explosions at a former French zinc factory killed a total of 11 workers. An international committee of experts who investigated the incidents in France identified up to 10 other major incidents at other sites attributable to sump drainage problems. The Monaca facility had suffered five documented column explosions prior to 2010, but none with fatalities, according to the CSB-commissioned report.