MARCH 11, 2015
WASHINGTON, D.C.
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 today by the U.S. Chemical Safety Board (CSB).
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.
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.
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.
“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.
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.
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.
The CSB is an independent federal agency charged with
investigating serious chemical accidents. The agency's board members are
appointed by the president and confirmed by the Senate. CSB investigations look
into all aspects of chemical accidents, including physical causes such as
equipment failure as well as inadequacies in regulations, industry standards,
and safety management systems.
The Board does not issue citations or fines but does make
safety recommendations to facilities, industry organizations, labor groups, and
regulatory agencies such as OSHA and EPA. Visit our website, www.csb.gov.