ZINC DISTILLATION COLUMN EXPLOSION — BLOCKED SUMP — 2 FATALITIES
Horsehead Corporation Zinc Distillation Column BLEVE
Horsehead Corporation
📍 Monaca, PA
Incident: July 22, 2010  •  CSB Report: March 2015
2
Fatalities
Zinc Vapor (Spontaneously Combustible — Superheated Liquid Zinc Column Eruption)
Chemical Involved
4
CSB Recommendations
📋 Incident Summary

On July 22, 2010, an explosion and fire at the Horsehead Corporation zinc refinery in Monaca, Pennsylvania, killed two employees and injured a third. Horsehead operated one of the last zinc distillation refineries in the United States, using the "New Jersey" distillation process — a method developed in the 1920s in which zinc ore is heated to produce zinc vapor that is condensed into refined metal.

The CSB investigation determined that the explosion most likely resulted from a buildup of superheated liquid zinc in a ceramic distillation column where a sump at the bottom had been replaced the previous month. The sump had become partially blocked, trapping liquid zinc that then explosively decompressed, releasing zinc vapor that ignited spontaneously in air.

The "New Jersey" zinc distillation process is no longer used in the United States. The Horsehead Monaca facility has been decommissioned. The CSB investigation documented the process hazards of this legacy technology and the mechanical integrity failures that contributed to the explosion.

🔎 Key Findings
Finding 01
Superheated Liquid Zinc Built Up in Blocked Column Sump
A sump at the bottom of the distillation column — recently replaced — became partially blocked, allowing superheated liquid zinc to accumulate. The trapped liquid zinc then explosively decompressed, releasing zinc vapor.
Finding 02
Zinc Vapor Ignites Spontaneously in Air
Zinc vapor released at high temperatures ignites spontaneously upon contact with air. The combustion of zinc vapor generated extreme heat and fire that killed the two workers in the vicinity.
Finding 03
Column Rebuilt One Month Prior — Sump Not Properly Installed
The zinc distillation column had been rebuilt approximately one month before the explosion, and the replacement sump was not properly installed. The installation deficiency created the condition for liquid zinc accumulation.
Finding 04
High-Temperature Molten Metal Requires Specific MI Controls
Zinc distillation operates at temperatures exceeding 1,000°C with molten metal and pyrophoric vapors — requiring specific mechanical integrity controls that were not adequate in this case.
Finding 05
Legacy Technology Without Updated Safety Standards
The "New Jersey" process was developed in the 1920s and had no modern PSM framework tailored to its specific hazards.
🔍 Root Causes
1
Column Sump Partially Blocked After Improper Installation
The sump at the base of the zinc distillation column was partially blocked following the rebuild one month before the explosion, allowing liquid zinc to accumulate and superheat.
2
No Inspection or Verification of Sump Function After Column Rebuild
After the column was rebuilt, no adequate inspection confirmed that the sump was properly installed and unobstructed before the column was returned to service.
3
Superheated Liquid Zinc Erupted and Released Ignitable Zinc Vapor
The trapped superheated liquid zinc explosively decompressed, producing zinc vapor that ignited spontaneously in the surrounding air.
☑ CSB Recommendations
→ High-Temperature Molten Metal Processors
Implement a formal pre-startup safety review for all equipment returned to service after maintenance in high-temperature molten metal operations; verify sump, drain, and overflow device function before restart.
→ OSHA / Industry
Develop process safety management guidance specific to high-temperature molten metal processes; address the unique hazard profile of spontaneously combustible metal vapors.
→ Zinc Industry (Legacy Processes)
Document and archive process safety lessons from legacy high-temperature metal distillation processes for use in educating process safety professionals about molten metal and pyrophoric vapor hazards.
💡 Lessons Learned
Pre-startup safety review (PSSR) is specifically designed to catch the type of deficiency that caused this explosion: a mechanical problem introduced during maintenance that was not identified before the equipment was returned to hazardous service. A PSSR for a rebuilt high-temperature zinc distillation column must verify that sumps, drains, and overflow paths are unobstructed and functioning.
High-temperature molten metal processes share common hazard characteristics: spontaneously combustible vapors, explosive decompression of trapped liquid metal, and catastrophic energy release potential. The specific failure mode at Horsehead (trapped superheated liquid metal eruption) has analogs in other molten metal operations and must be specifically addressed in PHAs and maintenance procedures.
Legacy industrial processes often operate without modern PSM frameworks tailored to their specific hazards. When a facility operates a process for which no modern guidance exists, it must develop its own hazard analysis from first principles. The age of a process is not a justification for an absence of hazard management.
Zinc vapor at high temperatures ignites spontaneously in air — there is no ignition energy threshold to manage. When a high-temperature molten metal process releases vapor, ignition is essentially certain. Process design and mechanical integrity for these systems must prevent the release, because once the release occurs, fire and explosion follow automatically.
MI: Mechanical IntegrityPSI: Process Safety InformationSOP: Operating ProceduresPHA: Process Hazard Analysis
🔨 Safety Meeting Toolbox Talk
►Do your pre-startup safety reviews for maintained or rebuilt process equipment include specific verification of sumps, drains, and low-point collection points in high-hazard service?
►If your facility operates high-temperature molten metal processes, have you conducted a PHA that specifically addresses spontaneously combustible metal vapors, trapped liquid metal eruption, and explosive decompression scenarios?
►After any maintenance or rebuild of high-temperature process equipment, what is your process for verifying that all installed components are correctly positioned and functional before restart?
Immediate Action Items
✓Review your PSSR procedure for high-temperature or molten metal processes to confirm it requires specific verification of sumps, drains, and overflow paths before startup.
✓Conduct a PHA for all high-temperature molten metal operations that explicitly addresses spontaneous combustion of metal vapors and explosive decompression of trapped liquid metal.
✓Verify that PSI for high-temperature processes documents the pyrophoric or spontaneous combustion properties of process vapors at operating temperatures.
✓Implement post-maintenance inspection requirements that include physical verification of sump, drain, and overflow device installation before return to service.
🔗 PSM Failures Behind This Incident

This incident traced to breakdowns across 4 PSM elements (MI · PSI · SOP · PHA). Each represents a documented gap that process safety documentation and consulting can close before a similar event occurs at your facility.

Mechanical Integrity (MI)
Equipment must be designed, inspected, and maintained to operate safely in its intended service. Mechanical integrity failures — degraded equipment, missed inspections, deferred repairs — contributed to loss of containment here.
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Process Safety Information (PSI)
Accurate, complete Process Safety Information is the foundation every other PSM element depends on. When PSI is missing or wrong — chemistry data, equipment specs, P&IDs — the entire hazard analysis is built on a flawed base.
Supporting documents in our library →
Operating Procedures (SOPs)
Operators cannot reliably hold safe operating limits without clear, current, enforced procedures. Deviation from acceptable operating conditions — a root cause here — is a direct consequence of SOP failure.
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Process Hazard Analysis (PHA)
A structured PHA or HAZOP study exists to identify exactly these scenarios before they occur. When PHA is absent, superficial, or overdue for revalidation, hazards operate unseen until they kill someone.
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