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Horsehead Holding Company Fatal Explosion and Fire

Overview

On 22 July 2010, the Horsehead zinc plant at Monaca suffered a fatal explosion and fire. The incident occurred on one of the New Jersey zinc refining columns at the Horsehead Monaca smelter, which used the Electrothermic Process to refine recycled zinc materials. Two workers were killed.

Incident Snapshot

Field Value
Facility / Company Horsehead Corporation
Location Monaca, PA
Incident Date 07/22/2010
Investigation Status The Board voted on January 28, 2015, to close the investigation and issue a technical analysis on the causes. The analysis was released on March 11, 2015.
Accident Type Chemical Manufacturing - Fire and Explosion
Final Report Release Date 03/11/2015

What Happened

  • On 22 July 2010, the Horsehead zinc plant at Monaca suffered a fatal explosion and fire.
  • The incident occurred on one of the New Jersey zinc refining columns.
  • The timeline shows B sump leaking, suggesting blockage, being strapped on 14 July and an air lance being applied to the sump.
  • Leaks occurred again on 15 July.
  • Further strapping took place on 19 July when the sump leaked.
  • Approximately 10 minutes before the explosion, an alarm indicated a high rate of temperature change covering the column waste gas temperature, and the operator cut the gas input flow.
  • Following a further alarm, gas flow was cut further.
  • Shortly afterwards, the column exploded.

Facility and Process Context

  • Horsehead Corporation operated a zinc smelter using secondary, recycled materials at Monaca, PA.
  • The Horsehead Monaca smelter used the Electrothermic Process, a pyrometallurgical process requiring a distillation plant for refining.
  • The refinery was also used for zinc oxide production.
  • The plant had seven electrothermic furnaces, of which five or six were normally operating.
  • At Monaca, sinter and pea coke were preheated to around 500ºC in drums fired with electrothermic furnace off gas, and charged to the electrothermic furnace by means of rotary distributors.
  • Some 50% of the zinc from the electrothermic furnaces was cast directly to meet Prime Western quality requirements.
  • The remainder was refined and combusted to zinc oxide, with a small production of SHG.
  • Refining of the remaining 50% ETF zinc at Monaca was carried out by the traditional New Jersey distillation method in columns fitted with silicon carbide trays.
  • Heat was provided by a mixture of Low Calorific Value gas, a by-product of the smelting furnaces, and natural gas.
  • A column is designed to be air tight, having liquid zinc metal seals at its three orifices, feed box, sump, and condenser sump on a lead or cadmium column.
  • The seal at the Monaca column sumps was 37/8 inches.
  • One important clearance in the sump to avoid blockages is the minimum vertical depth below the underflow and is, at most plants, 130-180 mm, described as “two bricks” or more.
  • The dimension shown for the B column underflow clearance was 25/8 inches, described as “1 brick,” about 65 mm.

Consequences

  • Fatalities: 2
  • Injuries: Major injuries occurred. The document does not specify a number of injuries for the 22 July 2010 event.
  • Environmental release: Not specified in the source extract.
  • Facility damage: The pressure relief panels were blown out into the workplace. The trays in the combustion chamber collapsed. The reflux tower was largely undamaged, having dropped a few feet such that the framework for the reflux section insulation was sitting on the combustion chamber roof. Damaged plant and equipment were covered with a zinc oxide ash. Bricks were blown everywhere and there was damage to steelwork.
  • Operational impact: The refinery was closed, but the remainder of the site remained operational. In January 2011 the plant restarted operations following rebuilding and safety improvements. In April 2014 the complete Monaca smelter had been permanently closed.

Key Findings

Immediate Causes

  • The sump at the bottom of the column partially blocked, allowing liquid zinc to back-fill and flood trays up the column.
  • The tray wall(s) failed, releasing a surge of vapour that blew out the combustion chamber wall.
  • The vapour and liquid zinc ignited and threw a flame across the workplace.

Contributing Factors

  • A sump design with restricted clearance.
  • A poorly executed column commissioning, allowing the formation of a sticky zinc oxide dross/liquid zinc emulsion that is known to promote blockages.
  • These factors were aggravated by a high and increasing rate of zinc throughput.
  • The feed arrangement through the needle valve was top down rather than bottom up, allowing dross and air to be more easily drawn down through the needle valve with the liquid zinc.
  • Air ingress can be a cause of dross, zinc oxide formation, and moisture in the air can make the dross sticky.
  • The sump burner was not operating properly.
  • The sump temperature was well below target during warm-up.
  • The column feed rate was being increased over this period.
  • The logged temperatures for liquation pots are frequently considerably higher than the target of not more than 450ºC.
  • The high and increasing rate of zinc throughput.
  • The absence of paper chart recorders for the main temperatures of each column.
  • CO gas BTU quality was varying.
  • The damper starts to open, probably as a result of the failure of the suction gauge in the combustion chamber.

Organizational and Systemic Factors

  • Human factors played a dominant role.
  • Because of a history of column blockages and explosions at the Monaca facility, hazardous conditions had been normalised.
  • Horsehead Monaca management should have put out a general warning that B column was functioning abnormally, that there was a potentially hazardous condition at the sump, and that extra care should be taken.
  • Process management had become desensitised to what was going on.
  • The absence of paper chart recorders for the main temperatures of each column meant that the subtle changes taking place may not have been observed on the computer screens by operators.
  • It is understood that the paper chart recorders for the main column temperatures no longer worked.
  • The number of individual combustion chamber thermocouples combined with their temperature movements due to gas quality changes would, however, have made it more difficult to see the clarity obtained from the penultimate chart.
  • Operators had not been given training to recognise these temperature trends as indicating possible blockage and backfilling.
  • The process is not easy to instrument and awareness of what is happening is vital.
  • The ground- and first-floor operators such as those tending the sumps can be the least-well trained.
  • Sufficient technical support was provided to the plant on a regular basis.

Failed Safeguards or Barrier Breakdowns

  • The sump burner was not present.
  • The sump burner flame, when the burner was present, was not being pulled into the sump, even with the cover tiles off.
  • The sump was not sufficiently clear to allow the sump burner to function correctly on heat-up.
  • The sump had to be strapped on several occasions.
  • The emergency procedure mentioned above clearly associates sump blockages with zinc oxide columns only.
  • No one inspected the nostril boxes.
  • The first time that the reflux temperature showed correctly in the life of the column was 12.10 on July 22 2010, just four hours before the explosion.
  • The key control thermocouple was not available for a matter of days.
  • The paper chart recorders for the main column temperatures no longer worked.
  • The column waste gas damper appears to be in response to a zero reading rather than to a real pressure change.
  • The relief bricks at the column top did not blow.

Recommendations

  1. Recommendation ID: Not provided
    Recipient: Not provided
    Status: Not provided
    Summary: No explicit recommendation text was provided in the source extract.

Key Engineering Lessons

  • Restricted sump clearance can promote blockage and back-filling in New Jersey zinc refining columns.
  • Commissioning quality is critical because a sticky zinc oxide dross/liquid zinc emulsion can promote blockages.
  • Loss of clear temperature trend visibility can prevent operators from recognizing developing blockage and backfilling conditions.
  • Sump heating and burner function are important safeguards during heat-up and abnormal conditions.
  • Column abnormality warnings and operator awareness are important when a history of blockages and explosions exists.

Source Notes

  • Consolidated from the provided structured extract for the final report (source_priority 1).
  • The source text identifies the event as a fatal explosion and fire at the Horsehead Monaca zinc plant on 22 July 2010 and states that two workers were killed.
  • No explicit recommendation text was provided in the extract.
  • https://www.csb.gov/horsehead-holding-company-fatal-explosion-and-fire/

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