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Gold & Silver Recovery Explosion

A waste reprocessing company specialized in the recovery of gold and silver from various electronic industry wastes. One of the initial steps in the recovery process was to dry the waste, which usually came in the form of aqueous slurry of metal oxides. The oven used stood about 6 feet high and was about 3 feet wide by 3 feet deep. The waste slurry was placed on steel trays that slid into the oven and were about 6 inches apart. 

oven after the explosion.
The figure above shows the lower portion
of the oven after the explosion.

The temperature of the oven was typically just slightly above the boiling point of water and drying times ranged from 24 to 48 hours. This was the first time the waste from a specific electronics company was being reprocessed. However two 55-gal drums of waste from the same company was successfully dried without incident days before.

After or during a drying cycle, two workers opened the oven, slid out a tray, and began to stir the waste. This was done to aerate the material and facilitate the drying process. On one such occasion, there was an explosion. One worker was serious injured and the fire department was called to put out the resulting fire. There were also significant business interruption losses.

The next Figure shows the one tray that had been slid out for stirring. Note the sharp bend.

Figure shows the one tray that had been slid out for stirring. Note the sharp bend.

This Figure shows the angle irons on which the individual trays rested. Note that they are bent downward. This angle iron downward bending was only from one tray location to the bottom of the oven. Above that location the angle irons were bent slightly upward.
angle irons on which the individual trays rested. Note that they are bent downward

The bending patterns of the one tray that had been pulled out for stirring and the bending patterns of the angle irons pointed to an isolated explosion on one single tray the tray that had been pulled out. The force of the explosion forced the tray down, bending both the tray and the angle irons on the way down. It was also noted that no other materials in the oven exploded. Each of the trays, except for the one that was pulled out and stirred, still held most of the contents with which they were originally loaded. Therefore, the oven was not full of shock-sensitive or explosive materials. If it was, the entire contents of the oven would have exploded and the oven itself would have been blown into pieces.

The physical evidence strongly suggested that whatever was on that one tray (that was pulled out) was somehow different than the remaining contents of the oven. Since that one tray was blown relatively clean by the explosion, no residuals could be measured. The major metal constituents of the waste from the other trays included: iron, gold, silver, sodium, titanium, aluminum and chromium.

The main issue in the investigation was: "What exploded." The methods used included:

  • NFPA 921 Fire & Explosion Investigative Methods,
  • Conventional chemical analysis (EPA methods),
  • Energy Dispersive Spectroscopy (SEM/EDS),
  • Thermal Gravimetric Analysis (TGA), and
  • Differential Scanning Calorimetry (DSC).

The DSC of residuals of the incoming waste indicated it contained some chemical energy, but that energy was quickly dissipated upon heating. Heating this material did not result in the formation of a shock-sensitive or explosive material.

Specs of material that were emitted from the oven and splattered on a wall across the room were analyzed by SEM/EDS and it was found that these specs contained elements that were not in the bulk of the waste that did not explode and remained in the oven. This further suggested that whatever was on the one tray that was pulled out and exploded, was different than the other wastes.

Dr. Fox is an explosion expert, fire expert, and chemical expert with extensive experience in OSHA, EPA and DOT chemical regulations and chemical safety.