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Phosphorous Pentasulfide (P2S5) Accident

A waste treatment plant that specialized in the incineration of waste oil and refinery byproducts would, over time, collect ash that contained too much lead (Pb). The treatment plant would then treat this High-Pb ash with phosphorous pentasulfide (P2S5) to form insoluble lead sulfide (PbS).

The addition of P2S5 to the ash would take place using a 3-level platform mixer as shown in the figure below.


The procedure started by adding 5000 lbs of ash to the mixer located on the 2nd Floor. This was accomplished by hoppers and conveyor belts. The next step was to add about 50 lbs of P2S5. The P2S5 was shipped to the plant in 55-gallon drums. The waste treatment plant had developed a makeshift funnel that would fit on the top of the 55-gallon drum. Once the funnel was in place, the drum would be lifted by a chain hoist on the 3rd floor, inverted, and then inserted into the chute leading to the mixer on the 2nd floor.

After the P2S5 was added to the mixer with the ash, a specified amount of water was added to mixer. Water reacts with P2S5 to produce hydrogen sulfide gas, H2S.

P4S10 + 16H2O 4H3PO4 + 10H2S

The H2S would react with the lead (Pb) to form insoluble lead sulfide (PbS). After the combination of ash, P2S5 and water mixed for 5 minutes it was dumped into a roll-off located under the mixer on the ground floor.

If there is any humidity at the time the P2S5 is packaged in the steel drum, a limited amount of H2S will form inside the headspace of the drum. The highest concentration ever measured in the drum headspace was 7,000 ppm. The incineration plant was very well aware of this headspace H2S and required the 3rd floor worker to wear a supplied air respirator when he opened the drum.

One day, three workers (A, B and C) on the 2nd floor were working on the valve that connected the chute to the mixer while at the same time the 3rd floor worker was removing an inverted drum from the chute. Worker B momentarily dropped to his knees and was helped outside by Workers A and C. About a year later Worker B sued the manufacturer of the P2S5 claiming he had been exposed to H2S gas and as a result subsequently developed a form of mental impairment. The specific claims were that the P2S5 packaging was inadequate and allowed humidity to enter the drum and form H2S and that the warnings about H2S were inadequate.

Chemaxx conducted an extensive amount of experimental testing and mathematical modeling and showed that it would have been physically impossible for Worker B to have been exposed to the concentration of H2S required to cause his collapse and/or the subsequent mental impairment (600 ppm). The animation below illustrates how the "Balloon Model" for gas dispersion was used. The model begins with a small sphere representing the headspace of a drum that contains 10,000 ppm of H2S, which was greater than any amount ever measured. The balloon model was further modified to favor Worker B by assuming that all the 5000 ppm H2S gas and air mixture would diffuse downward (which it wouldn't) in a ¼ spherical cone. In other words, the model was heavily biased to favor Worker B.


PRESS START BUTTON ON LEFT OF ANIMATION TO BEGIN

The result of the balloon model was that Worker B could not have been exposed to more than 10 ppm H2S whereas about 600 ppm would be required to cause even momentary unconsciousness, or "knock down." It was also noted that the other two workers were not seriously affected.

Rather than being exposed to H2S, Chemaxx proposed that Worker B had inhaled a particle of P2S5 dust that rained down when the 3rd floor worker removed the inverted drum before it was completely empty. The heavier particles of P2S5 floated down while the lighter particles remained suspended in the air on the 3rd floor. The animation below illustrates this scenario.


PRESS START BUTTON ON LEFT OF ANIMATION TO BEGIN

When P2S5 enters the lungs it reacts with the moisture in the lungs and forms H2S. Only a very small particle of P2S5 is needed to produce a toxic concentration of H2S once inside the lungs.

Unfortunately, the 3rd floor worker removed his Level B respirator and then walked through the area where he had handled the inverted drum (not shown). It is believed that he inhaled suspended P2S5 dust at that time and became unconscious before he could reach the ladder. He was later found collapsed on the 3rd floor but did not survive.

Chemaxx also did a significant amount of chemical simulation using actual 55-gallon drums of P2S5 and H2S. Opinions were not based on simple balloon models alone.

Chemaxx viewed the act of inverting a full drum of P2S5 as a catastrophe waiting to happen. For example, if the funnel was not secure and came loose during the drum inversion, over 500 lbs of fine P2S5 dust would have been released into the air. Since the floors are not solid, but steel gratings, anyone beneath the 3rd floor without effective respirator protection would face almost certain death as P2S5 is fatal by inhalation. Simple changes in procedures and the mandatory use of respiratory protection would have prevented this incident.

The case involving the fatality settled but the case involving the mental impairment was decided by the court in favor of the Defense.

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