A trained operator was blending water, sand, anhydrite, lime, cement, pulverized fly ash, and powdered aluminum in a mixing chamber to produce material for making concrete building blocks. In the blending process, sand and water are mixed to form a slurry, and then the powders are dispensed automatically into the mix by a computer-controlled system. Finally, a slurry of glycol-coated aluminum powder is added in the last few seconds before the mix is discharged into a car, and then molds are filled from the car. Adding aluminum to the mixture results in a small amount of hydrogen gas evolution, which disperses from the car into the surrounding ventilated area and out through roof vents. In addition to being an ingredient of the mix, water also helps to keep the mixture cool.
During a break in production, the operator took the opportunity to wash out the mixer in accordance with his work instructions. He did this by manually emptying out the water hopper, but he neglected to refill the hopper. When he switched the controls back to automatic, the program software did not realize that the hopper was empty since the digital readout still said "full".
During the subsequent mixing batch, the operator noticed an "excessive mix time" alarm on the mixer panel. This alarm stops the process and dumps the mixer contents. The operator noticed an inconsistency in the mixed material discharge, which was more like a sludge than a fluid concrete. He realized that there must have been a blockage and followed the written procedure by taking manual control and introducing as much water as possible to thin and cool the mix by opening the water hopper valves. During this process, a second alarm went off reading "mix temperature invalid", indicating that the mix temperature was not within the acceptable range of 0-99°C.
The operator went up to the mixer platform to investigate the reasons for the alarms and the mix inconsistency. He lifted the mixer inspection hatch and used an inspection lamp (later revealed to be faulty) to illuminate the interior of the mixer. The ensuing explosion resulted in temporary injuries to the operator, including loss of sight, burns, and cuts. He was wearing the correct PPE, including safety glasses and a hard hat, which were both blown off by the blast.
The company investigation revealed that the incident arose because insufficient water was added to the batch. This resulted in a rapid increase in temperature and evolution of hydrogen gas following the addition of aluminum powder in the last seconds of the mix. Despite the presence of a functioning level-control valve on the mixer, the hydrogen gas was ignited when the operator opened the hatch. The most likely source of ignition was the faulty lamp. The operator was acting in accordance with his training and following the company's written safety procedures.
The company took a number of measures to prevent a reoccurrence of this incident, including:
- provision of intrinsically safe lamps
- introduction of daily checks of the vent valve
- minor modification to LEV and increased venting throughout the mixing process
- lab testing by the aluminum supplier to evaluate system safety with regard to hydrogen generation for all reaction conditions and quantities of aluminum added
- reprogramming/development of the software to improve both the safety of the operation and operator understanding of warning alarms.