In the storage building at a chemical plant, an operation to transfer titanium tetrachloride (TiCl4) between two tanks had been in progress since 6:30 a.m. when a spillage occurred from the rupture disc on the receiving tank. Upon contact with the humidity in the air, the TiCl4 flowing into the retention basin formed opaque white smoke consisting of hydrochloric acid (HCl) and gaseous titanium dioxide (TiO2). The handling technician conducted an end of filling check and discovered the liquid leak and the smoke. He stopped the transfer pump and informed the control room, which in turn initiated the internal emergency plan. At the same time, HCl detectors triggered a local alarm and the containment of the building. The site’s firefighters covered the product contained in the retention basin with foam. The quality of the water/emulsifier mixture was carefully controlled to avoid exacerbating the hydrolysis reaction of the TiCl4, whereas the aim was precisely to minimise it by isolating the product from the humidity in the air. Treatment of the smoke in the building was activated (scrubbing of the HCl with sodium carbonate). A few fumaroles rose from the roof of the building, although no external impact was noted.
The accident was caused by the malfunctioning of two of the tank’s four load cells. The cells had shifted downward and were indicating a weight below the tank’s actual fill weight on the handling station’s control monitor. The packaging technician, relying on the weight displayed, had therefore programmed a filling volume greater than volume actually available. When the tank’s volume reached the high level safety sensor, it malfunctioned and did not stop the transfer as planned (blockage of the ring sliding on the vertical rod). The liquid product reached the tank’s rupture disc which eventually broke under the pressure created by the liquid. The TiCl4 then spilled out through the damaged disc and flowed into the retention basin, creating smoke after coming into contact with the humidity.
The operator is studying a change in technology used for the high-level sensors on the storage tanks and slaving the shutdown of the transfer operation to the HCl detection system. A regular inventory procedure was set up on the TiCl4 storage platform which would allow any deviation of the load cells to be detected since a direct operational check is impossible in light of their location.