YOKKAICHI, MIE PREF. – A heat exchanger that exploded at a Mitsubishi Materials Corp. silicon plant Thursday in Mie Prefecture, killing five people, had not been cleaned since its installation in January 2006 while a power generator had been placed nearby despite the risk of electric sparks, it was learned Saturday.
The revelations came as the Mie Prefectural Police on Saturday raided the firm’s plant, in an industrial district of Yokkaichi, on suspicion of professional negligence resulting in death and injuries. In addition to the five fatalities, the blast injured 12 others.
The police searched the plant and confiscated documents and other materials for their investigation into whether the firm had followed safety procedures at the plant. The police also conducted an on-site inspection at the facility Friday.
At a news conference at the plant, Mitsubishi Materials President Hiroshi Yao apologized for the accident but did not pinpoint its cause.
“Although the specific cause of the accident is still unknown, we will take thorough measures to prevent similar incidents,” Yao said.
The company admitted it depended on the experience of individual workers in operating heat exchangers, he said, noting its manuals were not good enough. He also vowed to review the existing work flow.
The explosion occurred at around 2:10 p.m. Thursday, when some 20 workers were cleaning a heat exchanger at the plant’s No. 1 facility. The device was being used to cool gas containing trichlorosilane, a material needed to manufacture silicon.
The heat exchanger exploded when workers opened its 300-kg lid, the company said. Four of the five workers killed were involved in that process.
Mitsubishi Materials officials said they suspect large amounts of chemicals inside the heat exchanger’s metal pipe may have produced hydrogen, resulting in the blast.