A storage tank at the Rubicon LLC chemical plant in Ascension Parish vented a white vapor cloud Thursday night that prompted roadblocks and orders for anyone within a mile of the facility to shelter in place, authorities said Friday.

The liquid released was identified as maleic anhydride, which can cause skin and eye burns and respiratory problems if inhaled, material safety data sheets say.

Mark Dearman, general manger of Rubicon, said Friday that workers saw the vapor cloud heading toward the fence line and in the direction of Rubicon’s parking lot and neighboring plants to the northwest when they asked for the order from parish emergency officials.

Rubicon, a joint venture of Huntsman and Chemtura corporations, is on 81 acres in Geismar along River Road near several other plants in Ascension’s chemical corridor along the Mississippi River.

But a subsequent preliminary estimate by the company of the air release suggests the vapor cloud was constituted of an amount of maleic anhydride so small it would not have been legally required to report the release, Dearman and state environmental officials said Friday.

Dearman said the cloud never went into the community and remained just outside the plant fence line, resulting in no exposures, injuries or first aid visits. But he added that Rubicon would rather err on the side of caution.

“We are on a small site, 80 acres, and with our neighbors close by, we just don’t take any chances,” Dearman said.

Ascension Parish officials, who at one point declared a general chemical emergency Thursday night, gave the all clear about 10:25 p.m., removing barricades and the shelter-in-place order.

The 1-mile order had extended to La. 73 between La. 30 and River Road, parts of La. 30 and surrounding plants.

State Department of Environmental Quality officials said the release from the Rubicon storage tank lasted for about seven minutes, starting shortly after 9 p.m. Thursday, when a pressure relief valve on the tank inside the maleic anhydride unit opened.

A DEQ first responder reported Thursday night to the agency that moisture infiltrated the tank and caused a chemical reaction that caused pressure in the low-pressure tank to build.

The relief valve is designed to open and prevent a tank rupture by releasing excessive amounts of gases or vapors before the valve closes.

Dearman said company officials are investigating if the valve operated properly.

The DEQ first responder added in his report that Rubicon had modeled the vapor cloud and estimated the plume would leave its site but noted then that the area of the shelter-in-place order affected nearby plants and not residences.

Dearman said about 75 workers were at the Rubicon facility at the time of the release.

Maleic anhydride is a chemical used to make fiberglass-reinforced resins for marine, automotive and construction products. The chemical also is used in lubricants, food additives and artificial sweeteners, Huntsman says in annual financial reports.

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