A storage tank containing 580 tonnes of hydrochloric acid split and leaked at a dock in Hull.
Concerns over a vapour cloud the leak caused has led to fire crews spraying a fine water mist to stop it spreading.
Twenty five firefighters were sent to United Molasses on the quayside of King George Dock at 22:15 BST on Monday.
The leak from the tank has been stopped, but an exclusion zone is in place within the port that has left the eastern access road shut.
Crews still on scene working with @ABPHumber site staff & other agencies to oversee the safe removal of all acid from leaking storage tank.— HumbersideFire (@HumbersideFire) September 19, 2017
Associated British Ports (ABP) said it was assisting the fire service and other authorities "to resolve the situation as quickly as possible".
A spokesperson said: "An exclusion zone within a section of the Port of Hull has been established.
"Please be assured nobody outside the exclusion zone should be concerned."
The fire service said it would remain there to assist ABP as the acid was transferred onto tankers to be removed.
Police said traffic in the area was unaffected and access to the ferry terminal and Hedon Road was unaffected.
The "significant" leak, at the east side of the dock near the Northern Gateway, earlier led to people in the area being advised to close windows and doors as a precautionary measure.
Crews wore gas-tight hazardous materials suits while dealing with the incident.
According to its website, United Molasses Group Ltd trades and markets molasses, vegetable oils and stores bulk liquids.