Both releases originated from the site's acid plant. This converts sulphur dioxide evolved during the sintering of zinc ore into sulphur trioxide before it is absorbed in sulphuric acid to produce stronger acid.
In March 1992, Pasminco was fined £2,000 for a release of sulphur trioxide which was caused by a failure to stop a pump feeding the gas to an absorber after the acid plant was shut down, and a second failure to ensure the inflation of a balloon at the base of the plant's stack which was intended to prevent emissions of unabsorbed gas (ENDS Report 206, pp 40-41 ).
The latest incident, last July, was triggered by the activation of three emergency buttons which stopped pumps feeding sulphur trioxide to the absorbers. The company said that this was a malicious act which was subsequently investigated by the police.
A heavy emission of sulphur trioxide began as soon as the absorber pumps stopped, and HMIP concluded that this was unduly prolonged because a fan continued to drive the gas through the plant. The plant's instrumentation prevents the closure of a damper to block off the gas flow until the fan slows to 150rpm. Pasminco believed that its control system would slow the fan to this speed within three minutes, but in fact it took ten minutes to do so under freewheel conditions before the damper was closed automatically.
Investigations subsequently conducted by the company at HMIP's request showed that the control system could be changed to enable the damper to be safely closed in about two minutes. The Inspectorate therefore decided that Pasminco had contravened the requirement in section 5(1) of the Health and Safety at Work Act 1974 that the "best practicable means" must be employed to minimise emissions.
The emission provoked a large number of public complaints from as far as two miles away, with some individuals claiming that they had suffered from prickling skin and breathing difficulties.
At a hearing before Bristol magistrates on 7 March, Pasminco pleaded guilty to a contravention of the 1974 Act, while arguing in mitigation that the incident had been caused by a malicious act by a person or persons unknown. But the court disagreed with its claim that the prosecution was unfair, imposing a fine of £15,000, with costs of £6,626.
Commenting after the hearing, Dr John Jackson, Head of HMIP's South West Region, said that the case "illustrates what happens when thorough proving of abatement equipment is not carried out before it is put to the test. In effect, this type of incident was waiting to happen and when it did many members of the public suffered." Since the incident, the operation has been sold by Pasminco to Britannia Zinc.