The oil trader Trafigura has been fined ¤1m (£840,000) for illegally exporting tonnes of hazardous waste to west Africa. It is the first time the London-based firm has been convicted of criminal charges over the environmental scandal, in which 30,000 Africans were made ill when the toxic waste was dumped in Ivory Coast.

A court in the Netherlands also ruled today that the firm had concealed the dangerous nature of the waste when it was initially unloaded from a ship in Amsterdam.

Eliance Kouassi, president of the victims' group in Ivory Coast, said: "Finally Trafigura has been called out in a court of law. It's a real victory for us." The fine is, however, only half the amount sought by the Dutch prosecutors.

Amsterdam district court judge Frans Bauduin also convicted a Trafigura employee and the Ukranian captain of the ship that carried the waste for their roles in the 2006 scandal.

The seven-week trial centred on Trafigura's initial attempt to get rid of the waste cheaply in Holland. Look Bougert, prosecuting, told the court that Trafigura had put "self-interest above people's health and the environment".

The prosecutor said Trafigura initially tried to conceal how dangerous the waste was, adding that the firm wrongly described it as routine slops from ordinary tank-cleaning. Residents complained about the foul smell. The company hired to dispose of the waste in Holland wanted more money for the job.

Trafigura then pumped the toxic waste back on to its tanker. The vessel, the Probo Koala, was sent to Ivory Coast, where the cost of getting rid of the waste was much lower. Instead of disposing of it properly, Trafigura "dumped it over the fence" in Abidjan, Bougert said. "Cheap, but with consequences," he added.

Last year, amid an international furore, Trafigura was forced to pay compensation totalling £30m to the thousands of Africans who needed medical treatment. The payments settled out of court a civil legal action brought by London lawyers Leigh Day. Three years ago, in another settlement, Trafigura paid £100m to the Ivorian government to help clean up the waste.

Trafigura has consistently denied wrongdoing, insisting that the waste could not have caused serious illness. After today's verdict, it said it would consider an appeal. "While Trafigura is pleased to have been acquitted of the charge of forgery, it is disappointed by the judge's ruling on the other two, which it believes to be incorrect. Concerning the delivery of dangerous goods, it is important that the court has noted that there was limited risk to human health from these slops, and indeed no damage occurred in Amsterdam."

The court convicted Trafigura employee Naeem Ahmed of leading the effort to dump the waste "while its dangerous nature was concealed". Ahmed was fined ¤25,000 and given a six-month suspended sentence. The Ukrainian captain of the Probo Koala, Sergey Chertov, also was sentenced to a five-month suspended prison term.

Greenpeace campaigner Marietta Harjono said: "This is a first step towards justice and a clear signal to other companies that the illegal export of waste to Africa will not go unpunished."

Trafigura has become notorious for threatening reporters who delve into its conduct. It has been paying for the services of lobbyists Bell Pottinger and libel lawyers Carter-Ruck. Last year, it was forced to back down when it tried to enforce a so-called super-injunction against the Guardian, gagging it from reporting proceedings in parliament.