Thirty-five people were injured on Thursday, three of them seriously, when a series of explosions ripped through a chemical recycling plant located on the southeastern outskirts of the Spanish capital, emergency services told EFE.

It took firefighters eight hours to bring the blaze under control after it erupted for as yet unknown reasons.

The explosions were so strong that they damaged the premises of four adjacent companies in an industrial estate and emergency services ordered the evacuation of five schools in the vicinity, officials said.

A large plume of dense, dark gray toxic smoke drifted over Madrid as the fire raged at the plant located in the Arganda del Rey neighborhood, 25 kilometers (15.5 miles) from the city center.

This is the second time in less than four years that a fire has erupted at the plant. In contrast to the blaze that broke out in June 2013, however, Thursday's fire seriously injured several people including two company workers, along with a female employee at a nearby plant, all of whom have been hospitalized.

One of the workers, a 37-year-old man, suffered burns on 30 percent of his body, while one of his colleagues, 42, was burned on 18 percent of his body. The woman, 22, broke her pelvis in a fall at a nearby plant as a result of the fire.

Ten other people were transported to various hospitals with slight injuries and two others were given first aid and released at the scene. Twenty others, also with minor injuries, went to the Sureste Hospital in Arganda on their own, emergency services authorities said.

The acrid tower of smoke issuing from the site - which could be seen from as far away as the city of Toledo, 85 kilometers (53 miles) to the southwest - prompted police to evacuate buildings within a 500 meter (1,640 foot) radius of the scene.

The plant belongs to Requimsa, a company specializing in the treatment and recycling of hazardous solvents and chemical products.