The accident in May 2014 killed 301 people when one of the pits of the Soma mine became engulfed by flames and carbon monoxide gas, trapping 800 miners working inside.
The tragedy sparked protests and raised new concerns about Turkey’s dire industrial safety record.
Relatives and the opposition denounced Wednesday’s verdicts, handed out on negligence rather than murder convictions, as outrageously lenient after prosecutors had initially demanded terms of 301 times 25 years for all the main suspects.
After a trial lasting over three years, the court in the western Turkish town of Akhisar jailed the former CEO of the Soma mine, Can Gürkan, for 15 years, the state-run Anadolu news agency said.
The mine’s general manager Ramazan Doğru and technical manager İsmail Adalı were handed prison sentences of 22 years and six months, and operations manager Akın Çelik and technical supervisor Ertan Ersoy 18 years and nine months, it added.
The chairman of the Soma Mines Company which owned the mine, Alp Gürkan, the father of Can Gürkan, was acquitted along with 36 other suspects.
Out of 51 suspects on trial, nine other lower-ranking mine managers were given jail terms of six to 11 years.
The Doğan news agency said the verdicts prompted victims’ lawyers and families to walk out of court in protest. Emergency services needed to be called as several collapsed due to stress, it added.
The opposition Republican People’s Party slammed the sentences, which it said were drawn up in advance as a result of pressure from the company and the authorities.
“Justice has not come for Soma and… the law has, once again, gone bankrupt,” its deputy chairman Veli Ağbaba said in a statement.
Alp Gürkan had denied responsibility for the disaster, asking when the trial opened in April 2015 to be freed “to do our own research and shed light on the accident”.
Turkey’s Confederation of Progressive Trade Unions expressed outrage that the convictions had been made on the lesser negligence charges, arguing the mine had failed to take necessary precautions and had overworked its employees.
“Three hundred and one of our worker brothers lost their lives before our eyes as a result as a slaughter,” said its chairwoman Arzu Çerkezoğlu.
“This is not called negligence or a mistake. It is a crime. We do not accept this decision,” she added in a statement.
The accident on May 13, 2014, exposed the lacklustre reaction of the government led by President Recep Tayyip Erdoğan, who was prime minister at the time.
Prosecutors said that the miners were killed after inhaling gas and toxic smoke from the fire which was caused when an abandoned pile of coal left next to an electrical transformer caught fire.
Erdoğan had notoriously appeared to play down the disaster, saying that “accidents are in the nature of the business” and comparing it to accidents in the Industrial Revolution-era UK.