The sale accompanied promises from SMS of investment in the 150-year-old company, including developing hydrogen power projects.
Trade unions OGBL and LCGB also received reassurances that there would be no redundancies among the 500-strong workforce before 2023.
The OGBL and LCGB said the deal was struck last week.
Currently the firm is 18.4% owned by the state investment bank (SNCI), the nationalised Banque et Caisse d'Épargne de l'État with 10.98% and the Luxembourg state directly with 10.98%.