The financial terms of the latest deal were not disclosed.
Under the three-year agreement, EGA will deliver several million tonnes of bauxite ore a year to the Chinese group.
This comes after EGA supplied Bosai with more than one million tonnes of bauxite this year under short-term agreements.
Bosai Minerals Group chairman Yuan Zhilun said: “We welcome EGA as a long-term supplier of bauxite to Bosai Group, and look forward to the reliable supply of Guinean ore over the years ahead to drive our business.”
EGA mines bauxite through its subsidiary Guinea Alumina Corporation (GAC), which operates a 690km² mining concession in north-west Guinea. The company is said to have invested around $1.4bn for the development of GAC.
All bauxite recovered from the concession is transported by train to GAC’s port at Kamsar for shipment to third parties.
EGA CEO Abdulnasser Bin Kalban said: “EGA is now a major player in the merchant bauxite market, and our strategy mirrors our approach for cast metal – consistent high quality, reliably supplied, and with a focus on building mutually-beneficial relationships with our customers.
“We are delighted that Bosai Minerals Group joins a group of long-term EGA bauxite customers that is geographically balanced in four key regions.”
Bauxite ore is refined into alumina, which is then used to produce aluminium metal.
According to a Reuters report, China is the largest aluminium producer in the world and counts Guinea as one of the main bauxite suppliers.