Traxys has agreed to buy 50pc of NioCorp's planned ferro-niobium (FeNb) output from the Elk Creek project in Nebraska for 15 years.
It will also act as exclusive marketer for scandium, titanium and rare earth products.
The binding agreement replaces earlier offtake arrangements. Neither company disclosed volumes or pricing terms.
ThyssenKrupp Metallurgical Products will cover the remaining 50pc of FeNb output for the first 10 years.
Traxys holds an exclusive option, but not obligation, to purchase all of NioCorp's remaining planned output, which includes scandium, titanium, and rare earth oxides such as neodymium-praseodymium (NdPr), dysprosium and terbium, NioCorp said.
The company's updated feasibility study projects average output over a 40-year mine life of 8,095 metric tonnes (t)/yr of FeNb, which would put Traxys' share at about 4,050t/yr. The study also projects 118t/yr of scandium oxide, 672t/yr of NdPr oxide, 67t/yr of dysprosium oxide and 17t/yr of terbium oxide.
The deal supports NioCorp's debt financing review by the US Export-Import Bank. Deliveries are subject to financing, construction and commercial production.
Traxys and NioCorp are discussing a potential equity investment of up to $30mn, which is not part of the current agreement.

