The deal with Yunnan Dian Huang Peony Industrial Group replaces one it had with the Yunnan TCT Yong-Zhe Company.
The initial contract ran for three years, with an option for a seven-year extensions.
The agreement with Dian Huang is for a full decade, ramping up to 1mln tonnes of the polyhalite fertiliser from its mine in North Yorkshire. The price remains confidential.
"There is great potential for our multi-nutrient product in both Yunnan and China so firming up this supply arrangement is another positive step for us, especially at such a competitive time for the fertiliser industry,” said chief executive Chris Fraser.
"We look forward to working with Dian Huang for many years to come and our POLY4 product playing a significant role in more effective, efficient and environmentally sustainable agriculture in the region."
First production from the Sirius project, near Scarborough, is expected to begin in 2021, with output hitting 10mln tonnes in 2023.
The recently-published definitive feasibility for the mine development underscored its “world class” potential, valuing the project at US$15bn currently and US$27bn once it is up and running. The after-tax internal rate of return is put at 26%.
The operation has the potential to generate underlying earnings (EBITDA) of US$1-3bn a year, depending on volumes and price.
The cash margins on the business are put at 70-85%, while operating costs are expected to be in the order of US$37.20 a tonne.
While the plan is for the mine to initially produce 10mln tonnes of polyhalite fertiliser a year, there is the capacity to double output.
The cost to deliver the first phase is put at US$3.56bn, with the financing done in two tranches.