The interest saw the company's shares hit levels not seen in four years.
Centrex is looking to produce 800,000 tonnes per annum, with Samsung to provide marketing services for annual sales covering 20% of total production, or 160,000tpa, whichever is lower.
Samsung may also assist Centrex with sales of any additional phosphate not taken by any other offtake partners.
It provides Centrex access to 43 nations around the world, although Samsung will be the exclusive representative in key markets such as South Japan, Indonesia, India, and Mexico.
Pricing will be based on market netbacks.
The deal is conditional on Centrex proceeding with the development of a planned 800,000tpa plant and commencement of production, and Samsung's internal approvals.
Managing director Robert Mencel said signing on Samsung was indicative of international interest in Ardmore.
It is still talking with a number of other Australian and international customers, with the hopes of securing deals in the near term.
It hasn't all been good news. Centrex is mired in a A$2 million dispute with royalty holder Southern Cross Fertilisers that was originally signed in 2017, over an extension fee SCF believes is due.
Centrex argues that more than 27,000t of ore mined during February, and subsequent commencement of sales into the Australian market means the fee is not due. It is seeking a meeting with SCF to resolve the issue.
Centrex had $2.7 million cash in August.
It updated its feasibility study in August finding higher capital costs of $78 million.
The stock has fallen from about 9c when the trials were suspended, to 4.1c, valuing the company at $15 million.
Centrex's shares had traded between 2.4c and 8.4c over the past year, but Samsung's interest saw the stock jump more than 180% to a peak of 13.5c, valuing it at $16 million.
It comes a day after Western Australian fertiliser player, Salt Lake Potash, collapsed into receivership and administration after start-up and funding issues.