Eastern Platinum (Eastplats), capitalised at over £930m, is one of the top five AIM listed companies by size.
Buchanan won a three-way competitive pitch for the business earlier this year and work begins this week.
Eastplats is also listed in Toronto and Johannesburg and produces around 150,000 ounces of platinum per year, making it the 5th largest producer of the metal in the world.
Buchanan partner Bobby Morse heads the account.
Earlier this year Buchanan Communications won a brief to advise a uranium company eyeing up a place in the FTSE 100 (PRWeek, 15 Jun).