The ore used to produce the result came from Mahenge. Only flotation methods were used, it said, adding that it believes the 99.3% percent purity is a world-first.
Black Rock said it used a double loss on ignition (LOI) method. A trial run realised 200kg, but that was reprocessed with a polishing circuit to achieve an ultra-high grade.
"Being able to generate ultra-high-grade product using flotation is of significant value to downstream processors, who are increasingly finding environmental constraints impacting their business," Black Rock Mining CEO John de Vries said.
"The value proposition of producing high-specification concentrate has been validated by Syrah Resources' recent announcement of planned increases of Balama concentrate grades to a targeted 97%-98% TGC (total graphite concentrate)."
He added that more than 55% of the ultra-grade concentrate reported to the large and jumbo fraction.
To get to its result, the company said it processed 90t of Mahenge ore: 50t from Cascades and 40t from Ulanzi. The Mahenge project consists of those two plus Epanko North.
A pre-feasibility study said Black Rock's Mahenge, which has a mineral resource estimate of 211.9 million tonnes, could return as much as 250,000t per year for 31 years of 98.5% graphite concentrate.
The miner will develop the project in three phases. Work on a definitive feasibility has commenced. A construction start is projected for late this year, with first production in 2019.