Diamond miner Petra Diamonds’ expan-sion plans for its Williamson mine, in Tanzania’s Shinyanga region, remain on track.

The aim is to ramp up tonnage throughput to about five-million tonnes a year from the 2017 financial year and, at a grade of about 6 ct for every hundred tonnes, delivery of 300 000 ct/y is expected.

“The Williamson mine is the world’s largest economic kimberlite mine by surface area at 146 ha and is Tanzania’s only important diamond producer. “Despite the mine having been operated continuously since 1940, the pit is only 90 m at its deepest point, owing to the large size of the deposit,” according to the company’s website.

The Mwadui kimberlite pipe, upon which the mine is based, contains a diamond resource of about 33.1-million carats.

The expansion plan follows the completion of the initial development programme at Williamson, which entailed a substantial rebuild of the existing plant and major pit-reshaping work. Production at the mine started in the fourth quarter of the 2012 financial year.

The company reported last year that these improvements “will be as a result of a focus on the plant design to deliver energy efficiency and water-recovery circuit improvements, resulting in the ability to increase throughput”.

Meanwhile, Petra Diamonds notes that output at the mine increased by about 15%, with production of 188 465 ct for the 2014 financial year. Production for the 2013 financial year was 164 376 ct.

However, the company notes that the run-of-mine grades decreased by 5%, “partly owing to excessive rainfall during quarter three of the 2014 financial year – which led to higher-grade production areas being made inaccessible – as well as plant constraints, which are being addressed”.

The Williamson mine has also been known for very rare pink stones, according to the company’s website. “Petra Diamonds recovered a high-quality 16.4 ct pink diamond at the mine in the first quarter of the 2015 financial year and the diamond was included in the company’s first sales tender of the financial year,” the company states. The diamond was sold for $2.2-million.

Petra Diamonds’ portfolio includes a full range of kimberlite operations, and combines major underground pipe mines with a low-tonnage, high-grade fissure mine, as well as one large high-volume opencast mine.