African Minerals, the developer and operator of the Tonkolili iron ore mine in Sierra Leone, is enhancing product quality. It has previously highlighted a number of technical interventions that it had proposed regarding materials management, particularly in the wet season. The most significant of those is the establishment of screening and de-sliming circuits, which are being installed to further treat the sub 10 mm fines material, which contains fine sticky clay and ultrafines.
Those circuits will be fitted to the 1B and 1D processing plants which are currently producing this fines product, and will be fitted to the new 1G process plant once it is commissioned in Q3 2014.
At present production consists of fines, lump and All in 32 (A32). This product mix has historically caused material handling problems in the port, where high clay levels have lowered throughput capacity, and has produced fines with high moisture content that requires significant re-handling and drying before it can be shipped.
The establishment of new screening and de-sliming circuits will create clean, free draining products with low moisture content and better product quality, which are expected to be shippable all year round.
Fines material below 10mm will be screened to +2mm to create a new intermediate product, a Group C cargo, that has no transportable moisture limit (TML) restrictions and can be shipped all year round.
De-sliming of the -2mm +0.15mm fraction will remove fine sticky clays and ultrafine material from these fines to create a clean free draining higher quality product, with low moisture content and enhanced material handling characteristics.
As a result, product mix at Tonkolili will in future include only de-slimed fines, intermediate product, and lump, until the establishment of production of the friable hematite concentrate.
In the screening stage the -10 mm fines material is classified into two fractions that are above 2 mm (now called ‘intermediate product’) and below 2 mm. The intermediate product is expected to be classed as a Group C cargo under the International Maritime Solid Bulk Cargoes Organisation code, and considered incapable of liquefaction. This product does not have a moisture limitation and can be shipped all year round.
The de-sliming process is centred around a series of constant density (CD) thickeners, which effectively separate the fine sticky clays and ultrafine material from the clean iron ore product. This low-cost and low-tech separator is inexpensive to build, inexpensive to run, and can be finely controlled by simply varying the water flow characteristics. The process creates a free draining product with enhanced moisture and handling characteristics. De-slimed fines will still be classed as a Group A cargo, but with a lower moisture content than that which is currently being produced.
The current fines have a TML of between 13.1% and 14.8% and have been shipped with an average moisture content of 11.6% in Q1 2014. Indications are that the de-slimed fines product will have a moisture level of below 10% and is also expected to be shippable all year round.
It is expected that the annual product split, once the DSO de-sliming circuits are fully commissioned, will be 49% lump, 17% intermediate product, and 34% of de-slimed fines. The mass yield from head feed to product is expected to be circa 85%, compared to the mass yield of 81% in Q1 2014.
Fine DSO material below 150 microns that reports to tailings after de-sliming will act as additional ore feed for the friable hematite concentrator stage, the full parameters of which will be announced shortly.
Bernie Pryor, Chief Executive Officer of African Minerals, said: “The establishment of these screening and de-sliming circuits will be a game changer for our wet season strategy, in that most if not all of our product will now be able to be shipped year round. Not only that, but the removal of fine sticky clay from our material, and of course the eradication of A32 itself, will greatly improve our port material handling characteristics. With a year round shippable product, and cessation of A32, we also expect our price realisation to improve meaningfully.”