Zulu nation – Riverside Mining’s anthracite resource in Zululand

Materials World magazine
,
1 Jan 2007

Anthracite has the highest carbon content of all coals and the lowest content of volatiles – above 80% and below 10% respectively. Some 456 million tonnes are mined globally of which 14Mt constitutes the seaborne trade (source: Energy Information Administration, International Energy Annual). Apart from use as a domestic smokeless fuel, it is ideal as a reductant when processing metal oxide ores and in graphitising material for electrode pastes. The coal is also used in soda ash and carbide manufacture, and in specific methods of cement-making.

Anthracite’s relative rarity compared to other coals is a reflection of the geological processes that convert organic matter from peat to brown coal or lignite, and bituminous coal and anthracite. Essentially, heat and pressure drive off water and volatiles to form a harder coal with a higher carbon content. These conditions are found when the basin in which the coal was formed is subjected to forces that depress the deposit deeper into the crust.

Peak condition

The distribution of anthracite is dependent on these geological conditions. There are substantial reserves in China, Vietnam, British Columbia, Australia and South Africa. In the latter, the Australian company Riversdale Mining is producing anthracite from the Zululand Anthracite Colliery (ZAC) at the rate of 740,000t/y from the coalfield in KwaZulu-Natal. Saleable production for ZAC this financial year is forecasted to be around 660,000t.
Listed on the Australian Stock Exchange, Riversdale Mining finalised the purchase of the operating mine in 2005 with its black empowerment partner, the Maweni Mining Consortium. This comprises an Employee Trust and a number of community groups. Changes in South African law require all mining organisations (and other industries) to open participation to the majority of the population. This has resulted in a number of changes in the ownership of companies and allowed junior miners to bring their entrepreneurial spirit to the region.

True anthracite

The Zululand coalfield, located in the northeast of South Africa, is part of the giant Karoo Basin that extends 200km east-west and 400km north-south. Zululand is in the north-eastern sector of a geologically complex region characterised by graben structures (fault-controlled troughs) where active rifting and downwarping took place. This rifting and subsidence resulted in thick sedimentation and high geothermal gradients, which, in turn, produced anthracite.

Other anthracite deposits within KwaZulu-Natal owe their formation to the intrusion of post-depositional dolerite sills, whose heat completed the process. These are local events that can result in variable metamorphism, whereas the ZAC deposits are a result of regional features leading to more uniform coals.
This structural feature has rendered ZAC anthracite as the only true anthracite in South Africa (as distinct from semi-anthracites). It has a volatile content of typically five per cent on an air-dry basis and a high vitrinite content that allows the mineral to be used in high-value niche markets. However, unstable tectonic conditions produced a number of recognisable coarsening-up sequences that have been separated into distinct zones, logged in the borehole record. Numerous small coal seams have developed within these upward cycles, but only the main one attains a thickness of more than 0.8m.

This deposit has been broken up into different mining blocks (Kwa-Sheleza, Mgeni and Maye) by north-south trending faults with displacements in excess of 100m. Within the Kwa-Sheleza block are three zones divided by northwest-southeast trending grabens with displacements of approximately 140m on the bounding faults. Many of the displacements have dolerite dykes that have exploited the weakened country rock fabric. Within the fault bounded resources areas, the main seam can be found at both subcrop and to more than 200m depth with increasing surface elevations from south to north.

In reserve

Zululand Anthracite Colliery was opened in 1985, following exploration in 1976 by Southern Sphere and the early 1980s by Trans Natal Coal Corporation (later renamed Ingwe Collieries Limited). It has been a continuously profitable operation supplying domestic and international markets. Nevertheless, reserves at the mine were limited and not of interest to the then owners, a multinational mining company that did not regard ZAC as a core asset.

Upon acquisition, the main task of Riversdale Mining’s Chairman (and now CEO) Michael O’Keeffe and his team was to increase the reserves. The Kwa-Sheleza region provided an important target, known as ‘Deep E Block’. Project development of this area began in the last quarter of 2005 and included drilling and the extension of stopes and drives from the existing infrastructure. The budget for the work was estimated at ZAR70 million (AUS$15 million) and production began in September 2006.

The company says that not only are all of these new reserves (at 47Mt) JORC-compliant but they will give a 17-year mine life. Production will also increase as Deep E reserves are integrated into the mineplan. Initial base production of 24,000t per month will be added to the mix of a continuous miner section (14,000tpm), three conventional sections (24,000tpm) and four shortwall section (13,000tpm). O’Keeffe notes, ‘should market conditions and our exploration programme prove as positive as currently indicated, Riversdale would have the flexibility to add additional equipment and bring forward production from this area at a much faster rate’.

Time saver

The company identified the optimum strategy for the development of Deep E as being a 10° decline from a box cut on the surface for 840m. In addition, its contractors, LTA-Grinaker, sank a 5m diameter vertical shaft for 195m and developed it 54m along the coal horizon from where a raise at 10º for 340m met the decline from surface. This reduces the risk of delays as the potential for both working faces being held up by bad ground lessens. The overall development rate was almost double that of a single face. Although expensive compared to traditional single-end sinking costs, the time saved in the long-term should more than offset this investment.

The main seam in Deep E lends itself to mechanisation using a combination of roadheaders, shuttle cars, continuous miners and scoops supporting conventional drill and blast sections in flexible bord and pillar mining. Square pillars are being used, some of which, where support allows, will be removed later to boost extraction rates from 80% to over 90%.

Riversdale Mining have developed a 10-year mining plan, envisaging a careful sequence of mining operations that blend the run of mine coal from the Deep E Block with that of other mining areas. This will allow maximum flexibility for planning and use of equipment. The competent sandstone roof and floor conditions in the Block permits a premeditated programme of stoping operations. Testwork reveals that the coal quality here is excellent, with low ash coal (8.5% ash on an air dry basis) produced at significantly better yields than the historical average onsite.

In March 2006, drill intersections revealed a ‘considerable resource’ in the Ngwabe block located within the ZAC mine area. Riversdale geologists used structural interpretation and an airborne survey to identify positive conditions for hosting coal seams across a strike length of 7km with a width of 800-1,000m.

Exploration for 2007 will include extensive drilling of this area with a budget of AUS$875,000. Drill holes N99 and NG001 in Ngwabe confirmed the presence of coal. Based on an average seam thickness of 2m, this could mean a resource of 25Mt. Perhaps more importantly, these thicker seams may allow low-cost mechanised mining.

 

Further information:

Riversdale Mining Ltd, GPO Box 5371, Sydney, New South Wales 2001, Australia. Tel: +61 28299 7900.