6.   METALLURGICAL PROCESSING

6.2.1  

Sishen Mine and SEP

Sishen Mine: The original ore handling plant at Sishen Mine was a dry crushing and screening plant commissioned in 1953. In 1963, the first dense medium separation (“DMS”) plant, South Plant, was commissioned. The North Plant, with a design capacity of 18Mtpa, was commissioned in 1976. In 1984, it was decided to rationalise production and the South plant was shut down.

Four types of hard high-grade ore are presently mined from the Sishen pit, namely massive, laminated, conglomerated and brecciated iron ore. The supply to the primary crusher of a suitable mixture of RoM ore largely controls the chemical quality of the final products. Presently only material with an iron content of greater than 60%Fe is fed to the plant.

Open pit ore is crushed via a primary gyratory crusher and two secondary cone crushers ahead of primary stockpiling. A smaller in-pit gyratory crusher is also available as required. Ore is withdrawn from the primary stockpile and sized into various fractions by washing and screening. Ore in the size range –90+25mm is beneficiated in the coarse dense medium (“DM”) drum plant. This circuit also includes a Larcodem dense medium vessel. Ore in the size range –25+8mm is beneficiated in the medium DM drum plant. Fine ore is split into two size fractions, –8+5mm and –5+2mm ahead of beneficiation in the coarse and fine DM cyclone plants, respectively. The –2+0.2mm fraction is forwarded to a new up-current classifier circuit for beneficiation. Product from the coarse drum plant undergoes quaternary crushing and screening to meet product size specifications, whilst the other circuits are correctly sized ahead of beneficiation. In total five products are produced:

• Sishen 66%Fe 27mm Direct Reduction Ore;
• Sishen 66%Fe 25mm Lumpy Ore;
• Sishen 66%Fe 20mm Lumpy Ore;
• Sishen 65%Fe 8mm Coarse Sinter Ore; and
• Sishen 65%Fe 5mm Fine Ore.

Dense medium rejects are stored on waste dumps whilst slimes are stored in tailings dams.

Capacity at the North Plant has been steadily increased to the current capacity of 28 to 29Mtpa.

Considering its age, the plant appears to be in a fair condition, both mechanically and structurally. With normal preventative maintenance and continuation of the refurbishment programmes already initiated, the plant can be expected to operate for the period scheduled in the LoM Plan.

Key historical processing statistics for the Sishen Mine Process Facility are summarised in Table 6.1.

The LoM Plan assumes an average RoM throughput of approximately 32.4Mtpa. The LoM yield is projected at an average of approximately 88% to yield total product of 29Mtpa. This is in line with current performance but somewhat higher than that achieved in recent years. The current performance has principally been ascribed to improved ore definition and the implementation of selective mining.

Average product quality achieved in recent years is summarised in Table 6.2.

It is evident that all other specifications have generally been met in recent years.

Sishen Expansion Project: Feed to the existing beneficiation plant is restricted to material with a grade of greater than or equal to 60% beneficiated Fe in order to meet the required product specifications. Included in Sishen’s growth strategy is the implementation of the brown field Sishen Expansion Project (“SEP”) aimed at beneficiating lower grade material in the range of 45% in situ Fe to + 60% in situ Fe to saleable product quality. This results in a significant increase in resource base and utilisation thereof.

Due to high separation densities required to beneficiate such material, DMS as currently employed at Sishen is not a suitable technology. Jigging however, is a viable option and a feasibility study incorporating this technology was completed in January 2005. Project start up is planned for July 2007, with capacity of 10Mtpa saleable product being realised by June 2008 and a further 3Mtpa by 2015.

A commensurate increase in Sishen Iron Ore’s allocation on the iron ore export channel capacity from 23.5Mtpa to 35Mtpa is planned.

Extensive laboratory and pilot plant testwork was undertaken through the various phases of investigation:

• Pre-feasibility characterisation of stockpile material and mine samples;
• Feasibility characterisation of ten mine samples with confirmatory pilot plant tests; and
• Feasibility optimisation on forty-eight mine samples.

In the interest of sample representivity, significantly large primary samples of up to 3,000t were taken from stockpiles and pit faces for pre-feasibility and feasibility investigations, with 80t primary samples being taken for feasibility optimisation studies. These in turn were crushed before secondary samples of approximately 3t each were split out for laboratory testwork.

The testwork programme focused on selection of the best relative cut density, the generation of beneficiation curves for various ore types and stockpiles that could be included in the geological model, prescription of the metallurgical flowsheet and determination of design parameters for engineering design.

A Mineral Density Separator (“MDS”) which is essentially a batch jig was used for laboratory characterisation of the various ore types and stockpiled material. MDS results were modified via a standard procedure to allow for process imperfection. Whilst each material type has a unique beneficiation curve, the modified results confirmed that at separation densities between 4.0g/cm³ and 4.2 g/cm³, lump and fine product at 64%Fe and 63.5%Fe, respectively, can be produced from feed between 50%Fe and 60%Fe at yields in excess of 60%.

The pilot plant tests were run in two campaigns as certain shortcomings were identified in the initial campaign. The second campaign confirmed the MDS beneficiation algorithms for the coarse and medium jigs but not the fine jig. Medium and fine jig capacity was also shown to be lower than originally anticipated. This was evaluated ahead of detailed design and subsequently a fourth jigging module was included. Pyrometallurgical testwork undertaken on the lump and fine products generally found both to compare well with current Sishen ore.

The SEP product specifications as summarised in Table 6.3 were determined through an iterative process between the resource beneficiation characteristics and market requirements. Laboratory and pilot test results confirmed that these specifications will be met in practice.

Letters of intent from existing Kumba clients support the demand for product of such chemical, physical and pyrometallurgical quality.

The proposed SEP flowsheet and process design criteria largely recognise the testwork findings. RoM ore will be fed to the primary gyratory crusher directly from the mine or from RoM stockpiles. Primary crusher product drops into a rock box ahead of a scalping screen and the secondary gyratory crusher. Scalping screen underflow and secondary crusher product drop into a rock box ahead of conveying to an intermediary stockpile. Material withdrawn from the stockpile is conveyed overland to the closed circuit tertiary crushing and screening plant. Screen underflow is conveyed to two pre-beneficiation blending beds. These principally serve to blend and homogenise the feed ahead of beneficiation. They also decouple the crushing and downstream beneficiation plant which in turn provides a maintenance buffer, improves mining equipment utilisation and allows for continuous feed to downstream beneficiation.

Material reclaimed from the pre-beneficiation blending beds is conveyed to three identical beneficiation modules comprising screening into a coarse (–25+8mm), a medium (–8+3mm) and a fine (–3+0.8mm) fraction. The three fractions report separately to coarse, medium and fine jigging. Product (sinks) from the coarse jigs is extracted via vibrating feeder ahead of screen dewatering and deposition on the lump product bed. Product (sinks) from the medium and fine jigs is extracted via vibrating feeders ahead of two stage screen and bunker de-watering before deposition on the fine product bed. Ore reclaimed from the product beds is conveyed to three existing load out stations for rail despatch to clients.

Waste (floats) from all jigs is dewatered before being conveyed to the discard dump. Each module has a degrit system comprising cyclones and dewatering screens. The –0.8+0.2mm fraction is combined with the plant discard and the –0.2mm fraction is thickened and pumped to tailings.

Presently the design excludes the processing of the –0.8mm fraction through up current classifiers. Such units will be tested and are likely to be included at a later stage.

Certain key aspects did require ratification before finalisation of the design. Firstly, a new generation screen with typical design specifications to those proposed for the SEP was tested in the Sishen washing and screening plant. The top deck did not meet efficiency claims under dry screening conditions. It is believed however, that sufficient screening capacity has been installed in the SEP tertiary crusher plant. This has been confirmed before finalisation of SEP screen selection. Secondly, utilisation of a bucket elevator to extract product from below the jig is more conventional than the proposed utilisation of a screen. Screen extraction has however, been successfully utilised in Australia at industrial scale. Screen extraction was selected on the basis of a visit to such an installation and has been ratified during final design.

Risk mitigation activities confirmed the jig throughput capacities and indicated that an additional jig beneficiation flow line was required to achieve the required product specifications and yields. These changes have been incorporated into the capital and operating cost estimates of the plant.

Annual plant throughput is planned at 15.6Mtpa RoM, comprising 13.3Mtpa low grade material and 2.3Mtpa high grade material at a planned yield of 64% to produce 10Mtpa of product (output relates to the first 10Mtpa expansion). The plant has however, been designed to process 16.7Mtpa RoM at an average design yield of 60% to produce 10Mtpa of product.

6.2.2  

Sishen South Project

Metallurgical testwork was undertaken on drill core samples of the various ore types that will be encountered at the Sishen South deposits. This included the following:

• Crushing testwork;
• Chemical analysis of screen fractions;
• Densimetric analysis of coarse screen fractions;
• Pyrometallurgical behaviour of coarse screen fractions;
• De-watering characteristics of fine screen fractions; and
• Settling characteristics of slime fraction.

Due to the decision to exclude beneficiation in Phase I of the project, the crushing and pyrometallurgical characteristics are the most relevant to the project. In general the crushing and pyrometallurgical behaviour of the Sishen South products were found to be comparable with those from Sishen Mine.

In the absence of beneficiation, RoM ore will have to be mined at product quality grade and the plant will also have to be fed at product quality grade. Material that does not meet product specification will be stockpiled for blending at a suitable stage, or for processing at such time as a beneficiation plant is introduced to the project. Current understanding of the geology indicates that the Sishen South deposit comprises high quality, clastic-textured (28.8% of total), laminated (52.9% of total), collapsed breccia (9.8% of total) and conglomeratic (8.6% of total) ores. The laminated and clastic-textured ores are of uniform quality and constitute the high-grade ores (Fe > 65%) at Sishen South, although the laminated ores tend to have a variable P content. The collapsed breccia and conglomeratic ores generally have a lower Fe content and significantly higher SiO₂, Al₂O₃, K₂O and P contents than the laminated and clastic-textures varieties due to higher clay content. Furthermore, the clay content of the Ploegfontein orebody was found to high at greater than 15% compared to levels below 3% in the Leeufontein/Welgevonden/Kapstevel orebodies. Based on this understanding of the ore types, a number of actions are proposed to ensure that the final products meet the required specifications.

• Inclusion of Leeuwfontein and Welgevonden/Kapstevel resources into Phase I of the project;
• Exclusion of Ploegfontein from Phase I of the project due to an inferior in situ quality in the collapsed breccia ores which are mostly in abundance;
• Exclusion of high clay-bearing ores that would otherwise impact negatively on the levels of contaminants and fines in the products;
• Implementation of effective grade control practices utilising blast hole sampling; and
• Reduction of the interdependence between the pit and the plant and minimisation of variation in plant feed grade by stockpiling and rehandling 100% of RoM ore. Opportunities to minimise the proportion of ore rehandled are however, currently being investigated.

Successful implementation of the abovementioned actions will be important in minimising any risk of not achieving product specifications.

Sishen South product specifications are more in line with SEP product specifications than those of Sishen Mine, as shown in Table 6.4.

Letters of intent from existing Kumba clients support the demand for product of such chemical, physical and pyrometallurgical quality. Aluminium levels are seen as disadvantageous for a stand alone project but should be acceptable when Sishen South product is blended with SEP product.

The proposed grade split between lump and fine ore of 64.0%Fe and 63.5%Fe, respectively, is considered to be conservative. In the event of this not materialising however, the following compensating steps may be necessary:

• Adjust the blend with SEP product at Saldanha to achieve the required combined specification;
• Decrease the specification of the fine ore; and
• Increase feed grade with associated impact on resource/reserve.

A review of the LoM mined grade shows an average of 64.3%Fe with occasional spikes above 64.5%Fe on an annual basis. This represents an opportunity to better utilise the resource through alternative scheduling and an improved stockpiling strategy.

The proposed Sishen South flowsheet and process design criteria largely recognise the testwork findings. The Sishen South plant will consist of five integral processing steps, namely primary crushing, secondary crushing, stockpiling, screening and tertiary crushing, product handling stockyard and load out.

RoM ore is dumped via trucks into the primary gyratory crusher. The crushed product reports to a rock box from which it is withdrawn using an apron feeder. Primary crushed product is screened, with screen oversize being fed to the secondary cone crusher. Screen undersize is conveyed directly to the secondary screening plant. Secondary crusher product is conveyed to the intermediate buffer stockpile, from which it is withdrawn for tertiary cone crushing. Tertiary crushing operates in closed circuit with the secondary screening plant. The +25mm fraction is recycled to tertiary crushing whilst the –25+8mm fraction and the –8mm fraction report to the lump and fine product stockpiles, respectively. The products are reclaimed with a single bucket wheel reclaim system and conveyed to a single load out station.

It is accepted that the plant which has been designed for the direct shipping ore operation will not be able to process wet or high clay containing ore. The following steps have been taken to lessen the impact of such occurrences:

• Low design utilisation of 64% (5,637 production hours per annum); and
• Over design of screen size by 25%.

Annual plant throughput is planned at 3.0Mtpa RoM at a planned yield of 99.8% to produce 3.0Mtpa of product.

6.2.3  

Thabazimbi Mine

The Thabazimbi process plant originally consisted of a washing and screening operation which was constructed in 1948. Since then, the plant has periodically been refurbished and upgraded. The dense medium drum section was added in 1954 and the cyclone section in 1970.

Generally, only material with an iron content of greater than 60% Fe is fed to the plant. When product quality allows however, a small proportion of lower grade ore at levels as low as 55% Fe is introduced but by-passed around the beneficiation plant. This has significant benefits in the better utilisation of the resource.

Open pit ore is crushed in one of two primary gyratory crushers ahead of stockpiling and conveying to the plant. Ore is sized into various fractions by further crushing, washing and screening. Ore in the size ranges –32+18mm and –18+8mm are beneficiated in static bath DM drums to yield a 62.5%Fe lumpy product. Ore in the size range –8+1mm is beneficiated in DM cyclones to yield a 63.0%Fe fine product. The –1mm fraction is de-watered and added to the fine product. Dense medium rejects are stored on waste dumps whilst slimes are stored in tailings dams.

Plant throughput is largely a function of plant utilisation but also of the feed size distribution. This latter aspect is important, as outside certain limits of the ratio of fine ore to lump ore, one or other of the plant sections will constrain overall throughput. With the present plant, the optimal proportion of fines is estimated at approximately 50%, under which circumstances the plant has a capacity of approximately 3Mtpa RoM feed.

Considering its age, the plant appears to be in a fair condition, both mechanically and structurally. A programme of structural refurbishment is in place in poorer areas of the plant. Thabazimbi operate a computerised maintenance management system although this has not yet been fully implemented on the plant. It is considered however, that with ongoing preventative maintenance the plant can be expected to operate for the period scheduled in the LoM Plan without the need for major refurbishment, other than that already identified.

Key historical processing statistics for the Thabazimbi Mine Process Facility are summarised in Table 6.5.

The LoM Plan assumes an average RoM throughput of approximately 3Mtpa. It is seen that this is accompanied by a sharp increase in the proportion of fines to 57% on average, which exceeds the indicated optimum of 50%. This is largely due to an increase in the proportion of Kwaggashoek ore, which is a lot finer than the other Thabazimbi orebodies. Fortuitously, the fine fraction of the Kwaggashoek ore is relatively clean and can in part be accepted on product beds without beneficiation. Currently approximately half of the Kwaggashoek ore is being screened at the pit. Screen undersize is transported directly to the fine blending beds, whilst screen oversize is combined with the balance of the ore and beneficiated as normal. It is proposed that this practice continue for the remaining LoM and that the plant not be upgraded to handle an increased proportion of fines. There may however, be the need to upgrade the slimes handling capacity to cater for future ores, for which the installation of a high rate thickener is proposed.

The LoM yield is projected at an average of approximately 84%, which is in line with recent achievements and considered to be sustainable on the projected feed ore.

Average product quality achieved in recent years is summarised in Table 6.6.

It is seen that product quality has generally been well within specification.