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Each and every step in the process where there is a transfer of material requires it to be evaluated for metal accounting and process control. Thus the material needs to be sampled and accurately analysed. The methods involved are numerous and many different analytical techniques are used.

The largest number of samples are analysed by fire assay techniques. These are used for low-grade materials such as ore, flotation tailings and concentrate. For higher grades and base metals, XRF (X-ray Fluorescence) is used. While for very high-grade materials often wet chemical digestion with measurement by ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry) is the most common. The final metals are analysed using Spark-OES (Spark-Optical Emission Spectrometry).

The majority of analysis is done at five fire assay laboratories at the various mining sites. The largest laboratory is the PF Retief laboratory in Rustenburg that analyses samples from the four concentrators, the smelter in Rustenburg and Bafokeng- Rasimone. The other four fire assay labs are at Union, Amandelbult, Potgietersrus and Lebowa. There are also laboratories at the refineries. In the past each mine has had its own laboratory for mining/exploration, process control and metal accounting analysis. The high cost of these laboratories and the rapid expansion of Anglo Platinum has resulted in samples from some sites being diverted to existing laboratories.

The last laboratory is at the Anglo Platinum Research Centre (ARC) where specialised analyses to service the research efforts at the centre are performed. The research centre is also involved with method development work and is where the research on fire assay and automation was carried out.

Analysis requirements at Anglo Platinum can be broadly categorised in three:

• Mining and exploration

• Metal accounting

• Process control

For metal accounting the highest accuracy and precision is required. Whether it is mining, flotation, smelting or refining, the analysis is used to calculate the metal produced and determines the profitability of the Business Unit within Anglo Platinum. These have very serious financial implications. As an example, if a flotation plant were to produce 20,000 tons of concentrate in a month with a platinum grade of 100g.ton-1 and the uncertainty of the analysis was 2%, the financial risk of the analysis would be

R5,500,0003. Anglo Platinum would be at risk of losing over five million Rand worth of platinum based on the uncertainty of the analysis. With metal accounting, only the most accurate and precise analytical methods are used, unfortunately these are costly. A typical nickel sulphide analysis done externally4 may cost R750 per sample [19]. With up to 500 flotation concentrate samples being analysed monthly this is a cost of R375,000.

Normally with transfers within the Group, analytical uncertainty would not result in an actual financial loss. But, the newest mines like Bafokeng-Rasimone and Modikwa (and others to come) are all joint venture (JV) partnerships to meet government requirements for Black Empowerment Enterprise (BEE) [20]. In a joint venture Anglo Platinum enters into a 50% partnership with other companies. The PGE containing concentrate produced from the concentrator plant needs to be analysed and there is financial settlement done based on the value calculated from the analysis. This is no longer an internal stock transfer as a payment is made to a second party. The analysis of the samples poses a financial risk to the company and the accuracy and precision requirements for metal accounting analysis have increased.

The mined ore is also subject to analysis and because of joint ventures; the sampling campaigns are more stringent resulting in increased numbers of samples. With the on- going expansion program, to understand the ore body, more exploration samples are being produced by drilling bore holes and taking samples. The problem that has arisen is that no new laboratory facilities have been built to handle this increased workload and these analyses are being outsourced to external laboratories. There are significant cost implications to the analysis as the cost can be estimated at about R300 per sample and if a conservative estimate of 20,000 exploration and mining samples per month is used, there is a cost of R6,000,000 spent on analysis every month.

These geological and mining samples are also considered metal accounting samples and need to be analysed reasonably accurately. The results are used to calculate ore reserves and again this has financial implications in the planning and production of a mine. These figures appear in the financial report every year as a “built up” head grade as well as reserves in ounces of platinum [21].

Process control analysis requirements are quite different in that although accurate and precise analyses are required, this is not the overriding requirement. Since flotation is a time dependent process, analysis is required at regular time intervals to monitor the process. If the efficiency of the process drops and the grade for the tailings from a

3 _{Based on US$500 per oz of platinum and an exchange of 1US$=8ZAR}

4 _{An external cost is used because internal costs often do not reflect overheads taken by other sections}

bank of cells increases, adjustments can be made to stabilise the process. In theory this is all very well, but because of the low grade and complexity of the ore, the only available analytical route has been fire assay. Unfortunately, the traditional fire assay procedure is a lengthy process.

In process control a timely assay of reasonable accuracy and precision is more useful than a highly accurate analysis after a long time. Timing is important as PGE will already be lost to the tailings dam, without the chance of recovery, if the analysis is received too late. An ideal situation would be to get an assay within 2 hours as this is the approximate retention time of a bank of flotation cells. If an analysis is received within this time, losses can be potentially avoided by adjusting the process to recover the PGE.

For process control, the sample is removed as slurry by a mechanical cross-stream sampler. It is then filtered, dried and transported to the laboratory for analysis. At the laboratory the sample has to be pulverised, weighed and mixed with flux. The fluxed sample is then fused and cast while molten. Once the melt has cooled and solidified the lead collector is then mechanically separated and cupelled. During the oxidative fusion the lead is oxidised and the molten oxide is absorbed into the porous cupel. After cupellation a PGE bead called a prill remains.

The prill is removed from the cupel and placed in a block cupel and subjected to another high temperature polishing step to remove any remaining lead impurities. The final prill is removed from the block cupel and flattened to remove cupel residue and finally weighed on a microbalance. This result is reported as a 4E grade for the sample. It has been convention to correct this figure with a factor due to losses in cupellation. This factor is calculated by comparison to quantitative chemical analysis methods. This is not an ideal solution to the problem as the accuracy of the factors used is often in question.

Then the result is finally reported to the plant. This procedure is called the gravimetric or 4E method, a name derived from the fact that the prill contains the sum of the platinum, palladium, rhodium and gold from the sample.

The only reason that this method is still used is due to its historical setting, its fast turnaround time and the fact that it is relatively inexpensive. Methods using wet chemical preparation and spectroscopic analysis would take longer and be considerably more expensive. An analysis performed in triplicate in our laboratories costs around R55 [22].

With one sample being taken every shift about 1400 samples would be analysed monthly at a cost of R77,000. The turn around in analysis could be a minimum of 8 hours, however, this is not achievable in practice and typically a result will take 24 to 72 hours to be reported. The reason for this is that flotation plants work 24 hours a

day, 7 days a week, while laboratories work a 12 hour day with a staggered shift and only 5½ days a week. Samples taken on a Saturday afternoon are only reported by Tuesday morning while most other samples are reported the day after they are taken. The net result of this is that true process control is not done on these plants but rather historical plant monitoring is achieved.

Some new innovations such as XRF slurry analysers [23, 24] have been implemented on some plants. These are capable of giving rapid turnaround analysis of only a few minutes of some higher-grade streams within the plant. This has given a modicum of process control – but ideally it is necessary to quickly analyse low grade feed and tailing streams to get true process control. With good process control it is possible to improve recovery of the concentrator plant and increase platinum production. With the saving of metal any potential process control laboratory would pay for itself in a short time.