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Recent advances in gold refining technology at Rand Refinery
Developments in Mineral Processing, Vol. 15 Mike D. Adams (Editor) r 2005 Elsevier B.V. All rights reserved.
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Chapter 27
Recent advances in gold refining technology at Rand Refinery P.J. Mostert and P.H. Radcliffe Rand Refinery Ltd, Germiston, South Africa 1. INTRODUCTION Demands on precious-metals refineries have increased significantly in modern times. Refineries are required not only to compete efficiently in an industry with excess global capacity, but also to be environmentally compliant on a wide range of issues. While in the past, economy of scale dominated, a relatively small refinery with modern technology can now prosper. Therefore, the well-established large refineries need to constantly review their processes and update their technology, automate and computerize where advantageously applicable, and, together with their economy of scale, continue to provide the most cost effective and competitive precious-metal refining service. Refining processes and technology need to be benchmarked against the following main criteria:
Process speed Process inventory Metal-recovery rates Recirculating loads Cost Risk of theft Accurate metallurgical accounting for receipts, in process material and final product Flexibility to treat a wide range of material Ability to meet market quality requirements Compliance with environmental, health and safety standards. DOI: 10.1016/S0167-4528(05)15027-3
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While this chapter focuses on advances in the refining of gold at the Rand Refinery, much of this information applies to the other commercial refineries that collectively produce the majority of the world’s gold bullion. Further information pertaining to the refining of gold along with base metals and platinum group metals (PGMs) may be found in Chapters 35 and 36, respectively. Having successfully used and proven its evaluation process over some 85 years of operation, Rand Refinery remains convinced that the melting of gold and silver dore´ receipts in an induction furnace is imperative in order to obtain a completely representative sample of gold and silver content for analysis. Of special importance to gold refineries is the segregation in dore´ bars of the gold/silver/lead/zinc alloys produced in cyanide mills, shown in Fig. 1. The classic and earliest illustration of this is of an investigation by Matthey (1896), where one dore´ bar of 120 oz assayed 662 ppt gold at the bottom corners, and only 439 ppt at the top. This early experience confirms that only when melting, and with the excellent mixing action of an induction furnace, can a completely accurate sample for analysis be obtained. Since melting each dore´ receipt, which is mainly in the form of rough bars, is a prime requisite for evaluation, it follows that the classical Miller process (shown in Fig. 2) continues to be rated the best subsequent process for the removal of base metals, primarily due to the speed of the process and its low
Fig. 1. Dore´ bars arriving from a mine.
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Fig. 2. Refining furnace in the Miller refining circuit.
inventory requirement. The process has been continuously optimized by Rand Refinery over many years. The Miller process can produce marketable 9950 grade gold and meets the majority of the above-mentioned criteria. However, the subsequent process of electrorefining to refine 9950 grade gold to 9999 grade does not meet all the criteria and has therefore been carefully reviewed and investigated over the past 5 years. This led to the decision to purchase a high-speed silver-electrolysis plant (HSSE) that was commissioned in August 2000 and is depicted in Fig. 3. This plant was installed to prove the practicability of the process principle, with the intention, if proved to be satisfactory, of applying it to improving the gold-electrolysis operation. The electrorefining process (see also Chapter 26) is still considered the process of choice, for several reasons: electrolytic dissolution is superior to chemical dissolution for numerous
reasons, speed being the most important;
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Fig. 3. The HSSE plant.
simultaneous dissolution and deposition results in not only high speed, but
also requires much smaller solution volumes (and hence, a significantly smaller plant size) than wet-chemical refining processes, especially when the process is catering for tonnage quantities of gold; and the electrolytic refining process uses very few chemicals or reagents, while effluent arisings are almost negligible, which, in turn, practically ensures 100% gold recovery.
2. EVALUATION The accurate and precise evaluation of incoming customer dore´ and smelter material forms the first vital step in the refining process. Dore´ is delivered to the refinery by helicopter or road and is received into the refinery through a double door airlock arrangement. The dore´ is removed from the packaging and is weighed in its as received form to give a ‘wet’ weight. The dore´ is then dried in a two-stage drying process that is resistance heated to a temperature of 120 1C to dry off any entrained moisture. Following this, the dore´ is weighed on three separate scales, each accurate to 0.10 g. The mean of these three values is automatically calculated and
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Fig. 4. Pouring a gold button prior to analysis by fire assay.
recorded in the database of the metallurgical-accounting system as the official delivered weight of the customer’s dore´. Following the weighing process, the dore´ is melted in an induction furnace. Four sets of dip samples are taken from the molten metal, as shown in Fig. 4. This allows for quadruple analysis by each of two X-ray spectrophotometry streams and two fire-assay streams, i.e., a total of 16 determinations. The X-ray result is known within 12 min and this value is used to price the customer’s deposit on the same day that it was delivered. The fire-assay results are used to effect payment on the second day after delivery. This important step in the process has received a high level of attention of late, to the extent that Rand Refinery’s analytical laboratory is capable of achieving a fire-assay precision (as measured by the standard deviation from the mean) of less than 0.01% and a fire assay accuracy (as measured by the divergence from the mean) of less than 0.005%. These exacting standards were achieved as a result of the stringent two-year process of accreditation as a good-delivery referee for the London Bullion Market Association (LBMA), an accolade that Rand Refinery achieved in January 2004, placing it amongst the top five refiners in the world.
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Rand Refinery’s analytical facility is considered to be state of the art in that it is highly instrumented, utilizing X-ray spectroscopy, spark-emission spectroscopy and inductively-coupled plasma spectroscopy in addition to the traditional fire-refining techniques. This provides the ability to analyse precious metals for virtually every element on the periodic table. Material being delivered into the smelter plant also benefits from the high levels of expertise and technology in the evaluation department. The smelter treats a wide range of material including borax slag, gravity concentrates, electronic scrap and catalysts. These materials are subjected to the same high levels of sampling accuracy and assay precision as for refinery dore´. The smelter treats 3800 t of material per year which is all weighed, sampled and assayed in a purpose-designed plant ahead of the arc furnace (as described in Section 5).
3. THE HIGH-SPEED SILVER-ELECTROLYSIS PLANT OPERATION As the process is patented and Rand Refinery has signed a non-disclosure agreement with Prior Engineering AG, permission has been granted to describe the plant and process in general, while certain details and know-how are withheld. The process is based on the Moebius and Balbach Thum electrolytic processes. The main change is the use of an anode basket (see Fig. 5) for containing silver granules instead of the conventional cast-silver anodes. The design allows continuous operation by continuously separating anode sludge, or essentially high-grade gold slime, from the bottom of the anode basket. By using granules and a specific operational technique, a 10–15-fold increase of effective anode surface area is achieved compared to conventionally cast silver anodes of similar dimension. This allows cell operation at multiples of the maximum current of the conventional cell, increasing productivity per cell significantly, while, with the lower anode current density, the quality of cathode is improved to at least 9999 quality silver. In addition, the process can accommodate significantly higher levels of anode impurities than the conventional Moebius or Balbach Thum electrolytic cells, of up to 20% gold, copper, base metals or palladium, and still produce 9999 silver quality product. Impurity levels quoted are the individual maxima, as anode silver should exceed 80%. Compared to Rand Refinery’s existing 23-cell Moebius electrolytic plant with a capacity of 120 t of 9990 silver capacity per annum, the HSSE plant has a capacity of 80 t of 9999 silver capacity per annum requiring only four cells, and the plant footprint is very much smaller, as shown in Fig. 6.
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Fig. 5. Silver granules as used in the anode baskets of the HSSE plant.
The cathode design is essentially conventional with only minor changes. Silver crystals deposited on the cathode are continuously removed with mechanical scrapers, recovered from the bottom of the cell mechanically, automatically two-stage washed, and enter a centrifuge where they are pre-dried. The electrolyte silver tenor is maintained by making up with solution produced from anode-quality silver in the integrated electrolyte-recycle system (hydrolysis unit), which removes impurities from the electrolyte by precipitation under elevated temperature and pressure. This arrangement results in a zero-effluent operation and NOx emissions are almost totally eliminated, while base-metal impurities and PGMs are removed as metal oxide solids. The electrolysis, hydrolysis and dissolution units are all automatically controlled by a single computer, which, together with the continuous operation of the system, reduces labour requirements to a minimum.
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Fig. 6. An electrolytic cell in the HSSE plant.
Due to the high speed and continuous operation of the process, gold inventory when refining silver containing up to 20% gold is minimized. Also, the process is far more suitable for treating silver containing such high quantities of gold than conventional silver electrolytic processes. The Miller process is unsuitable for dore´ with a silver content higher than 50%. As a rule, all new technology is not without problems and this plant proved not to be the exception. (The first plant was installed at Western Mining Corporation’s Olympic dam expansion project, Australia, and commissioned in August 1999. However, it was decided to continue using their old plant. Another plant was installed at Ohio Precious Metals Inc., USA, and has been commissioned and operated since August 2002.) The initial use of a single transformer servicing all four cells in series resulted in silver being deposited in the silver-crystal conveyor system, causing intermittent breakdowns. However, once the supplier replaced the single transformer with a transformer serving each cell, this problem was resolved. It was learnt that a single transformer could only be used with multiple cells in series at a low-power input. The second major problem experienced was that, while design power on the cell was initially achieved, over time the power input progressively dropped due to very fine anode slime blocking the outer anode-basket diaphragm. Once the blocked outer diaphragm was replaced, design (rating) power was again achieved but again deteriorated over time.
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Fig. 7. 1,000 oz good-delivery silver bars.
This problem was resolved by changing the type of diaphragm that is used. Various improvements and design changes to the anode underflow or extraction system have also assisted in resolving this problem. Generally, it took longer than anticipated for operating and maintenance personnel to become accustomed to the operation of the new computerized plant, resulting in lower than design plant utilization being achieved. However, the mechanical and control problems have been resolved and close to 100% utilization is now achieved. A large proportion of the equipment, such as pumps and valves, has been replaced with locally manufactured equivalents, resulting in a marked improvement in spares availability, and, consequently, a much improved plant utilization. The HSSE plant is now achieving design output rate, plant utilization and product quality (Fig. 7). 4. GOLD ELECTROLYSIS The well-known Wohlwill electrolytic process has served the industry well for over 100 years. Little change or innovation has, however, occurred during this period. Although most of the process criteria are met, conformance with
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three critical elements, viz., process speed, as well as minimal process inventory and recirculating loads, is less than acceptable and needs to be improved. Rand Refinery has, since its inception, used the aqua regia-based electrolytic operation, shown in Figs. 8–10. To maximize cell production rate in order to minimize electrolyte gold inventory, the practice at Rand Refinery is to operate each electrolytic cell at well over the critical current density normally required to maintain equilibrium operating conditions. Above this critical current density, gold cathode deposition rate exceeds anode dissolution rate and electrolyte gold tenor decreases. The current practice is to make a daily addition of nitric acid to each cell to maintain electrolyte gold tenor or cell equilibrium. Although the addition of nitric acid increases the anode dissolution rate and maintains electrolyte gold tenor, the production of metallic gold sludge also occurs. Gold sludge make is in the order of 10% of anode weight charged. This has to be manually removed from the cell each day, before washing to remove entrained electrolyte, and then has to be melted, the silverchloride slag removed, cast into anodes and returned to the electrolytic cell. Investigations aimed at reducing or eliminating gold sludge make have been unsuccessful. Gold sludge make is attributed to the formation of monovalent gold as aurous chloride (AuCl) rather than the more stable acidic gold chloride (HAuCl4) or trivalent gold chloride, which ionizes as follows: þ HAuCl4 ! AuCl 4 þH
Fig. 8. Electrolytic cells used to produce 9999s purity gold.
(1)
Recent advances in gold refining technology at Rand Refinery
Fig. 9. 9999s sponge gold on a cathode plate.
Fig. 10. Electrolytic cells showing the titanium cathode plates.
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The electrolytic process is therefore: At the cathode: 3þ AuCl þ 4Cl 4 ! Au
(2)
Au3þ þ 3e ! Au
(3)
At the anode: Au ! Au3þ þ 3e
(4)
Au3þ þ 4Cl ! AuCl 4
(5)
Gold sludge is invariably found directly below the anode and in higher purity. The cathode gold current efficiencies based on trivalent cathode gold and monovalent gold sludge makes, also confirm a chemical mechanism for sludge precipitation. As the electrolyte is aqua regia, it would be expected to dissolve the metallic gold sludge arisings. However, this does not typically occur because the electrolyte aqua regia concentration is too low. The normal aqua regia reaction is as follows: HNO3 þ 3HCl ! 2H2 O þ Cl2 þ NOCl
(6)
NOCl þ H2 O ! HNO2 þ HCl
(7)
3HNO2 ! HNO3 þ 2NO þ H2 O
(8)
Thus, the liberated chlorine dissolves gold to form the trivalent auric chloride, while the nitrosyl chloride, having a boiling point of 6 1C to 8 1C, is readily hydrolysed by water to nitrous acid and hydrochloric acid (Eq. (7)). Nitrous acid, being very unstable, decomposes to nitric acid, nitric oxide and water (Eq. (8)). The total dissolution reaction is therefore the evolution of chlorine, with the formation of hydrochloric acid and nitric acid, products useful to the initial reaction (Eq. (6)), with the liberation of nitric oxide. The electrolyte aqua regia concentration and conditions are not favourable for dissolving the metallic gold sludge formed, but with the positive electrode influence or field of the anode it would appear that anode gold will chemically dissolve to restore the electrolyte gold-tenor equilibrium, due to the liberation of chlorine. However, the nitrosyl chloride produced reacts with the trivalent auric chloride in solution to form monovalent gold or aurous chloride: 2NOCl þ AuCl3 ! AuCl þ 2Cl2 þ 2NO
(9)
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Aurous chloride is, however, very unstable and readily breaks down to the stable auric chloride and metallic gold, resulting in metallic gold sludge being produced: 3AuCl ! 2Au þ AuCl3
(10)
The foregoing is the most likely explanation of how nitric acid addition to the cell allows critical anode current density to be exceeded, electrolyte goldtenor equilibrium to be maintained and of how the undesirable metallic gold sludge is formed. Testwork to reduce or eliminate gold-sludge production in the electrolytic cell has indicated that to maintain electrolyte gold tenor at higher than equilibrium current densities, the addition of gold solution is preferable to nitric acid addition to the cell, while nitrate-free gold solution further improves the operation. Rand Refinery is currently planning to install gold-dissolution equipment to allow efficient production of auric chloride solution, utilizing the hydrochloride and chlorine-gas process. While the addition of auric chloride solution to the cell will reduce gold sludge make, it will also allow operation at even higher current densities. However, these improvements will not fully optimize the gold-electrolysis operation. To further optimize the process speed, minimize gold inventory and considerably reduce circulating loads, it is planned to pilot the application of high-speed gold electrolysis (HSGE). Numerous exercises have been completed by Rand Refinery technical staff, independent consultants and another gold refiner to compare the operating cost of the electrolytic process, the classical wet-chemical process, and the more recent wet-chemical/solvent-extraction process (Feather et al., 1997). The studies confirmed that Rand Refinery’s current electrolysis was the most cost-effective process, even before conversion of the current electrolytic operation to a chloride-only operation. However, should the HSGE process prove to be successful, the improvements envisaged will make a further significant improvement. While the equipment required to produce nitrate-free gold-chloride solution will be used for improving the current operation, it is also required for the HSGE pilot-plant operation; and if piloting is successful, it will also be used for the HSGE operation. The three main criteria that conventional electrorefining does not meet are largely achieved by utilizing the Prior HSGE technique: Speed – electrolytic gold cathode is available 12 h sooner, the electrolytic
deposition rate being increased some sixfold. Inventory – the gold content of electrolyte inventory is reduced by over
80%, owing to the increased productivity per cell or more efficient electrolyte utilization.
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Circulating loads – anode stub and anode sludge recirculating loads are
eliminated. Since the application of the Prior HSSE process has now been successfully completed, it is intended to commence the piloting of the process for gold electro-refining. The advantages of the process outlined could possibly be applied to other metallurgical applications, such as the electrorefining of copper. In copper electrorefining, the productivity per cell is conventionally dictated by the maximum current density of the cathode. This is considered a limitation because when the critical current density is exceeded, three factors usually result in the equilibrium of the operation being disturbed. First, the electrolyte tenor drops and the anode is now considered to be passivated. Second, with the longer copper-cathode plating cycles (714 days), nodule growth on the cathode increases; resulting in, third, lower power efficiencies. Trials with anode baskets to contain copper granules have also been unsuccessful owing to excessive corrosion of the basket in the powerful electrolytic field environment. The Prior design of anode basket successfully obviates this corrosion problem. Maintaining electrolyte copper tenor equilibrium by supplementation with copper sulphate solution, together with mechanical removal of cathode nodules, could possibly result in the process being applied to electrolytic copper-refining operations. With the massive scale of copper electrorefining plants, it might be difficult for operators to justify conversion to the high-speed copper-electrolysis process. However, the economics for using the process for extensions or new plants should be distinctly favourable. 5. RAND REFINERY SMELTER OPERATIONS The Rand Refinery smelter receives a large variety of gold- and silverbearing material from South African and international gold producers, gold and silver processors and scrap-recovery operations. Gold, silver and PGMs are efficiently and cost-effectively recovered from these materials using pyrometallurgical processes. In 1966, the existing facilities were modernized by the installation of six basic operations – sampling, blending and sintering, blast-furnace smelting, cupellation, pan-furnace smelting and fume collection. At that time, this was considered to be state-of-the-art technology. In 1986, the sinter plant, blast furnace and cupellation units were replaced by a 2.2 MVA electric-smelting furnace and a top-blown rotary converter. This was done not only to improve efficiencies and reduce costs, but also to reduce the cost of meeting both environmental and health and safety requirements. In 1992, a new feed-blending plant was installed and the
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top-blown converter replaced by the more efficient bottom-blown oxygen converter for cupellation, depicted in Fig. 11. The twin-stream blending and continuous-sampling plant includes a single conical and three ribbon blenders together with continuous cross-belt samplers for sampling customer deposits. Considerable effort over numerous years has been put into developing reliable and accurate primary and subsampling methods for the very wide range of materials that are received. It has been found that screening out the metallics from the whole sub-sample is essential to leave a fines bulk sample that is more uniform and can subsequently be accurately split. The total metallics sample, if large, is smelted, fluxed if required and the molten sample sub-sampled or, if small, the total assayed. The metallic and fines assays are combined according to their masses to calculate the overall deposit lot analysis. Smelter recoveries of gold, silver and PGMs are high due to the twin matte and lead collection phases used. Gold, silver and PGMs report predominantly to the lead phase, while base metals report to the matte phase. Lead and matte are tapped separately (shown in Fig. 12), lead being charged to the bottom-blown oxygen converter for cupellation and matte, solidified, crushed and recirculated back to the electric furnace. Dore´ with +99%
Fig. 11. Preparing to pour dore´ from the bottom-blown oxygen converter.
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Fig. 12. Slag tapping from the arc furnace.
gold, silver and PGMs produced by cupellation is sent to the refinery. When base metals in matte reach sufficiently high values, excess matte is bled from the circuit for recovery of both base and precious metals overseas. To provide a full service to customers, activated-carbon fines receipts are incinerated in a fully autogenous, continuous, fluid-bed incinerator and the ash produced is included in the electric arc-furnace feed. This facility also provides a cost-effective service for small gold-mine operations without activated-carbon elution facilities for the recovery of gold and silver. 6. SMALL-BAR PLANT A new fully automated production facility was commissioned in 1998 to produce small bars, mostly in the 100–1,000 g range. The facility automatically weighs out gold granules to the exact bar weight. The weighed granules are induction melted to produce a full range of high-quality gold bars (Fig. 13). Bars are packed and shipped directly to customers via Rand Refinery’s vault facility at Johannesburg International Airport. This vault serves the total South African gold and platinum industry, by handling receipts from international customers, and despatching refined precious-metal products worldwide.
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Fig. 13. Kilobars after check weighing.
7. CERTIFICATION AND ACCREDITATION Rand Refinery is certified compliant with the requirements of ISO 9001 (quality) and ISO 14001 (environmental) and has recently been awarded certification under OHSAS 18001 (Heath and Safety). Furthermore, the refinery has held good delivery accreditation to the LBMA since 1921, and has recently been accredited as a good-delivery referee, as mentioned earlier. The high standard of engineering maintenance at the company was rewarded in 2003 when Rand Refinery was declared a gold medallist by the South African Maintenance Association, in addition to being the national champion company for maintenance excellence. REFERENCES Feather, A.,TMSole, K.C., Bryson, L.J., 1997. Gold refining by solvent extraction – the Minataur Process. J.S. Afr Inst. Min. Metall. 97(4), 169. Matthey, E., 1896. Proc. Roy. Soc. 1, 21.
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Peter H. Radcliffe was born in England and graduated in metallurgy at Cambridge University. His career has been almost entirely with Anglo American and Anglogold Ashanti in various operating, technical and managerial capacities. He relinquished his position Head of Metallurgy with Anglogold Ashanti to become Managing Director of Rand Refinery and has recently retired.
Phil J. Mostert graduated in chemistry from the University of Cape Town and during his subsequent career with Anglo American served on the Zambian copper belt and later as consulting metallurgist for Zimbabwe. Shortly after his return to South Africa he was appointed general manager of Ergo, and then completed his career as managing director of Rand Refinery from 1994 to 1999, where he currently consults.
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Recent advances in gold refining technology at Rand Refinery P.J. Mostert and P.H. Radcliffe Rand Refinery Ltd, Germiston, South Africa 1. INTRODUCTION Demands on precious-metals refineries have increased significantly in modern times. Refineries are required not only to compete efficiently in an industry with excess global capacity, but also to be environmentally compliant on a wide range of issues. While in the past, economy of scale dominated, a relatively small refinery with modern technology can now prosper. Therefore, the well-established large refineries need to constantly review their processes and update their technology, automate and computerize where advantageously applicable, and, together with their economy of scale, continue to provide the most cost effective and competitive precious-metal refining service. Refining processes and technology need to be benchmarked against the following main criteria:
Process speed Process inventory Metal-recovery rates Recirculating loads Cost Risk of theft Accurate metallurgical accounting for receipts, in process material and final product Flexibility to treat a wide range of material Ability to meet market quality requirements Compliance with environmental, health and safety standards. DOI: 10.1016/S0167-4528(05)15027-3
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While this chapter focuses on advances in the refining of gold at the Rand Refinery, much of this information applies to the other commercial refineries that collectively produce the majority of the world’s gold bullion. Further information pertaining to the refining of gold along with base metals and platinum group metals (PGMs) may be found in Chapters 35 and 36, respectively. Having successfully used and proven its evaluation process over some 85 years of operation, Rand Refinery remains convinced that the melting of gold and silver dore´ receipts in an induction furnace is imperative in order to obtain a completely representative sample of gold and silver content for analysis. Of special importance to gold refineries is the segregation in dore´ bars of the gold/silver/lead/zinc alloys produced in cyanide mills, shown in Fig. 1. The classic and earliest illustration of this is of an investigation by Matthey (1896), where one dore´ bar of 120 oz assayed 662 ppt gold at the bottom corners, and only 439 ppt at the top. This early experience confirms that only when melting, and with the excellent mixing action of an induction furnace, can a completely accurate sample for analysis be obtained. Since melting each dore´ receipt, which is mainly in the form of rough bars, is a prime requisite for evaluation, it follows that the classical Miller process (shown in Fig. 2) continues to be rated the best subsequent process for the removal of base metals, primarily due to the speed of the process and its low
Fig. 1. Dore´ bars arriving from a mine.
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Fig. 2. Refining furnace in the Miller refining circuit.
inventory requirement. The process has been continuously optimized by Rand Refinery over many years. The Miller process can produce marketable 9950 grade gold and meets the majority of the above-mentioned criteria. However, the subsequent process of electrorefining to refine 9950 grade gold to 9999 grade does not meet all the criteria and has therefore been carefully reviewed and investigated over the past 5 years. This led to the decision to purchase a high-speed silver-electrolysis plant (HSSE) that was commissioned in August 2000 and is depicted in Fig. 3. This plant was installed to prove the practicability of the process principle, with the intention, if proved to be satisfactory, of applying it to improving the gold-electrolysis operation. The electrorefining process (see also Chapter 26) is still considered the process of choice, for several reasons: electrolytic dissolution is superior to chemical dissolution for numerous
reasons, speed being the most important;
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Fig. 3. The HSSE plant.
simultaneous dissolution and deposition results in not only high speed, but
also requires much smaller solution volumes (and hence, a significantly smaller plant size) than wet-chemical refining processes, especially when the process is catering for tonnage quantities of gold; and the electrolytic refining process uses very few chemicals or reagents, while effluent arisings are almost negligible, which, in turn, practically ensures 100% gold recovery.
2. EVALUATION The accurate and precise evaluation of incoming customer dore´ and smelter material forms the first vital step in the refining process. Dore´ is delivered to the refinery by helicopter or road and is received into the refinery through a double door airlock arrangement. The dore´ is removed from the packaging and is weighed in its as received form to give a ‘wet’ weight. The dore´ is then dried in a two-stage drying process that is resistance heated to a temperature of 120 1C to dry off any entrained moisture. Following this, the dore´ is weighed on three separate scales, each accurate to 0.10 g. The mean of these three values is automatically calculated and
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Fig. 4. Pouring a gold button prior to analysis by fire assay.
recorded in the database of the metallurgical-accounting system as the official delivered weight of the customer’s dore´. Following the weighing process, the dore´ is melted in an induction furnace. Four sets of dip samples are taken from the molten metal, as shown in Fig. 4. This allows for quadruple analysis by each of two X-ray spectrophotometry streams and two fire-assay streams, i.e., a total of 16 determinations. The X-ray result is known within 12 min and this value is used to price the customer’s deposit on the same day that it was delivered. The fire-assay results are used to effect payment on the second day after delivery. This important step in the process has received a high level of attention of late, to the extent that Rand Refinery’s analytical laboratory is capable of achieving a fire-assay precision (as measured by the standard deviation from the mean) of less than 0.01% and a fire assay accuracy (as measured by the divergence from the mean) of less than 0.005%. These exacting standards were achieved as a result of the stringent two-year process of accreditation as a good-delivery referee for the London Bullion Market Association (LBMA), an accolade that Rand Refinery achieved in January 2004, placing it amongst the top five refiners in the world.
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Rand Refinery’s analytical facility is considered to be state of the art in that it is highly instrumented, utilizing X-ray spectroscopy, spark-emission spectroscopy and inductively-coupled plasma spectroscopy in addition to the traditional fire-refining techniques. This provides the ability to analyse precious metals for virtually every element on the periodic table. Material being delivered into the smelter plant also benefits from the high levels of expertise and technology in the evaluation department. The smelter treats a wide range of material including borax slag, gravity concentrates, electronic scrap and catalysts. These materials are subjected to the same high levels of sampling accuracy and assay precision as for refinery dore´. The smelter treats 3800 t of material per year which is all weighed, sampled and assayed in a purpose-designed plant ahead of the arc furnace (as described in Section 5).
3. THE HIGH-SPEED SILVER-ELECTROLYSIS PLANT OPERATION As the process is patented and Rand Refinery has signed a non-disclosure agreement with Prior Engineering AG, permission has been granted to describe the plant and process in general, while certain details and know-how are withheld. The process is based on the Moebius and Balbach Thum electrolytic processes. The main change is the use of an anode basket (see Fig. 5) for containing silver granules instead of the conventional cast-silver anodes. The design allows continuous operation by continuously separating anode sludge, or essentially high-grade gold slime, from the bottom of the anode basket. By using granules and a specific operational technique, a 10–15-fold increase of effective anode surface area is achieved compared to conventionally cast silver anodes of similar dimension. This allows cell operation at multiples of the maximum current of the conventional cell, increasing productivity per cell significantly, while, with the lower anode current density, the quality of cathode is improved to at least 9999 quality silver. In addition, the process can accommodate significantly higher levels of anode impurities than the conventional Moebius or Balbach Thum electrolytic cells, of up to 20% gold, copper, base metals or palladium, and still produce 9999 silver quality product. Impurity levels quoted are the individual maxima, as anode silver should exceed 80%. Compared to Rand Refinery’s existing 23-cell Moebius electrolytic plant with a capacity of 120 t of 9990 silver capacity per annum, the HSSE plant has a capacity of 80 t of 9999 silver capacity per annum requiring only four cells, and the plant footprint is very much smaller, as shown in Fig. 6.
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Fig. 5. Silver granules as used in the anode baskets of the HSSE plant.
The cathode design is essentially conventional with only minor changes. Silver crystals deposited on the cathode are continuously removed with mechanical scrapers, recovered from the bottom of the cell mechanically, automatically two-stage washed, and enter a centrifuge where they are pre-dried. The electrolyte silver tenor is maintained by making up with solution produced from anode-quality silver in the integrated electrolyte-recycle system (hydrolysis unit), which removes impurities from the electrolyte by precipitation under elevated temperature and pressure. This arrangement results in a zero-effluent operation and NOx emissions are almost totally eliminated, while base-metal impurities and PGMs are removed as metal oxide solids. The electrolysis, hydrolysis and dissolution units are all automatically controlled by a single computer, which, together with the continuous operation of the system, reduces labour requirements to a minimum.
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Fig. 6. An electrolytic cell in the HSSE plant.
Due to the high speed and continuous operation of the process, gold inventory when refining silver containing up to 20% gold is minimized. Also, the process is far more suitable for treating silver containing such high quantities of gold than conventional silver electrolytic processes. The Miller process is unsuitable for dore´ with a silver content higher than 50%. As a rule, all new technology is not without problems and this plant proved not to be the exception. (The first plant was installed at Western Mining Corporation’s Olympic dam expansion project, Australia, and commissioned in August 1999. However, it was decided to continue using their old plant. Another plant was installed at Ohio Precious Metals Inc., USA, and has been commissioned and operated since August 2002.) The initial use of a single transformer servicing all four cells in series resulted in silver being deposited in the silver-crystal conveyor system, causing intermittent breakdowns. However, once the supplier replaced the single transformer with a transformer serving each cell, this problem was resolved. It was learnt that a single transformer could only be used with multiple cells in series at a low-power input. The second major problem experienced was that, while design power on the cell was initially achieved, over time the power input progressively dropped due to very fine anode slime blocking the outer anode-basket diaphragm. Once the blocked outer diaphragm was replaced, design (rating) power was again achieved but again deteriorated over time.
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Fig. 7. 1,000 oz good-delivery silver bars.
This problem was resolved by changing the type of diaphragm that is used. Various improvements and design changes to the anode underflow or extraction system have also assisted in resolving this problem. Generally, it took longer than anticipated for operating and maintenance personnel to become accustomed to the operation of the new computerized plant, resulting in lower than design plant utilization being achieved. However, the mechanical and control problems have been resolved and close to 100% utilization is now achieved. A large proportion of the equipment, such as pumps and valves, has been replaced with locally manufactured equivalents, resulting in a marked improvement in spares availability, and, consequently, a much improved plant utilization. The HSSE plant is now achieving design output rate, plant utilization and product quality (Fig. 7). 4. GOLD ELECTROLYSIS The well-known Wohlwill electrolytic process has served the industry well for over 100 years. Little change or innovation has, however, occurred during this period. Although most of the process criteria are met, conformance with
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three critical elements, viz., process speed, as well as minimal process inventory and recirculating loads, is less than acceptable and needs to be improved. Rand Refinery has, since its inception, used the aqua regia-based electrolytic operation, shown in Figs. 8–10. To maximize cell production rate in order to minimize electrolyte gold inventory, the practice at Rand Refinery is to operate each electrolytic cell at well over the critical current density normally required to maintain equilibrium operating conditions. Above this critical current density, gold cathode deposition rate exceeds anode dissolution rate and electrolyte gold tenor decreases. The current practice is to make a daily addition of nitric acid to each cell to maintain electrolyte gold tenor or cell equilibrium. Although the addition of nitric acid increases the anode dissolution rate and maintains electrolyte gold tenor, the production of metallic gold sludge also occurs. Gold sludge make is in the order of 10% of anode weight charged. This has to be manually removed from the cell each day, before washing to remove entrained electrolyte, and then has to be melted, the silverchloride slag removed, cast into anodes and returned to the electrolytic cell. Investigations aimed at reducing or eliminating gold sludge make have been unsuccessful. Gold sludge make is attributed to the formation of monovalent gold as aurous chloride (AuCl) rather than the more stable acidic gold chloride (HAuCl4) or trivalent gold chloride, which ionizes as follows: þ HAuCl4 ! AuCl 4 þH
Fig. 8. Electrolytic cells used to produce 9999s purity gold.
(1)
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Fig. 9. 9999s sponge gold on a cathode plate.
Fig. 10. Electrolytic cells showing the titanium cathode plates.
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The electrolytic process is therefore: At the cathode: 3þ AuCl þ 4Cl 4 ! Au
(2)
Au3þ þ 3e ! Au
(3)
At the anode: Au ! Au3þ þ 3e
(4)
Au3þ þ 4Cl ! AuCl 4
(5)
Gold sludge is invariably found directly below the anode and in higher purity. The cathode gold current efficiencies based on trivalent cathode gold and monovalent gold sludge makes, also confirm a chemical mechanism for sludge precipitation. As the electrolyte is aqua regia, it would be expected to dissolve the metallic gold sludge arisings. However, this does not typically occur because the electrolyte aqua regia concentration is too low. The normal aqua regia reaction is as follows: HNO3 þ 3HCl ! 2H2 O þ Cl2 þ NOCl
(6)
NOCl þ H2 O ! HNO2 þ HCl
(7)
3HNO2 ! HNO3 þ 2NO þ H2 O
(8)
Thus, the liberated chlorine dissolves gold to form the trivalent auric chloride, while the nitrosyl chloride, having a boiling point of 6 1C to 8 1C, is readily hydrolysed by water to nitrous acid and hydrochloric acid (Eq. (7)). Nitrous acid, being very unstable, decomposes to nitric acid, nitric oxide and water (Eq. (8)). The total dissolution reaction is therefore the evolution of chlorine, with the formation of hydrochloric acid and nitric acid, products useful to the initial reaction (Eq. (6)), with the liberation of nitric oxide. The electrolyte aqua regia concentration and conditions are not favourable for dissolving the metallic gold sludge formed, but with the positive electrode influence or field of the anode it would appear that anode gold will chemically dissolve to restore the electrolyte gold-tenor equilibrium, due to the liberation of chlorine. However, the nitrosyl chloride produced reacts with the trivalent auric chloride in solution to form monovalent gold or aurous chloride: 2NOCl þ AuCl3 ! AuCl þ 2Cl2 þ 2NO
(9)
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Aurous chloride is, however, very unstable and readily breaks down to the stable auric chloride and metallic gold, resulting in metallic gold sludge being produced: 3AuCl ! 2Au þ AuCl3
(10)
The foregoing is the most likely explanation of how nitric acid addition to the cell allows critical anode current density to be exceeded, electrolyte goldtenor equilibrium to be maintained and of how the undesirable metallic gold sludge is formed. Testwork to reduce or eliminate gold-sludge production in the electrolytic cell has indicated that to maintain electrolyte gold tenor at higher than equilibrium current densities, the addition of gold solution is preferable to nitric acid addition to the cell, while nitrate-free gold solution further improves the operation. Rand Refinery is currently planning to install gold-dissolution equipment to allow efficient production of auric chloride solution, utilizing the hydrochloride and chlorine-gas process. While the addition of auric chloride solution to the cell will reduce gold sludge make, it will also allow operation at even higher current densities. However, these improvements will not fully optimize the gold-electrolysis operation. To further optimize the process speed, minimize gold inventory and considerably reduce circulating loads, it is planned to pilot the application of high-speed gold electrolysis (HSGE). Numerous exercises have been completed by Rand Refinery technical staff, independent consultants and another gold refiner to compare the operating cost of the electrolytic process, the classical wet-chemical process, and the more recent wet-chemical/solvent-extraction process (Feather et al., 1997). The studies confirmed that Rand Refinery’s current electrolysis was the most cost-effective process, even before conversion of the current electrolytic operation to a chloride-only operation. However, should the HSGE process prove to be successful, the improvements envisaged will make a further significant improvement. While the equipment required to produce nitrate-free gold-chloride solution will be used for improving the current operation, it is also required for the HSGE pilot-plant operation; and if piloting is successful, it will also be used for the HSGE operation. The three main criteria that conventional electrorefining does not meet are largely achieved by utilizing the Prior HSGE technique: Speed – electrolytic gold cathode is available 12 h sooner, the electrolytic
deposition rate being increased some sixfold. Inventory – the gold content of electrolyte inventory is reduced by over
80%, owing to the increased productivity per cell or more efficient electrolyte utilization.
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Circulating loads – anode stub and anode sludge recirculating loads are
eliminated. Since the application of the Prior HSSE process has now been successfully completed, it is intended to commence the piloting of the process for gold electro-refining. The advantages of the process outlined could possibly be applied to other metallurgical applications, such as the electrorefining of copper. In copper electrorefining, the productivity per cell is conventionally dictated by the maximum current density of the cathode. This is considered a limitation because when the critical current density is exceeded, three factors usually result in the equilibrium of the operation being disturbed. First, the electrolyte tenor drops and the anode is now considered to be passivated. Second, with the longer copper-cathode plating cycles (714 days), nodule growth on the cathode increases; resulting in, third, lower power efficiencies. Trials with anode baskets to contain copper granules have also been unsuccessful owing to excessive corrosion of the basket in the powerful electrolytic field environment. The Prior design of anode basket successfully obviates this corrosion problem. Maintaining electrolyte copper tenor equilibrium by supplementation with copper sulphate solution, together with mechanical removal of cathode nodules, could possibly result in the process being applied to electrolytic copper-refining operations. With the massive scale of copper electrorefining plants, it might be difficult for operators to justify conversion to the high-speed copper-electrolysis process. However, the economics for using the process for extensions or new plants should be distinctly favourable. 5. RAND REFINERY SMELTER OPERATIONS The Rand Refinery smelter receives a large variety of gold- and silverbearing material from South African and international gold producers, gold and silver processors and scrap-recovery operations. Gold, silver and PGMs are efficiently and cost-effectively recovered from these materials using pyrometallurgical processes. In 1966, the existing facilities were modernized by the installation of six basic operations – sampling, blending and sintering, blast-furnace smelting, cupellation, pan-furnace smelting and fume collection. At that time, this was considered to be state-of-the-art technology. In 1986, the sinter plant, blast furnace and cupellation units were replaced by a 2.2 MVA electric-smelting furnace and a top-blown rotary converter. This was done not only to improve efficiencies and reduce costs, but also to reduce the cost of meeting both environmental and health and safety requirements. In 1992, a new feed-blending plant was installed and the
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top-blown converter replaced by the more efficient bottom-blown oxygen converter for cupellation, depicted in Fig. 11. The twin-stream blending and continuous-sampling plant includes a single conical and three ribbon blenders together with continuous cross-belt samplers for sampling customer deposits. Considerable effort over numerous years has been put into developing reliable and accurate primary and subsampling methods for the very wide range of materials that are received. It has been found that screening out the metallics from the whole sub-sample is essential to leave a fines bulk sample that is more uniform and can subsequently be accurately split. The total metallics sample, if large, is smelted, fluxed if required and the molten sample sub-sampled or, if small, the total assayed. The metallic and fines assays are combined according to their masses to calculate the overall deposit lot analysis. Smelter recoveries of gold, silver and PGMs are high due to the twin matte and lead collection phases used. Gold, silver and PGMs report predominantly to the lead phase, while base metals report to the matte phase. Lead and matte are tapped separately (shown in Fig. 12), lead being charged to the bottom-blown oxygen converter for cupellation and matte, solidified, crushed and recirculated back to the electric furnace. Dore´ with +99%
Fig. 11. Preparing to pour dore´ from the bottom-blown oxygen converter.
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Fig. 12. Slag tapping from the arc furnace.
gold, silver and PGMs produced by cupellation is sent to the refinery. When base metals in matte reach sufficiently high values, excess matte is bled from the circuit for recovery of both base and precious metals overseas. To provide a full service to customers, activated-carbon fines receipts are incinerated in a fully autogenous, continuous, fluid-bed incinerator and the ash produced is included in the electric arc-furnace feed. This facility also provides a cost-effective service for small gold-mine operations without activated-carbon elution facilities for the recovery of gold and silver. 6. SMALL-BAR PLANT A new fully automated production facility was commissioned in 1998 to produce small bars, mostly in the 100–1,000 g range. The facility automatically weighs out gold granules to the exact bar weight. The weighed granules are induction melted to produce a full range of high-quality gold bars (Fig. 13). Bars are packed and shipped directly to customers via Rand Refinery’s vault facility at Johannesburg International Airport. This vault serves the total South African gold and platinum industry, by handling receipts from international customers, and despatching refined precious-metal products worldwide.
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Fig. 13. Kilobars after check weighing.
7. CERTIFICATION AND ACCREDITATION Rand Refinery is certified compliant with the requirements of ISO 9001 (quality) and ISO 14001 (environmental) and has recently been awarded certification under OHSAS 18001 (Heath and Safety). Furthermore, the refinery has held good delivery accreditation to the LBMA since 1921, and has recently been accredited as a good-delivery referee, as mentioned earlier. The high standard of engineering maintenance at the company was rewarded in 2003 when Rand Refinery was declared a gold medallist by the South African Maintenance Association, in addition to being the national champion company for maintenance excellence. REFERENCES Feather, A.,TMSole, K.C., Bryson, L.J., 1997. Gold refining by solvent extraction – the Minataur Process. J.S. Afr Inst. Min. Metall. 97(4), 169. Matthey, E., 1896. Proc. Roy. Soc. 1, 21.
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Peter H. Radcliffe was born in England and graduated in metallurgy at Cambridge University. His career has been almost entirely with Anglo American and Anglogold Ashanti in various operating, technical and managerial capacities. He relinquished his position Head of Metallurgy with Anglogold Ashanti to become Managing Director of Rand Refinery and has recently retired.
Phil J. Mostert graduated in chemistry from the University of Cape Town and during his subsequent career with Anglo American served on the Zambian copper belt and later as consulting metallurgist for Zimbabwe. Shortly after his return to South Africa he was appointed general manager of Ergo, and then completed his career as managing director of Rand Refinery from 1994 to 1999, where he currently consults.