The importance of gold mining to South Africa
Gold mining in South Africa, from its humble beginnings in the first recorded mine in Eesterling in the Northern Province in 1871 to its pre-eminence as the largest gold mining industry in the world, has played a significant role in the economic development of the country over the past 120 years. Through gold mining, many towns and cities have come into being. Notable examples include Johannesburg, Welkom, Orkney, Springs, Benoni, Witbank and Klerksdorp. Much of the infrastructural development of roads, electricity generation, water reticulation, telecommunications, housing and the development of industry to provide the inputs to the gold mining industry have resulted directly from gold mining.
In the SA gold sector, 2006 was characterised by higher prices, consolidation and a focus on costs and productivity. Local gold production fell by 7.5% to 275 tons in 2006, but this rate of decline was just over half the rate of decline in 2005. Nevertheless gold production, at 275 tons in 2006, was the lowest level of production since 1922.
Despite a further 7.5% decline in production in 2006, the sector remained the world’s largest single producer and accounted for 11.2% of total global production. This top position could be lost within the next few years, as other large-scale producers in the form of China, could overtake South Africa. The gold mining sector remains critical to the South African economy. In 2006, the sector accounted for R36.7-billion or 8.4% of South Africa’s total merchandise exports, about 1.1% of GDP.
2006 saw the welcome improvement in the rand/gold price with the spot price rising by 44.6% to R131 323/kg as a result of the 35.8% improvement in the US$ gold price and the 6.4% depreciation in the rand exchange rate. The improvement in the rand price helped provide the resources for an improvement in capital expenditure. Capital expenditure increased by 61.9% to R5.9-billion in 2006, which bodes well for sustaining future production.
The 36% acceleration in the US$ gold price in 2006 was on the back of good market fundamentals, a decline in new mine supply, continued de-hedging by mining companies, strong investor demand and a continued weakness in central bank gold sales.
The US$ gold price, in averaging US$604 per ounce in 2006, was the second highest annual price level ever after the 1980s level of US$615 per ounce. The 36% acceleration in the US$ gold price in 2006 represented a fairly significant improvement in the price on the basis of good market fundamentals, a decline in new mine supply, continued dehedging by mining companies, strong investor demand and a continued weakness in central bank gold sales. Swings in speculator interest were the discerning factors in relation to short-term moves while jewellery demand was clearly a price taker and fell sharply in earlier 2006, before recovery in the latter part of the year.
According to GFMS, total new mine production fell by 3.1% to 2 471 tons. While old gold scrap was at its highest level in eight years at 1 108 tons, official sector sales were the lowest level in nine years at 328 ton. Total supply in 2006 fell 5% to 3906 tons mainly as a result of the decline in official sector sales.
The decline in global production in 2006 resulted in the posting of a 10-year low in terms of the actual physical volume of production. Higher prices translated into lower demand specifically for jewellery. Total demand for gold fell by 5% mainly as a result of the 427 ton decline in jewellery off take, despite the rise in gold mine producer de-hedging which rose by 287 tons.
Implied investment demand remained reasonably robust at 388 tons while coin demand rose by 16%. The balance between the fabrication demand for gold and new mine supply has remained in deficit since 1988 and this has meant that 835 tons a year of above surface gold holdings have been added to the supply equation to satisfy demand. The gradual right sizing of the official sector above ground stocks will continue to support the gold market. The continued reduction in the mining sector’s propensity to hedge forward, when combined with increased research into possible industrial uses for gold and better marketing of gold jewellery, will also support the positive market dynamics.
A combination of the still relatively strong rand exchange rate and the depth of operations and various challenges related to the inputs from supply industries meant that pressure on costs remained.
Pressure on costs is not a unique gold mining phenomena and has been identified as one of the key challenges facing the entire mining sector. In 2006, cash production costs increased by 7.4% to R78 447 per kilogram and total production costs per kilogram (excluding capital expenditure) rose by 11.9% to R99 725 per kilogram. Pressure on areas where either import parity pricing or administered pricing are in play, such as steel prices, water prices and electricity prices, translated through into the pressure on pricing that the industry experienced. Diminishing gold production also impacted on unit production costs as certain fixed costs still had to be met.
While some of the pressure that the gold mining industry had been under between 2004 and 2005 had begun abating into 2006, the number of people employed in this sector at 159 984 was a modest 0.4% down on the comparable figure of 2005. R12.8-billion was paid in the form of earnings to workers in 2006 meaning that gold mining retained its leading share in terms of contribution to earnings of employees in the mining sector.
Annual rate of growth (decline) in gold production from key countries
The importance of gold to South Africa’s economy
The South African mining industry has been the mainstay of the South African economy for over a century. Gold and diamonds are the two highly valued commodities which were largely instrumental in the development of the country’s infrastructure and the establishment of secondary industry during the first half of the twentieth century.
With the stabilisation in world mine production and central bank sales, the prospect of any major sources of new supply of gold have diminished. The continued focus on productivity and consolidation in the South African gold mining industry will result in a decline in production - although replacement tonnage from new mines will slow the rate of decline. The fact that South Africa has become the cheapest major producer of gold on a total cost an ounce basis will be an important factor contributing to the prospects for the industry in the year ahead.
Although the relative importance of gold mining has fluctuated over the last decade with the performance of the gold price, gold mining still contributes just under 4 percent directly to GDP. Taking into consideration the indirect contribution to the economy and the multiplier effects, gold mining’s total contribution to GDP is closer to 10 percent.
Special characteristics of gold mining in South Africa
The development of the technical capacity to mine deep-level gold ore bodies has seen South Africa become a world leader in deep-level mining technology. This has led to gold mining becoming even more capital intensive. This is because of the massive capital required for ventilation, cooling, hoisting, underground tunnelling and surface processing plants, and the need to have the mines operated by large numbers of workers, particularly with respect to hard rock mining. In the case of gold mining the depth of operations has increased to levels of four kilometres below the surface in some cases.
How gold is mined in South Africa
|The Witwatersrand basin and its goldfields were laid down many millions of years ago. Geologists say that a great lake stretched 250 kilometres from the Klerksdorp area to the present Springs Nigel region east of Johannesburg. Several rivers, they believe, flowed into this lake from granite mountains to the north and south west, carrying with them sand, silt and pebbles and fine particles of gold. |
This prehistoric land was carved and shaped by these rivers, which deposited pebbles and gold in fan shaped deltas near the shores of the lake. Over aeons there were alternating periods of deposition, of activity and calm. Sometimes fast flowing rivers and streams brought rock particles from northern mountains to the lake, rolling them out across the braided (laced with gravel and sand bars) deltas, piling them up, layer upon layer. At other times the streams ran slowly and carried only sand and silt with very fine gold: Eventually the lake was silted up, filled with vast sedimentary deposits containing gold bearing reefs rather like jam layers in a cake. The mud, pebbles, sand and gold were fixed and preserved in the sedimentary deposits which became very hard rock now known as the Witwatersrand System - in places 7 500 metres thick
The earth’s formative turmoil continued and the Witwatersrand rock became warped as the nascent formations were thrusted upwards and slipped downwards, sometimes a few centimetres, sometimes thousands of metres. Then volcanic action forced up great dykes of igneous material and lava flowed across the country. Deposits of dolomite rock were laid down later when the area was covered by the sea or at least another great lake. The whole bed of Witwatersrand rock was tilted at some stage; it became like a giant saucer with one edge deeply buried and the other thrust to near the surface. It was on the up-thrust northern lip of the saucer that the Witwatersrand gold deposits were discovered in an outcrop in 1886.
Sedimentologists agree that the main deltas on the prehistoric lake, along with the river systems and their flood-plains, were the areas of greatest concentration of deposits of gold-bearing particles. Today, these ancient river deposits, now covered by kilometres of gully rock, form the rich gold-bearing reefs of South Africa’s ’golden arc’ where the main gold-mining activity has been concentrated for more than a century.
Mining the gold
South Africa’s thin but extensive gold reefs often lie several kilometres beneath the earth’s surface and usually slope through the ground at up to 20 degrees. The country’s gold mining industry has to sink the deepest mine shafts in the world sometimes close to four kilometres in depth - in order that miners can reach and extract these reefs.
It can take several years to drill, blast out and equip a shaft. Once the excavation is complete, the tremendous depth of mining still presents problems in the form of the dual hazards of high rock pressures and the great heat bound up in deep rocks.
Just like any other lift system, the shaft’s cages stop at numerous ’floors’ or ’work stations’ in mining terminology, on different levels near the shaft bottom, allowing men and materials access to various points of the sloping reef ore-body. Often the distances from the shaft are so great that trains are used to transport miners. Mines are so deep today that shaft systems frequently have to be split into two or three stages, one above the other. The lower (sub vertical) shafts have their own underground hoist systems.
Tunnels are driven from the shaft to reach the underside of the reefs, whereupon a network of access and transportation tunnels is constructed beneath the ore-body to service the mining operations. Entry is made upwards into the overhead reef layers where systematic drilling and blasting of ore take place in low roofed working areas called stopes. The stopes are planned to ensure the progressive extraction of optimum ore values over extensive areas. Broken ore is gathered and dropped down a number of short shafts, called ore-passes, often now cut mechanically with a blindhole-boring machine to a railway or trackless system below.
Cross section of a gold mine
Roof supports are placed in the void left by mining to temporarily hold up the stope roof. Mined ore is hauled away towards the main shaft and tipped down a series of inter-connecting ore-passes to gather at the foot of the main shaft. From here, large skips hoist the ore to the surface in a continuous, automated cycle.
The main functions of a working mine are augmented by a multiplicity of essential auxiliary activities. These include the use of ice and refrigeration (to chilled water used to cool down air in the mine’s depths where rock temperatures can be as high as 45’C); pumping to surface of large volumes of waste water; maintenance services; the provision of electrical power; and compressed air.
Today, with the tremendous pressure on profit margins in the gold mining industry, which is mining steadily declining grades at ever greater depths, there is more emphasis on mechanisation than ever before. Mechanisation should extend the mines’ longevity as it will enable them to mine lower-grade ore, and employ a smaller though more skilled, better-paid and, therefore, more productive work force. Among the many aspects of mechanisation which are the focus of ongoing research are technologies like trackless mining, backfilling and hydro-power.
The mechanisation techniques involving the use of trackless mining equipment - the use of tyred vehicles on roadways instead of conventional hoppers on rails in underground haulages - being introduced to South Africa’s gold mines have been adapted from base-metal mines where they have been employed for several years.
But, unlike many collieries and base metal mines, where very wide ore bodies are exploited, the reefs mined in South African gold mines - apart from exceptions like Western Areas - are narrow and generally less than a metre in thickness, This necessarily impacts on productivity. Whereas over the last decade or so South African coal mines have shown healthy productivity increases, it is more difficult to increase productivity in the narrow-reef, deeper gold mines which less readily lend themselves to mechanisation. Until recently, drilling operations always had to be carried out in such narrow stopes utilising traditional labour intensive methods with crews of workers using hand held drilling machines. Today, however, after years of stalled research until the rising labour costs of traditional gold mining methods started to make the economics of mechanisation attractive, a mechanised drilling machine has been developed that is small and mobile enough to work in such confined spaces The world’s smallest, twin-boom, hydraulic drilling machine is being used increasingly on a number of gold mines. The rig is of most use in shallow mines, since it requires a fairly large unsupported span near the face; in deep mines it would interfere with essential close face support.
Gold mines are making increased use of backfill - the filling-in of worked-out slopes with waste material which is ground into fine particles to give roof support and to reduce the heating up of ventilation air caused by rock at high temperatures. The backfill process dramatically reduces the damage caused by seismic disturbances, especially rockbursts, and should ameliorate conditions in slopes through the reduction of stress on lateral supports at the stope face.
The feasibility of hydro-power using a single energy source, high pressure water, to provide cooling and hydraulic power in deep mines, has been confirmed. A comprehensive range of hydro-powered equipment has already been developed, while a fully-water-powered rockdrill, developed by the Chamber of Mines Research Organisation (Comro) along with a number of manufacturers, came into operation in the early 1990s. Availability of hydro-power will mean the end of compressed air under-ground. Looking further into the future, it is hoped that much equipment, now electrically driven, will be run off high-pressure water.
Winning the gold
On average, only five parts per million of every ton of ore mined are actually gold. It is therefore necessary to separate the precious metal from the more than 100 million tons of ore milled each year in South Africa. This is carried out in the mine’s gold plant. Initially, ore hoisted from the mine is broken into smaller pieces by a primary crusher; secondary crushers break it down further, followed by a milling stage to produce a fine rock dust.
At this point most of the tiny particles of gold contained in the ore have been exposed. A cyanide solution chemically dissolves away the exposed gold particles and the resultant liquid follows one of two routes to recover the gold now held in solution.
In the first and older method, zinc dust is added to the gold bearing solution after unwanted rock fines have been filtered out, Zinc has the effect of taking goldís place in the cyanide solution, allowing gold to fall out and be gathered in solid form prior to smelting.
The carbon-in pulp (CIP) method, which is increasingly widely used (some 40 CIP circuits have been installed already). makes use of the tremendous physical affinity ’activated’ carbon has for gold, which it readily attracts to its surface in cyanide solution. The CIP process consists of three essential stages: absorption in which the dissolved gold in the pulp is loaded on to aerated carbon: elution, in which the gold is removed from the carbon into an alkaline cyanide solution, and electro-winning, in which the gold is removed by an electrical process from the alkaline cyanide solution and deposited on steel wool electrodes.
The carbon is then treated with acid to remove contaminants, after which the acid itself is treated. Both are then recirculated into the adsorption-elution circuit.
After smelting, which takes place on individual mines, bullion bars containing about 85 per cent gold are taken to the Rand Refinery near Johannesburg and processed to either 99,5 per cent purity (the accepted standard for coinage) or 99,9 per cent purity to meet specialised demands from certain industries.
Considerable research and development work is taking place to find more efficient gold extraction methods. The modern carbon-in leach (CIL) method pioneered and adopted by Ergo is in use at certain gold plants. At Ergo a pyrite concentrate is recovered through flotation of slurry from slimes dams and milled sand and rock waste dumps. The concentrate is acid leached for uranium recovery, roasted to produce sulphuric acid and the resulting calcine cyanide leached for gold. The flotation railings are further processed in the CIL plant for the recovery of additional gold. Certain mining operations like West Witwatersrand gold mine and the Drylands opencast project use the heap-leaching process whereby cyanide in solution with water percolates through highly leachable, heaped, low-grade ore, with the resultant dissolved gold being adsorbed on to carbon in absorption tanks. Finally, at Hartebeestfontein, the use of radiometric sorting equipment to extract low grade gold bearing ore from waste rock has yielded encouraging results
The science of gold extraction
The origins of gold in South Africa
The Thulumela archaeological project in the Kruger National Park revealed that gold has been exploited in South Africa for over 500 years for commercial and decorative purposes. It is also known that gold was actively traded between African and Arab counterparts in that period. Many surmise that the Great Zimbabwe Ruins were at the heart of this trading activity. In the seventeenth century, gold was exported in significant quantities from the ports of Sofala and Delagoa Bay.
Modern South African history records that gold was first discovered on the Witwatersrand in 1834 when Carel Kruger apparently discovered gold during a hunting expedition to the interior, and took samples back to Cape Town. A more reliable account is the discovery of gold in 1852 on the farm Paardekraal (now Krugersdorp), by English mineralogist John Henry Davis. Many believe that the subsequent discovery of gold in 1886 by the Australian digger, George Harrison, that opened up the Witwatersrand Gold Fields Limited as we know them today, was from the very same ground prospected by Davis.
Gold was also found in Natal in 1853. The first gold was found in the form of nuggets or veins and was recovered fairly easily by hand. The discovery of larger-scale alluvial deposits with fine gold grains required concentration of the material before handling. Where gold occured in conglomerates, some form of milling was necessary to separate the gold prior to concentration using strakes and vanners. Mercury amalgamation was then widely adopted as a supplement to gravity concentration. This was followed by dissolution in cyanide in conjuction with zinc precipitation. More recently, the carbon-in-pulp process has become an important method to win gold.
Gravity concentration has been practised for at least 4 000 years, allowing fine gold grains to be recovered from sand or river gravel. This was effected by washing the material in a stream of water across flat, sloping rocks, along inclined troughs, or, over animal hides pegged to the bottom of a ditch (the Golden Fleece). Using these techniques, the heavier grains of gold were retained as a concentrate whilst the lighter sand was carried away by the flowing water. The difference between the specific gravity of gold (r.d.19,3) and that of the gangue (2,6 - 2,75) resulted in a satisfactorily high degree of concentration and of extraction. (Note that gravity concentration does not use buoyancy differences but inertia differences related to relative density [r.d.].) Where gold is found in particle sizes between 600 and 30 microns, gravity concentration remains effective. Therefore, it can be applied to crushed conglomerates such as South Africa’s.
This is made problematic where ores are refractory, especially in the Barberton district in Mpumalanga. Although Witwatersrand and Free State ores are classified as non-refractory, about half the gold remaining after cyanidation of the ore, was gold locked in pyrite (r.d. 5,0). Generally, it is held that gravity concentration is unviable in treating ore grades below 10g Au/t or where a high proportion of gold is locked in sulphides. In non-refractory ores, the ore was concentrated in Johnson drums or plane tables, followed by re-dressing on endless riffle belts or shaking tables.
Amalgamation was the principal gold recovery method used in South African gold mines last century. Crude amalgamation is still practised by some artisanal gold miners in South Africa, mainly in the Mpumalanga province. Unfortunately the method is in widespread use in other African countries, with devastating consequences for human health and the environment as mercury is released into rivers during processing.
In large-scale commercial applications of amalgamation, ore was crushed and ground in stamp mills, and later in tube mills which represented a significant technological improvement. The crushed ore, as fine as talcum powder, was hydrated to form a viscous pulp that was passed over copper plates (4,57 metres long by 1,52 metres wide, 18% slope) coated on their upper surface with mercury. By virtue of its high relative density, the gold sank through the pulp into contact with the mercury, by which it was caught, and converted into amalgam. This amalgam, (a solution of gold in mercury that took the form of a thick paste) was scraped off the plates, and retorted to yield gold sponge. Despite regular scraping, hard layers of gold amalgam accumulated on the plates. This was removed by softening the amalgam with steam which allowed it to be scraped off without exposing the copper.
By 1922, amalgam plates were replaced by corduroy strakes that were positioned in the discharge streams of tube mills. Using this process, pulp was passed over a flat, inclined surface covered with strips of corduroy fabric laid across the surface at right angles to the pulp flow. The gold and other heavy constituents were trapped in the corduroy riffles. The cloths were then removed and washed off into receptacles for amalgamation followed by retorting.
Amalgamation was still retained after the introduction of cyanidation in 1890 because it allowed the recovery of as much gold as possible at an early stage in processing. However, the introduction of the tube mill forced a move away from amalgamation since the slurry was coarser and less suitable to such treatment.
|Flotation was initially ignored in South Africa because of the efficiency and effectiveness of cyanidation. In flotation, reagents are added to flotation cells through which the finely ground ore is passed. Certain reagents will cause certain minerals to float to the top in a foam, and this can be collected as a concentrate for subsequent processing. By the early 1900s, flotation was an important recovery process in the South African gold mining industry, although it was mainly confined to refractory ores such as those of the Barberton area. However, the recovery of uranium oxide from gold ores from 1952 had a significant impact on its application in South Africa. This resulted in a trend toward using flotation as a complementary process to cyanidation. On South African gold mines, gold and the gold bearing minerals are floated as they constitute the smallest component of the ore. Common practice favours high pyrite recovery.|
In 1890, JS MacArthur and the Forrest brothers of Glasgow, introduced the cyanide process to the Witwatersrand gold mines. The original process involved dissolution of the gold from the milled conglomerate by a weak cyanide solution in the presence of oxygen and its subsequent precipitation from the solution by means of zinc shavings. In modern plants, gold is precipitated out of the ’pregnant solution’ using zinc powder or absorbed onto activated charcoal. The precipitate is then filtered out of the solution, and the filtrate (which looks like black mud at that point) is then melted with fluxes at the mine to recover the gold bullion as doré bars (typically 84 per cent gold, 10 per cent silver and 6 per cent base metals), which are refined to 99.5% or 99.99% purity by Rand Refinery Limited. Gold absorbed onto charcoal is recovered by elution and submitted to the same pyrometallurgical process.
The introduction of cyanidation certainly saved the industry from stagnation and possible demise although it was not without great struggle as the Chamber of Mines (then the Witwatersrand Chamber of Mines) fought the excessive royalties demanded by MacArthur’s African Gold Recovery Company. In 1896, the Chamber challenged the validity of two MacArthur patents - No. 47 for cyanidation and No.74 for zinc precipitation. Judgement was given in favour of the Chamber on the grounds that in 1885, gold had been extracted using a solution of potassium cyanide after which it had been precipitated on zinc plates.
Recovery by means of amalgamation had declined steadily and attempts to improve recovery using chlorination proved costly and ineffective. When the first commercial cyanidation plant was established in 1890, 75% of the gold contained in amalgamation tailings was recovered, resulting in an overall recovery (by amalgamation and cyanidation) of nearly 90%. In contrast, crude amalgamation had recovered less than 60% of gold from the sulphide ores encountered underground. The introduction of frue vanners and corduroy strakes produced a concentrate from which gold was recovered by roasting and chlorinating. This process achieved an overall recovery rate of 74%.
The MacArthur method was challenged briefly from 1894 to 1898 by the Siemens-Halske electrolytic precipitation method. However, MacArthur’s patented zinc-lead couple proved more effective and less costly, and became the dominant extraction process. Counter current decantation cyanidization remains important for ore that is not amenable to heap leaching. The operating cost is much higher than heap leaching since the ore is crushed and then ground to perhaps 80% minus 200 mesh (or even finer), prior to gold recovery.
The heap leaching process involves placing ore in a stationary heap and a solvent (cyanide) percolates through the solid. The primary benefit of heap leaching is that it can be done with little reduction and hence significant cost savings.
Generally, ore of selected rock size is placed on a leach pad, which is saturated with a weak cyanide solution. The pads are constructed by laying impermeable plastic sheeting over a gently inclined, compacted graded soil. Before stacking the ore onto the leach pads, a thin, protective layer of sand is placed on the plastic as in insulator. The gold (and other metals) is then leached from the stack where they it is gathered in a recovery pond for subsequent processing. For single stage leaching, two steps are involved:
The extract is the solvent phase and the raffinate is the solid material and its adhering solution. The solute in the raffinate is in both dissolved and undissolved forms.
Heap leaching is not widely used in South Africa because of the nature of the ore bodies. A derived process for refractory ores involves bacterial leaching to oxidize sulphide minerals.
Leaching gold ores and concentrates with cyanide results in a solution containing anionic metal cyanide complexes from which gold must be recovered. The traditional method used was the separation of solution prior to gold recovery. The newer method is the carbon-in-pulp technology.
The most important method of recovery from dilute solutions arising from liquid-solid separation is zinc precipitation. The early use of zinc shavings was superseded by zinc dust from 1911 (the Merrill process) which offered a greater surface area per unit mass and required less handling than shavings. Aluminium and charcoal were also applied as precipitants but never achieved the efficiency of zinc dust.
Zinc leaching plants initially consisted of a series of gently sloping long, narrow and shallow tanks in which bundles of zinc shavings were stacked. Gold bearing slurry was introduced at the raised end of the box and pased slowly through the shavings to overflow at the lower end. To maximise contact with the zinc, five or six vertical plates were introduced. With the introduction of the zinc-lead couple in 1894, the shavings were dipped in a 5% solution of lead acetate. As gold precipitated on the zinc, the shavings would break up as some dissolved in the cyanide solution and some became heavily coated with gold. The boxes were typically cleaned once per month. The concentrates were cleaned in sulphuric acid vats to dissolve the bulk of the zinc. The insoluble residues were then smelted to produce gold bullion. The primary weakness of this process was the lock-up of significant quantities of gold.
The process was improved using zinc dust, pioneered by CW Merrill in Montana between 1907-1908 when he added zinc fume to the pregnant solution and pumped the emulsion through a filter press. The precipitated gold and excess dust remained in the filter frames while the barren solution passed through the canvas filter cloth into a storage tank for re-use in the cyanide plant.
Further improvement took place in the form of the Crowe-Merrill zinc dust precipitation process invloving the replacement of pressure-filter presses by suction-operated canvas frame filters (1,22 metres wide by 1, 83 metres high) comprising 36 or 48 leaves set radially in a circular tank. The main feature was the vacuum de-aeration tank invented by TB Crowe. This allowed dissolved oxygen in the pregnant solution to be removed just before the addition of zinc dust. The leaves of the filters were connected to a header pipe leading to the suction of a centrifugal pump. Solution gravitated to the Crowe tank, where zinc dust and lead nitrate were added continuously, and then into a circular tank in which the canvas frames were completely immersed.
To overcome the labour intensivity of the Merrill process in terms of cleaning the filters, enclosed pressure filters (mainly Stellar candle filters) were introduced in 1960 that allowed a measure of automation using back-washing. These filters were based on water clarification techniques developed during World War II.
Activated carbon (Carbon-in-pulp)
The application of activated carbons to gold recovery has its origins in the patented use of wood charcoal for the recovery of gold from chlorination leach liquors in 1880. However, its use in precious metal recovery is a small element of activated carbons’ potential and existing applications in any number of fields.
The carbon-in pulp (CIP) process, which was developed to its present form in South Africa during the 1970s, is considered to be the most significant advance in gold recovery technology in recent years. The CIP method, which is increasingly widely used (by 1995, there were 42 CIP circuits installed in South Africa). CIP makes use of the tremendous physical affinity ’activated’ carbon has for gold (it can attract 7% of its weight in gold), which it readily attracts to its surface in cyanide solution. The finely ground ore (typically about 75 µm particle size), and the slurry of fine ore and water (the ’pulp’) are treated with cyanide in large tanks that are stirred mechanically or by air-agitation. Activated carbon is used to adsorb the gold directly from the cyanided pulp which flows continually from the first vessel to the last in the series, and the carbon is transferred intermittently by pumping in the opposite (countercurrent) direction. The gold value of the pulp decreases downstream, and the gold loading on the carbon increases upstream, with the highest value in the first tank.
The CIP process consists of three essential stages: adsorption in which the dissolved gold in the pulp is loaded on to aerated carbon: elution, in which the gold is removed from the carbon into an alkaline cyanide solution, and electro-winning, in which the gold is removed by an electrical process from the alkaline cyanide solution and deposited on steel wool electrodes. The carbon is then treated with acid to remove contaminants, after which the acid itself is treated. Both are then recirculated into the adsorption-elution circuit. When the leach and absorption circuits are combined, the process is described as carbon-in leach.
|The final stage of recovery is the conversion of gold concentrate to bullion. Generally, South African mine doré contains about 10% silver and 2 to 3% of copper, iron and other base metals. The removal of the non-gold metals is achieved in refineries. As a result, smelting in mine metallurgy plants is restricted to producing a suitable quality bullion that can be accurately sampled and assayed.|
|With amalgamation, the gold amalgam was retorted in cast iron ’retort boats’ set horizontally in coal-fired furnaces. Pressed amalgam, containing 30-50% gold was heated to above the boiling point of mercury. The vapourised mercury was then distilled by passing it into a water-cooled condenser for re-use on the amalgam plates. Distillation is usually complete after 4 hours at 800°C. ’Sponge gold’ then remained in containers inside the retort cylinder. The sponge was then melted in plumbago crucibles and poured into moulds. Retorting was simple and efficient, improved only by oil and then electric furnaces instead of coal or coke firing. Handling of mercury presents a serious hazard and considerable care has to be taken to avoid any inhalation of mercury vapour or even skin contact with it.|
|With the zinc-lead couple process, insoluble lead sulphate and other impurities could be separated from the bullion only by pyrometallurgical means, namely, calcination and fusion. Calcining refers to the removal of moisture, burning off of combustible impurities and the oxidiation of base metals such as lead and zinc prior to smelting which provides higher-grade bullion and cleaner slag. Oxidiation was achieved by placing the moist filter cakes of gold slime on flat clacine trays and then placing them in ovens where they were roasted at a dull red heat for several hours. The ideal cake thickeness is 75mm with 16 hours of roasting at between 550°C and 700°C.|
|After cooling, the calcined material was mixed with a suitable flux (usually borax and sand). The mixture was then placed in plumbago crucibles that were heated in reverberatory furnaces for two to three hours at approximately 1 200°C (gold melts at 1063°C). A fluid slag developed made up of borosilicates formed from PbO and ZnO2 combined with molten sand. After fusion, the fluid slag in each crucible was poured into moulds with the gold and silver settling to the bottom and slag forming a supernatant cover. The slag and bullion were separated by tapping with a hammer. The bullion was then re-melted into bars of approximately 1 000 Troy ounces (31kg) for delivery to Rand Refinery. The reverberatory furnaces have been largely replaced by submerged arc furnaces, particularly for smelting calcined slime and melting gold sponge.|
|Flotation concentrates have to be subjected to roasting to render the gold amenable to extraction by cyanidation. This is a major factor in the Archaen sulphidic ores of the Barberton and Pilgrims Rest areas. The process has to be conducted slowly and at controlled temperatures, usually between 450 and 800°C. The roasting process has environmental implications, particularly because it produces sulphur dioxide which is used to manufacture sulphuric acid. Timothy Wood 8 October 1996|
A brief history of gold
Eternally fascinating gold
Historians believe that gold was the first metal known to man, possibly as long ago as 6000 years. It was love at first sight; ever since, man has desired gold for its sheer beauty, and for the ease with which he can make it into beautiful objects.
Because they have survived in situations where objects made of other materials have perished, gold artefacts are a source of much information about ancient civilisations. In such civilisations gold was not used only in jewellery. It gradually became a major symbol of wealth and power. It even became a standard of value, hence the expression ’as good as gold’.
Over the centuries, gold has been used as money and in modern times, it has come to be used in industry such as dentistry, computers, electronic circuits, and even in the aero-space industry.
Gold and the Egyptians
Early Egyptians associated gold with immortality. The ancient Pharaohs were believed to be sons of the sun-god, Ra. They wore great quantities of gold in life, sat on gold thrones and carried gold into their graves to assist them in their afterlives; it was a constant reminder of their immortality, wealth and position. Ancient Egypt was one of the richest gold producing areas in those days, its gold being obtained by slaves from river beds or shallow excavations, usually under wretched conditions. Here is a description of these slaves by the historian Diodorus:
Their plight was as bad as that of those who were forced to toil in the making of the pyramids. Over the years, the treasures
buried with many of the Pharaohs were plundered by grave robbers. One of the most important archaeological finds was the tomb of the young Pharaoh, Tutankhamun, discovered by Howard Carter in 1922. Although the outer rooms had been plundered by thieves, the actual tomb was intact. It had remained undisturbed for more than three thousand years, since the death of the young king in the 14th century B.C. The treasures Carter found reveal the very great skills of the Egyptian goldsmiths.
A love of gold was widespread throughout the ancient Mediterranean world. The civilisations of Crete, Persia, Rome and Byzantium, as well as those of the Nordic peoples as far apart as Scandinavia and Siberia, produced gold and valued it highly.
Gold in the Americas
After the fall of the Roman Empire, gold became scarce in Europe until the discovery of America in 1492 by Christopher Columbus. This raised hopes in the courts of Europe that the newly found continent would yield vast riches, including gold.
In the 16th century, the Spanish explorers, Cortes and Pizarro, landed respectively in Mexico and Peru where gold was found to be used lavishly.
Cortes made war on the Emperor Montezuma and seized the vast Aztec treasure. This sad episode was repeated in Peru where Pizarro captured the Emperor Atahualpa. The Inca king promised to fill his prison cell with gold in return for his freedom, but despite his fulfilling this promise, Pizarro had him killed. The story of the Inca civilisation and the role gold played in it is a fascinating one. The Peruvians were highly sophisticated. They had royal gardens of exotic plants, and their capital, Cuzco, contained a Temple of the Sun considered one of the marvels of its age. It can still be visited, and here is the impression it left on a Spanish contemporary:
Unfortunately for posterity, Pizarro had all the beautiful gold objects melted down and shipped back to Spain. For a period, South America became the chief source of gold in Europe, where the metal from Aztec and Inca treasures was refashioned into objects of adornment for palaces and churches, as well as magnificent jewellery for European kings and queens.
However, the quest for gold continued. The search for Eldorado, the legendary land where gold was assumed to be as common as sand, was to lead to the settlement of new continents.
A modern Eldorado: recent gold rushes
Most of the gold found up until the last century was alluvial, i.e. gold found in river beds. Panning for such gold was relatively easy. Using a pick, shovel and pan, a prospector would dig out sand and gravel from the river bed, put them into the pan and then rotate it until the water and the lighter mud and gravel spilled over the edge. Because gold is a very dense metal, it remained at the bottom of the pan.
Although gold mining in Russia began in the Urals in 1744, most of the world’s major finds occurred in the second half of the 19th century. It is interesting to note that the world output for this period was 10000 tons compared to a mere 750 tons for the entire first century after the discovery of America! Some of the most important gold discoveries of recent times are described below.
The Californian Gold Rush of ’49
|In 1849, James Marshall, a carpenter, found gold at Sutter’s Mill in the Sacramento Valley of California. Diggers from as far afield as China, Australia and, of course, Europe, swarmed to the workings and three years later, these "forty-niners", as they were called, had already dug 820 tons of gold. During the gold rush, almost half a million people were attracted to the area, and those who stayed behind developed it into a new and prosperous state.|
The Australian Gold Rush of 1851
|An unsuccessful "forty-niner", Australian Edward Hargraves, noticed a telltale similarity between the geological features of the Sacramento Valley and certain areas in Australia familiar to him. He returned to his home country, in the hope of making a gold find, which he did at Ballarat in Central Victoria in 1851.|
The Klondike Gold Rush of 1896
In 1896, gold was found on the border between Northern Canada and Alaska by an ex-sailor named George Carmack, who had married the daughter of an Indian Chief. His story of the find has been told as follows:
The Witwatersrand Find of 1886
t was South Africa that became the scene of the greatest gold find of all. However, the gold was embedded in rock and could not be recovered by simple panning. As a result the whole nature of gold mining changed, in that it could not easily be pursued by individuals, but had to be carried out by large corporations. The great mining houses of modern times were born. Still, the dream of Eldorado persists and is pursued by individual men and women.
The story of gold in South Africa
South Africa: gold country
|South Africa, with a production of about 60 per cent of the gold mined in the world today, is the world’s largest producer. Many South Africans depend on gold, which is the country’s main export and the nation’s largest single industry and second largest employer (after the agricultural sector), for their livelihood.|
The first significant discoveries of gold in South Africa were made in the then Eastern Transvaal (now Mpumalanga Province) in the 1870s. They gave rise to "gold rushes", particularly to the Pilgrim’s Rest and Barberton areas. Although gold is still mined near Barbeton today, the deposits of the metal in these two areas ultimately proved small in relation to those of the gold bearing Witwatersrand reefs.
An outcrop of these was first discovered in 1885 by an Australian handyman/prospector, George Harrison, on the Oosthuizen farm, "Langlaagte", on what are the western outskirts of present day Johannesburg.
The full extent of these reefs has emerged only gradually over the years. At present they are being mined over an arc which is about 500 kilometres in length and extends from beyond Virginia in the Free State Province, through Klerksdorp in the North West Province, Carletonville, Krugersdorp and Johannesburg (Gauteng Province) to Kinross in Mpumalanga Province. They constitute by far the largest known deposits of gold in the world and are still the source of considerably more than half of the annual world production of newly mined gold.
The most productive mines are situated in the ’West Wits’ and Free State sections of the arc (the "shoreline" of the Witwatersrand Basin), where there are a number of mines each producing more than 20 tons of gold per annum.
The Witwatersrand Basin
Experts believe that about 250 000 000 years ago a great inland sea existed in what is now the Highveld (parts of North West and Gauteng Provinces) and the Free State plains. Successive layers of conglomerate containing pebbles and gold were washed down into the sea and spread over the bottom by wave action. The gold particles subsequently settled in successive layers of pebbles along the shoreline of this sea which later silted up. Forced into rock formations by the buckling of the earth’s crust in past ages, the gold-bearing pebble layers, in turn, formed the golden reefs of the Gold Fields Limited.
The great inland sea which it is estimated was formed about
How gold is mined in South Africa
Modern prospecting culminates in the boring (drilling) of holes into the earth at selected spots in order to locate the gold reef precisely. These holes (boreholes) may be no more than centimetres in diameter and extend to depths of many thousands of metres.
When payable deposits are found, a mine is developed. After headgear and othe r equipment have been installed, a shaft is sunk to reach the areas of gold-bearing rock. Tunnels (called "cross-cuts" are then driven at various levels from this shaft, until they strike the inclined plane of the gold bearing reef.These "cross-cuts" are excavated by drilling patterns of holes at various angles into the "face" of the tunnel.These holes are then filled with explosives and the rock is blasted out.
When the cross-cuts reach the reef (the gold bearing conglomerate), other tunnels are developed along the plane of the reef. This is called ’reef development’ and exposes the payable ore for mining. The reef is mined by a process of drilling and blasting known as ’stoping’. Barren rock and reef are transported to tipping stations, dropped down a rock chute (ore pass), and hoisted up the shaft. The barren rock is sent to waste dumps and the gold-bearing ore is sent to the reduction works for processing and the recovery of gold.
Marketing of South Africa’s gold
All the South African gold mines forward the bars of crude bullion which they produce to the Rand Refinery. The Refinery processes these bars and produces from them the 400 ounce ’good delivery’ bars of at least 99.5 per cent gold content (and is able, on demand, to produce bars with a purity of 99.99 per cent). These bars are stamped with a serial number, the Refinery’s stamp and their percentage purity.
With the exception of the gold which is used for the minting of gold coins (e.g Krugerrands) and small amounts sold to the South African Mint for supply to local jewellery manufacturers and other industrial users, virtually all of the Rand Refinery’s production is sold to the South African Reserve Bank. The Bank, in turn, sells South African gold bullion on the international gold markets. The major gold market is in Zurich with other important centres in London, New York and Hong Kong.
The big companies that process precious metals are important purchasers of gold on the world’s gold markets. They supply gold, either as such or in alloyed form, and often semi-fabricated, to jewellery manufacturers, other industrial users and investors in their areas. They also channel gold throughout most nations in the Middle and Far East and South America. In this manner, gold has become an internationally traded product accepted and demanded everywhere in the world.
Gold in our daily lives
|Gold has been with man since the dawn of time and has always been highly prized. In the past, this was chiefly for its beauty but in recent years, gold has become more and more extensively used until today there is almost no one in the western world, and few in the developing world, whose lives are not somehow touched by gold each and every day.|
Characteristics of gold
Durability: The foremost characteristic of gold is its durability. Unlike other metals, it does not tarnish or corrode under normal circumstances. That is why ancient gold, like that found in tombs, and gold coins recovered from the sea bottom, gleam as brilliantly today as on the day they were made. Heat does not corrode or oxidise gold so it therefore can be used over and over. In fact, it is not unlikely that some of the gold in the gold jewellery worn today could have originally from ancient Egypt or the land of the Incas!
Malleable and ductile: The second important characteristic of gold is that it is the most malleable and ductile of all metals. It can be beaten into gold leaf so thin that one ounce of it will cover 16 square metres. The domes of the Kremlin and other Moscow buildings are
adorned with such gold leaf, as is the famous Mosque of Omar in Jerusalem from which Mohammed is said to have ascended to heaven.
Gold can be drawn into such fine wire that a gold thread drawn from one ton of gold would stretch the distance to the moon and back. Its versatility makes gold an ideal component of such diverse items as objets d’art and umbilical cords for space walkers.
A good conductor: Yet a third property of gold is that it is an exceedingly good conductor of electricity and heat. Only silver is a better electrical conductor, but it lacks gold’s resistance to tarnish and corrosion. This accounts for the widespread use of fine gold wire and thin gold films in transistors, minute computer circuits, telephone exchanges, etc. Many hundred millions metres of fine gold wire are used each year in the production of micro-circuitry.
An efficient heat reflector: A fourth characteristic of gold is its ability to reflect heat rays efficiently. This is what makes it so valuable as a heat protector in space suits and vehicles. Thus films of gold so thin that they are translucent are applied to the glass of windows in both hot and cold climates. Under hot conditions, the gold eliminates glare, reflects heat from the sun and thus reduces cooling costs. Conversely, the film reflects back heat radiating from within the buildings and so reduces heating costs. Both ways are important in helping to conserve our precious energy resources.
The colour: A fifth significant property of gold is its colour. It is one of the very few metals (copper is another) which is coloured and this is another reason, in addition to its resistance to tarnish and its malleability, for its extensive use in jewellery.
A good mixer: Gold readily mixes with a wide range of other metals to form alloys. Many of these have exciting properties which are very useful throughout industry. For example, gold alloys form the perforated plates through which many of the synthetic fibres for fabrics and clothing are extruded. Again, many of the instruments used in research and for industrial control use gold alloys. Thus gold alloys are used in the temperature recorders of jet engines.
Nevertheless, the most important alloys of gold remain those in use in jewellery and dentistry.
Gold in jewellery
Gold in its pure state is soft and easily deformed, and therefore unsuitable for use in jewellery without some modification of its properties. Such modification can be achieved by mixing or alloying it with other metals. The extent to which pure gold is alloyed or mixed with other metals is controlled, in most countries by tradition, but by law in others. For this purpose, the gold content of gold alloys is expressed in terms of either caratage or fineness.
Caratage indicates the number of parts of gold present in each 24 parts of alloy; the fineness indicates the number of parts of gold present in each 1000 parts of alloy. The relationship between the caratage, fineness and percentage of gold in common gold jewellery alloys is shown in the following
Some countries require all gold jewellery to be stamped with its caratage before being sold. A designated organisation does the assaying and hallmarking. Purchasers are thereby assured of the caratage of their jewellery. In countries where there is no such compulsory marking, manufacturers usually stamp their products with either caratage or fineness. It is to the buyer’s advantage to buy only gold jewellery with such a stamp or hallmark. As to the relative merits of the different caratages, here is a rule of thumb comparison:
The metals mixed with pure gold in the making of carat gold jewellery alloys are copper, silver, nickel, palladium and zinc. The proportions in which they are added to the gold determine both the properties and the colours of the final alloys.
Much jewellery is still manufactured today using classic techniques dating back centuries. Thus many rings worn today are cast using the "lost wax" method which has been known for at least 4000 years.
In 1982, the jewellery industry consumed almost 65 percent of the total world gold supply, while since 1990 the demand for gold for jewellery manufacture has every year outstripped the supply of gold by a considerable margin. Although gold is not the only available material for the manufacture of jewellery, it is the only material known to us which possesses the four characteristics of lustrous beauty, virtual indestructibility, extreme rarity and easy workability - a magical combination that seems to satisfy some deep inner desire in the human psyche.
Gold in dentistry
Gold was used at least 2000 years ago in historical dental practice; the Etruscans used to wire loose teeth together with strips of gold, and the ancient Greeks and Romans used gold in their crude prostheses.
It was not until 1907 that the "lost wax" casting process was applied to dentistry by an American dentist, W.H. Taggert. In this technique, a crown or inlay is first shaped from wax and then surrounded by a heat resistant plaster. After the plaster has hardened, it is heated so that the wax melts and runs out. Molten gold alloy is then poured into the resulting cavity. On cooling, the plaster is broken open, leaving an exact duplicate in gold alloy of the model wax restoration. Gold crowns, inlays and other dental restorations may be made in this manner. The properties of the gold are modified to suit the various applications by alloying it with other metals.
More than 60 tons of gold are used every year by American dentists alone.
The properties which make high quality gold alloys so popular for dental work are their non-tarnishing and non-corrosive qualities. They have no injurious effect on gums and other tissues. They take a high polish, are malleable and can easily be shaped without breaking. This last quality also ensures a tight fit around the edges of the cavity, sealing it perfectly. Additionally, gold alloys are available which do not fracture under the powerful pressures of biting. And of course, gold is everlasting.
Today’s standardised gold casting alloys come in different hardnesses, melting points and expansions. The dental technician has therefore a variety of materials with which to work. And although the price of gold is itself usually high, the main costs in dentistry are labour and services, not the metal. In the average crown, one twentieth of an ounce or less is gold.
Gold usage in dentistry is almost as old as its usage in jewellery, with the result that there is a long experience of the performance and bio-compatibility of gold alloys in dentistry. This is not the case with many of the base-metal alloys which have been introduced into dentistry in recent years.
Gold and money
|Gold, that lustrous metal of kings, has not always been available to the common man. Somewhere around 700 B.C., however, kings began to stamp gold into coins, and in that form they became more widely acceptable. Gold coins became available to the soldier, the merchant and to families who had never possessed a gold bracelet or necklace. They were zealously hoarded against bad times.|
Assets and coins
Because gold is regarded as a valuable asset throughout the world, it enjoys international confidence as a store of value and in many places, people hold at least part of their wealth in gold rather than other investments. This is particularly true of the Middle and Far East where gold jewellery and coins constitute a major medium of investment.
In the western world too, many investors are putting a portion of their wealth into gold. Gold coins are a popular modern means of investment and perhaps the best known gold coin of all is the Krugerrand which is a fascinating story on its own.
Some other uses of gold
There are a multitude of other uses to which gold is put in the modern world. It is used extensively in the electrical, electronics and communications industries. Engineers and architects use it, both for ornamental applications and for its heat resistant and reflecting qualities.
For decorative purposes, the technique of electroplating is the one most widely used. Coating articles with gold or gold alloys is done either from cyanide or sulphite baths. A significant quantity of gold is converted each year to potassium gold cyanide, the form in which gold is most extensively used in electroplating baths. Mechanical cladding is also used on a large scale to apply gold alloy coatings to other metals. The resulting products are called rolled gold, doublé or gold-filled. Gold-filled sheet, wire and tubing are the raw materials for making such final products as pens, lighters, spectacle frames, etc.
In medicine, gold compounds have been used in the treatment of rheumatoid arthritis since the 1930s. Implants of gold grains containing a radioactive isotope of gold are used as a source of gamma or X-ray radiation for the treatment of malignant growths.
Gold is an essential component in a wide variety of scientific and technical instruments because, for one thing, its stability guarantees that the instruments will not deteriorate in storage or use. Thus gold-palladium alloy wires are used to measure high temperatures in devices known as thermocouples, which are used extensively in aircraft engines for measuring temperatures up to 1 000 degrees C.
Instruments used in geochemical prospecting (the search for ore deposits) capitalise on yet another of gold’s unique properties - its ability to absorb mercury vapour from the air. As traces of mercury are often found in association with ores such as copper, nickel, zinc and lead, the presence of mercury vapour ’halos’ in the soil or atmosphere can act as a pointer to the possible existence of an underlying ore deposit. The same principle is utilised in monitoring devices that have been developed for wear by people who work in areas possibly contaminated by mercury in the air.
In the ceramic industry gold is used to obtain pink and reddish-purple shades in glazes and enamels for chinaware. Ruby colours in glass are also achieved by the use of gold.
Although only small quantities of gold are used in photography, it is of importance. Gold salts are added in small amounts of silver bromide emulsions used in black and white and colour films, increasing their sensitivity and usability in ’available light’ photography. Gold is also used in phosphors which generate the colours on the screens of television sets. It has been estimated that between 15 and 25 percent of all the gold produced each year is used for industrial purposes.
|A thin, almost invisible film of gold, when applied to the faceplate of a firefighter’s mask, reflects infrared light (the part of the spectrum we sense as heat) but lets the visible part pass through. This protects the firefighter’s face from the heat of the flames while allowing him to see clearly.|
The first American to walk in space was attached to his spacecraft by an umbilical cord coated with gold to protect it from the radiant heat of the sun. Other space applications abound, such as the high energy radiation shields on space suits, capsules, rocket engines, helmets, visors and tether lines to protect men and equipment from the sun’s heat, rocket blasts and re-entry. Additionally, gold’s low coefficient of friction makes it useful for lubrication of sliding or rolling surfaces in the vacuum environment of space where ordinary lubricants vaporise. Nearly 40 tons of gold were used in the construction of space shuttle "Columbia" in preparation for its maiden flight in April 1981.
It would seem that in addition to being so beautiful and rare, gold is also one of the most useful substances in our world and will continue to be so in the future.