Of all the methods of extracting gold & processing it from its ore, I used a few to evaluate two principal flowsheets in this case study. The flowsheets utilized operations that involved flotation,cyanidation and gravity concentration. Tests that mirror each of these unitoperations were utilized to evaluate the principal flowsheets. This page offers a comparative review of gold recovery methods:
The feed gold and silver was amenable to gravity separation andflotation. However, the subsequent cyanidation of the concentratesdid not provide any significant improvement, on average, overWhole-of-Ore CIL testing. The gravity/flotation flowsheet withcyanidation of the respective concentrates recovered between 30and 77%of the gold in feed, with an average of about 59percent across all eight lithology composites. Whole-of-ore CILextracted between 28 and 79 percent of the gold in feed, with anaverage of about 61 percent. These results would indicate there is some degree of refractory gold, which would requirealternative processing to recover.
Silver recoveries were higher using the flotation flowsheet withsubsequent cyanidation of the flotation concentrates. Recoveriesvaried between 59 and 81 percent of the silver in the feed, with anaverage of about 69 percent. Whole-of-ore cyanidation extractedbetween 29 and 47 percent, for an average of about 38 percent.
Cyanide is a lixiviant, or reagent that is used to leach, often in tanks, gold from a solid matrix and form a gold cyanide complex. The gold cyanide complex is then extracted from the pulp or slurry by adsorption onto activated carbon. CIL stands for carbon-in-leach. This is a gold extraction process called cyanidation where carbon is added to the leach tanks (or reaction vessel) so that leaching and adsorption take place in the same tanks. CIL is slightly different from another gold extraction process called CIP or carbon-in-pulp process. In the latter case leaching takes place in tanks dedicated for leaching followed by adsorption onto carbon in tanks dedicated for adsorption.
Leaching can be assimilated to the principle of preparation of tea. When we add tea in hot water, the aroma of tea will dissolve in the water and not the tea leaves. So the aromas is found in liquid form and tea leaves remains in solid form in the hot water. Thus we can separate (solid-liquid separation) tea aromas from tea leaves (filtration for example). At this stage , we have been able to extract aromas of tea from the tea leaves by changing the physical shape of the tea aromas.It is the same principle used in the leaching of gold. The gold that is in solid form in the ores turned into liquid form with cyanide in the presence of oxygen. Thats the way gold is separated from the majority of its gangue. The second step is the adsorption of gold on the surface of the activated carbon. Following a natural phenomenon known around the world (positive charges attract negative charges) Gold sticking to coal. the coal loaded with gold and some impurities is then transferred to the elution where he undergoes a cold wash which removes certain impurities and then a hot wash. The solution from the hot wash is transferred to the electrolysis where pure gold is recovered.
Carbon-in-pulp (CIP) is the sequential leach then absorption of gold from ore. During the CIP stage, pulp flows through several agitated tanks where sodium cyanide and oxygen have been added to dissolve gold into solution. In the absorption stage, this solution flows through several agitated tanks containing activated carbon. Gold absorbs onto the activated carbon, which flows countercurrent to the pulp, while screens separate the barren pulp from the gold-loaded carbon. Carbon-in leach (CIL) is a simultaneous leach and absorption process. The simultaneous leach and absorption phases of the CIL process were developed for processing gold ores that contain preg-robbing materials such as natural absorptive carbon. These reduce the gold yield by attracting gold meant for the activated carbon. Simultaneous leaching and absorption help minimize the problem.
Process Description. The first industrial CIP plant was installed inSouth Dakota. A CIP circuit utilizes the same flowsheet as an agitated leach circuit up to the point where the slurry discharges from the final agitated leach tank. At this point, the precious metal values arc recovered directly from the slurry onto granulated activated carbon in agitated CIP tanks The carbon is retained in the tanks by any one of several different types of screens through which the slurry discharges CIP circuits are typically designed with at least four tanks to prevent short-circuiting of sluny and allow sufficient retention time for recovery of all the metals. Although agitated leach tanks are used before CIP tanks, CIP and CIL circuits are considered as a separate process.
CIL circuits are similar to CIP circuits with the exception that leaching and extraction occur simultaneously in agitated leach tanks that also contain carbon and are equipped with carbon retention screens
The evaluation of agitated tank leaching verses CIP and CIL circuits is not as complex as the heap leach-agitated tank leach analysis. CIP and CIL circuits generally have lower capital and operating costs for gold ore bodies than agitated tank leach circuits. Silver ore bodies show better economics with agitated tank leach-Merrill Crowe circuits. This is because the volume of carbon that would have to be processed to recover economic levels of silver would increase the capital and operating cost of a CIP or CIL circuit above that of an agitated tank leaching/CCD/Merrill-Crowe circuit.
Counter-current leaching. leaching efficiency can be enhanced by the application of Le Chateliers principle. In summary, the lower the concentration of gold in solution, the greater the driving force for gold dissolution to occur, although in a mass transport controlled reaction it is debatable what role this plays in gold leaching. An alternative explanation for this phenomenon is the reversible adsorption of Au(I) cyanide onto ore constituents. The gold adsorption is reversed when the solution is exchanged for a lower-grade solution or when a material (such as activated carbon or suitable ion exchange resin) is introduced into the slurry, which actively competes for the Au(I) cyanide species. This effect can be exploited in practice by performing intermediate solid-liquid separation steps during leaching to remove high-grade gold solutions, and re-diluting the solids in the remaining slurry with lower-grade leach solution and/or with freshwater plusreagents. Successful applications of this principle have been used at the Pinson and Chimney Creek, Nevada (United States), and East Driefontein (South Africa) plants, and at other operations.
At many operating gold plants, an increase in gold dissolution is observed when a leach slurry is transferred from one type of process equipment to another (i.e., between leach tanks, thickeners, filters, pumps, and pipelines). This is explained by the different mixing mechanisms in the different equipment, coupled with other factors, such as changes in slurry percent solids, changes in solution composition, and the effects of pumping transfer (i.e., plug flow mixing).
Likewise, the benefits of the carbon-in-leach (CIL) process compared with leaching and carbon-in-pulp (CIP) have been clearly demonstrated both experimentally and in practice, even without the presence of interfering preg-robbing constituents in the ore. The CIL process results in improved conditions for gold dissolution as a result of the lower gold tenor, albeit at a cost of lower gold-on-carbon loading.
The process technology and equipment design are described in detail for the carbon-in-pulp process. A typical process flowsheet is given with a description of appropriate design criteria. Technical advantages and disadvantages as compared to the traditional countercurrent decantation process are discussed including some illustrative comparisons of capital and operating costs.
In the carbon-in-pulp process activated carbon is mixed with a ground ore plus water slurry in which gold and silver cyanides are dissolved. After the precious metals are adsorbed onto the carbon, the loaded carbon is separated from the pulp and stripped. The barren pulp is disposed of as tailings and the precious metals are recovered from the strip solution by electrowinning or zinc precipitation.
The carbon-in-pulp process is used to treat low grade gold and/or silver ores. The ore is first ground in a ball mill which operates in closed circuit with a cyclone or similar sizing device. This is done to produce a feed suitably sized so that subsequent leaching is rapid.
The ground ore overflowing the sizing device generally runs at 25 to 35 percent solids. First it is passed through a trash screen to remove tramp oversize, plastics, wood, and other debris. It is then thickened to a requisite 40-50 percent solids prior to leaching. This range of pulp density keeps the activated carbon suspended in the pulp and is suitable for the subsequent leaching operation.
Following leaching, the pulp flows into the carbon-in-pulp circuit which operates in counter-current fashion. Usually five stages of adsorption are employed. Traditionally, 6 by 16 mesh activated carbon is added to the number 5 carbon-in-pulp tank and advanced to the number 4 tank and so on by air lift. The pulp is advanced from the number 1 tank to number 1 tank and so on by a second series of air lifts.
One of the most important factors in the carbon-in-pulp process is the minimization of gold losses on fine carbon reporting to the tailings. To date, the most abrasion resistant plus surface-active carbon has been found to be coconut shell carbon. Normally, the carbon consumption will average between 0.1-0.02 kg carbon per tonne of ore feed.
The traditional process for gold recovery from cyanide leach liquors has been the countercurrent decantation process. Following leaching, countercurrent decantation takes place in a series of thickeners in which the leach pulp is washed by the countercurrent flow of barren solution. The classified pregnant solution is treated for gold recovery with zinc dust. The precipitate may be acid digested. It is then smelted into bullion bars. The barren solution from zinc precipitation is returned to the last thickener.
The most well known carbon-in-pulp plant remains the prototype Homestake plant in Lead, South Dakota. Operating since 1973, this facility treats 2130 tonnes of ore per day averaging 2.7 g gold per tonne. Homestake has also operated small silver carbon-in-pulp plants at Creede and Cripple Creek, Colorado since 1979.
The slowing of meat processing at some of the nations largest packing plants due to COVID-19 has spotlighted the need for smaller regional packing plants and locally owned locker facilities, according to Kim Ulmer of South Dakota. He is one of 13 investors that is pleased to be bringing a meat harvesting locker in Fort Pierre back online this May.
We saw it happen in the 80s with the pork industry where the large companies controlled the kill space and over 400,000 hog producers were wiped out. Now in the cattle industry, there are four packers that own over 85 percent of the kill space. Major corporations want producers to become employees and we cant have that, Ulmer said.
Once known as Bad River Pack, owned by Don and Mary Ward, the now updated facility is called U.S. Beef Producers and has investors from Minnesota, North Dakota and South Dakota. We are all cattle producers either cattle feeders or ranchers that have invested in this, Ulmer said.
With help from the Governors Office of Economic Development, U.S Beef Producers was able to secure a $100,000 loan at a low-interest rate. When I learned that the state was willing to help us with funding and the process of getting certified to operate it was a go from then on and I really started putting the word out, Ulmer said. I cant say enough how important it was for the state to be involved. It is important to give credit to where credit is due and Kyle Peters with Economic Development and David Bonde with Fort Pierre Economic Development have been very helpful. It is important to know this system and team in place in South Dakota really want to provide options to producers.
He said understanding and meeting the regulations in place has been the toughest challenge. I can see why more cattle producers dont do this. The manual of requirements is 503 pages long. Im glad the people that wrote the manual were thorough but if at any point in this they could have offered a couple of hours of coaching, to us that would have been helpful.
The locker plant has a three-legged approach to how the facility will harvest and market the cattle. First, the locker will be providing a custom harvest for cattle producers wanting to butcher their own livestock. Second, the initial investors and other cattle producers wanting to harvest and market their beef can do so under the U.S. Beef Producers label direct to households. Third, the U.S. Beef Producers will be pursuing supplying their country of origin, source-verified labeled beef in grocery stores. Currently, the facility is looking for cattle producers in each South Dakota county to act as retail sale reps to help get U.S. Beef Producer labeled meat in stores, restaurants and schools.
This year has proven that the big four meatpackers cant continue to handle everything. We want to take this model of small, clean processing plants that are producer-owned and duplicate them all over the country. This is going to be producer to plate. Traditionally getting meat to the retail level has been a five-step process. Now it is less with cattlemen and women able to harvest cattle and market them direct, Ulmer said.
For thousands of years the word gold has connoted something of beauty or value. These images are derived from two properties of gold, its colour and its chemical stability. The colour of gold is due to the electronic structure of the gold atom, which absorbs electromagnetic radiation with wavelengths less than 5600 angstroms but reflects wavelengths greater than 5600 angstromsthe wavelength of yellow light. Golds chemical stability is based on the relative instability of the compounds that it forms with oxygen and watera characteristic that allows gold to be refined from less noble metals by oxidizing the other metals and then separating them from the molten gold as a dross. However, gold is readily dissolved in a number of solvents, including oxidizing solutions of hydrochloric acid and dilute solutions of sodium cyanide. Gold readily dissolves in these solvents because of the formation of complex ions that are very stable.
Gold (Au) melts at a temperature of 1,064 C (1,947 F). Its relatively high density (19.3 grams per cubic centimetre) has made it amenable to recovery by placer mining and gravity concentration techniques. With a face-centred cubic crystal structure, it is characterized by a softness or malleability that lends itself to being shaped into intricate structures without sophisticated metalworking equipment. This in turn has led to its application, from earliest times, to the fabrication of jewelry and decorative items.
The history of gold extends back at least 6,000 years, the earliest identifiable, realistically dated finds having been made in Egypt and Mesopotamia c. 4000 bc. The earliest major find was located on the Bulgarian shores of the Black Sea near the present city of Varna. By 3000 bc gold rings were used as a method of payment. Until the time of Christ, Egypt remained the centre of gold production. Gold was, however, also found in India, Ireland, Gaul, and the Iberian Peninsula. With the exception of coinage, virtually all uses of the metal were decorativee.g., for weapons, goblets, jewelry, and statuary.
Egyptian wall reliefs from 2300 bc show gold in various stages of refining and mechanical working. During these ancient times, gold was mined from alluvial placersthat is, particles of elemental gold found in river sands. The gold was concentrated by washing away the lighter river sands with water, leaving behind the dense gold particles, which could then be further concentrated by melting. By 2000 bc the process of purifying gold-silver alloys with salt to remove the silver was developed. The mining of alluvial deposits and, later, lode or vein deposits required crushing prior to gold extraction, and this consumed immense amounts of manpower. By ad 100, up to 40,000 slaves were employed in gold mining in Spain. The advent of Christianity somewhat tempered the demand for gold until about the 10th century. The technique of amalgamation, alloying with mercury to improve the recovery of gold, was discovered at about this time.
The colonization of South and Central America that began during the 16th century resulted in the mining and refining of gold in the New World before its transferal to Europe; however, the American mines were a greater source of silver than gold. During the early to mid-18th century, large gold deposits were discovered in Brazil and on the eastern slopes of the Ural Mountains in Russia. Major alluvial deposits were found in Siberia in 1840, and gold was discovered in California in 1848. The largest gold find in history is in the Witwatersrand of South Africa. Discovered in 1886, it produced 25 percent of the worlds gold by 1899 and 40 percent by 1985. The discovery of the Witwatersrand deposit coincided with the discovery of the cyanidation process, which made it possible to recover gold values that had escaped both gravity concentration and amalgamation. With E.B. Millers process of refining impure gold with chlorine gas (patented in Britain in 1867) and Emil Wohlwills electrorefining process (introduced in Hamburg, Ger., in 1878), it became possible routinely to achieve higher purities than had been allowed by fire refining.
The major ores of gold contain gold in its native form and are both exogenetic (formed at the Earths surface) and endogenetic (formed within the Earth). The best-known of the exogenetic ores is alluvial gold. Alluvial gold refers to gold found in riverbeds, streambeds, and floodplains. It is invariably elemental gold and usually made up of very fine particles. Alluvial gold deposits are formed through the weathering actions of wind, rain, and temperature change on rocks containing gold. They were the type most commonly mined in antiquity. Exogenetic gold can also exist as oxidized ore bodies that have formed under a process called secondary enrichment, in which other metallic elements and sulfides are gradually leached away, leaving behind gold and insoluble oxide minerals as surface deposits.
Endogenetic gold ores include vein and lode deposits of elemental gold in quartzite or mixtures of quartzite and various iron sulfide minerals, particularly pyrite (FeS2) and pyrrhotite (Fe1-xS). When present in sulfide ore bodies, the gold, although still elemental in form, is so finely disseminated that concentration by methods such as those applied to alluvial gold is impossible.
Native gold is the most common mineral of gold, accounting for about 80 percent of the metal in the Earths crust. It occasionally is found as nuggets as large as 12 millimetres (0.5 inch) in diameter, and on rare occasions nuggets of native gold weighing up to 50 kilograms are foundthe largest having weighed 92 kilograms. Native gold invariably contains about 0.1 to 4 percent silver. Electrum is a gold-silver alloy containing 20 to 45 percent silver. It varies from pale yellow to silver white in colour and is usually associated with silver sulfide mineral deposits.
Gold also forms minerals with the element tellurium; the most common of these are calaverite (AuTe2) and sylvanite (AuAgTe4). Other minerals of gold are sufficiently rare as to have little economic significance.
Of the worlds known mineral reserves of gold ore, 50 percent is found in South Africa, and most of the rest is divided among Russia, Canada, Australia, Brazil, and the United States. The largest single gold ore body in the world is in the Witwatersrand of South Africa.
It offers an unbeatable combination of innovative process technology, efficient handling, high flexibility and safety. We also offer a full suite of support services to help you maximize plant uptime, including spares delivery and installation, and remote assessment of equipment status.
The Metso Outotec Gold Refining Plant is the result of more than 30 years experience in developing robust and cost-effective solutions for precious metals recovery and refining. The process has been successfully applied worldwide in installations with annual gold refining capacities from 1 to 50 tons. The gold refining process is purely hydrometallurgical, i.e. an oxidative leaching of gold bullions or concentrates followed by selective precipitation of fine gold. In contrast to conventional pyrometallurgical chlorination of crude bullion with subsequent electrolytic refining (Miller and Wohlwill processes), our technology offers very high direct recovery, short process time, low inventory of gold bound in the process and ensures good and safe working environment. The process produces fine gold and separates valuable impurities such as platinum group metals (PGM) and silver for recovery and further refining.
The feed material received from gold cyanidation plants is high-grade gold bullion bars. Before being processed at hydrometallurgical operation the bars are melted and atomized to fine powder. This offers the benefits of a much larger contact surface area available for the chemical attack and increased reaction rate of the process. Shorter residence time allows in turn to reduce the volume of leaching reactors.
Next step of the refining process is an oxidative leaching step. Gold and platinum group metals (PGM) containing in a bullion/slime are leached in a solution of hydrochloric acid. Dissolution of the precious metals is facilitated by addition of an oxidation agent to the slurry. In contrast to gold and PGMs, silver forms a low soluble silver chloride that precipitates during oxidative leaching. The leaching operation is performed in either a glass lined or titanium reactor.
The major part of the dissolved gold is selectively precipitated by addition of a reducing agent to the solution in the precipitation reactor. The precipitate of fine gold sand is filtrated, carefully washed and then dried.
The filtrate is pumped to the second precipitation reactor where reduction of the remaining dissolved gold takes place. Gold received after the second precipitation has lower quality and is recycled to the leaching tank for refining. This way the purity of the precipitated fine gold is maintained at a high level.
The dried gold sand is melted in a furnace and cast into bars. Depending on the capacity of the Gold Refinery installation, casting of 400 troy ounces (~12 kg) fine gold bars is done either manually by using a removable crucible and a casting table or by using a Metso Outotec Gold Bar Casting Wheel. The gold bars are then cooled in a water bath. Alternatively, gold granules can be produced.
The off-gases from the leaching and precipitation reactors are scrubbed in a gas cleaning system to absorb the fumes of acidic gases. The gas cleaning system consists of a washing tower, a condenser, and a circulation tank. The treatment of the off-gases also ensures the high recovery of precious metals.
The filtrate received after PGMs recovery undergoes an additional treatment step to ensure an essentially precious metals free solution. The solution received from the recovery of the precious metals is sent to the wastewater treatment.
All the features of the Metso Outotec Gold Refining Solutions are designed to increase the efficiency of the process and the amount of gold recovered, decrease manual work and improve working environment. The modular design makes it easy to scale the solution for your desired capacity. Metso Outotec offers comprehensive technologies and equipment packages for the whole production chain from the mine to refined metal. With extensive research facilities and resources, we ensure that the latest advances in your chosen technology are continuously available when needed, and when they benefit you most. The core of our design lies in the environmentally sustainable technology, which enables economically feasible silver refining technology.
Through Metso Outotec global network of service centers in more than 25 countries we provide lifecycle services from spare parts, maintenance and technical services to modernizations and O&M agreements. Our service experts provide punctual spare parts logistics maintenance and training, as well as assessment and consulting services that generate a competitive advantage for you. We can tailor services efficiently to your specific needs, when considering the necessary operational and maintenance services already from the beginning of the project. As a trusted service partner, Metso Outotec ensures an optimized operation for the entire lifetime of your plant.
Obtaining all spare parts and maintenance from the same organization as process and equipment design ensures that your plant runs optimally. We make sure that you receive only the highest quality parts, consumables and upgrades that can be seamlessly integrated into your plant.
Prominer maintains a team of senior gold processing engineers with expertise and global experience. These gold professionals are specifically in gold processing through various beneficiation technologies, for gold ore of different characteristics, such as flotation, cyanide leaching, gravity separation, etc., to achieve the processing plant of optimal and cost-efficient process designs.
Based on abundant experiences on gold mining project, Prominer helps clients to get higher yield & recovery rate with lower running cost and pays more attention on environmental protection. Prominer supplies customized solution for different types of gold ore. General processing technologies for gold ore are summarized as below:
For alluvial gold, also called sand gold, gravel gold, placer gold or river gold, gravity separation is suitable. This type of gold contains mainly free gold blended with the sand. Under this circumstance, the technology is to wash away the mud and sieve out the big size stone first with the trommel screen, and then using centrifugal concentrator, shaking table as well as gold carpet to separate the free gold from the stone sands.
CIL is mainly for processing the oxide type gold ore if the recovery rate is not high or much gold is still left by using otation and/ or gravity circuits. Slurry, containing uncovered gold from primary circuits, is pumped directly to the thickener to adjust the slurry density. Then it is pumped to leaching plant and dissolved in aerated sodium cyanide solution. The solubilized gold is simultaneously adsorbed directly into coarse granules of activated carbon, and it is called Carbon-In-Leaching process (CIL).
Heap leaching is always the first choice to process low grade ore easy to leaching. Based on the leaching test, the gold ore will be crushed to the determined particle size and then sent to the dump area. If the content of clay and solid is high, to improve the leaching efficiency, the agglomeration shall be considered. By using the cement, lime and cyanide solution, the small particles would be stuck to big lumps. It makes the cyanide solution much easier penetrating and heap more stable. After sufficient leaching, the pregnant solution will be pumped to the carbon adsorption column for catching the free gold. The barren liquid will be pumped to the cyanide solution pond for recycle usage.
The loaded carbon is treated at high temperature to elute the adsorbed gold into the solution once again. The gold-rich eluate is fed into an electrowinning circuit where gold and other metals are plated onto cathodes of steel wool. The loaded steel wool is pretreated by calcination before mixing with uxes and melting. Finally, the melt is poured into a cascade of molds where gold is separated from the slag to gold bullion.
Prominer has been devoted to mineral processing industry for decades and specializes in mineral upgrading and deep processing. With expertise in the fields of mineral project development, mining, test study, engineering, technological processing.