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Considerations and Potential Economic Advantages for the In-Situ Recovery of Gold from Deep, Hard-Rock Deposits,
... ISR has sometimes been referred to as 'keyhole mining': rather than removing ore from the ground, a solution is pumped underground through boreholes to dissolve the target mineral in place (hence in-situ) then the solution is pumped back to the surface where the metal (e.g. copper) is recovered (Kuhar et al., 2018a;Robinson and Kuhar, 2018). Because it is not necessary to remove the ore, there is considerably less surface disturbance compared with open pit mining, which avoids the creation of large voids, reduces dust and noise, removes the need for tailings dams and requires minimal infrastructure. ...
... Therefore, careful solution flow control and containment is required to retain that solution within the mining area and prevent leakage. ISR mining is used commercially in Australia but has not yet been used for copper (Kuhar et al., 2018a;Robinson and Kuhar, 2018). ISR technology has been around since the mid twentieth century. ...
Mining legacies are manifest in multiple ways. One of these relates to heritage values of former mining places which may contribute to regional identity and potentially tourism. Changes in mining methods and technologies make it economically feasible to re-open some mines that previously closed due to market factors, ore grades or difficulties in accessing residual mineral deposits. In this paper, we consider the factors affecting the social licence for proposed renewed mining activity in a post-mining town (Kapunda, South Australia). We examined how residents of Kapunda view the mining sector in general and how it fits with town identity. Focus groups and interviews explored the extent to which a potential renewed mining industry would align to community values, considering the possible application of copper in-situ recovery (ISR) for an historic deposit in the town. The research found that participants were open to the prospect of a new copper ISR operation, provided it was well managed, environmentally responsible, and able to align with tourism. The paper concludes that an important dimension of social licence for re-opening mines is whether new mining could occur in a way that doesn't undermine the heritage values associated with a former mining legacy.
... The conventional mining industry currently faces many challenges i.e. reduced commodity prices; declining ore grades; increased mining depth; increased capital and operating costs; and environmental risks (6) . Under the circumstance of high in-situ stress, temperature, and high pore pressure, the mining activities will cause environmental (i.e. ...
... Additionally, the extra hand pumps and oil tank may be required for pressurization/depressurization, filling up/refilling the system respectively (3,6) .The needle value accurately control the injection rate (1,7) to avoid catastrophic failure, especially for laboratory scale experiments The wellbore pressure were monitored by pressure sensors and recorded in Control Centre Series 30 software. ...
Ore deposits are expected to be permeable (e.g. porous media) to grant the success of In-situ recovery (ISR). However, the ISR from hard deposits which lack of porous media is still challenging, due to poor understanding of the fracability i.e. connectivity between pre-existing and artificial fractures. Therefore, in this study, extremely hard Foliated Silicified rock (Uniaxial compressive strength (UCS)=55kpsi, Density=2.8gr/cc, Young's Modulus (E) =15.7 million psi) was acquired from subsurface of 3900ft, meta-sedimentary deposit, Western Australia. The irregular sample was moulded into a 15cm cubic polyester resin which can fit our in-house built 1600KN TTSC (True Triaxial Stress Cell). To investigate the potential of stimulated orebody volume, we conducted three hydraulic fracturing experiments (i.e. initial fracturing and two additional refracturing) associated with reopening tests in the TTSC. The magnitude of field stress state was initially simulated as σ1=5100psi, σ2=3700psi, and σ3=2500psi but the direction was changed during refracturing tests. Meanwhile, wellbore pressure and microseismicity were simultaneously monitored and recorded. High energy CT scanning (Medical CT, 140KV, 1000MA) was conducted on the pre and tested sample. CT image was then processed to demonstrate the characterization of pre-existing cracks and induced fractures. Base on CT images (pre-tests), one pre-existing longitudinal crack nearly cross the entire rock. The breakdown pressure of 5800psi, 5600psi, and 5950psi were recorded respectively during initial fracturing and two subsequently refracturing tests. Based on the hypocenter localization of microseismic events and CT images (after tests), both longitudinal (double wings) and transverse fractures (double wings) were generated and extended to the boundary of rock sample. The induced hydraulic fractures propagated as approximately two dimensional geometry governed by minimum principle stress. Furthermore, no secondary branch of fracture was observed from microseismicity and CT images, which indicated that the complexity of fracture network is low. Therefore, we elucidate that the stimulated volume of Foliated Silicified rock is limited and hydraulic conductivity can only be partly improved by the main fractures.The good correlation between hypocentre localization of microseismic events and CT images of induced fractures illuminate that the field engineers can infer the geometry of induced fractures based on microseismicity, which is essential for recovery well placement and up-scaled modelling in such type of ore reservoir.
... Established mining technologies are identified based on the current and well established history of their use over a long time period. Emerging technologies in this study are those that have been used over a comparatively shorter time period in Australia and there are existing commitments or research to explore new or extended applications of these technologies 1 (Finkel et al., 2017;Kuhar et al., 2018). It should be noted, the emerging technologies in this study are proven technologies that have been used in Australia over several decades. ...
... excavation is not required. In situ leach mining is currently used to recover uranium in Australia and there is potential to apply this technique to gold and copper mining (Kuhar et al., 2018;Martens et al., 2012). ...
Technology plays a central role in mining activities throughout Australia and is critical to achieving greater economic and environmental sustainability. Our choices about which technologies to develop, adopt and deploy in the landscape reflect one of the most critical interfaces between mining and society. There have been numerous reviews and studies of the social licence of the mining industry, which have examined the way in which public perceptions influence the broad acceptance and approval of mining activities. However, very few studies have examined public perceptions of the technologies and extractive methods used by the mining industry. This paper therefore contributes to expand the scope of this mining-society scholarship by understanding the drivers that shape public perceptions in relation to established and emerging mining technologies. We present findings from a survey of Australian citizens (N=476) that tested their general awareness of and response to different types of mining technologies and extraction methods that are currently in use. These included comparisons between three broad methods of resource extraction technologies including open cut, underground and in situ leach mining. Hydraulic fracturing, a technology that is used in conjunction with some forms of resource extraction, was also included. In this paper, we examine the relationships between the public’s self-rated knowledge of these four mining technologies, their perceptions of the environmental and safety impacts of those technologies, and their level of acceptance of each mining technology using descriptive statistics and path analysis. Our research found that higher levels of overall acceptance were expressed for established technologies such as open cut and underground mining. However, our results also reveal a nuanced role for the type of knowledge that citizens may have about novel and emerging technologies in determining their acceptance of these technologies.
... In 1994, the Gagarskoye gold deposit was developed using ISR in the Ural Mountains region (Russia) (Seredkin et al. 2016). Despite examples of prior application of ISR to gold deposits, many gold deposits remain uneconomical to process using ISR because the gold is inaccessible (located in hard rock), which yields low metal recoveries (Kuhar et al. 2018). ...
Rising mining costs and fewer high-grade ore deposits have necessitated a search for alternative methods for the recovery of metals from deposits that are no longer economically or environmentally exploitable by conventional mining. One such alternative method is in-situ recovery (ISR). Although ISR has typically been used for mining uranium ores that are not economic to mine with conventional methods, it has also been used less frequently for the treatment of other low permeability rocks, such as hard rocks containing copper, nickel, and gold. The reason for the limited uptake of the technology for hard rock mineralization is primarily due to the low natural rock porosity and permeability and hence limited ability of a lixiviant to permeate the rock and contact the minerals. This review focuses on three novel potential access creation methods: microwaves, high-voltage pulses, and cryogenic fracturing procedures. Most research studies of microwaves and high voltage pulses have focused on energy savings in comminution, but little research has been done on their application of in ISR. In situ cryogenic fracturing has been used in the petroleum industry and may be a potential novel option for use for the in situ recovery of minerals in the mining industry. The aim of this review is to summarize available information on these three methods for increasing the permeability of hard rocks and thereby improving the rate of lixiviant-mineral contact and mass transfer in in-situ recovery. The review will start with an overview of considerations for the use of ISR. The mechanisms of microwave, high-voltage pulse, and cryogenic fracturing methods will then be discussed.
... Three methods are discussed. Firstly, In situ recovery is well known and used for uranium (Sarangi & Beri, 2000), copper, and gold (Kuhar, Breuer, Robinson, & McFarlane, 2015;Kuhar, Breuer, Haque, & Robinson, 2018;Sinclair & Thompson, 2015) and other minerals (Seredkin, Zabolotsky, & Jeffress, 2016). The novelty is provided by recent advances in the lixiviants that are required to extract the ore. ...
... This mining technique, the first evidence of which dates back to Roman antiquity, is also used to extract evaporites (potash, natron, and halite). In addition to uranium, ISR is also used for other metals such as copper and gold (Kuhar et al., 2018;Martens et al., 2012;Seredkin et al., 2016;Sinclair and Thompson, 2015). This mining method consists of dissolving the uranium contained in permeable, mainly sandstone, geological formations "in situ" using leaching solutions. ...
This article presents the results of groundwater monitoring over a period of six years and the interpretation of these results by a reactive transport model, following an In Situ Recovery (ISR) test on the Dulaan Uul uranium deposit in Mongolia. An environmental monitoring survey was set up using 17 piezometers, from which it has been possible to describe the changes in the water composition before, during and after the ISR test. The water quality before the start of mining activities rendered it unfit for human consumption. During and after the test, a descent of the saline plume was observed, resulting in a dilution of the injection solutions. After a rapid decrease to pH = 1.13 during the production phase of the ISR test, the pH stabilized at around 4 in the production area and 5.5 below the production cell one year after the end of the test. Uranium and radium were being naturally attenuated. Uranium returned to background concentrations (0.3 mg/L) after two years and the measured 226Ra concentrations represent no more than 10% of the expected concentrations during production (75 Bq/L). The modeling of the contaminants of concern mobility, namely pH and concentrations of sulfate, uranium and 226Ra, is based on several key complementary mechanisms: density flow, cation exchange with clay minerals and co-precipitation of 226Ra in the barite. The modeling results show that the observed plume descent and sulfate dilution can only be predicted if consideration of a high-density flow is included. Similarly, the changes in pH and 226Ra concentration are only correctly predicted when the cationic exchanges with the clays and the co-precipitation reaction within the barite using the solid solution theory are integrated into the models. Finally, the proper representation of the changes in water composition at the scale of the test requires the use of a sufficiently fine mesh (1m x 1m cell) to take into account the spatial variability of hydrogeological (permeability distribution in particular) and geological (reduced, oxidized and mineralized facies distributions) parameters.
... In-situ recovery or ISR (also referred to as in-situ leaching) is an environmentally friendly approach used for the recovery of valuable metals [such as uranium (U) and copper (Cu)] and minerals [such as halite (NaCl), potash (such as potassium hydroxide, carbonate, chlorate, chloride, nitrate, sulphate and permanganate), boron and magnesium minerals] from ore deposits by the circulation of a fluid underground (Bartlett, 1998;Kuhar et al., 2018). ISR is considered beneficial for many reasons: a minimal surface disturbance, a lack of de-watering of the groundwater system -above or around the deposit-and a minimum distortion of the hydrological/environmental system (Borch et al., 2012). ...
A uranium-mineralized sandy aquifer, planned for mining by means of uranium in situ recovery (U ISR), harbors a reservoir of bacterial life that may influence the biogeochemical cycles surrounding uranium roll-front deposits. Since microorganisms play an important role at all stages of U ISR, a better knowledge of the resident bacteria before any ISR actuations is essential to face environmental quality assessment. The focus here was on the characterization of bacteria residing in an aquifer surrounding a uranium roll-front deposit that forms part of an ISR facility project at Zoovch Ovoo (Mongolia). Water samples were collected following the natural redox zona-tion inherited in the native aquifer, including the mineralized orebody, as well as compartments located both upstream (oxidized waters) and downstream (reduced waters) of this area. An imposed chemical zonation for all sensitive redox elements through the roll-front system was observed. In addition, high-throughput sequenc-ing data showed that the bacterial community structure was shaped by the redox gradient and oxygen availability. Several interesting bacteria were identified, including sulphate-reducing (e.g. Desulfovibrio, Nitrospira), iron-reducing (e.g. Gallionella, Sideroxydans), iron-oxidizing (e.g. Rhodobacter, Albidiferax, Ferribacterium), and nitrate reducing bacteria (e.g. Pseudomonas, Aquabacterium), which may also be involved in metal reduction (e.g. Desulfovibrio, Ferribacterium, Pseudomonas, Albidiferax, Caulobacter, Zooglea). Canonical correspondence analysis (CCA) and co-occurrence patterns confirmed strong correlations among the bacterial genera, suggesting either shared/preferred environmental conditions or the performance of similar/complementary functions. As a whole, the bacterial community residing in each aquifer compartment would appear to define an ecologically functional ecosystem, containing suitable microorganisms (e.g. acidophilic bacteria) prone to promote the remediation of the acidified aquifer by natural attenuation. Assessing the composition and structure of the aquifer's native bacteria is a prerequisite for understanding natural attenuation and predicting the role of bacterial input in improving ISR efficiency.
... The first path involves brownfield deposits and shallower mineralizations. There is rapid development of enabling technologies that allow solution mining (or ISR) practices to extend into a broader range of ore bodies [24,26,[34][35][36][37]. The challenges are numerous but include the following: advanced in situ ore body characterization tools, robust self-sufficient and wireless metal-specific sensors for environmental and production monitoring, controllable fracturing technologies and improved in situ target mineral liberation, and advanced lixiviants (more robust but environmentally benign and target metal selectivity). ...
The history of mineral processing in general and flotation in particular is long and has always been tied to mining methods of the day. Building on the ever-improving fundamental understanding of the underlying science, the most significant trend in flotation has been the putting into practice the learnings from trailblazers such as Professor Fuerstenau that has given the confidence that enabled an ever-increasing scale of operations. There is, however, doubt this ongoing trend is enough to maintain the economics against global trends such as that of falling grades, increasing mining costs, pressure on water supply and demand, rising energy demands needed for mineral processing, and a focus on whole of life value and mine legacy issues.
In-situ recovery (ISR) is accepted and applied across many geographies, and approximately 50% of the world’s uranium is currently extracted by using an ISR method (World Nuclear Association, 2014). The low environmental impact, the elimination of risk for mine workers and the fact that all of the lowest-cost producers of uranium use an ISR approach should raise interest from traditional miners (IAEA, 2014). However, the broader application of ISR to other commodities and mineral systems remains limited. A number of demonstration (experimental) copper projects have been undertaken (Sinclair and Thompson, 2015) with a few copper projects in Arizona being close to demonstration (Florence, 2018 and Gunnison, 2018). Evidence of very few economically operating ISR copper projects can be found (for example, the Russian, Uralgidromed OAO, built by the Russian Copper Company in 2005 and operated as an ISR facility from the Gumeshevskoye deposit (Russia Mining, 2018). A number of gold deposits, particularly paleochannel deposits, have also been considered for ISR, but, to the best of our knowledge, only limited demonstration and no commercial operations have been established, yet (Kuhar et al, 2018 and references therein).
While the broader uptake of an ISR approach in new or greenfield projects may be emerging (Kuhar et al. 2016), we would contend that the application of an ISR approach (or ISR enabling technologies) to brownfield sites has massive potential (in a shorter time-frame) for increasing total values recovery from operating mines; extending mine-life, production and jobs; and for positive economic impact in rural Australia.
Over recent years, the combination of cost (profit margin) pressure and major advances in a number of relevant technologies has made researchers, companies and regulators realise the potential benefit for the broader use of ISR and the potential for broader application of ISR-enabling technologies (Batterham and Robinson, 2018). In some instances this has involved a consideration of the application of ISR-enabling technologies into remediation and the revisiting of old heaps/dumps for additional value recovery.
Potential additional targets include stranded, small and otherwise uneconomical ores that remain when mines are closed/abandoned and miners move to more attractive targets. These targets would include residual values in the floors and walls of pits or underground operations, material in rims of or between pits, otherwise inaccessible mineralisation (e.g., below established infrastructure) and less attractive (e.g., low-grade) mineralisation that does not justify conventional (open-pit or underground) workings.
While we work with companies to identify and progress a number of immediate opportunities, we are working in parallel programs on the critical enabling technologies. In particular, ISR opportunities are being sought where improved mining outcomes (economic, societal, environmental and health and safety) can be achieved while allowing developing technologies to be demonstrated and tested in “the real world” and further optimised. Within an “In-Place Mining” strategic research focus (Mining3, 2018) involving “In-Line”, “In-Mine” and “In-Situ” Recovery, Mining3 are leading a coordinated and collaborative thrust to progress this portfolio of projects and research directions in Australia. Specific challenges that have been identified with industry are being targeted, these include:
- Improving ore body characterisation with a focus on in-situ target mineral porosity/permeability liberation, and access.
- Increasing and sustaining in-situ access for lixiviants through target ore bodies.
- Understanding and modelling new and conventional fracturing technologies and outcomes.
- Design of more selective and active lixiviant systems and improving lixiviant–mineral (value and gangue) interactions (i.e., improved understanding of in-situ chemistry).
- Development of improved containment strategies including commercially available and novel barrier technology.
- Development of well field optimisation strategies based on individual well hole production and flow models.
- Improving awareness of ISR and communication strategies to facilitate social licence to operate.
- Developing and testing robust, stand-alone sensors for real-time downhole environmental and production monitoring and optimisation.
The opportunity has been recognised by many in our industry and in parallel with the above research and technology development effort, real ISR projects are being progressed within many companies that will take up these technologies and “real-world” test them almost as soon as they are ready. While the focus of these projects appears to be largely copper, gold or copper and gold, they still range from abandoned/historical mines, to near end-of-life mines and new prospects altogether. We remain convinced, however, that the number and range of opportunities is far greater than generally realised, and an early demonstration of full ISR extraction or application of enabling technologies to extend mine life (for example) will see a further increase in the level of interest mining companies pay to these developments.
The history of mineral processing is indeed long and has always been tied to mining methods of the day. While many new processes and devices have been developed, the most significant trend in recent times has been the ever increasing scale of operations. There is serious doubt this ongoing trend is enough to match the global trend of falling grades and rising energy demands needed for mineral processing.
As well we see that globally, the social licence to operate for mining (and hence mineral processing) is far from secure. Indeed, some argue that mining as we know it is dying and in the future, mining will be much more environmentally secure, even to the extent of insisting upon “in place” recovery and only bringing to the surface the valued product.
In this paper we comment on the ramifications of broader uptake of more sustainable mining for mineral processing methods and address the specific question of whether recent progress in in place recovery will lead to a timely breakthrough. The answer proposed is that this is possible in the next 10 - 20 years but in the meantime, the focus is just as likely to be on removal of waste much earlier in circuits. Either way, mineral processing as we know it will change significantly.
Increasing global demand for commodity metals and capital and the greater operating costs of mining and metal production present significant challenges for the metals mining industry. Current conventional technologies may not be feasible for processing deposits of declining grade, challenging mineralogy or those that occur at increasingly greater depths. To overcome these issues, the development of a step-change technology such as in situ recovery (ISR), which reduces mining and avoids milling costs, may be vital in ensuring future economic metal production. ISR has been used extensively to treat porous soft rock deposits such as the water-soluble salts, sylvite and halite. Although ISR has typically been used for mining conventionally uneconomic uranium ores, it has also been used less frequently for the treatment of other restricted permeability hard rock deposits, such as those containing copper and gold. The reason for the limited uptake of the technology for hard rock mineralisations is primarily due to low natural rock porosity and permeability and hence poor response to available lixiviant. However, additional challenges include solution containment, management of groundwater and regulatory, ecological and societal requirements and expectations. CSIRO has focused on understanding the challenges of hard rock ISR and identifying global capabilities that can contribute to developing the essential enabling technical areas required to realise ISR. Specific focus areas include geochemistry and advanced resource characterisation; drilling and controlled rock-breaking technologies; solution chemistry and environmentally benign lixiviant systems; lixiviant delivery systems and ‘subsurface mixing’; subsurface hydrology and reactive transport mechanisms and models; downstream processing options; legal, social and environmental aspects; and techno-economic modelling. This paper will discuss ISR challenges including industry feedback focused on addressing this issue, gaps preventing ISR from being applied in practice, capabilities to overcome these gaps and the development of a collaborative approach to making hard rock ISR a reality.
In situ leaching tests with thiourea as the lixiviant were carried out in two worked-out stopes at the No. 6 shaft at Rand Leases Gold Mining Company. Dissolution rates of 0.9 and 0.7 g of gold per week per 100 m2 of footwall area were achieved. A major disadvantage of the method seems to be the slow rate of dissolution of the gold, which is probably caused by poor contact and slow mass transfer between the lixiviant and the gold surfaces. Under full-scale application, the loss of lixiviant by leakage could also be a very important factor. Furthermore, the oxidation products of thiourea may 'poison' the activated carbon that is used for the recovery of the gold. The method may therefore be economically viable only in stopes that contain high values of residual gold.
Platinum Group Metals (PGMs), often with associated gold, have very few occurrences where they are present in an ore deposit at economically extractable levels. They are classified as both precious and critical metals due to their scarcity and their wide industrial use. With deteriorating socio-political environments in most primary PGM producing countries, PGM deposits that are smaller but in less risky jurisdictions have to be evaluated. However, the lower PGM grades, increased mineralogical complexity of the ores, capital intensity and strict environmental regulations in other international jurisdictions, limit the implementation of conventional metallurgical processing options, particularly smelter-based operations. The conventional smelter-based process options are justifiable for high grade, low chromite, large resource and long life-of-mine operations.
Stimulation principles, construction techniques, equipment requirements and technical features of liquid carbon dioxide fracturing were summarized, and the existing problems and development trend of this technology were discussed. Compared with the conventional hydraulic fracturing technology, it has several advantages including high flowback, small damage in reservoir, outstanding stimulation effect and so on. There are five main problems existing in this technology: friction of liquid carbon dioxide is very high; liquid carbon dioxide has an extremely low viscosity, poor proppant carrying ability and a large amount of fluid loss, thus behaves poorly in fracturing; the phase behavior of carbon dioxide is very complex in the process of fracturing, it is hard to realize accurate prediction and control for phase transition of carbon dioxide; fracturing equipments, especially blenders have obvious defects and should be improved further; computational methods for operation parameters in liquid carbon dioxide fracturing is still lacking. Supercritical carbon dioxide fracturing technology succeeds almost all the advantages of traditional liquid carbon dioxide fracturing technology, and has a better stimulation effect, smaller pump pressure and fewer requirements for blenders, thus is the trend in carbon dioxide fracturing.
Cyanide has been recognized for a long time as a powerful lixiviant for gold and silver, forming very stable cyano complexes with both metals. While cyanide is very effective in leaching free milling ores, there are certain classes of gold and silver ores (i.e., carbonaceous, pyritic. arsenical, manganiferous, cuperferous) that are considered refractory to conventional cyanidation dissolution. Recently there has been considerable effort directed towards new and improved reagents for leaching these difficult-to-treat ores and concentrates. A large portion of this effort has been devoted to finding alternative lixiviants that might compete with conventional cyanidation. Furthermore, there is a general interest in developing non-toxic environmentally safe substitutes for cyanide.There are a number of reagents that form stable complexes with gold and silver e.g., thiourea, thiosulfate, halides, malononitrile, acetonitrile and polysulfides. The chemistry of gold and silver dissolution using alternative lixiviants is discussed in this paper. Special emphasis is given to the application of Eh-pH diagrams to interpret the dissolution behavior.
The solubility of Au has been measured at 25°C in aqueous solutions in the presence of various organic ligands (acetate, benzoate, oxalate, phthalate, salicylate, and thiosalicylate). These ligands were chosen as simple analogs of humic acid moieties in order to model the complexation of Au by humic and fulvic acids in natural waters. With the first five ligands (ΣL = 0.1 M), solubilities were generally below 25 μg/l, whereas in the thiosalicylate solutions (ΣL = 0.45 M), a maximum Au concentration of 680 mg/1 was measured. Acetate and benzoate complexes are too weak to detect by the solubility method. Oxalate appears to have a reducing effect on Au in solution, and both oxalate and phthalate complexes of Au(I) may be coordinatively unfavorable. It was only possible to identify one salicylato complex (logβ2 = 17.5 ± 0.5) and two thiosalicylato species (logβ1 = 29.9 ± 0.3 and logβ2 = 31.7 ± 0.3). In addition, stability constants for a number of O-, N-, and S-donor complexes of Au(I) were estimated from linear free energy relationships with Cu(I), Ag(I), and Hg(II).General trends in stability constants of Au-organic complexes with various donor atoms are S ⪢ N ≥ O. Calculations based on a simple model of a fulvic acid suggest that Au is almost exclusively bound to S-donor sites under reducing conditions, but AuOH(H2O)0 and complexing by organic O-donors are more important in oxidizing environments.
Engineering data gathered from 26 operations indicate that most ores are leached in heaps following crushing and distribution on pads. Metal values are recovered from cyanide leach solutions using either zinc precipitation or charcoal adsorption. Potential problems that may hamper block development of a leaching operation are poor percolation characteristics of the ore, calcium salt buildup, low temperatures, and solution losses. An extensive bibliography on gold and silver leaching is appended.
In situ leaching offers a potentially attractive way to extract copper from the subsurface without costly fragmentation and processing. Applicability of in situ leaching is limited to deposits where sufficient permeability exists and where the copper and gangue mineralogy is amenable to leaching. A key challenge from past projects is establishing uniform contact between the fluid and the formation in fractured environments, particularly if fractures become blocked by gypsum and jarosite precipitation during leaching. Previous projects have demonstrated that in situ leaching of copper sulfides is feasible by regenerating ferric iron using atmospheric bacteria cultures, pressurized oxygen gas, or chemical oxidants. Oilfield technologies, including polymer injection for permeability modification and nanoparticle tracers, may have future applications for diagnosing and mitigating short circuiting between wells. Geophysical techniques such as electrical resistance tomography have the potential to provide real-time data on lixiviant movement in the subsurface, thus aiding in both recovery optimization and environmental control. In this review, data from past copper in situ leaching projects are assembled, with a focus on recovery without prior permeability enhancement. The resulting database includes key operating variables from copper in situ leaching projects ranging from field scale pilots to commercial operations.
A novel ore pre-concentration technique using high voltage pulses is proposed in this study. The technique utilises metalliferous grain-induced selective breakage, under a controlled pulse energy loading, and size-based screening to separate the feed ore into body breakage and surface breakage products for splitting of ores by grade. Four copper ore samples were tested to demonstrate the viability of this technique. This study consists of two parts: Part 1 presents the principle, the validation and the major findings; Part 2 discusses the new opportunities and challenges for the mining and mineral industry to take up this technique.
It has been established that gold can be dissolved under conditions typical of those encountered during the pressure oxidation of refractory gold ores provided that the solutions contain chloride ions and that a high ratio of iron(III)/iron(II) is maintained. This study has been conducted in order to understand the behaviour of gold under such conditions and has comprised both thermodynamic and kinetic measurements over a wide range of temperature and chloride concentration.Preliminary electrochemical studies have demonstrated that the effective oxidant for gold under pressure leach conditions is iron(III) and not dissolved oxygen. Measurements of the equilibrium potentials of the gold(III)/gold and iron(III)/iron(II) couples over a temperature range from 25 °C to 200 °C in acidic sulphate solutions containing various concentrations of chloride ions have confirmed thermodynamic predictions that the equilibrium solubility of gold increases with increasing temperature, chloride concentration and iron(III)/iron(II) ratio.Detailed electrochemical studies of the anodic behaviour of gold and the cathodic reduction of oxygen and iron(III) have been made under the above conditions in an autoclave and have been combined with mixed potential measurements to develop an electrochemical model for the dissolution of gold. The model has been verified by measurements of the actual rate of dissolution of gold as a function of the chloride ion concentration and the temperature.The implications of these results as they relate to the deportment of gold during pressure oxidation and also to the possibility of an alternative process to cyanidation are discussed.
The presence of copper minerals with gold is known to lead to many challenges during the cyanidation of gold ores, such as high consumption of cyanide with low gold extraction and undesirable impacts on gold recovery during the downstream processes. An alternative selective leaching process for copper minerals from copper-gold gravity concentrate (3.75% Cu, 11.6% Fe, 11.4%S and 0.213% Au) using alkaline glycine solutions was studied and evaluated. The lixiviant system containing glycine and peroxide showed that total copper dissolution of 98% was obtained in 48 hours at ambient conditions and a pH of 10.5-11. The results show that 100% of chalcocite, cuprite, metallic copper, and about 80% of chalcopyrite in the concentrate were also dissolved. Pyrite remained intact during the leaching time and iron concentration in the final pregnant solution was found to be 12 mg/L when copper is solution is at 4745 mg/L, while the gold concentration was limited to 0.8 mg/L Au. QEMScan analysis indicated that unleached copper in the leach residue was mostly distributed amongst larger chalcopyrite grains and covellite. The effects of single versus two-stage leaching, pH, oxidant concentration, pulp density and glycine concentration on copper extraction rate and extent were explored.
The experiments on gold solubility in amino acid solution indicate that gold is very intensively soluble in amino acid (or other organic acids), which is extensively present in geological bodies, and is most soluble in histidine. The temperature and concentration, acidity and type of amino acid in the solution are important factors affecting gold-amino acid complexing. The solubility of gold in amino acid is different under different conditions of temperature, amino acid concentration and pH value of the solution. At 80°C and pH=6–8, gold is most soluble in amino acid. Gold dispersed in water and rocks could be concentrated and transported by amino acid and then precipitated in favorable loci. Amino acids might have played an important role in metallogenesis as well as in the formation of source beds of gold. Nitrogen, oxygen and sulfur in amino acid might have reacted with gold to form soluble complex ions.
Over 25 alternative lixiviant processes to cyanide have been tested in the laboratory; some of which have been successful for niche applications. The process conditions, applications and current status of the most attractive are reviewed with an emphasis on publications since 1995. Most work has focussed on thiosulfate, thiourea and halide leach systems. Pressure oxidative chloride, sulfide and ammonia leaching processes are generally more applicable for the extraction of gold or platinum group metals as a by-product from base metal sulfide concentrates. Despite the research interest and pilot plant trials on many non-cyanide gold lixiviants a viable alternative gold process is still at the early developmental stage. The most advanced is the thiosulfate heap leaching of low grade carbonaceous preg-robbing ores by Newmont Mining Co. In most alternative cyanide leaching processes, reductions in the quantity of reagents used, reagent consumption and improvements in recovery technology are required. Some chemicals used such as ammonia also pose health, safety and environmental concerns. Consequently proper disposal of wastes and sustainable development issues will have to be addressed by mining companies.
Since the initial report of thiourea as a complexing reagent for gold leaching, considerable research has been directed toward the use of thiourea as an alternative to cyanide for gold extraction from different auriferous mineral resources. At the same time, some fundamental investigations of the system have been reported. In this article, a review of both applications and fundamental research is made, including a review of the recent results from laboratory studies at the University of Utah. Recent research results demonstrate that thiourea decomposition is quite slow in the presence of ferric sulfate for simple solutions. Ferric sulfate and formamidine disulfide (FDS) are effective oxidants with fast kinetics. No passivation of the gold surface is observed in simple acidic solutions. However, in actual leaching systems, some sulfide minerals significantly catalyze the redox reaction between thiourea and ferric ion, causing high thiourea consumption if ferric ion is present in excess. The presence of copper has a deleterious influence on this leaching system.
Experimental data are presented to show that humic acid can dissolve, complex and transport Au. As much as 330 ng Au/ml (ppb) were taken into solution from 0.07 to 0.15 mm Au particles by 500 μ/ml (ppm) solutions of humic acid during a 50 day period. Au was mobilised in electrophoresis experiments in the presence of humic acid, whereas Au alone remains immobile. The formation of a complex between Au and humic acid was indicated by the results of polarographic. solvent extraction and X-ray diffraction investigations.It is apparent that the formation of a humic acid complex of Au may be important in the mobilisation of element at the Earth's surface.
Gold(0) dissolves in a wide range of aqueous amino acid solutions exposed to dioxygen. In particular, the thiol-containing molecules cysteine, penicillamine and glutathione give solutions of gold complexes which can be identified by circular dichroism. Animal experiments suggest that gold complexes can be absorbed readily through skin. Gold concentration in human skin which had been in prolonged contact with gold and concentrations from the skin of patients treated with Myocrisin, a gold(I) compound, are reported. The possible significance of these results in terms of erosion of jewelry and skin-irritant reactions is discussed.
Selection of a leaching system for gold involves consideration of ore texture and mineralogy, chemical requirements, leaching techniques, the development of flowsheets, and environmental management. Aqueous dissolution chemistry for alkaline, neutral, and acid systems is mainly considered here. All systems require an oxidant to oxidise gold and a ligand to complex with gold in solution. Adjustment of pH is usually necessary.Alkaline lixiviant systems (pH > 10)include cyanide, ammonia-cyanide, ammonia, sulphide, nitriles, and a few other minor possibilities. Oxygen is the main oxidant. Cyanide, which is the main ligand in these systems, forms an anionic complex, “Au(CN)2”, with Au(I). Gold dissolution rates are controlled by oxygen solubility in solution.Neutral lixiviant systems (pH 5-9)include thiosulphate, halogens, sulphurous acid, and bacteria plus natural organic acids as the ligand. Oxygen is the normal oxidant and either Au(I) or Au(III)complexes are formed.Acid leaching systems (pH ⩾ 3)may contain thiourea, thiocyanate, chlorine, aqua regia, or ferric chloride. Chloride is the ligand in the last three systems and the oxidants include chlorine, ferric chloride, hydrogen peroxide, and nitric acid which produce Au(III) anionic complexes, e.g. [AuClJ". Fast gold dissolution is possible but reagent consumptions are high. Thiourea is unusual in producing a cationic Au(I)complex, “Au(NH2CSNH2)2” and gold dissolution is slower.For treating simple auriferous oxide-silicate-carbonate ores, and many otfier materials, cyanide remains the preferred lixiviant.Most non-cyanide leaching systems appear to have little wide-spread practical application. Possible niche applications include the use of chlorine or aqua regia to dissolve coarse gold from gravity concentrates, oxidising acid chloride solutions for die treatment of auriferous base metal sulphide concentrates, thiosulphate for dissolving gold from gold-copper ores, and thiourea for auriferous hydrometallurgical intermediates.
Metallurgists have long sought cost-effective techniques for recovering precious metals from their ores. They are increasingly called upon to design processes for ores refractory to conventional recovery techniques. Adding environmental costs (including site remediation) to the total cost of mining has also stimulated a search for alternative to conventional processes. This paper describes progress in development of an alternative to cyanidation, bisulfide leaching, for extracting gold, silver from refractory ores and concentrates. While bisulfide leaching can be implemented abiolically, significant benefits accrue from its practice as a bioprocess integrating precious-metals liberation and extraction steps. Bisulfide leaching appears to offer advantages over the traditional cyanidation process, including lower reagent costs and toxicity as well as an ability to leach “preg-robbing” ores and other ores not amenable to cyanidation. Bisulfide leaching may also offer advantages over cyanidation for selective dissolution of precious metals from base-metal concentrates.
Ferric EDTA and ferric oxalate complexes are both effective oxidants for the aerobic and anaerobic dissolution of gold in thiosulfate solutions, and therefore are potential candidates for the development of an in situ leaching system. The thiosulfate and polythionates were quantified during leaching using HPLC with perchlorate eluent and an anion exchange column, and it was found that both the iron EDTA and oxalate complexes have a low reactivity with thiosulfate, and they do not react with thiourea when it is added as a leaching catalyst. Anaerobic leaching experiments showed that both systems were still active after seven days leaching, and when 1 mM thiourea was present, there was significant gold dissolution. However in the absence of thiourea, the gold leaching was very slow, and hence the addition of thiourea as a gold oxidation catalyst is required for the iron(III) leaching systems. When anaerobic leaching was carried out in the presence of finely ground pyrite, the iron(III) complex was rapidly reduced to iron(II) as a result of the pyrite catalysed oxidation of thiosulfate. Pyrrhotite was also found to be problematic as it directly reduced the iron(III) complex, and therefore the quantity of gold leached was significantly lower in the presence of both these sulfide minerals. These problems associated with the presence of sulfide mineral need to be overcome if such a system is to be used in an in situ leach environment.
The thermodynamic equilibria and kinetic aspects of gold dissolution in iodide electrolytes have been studied with emphasis on the effect of different oxidants on the system. In conjunction with kinetic measurements, the chemix computer program was used to predict the concentration profiles of the predominant species at equilibrium in different solution conditions for the systems Au-I−-I2-H2O and Au-I−-OCl−-H2O.The thermodynamic study showed that I3− is the predominant oxidants species in both systems. However, if the concentrations of OCl− and I− are equal, solid iodine is formed. In these systems iodide (I−)_is used to form I3− (responsible for the gold oxidation) and more free iodide needed for the gold complexation is destroyed in the I−-OCl− system than the I−-I2 system. The formation of solid AuI also explains the lower rate of gold dissolution determined for certain conditions in the kinetic study.The thermodynamic modelling supports the kinetic measurements which show that, although the I−-OCl− system has a higher oxidation capacity, it does not extract gold as well as the I−-I2 system. In all cases there exist optimum oxidant/iodide ratios for achieving maximum gold extraction rates. A mixture which has the highest I3− and free I− concentration will attain the best gold extraction rate.
Australia is now an important gold producer in the world. The nature of Australian gold production is briefly reviewed and the hydrometallurgy of gold extraction is considered. The choice of processing routes for free milling, complex and refractory ores is discussed. For free milling ores, cyanidation and recovery by the Carbon-in-Pulp/Carbon-in-Leach process (CIP/CIL) is the primary and proven treatment process. Copper containing ores are discussed in some detail as they interfere in the CIP/CIL process. Oxygen consuming and preg-robbing ores are also described. Five different classes of process options for pretreating refractory ores are considered in detail. These options include: ultrafine grinding; acid and alkaline pressure oxidation; and a variety of chemical pretreatments such as: Activox process, HMC process and Electrolytic oxidation process. In Australia, the two pre-eminent options for refractory gold ore pretreatment are roasting and biooxidation and this development is reported. The trial of pressure cyanidation of stibnite concentrates at the Golden Spec mine in Australia is described. Pyrolysis, the Nitrox/Redox process, the Artech/Cashman process and the Caro's acid process have not gained commercial status so far.The potential for resin-in-pulp (RIP) to replace CIP/CIL is discussed. The use of cyanide has generated environmental concerns because of its toxicity and therefore research on alternative gold recovery processes using non-toxic reagents is considered. The key candidates are: ammoniacal thiosulphate, thiourea and halide solutions. The chemistry of these leaching systems is briefly described and proposed flowsheets are referenced. The future prospect for biohydrometallurgical gold recovery is indicated.
Of the halogens, the gold iodide complexes are the most stable in aqueous solutions. A series of experiments were performed to investigate the kinetics and mechanism of the leaching reaction between gold and iodide. Using a rotating disk technique, the effects of rotation speed, iodide and iodine concentration, temperature, pH and the presence of different electrolytes were measured. Oxygen and hydrogen peroxide were also examined as oxidants in the iodide system. A first order reaction rate was found with respect to I3− and half order reaction rate with respect to I−. A comparison of gold leaching between iodide and cyanide is also presented, in which a rate of about 2.6 × 10−9 mol/cm2 sec for 10−2M Nal and 5 × 10−3M I2 was obtained. This value is close to that for typical cyanidation.
Several oxidants (hypochlorite, iodine and hydrogen peroxide) were used to evaluate the dissolution of gold in iodide electrolytes at ambient temperatures. Evans diagrams constructed for the two half cells involved in the dissolution process show that hydrogen peroxide is not a suitable oxidant for the iodide system. The gold dissolution rate in an iodine-iodide mixture is dependent upon the concentration of iodide, iodine and the solution pH. An optimum iodine-iodide mole ratio of 0.35–0.4 was found for 0.1 M KI solutions, with pH ranging from 2.7 to 11.5; these are capable of dissolving gold at a maximum rate of 17 mg/cm2h. If a small addition of hypochlorite is made (< mM for 0.1 M KI solutions), gold will be dissolved faster than with iodine as oxidant. However, the gold dissolution rate in a hypochlorite-iodide mixture is strongly dependent upon the solution pH. The optimum hypochlorite-iodide mole ratio is 0.25 for KI solutions of 0.02 to 0.1 M at pH 2.7. Cyanidation, using the same concentration of cyanide yielded gold dissolution rates within the range of 1.3 (at pH 12.5) and 3.5 mg/cm2h (at pH 8.5).
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