Full length articleProducing metals from common rocks: The case of gold
Introduction
Because of its extraordinary properties, such as electrical conductivity, stability at environmental conditions and meaning of nobleness in ornaments. Due to its stability, gold has a number of applications in medical implants as reported in (Higby, 1982; Pricker, 1996; Dykman and Khlebtsov, 2011), and activity in catalysis as investigated in (Hashmi and Hutchings, 2006; Gold-Catalyzed Organic Reactions, 2007; Yang and Hashmi, 2014; Pflästerer and Hashmi, 2016; Asiri and Hashmi, 2016). Therefore, gold has been exploited for centuries. In 2015, main gold producers were: China 20%, Australia 12%, Russia 11% and Canada 7%. Peru, South Africa, and Mexico had 6% of world mine production. In Fig. 1, the production of gold from 1998 to 2015 by country is shown based on statistics and reports of the US Geological Survey (USGS) (Matos, 2019; U.S. Geological Survey Mineral commodity summaries, 2018).
The evolution of the decline of high-grade deposits has been investigated by different authors, such as Mudd (2007a, 2007b, 2007c, 2008, 2010; Craig et al. (2014); Norgate and Jahanshahi (2010) or Calvo et al. (2016). In this respect, and although the recycling of gold accounted for approximately one-third of the total supply from 1995 to 2014 (Hewitt et al., 2015), Mudd (2007a) observed a clear decline tendency of the ore grades (Mudd, 2007a) of gold-producer countries, such as Brazil, Australia, South Africa, Canada, and the United States,. For instance, in Australia in 1859 ore grades in gold deposits were 37 g/t and nowadays the concentration is found at 2 g/t.
The need for renewable energy technologies for a decarbonized society will imply the use of more metals (Sawyer et al., 2016; International Energy Agency Energy Technology Perspectives, 2010; World Steel Association Steel´s contribution to low carbon future and climate resilient societies - worldsteel position paper, 2017). In a recent publication by Valero et al. (2018) on material restriction for the manufacture of renewable energy technologies, it was pointed out that moving towards a low carbon economy would cause a deeper reliance on non-fuel minerals.
Minerals are essential for keeping today´s standard of living (United Nations (UN) Plan of Implementation of the World Summit on Sustainable Development Contents, 2019), and high metal-content deposits are gradually being depleted. As a consequence, today, man needs to go deeper and to more remote places to satisfy the increasing societal need for raw materials. Therefore there is an urgent necessity to manage the mineral capital effectively, and to do that; the first step is to assess it accurately. This assessment can be performed merely on a tonnage perspective. When the evaluation of minerals is done on a tonnage approach, substantial aspects of minerals are ignored, such as scarcity in the Earth´s crust (Valero and Valero, 2014; Domínguez and Valero, 2013; Palacios et al., 2018). For instance, one ton of iron is equal to one ton of gold, on a mass basis, however iron is more abundant than gold, and the energy required for the processing and refining of the metal from the ore (embodied energy) is higher for gold than for iron. This comparison reveals that a tonnage approach is no longer valid for the proper evaluation of minerals.
Another way to assess minerals is considering their market prices. Metal prices are linked to the effort made to extract metals from ores and extraction costs (Valero and Valero, 2014; Henckens et al., 2016). Nevertheless, prices are strongly influenced by many other factors, such as, supply-demand, geopolitics or speculation, that is why its behavior is volatile (Henckens et al., 2016), as can be seen in Fig. 2, where historical gold prices since 1900 are shown (Kelly and Matos, 2019).
Alternatively, minerals can be assessed considering their physical quality through the Second Law of Thermodynamics. Valero and Valero proposed the concept of the exergy replacement cost (ERC) and thermodynamic rarity for the evaluation of mineral resources (Valero and Valero, 2014; Valero et al., 2013). The ERC characterizes the energy required to concentrate minerals at an average ore-grade found in Thanatia until the concentration currently located in the mines, while thermodynamic rarity is the sum of the ERC and current mining and beneficiation energies for obtaining the different mineral commodities. Thanatia is an idealization of complete mineral dispersion in the Earth´s crust (Valero and Valero, 2014) and represents the common rock from which minerals would be concentrated when mineral deposits no longer exist. The ERC can be seen as an avoided cost that man saves for having minerals concentrated in mines and not dispersed throughout the crust. In the limit, this free bonus would become lost, and mining would need to take place directly from common rocks. Several authors also examined the idea of crustal mining as a limit. Henckens et al. stated that the maximum cost of extraction of commodities would be achieved when mining would come from low concentration deposits, common rocks and seawater (Henckens et al., 2016). Skinner mentioned that the extraction from common rocks is technically feasible, but it would require more energy than from rich metal ores (Skinner, 1976). Authors like Harmsen et al. (2013), Bardi (2014), supported Skinner´s statement about the increment of specific energy for the extraction of metals from low-ore grade deposits. Steen and Borg published considerable increases in the production cost of metal concentrates, such as copper, cadmium, manganese, etc. (Steen and Borg, 2002).
Valero and Valero (2014) made an in-depth analysis of such crustal mining energy values characterized by ERC. This was estimated by analysis of statistical trends, estimations or mathematical models, supported by thermodynamic assessments considering that the energy consumption as a function of the ore grade is ruled by the entropy law and hence as the ore grade tends to zero, the energy consumption tends to infinity.
In this research, we show a novel method to estimate the specific energy needed to produce metals; in this case, gold, from Thanatia. The methodology is based on a computational model with HSC Chemistry software (Garcia et al., 2018) and hence is more exact than the values provided by Valero and Valero (2014). Our investigation aims to contribute to a deeper discussion about the sustainable production of metals and the loss of the natural patrimony of nations.
Section snippets
Production of gold from Thanatia
In the first part of this section, the concepts of thermodynamic rarity and exergy replacement costs are explained. Moreover, the main features of the production of gold for our model are described. Finally, the procedure to estimate the energy required to produce gold from Thanatia is explained.
Methodology
Due to the nature of Thanatia, the design of different processes to obtain gold has been done based on a literature review and an in-depth analysis of flowsheets. Basics about gold processing were studied from publications by Marsden and House (2019), Yannopoulos (1991), and mineral processing by Wills and Finch (Wills and Napier-Munn, 2006). Technical reports for gold processing plants were also studied, such as Éléonore Project in Canada (Christine et al., 2014), Fruta del Norte in Ecuador (
Results and analysis
In this section results from the modeling and simulation campaign are shown. The validation of results is determined through a comparison of main parameters, such as energy for the comminution process and flotation with those found in the literature. Finally, from the methodology presented in this work, the calculation of the specific energy to produce gold from common rocks, Thanatia, is shown accompanied by a sensitivity analysis.
Conclusions
With the methodology described in this paper based on a computational model developed with HSC Chemistry software, it has been possible to estimate the specific energy that would be required for the production of gold from common rocks. Since Thanatia has been an idealization of mineral dispersion in the Earth´s crust, it has been selected as our common rock. Due to this fact, the specific energy was determined by applying Bond´s equation, and not by other more precise methods which require
Author contributions
Palacios, J.L. performed the literature review, model, and simulation campaign and writing the first stages of the paper. Alejandro Abadias supported stages of modeling and simulation. Valero, An. and Valero, Al. supervised the research on the calculation of energy consumption to upgrade ERC for copper. Reuter, M. contributed with metallurgical advice and supervision of the model and results.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgments
We thank the National Secretary of Science and Education of Ecuador (SENESCYT) for the support for research activities and for the financial support of the Spanish Ministry of Economy, Industry, and Competitiveness through project ENE2017-85224R. We want to thank Prof. Dr. Oscar Restrepo of Universidad Nacional de Colombia for providing a valuable bibliography. Thanks to Ivan Fernandes of Helmholtz Institute Freiberg for Resource Technology for the revision of the first versions of this paper.
References (88)
Removal of cyanide from solution using activated carbon
Miner. Eng.
(1994)- et al.
Benchmarking comminution energy consumption for the processing of copper and gold ores
Miner. Eng.
(2014) - et al.
The direct electrowinning of gold from dilute cyanide leach liquors
Hydrometallurgy
(1987) - et al.
Treatment of gold e telluride ores
- et al.
HSC Chemistry
(2018) - et al.
The impact of copper scarcity on the efficiency of 2050 global renewable energy scenarios
Energy
(2013) - et al.
Mineral resources: geological scarcity, market price trends, and future generations
Resour. Policy
(2016) Global trends in gold mining: towards quantifying environmental and resource sustainability
Resour. Policy
(2007)An analysis of historic production trends in Australian base metal mining
Ore Geol. Rev.
(2007)Gold mining in Australia: linking historical trends and environmental and resource sustainability
Environ. Sci. Policy
(2007)
Radon releases from Australian uranium mining and milling projects: assessing the UNSCEAR approach
J. Environ. Radioact.
Global trends and environmental issues in nickel mining: sulfides versus laterites
Ore Geol. Rev.
Solvent elution of gold from C.I.P. carbon
Hydrometallurgy
Using life cycle assessment to evaluate some environmental impacts of gold production
J. Clean. Prod.
Low grade ores – smelt, leach or concentrate?
Miner. Eng.
Gold – a key enabler of a circular economy
An estimation of the cost of sustainable production of metal concentrates from the earth’s crust
Ecol. Econ.
Chemical exergies of the elements
Appl. Energy
Depletion of the non-renewable natural exergy resources as a measure of the ecological cost
Energy Convers. Manage.
Exergoecology: a thermodynamic approach for accounting the Earth’s mineral capital. The case of bauxite-aluminium and limestone-lime chains
Energy
Exergy of comminution and the Thanatia Earth’s model
Energy
What are the clean reserves of fossil fuels?
Resour. Conserv. Recycl.
The crepuscular planet. A model for the exhausted continental crust
Energy
Using thermodynamics to improve the resource efficiency indicator GDP / DMC
Resources, Conserv. Recycl.
(Canada) Ltda Pueblo Viejo Gold Project Dominican Republic Technical Report Final V 4
Gold-catalysed reactions of diynes
Chem. Soc. Rev.
Complexity and Wealth Maximization
Extracted
Thermal Design & Optimization
ZADRA elution circuit optimisation and operational experience at the CIL plant of Gold fields Ghana limited
4th UMaT Bienn. Int. Min. Miner. Conf.
Physical assessment of the mineral capital of a nation: the case of an importing and an exporting country
Resources
Decreasing Ore Grades in Global Metallic Mining: A Theoretical Issue or a Global Reality?
Resources
Thermodynamic approach to evaluate the criticality of raw materials and its application through a material flow analysis in Europe
J. Ind. Ecol.
Concentración gravimétrica de menas auríferas
Rev. Met.
Thermodynamics: An Engineering Approach
Metal Resources and Energy
Éléonore Gold Project Quebec, Canada NI 43-101 Technical Report; Québec, Canada
ISBN
Earth Resources and the Environment
In Renewables-Based Technology
Global Gold Mining: is technological learning overcoming the declining in ore grades?
J. Environ. Account. Manag.
Gold nanoparticles in biology and medicine: recent advances and prospects
Acta Naturae
Treatment of gold-telluride ores
Froth Flotation: A Century of Innovation
Cited by (14)
The influence of ore grade decline on energy consumption and GhG emissions: The case of gold
2022, Environmental DevelopmentCitation Excerpt :Due to the considerable number of assumptions to simulate gold production from Thanatia, only the main ones are summarised in this paper. The rest can be found in a previous study (Palacios et al., 2019b). The consumption of energy, particularly diesel, used for ore-handling, meaning the transport of material from the mine to the production site, is significant and cannot be disregarded.
The increase in embodied exergy to produce metal proportional to the decrease in mineral concentration
2022, Cleaner Engineering and TechnologyCitation Excerpt :Authors explain the need to develop sustainable iron assessment methodologies to attend current and future generations. In the same year, Palacios et al. (2019a) evaluate exergy required in gold production, using software and applying the Thanatia model with ERC and thermodynamic rarity. They claim that minerals’ consumption increases whilst ore grades decrease.
Recovery of scattered and precious metals from copper anode slime by hydrometallurgy: A review
2020, HydrometallurgyCitation Excerpt :Au is a strategic precious metal with special properties and important applications (Dai et al., 2012). As global consumption of Au continues, its reserves are being rapidly depleted (Palacios et al., 2019). At present, Au as a precious metal is inherently in short supply, with the grade of primary resources typically less that 1 g/t. Therefore, secondary sources such as CASs are valuable additional resources (Dönmez et al., 1999).
The thermodynamic rarity concept: A systematic review
2020, Ecological IndicatorsCitation Excerpt :Palacios et al. (2018b) uses the ERC to evaluate the depletion of minerals in Latin America, stating that fairer measures are required for the production of non-combustible minerals in the region. Palacios et al. (2019a,b) investigates the concentration of iron in common rocks by means of a computational model, confirming that the specific energy for metal concentration is greater than the energy required by modern processes. Palacios et al. (2019a,b) evaluates the energy that may be required to produce gold from the common rock through the same computational model, showing that the natural bonus (ERC) avoids a huge energy intensity for obtain the metal.