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Decreasing Ore Grades in Global Metallic Mining: A Theoretical Issue or a Global Reality? [1]
['Calvo', 'Mudd', 'Valero', 'Guiomar Calvo', 'Gavin Mudd', 'Alicia Valero', 'Antonio Valero']
Date: 2016-12-21
Therefore, this paper presents a study at the world level of the most important mines that extract economically relevant minerals, such as gold, lead, zinc and copper, using information from several mining companies distributed in different countries. With data regarding ore mined and milled, ore grade evolution and energy use over the last decades, the energy intensity use has been analyzed, as well as the possible links between those factors.
Whilst some studies show that the ore grade is decreasing over time [ 6 9 ], other studies state that the declining ore grades must neither be interpreted as a sign of depletion nor as an indicator of resource availability [ 10 11 ]. This is because changes in ore grade can be attributed to other factors such as innovation and improvements in extractive technologies, extending the life of older mines over finding new ones, among others [ 12 ]. Moreover, there is evidence that for most metals, as ore grades decline, the deposit size grows faster than the very decline in ore grade—meaning greater contained metals (e.g., uranium [ 8 ]; nickel [ 13 ]; zinc [ 14 ]). This is why a deeper quantitative analysis is crucial to better understand the relationship between ore grade changes and energy intensity in mines.
Demand for the main metals that modern society needs to produce goods has increased dramatically over the past few decades, thereby increasing the extraction to meet this demand. Between 1998 and 2014, world material extraction of the main commodities increased by a factor of 1.7 [ 1 2 ], a very significant number if we compare it to the 8-factor increase observed by Krausmann et al. [ 3 ] from 1900 to 2005. For instance, in the past 10 years, approximately one quarter of the total historic mine production of copper was produced, showing that global copper production has doubled every 25 years since data started being recorded [ 4 ]. Meeting the increasing demands for metals makes the mining industry one of the most energy-intensive industrial sectors. According to the International Energy Agency, between 8% and 10% of the world total energy consumption is dedicated to the extraction of materials that the society demands, and that number does not take into account metallurgical processes, transport and other mining-related activities [ 5 ].
Gold mining takes place both in surface, such as open cut or placer mining, and underground. Currently there are several processes to extract gold, with gravity concentration, flotation, pyrometallurgy and hydrometallurgy (cyanidation) the main processes. Still some small mines in third world countries use amalgamation with mercury due to its simplicity, but as it is a very toxic process and has inferior performance, it is not widely used anymore [ 37 ]. At the world level, cyanidation is the standard method used for recovering most of the gold extracted today, including agitated tank leaching, heap leaching and carbon adsorption recovery (carbon-in-pulp, carbon-in-leach, carbon-in-column) [ 32 ].
As gold usually appears in very low concentrations in mineral deposits, it is usually measured in grams per tonne, and the average ore grade can vary significantly from one mine to another. Usually open cut mines have lower grades, around 1 to 4 g/t, while underground mines can reach up to 8 to 10 g/t [ 36 ]. Mudd [ 6 ] estimated the average ore grade for Canada, Australia and South Africa as 7.15, 2.65 and 9.83 g/t respectively.
The gold production in 2015 was 3000 tonnes, with China, Australia and Russia being the main producers with production amounts of 490, 300 and 242 tonnes, respectively [ 15 ]. According to that same report, there are 56,000 tonnes of world gold reserves, while other authors state that the amount of recoverable reserves is 135,000 tonnes [ 31 ]. The production peak for gold using Hubbert’s diagram, where neither recycling nor trade are taking into account, is situated around 2010–2015, while using a full dynamic simulation model that incorporates recycling among other factors, the gold mines become exhausted after 2030 [ 31 35 ].
Lead and zinc ores are mined mostly by underground operations as the most common form of mineralization are veins where an association of different minerals can be found. In general, lead and zinc ores are processed with differential froth flotation to produce separate lead and zinc concentrates, with lead concentrates (usually ~50%–70% lead) then smelted using pyrometallurgy and zinc concentrates (usually ~50% zinc) are most commonly refined using hydrometallurgy, although some 15%–20% of world zinc production is derived through pyrometallurgical techniques [ 32 ]. The energy requirements for the primary production of lead are between 10 and 20 GJ/t and between 24 and 48 GJ/t for the case of zinc [ 33 34 ].
Lead and zinc appear almost always associated in mineral deposits, so mines that produce zinc also produce smaller amounts of lead and vice versa. In 2015, 4.7 million tonnes of lead were produced at world level along with 13.4 million tonnes of zinc [ 15 ]. According to the United States Geological Survey, there are 89 million tonnes of reserves and more than 2 billion tonnes of resources for the case of lead and 200 million tonnes of reserves and 1.9 billion tonnes of identified resources for the case of zinc. Meanwhile, other studies estimate that the extractable reserves are 2.3 and 2.6 billion tonnes for lead and zinc respectively [ 22 ]. Although there are no data available on global average mined ore grades for zinc mines, the average ore grade in the mine () estimated by Cox and Singer [ 29 ] is 6.05% Pb + Zn. There are also complementary studies regarding the ore grade of specific countries [ 30 ]. Using the available information on reserves and resources, the peak production year has been estimated for both metals, namely 2018 for lead and 2030 for zinc [ 31 ].
In the pyrometallurgical process, the ore is mined, concentrated, smelted and refined. The sulfides are separated from the gangue material using flotation to form a concentrate containing 25% to 35% of copper [ 26 ]. During smelting, the concentrate is fed to a smelter, along with oxygen and a reductant such as coking coal, where sulfides are oxidized producing a blister of 97%–99% of molten metallic copper that is later purified by electrolytic purification to pure copper [ 27 ]. From the 1980s a new technology emerged, commonly known as the heap leach–solvent extraction/electrowinning process (HL-SX/EW), although only 10% of the primary production comes from this hydrometallurgical route [ 28 ]. This process operates at ambient temperatures and the copper is dissolved to produce a copper sulphate solution through heap leaching, after which the copper is recovered through SX-EW to produce pure copper cathode.
Copper is currently mined using both underground and open cut methods and there are basically two main processing routes, depending on the type of ore present, sulfides or oxides. After ore sorting and grinding, used in both types of ore, the main technique used for concentration of sulfide ores is froth flotation followed by smelting and refining (pyrometallurgical process) while in the case of oxide ores (and some low grade sulfide ores) a heap leaching process is combined with solvent extraction and electrowinning (SX-EW) (hydrometallurgical process) [ 25 ].
Demand for copper has increased dramatically over the last few decades. From 1991 to 2015, the world total extraction has doubled, going from 9.3 million to 18.7 million tonnes. Available data suggest that the world copper reserves are 720 million tonnes and the identified resources approximately 2.1 billion tonnes [ 15 ] while other studies state that the ultimately recoverable reserves could be 2.8 billion tonnes [ 16 ]. At present, the global average copper ore grades for copper mines is approximately 0.62% of Cu content [ 17 ] and this number is expected to decrease as mines with higher ore grades become exhausted [ 17 19 ]. Starting from this information, the concern over the future availability of copper is on the rise, and several studies have focused on estimating the global copper peak production using Hubbert’s model, which has been estimated to be between 8 and 40 years from now [ 16 24 ].
In this section the different stages of the extraction and processing of the main commodities analyzed in this paper (copper, zinc and gold) are going to be described. Only the in-mine processes are considered, namely beneficiation, concentration and smelting, excluding the smelting and refining processes that takes place outside the mine and mining facilities.
The quality and consistency of the reports varied significantly between companies and even between different years in the same company. Some of the companies, but still a little portion of the total, have already adopted the Global Reporting Initiative (GRI) protocol, a coalition of the United Nations, industry, government and civil society groups [ 38 ]. The aim of this initiative is to provide guidelines to achieve uniform and consistent reports on sustainability performance for different sectors, including mining and metals [ 39 ]. The main drawback is that these reports only require general data regarding different social, economic and environmental aspects. For instance, a company that has several mines in operation can provide only aggregated data for the energy consumed within all the owned mines, fulfilling the requirements of the GRI but at the same time making the data of little value to analyze with respect to ore grades, project configuration and other key factors which are known to affect energy intensity. Hence, the extent of quality of data can vary considerably, discrepancies and between different reports have been corrected when possible, but due to the specific mines chosen and the consistency of their reporting, there is expected to be only a minor degree of uncertainty in the data collected herein.
Using the information available on the reports of the aforementioned companies, the following data have been compiled when possible for each year and for each individual mine:
A preliminary analysis of several mines and companies has been accomplished to select the ones that provide both consistent and disaggregated information related to energy use on a site by site basis, leading to a total of 38 mines chosen for subsequent analyses. Data have been sourced from numerous companies’ reports, including financial, annual, quarterly and sustainable reports. Years available for each of the mining companies analyzed are shown in the following list:
Many mining companies have started to report annually their sustainability and social performance along with their financial results. These reports vary substantially from one company to another but they can be used as an approach to have real information on their performance. Using the reported data it is possible to analyze links between different factors, such as energy consumption, ore grade, mineral production, greenhouse gas emissions, and solid wastes, among others.
4. Energy Intensity Factors
The main data and results for the selected mines, with information of the main metal extracted, mine type and process can be found in Table 1 . Each mine site has been categorized based on the metals extracted and the major mining and extraction methods used, separating between underground (UG), open cut (OC) and OC + UG mines. The main methods differentiated in the mining process are mine (M), concentration (C), smelting (S), refining (R) and leaching (L). Additionally, the production of the main metal (in bold in the table) has been included for 2013 when possible.
The main focus of this study has been obtaining information of energy consumption as a function of ore grade, as well as to have a better knowledge of energy intensity use in mining. For this task, the following substances have been included in this study: gold, silver, copper, lead, zinc and nickel. As countries such as Chile, Australia and Peru have been incorporated in the study, approximately between 30% and 40% of the global copper production is being taken into account, as well as half of the Australian gold production.
(a) Electricity use (kWh per tonne of total ore mined) as a function of ore grade (b) Litres of diesel per tonne of rock (including waste rock and ore) as a function of ore grade (note that underground rarely report waste rock extracted) (c) Total energy consumption (GJ per tonne metal), excluding explosives forces. With the information available, different energy intensity factors have been defined as follows:
For the first two cases, the average data for each mine have been included in the table, as well as the standard deviation and the number of data points available. These data only include information regarding energy consumption inside each mining facility. As for total energy consumption, explosive forces, essential for blasting, have not been included. This is because, even if some mines reported the consumption of ANFO (Ammonium Nitrate-Fuel Oil) its chemical exergy is very low compared to the total energy consumption (less than 0.1% in those mines where information was available). Therefore it can be considered almost negligible.
The electricity intensity use, kWh/t ore as a function of the ore grade, for all the commodities is represented in Figure 1 . In the left diagram, there are two series of data sets, as the copper mines usually have lower average ore grades than the lead-zinc mines. There seems to be a link between the amount of electricity used per tonne of ore mined in open cut and underground mines, as the latter increases when the ore grade decreases. In the case of mines that have both open cut and underground facilities, represented in grey in the figure, the data are quite clustered in the lower part of the diagram, and this decreasing tendency is not clear. However, this could be easily explained by the way each mine reports their data as data from underground and open cut mines are reported together and both have very different electricity requirements. Usually, considering the extraction process, underground mines are more intensive in electricity than open cut mines because of mine depth and ventilation [ 40 ]. Still, open cut mines have also a considerable amount of electricity demand and it has not been possible to assess the percentages that correspond to each facility in these set of data. Regarding electricity use and type of process configuration, the dependency is not very clear when analyzing the data but there are many other factors that could be influencing this, from each individual process to the equipment used. For instance, in the case of Sepon mine (Laos), the electricity consumption per tonne of ore is very high, and in 2014 more than 20% of the total energy used by the mining company was consumed at this facility (MMG Limited). Sepon mine has unusual extraction and processing methods, as after the ore is extracted, crushed and milled, the ore is directly leached without undergoing any concentration process.
As for electricity intensity use in gold mines (right diagram), only information related to a selected group of Australian gold mines is represented with the average ore grade measured in grams per tonne. Data seem quite dispersed when observing only the ore grade variations but the electricity requirements remains approximately within the same levels, between 25 and 150 kWh/t ore. There is an exception with the Granny Smith mine, as in 2007 the mine changed from an open cut mine only to an underground mine only, the energy use being higher in the latter case. The open cut mine had requirements of 116 kWh/t ore while the underground mine has requirements of 414 kWh/t ore as average, almost four times higher.
Another factor that can be analyzed is the influence of the mining process and configuration in the electricity use per tonne of ore mined. Mines that extract lead and zinc usually have an MC configuration (mine + concentrator). The majority of copper mines also have MC configurations, very few have a smelter and even fewer have MCSR, while a modest number also include an HL-SX/EW circuit. There is only one mine that has an ML configuration (mine + leaching), Sepon (Laos), and as this mining process is quite particular the data regarding electricity use are very different. Regarding gold mines, they all have an MCL configuration (mine + concentrator + leaching).
Diesel intensity use, in liters of diesel per tonne of rock mined, is represented in Figure 2 as a function of the ore grade. Usually diesel is used in mines for transport and machinery and sometimes for electricity production. In the case of diesel used for transport there are two main distinctions, diesel used for transport inside the mine and diesel used for transport outside the mine, although the reports rarely differentiate between each different use in the mine. This could become significantly relevant in the case of older mines, as trucks have to go deeper and further.
In this case, the information of copper, lead, zinc and nickel mines is represented distinguishing between open cut, underground mixed mines (mines that have both open cut and underground facilities). As the mining reports vary considerably regarding this issue, it has not been possible to obtain as many values as for electricity intensity use, but it can still be used to get an overall picture of the diesel consumption in these mines.
Concerning the ore grade and the diesel consumption, it seems that the ore grade has a certain influence on the diesel consumption, although there is not a clear relationship between these two factors in the case of the selected period and mines. Again there is a cluster of data corresponding to the copper mixed mines in the lower part of the diagram, represented in grey, but this could be explained by the way each mine reports the information on diesel use.
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