Emission contribution in primary copper production

Emission contribution in primary copper production

Exploring the emissions from a typical copper operation

Greenhouse gas (GHG) emissions through the copper production process are typically associated with the consumption of fuel in the mining and materials transport process, and indirect emissions from electrical energy use in extractive and beneficiation processes.1

Northey et. al. assessed a wide variety of copper producing mines to determine the average energy intensity and GHG intensity with an average of 2.6 t CO2-eq per tonne of copper produced, as shown in Table 1 below. Energy intensity was categorised into direct and indirect energy categories, aligned with the recommendations from the Global Reporting Initiative (GRI) guidelines. GHG emissions were calculated as the sum of Scope 1 and Scope 2 emissions.

Table 1: Summary of greenhouse gas emissions and energy intensity of a typical copper process, obtained by an assessment of sustainability reports of several copper producing mines around the globe. 2

RANGEAVERAGE
Energy Intensity (GJ / t Cu produced)TOTAL10 – 7022.2
DIRECT2 – 5113
INDIRECT1 – 2312
Greenhouse gas emissions (t CO2-eq / t Cu produced)1–92.6

These findings indicate that while there is a broad range of emission intensity within the industry, the average is skewed toward the lower end of the range.

A note on sulphur dioxide

Of note, in addition to managing CO2 emissions, the copper industry also addresses emissions from sulphur dioxide (SO2). Sulphur dioxide (gas) is typically produced in smelting, leaching, and electro-refining. The smelting-converting process produces 2t SO2/ t Cu produced (energy and process emissions).3 These emissions are typically captured and reused for sulfuric acid production on site. The copper industry has established processing pathways to manage other gas pollutants such as SO2 as a product within the copper extraction process.

Division by countries

Additional data from Northey et. al. is summarised in the Figure 12 below, showing the energy intensity and GHG emissions intensity of primary copper production by country. It should be noted that for all countries except Australia, Canada and Chile, only a single mine was in the data set.

A review of the carbon footprint of copper mines by country and metallurgical process was carried out by Nilsson et. al. (2017)4, as shown in Figure 13 below.

It is indicated that energy intensity of the primary copper production process will vary greatly, as most likely a result of factors such as:

  • Variation in deposit type, ore grade and composition

  • Geographic location and number of operating days per year

  • Mining method and equipment requirements

  • Material movement methods

  • Waste to ore ratios

  • Type of copper produced

  • Processing method and output (concentrate or cathode),

Whereas the GHG intensity is linked to the available energy sources.5,6,7

The impact of declining ore grade

The trend towards declining orebody grades and continued development of the pursuit of existing operation to exploit lower grade deposits is likely to continue, in the absence of high-grade project discovery. A decline in ore grade results in higher operating cost due primarily to the amount and depth of material required to be mined and processed to produce the same amount of copper product. It is no surprise that both GHG emission intensity, refer Figure 148, and energy intensity, refer Figure 159, increase as ore grade decreases. There is a point of inflection, where below an ore grade of around 0.5% Cu the intensity of both metrics rises sharply.

Figure 14 - GHG Intensity as a function of ore grade for 28 copper operations, with each data point representing a year of production. 10

Figure 15 - Energy Intensity as a function or ore grade for 31 copper operations, with each data point representing a year of production. Source: Northey et. al. 11

Where are the emissions from a typical copper operation?

There are a number of factors that drive and influence the energy and emission intensity of the copper mining process. The ore grade and geochemical composition will determine the extraction methodology that will influence both the intensity and quantity of emissions from the mining and mineral processing stages of production.

In general, the energy consumption in the primary copper process is dominated by the earlier stages of beneficiation. This is due to the high energy demand requirement to crush and grind ore. Within the mining process loading and hauling, blasting, and ventilation (in the case of underground mining) all consume a higher proportion of energy to other aspects of the mining process.

GHG emissions are in general comparable for underground operations versus open cut mines when viewed in a global context. There are differing opinions as to whether pyrometallurgical processes have a higher energy intensity than electrowinning and waste heat recovery.12 There is however consensus that the extractive processes are the most energy intensive stage of the entire copper production process.

While not extensive, several journal articles and government reports have estimated the distribution of emissions in the process chain. These estimations have been summarised schematically below, noting the variation between sources related to process assumptions and ore grade variability.

Emission comparison between Underground and Open Cut Mining

The average emissions and energy intensity for underground, open cut, and combined mines have been compared (data obtained from Northey et al.) It can be seen that open cut mining methods have a higher energy intensity, underground mining produces a comparable emissions per tonne of copper produced.

The increased energy requirement of a combined mine is due to integrating the operating parameters of an underground operation. In addition to hauling and ore transportation that is required energy in an open-cut mine, underground mining is often at depth and requires significant energy demand for hauling in addition to ventilation, lighting, water pumping, increased transfer points for materials handling and other necessary activities.13

As the industry seeks to exploit lower grade and deeper deposits, the incorporation of underground operations to existing open cut operations could result in an increased number of combined operations. The outcome is that GHG emissions are approximately 35% greater when an underground mine is combined with an open cut site.

Table 2 - Summary of greenhouse gas emissions and energy intensity of underground, open cut and combined mines, obtained by an assessment of sustainability reports of several copper producing mines across the globe.14

UndergroundOpen CutCombined
Energy Intensity (GJ / t Cu produced)TOTAL21.926.828.3
DIRECT9.316.112.8
INDIRECT12.69.117.5
Greenhouse gas emissions (t CO2-eq / t Cu produced)2.472.523.42

Energy distribution within the mining process chain

The energy distribution for underground and open cut mining extraction processes are represented in Figure 16 below, which was adapted from information sourced from Norgate and Hague (2010). It assumes an ore grade of 1.8 % and the production of concentrate, grade 27.3%.

Within the mining process, loading and hauling account for a greater proportion of energy consumption due to heavy reliance on diesel operated vehicles. Diesel is required to transport large quantities of waste rock from an operation to ensure only copper bearing ore of grade is sent to the mill for processing.

Underground mining also requires ventilation, lighting and water as essential services. The confinement of underground mines creates a potentially toxic and explosive environment that needs to be mitigated to provide a safe working environment for mine site operations personnel. Ventilation is required to remove and dilute airborne contaminants with the use of underground fans which justifies the high energy demands. 15, 16

Water and lighting are essential services to ensure efficient drilling practices. Water is used in drilling, water spray is used to minimise airborne particles, and water is used as a coolant in deep operations. Water generated through underground mining operations as well as water inflow from operations below the water table is pumped to the surface as part of dewatering activity to ensure a safe working environment.

Figure 16 - Percentage distribution of energy within the copper production process for open cut and underground mining operations

Energy Distribution in the Beneficiation Processes

Moreno-Leiva et. al. determined the comparative emissions (global warming potential) for pyrometallurgical and hydrometallurgical processes.17 The distribution of these emissions in the process chain are shown in the Figure 17. Norgate et. al. (2007), determined converse results between the two processes, and therefore it is difficult to conclude a comparative advantage of one process over the other, except to conclude that the overall processing phase, regardless of method, holds the greatest global warming potential when compared to other aspects of the mining process.18