What drives GHG emissions from copper production?

/

November has been a big month for climate change. The Paris Agreement went into effect on 4th November. The Marrakech COP 22 climate talks kicked off a few days later, then unexpectedly, on 9th November the world awoke to the news that Donald Trump was president elect of the USA, after he promised, while campaigning, to kill the Paris Agreement and bring back coal. This week, the director of NASA's Goddard Institute for Space Studies personally wrote to Australian Senator Malcolm Roberts of the hard right One Nation party to correct misconceptions held by the climate change skeptic. In recent days, there are reports of unprecedented low sea ice levels at both poles.

Despite the political turbulence, most scientists agree that climate change is occurring, and mining companies are recognizing a need to measure and reduce GHG emissions from their operational activities. I’m sharing analysis about GHG emissions associated with production of my favourite metal, copper, and looking at ways these might be reduced.   Copper, due to its superior electrical and heat conductivity, is a key metal for a green and high tech future. However, its extraction can come at considerable environmental impact. For those not that familiar with copper production and use, check out the International Copper Study Group World Copper Factbook for some background.

About 80% of global copper mine production comes sulphide ores that ore processed using grinding and flotation plants to produce concentrates for smelting. The rest comes from mines using dump or heap leaching, solvent extraction and electrowinning to produce cathode copper. Most copper mines are large, low grade, open pits. Typically, these operations treat over 50,000 t/d of ore with copper head grades below 1%, and sometimes as low as 0.3%. The economic viability of these mines depends on keeping operating costs per tonne of ore below the payable revenue per tonne. 

For these mines, energy is a significant fraction of operating cost and the main source of GHG emissions. The energy use for these operations is primarily in two forms:

  • Diesel (or other liquid fuels), primarily for movement of ore and waste rock from pits
  • Electricity
    • For crushing and grinding; pumping water and slurries; and operating flotation machines in the case of sulphide ores
    • For crushing; conveying; pumping solutions; and electrowinning copper metal for oxide ores 

In addition, a mine often uses consumables, such as steel grinding media, which produced GHG emissions in their manufacture.

I use a driver tree methodology to understand GHG emissions. Here’s an example that shows emissions from diesel, electricity and grinding media for an open pit copper sulphide mine with a concentrator.

The driver tree methodology can also be used to analyze and improve profitability of a copper mine, as many of the same drivers that reduce environmental impact (less energy, water and steel grinding media consumption) also reduce operating cost. Sustainable business is good business.

What determines the GHG intensity of copper production (t CO2 equivalent/t Cu)? To use a classic metallurgist’s answer, “it depends”. The biggest factors are:

  • Ore head grade (i.e. copper and valuable by-product content) as this determines the mass or ore that must be moved and processed to recover a given unit of copper.
  • The mining method and geometry, especially for open pits as this determines the quantity of waste that must be moved (determined by stripping ratio), and the length and elevation of ore and waste movement. These in turn drive diesel consumption for haul trucks.
  • Mineralogical properties of the copper ore such as level of oxidation, mineral textures, grain size etc., as these determine the type and intensity of processing needed to extract the metal.
  • Ore hardness, as this determines how much energy is needed for breakage.
  • Geographical factors, as the proximity to power and water sources is closely tied to the GHG intensity of electricity and the energy requirements to deliver water.

We can relate the basic environmental and economic drivers in a 3D display, using the example of power consumption and costs. From an economic perspective, the profitability of a mine relates to:

  • Head grade – drives tonnes of ore per unit metal produced
  • Ore hardness – drives power consumption per tonne of ore
  • Power unit cost – the power unit cost multiplied by power consumption drives power cost per unit ore treated

Hence, a mine with high head grade (hence high revenue per tonne of ore), low hardness and low power cost (hence low operating cost per tonne of ore) will tend to be more profitable, while a low grade mine with hard ore and expensive power will be marginal or uneconomic. The analogy can be extended to GHG emissions, if we substitute power unit cost with GHG intensity of electricity. Hence a high head grade and low hardness ore at a site supplied with renewable power will have very low GHG emissions. In contrast, a low head grade mine with hard ore at a site with coal fired power will have very high GHG emissions. I’ve calculated the extreme scenarios below for demonstration. It makes a big difference – both for economics and GHG emissions.

The table above shows a worst case scenario, a typical low grade copper mine using coal fired electricity would produce in the order of 3 – 5 t CO2e/t Cu produced. A large open pit copper mine may use in the order of 200,000 L/d of diesel, with an emissions factor of about 2.7 kg CO2 e/L, resulting in 200,000 t CO2 e/y of emissions. That’s typically around 2 – 4 t CO2e/t Cu produced. The grinding media use is typically 1 kg steel/t ore, and assuming 2 t CO2e/t steel consumed, this results in about 0.3 – 0.6 t CO2e/t Cu produced of embedded emissions.

This means that copper in concentrate production at an open pit mine site typically results in around 5.5 – 9.5 t CO2e/t Cu if powered by diesel and coal fired power, and with readily accessible water supply. Significant reductions (>50%) could be achieved if:

  • The open pit mine transitions to a higher grade underground mine with less waste movement (reduces both power and diesel related emissions)
  • Coal power is converted to lower GHG intensive sources, e.g. combined cycle natural gas, or renewables
  • Diesel for haul trucks is replaced with low GHG energy sources, e.g. biofuels or low GHG electricity
  • Modifications are made in the process area to significantly reduce energy and steel media consumption, e.g. ore sorting to separate low grade, uneconomic material from ore stream; SAG mills are replaced with high pressure grinding rolls; higher quality and lower wearing media is used in grinding

For sites with no nearby water, a significant amount of power may be required to treat and pump water to the mine site, this is now commonly the case in Chile. For such mines, it becomes critical to consider both the GHG intensity of power for water treatment and conveyance, and practices to minimize water consumption (e.g. maximizing water recovery and reclaim from tailings) in order to manage costs and GHG emissions. 

The economic, environmental and social factors for the above alternatives need to be considered in order for copper mining to stay viable. Continued efforts by industry to optimize and advance the improvement initiatives discussed will be important to reduce GHG emissions in metals production, even in the face of declining ore grades. The past paradigm of "bigger is better" won't be sufficient in the face environmental constraints for the mining industry, whether these be GHG emissions, water consumption or disturbance footprint.