TECHNOLOGIES, ENERGY CONSUMPTIONS, COSTS AND ENVIRONMENTAL IMPACTS IN THE EXCAVATION, PROCESSING AND TRANSPORTATION OF LIMESTONE AND OLIVINE

Abstract

Recent research shows that it is no longer possible to achieve net-zero emission target without removing CO₂ from the atmosphere. Some Carbon Dioxide Removal (CDR) techniques such as Ocean Alkalinization (spreading of alkaline materials over seawater) and Enhanced Weathering (accelerated dissolution of minerals such as olivine in the ground) may sequester from millions to billions of atmospheric CO₂ per year (Caserini et al, 2021; Beerling et al, 2020). Worldwide implementation requires extracting and processing equally large amounts of raw materials, mostly given by limestone and olivine-rich rocks, with high costs and energy consumptions. Hence, three main stages of the process chain of such raw materials, have been analysed so as to elucidate components and processes leading to the higher-than-average energy consumptions. Specifically, mining, comminution and long-haul transportation have been surveyed in each of their sub-stages. Mining has been broken down in drilling, blasting and material handling and hauling whereas comminution in crushing, grinding and milling. Environmental impacts in the excavation have also been discussed. Overall, it has been found that Material handling and hauling is the crucial stage in the extraction. The energy consumption is of ~3.8 kWh t^-1 against the total which is of ~5 kWh t^-1. Its energetic impact is thus of 80% over the total and at least 50% of the total energy consumed for ton of ore mined is caused by diesel engines. Long-haul transportation has been analysed through four transport modes: road, train, maritime and inland waterways transportation via large ships and barges/small vessels respectively. It appears that land transportation, if carried out by diesel-powered trucks and for long distances, is the most energy intensive transport mode; maritime transportation is the cheapest; rail transportation is the most efficient. Road transportation, if distance is greater than 100 km, may exceed 40 kWh t^-1. The other pivotal stages are grinding and milling. Here, the energy demand can greatly vary with the kind of equipment used, the geological hardness of the rock and most of all, with the diameter of the end-product. Calculations show that energy consumption is ~ 3 kWh t^-1 for diameter grain size of ~ 1 mm and ~ 822 kWh t^-1 for ~ 2μm (Strefler et al, 2018). These processes never produce particles with a single size but distribution of particles with different sizes. The micronized scale can be necessary since dissolution in the water of alkaline materials is largely controlled by particle grain size; the lower the diameter, the fastest the dissolution (Rinder et al, 2021). Electricity is the primary source of energy used to power equipment. Another extremely energy intensive operation is calcination. It is required if slaked lime (Ca(OH)₂) is used instead of mined and ground limestone (CaCO₃). Thus, a comparison has been done. It has been found out that, in calcination, energy consumption may require up to ~988 kWh tCaCO₃^-1, which is a contribution reached only in the ultra-fine milling. For what concerns operating costs, in the excavation the total cost is about ~ 1.5 €t^-1 whereas in comminution, similarly to the energy consumptions, costs are much more variable, ranging from ~2 € t^-1 for diameter grain size of 50 μm and ~80 € t^-1 for 2 μm. (Strefler et al, 2018). Other influencing factors are the cost of electricity, labour costs and fluctuations in the market prices. Nevertheless, both material handling, hauling and milling can lower their current energy consumption of at least 37%, if investment on new technologies, policies to upscaling renewables, energy efficiency and electrification could take place at a fast-enough pace and within a wider system approach.