For aluminium smelters operating with emissions free power like hydroelectric, nuclear and renewables, the carbon raw materials supply chain accounts for around 15 % of the total smelter CO2 footprint. The calcined petroleum coke supply accounts for 85 % with the balance from coal tar pitch. Work aimed at reducing CPC related emissions can have a meaningful impact on the smelter CO2 footprint and this paper provides a review of calciner CO2 emissions including a carbon capture solution.

The two primary contributors to CPC emissions are green petroleum coke (GPC) production (40 %) and calcination (60 %). Rain Carbon (RC) has done a substantial amount of work to quantify calciner process emissions. A key enabler was the development of a method which utilizes online CO2 concentration and flowrate analyzers to quantify emissions in real time. Reducing GPC fines carryover during calcination is a key means of reducing CO2 emissions. The calciner technology, operating conditions and GPC quality also play a key role.

CO2 capture and storage can be used as a final reduction method. RC has undertaken a detailed capital and operating cost analysis to add a CO2 capture system to its Lake Charles Calciner. The plant is located less than 20 km from a qualified CO2 sequestration site in Louisiana and would qualify for US CO2 sequestration tax credits. Relative to a smelter, CO2 can be captured more efficiently at a calciner due to higher CO2 concentrations. The technology exists today to execute a project like this, but the primary challenge is achieving a satisfactory return on investment. Without a price premium for low-CO2 CPC, the investment return remains a major hurdle.

The petroleum coke rotary kiln calcination process has not changed considerably in recent decades. The only exception is improvements in instrumentation with technological advancements. Calcination is a mature process which is well understood, and operation is reasonably well controlled. Nevertheless, in 2020, a combustion blast occurred after a restart following a short stop of the rotary kiln at a Rio Tinto calcination site. This highlighted a risk that had not been previously identified through the Process Hazard Analysis used by Rio Tinto.

The investigation identified the presence of residual volatiles in the coke bed between the feed end and the air injection nozzles as the cause for the incident. Thermogravimetric analysis indicated the presence of residual volatiles in the coke bed when the kiln rotation was restarted after 12 minutes. This study allowed Rio Tinto’s calcination plants to improve the robustness of their operations through procedures adapted to the risk.

The metal plant will have major and minor process streams of materials. Materials purchased for the production are well characterized, with a certificate from the supplier, and there will be an onsite laboratory for critical quality checks - and underneath all this is an extensive support structure and knowledge base including instrument suppliers, ISO standards and suppliers of Reference Materials.

But for some minor process streams, some recurring waste, and incidental unknown materials this support structure might be not-so-helpful, or maybe non-existing. Hydro Aluminium has addressed this issue. Starting with experience from our onsite analysis of materials for production, principally quality control, Hydro Aluminium has established a practical approach to what we call Complex Analysis. Some of the methodology will be described and examples will be given that include random dust, new and used refractory, new and used cell materials, and electrolyte bath mixed in with other material in several forms.

Complex Analysis uses smart sampling, imagery and XRD phase analysis supported with XRF to understand the material. The XRF matrix modelling is supported with combustion analysis, electrochemical titration and XRD to include the light elements. Risk evaluation and HSE concerns are addressed in all these steps.

Hydro Aluminium has established an effective operational procedure for handling a wide and increasing range of materials from production. And an added benefit is that when materials begin to be well characterized, they can be established as internal Reference Materials, and used when building calibrations.

The carbon anode, which is an important raw material in the production of aluminium, is conventionally produced by mixing granular calcined petroleum coke (CPC) and coal-tar pitch (CTP), which acts as a binder. The mixture is compacted and then baked to produce carbon anodes. With the continued rise in the global demand for aluminium, the demand for carbon anode is expected to increase significantly. The aluminium industry is exploring methods to lower its carbon footprint on the path to achieving net zero greenhouse gas emissions by 2050. One potential approach involves using biomass-based binders instead of CTP-based binders. While this technology is in its early stages and not yet in industrial production, it still generates emissions despite using biomass as a raw material. The current study, which supports the ongoing global efforts to mitigate industrial processes' environmental impact, delves into the comprehensive life cycle assessment (LCA) of CTP-based carbon anode production. By utilizing the LCA methodologies, the results of this study will serve as a baseline for comparison with the potential impacts of bio-pitch-based anodes. A baseline process tree, which represents a cradle-to-gate life cycle inventory process system, has been built to produce 1 t of CTP-based anode and assess its environmental impacts in Quebec. The study utilized the ReCiPe midpoint (H) environmental impact method for the assessment. The results from the study show that CTP and CPC have the major environmental impacts in all environmental impact categories. CTP and CPC accounted for about 69.5 % of the climate change emissions, and 82.7 % of the emissions from human toxicity. The two sensitivity analyses proved that an increase in the total mass of CTP within the accepted range of the mixing ratio from the industry causes a consistent increase in the total emissions in each environmental impact category.