Case Study for Utilisation of CO2 from Flue Gases in PCC Production from Slags

Synthetic calcium carbonate, commonly known as precipitated calcium carbonate (PCC), is used as a filler and coating pigment in papers and plastics. Although the production of PCC binds CO₂ as carbonate, more CO₂is emitted when the limestone that provides lime as raw material for the process is calcined. An alternative PCC production concept enables extraction of calcium from steelmaking slag using ammonium salt solutions, which omits the need for fresh limestone and its calcination. This PCC production concept has the potential to lower the carbon footprint from PCC production and simultaneously reduce the mass of slag generated.

This article presents a techno-economic assessment for producing PCC from steelmaking slag. The studied case consists of a stainless steel plant generating slag and a paper plant consuming PCC situated at a relatively close distance. The process uses ammonium salts, such as ammonium chloride, as solvent for extracting calcium from the slag and subsequently precipitating calcium carbonate using lime kiln flue gas. The process was modelled using Outotec HSC Chemistry software. The environmental footprint of the process was calculated with GaBi software. Additional experiments, such as calcium extraction from steelmaking slag and filterability of PCC, were performed for acquiring a better estimate of certain process parameters.

The results from this work indicates that the process could in its current stage of development be economic for producing calcium carbonate to replace ground calcium carbonate (GCC) used by paper mills. At the same time, the process would avoid 0.3 t CO₂ emissions per tonne PCC produced. Although the operational costs for the process were found to be similar to that of a conventional PCC process, the investment is more than twice the cost of a conventional PCC process. The main reason for the high investment costs is that the studied process uses an ammonium salt solution needs to be recovered for reuse and removed from the PCC product, while a conventional PCC process uses only water as a process solution.