Preliminary assessment of a method utilizing carbon dioxide and steelmaking slags to produce precipitated calcium carbonate

Abstract

One of the options that can contribute to the reduction of carbon dioxide emissions for climate change mitigation is the so-called CO₂ sequestration by mineral carbonation, or CO₂ mineral sequestration. Steel manufacturing could benefit from this option by utilizing its own by-products, i.e. steelmaking slags to combine with CO₂. We have recently studied a method, where aqueous solution of ammonium salt (e.g. ammonium acetate, ammonium nitrate and ammonium chloride) is used to extract calcium selectively from the steel converter slag, followed by precipitation of pure calcium carbonate by bubbling CO₂ through the produced solution. The ammonium salt solution is recovered and re-used. The purpose of this research was to determine if the economic potential of the method warrants moving forward to large-scale application. Despite the small solvent losses, the method was found to have economical potential. In addition, it has significant CO₂ emission reduction potential as well. Scaling up the reactor from the small laboratory scale will allow more detailed design for the process to be made followed by a full economical evaluation including all of the important operational and capital investment costs.

Introduction

Carbon dioxide capture and storage (CCS) is recognized as one option to tackle the increase of atmospheric concentration of CO₂ for climate change mitigation [1]. While geological storage of CO₂ is the only storage option that has been fully demonstrated [2], storage of CO₂ as mineral carbonate has a larger storage potential [3] and does not require use of expensive monitoring equipment to ensure that CO₂ does not escape into the atmosphere. In mineral carbonation, carbon dioxide is fixed with magnesium or calcium oxide of a certain mineral forming solid and stable carbonate as an end product [1], [4]. Steel manufacturing which is one of the biggest industrial sources of CO₂ emissions [5] could benefit from this option by utilizing its own by-products, steelmaking slags for sequestering CO₂. Steelmaking slags such as steel converter slag and blast furnace slag have a high calcium content and are therefore suitable raw material for carbonation.

Few studies concerning utilization of steelmaking slags as raw material for carbonation have been made [6], [7], [8], [9], [10], but none of these has yet been commercialized. Indirect mineral carbonation methods, where reactive component is extracted from the mineral prior carbonation seem attractive; the end product can be a pure carbonate that has a high market value. The main challenge in mineral carbonation is the slow reaction kinetics. In indirect carbonation, an acidic environment enhances the extraction of reactive compound like calcium, the following carbonation reaction step, on the other hand, is dependent on a basic (alkaline) environment [11], [12], [13], [14]. Specifically, while acidic conditions can be neutralized by base addition, this prevents efficient recycling of the chemicals making the method expensive. In our previous research the possibility to dissolve calcium selectively from the steel converter slag was investigated using various different solvents [15]. We found that ammonium salt solutions were the most promising solvents amongst the tested ones. Besides the two weak base-strong acid salts (ammonium chloride and ammonium nitrate) studied also by Yogo et al. [16] and Kodama et al. [10], ammonium acetate, which is a salt of a weak base and weak acid, dissolved calcium efficiently and selectively. After removal of the slag residue, carbon dioxide was introduced into these solutions resulting in the precipitation of pure calcium carbonate (∼100 wt.%). These ammonium salt solvents were found to be recyclable [17], although minor losses of ammonia were noted.

The objective of the research reported here is to evaluate whether the studied method, which enables production of pure calcium carbonate from steel converter slag by using ammonium salts as a solvent (Fig. 1), has enough potential for scale-up. First the method’s input and output streams were calculated on the basis of earlier experimental work. Preliminary economical evaluation based on these streams was then made. Finally CO₂ emissions reduction potential of the method was evaluated by taking into account possible indirect CO₂ emissions of the method. Two new experiment sets were made in order to enable these evaluations.