Rein Kuusik, Senior Researcher, Laboratory of Inorganic Materials

It all began with a series of fortuitous circumstances that support development. As the head of one of the working groups of the research Laboratory of Inorganic Materials, I had been working on reducing the sulphur dioxide content in the flue gases of oil shale power plants (using basic oil shale ash) for a long time, when I came across an article by Jaan-Mati Punning. In the article, the hypothesis was presented that oil shale ash could be a binding sorbent for carbon dioxide (CO2). The results of the initial test added enthusiasm and a bachelor’s thesis confirmed its validity. When one fine day this same student – Mai Uibu – entered my office and said she was writing a master’s thesis, choosing the topic was easy.

That, however, was not all: a previous partner from LUT University called to ask for my recommendations to find a cooperation partner in Estonia on the subject of CO2 sequestration. The main focus of this project (Nordic CO2 Sequestration) was the comprehensive refresher training of doctoral students and in-depth supervision of their work. After that, the study and research activities followed their logical path and ended with Mai Uibu defending the first (and highly praised) doctoral thesis on this topic in the Baltics in 2008.

Basic research at the University of Technology

At first, we focused only on CO2 sequestration/separation from the flue gases of oil shale power plants. We analysed, compared, and evaluated the possibilities of several sub-variants of the process – carbonisation of alkaline circulation water and dry ash and ash aqueous suspension with flue gases –, as well as the nature and perspective of natural binding of atmospheric CO2 in ash fields. We also worked on possibilities for intensifying the process. A concept was proposed to reduce CO2 emissions in the system of AS Eesti Energia (Figure 1 and Uibu, M., Velts, O., Kuusik, R., Developments in CO2 mineral carbonation of oil shale ash. J. Haz. Mat. (2010) 174, 209–214).

All variants of technological sequestration were worked out and several innovative solutions were proposed and patented. We also described the calcium carbonate (PCC, international abbreviation for precipitated calcium carbonate) that precipitates during the process, finding, among other things, that rhombohedral calcite preferentially precipitates from an aqueous solution (Figure 2). Largely thanks to the expertise and contacts of Juha Kallas, who retired from LUT University and joined the laboratory, the cooperation on this topic with UPM-Kymmene OY, a large company in the Finnish forest and paper industry, developed.

The next logical goal was to develop the basic knowledge for the binding of Ca ions in ash water to PCC using CO2.

For this, it was necessary to first understand the mechanism, kinetics, and dynamics of this multi-step process, and then to be able to describe these processes mathematically, as well as to describe the properties of the resulting crystalline product. These tasks were solved in the doctoral thesis of Olga Velts-Jänes (2011). By that time, we had realised that the process of leaching oil shale ash is of key importance in the context of the disposal of this waste. In the doctoral thesis of Kadriann Tamm (2016), the characteristics of portlandite (Ca(OH)2), calcium sulfate dihydrate (CaSO4×2H2O), and oldhamite (CaS) dissolution and their kinetics were determined, which were used for the mathematical description of processes and phenomena in the water-ash system, as well as for the calculation of the corresponding reactors on the ASPEN Plus platform.

As we were practically pioneers in the field of using large-scale industrial waste as a raw material in the production of PCC, our work attracted attention and gave impetus to similar research in the world, including the use of iron slag. However, as Ca compounds are mostly not contained in these wastes – unlike oil shale ash – in a water-soluble form, attention was focused on the use of Ca-selective solvents/salt solutions. Our research group did the same. The first student theses were completed and the first articles were published (Tamm, K.; Viires, R.; Kuusik, R.; Uibu, M. (2017). Calcium extraction from Estonian industrial wastes based on Ammonium solvents. WIT Transactions on Ecology and the Environment, 224, 465−476. DOI: 10.2495/ESUS170431). The perspective of oil shale ash as a raw material increased, because by using solvents, especially aqueous solutions of ammonium salts, instead of pure water, it was possible to increase the Ca ion content in the solution several dozen times, achieving a Ca extraction degree of 90–95%.

Cooperation with the company R-S OSA Service AS began

The Swedish family company Ragn-Sells Grupp, which specialises in the development and implementation of solutions supporting recycling and the circular economy, took advantage of the opportunity more than five years ago. The company operates in four countries; in Estonia, they employ nearly 300 people.

They learned about the unique composition of Estonian oil shale ash and the problems with valorisation. The new results of Tallinn University of Technology, which were checked in the laboratories of the company, gave a positive input for refining oil shale ash by first obtaining PCC. A subsidiary called R-S OSA Service AS focusing on development work in this field was established and in-depth applied research was launched in partnership with the Department of Materials and Environmental Technology of TalTech, the University of Tartu, and the laboratories of the company. Partners in various aspects include Eesti Energia, the French company Tarkett, and the German company Gealan.

At first, the company itself contributed to the innovative works. Later, the state of Estonia has supported the various stages of this project (the largest recycling project in the Baltic and Nordic countries) through Enterprise Estonia and the European Regional Development Fund with a total of almost two million euros. Alar Saluste coordinates the extensive and intensive cooperation as a professional and dedicated project manager. On behalf of Tallinn University of Technology, Mai Uibu, Kadriann Tamm, Marve Einard, Andre Gregor, Can Yörük, Inna and Sven Kamenev, Andres Trikkel, and others have made a great contribution, as have a number of students. The University of Tartu team is led by Riho Mõtlep.

New solutions have been developed and patent applications have been submitted. Oil shale ash valorisation plans have been repeatedly presented in the municipalities of Ida-Viru County, as well as internationally, including at the ACEME2021 conference initiated by the author in Tallinn and at the International Chamber of Commerce at the COP26 world climate conference in Glasgow on 1 November 2021, in the Nordic countries on 2 November, in the European Union in the Estonian-Finnish joint programme on 6 November, and on the world’s largest climate-themed platform We Don’t Have Time on 7 November. The project was also introduced to the Vice-President of the European Commission, Frans Timmermans, during his visit to Ida-Viru County in May of this year, who called it significant in all of Europe on his Twitter account.

A success story for researchers of TalTech as well?

Ragn-Sells plans to start production in 2028 and the so-called demo plant will be launched in 2024. In accordance with the plans, the plant will process about 1.3 million tonnes of oil shale ash per year and will bind 250,000 tonnes of CO2 in the product – ultra-pure calcium carbonate, mainly in the form of its crystal modification vaterite (Figure 3). The company estimates to produce almost half a million tonnes of ultra-pure calcium carbonate per year. As the process can be controlled in such a way that the particle size can vary from a few microns to a few dozen microns, the list of areas of use of the product is wide: as a filler and surface material for paper, in the production of food and medicinal products, in the production of paint and plastic products, as well as in the cement, glass, and ceramic materials industry.

The oil shale ash valorisation plant will bring an estimated 250 million euros of investments to the region, create almost 100 new jobs, and indirectly employ up to 400 more people.

As the internationally recognised working group of the inorganic materials research laboratory of Tallinn University of Technology was the first initiator of the relevant basic research in Estonia and has made and continues to make a significant contribution in all subsequent stages of this project, we can ask: is this a success story of environmentally friendly recycling of oil shale ash? Is this a success story of public and private sector cooperation? Is this the first climate-positive PCC project in the world? We will have to wait and see!