Risto Pajarre & Pertti Koukkari
During the two last decades an unprecedented emergence of tens of chemical elements has occurred both in industrial and consumer products. While pursuing to re-use these materials a vast number of new chemicals and their mixtures will appear in the recycling processes. However, ignorance of the property data of the new mixtures hampers the design of the recovery processes and raises recycling cost. Mapping of the material properties of such new chemical compositions will be an unavoidable task in the future circular economy. Closeloop project is answering to this challenge by generating mixture data with new computational methods.
In addition to thermodynamic equilibrium methods, algorithms that are based on the minimisation of Gibbs free energy, but enable simulation of kinetically constrained dynamic systems have been developed. Thermodynamic modelling provides a possibility to create virtual descriptions of complex processes often with startling accuracy and has shown to be a most efficient tool for process development. Programs developed at VTT during the last two decades (ChemSheet and Kilnsimu) have been taken into use in six continents, mostly in solving industrial problems related to more efficient use of raw materials and to energy and materials recycling. Practical applications span from controlling sulphate emissions of Talvivaara mine in Finland to cutting the consumption of fossil carbon by 50 per cent in recycling processess in Japan or in Germany.
Thermodynamic simulation is based on the thermodynamic property data. In addition to properties of pure substances, a process chemist will also need the data on how the elements and compounds behave when mixing and reacting with each other. The progress in chemical thermodynamic simulation is founded in knowing the formation energy parameters for both pure substances and mixtures. The same basic data is generally applicable to many different practical processes. For example, the mine water simulation tools developed by VTT include these properties for approximately ten metal ions and five anions (Table 1). The same ions are important and have been applied in both mining and paper industry neutralisation and wastewater management purposes (the more economically valuable metals having been captured in earlier processes stages at a mine)
The aim for sustainable circular economy however creates new challenges. During the last few decades, more and more chemical elements have found use both in industry and in consumer products. In 1990’s the number of elements in a computer circuit boards was 12, nowadays in a corresponding product over 50 (Picture 1). For the great majority of these only the property data of pure substances have been determined while the mixture parameters are for most part unknown. A typical example is one of the topics of the CloseLoop project, lithium and its compounds, both in high temperature salt and and slag mixtures as well as in aqueous solutions. The lack of adequate property data hampers both the design of primary extraction processes and development of recycling technologies. Mapping of such data has traditionally been possible to solve only by added experimental research.
Study of property data is emerging as a key challenge in development of circular economy. At least some extent the traditional thermodynamic experimental measurements can be replaced by computational methods. The ab-inito methods in combined with thermodynamic continuum models are applicable to relatively simple high temperature systems. More complex cases such as aqueous solutions may susceptible to machine learning (artificial intelligence) methods for deducing interaction parameters for new systems based on previously studied cases.
Whereas physicists and materials scientist have harnessed the properties of ever more elements and compounds for producing various high-tech products, the chemists must accept as a challenge the interactions of these substances when recovering and recycling them.
Table 1. Key ionic species in the VTT aqueous database
|Major species||Na+, K+, Ca2+, Mg2+, H+||Cl–, SO42-, OH–, CO32-|
|Minor species||Al3+ ,Fe3+, Cu2+, Mn2+, REE3+||SiO44-|