Potassium is an essential element for the growth of plants. Soluble potassium salts, known as potash, are used as fertilizers to fulfil plant nutritional needs and to boost crop production. It is projected that the demand for potash in the agriculture industry would increase at a compound annual growth rate of 3.2% between 2024 and 2029. Currently, potash is commercially mined from sea brines and buried ancient seas. This makes potash extraction geographically confined, with substantial commercial reserves concentrated in a few countries, including Canada, Russia, Belarus, China, and Germany. Due to the limited number of suppliers, the global potash market is highly sensitive to international relations, trade policies, and even conflicts. 

Potassium can be found abundantly in certain clays and rock-forming minerals. K-bearing feldspars are estimated to account for approximately 12 wt. % of the earth’s crust and they can contain up to 14 wt.% of potassium. This abundance makes K-feldspar an attractive potential potash source. The widespread availability of K-feldspar across various regions of the world suggests that, if economically viable extraction methods can be developed, it could contribute to diversifying the potash supply chain and to meet the growing demand for this mineral. However, still, economic extraction of potash from these minerals is not feasible.

This work is part of the ERA-MIN project POTASSIAL, which aims to develop a zero-waste process for the treatment of K-feldspar in order to provide an alternative source of potash and alumina. The main processing steps suggested to produce potash and alumina involve intensive grinding, HCl leaching, separation and purification processes, crystallization, and roasting. This doctoral project is part of the work package aimed at developing a suitable separation and purification approach for treating the leaching solution, with an emphasis on potassium recovery. Two different recovering/purification methods, namely anti-solvent crystallization and solvent extraction have been included in this work for the recovery of potassium. The selection of these methods was based on the following requirements set for the formulation of the purification processes: (i) minimal neutralization of the leaching solution, (ii) maximum separation of impurities, and (iii) feasible concentration of potassium. 

Anti-solvent crystallization was selected for direct and selective recovery of potassium in the form of muriate of potash. The aim for using this process is to combine both purification and crystallization steps for potassium recovery. Initially, screening experiments were carried out with the aim of analysing the crystallization behaviour of key components. Screening experiments were performed using five anti-solvents, namely methanol, ethanol, acetone, 2-propanol, and ethylene glycol. Acetone and 2-propanol were viable options for crystallization of potassium chloride. Using acetone and 2-propanol, the effects of anti-solvent ratio, time, and anti-solvent addition rate on potassium-chloride crystallization were then further investigated. A recovery of 83% of potassium was achieved when using acetone at the O/A of 5 with the addition rate of 10 ml/min, at room temperature with a hold time of 180 minutes. The optimum conditions for 2-propanol were determined to be similar, except for using a 5 ml/min addition rate for 79% recovery. The final muriate of potash products had a purity of over 99.9% using either of the anti-solvent. However, differences in morphology and crystal size of the products were observed.

Experiments were followed by determining the efficiency of solvent extraction using crown ethers for possible purification and concentration of potassium from the leaching solution. This approach was chosen for situations in which the potassium content in the leaching solution is inadequate for direct crystallization and crystallization methods other than anti-solvent are applied. Crown ethers were chosen as the extractant due to their ability to function in highly acidic environments. Accordingly, the effects of HCl concentration, extractant type, diluent, extractant concentration, and organic-to-aqueous phase ratio on potassium extraction efficiency was examined. Dibenzo-18-crown-6 diluted in m-cresol showed to preferentially extract potassium (85% recovery) from highly acidic HCl solutions (2 to 6 M), with minimal co-extraction of impurities, such as aluminium and sodium.

Finally, two scenarios were considered for the purification of K-feldspar leaching solution to recover potassium: 1- Anti-solvent crystallization followed by distillation (for the recovery of the anti-solvent) and 2- Solvent extraction with crown ethers followed by evaporative crystallization (to recover potash). Economic factors associated with these scenarios were examined to establish the conditions under which each treatment approach is preferred for recovering potash from a K-feldspar leaching solution.