Development of a new process to produce gypsum products from potash mining

1 Introduction

With the phase-out of coal-fired power generation and the subsequent loss of gypsum from flue gas desulphurization plants (FGD gypsum) in Germany, the gypsum-processing industry faces a huge challenge. The reason for this is that up to 55 % of German gypsum demand has been met with FGD gypsum in the past [1]. For the opening “gypsum gap”, alternatives must be found to satisfy the existing raw material demand, so as not to fall back exclusively on an increased use of natural gypsum. Even if the general shortage of gypsum as a resource can be alleviated in the short term on account of the current geopolitical action in a temporary period, Germany is sticking to its original plans to phase out coal-fired power generation and the associated decarbonization [2].

On federal and state level, a large number of research activities are underway to find short-, medium- and long-term innovative solutions to increase the availability of recycled gypsum (RC gypsums) as well as gypsum from alternative raw material sources. In this context, for example, the WIR! Alliance “Gypsum Recycling as an Opportunity for the Southern Harz Region” [3] and the Thuringian research alliance “Resource Management and Sustainable Construction” [4] should be mentioned. The WIR! Alliance “Gypsum Recycling as an Opportunity for the Southern Harz Region” is funded in the programme “WIR! – Wandel durch Innovation in der Region” by Germany’s Ministry of Education and Research (BMBF). The four alliance projects in the Thuringian Research Alliance “Resource Management and Sustainable Construction” are supported with funds from the Free State of Thuringia.

The investigation of alternative gypsum sources is also in the focus of the researchers. The use of industrial gypsums in the gypsum industry is established. Gypsums from industrial processes, e.g. from the production of citric acid, hydrofluoric acid, lactic acid and tartaric acid, are already used today. The total volume of synthetic gypsums from chemical or industrial processes is, however, currently at a low level of around 35 000 t/a [5; 6].

For this reason, other industrial processes are sought as alternative sources for the production of gypsum as a by-product. This is where the research project “Production of gypsum products from potash mining” with the acronym PolyGips comes in. The project is funded by Germany’s Federal Ministry for Education and Research (BMBF) in the scope of the funding programme “WIR! – Wandel durch Innovation in der Region” with the funding number: 03WIR0307 (programme time: 11/2020 to 10/2023). In the alliance project between K-UTEC AG Salt Technologies and Nordhausen University of Applied Sciences/ThIWert, the project partners are working to establish whether gypsum can be produced as a by-product in the processing of potash. Specifically, the mineral polyhalite, which is composed from the compounds potassium, magnesium and calcium sulphate, is investigated.

2 PolyGips research project

The overarching goal of the PolyGips project is the production of gypsum from a previously unused secondary gypsum source, the mineral polyhalite. The project researchers are investigating whether it is possible to apply a newly developed processing method to extract gypsum from polyhalite and to send this for higher-value utilization in the gypsum-processing industry. Fig. 1 provides an overview of the work packages and the project workflow.

2.1 Processing of the mineral polyhalite

The state-of-the-art processing methods only foresee mechanical processing of the polyhalite. The mineral is comminuted to a particle size < 2 mm and used as a long-term fertilizer in agriculture. The required fertilizer effect is achieved specifically by the potassium and magnesium sulphate contained in the mineral. The calcium sulphate contained only plays an insignificant role.

The newly developed processing method is aimed at producing potassium sulphate as fertilizer with calcium sulphate as a by-product. Fig. 2 shows a simplified flowchart of the intended process.

The new process entails various process steps:

First, any existing non-relevant minerals are to be removed from the crude salt. For dissolution of the chloride constituents (especially sodium and potassium chloride), first cold leaching is performed, which is followed by a hot dissolution process for removal of the easily soluble sulphates. The remaining leaching residue, which consists essentially of polyhalite, anhydrite and small quantities of other insoluble components, serves as a basis for further processing.

This is followed by thermal activation of the polyhalite to increase its solubility in water.

Following a further hot leaching step, a residue containing calcium sulphate remains.

The process is concluded with several cleaning cycles until the finished calcium sulphate product is obtained.

The process parameters derived from the process, e.g. calcination temperature and dwell time, all have a significant influence on the quality of the ultimate calcium sulphate product. Other influencing process factors are the conditions during the hot and cold leaching steps. Finally, the formation of potential double salts, like, for example, of syngenite (K₂SO₄ x CaSO₄ x H₂O) is to be analysed and prevented as far as possible. The formation of the double salt during processing would be obstructive as it could impair the yield rate of the products potassium sulphate (K₂SO₄) and gypsum (CaSO₄) as shown in Fig. 2.

2.2 Laboratory analysis of the specimen material

Besides the development of the processing method, analysis of the specimen material was another central objective of the project work. At the beginning of the laboratory work, the task was to define comparable requirement parameters. For this, the reference values defined by the Federal Association of the German Gypsum Industry in the information leaflet “Initial Tests for Recycling Plants, Quality Management, Quality Requirements and Analysis Processes” (date issued 06/2020) [7] were taken as a basis.

Fig. 3 shows the calcium sulphate product produced in the processing process from the polyhalite mineral, which originates from potash mining. With selected analysis methods, the material was tested and compared with the defined quality parameters.

In the scope of the laboratory analysis, the purity and moisture content were determined and foreign minerals such as potassium, magnesium and sodium salts were analysed. Table 1 provides an overview of the methods used to test the parameters.

The findings obtained so far show that the calcium sulphate product extracted from the polyhalite mineral with the innovative processing method allow expectation of good gypsum quality and therefore high utilization potential. In view of the high purity, it is clear that the gypsum produced does in principle have the conditions for material utilization. The purity of the material that can be classed as high as it is greater than or equal to 96 % allows consideration and adjustment of the specification parameter purity (85 %) applied by the Federal Association. With a view to practical application, the targeted reference value was upped to 95 %, which roughly corresponds to the purity of FGD gypsum [8]. The low moisture content determined and the measured particle size distribution (median value: 18.56 μm) can also be rated as positive with regard to utilization. In contrast, there is still a need for optimization with regard to the extraneous minerals present, e.g. magnesium and potassium salts. The extraneous minerals can be reduced with further adjustment of the processing method. Selected laboratory findings are shown in Table 2.

3 Outlook

The remaining project processing time is focussed on concluding laboratory tests and the investigation of potential material utilization possibilities for the gypsum extracted from the polyhalite. In addition, a feasibility study with regard to the extended processing method and the energy required is planned. Moreover, assessment of the transferability of the developed process method to other deposits is an integral component of the project. A potential industrial production of PolyGips and the transferability of the processing method to other national and international polyhalitic deposits (e.g. in Great Britain and Eastern Europe) are preconditions for the use of polyhalite mineral as a secondary source of gypsum raw material.