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Simulation of the crystallising

Flow Simulation boosts Efficiency in the Salt and Potash Industry

Minimise Production Times and Costs

The salt and potash industry have not been dragging its heels when it comes to the ongoing improvement of mining and production techniques. Modern software tools offer efficient numerical methods for flow simulation that can identify and develop optimisation potential, boost efficiency and help introduce innovative operational procedures.

Simulation modelling provides a better understanding of the operating processes and ensures a high degree of transparency when managing technical characteristics and cause-and-effect mechanisms. Its application not only helps secure existing production technology but also supports a sustained increase in competitiveness.

Potassium chloride (KCl) is required in huge quantities around the world as an important mineral fertiliser for the agricultural sector. It also serves as a raw material for a wide range of industrial products – from plastics to pharmaceuticals. This market demand is met by naturally occurring potash salt that is extracted from underground deposits.

High Quality and Purity required

If the high quality and purity requirements are to be met it is vital that all unwanted associated minerals (especially sodium chloride – NaCl) are separated out. The crystallisation process is particularly suitable for the production of very high-purity products and the salt and potash industry has therefore been using this particular processing technique for many years. 

K+S AG, which is one of the world’s largest suppliers of fertilisers and industrial salts, has recently commissioned a new mine near the community of Bethune in the Province of Saskatchewan in Canada [1, 2]. Because of the favourable climatic conditions in this area this new facility uses a cooling pond (Fig. 1) for recovering the crystalline potassium chloride. The pond’s operating principle is based on the cooling of a salt solution in a meandering channel. Taking up an enormous 500 m in width by 600 m in length the pond has been set up for high-volume production.

Analysis of physical Processes using Simulation

While the fundamental principles of the system are fairly simple the underlying physical processes are in their detail quite complex and are often difficult to reconstruct in full by experimental means. For this reason K+S has opted to employ Ansys Fluent, a high-performance CFD tool (computational fluid dynamics) designed for flow simulation.

This software solution is able to model the effects that take place as the process develops. This ranges from the simple simulation of the flow and the coupling with temperature fields through to the modelling of crystallisation. Working with simulation specialists CADFEM GmbH a model for calculating the actual production process has also been developed for the Bethune cooling pond, the aim here being to illustrate the various options available for numerical simulation. 

This work commenced with a two-dimensional profile model of the channel in the cooling pond. This time-dependent simulation is able to answer a number of questions, one of the most important of which concerns, without doubt, the quantity of crystallised salt being produced. However, it can also investigate various uncertainties relating to the composition of the crystallising salts, the fluid in question being obtained using a special technique known as ‘solution mining’ (Fig. 2)

Saturation limit of potassium chloride

This involves the injection of fresh warm water through a borehole into the dissolvable potash-bearing salt rock, a process that creates salt solution-filled chambers known as caverns. In this particular case the caverns can be as much as 1,500 m below the surface. In the second phase of the operation the saturated solution is pumped back up to the surface via another borehole and then processed further, which involves transference to the cooling pond.

This is where the KCl is recovered by the cooling of the mixed NaCl/KCl solution. The drop in temperature gradually takes the solution down to the saturation limit of the potassium chloride. As soon as this point is exceeded the KCl begins to crystallise while the NaCl remains in solution. This means that small salt crystals start to form in the liquid solution, and these then sink to the bottom of the pond and constitute the end product. When a sufficient quantity of crystals has built up on the floor of the pond they are recovered by a floating dredger and then pumped to the processing plant for final treatment. The salt is then used for example as fertilizer.

Parameters affecting Crystallisation

Crystallisation is a very complex phenomenon and is dependent on a number of parameters. Crystallisation in itself involves a mass transfer between two distinct phases. Ansys Fluent is able to model this process by selecting an appropriate multi-phase model. In this particular instance the components dissolved in the fluid (NaCl and KCl) play a decisive role. The crystallisation process is strongly influenced by the solubility of these two substances.

The solubility of KCl is highly dependent on temperature, whereas that of NaCl is almost insensitive to temperature changes. This is the foundation of the entire production process. Climatic conditions and the resulting exchange of energy with the cooling pond therefore have a key role to play in the operation. In order to correctly describe the heat balance a number of relevant effects have to be taken into account, such as the enthalpy (heat of reaction) generated during the crystallisation process. Other parameters of significance include convection, radiation and evaporation on the surface of the water. Fig. 3 shows by way of example the heat flux associated with these three contributions and their relationship to temperature.

The simulation exercise can also investigate the impact of the surrounding soil on the thermal behaviour of the cooling pond. In order to obtain the most accurate picture of the crystallisation process K+S specified both temperature- and concentration-dependent solubility curves and fluid characteristics. These were implemented as part of the Fluent modelling process with the help of UDFs (user defined functions). UDFs are able to expand the capabilities of the software in order to manage client-specific issues.

Influence of temperature on solubility

Various key attributes associated with the system under analysis, such as the interaction of the fluid properties, the thermal boundary conditions and the crystallisation processes, were implemented using UDFs. The model that was created can for example investigate the effect of changing climate conditions on the quantity of crystallised product obtained. In the cycle of the seasons the cooling pond experiences a wide range of conditions that have an impact on product quantity and quality.

It is also interesting to investigate monthly fluctuations in temperature and their influence on the solubility of the components. What is more, the system can also be influenced by variables such as average wind speed and relative air humidity. In strong winds, for example, more heat will be released from the surface of the pond and this in turn has a positive effect on the cooling process. The model-generated temperature profile shown in Fig. 4 is representative of a characteristic climate status.

Composition of the mixed solution

Changes in brine composition can also be virtually investigated, along with different profile forms and associated fill levels. Moreover, the model was extended to the third dimension in order to study temperature and flow behaviour in a more global manner. Due to the inability of a 2D model to capture three-dimensional resolved eddies, the 3D model is well suited to analyse turbulent mixing and its impact on the overall temperature performance.

The degree of mixing, in other words the extent to which as constant as possible a temperature profile is maintained from the surface of the solution down to the bottom of the pond, is in particular of special importance for the uniform precipitation of crystallised product. Fig. 5 depicts by way of example the temperature profile at a selected depth. It is readily apparent just how intensely the solution interacts with the cold environment. Even by the end of the first lane it can be seen that the temperature has fallen by nearly 15 °C.

Minimization of production times and costs

In summary, numerical simulation has many benefits to offer as far as the production of industrial salts and raw materials for fertilisers is concerned. The possibilities available range from all kinds of climate scenario investigations through to comparative analyses of different pond designs.

This means that different versions of proposed construction projects can be analysed and assessed early on so that the most appropriate solution can be selected and further optimised right from the planning stage. This may relate, for example, to measures for increasing the rate of mixing or altering the composition and may also involve studies of other potential operating sites.

These three examples illustrate how simulation can help improve production efficiency at K+S so that expenditure, production times and production costs can be kept to a minimum.


K+S Aktiengesellschaft
Thomas Radtke
thomas.radtke@k-plus-s.com
www.kpluss.com

© Images: K+S Aktiengesellschaft

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First publication of these articles

This article was first published in the journal GeoResources (3-2020).
Radtke, T.; Ziegler, L. (2020): Strömungssimulationen für mehr Effizienz in der Kali- und Salzindustrie. GeoResources Zeitschrift (3-2020), S. 58–60.
Online: www.georesources.net/download/GeoResources-Zeitschrift-3-2020.pdf.

Authors and References

Dr. Thomas Radtke is Senior Scientist at the K+S Analytics and Research Centre, Unterbreizbach, Germany | Contact: thomas.radtke@k-plus-s.com
Lucas Ziegler, M.Eng., is Computation Engineer for Professional Development at CADFEM GmbH, Grafing b. München, Germany | Contact: lziegler@cadfem.de
[1] Elfferding, M.; Grommas, J.; Stax, R. (2015): Das K+S Legacy Projekt – Solution Mining auf Kali in Kanada. In: GeoResources Zeitschrift (4-2015), S. 36–46. Online:  www.georesources.net/download/GeoResources-Zeitschrift-4-2015.pdf
[2] Elfferding, M.; Grommas, J.; Stax, R. (2016): The K+S Legacy Project – solution mining for potash in Canada. GeoResources Journal (1-2016), pp. 42–51. Online: www.georesources.net/download/GeoResources-Journal-1-2016.pdf

 


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