The agitated nutsche filtration process begins when the slurry is introduced to the vessel, either in bulk or in gradual volumes. Agitation generally doesn’t begin until this initial step is complete. The agitator, which is a bladed drive assembly within the vessel, is then initiated. This begins a low-speed high torque mixing action close to the filter media to ensure the cake height doesn’t inhibit the filtration rate. A gas pressure is subsequently applied and solid—liquid separation begins. As filtration advances, the agitator blades rise to the top of the product cake to smooth out cracks and/or preferential channels. This process is quicker than relying on gravity, and allows for faster filtration rates.
Once the API crystals have been fully separated, any remaining filtrate and impurities must be washed-out. This can be carried out via a traditional or pre-slurry wash—the latter generally being preferred. A traditional wash introduces a solvent which seeps through the product cake to displace impurities or mother liquor residual traces. This is often conducted multiple times for best results.
A re-slurry wash may be conducted, which is a similar process albeit with the agitator ploughing the solvent into the cake before pressing. Solids are fully resuspended which increases the contact between solid surfaces and solvent, therefore reducing the total volume of solvent needed to reach equivalent purity levels. At the end of the process, persistent liquid is pressure filtered from the system.
After washing, the drying process can begin. Where it is necessary, there are two main options: pressurised gas heating, or vacuum-assisted heating. Both methods depend on optimal heat transfer within the vessel; so, heat transfer media is typically applied to the sidewall, filter base, and agitator blades. Additionally, the agitator continues to serve a critical purpose by providing gentle mixing and drying homogeneity.
The agitator ensures vertical homogeneity in the cake layer by raising/lowering as required. Even more difficult products can be broken down by temporarily reversing direction. This level of bespoke control extends to the temperature-controlled zones too. These can be cooled down at the end of the process to bring the product temperature down to a safe level for manual intervention during discharge.
This is a crucial step, and often the most difficult as the mixing action can be precisely and uniformly programmed throughout the process. Convective drying can dramatically accelerate process times, provided it is carried out effectively.
Dispensing the product
Finally, the agitator again serves a critical role in product discharge, gradually forcing the cake to the vessel wall and discharge plug. A funnel is often used for optimised packing, and heel recovery can be performed using an additional product rake.