What are They?
Filtration Aids used for the reduction of moisture content in filter cakes are typically based on surfactants and polymers.
These products work in all or a combination of the following mechanisms:
• Improved drainage of filtrate (usually water) from the void spaces in a filter cake by lowering the surface tension and increasing the particle surface hydrophobicity.
• Reduced capillary action to encourage removal of filtrate from the ‘inside’ of the particles in the cake.
• Reduction of ‘micro bubbles’ or foam within the filter cake that can interfere with the transfer of filtrate.
• Agglomeration of fines in the cake to prevent them from blinding the filter cloth/wire, medium or cake.

Why do Plants Use Them?
Dewatering aids provide benefits by improving filter cake and/or filtrate quality.

• Reduces moisture content of the cake thereby reducing dryer/calciner costs through fuel saving.

• Lowers the weight of the cake, reducing transportation and disposal costs.
• Improves washing efficiency of the cake to recover valuable filtrate streams.
• Lowers wash water requirements reducing both water and wash water heating costs.

• Improves cake handling and release from the filter cloth
– increasing throughput of filter table.
• Reduces blinding of the filter cloth/wire reducing cloth costs, and maintains filter performance.
Application
Many processing plants add filtration aids to the filter feed; however, this is often not the optimum addition point. Sometimes a combination of products may be required in order to combat and address the mechanisms on the filter plant that are limiting performance.
Often, flocculants (dosed to the feed) can be used to agglomerate fines and dewater the cake. Surfactants are dosed via wash and spray bars, dewatered to ‘defoam’ the cake and improve drainage and reduce the capilliary action. A simple positive displacement metering pump (LMIs for example) are used to dose the product.

Dosage
Dosage may range from 25g/mt to 70 g/mt depending on the nature of the slurry feed, the operating conditions and the equipment being used. It is recommended that dosage curves and
optimum operating conditions be simulated with a filter leaf or a Buchner funnel. A typical moisture vs dosage curve is demonstrated in the graph below:

There are many factors that can affect the dosage of the dewatering
aid. Some of these are:
• Particle size – Typically the coarser the particle the better the dewatering and hence the lower the dosage.
• Level of vacuum – Too high or too low a vacuum is detrimental. Too high a vacuum tends to compress the cake while too low a vacuum does not provide enough force to remove the water.
• The condition of the filter cloth – old, hardened filter cloth or cloth with holes – will give poor results, as will blinded cloths.
• High wash water temperature reduces the viscosity of the liquid in the void spaces and hence improves dewatering. It also allows for improved dispersion of the dewatering aid, especially in the case of AERODRI® dewatering aids. Hence, the higher the wash water temperature, the better. Some plants inject steam into the wash water line to the filter.