The particle size characteristics evaluated are mean particle size and the fine percentage of the distribution. While neither is a real concern from a blinding standpoint as compared to a conventional fabric, it is a factor regarding long-term baghouse pressure drop levels. Once fines are introduced into a fabric filter system – whatever the cleaning method employed – they are extremely difficult to get out of the system. Even with off-line cleaning, a disappropriate percentage of fines will remain in the bag mass and will bias the permeability of the generated filter cake to the lower end, which leads to a higher initial pressure drop. While this will happen eventually, the use of the precoat filter aid will delay the impact of the introduction of the fines.
The belt filter is often fabricated in modules to segregate the several stages of filtration. It utilizes a continuous rubber drainage belt that supports a continuous filter fabric belt upon which a vacuum is drawn. They travel under vacuum through the various zones down the length of the machine.
Zone 1 – Slurry Pooling:
Either a weir-controlled feed box or a slurry distribution tray gently delivers the process feed to the filter deck, uniformly distributing and pooling the process slurry on the full belt width. The vacuum, if applied at all, is flow-controlled in this initial zone.
Zone 2 – Cake Formation:
Vacuum is applied to fully set and form the filter cake. The mother liquors drain through the filter fabric belt, into the grooved channels integral to the rubber drainage belt, and finally into drain holes located directly over the vacuum pan. The mother liquor filtrate flows into the vacuum pan and into a filtrate manifold located along the length of the filter.
Zone 3 – Cake Washing:
Co-current or counter-current wash solutions are fed onto the filter cake, flowing across the full width. Wash solutions are delivered in either adjustable weir wash boxes or with spray nozzles located in wash bars mounted on the frame. Several wash stages are possible in addition to several recycle loops of wash filtrates for economy. There is further field adjustment of the cake wash devices’ locations to achieve optimal results.
Zone 4 – Cake Dewatering:
Final dewatering after washing is achieved to decrease final cake moisture.
Zone 5 Cake Discharge:
The filter fabric belt travels off the rubber drainage belt to around a helical discharge roll, followed by a cake doctor blade.
After cake discharge, belt cleaning or washing occurs in an enclosed wash box to remove solids residue.
Belt Tensioning, Belt Alignment, and Belt Steering are separate assemblies designed to ensure reliable tracking of the filter fabric belt at belt speeds up to 70 feet per minute.
The Micronics Engineered Filtration Group offers several different support decks upon which the rubber drainage belt travels. Air deck, water lube deck, or roller deck are designed to reduce the friction and therefore the power consumption of the belt drive assembly. The vacuum seal between the drainage belt and the stationary vacuum pan is typically designed with narrow, sacrificial wear belts that travel on plastic tracks – components that can be easily replaced.
The Micronics Engineered Filtration Group offers modular as well as fully assembled horizontal belt vacuum filters, to best manage the installation cost and available area for the filtration equipment.
For medium filtering, fast-settling slurries with the following characteristics:
- Slurry densities > 1.8
- P80 Particle Size ranges from 20µm to 200µm
- Cake Thickness ranges from 6mm to 80mm
- Cake Formation Time ranges from 10 seconds to 100 seconds
- Cake Wash Displacements are typically greater than three, often with Counter-current Washing
- Producing solids at 50 – 300 dry pounds per hour per square foot
- Producing filtrate at 0.2 – 5 gallons per minute per square foot
- Producing cake moistures as low as 5%, depending upon the cake porosity