Rotary Drum Vacuum Filters - Belt, Scraper, Precoat, Roll Discharge
The NFM Rotary Drum Vacuum Filter is a continuous filter for low-to-medium production rates with medium filterability. It is used typically with medium to low resistance filter cakes. However, the NFM Precoat Discharge Rotary Drum Vacuum Filter uses filter aids such as dicalite or perlite as a precoat for high resistance filter cake conditions.
A horizontal drum floats in a vat into which process slurry is pumped. The drum rotates through then out of the slurry, with the following sequence:
Zone 1 Cake Formation: with the drum submerged in the process slurry, the suspended particles are pulled by vacuum on to the deck.
Zone 2 Cake Drying Step #1: the drum has rotated out of the slurry and the continuous vacuum begins to dewater the filter cake by drawing liquid out of the cake and into filtrate pipes internal to the drum.
Zone 3 Cake Washing: at approximately 10 o’clock position, cake washing can occur by spraying a wash solute on the filter cake.
Zone 4 Cake Drying Step #2: after cake washing, the cake dewaters further as the drum rotates toward the 12 o’clock position
Zone 5 Cake Discharge: after final dewatering, the filter cake is discharged from the drum deck, after which the drum rotates back into the process slurry.
The drum deck is sectionalized into individual division strips to control vacuum without losses. Each section is an integral vacuum zone that prevents the leaking of vacuum from one zone to the next. Each section has its own filter fabric, support grid, and filtrate piping often positioned in leading and lagging parts of each section. This design promotes high hydraulic rates out of the filter.
The filtrate pipes welded to each section are then turned through the inside of the drum to the the non-drive side drum trunnion called the filtrate end. The filtrate pipes are welded in an equally spaced pattern to a pipe plate. The pipe plate is in effect a manifold for all of the filtrate pipes.
Mounted tight against the rotating pipe plate is a stationary rotary valve that has a sacrificial wear plate which takes the friction between the pipe plate and the valve body. The valve body not only directs the liquid filtrate away from the drum. It also allows for vacuum control in the sections of the drum, to segregate cake formation from cake drying from cake washing to cake discharging. This vacuum control is called the “timing” of the vacuum with the use of adjustable bridge blocks within the valve’s internal cavity. The location of the bridge blocks allows control of vacuum where it is most useful and then, with certain designs, to cut off the vacuum when the cake must be discharged.
To segregate cake formation from cake drying, the drum is commonly submerged from 10% up to 37.5% in the slurry. During cake formation, the vacuum applied deposits suspended slurry solids on top of the filter fabric. During cake drying, the vacuum continues to be applied to the filter cake. The cake now creates a pressure differential that dries the cake to low moistures. During cake discharge, the vacuum is released, and the cake is discharged several different ways: belt discharge, scraper with air blow-back discharge, precoat knife discharge, roll discharge, string discharge.
In the vat is an oscillating agitator that sweeps the bottom of the vat to uniformly suspend and distribute.
Filtrate discharging from the rotary valve flows to a vacuum receiver tank. In this tank, the air-filtrate velocity is significantly reduced to allow for separation. The air discharges to the vacuum pump through the top outlet of the tank while the filtrate discharges to a self-priming pump located at the bottom outlet of the tank.
The drum filter commonly utilizes VFD drives on the drum, on the discharge roll, and on the agitator. The production rate is directly determined by the drum speed (measured in “MPR” or minutes per revolution) and drum submergence (or vat level). However, the discharge roll and the agitator can indirectly help or hinder the rate.
Operating trade-offs often pose dilemmas for operators. High drum submergence will generally yield thick cakes. However, the moisture content may be high due to the shorter dry time. When the low cake moisture is required, then the drum submergence is reduced to offer more dry time. But then the cake thickness will be low resulting in a lower production rate.
Form time versus dry time can be precisely managed with the timing position of the bridge blocks internal to the rotary valve. High vacuum levels during cake formation increase cake thickness and yield. Compressible cakes, however, may have a high pressure differential which will reduce the production rate. If cake washing is required, the chances of cake cracking from premature dewatering is often by the vacuum level. If cake washing is critical, the wash solute will diffuse through the filter cake if the cake is thin and the vacuum flow is high.
The filter fabric selection is an important consideration. The trade-offs for operators are chemical and temperature compatibility, fine particle capture for high filtrate clarity, low blinding tendency, durability with abrasive solids, cleanability, and of course cost. This also applies to filter aids such as diatomite, perlite, cellulose, fly ash, and some process derivatives.
The NFM Rotary Drum Vacuum Filter is used on the following process conditions:
- For slow filtering, moderately settling or not settling slurries
- P80 Particle Size ranges from 5µm to 100µm
- Cake Thickness ranges from 3mm to 50mm
- Cake Formation Time ranges from 30 seconds to 300 seconds
- Cake Wash Displacements cannot exceed three
- Scraper Discharge
- Simple air blowback discharges cake
- P80 Particle Size ranges from 30µm to 100µm
- Cake Thickness ranges from 10mm to 50mm
- String Discharge
- Cohesive cake that will not accept air blowback
- P80 Particle Size ranges from 20µm to 100µm
- Cake Thickness ranges from 5mm to 30mm
- Roll Discharge
- Thixotropic, tacky cake
- P80 Particle Size ranges from 3µm to 30µm
- Cake Thickness ranges from 2mm to 20mm
- Belt Discharge
- Cake requires assistance to discharge; filter media must be washed
- P80 Particle Size ranges from 5µm to 20µm
- Cake Thickness ranges from 2mm to 15mm
- Precoat Discharge
- Polished Filtrate Liquor is the product with solids that blind a filter cloth on contact
- P80 Particle Size ranges from 1µm to 10µm
- Cake Thickness is less than 2mm
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