Rotary Vacuum Drum Filter Basics

As the drum emerges from the slurry tank, spray bars wash the cake, allowing air, water, and the remaining liquor to travel through the drum section into piping, through the valve, and into a vacuum receiver.

As the rotary vacuum drum filter begins to rotate facing the ceiling, it enters the drying zone. The drum remains under vacuum, pulling air through the cake for more dewatering.

The drum then reaches its discharge zone as it roll to the position just over the slurry tank. Drum filters discharge in a variety of ways; scraper discharge, belt discharge, string discharge, roll discharge, and pre-coat discharge.

Scraper discharge is best for cake that easily separates and for cloth that only requires periodic washing. As the cake approaches the scraper blade, a low pressure burst blows the cloth up, allowing the blade to scrape away the cake.

Belt discharge is ideal if the industrial filter media rapidly blinds. Instead of a fabric caulked into place on the rotary vacuum drum filter, a traveling endless belt leaves the drum face after drying, passes over a demooning bar, then discharges on a roller that turns the belt at least ninety degrees. The belt then runs through a wash trough and spray bars on both sides prior to rejoining the drum and reentering the filter tank.

String discharge filters are best for discharging gelatinous cakes and cakes composed of cohesive particles. Parallel strings wrap the entire face of the drum and discharge rollers similar to those of a belt discharge vacuum drum. Unlike the belt discharge drum, a string discharge drum has industrial filter media caulked on, allowing for good filtrate clarity.

Roll discharge filters are great for very thin and tacky cake. As the cake approaches discharge, a light blow back occurs allowing the cake on the drum to make contact with, and stick to, cake on the roll. A scraper blade or wire then discharges excess cake from the roll. This discharge method is great for fabric filter wear life, since there is no mechanical wear.

Pre-coat discharge is used if slimy, sticky, oily, or blinding solids are present. Cloth covers the drum, then around four inches of diatomaceous earth, perlite, cellulose, carbon, or some combination form the ideal filter cake. Next, the cloth runs through the slurry, picks up and dries the problem solid. Next, a scraper blade removes the solid along with a thin layer of the pre-coat. This pre-coat removal gives every cycle the ideal filter cake to pass through.

The drum completes its rotation in a no vacuum zone prior to reentry into the slurry. This dead zone is necessary to maintain vacuum across the surface of the rotary vacuum drum filter. While we just went through one panel’s rotation, the rest of the drum continued its own function. There is always a section of drum going through each step of the process.