Ceramic Filter Recovers Metal Dust

At its plant in Wales, Mountstar recovers metallic granules from scrap in a furnace operation. In 1991 The Environmental Protection Act required cleaner exhaust gas emissions from the 40-year-old plant. KC Cottrell undertook designing a gas cleaning system for the facility which offered a few challenges. The exhaust gases were potentially explosive. Burning material could be present in the gases at temperatures up to 400°C. The gases were acidic. Much of the particulate was small in size and it was desirable to separate the metallic particulate.

These factors made a bag filter unsuitable, and ceramic filters became likely candidates for the job. The key elements of the gas cleaning plant design were (1) the use of horizontal ceramic filter elements to ensure sufficient removal of the large proportion of submicron particulate and to give security against occasional temperature excursions/incandescent materials; (2) the fitting of a prefilter cyclone to remove the larger metallic particulate such that they could be recycled; (3) the inclusion of explosion reliefs; and (4) the addition of powdered limestone to reduce the levels of acidity.

The final design of the gas cleaning system  met these specifications. Powdered limestone is fed into the gas stream in the duct before the gases enter the cyclone. The cyclone is designed to remove only those particulates greater than 50 microns in size, which in this case are metallic particles recycled into the recovery process. The limestone and particulate accumulate on the outside of 1296 horizontally-installed, one meter long ceramic filter elements. The elements are configured in back-to-back banks, facilitating maintenance. Pulse cleaning partially removes the lime/dust cake at intervals to ensure some dust cake is always present for continuous acid gas removal.

KC Cottrell put the gas cleaning system  into operation in 1992. The system continues to  exhaust low particulate of less than 5 mg/m³. The original filter elements remained in use for over five years.