Factors Influencing Filter Cake Moisture Content

The -28 mesh coal fraction has a major influence on the coal preparation plant product. This is due to the much higher specific surface area of this fraction which of course is filmed with water (unless themally dried). While 15-25% of the plant product might be -28 mesh, proportionate more water is contributed. As a large percentage of this material is dewatered by disc filters in the United States the most influential variables affecting moisture content of discharged filter cake are discussed.

Particle size distribution of the -28 mesh fraction is a major factor. In this regard, the -200 mesh is of greatest importance. If there is more than 10% -200 mesh, almost 100% of the specific surface area will be found in this fraction. As the bulk of floation coal producted in the U.S. ranges from 30-40% -200 mesh for example, there is a relatively high specific surface area.

It is also important to minimize the pressure drop from the bottom of the filter media to the vacuum pump suction in order to maximize this value across the cake. This can be done by proper design of the filter drainage network and piping to minimize hydraulic friction and elimination of air leaks in the connection of the disc sectors to the filter center barrel. Another factor is proper maintenance of the disc sectors to eliminate holes in filter bags.

Agidisc Filter Design Developement

In order to significantly increase the cross sectional area of the internal channels inside the center barrel, a fabricated steel construction was employed instead of cast iron. Accordingly, “pie-shaped” channels of twenty square inches cross sectional area each was obtained for the twelve foot six inch diameter disc as compared to the former 8.3 square inch cross sectional area for the circular cast iron ports. Thus, for the same quantity of air and filtrate, velocity was reduced by 58.5%.

The ferrule completes the contact between the individual sector and the center barrel. The previous design was steel and required a gasket. Hydraulic studies showed that there was a significant pressure drop approaching one inch of mercury in the steel ferrule.

Blow Discharge

A “sudden blow” has been developed using a low pressure blower with a 0.21 to 0.35 kg/cm² gage discharge pressure (3-5 psig) for cake discharge. Large diameter solenoid valves operating off a cam rider provide the sudden burst of air to dislodge the cake. Thus, a lower horsepower compressor is required saving energy costs.

It will be noted that not only is the open area doubled but flow area is increased 2.84 to 5.7 times. Thus, sector life should be increased and there is far less opportunity for plugging of drainage channels as experienced with the redwood sectors.

Performance Results in Operating Plants

A complete disc of PIPPED sectors were installed on an old style 8′ 10″ diameter by 8 disc Agidisc filter at a preparation plant in West Virginia. Feed was from flotation employing DOWEL M222 reagent with fuel oil. The PIPPED sectors were installed about one year ago on a single disc and were fitted with 40 x 60 mesh stainless steel screens. The other seven discs employed redwood sectors with similar stainless steel screen.

Discharge was practically complete with the PIPPED sectors. On the other hand, discharge from the redwood sectors is only achieved by cutting of the discharge by the scraper blades resulting in appreciable cake left on each sector. Accordingly, capacity per unit area is substantially increased on the PIPPED sectors and the redwood sectors show blinding tendencies.

More complete discharge was experienced with the PIPPED sectors and many times the cake would not discharge on the redwood sectors. It was also indicated by the management that redwood sectors had to be periodically unplugged in the drainage groves which did not occur with the PIPPED sectors.