Vertical Pressure Leaf Filter
Vertical Pressure Leaf Filter
Vertical Pressure Leaf Filters are quite similar to the Horizontal Plate Filters except for the orientation of the filter elements which are vertical rather than horizontal. They are applied for the polishing slurries with very low solids content of 1-5% or for cake filtration with a solids concentration of 20-25%.
Just like the Horizontal Plate Filter the Vertical Leaf Filters are also very well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved. These features are not possible on Filterpresses which require the opening of plates to the atmosphere and shifting them one by one to allow cake discharge at the end of each cycle.
The largest Leaf Filters in horizontal vessels have a filtration area of 300 m2 and vertical vessels 100 m2 both designed for an operating pressure of 6 bar
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In most of the fine chemicals processes the leaves are fitted into vertical vessels whilst horizontal vessels are used in the heavier process industries such as the preparation of sulfur in phosphoric acid plants. The leaves inside horizontal tanks may be positioned either along the tank axis or perpendicular to the axis.
In order to utilize the tank volume for maximum filtration area the width of the leaves is graduated so they fit to the circular contour of the tank. This also reduces the slurry heel volume that surrounds the leaves.
The vessels are fitted with highly secured cake discharge openings to ensure safe sealing of the tank under pressure. The cake that accumulates on the leaves may be discharged as a wet thickened sludge or as a dry cake. For wet cakes the vessel will normally have a small outlet that is fitted with a valve whilst for dry cakes the opening is large and the closure locks up electrically or hydraulically with a bayonet wedge.
The head cover of vertical vessels is often pivoted so that it is swung away to allow the upwards removal of the leaves in the stack. It is good practice to design a special rig that will support a leaf that is removed from the vessel.
Special quick opening bolts are fitted around the cover so that tightness is secured during operation but enable easy opening when access to the stack is required. |
The slurry is pumped under pressure into a vessel that is fitted with a stack of vertical leaves that serve as filter elements. Each leaf has a centrally located neck at its bottom which is inserted into a manifold that collects the filtrate. The leaf is constructed with ribs on both sides to allow free flow of filtrate towards the neck and is covered with coarse mesh screens that support the finer woven metal screens or filter cloth that retain the cake.
The space between the leaves may vary from 30-100 mm depending on the cake formation properties and the ability of the vacuum to hold a thick and heavy cake to the vertical leaf surface. The space is set by the filtrate necks of the leaves at the bottom end and with spacers at the top end brackets.
For fast filtering slurries the space may be doubled by removing every second plate so consequently the cake space doubles but the filtration area is cut in half.
