- 13 December 2006 -

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Efficiency of operation

Factors contributing to improved efficiency of filtration are slurry characteristics (solid content, temperature and free acid (SO3) content); a low-acceleration tilting track which can increase the capacity of the filter; an automatic system to keep the pans horizontal; a waterproof cloth-fixing system whereby the cloth can be replaced in a very short time; a compact vacuum box for pre-separation of gas and filtrates; and leak proof vacuum lines almost free from scaling.

The trays with sloping bottoms for fast evacuation of filtrates are each fitted with a roller arm assembly that tilts and maintain the level of the tray as it passes trough the inverting track. The filter cloth is held in position with the help of circumferential fasteners which can be removed easily for quick replacement of filter cloths.

The wear plate of the main valve is susceptible to erosion since its large diameter makes it vulnerable to loss of vacuum. The twisting rubber hoses or "O" ringed elbows connect the rotating pan necks with its pipe and wear plate to the face of the stationary valve. The condition of the high impact nozzles mounted on the cloth wash manifold and, for wet cake discharge, the sluicing manifold both require frequent inspection.

The support rollers carry the vertical load of the entire machine and the horizontal thrust rollers keep the rotating frame concentric. The toothed rim and sprocket drives the filter.

As the tray containing gypsum slurry travels through every quarter of the filter, it is washed with dilute acid from the successive stages. Thus acid with concentration at four levels i.e. 30%, 18%, 12% and 5% is produced. The 18% solution is recycled to the attack tank.

A central valve with internal compartments for the mother and wash filtrates and bridge setting controls the various zones throughout the filtration cycle. Feed and wash boxes are set in a slight slope so that the distribution over the trapezoidal surface is proportional to the tray area and the cake formed has an even thickness and wash water is distributed evenly.

The main distributor controls the cycle segments with bridges that open and close the port of each tray as it passes from high vacuum through cake blow-off to low vacuum. Compartments within the valve separate the various zones to ensure that the separation is sharp and no mixing of mother, strong, middle and weak filtrates occurs during the cycle and the filtrate is collected in the four compartments of the filter seal tank fitted with agitators.

30% acid solution is separated from the process flows to a settler from which it is used either directly in the process plants manufacturing complex grades or is further concentrated to 54% =to be sold as merchant grade acid. The filter media is polypropylene cloth laid over a grid in the tray. Around 1% phosphoric acid as P2O5 (around 0.2% water soluble and the remaining 0.8% from unreacted rock phosphates) is lost to the gypsum.

The filter is usually fitted with a variable speed drive. The speed of rotation determines the cycle time and this, being an important design parameter, is limited mechanically by the mass inertia of the swinging pan and its wet cake load at the point of discharge. Typical cycle times for tilting pan filters are 2½ to 4 minutes depending on the size of the filter and the design of the peripheral cam that controls the tilting velocity.

The filter speed is regulated manually to maintain optimum cake thickness, improve crystal characteristics in the slurry and maintains a constant pressure drop across the filter cloth. All these aspects help to improve recovery of water soluble P2O5 and reduces scale formation inside the trays' vessels.

The fumes emanating from the filter containing hydrogen fluoride (HF) are sucked by a fan through a ventury scrubber and, after washing, is vented to atmosphere. The effluent scrub liquor is treated in an effluent treatment plant (ETP).

Construction material

Most of the wetted parts are constructed from UNS N08904 or equivalent and other structure and filter bodies out of carbon steel. The acid lines are either rubber hoses or rubber lined mild steel pipelines. The trays and wetted parts of the filter are fabricated from specialty alloys like AISI 904 L, UB6 or Sanicro 28 to survive the harsh phosphoric acid conditions.

Inspection and maintenance

These filters have a high reliability with on-stream factor greater than 95%, an excellent ratio between capacity, extraction yield and drying of the cake and are extremely energy-efficient.The proper upkeep of the equipment is ensured through a regular onstream maintenance. Periodic maintenance includes the replacement of filter cloth and occasional maintenance involving correction of mechanical components and alignment to ensure reliable operation of the filter. Consequently, the equipment causes little downtime in the plant and, if looked after, works well in the operating environment of the phosphoric plants.

Ensuring effective washing between different sections and maintaining the desired vacuum can facilitate easy filtration. A weekly wash with hot water ensure that deposits are removed. Usually, mechanical upkeep of the filter include routine maintenance, cloth replacement and cleaning of filter trays and grids. The major parts requiring frequent inspection and preventive maintenance during filter operation include filter trays, wear plates, rubber hoses and joints, washing nozzles, and the mechanical framework.

As well as this, the operator has to ensure proper alignment of the connected parts, lubrication etc. During annual turnaround of the plant, all the trays are aligned, and the filter hood is inspected and lubricated. It is desirable that cloths are changed as per a standard schedule depending on plant characteristics so that loss of P2O5 does not occur. Occasionally hot water washing may be done to remove insoluble materials sticking to trays.

With the development of new generation rubber material horizontal belt filters with large filtration areas have come of age. Modern large capacity plants prefer belt filter in place of tilting pan filters. It is a straight belt conveyor and slurry is fed to the front end. The belt section is divided into four chambers that are subjected to vacuum. As with the tilting fan, the filtration and collection of the filtrate as well as the discharged gypsum takes place. A major advantage of belt filtrates over tilting ones is that they have fewer moving parts and consequently lesser expenditure to maintain. The efficiency of operation is also comparable.

About the author
Dr MP Sukumaran Nair, FIE, is currently with The Fertilizers and Chemicals Travancore (FACT) Ltd, India 's pioneer fertilizer and chemicals manufacturing, engineering design and consultancy organization. He is a former Chairman of the Institution of Engineers ( India ), Cochin.

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