Precoat filtration is used in order to prevent filter elements from premature clogging and to secure fine filtration efficiency. Filter elements are protected by depositing a layer of precoat material (normally cellulose, diatomaceous earth, or similar material) on their surface prior to filtration. For this purpose a precoat slurry is introduced into the filter vessel prior to the start-up of the filtration process.
Generally precoat filtration is used in the treatment of lean amine slip streams, saturated brine for chloralkali plants and turbine condensate return streams.
The Unidro candle precoat filters consist of a vertical cylindrical vessel with a conical bottom. The filtering candles are fixed at the top on a horizontal nozzle sheet or collecting manifold.
The backwash procedure may be specific for the application and project requirements. The Unidro system can be designed for slurry discharge or dry discharge where the latter involves a drying gas stream which facilitates delivering a dry cake for disposal.
The Unidro leaves precoat filters consist of a horizontal cylindrical vessel equipped with circular filtering leaves installed on a central internal collecting header.
The filtering elements are vertical round leaves which can filter inward through each side. They are held by a central shaft which also acts as a filtered liquid collector.
When the washing procedure starts, the shaft rotates the filtering leaves. A strong blade jet on the rotating filtering leaves cleans them effectively.