DICALITE FILTRATION AND FILTER AIDS

PERLITE -  DIATOMITE -  DICAFLOCK  

Filtration is an integral part of plant operation. In today's competitive environment, it is essential to keep the cost of filtration down and flowrates at a maximum while maintaining clarity. Finding this balance is becoming increasingly difficult. For over 60 years, Dicalite has been engaged internationally in the manufacture and sale of diatomite, perlite and cellulose filter aids and fillers. Our emphasis on customer service has proven to be the most important driving force in the company's ability to produce high performance filter aids. Dicalite is the only filter aid filter aid manufacturer that produces both salt water and fresh water diatomite and perlite filter aids. Our products are backed by a staff of professionals to assist you in operating your process optimally.

Technical Support

Dicalite will evaluate filtration processes for its customers. This is sometimes done to optimize an existing process even though it appears to be running well or to solve a particular problem. Similar tests are often conducted for new installations to help in the design process and eventual plant start-up.

Filtration tests can be done on a laboratory bench filter, using pressure or vacuum types. This is a good way to determine the effect of the various operating parameters without the risk and cost of using the plant filter. Vacuum tests can be conducted on the Dicalite Rotary Vacuum Test Leaf. Both provide scale-up data which correlates closely with results obtained in plant production. For pressure tests, the Dicalite Pressure Test Filter is used.

Variables easily determined in a few hours are:

  1. Optimum type and grade of filter aid for precoat and bodyfeed.
  2. Optimum Bodyfeed Dosage.
  3. Throughput.
  4. Clarity of Filtrate.
  5. Cycle Length.
  6. Effect of operating variables which may be of specific concern to the particular plant such as pH, temperature or polymer concentration.

General Considerations

Filtration is the separation of solids from liquids by forcing the liquid to flow through a porous medium and depositing the solids on the medium. A filter aid is a finely divided material which, when added to the liquor to be filtered, helps control flow and solids removal. The septum, usually screen or cloth, serves principally as a support for the cake. The filter aid forms a porous layer on the septum and thus the filter aid is the filtering medium that traps the solids being removed and prevents them from blinding the septum. Filter aid filtration is mechanical, not chemical in nature. Irregularly shaped particles interlace and overlay in a fashion that leaves 85 to 95 % voids or open spaces. These voids form billions of microscopically fine interstices between the filter aid particles. The size of these openings is so minute that the unwanted solids are strained from the liquid. The vast number of openings compensates for their small size resulting in fast flowrates and brilliant clarity.

Good filter aids are light in weight, chemically inert and form high porosity filter cakes to maintain free flow of the liquid. The structure of the particles must be such that they will not pack too closely. This not only permits high initial liquid flow but also provides pore spaces to trap and contain the filterable solids and leave a high percentage of channels remaining open for flow. Particle size distribution must be tailored to permit precoating on coarse wire and yet give the desired flow rate and clarity. Dicalite perlite, diatomite and cellulose filter aids meet these criteria. They are available in a variety of grades to suit the solids removal requirement of any application.

Pressure Filtration

General Operating Notes

The filter septum serves principally as the support for the filter aid cake. However, the effect of the septum on performance is sufficient to warrant careful selection. The size of the opening should be fine enough to retain the filter aid particles and allow a firm cake to be formed quickly while at the same time giving a minimum resistance to flow. The material must be able to withstand chemical, pressure and temperature conditions existing during filtration. The two most widely used septa are metal and cloth. The most common metal screen is 24 x 110 Dutch Weave. Addition of Dicacel will improve the precoatability of finer grades.

Precoat

In most cases, the first step is formation of the precoat. It is a thin layer, 1.5 to 3.0 mm (1/16 - 1/8 inch), which protects the septum and ensures clarity by stopping the solids at the surface. The filter aid grade used for precoat must be carefully selected to allow the fastest possible flow yet trap the solids. A slurry is made from filtered liquid, or sometimes water, and filter aid. The concentration should be low as possible, 0.5 % is typical. Agitation in the precoat tank should be sufficient to keep the filter aid in suspension. Excessive agitation for extended time may break down the particles. Filter aids should be added at 500 to 1200 g/m² (10 - 25 lbs/100 ft²) of filter area. The precoat is formed by recirculating the filter aid slurry through the filter. The coarser particles deposit themselves first on the screen followed by smaller ones. Precoating rate of 40 litre/m²/min. (1 GPM/ft²) is normal. Much lower rates are used with higher viscosity liquids. There should be at least 0.07 kg/m² (1 psi) differential pressure during the precoat process. Precoating liquor should clear up within 10 to 15 minutes.

Bodyfeed

Addition of filter aid to the liquid to be filtered is referred to as bodyfeed. The type and grade as well as quantity to be added is vitally important to obtain the highest filtration flowrate consistent with the clarification required. Filter aid dosage varies with the solids content and other variables specific to each application. In general, a dosage of ½ of the percent solids by weight is close. Bodyfeed can be added directly to the tank of liquid to be filtered, or dosed from a slurry tank into the filter inlet.

Grade Selection

Clarity is considered by many engineers to be the most important measure of efficiency in filter aid filtration. A high quality filter aid is most important for uniform results day after day. Selection of the particular type and grade of filter aid having the correct particle size and distribution is a major factor. After these come many considerations such as the quantity of filter aid to be used; flowrate needed to meet plant production schedules, equipment limitations and general filtration conditions. All of these can best be resolved by tests using the actual liquid involved.

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