The clean filtrate returns to the precoat tank. Typical precoat thicknesses are in the 1—3-mm range. Using precoat in a clarification process offers several advantages.
It helps prevent the filter medium from becoming clogged by the slurry fines, thus extending filter medium life. This is especially critical when filtering amorphous or slimy solids. Precoating also helps provide immediate filtrate clarity. The precoat layer acts as a depth filtration bed for trapping fine particles that typically would pass through the filter medium. In addition, precoat helps filter cake solids discharge at the end of a filtration cycle by providing a cleavage plane from the filter medium.
Body feed. This technique involves blending the filter aid with the slurry feed either by dosing concentrated filter aid suspension into the slurry feed and in-line mixing or by mixing the filter aid into the entire slurry batch and maintaining agitation.
The filter aid solids and the native solids continuously build a filter cake until either the maximum cake height or maximum operating pressure is reached.
By adding filter aid into the slurry feed, the resulting filter cake is more porous, allowing higher and longer sustained flow rates. Body feed also helps restrict solids movement, which improves filtrate clarity. The goal of the optimal body feed concentration is to maximize the flow rate of a filtration cycle while avoiding feeding too much filter aid solids.
Using too much body feed will cause the filter to reach the maximum filter cake height faster than necessary. Employing a body feed without precoat can lead to an initial turbid filtrate while the cake begins to build on the filter medium. This initial turbid bypass generally will be much shorter than that observed filtering the raw slurry.
You can recycle this turbid filtrate back to the slurry feed tank to ensure no solids bypass occurs.
Whether the process is precoating or body feeding, the filter aid slurry tank and pump are critical to the operation. The precoat mix tank should be a round vertical tank whose height is twice its diameter. Avoid rectangular or horizontal tanks. The precoating pumps almost always are centrifugal pumps because they produce no pulsations to disturb precoat formation.
Such pumps typically are low speed 1, rpm and low pressure as the precoating rate is much faster than the filtration rate with low pressure drop. Also, because the precoat pump is exposed to filter aid particles, its internal parts usually have hardened surfaces and open impellers to reduce wear.
Bleed-through is common where the filter aid is bypassing the filter media. It may stem from mechanical, operational or process causes. Check a couple of mechanical points: Is the filter medium secured to the filter correctly? If it is perfectly distributed, 10 lbs. The use of baffles of precoating at a different rate may be necessary for an even precoat at lower precoat amounts.
Precoat slurry concentration will depend primarily on the ratio of filter area to the liquid volume of the filter and piping. If the concentration is much below 0. The precoat pumping rate will depend mainly on the viscosity of the liquid used. The rate should be sufficient to keep all the filter aid in suspension but should not be fast enough to cause erosion of precoat in the filter. For water, a typical rate is from 1 to 2 gals. For viscous liquids, the rate may be as low as 5 gals.
A general rule for precoating is to precoat at that rate which gives a differential pressure of approximately 2 lbs. The suspension of filter aid can be improved in the tank type, or pressure leaf filter, by recirculating part of the inlet flow from the top of the filter back to the precoat tank. Precoating filtrate should clear up in 2 to 5 minutes. However, this does not mean the precoat is all in place.
Continue precoating until the liquid in the filter shell is relatively clear. This usually takes place in 10 to 15 minutes at most. How Filteraid Works How do I know which type of filteraid to use? Why doesn't one grade of filteraid handle all applications?
For a given liquid, clarity of filtrate is governed principally by: 1 grade and amount of filter aid for body feed 2 grade and amount of filter aid for precoat 3 length of cycle 4 filtration rate Desired clarity, or the amount of acceptable suspended solids in the filtrate, can be determined in a number of ways: 1 visual examination of a sample of filtrate 2 comparing a sample of filtrate with a standard 3 the use of electronic turbidity instruments 4 filtering a sample of filtrate on a fine while of black filter paper 5 chemical or biological analysis 6 gravimetric analysis.
The purpose of the precoat is threefold: 1. To give immediate clarity. What are some signs and symptoms from a brief exposure to diatomaceous earth? If breathed in, diatomaceous earth can irritate the nose and nasal passages. If an extremely large amount is inhaled, people may cough and have shortness of breath. On skin, it can cause irritation and dryness. What is De used for? DE can be found in snail and slug repellents and is used to control cockroaches and fleas. In the garden, DE can be a lethal dust to aphids, ants, thrips, mites, and earwigs.
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Learn More About filter aid. Statistics for filter aid Look-up Popularity. Style: MLA.
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