Filter aids may be placed on the filter prior to filtration or mixed with the liquid. The aids can help prevent the filter from clogging and can increase the porosity of the "cake" or feed into the filter. A related separation technique is sieving. In contrast, during filtration, the filter is a lattice or has multiple layers.
Fluids follow channels in the medium to pass through a filter. There are more effective separation methods than filtration for some applications. For example, for very small samples in which it's important to collect the filtrate, the filter medium may soak up too much of the fluid.
In other cases, too much of the solid can become trapped in the filter medium. Two other processes that can be used to separate solids from fluids are decantation and centrifugation. Centrifugation involves spinning a sample, which forces the heavier solid to the bottom of a container. In decantation , the fluid is siphoned or poured off of the solid after it has fallen out of solution. Decantation can be used following centrifugation or on its own. Actively scan device characteristics for identification.
Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. The fluid is poured or siphoned off, leaving the solid. An advantage of decantation is that no material is lost on the filter medium.
Centrifugation uses centrifugal force to separate mixture components based on size, density, and viscosity. After centrifugation, solids form a firm cake. Decanting the liquid results in less loss of either liquid or solid after centrifugation. The method of filtration is the same as in the central feed machines. The pressure obtained by the force pump may be only a few pounds, or it may rise to several hundred pounds per square inch.
The filter press may contain from a dozen to fifty or more cells, and these cells may be as large as four feet in diameter. For many purposes the press is surrounded by coils or jackets, through which steam or refrigerating solutions may be circulated, according as hot or cold filtration is desired. The filter press is very rapid in its action and is extensively employed in industrial chemical work. For use with acid or corrosive liquids, the plates and distance frames are often covered with lead or some alloy which is not easily corroded.
The centrifugal machine Fig. This furnishes the most rapid method and leaves the substance almost dry. The centrifugal machine is a cylindrical box or basket A of wire gauze or perforated sheet metal, fixed a vertical shaft B , which rotates at a very high speed. The contents of the box are driven to the outer wall by the centrifugal force, the solid matter being retained by the gauze or screen.
The liquid passes through and is caught in a fixed shell C , surrounding the rotating basket. These machines are of various sizes from 12 to 60 inches diameter, and 8 to 36 inches, depth of basket.
Two general forms are in use: the over-driven type, in which the driving pulley P is fixed at the upper end of the shaft, above the basket; and the under-driven type, in which the basket is placed on the upper end of the shaft, and the pulley below. In the over-driven type it is frequently customary to suspend the shaft in flexible bearings.
Thus the basket is enabled to adjust itself to any change in the centre of gravity, caused by unequal loading, and runs without vibration. Sand filters are sometimes used for work on a large scale. These are made as follows; Into a box having a perforated bottom, is put a layer of coarse gravel; this is covered with finer pebbles; these by sand, and a jute or canvas cloth covers the whole.
A wooden or iron grating is added to protect the filter, when the sediment is shoveled out. The filter is often placed over a receptacle from which the air may be exhausted, thus affording pressure filtration if necessary.
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