Filtering Devices

The separation of comparatively small amounts of solids from larger amounts of liquids is done by settling, followed by decantation of the supernatant clear liquid, or by the use of a filtering device. The separation of very small amounts of liquids from large amounts of solids is done by draining, or by centrifuging.

Settling out the solid is the cheapest method, and is frequently used. If the operation is extensive, a large capacity in settling tanks is required: these act also as storage tanks. If the liquor is hot, the tank may be insulated by strips of wood. As a rule, this process is too slow; also the mud collected in the bottom of the tank still contains too much water, and is usually sent through a filter, a combination of settling and filtering.

Filtering devices include gravity filters, pressure filters, more commonly called filter-pressers, and suction filters. Several points must be considered when studying a filtration problem: whether the cake or the filtrate is the valuable part, whether or not washing is required, and whether the operation should be continuous or intermittent. These considerations will have weight in the choice of the device.

The suspension to be filtered is called a slurry, if its content in solids is not so high that it does not flow and cannot be pumped; slurries contain from less than 1 per cent to 40 per cent of solids are more properly called sludges. The solid separated in the filter device is called the cake, the clear liquor running off is the filtrate. filtering devices such as filter-presses and suction filters are really frames for the support of the filtering medium, the filter cloth, which may be cotton duck of various thicknesses, muslin, paper, wool flannel, or metal wire woven into cloth, such as iron wire, nickel wire, and monel wire.

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Dust Collectors

The weight of quantity of solids which air or gas can carry varies directly as the velocity; the higher velocity, the greater the carrying capacity. The velocity of a dust-laden gas travelling in a pipe may be lowered by enlarging the pipe; at such enlarged places, the gas will drop part or all of its load. The enlargement need not be very great to produce a considerable reduction, for example, if a 6 inches pipe is widened to 24 inches a velocity of 160 feet per second becomes 10 feet per second. The dust collector is such an enlargement. It is made up of two truncated cones; the upper cone flares out downward; the lower cone meets the upper one and flares in downward, so that the box is widest half-way up (посередине). The pipe bringing the gas is so placed that the air is delivered along the wall and acquires a circular motion as it travels spirally around and down the box. The dust drops out and collects at the bottom, while the comparatively clean gas leaves by an internal pipe set halfway down the collector. The size of the dust collector must fit the volume of air or gas handled, so that the required reduction of velocity is reached.

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