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Abstract:
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Utilizing Ruth's equations, the filtration resistance
of cotton filter cloths is studied for systematic variations
of cloth construction,factors and variations of many
filtration factors in constant-pressure filtrations of an
aqueous calcium carbonate slurry. The clarifying properties
of the cloth are evaluated in terms of the time required
to produce a clear filtrate, the amount of filtrate
flowing in this time, and the amount of solids contained in
the filtrate.
The behavior of a fabric in a given filtration is expressed
in measurable quantities which do vary for different
fabric construction properties. For a wide variety of
cotton filter cloths and filtration conditions, it is
found that Ruth's proposals for linear, inverted-rate plots
hold.
The filtration resistance of the fabric includes not
only the resistance to flow of filtrate but also the resistance
resulting from the solids having throttled the fabric pores. Both of these resistances varied with the construction
of the fabric. In general, the filtration resistance
is found to be related to both the nature of the wet fiber
surfaces in yarns and the fabric density. The following
fabric properties are found to contribute to a low fabric
resistance: 1) long floats and a small number of yarn interlacings per unit area of fabric, 2) low yarn twist,
3) loose texture, and 4) the presence of coarse, mature
fibers rather than fine, immature ones. Initial use of
the fabric, use of previously unwet fabrics, high filtration
pressures, and fine particles of solids were also
found to effect a low filtration resistance of the fabric.
The amount of a given particle size of solids passed
by a fabric appears to be related to the size and distribution
of pores lying between the yarns of the fabric.
Those factors which in general give low filtration resistances
also effect an excessive passage of solids through
the fabric except in the case of low yarn twist. In this
case, a low resistance is obtained for 2/1 twill fabrics
with little change in clarity of filtrate. The following
fabric properties are found to contribute towards a small
amount of solids being passed by the fabric: 1) short
floats and a large number of yarn interlacings per unit
area of fabric, and 2) close weave, Prewetting of fabrics,
prolonged usage of a given piece of fabric, low filtration
pressures, and coarse particles of solids also effect a
low amount of solids being passed by the fabric.
In general, twill fabrics are found to give lower fabric
resistances but permit the passage of more solids than
duck fabrics for similar fabric weights. Certain twill
fabrics can be chosen, however, which offer low resistance
and yet permit only a slight passage of solids.
A mechanism for the theory of fabric behavior in filtration
is presented in terms of the influence of fabric
construction factors and filtration factors on flow
through the fabric, resistance offered by the fabric,
amount of solids passed, and the rate of clarification. It is concluded that fluid flow occurs through all portions
of the fabric, i.e., through pores lying both between
yarns and those lying within the yarns.
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