Factors Affecting Filtration Rates and Cake Moistures

Factors Affecting Filtration Rates and Cake Moistures

Here is a list of all Factors Affecting Filtration Rates and Cake Moistures in plants or laboratory de-watering systems and processes.

Particle Size of Solids

Generally the large the particle size, the higher the filtration rate in Kg/m2/h and the lower the cake moisture. However, the validity of the last statement depends on other factors, e.g. distribution specific gravity of solids, absence of slimes and feed concentrates. For instance, a small average particle size, but with a narrow distribution range and no slurries, will have a high rate.filter cake

Ratio of slimes to coarser particles

The slimes or extreme fines in a filter feed slurry affect filtration rates to a vastly greater extent than their percentage. The residual cake moisture is similarly seriously affected. A particularly difficult slurry is one that contains relatively coarse particles and a number of very fine or slimy particles with little or no intermediate size.

Filter aids

Filter aids like diatomaceous earth, perlite, powdered coal, fly-ash or paper pulp may be added to the flurry to increase its filtration rate and cake porosity.

Feed solids concentration

In general, the greater the percentage of suspended solids in a given slurry, the higher the cake filter rate in Kg/m2/h and the lower the filter rate in m3/m2/h. Where maximum solids capacity is desired it is advisable to consider thickening the slurry by gravity. In some applications involving thickening with sludge recycle, particle size is actually increased and both cake and filtrate rates can increase.

Filter Thickening

Filter thickening normally occurs in a continuous filter rotating in a tank containing slurry wherein the solids in the filter tank increase in concentration and shift to a coarser size distribution. While an equilibrium concentration and size distribution is usually obtained, it may sometimes be necessary to dilute the pulp.

Slurry pH

Since slurry pH and particle dispersion are closely related, changes in pH could be one of the most effective methods to achieve flocculation and improved filterability, if the process can tolerate it.

Flocculation/Dispersion of fine solids

Flocculation is generally desirable for slurries of fine solids which are in a dispersed state and generally filter poorly. The wide variety of polyelectrolyte flocculants provides room for a substantial improvement in filtration rates. Effective use of flocculants, especially polyelectrolytes, on moderately high concentration filter feeds requires strong agitation to get good solids-flocculant contact. A minimum of further agitation and minimum aging are important.
Some slurries may be so viscous as to create filtering problems and a dispersant may be a better way to gain fluidity than dilution.

Slurry Age

Sometimes processes involve detention times whether international or not, which provide a conditioning effect, modifying filter performance. Samples shipped for testing involve a risk that excessive aging may have some effect on filterability.

Viscosity of liquor and temperature

This is one of the most significant variables both in filtration rate and rate of dewatering to a minimum moisture. Viscosity is closely related to temperature. As temperature is increased, viscosity is decreased resulting in a higher filter capacity and lower cake moisture.
As the same time, increased vapour pressure will help reduce moisture.

Agitation Speed

Some slurries, particularly with a wide particle size range, tend to classify in the test slurry container or the filter tube. Increasing the agitation speed (or stirring) to a point that the coarse and fine particles are always thoroughly mixed may be desirable although too high a speed could limit cake thickness, prevent coarser particles from forming in the cake or cause delicate flocs to break down.

Type of Filter Medium

Filtering characteristics of fabrics depend mostly on the type of yarn and weave. Yarns can be momo-filament, multi-filament, spun from staple fibre, or a combination of the latter two. A high twist can make a multi-filament perform more like a mono-filament.
Permeability and porosity are prime qualities in cloth selection. The Frazier permeability rating, expressed as cfm/sq ft, is a measure of air flow at one-half inch water pressure through a dry cloth, and is comparable to per cent open area.
Porosity and particle retention may not be accurately indicated by permeability; there being no direct measure of porosity.

Filter cloth condition

Cloth conditioning refers to the reduction of pore size or open area due to entrapment of fine solids in the interstices.

Applied vacuum
The applied vacuum creates the pressure differential which is the driving force for filtration and dewatering. High vacuums give somewhat higher rates and lower moistures in all cases except for unusually incompressible cakes or where the filter medium becomes more conditioned. With coarse particles and porous caked lower vacuums (e.g. 5”-10” Hg) may be adequate. With such porous cakes, the vacuum capacity (m3 / m2 ) could be the important factor.

Cycle Time

Cycle time of leaf tests is analogous to the filter drum speed, generally expressed in seconds or minutes per revolution. Generally the faster the drum speed, the higher the output. However, under these conditions, the cake is thinner and sometimes wetter, so discharge may deteriorate. At all times, a dischargeable cake must be produced. Any final selection of cycle or drum speed is a compromise of these conditions.

Surface Tension

Lowering of surface tension through higher temperature or surfactants can significantly improve cake moisture of some materials. Its benefits are not predictable and it has no identifiable effect on filtration rate. Where surfactants are sued, vacuum capacity may be greatly reduced.

Cake Compression

Cake compression is normally achieved as an adjunct to the filtration step to reduce cake moistures of compressible cakes.