The following tests are required on all slurries: total solids and slurry density. These are the absolute minimum needed to characterize the slurry and for sizing a filter press.
Additional tests may be required to properly interpret the test results. The more common of these tests are conductivity, viscosity, pH, concentration of specific soluble species, total suspended solids (filtrate), and particle size and shape.
The most common screening test available to our customers is the Buchner funnel. The Buchner funnel test is used for initial screening and general chemical conditioning but it is vacuum filtration method not a pressure filtration method. It does not mimic the basic operation of the filter press, which is a pressure filter.
The Buchner funnel uses a vacuum to remove the water from the slurry through a filter paper. The time for all the liquid to be removed from the funnel is measured and various samples can be run on the funnel. The major problem with this test is that some slurries that do not filter well under vacuum will filter reasonably well with pressure filtration.
For the Buchner funnel, the shortest time indicates the best filtration of the slurry and the treatment required, if any, to achieve this is determined. The doses determined are then used to treat the slurry when the filter press tests are done. However, it does not predicting actual filter press performance such cycle time or cake solids.
The Buchner funnel has problems with very “thin” (very low solids) slurries. The basic problem is these slurries may have tendency to “blind” – form an essentially impervious cake layer – when sufficient solids have been filtered. Sometimes several samples are filtered through a cake layer to see if thickening the cake layer causes blinding.
High viscosities will cause slower filtration rates than would normally be expected. High slurry viscosities often caused by stable emulsions, which are broken during slurry treatment. Once the emulsions are broken, the viscosity of the treated slurry is generally much lower. However, some liquids and solutions have a naturally high viscosity (for example: sulfuric acid, phosphoric acid, molasses, and some sugar solutions) and their filtration will be slower than watery slurries and solutions. There is a theoretical analysis of this available in the scientific literature if one wishes to pursue this further.