Generally washing of the filter cake sets in immediately after cake formation, ideally just when the filter cake formation is finished, i.e., at saturation one. Sometimes an intermediate deliquoring stage is allowed for before washing liquid is provided. While the mother liquor remains undiluted during this stage, the rewetting of the filter cake may pose a problem for subsequent washing. Sometimes re-slurrying of the filter cake with wash liquid and renewed cake formation can be an option. Washing is a cross-flow operation where the filter cake moves horizontally while the wash liquid flows through the filter cake vertically. When the wash liquid is collected stage-wise, it can be re-used in co- or counter-current operation mode or as a reflux.
Washing aims for the removal or exchange of the mother liquor by another liquid. The targeted product could be the clean and maybe ultimately mechanically dry solid product, the mother liquor itself or its dissolved constituents. While filter cake deliquoring is based upon liquid displacement, a combination of displacement and diffusion/dispersion mechanism is relevant for cake washing. Liquid displacement is not a perfectly homogeneous process but occurs predominately at larger, through-pores while narrow, dead-end pores are hardly accessible. Clearly excessive washing in terms of more wash liquid and/or washing time will improve the situation but the gain is sometimes comparably small. Systems that are particularly affected by this phenomenon can be improved by re-slurrying with the penalty of more wash liquid and higher expenses and complexity in set-up.
Conventional, discontinuous filter tests are carried out consisting of...
Session: L7 - Cake Filtration – Particle Washing
Day: 23 October 2019
Time: 13:00 - 14:15 h