El Aviador

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THE EFFECTS OF SCALE AND PROCESS PARAMETERS IN CAKE FILTRATION E.S. Tarleton(e.s.tarleton@lboro.ac.uk) and S.A. Willmer Department of Chemical Engineering, Loughborough University, Loughborough, Leics. LE11 3TU, UK.

ABSTRACT A well controlled and instrumented apparatus has been used for a systematic investigation of cake growth in pressure leaf filtration. Both incompressible and compressible systems have been filtered over a range of constant pressures to evaluate the effects of appliedpressure, initial suspension concentration, time of filtration, particle surface charge (interpreted through suspension pH) and scale of filtration. Sample data show how aqueous calcite and zinc sulphide systems behave in different manners dependent on both the relative magnitudes of the process parameters and apparently on the scale at which a filtration is performed. The data, analysed byconsistent procedures through the general filtration equation, indicate that the scale-up ‘constants’ frequently used in filter design may vary with scale and how variations in measured cake properties can be seen as scale is altered. The results are discussed in the context of filter design.

KEYWORDS Cake filtration; Compressibility; Scale-up; Filter design; Process parameters

INTRODUCTION The useof cake filtration is widespread in many of the chemical, food, brewing, ceramic and mineral processing industries. Within these areas the objective of filtration may be to recover a clear filtrate, to recover the suspended solids, or, in some cases, to recover both the solids and the filtrate. Referring to Figure 1, these operations are achieved by imposing a pressure gradient across asemi-permeable medium such that particulate material is separated from a suspending fluid to produce a cake upon the filtering surface and a clear filtrate downstream of the septum. The suspension is fed toward the filter medium where permeable layers of particulates form initial layers, effectively filtering the remaining suspension as the cake grows with time. The flow of liquid through the filter cakeinduces viscous drag forces on the constituent particles that arise from the interfacial transfer of momentum from the liquid to the solid particles. If the cake structure is able to withstand the cumulative effect of these forces throughout a filtration to give a uniform and timeindependent cake porosity, then the cake is regarded as incompressible. Otherwise, particle rearrangement occurs toinduce a porosity gradient through the cake height and the cake is thus considered to be compressible; the maximum solids concentration generally occurring at the cake/filter medium interface. Despite the importance of cake filtration to many industries, and the relative wealth of research in the subject area, there is still much reliance on heuristics and conventional (or classical) cake filtrationtheory1,2 to analyse and scale-up filtration operations. Ruth2 showed that the resistance of a cake formed during filtration is proportional to the amount of cake deposited at the filter medium provided cake porosity and filtration pressure are assumed constant. This principle, which infers that conventional theory is only valid when a cake is incompressible, is manifested in the now widely usedgeneral filtration equation. Grace3 apparently validated Ruth’s work using compression permeability (C-P) cells through the so called ‘perfect parabolas’ for volume-time discharge curves. Tiller4 later showed that the discharge curves for constant pressure filtrations were not always ‘perfect parabolas’ but that cake resistance was seemingly dependent on a host of process parameters. Since the...