Table of Contents
The yield of most types of dewatering installation has been shown to be related to cake specific resistance. Any sludge conditioner which significantly reduces the specific resistance will have a marked effect on the yield, and hence the size of the dewatering equipment required. Results from the current research show that the specific resistance behavior is analogous to the behavior of electrophoretic mobility and cake moisture when pH is changed. Thus, specific resistance can be reduced by modifying the coal surface with metal ions precipitated from solution as surface active metal hydroxy species by controlling pH to yield low moisture cakes, which in turn will be beneficial in plant design and scale-up.
The fine size clean coal utilized for this study was obtained from the waste stream of a southern Illinois preparation plant using the ‘Ken- Flote’ column. The column product contained 20 percent solids (7.1 percent ash) with an average particle size of 21 microns.
The conditioned slurry was subjected to vacuum filtration using Whatman No. 1 filter paper at a constant pressure drop of 25 inches Hg. The rate of filtration was monitored with time. The kinetic data thus obtained were used in calculating fine coal filter cake parameters.
Results and Discussion
Coal surface is modified in an attempt to achieve low moisture cakes and the behavior of cake specific resistance is analyzed under these conditions. Reduction in specific resistance achieved by means of controlling coal surface is expected to be beneficial in performance and scale-up of dewatering equipment. Laboratory vacuum filtration data on fine clean coal are used in calculating cake parameters based on Darcy’s law by employing computer spread sheets.
The integrated form of Darcy’s law under conditions of constant pressure and cake permeability is as follows:
t/V = (µβ/2 ΔP A² k) V + (µ Rm/ A ΔP)……………………………………………….(1)
t : filtration time, sec
V : filtrate volume, m³
The slope and intercept of the above linearized rate equation were used in calculating cake permeability, k, filter medium resistance, Rm, and specific cake resistance, α. The cake concentration by volume fraction, C, and volume ratio of cake to filtrate, β, are other ways of expressing cake moisture, and all are interrelated. Relationships of different parameters in rate equation are given below. From equation (1),
Slope = µβ/2 ΔP A² k………………………………………………………(2)
µ : Liquid viscosity, Pa. s
β : Volume ratio of cake to filtrate
ΔP : Pressure drop across cake and medium, Pa
A : Filter area, m²
k : Cake Permeability, m²
The volume ratio of cake to filtrate, β, can be calculated from a mass balance on the solids which gives,
β = s pl ((1-s) C ps – (1-C) sp)…………………………………………………….(3)
s : Mass fraction of solids in the slurry to be filtered
p : Liquid density, kg/m³
ps : Solid density, kg/m³
C : Cake concentration by volume fraction
C, volume fraction = Dry cake volume / Wet cake volume = (Dry cake weight/Coal density)/((Dry cake weight/Coal density) + (Moisture in wet cake/Water density))
Influence of Copper Ions on Fine Coal Filter Cake Parameters
The changes in behavior of α and k by varying experimental conditions such as introducing copper ions and controlling pH are presented in this section. The electrophoretic mobility and filter cake moisture data with respect to pH in the presence of 10 -4 M/L (=50 ppm) copper (Cu+²) ions from CuCl2·2H2O are shown in Figure 3. Charge reversal on the solid surface occurs as pH is increased, showing point-of- zero-charge (PZC) at pH 4.8 and 6.3. Note that filter cake moisture is significantly lower in the region of PZC’s. A third charge reversal point was approached near pH 10.0. In the presence of copper ions, these charge reversal points occur near the pH of formation of the first copper hydroxy species (pH 6.3) and the PZC of Cu(OH)2 (pH 10.0). These results are consistent with the charge reversal model proposed by James and Healy, later confirmed by Parekh. The effect of other metal ions and surfactants on filter cake moisture is given elsewhere.