Settling of Iron Blue Pigment in Water Yatish T. Shah’ and Naresh Kakar Department of Chemical and Petroleum Engineering, (7?ziversity of Pittsburgh, Pittsburgh, P A 15113
An experimental investigation was carried out to study settling of an industrial slurry of iron blue pigment in water. This slurry is physically and chemically markedly different from those studied earlier by Michaels and Bolger ( 1 9620,b) and Bodman et al. (1 972). The applicability of Michaels and Bolger’s correlations for the settling rate of flocculated suspension to iron blue-water system is examined in a manner similar to the one reported by Bodman et at. for the settling of titanium dioxide and alum mud in water. In general, Michaels and Bolger correlations have been found to apply well to the iron blue-water system.
Recently, Bodman et al. (1972) have experimentally analyzed the applicabilitj of the quantitative correlations of Michaels and Bolger (1962a,b) for the settling rate of flocculated particles in a liquid-solid slurry to the settling of titanium dioxide and alum mud in water. This paper reports a similar type of analysis on another industrially important slurry. The slurry examined here is the suspensions of ferric ammonium ferrocyanide commonly known as iron blue in water. Iron blue is an important pigment in some grade of paints and it is chemically and physically markedly different from the kaolin suspensions studied by Michaels and Bolger (1962a,b) as well as titanium dioxide and alum mud systems investigated by Bodman et al. The iron blue slurry used here was obtaihed directly from industrial process. Results of this study should be useful for the design of large-scale industrial thickeners as shown by Fitch (1966). Experimental
The iron blue used here was Milori iron blue supplied in slurry form by American Cyanamid Co., Willow Island, WV. The average diameter of iron blue particles in the slurry was -0.075 p. pH of the slurry was approximately 1.5. The flocculating agent was supplied by the American Cyanamid Co., Linden, NJ, with the trade name Superfloc 16. Exact chemical composition of the Superfloc 16 was not available, but it is a substance with an approximate molecular weight of 3000 and has a structure of polyacrylamate of the type CH=CH2.
I
CONHz Experimental details on the settling rate determinations were similar to the ones reported by Bodman et al. (1972). These details are also given by Kakar (1972). Results and Discussion
Results of a settling experiment are conventionally presented as a graph of the height of the interfacial plane between the slurry and the supernatant liquid as a function of time. Michaels and Bolger (1962a) reported t h a t there are basically three different types of settling plots obtained depending on the solids concentration in the slurry. Thus, three types of settling plots can be associated with three different solid concentrations r e g i m e d i l u t e , intermediate, 1
To whom correspondence should be addressed.
308
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 2, 1972
and concentrated. The niain difference in the nature of the settling plots for dilute and intermediate concentration regimes is t h a t the initial settling rate is constant for dilute regime while the initial settling rate is low and it is increased with time reaching a maximum value for the intermediate regime. Bodman et al. (1972) asserted this difference in the nature of the settling plots for the settling of titanium dioxide and alum mud in water. However, as shown in Figure 1, dilute and intermediate concentration regimes were not distinctly differentiated by the nature of the settling plots. The main reason for this is believed to be an abrupt behavior of liquid-slurry interface in the initial period of settling. Dilute and intermediate concentration regimes were differentiated in this study with the help of empirical correlations for the settling rates developed by Michaels and Bolger (1962a). The important system parameters which could affect the settling of iron blue suspensions are investigated with the help of the following experimentally measurable and/or controllable quantities: concentration of solids in the slurry, settling container size, slurry temperature, mixing variation prior to the settling process, and chemical treatment to the slurry. Effects of these quantities on maximum settling rates in dilute and intermediate concentration regimes are analyzed. The different chemical treatments to the slurry were accomplished by the variations of pH, salt content, and the flocculating agent concentrations of the slurry. Solid Concentration in the Slurry. The settling rate, in general, should decrease with an increase in solid content of the slurry. This is shown in Figure 2 for a typical case of settling of iron blue. Just like in titanium dioxidewater system, the results of Figure 2 indicate t h a t the break points betueen the settling curves for two different concentration regimes are not as clearly defined as they appear to have been in the kaolin system studied by Michaels and Bolger (1962a). I n the dilute concentration regime, the floc diameter, the volume fraction of iron blue in aggregate-Le., volume of aggregates/volume of iron blue in aggregates-and the Stokes settling velocity of a single aggregate can be calculated from Michaels and Bolger’s empirical correlation for the settling rate. These types of calculated results obtained from the experimentally measured settling rates under typical system conditions are listed in Table I. Observation of a floc network of a typical slurry sample under a microscope indi-
Table 1. Aggregate Characteristics in Dilute Concentration Regime Stok-r srttlinn
~
C&, ml aggregate/ ml iron blue
velocity of (I single aggregate VSA. cmlhr
135 185 221
42.0 26.5 68.0
152.4 248.7 185.6
63
6.2
223.3
Aggregate
PH
Temp, 'C
2.30 6.92 7.50
28 27 25
2.48
26
Slurry characteristics Salt oddition
diam
ar, P
FA addition
...
...
... ...
NaHCOi (0.5 mol/l.)
...
8(ml FA/g of iron blue)
cated t h a t the floc structure was very nonuniform (Figure 3) similar to the one obtained hy Bodman et al. for the titanium dioxide-water system and had aggregate size which varied from 50 to 400 p. As shown by Michaels and Bolger for the kaolin system and Bodman e t al. for the titanium dioxide system, the settling rate in the dilute concentration regime in this system was found to be essentially independent of initial slurry height (Figure 4) and container diameter. Scattering in the data points for the plot of settling rate vs. reciprocal of initial slurry height in the dilute concentration regime shown in Figure 4 is believed to be due mainly to experimental errors caused by the cloudy supernatant liquid and an unclear liquid-slurry interface. Slope of the plot indicates trend of the data rather than best fit through all experimental points.
For the intermediate concentration regime, Michaels and Bolger (1962a) indicated that the settling rate should be increased with an increase in the initial slurry height. This is because there is less time available for channel formation, and the slurry settles slowly in the short height runs. Also, in the intermediate concentration regime, maximum settling rate should be increased with an increase in container diameter. Both of these relationships were found to be valid here. For very large container diameter, Michaels and Bolger indicated t h a t maximum settling rate should vary linearly with the reciprocal of initial slurry height. As shown in Figure 4 this relationship was also found to he valid for the present system. Slurry Temperature. I n general, temperature could have a n effect on aggregate microstructure as well as on fluid properties such as viscosity and density. If density of t h e slurry and t h e floc microstructure are assumed to be independent of temperature, t h e empirical correlations of Michaels and Bolger for the settling rates in dilute and intermediate concentration regime indicate t h a t settling rates should vary as reciprocal of viscosity over a range of temperatures. Figure 5 shows typical plots of settling rate vs. reciprocals of viscosity obtained here. These plots indicate
r1 .
-1
x
Figure 1. Examples of settling plots in three different concentration regimes
Figure 2. Maximum settling rate as a function of the volume fractionof iron blue
Figure 3. Photomicrograph of aggregates at 50X p H = 2.3; temp = 28'C;
= 0.00651; motionless fluid
Ind. Eng. Chem. Process Der. Develop., Vol. 11, No. 2, 1972
309
Table II. Effect of M i x i n g Prior to Settling on the Settling Rate Concn region
Mixing type
Temp,
O C
Settling rote, cm/hr
Inverted Blended Inverted
Dilute + i = 0.00627 Dilute $ i = 0.00627 Dilute qi = 0.00627
2.5 2.5 2.5
27 27 27
73.92 84.48 75.00
Inverted Blended Inverted
Intermediate +{ = 0,00791 Intermediate 9~ = 0.00791 Intermediate $i= 0.00791
2.41 2.41 2.41
28 28 28
54.80 63.36 58.10
Inverted Blended Inverted
Intermediate 9i = 0.01093 Intermediate @ i = 0.01093 Intermediate q ~ i= 0.01093
9.0 9.0 9.0
27 27 27
24,83 22.36 22.74
60
CONC. REGIME
INTERMEDIATE
A
pH
o.01046
2.2
0.02
0.03
Ternp”C
e
2 0
20
0
0.01
l/zo
0.04
(rm-l)
Figure 4. Maximum settling rate Q vs. reciprocal of initial slurry height
Z O
101
1
I
0.9
1.0
1.1
1.2
1.3
1.4
I/F (centipoise-1)
Figure 5. Correlation between settling rate and fluid viscosity
t h a t unlike for titanium dioxide and alum mud systems examined by Bodman et al., in this system a correlation between settling rate a t various temperatures and the reciprocals of viscosity is only approximately linear. Since density of iron blue-water slurry does not change significantly with temperature, the floc microstructure for iron blue-water system should be affected significantly with the variations in slurry temperature. The temperature variation could cause a variation in the viscous force betneen the liquid and the solid within an aggregate. This could, in turn, change the floc microstructure. Microscopic observations of the flocs 3 10
PH
Ind. Eng. Chern. Process Des. Develop., Vol. 1 1 , No. 2, 1972
inside the various temperature slurries indicated this t o be the case. Mixing Prior to Settling. &fixing prior t o settling could affect t h e floc inicrostruct,ure and hence t8hesettling rate. I n kaolin system, Michaels and Bolger reported t h a t mixiiig affects t,he settling rate in the basic region. Bodman e t al., on the ot8herhand, reported t h a t mixing prior t o settling does not have significant effect on the settling of titaiiium dioxide in water. As shown in Table 11, mixing prior to the settling was found to affect the settling rate in low p H slurry and had essentially no effect on the settling rate in the basic region. I n Table 11, “Inverted” means the settliiig tube was turned end over end by hand approximately 10 times to start the settling run. “Blended” means the sample was mixed for 5 min in a blender just before being poured into the settling tube. The results shown in this table indicate t h a t the nature of the cohesive forces holding together the aggregates of iron blue particles are basically different than ones in kaolin and titanium dioxide-water systems Since in the industrial operations, p H of the iron blue slurry is nornially very low (approximately between 1 and 2 ) , these results imply t h a t a strong mixing prior to the settling operation is very desirable for the optimization of the industrial settling. Just as shown by Michaels and Bolger (1962a) for the kaolin system and Bodman et, al. for the ,titanium dioxide system, the data of Table I1 also show t h a t the settling rates are reproducible for each of the cases of inversion of t h e settling tube and t h a t the aggregates can recover their former size after severe agitation in the blender. Chemical Treatment to the Slurry. Optimization of t h e set,tling process would, in general, require knowledge of the effect’s of a variety of standard chemical treatments t o t h e slurry on aggregate structure and fluid properties. As indicated by Bodman e t al., the response of the settling process to the chemical treatment of the slurry, depends on the zeta potential of the slurry. A slurry with smaller zeta potential will respond more readily t o the chemical treatments. Since t,he zeta potential of the industrial iron bluewater slurry in the acidic region (between pH 1.3 and 7.0) is very close t o zero (Smith, 1971), the settling rate of iron blue should be changed considerably by the various chemical treatments to the slurry. Effect of pH on the settling rate of iron blue is shown in Figure 6. This effect is considerably different from the one reported by Bodman e t al. for the settling of titanium dioxide in water and it is somewhat similar (in the fact t h a t both effects show maxima) t o the effect of flocculating agent on
e
-I
70-
*i
p~
TEMP.Y
3.0 7.5
24 23
0 60EU
4
*
0.00644 0.00644
50-
0
z
40-
m
301
'
I
'
6
10
8 PH
12
14
Figure 6. pH effect on settling rate of iron blue
---I
0
0.25 0.50 0.75 SALT CONCENTRATION (rnolei/liter)
1.00
Figure 8. Effect of salt concentration on settling rate of iron blue in dilute concentration regime
2 so
a Tern 2.4 0
1
0
1
f
2
4
2.45
2 0.00664
.'C
24
0.01
I
I
6 8 10 12 FA/a OF SOLID IRON BLUE)
14
1
F
I
0
0.1
0.2
I
I
,
I
I
0.3
0.4
0.5
0.6
0.7
0.8
LOBi k m )
Figure 7. Effect of flocculating agent concentration on settling rate of iron blue
Figure 9. Correlation between final and initial sediment heights
the settling rate of iron blue shown in Figure 7 . Variations in both pH and flocculating agent concentration are believed to affect t h e aggregate density more significantly than aggregate size (Table I). These results are soniemhat different than those for titanium dioxide-water system. Unlike for titanium dioxide-water system, here, the nature of the settling plot was unaffected by increase in pH arid flocculating agent concentration. The increase in sett'liiig rate with flocculating agent concentration was more pronounced for t h e smaller values of the solid concentration. At high concentration of t h e flocculating agent, t h e set'tling rate reaches a maximum value and then starts decreasing. The decrease iii settling rates is probably due to breaking of the aggregat,es caused by t'he interparticle repulsion generated by the self-induced double layer created by high concentrations of flocculating agent a t particle surface. It should be noted t h a t since iron blue decomposes a t high slurry pH (pH greater t h a n -7.5) into ferric and ammonium compoiients, the settling data a t high slurry p H are of no practical values. The effect of flocculating agent concentration on the settling rate obtained here was very similar to the one reported by Eodman et al. for the titanium dioxide-water system. It should be noted t h a t t h e viscosity of the iron blue-water slurry is essentially unaffected with increase in pH and flocculating agent concentration. Additions of various types of salts in t h e iron blue-water slurry were found to have somewhat unexpected effect on the settling rate of iron blue. Additions of each of four different' types of salt-sodium chloride (KaCl), sodium sulfate (KazSod. IOHzO), ammonium sulfate [ (NHq)zSO1], and sodium bicarbonate (NaHCOJ-were found to cause the graduate decrease in the settling rate (Figure 8). As in the titanium dioxide system, the settling rate did not show noticeable increase with initial additions of salts. The decrease in the
settling rates with increase in salts additions is believed to be mainly due to increase in slurry viscosity and decrease in aggregate density (Table I) with increase in salts additions. Settled Bed Density. Rlichaels and I3olger (1962a) derived the following relationship between the height of the settled bed and t h a t of the initial slurry:
In this study, the validity of the above equation was examined for a typical set of conditions by plotting 2, vs. Zo@i(for constant C p i ) . This type of result, showii in Figure 9 indicates t h a t Equation 1 applies well t o t,he iron blue-water system. h small value of b (approximately 1.4) shown in Figure 9 iiidicates t h a t the settled bed in iron blue-mater system is of more uniform deiisitj- t,han the one obtained in titanium dioxide-water system. Conclusions
It is concluded froni this study that for settling of iron blue in water, the dilute and intermediate coilcentration regimes are not, as clearly differentiated by the iiat'ure of t h e settling plots as they were in titanium dioxide, aluni mud, and kaolin systems. The effects of variations in slurry pH and salt concentrations as well as mixing prior to the set'tling and t h e temperature of t h e slurrp on the settling rate of iron blue are somewhat, different than ones reported for the settling of titanium dioxide iii water. 111 general, Michaels and Bolger correlations would be applied well to the settling of iron blue in water. Nomenclature
6 Ca,
=
=
ordinate intercept of straight line of Equation 1, cm ratio of volume concentration of aggregate t o solid iron blue
Ind. Eng. Chern. Process Des. Develop., Vol. 1 1 , No. 2, 1972
31 1
CF1 = da
D F
Q
Vsa Zo 2,
= = =
= = =
=
ratio of volume concentration of floc t o solid iron blue average (equivalent) aggregate diameter, p diamet’er of settling tube, cm volume ppm of flocculating ageiit/g of iron blue initial or masinium settling rate, cm/sec Stokes settling velocity of a single aggregate, cmjsec initial height of slurry column, cni final height of sett’led bed, cm
GREEKLETTERS 1.1
+i
= =
literature Cited
Bodman, S. W., Shah, Y. T., Skriba, hf. C., Ind. Eng. (:hem. Process Des. Develop., 11,46 (1972). Fitch, B., Ind. Eng. Chem., 58 (lo), 18 (1966);, Kakar, K., “Settling of Iron Blue in Water, AIS thesis, University of Pithburgh, Pittsburgh, PA (1972). RIichaels, A. S., Bolger, J. C., Ind. Eng. Chem. Fundam., 1, 24 (1962a).
Michaels, A. S., Bolger, J. C., ibid., I53 (1962b). Smith, J., personal communication, American Cyanamid co., Willow Island, WV (1971).
viscosity of solution, cP solid iron blue volume concentration
RSCEIVCD for review September 14, 1971 ACCEPTED December 27, 1971
COMMUNICATIONS
Mass Transfer in Through-Circulation Drying of Packed Beds
Experimental results on mass transfer obtained during the constant rate period of the through-circulation drying of packed beds, were analyzed. The results, expressed as j factors, were correlated using a theoretical model for fluid-particle mass transfer in fixed beds. A good agreement was found, which allows us to deduce that a laminar boundary layer exists over the range of Reynolds numbers investigated.
I n a through-circulation dryer a material generally dries at a constant rate until a critical moisture content is reached; then it dries at a progressively slower rate until drying is complete. This work is limited t o study the constant-rate period in which the flow of air through the bed provides only gas-film resistance to heat and mass transfer. The present investigation measures overall integral coefficients for a packed bed in the low range of air velocities commonly used in through-circulation drying. The results are correlated using Carberry’s model (1960) for fluid-particle mass transfer in fixed beds. Experimental Apparatus
The laboratory through-circulation dryer consists of a centrifugal fan which blows the air over 12 1-kW bar elements into a chamber at the base, and thence upward through a vertical duct. A removable basket containing the wet bed rests at the upper end of the duct. The vertical duct had a flow-smoothing section of 5 cm of small glass spheres. The air velocity was measured by a n orifice plate connected to a n inclined manometer. The inlet dry-bulb temperature was regulated by a thermostat and relay which controls one of the heaters. The humidities at inlet and outlet of the bed were determined with dry- and wet-bulb thermometers. The bed was made of ceramic cylinders 1.50 0.03 cm nominal size; these cylinders were capable of absorbing 31 2 Ind.
Eng. Chem. Process Des. Develop., Vol. 1 1, No. 2, 1972
sufficient quantities of water to exhibit constant drying rates. The bed was 500 cm2 in area and 7 cm in thickness; it had a void fraction of 37y0 calculated from the dimensions of the empty basket and the number and average size of the cylinders. To measure the temperatures of the surface of the cylinders in the bed, fine wire thermocouples (0.02-cm diam) were installed in several cylinders by drilling and inserting the wire through the hole until the couple junction reached a point just below the opposite surface. Experimental Procedure
The experimental technique applied to these studies represents a quantitative analysis of water evaporation rates from the available transfer surface in the bed of porous cylinders. All experimental measurements were restricted to the predetermined constant rate period to ensure transfer under steady-state conditions. T o provide a margin of safety, the runb were restricted t o a maximum of 75% of this predetermined constant rate period. The cylinders were soaked for 3 hr in distilled water. Then the water was poured off and surface droplets were removed with a cloth prior to placing them in the bed. An initial period of operation was allowed for the system to reach the steady state (about 10 min); the bed was then quickly removed, weighed, and returned to the dryer. After a time interval, the bed was again removed and weighed;
