Efficacy of dispersion of magnesium silicate (talc) in papermaking
S1065
Table 4 Effect of dispersion mechanism of different talc fillers on filler retention, and light scattering coefficient of talc filler and paper.
Colloidal charge demand, µeq/g FPAR, %
41.1
40
28.8
28.9
Light scattering coefficient of talc, m 2 /kg
Particular
FPAR, % Light
30
25.0
24.2
scattering coefficient of paper, m 2 /kg
23.4
20
A a B b A a B b A a B b A a B b A a B b
50.1 45.3 42.9 41.7 39.3 40.8 29.1 23.4 20.1 21.5
44.9 46.5 48.8 50.1 52.4 44.5 47.4 48.2 50.3 51.2
88.3
Talc-1
110.2 126.8 137.5 148.1
10
3.3
Talc-2
2.0
0
Talc-3
With water only With wetting agent (7 kg/t)
With dispersant (3 kg/t)
With wetting agent (7 kg/t) & dispersant (3 kg/t)
89.3
Talc-4
108.9 127.1 140.0 151.4
Talc-5
Figure 6
Effect of wetting and dispersing chemicals on colloidal
charge and first pass ash retention of Talc-3 filler.
a With water only. b With wetting agent, 7 kg/t filler and dispersant, 3 kg/t filler.
coefficient of both filler and paper were not affected by the change in the dispersion time. In all cases, the FPAR was 45.0–46.5%. The scattering coefficient of paper having 15.5% filler was 49.5 m 2 /kg. The calculated scattering coeffi- cient of Talc-3 filler was 123.0 m 2 /kg (calculated using Eq. (3)). The scattering coefficient of paper sheet with no filler con- tent was 36.0 m 2 /kg (Fig. 7).
This showed that the particle size of filler has a great role in papermaking. If PSD of filler is broader, more fillers can be re- tained in paper however the light scattering will be decreased and vice versa. The effect of PSD of talc fillers was examined through their dispersion with water only and the optimized dosage of wet- ting agent along with dispersant. In these experiments no retention aid was used. The reason was to observe the effect of particle size and dispersion medium on as such filler reten- tion. The filler addition levels were selected to get around 15% ash content in paper. The FPAR vis-a` -vis ash content in paper was lower in the case of fillers wetted and dispersed as compared to those dispersed in water only. The FPAR of Talc-1, Talc-2, Talc-3, Talc-4 and Talc-5 fillers dispersed in water only was 50.1%, 45.3%, 42.9%, 41.7% and 39.3%, respectively which decreased to 40.7%, 29.1%, 23.4%, 23.0% and 21.6%, respectively when the fillers were wetted and dispersed (Table 4). In order to get the same ash content in paper, the filler addition levels were increased in the case of fillers wetted and dispersed. Under our experimental condi- tions, there was no change in scattering coefficient of paper and filler in both the dispersion techniques. This was true for all talc fillers. The scattering coefficient of filler vis-a` -vis paper increased on decreasing the particle size of filler due to more number of solid-air interfaces. In order to observe the effect of retention aid polymer on the retention of talc filler dispersed using different methods, the Talc-3 filler was added in refined pulp along with cationic polyacrylamide retention aid (200 g/t pulp). The ash content in paper, FPAR and scattering coefficient of paper were mea- sured. The target ash content in paper was around 15–16%. When no retention aid was used the FPAR of Talc-3 filler dis- persed with water, and wetting and dispersing agent was 42.9% and 23.4%, respectively which increased to 49.2% and 50.0%, respectively with the addition of retention aid polymer. The cationic retention aid was helpful to maintain the charge chemistry of the papermaking slurry which, in turn, was helpful to maintain the same FPAR for differently dis- persed Talc-3 fillers. Under our experimental conditions, the
3.4. Effect of particle size of talc filler on FPAR and scattering coefficient
The retention of filler particles increases on increasing the par- ticle size of filler and vice versa. Our study also showed a sim- ilar trend. The filler retention decreased with decreasing the particle size of talc filler. It was highest in the case of coarsest filler i.e. Talc-1 followed by Talc-2, Talc-3, Talc-4 and Talc-5. The reduction in FPAR on wetting and dispersion of filler was also dependent upon their particle sizes. The rate of decrease in FPAR for Talc-1, Talc-2, Talc-3, Talc-4 and Talc-5 fillers was 18.8%, 35.8%, 45.4%, 44.8% and 45.0%, respectively (Ta- ble 4). There was an enormous difference in the rate of reduc- tion in FPAR of Talc-1, Talc-2 and Talc-3 fillers whereas that of Talc-4 and Talc-5 filler was almost comparable to Talc-3.
FPAR, % Scattering coefficient, m2/kg
140
124.5
122.8
123.7
123.0
120
100
80
60
46.5
45.0
45.6
45.6
40
20
0
30
60
90
120
Dispersion time, min.
Figure 7
Effect of dispersion time of Talc-3 filler in water on first
pass ash retention and scattering coefficient of talc.
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