1. Introduction - Firenze University Press

Table 6. Experimental conversions of steel slag into PCC
Experiment 1 2 3 4 5
Reactive calcium feed g/h 5.8 4.5 4.3 3.7 3.7
PCC product (carbonator) g/h(% of total) 2.5(100) 4.2(75) 1.9(51) 1.4(31) 2.0(35)
PCC product (pH adjustment) g/h(% of total) - - 0.8(20) 2.3(51) 3.5(60)
CaCO3 (other system) g/h(% of total) - 1.4(25) 1.1(29) 0.8(18) 0.3(5)
CaCO3 (total) g/h 2.5 5.6 3.8 4.5 5.8
Conversion of reactive Ca % 17 49 35 48 62
Conversion of total Ca % 8 22 16 21 28
Table 7. Experimental Ca 2+ concentrations after and before extraction reactor
Experiment 1 2 3 4 5
Ca 2+ after extraction reactor mol/L 0.026 0.041 0.012 0.015 0.018
Ca 2+ before extraction reactor mol/L 0.007 0.019 0.004 0.007 0.004
226
Fig. 6. pH during experiments 3-5 in extraction
(up left), carbonation (right) and pH adjustment
(down left) units.
A remark that can be made based on Fig. 6 is that the pH in the pH adjustment unit has been quite
unstable during the experiments. Thus, applying a process control device to steer the bypass rate as
proposed in the previous section would improve the performance of the experimental setup.
4. Energetic properties of the process
Based on the presented Aspen model, heat duties of different process steps can be estimated. In
Table 8 values are shown for the case with a 15% bypass, which was in above discussion found to
result in highest production rate of pure PCC. It can be observed that all process steps, except for
the flashing units, are exothermic, i.e. release heat. However, as the process is operated at low