LC-refining is a process where the interpretation of the results is complex. Different pulps show different responses and in spite of all trials performed no general conclusions have been developed. This lack of understanding of the refining process is mainly due to the fact that the refiner set up and the pulp interact to create the conditions of refining.
In the literature discussing pilot scale refining a large emphasis is put on refining intensity when comparing refining conditions. Very little concern is put on how other variables are changed to reach a certain refining intensity. Variables like flow-rate, consistency and rotational speed and the level of loading of the refiner (power input) are not usually discussed.
The refining pilot plant at Innventia has a large degree of flexibility and many trial combinations can be evaluated at relevant conditions. We have also found that recording the true gap clearance during LC-refining can be a way improve the understanding of the interaction between the refiner and the pulp suspension.
When planning a trial it is mainly three trial types to consider:
| Trial type |
Variable to obtain
refining curve |
Evaluate sensitivity to refining conditions |
Constant intensity 1
Multistage refining
Constant power |
Multistage refining
from chest to chest
The pulp quality entering the refiner is changed from stage to stage, affects the gap clearance at constant power |
Two intensity levels
Two levels of power +
Two levels of through-put to obtain the same energy input per stage |
|
Constant intensity 2
Single stage
Constant power |
Variable: throughput
Flow-rate affects gap clearance at constant power |
Two intensity levels
Two levels of power |
|
Mill-like
Single stage
Constant throughput |
Variable: power (gap clearance)
Varying intensity |
Two (three) levels of flow-rate |
Mill-like trial
The advantage of the mill-like single stage constant throughput refining trials is that besides it being the most relevant case for most applications the only variable within one refining curve is the gap clearance. The quality of the ingoing pulp is the same for all refining points. The refiners are equipped with Dametric gap clearance sensor in order to record the absolute gap clearance.
Constant intensity – single stage refining
This trial type requires that throughput is changed at constant power. This is easiest done by changing the flow-rate at a given concentration. The draw back with this layout is that the retention time in the refiner is changed and this can affect the homogeneity of refining. Refining at constant power and varaible flow-rate is not the same as that refining is done at constant gap clearance.
Constant intensity – multistage refining
In this trial type the pulp is pumped from one chest to the other and back at a given power, concentration and flow-rate. This means that the quality of the pulp entering the refiner will vary and this may affect the gap clearance and the refining effciency. To make comparable trials at different refining intensities the specific refining energy/stage should be similar. This trial type reqires less pulp than the single stage refining trials.
There is also a forth possibility and that is to recirculate the suspension in a similar manner as done in the industrial-like laboratory refiner, Voith LR 40. In this case the refiner is run at constant power and samples are collected at the time intervals to reach the target specific refining energies.
The refining pilot plant at Innventia allows for many different trial layouts. Our own research is mainly performed in a mill-like manner, i.e. refining at constant throughput. This is different from most pilot scale studies reported in the literature, where the refining intensity is kept constant for a refining curve. The design of the trial can have a considerable impact on the result, i.e ranking of different alternatives.
Examples of how the trial layout may effect the refining results.