Dispersion And Kneading

Question 1

What are the main objectives of dispersion

Answer 1

Reduce dirt specks below visible limit and distribute them finely or make them floatable it is a similar concept with stickies and wax. Additionally to detach ink particles and mix bleaching agents in. They can also increase the bulk and strength characteristics of fibres due to the mechanical treatment. The process can also help destroy catalase enzymes which decompose hydrogen peroxide.
THE MAIN GOAL IS NOT TO REMOVE CONTAMINANTS BUT TO DISINTEGRATE THEM

Question 2

What are the steps of dispersion process, process parameters and the types of unit

Answer 2

Stock is dewatered to 30% and then heated with steam, temperatures 85-120 in a heater screw. The fibres are then fed into the dispersion unit where they experience high friction and shearing forces. The stock is then diluted again for further processing.
Goal is to introduce shear forces that will bread down and distribute contaminants.
Process parameters include viscosity of pulp and contaminant (reduce viscosity) and intensity of sheer and time
Can have disk disperser or kneading disperser

Question 3

Compare kneaded to disk disperser : which system is better for dirt and stickies reduction, for strength properties, and for bulk
How can you influence the above parameters on a disk disperser

Answer 3

Disk disperser is filled with teeth which the stock is forced through, very high shear forces are generated
Kneading dispersers are often much slower and have wider spacing
Disk dislersers have good effect on direct and stickies reduction whereas kneaders are not so good with stickies. Disks have good impact on strength properties whereas kneaders had no effect.
On dis, disperser increase in consistency, dispersion energy and temperature will have a positive effect. But heat needs to be balanced not to damage the fibres