Flotation Flashcards
(11 cards)
- Explain (incl. sketch) the principle of flotation deinking!
Flotation is a separation process where air is injected into the stock suspension. Contaminants such as ink and fillers are adhered to air bubbles which rise to the surface where they can be removed. This principle allows us to remove particles which have the same density as water.
Involves mechanical and chemical aspects. Major steps are particle detachment, collision of particle and air bubbles, attachment and then removal.
- What are typical operating conditions in a flotation cell?
Stock consistency around 1%. Flotation works best with low consistency – needs to be easy for bubbles to rise, but of course lower consistency = lower productivity so need to balance, ~1% is good.
Temperature 40-70°C Reduces water viscosity, also helps with particles adhering to the bubbles.
pH value 7-9
Water hardness 5-30°dH You need enough calcium 2+ in the system! This is so you can form the right soap.
- Explain the working principle (incl. sketch) of a Voith Paper EcoCell Flotation Unit? How is the reject flow handled with this system?
Has an oval shape which is optimised compared to older round versions. The shape means the surface area is increased so we can form and remove foam better. Reject is allowed to overflow
- What is the effect of consistency (low vs high) on brightness and dirt speck reduction (same raw material composition)?
Low consistency is best, much easier for the bubbles to pass to the surface and allow for effective cleaning. However, this needs to be balanced as the lower the consistency the lower the throughput. Usually around 1% is used. The principle is the same for dirt specks.
- If you have a two stage flotation cell system, what are the advantages of cascade flow compared to a forward flow concept? What are the disadvantages?
Advantage is that often in the first cell there is a large filler and fibre content. However there is so much foam that you need a foam breaker which needs to be removed with a cyclone (similar to cleaners).
- What are standard deinking chemicals and which optional chemicals can you add in addition?
Standard: NaOH
Waterglass/sodium silicate (supports the bleaching of fibres & stabilises hydrogen peroxide)
H2O2 (whitening agent)
Surfactant (soap which later transfers to calcium soap which helps bind ink particles to bubbles)
Optional:
Chelating agent (protects hydrogen peroxide from heavy metals)
Calcium chloride (if there is low water hardness)
- Explain the working principle of caustic soda for flotation deinking! Why do you need hydrogen peroxide?
The caustic soda promotes fibre swelling which helps loosen the attached ink. It also causes ionisation of surface groups and contributes to saponification of ink binders. The hydrogen peroxide minimises darkening of mechanical pulp. Bleaches cellulose fibres. Oxidise bonds in polymer network to help ink detachment.
- How does “sodium soap” work in flotation deinking, what is the function of “calcium soap”?
Sodium soap is not very hydrophobic it can only attach to one chain. Whereas, the calcium soap has 2 hydrophobic ends so can attach to multiple hydrophobic inks. Since it is very hydrophobic it acts as an ink collector which can then be removed to the surface.
- What is saponification during deinking?
Saponification process helps to transform hydrophobic substances into soap which is hydrophilic. This transformation makes it much easier to remove.
- What are the functions of the following chemicals in the deinking process: NaOH, H2O2, Na2SiO3, CaCl2, fatty acids, calcium soaps, surfactants
NaOH – adjust pH to alkaline region. Saponification of ink. Fibre swelling. Induces ionisation of carboxylic groups (creates more negative charges which is beneficial for repulsion)
H2O2 – minimise darkening of mechanical pulp. Bleaches cellulose fibres. Oxidise bonds in polymer network to help ink detachment.
Na2SiO3 (water glass) – avoids dissociation of peroxide (acts as chelating agent). Improves brightness and reduces yield loss. Works as pH buffer, also acts as a dispersion agent which helps with deinking.
fatty acids – act as surfactants. They precipitate with calcium ions to form highly hydrophobic soap flakes which act as ink collectors
calcium soaps – calcium soap is very hydrophobic so can attach to hydrophobic inks and allow them to be removed to the surface.
surfactants – Help with emulsification, they release oil from the fibres to form an oil in water emulsion.
- What are the influencing parameters for peroxide decomposition and how can you avoid them?
Heavy metal ions can dissociate peroxide turning it to water and oxygen. To avoid this, you can add chelating agents to trap the metal ions and waterglass. The water glass also helps to improve brightness and reduce yield loss.
Another parameter is the presence of catalase in the pulp. This enzyme decomposes hydrogen peroxide. To destroy this can do a chemical treatment or thermal (>80C) – thermal is preferred.
