The range of possible applications for bentonite in the paper industry is very board .Because of its unique properties bentonites can be used in many different ways to create benefits in the paper making process.We should like introduce the product "bentonite" and its main properties and advantages in this brief overview:
Bentonite is a nature clay, which mainly consists of smectite clay minerals.It is excavated in extensive mining activities. Most bentonites have been transformed by decomposition of volcanic pyroclastic rocks over millions of years. The particles of these minerals are very small(＜2μm) and extremely thin (about 1nm). Usually bentonites are characterised by their swelling behaviour and their ability to adsorb tremendous amounts of water.
Fig.1 shows the chemical structure of montmorillonite,the most numerous smectite mineral. Every single platelet of this special layered alumosilicate is formed by two tetrahedron layers and one octahedron layer in between. By substitution of atoms in the crystal structure (Mg2+ for Al3+/Al3+ for Si4+) montmorillonite exhibits a negative surface charge.This charge is compensated by cations in the intermediate layers, predominantly Ca,Mg or Na.Additionally the edges of the platelets can provide positive charges depending on the pH.
In dry state the electrostatic forces keep the layers together.But actually the interaction is so weak,that the cations in the intermediate layers are exchangeable.Furthermore water molecules can be incorporated between the layers,which enable an expansion of the material.Bentonite is able to adsorb up to 10 times of his own weight of water.
If bentonite is added in water the stacks of platelets can expand until the whole material is completely dispersed into very thin single sheets. Therefore bentonite probides an available surface area of 700-800m2/g in dispersion. The very high surface area in connection with the charge of the material forms the outstanding property of bentonite for applications like adsorption, flcculation and dewatering.
Most of the natural reasources are Calcium-bentonites.They predominantly have Ca2+ or Mg2+-ions in their interlayer and the sewllling properties are low.The most common products on the market are activated bentonites. The cations of these procedure is well known as "alcaline activation". Natural sodium-bentonites, Known as Wyoming-bentonites (but also existing in other deposits), are much rarer.They are dominated by Na+-ions in their intermediate layers but often comprise Ca2+ or Mg2+-ions in variable quantities,too.
Due to the high surface area and the charge balance bentonite can function as an excellent adsorber of disturbing material in the process streams.IT can be utilizedfor cleaning internal or external water streams.for example together with polymers in a microflotation unit or during waste water treatment.Thus bentonite forms also a good "trash catcher", adsorbin disrupting substances like sresin,stickies or pitch and takes them outof the process.
Bentonite is able to flocculate papermaking material in a suitable micro-structure that maximizes paper quality and improves paper performance. Additives like sizing agents or dyes are proved to be more efficiently retained in the sheet, a fact that secures quality while it saves costs.Moreover the retention of fines, fillers and colloidal material increases through addition of bentonite to the pulp.
The unique structure of the bentonite particles leads to improvement of dewatering. The drainage time of furnish on the wire increases if a bentonite microparticle system is used. In the same way bentonite can ease the sludge dewatering tremendously.
Bentonites, often very bright types, can be used to improve opacity in the finished paper productor to improve printability by their extremely well adsorption properties.
Some examples of possible applications in the paper industry (Fig.2):