Calcium Carbonate

Ok, so since I mentioned it briefly in my last post I figured I'd go ahead and talk about it some more. Calcium carbonate is naturally formed as limestone. It can also be manufactured through a precipitation process. The addition of calcium carbonate offsets the amount of wood fiber necessary to produce a certain weight of paper. The calcium carbonates also improve some properties of the paper depending on the shape and characteristics of the particles.

Precipitated calcium carbonate (PCC) is manufactured so the size and shape of the particle can be controlled. Some particles are scalenohedral in shape, they look like spiky balls. These particles can bulk up the sheet. They make the sheet thicker to some extent. There are also prismatic particles. These particles are rice shaped. They are used to improve the appearance of the sheet and porosity. It also strengthens the sheet with improved bonding. PCC is generally added at the wetend of the paper machine. PCC levels should be looked at from time to time on different grades. A good process engineer should look at opportunities to increase the amount of PCC added. To justify the addition compare the cost of the increase in PCC with the reduction in fiber. Also look at the reduction in refiner load due to the reduction in fiber usage.

Ground calcium carbonate (GCC) is naturally and as the name states is ground. The shape of the particle is rhombehedral. The particles are less uniform than PCC. They also differ much more in size from smallest particle to largest particle than PCC. GCC can be added at the wetend, but is generally added at the size press. GCC, aside from offsetting fiber, also improves sheet appearance and print qualities.

The addition of PCC should be looked at as a low hanging fruit. Running trials at higher rates if carefully planned run little risk of reject paper. Increasing the addition rates can push paper ash levels above 20%. So if you haven't looked at your calcium carbonate addition rates lately, it might be time to take a look. There's also new technology and process innovations being made that can help paper machines achieve higher addition rates. So also talk to your local vendor, they can be very helpful at times.