Overview
Specialty Minerals Inc. (SMI) precipitated calcium carbonate (PCC) filler pigments
permit papermakers manufacturing wood-containing papers to cost-effectively
attain high brightness and opacity demanded by today’s publishers. Traditionally,
paper utilizing virgin or deinked mechanical pulps was made under acidic conditions
in part due to fiber darkening. Mechanical (groundwood) pulps used in the wet end
contain lignin, a naturally occurring material that darkens with increasing pH.
This alkaline darkening reduces the effectiveness of PCC at increasing paper brightness.
If the pH of a mechanical papermaking system is controlled to about 6.5 to 7.5, alkaline
darkening is minimized and PCC is much more effective. The problem with this pH range
is that calcium carbonate is soluble and significant amounts of PCC will dissolve.
SMI offers the AT® PCC (acid-tolerant) system utilizing a patented technology that allows PCC to exist in weakly acidic to weakly alkaline conditions without significant dissolution or stability problems. The AT® PCC system can be used with a number of different PCC fillers that outperform clay and other fillers. The AT® PCC system allows mechanical papermakers to realize brightness and opacity characteristics compared to PCC in woodfree papers, which are typically made under alkaline production environments.
The AT® Precipitated Calcium Carbonate System
The AT® PCC system from SMI is a three component system
consisting of appropriate amounts of calcium carbonate (precipitated or ground
calcium carbonate and/or chalk), a weak acid, and a chelant. The weak acid
buffers the water of the paper machine at the desired pH level (typically 6.8 to
7.2). When stable, the buffer system will resist pH changes in either direction
because buffering species are capable of consuming both H+ and OH- ions.
PCC is prevented from significantly dissolving in the system because of the
common ion effect. Once the system reaches equilibrium, the presence of
dissolved calcium ions in the water inhibits further PCC dissolution.
While increased dissolved calcium levels help inhibit calcium carbonate dissolution, they also accelerate the decomposition of soluble buffering species which can precipitate as calcium salts. The chelant addresses this issue by serving a dual function. First, the chelant stabilizes the buffering species by binding calcium ions. The chelant has a higher affinity for calcium ions than the soluble buffering species which allows for greater increases in dissolved calcium before the buffering species precipitate as calcium salts. Second, the chelant acts as a scale inhibitor. The chelating agent, in addition to binding calcium ions, inhibits the formation of calcium salts by reacting with the surface of calcium salt crystal inhibiting further crystal growth. This action prevents deposit formation on the paper machine. The chelant is usually added to the filler either separately or as part of the filler dispersant package.
Pointers for Understanding the Chemistry of the AT® Precipitated Calcium Carbonate System
- When running the AT® PCC system, it is important to consider the entire papermaking system. Chemical dosages will be determined by the degree of unbuffered fresh water entering the system although filler loading and alkalinity will affect acid demand.
- If the AT® PCC system is not run properly, poor brightness response or scaling issues may result. Warning signs that the system may not be running properly are large variations in pH, increases in acid demand, and sharp increases in dissolved calcium.
- Chelant use is critical for this application to run properly. SMI has identified several preferred chelating agents for this application.
The AT® PCC system does not stabilize PCC against dissolution by strong acids. Contact with strong acid (e.g. HCl or H2 SO4) should be avoided throughout the paper mill to minimize dissolution and calcium ion buildup.
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