Paper making and fiber liberation – Processes and products – Non-fiber additive
Patent
1994-05-05
1996-11-26
Chin, Peter
Paper making and fiber liberation
Processes and products
Non-fiber additive
162183, 162190, D21H 2106
Patent
active
055781691
DESCRIPTION:
BRIEF SUMMARY
Several changes have taken place during the past few years in the water circulations of processes dealing with wood-based fibre suspensions, especially in the case of water circulations connected to papermaking:
Environmental and social pressures call for reductions in effluents to the water system. This leads to shortened water circulations and their partial closing off.
The need to save expensive fibre raw material calls for increasing use of recycled fibre. The use of recycled fibre has increased significantly and there are countries where the legislation makes it compulsory to use such material. For the same reason the use of groundwood and other mechanical pulps is on the rise.
As a means of cutting costs, increasingly higher proportions of the more expensive filler and coater pigments in the manufacturing of paper and board are being replaced by CaCO.sub.3. This requires a shift over from acidic processes to neutral processes.
Due to all the above reasons, increasing amounts of what may be called interference chemicals are becoming concentrated in the process water circulations. The interference materials are chemically a highly heterogenous group of compounds. Typical examples of these are wood extracts, neutral or anionically charged, substances in dissolved or colloidal form arising during cellulose production or originating from the wood.
In addition, coating binder residues, synthetic polymers, anionic emulgators, etc., end up in the water circulation of the paper machine in the course of manufacturing of coated papers.
De-inked recycled material can never equal virgin fibre raw material in terms of purity as it always contains de-inking chemical and ink residues.
The aforementioned interference substances impair i.a. the runnability of the paper machine by increasing the frequency of web breaks, by blocking up the wires, by contaminating the press rolls and by reducing paper quality in terms of increasing number of holes. The interference substances reduce retention by reducing the efficiency of the available retention system as impurities bind to the retention substance and thereby bind part of the functional groups of the said retention substance.
Broadly defined, all chemicals used in the above processes become interference substances once they become concentrated in the water circulation. In its more emphasised form, this problem occurs expressly in paper machines.
Traditionally, paper manufacturers have used alum to combat the effect of interference substances. Alum efficiently binds interference substances to the fibres and thereby inactivates them. The effect of alum is based on the fact that within the pH range of 4.2-5.0 alum forms an abundance of positively charged cations which bind and neutralise mainly negatively charged interference substances. In neutral paper manufacturing processes alum is ineffective and this is why other solutions must be resorted to.
Another method is to use talc. Talc binds impurities to its surface and in favourable conditions the interference substances leave the system along with the paper web. However, talc does not work very well if the concentration of impurities is high.
Dispersing agents are used with the purpose of dispersing the interference substances into the circulation water. A shortening of the water circulations leads to situations in which the concentrations of the interference substances rise to high levels in the circulation water, which in turn reduces retention and on the other hand makes it possible for fresh accumulations to occur, thereby reducing product quality.
As alum is a cationic product, there is a trend in neutral process papermaking to replace it with short-chained cationic polymers; the best known among these are the poly-DADMAC products (i.e. poly-diallyl-dimethylammoniumchloride). These polymers typically possess a high cationic charge (meqv/g) and their molecular weight is relatively low when compared to synthetic products traditionally used as retention polymers. Corresponding other cationic binding chemicals include va
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Chin Peter
Raision Tehtaat Oy AB
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