Opportunities in Wet-End Chemistry: Feature Essay, Posted Nov. 2003
"How "Good" Is Your Process Water?"
Martin A. Hubbe
Dept. Wood & Paper Sci., N.C. State Univ., Box 8005, Raleigh, NC 27695-8005
Citation (public domain): http://www4.ncsu.edu/~hubbe/new
Anyone can make paper with an unlimited supply of crystal pure water. Trouble is, nobody has an unlimited supply of crystal pure water. That's where things start to get interesting.
Things that may make your process water a bit different from the water in the next nearest paper mill, or even on the adjacent paper machine include the following:
Given enough time, a good papermaker will figure out how to cope with just about any type of process water. For example, let's consider the issue of hardness. Water is called "hard" when it contains a lot of calcium or magnesium ions. These ions tend to precipitate wood pitch. They tend to compete with alum, making it more difficult to get good sizing with rosin products. In alkaline papermaking systems the hardness can combine with the hydrolyzate of ASA size, making a very sticky type of deposit. In extreme cases the hardness ions can participate in the formation of calcium oxalate, carbonate, or sulfate scales. Papermakers in the US Midwest have learned over the years to cope with water hardness exceeding 100 in the fresh water.
It gets even more interesting when papermakers introduce calcium carbonate into "acidic" papermaking processes, i.e. those that use rosin and alum for sizing. Just like an Alka-Seltzer® tablet, the calcium carbonate releases CO2 bubbles and dissolves in the acid solution, releasing calcium ions. Hardness levels then get artificially high, enough to precipitate scale in some cases. Yes, scale-control agents are available. And yes, some major suppliers of calcium carbonate filler have ways to slow down the dissolution of CaCO3 filler. However, as the amount of calcium carbonate entering the system increases (for instance, in coated broke or in waste paper) there may come a point where it is just simpler to avoid hardness problems by alkaline papermaking.
"Alkalinity" is a quantity found by titration. You cannot measure it with a pH meter alone. Rather, it is a measure of how much acid it takes to reduce the pH to a low value. You might want to run alkalinity tests over a period of time if you face a situation similar to the one described in the previous paragraph. Find out if the alkalinity is highly variable, and whether it is consuming an excessive amount of alum, PAC, or sulfuric acid. "Acidity" or "total acidity" is based on a similar titration with sodium hydroxide or other suitable alkali titrant. Papermakers who use alum can use total acidity tests to quickly estimate whether soluble aluminum is accumulating in their process as the water recirculates. If the measured total acidity of filtered process water is several times higher, compared to what you calculate based on an assumption of 100% retention of alum, then you ought to consider cautiously raising the pH of the system.
Hmmm, we haven't nearly gotten through
our list, but now we're out of space. The point of the list is that there are
quite a few good tests that can be used to describe process water is more useful
ways than "good" or "bad." Perhaps in the mill or mills
you work with none of these tests is needed. But in another case a test for
hardness, or total acidity, or cationic demand might for the basis of a strategy
for smoother, more efficient operations, better sizing, or less felt-filling,