Building a better wet end: to improve wet end chemistry, consider some new concepts.
* broke coagulation for deposit control
* on-line consistency and retention instrumentation
* microparticulate retention systems
* alum in alkaline papermaking
* charge measurement for process stability
* internal sizing technology advancement
* on-site carbonate generation
* improved and safer biological control technology.
These major developments have improved papermaking, but there are several other unique and underutilized wet end chemical concepts and approaches that mills should consider. The discussion below covers selected concepts that are either new, novel in approach, or used infrequently. These concepts are:
1) controlling white water solids instead of retention
2) white water coagulation
3) turbidity as a measurement tool
4) starch residual measurement
5) split colorant addition
6) controlling process inputs.
White water solids control: Instead of running to a retention target, mills should consider using a white water solids target. Machine cleanliness and up time depend on white water cleanliness, not retention. Chemical and biological deposits develop as concentrations build in white water loops. While retention is a measurement of white water cleanliness, it can be misleading.
Consider the following example where two machines are running to a target of 85% first pass retention. Machine A is running head box (HB) solids at 0.45% and white water solids at 0.07%. Machine B is running a HB solids rate of 0.75% and white water solids of 0.11%. Both sets of numbers yield 85% first pass retention, but machine A is nearly 40% cleaner than machine B because its white water is cleaner.
This technique also eliminates basis weight swings on a given machine since mills maintain white water solids when head box consistencies vary with weight changes. One final benefit lies in the reduction of lab testing and the ability to use freed-up lab time for higher priority items.
White water coagulants: Mills have used coagulants or low molecular weight polymers with high charge densities for many years to neutralize contaminant charge, fix dyes, promote high molecular weight retention flocculants, increase drainage, remove color, treat broke, neutralize wood containing stock, and neutralize recycle paper. These coagulant polymers include alum and can be used effectively to neutralize soluble chemicals and particulates in a machine's white water. These polymers increase particle size and bring some solubles out of solution.
The result is that these "bad actors" are easier to retain and carry out of the system. Since an increase in machine retention flocculent is not involved, mills can increase retention (white water solids are decreased) while maintaining or improving formation.
This technique has also led to mills using less high molecular weight retention flocculant while maintaining the same first pass retention target. Mills can choose from a wide variety of coagulants that will likely work for any given white water. These coagulants are selective in what they will or will not work on, so mills should screen them before machine application.
Turbidity as a measurement tool: Turbidity is an inexpensive and rapid test that can be used to monitor machine changes. The current lines of on-line consistency meters use various forms of transmitted, reflected, and scattered light to measure solids content. Papermakers can use this same concept with a small portable or lab-sized turbidity meter to monitor trial results or to control machine process parameters, They can measure white water turbidity directly on the machine or in the lab and relate it to the machine's white water solids and retention level.
This technique is useful for relative changes, but should not be used for absolute numbers or comparisons between two different machines. Pigments and fillers can also affect the values, so the test works best where pigment and filler levels are relatively constant. The rapid test results are useful for monitoring trial changes and for fast operator response.
Mills can use a similar technique to monitor the effectiveness of a white pitch coagulant on latex containing broke. The better the coagulant is working, the clearer the broke filtrate. The polymer ties up the opacity causing latex and pigment and the filtrate will be almost clear at 100% effectiveness.
Starch residual test: Soluble wet end starch is present in all processes using size presses, coolers, and wet end starch. The residual soluble starch in the wet end comes from either broke, unretained wet end starch, or both.
This soluble starch is an interfering substance in that it can cause foam, act as a loud for biological deposits, interact with the other cationic additives, or block surface adsorption by desired additives Papermakers should try to minimize these starches.
A simple starch iodide colorimetric test will tell the quantitative level of these soluble starches. This test will not distinguish between size press, coating, or wet end starches, but that is not important since all soluble starch should be maintained at the minimum possible level.
To run this test, take a standard colorimeter and run a starch calibration curve with different starch concentrations using a potassium iodide solution as the color indicator. Follow the same procedure with the filtered machine white water and compare the adsorption on the colorimeter to the standard curve. Finally, alter the process to minimize the soluble starch in the white water system.
Split colorant addition point: Mills frequently lose paper from their machines that is off color or off shade. The paper industry has discussed extensively the need to split flow starch and other wet end additives, but so far with little emphasis on colorants. Paper mills can significantly reduce off color and off shade paper losses by splitting the flow of dyes and fluorescent whiteners.
To accomplish this, base load about 80% of the colorant to the thick stock at a point of good turbulence with proper dilution and separated from other additives. Then, trim the flow with the remaining 20% to the thin stock, again with good turbulence, plenty of dilution, and away from other additives. As color varies, it is now possible to adjust this thin stock stream dye flow in a way that more rapidly reacts to color swings.
This is an improvement over waiting 45 minutes or more for the thick stock dye change to work its way through the entire furnish system. Faster response means less lost paper.
Process Variation Reduction: Paper mills should consider using a process that allows improved control of process input parameters. I will discuss this process in a column in next month's "Worldwatch" section of Solutions! magazine.
About the author: C. A. (Kasy) King is principal of Papermaking Process Consulting LLC, Appleton, Wisconsin, USA, and a member of the TAPPI Editorial Board. He can be reached at +1 920/991-9102, or by email at email@example.com
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|Title Annotation:||Wet End Chemistry|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Apr 1, 2002|
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