Wet-end starch use has become nearly universal in the packaging and graphic papers manufacturing process. It is generally accepted that wet end starch is good for drainage, retention, and strength development. What is less commonly appreciated is the extent of wet end starch’s impact on machine runnability, paper quality and cost. There are two important, frequently overlooked aspects of solids consistency that can mean, literally, hundreds of thousands of dollars to a mill. The first of these, addressed here, is simple loss through over-use. The second issue will be addressed separately as “the impact of starch consistency on paper machine runnability”.
Lost starch and increased costs through weak solids control
Wet end starch is normally prepared, stored and used between 1% and 3% solids. But normal variation can be as much as 20%. How is this possible?
1. If using starch at 1% solids, for example, it is not uncommon to see readings of ±0.2% (i.e. 0.8% to 1.2%) simply due to reading error, improper zeroing of an instrument, inadequate temperature compensation, sample evaporative loss or simple lack of appreciation for the significance of a seemingly small variation. On a standard 0 – 10% gradient refractometer the smallest discreet unit is only 0.1%. Thus, there is an inherent experimental error of 0.05% (5% of target at 1% starch consistency) and that is if the instrument is correctly zeroed and read.
2. Drying ovens, moisture balances or in-line densitometers can improve the accuracy and repeatability of solids measurement, but even these are not without their limitations.
a. Care must be taken to minimize evaporative loss from the sample.
b. Periodic testing, like standardizing any instrument should be done regularly to insure accuracy and repeatability.
3. Experimental error can be reduced by using starch at higher solids, say 1.5% to 3.0%, but care must be exercised to ensure good dispersion of the starch into the stock stream to avoid over-flocculation and machine direction inconsistencies due to poor dispersion or localized concentration of the charge. 1% is the standard concentration for good reason.
4. By extension, if the delivered cost of wet end starch is $0.45/lb. ($1000/metric ton) on a 100% dry substance basis (normally received
at 12% to 18% moisture), the potential variability is as much as $45,000 (i.e. ±10%) per year for each one million pounds (454 metric tons) of wet end starch purchased (100% ds).
5. Excessively broad mill standards for targeted starch solids can also lead to significant excess cost. For instance, one recently observed target specification was 3% ± 0.5%. That is an acceptable range of variation of 1% (33% of target), even forgetting about experimental error!
In this case, if the target solids is 3% and daily use is within the mill specification at 3.5%, the over-use of starch is potentially 16.7%, or as much as $75,000 annualized for each million pounds (or 454 metric tons) of starch with the same cost assumptions as above. Further, if acceptable runnability and quality are attainable at the permissible 2.5% minimum, then the extra cost incurred is as much as double, or nearly $150,000 annually per million pounds of starch used!
All of these details are tedious and no alarms will go off if either accuracy or consistency is lacking. But the cost to a mill is real. The key to minimizing these costs is to accurately control starch solids with precision to the tightest reasonable specification.
Starch Performance Services can help your mill maximize the value from your starch.
