“Sticky” Issues Facing the Pulp and Paper Industry
Recycle fiber is being recovered at a rate of 40MM tons annually. Currently 75% of paper mills utilize recovered fiber and 200 mills are 100% recycle. One industry source estimates the economic impact of stickies to be in excess of $700 million a year. The economic cost of stickies will rise as lower quality recycle fiber is utilized to meet the demands of recycle rate increases. The economic costs of stickies can be categorized into the following areas.
Raw material downgrade
Mill equipment replacement costs
Water reuse and quality costs
What Are Stickies?
Stickies are the broad class of deposits that contain synthetic contaminants from the break down of adhesives in wastepaper. Hot melts, contact adhesives, and xerographic inks comprise the three major sources of stickies. Figure 1 shows the sources and components of stickies.
Figure l shows that there is an overlap of components in the 3 major classifications. Typically the analysis of a stickies deposit will show a mixture of adhesive components along with inorganic and organic compounds not related to wastepaper contaminants.
Stickies can be formed from the direct breakdown of adhesives in wastepaper (primary stickies) and/or from the interaction of chemicals (secondary stickies). The tackiness of stickies particles is greatly influenced by chemical interactions within the system. Wood resins and hydrocarbons from additives both greatly increase the tackiness of stickies. Laboratory studies have shown that pulping at high pH increases stickies formation and tackiness. Peroxide bleaching of wood containing wastepaper has also been shown to increase tackiness of stickies.
Stickies retained on a 6 cut screen (>l 50um) are classified as macro stickies, whereas micro stickies are smaller than l 50um in diameter. Numerous test procedures have been developed to measure the quantity of micro stickies. These test procedures are typically based on some variation of collecting the micro stickies on a pre-weighed hydrophobic material. Within the mill environment macro stickies can be broken up into micro stickies by process equipment such as pressure screens and micro stickies can become macro stickies through agglomeration.
The surface properties of stickies, roughness and surface properties of the material on which they are being deposited, system, temperature, chemical interactions, and the electrostatic environment of the white water all play important roles in the deposition of stickies. The complex nature of these interactions makes the accurate prediction of stickies deposition potential very difficult.
"Stickies Aren’t Stickies Until they Are Stuck"
Mills utilizing Old Corrugated Cardboard (OCC) typically face the largest problems with stickies. The following table shows the relative quantity of stickies for various wastepaper grades.
|The Relative Quantity of Stickies in Waste Paper by Source
The strategies used by mills to minimize stickies related problems include the following.
Sorting and source control of wastepaper
High Tech Surface Coatings
Pulping pH and Temperature
Wet End Chemistry Control
Chemical Treatment Programs
To achieve effective control of stickies, most mills use a combination of all three above methods.
As wastepaper utilization rates rise, the economic impacts of stickies can be expected to increase. The complex nature of stickies and their deposition will require mills to balance capital equipment expenditures and chemical costs to maintain runnability and paper quality. Innovative equipment and chemical suppliers will be needed to develop new solutions to address the specific needs of individual mills.