A novel unit operation to remove hydrophobic contaminants

Abstract

For mills making paper with recovered fiber, removal of hydrophobic contaminants is essential for trouble-free operation of paper machines. Significant cost savings on paper machine operation can be achieved by reducing deposits, which results in better quality, reduced downtime, increased fiber yield, and reduced energy consumption. Bubble nucleation separation (BNS) is a relatively new process for removing hydrophobic particles. When vacuum is applied to a slurry, dissolved gas bubbles nucleate on hydrophobic particles and drag them to the surface for easy removal. We constructed a 16-L batch unit to evaluate the effect of operating parameters on removal of hydrophobic particles, using statistical design of experiments. These results were used to guide our design of a 16-L continuous unit. We tested this unit on laboratory and mill samples. The removal of 60%–80% of hydrophobic particles was achieved with a low reject rate of < 2%. Following on this success, we built a 200-L pilot unit and tested it in our pilot plant. With promising results there, we installed the pilot unit at a commercial paper recycling mill. Over the course of several mill trials, we showed that it was possible to remove a considerable amount of suspended solids from paper machine white water with less than 2% rejects. Unfortunately, due to the unit only treating 50 L/min and the mill flow being 12000 L/min, we were not able treat a sufficient portion of the white water to know whether a large-scale implementation of BNS would improve paper machine runnability. Application: If implemented in a paper machine white water loop, this technology could provide a way to control stickies and dirt, without adding any chemicals. According to the American Forest and Paper Association (AF&PA), 77% of paper mills in the United States use some form of recovered fiber [1]. In spite of advances in screens and cleaners, we do not have a satisfactory process for the removal of hydrophobic contaminants, especially small particles that pass through slotted screens. Hydrophobic particles in paper recycling, also called stickies, arise from pressure sensitive adhesives, natural pitch, book bindings, postage stamps, envelopes, inks, etc. Often, stickies are comprised of mixtures of materials including, wax, polystyrene butadiene, polyvinyl acetate, silicones, wood extractives, and any other polymers in the recovered paper. In paper recycling, the removal of stickies is of paramount importance for trouble-free operation of paper machines. Stickies accumulate in the paper machine white water and eventually agglomerate and deposit on wires and felts. These deposits have drastic effects on the paper machine productivity. Cost savings on paper machines due to reducing stickies deposits can be substantial. Tom Friberg estimated in 1996 that the cost impact of stickies to all paper recycling mills combined in the United States is about $700 million per year [2]. Flotation processes are widely used in the industry to selectively or completely remove suspended solids from slurries. Two widely used flotation processes are froth flotation (FF) and dissolved air flotation (DAF). In this project, the bubble nucleation separation (BNS) process is introduced. Important features of these three flotation processes are shown in Table I. Froth flotation is widely used in ore processing and pulp deinking for selective separation by dispersing air through the slurry. Surfactants are added to enhance particle attachment to bubbles. Relatively large bubbles, 0.1 to 1.0 mm2, collect hydrophobic particles and float to the surface of the slurry, where the foam is removed. Controlled turbulence is maintained to minimize fiber loss. Dissolved air flotation completely removes suspended solids from slurries for water clarification. In DAF, air is dissolved at high pressure (about 65 psi). Coagulation and flocculation chemicals are added to flocculate suspended solids. When the pressure is released, dissolved air comes out of solution as fine bubbles that are entrapped in the flocs, which causes everything to float to the surface where it is skimmed off. Relatively quiescent condition is maintained to avoid breaking up flocs. In most industrial water streams, there are sufficient dissolved gases, including air and carbon dioxide. When vacuum is applied to a tank containing a slurry, dissolved gases form bubbles that nucleate on hydrophobic particles and float to the surface. In this respect, BNS is somewhat similar to DAF except that no agglomeration chemicals are added. There are other types of flotation processes like electroflotation, centrifugal flotation, and cavitation air flotation as reviewed by Rubio et al. [3]. However, at present these processes are not commercially used. APRIL 2020 | VOL. 19 NO. 4 | TAPPI JOURNAL 207