The present invention relates to a system and method for removing particulate contaminants from a fluid stream. More particularly, the present invention is directed to a system and method for removing particulate contaminants from a fluid stream where the particulate contaminants have densities equal to or approximating the densities of the carrier fluid.
Purification of fluid streams through removal of particulate matter is important in several industrial processes. As it has become popular and necessary to recycle paper and paper related products, the recycle content in most products has increased. However, the nature of recycled material introduces many contaminants to manufacturing processes that wreak great havoc if not removed or otherwise dealt with properly. Increased recycle content and the push for further water system closure is resulting in higher concentrations of contaminants in mills. This has added a new degree of complexity to the already daunting task of providing uniform and clean recycle furnish.
Stickies are contaminants in recycled pulp that arise from the repulping of the many different types of adhesives associated with newspapers, advertisements, junk mail and a host of other sources. Stickies cost the pulp and paper industry between $700-$800 million per year.
Stickies-generating materials come from many different chemical sources in recycled fiber. The predominant contributors are pressure-sensitive adhesives, nonpressure-sensitive adhesives and packaging adhesive products, various paper coatings, waxes, and thermoplastic, nonimpact print toners. Chemical analysis of stickies particles taken from commercial deinking equipment show the particles are actually an agglomeration of many different chemical products. Stickies are not generally derived from a single contamination source. This further complicates the problem since the chemistries of several different materials must be addressed to effect their efficient removal.
One approach in controlling stickies is to not let them enter the mill in the furnish. By careful inspection of the wastepaper being brought into the mill, stickies problems can be prevented. This sorting method is manual labor intensive to effectively reject wastepaper with a high degree of stickies contamination, increasing the cost of raw material. Also, this does not solve the problem of stickies in a global sense, it just transfers them to someone else. Most mills have increased spot-checks and rejections of suspect raw materials.
Another approach for controlling stickies is directed to process improvements. By improving the process of pulping and deflaking, fewer of the stickies will be broken down into smaller particles that can travel through screens and cleaners. Optimizing pulping parameters such as pulper configuration, temperature, intensity and consistency can improve process performance. It is difficult to optimize these parameters, as the nature of recycled wastepaper does not allow for the quality control tolerance of virgin pulp. Therefore, the variation in furnish is not receptive to one optimum set of pulping parameters. Auxiliary pulping equipment is available for contaminant removal, but there effect on removing stickies has not been clearly demonstrated.
The majority of stickies removal occurs during screening and cleaning. Fine screens are very effective in removing macro stickies. However, as the slots in the screens are made smaller in order to remove more contaminants, fiber fractionation (fiber loss) occurs. At this point in the process the stickies are small (&lt;0.15 mm), as long as they do not agglomerate, but may still be visible in the final product. Dispersion will break the stickies up further and is dependent on consistency, temperature and pressure. Since the ink particles in the deinked pulp tend to be associated with the stickies, dispersion of the stickies-ink complex can lead to a decrease in brightness.
Finally, another approach to controlling stickies includes the use of chemical additives. There are many different chemicals used for stickies control including talc, solvents, dispersants, cationic polymers synthetic fibers, zirconium compounds and alum sequestering agent. The additives use dispersion, detackification, fixation, passivation or cleaning mechanisms, either alone or in combination. Additives can be costly and usually act as a band aid and not a solution. Talc, synthetic fibers, and zirconium compounds stabilize stickies by detackifying them, but the particle size is not reduced. These complexes are shear sensitive and additional stickies surfaces can be exposed. Dispersants break down the particle size and prevent reagglomeration. Dispersants are pH and temperature sensitive, and compatibility with existing chemistry as well as environmental issues may be encountered.
Removal of many type of contaminants is commonly effected through centrifugal cleaning. In centrifugal cleaning, differences in density between the particulate matter and the fluid are exploited to separate particles from the fluid. In the particular case of paper recycling, the density of some contaminants is very similar to that of the process water. In paper recycling, contaminants of specific gravity significantly higher than unity (the specific gravity of the process medium) are rejected through their passage through forward cleaners. Contaminants of specific gravity significantly lower than unity are removed through reverse cleaners. Various configurations of centrifugal cleaners have been disclosed in U.S. Pat. No. 5,566,835 to Grimes, U.S. Pat. No. 4,259,180 to Surakka et al., U.S. Pat. No. 4,226,707 to Boivin, U.S. Pat. No. 4,309,283 to Vikio et al., and U.S. Pat. No. 4,622,132 to Chupka. Both forward and reverse cleaners are expected to be transparent to contaminants of specific gravity very close to or equal to unity.
In a paper recycling operation, the specific gravities of some contaminants will usually approximate unity. Even if the specific gravities of the individual contaminants are either greater than or less than unity, the individual contaminants can combine, and the specific gravity of some combinations may approximate unity. The removal of these contaminants, or combinations thereof, is presently impossible through centrifugal cleaning technology.
It is an object of the present invention to provide a system and method for removing particulate contaminants from a fluid stream where the particulate contaminants of the fluid stream have densities equal to or approximating the density of the carrier fluid.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.