Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
1999-08-23
2001-07-10
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S735000, C210S917000, C210S928000, C162S189000
Reexamination Certificate
active
06258279
ABSTRACT:
TECHNICAL FIELD
This invention concerns a method of removing color bodies from paper mill waste water using a hydrophilic cationic dispersion polymer.
BACKGROUND OF THE INVENTION
Color removal from the effluent streams of paper mills continues to be a problem within the pulp and paper industry. It is necessary that these downstream waste waters be treated for color removal prior to disposal.
The United States wood pulp production capacity is approximately 60 million tons per year. Since the average cellulose content of wood is about 40%, 150 million tons of wood is needed to produce this 60 tons of pulp. The difference between these two numbers represents the lignin and hemicellulose that must be removed or separated in the pulping process in order to free the cellulose fibers.
The pulping process, however, does not remove 100% of the lignin present in the wood, with approximately 5% remaining after either Kraft or sulfite pulping (for mechanical pulping the amount is considerably higher). If a high grade paper is the desired end product, then this 5% residual lignin must be removed by bleaching the pulp.
Since over 35% of the pulp produced in the United States is bleached, there are about one million tons of lignin removed each year at the bleach plant, and most of this in the caustic extraction stage. This number is significant because in the residual lignin is solubilized. This solubilized lignin is a strong absorber of visible radiation resulting from the conjugation of unsaturated and quinodal moieties formed during the oxidation step in the bleach plant. Consequently, the bleach plant effluent is highly colored. Although there are other sources of color in paper mill waste effluent, it is readily apparent that where bleaching is performed its effluent can be expected to be the major contributor of waste color. Indeed, at Kraft, bleach mills the effluent from the first caustic extraction stage accounts for at least 70% of the waste color.
The goal of the pulping and bleaching operations is the removal of lignin and hemicellulose from the cellulose fiber in wood. The 95% removed by pulping is often burned as fuel in the process of recovering the inorganic chemicals present in the black liquor. In the bleaching operation, the 5% residual lignin is separated from the fibers by degradation and solubilization and ends up in the wastewater. Chemical removal can, therefore, only be accomplished by reducing this solubility, which has proved to be a difficult task.
Therefore, the primary source of color in pulp is lignin. It has also been suggested that Kraft color is due to ketoenols produced from carbohydrates during the Kraft cook stage in the papermaking process. Chlorination of the pulp during the bleaching operation results in the formation of color bodies which are leached from the pulp by caustic alkali solutions. Thus, the caustic extract effluent contains a major proportion of the color bodies and the other organic materials which have to be disposed during the waste water treatment.
The process of color removal from the effluent stream is further complicated by the presence of lime, solid particulate matter like pulp, clay, dispersants/surface active materials and polymers used during various stages in the papermaking process. The solid particulate matter is commonly referred to as anionic trash.
Most governmental regulations pertaining to color removal from the effluent stream of a papermaking process are directed to true color, i.e., the color at pH of 7.6 after filtration through a 0.8 micrometer filter paper and expressed as Pt Co color units (i.e., platinum cobalt color using a DR2000 spectrophotometer). Nevertheless, there is increasing pressure on pulp and paper mills to lower the apparent color of the effluent water because that is the color visible to the naked eye. There are occasions when the true color of a system that has undergone treatment is low, but the corresponding apparent color is high. This problem is commonly caused by the presence of suspended particulate matter that causes an increase in the turbidity of the system. Therefore, it is important that any new treatment for color removal should not only remove the true color of the effluent, but should also lower the apparent color as well.
It has been shown that by-products are water soluble, and that a significant amount is produced. This puts severe demands on chemicals to be used for color removal. There are techniques already available, however, that can remove greater than 90% of the color from either total mill effluent or isolated waste streams, such as from the caustic extraction stage of the bleach plant. These techniques include chemical (e.g., alum, ferric, lime or polyelectrolytes), biological (e.g., white rot fungus) and physical processes (e.g., ultrafiltration, ion exchange and carbon absorption). None enjoys widespread use because of unfavorable economics.
The demands on a product used in a color removal application are quite severe, i.e., the product must be capable of reacting with the color bodies in a manner which results in their becoming insoluble and, because of the extremely large amount produced, the color removal product must work at very low weight ratios relative to the material being removed or its use will be precluded by prohibitive costs.
Conventional treatments for color removal include use of ferrous sulfate and a water-soluble cationic amine polymer as disclosed in U.S. Pat. No. 5,200,089; use of hydrophobic polyelectrolytes in U.S. Pat. Nos. 5,338,816 and 5,314,627; and use of hydrophobic dispersion polymers in U.S. Pat. No. 5,435,922.
A common problem associated with conventional chemical treatment methods, such as epichlorohydrin/-dimethylamine (Epi/DMA), is the fact those polymers cannot lower the color of a system below a certain value beyond which they tend to re-disperse the color. This problem is commonly referred to as “overdosage.”
SUMMARY OF THE INVENTION
We have discovered that certain low molecular weight water soluble cationic dispersion polymers can be used to successfully remove color from pulp and paper waste water effluents. These dispersion polymers are excellent agents for the removal of both “apparent” and “true” color pulp and paper mill waste water. The color removal characteristics of polyacrylamide is significantly improved by imparting a certain degree of hydrophilicity to the polymer. This modification is accomplished by copolymerizing acrylamide with certain hydrophilic cationic monomers. The resulting hydrophilic cationic dispersion polymers display excellent replacement ratios, while avoiding the problem of “overdosage” which frequently arises when conventional polymers are used to remove color. These hydrophilic cationic dispersion polymes have a unique mode of action resulting in an all organic treatment for removal of color in pulp and paper mill waste water.
The hydrophilic cationic dispersion polymer described herein confers additional advantages for use in a paper wastewater treatment process. Specifically, the hydrophilic dispersion polymers of the invention show improved or equal activity with respect to color removal performance without the unwanted addition of oils and surfactants as compared to conventional cationic latex polymers. Additionally, these hydrophilic cationic dispersion polymers require no inverter system and can be introduced to the papermaking process using simple feeding equipment.
Another advantage concerns the mode of addition of the hydrophilic cationic dispersion polymers. In most cases, conventional water-soluble polymers are now commercially available in a powder form. Prior to use, the polymeric powder must be dissolved in an aqueous medium for actual application. The polymer swells in aqueous medium, and the dispersed particles flocculate. It is typically very difficult to dissolve the conventional polymers in an aqueous medium. By contrast, the hydrophilic cationic dispersion polymers of this invention, by their nature, avoid dissolution-related problems.
Furthermore, the hydrophilic cationic disper
Metzgar Richard E.
Shah Jitendra
Breininger Thomas M.
Hruskoci Peter A.
Martin Michael B.
Nalco Chemical Company
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