Combined halogens: new products to combat an old problem, Solutions!, Online Exclusives, March 2003

Combined halogens:
New products to combat an old problem

By Bruce Urtz, Ondeo Nalco Company

New combined halogen products offer papermakers several potential advantages over the use of traditional oxidants in the battle against slime.

As long as papermakers have been making paper, microorganisms have been interfering with the process. Microorganisms such as bacteria and fungi are capable of forming slime on the paper machine and other areas, such as the saveall. Slime can create various problems such as sheet defects, holes, breaks, and plugged shower nozzles or screens. Figure 1 illustrates a sheet hole caused by slime.

One way papermakers have controlled slime is through the use of oxidants. Products like bleach (sodium hypochlorite), sodium hypobromite (activated bromide), and chlorine dioxide have been around for many years. These oxidants can be relatively inexpensive, and very effective against slime-producing organisms. However, the use of oxidants does not come without problems. Oxidants can be corrosive, damage felts, and react with various papermaking chemistries—for instance, optical brighteners and dyes.

Over the past several years, a number of new oxidant products have entered the marketplace. These products consist of halogens, bromine and/or chlorine, combined with an organic or inorganic carrier. One key advantage to combining the halogen is that it can often reduce the negative impact of the oxidant while maintaining its biocidal properties.

An old idea
The idea of combining a halogen with another molecule so that the halogen is less aggressive but still biocidal is nothing new. In the 1930s and 1940s, mixing bleach (or chlorine gas) with ammonia to make chloramines for microbiological control in papermaking systems was very common (1, 2). Even though chloramine treatment was often touted as being more effective than chlorine, eventually the chemistry was abandoned. Some of the reasons for abandoning chloramine treatment were increased corrosion and increased microbiological activity, presumably due to the ammonia, a good source of nitrogen for bacteria (3, 4). In the 1960s and 1970s, bleach in combination with sulfamic acid (chlorosulfamate) was proposed as a biocide with low reactivity to process equipment and chemistries (5, 6). However, the product never obtained commercial success in paper probably due to the weak biocidal activity of chlorosulfamate.

Many shades, but only a few colors
In the past several years, interest in using combined halogens has been rejuvenated. Papermakers are currently using a number of combined halogen products for microbiological control in papermaking systems (7, 8, 9, 10). The various trade names often make it confusing to determine the number and nature of the products available. In reality, papermakers currently use only four types of chemistries: hydantoin, sulfamate, ammonia/ammonium, and isocyanurate.

Hydantoin: This group consists of bromine, chlorine, or both attached to a hydantoin molecule. Halohydantoins are not very stable in liquid form so they are manufactured and sold as a solid product either as powder, granules, or briquettes. Feeding halohydantoins requires the use of a powder feeder or a brominator. A brominator consists of a vessel with granules or briquettes. Water flows through the vessel, dissolves product, and is sent to the process. Powder feeders work by making a slurry and delivering that slurry to the process. Once in the process, the product completely dissolves.

Sulfamate: This group consists of bromine or chlorine attached to a sulfamate molecule. Unlike the halohydantoins, halosulfamates can be manufactured as stable liquid products. Currently, only bromosulfamate is used for paper mill water treatment. Chlorosulfamate is used in some cooling tower applications.

Ammonia/ammonium: Haloamines can be generated by mixing chlorine or bleach with ammonia or an ammonium salt. The two can be mixed in the process water or pre-mixed before application to the process water. Like halohydantoins, these types of haloamines cannot be manufactured as stable liquid products. Recently, bleach in combination with ammonium bromide has been introduced as a way to produce a new haloamine oxidant (7).

Isocyanurate: This group consists of chlorine attached to an isocyanurate molecule. However, sodium bromide may also be present, allowing for the formation of hypobromite. Like halohydantoins, halocyanurates are produced in solid form. One halocyanurate product, which is applied using a specialized powder feeder, has enjoyed commercial success in cooling tower applications and utilities, but has seen only limited application in paper (9).

Potential benefits of combined halogens
Persistence: One potential benefit of using a combined halogen is persistence. Oxidants like bleach and sodium hypobromite are quick reacting but usually not persistent. In contrast, combined halogens, in general, are slower acting but more persistent. Since all mills recycle their process water to some extent, persistence can be beneficial. It allows the oxidant to carry farther through the system and provide more long-term control.

Figure 2. Persistence of halohydantoin and bromosulfamate vs. bleach in white water from an uncoated free-sheet mill. Note how bleach is more effective in controlling bacterial counts initially, but the halohydantoin and bromosulfamate are more effective after repeated challenges.

Figure 2 provides an example of this. Three oxidants at equal concentration were added separately to white water samples. Every 30 minutes, non-treated white water was added to the treated white water samples to see if any residual oxidant was left to handle the additional microbial challenge. Initially, bleach was the most effective oxidant; however, after several challenges, the combined halogens were found to be more effective due to their persistence.

Increased efficacy in high oxidant demand systems: When oxidants are added to process water, not only do they react with the microorganisms they are supposed to kill, they also react with the various substances in the process. These other components create an oxidant demand that neutralizes the oxidant and prevents it from being biocidal. Oxidant demand can vary dramatically from mill to mill. One advantage of using a combined halogen is that it may have less reaction with this demand and, therefore, be a more effective biocide than its non-combined counterpart. However, this is not true of all combined halogens, and will vary from mill to mill.

Better slime penetration and removal: What papermakers refer to as slime, microbiologists refer to as biofilms. There is some evidence that combined halogens are more effective at killing bacteria in biofilms than non-combined halogens (11, 12, 13). However, very little work has been done in paper systems, which typically have more complex biofilms due to the presence of other components such as fillers and fibers.

Better compatibility with papermaking chemistries: As stated earlier, one proposed advantage of using a combined halogen is better compatibility with papermaking chemistries. We have conducted a number of experiments in this area looking at various chemistries such as size, wet strength, dyes, and optical brighteners. In general, combined halogens are more compatible than their counterparts, bleach and sodium hypobromite. Still, there are exceptions. For example, the compatibility of three combined halogens was tested on an optical brightener (Figure 3). Two of the combined halogens were less reactive than bleach and sodium hypobromite. The third, a halohydantoin, was more reactive than bleach. Therefore, you cannot assume that a combined halogen is always going to be more compatible than its non-combined counterpart.

Figure 3. Effect of various oxidants on an optical brightener.

Better compatibility with papermaking equipment: If combined halogens are more compatible with papermaking chemistries, you might expect that this would be true for equipment as well. In our laboratory, as well as others, we have found that combined halogens such as bromosulfamate and halohydantoins are more compatible with felt fibers than bleach or sodium hypobromite (14). However, when we looked at the effect on vapor phase corrosion, the results were mixed (Figure 4).

Figure 4. Effect of various oxidants on vapor phase corrosion of mild steel.

In the case of monochloramine, a haloamine we found it to be more corrosive than other oxidants including bleach and sodium hypobromite. This is likely due to the high volatility of chloramines. Therefore, as with chemistry compatibility, you cannot always assume that a combined halogen is more compatible than a non-combined one.

At what cost?
For papermakers, one of the biggest concerns is cost. Oxidants like bleach are commodity chemicals; as such, they are relatively inexpensive. Combined halogens are more expensive. However, in weighing the economics of the application, one has to consider the hidden or not so hidden costs of oxidant application. This can include dye and optical brightener consumption, corrosion, and felt damage. Also, if the combined halogen is more effective on an actives basis than its non-combined counterpart, the cost difference is reduced.

Another potential drawback of using combined halogens is feeding and controlling dosages. Most combined halogen products discussed here require some sort of specialized feeding equipment. Controlling the dosage of some of these products can be challenging, especially the solid products.

Choosing the right product
Combined halogens are no different than other biocides in that there is no “one size fits all” product. Therefore, it is important to understand not only the oxidant, but also the mill in which it will be applied. Screening the product is essential. If the screening results look good, other factors should be considered. For example, is there concern about optical brightener destruction, dye consumption, or corrosion? In 1929 when discussing the use of chlorine, C. M. Baker came to the conclusion that “…owing to differences in the conditions in different mills, each mill is a separate problem and should be studied individually”(15). The same holds true more than 70 years later when applying a combined halogen.

About the Author:
Bruce Urtz is a SeniorR esearch Microbiologist, Paper Services Division, for Ondeo Nalco Company. He holds a Ph. D. in Microbiology from North Carolina State University and has worked for Ondeo Nalco for 5 and 1/2 years.

Acknowledgements
The author wishes to acknowledge Dr. Michael Enzien for his help on the corrosion testing.

References

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