Features Research & Innovation
Reduce Slime and Biocide Stress with Worry-free Chlorine Dioxide

Tighter control of the microbiological activity in incoming and process waters will always bring higher efficiency in pulp and paper production, as well as contributing positively to sheet quality and printability.

A simple conversion to chlorine dioxide (ClO₂), delivered by a compact generator, addresses these issues, and consequently eliminates persistent deposits on forming fabrics, press felts and equipment. Foul odors, increased calcium levels, and high conductivity can also be drastically reduced.

“Compact ClO₂ units are a wise move for the Canadian papermaker; they are the most cost effective slime control option for paper machines,” says Jim Anderson, market specialist Purate, Eka Chemicals. “Typically, payback is less than six months because Eka Chemicals takes responsibility for installing and operating the generator. The initial cost for the customer is limited to PVC piping, electrical and DCS connections, including tote bin handling and containment.”

Scott Auger, market manager for Purate, continues, “When you form your sheet with cleaner, pretreated water, the focus is on optimizing performance, not on the interferences caused by traditional biocides. In addition, downtime caused by corrosion is not an issue.”

While treating papermaking process water for slime control offers the greatest benefits, applications such as fluorescence reduction, broke bleaching, and incoming water treatments can also be of considerable value.

Microorganisms enter process water with fresh water, various additives, and other raw materials. Under alkaline/neutral papermaking conditions, at temperatures of 40°-50°C, and in closed white water systems, slime can seriously interrupt operations.

When the growth takes off, corrosion, sheet holes, odor, and poor machine performance can seriously threaten wet-end stability and machine runnability.

The slime-like deposits, which attach to surfaces on machines, piping, and tanks are caused by aerobic bacteria that produce a mucous capsule consisting of polysaccharides. The capsule protects the cell against harmful conditions such as variations in temperature, pH, and lack of nutrients.

Fiber fines and colloidal particles adhere to biofilm and contribute to rapid growth of deposits. When the slime layer thickens, parts of that layer can break loose and cause runnability and paper quality problems. Anaerobic conditions that can occur under thick layers of slime also can cause corrosion problems. ClO₂ attacks and penetrates these deposits in their formation stages, and eliminates issues that are caused by “slough off”.

What used to be a troublesome and costly process has now become worry-free.

ClO₂ can be made available to all users safely, efficiently, and at a much lower cost than ever before. What’s more, the paper industry version of Eka Chemicals’ Purate has gone one step further by introducing a state-of-the-art control system that can distribute ClO₂ safely to four different feed points in relation to known parameters. Each point is controlled individually and can dose ClO₂ on a continuous, shock, or proportional mode. All of this can be integrated with the mill’s DCS, or function as a stand-alone unit.

The negative effects of deposits on paper machine clothing are well known. Often, sheet defects and process upsets are the direct result of build-up on forming fabric and felts. Because ClO₂ is highly reactive and can penetrate biofilm, it’s only logical that fewer deposit issues are the result of its application. In addition, longer clothing life is achieved, as wear is reduced by a smoother, less contaminant-ridden flow across the wet-end and beyond.

Chlorine application can be quite harmful to clothing, especially because a higher dosage is needed to have the desired killing effect.

Because of chlorine dioxide’s ability to minimize slime at the onset, the important interactions between nanoparticle retention and drainage systems, minerals, additives, and sizing agents can occur with less interference. Not only can chemicals function more effectively, but also they can reach high levels of performance at potentially lower dosages, because traditional biocides are not clouding their effectiveness.

Strong unappealing odors are not only undesirable but they are also potentially explosive, and are usually a sign of harmful substances in the air or on surfaces. ClO₂ also minimizes slime in mist areas, and improves the cleanliness of railings, flooring, and steps. This can result in a big improvement in safety.

A wide range of papermakers now employ compact chlorine dioxide generators. Most applications are in Europe, but the technology is catching on in the Americas and Asia.

One paper and board producer in Northern Europe was losing productivity because of excessive sheet holes, especially at high grammage. Slime was suspected as the main cause of the sheet holes problem. A non-oxidizing biocide was added to the whitewater system and evaluated as a means to control the microbiological activity and improve the productivity.

Significant amounts of slime had accumulated in the clear filtrate tank. Measurements also indicated a high level of bacterial activity in the broke tower, probably due to long residence time. The non-oxidizing biocide was ineffective at controlling slime and keeping the broke from “rotting”.

The chlorine dioxide treatment at the chosen dosage points was effective in the water system and on the paper machine. Measurements on microbiological activity in process waters and the broke tower showed a significant reduction in aerobic count and ATP (adenosine triphosphate) activity. In the white water tower, one effect of the treatment was a visible difference in the clear filtrate tank. Over a period of time this addition of chlorine dioxide kept the tank slime free.

The most important observation with chlorine dioxide treatment was the effect on the productivity and quality. The improvements following chlorine dioxide addition were significant. Sheet holes on average were reduced by more than 50%. Treatment cost of the system was comparable to the non-oxidizing biocide system and delivered significant additional quantifiable value to the papermaker.

The MD Papéis’ Santista mill, located in Cubatão, São Paulo State, Brazil, decided to switch from a monochloramine-based system. The mill produces 60,000 tpy of printing and writing grades and flexible packaging.

Caio Mori, Eka’s business development manager in Brazil, explains, “Because monochloramine is a persistent chemical and can be harmful to waste treatment, MD Papéis Santista began looking for a way to reduce the toxicity of their effluent. Purate, now in place for nearly two years, provided the level of treatment efficiency desired without the persistency problems in the effluent, and also helped with paper machine runnability.”

Several units are employed in trials in the U.S., and look to be fully commercialized in the near future. PPC

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