Microbial contamination and control in papermaking process

With the large-scale recycling of recycled fibers, the requirement of "zero" discharge of paper machine white water, and the emphasis on chemicals in the wet end of paper machines, the pulp fluid is rich in carbon sources, nitrogen sources, and minerals, plus a suitable temperature. Create an ideal breeding environment for the growth of microorganisms. The common types of microorganisms in the papermaking process are bacteria, fungi and yeasts. Therefore, the production water of the paper mill must be precipitated and purified to control the number of microorganisms in the water body, otherwise it will have a great impact on the production operation and the quality of the end product. Common types of microorganisms are bacteria, fungi and yeasts. As can be seen from the table, the common feature of the growth of various microorganisms is to have sufficient carbon sources, nitrogen sources and minerals, followed by a suitable temperature, preferably room temperature. The difference is that various microorganisms grow at different pH values, the pollution products are different, and the demand for oxygen is also different. Therefore, for alkaline papermaking, the types of microorganisms are mainly bacterial species; for acidic papermaking, fungi and yeasts are mainly used, so that when choosing fungicides, they must be treated separately.

1 The production and harm of microorganisms in the papermaking process

The microorganisms in the slurry exist in two ways, one is floating and the other is fixed. Biofilm is formed by the interaction of various fixed microorganisms, mainly using nutrients and metabolites in the surrounding environment to grow and reproduce. Crecules and Marshall pointed out: For microorganisms, attaching to the surface of objects is one of their competitive advantages. Because it avoids being washed away by the liquid, the probability of microorganisms coming into contact with multiple nutrients increases as the fluid changes. In biofilms, there are different oxygen concentration gradients. This varying oxygen concentration gradient can meet the oxygen requirements of different types of microorganisms. In addition to the problem of deposits on the paper machine, the action of microorganisms can also cause the corruption of paper stock, pulp and chemical additives. For example, uncoated paper sometimes emits an unpleasant smell when copying. This problem originated from the improper anti-corrosion method of the fiber in the slurry tank and the growth of anaerobic bacteria. These anaerobic bacteria produce large amounts of volatile fatty acids, such as propionic acid and butyric acid. The anaerobic bacteria in the slurry tank also produce harmful and explosive gases, such as hydrogen and hydrogen sulfide, while growing. Therefore, such microbial contamination can cause serious casualties. At the same time, the acidic products produced by the biofilm cause the acid-base fluctuation of the slurry, and at the same time corrode the solid surface under the biofilm, causing scars and unevenness on the solid surface.

When the microbial polymer matrix is deposited on the paper machine or tube wall, it will capture various organic and inorganic particles, and even hook fibers. These sources of microbial contamination can reduce the physical properties of pulp and make various additives ineffective. When these sediments are loosened and mixed in the pulp, it will cause paper defects, such as holes, stains, pinholes, etc., and in severe cases will continue to cause dirty material to break on the paper machine. Foreign studies have shown that most of these asphalt-like sediments are caused by microorganisms. In addition, the negative effects of microorganisms on the performance of the paper machine include: clogging of the blanket, reducing the service life of the blanket; increasing the pinholes on the paper surface (mainly caused by Yeast); produce dirty slurry, etc.

2 Causes of pollution in the papermaking system

With the large-scale recycling of recycled fibers, the requirement of "zero" discharge of paper machine white water, and the emphasis on chemicals in the wet end of paper machines, the pulp fluid is rich in carbon sources, nitrogen sources, and minerals, plus a suitable temperature. Create an ideal breeding environment for the growth of microorganisms.

2.1 Papermaking water

Papermaking water is an important factor leading to microbial contamination of pulp. Water for papermaking is mostly taken from rivers or lakes, which contains a certain amount of metal elements such as calcium, magnesium, sodium, iron, and manganese, as well as a large amount of impurities such as organic matter and bacteria. Therefore, the water entering the production system must be purified and treated before it can be used. Although the number of aerobic bacteria in the treated clean water is much lower than that in the papermaking system, the number of aerobic bacteria in the latter is usually above 106CFU/ml, but if there are still a certain number of aerobic bacteria in the water (102CFU/ ml or so), it will also bring microbial contamination to the paper machine.

2.2 Raw and auxiliary materials for papermaking

The raw and auxiliary materials used in papermaking are also the main causes of microbial contamination in the papermaking process. Cellulose and hemicellulose are natural plant components that provide rich nutrients for the reproduction of microorganisms; and related additives such as starch, rosin gum, carboxymethyl cellulose, talc, calcium carbonate, titanium dioxide, acid-base dyes It is also extremely susceptible to microbial contamination during manufacturing, transportation and storage.

2.3 Piping and equipment in the production process

Pipes and equipment in the production process are also a cause of microbial contamination. Because there are always dead spots in the pipelines and equipment, which are suitable for the growth of bacteria, and many factories seldom adopt fully enclosed processes, so that the air and airborne molds can easily enter the pipelines, which can also lead to microbial contamination of the slurry fluid.

2.4 Additives

Common chemical additives include starch, clay, protein, titanium dioxide, calcium carbonate, sizing agents and defoamers. In the process of manufacturing, transportation and storage, these raw materials may be contaminated by microorganisms. Generally speaking, when the paper mill receives these additives to be diluted or formulated, the paper mill or the special chemical supplier of the paper mill should be responsible for its anti-corrosion work.

2.4.1 Starch

Starch is one of the ideal nutrient sources for microorganisms. Microorganisms use their enzymes to convert starch into sugars for their growth needs. Starch is easily hydrolyzed into glucose after being dissolved in water, which provides nutrients for the growth of bacteria. After a period of time, the glucose is metabolized into a large amount of low-molecular fatty acids, butyric acid, lactic acid, propionic acid and acetic acid. These volatile components are caused by The reason for the smell around the production workshop and large alkali recovery workshop.

2.4.2 Protein

Protein is often used as a coating with other binders (such as starch, etc.). Microbes can easily degrade these protein macromolecules into amino acids, and use them as a source of nutrients to grow and reproduce. When microbial contamination occurs, the viscosity of the protein solution drops drastically, but it is often accompanied by a slight decrease in pH. Ludi and May reported that shortly after the initial pH drop, proteins began to precipitate, and the solution stinks and changed color.

2.4.3 Synthetic adhesive

Styrene-butadiene, vinyl acetate and propylene compounds are commonly used synthetic binders, and they are often used in combination with starch and protein. The dispersant and stabilizing ingredients in latex preparations are often decomposed by microorganisms. Because decomposition does not accompany changes in pH and viscosity, this type of microbial decomposition is more difficult to detect. However, the poor adhesion of the paint to the base paper, the reduction of the pigment adsorption capacity, and the poor supercalendering effect can be used as indicators to judge microbial degradation.

2.4.4 Dyes

If microorganisms contaminate the dye, the dye measuring instrument will not work properly. Once the biofilm is formed, it will cause blockage of the supply line and uneven supply of the toning dye. Although the degradation of papermaking dyes by microorganisms does not seem to be a prominent problem, the azo dyes used in the production of colored paper are extremely easily contaminated and degraded by microorganisms.

2.4.5 Packing

White clay, precipitated and ground calcium carbonate, and titanium dioxide are often used as fillers in papermaking or as ingredients in coatings. Generally speaking, calcium carbonate and clay should have been properly treated with anti-corrosion on the way to the paper mill by train and tank truck. But in fact, its anti-corrosion treatment is often improper or ignored. Unlike starch and protein, these raw materials themselves are not the best nutrition for microorganisms, but they contain dispersants that are critical to their functions. If these dispersants are degraded by microorganisms, the viscosity of the material will change accordingly, and particles will be produced in the filler, and then micro-aggregation will occur to form micro-agglomerates, causing fine scratches on the coated surface.

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