Filtration in Pulp and Paper Mills
Updated: Oct 20, 2020
Water is one of the most important process material in pulp and paper industries. These industries are the largest industrial consumer of water, and uses more water per tonne of product than any other industry.
For example, about 100 litres of fresh water is needed to make 1kg of paper. Filtration and other separation equipment have been helping paper makers reduce their environmental impact for some time. Proper waste water treatment plants and closed water circulation systems allow 90% of this water to be recycled.
Liquid filtration application areas for these industries include:
Modern mills have moved away from using chlorine as a method of bleaching the pulp stock, and have adopted methods that produce the powerful oxidizing agent Chlorine Dioxide (ClO2). The ClO2 production process involves the reaction of Sulfuric Acid, Sodium Chlorate, and Methanol, diluted into water to 8 to 10 g/L.
This process is very reactive and has to be very carefully controlled in a vacuum. Undesirable impurities, which include oxidizable substances such as reactive metals, rust, dust and organic contaminants such as hydrocarbon greases, oils and rubber, can make the reaction unstable and more sensitive to "puffs", (essentially mini explosions in the reaction vessel).
Sulfuric Acid is typically delivered in tank cars and stored in tanks where it can pick up rust and scale. If these contaminants are not removed, it can lead to "puffs" in the reaction chamber. Some mills will use string wound, but because string wounds are notorious at unloading contaminant, they can lead to slugs of captured contaminants entering the reaction, which can lead to ‘puffs’. High efficiency melt blown filters provide improved contaminant removal. Sodium Chlorate and Methanol also pick up tank rust, scale, and other debris and should be filtered before use for the same reasons.
Methanol: Stratum A 1 micron
Copious volumes of water are used in the paper mill every day. Many mills draw their water from a surface source. Those plants located in colder climates see an increase in the amount of silt in the water during the spring runoff or snow melt period, resulting in an explosion in filter usage. Many paper mills will employ RO membranes. These are typically protected by multistage prefilters which may include baskets, bags, and melt blown filters.
We recommend: MBF Series 5 micron
The mill uses water knives to trim the rough edges off the paper (Trim Squirt application) and also to cut off at the end of a roll (Tail cutters). These water knives feature fine orifices which can easily clog if particulate is not removed from the water, causing plant downtime, and wasted pulp stock. In addition, finer particulate, that does not plug the nozzle, could cause erosion of the orifice, necessitating premature replacement of the nozzle.
As the newly formed paper moves down the line it must be kept wet so it does not break. Spray nozzles are used to apply water to the paper. If the water contains particulate, the nozzles can plug, causing the paper to dry and break. The result is loss of production time, replacement of nozzles and difficulty restarting the line. Point-of-use filters should be employed to remove contaminant that can plug nozzles.
We recommend: Stratum C 10 to 50 microns
Paper mills are extremely safety conscious and eyewash stations are located throughout the plant site. A medium size mill could have many stations and filters are typically changed monthly or quarterly on a maintenance schedule. Some mills have been using resin bonded filters for this application, an extremely poor choice since fiber and resin particles can release from the filter and be projected into the employees’ eyes.
We recommend: MBF Series 1 micron
Paper machines run at speeds up to 75 mph thereby putting a lot of stress and wear on bearings and moving parts, in addition to generating a lot of heat. The cooling water that is used must be filtered to prevent contaminants from eroding the seals of rotating equipment such as pumps and refiners. The filters ensure that particulate in the cooling water, such as sand or silt, is removed to prevent wearing of the packing sleeve, and pre-mature failure that leads to downtime.
We recommend: Stratum C 20 to 50 microns
Many additives are used in paper production, especially when higher quality papers are being produced, such as for magazines or inkjet printing. These additives may agglomerate and can mar the smooth finish that is desired.