Cat-ionic Starch - Technical Papermaking

Paper makers hold starch in high regard as one of the most important papermaking chemicals in producing high quality paper.

Cat-ionic starch is made up of modified maize starch that now has a slight cat-ionic charge (positive charge). Cat-ionic starch is added at the wet end of the machine. The slight cat-ionic demand of the starch allows it to bond with the anionic (negatively charged) fibers, fillers, fines, and other small unusable materials to improve dry strength in the sheet and even improve the retention of the sheet.

Cat-ionic starch is produced when a reactant chemical is treated with slurry of partially swollen starch granules.

Typical starch addition points in a approach flow system occur either before the machine chest in the thick stock system or after the machine chest at the start of the thin stock flow to the head box, Arguably both addition points allow for high strength, depends on the grade and the dosing quantity of starch. 

The problem with adding too much is that it will exceed the adsorption capacity of the surface, based on either the surface area or the limited extent of negative charge of the surfaces of fibers and other solid surfaces in the furnish. Excess starch beyond what adheres to the fibers in likely to cause foam, high biological oxygen demand (BOD) levels in the effluent, and poor retention and drainage. The performance of cationic starch as a strength agent sometimes can be improved by raising the pH; this will tend to make the fibers slightly more anionic and better capable of interacting with the starch.

When adding cat starch in a system with a high level of anionic trash the starch strength qualities can be improved with pretreating the stock with a highly charged cationic material to neutralize the anionic trash. 

Papermachine Cleaning Chemicals - Technical Papermaking

Wire conditioner

Wire conditioner is sprayed on to the wires of the fourdrinier machine. The job of the wire conditioner is to coat the wire and prevent stickie’s, glues, pitch and other tacky substances from binding with the wire plugging the holes in the mesh. Blocked parts on the wire will inhibit drainage leading to weight and moisture deviations as well as holes/ weak spots in the sheet. these will cause a lot of issues on the paper machine.

When problems occur on the paper machines wire like marks from stickie’s and hot melts a caustic chemical clean on the headbox can remove the hot melts. Caustic soda is used because it breaks down inorganic materials like chalk, ash and stickie’s.

Felt conditioner

Felt conditioner works in a similar way to the wire conditioner helps prevents stickie’s and pitch from attaching to the felt and blocking the felt. An excessive amount of pitch on the felts not only hinders dewatering but sticks off a small amount of fibers. After time this will build up and cause defects in the sheet, potentially leading to breaks.

Chemical felt cleaning and conditioning

Chemical precipitates found in the press section are inorganic as well as organic. The organic or hydrophobic types include rosin size, wet strength resin, and pitch and hydrocarbon oil. Inorganic precipitates include clay, calcium carbonate.  

Generally speaking, chemicals for cleaning felts are applied using either a continuous or a shutdown cleaning method. Continuous cleaning effectively keeps the felt open during its run rather than relying on the more difficult job of cleaning a plugged felt.

Chemical felt washes are done more infrequently on PM3/6 than on PM4, usually if the machine has been shut for a while a chemical wash is usually done but on crawl speed to allow better penetration of the chemicals in the felt. Using a lower speed allows the chemicals to bed in and reacts with the dirt and contaminates.

 Acid

Acid is used in two ways on the machines, continuously dosing and shock dosing. The acid is used continuously on all of the machines to help keep the felt open, rather than relying on the shock cleaning on a closed felt.

The Shock dosing is done at a high concentration of acid, it is important to ensure the water is running, neat acid will melt the felts.

Acid is used to clean the felts and to neutralise the caustic that has been previously dosed. The acid is dosed through the chemical sprays on the felts. Acid is dosed for ten mins with 5 mins flushing time after. The acid removes the broken down pitch, fines etc. in the felts. The acid is delivered in a higher concentration when shock dosed and more dilution water is added when the acid is used continuously to keep the felts cleaned. The caustic id a alkaline which is on the opposite spectrum to the acid.

Caustic Soda

Caustic is used to treat the felts. The chemical is shock dosed through the chemical sprays onto the felts to break up the pitch, ash, fines and dirt collected in the felts. The caustic is applied for 10 mins on each of the felts. The issue with caustic being used on the felts is the chemical can cause the felt to close up, reducing the efficient of water removal. After the chemical dosing the lines are flushed with fresh water to clean the pipe work and sprays.

 Anti-Scale Chemical

Anti-Scale chemical is important chemical because the papermaking process is water based. Scale is made up of minerals (mainly chalk and limestoneare composed mainly of calcium carbonate (CaCO₃), magnesium hydroxide (Mg(OH)₂), and calcium sulfate (CaSO₄)) within the water precipitating out and building up within water systems.

Industrial water systems using hard water can experience breakdowns as the scale builds up in pipes, boilers, water tanks etc. To maintain the level of scale within the water systems a chemical is doses at specific points to dissolve/ break down the chalky deposits preventing build up and ultimately costly breakdowns.

The effects of scale

Can restrict the flow through pipes as the internal diameter decreases as the scale builds up in the pipe.

Scale impairs the heat exchange between metals into the water. this reduces the cooling/ heating efficiency and can lead to the metal components over heating. This causes issues with the drying cylinders requiring more steam to dry the paper because not al heat can be transferred through the cylinder if a build up of scale occurs.

Below is a list of all the dosing points in the site. There are problematic areas that scale is likely to build up and can cause catastrophic breakdowns for example. If the dosing to the pump seal water failed the pumps can clog up with scale, pumping would be reduced and the mechanical could seize. 

Wet end polymer addition - Technical Papermaking

Polymer (cationic polyacrylamides)           

Flocculation, retention, and drainage is affected by the quality of the backwater as well as the polymer characteristics, these characteristics affect the electrokinetic energy between the fibers namely the charge density, size of the particles, the weight of the molecules etc.

                                                                  

A balance has to be made between the flocculation of stock and the drainage of the stock. This balance results in a need for fiber flocculation to be limited but the flocculation of fine particles and retention additives should be maximized. As the fibers and fillers flock together to form bonds the drainage on the wire decreases. The molecular weight of the polymer affects the drainage and flocculation. With higher weight flocks are better formed. To improve the drainage the weight of the molecule has to be reduced. It is very important to choose the right polymer to keep the balance.

The total branching of the polymer affects the flocculation. Branching is the term used to describe the polymers ability to bond with multiple chemicals, fibers, fines etc. with industrial water (Backwater system/ dirty water) it was found that polymer with a high degree of branching and a higher weight had more resistance against shear forces and held overall better bonds. This made a positive effect on the retention and drainage of the stock. The dosing point of the polymer is before the primary screen where it will experience high shear forces. Using a polymer with both of these qualities will benefit the formation/ retention.

  Polymer used is a dry chemical mixed with dilution water and stored in a chemical tank before being dosed. The issue with the liquid polymer is that it can affect the chemical balance in the thick stock system (flocculation/ retention is reduced) a higher dosing rate is needed to achieve the same parameters as the dry polymer.

The Diluted polymer is added pre-Primary screen and used to create flocks of fibres by reducing the negative charge between the fibres. The polymer is made up of Nano particles and chains which bond together fillers and fines to create flocks. Polymer bonds between the individual fibres forming hydrogen bonds between them. The screen breaks these down into smaller flocks/ chains. This aids in the formation and will improve strength. 

Fourdrinier Retention Mechanisms - Technical Papermaking

Mechanisms of retention/ drainage

Retention aids traditionally based on Alum, Alum neutralizes the charge of the paper making furnishes and was seen a "fix all" regarding wet end chemistry problems. 

Modern Retention aids based on PEI single polymers used "bridging" as the dominant mechanism for retention. the first type of polymers were of high molecular weight which brougfht fibers and filler together and formed "bridges". Nowa days new micro particle systems follow a complex flocculation system to improve wire retention.

What is of interest are the flocculation properties of polymers (retention aids) because many components of the stock furnish (sludge, fillers, fines etc) are too small to be mechanically retained on the wire and need to be bound to the larger fibers through flocculation. The ideal scenario would be to restrict fiber to fiber flocculation and encourage the smaller particles and additives to flock to the fibers. This would give the best retention and dewatering of the sheet.

Bellow is a table that describes the elements of papermaking that will affect the retention on a fourdrinier machine, they can be catagorised into, Pulp conditions, Wire conditions and the additives added to the stock/ furnish

Stock Factors	Conditions of Wire	Additives
pH	Sheet grammage	Types and amounts of fillers
Consistency	Sheet formation	Shape and density of mineral particles
Temperature	Fabric characteristics	Types and amounts of other additives
Fiber characteristics	Type of dewatering elements	Order of addition
Degree of system closure	Machine speed	Ionic balance
	Shake (if used) – old technique used on high quality paper machines	Level of anionic trash

Improving retention on the wire has many benifits, primarly cost. Papermachines producing News print have a retention of about 50%, increasing this retention to 55% for example will reduce the amount of primary stock needed to an extra 45%. for example at a retention of 50% producing 25t/hr throughput the mass of stock through the headbox needs to be 50t/hr, by increasing the retention to 55% the mass through the head box is reduced to 45T/hr

Colloid Chemistry - Technical Papermaking

Colloid chemistry

The word "colloids" (used in science) describes materials that have at least one dimension that is smaller than 1 micro-meters. The word colloid does not give any indication on the chemical makeup of the particle.

Almost everything the papermakers deal with can be considered to be colloidal. Although fibers are larger than the classical definition, the fiber surface is highly porous, and micro fibrils of colloidal dimensions extend out into solution from the surface of a refined cellulose fiber.

Other colloidal particles common in papermaking furnish include fiber fines, filler particles, sizing emulsion particles, and retention aid molecules (for example; molecules so big that they no longer behave like regular molecules – high molecular weight polymers).

The average end-to-end distance of a retention aid polymer (500 nm) is much larger than the size of a typical colloidal particle (2 to 5 nm ).

When papermakers refer to colloids, they usually are most interested in the colloidal organic materials, including fatty acids, lignin by-products, and oxidized hemicellulose. These are often called "DSC" for "dissolved and colloidal materials," or "anionic trash."

Deposits form on papermaking equipment due to the “thermodynamic instability” of many materials suspended in water (oils, pitch, hot melts etc.). We can combat this by getting those materials to deposit/ bind onto fibers, thereby keeping their concentration low in the liquid, ergo less deposits on the machine.

Retention of colloidal materials is best achieved by a combination of coagulation (treatment to neutralize charges (conductivity), causing the particles to come out of the suspension) and flocculation (treatment with polyelectrolytes (high molecular polymers) so large that they can bridge between the surfaces).

Consistency Measurement - Technical Papermaking

Consistency Measurement


Testing the consistency of a stock is the same as consistency of water solution. Consistency is the term given to the amount of solid matter within a body of liquid. The higher the consistency the more solid matter is present within the same volume of liquid. Consistency is measured in percentage; it’s a percentage/ ratio of solid matter to water in a certain volume.


To find the consistency we need to test the amount of dry content within a set volume of water. For example if we wanted the consistency of 100ml of water, and tested 25 gram of dry solids the consistency would be about 25%.

To capture the solid content within a solution a filter paper is used. Firstly the filter paper is weighed at 0% moisture (previously oven dried) and recorded. The stock sample is weighed (1g = 1ml) and recorded. The weighed stock sample is filtered through a vacuum and rinsed making sure all of the solids from the sample are on the sheet. The filtered solids and the filter paper go into the oven to remove all of the remaining moisture.

Once the sample leaves the oven it is weighed and recorded. This formula is used to calculate the consistency below.

Consistency=  (Mass of Solids)/(Volume of Liquid)

We have to calculate the mass of the solids captured on the filter paper, this is calculated below;

Mass of Solids=Total dry weight-Filter paper weight

Here are a list of results that I took from measuring the consistencies of a pulp preparation plant's stock flows through two fractionators and Long fiber screens.