Hand Sheet Making - Paper Testing

Hand sheet making

Hand sheets are created for a variety of uses; to determine the composition of stock/ pulp in a tank, to look at the cleanliness of papermaking stock, to test the strength properties of the pulp etc. There are a variety of ways to calculate the hand sheet weight and volume of stock needed to form a specific hand sheet.

Consistency method

Hand sheets are made to a grammage of 120gsm for testing to achieve this weight the stock volume needed is calculated. The hand sheet weight should be about 2.4g. (this depends on the diameter of the final hand sheet) Firstly the consistency of the stock sample is calculated. Once this is known a simple calculation can be made to work out the stock volume.

  

There will be a tolerance error using this calculation as the consistency of the stock sample can be inaccurate due to pouring the sample/ collecting the sample. A more accurate method can be used.

A good consistency measurement example can be found here; Technical Papermaking Consistency

Test sheet Method

Before making a hand sheet a test sheet is created to determine the amount of stock need to make a specific weight hand sheet. This method is ideal if the consistency is unknown.

First the stock needs to be diluted to around 0.3% consistency in a bucket. Using the same calculation above a volume of stock is calculated for the test sheet. The test sheet is formed and dried. Once the sheet has been dried the test sheet is weighed and recorded.

The weight is used in another calculation to determine how much extra or less stock is needed to achieve the desired weight. This step removes the errors made when creating the 0.3% solution.

 

This calculation will give you the new volume needed to make the hand sheet.

Forming Hand sheets for physical test of pulp

Using the test method as described by Tappi T205 hand sheets can be made from pulp ready for physical testing.

The stock samples are taken. The first step is to identify the amount of stock needed to make a standard 80GSM hand sheet. This can be achieved in two ways, either by conducting a consistency test on the pulp sample, which takes time and not always accurate.

The other method requires making a test sheet before the hand sheets.

Creating a hand sheet

With the hand sheet cylinder down (open) the hand valve is opened to allow water to pass through the wire, gently rubbing the surface allows any remaining fibres to be removed. The wire is now clean.

The cylinder is locked back in place. The valve is opened slightly to allow some water to fill up, next add 500ml of 0.3% stock solution. The water is filled up to 350mm from the wire to the inscribed line.

Insert the perforated stirrer 5 times for about 6 seconds trying not to spin the stirrer and remove the perforated disk from the liquid. Carefully remove the stirrer on the last up movement and wait for 5 seconds allowing entrapped air to leave and the fibres to settle. Fully open the drain and let the water drain under the vacuum from the water leg. The hand sheet will now form on the wire.

Blotting paper is placed on the formed hand sheet as well as a couch plate on top; the couch roll is applied with no extra force apart from the rolls weight. The couch roll is rolled five times across the plate. With the movement likened to opening a book, the disk and blotting paper is removed and placed onto the drying rings or hot plate.

Cleanliness

Creating hand sheet can be a good indication on the cleanliness of the stock; they can be used especially when troubleshooting quality problems on the machine. For example if a screen was passing/ the slots or holes were damaged you can see an increased level of contaminates in the hand sheet of the accepted stock line. It is a good idea to have a base line hand sheet of the process to be used as a comparison when trouble shooting appearance quality issues. 

Paper Machine Water Chemistry - Technical Papermaking

Paper Machine Water Chemistry

Process water is the name given to the back water on the paper machine, water that is reused within the papermaking process.

The backwater is a mixture of chemicals and fines that were not used the first time around (one pass retention). The back water can have detrimental effect ts to the process if the parameters of the water fall outside of specific ranges/ conditions. For example if the conductivity of the back water became too low or outside of normal operating conditions it would mean the retention on the wire is reduced.

Low conductivity (-18mV) shows there is a high volume of anionic trash within the system which when mixes with the cationic starch or polymer, hydrocol will bond with the trash rather than the fibres leading to poor formation on the wire, pick outs and deposits forming around the machinery and a reduction in run ability. 

pH of process water

pH can be described as the single most important aspect of wet end chemistry, this is because most/ all aspects of the chemistry relies on in some aspect the pH.

When the pH of the water increases (becomes more alkaline), the surface charge of the fibres also increases. This will affect the attraction of the retention aids and other cat-ionic substances to the fibres. The most undesired effect would be the substantial increase of bacteria in the system.

Another effect of high pH ius fiber swelling, fiber swelling is useful during refining because the higher surface area and increased flexability of the fiber leads to high de-fibrilation (versus cutting of the fiber). Caustic acid is added to create this effect.

fiber swelling is also used within de-inking plants. swelling of the fibers pre floatation allows the inks and binder to split and break off when the fiber swells this aids in the  bleaching and deinking process leading to high brightness of the finished pulp.

pH can affect quite strongly the dissolving ability of wood components and to changes in the dissolved substances. Increase in pH improves wood components dissolving ability in the water system and thus the amount of anionic particles dissolved and colloidal substances.

Because of the undesirable fractions within water, the water needs to be cleaned at some stage with different kind of methods like for example with disc-filtering and or chemicals. The wet-end of the paper machine contains the highest amount of water and its chemistry has to be controlled by a variety of chemicals like, retention chemicals, fixatives, de-foaming agents and biocides.

A decrease of the pH value leads to deposits on the machine as the precipitation of non-wood materials increases.  A fine balance has to be made; typically a pH of 7 is achieved within the water loop.

Machine Issues caused by change in pH
Increase in pH	Decrease in pH
Level of bacteria in water increases	Deposit precipitation increases – deposits on the machines
Higher amount of anionic trash in the system
Surface charge of fibres increase

Bugs/ Bacteria - Biocides

Paper machines run well when the operating conditions remain constant. Water plays a large part in paper production and any slight disturbances in the water can cause negative effects on the machine.

Bugs, bacteria thrive in water and damp conditions. Bacteria build up within the water system will cause a variety of issues. For example; Slime build up, lowering PH within the water (acidification) and upsetting the chemical balance within the water loops.

Biocides are used to control the bacterial problem. The main goal of the biocides is to limit the growth of sessile bacteria, i.e. those that are attached to surfaces. These are the bacteria that tend to build up, cause slime deposits and holes, hurt productivity, and hurt product quality.

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.