Opacity of paper – printing opacity

Posted on March 31, 2020 by admin

Opacity is an important optical property of paper. The opacity of paper is also known as printing opacity. It is expresses as a percentage. It means as the percentage of visible light that does not transmit through a sheet of paper. The most of the printing papers fall within the range from 80 to 98 percent. The 100% opacity means that 100% of the light cannot transmit through the sheet, and is absorbed in or reflected by the paper’s surface. The lower opacity containing paper have show-through problem; therefore the printing images onto the reverse side or following sheet of a book, magazine or any papers can visible. The tracing paper is the excellent example for low opacity. On the other hand, the higher opacity has less show-through character which allows us to read a page without disturbing by print images on the back side or subsequent sheet.

Opacity of paper - printing opacity

How can you improve the opacity of paper?

The opacity of paper will be improved when the paper achieved high light-scattering efficiency. It can be done by the amount and kind of filler materials, higher basis weight, unbleached pulp, coating and certain chemicals.

There is a correlation of density with the opacity of the paper. The air spaces in the paper scatter the light. This is why the opaque value of a low density (bulky) paper sheet is much higher than the high density sheet. As a result of refining, the bonding capacity of the fiber increases, so the paper density increases and the opacity is reduced. Although low density paper is less strong than high density paper due to the poor tendency of fiber to fiber bonding.

Wet-pressing and calendering can affect on opacity. Wet-pressing increases the paper density, thus reducing opacity. So to increase opacity, extra wet pressing and calendering should be avoided.

The paper produced from the unbleached pulp has a higher opacity. But this type of paper is less used for printing purposes, because brightness and some other features are more important than opacity for most paper.

The opacity value of paper with higher GSM is higher than that of paper with lower GSM, but in this case the printing costs increase with the purchase of paper.

Another way to increase paper opacity is to add filler material. Again, the opacity values of filler materials of different grades are different; in this case the opacity values of titanium dioxide are higher. On the other hand, the high surface area and irregular shape of the tiny particles of the pulp increase the opacity of paper. Retention aid has a positive effect on opacity due to the filler particles and pulp particle aggregation capabilities.

Determination of printing opacity

The determination of opacity is very importance to both the paper manufacturer and the consumer like printing press. It indicates the ability to hide what is behind it. The paper that has high opaque value will have the better hiding power. The opacity of paper is defined as the ratio of the two reflectance factor of a single sheet with black backing to that with a backing consisting of a pad of the same paper, multiplied by 100. [TAPPI Standard T 519]

Printing opacity=R0/R∞ ×100%

TAPPI opacity=R0/R0.89×100%

Where,

R∞ = Reflectivity of thick pad of paper

R0 = Reflectance of a single sheet of paper backed by a blackbody

R0.89 = Reflectance of a single sheet of paper backed by material with reflectance of 0.89

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Dispersing Agent for Paper Making

Posted on September 21, 2019 by admin

Dispersing agent is used in paper making process as additive to provide uniform stock by separating fiber or particles and to prevent clumping and setting. It is a polymer compound with high molecular weight. Normally it is white fine granulated powder. Dispersing agent is water soluble and has high viscosity in aqueous solution. It is also known as dispersant agent. Very low dosage (approximately 0.2kg to 5kg/ton) of it works properly for separation. There are different types of dispersant chemicals (such as sodium salt of polyphosphate, polyacrylate or silicate) are available in the market but the anionic polyacrylamide is very popular.

Dispersant agent

Effects of dispersing agent

The paper dispersant has high dispersion efficiency.

  1. It improves evenness, softness and viscosity of the pulp.
  2. It increases the wet and dry strength and stiffness of the paper.
  3. It makes the final paper product more uniform formation, smooth and flexible without creating any pinholes. It improves the opacity, brightness and printability of paper.
  4. The Dispersing agent also improves the machine speed and increase production. It reduces energy consumption and save pulp beating time.
  5. Overall, it has economical benefits.

Usage and preparation

Temperature and stirring time is important for the effectiveness of the dispersant agent. The longer stirring time will damage the action of this agent. Always use freshly prepared aqueous dispersant solution; otherwise it will lose its dispersing effect after one or two days. The small amount of this chemical can works properly. The actual dosing of it can vary depends on the various paper product (printing paper, writing paper, tissue paper, newsprint paper, paper board). It may be 0.2kg to 5kg/ton. Use proper amount of the dispersant agent. If you use less amount then it will not works properly. On the other hand, high dosing will create unnecessary waste.

Mechanism of Dispersant agent

The dispersing agent should not a barrier for using other paper making aid or it should not be affected by other agents. It has a surface tension activity. It will wet the solid particles, which is advantageous for the dispersion of the fibers, fillers and other solid particles. The dispersing agent creates a negative attraction between particles thus reduces the interaction among the particles and therefore the fibers, fillers and other solid particles remains separately in the colloidal system which has a better flow properties. Then the uniform stock remains stable for longer time. The dispersant agent is widely used in various paper making process like writing and printing paper tissue paper, newsprint paper, paper boards etc.

Dispersing agent mechanism

Specification:

The specification of Dispersing agent for paper making may be as follow:

  1. Physical Appearance: Powder.
  2. PH (1% Dilution): 6- 10.
  3. Solubility: readily soluble in cold & warm water.
  4. Effectiveness:- Must be effective for dispersing of the pulp fiber bundles in both acid sizing & alkaline sizing.
  5. Self life: To be declared by bidder.
  6. Moisture:  ≤1.0%
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Rhodamine b dye

Posted on April 14, 2019 by admin

Rhodamine B is a organic compound and it is a basic dye. In paper mill it is used as dye for making pink color paper. The chemical formula of Rhodamine B is C28H31N2O3Cl. The IUPAC name of the compound is [9-(2-carboxyphenyl)-6-(diethylamino)xanthen-3-ylidene]-diethylazanium-chloride. It contains amino, imino and carboxylic group and has a xanthene structure. Physically it looks like dark reddish purple or brown powder. The pKa value is about 3.1. It is also known as Basic Violet 10, Brilliant Pink B, Basic rose red etc.

Rhodamine b

Solubility of Rhodamine b dye

Rhodamine B is water soluble compound. The solubility of it in water is approximately 15g/l.

Lab test

There are three tests done in the lab to see the quality of the colors. The three tests are Back water Number, Water fastness and Light fastness. To test the color at first we need to make dye solution. Take 0.25g dye sample into the 250 ml volumetric flask and fill up to the mark with distill water for making 0.1% solution. Then we make hand sheet with adding the color and sizing chemical.

Back water Number

To test Back water Number take the specific amount of pulp and add the sample Rhodamine B dye and sizing chemicals. Then filter the some portion of the stock before making the hand sheet and collect the filtrate.  By the coloration amount of the filtrate we calculate the Back water Number.

Rhodamine b back water Number

Water fastness

Firstly, cut off a part of the created hand sheet just similar to filter paper size. Then, this sheet is soaked by keeping into two filter papers. After that it is placed between two glasses sheet for overnight. Then the two filter papers are dried in oven and calculate the Water fastness by the coloration of the filter papers.

Light fastness

Cut a half of the created hand sheet and placed in sunlight for 12 hours. Then, calculate the Light fastness by the coloration of the sheet.

Environmental effects


Rhodamine B is an organic dye, so it can creates environmental and biological problems. It has limited effects on human – may cause skin irritation and damage to eyes, whereas has some evidence of carcinogenicity in animals. The wastewater that contains Rhodamine B dye must be treated before discharge to the environment; otherwise it will damage ecological environment.

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Bursting strength of paper

Posted on November 12, 2018 by admin

Bursting strength machine is used to test strength of the different kinds of paper and paperboard. The operating system of the tester is very easy. It is very important and widely used measurement. It is defined as the required minimum amount of hydrostatic pressure to rapture the paper or paperboard (maximum resistance (in kPa) of a specimen). The pressure is applied through a rubber diaphragm. This paper property is depends on many things like kinds of fiber, formation, chemicals, basis weight. It increases by using of long fiber, wet and dry strength additives, sizing chemicals etc. Bursting properties also increases with the degree of refining due to fiber to fiber bonding improvement. On the other hand it decreases by increased filler materials, secondary fiber and dried pulp. The test is also known as Mullen test because it is design by Mullen about 100 years ago. It is an oldest method for test the paper strength.
Bursting strength of paper

How to test the bursting strength

The paper bursting machine can be Hydraulic and pneumatic. It can be computerized or manual operating system. The tester has two clamps the lower clamp and upper clamp. The upper clamp has a diameter about 95 mm with a circular opening 31.5 mm in diameter whereas the lower clamp has circular opening 31.5 mm in diameter. The upper clam connects with a pivot joint so that it can create pressure to strongly hold the paper sheet. To perform the test, a sample of paper sheet is placed firmly between the lower clamp and upper clamp, and then provides pressure on the sample. The pressure is increased though the rubber diaphragm and hydraulic pressure until the sample burst. The pressure is generated though a motor, rotary pump or other suitable methods. Then record the reading of the pressure. The bursting strength of paper represent as kPa (kilo-pascal).

Burst index and burst factor of paper

The reports of the bursting strength of paper need to be submitted as burst index and burst factor. These two parameters are very important in packaging industries. According to ISO 536 the burst index means the bursting strength in kilopascals, divided by the basis weight in gsm. On the other hand the burst factor can be calculated with the bursting strength in gram per centimeter squire, divided by the basis weight in gsm.
Burst index = bursting strength/ basis weight = kPa/g/m2 = kPa.m2/g
Burst factor = bursting strength (g/cm2) / basis weight (g/m2)

Conclusion

We know that bursting strength indicates very significantly the quality of the paper and packaging materials. It also tells us, how much stress can be bear by the materials when we applied an external pressure. So in packaging industries, it is very important.

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Secondary fiber

Posted on September 19, 2018 by admin

The fibers that are already used for paper manufacturing process are known as secondary fiber or recycled fiber. Different grade of waste paper are the sources of it. Even, dry end broke, cutter and finishing section broke are also included in secondary fiber. This fiber is an important part for paper productions. The wastepaper may be printed, unprinted, colored, coated which has to be shorted, graded and baled as a part of better utilization. Considering economical issue Recycled fiber based paper mills usually established in the areas where they can easily collected and transported.
secondary fiber

Types of waste paper

There are different grade of secondary fiber such as mixed paper, old news paper, OCC (old corrugated containers) etc. Some of them are adulterated in many ways such as printing, coating, or any other ways. These types of waste paper should be clean or chemical processing to remove contaminants before using as fiber. We can utilize printing grade wastepaper with the help of deinking process. On the other hand, some of them are not adulterated; they can be used directly as pulp substitutes. Office wastepaper, magazines, telephone directories, catalogs, boxboard cuttings etc. includes in mix papers.

The secondary fiber can also categorize as pre-consumer and post consumer basis on their sources. Different plants like converting and printing press are the sources of the pre-consumer paper whereas home, office etc are the sources of the post-consumer waste paper. This wastepaper could be sorted and baled after collection as they are higher contaminant. Internal broke such as repulp, dry end, cutter and finishing broke are also one kind of recycled fiber; while some people don’t consider them as that.

Contaminants of waste paper

There are so many types contaminant like plastic, laminates, adhesives, coating, glues, waxes, asphalt, dyes, dirt etc. may contains in waste paper. To removes contaminant different types of process are used such as screening, cleaning, dispersion, washing, bleaching deinking. To utilize the secondary fiber effectively the contaminant should be removed from the wastepaper. Most of the contaminants are removes in pulper and deflakers.

Properties of secondary fiber

The strength, bonding potential, tensile strength, burst and tear factor will be lower with the times of recycle. Drying is the main factors for reducing the strength properties. Moreover, the fiber secondary also losses it’s flexibilities and permeable to water. They have a lower yield percentage. Moreover, they can increase slime accumulation.

Secondary fiber utilization

The characteristics of virgin fiber and secondary fiber are different, so it should be considered as different furnished. Generally, recycled fiber may contain some contaminants like waxes, glues, inks or different dissolved solids. They will impact on papermaking process. They can precipitate on wire clothes, dandy roll, press felt, press rolls, dryer screen, dryer cylinder or calender surfaces which will affect on runnabiliy and paper quality. Different types of small and larger particles in white water systems may visible as dirt and spots or creates holes in the paper products. Recycled fiber consumption increases in some countries particularly in North American countries and European countries, Japan, China but rest of the countries utilizes minimum amount.

Impact on Environment

The using of waste paper plays an important role to protect our environment. It helps the environment by conserving the natural plants. It keep clean the earth, air and water.

How many times can paper be recycled

The quality and effectiveness of the fiber remains up to 4 or 5 times recycle. After four or five times recycles, the fiber losses its physical properties greatly and increases the fines content rapidly which is responsible for reduction in stock drainage. But some people believed the recycle times can be five to seven times which is depends on recycling process.

Conclusion

As the secondary fiber losses its strength related properties, bonding capacity, purity and quality; so it is better to mix with virgin fiber for producing quality paper.

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Paper mill wood chipper

Posted on August 1, 2018 by admin

Wood chipper is used in paper mill to produce chips from pulpwood so that cooking chemicals penetrate the wood quickly, completely and uniformly during manufacturing of chemical pulp or mechanical pulp. The process from pulpwood to chips is known as chipping process. The sizes of the good quality chips can be 5/8 to 5/4 inch long and 3-6 mm thickness. Oversize chips can create cooking problems which increases screen rejection. On the other hand, fine chips and rotten chips cause lower yield and strength problem. The available chippers are many sizes and styles. They are portable or stationary. Both types’ chippers have almost the same functions. Often, a truck or van is attached behind the portable wood chipper.

The pulpwood enter into the chute and then the inside blade cut the woodpulp into chips. The pulpwoods are feeded into the chipper as logs or as whole tree. Before entering into the chipper, it should be free of bark. So a debarker may be used before the chipper. Some chipper has both the debark and chipping system. The efficiency of the chipper depends on productivity, power demand, fuel consumption and product quality.

Types of wood chipper

The wood chipper or tree chipper can be different types such as disc type, drum type, high torque roller, screw type etc. Among them the disc chipper and drum chipper are most common chipper in pulp and paper industries. They have some advantages and disadvantages. So you have to choose which is right for you.

Disc chipper

The disc chipper consists of steel disc, spout and chopping blades or knives. The knives are mount on the disc. The number of knives may be 4, 8 or 12. The older chipper had 4-knifes which was lower capacity and produce more waste. Therefore, comes in multi-knives chipper containing 8, 10 and 12 knives which are higher capacity and produce less waste and quality chips. All of them have almost same function. The spout of the chipper may be rectangular or oval shaped. The rectangular shape spout produces more uniform chipper. Usually, the logs are fed to the disc by a sloping spout so that they remain at an angle. After some hours the knives may be changing for sharpening. This process consumes huge energy but produces uniform woodchips. When the disc spins, then the blades cut the pulpwood into chips. The sizes of the chipper depend on disc diameter, number of knives, spout etc.

Drum chipper

The drum chipper consists of large steel drum and blades or knives. The drum is parallel to the hopper. It has some drawback. For example, it creates large uneven chips and has safety issue. But it is more energy efficient compare to disc type chipper. It produces more productive chips.


After the wood chipper, the chips are passing through screens for separate acceptable sizes chips from fine and oversized chips. The chips should be free of dirts, sand, mud, stone and other foreign materials. Hence the chips are wash after screen scetion. Then they are send to storage tank with conveyor velts or pipe line for storage like silos. The outside storage has some disadvantages. The surfaces chips can contaminant with air borne pollutant.

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Optical brightening agents in paper

Posted on April 6, 2018 by admin

Optical brightening agents are additives which are used in the paper industries to enhance whitening effects of papers. These chemical compounds absorb light in the ultraviolet and violet region (usually 340-370nm) and re-emit light in the blue region (typically 420-470 nm). It gives a fluorescent effect that masks the inherent yellowness of the fiber and enhances the brightness of the paper product. They not only used in paper but also used in plastics, textiles, laundry detergents. They are also known as optical brighteners, artificial whiteners. It is one kind of coating agent.
Optical brightening agents in paper
The optical brighteners can be applied in either the wet end or dry end or both end. If you want to internal brightness then you have to add them to the stock in wet end. Many paper manufactures use them in dry end at the size press or calender stack as surface coating. The dry end application is more economical compare to the wet end because in the dry end the chemical are used on the outer surface fiber in lieu of whole fiber content for reflects the ultraviolet light. However some manufacturers use a combination treatment; they use both the wet and dry end.

Types of optical brightening agents

The optical brighteners that are used in paper industries can be sort into three types based on the sulfonic groups. All of them contain stilbene structure.

Disulfonated OBAs

This OBAs contains two sulphonic groups. They are hydrophobic and have a very good affinity. The solubility is very low. Normally it is used in wet end.

Tetrasulfonated types

This OBAs contains four sulphonic groups. It has medium affinity and good solubility so ideal for paper industries both in wet end and dry end. They are suitable for neutral or alkaline pH medium. It is most common type of OBAs that are used in paper and paper board.

Hexasulfonated OBAs

hexasulfonated OBAs contains six sulphonic groups. It is special type of brightener which has excellent solubility. Mostly it is used in those papers where high brightness is required such as photographic paper. They are used in dry end as coating.

Optical brightener’s chemistry

We know that optical brightening agents are stilbene derivatives. The stilbene is a diphenylethene which consist two stereo-isomer – trans-isomer and cis-isomer. Between these two configurations, the trans-isomer can exhibits strong fluorescence whereas the cis-isomer does not exhibit fluorescence. The trans configuration is more stable than cis configuration but when the uv light applied on the trans configuration then it become electronically exited and converted into cis configuration. Consequently the fluorescence phenomenon occurs in. The visible blue light effectively neutralizes the cream color or yellowish hue of the paper fiber.
Optical brightener’s chemistry

Disadvantage

All the optical brightening agents are dyestuffs. As like most of the other colorants they also degraded by oxygen in air slowly. So after several years or months, the increasing brightness of the paper with optical brighteners will decrease. As a result the printed paper will not look good. Thus some people does not like print on the paper with artificial brightness. Similarly it is not useful in photographic or art applications. On the other hand in some places or some printers the OBAs would not work. Therefore the paper will appear its normal brightness. This is the reason why some printed papers have a yellowish hue. Although the OBAs are efficient on bleached chemical pulps but it is ineffective on unbleached pulp due to lignin is also an ultraviolet absorbing agent.

Finally

Normally the consumers expect higher brightness paper and paper boards. But all the time the brightness of pulp and fillers cannot fulfill the targeted brightness. Therefore the manufactures use different type of optical brightening agents. Most of the paper manufacturers add these chemicals in order to make paper appear brighter.

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Paper machine headbox

Posted on December 16, 2017 by admin

The paper machine headbox is an equipment that supply very low consistency stock into the forming section uniformly. The formation and uniformity of the final paper product depends on the dispersion of the fiber and fillers. The furnish delivery system varies on the construction of the headbox. It has a significant effect on paper qualities. All the headbox are not suitable for all grade of paper and machine speed. A well constructed Headbox can provide high quality board and paper and higher productivity. So the design should be properly. Now a days Hydraulic type headboxes are commonly used in the pulp and paper industry. The headbox is also known as flowbox.

Paper machine headbox types

Normally the headbox is categorized into three types such as open type, air pad type and hydraulic type. Among them the hydraulic headbox is designed for a high speed modern paper machine (above 300 m/min) whereas open headbox is used in low speed paper machine. In hydraulic and Air pad headbox , the discharge velocity from the slice depends directly on the feeding pump. For this reason they commonly known as pressurized headbox. The hydraulic headbox has no air pad, it is fully enclosed.

In case of air pad headbox a pond level is maintained. Above the level it contains pressurized air. The stock level is known as head, which is controlled by air pressure. The head determines the slice jet speed. It contains two or three Rectifier roll.

Moreover multi-layer hydraulic headbox is available which is designed for making tissue paper and lightweight containerboard. Each layer has different furnish compositions and delivery system.

Headbox jet velocity calculation

The headbox jet velocity is very important factor for manufacturing high quality paper. You should accurately maintain the jet-to-wire velocity ratio; it should be in the range of 0.97 to 1.03. The headbox jet velocity can be calculated from the following Bernoulli’s equation:

V2 = 2gh
Where,
‘V’ is the jet velocity (m/s)
‘g’ is the acceleration constant, due to gravity (9.81 m/s2)
and ‘h’ is the Height of stock in the headbox. (m)

Functions

The headbox has many functions among them the following three functions are very important.

  1. It spread the stock uniformly along the width of the forming fabrics.
  2. It provides the stock at constant velocity according to the paper machine speed.
  3. Create control turbulence and uniform suspension to eliminate fiber gathering.

If the headbox does not perform these functions correctly then the quality of the paper will be greatly hampered.

Benefits

You will get many benefits from the well constructed headbox. It can improve formation and paper quality, reduce number of breaks and downtime, fewer pinholes, dirt spots, improve runnability, increase machine speed, stabilized paper caliper, improve water removal, and reduce chemical cost. All these properties are achievable, if the CD basis weight profile, main fiber direction, uniform formation, flatness, etc. are perfect.

Rectifier roll or evenor roll

This roll are used in paper machine headbox to control both the even out flow irregularities and to create turbulence to keep the fiber dispersed up to the slice opening. It is also known as hollow perforated roll or holey roll. Normally the hole diameter is about 2 to 4 cm.

Paper machine headbox slice

The paper machine headbox slice is a full width nozzle with a completely adjustable opening to give the desired rate of flow. The slice contains two lips; top lip and bottom lip. Among them the top lip is adjustable; you can up or down them with hand operating or motor driven. On the other hand the bottom lip is fixed. The distance between the two lips are called slice opening. This slice opening controls the stock jet velocity which is vital for paper formation.

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Kappa number of pulp

Posted on November 25, 2016 by admin

The Kappa number of pulp is an indication of the lignin content or bleachability of pulp. We use it to estimate the required amount of bleach chemicals to achieve targeted brightness. If the kappa number value is higher then, the required bleaching chemicals are also higher. On the other hand low kappa pulps are easier to bleach. The KMnO4 not only oxidized the lignin, but also oxidized the others compound. Therefore it will raise the consumption of KMnO4. Consequently raises the Kappa number of pulp. It is almost proportional to the residual lignin content of the pulp. The kappa number depends on digestion system, wood species and delignification procedure. Normally the kappa number of soft wood is higher than hard wood. Oxygen delignification reduces the bleaching consumption.

Principle

The kappa number is one of the significant parameters for pulp production. It is determined as the volume of N/10 KMnO4 solution consumed by one gram of air-dried pulp in an acidic medium. The KMnO4 solution is spent due to reaction with lignin and others oxidized compounds within a given time.
MnO4– + 8H+ → Mn2+ + 4H2O
MnO4– + 4H+ → MnO2 + 2H2O

The KMnO4 solution is added such an amount so that about 50% permanganate is left unconsumed before adding the thiosulfate solution and potassium iodide. The excess volume of KMnO4 solution is measured by titrating with standard thiosulfate solution after adding an excess of potassium iodide.
2 MnO4– + 10I– + 16H+ → 2Mn2+ + 5I2 + 8H2O
MnO2 + 4H+ + 2I– → Mn2+ + 2H2O +I2
2S2O32- + I2 → S4O62- + 2I–

Test procedure:

  1. Collect the pulp sample and air dry them.
  2. Take 1gm air-dried pulp sample and 650ml distilled water into a two liter beaker. Disintegrate the pulp with this water so that no fiber bundles exist.
  3. Add 25ml 4/N H2SO4 acid.
  4. Add 25 ml N/10 KMnO4 solution then wait for 5 minutes.
  5. Add 5ml 1N KI.
  6. Then, titration free iodine with N/10 Na2S2O3 solution using starch indicator.
  7. Carry out a blank titration following the same procedure as above omitting the sample.

Kappa number calculation

Kappa number calculation
Here,
K = kappa number.
p = amount of permanganate actually consumed by the sample.
f = factor for correction to a 50% permanganate consumption.
a = amount of the thiosulfate consumed by the sample.
b = amount of the thiosulfate consumed in blank titration.
N = Normality of the thiosulfate.

The disadvantages of this method are that:-

  • The iodine is volatile compound which can create an error. So a blank experiment is needed to reduce error due to high volatility of iodine.
  • The reaction should be kept under weekly acidic condition.
  • The reaction time may causes errors in determining kappa number for various pulps.
  • Without lignin other compound like hexenuronic acid in bleached pulp also oxidized.
  • Reaction temperature should be kept 25⁰C throughout the test of kappa number of pulp. Otherwise it can create errors.
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Hydrogen peroxide bleaching

Posted on July 4, 2016 by admin

Hydrogen peroxide is a total chlorine free (TCF) bleaching agent. It is appropriate for chemical, mechanical and recycled pulp to achieve a wide range of brightness. It has both oxidizing and reducing properties which is pH depended. The chemical formula of this bleach is H2O2. Pure hydrogen peroxide is very light blue color but colorless in solution. Recently, the use of this bleaching agent increased significantly in the pulp and paper industry due to easy to use, lower production costs, improved paper quality, increased yield percentage and environmental friendly. The brightness of the paper that is made from hydrogen peroxide bleach is more stable. A color reversion of about 1% is obtained over a long period. The results obtained from hydrogen peroxide bleaching depend upon the species of wood, age of wood, lignin content etc.

Hydrogen peroxide bleaching is a single-stage or two-stage process. This bleaching agent can be used in extraction stage of the bleaching sequence. However, it is also useful as a terminal stage. When it is used, it can be saved a great amount of ClO2 chemical in the subsequent stage. Some bleaching plant design the system as the first tower is at medium consistency whereas the second tower at high consistency. Ours bleaching plant is two stages and medium consistency system. We used it in extraction stage. At both stages we used 3-4 kg/ton whereas the magnesium sulfate is 1-1.5 kg/ton. At first extraction stage we also used oxygen with it. A two-stage process has an advantage in providing a cleaner pulp. The chemistry is very similar to that involved in oxygen delignification.

HOO– ion is the active agent of hydrogen peroxide bleaching. This ion is originated by the ionization process.

hydrogen peroxide bleaching

It is a reversible reaction. As the front reaction produce H+, so high pH will increase the HOO– ion. This ion little effect on cellulose, but selectively reacts with the organic coloring matter. It reacts with lignin through the carbonyl groups.

Peroxide is catalytically decomposed in the presence of metal ions such as iron, copper, lead and manganese. In order improvement the efficiency of hydrogen peroxide bleaching sodium silicate and magnesium sulfate are added during preparation of the peroxide bleach. It controls the metal levels.

Peroxide bleaching introduces in the year 1907, but commercial development took place at 1941. After that the development comes in rapidly.

Effect of pH
The pH is an important factor for hydrogen peroxide bleaching. Maximum brightness is obtained at pH 10.5 or higher. The consumption rate of peroxide is also maximum at this pH because of OH– ion. This ion forwards the reaction rate into right side and helps to produce more perhydroxyl ion (HOO–).
H2O2 + OH– → HOO– + H2O

Usually the pH level is maintained by addition of sodium hydroxide and sodium silicate. Here, the sodium silicate acts as both a stabilizer and a buffering agent. It is usually added with magnesium sulfate. Higher pH has one drawback that it can cause the development of yellow color and reversion of brightness. This drawback can be arise if the pulp left alkaline after the peroxide is all consumed; otherwise not. So, to get better benefit from hydrogen peroxide bleach, it is requires a careful control of alkali application to the pulp. Generally, it is applied so that the pulp will be neutral or slightly acidic at the end point of this bleaching.

Higher concentrations would harmful to human skin and corrode many materials. Over 75 ppm of its vapor is risk to human health. It should be stored in a cool, dry, well-ventilated area and away from any flammable substances. The stored container materials should be non-reactive.

Effect of temperature
Temperature plays an important role on the rate of bleaching action. Lower temperature can increase the reaction time. On the other hand, higher temperature accelerates the reactions rate but can lead to a slight color reversion due to undesirable side reactions. Moreover, in the presence of heavy metal ions the stability of peroxide is badly affected. Generally, the temperature is maintained from 40 to 6o⁰C to obtain maximum brightness. At moderate temperatures hydrogen peroxide bleach improving the brightness of pulps without significant yield loss.

Effect of Consistency
The consistency of the pulp has noticeable effects on the reaction rate during hydrogen peroxide bleaching. High consistency can reduce reaction time and effectiveness of the bleaching. It is also economical. At first, most of the bleaching plant installed lower consistency (3 to 6%), but at present higher consistency is desirable. Now, most of the bleaching plant operates at 10 to 15% consistency whereas some are operate at 40 to 50% consistency. It reduces the consumption of the bleach and steam requirement. Lower consistency is mostly carried out as a batch wise in a beater, pulper or circulating tank. High consistency is great for maximum brightness, retention time and lower chemical dosage, but it can create maintenance problems and the equipment costs are higher. So it is better to keep medium consistency from 10 to 15%. It requires the retention time 45 minutes to 5 hours and 40 to 6o⁰C temperature.

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