The Role of Magnesium Sulphate in Alkaline Peroxide Brightening of Mechanical Pulp

1. Peroxide stabilizer
2. Scale control

It has been well established that soluble magnesium sulphate can be used to trap and deactivate transition metals under alkaline conditions, through the formation of hydrous magnesium hydroxide gels and precipitates.

Most commonly, magnesium sulphate is used in combination with sodium silicate to enhance the transition metal deactivation characteristics of both chemicals. Metal silicates that are rich in magnesium do not adhere to equipment surfaces. The silicates therefore help to minimize the risk of silicate-related scaling.

The Role of MagnaBrite™ (Magnesium Sulfate) In Oxygen Delignification

1. Cellulose Protection
2. Enhanced Delignification

PQ and its Canadian subsidiary, National Silicates, have assisted in implementing the use of magnesium sulfate in more than 30 kraft mills in North America.

Please contact our Technical support if you would like to discuss your application and develop a customized trial.

Reaction Mechanisms

Although the reaction mechanisms that explain the role of MgSO4 in protecting pulp viscosity during bleaching with oxygen-containing compounds are not yet fully determined, there are two widely accepted hypotheses:

The Role of MagnaBrite™ (Magnesium Sulfate) In Kraft EOP Bleaching

1. Peroxide Stabilization
2. Cellulose Protection
3. Enhanced Delignification

The PQ Corporation has demonstrated the effectiveness of using liquid magnesium sulfate to reduce costs in EOP bleaching. Bleach savings of $0.25 to $1.00 per ton and even more can be realized as a result of reduced chlorine dioxide usage and/or reduced peroxide application addition rates. The precise saving is very mill-specific and can be determined only by mill trials. ISO brightness gains can range from 3-10 points. From the technology description that follows, improved delignification and pulp viscosity benefits may also be obtained.

Please contact our Technical Support if you would like to discuss your application and develop a customized trial.

Technology Description
In oxygen/peroxide-enriched alkaline extraction, several reactions take place:
1. Solubilization of chlorinated lignin fragments.
2. Pulp delignification.
3. Pulp brightening.
4. Cellulose degradation.
5. Peroxide decomposition.

To maintain pulp viscosity at acceptable levels and to obtain maximum efficiency from the peroxide in this stage, the first three reactions need to be maximized and the latter two minimized. The addition of fresh MgSO4 to the EOP stage protects pulp viscosity and prevents peroxide decomposition. These functions result in improved pulp strength, increased delignification, and increased pulp brightening.

The reaction mechanisms through which MgSO4 acts are best understood by the following hypotheses:

generation of free radicals. Since free radicals degrade cellulosics, controlling their rate of generation can improve the selectivity of the process. The direct consequences of improved selectivity are increased pulp viscosity, increased delignification, and increased pulp brightness. This method of transition metals control is believed to be superior to other chemistries based on chelation agents because the metal ions are completely encapsulated and are physically incapable of coming into contact with organic peroxides.

Pulps that have been bleached with chlorine-containing chemicals, in acidic conditions, usually have low levels of metals. While this situation is desirable with regard to transition metals, however, it is not desirable with regard to magnesium ions. To maximize the selectivity of the process, a certain level of magnesium should be maintained, even if the concentration of transition metals is low. These factors explain (1) why, in most cases, acid washing and chelation of the pulp before oxygen or peroxide bleaching do not completely solve the problem of viscosity loss, and (2) why soluble magnesium compounds must be added in these bleaching stages.

The Role of Sodium Silicate in Deinking of Wastepaper

Overview

Repulping
1. Dispersant
2. Buffering
3. Emulsification
4. Anti-Redeposition

Flotation
1. Ink Collection
2. Fiber Yield Loss Prevention

Peroxide Postbleaching
1. Peroxide Stabilizer
2. Buffer
3. Prevent Alkali Darkening

Overview-The Role of Soluble Silicate in Deinking

PQ sodium silicates are key ingredients in wastepaper deinking formulations. They help lift inks from paper fibers and aid in suspension and dispersion of inks, thereby preventing ink particles from redepositing on the fibers. Silicates also contribute alkalinity to the deinking operation and allow the process to be carried out at a lower pH than possible when using caustic soda alone. Deinking at a lower pH minimizes alkali darkening, which tends to be a problem with mechanical pulps.

Silicates also stabilize hydrogen peroxide, which may be added to a deinking formulation. Silicates work efficiently in both washing and flotation deinking processes, and with a variety of inks and papers, including newsprint, colored or varnished magazine stock, and rotogravure stocks.

Deinking wastepaper, or secondary fiber, is essentially a laundering and bleaching process. Chemicals, heat, and mechanical energy combine to dislodge oil-based printing inks from the paper fibers and disperse them in an aqueous medium. Ink and other contaminant particles are then separated from the stock by washing, flotation, dispersion, or a combination of the three.

Sodium silicates from PQ, with their multifunctional detergency and deflocculating properties, are routinely used to improve ink removal during repulping and to reduce alkali reversion of wood-containing pulps. In the repulping and postbleaching processes, silicates stabilize peroxides to enhance the final brightness values of the recycled stock.

The use of sodium silicate in wastepaper deinking is sound economically and environmentally. No other single chemical contributes alkalinity and enhances bleaching, ink collection, detergency, and brightening reactions. Unlike many competing products, silicates do not contribute to the creation of pollutants or carcinogens in plant effluents.

A typical deinking operation consists of three main steps:

1. Repulping
2. Ink removal
3. Postbleaching

In some cases a dispersion step may also be included in the process.

The specific types of inks, coatings, adhesives and other contaminants will determine the best combinations of chemicals and operations for each of these steps.

Repulping

During repulping, sodium silicate contributes detergency. That is, it functions as a dispersant and contributes buffering, emulsification, and anti-redeposition properties. Sodium silicate keeps inks and other particles suspended in solution, rather than allowing them to redeposit on the fibers, where even trace amounts will discolor recycled stock. The suspended particles can be removed later by washing or flotation.

Buffering

When sodium silicate and other alkali sources are used together, they will enhance the total performance of an alkali system. The optimum ratio of sodium silicate to other alkalis depends on the composition of the furnish and on processing parameters.

Sodium silicate typically flattens the pH profile of the bleaching reaction in the repulper and dump chest system and/or in postbleaching operations: initial reaction pH is slightly lower and final reaction pH is slightly higher, compared with bleaching without silicate. This method of controlling free alkali concentration gives the best possible balance between the rates of pulp brightening, hydrogen peroxide decomposition, and pulp darkening reactions.

Detergency

Sodium silicates also help to disperse inks, stickies, and other contaminants in the repulping/washing and deinking processes. Doing so promotes better cleaning at later stages. As in all types of detergent and cleaning applications, soluble silicates contribute to the wetting, dispersion, emulsification, and suspension of soils. Sodium silicate also enhances ink removal in alkaline systems to a greater extent than achieved with alkali alone.

Flotation

In the presence of soap (fatty acids), the addition of soluble silicate in the repulping step enhances the agglomeration of ink into particles large enough for removal by flotation. When silicate is added to the repulper, significantly fewer ink particles remain after flotation.

Studies at PQ's technical center indicate that soluble silicates contribute to better fiber yield in flotation. Laboratory results show that a reduction of 0.5% to 2.0% in flotation fiber yield loss often can be achieved with optimum addition of sodium silicate. A complete analysis of the results of numerous laboratory studies of repulping/flotation deinking suggests that the flotation yield benefit of soluble silicate results from the ability of silicate to remove permanently traces of ink from fiber surfaces-surfaces that otherwise would have enough remaining hydrophobic sites for bubble attachment and flotation.

Postbleaching with Sodium Silicate

Some recycling plants add a postbleaching or brightening step to compensate for insufficient brightness development during the repulping and ink removal stages. The postbleach can be either oxidative (peroxide) or reductive (hydrosulfite). In peroxide postbleaching, soluble silicate works in much the same way as it does in peroxide brightening of virgin mechanical fiber.

Extensive research has demonstrated conclusively that both the deactivation of transition metals and the buffering of reaction pH are critically important to the unique role of sodium silicate in alkaline peroxide brightening of mechanical pulps:

Even for pulps that contain only a small fraction of mechanical fiber, these properties of soluble silicate contribute significantly to the efficient cleaning and brightening of the fiber and help minimize consumption of hydrogen peroxide.

Scale Management

N® sodium silicate and MagnaBrite™ magnesium sulfate solutions are crucial elements in today's metals management programs. PQ's expertise with these two versatile chemicals, combined with our mill experience, will help you avoid scale formation problems-without extensive process modification-in conventional and closed-effluent systems.

Scale formation is often attributed to use of sodium silicate. In fact, its use is not usually the causative factor. While nonprocess elements (NPEs) can sometimes contribute to scale formation in bleaching and deinking processes, scale formation is directly related more often to such factors as soluble water hardness, pH and/or temperature shock, and modes of chemical addition.

Mills using naturally soft, lime softened, or recovered distilled water usually have little or no scale problem in the bleach liquor make-up system. Mills that operate with hard water are more susceptible to scaling.

Mills using naturally soft, lime softened, or recovered distilled water usually have little or no scale problem in the bleach liquor make-up system. Mills that operate with hard water are more susceptible to scaling.

Scale Formation can be managed effectively in two ways: first by avoiding pH shock and second by increasing the ratio of magnesium sulfate to sodium silicate.

The addition of concentrated caustic to a diluted silicate solution can cause pH shock, thereby destabilizing silicate in the make-up tank. Dilution of the caustic before silicate addition, preferably with soft water, will reduce or eliminate scale formation in the bleach liquor system.

Properly locating the peroxide addition point also is effective in avoiding pH shock. Ideally, add peroxide to the bleach liquor line and follow with in-line mixing close to the pulp-stream feed point. Alternatively, add peroxide directly to the pulp line just before the alkaline bleach liquor.

The dilution of sulfuric acid with pulp pressate or process water after bleaching is an effective way of preventing pH shock during souring.

The addition of MagnaBrite™ magnesium sulfate solution can modify the soluble domain of silica, forcing it into an area where it forms a soft, nonadherent, talc-like scale that has a low affinity for metal surfaces and is easily removed.

Knowledge of the reactions among bleaching and deinking chemicals leads to a better understanding of the role of NPEs in your process. By understanding the effect of the NPEs and the role of MagnaBrite™ you can effectively eliminate the cost and performance issues associated with scaling.

PQ Technology Group Capabilities

PQ Corporation, through its Canadian subsidiary, National Silicates, operates a modern, fully equipped technical center in Toronto for pulp and paper applications. To ensure efficient use of PQ's soluble silicate and magnesium sulfate products, our team of trained technical professionals stands ready to assist customers around the world with information, analytical services, and processing studies:

Wastepaper Deinking and Recycling

• Recycle pulp analysis: image analysis (speck count), ERIC, brightness, color, fluorescence
• Silicate feasibility studies
• Deink plant simulation: laboratory repulping, flotation, washing, etc.
• Laboratory optimization of deinking chemistry for washing and/or flotation
  (that is, silicate, peroxide, caustic, DTPA, surfactant, etc.)
• Brightening and color stripping studies
• Silica mass balance studies
• Fiber loss analysis
• Metals analysis, etc.
• Scale analysis and control studies
• General process design audit
• Brightness reversion studies
• General bleach plant design audits
• Bleach plant control audits
• Safety audits
• In-plant trials involving all the operations above

Oxygen Delignification

• Laboratory optimization of delignification conditions and magnesium sulfate dosage
• Pulp washing evaluations

Linerboard Enhancement (Under Development)

• Laboratory optimization of wet-end chemistry for optimum silicate addition
• Contaminant removal and wet-strength additives for recycling

Mechanical Pulp Brightening

• Laboratory optimization of bleach liquor formulations (that is, silicate, peroxide, caustic, DTPA, surfactant, etc.)
• Silicate feasibility studies
• Magnesium sulfate feasibility studies
• DTPA elimination studies
• Brightness reversion studies
• Metals analysis, etc.
• Scale analysis and control studies
• Mass balance studies
• Bleach liquor preparation system design
• General bleach plant design audits
• Bleach plant control audits

Pulp and Paper

APPLICATIONS

Mechanical Pulp Bleaching
Kraft EOP
De-Inking
Oxygen Delignification

RECOMMENDED PRODUCTS: Sodium Silicate; Magnabrite Magnesium Sulfate Solutions

RECOMMENDED LITERATURE:

De-inking Bulletin 6-9
Without a Trace, PP601
Sodium Silicate in Peroxide Brightening of Mechanical Pulp, PP602
PQ Products for the Pulp and Paper Industries, PP601

Please contact Technical Service for more information on this application.