Pulping/Bleaching

Achieving the best pulp quality requires understanding the interactions of new bleaching chemicals with all pulp components

BY Christine Chirat and Dominique Lachenal

New Chemicals Cause Dramatic Changes in Bleaching Sequences

Chemical pulp bleaching sequences have undergone unprecedented changes in the past few years, largely as a result of environmental pressures (Table 1). One consequence of this has been a considerable increase in the number of bleaching sequences that are used in today's pulp industry. For those not specialized in bleaching, it can be difficult to follow and understand all the changes that have taken place. And added to that, new bleaching agents have made it an even more complex task to choose a bleaching sequence for new lines or to decide which modification to utilize on an existing line when it has to be modified.

The objectives in this article are not only to outline the general trends in bleaching, but also to show that all bleaching processes are not equal. As most people know, the aim of bleaching is to remove the residual lignin after cooking to reach a target brightness level. But even in aiming for this simple goal, there is the question of whether this target level should be very high (a brightness of above 90% ISO) or only moderately high.

Before reviewing the current trends in bleaching, it is important to list the incentives for modifying an existing bleaching line or the requirements that must be considered for a completely new bleaching sequence. Since each mill represents a particular case, the likely factors to be considered listed below are not in order of importance, but are some of the elements that will be involved in the decision-making process:

• Reduction of bleaching cost
• Increase in pulp yield
• Reduction of the pollution load (BOD, COD, AOX, toxicity) and water use
• Improvement in pulp quality (pulp strength properties, brightness, cleanliness, opacity)
• Flexibility and easy control to ensure the production of a consistent pulp quality
• Compatibility with future developments-e.g., evaluating moves toward a partially or totally closed cycle mill
• Simplification of the process-i.e., creating an even temperature or pH profile throughout the sequence
• The mill's specific circumstances-e.g., a mill may be located in a remote area, making it difficult to get supplies of some chemicals.

For example, basic research has enabled us to show that:

Chlorine (C) is capable of reacting with all types of aromatic structure in residual lignin, which explains why it is the most efficient bleaching chemical available so far. Ozone can be ranked in the same category when it comes to delignification efficiency (it reacts with all aromatic rings).

Chlorine dioxide (D) is known to react primarily with free phenolic groups in lignin, leaving the other phenolic groups almost intact, unless the ambient conditions are made more extreme. Oxygen (O) also reacts exclusively with free phenolic groups, which makes it similar to chlorine dioxide as far as delignification is concerned. Thus the use of oxygen and chlorine dioxide, such as in the O-D-E-D-type sequence, is far from being optimized. (DC) or (CD) combinations, on the contrary, are efficient.

Hydrogen peroxide (P) itself is not very efficient when used at moderate temperatures (lower than 70C). It can react only on carbonyl-containing structures (such as quinone-type compounds), and its use is limited to color removal. The formation of radicals such as OH and O2o- seems to be needed to achieve a substantial delignification. This can be done by raising the temperature. The role of radicals as intermediates during most of the bleaching stages on delignification is still a matter of debate. One drawback of the occurrence of radicals species such as OH is that they also react readily with cellulose and lead to degradation of the fiber.

Metal ions in pulp must be controlled. Some metal ions, such as iron and copper, favor the formation of radical species during P, O, or Z treatments, leading to subsequent cellulose degradation. They can also be a source of chemical loss. For example, manganese, iron, and copper cations catalytically decompose hydrogen peroxide. Magnesium, on the other hand, is beneficial to oxygen and peroxide stages. Metal ions can be removed and/or controlled by acid or chelation stages.

PHASING OUT THE CHLORINE. Elemental chlorine is the most efficient and universally used bleaching chemical, but in less than five years it has been phased out in all European countries as a result of environmental pressure. Its use is also constantly decreasing in all the other countries.

The main effect of eliminating chlorine has been a large decrease in the formation of chlorinated organics (on average, this has dropped by three quarters) and the reduction in the content of chlorinated dioxins in the bleaching effluents and in bleached papers. These have both fallen to undetectable levels. Whether or not this change has contributed to any extent in improving the environment is still a matter of debate.

Most mills have opted for the partial or total replacement of chlorine by chlorine dioxide. The net result is an increase in chemical costs of 20% to 30%. Some mills have taken the opportunity to install oxygen delignification ahead of the bleaching line, which makes it possible to maintain the chemical cost at about the same level, but this also represents a $30 million investment in most cases.

These drawbacks are the direct result of poorer delignification power of chlorine dioxide compared with chlorine.

MORE OXYGEN. Today, oxygen is the cheapest bleaching chemical, but capital cost is still the major drawback of oxygen delignification. The problem is not only the level of investment involved, but also the fact that the money is spent to achieve a rather poor result, say 40% to 50% delignification. Despite this, efforts have been made to use oxygen much more extensively.

More and more mills are installing a second oxygen stage-with or without intermediate washing-and the gain in delignification is usually rather small at 10% to 15%. The last three to four years have also seen the appearance of hydrogen peroxide stages under oxygen pressure (PO) at high temperature (90C to 110C) used either at the beginning of a bleaching sequence or at the end. For instance, sequences such as O-Q-(PO)-D-E-D, O-Q-(PO)-D-Q-(PO)-D-Q-(PO), or O-O-Q-(PO)-Q-(PO) are used today.

If (PO) is used as a modified O stage, then delignification increases by about 20%. As a final stage, though, (PO) performs more like a modified peroxide stage. One advantage of using (PO) instead of a P stage is that the process is more efficient, requiring reduced retention times and chemicals. The use of hydrogen peroxide in tough conditions (high temperature and oxygen pressure) makes it very important to have proper control of the metal ions with chelating stages (Q), for example.

Again, the limited performance of oxygen and oxygen peroxide combinations is a result of the weaker delignification power of these chemicals compared with chlorine. They also have too many similarities in their chemical reactions with lignin.

CHLORINE-FREE BLEACHING AGENTS. The incentive to go for totally chlorine-free (TCF) bleaching has faded during the past two years. Even though bleach plant closure should be somewhat easier with TCF than with elemental chlorine-free (ECF) processes, there are still problems which remain to be solved.

The superiority of TCF over ECF bleaching in terms of environmental impact is questionable. There are some significant disadvantages in TCF bleaching, which explains the lack of interest still being expressed by most pulp producers. The most important of these is that bleaching a kraft pulp to high brightnesses (90% ISO) is not possible without sacrificing some strength properties. The problem of cellulose degradation during TCF bleaching has been extensively studied.

Taking an O-P-Z-(EO)-P sequence, for example, it was shown that each stage might contribute to some cellulose depolymerization. One critical factor is the amount of ozone introduced in the sequence. For charges higher than 5 to 6 kg/ton, the cellulose may be slightly depolymerized and oxidized. This last effect makes the pulp sensitive to any alkaline environment, such as (EO)-P, which leads to further chain cleavage by a mechanism that has already been well documented.

Consequently, despite the fact that such a sequence was close to optimum efficiency in terms of delignification (ozone is ranked in the same category as chlorine) and bleaching power, it is penalized by the occurrence of several degradation mechanisms taking place on cellulose in a synergistic way.

One possible solution to the problem of cellulose degradation during TCF bleaching to 90% ISO is to limit the charge of ozone and to introduce some non-degrading bleaching agents in the sequence. The only reagents that demonstrate this property so far are the peroxyacids (peroxyacetic, peroxymonosulfuric acids). But more research is needed to reduce the chemical cost to acceptable levels when peroxyacids are used.

MODIFIED ECF SEQUENCES. One promising trend today is the use of modified ECF sequences, i.e., sequences that still use chlorine dioxide but not in the classical manner of D-E-D-E-D or O-D-E-D sequences. The key here is to make them as efficient as chlorine-containing sequences. The use of chlorine dioxide and ozone in combination-in (DZ) or (ZD) stages-has started to appear during the past three years. Indeed, basic chemistry tells us that the reactions of these two chemicals on lignin complement each other, as with the case of chlorine and chlorine dioxide combinations (DC), for example. It makes the process more efficient than D0 or Z alone.

The other interest in combining the use of ozone and chlorine dioxide lies in the fact that the required operating conditions (temperature, pH) are similar for the two chemicals, making it possible to run (DZ) or (ZD) stages with no intermediate washing. The criterion in the process is the replacement ratio, i.e. the amount of chlorine dioxide replaced by 1 kg of ozone. With ozone and chlorine dioxide (as pure chlorine dioxide) being of the same order of cost today, a replacement ratio higher than one means a reduction in chemical cost.

Table 2 shows an example of the (ZD) and (DZ) processes applied to a softwood kraft pulp. This type of combination has been thoroughly studied at CTP (Centre Technique du Papier), both in the laboratory and at pilot scale to optimize the process. The (ZD) process is already in use in a couple of mills, and several other projects announced recently intend to utilize it. This process provides a good example of how a relatively simple retrofit in an existing mill can meet most of the requirements outlined earlier in the article.

FINAL BRIGHTNESS. It is generally recognized that the last few points of brightness increase in a bleaching sequence are the most difficult to achieve, requiring long residence times and significant chemical usage. Even though strict process control and optimization should enable mills to reach high final brightness levels, it is a daily challenge for some mills to obtain constant high final brightness levels.

The question of whether the final brightness of pulp and paper should be more than 90% ISO is another matter for debate. This discussion must take into account many factors, including the demands of the end user, cultural expectations, and environmental impact. Even though these considerations are outside the scope of this article, some technical thought can be expressed which might have a positive influence on the development of processes such as TCF, which cannot meet the very high brightness target without diminishing strength properties.

Consider the case of wood containing papers. Figure 1 gives the resulting brightness of mixtures of a chemical pulp of varying brightness (from 80% to 90% ISO) with a mechanical pulp of increasing brightness (from 60% to 80% ISO). Each curve represents all the brightness combinations leading to a blend of given brightness. From the shape of the curves, it appears that the brightness of the chemical pulp fraction has a minor effect on the resulting brightness. Raising the brightness of the chemical pulp from 86% to 90% results in a one point brightness gain for the blend containing 20% mechanical pulp.

This phenomenon is even more pronounced in the case of blends with higher mechanical pulps contents. At 50% mechanical pulp, any increase in the brightness of the chemical pulp beyond 86% has only a marginal effect. Consequently, 85% to 86% kraft pulps are perfectly suitable for these applications, which is a level easily reached with modern TCF processes. Rationalization should also take such aspects into consideration.