The makeup of papermaking fibers is changing at a rapid pace, but it is also affecting wet end chemistry issues

By Leslie Webb

Mill materials for the new millennium

It may seem surprising, but accurately breaking down this figure into the main papermaking raw materials is not easy (Figure 1). The assumptions in Figure 1 are based on approximate round percentages for moisture contents and losses to give a reasonable input figure for retained non-fibrous raw materials. Even so, this is probably on the high side as the two biggest materials (fillers and starches) only total about 25 million tons.

Figure 1 - Mass Balance for Global Papermaking in 1998

It is evident from this data that the contribution of virgin pulp to paper continues to decline, having dropped from 63% in 1995 to 58% in 1998 (calculated on a normal air dry basis). About 130 million air-dried tons of used paper were recovered and recycled globally in 1998, corresponding to an overall rate of about 43%. Once the processing losses have been excluded, the real utilization rate (ie based on dry materials reaching the paper machine) cannot be any greater than 33-34%.

Figure 2 - Framework for Looking at Papermaking Materials

Papermaking materials can be classified in various ways, but a useful generic framework is shown in Figure 2. The key property is that the materials should express their desired functionality - at the wet end in the case of process additives or in the paper for product chemicals. Fibers provide the backbone for all paper products, usually expressed as some strength parameter. But their configuration in the sheet also has a big impact on structural characteristics such as density and permeability. The shift from virgin to recycled pulp and the unrelated, but steady, increase in the use of mineral fillers and pigments impacts papermaking chemistry in several ways, including:

• more fine particulate material, increasing particle numbers and their surface area

• different sorts and perhaps higher levels of dissolved and colloidal material causing problems, some old and some new

• greater use of wet end additives for maintaining sheet strength.

Simply measuring the mass contribution of fines fails to reveal their full impact. Fines make a much larger contribution to wet end surface area in the thin stock and have an overwhelming impact on particle numbers, even when the fines content in the incoming pulp is very low. More recycled pulp and fillers spell bad news for wet end chemistry in terms of particulate retention, but the increased surface area may be good or bad depending on circumstances. If a mill wants to incorporate more of a water-soluble material into the paper (such as a strength additive), a high surface area is useful. But if the mill has to cover the available surface in order to develop some desired result (such as color or sizing), high chemical consumption is needed.

Solutions are not the answer

As Figure 2 shows, solubility at the wet end is also an important characteristic of papermaking materials. This is because the single pass retention of dissolved solids is very low (about 1%) and their total retention also tends to be minimal (no more than about 20%) except on machines with closed water circuits. As a result, dissolved solids can build up to high levels, which may cause a range of problems including deposition, foam, interference, etc. For these reasons, one of the objectives in optimizing wet end chemistry is to minimize the concentration of dissolved solids, which originate from three main sources:

• fresh water, which contributes largely inorganic electrolytes such as hardness salts

• dissolution from largely insoluble papermaking materials such as pulp, broke and filler

• residues of papermaking chemicals added at the wet end, but which have not been taken up (adsorbed) by the particulate surfaces.

Water is unlikely to be displaced as the most pervasive material in papermaking, although its impact on wet end chemistry will hopefully become better appreciated. In time, climate change as a result of global warming could bring about different patterns of water availability, particularly in terms of confidence in it being there in sufficient quantities at all times.

Outsourcing has been a strong trend in recent years, most notably for energy supply. However, as water is a more integral part of papermaking, it is likely to continue to be obtained directly from its natural source (river/lake or ground) wherever possible. With the increasing re-use of treated wastewater in place of fresh water and the consequent need for a reliable recycled quality, outsourcing of wastewater treatment is also likely to remain something that is talked about rather than much practiced.

It is also likely that contracted-out water supply increases the risk of variable water chemistry. This is undesirable as the quality of fresh water establishes what might be termed the baseline chemistry of the paper machine. It might be argued that there are so many other, and probably larger, sources of chemical variability that this one is insignificant. But variability must be minimized as it is often the reason for wet end chemistry problems and demands greater sampling/analysis efforts to characterize the system.

When the machine water system is open, its wet end chemistry is similar to that of the fresh water. As closing up proceeds, the fresh water baseline stays where it is, but materials from other sources build up depending on their wire retention characteristics. The ratio of the machine concentration of a substance to its fresh water concentration is often incorrectly used as an index of the machine's degree of closure, but this ratio is not only affected by fresh water consumption. It also depends on the relative inputs from fresh water (which do not build up on closure) and from other sources (which do).

Any particulate solids in the fresh water should normally have been removed during pre-treatment, so its baseline is always close to zero. Dissolved solids in the raw water are not normally removed before use, but the natural level does vary according to the source and season. One set of materials that has to be removed from some surface waters is the colored humus-derived acidic substances, but this is easily achieved with metal coagulants followed by settlement and/or filtration. Sometimes, the inorganic salts associated with fresh water can cause scaling problems, the most common being due to precipitation of temporary hardness salts (mainly calcium carbonate). Incidental chemical reactions of any sort such as precipitation are generally undesirable in papermaking, but their severity does depend on where and how they take place.

Precipitation of salts in an ordered, crystalline form tends to produce hard deposits that can sometimes be difficult to remove by anything other than physical force. In contrast, unstructured amorphous deposits are usually quite soft and removable by normal shear forces. Some natural polymers in papermaking can interfere with crystallization processes and this effect can be augmented by adding small amounts of dispersant chemicals such as phosphonates. If precipitation occurs within the stock at the wet end of the machine, some free filler is produced, but if it occurs in shower systems, blockages can result. A downward adjustment of the pH level is one way of avoiding this problem, but some mills have experienced success with magnetic treatment of the water.

Additional salts enter the papermaking system via the pulp (residual pulping and bleaching chemicals) and from a limited number of chemicals such as alum. An additional source of dissolved calcium is calcium carbonate fillers through their interaction with a source of acidity such as alum or microbial activity. Recycled pulp contains a wide range of materials, particularly non-deinked (direct-entry) grades and surface-treated broke. The materials are mostly organic such as starches, brighteners, etc, that can dissolve at the wet end. Rather than poor pickup of wet end additives by fiber and filler surfaces, this dissolution is the main source of dissolved organics at the wet end and is a major handicap when trying to close up any machine's water system. Dealing with this material either by stopping it getting there in the first place or by coping with its presence, will remain a key element in optimizing wet end chemistry in future years.

Recycling limits

Although the fiber fraction of many grades of paper is 100% recycled, the age structure within that set of fibers depends on many factors. For example, some fibers will not have not have been recycled before, while others will have been around the loop many times, perhaps in very different grades of paper. Some fibers may not have seen a paper machine for many years, while others may have been used in the process earlier that week, perhaps even on the same machine. Some work has been done to model the variables in this complex situation, but predictions depend critically on assumptions regarding the homogeneity of waste streams and how they are split regionally.

To date, actual recovery and recycling rates have been driven by economic and environmental factors, the latter sometimes with the added push of legislation such as the European Union Packaging directive. Although we know that the extent of recycling does affect (usually adversely) both the macro quality of a batch of recovered paper and the micro quality of individual fibers, this has not so far exerted any sort of deterrent to recycling.

Problems in any recycling system usually get much worse as the system gets nearer and nearer to being fully closed. So it is not too surprising that with a global recovery/recycling rate of about 43%, technical issues about quality have yet to constrain recycling at a global level.

Some paper sectors are, of course, well advanced in recycling terms. The best examples are the recycled liner/fluting mills, many of which are based exclusively on recycled materials. These mills have the most difficult wet end conditions, not only due to the high input of fines and soluble materials, but also to their water systems, which are usually very closed. Wet end temperatures of around 50°C are not uncommon, even with quite low energy inputs, and neither are dissolved solids up to 30-40 g/l, ie concentrations that are much higher than thin stock consistencies. These conditions are a good incentive to move additions of product chemicals (such as sizes in the case of liner grades) away from the wet end to the surface of the paper. This does depend on the machine having some means of surface application (usually a size press), but many mills with a size press would like to be able to run without it due to the constraints placed on machine productivity.

An increased level of soluble materials originates not only from pulp, but also from changes in the use of fillers. The last 40 years have seen a tremendous change in the balance of papermaking minerals away from clays to calcium carbonates for a host of now well known benefits. More recently, traditional fillers supplied in a powder form are giving way to high solids content slurry forms for ease of storage and application. In order to maintain a high solids content, it is necessary to incorporate low levels of dispersant in the slurry, typically an anionic polymer.

Slurry pigments have been the norm for coating for some time, but even the introduction of a (hopefully small) proportion of the polymer to the wet end via the coated broke causes problems. For woodfree mills making papers with 20-25% ash content, a slurry filler would be the main source of wet end "anionic trash". One of the main ways around this is the addition of a compensating source of extra cationicity. In recent years, a substantial market for strongly cationic chemicals like polyaluminium chloride (PAC), polydadmacs, polyethyleneimines and super-cationic starches has developed. However, most of them are used in recycled and wood-containing grades, which have higher levels of anionic trash, rather than in uncoated woodfrees.

Monitor and control

Most of these chemicals have traditionally been sold as retention/drainage aids and one of their adverse effects is on wire retention. As a result, dealing with anionic trash has become an important element of retention chemistry. Most retention chemicals are based on synthetic organic polymers (including the above chemicals plus polyacrylamides and polyethyleneoxides) or modified natural minerals (silica and bentonite) and organics (cationic starch). These systems have become very efficient, resulting in good capture of fine particles and micro-scale flocculation, which is beneficial for machine efficiency and product quality.

Despite the significant advances seen in the last 20 years, one of the big questions about optimizing retention remains the best method of control. Whereas most new paper machines are equipped with a whole range of dry end sensors as a matter of course, the inclusion of wet end sensors is often still a hard sell for suppliers.

The benefits of measuring paper quality, either in real time on the machine or offline in the laboratory, are obvious. But the contribution of variations in wet end chemistry to poor paper quality and machine runnability is still not always recognized. Or at least the required investment in equipment and in people to gain control in this area is not recognized as being cost-effective in comparison with other investments, which may have a more guaranteed payback.

Putting this right does not always mean coughing up what may seem large sums of money for the latest online gadget (as many useful measurements can be done offline), but it does at least involve training somebody to understand and interpret the measurements being made. The sad fact is that too many mills measure little other than the odd retention or pH levels and yet expect to be able to produce spot-on paper quality with no deposits, foam or any of the other problems that can upset paper machines (and production managers).

All of the parameters shown in Figure 3 can now be measured online with sensors from companies like Eka Chemtronics, Neles Automation (formerly Valmet Kajaani), BTG, Mutek, BCL Sonica, etc, which tend to specialize in one type of sensor such as consistency/retention, charge or gas content. An exception to this is Raisio Chemicals, which has put together a monitoring system (WIC) for measuring a range of parameters such as pH, conductivity, alkalinity, calcium, aluminum, silicate, organic carbon, turbidity and cationic demand. All of these parameters can be measured with individual sensors from other companies, but the WIC system has been configured specifically for paper machines and comes with its own sampling system for up to 12 locations, sample preparation, guaranteed accuracy/measurement frequency for up to seven parameters and evaluation software.

Over 20 WIC systems have been installed on machines producing coated woodfrees, liquid packaging, folding boxboard, lightweight coated paper (LWC) and newsprint from both virgin and recycled furnishes. Payback times of 4-8 months have been reported by large machines making fine papers, largely due to a combination of reduced wet end breaks, higher yield (typically 0.5% extra) and chemical savings of 10-15%.

Hard choices

On top of the product specification, many process-related factors affect the choice of papermaking raw materials. Arguably the biggest change in recent years though, has been the inclusion of criteria related to their environmental impact. Whereas the mill can choose what product it makes and to some degree how it is made, it has little control over the regulated aspects of its business. This is therefore an area where pro-active action can pay dividends.

The eco-evaluation of raw materials started with virgin pulps but only concentrated on the impacts associated with the bleaching stage. Today, the eco-profile of raw materials is viewed more broadly, although supposedly environmentally-sound products still tend to be marketed behind a catchy phrase such as ECF or TCF pulp, low-AOX wet strength resins, solvent-free AKD or tree-free paper.

Environmental attributes of raw materials need to be viewed across their life-cycle. For paper products, this places pressure on the manufacturing (including extraction) of raw materials, the papermaking stage and what happens to the product after it has been used (Figure 4).

Figure 4 - Ideal Environmental Profile of Papermaking Raw Materials

The simplest terminology to denote overall environmental-soundness would be to apply the word "sustainable". But so far, few manufacturers are in a position to do this with any confidence for anything other than perhaps their wood. All the points in Figure 4 ask questions from the viewpoint of the environment about the molecules in paper, which has nothing to do with their functionality in the product or in the process. The four dominant raw material types used in papermaking (water, fiber, mineral fillers and starches) are all natural renewable materials, albeit changed somewhat during preliminary processing, while most of the remainder are synthetic and non-renewable.

One of the three main wet end sizing chemicals (rosin) also has a natural origin, while the other two are partly natural (alkyl ketene dimer or AKD) and wholly synthetic (alkenyl succinic anhydride or ASA). However, none are perfect wet end sizes as rosin does not work efficiently at neutral pH and requires a source of aluminum. At the same time, AKD does not always cure well due to its slow "reactivity" and ASA has to be prepared on-site due to its fast "reactivity". Because of this last characteristic, ASA's sizing mechanism is fairly well understood, but this cannot be said of AKD and rosin, despite the latter having been used for sizing for nearly 200 years.

Dry end chemistry is important to the efficiency of all three sizes, as it is for wet strength chemicals. The drying section is the only place in papermaking where some genuine chemical reactions are supposed to take place and without which the production of sized and wet strengthened papers would be difficult.

On the process control side of paper chemicals, much of the equipment is directed at stopping potentially problematic chemical or biochemical reactions. Synthetic organic chemicals are the mainstay of this business and we have yet to see any big breakthrough from the most natural of chemicals, namely those produced by micro-organisms, enzymes. Some success has been achieved with slime and pitch control as well as for drainage improvement, but enzymes in papermaking still generally cause more problems (such as material degradation) than they cure.

Future directions

It should be possible to apply some of the characteristics in Figure 2 to individual suppliers. Stability is not a strong point in the industry as suppliers seem to be following one of the good practices of wet end chemistry in aiming for maximum flocculation, or maybe coagulation, to increase the size of individual companies. Last year was no different from the previous few years in this respect, although most of the action came from an unexpected quarter. The Degremont arm of France's Suez Lyonnaise des Eaux group first swallowed up the relatively small Calgon business of ECC International (after it had been taken over by another French group, Imetal) for about $400 million. But this deal was later dwarfed by the $4 billion acquisition of Nalco, which will now form the platform for the vastly expanded water treatment business of this group. The year 2000 will almost certainly see other familiar names disappear and new ones appear among both suppliers and papermakers.

At the start of the new millennium, both suppliers and producers might like to take the opportunity to look at the areas of improvement in paper chemistry. At the mills, adequate monitoring of the relevant chemical parameters at the wet end and training the workforce to an appropriate level of chemical understanding should be priorities. At the same time, mills need to maintain some chemical independence from suppliers.

For raw material suppliers, improvements should focus on assessing and optimizing life-cycle characteristics of materials in a bid to achieve sustainable chemistries. They could also concentrate on selling added value and expertise, not just maximum tonnage, as well as improving dialog with machinery makers. The latter could benefit from incorporating wet end chemistry sensors at the design stage as well as getting a boost from improved relations with chemical suppliers.

Green chemistry is certainly something that all manufacturers should bear in mind (not just dye producers). But hopefully, the bottom line will remain black in the next millennium for both suppliers and their customers.

Leslie Webb directs the activities of Envirocell and can be contacted in the UK by telephone/fax on +44 1372 276599, via email on leswebb@cwnet.com or at www.envirocell.cwcom.net

Stories

Columns