Börje Kyrklund
BÖRJE KYRKLUND is a pulp and paper engineer and a staff member of the FAO Forestry Department.
The technology exists and the economics can be worked; what is needed is the will
The term "mixed tropical hardwoods" when used in the context of making pulp for paper refers to a more or less natural mixture of hardwoods from a tropical forest. But there is considerable variation in mix with regard to the number of species, the frequency of occurrence of each species and consequently also in the average wood properties of a tropical hardwood mixture. In some countries a mixture of 15 to 20 species is defined as mixed tropical hardwoods, whereas in others the number of species in the mixture may be counted in hundreds.
As regards wood properties, the average basic density of mixed tropical hardwoods is generally considered to be high - in the range of 700-800 kg/m3 - but it can be considerably lower in some areas (1). The range of basic densities of individual species within one forest area may be as wide as from 150 kg/m3 to 1300 kg/m3.
It is evident that the technical problems in connection with pulping of mixed tropical hardwoods and the pulp quality arrived at will vary with the properties of the hardwood mixture: used and they in turn depend on the actual forest from which the mixture: is obtained, with variation even within the same forest area. Accordingly, no generally valid prediction can be made about the suitability of mixed tropical hardwoods for the production of pulp and paper, in terms of expected strength and other characteristics as it is possible to do with single wood species.
The mixed tropical hardwoods thus constitute a heterogeneous raw material for pulping - as heterogeneous as the forests from which the woods are obtained - and special consideration must be given to this both when planning a pulp and paper industry based on this raw material and when assessing the quality of the pulp which can be obtained. It is therefore essential that a study of the economic and technical feasibility of a pulp and paper project based on mixed tropical hardwoods refer to a well specified forest area which is envisaged to be used as a source for pulpwood rather than to a general area in a country.
The harvesting of mixed tropical hardwood forests should be based on intensified fellings rather than extraction of a few selected species of suitable size for mechanical wood products should be marked separately. This means that the species used for pulping, in addition to young trees of commercially valuable species, will be those which traditionally are of no commercial importance. The establishment of a pulp mill can, therefore, contribute significantly to a complete and more economic utilization of mixed tropical hardwood forests which otherwise would go unutilized or would be poorly utilized through creaming of a few high value species.
A considerable amount of research has been carried out on the pulping and paper-making characteristics of individual species of tropical hardwoods (3). The fibres obtained from tropical hardwoods are always short in comparison with fibres from coniferous species, but there is great variation in fibre length, fibre thickness, and cell wall thickness among the species in a tropical forest, even within a limited forest area. Accordingly the average fibre characteristics and their distribution in the pulpwood mix will govern the general characteristics of the pulp obtained.
The most generally applicable processes for pulping of mixed tropical hardwoods are the sulphate process and the semi-chemical (NSSC), cold soda or semi-chemical sulphate processes. Fully bleached sulphate pulp can be expected to have good opacity and formation. In that respect a pulp made from mixed tropical hardwoods compares well with beech pulp. As regards the overall strength properties, they compare favourably with those of pulps from single temperate zone hardwood species but the actual strength characteristics depend on the pulpwood mix used, that is, on the composition of the pulpwood obtainable from a given forest area.
The quality of bleached sulphate pulp made from the mixed tropical: hardwoods thus corresponds to grades already manufactured commercially from a single species or a few species and consequently there is no reason why this type, or rather, these types of pulp could not be used for the same purpose as those of corresponding types already manufactured commercially. Therefore, they should be able to compete successfully on the international markets with pulp from more traditionally used species of trees.
The quality of high-yield or semi-chemical pulps made from mixed tropical hardwoods has been proved to be high. However, the quality of the pulp produced even by these processes depends on the pulpwood mix used, although perhaps not quite to the same extent as in the case of bleached sulphate pulp.
Bleached sulphate pulps from mixed tropical hardwoods can be expected to be used successfully for the production of printing and writing grades of paper with an addition of long-fibre pulp which may vary from 40 down to 10 percent of the total fibre furnish. A good opacity and good formation on the paper machine is to be expected with mixed tropical hardwood pulps.
Newsprint manufacture based on refiner-type mechanical pulp made from mixed tropical hardwood would be limited to low-density species, preferably where they exist in fairly pure stands or if they can be separated by mechanical means from a chip mixture. In the latter case some use must be found for the remaining part of the chips - for sulphate or semi-chemical pulping or as process fuels. Application of a modified thermo-mechanical process with addition of chemicals may also increase the range of wood densities to which such a pulping process would be applicable.
Industrial grade papers such as sack and bag paper, as well as liner board have been produced successfully for some years with a substantial percentage of high-yield sulphate and semi-chemical mixed tropical hardwood pulps. The quality obtained is acceptable at least for domestic markets and even for manufacture of banana boxes, which must have good stacking strength.
ECUADOR the potential value is great
The most promising paper product, together with printing and writing papers, is corrugating medium which also has been made on a commercial scale with a mixed tropical hardwood pulp content of up to 70 percent. The quality which can be obtained is usually good enough to make the product competitive on an international market.
Considerable progress has been made during the past decade with respect to the technology of pulping of mixed tropical hardwoods. As opposed to the attitude in the past that the technical difficulties - real or imagined - should be avoided by selective extraction of "suitable" species, the opinion today is that the pulping should be based on natural mixtures of pulpwood available in the forest, excluding only species which are too valuable for pulping or definitely unsuitable from the processing point of view. Even clearly unsuitable species are excluded only if they are present in the forest in large enough quantities to really affect the processing of the overall pulpwood mixture.
The main, but not insurmountable, problems involved in pulping of mixed tropical hardwoods come down in the end to three, the continuity of supply of pulpwood throughout the year, the processability of the species, and the uniformity of the resulting pulp quality.
In some tropical forest areas the forest or part of the forest is inaccessible during the rainy season, but this is not a general characteristic of tropical forests as such. The conditions vary considerably with subsoil characteristics and peak rainfall, rather than according to the total annual rainfall.
It is self-evident that a pulp mill must be able to operate throughout the year and so requires a continuous supply of all raw materials. In some cases, no pulpwood can be extracted from the forest during the rainy season. The reasons for this may be either that forest roads will be impassable during that season or that certain areas of the forest are flooded due to heavy rainfall. The length of the period when no pulpwood can be extracted may in some instances be only a few days at a time, whereas in others it can be considerably longer, up to several weeks.
In order to ensure continuity of operation of a pulp mill in such areas it is essential that buffer supplies of pulpwood be kept at the mill or along the sides of roads which are carriage-able throughout the year or stored in the form of chips at the mill. However, storage of wood either as logs or as piles of chips poses certain problems under tropical climatic conditions.
Storage of pulpwood in log form has the advantage of protecting the bulk of the wood from attack by outside fungi and bacteria, although some species exhibit rapid growth of certain fungi on the outside of the billets. In some instances this is noticable even after a few days. In addition, there is some discoloration of the wood itself, as well as some penetration by fungi and bacteria from the outside.
Storage in chip form in piles has been extensively studied in the major pulp producing countries. Conditions for chip storage in tropical regions, however, is another matter. The problem is different from place to place depending on climatic conditions. So far, the only published information about storage of chips under tropical conditions is from Papua New Guinea (2, 3, 4) which has an annual rainfall of 2000 to 3000 mm. Storage of chips under these conditions for four months showed considerable microbial discoloration and there was an average wood material loss of about one percent. Nevertheless, storage did not seriously affect the pulping, bleaching and paper-making properties (5). However, these results apply only to Papua New Guinea, and further research is required into chip storage under more severe climatic conditions and the effects there may be on the pulp.
About 50 percent of the wood losses during chip storage are losses of constituents, such as low molecular carbohydrates, resins and acetic acid. These are lost during pulping and bleaching in any case. The other 50 percent loss is wood material itself. To control the deterioration of the wood material such methods such as spraying with water, acids and pulp mill effluents have been suggested as well as treatment with chemicals or antagonistic or selective micro-organisms (7). But so far none of these methods have been tried under tropical conditions which have their own specific variety of microorganism flora, chemical compositions of wood material, and climatic conditions. This a field where further research is required in different regions and areas of the tropics.
GABON, FORESTRY INSTITUTE LABORATORY behind the pulping scenes
Until problems of chip storage are solved it is probably better to store wood in the form of logs to ensure a continuous supply of pulpwood even during rainy seasons. Chip storage at the mill should be restricted to a minimum - probably not more than a few weeks at the most:
The processability of mixed tropical hardwoods boils down to five main points:
debarking; chipping; chip impregnation and pulping; bleaching; and chemical recovery.
Mechanical debarking can be employed, except with extremely crooked logs or if the bark has become very hard during storage. Such logs may have to be separated and debarked manually.
Chipping of mixed tropical hardwoods does constitute a problem since, due to the extreme hardness of some of the wood, there is considerable wear of the chipper knives, so that the knives must be sharpened and replaced frequently. Using knives of higher quality - and cost - the frequency of chipper maintenance can be reduced somewhat. The problem is more economic than technical and the right balance has to be found in each case between the cost of maintenance and the cost of knives.
Chip impregnation has often been quoted as being a severe problem with mixed tropical hardwoods. However, this varies considerably with the wood mixture used. Even if impregnation presents a problem, it can be overcome by prolonging the temperature rise period during chemical pulping or by using' pressurized impregnators in semi-chemical pulping.
The consumption of chemicals is usually in the range normal for temperate zone hardwoods. An interesting feature of mixed tropical hardwoods is that they can often be pulped to a higher pulp lignin content than conventional hardwoods without any serious increase in the content of screenings in the pulp. This usually brings about improved strength characteristics, but at the same time the consumption of bleaching chemicals will increase. Therefore, to improve the quality of the pulp in this way may prove uneconomical. However, if the pulp is to be used as unbleached such an approach is usually recommendable.
If species containing large quantities of silica or latex are abundant in the pulpwood mix, this tends to cause problems in the recovery process. However, it can be overcome by reducing the overall content of these substances in the pulpwood mix through exclusion of some of these species at the harvesting stage.
In principle it may be said that all tropical hardwoods can be successfully processed (8). But we feel that, for technical and economic reasons, some species may have to be excluded from the pulpwood mix during harvesting in order to avoid problems, especially in the recovery of chemicals and in order to reduce wear on the chipper knives to an economically acceptable level. However, these excluded species will usually constitute only about 10 percent or less of the number of species present and only a small percentage of the total tonnage of pulpwood obtainable from an area.
The problems in processing of mixed tropical hardwoods are, therefore, more economic than technical.
As was mentioned at the beginning of this article, the mixed tropical hardwood forest is in itself heterogeneous in the sense that within the same forest area, several different types of forests can be distinguished. However, there are instances when the entire supply area of a pulp mill is uniform in composition - a homogeneously heterogeneous forest. In that case the quality variations in the pulp mill will not constitute a major problem. On the other hand if several types of forests are distinguishable within the supply area, the quality of the pulp will vary as the harvesting proceeds from one part of the forest to another.
In order to avoid both short-term and long-term variations, the first being due to variations in the composition of the pulpwood from the part of the forest being harvested and the second to differences in pulpwood composition between different parts of the forest area, several different approaches have been suggested. It is evident that separation of the trees by species into separate piles and then maintaining a mix of constant composition by species in connection with the feed to the chippers is completely unfeasible. Utilizing buffer storage in the form of logs or chips for control of the quality variations has the disadvantage of diminishing only the short-term variations, unless the time of storage in these buffers is extremely long, something which under tropical conditions is apt to increase wood decay.
Another approach suggested (9) is to divide the forest area into sectors of roughly homogeneously heterogeneous vegetation, and to make a statistical estimation of the average pulping and paper-making characteristics of the pulpwood mix obtainable from each sector. Following this, the harvesting is planned and carried out in such a way that by mixing pulpwood from four to eight different sectors a constant average pulp quality is obtained, at the same time keeping under control the limits within which the quality is allowed to vary. According to this system of pulp quality control, the amount of wood stored in the form of logs or chips can be kept to a minimum inasmuch as its purpose is mainly that of mixing logs or chips. The system has the advantage of keeping both the long-term and short-term variations under control. Although this system seems promising it still needs to be tried out under actual conditions, for instance as a pilot study in a tropical country. Such a study should aim at practical aspects, both technical and economic. This approach also puts certain requirements on the forest inventory, and, in addition to control of quality, it can also be used as a tool for harvesting management in fact it puts very high demands on the control of the forest operations and transport to the forest landings and/or to the mill site.
There are no technological reasons why bleached sulphate pulp and corrugating medium of internationally competitive quality cannot be made from a natural pulpwood mixture from most tropical hardwood forests so these products have a potentially bright future.
However, due to the fact that the raw material which would be used is still comparatively unknown internationally, it may be necessary for an export pulp mill to allow for initial discounts for products made from these woods.
From a pulp and paper point of view the purpose of the forest inventory is to provide the following information:
- Identification of sectors of roughly homogeneously heterogeneous vegetation within the forest area visualized to be used as a source of pulpwood.- Accessibility of each sector in the whole forest area.
- Quantity of pulpwood obtainable from each sector per unit area and the variance of this quantity.
- Species distribution in each sector.
After sampling of the species in the forest area, in connection with the forest inventory, determination is made of the pulping and paper-making characteristics of pulp which can be made from pulpwood obtainable from each homogeneously heterogeneous vegetation sector in the forest area as well as the variation of each characteristic within each sector.
A more detailed account of how this is achieved is given elsewhere (see Reference no. 9).
A MODERN EUROPEAN PAPER MILL in the future, in the tropics?
Once all the data are available, an assessment can be made of how pulpwood from different sectors in the forest should be mixed in order to maintain a uniform pulp quality. At the same time it provides the basic information for a harvesting plan. In some instances the quality of the pulpwood obtainable from some sectors may be of such poor quality that it cannot be used even in combination with pulpwood from other sectors. In that case a decision has to be made as to whether it is going to be left unharvested, used for production of products other than pulp and paper or as source of fuelwood.
The selection of the right pulping process has to be based on the results of market studies and on preliminary trials with some natural mixtures of pulpwood from the forest area. These trials should be linked to the forest inventory.
Once the process has been selected and the influence of process variables on the pulp quality have been studied with some natural mixtures of pulpwood in order to establish the process conditions, pulping tests in connection with the forest inventory can be carried out. This provides the basic information on the kind of average quality which can be produced and the limits within which it can be made when pulpwood is obtained from the entire forest area in accordance with an appropriate harvesting plan.
The harvesting plan for the forest area should be based on the following information:
- Accessibility of each homogeneously heterogeneous sector in the forest area.- The pulping and paper-making characteristics and their variations for pulpwood obtainable from each sector in the forest.
- The soil characteristics and terrain conditions in different parts of the forest in order to allow establishment of road construction routes and the time and cost of road building.
The aim of the harvesting plan should be to facilitate production of pulp of uniform quality by mixing pulpwood from 4 to 8 different homogeneously heterogeneous vegetation sectors in the forest area.
The first question to be answered with regard to reforestation is whether it will be based on plantation or on natural regeneration. If the thinking is in terms of plantations the utilization of mixed tropical hardwoods for pulping is considered as only a temporary measure which will provide economic support for future plantation activities, after which pulp manufacture will be based on plantation species alone. But plantations are not always possible due to the local conditions, so natural regeneration may be the only alternative.
It is important that a study be made at an early stage at least of the possibilities of plantation and also of what species or mixture of species can be introduced into the area. At the same time the possibilities and implications of natural regeneration should be given at least preliminary study. On the basis of the conclusions arrived at, a tentative management plan for reforestation can be established.
If the market, domestic, intra-regional or international, is sufficient to warrant the establishment of an integrated pulp and paper mill, there is no doubt that this should be taken into account when planning the utilization of the mixed tropical hardwood forest area. Additional integration with a mechanical wood products plant will naturally further improve the viability of the operation, first because the forest often contains commercially valuable species which should be excluded from the pulpwood volume extracted and second because certain sectors within the forest area may have pulpwood of unsatisfactory quality. The wood from these sectors may be utilized, for instance, for hardboard or for particle board manufacture. A third argument in favour of this type of integration is that the pulp mill can use residues from the mechanical wood products plant as an additional source of raw material. All of which brings us to four main conclusions about the use of mixed tropical hardwood for pulp and paper.
1. Mixed tropical hardwoods can be used successfully for the manufacture of pulp and paper and certain grades of these products can be competitive even on international markets provided the problem of quality variation is solved.
2. The problems of mixed tropical hardwood pulping are less technical than economic in nature.
3. The quality of pulp produced can in general be expected to be adequate for use in manufacturing the same grades of paper as conventional short-fibre pulps. In fact, the pulp from mixed tropical hardwoods may even offer some advantages due to the mixture of thick-walled and thin-walled fibres, thick and thin fibres and a wide range of fibre length, which can help to give the pulp special characteristics which conventional hardwood pulps do not have.
4. An important question with regard to pulping of mixed tropical harwood is that of reforestation by plantation - either exotic species or natural or related species - or, alternatively by natural regeneration. Usually the secondary forest growing up after clear-cutting is of lower quality than the virgin forest for commercial timber but most likely suitable for pulp and paper. In some areas where plantation cannot be made, natural regeneration is the only possibility. Nevertheless, this is a field where serious research is required, especially with regard to management of the natural regeneration. There is no doubt that the mixed tropical hardwood forest is a promising source of pulping material.
