The present invention relates to a method of producing pulp which has high light-scattering properties and a low shive content, by refining wood material.

Chemithermomechanical pulp (CTMP) is normally produced by pre-treating wood chips with a sulphite solution in an impregnating process which is followed by a pre- heating stage under pressure, prior to the refining stage. In comparison with the similar thermomechnical process (TMP) , the primary advantages of the CTMP- process are that by subjecting the wood material to sulphonation there is obtained a pulp which has a much lower shive content with comparable drainage ability and energy consumption, and improved strength proper¬ ties at least at a given drainability. The most serious drawback with the CTMP-process, in comparison with the TMP-process, is that the light-scattering coefficient of CTMP-pulp at a given drainability and given energy consumption (in manufacture of the pulp) is consider¬ ably lower than in the case of the TMP-pulp. This drawback is of particular significance when the pulp is intended for the manufacture of newsprint of a kind where the light scatterability is one of the most important requirements of printability.

Furthermore, known sulphonation processes have the disadvantage that only about 25-50% of the sulphite charged to the process (when the sulphite charge is

1-4% of the weight of the wood present) reacts with the lignin present in the wood to form sulphonate groups. The remaining part of the sulphite remains in the pulp suspension, mainly in the form of inorganic sulphite or sulphate. It is well known that this results in waste handling problems, particularly when anaerobic purifi-

cation is employed.

The Norwegian patent 110 849 discloses a process at the production of Chemi Mechanical pulps, in which chips impregnated with sulphite solutions are heated in saturated steam (water vapour) under superatmospheric pressure to a temperature above 100°C and are there¬ after defibred under superatmospheric pressure. According to this Norwegian patent the chips are firstly impregnated at a temperature below 100°C with a diluted alkaline metal hydroxide or carbonate solution without essentially chemically affecting the chips, the chips thereafter being continuously fed into a pressu¬ rized tank to which fresh steam is introduced to main- taining the pressure together with free S0 ₂ in such an amount, that the base is transformed to sulphite, bisulphite or mixtures thereof. This treatment must be considered as carried out in a liquid phase, since the sulphur dioxide is dissolved in the saturated steam (drops of water) and, furthermore, the chips are sub¬ stantially impregnated (filled) with liquids.

It has now been found that these drawbacks can be avoided by using a gas-phase sulphonation process instead of liquid-phase sulphonation process. This results in a novel type of CTMP, in which the high light-scattering ability of a TMP-pulp can be combined with the low shive contents of a CTMP-pulp. It avoids also the problem of handling sulphite and sulphate when purifying or cleansing the waste water.

Accordingly, the inventive method is characterized by sulphonating the wood material in gas-phase prior to complete refining of the wood material, preferably prior to the refining stage as such; and by subjecting the wood material prior to or subsequent to the sul-

phonation stages to a treatment introducing alkali metal ions as counter ions to the acidic groups of the wood. The purpose with this introduction of alkali metal ions is to increase the buffer capacity of the wood so that acid hydrolysis of the carbohydrates of the wood and the condensation of the wood lignin are inhibited. It is within the skill of the man in the art to control the amount of alkali and other reaction conditions required. An excess of alkali beyond the amount required to transform acidic groups of the wood to alkaline form may be required in order to neutralize gaseous sulphur dioxide depending on the technique used.

When treating in gaseous phase sulphur dioxide in the form of gas flows through the cavities of the wood material (large pores, fiber lumen, rays, resin canals) , the gas during its passage being solved in the water of the wood therewith sulphonating the wood lignin. The reason for the fact that the treatment in gaseous phase provides maintained light-scattering properties the shive content being reduced at the same time, is according to our present knowledge that the gas is solved in such parts of the fibers which are essential from the view of defibring. In this way low shive content is provided at a very low degree of sulphonation; the light-scattering property is thereby maintained.

At conventional production of CTMP a liquid impregna¬ tion of the chips being as homogenous as possible is desired in view to obtaining the effect desired (i.a. a low content of shives) ; the degree of sulphonation will then be considerably higher. Therefore, it is probable that the difference in transportation (sulphonation in liquid phase and gaseous phase, respectively) of the

sulfonating agent in the chips provides the differences in pulp properties between conventional CTMP and the pulp according to the invention.

A first embodiment of the invention comprises the steps of impregnation, the chips (wood) with a solution that contains a complexing agent in an alkali form, e.g. a sodium form, and/or alkali; pressing solution out of the chips subsequent to impregnation to such an extent that at least some free space is provided for gas transport, said chips being then treated with gaseous sulphur dioxide; and introducing the chips into a pres¬ surized reaction vessel for instance, said reaction vessel containing sulphur dioxide gas, optionally together with an inert gas, such as nitrogen gas or air. The chips are then heated with steam and refined in a known manner. Alternatively, rather than impregna¬ ting the chips with said solution, said solution is supplied in such an amount e.g. by spraying, that free space for gas transport is left and so that gas sulpho¬ nation can be carried out without intervening measures.

Thus, the treatment of the wood material is carried out in gaseous phase, the amount of water (steam) of the gas being so small that liquid does not essentially precipitate on the chips. By means of data from tables for air-water-systems is it easy to calculate if water precipitates and in such a case how much. For instance, if that temperature of the air is 60°C and drops to 55°C during the treatment, the initial relative humidi¬ ty of the gas must not be more than 80% in order that liquid should not precipitate on the chips. However, it is possible to permit some amount of liquid to precipi¬ tate, but this amount must not be so great that the transport of the gas through the chips is inhibited or essentially disturbed. According to the present inven-

tion, steam can not be considered as an inert gas, since it partly condensates on the chips and in the cavities of the chips, whereby the transport of gas through the chips is disturbed or inhibited, partly solves gaseous sulphur dioxide (in the water drops of the steam) . The transport of the sulphur dioxide and the reaction in the chips differs, as far as now seems to be the case, between gaseous sulphur dioxide being solved in the water drops (steam and condensed water) and substantially dry gas. Thus, the essential feature of the invention is that the sulphonation is carried out in gas phase which means that the dry gas disolves in the wet parts of the fiber, while according to prior technique liquid sulphur dioxide dissolves in the dry parts. Accordingly the atmosphere of the reaction vessel should be free from steam in as high a degree as possible when feeding the gaseous sulphur dioxide. According to another embodiment of then invention, the chips are treated with sulphur dioxide, suitably in the same manner as that aforesaid, prior to impregnating the chips with a solution which contains a complexing agent in alkali form, e.g. in sodium form, and/or alkali.

Both of these embodiments can be carried out in present-day TMP and CTMP mills or plants, when the plants include a continuous pressure vessel that has a continuous infeed and outfeed facility for treating wood material with sulphur dioxide, upstream or down- stream of an impregnating vessel in which the chips are impregnated with alkali, complexing agent or both.

In addition to containing complexing agent, the impreg¬ nating solution may also contain alkali, by which is meant primarily hydroxide, although, for instance, the solution may also contain carbonates of alkali metals.

such as sodium and potassium, and also ammonia (ammo¬ nium hydroxide) . The alkali, such as alkali hydroxide, may also be used as such in solution. The alkali is primarily used to neutralize the acidic groups of the chips or wood material. This neutralization can also be effected with complexing agents used in alkali form. In addition to alkali, the impregnating solution can also contain an alkaline metal or ammonium salt, whereby an ion exchange of the acidic groups of the wood into the form of alkaline metal or ammonium ions is carried out. The treatment is in this case equivalent with an impregnating process with alkali, as alkaline metals or ammonium is introduced as counter ions to the acidic groups of the wood, the buffer capacity of the fibers being increased in this way. These treatments of in¬ troducing alkaline ions as counter ions are known per se. In some cases it may be convenient to treat wood with acid (acidic solutions) in order to transform the acidic groups of the wood to protonic form before said treatment with complexing agent and/or alkali, optionally including alkaline metal or ammonium salt. In this way the ion exchange to the alkali form of the acidic groups is to carry out.

The sulphonation process is suitably carried out in gas phase on the chips as they are obtained from the chip production plant, or chipper, suitably after steaming the chips. The chips will then preferably have a dry solids content of 25-90%, such as 35-80%, for example 50-75%, although the sulphonation process can also be carried out on partially refined wood, for example subsequent to coarsely disintegrating the wood. The dry solids content of the chips should be such that free space is left in the chip. This is as distinguished from liquid impregnation at which the chip is essen¬ tially completely filled with liquid. The proportion of

cavities in the chip is dependent on the wood density. The dry solids content at which the cavities begin to be filled with liquid is dependent of the fiber satu¬ ration point of the wood species (defined as the water content of the wood after the removal of all the capil¬ lary water, but before loss of water from the cell wall) ; usually this loss occurs at a dry solids content of 67-80 %. Research has shown that transport of gas through coniferous wood easily still occurs up to a filling with liquid of about 60 % of the cavities in sapwood and about 40 % in heartwood. If the filling with liquid is further increased, the transport of gas is rendered subtantially more difficult. At a filling of 60 % that the transport of gas in sapwood takes place five times easier than transport of liquid. Assuming a fiber saturation point at dry solids contents of 75 % and a liquid filling of 50 % of the cavities the following formula is obtained for the limit of the dry solids content at which transport of gas still can be considered to take place to a great extent through the chip:

th=100/(l,00 + 500/p wood) % (valid for coniferous wood) .

This formula gives the following limits of dry solids content, for instance:

pwood (kg/m ³ ) Dry solids content (%)

400 44

350 41

300 37

250 33

It is possible to carry out both the embodiments while

applying divided defibration at high concentrations and refining at low concentrations, such as described, for instance, in our Swedish Patent Application No. 8903710-5 filed on the 6th November, 1989. The gas- phase sulphonation process can either be carried out before or after coarse defibration at high pulp consistencies. The pulp consistency will preferably be above 20%.

The amount of complexing agent added to the wood material is normally within the range of 3-50 kg per tonne of wood, and preferably 7-20 kg per tonne of wood. A particularly suitable interval is 8-12 kg per tonne of wood, for example 10 kg. The complexing agent used will preferably have the form of an alkali metal, wherewith a sodium form being particularly suitable in this regard. From an economic aspect, it has also been found convenient to use a complexing agent in a potassium form, and also in an ammonium form, whereas other alkali metal forms would be too expensive in normal operations.

The amount of complexing agent added can also be calcu¬ lated on the basis of the amount of calcium and other multivalent metal ions present in the wood material and by determining the mole quantity hereof and using the complexing agent in an amount of ± 50% thereof. An appropriate range is ± 30%. As will be understood, essentially equimolar quantities also are used.

Given below is a list of suitable complexing agents, and examples of specific complexing agents (chelating agents) .

Amines-ethylamines. Imines (Carboxylate. Phosphonate. Sulphonate) :

Designation

DTPA Diethylene-triamine-pentaacetic acid

EDTA Ethylene-diamine-tetraacetic acid

HEDTA Hydroxy ethylethylene-diamine-triacetic acid

NTA Nitrilo-triacetic acid

DHEG N, N-di (2-hydroxyethyl) glycin

TEA Triethanol amine

NTP Nitrilo-trimethylene phosphonic acid

MIDA N-methylimine-diacetate

IDA Imine diacetate

HEIDA Disodium-hydroxy-ethylimine-diacetate

DTPMPA Diethylene-triamine-pentamethylene- phosphonic acid ("Dequest")

EACDA Ethyl amine-cyclopentene-1-dithiocarboxylic acid

CDTA Cyclohexylene-diamine-triacetic acid

Poly-carboxylates (including polyphosphonate and polysulphonate) :

POC Poly (hydroxycarboxylate) , M up to about

6000

Multivalent carboxylate:

Na-citrate

Gluconic acid lactone - Na-tartrate

Multivalent phosphate:

STPP Na-tripolyphosphate

Remainder:

MTPP Bis-phosphonyl methyl phosphonic acid

- Poly (sodium-α-hydroxy aerylate)

The addition of a complexing agent in alkali form or alkali neutralizes the acidic groups in the wood. The use of such complexing agents also convert the wood acids from an counter-ion form which consists of a com¬ bination of multivalent metal ions and proton form (undisassociated state) to an counter-ion form which is dominated by alkali, preferably sodium. This is neces¬ sary in order to prevent the wood material from becom¬ ing brittle and from having poor swellability when refined, despite the sulphonation. This also avoids, to a large extent, acid hydrolysis in the carbohydrate chains and condensation reactions in the lignin. The refinement of wood material which has been sulphonated in gas phase and which has not been treated with a complexing agent and/or alkali and/or with other equi- valent methods according to the invention (ion exchange) either before or after the sulphonation process results in a very poor pulp quality.

The inventive method can be carried out, for example, such that the refining process will involve coarse disintegration of the wood in the form of chips at high pulp consistency, suitably a consistency of more than 20%, with a low energy input, suitably an input of at most 800 kWh/t wood and with the wood material diluted with water whose temperature corresponds to the soften¬ ing temperature of the lignin, i.e. 40-95°C, at low ion strength, suitably at most 0.05 mol/1, and then beating the wood material to a concentration of 1-10% with a low energy input, preferably an energy input of at most 500 kWh/t.

According to a further embodiment of the invention, sulphonation in gas phase and treatment with complexing agent is carried out prior to treating the wood mate¬ rial in one or more screw presses in which the wood material is subjected to shear forces, or in an ex¬ truder which includes alternate expansion and compres¬ sion zones, wherein subsequent to being pressed or extruded, the pulp is subjected to conventional refine¬ ment process or a conventional beating process at low pulp concentration.

According to our present experience it is preferred to carry out the sulphonation of the chips, optionally after a slight steaming, with sulphur dioxide gas being substantially free from steam. Atmospheric pressure provides excellent sulphonation and requires no speci¬ fic arrangement for providing subpressure or super- pressure. The temperature at the gaseous treatment should normally be below 100°C. A preferred dry solids content of the chips is 50-75 %. It is preferred to use a complexing agent, most preferably NaδDTPA as impregnating agent. This impregnation is preferably preceded by steaming.

Thus, the most preferred embodiment is at present: sulphonating the chips, optionally after slight steaming. The sulphonation is carried out by causing gaseous sulphur dioxide substantially free from steam to flow through a vessel at atmospheric pressure for 4-10 minutes. The temperature at the gas treatment should be below 100°C, preferably in the range of 40- 70°C- The dry solids content of the chips should be 50- 75 %. Before refining the chips are impregnated with the complexing agent NaδDTPA in a charge of 8-12 kg/tonne wood, preferably 10 kg/tonne wood (based on pure NaδDTPA) , this impregnation being preceded by a

steaming at 100-130°C for about three minutes. The chips are heated before refining.

The invention will now be described in more detail with reference to a number of examples. Of these examples, two are concerned with the sulphonation in gas phase of spruce, either before or after impregnating the wood material with a complexing agent, whereafter the wood was refined conventionally to a desired drainability. One example relates to sulphonation in gas phase prior to coarsely refining the wood at high pulp concentra¬ tion; the pulp was then beaten at low pulp concentra¬ tion, whereby the energy consumed was, in total, lower than the energy consumed in conventional refining processes. One example relates to sulphonation in gas phase in pulp shearing and pressing apparatus, a so- called BiVis; whereafter the fibre material can be refined in a conventional high-consistency refiner or can be beaten at low pulp concentration to obtain low energy consumption.

The results obtained from the tests carried out in these examples are set forth in the accompanying draw¬ ings, in which

Figure 1 shows the light-scattering coefficients as a function of the shive content of a pulp where, during the manufacture of the pulp, the wood was sulphonated in gas phase subsequent to being impregnated with a complexing agent; two comparison pulps are also shown in Figure 1;

Figure 2 illustrates the shive content of the same pulps as those of Figure 1 as a function of the amount of chemically-bound sulphur; Figure 3 is a diagramme which shows tensile index as a function of the energy consumed when manufacturing the

pulps of Figure 1;

Figure 4 shows the light-scattering coefficient as a function of the shive content of a pulp in which the wood was sulphonated in gas phase prior to being im- pregnated with a complexing agent; comparison pulps are also shown, analogous with Figure 1;

Figure 5 shows the shive content as a function of the amount of chemically-bound sulphur in the same pulps as those of Figure 4; and Figure 6 shows tensile index as a function of the energy consumed when producing the pulps shown in Figure 4.

In said figures 1-3 x denotes pulp according to the invention having 50 % TH during the gas phase sulphonation -{ denotes pulp according to the in¬ vention having 70 % TH during the gas phase sulphonation

In said figures 4-6 E denotes pulp according to the invention having 50 % TH during the gas phase sulphonation o denotes pulp according to the in¬ vention having 70 % TH during the gas phase sulphonation

In all said figures denotes said WTMP reference D denotes said CTMP reference

The pulps shown in figures 1, 2, 3 and 5 all required an energy consumption of 2200-2300 KWh/tonne.

EXAMPLE 1

A TMP-process which included a water-impregnation step

prior to pre-heating (= so-called WTMP, which has a slightly lower energy consumption than TMP in which water-impregnation is not included) was modified such that the chips were impregnated with complexing agent (NaDTPA-solution) subsequent to the steaming stage. The chips were then sulphonated in gas phase, by causing SO_ (gas) to flow through a reactor vessel containing said chips, for 10 minutes at about 70°C. Subsequent to treating the chips with S0 ₂ , the chips were impregnated with water and heated to a temperature of 130°C. over 3 minutes, whereafter the chips were refined under pressure in the same manner as when producing pulp in accordance with the TMP-process.

It will be seen from Figure 1, which shows the light- scattering coefficient (s) as a function of the shive content, that sulphonation in gas phase in accordance with this example, produces a pulp whose shive content is equally as low as that of a comparison pulp (CTMP) , while the light-scattering coefficient of the pulp is, at the same time, equally as high as that of another pulp (WTMP). It will be seen from Figure 2, which shows the shive content as a function of the degree of sul¬ phonation (given as chemically-bound sulphur) , that the low shive content has been obtained in spite of the fact that the degree of sulphonation of the pulp sul¬ phonated in gas phase is considerably lower than in the case of the comparison pulp produced by the CTMP-pro- cess. The energy consumed when producing pulp which has been sulphonated in gas phase is approximately the same as the energy consumed when producing the comparison pulp by the WTMP-process, when sulphonation in gas phase has been carried out at normal chip dryness, and is approximately the same for the comparison pulp produced by the CTMP-process when sulphonation in gas phase has been carried out at a slightly higher dry

solids content (TH) of the chips, as can be seen from Figure 3.

EXAMPLE 2

A TMP-process was modified such that the chips were sulphonated in gas phase with SO_ in the same manner as in Example 1, although in this case the chips were treated prior to the steaming stage. Subsequent to being steamed, the chips were impregnated with NaDTPA- solution, whereafter the chips were pre-heated for 3 minutes at 130°C and then refined under pressure in the same manner as when producing TMP.

Figure 4 shows the light-scattering coefficient (s) as a function of the shive content. It will be seen from this Figure that sulphonation in gas phase provides a pulp whose shive content is equally as low as that of a reference CTMP-pulp and, at the same time, a light- scattering coefficient which is equally as high as that of a reference WTMP-pulp. Figure 5 shows the shive con¬ tent as a function of the degree of sulphonation. It will be seen from this Figure that the low shive con¬ tent has been achieved in spite of the fact that the degree of sulphonation of the pulp that was sulphonated in gas phase during its manufacture is much lower than the shive content of the reference pulp CTMP. The energy consumed when producing pulp which has been sulphonated in gas phase during its process of manufac- ture is approximately the same as the energy consumed for WTMP and somewhat greater than for CTMP. This is evident from Figure 6, which is analogous with Figure 3.

EXAMPLE 3

A TMP-process was modified such that the chips were sulphonated in gas phase, as in Example 2. The chips were then, however, refined coarsely in a refiner under pressure with low energy input, whereafter the pulp was beaten at low pulp concentration (a concentration range of 1-10%) . The energy consumed was much less than the energy consumed by a conventional refining process. The

physical properties of the pulps obtained being com- parable, however.

EXAMPLE 4

Mechanical pulp was produced by sulphonating chips in gas phase in apparatus which, simultaneously with the chemical reaction taking place, presses and shears the chips into free fibres and shives, a so-called BiVis. In this case, it was possible to introduce gaseous SO _? immediately before or immediately after adding NaDTPA or alkali in combination with complexing agent to the chips contained in the apparatus. Subsequent to leaving the apparatus (BiVis) , it was possible to refine the fibre material in a conventional refiner under pressure or, alternatively, to beat the fibre material at low pulp concentration, in order to obtain low energy consumption.