Lignocellulosic material means in this connection all material of plant origin used as raw material in a chemical pulp cook, such as wood, straw, grass, bagasse, etc.

The lignocellulosic material used in the method is, when needed, in a form cut in a conventional manner for delignification, for example, wood material as chips or sawdust.

In this connection, pulping refers to the pulping of lignocellulosic material, which includes some degree of delignification of the lignocellulosic material which takes place by means of chemicals and heat. The delignification accomplished with chemicals and heat may be supplemented with mechanical defibring to achieve the desired degree of defibring.

The treatment of lignocellulosic material in a superheated steam atmosphere included in the invention can also be applied to the loosening up of the structure of lignocellulosic material for mechanical pulping. A similar procedure has been used in state-of-the-art applications, for example, in the so-called TMP process, but under saturated steam treatment conditions. If, on the other hand, steaming is carried out by using superheated steam, there arises in the fibre material a situation in which the moisture within it tends to vaporise and force its way out of the fibre because of the drying effect produced by the superheated steaming conditions. As a result of this treatment, the lignin contained in the fibre softens and, at the same time, the internal structure of the fibre opens and loosens up, which in itself is helpful to the subsequent mechanical pulping stage. After the

treatment, within the fibre (if it has not been overdried) there prevails a situation where there is steam in a saturated state within the fibre. If the fibre in this state is brought under conditions in which steam is condensed, and in addition relatively quickly, this gives rise to a collapse phenomenon in the fibre structure, which further loosens up the structure. This assists defibring to a substantial extent in subsequent mechanical pulping. The defibring process requires less energy, and harmful breakages are reduced.

By delignifying chemicals are meant in this connection all chemicals commonly used for the purpose in question, which chemicals are absorbed into lignocellulosic material and which dissolve or soften the lignin contained in the material and by which delignification is accomplished at a temperature higher than ambient temperature. As examples, sulphate and sulphite pulp cooking liquors may be mentioned.

The reactions of the lignin contained in fibre material with dissolving and/or softening chemicals take place in the desired manner only if the chemicals have penetrated uniformly and thoroughly into the material which is being pulped. The absorption of chemicals is influenced by two mechanisms, penetration and diffusion. In natural penetration, cooking chemical liquor penetrates into fibre material through its capillary structure by the action of capillary forces. The penetration process may be assisted by providing a pressure difference across the capillary structure.

The speed and thorough accomplishment of the penetration process is greatly deendent on air which is contained in fibre material and which should be removed before treating the material with delignifying chemicals. One method used is the deaeration of fibre material by steaming. During steaming, the air contained in fibre material expands through heating, and partly escapes from the pore spaces of fibre material. The steam used in the treatment penetrates, in turn,

into the pore spaces of fibre material where it contributes to the exit of air from the pores of fibre material through condensation and vaporisation processes.

Prior art steaming processes have used saturated steam. The upper limit of the temperature of steam has been considered to be 120-126 °C and, at temperatures higher than this, the hydrolysis of fibre materials has been considered to accelerate to a detrimental extent. The condensation of lignin at higher temperatures has also been considered to be a problem.

Steaming of lignocellulose containing fibre material in accordance with the prior art with saturated steam inevitably leads to an increase in the moisture content of the fibre material, because substantially the entire steam amount used in steaming condenses on the fibre material and penetrates into its structures. The condensation energy of steam correspondingly increases the temperature of the fibre material that is being treated. On the other hand, the steam condensing on the surface layers of the fibre material prevents escape of air.

In accordance with the basic idea of the invention, steaming of lignocellulose based fibre material is carried out in a steam atmosphere under superheated conditions, and the lignocellulosic material is maintained substantially at the steaming pressure up to at least an impregnation stage. The steaming is advantageously accomplished by passing superheated steam into contact with the fibre material. Also other means of introducing energy, such as indirect heating or an inert energy carrier (such as fluidised bed material), are feasible enabling a steam atmosphere under superheated conditions to be maintained during steaming.

When using superheated steam passed to the fibre material, the inlet and the final temperatures of steam are determined according to the material of the fibre material being processed. In treatment tests of bio-mass, the temperature of incoming steam has varied in a range of from 270 °C to 350 °C and the temperature of exiting steam has been determined according to saturated steam

pressure corresponding to operating pressure (max. 23 bar (abs) ) such that its superheating has been by about 15 °C-25 °C higher than the saturated state.

Superheated steam has a strong effect on the fibre material to be treated. Firstly, superheated steam binds into itself water from the fibre material and, secondly, causes an increase in the temperature of the fibre material. The water present in the fibre material vaporises, passes to the surface of the material and exits with superheated steam out of connection with the fibre material. At the same time, vaporising water drives the air present in the fibre structure out of the fibre. An essential factor is also that the vaporising water most evidently opens the capillary and pore structure of the fibre.

From the viewpoint of carrying out of the invention it is essential that the fibre is maintained under conditions where this open space of capillaries and pores is preserved as far as possible into the absorption process of delignifying chemicals.

In this respect, the residual moisture of fibre material and the ambient atmosphere are crucial.

Attempts are made to maintain the fibre material in such a state that there is residual moisture at least in a vapour state in its structure. The best situation is achieved if there is moisture in such an amount and in such a state that it still tends to vaporise and force its way out of the structure. This assures that the pore structure remains open. On the other hand, the moisture vaporising in the fibre structure binds heat, which prevents the temperature of the fibre material from rising so high that detrimental degradation and condensation reactions of the fibre material are initiated. In respect of residual moisture, the crucial factor is the time during which the fibre material is in contact with superheated steam, in addition to the superheating degree of the ambient steam atmosphere. The state of the ambient atmosphere is in turn determined by the pressure prevailing in it.

When fibre material which has been steamed and the pore structure of which is in an open state (containing vaporised moisture and being at least at a pressure of saturated steam corresponding to ambient temperature) is brought into contact with a solution containing delignifying chemicals, the solution will penetrate into the fibre structure replacing the moisture which has escaped from there. The penetration of the solution into the fibre structure can be promoted by creating a situation where the moisture present in the fibre structure is condensed, whereby a corresponding underpressure is produced between the pores of the fibre structure and the environment. This situation can be created, for example, by using a delignifying chemical solution that is slightly colder than the saturated state of the fibre structure.

Said arrangement has been found to allow rapid absorption of delignifying solutions uniformly through the entire fibre material. The arrangement also makes it possible that the impregnated fibre material is substantially at delignification temperature after impregnation. The detrimental reactions of fibre raw material and delignifying chemicals associated with slow absorption and with slow heating to delignification temperature can thus be eliminated.

In the steaming of lignocellulosic material accomplished by superheated steam, the contact time between the material and steam is substantially short, of course, depending on the particle size of the material. For example, in the case of fine wood raw material (classified with the definition"sawdust", the particle size being about 3-5 mm) an appropriate dwell time is of the order of seconds.

The steaming stage of the invention can be accomplished in a processing apparatus of the so-called flash type in which fine lignocellulosic material is supplied to be carried with a superheated steam flow, and is separated from this flow after an appropriate dwell time. The apparatus is illustrated in the accompanying drawing, in which

Figure 1 shows one embodiment of the steaming and impregnation apparatus in accordance with the invention, and Figure 2 shows an apparatus used for a similar purpose, as an alternative embodiment in respect of the impregnation apparatus.

In the embodiments shown in the drawing figures the steaming apparatus comprises a steam flow circulation 1 which includes a fan 3 maintaining the circulation, a superheating device 2 (boiler), a steaming stage 4 situated after the superheating device and having at its lower end feed devices 5 for feeding lignocellulosic material, a steam separator 6, excess steam exhaust 7, as well as a seal type feeder 8 for lignocellulosic material. When needed, the apparatus also includes a return circulation 10 for steamed lignocellulosic material.

In the embodiment shown in Fig. 1, the steamed lignocellulosic material is shown to be fed through the seal type feeder 8 into an impregnation device 11, into which a solution containing delignifying chemicals is fed from a tank 9. The impregnation device may have a structure known in the art. Inside it there is usually a screw feeder with a gentle pitch, which conveys lignocellulose containing fibre material through the impregnation device along an obliquely upwards rising path. In the impregnation device, the delignifying solution is usually maintained at a level that extends to about half the length of the impregnation device. Impregnation takes place in the lower part of the device and discharge of excess solution takes place in the upper part by draining. The excess solution can be passed into the solution tank 9, possibly monitored in respect of the chemical content.

At the discharge end of the impregnation device 11 there is advantageously also a seal type feeder 12, by means of which an independently adjustable pressure can be maintained in the impregnation device 11. The lignocellulosic fibre material is fed from the seal type feeder 12 to a delignification vessel 13 proper, in which

delignification reactions are performed to the desired delignification degree. In the delignification vessel 13, a suitable pressure and a corresponding temperature of saturated steam are maintained at which delignification reactions have been found to occur most advantageously from the point of view of fibre material.

Temperature can be maintained, for example, in a knows manner by means of a liquid circulation provided with heating 15.

The impregnation after steaming can be alternatively accomplished by operating as shown in Fig. 2. In this embodiment, a press can serve as the seal type feeder 8.

A solution containing delignifying chemicals can be fed into the fibre material in connection with the press and excess solution can be squeezed out in the press.

After that, the impregnated fibre material is passed to the delignification vessel 13, in which the fibre material is/is maintained at delignification temperature in the ambient steam phase. The temperature is maintained by passing direct steam into the vessel from a conduit 16. Alternatively, the temperature can be maintained by indirect heat transfer by means of steam passed from the conduit 16. This apparatus arrangement allows the liquid volume of the process to be minimised.

The fibre material is passed from the delignification vessel 13 through a seal type feeder 14 to possible mechanical defibring, and further to conventional washing and bleaching stages.

The arrangements shown in Figs. 1 and 2 make it possible to use different conditions in different process stages. From the point of view of the total flow of the process it is advantageous that the fibre material is maintained throughout the entire process substantially at a pressure that corresponds to the pressure of the saturated steam of the temperature used in the delignification stage. As a variant of this operation model, it is also possible to operate so that the pressure drops slightly during transition from steaming to impregnation and further during transition from impregnation to the delignification stage. Also other alternatives

of pressure and corresponding temperature conditions are feasible when carrying out the invention taking into account the fibre material (the source of material as such, the particle size, moisture content), the chemicals used and the desired delignification degree.

The moisture escaping from the lignocellulosic material which is in contact with superheated steam in the steaming stage 4 is carried with steam into the separator device 6, from which excess circulation steam is discharged through the conduit 7.

The energy level of this steam is high, and it can be utilised in later stages of the pulping process, for example, in heating delignifying chemicals, in regeneration stages of the chemicals circulation, in bleaching, in washing, etc. Control of the exhaust steam 7 can also be used for regulating the heating stage 2 of the superheating circulation so that there should prevail an inert gas state in the circulation 1.