Specifically, the invention relates to a novel method for organizing in a sequence biological, chemical and physical processes which are in part known in the art.

The invention will be named hereafter as Anaox process which is a short form for"ANAEROBIC"and wet"OXIDATION"the two main technologies on which the present invention is based.

BACKGROUND ART The world paper production should be duplicated only considering the increase in China and in the South Eastern Asia of the current yearly pro-capita consumption from 30 kg to 200 kg of the European Community.

Due to the lack of wood and of areas suitable for fast growing plantations, in the mentioned Countries the recourse to agricultural residues and annual fiber is a must.

The high specific volume of these wood substituting materials and the related difficulties in harvesting, storing and transporting represent a limit to the mill size.

Moreover, the environmental impact, particularly dramatic in pulping annual fiber because of their chemical composition and the high NPE (non prosecutable element) content, should be minimized for a future mill.

It has been evaluated that in the next ten years, 2000 mills of the average allowed capacity of 80 100 Vd should be installed to cope with the increase in consumption.

The invention intends to provide the pulp and paper industry with a novel method for the setup of a pulp mill of a small and medium capacity in order to process economically and with an environmental acceptable impact fast growing cellulosic materials which are required to support the fast growing demand of paper in the whole area lacking of forest and trees plantations.

The pulping technology to separate the cellulose from other organic compounds makes use of an association of chemicals which most frequently is formed by NaOH + Na2S which characterizes the kraft process.

Sulphite and disulphite characterizing the acid process are used in a minor degree.

For open structured raw material like straw and annual fiber the soda process, associated with anthraquinone or with 02 as in the most advanced technologies, is used.

The kraft and soda process produce pulp which requires long bleaching sequences based, in the past, on chlorine and chlorine compounds which, because of the high toxicity of the resulting organic chlorinated material have been substituted by 02, 03 and H202 in a multistage treatment.

The NaOH + 02 process according to the present invention which is used for the open structured materials as agricultural residues and annual fiber, produces easily bleachable pulps using chlorine free sequences. For this mill the delignification process should be completed by a weak black liquor (W. B. L. ) recovery which should adapt to the peculiarities of a low heat value and high content of NPE frequently containing high percentage of Si02 and Cl-.

One of the objects of the present invention is to provide a recovery and recycling process for chemicals, energy and water from the W. B. L. and bleaching effluent of a NaOH + 02 chlorine free process, in order to install profitable and environmental acceptable mills also of small and medium capacities for pulping agricultural residues, annual fiber and fast growing cellulosic materials.

A second object of the present invention is to minimize the pollution caused by the kappa reduction and bleaching sequence of the standard size wood based mills, minimizing the difficulties in water

recycling which relate to the presence of organic salts (oxalate, acetate, etc.).

A further object of the present invention is to facilitate the effluent treatment and water recycling in the high yield pulping processes (TMP, CTMP, semi-chemical and secondary fiber pulp) in which the organic materials in the effluent are present in a low concentration which cannot justify investment in conventional recovery.

DISCLOSURE OF INVENTION The process according to the present invention is characterized in that it comprises the steps of: a) causing the black liquor resulting from a delignification process to undergo one or more anaerobic treatments in order to produce biogas from the organic substance to be used as an energy source and to convert, by such methanation, the sodium salts of the organic acids into Na2CO3, b) evaporating the liquor resulting from the anaerobic treatment in order to recover water condensate and concentrate the liquor, c) causing the concentrated black liquor to undergo an oxidation treatment in order to destroy the organic compounds with C02 production and recover chemicals, heat and water, d) recovering the alkaline sodium salts by means of different solubility, e) causing the separated alkalis to undergo a decarbonation in order to transform them in carbonates ; and

recausticizing part of carbonates in sodium hydroxide, a reactant contained in the liquor of the delignification.

According to a feature of the present invention, the concentrated effluent from the evaporation of the water recovery system from the kappa reduction and bleaching stage is added to the W. B. L. of the delignification step.

According to another feature of the present invention, the biogas from the anaerobic treatment is conveyed to the power generation plant to cover the process power requirement.

According to another feature of the present invention, the condensate from step b) of the process-clean water-is recycled to the process.

Further features of the present invention are the following : i) the C02 developed in the wet oxidation step and in the decarbonation step is recycled to adjust, specifically lower, the pH of the W. B. L. to the anaerobic treatment and the water recovered from the wet oxidation is recycled to the pulping line, ii) the NaOH from recausticizing, in the ratio required by the process, is recycled to the delignification step.

BRIEF DESCRIPTION OF DRAWING The invention will be better described in the following referring to the attached block diagram in which the interconnections and interactions are shown.

In the diagram the arrows are used to indicate the flow directions, while the nature of the substances and their recycling destinations are identified by: RC when it relates to the different forms of energy RA when it relates to water or clear condensate RR when it relates to chemicals, regardless to their gaseous, solution or solid form.

BEST MODE FOR CARRYING OUT THE INVENTION The invention in its complete structure is shown in the block diagram of Fig. 1 which is divided in sections from 1/a to 1/e; each section relating to the single process and treatment steps, their interconnection and their sequence in the process.

Section 1/a shows the pre-treatment and delignification step which in different form are part of any conventional pulping process. In these steps the non cellulosic components of the raw material (lignin, hemicellulose, inorganic components) are partially separated from the cellulose producing pulp with yields (ratio between pulp and raw material on BD basis) in a range from 85-90% to 38-40%.

The higher yield characterizes pulp like GW, TMP, CTMP mainly used for newsprint production, while the pulps with lower yield are

bleached pulp for printing and writing paper, or more sophisticated paper as security banknotes.

The lowest yield pulps are classified as"dissolving pulp"which by different physical and chemical processes (acetilation, nitration, viscose) produces chemicals, films and yarns for the textile industry.

The non cellulose components in the raw material are dissolved in the W. B. L. which will have different concentration according to the different process conditions and yields. These liquors are normally concentrated in strong black liquor for their consequent processing.

In the drawing : in the section 1/a an agricultural residue (block 10) is pretreated with water and weak caustic liquor (block 11) to minimize its Si02 and Cl-contamination before being delignified (block 12).

In the alkali/02 process the alkali is mainly sodium carbonate recovered and recycled (line L1) from the decarbonation step. The loss of chemicals in the recovery cycle is integrated by NaOH make up (line L2).

In this preferred but not exclusive embodiment of the process according to the invention, the process is optimised by the choice of a proper ratio NaOH/Na2CO3 to guarantee the OH ion concentration required to catalyse the lignin decomposition.

The alkali is the main agent in lignin solubilization, while the 02 causes the : - demolition of part of the lignin chain producing fragments of low molecular weight organic acids and corresponding sodium salts (sodium acetate, sodium formate, sodium oxalate, etc.),

- lignin decolourization by reacting with the chromophore groups in a reaction similar to the peroxide reaction.

As a consequence of these reactions, pulps are less dark and more easily bleachable than the kraft and soda pulps, by means of only one pressurised peroxide step, strengthened by the presence of 02, a 80-83 ISO bleaching grade can be reached.

The use of 02 in the delignification step or in the following kappa reduction and bleaching step makes part of the lignin biodegradable due to the chain demolition and low molecular weight acid formation.

(Kappa index indicates residual lignin in a delignified pulp).

The modification of the lignin in biodegradable substance is relevant for the total process efficiency. Specifically, the efficiency of the anaerobic treatment of the W. B. L. from the alkali/02 delignification and from kappa reduction becomes significantly higher than that obtained processing kraft/soda liquors.

The unbleached pulp from the delignification step is separated from the liquor by filtration and washed in a multistep brown stock washing system (block 13) and then transferred (line L3) to the kappa reduction and bleaching step (block 14 and line L14). From the effluents of the bleaching step the water is recovered by evaporation (block 15) and recycled to the process (line L11).

In the section 1/b the weak liquor from delignification step (block 16) and the concentrated effluent from the kappa reduction and bleaching step based on 02, Os in a mild alkali medium, are fed to the anaerobic step.

The energy for the anaerobic micro organisms is supplied by the organic substance degradation in reduced atmosphere free from 02.

In this step the biogas (prevailing CH4 + CO2) is produced by the modification of biodegradable liquor components.

The W. B. L. from the delignification step, which has a high organic content normally evaluated as"chemical oxygen demand"COD, in one of the preferred but not exclusive application of the invention, is mixed (line L4) with the concentrated residues from the evaporation of the kappa reduction and bleaching effluent, having lower COD.

The ratio of this mixture is adjusted to obtain COD, temperature and pH of the mixed effluent to guarantee the best efficiency of the anaerobic step.

The final adjustment of the temperature in the range of 37-38°C (block 17) is obtained by thermal exchange with the fresh water for cycle integration, while the pH fine adjustment to 7 8 (block 18) is obtained by recycled C02 (line L5) from wet oxidation step (section 1/d).

The anaerobic treatment is frequently used as the first step of an effluent treatment because of its capability to transform 60-70% of organic components into biogas with minimum of bio sludges production (line L6).

The anaerobic treatment (section 1/b block 19) is of decisive importance in the process according to the present invention because, together with the above mentioned effect, it has the capability to convert, as an additional result of methanation, organic

sodium salts (acetate, oxalate, formate, etc. ) into inorganic sodium carbonates.

Sodium carbonate and sodium hydroxide are the alkali for the solubilization of the non cellulosic components, in combination with 02 in the most advanced technologies.

The conversion of the mentioned sodium organic salts into sodium bicarbonates/carbonates, taking place at 37-38°C in the anaerobic treatment of the present invention, requires instead temperatures around 1000°C obtained by the combustion of the organic components of the strong black liquor in the conventional technologies for chemicals and heat recovery.

At the temperature of 300-320°C, which is the maximum temperature reached in the wet oxidation step (section 1/d in the block diagram fig. 1) of the process according to the present invention, the conversion from organic into inorganic salts will not proceed.

Consequently, sodium acetate, oxalate, formate, etc. will be present in a mixture with sodium carbonate if not converted by the anaerobic treatment.

The similar solubilities of all the mentioned salts in the mixture, will not allow the separation of sodium bicarbonate/carbonate which are the salts to be recycled to the delignification step.

As said, the degradation, due to the 02, of part of the lignin present in the black liquor in biodegradable compound is essential for optimising the efficiency of the methanation (COD reduction) in the anaerobic step which may reach 80-85% compared to 60-70% characterizing

the treatment of the liquor (from soda and kraft process) in which the lignin is not partially degraded.

The biogas production (energy recovery) is also improved, since it is proportional to the COD reduction.

About 400 Nmc of biogas are originated from each ton of destroyed COD independently from the technology used for the anaerobic treatment.

The high efficiency in anaerobic treatment, which characterizes the invention, can be seen in that the heat recovery in biogas is characterized by a content of 65-70% of methane.

In one of the preferred applications of the invention, the biogas is converted into electric power (line L7) by a special diesel generator designed for this type of gas (block 20).

The heat balance of the process of the invention, taking into account the combination of: - the high efficiency of anaerobic treatment and biogas production, - the wet oxidation, which completes the destruction of the organic compound operating at maximum liquor concentration of 12. 15% TDS (with a corresponding ODS concentration of 3=4%), - the recycling of water by the evaporation of W. B. L. from the anaerobic and from the bleaching step, should be positively compared with the heat balance of the conventional more efficient processes which are negatively affected

by the higher power and heat demand to evaporate the black liquor at the concentration of 60 65% for their combustion in a standard recovery boiler.

The section 1/c relates to the evaporation (block 21) of the weak liquor with reduced ODS after the anaerobic step (block 19) in order to enter the proceeding wet oxidation process (section 1/d).

For a better understanding of the invention process concerning the evaporation step, the following exemplification is provided.

A weak black liquor from the delignification (block 12) with 10% TDS will be reduced to 8% TDS by mixing with the concentrated effluent from the kappa reduction and bleaching step (block 14) in a ratio of 50% organic/TDS substance.

The anaerobic treatment, transforming in biogas the 85% of the organic substance, will reduce the 4% ODS of the incoming W. B. L. to 0,6% ODS in the liquor after the anaerobic step.

The evaporation step (block 21) of the liquor with the 0,6% ODS content is intended to increase the ODS% to the minimum organic content 2, 5-3% for self supporting the oxidative reaction.

The low concentration level of the liquor in the described process will guarantee a condensate from the evaporation step with a negligible contamination by organic substances, much lower than that of the condensate of the conventional recovery process which requires a liquor concentration up to 60-65%.

The cleanness of the condensate from the evaporation, which is another positive feature of the invention, allows the condensate to be

recycled (line L8) in all the steps of delignification and bleaching processes.

In addition, the possibility to operate the evaporation plant to a much lower liquor concentration practically eliminates the well known difficulties due to scaling in the multistage evaporators and the difficulties due to high viscosity which negatively affects the pumping and micronizing of the liquor in the recovery boiler.

The section 1/d relates to the wet oxidation step (block 22).

The liquor at 12-15% TDS is fed to the reactor together with the oxygen in a ratio 1 : 1 to the liquor COD. Up to 99, 5% of the incoming organic substances is oxidised (burnt) in this process step.

The liquor from the evaporation with a content of 2,5% of organic substance and about 12,5% of inorganic salts will be pre-heated at 270-280°C by the heat from the oxidative reaction step (block 22) and then injected in the reactor operating at a pressure of 100=110 bar in which the 02 is simultaneously added.

The heat from the oxidative reaction is in excess over the demand for the liquor preheating and for reaching and maintaining the process temperature at 320°C.

To control the temperature of the reaction, the mentioned excess of heat is used to produce steam at a pressure of 10 bar and recycled to the process (line L9).

The reduction (80-85%) of the COD in the W. B. L. by the anaerobic treatment reduces the 02 required by the wet oxidation to only

15v20% of the 02 consumption to oxidize the total ODS of the W. B. L. from the delignification step.

Particularly in a mill based on advanced technology for pulping agricultural residues and annual fibers, the 02 required for the delignification step (block 12), for the kappa reduction and bleaching step (block 14) and for wet oxidation step of the recovery system (block 22) can be economically produced in the pulp mill by means of the molecular sieve technology.

It should be emphasized that a pulp mill based on alkali/02 delignification may depend for its chemicals from the external sources only for the alkali make-up.

This feature, together with the minimised fresh water requirement by the recycling (line L8,10, 11) of condensate from evaporation step (block 21), significantly improves the possibility to locate the mill where the non wood materials can be economically concentrated.

In section 1/e, conventional processes and operations, which are not essential for the invention but which may be an aid to its industrial application, are included.

A solution of salts concentrated at 20 25% will be produced in the wet oxidation step (block 22).

The salts, due to the conversion operated by the anaerobic treatment, will be substantially sodium bicarbonates which may be contaminated by a low percentage of NPE mainly sodium chloride (Cl- ) and silica (Si02) present in many of the agricultural residues.

Most of the contaminants are washed out in the wet pretreatment system which is normally included in the design of a new mill.

In the case of wheat straw, the amount of 6-10 g of Cl-and 4 6 9 of Si02 per kg BD of straw will be respectively reduced by 90% and 50 60% in the wet pretreatment.

The residual quantity will be present in the weak black liquor from delignification and will be transferred in the bicarbonate/carbonate recovered solution.

Their building up over the amount which can be tolerated by the process can only be controlled by taking care of their excess.

The different solubilities of the contaminating salts (section 1/e) compared with that of sodium bicarbonate gives the possibility (block 23) for evacuating the Cl-with the"mother waters" (bittern) (line L13).

The bicarbonate, practically free from Cl-, is recovered as carbonate by decarbonation with C02 release (block 24).

The silica (Si02), which is solubilized in the high pH condition of the alkali delignification, precipitates (line L12) due to the reduction of the pH below 10,2 produced by the organic acids from the 02 degradation of the hemicellulose and the lignin, followed by the pH adjustment to 7. 8 for the anaerobic treatment obtained by the C02 recycled from the wet oxidation step (line L5).

In annual fibers and agricultural residues pulping, based on alkali/02 delignification, the sodium salts (block 22) can be recycled as sodium carbonate (line L1), after the separation from the contaminants (block 23).

Its recausticizing (block 25) may be limited to the only quantity which should be recycled (line L2) as hydroxide for kappa reduction and bleaching step (block 14).

The invention advantages can be summarised in the following : 1) energy recovery by the bio-reduction and combustion of the organic matter in effluents ; 2) recovery and recycling of the chemicals and of the process water ; 3) the to-day more restrictive environmental regulations relating to air, water and solid pollution limits can be satisfied; 4) small and medium capacity pulp mills for the economic utilization of the annual plants, agricultural residues and short cycle trees plantation can be installed ; 5) the mills of limited capacities can be more easily positioned on the territory facilitating the harvesting collection and transport of the agricultural residues and annual plant characterised by low bulk weight ; 6) the construction of new mills and updating of existing mills, even of small capacity, on the line of the"close loop mill" concept.