ITMI20012119A1 - PROCEDURE FOR THE RECOVERY AND RETURNING OF THE COMPOUNDS CONTAINED IN THE EFFLUENTS OF THE PROCESSES OF DEGLIGNIFICATION AND IMPIANTING OF IMP - Google Patents

Description

DESCRIPTION

The present invention refers to a new concept process for carrying out the recovery and recycling of the substances contained in the effluents of the delignification and bleaching processes of cellulose production plants, for the recovery of energy and chemical reactants as well as of the water through a treatment or rather a sequence of biological, chemical and physical treatments, partly known individually, but which are associated and organized in a new and original way.

For simplicity and repetition of definition, the process according to the invention is referred to here and hereafter with the expression "ANAOX process" as this expression is nothing more than the simplification by abbreviation and merger of the terms ANAEROBIO wet OXIDATION which represent the two technologies mainly used for carrying out the procedure.

The inventive idea arose from the fact that cellulose is essentially used to meet the needs of the paper industry and from the observation that world paper production would have to be doubled in order to bring the current annual per capita consumption of 30 kg to China and beyond. East and Southeast Asia to 200 kg of the European Community countries. Since the availability of wood in these areas is minimal, the only way to reach this goal would lie in the use of cellulosic raw materials, such as agricultural residues and annual plants, to replace wood. The inventive process would allow the construction of small and medium capacity plants in these areas, such as those required to cope with the high specific volume and related difficulties in collecting, storing and transporting the aforementioned wood replacement materials. As an example, it can be estimated that in the next 10 years, in order to cope with these increases in consumption, at least 2000 factories with a capacity of 80-100 tons / day should be built.

Another factor that led to the inventive idea is represented by the increasingly restrictive ecological regulations concerning both the gaseous and liquid and solid effluents of the cellulose production plants, which still represent one of the major sources of pollution.

As is known, cellulose production plants are used to extract cellulose from wood and isolate it from all those substances that accompany it. From a strictly technical point of view, this is not difficult because cellulose itself is relatively inert to the chemical attack of solvents, while the other substances present in the wood are soluble, in suitable conditions, and therefore easy to remove.

The delignification of raw materials can be obtained with the use of different chemical reagents and / or with the association of them.

The most widely used combination of reactants is that of the sulphate process (NaOH Na2S) which characterizes the kraft process; to a lesser extent sulphites and bisulfites of soluble and insoluble bases are used which characterize the acid processes. For plants with a more open structure, NaOH is associated with anthraquinone and, in the most advanced technologies, with O2 (see known procedure 3).

The kraft and soda processes give rise to crude cellulose that require complex and long bleaching sequences, in the past made with the use of elemental chlorine, chlorine dioxide and hypochlorites, today no longer usable due to the toxicity of the organic chlorinated products. The alternative sequences in use today consist of multistage treatments based on the use of O2, O3 and H2O2.

When the structure and chemical characteristics of the raw material allow it (agricultural residues or annual plants or fast-growing woods), the most advanced technology is that which sees soda associated with oxygen and which gives rise to pastes that can be more easily whitened with sequences free from chlorine.

It is therefore evident that the processes that resort to the use of O2 and its derivatives are of primary importance both for the total delignification of raw materials with a more open structure (agricultural residues and annual plants) with easier access to liquid and gaseous reagents, and to complete the delignification of raw wood cellulose obtained with the above-mentioned procedures. These procedures have highlighted the need to be completed, in the context of the economy of the activity and respecting the environment, with the most advanced systems for recovering the reactants and the energy inherent in the organic substance solubilized in black lyes, associated to the substantial reduction of water consumption.

Therefore, an object of the present invention is to carry out a process of recovery and recycle of the substances contained in the effluents of the delignification and bleaching processes of cellulose production plants that allows to make plants, even small ones, ecologically and economically acceptable. for the production of cellulose and paper pulp from agricultural residues, annual plants and fast-growing tree essences, in particular in the case of plants in which the delignification is substantially entrusted to alkali / oxygen processes and the like.

Another object of the present invention is to provide a process of the aforesaid type capable of facilitating the solution of the pollution problem even of the largest plants commercially existing today, connected to the treatments based on O2, O3 and H2O2 used for the partial delignification downstream of the main delignification.

Yet another object of the present invention is to provide a process of the aforesaid type capable of solving the ecological problems that characterize plants for the production of high and very high yield paper pulps (TMP CTMP), semi-chemical pulps, etc. , which originate effluents where the organic substance deriving from the partial dissolution of the cellulosic materials and the chemical reactants used are present in very low concentrations.

More specifically, the process according to the invention is characterized by the fact that it consists of:

a) in subjecting the black liquor coming out of the delignification process to one or more anaerobic treatments to produce biogas from the organic substance as an energy source and to convert, through this methanization, the sodium salts of organic acids into Na2CO3 and, when the circumstance occurs, to reduce the amount of sludge in subsequent aerobic treatments;

b) concentrating the leachate coming out of the anaerobic treatments in order to recover the water and concentrate it;

c) in subjecting the concentrated liquor to an oxidation treatment to obtain the destruction by combustion of the organic components with recovery of the reactive, production of CO2 and recovery of water;

d) recovering the alkalis by separating their salts by difference in solubility;

e) subjecting the separated alkalis to a decarbonation step to transform them into carbonates; And

f) in deriving a part of the carbonates in caustic soda, that is in a reactive contained in the delignification liquor.

According to a feature of the present invention, in the case of the production of bleached cellulose, the evaporation concentrate of the system for the recovery of water from the reduction of the kappa index and bleaching phase is added to the liquor leaving the delignification phase.

According to another feature of the present invention, the biogas obtained in the anaerobic treatment phase is sent to energy production plants to be used for conducting the process.

According to another feature of the present invention, the condensate coming out of step b) consists of water which is recovered and reintroduced into the process cycle.

According to another feature of the present invention, the CO2 formed in the oxidation treatment and partly in the alkali decarbonation phase is reused in the anaerobic treatment and the water coming out of the oxidation phase is recovered and reintroduced into the process cycle.

According to yet another feature of the present invention, the NaOH coming out of the recalculation phase f) is sent back to the delignification treatment.

The present invention will now be described in more detail in relation to the attached drawing which shows a block diagram of the process according to the present invention and in which the directions of the flows are indicated by arrows, while the species and destination of the recoveries are marked with:

RC when referring to energies in different forms, RA when referring to water and evaporated similar to it,

RR when referring to chemical reactants whether they are in the form of solution or gas.

The overall scheme of the invention process represented in Figure 1 is divided into boxes 1 / a - 1 / e which include and highlight each of the treatments, their application sequence and specific function within the scope of the invention.

Box 1 / a includes the phases of pre-treatment of materials and delignification common to each process for the production of cellulose and / or pulp, in which the components of the raw material: lignin, hemicellulose, inorganic materials, etc., are separated from the cellulose, and give rise, with different yields varying between 85-90% and 38-40%, to pulp and cellulose.

The higher yields correspond to the pastes of the GW, TMP, CTMP type, intended for example for the production of newsprint, while the lower ones correspond to the bleached cellulose, to produce paper, for example for printing and writing, and, to the yields lower, the cellulose from dissolution which, through subsequent chemical-physical modifications (acetylation, nitration, viscose, etc.), give origin to chemical products and films or yarns for the textile industry.

The substances dissolved from wood in different proportions due to the action of chemical reactants in the specific conditions of the process are present in solution at low concentration in the lyes currently called weak black liquor, which are normally transformed into concentrated lyes (strong black) liquor) for their subsequent treatments.

In the form exemplified in box 1 / a, a raw material consisting of an agricultural residue (block 10), is subjected to a pre-treatment with water or weak lyes (block 11), by which it is freed from most of the inorganic contaminants, such as silica and sodium chloride, and subsequently delignified (block 12) with the alkali / 02 process in which the alkali consists of a mixture in which the sodium carbonate recovered and returned to the cycle (line L1) by subsequent steps which constitute the process of the invention. The reactive cycle is integrated with a quantity of NaOH corresponding to the make-up (line L2) necessary to compensate for the cycle losses.

In this preferred, but not exclusive form of delignification, to optimize the results of the invention, soda (NaOH) and sodium carbonate (Na2C03) are used in such ratios as to ensure the necessary concentration in OH ions which catalyze the reactions of decomposition of lignin.

The alkali is the main agent in the solubilization of lignin. The presence of O2 determines:

- the demolition of part of the lignin chains producing fragments consisting of low molecular weight organic acids and their corresponding sodium salts (sodium acetate, sodium formate, etc.), - the discoloration of lignin, reacting with the chromophor groups in a reaction similar to that of peroxides.

As a consequence of these reactions the pulp and cellulose produced are less dark when compared to the kraft and soda ones and are more easily whiteable.

For degrees of whiteness limited to ISO 80-83, a single pressurized peroxide stage reinforced by the simultaneous presence of 02 (EOP) is sufficient.

The use of 02 in the first and / or subsequent delignification phase (reduction of the kappa index and bleaching) means that the lignin, which as such is not biodegradable, becomes so with the demolition of the chains and the formation of low molecular weight acids. (The kappa index defines the amount of lignin still existing in the cellulose in the different stages of their delignification).

This fact is decisive for the treatments of the invention. In particular, it differentiates the result of the anaerobic treatment of black lyes deriving from traditional processes (kraft and soda) and those deriving from more advanced processes that take advantage of the action of O2 in the delignification or in that downstream of the reduction of the kappa index.

The cellulose obtained from the delignification phase is separated by filtration, washed (block 13) and sent (line L3) to the step of reducing the kappa index (block 14) to obtain bleached cellulose (line L14). The water to be recycled from the effluents of this bleaching phase is separated by evaporation (block 15) (line L11).

Box 1 / b illustrates the anaerobic treatment phase of black lyes (block 16) from the delignification and kappa index reduction phase in the pre-bleaching and bleaching stages, which use reagents such as O2, O3- With anaerobic treatment transformations are produced in the liquor which occur in the absence of O2 by microorganisms. In these anaerobic microorganisms, energy is provided by demolition reactions without the intervention of atmospheric oxygen.

With this treatment, the biodegradable organic substances are transformed into biogas, mainly as methane.

The black lyes of the first delignification phase which have a high content of organic substance (normally expressed as chemical oxygen demand (COD)) are mixed (line L4), in one of the preferred applications of the invention, with the low concentrated effluents. COD deriving from the evaporation of the effluents of the treatments to reduce the kappa index and bleaching for the recovery of process water. This evaporation is pushed until a concentrate is obtained which, mixed with the black delignification lyes, produces a lye with an optimal COD for anaerobic treatment.

This mixing also produces a reduction in the temperature and possibly in the pH of the liquor. The temperature is subsequently controlled at 37-38 ° (block 17) through a heat exchange with the cycle integration waters. The pH, on the other hand, is controlled at a level of 7-8 (block 18) by putting back into the cycle the CO2 (line L5) produced in the subsequent oxidation phase (box 1 / d).

In its most frequent applications, anaerobic treatment is used as the first stage of the biological treatment of effluents with COD beyond the acceptable limits, due to its ability to transform 60-70% of the organic substance in the treated lye into biogas.

The anaerobic treatment illustrated in box 1b (block 19) of the sequence of process phases of figure 1 is a determining element, since among the chemical transformations it determines, its ability to convert, through methanization, the sodium salts of acids is fundamental. organic (acetates, formates, etc.) in sodium carbonates.

In the form of carbonate or in the form of hydroxide, these salts constitute the alkali which in known processes forms the reagent needed for the solubilization of non-cellulosic components and which, in the most advanced technologies, see the participation of oxygen.

In the traditional processes of recovery of the reactants and of incineration of the organic substance, the transformation of the aforementioned organic salts, instead of at 37-38 ° C of the anaerobic treatment, takes place due to the high temperatures (around 1000 ° C) that are reached in the combustion of concentrated lyes, as already mentioned in the introductory part.

The temperature of 300-320 ° which, as better explained below, characterizes the further oxidation phase illustrated in figure 1 in box 1d, by which the organic substance is destroyed, would not allow this transformation; the organic salts, in the forms of acetates, formates, etc., would remain so when mixed with sodium carbonates.

The practically equal solubilities between the salts present in the mixture would make it difficult to separate, which is necessary for their reuse in the delignification process.

The transformation of part of the lignin into a biodegradable substance, due to the action of O2, present in the white liquor, is relevant for the purpose of optimizing the yield in the methanization of the organic substance (reduction of COD) of the lyes subjected to anaerobic treatment. This yield reaches 80-85%, compared to normal oil 60-70% obtained with the treatment of lyes containing non-biodegradable lignin.

This higher yield results in a corresponding production of biogas directly proportional to the reduction of COD.

The methanization of the organic substance of the lyes produced in the delignification of cellulosic materials gives rise to a quantity of biogas of the order of 400 Nm3 per ton. of COD destroyed almost independent of the type of anaerobic treatment used.

The aforementioned greater efficiency in the methanization of the organic substance that characterizes the phases that are part of the invention corresponds to a recovery of energy with a high caloric potential, consisting of the methane gas contained in the biogas, in the ratio of 65-70%.

In one of the preferred versions, the biogas is converted directly into electrical energy (line L7), feeding generating sets with diesel engines (block 20) designed for the use of this type of gas.

The high efficiency with which most of the organic substance is converted into gas and the choice of the oxidation phase to complete the destruction of the organic substance present in the lyes, allow to limit to 12-15% of total solids (which corresponds normally 3-4% of organic substance) the concentration of the lyes.

The set of these conditions that characterize the process of the invention, makes it possible to make the thermal balance of the inventive process (which includes the recovery of water) comparable with that of the more efficient traditional processes, also because these require a greater quantity of electrical energy. and heat to concentrate the lyes at 60-65% for combustion in the recovery boilers, as mentioned in the introductory part.

By way of example, box 1 / c shows the evaporation phase of the weak liquor (block 21) which has undergone anaerobic treatment (block 19) to make it suitable for the subsequent oxidation process (box 1 / d).

It is conceivable in the context of the example referred to that the black liquor produced in the delignification phase (block 12) has a total content of dry solids (TDS) of the order of 10%, which is reduced to 8 %, for mixing with the concentrate of the effluents in the phase of reducing the kappa index and bleaching (block 14).

Of this percentage, 50% consists of inorganic salts and the remaining 50% of organic materials deriving from the dissolution of the raw material.

As a result of the conversion into biogas, with a yield of 85%, the organic part (COD) present in the liquor is reduced from 4% to 0.6%.

This content is less than the 2.5-3% necessary for the liquor activated with the appropriate amount of 02 to ensure the maintenance of the temperature during the oxidation phase.

The recovery, after anaerobic treatment, of a minimum of 2.5-3% of organic substance in the lyes, is obtained by subjecting the lyes to evaporation (block 21).

The evaporation of this operation, little contaminated by the entrainment of organic matter (COD) due to the limited concentration of the lyes, allows it to be put back into the cycle (line L8) in many points of the delignification and pre-bleaching sequence.

The minimum contamination of this evaporated product is a positive feature compared to the level of contamination of the evaporated product of traditional processes, in which the concentration of lyes must be pushed up to 60-65% of TDS.

The possibility of operating at low concentrations of the lyes that characterizes the invention also minimizes the difficulties, present in the other recovery processes, related to the encrustations of the exchangers of the multistage evaporators and to the high viscosities of the concentrated black liquids which make pumping and pumping difficult. the micronization for their combustion.

Box 1 / d shows the oxidation phase of the lyes (block 22) according to the invention, in which the 12-15% concentrated lyes are oxidized by the introduction of the lye and at the same time of the oxygen in the ratio of 1: 1 with respect to to the COD present, in order to determine its destruction, by combustion, with an efficiency of up to 99.5%.

In the case considered for the purposes of the example, a concentrated liquor containing 2.5% of organic substance and 12.5% of inorganic salts is preheated to 270-280 ° C using the heat resulting from the oxidation (combustion) reaction ( block 22) and injected into a reactor at a pressure of 100-110 bar, into which 2 is introduced.

The oxidation reaction (combustion) generates heat in quantities greater than that necessary to preheat the liquor and maintain the reaction temperature around the 320 ° C required for the reaction to proceed.

This temperature is controlled by using the excess heat in a boiler producing steam at 10 bar, as shown with line L9. The quantity of O2 necessary for the reaction is by weight equal to the organic substance to be destroyed and therefore reduced to about 15-20% of that necessary if the liquor was not previously subjected to the anaerobic treatment.

In advanced technology plants and in particular for the transformation of agricultural residues, annual plants, etc. into cellulose, the O2 requirement required in the delignification phases (block 12), reduction of the kappa index and bleaching (block 14), and that necessary for oxidation (block 22), it can be economically covered with self-production in the same pulp factory.

It should be noted that in this way the cellulose production plant, as regards the supply of reactants, remains dependent on external sources only as regards the integration of alkali losses in the cycle.

It is clear that this condition, to which is added the possibility of putting a high percentage of process water back into the cycle (line L8, L10, L11), makes the factory easily "distributable" in the area, in locations where it is possible to concentrate the non-woody raw material.

Box 1 / e shows the operations entrusted to conventional procedures which are only partially related to the invention and which can be conveniently used for its industrial use.

The resulting salts in the oxidation step (block 22) are rendered in the form of concentrated solutions, preferably at 20-25%.

As previously described, due to the action of the anaerobic treatment, almost all of the salts present in the black liquor of the delignifications are converted into sodium carbonates.

There are cellulosic raw materials and in particular straws, which contain among their inorganic components chlorides, mainly sodium chloride, and silica (SiO2).

The new known processes provide for pre-treatment of raw materials aimed at reducing these contaminants and in particular those described above which, once they have entered the delignification processes, come out partially accompanying the cellulosic fibers (product), but in large part are transferred to the black lyes.

By way of example, we cite the case of wheat straw, with the pre-treatment of which the chlorides are reduced by 90% (6 - 10 gr / kg dry) and by 50-60% the silica (4 - 6 gr / kg. dry) in the vegetable transferred to the subsequent delignification.

Since the sodium salts in prevalent form of carbonates obtained through the sequence of phases according to the invention are carried over into the cycle, it is evident that, even at a reduced level, the contaminants can be tolerated in the steady-state system in specific percentages and therefore the quantities excesses must be extracted in order to avoid their progressive concentration in the cycle.

To this end, see box 1 / e, with treatments mainly based on the different solubility between the contaminating salts and sodium bicarbonate (block 23), the chlorides present in the mother liquors discharged from the cycle are eliminated (line L13). The recovered reactive is subjected to a decarbonation operation (block 24) to transform it into carbonate with evacuation of CO2.

As regards the silica, which is solubilized in the high pH alkaline phase of the delignification treatment, it is separated by precipitation (line L12) when, as organic acids are formed due to the action of O2 on the lignin, the pH is lowered. below the 10.2 level.

This separation proceeds in correspondence with the subsequent reduction of the pH to 7-8, necessary for the anaerobic treatment and obtained with the recycle of the CO2 produced in the oxidation phase (line L5).

In more advanced technology processes that use alkali oxygen, the reactants obtained from the oxidation step (block 22) after the purification treatments (block 23) are recycled mainly as carbonate (line L1).

The justification (block 25) is limited only to a part of the carbonates present to make it possible to reuse them (line L2) in the stages of reducing the kappa index and bleaching (block 14).

The advantages represented by the process according to the invention are the following:

- destruction of the organic substance present in the effluents, with energy recovery,

- recovery and re-cycling of chemical reactants and process water,

- observation of the most restrictive ecological regulations, referring to both gaseous and liquid and solid effluents,

- possibility of creating small production capacity plants required for the use of cellulosic raw materials consisting of agricultural residues, annual plants or tree crops with a short vegetative cycle, - possibility of distributing small-capacity factories, intended for the use of agricultural residues, annual plants or in any case cellulosic materials that are difficult to collect, transport and store, in the area near the sources of raw material,

- possibility of transforming existing plants and building new ones, even of small capacity, so that they conform to the definition of “closed loop mill”.

Claims (21)

CLAIMS 1) Process of recovery and recycle of the compounds contained in the effluents of the delignification and bleaching processes of cellulose production plants, characterized by the fact that it consists of: a) in subjecting the black liquor leaving the delignification process to one or more anaerobic treatments to produce biogas from the organic substance as an energy source and to convert, through this methanization, the sodium salts of organic acids into Na2C03; b) concentrating the leachate coming out of the anaerobic treatments in order to recover the water and concentrate it; c) in subjecting the concentrated liquor to an oxidation treatment to obtain the destruction by combustion of the organic components with recovery of the reactive, production of CO2 and recovery of water; d) recovering the alkalis by separating their salts by difference in solubility; e) subjecting the separated alkalis to a decarbonation step to transform them into carbonates; And f) in deriving a part of the carbonates in caustic soda, ie in one of the two reactants contained in the delignification lye. 2) Process according to claim 1, characterized by the fact that the evaporation concentrate for the recovery of process water from the kappa reduction and bleaching cellulose phase is also added to the liquor leaving the delignification phase. 3) Process according to claim 1, characterized in that the biogas obtained in the anaerobic treatment step is sent to energy production plants which can be used to carry out the process. 4) Process according to claim 1, characterized in that the condensate leaving phase b) consists of water which is recovered and reintroduced into the process cycle. 5) Process according to claim 1, characterized in that the CO2 formed in the oxidation treatment is partially reused in the treatment of the liquor for the anaerobic process, while the water leaving the oxidation phase is recovered and reintroduced into the cycle. 6) Process according to claim 1, characterized by the fact that the NaOH coming out of the recalculation phase f) is recycled to delignification. 7) Process according to the preceding claims, characterized in that, in the case of treatment of delignifications obtained with the NaOH 02 processes in which oxygen, by decomposing part of the non-biodegradable lignin into organic acids and the corresponding biodegradable salts, allows the destruction and the transformation into biogas of 80-85% of the organic substance present in the lyes, against the 60-70% obtained in the anaeorbic treatment of lyes from kraft and soda processes. 8) Process according to the preceding claims, characterized in that the biogas is used as a fuel in electrical energy production plants with generating sets driven by suitable diesel engines. 9) Process according to claim 1, characterized in that in the oxidation step a destruction of up to 99.5% of the organic substance contained in the black lyes is obtained. 10. Process according to claim 1, characterized in that the inorganic salts in the form of sodium carbonate released from the organic substance accompanying them in the black liquids are recovered as concentrated solutions at 15-30%. 11) Process according to the previous claims, characterized by the fact that the recovery of energy from the organic substance and of the chemical reactants in the forms that can be reinserted into the cycle, takes place at temperatures not exceeding 350 ° C. 12. Process according to claim 1, characterized in that the oxidation step uses 12-15% dry matter lyes, against the 55-60% concentrations required for combustion in conventional recovery boilers. 13. Process according to claim 1, characterized by the fact that the C02 produced in the oxidation step is reused to reduce the pH of the black liquids, adapting it to that required in the anaerobic treatment. 14. Process according to the preceding claims, characterized in that the excess heat, developed in the oxidation reaction of the organic substance, is recovered in the form of vapor at 10 bar. 15) Process according to the preceding claims, characterized in that, to reduce and / or avoid the formation of encrusting products and insoluble contaminants (for example glass) which characterize the exercise of conventional recovery processes which operate at temperatures of 1000-1100 ° C, it is operated at a temperature not exceeding 350 ° C. 16) Process according to the preceding claims, characterized in that the silica present as impurity in the cycle is reduced and maintained at an acceptable level by evacuating it partly with the cellulose produced and partly by precipitation and reduction of the pH in the black and / or through the recalculation phase. 17) Use of the process according to the preceding claims in plants with a small production capacity required for the use of cellulosic raw materials consisting of agricultural residues, annual plants or tree crops with a short vegetative cycle. 18) Use of the process according to the previous claims in small-capacity factories intended for the use of agricultural residues, annual plants and / or cellulosic materials that are difficult to collect, transport and store, and which can be distributed throughout the territory near the sources of raw material. 19) Use of the process according to the preceding claims in small-capacity factories which, using the oxygen obtained from the air in the plants themselves as a reactive, remain dependent on external sources only for the alkali necessary for the integration of cycle losses. 20) Use of the process according to the preceding claims for the treatment of concentrated effluents from evaporation systems for the recovery of process water in plants for the production of large capacity cellulose which use wood as raw material. 21) Use of the process according to the preceding claims in plants for the production of high-yield and semi-chemical pulp or plants that use raw materials with high percentages of cellulose, for example cotton linter, hemp cortical fibers and similar vegetables, for the treatment of effluents with a low concentration of dry matter.