Technical Field The present invention relates to the recovery of toluene diamine from high- boiling tar residues discharged from toluene diisocyanate preparation processes.

More particularly, the present invention relates to a method for recovering toluene diamine from high-boiling tar residues discharged from the bottom part of the distillation tower in the toluene diisocyanate preparation process, in which after free toluene diisocyanate contained in the high-boiling tar residues is separated and recovered, the resulting solid residue is subject to hydrolysis in the presence of catalyst at a high temperature near critical point of water under a pressure higher than a vapor pressure of that temperature (i. e. , liquid phase region of water) to produce toluene diamine, and then the thus-produced toluene diamine is recovered with a high yield. The present invention is also concerned with economic benefit and environmental friendliness by recycling the water and catalyst used in the toluene diamine recovery.

Background Art Convenience and economic advantages have made land burial or ocean disposal one of the most attractive solutions to the treatment of the liquid or solid industrial wastes generated. However, since serious problems, such as the production of soil and ocean pollution, exist with land burial or ocean disposal, increasingly severe regulations have been enforced with the development and

enlargement of industries. It is anticipated that the implementation of various international protocols regarding environmental pollution and the exhaustion of available burial sites force the aforementioned disposal methods to soon be abandoned.

Incineration, by which wastes are decomposed by combustion at high temperatures, has emerged as a favorable substitute for land burial and ocean disposal, especially for the treatment of solid wastes. Incineration, however, suffers from economic disadvantages: incineration furnaces are generally operated at 1,000 °C or higher, which requires high operation costs. Further, since high- temperature combustion produces secondary pollutants including nitrogen oxides (NOX), sulfur oxides (SOx), dioxin, etc. , additional facilities are required for removing them.

Therefore, there is needed an alternative which avoids the problems of the conventional burial and incineration treatments. Recent research has been focused on the recovery of reusable materials from wastes and on the conversion of wastes into new usable materials in order to protect the environment and take maximal advantage of resource materials.

Toluene diisocyanate (TDI) is an industrially important material, which is used for the production of polyurethane. TDI is produced from toluene diamine through phosgenation. As a result, there remain various compounds (such as TDI, reaction intermediates, hydrogen chloride, phosgene, other by-products, reaction solvent, etc. ) in the product mixture.

Advantage is usually taken of a distillation tower to isolate and purify TDI from the product mixture. From the bottom part of the distillation tower is discharged high-boiling residues which are in a tar state having some degree of flowability at room temperature and contain significant amounts of TDI, by- products and various impurities.

Hydrolysis techniques for treating tar wastes generated from TDI preparation processes are well known in the art. For example, hydrolysis is

carried out at 350 °C or less in a gas-liquid phase or in a liquid phase with the addition of an aqueous ammonia solution or an aqueous solution containing bases such as hydroxides of alkaline earth metal or acids such as inorganic or organic acids. A superheated steam of 200-400 °C may be also used for the hydrolysis under a pressure as low as 1-5 atm. As for tar which shows fluidity at 100 °C or less due to its high content of low-boiling components such as toluene diisocyanate, it can be limitedly decomposed by using supercritical water or subcritical water.

U. S. Pat. No. 3,331, 876, DE Pat. Publication Nos. 2 942 678 and 1 962 598, and Japanese Pat. Laid-Open Publication No. Sho. 58-201751 disclose batch- type methods where, following the rapid transformation of TDI residues and water into solid phase, gradual re-liquification is carried out with the progress of hydrolysis. However, the solid phase material impedes the continuous operation of the process. In addition, since fluid residues abundant in free TDI are employed as raw materials, the conversion of the TDI produced through phosgenation to toluene diamine reduces economic benefits.

Korean Pat. Laid-Open Publication No. 2001-52948 discloses a hydrolysis technique by which tar wastes are hydrolyzed at 170-230 °C under 25-50 atm in a continuous or semi-continuous type reverse-mixing reactor. However, this method suffers from problems that, due to the limited characteristics of mass transfer, e. g. , penetration, diffusion speed, etc. , which solid wastes possess in a hydrolysis medium under the low or moderate pressure conditions, not only is the process time extended, but large-scale treatment facilities are required.

In U. S. Pat. No. 6,255, 529 and Korean Pat. Laid-Open Publication No.

2001-1488, supercritical water or near critical water of high temperatures and pressures is used as a medium for the hydrolysis of tar wastes. Particularly, U. S.

Pat. No. 6,255, 529 discloses a hydrolysis method of TDI residues by using high- temperature water. However, this method is not economically favorable because fluid tar with a high content of free TDI is hydrolyzed. Moreover, when applied to tar from which low-boiling residues are removed by use of thin film distillation

apparatus as in the present invention, the method shows a very low conversion rate into toluene diamine due to the absence of any catalyst.

Korean Pat. Laid-Open Publication No. 2001-1488 discloses a method where ammonia water is employed as a catalyst for hydrolyzing tar wastes at 350- 600°C at 218-400 atm in supercritical water. Under such high-temperature conditions, part of the TDI prepared is thermally decomposed or oxidized to benzene diamine, an undesired product. In addition, a large quantity of the ammonia is required as. a catalyst, amounting to twice the weight of the tar. Also, the ammonia combines with carbon dioxide which is usually generated during hydrolysis, to form ammonium bicarbonate, ammonium carbonate and organic polyamine salts. Then, these compounds are converted into salts of complex hydrate which act as impediments in continuously operating the facilities.

Moreover, in the course of recovering TDI, the complex hydrates contaminate the product or are discharged as an admixture with the final wastes to produce secondary environmental pollutions. The use of ammonia water may provide ammonia or ammonium salts to the final waste matter which increases the total nitrogen concentration to the leachate or wastewater, resulting in the production of environmental pollution. Furthermore, treatment facilities experience various problems associated with the use of ammonia water. The ammonia solution needs a large-scale treatment facility due to its large quantity and compels the use of very expensive, specially-designed apparatuses due to its high pressure and temperature.

Supercritical water not only causes the facilities to be corroded, but lowers solubilities of various salts to plug the pipes of the facilities.

Apart from the aforementioned methods, the recovery of toluene diisocyanate from fluid tar may resort to a thin film evaporator or a rotary evaporation granulator in which free toluene diisocyanate-containing fluid tar is re- treated under a high-temperature vacuum condition of 250°C and around 10 mmHg.

Left after the re-treatment, the resulting solid wastes are buried or incinerated.

However, the solid wastes are found to contain a significant amount of components

which are conversable into toluenediisocyanate by hydrolysis. Further, the incineration or burial of the solid wastes runs counter to worldwide environmental protection policies.

As mentioned above, conventional methods of recovering toluene diamine from tar residues generated from toluene diisocyanate preparation processes suffer from problems including the poor reactivity attributed to mass transfer resistance at 250°C or lower, the salt generation according to the use of improper catalysts, the use of excess water, the secondary pollution caused by nitrogen components contained in waste water or discarded matters, the extension of reaction time, and the pyrolysis of toluene diamine at 400°C or higher. The conventional methods are also costly in that since tar residues, as being not sufficiently removed of the free toluene diisocyanate contained therein to keep its fluidity, are subject to hydrolysis, a part of the toluene diisocyanate produced during the processes should be converted to toluene diamine.

Description of Drawings Fig. 1 is a schematic view illustrating processes of recovering toluene diamine by treating the solid residues derived from the fluid, high-boiling tar residues discharged from toluene diisocyanate preparation processes in accordance with an embodiment of the present invention ; and Fig. 2 is a schematic view illustrating processes of recovering toluene diamine by treating the solid residues derived from the fluid, high-boiling tar residues discharged from toluene diisocyanate preparation processes and recycling the catalyst and water used in the hydrolysis, in accordance with another embodiment of the present invention.

Disclosure Technical Problem Leading to the present invention, the intensive and thorough research on the recovery of toluene diamine from fluid, high-boiling tar residues discharged from TDI preparation processes, conducted by the present inventors, resulted in finding that solid residues remaining after the separation and recover of free TDI from the fluid, high-boiling tar residues, contain TDI oligomers in a significant amount, and in developing a novel process in which the oligomers are hydrolyzed into toluene diamine and the resulting toluene diamine is effectively recovered. In addition, the generated waste water and other wastes (such as a spent catalyst) can be recycled in the present process.

Accordingly, it is an object of the present invention to provide a process for recovering toluene diamine with a high yield from fluid, high-boiling tar residues discharged from TDI preparation processes.

It is another object of the present invention to provide a process for recovering toluene capable of recycling the spent catalyst and water generated during the recovery of toluene diamine so as to enjoy the advantage of being environmentally friendly and economically favorable.

Technical Solution In accordance with an embodiment of the present invention, there is provided a process for recovering toluene diamine from a fluid, high-boiling tar residue discharged from toluene diisocyanate preparation processes, comprising the

steps of : a) providing a solid residue, said solid residue being derived from the substantial reduction of free toluene diisocyante contained in the fluid, high-boiling tar residue; b) pulverizing the solid residue into particles; c) slurrying the particles of the solid residue with water and subjecting the slurry to hydrolysis treatment in the presence of a catalyst under the condition of a pressure of 40-250 atm and a temperature of 200-370 °C to produce toluene diamine, said hydrolysis condition being maintained within the liquid phase region under a critical point of water; and d) recovering the resulting toluene diamine from the hydrolysis-treated slurry.

It is preferred that the hydrolysis condition is reached by pressurizing the slurry to 40-250 atm and heating the pressurized slurry to 200-370 °C within the liquid phase region under a critical point of water. Further, the temperature elevation of the slurry is economically achieved by heat exchange with the slurry resulting from the previous hydrolysis treatment, followed by an additional heating.

In accordance with the preferred embodiment of the present invention, the step d) comprises subjecting the hydrolysis-treated slurry to a reduction of temperature and pressure, and then conducting separation to provide a first overhead fraction of gas phase containing water vapor and light gaseous components (e. g. , low boiling organics, carbon dioxide, ammonia, etc. ), and a first bottom fraction containing toluene diamine, the spent catalyst and other tar residues, in a distillation tower, and separating and recovering the resulting toluene diamine from the first bottom fraction through pressure-reduced evaporation.

In another embodiment of the present invention, the first overhead fraction is reacted with oxygen in the presence of an oxidative catalyst to efficiently remove pollutants contain therein. In addition, this embodiment further comprises subjecting said oxidation-treated first overhead fraction to temperature reduction and then to separation into a second overhead fraction of gas phase and a second bottom fraction of liquid phase (typically, composed of condensed water) in a gas-liquid separator; mixing the residues or portions remaining after the recovery of toluene diamine from the first bottom fraction with the second bottom fraction; and filtering the mixture to give a catalyst-containing filtrate and recycling the filtrate to said hydrolysis reaction, thereby the waste water and the spent catalyst remaining after the toluene diamine recovery can be reused, and thus economic benefits are guaranteed.

Advantageous Effects In the present invention, solid residues left after the removal or substantial reduction of free TDI in fluid, high-boiling tar residues discharged from TDI preparation processes were subjected to a hydrolysis in the presence of a catalyst under the condition of liquid phase region of temperature and pressure near critical point of water. As a result of the reaction, toluene diamine useful as raw material for the preparation of toluene diisocyante, can be recovered on the level of about 55- 85 wt% of the solid residues used. Also, according to the present invention, the waste water and spent catalyst remaining after the hydrolysis reaction can be recycled, whereby the amount of the solid waste finally discarded can be reduced by 80-95 wt% compared with that according to the conventional methods. Particularly,

because alkali metal hydroxide and/or alkali metal carbonates, which does not cause environmental pollution, is employed as the hydrolysis catalyst, the present invention does not aggravate the negative influence of the final solid waste on the environment.

In consequence, the present invention is economically favorable and environmentally friendly because of recycling the waste water and spent catalyst as well as recovering toluene diamine.

Best Mode In the present invention, solid residues left following the separation and recovery of free TDI from fluid, high-boiling tar residues are pulverized and slurried, and then the slurry is hydrolysis-treated in the presence of the catalyst at a temperature near critical point of water under a pressure higher than a vapor pressure corresponding to that temperature (i. e. , within the liquid phase region of water), so as to produce and recover toluene diamine with a high yield. Further, the waste water and the spent catalyst remaining after the toluene diamine recovery can be effectively recycled.

Generally, fluid, high-boiling tar residues resulting from the side reactions of TDI preparation processes are not vaporized in distillation towers for the TDI purification. For instance, such high-boiling tar residues are not vaporized even at 200°C and 50 torr. The high-boiling tar residues which are discharged from the bottom of distillation towers are found to contain ones to tens weight %, typically as much as 20 to 40 weight % of free TDI, and thus to be fluid.

In accordance with the present invention, free TDI is recovered from the high-boiling tar residues through the separation, e. g. , vacuum evaporation and thin film evaporation, which leaves a solid residue deprived of fluidity due to the substantial reduction of free TDI to a level as low as hundreds ppm.

Below, a detailed description will be given of processes recovering toluene

diamine from the solid residues, with reference to drawings.

Referring to FIG. 1, there is a schematic view illustrating processes of recovering toluene diamine by treating the solid residue derived from the fluid, high-boiling tar residues discharged from TDI preparation processes in accordance with an embodiment of the present invention.

In FIG. 1, a solid residue 10 is pulverized into particles with the aim of being efficiently hydrolyzed. In this regard, a pulverizer 13 and a screen separator 14 are used to adjust the particle size to 1,000 pm or less in diameter, and preferably, to 100 um or less.

In a mixer 15, the particles are mixed with water 11 and a hydrolysis catalyst 12 to be slurried. At this time, the hydrolysis catalyst 12 should serve to promote the hydrolysis of the organics present in the solid residue at high reaction efficiency while causing little corrosion on facilities. For this, suitable is the hydrolysis catalyst selected from the group consisting of alkali metal oxides, alkali metal carbonates and a combination thereof. Preferred alkali metal is sodium or potassium. Sodium carbonate is the most preferable as the hydrolysis catalyst since it can perform catalytic action with the least corrosive influence on the facilities.

The slurry contains the particles of the solid residue, preferably in an amount of about 0. 1-50 wt%, and the hydrolysis catalyst in an amount of about 0.1- 5. 0 wt% and preferably in an amount of about 1.0-3. 0 wt%. The hydrolysis in the slurry may be accelerated with pH increasing. As such, the pH of the slurry is preferably maintained at 7 or higher.

Before entering a hydrolysis step, the slurry is pressurized by use of a high pressure-feeding pump 21 to a level in which water is not converted to vapor (i. e., about 40-250 atm). Further, a temperature elevation is required for a sufficient hydrolysis. In the present invention, the pressurized slurry is heated to about 200- 370 °C. Because much heat energy is needed to reach such a temperature, the pressurized slurry passes through a heat exchanger 22, in which the fresh slurry is

heated by heat exchange with the hydrolysis-treated slurry effluent of high temperature resulting from the previous hydrolysis, and then the additional heat is provided by use of a heater 23 to reach the desirable hydrolysis temperature (i. e., 200-370 °C). The economic benefit is attained in that there is taken maximal advantage of the heat energy generated in the previous hydrolysis.

Then, the resulting slurry is subjected to hydrolysis in a reactor 24. Any type reactor may be employed without limitation. For example, a cylindrical reactor, a tower reactor, a tubular reactor, a stirred tank, and/or a fluidized bed reactor are available. When employed, two or more of the same or different type reactors may be arranged in series or in parallel.

As mentioned above, the hydrolysis temperature and pressure fall within the range of about 200-370 °C and about 40-250 atm, respectively. Preferably, the hydrolysis is conducted at about 280-320 °C under a pressure of about 100-200 atm. A higher reaction temperature gives contribution to an improvement in the mass transfer between reaction medium and tar, resulting in a faster reaction rate.

At excessively high temperatures, however, the methyl moiety of the toluene diamine produced by the hydrolysis undergoes thermal decomposition to produce by-products such as benzene diamine, which degrades the toluene diamine product.

In the present invention, it is particularly important to conduct the hydrolysis in the liquid phase region under the critical point of water because, with sharply decreased solubility in supercritical water, the catalyst cannot perform its function under supercritical conditions. The average reaction time or the residence time in the reactor is determined depending on the properties of the slurry, the solid residue amount in the slurry, etc. , typically within the range of about 0.1-60 min and preferably about 1-5 min.

The hydrolysis-treated slurry of relatively high temperature is discharged from the reactor 24 to the heat exchanger 22 in which the hydrolysis-treated slurry is cooled (e. g. , about 80-200°C) while the heat it retains is transferred to the subsequent fresh slurry incoming to the heat exchanger 22. Before the cooled

slurry is subjected to toluene diamine recovery therefrom, its pressure is preferably reduced (e. g. , to about 1-30 atm) with a pressure-reducing valve.

According to this embodiment, the toluene diamine recovery equipment, as seen in Fig. 1, includes a distillation tower 31 and a pressure-reducing evaporator 41. The cooled, pressure-reduced slurry is subjected to separation in the distillation tower 31, and then discharged as a first overhead fraction of gas phase and a first bottom fraction, respectively. The first overhead fraction contains water vapor and light gaseous components (such as low boiling organics, reaction products such as carbon dioxide and ammonia, etc. ). The term"low boiling organics", as used herein, indicates organic compounds with a boiling point less than 100°C. After being cooled or condensed preferably to about 0-80 °C by a condenser 32, the first overhead fraction is driven to a gas-liquid separator 33 whereby the first overhead fraction is separated into gas 34 and waste water 35, and then finally discarded.

On the other hand, the first bottom fraction, discharged from the lower part of the distillation tower 31, containing toluene diamine, the spent catalyst and the other tar residues, is transferred to the pressure-reducing evaporator 41 wherein toluene diamine is recovered in a gas phase 42 while the spent catalyst and the other tar residues are solidified and discarded in a solid phase 43.

As for the conditions of the distillation tower 31, it is preferred that an internal pressure is within the range of about 1-5 atm (absolute), while a internal temperature for the upper part and the lower part is within the range of about 100- 150 °C and about 180-250 °C, respectively. The internal pressure and temperature of the pressure-reducing evaporator 41 is preferably controlled to about 0.01-1. 0 atm (absolute) and about 100-320 °C, respectively, for the purpose of the prevention of toluene diamine from thermal decomposition and the improvement of purification.

Fig. 2 is a schematic view illustrating processes of recovering toluene diamine by the hydrolysis treatment of the solid residues derived from the fluid,

high-boiling tar residues discharged from TDI preparation processes and of recycling the catalyst and water used, in accordance with another embodiment of the present invention.

Like the embodiment of Fig. 1, this embodiment starts with a solid residue 110 which remains as a result of recovering free TDI contained in the fluid, high- boiling tar residues discharged from TDI preparation processes. The solid residue 110 is pulverized into particles with a size of 1, 000 um or less and preferably 100 m or less by use of a pulverizer 113 and a screen separator 14. In a slurry mixer 115, the particles of the solid residue are slurried with a mixture, provided typically in an aqueous solution state from a catalyst mixer 145, of water 111 and a hydrolysis catalyst 112. In this regard, the mixture (of the water and the catalyst) to be transferred to the slurry mixer 115 is a combination of freshly supplied mixture of water 111 and catalyst 112 from external sources (not shown) and a catalyst-containing filtrate recycled from a previous process mode. The same as in the embodiment of Fig. 1 are applied for the hydrolysis catalyst and the slurry's solid residue amount, the catalyst amount, and the pH range useful in this embodiment.

As in the embodiment of Fig. 1, the slurry is pressurized by a high pressure-feeding pump 121, then heat-exchanged in a heat exchanger 122 with the hydrolysis-treated slurry transferred from the reactor 124 of the previous hydrolysis, and let to undergo an additional heating in a heater 123 before being hydrolysis-treated in the reactor 124. The conditions for the hydrolysis reactor 124 adopt those set forth in respect of the reactor 24 of Fig. 1.

Features provided by the embodiment of Fig. 2 reside in the oxidative removal of the pollutants from the first overhead fraction and simultaneously the recycling of the spent catalyst and waste water discharged from toluene diamine recovery processes.

As shown in Fig. 2, the hydrolysis-treated slurry goes through the heat exchanger 122 and a pressure-reducing valve 125, and to a distillation tower 131,

during which the slurry is cooled in the heat exchanger 122 and reduced in pressure by the pressure-reducing valve 125. In the distillation tower 131, the slurry is separated into a first overhead fraction of gas phase containing water vapor and gaseous light components (e. g. , low boiling organics, reaction products such as carbon dioxide and ammonia, etc. ) and a first bottom fraction containing toluene diamine, the spent catalysts and other tar residues, and the respective fractions are discharged from the distillation tower.

As such, since the first overhead fraction contains the pollutants such as the low boiling organics and ammonia inevitably under operation condition of the distillation tower, the removal of such pollutants serves to lower the extent of pollution of the waste water to be subsequently discarded.

For this, the first overhead fraction, as shown in Fig. 2, is subjected to oxidation in the presence of an oxidative catalyst in a reactor 138 to which oxygen 136 is provided. The reactor 138 is preferably operated at about 100-250 °C under the same pressure as the inside of the distillation tower, i. e. , about 1-5 atm (absolute).

As a result of the oxidation, the first overhead fraction removed of the pollutants is cooled by use of a condenser 132, and driven to a gas-liquid separator 133. In the gas-liquid separator 133, the resulting first overhead fraction is separated into a second overhead fraction and a second bottom fraction. The second overhead fraction of gas phase 134 is discharged from an upper part of the gas-liquid separator 133 while the second bottom fraction of liquid phase is transferred to a condensate-recovering tank 137. Typically, the second bottom fraction is composed of condensed water, which can be reused because organics are sufficiently removed through the oxidative reaction.

On the other hand, the first bottom fraction, discharged from the lower part of the distillation tower 131, containing toluene diamine, the spent catalyst and other tar residues (particularly, polymers not converted to toluene diamine by the hydrolysis reaction), is transferred to the pressure-reducing evaporator 141 wherein

toluene diamine is recovered in gas phase 142 while the remainder is mixed with the second bottom fraction in a sludge mixer 146. The resulting mixture is filtered in a catalyst recycler 144 after which the catalyst-containing filtrate thus obtained is recycled into the catalyst mixer 145. The sludge residue is treated as a waste 143.

The same as in the embodiment of Fig. 1 are internal pressures and temperatures of the distillation tower 131 and the pressure-reducing evaporator 141.

Further, the reactor 138 is filled with an oxidative catalyst. Suitable is the catalyst comprising an oxide of transition metal as a catalytic ingredient in an amount of about 0.01-10. 0 wt% on an alumina support. More preferably, the transition metal is selected from the group consisting of vanadium, chrome, manganese, copper and a combination thereof. This transition metal/alumina catalyst may further comprises a precious metal selected from the group consisting of platinum, silver, rhodium, palladium, ruthenium, gold and a combination thereof, in an amount of about 0.01-1. 0 wt%. The oxidative catalyst may be used in the form of the combination of the transition metal/alumina catalyst with the transition metal/precious metal/alumina catalyst.

Mode for Invention A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as the limit of the present invention.

COMPRATIVE EXAMPLE 1 The tar residue resulting from the side reaction of the phosgenation of toluene diamine to TDI were treated for about one hour in a rotary evaporation granulator maintained at 260 °C and 10 mmHg, to remove low boiling materials including TDI. The resulting solid residue was analyzed to have free TDI in an amount of 500 ppm or less and found to be of no fluidity.

After being pulverized to particles of 100 meshes or less, the solid residue was mixed in the weight ratio of 200: 600 with water. The aqueous mixture was provided to such a continuous reaction apparatus consisting of a pre-heater and a tubular reactor as shown in Fig. 1. A hydrolysis reaction was conducted at 400 °C under a pressure of 250 atm for one min, followed by quenching. Analysis results of the reaction product are given in Table 1, along below, with reaction conditions.

COMPARATIVE EXAMPLE 2 The same hydrolysis reaction as in Comparative Example 1 was carried out with the exception of extending the reaction time from 1 min to 5 min. Analysis results of the reaction product are given in Table 1, below, along with the reaction conditions.

COMPARATIVE EXAMLE 3 The same hydrolysis reaction as in Comparative Example 1 was carried out with the exception of increasing the reaction temperature from 400 °C to 460 °C.

Along with the reaction conditions, analysis results of the reaction product are given in Table 1, below, indicating the production of 1,3-benzene diamine in an amount of 2 %.

COMPARATIVE EXAMPLE 4 The same hydrolysis reaction as in Comparative Example 1 was carried out with the exception of using an aqueous 3 wt% sodium hydroxide solution instead of water. Analysis results of the reaction product are given in Table 1, below, along with the reaction conditions. The hydrolysis product was found to have chrome and molybdenum in amounts of 30 ppm and 50 ppm, respectively, as measured by an analyzer for inorganic elements, which indicated that salts might be deposited inside of the reactor after completion of the reaction.

EXAMPLE 1 The same apparatus and high-boiling tar residue as that of Comparative Example 1 were used. 200 g of solid residue was dispersed in 600 g of an aqueous 5 wt% sodium carbonate solution, followed by conducting a hydrolysis reaction at 300 °C and 100 atm for one min. Analysis results of the reaction product are given in Table 2, below, along with the reaction conditions. The reaction product was found to have chrome and molybdenum in amounts of 5 ppm and 7 ppm, respectively, as measured by the same analyzer that used in Comparative Example 4.

EXAMPLE 2 The same procedure as in Example 1 was carried out with the exception that an aqueous 3 wt% sodium carbonate solution, instead of a 5 wt% solution, was used. Analysis results of the hydrolysis product are given in Table 2, below, along with the reaction conditions.

EXAMPLE 3 Water was recovered from the decomposed mixture obtained in Example 2 in a rotary vacuum evaporator and added with sodium carbonate to a concentration of 3 wt%. The aqueous solution was used in conducting the same hydrolysis treatment as in Example 2, except that the reaction time was maintained for five min. Analysis results of the reaction product are given in Table 2, below, along with the reaction conditions, demonstrating no problematic effects on the reaction, and thus an economic benefit of the recovered water.

EXAMPLE 4 The same procedure as in Example 1 was carried out except that the solid residue was dispersed in a weight ratio of 200: 800 with an aqueous 3 wt% sodium carbonate solution and the hydrolysis reaction was conducted at 320 °C. Analysis

results of the reaction product are given in Table 2, below, along with the reaction conditions. The reaction product was found to have chrome and molybdenum in amounts of 3 ppm and 5 ppm, respectively, as measured by the same analyzer that used in Comparative Example 4.

EXAMPLE 5 The same procedure as in Example 1 was carried out except that the solid residue was dispersed in a weight ratio of 200: 800 with an aqueous 3 wt% potassium hydroxide solution and the hydrolysis reaction was conducted at 320 °C. Analysis results of the reaction product are given in Table 2, below, along with the reaction conditions.

EXAMPLE 6 The same procedure as in Example 1 was carried out except that the solid residue was dispersed in a weight ratio of 200: 800 with an aqueous 3 wt% potassium carbonate solution and the hydrolysis reaction was conducted at 320 °C.

Analysis results of the reaction product are given in Table 2, below, along with the reaction conditions.

EXAMPLE 7 The water recovered in Example 3 was subjected to oxidative reaction with oxygen at 125 °C under the atmospheric pressure in the presence of an oxidative catalyst in a fixed bed reactor to remove remaining organics and ammonia therefrom. The catalyst comprised an aluminum support with 5.0 wt% of manganese oxide and 0. 05 wt% of platinum impregnated. The initial concentrations of the organics, on the basis of the total amount of organic carbon, and ammonia in the water were measured to be 3,000 mg/1 and 2,000 mg/1, respectively. Removal was very efficiently achieved with 97% for organics and 94% for ammonia.

TABLE 1 Comparative Examples 1 2 3 4 Catalyst---NaOH Catalyst Conc. (wt%)---3 Wt Ratio of Water : particle 3 3 3 3 Rxn Time (min) _ 5 5 1 Rxn Temp. (°C) 400 400 460 400 Rxn Press. (atm) 250 250 250 250 TDA Yield (%) 20 25 21 55 BDA2 Content in product (%) - - 2 - Cr content (ppm)---30 Mb content (ppm) - - - 50 Note--Salts deposited- TABLE 2

Examples 1 2 3 4 5 6 Catalyst Na2CO3 Na2CO3 Na2CO3 Na2CO3 KOH K2CO3 Catalyst Cone. (wt%) 5 3 3 3 3 3 Wt Ratio of Water: particle 3 3 3 4 4 4 Rxn Time (min) 1 1 5 1 1 1 Rxn Temp. (°C) 300 300 300 320 320 320 Rxn Press. (atm) 100 100 100 100 100 100 TDA Yield (%) 75 78 81 85 75 73 BDA Content in product (%) - - - - - - Content (ppm) 5 - - 3 - - Mb content (ppm) 7 - - 5 - - Note--Recycled water--- was re-used for the hydrolysis 1 toluene diamine 2 benzene diamine

Industrial Applicability In the present invention, solid tar residues left after the removal of free TDI from fluid, high-boiling tar residues discharged from TDI preparation processes were subjected to a hydrolysis reaction in the presence of a catalyst under the condition of liquid phase region near critical point of water. As a result of the reaction, toluene diamine, a toluene synthesis material, can be recovered in an amount of 55-85 wt% of the solid residues used. Of course, the recovered toluene diamine can be used for the preparation of TDI. Also, according to the present invention, the waste water and the spent catalyst discharged after the hydrolysis can be recycled, so that the weight of the solid waste to be finally discarded is reduced by 80-95 wt% compared with that according to conventional methods. In consequence, the present invention is economically favorable and environmentally friendly because of recovering toluene diamine as well as recycling the water and catalyst used.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.