GB2240872A - Process and an apparatus for the treatment of liquid organic waste - Google Patents

Abstract

Into the reactor (1) containing concentrated sulphuric acid (5) are introduced the liquid waste to be treated (by 7) and nitric acid or hydrogen peroxide (by 11) at flow rates such that in the reactor there is a carbonization of the liquid waste introduced by the sulphuric acid and an oxidation of the carbon and SO2 formed during the carbonization by simultaneously regenerating the sulphuric acid. A continuous measurement takes place of the optical density of the liquid medium (5) for stopping the liquid waste supply or controlling flow through 11 when the C content of the liquid medium exceeds a set value. The liquid waste can be an organic solvent such as tributyl phosphate contaminated by radioactive elements. <IMAGE>

Description

PROCESS AND APPARATUS FOR THE TREATMENT OF LIQUID ORGANIC WE BY SULPHURIC MINERALIZATION The present invention relates to a process and an apparatus for the treatment of liquid organic waste.

It more particularly applies to the treatment of liquid organic waste constituted by spent organic solvents contaninated by radioactive or toxic products e.g. caning fran nuclear installations or research laboratories of the chemical industry, which have to be conditioned in a stable manner.

Among the knows waste conditioning processes, coating in bitumen is inadvisable, because liquid organic products lower the softening point of the bitumen. Coating in concrete is also inadvisable, because the dilution rust be very considerable to avoid euudation phenomena.

Incorporation into a polymerizable product is of interest, but is difficult to apply to liquid organic products.

Thus, the most reliable treatment processes for organic waste consist of transforming the latter into a mineral substance in order to bring about its long term conditioning. This can be carried out by incineration or dry pyrolysis of the waste. The incineration of organic liquid waste contaminated by radioactive elements causes certain problems, because it is difficult to treat in this way waste with an cc activity higher than 10 3Ci/m3 and a ss or Y activity higher than 10 1Ci/m3 Moreover, when the liquid waste is constituted by organic phosphorus compounds, combustion produces phosphoric acid droplets which nust be neutralized by the addition of calcium formate to the flame. Calcium pyrophosphate is then produced, which considerably increases the ash volune and clogs filters. In addition, in order to avoid exceptional operating conditions, the phosphorus ccntent of the solvent must be limited to 0.1%.

Dry pyrolysis processes are difficult to perform with highly contaninated liquid waste, because they lead to the handling of very active ash.

Another process for the treatment of organic waste is sulphuric mineralization, which has only been used up to now for the treatment of solid organic waste. This process consists of carbonizing the waste by sulphuric acid and oxidizing the carbon formed by nitric acid and/or hydrogen peroxide.With an organic waste of formula CmHn, this corresponds to the following reactions: CmHn+fl/2H2SO4 - > nH2O+n/2SO2+S (1) C+2H2SO4- > 2H2O+2SO2+CO2 (2) 3C+4HNO3 b 2H2O+4NO+300 (3) 3C+2HNO39 H2O+2NO+300 (4) C+H2So4 - > H2O+SO2+CO (5) The rate constants of reactions (2) to (5) at 2550C are as follows: K2 = 0.0016 K2+K3+K4+K5 = 0.016 min-1 K3 = 0.011 min-1 K4+K5 = 0.0029 Oxidation of carbon is faster by nitric than sulphuric acid (K3 K2).

Thus, there are two stages in this process: 1. Carbonization by H2S04, 2. Oxidation of the carbon into Oo and CO2 by nitric acid.

The gases formed during these reactions are sO2I CO2, NO and CO, but SO2 can be converted into sulphuric acid by oxidation with the nitrogen oxides formed and nitric acid. It is therefore possible to regenerate the sulphuric acid necessary for carbonization.

Processes of this type are described by Lerch et al in Nuclear and Chemical Waste Management, vol. 2, 1981, pp 265-277.

The present invention relates to a process for the treatment of liquid organic waste by sulphuric mineralization making it possible to obviate the disadvantages of knçn dry pyrolysis and incineration processes and whilst also having the advantage of being continuously performable.

The inventive process for the treatment of a liquid organic waste earprises: introducing into a reactor kept at a tgnperature of at least 1500C and containing concentrated sulphuric acid, a) the liquid organic waste to be treated and b) nitric acid and/or hydrogen peroxide at flow rates such that in the reactor is brought about a carbonization by sulphuric acid of the liquid waste introduced and an oxidation of the carbon and S02 fonned during carbonization by simltaneously regenerating the sulphuric acid, measuring continuously the optical density of the liquid medium present in the reactor and regulating the liquid organic waste flow rate and/or the flow rate of nitric acid and/or hydrogen peroxide introduced as a function of the measured optical density.

Tills, according to the invention in the same reactor are sinultaneously performed the two stages of carbonization and oxidation of the sulphuric mineralization process and the stage of regenerating the sulphuric acid by oxidation of sO2 by means of nitric acid and/or hydrogen peroxide introduced into the reactor. There is also a regulation of the flow rates of the liquid waste and/or the oxidizing agent introduced as a function of the measured optical density.

Thus, the optical density varies as a function of the elementary carbon concentration of the reaction mediun. In addition, when said optical density exceeds a set level corresponding to the elementary carbon concentration which can be accepted in the reaction mediun, there is either a reduction or stoppage to the organic liquid waste supply rate, or an increase in the nitric acid and/or wdrogén peroxide supply rate in order to oxidize the carbon excess present in the reaction median. As soon as the optical density again assumes a value below the set level, the liquid waste or nitric acid and/or hydrogen peroxide supply rate is restored to its starting value. This, the inventive process can be performed continuously without adding sulphuric acid.

Preferably, according to the invention, the flows of liquid organic waste, nitric acid and/or horogen peroxide introduced into the reactor are fixed to predetermined values and the introduction of the liquid waste into the reactor is interrupted when the measured optical density is above a set value and this supply is automatically reestablished after a stoppage time more particularly dependent on the value chosen for the liquid waste flow. When the flow rates are low, the stoppage lasts a few seconds, e.g. 1 to 3 seconds. However, the stoppage is longer when the flow rates are higher.

The sulphuric acid used for the carbonization of the waste is generally concentrated sulphuric acid, e.g. 16 to 18 M sulphuric acid at 250"C.

The nitric acid used for the oxidation of the carbon and the sO2 is also preferably concentrated nitric acid, e.g. 12 to 14 M nitric acid.

When hydrogen peroxide is used, the latter is preferably 3 hydrogen peroxide (110 volumes).

The inventive process is more particularly applicable to the treatment of liquid organic waste constituted by organic solvents such as those used in irradiated nuclear fuel reprocessing installations.

Examples of such solvents are those having at least one cadiz chosen fran anong organophosphorus coopunds such as tributyl phosphate and nines such as trilauryl amine and mixtures thereof.

When the liquid organic solvent to be treated is an organic solvent possibly containing an organic diluent, it preferably undergoes a preliminary neutralization treatment (to eliminate the acids and nitrate ions which may be present) and concentration by distillation in order to reduce the liquid volume to be treated, with the elimination of the diluent and alcohol, which are light products which would tend to partly volatilize during mineralization.

The neutralization treatment can be carried out at ambient temperature by contacting the solvent with a sodium carbonate solution. This solution makes it possible to eliminate the acids which may be present and the nitrate ions, which would produce undesirable reactions during distillation. It also makes it possible to solubilize cations, such as those of uranium or plutonium.

The distillation concentration operation is carried out under reduced pressure in nost cases. However, when the liquid waste contains highly volatile products, such as ethanol, praaanol and propionic acid, preference is given to the performance of distillation at ambient pressure. This distillation makes it possible to reduce the solvent volume to be mineralized, the distilled part being decontaninated and to eliminate the diluents and chlorine derivatives, which are generally very volatile, in order to avoid the production of undesirable wdrochloric acid in the reaction enclosure.

This distillation can be carried out in a tight enclosure using an apparatus with a short residence time to avoid the deterioration of the solvents and to limit the formation of tar. This apparatus can be equipped with a rotary belt for discharging deposits left during evaporation, so that there is no build-up of cleaning effluents. It is possible to recover the concentrated organic residue obtained in a vessel, which is reheated if necessary in the case where it has a high viscosity and7Or in order to avoid precipitation.

The condensates are discharged into a separate tight enclosure to avoid any radioactive repollution during the checking thereof.

If the solvent canes fran an irradiated fuel reprocessing installation, all these operations are carried out in a glove box under a nitrogen atmosphere.

In order to then carry out the treatment according to the invention, the organic residue is introduced into the reactor heated to a ttiper ature of an appropriate level and above 2000C, when the oxidant used is nitric acid.

Thus, organic nitro derivatives cannot ftnn above 200"C. In this case, it is preferable to operate at a teTlperature of 220 to 2700C, e.g. 2500C.

In the case of hydrogen peroxide, a ti3rsperature between 150 and 2000C, e.g. 1700C is Adequate.

The invention also relates to an apparatus for the treatment of liquid organic waste caiprising, as in the prior art apparatuses, a reactor, heating means for said reactor, means for introducing into the reactor the organic liquid waste to be treated and means for introducing into the reactor nitric acid and/or hydrogen peroxide, but which also comprises means for measuring the optical density of the liquid medium present in the reactor and means for regulating the flow rate of the organic liquid waste and/or the flow rate of the nitric acid and/or hydrogen peroxide introduced as a function of the measured optical density.

Preferably, the optical density measuring means comprise a first light guide and a second light guide reciprocally arranged in such a way that a light beam guided into the first light guide can be guided into the second light guide after perfonning a path in the liquid medium of the reactor, means for introducing a light beam into said first light guide and means for measuring the intensity of the light bean leaving the second light guide.

The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings, wherein show: Fig. 1 A general diagram of the installation.

Fig. 2 A diagrammatic representation of an apparatus according to the invention.

Fig. 3 A constructional variant of optical density measuring means.

Fig. 1 shows that the installation canprises a reactor 1 into which is introduced via pipe 7 equipped with pump 9, the organic solvent to be mineralized and by tube 11 provided with pump 13, concentrated nitric acid. The reactor can be heated by heating means 14. The gases fran the reaction are discharged by a pipe 19 connected to a first rectification column R associated with a condenser C into which is introduced the air.At the outlet fran the condenser, dilute HNO3 and H2504 are recovered, which are stored and gas is recovered containing nitrogen oxides and air which are recanbined catalytically in the catalytic recanbination column 20 by forming nitric acid with water introduced in the upper part of said column 20. The gases freed fran nitrogen oxides then pass into the sodium trap 22, where the and the last traces of impurities (HNO3, HN02 and H2SO4) are trapped by soda.

Fig. 2 shows that the apparatus canprises a reactor 1, e.g. made fran pyrex glass, which is sealed in its wer part by a cover 3, e.g. made fran solid polytetrafluoroethylene.

Reactor 1 is partly filled with sulphuric acid 5 and the organic liquid waste to be treated is introduced by a pipe 7, which is not imeersed in the sulphuric acid 5. The liquid waste flow rate is regulated by a gear purp 9, which also permits the introduction of a small air flow to avoid any clogging of the pipe 7 by carbonization of organic matter.

The nitric acid or hydrogen peroxide used for the reaction is intro- duced by a tube 11 irmsersed in the sulphuric acid 5 and its flow rate is regulated by a purtp 13. The reactor is also equipped with heating means 14, stirring means constituted by a turbine 15, e.g. made fran polytetrafluoroethylene, a probe 17 for measuring the temperature of the liquid medium and optical density measuring means 21, 23. The gases produced by the reaction can be discharged fran the enclosure by a pipe 19 connected to the ventilation circuit of the confined enclosures of the nuclear installation after passing over a gas treat merit assembly, like that shown in fig. 1.

In the Bnbodlment shcsn in fig. 2, the optical density is measured by a photoelectric sensor 21 located at the end of the reactor facing a light source 23 located in a transparent tube 25 for isolatian fran the liquid medium. The light source can be a quartz iodine lard.

Generally the light source 23 is approximately 1 an fran the wall of the reactor 1, which is in this case obviously transparent.

The photoelectric cell 21 indicates the optical density of the solution and it is connected to a microprocessor 27 controlling the px 9 for supplying the liquid waste to be treated.

The microprogranmer 27 is programmed in such a way that when the optical density exceeds a set level, e.g. corresponding to a carbon content of the reaction medium of 1 to 5 g/l, it actuates the puup 9 in order to provisionally stop or reduce the supply flow of organic waste to be treated.

In this anbodiment, the flow of nitric acid or hydrogen peroxide introduced by tube 11 is regulated by pump 13 to a fixed value.

Obviously, pump 13 could be made dependent on the microprocessor 27, in order to increase the nitric acid or hydrogen peroxide flaR when the optical density exceeds the set level.

Fig. 3 shows another embodiment of means for measuring the optical density of the liquid medium present in the reactor. In this case, the descending or immersing tube 25 of fig. 2 is replaced by the assenbly 30 of fig. 3. This assembly comprises a holder or envelope 29, e.g. made fran polytetrafluoroethylene and having orifices 31 enabling the reaction medium to reenter the envelope 29. Within the latter is provided a first light guide 33, whose lower part 33a is curved and is then extended level with the orifices 31 by a second light guide 35, which is positioned above light guide 33a by providing a space between the two guides corresponding to an optical path T in the reaction medium.A light generator outside the reactor not shown in the drawing is positioned in such a way as to introduce a light bean into the first light guide 33. This light bean then traverses the liquid mediun on path T and is then guided by the second light guide 35, which is associated with a photoelectric cell outside the reactor, in order to determine the intensity of the outgoing light bean and for deducing therefran the optical density of the liquid medwn present on optical path T. The photoelectric cell associated with light guide 35 is connected to the microprocessor 27 which, as hereinbefore, controls the introduction rate of the liquid waste to be treated. As hereinbefore, the optical path T corresponds to approximately 1 em of solution.

This second system is particularly adapted to working in a tight enclosure, e.g. a glove box, because the light guide and detection photoelectric cell can both be located outside the glove box.

The apparatus according to the invention can be provided with security or standby systems to prevent any incident in the case of a failure of the light scurce, the photoelectric cell or the nitric acid supply.

In all cases, the organic liquid waste supply is stepped. In the case of a tarperature drop, it is possible to stap the organic liquid waste supply.

The following examples are given to illustrate the process of the invention.

Example 1: Treatment of tributyl phosphate This example uses a 1 1 reactor containing 0.5 1 of 18 M sulphuric acid heated to 2500C and through pipe 11 is introduced 14 N nitric acid at a flow rate of 197+3nl/h, whilst making turbine 15 operate at a speed of 300 r.p.m. The tributyl phosphate (TBP) is introduced at a flow rate of 100 ml/h through pipe 7 and the microprocessor 27 is regulated in such a way as to stop the tributyl phosphate supply when the optical density exceeds a value corresponding to 1 g/l of carbon in the liquid mediun. The latter is autanatically reestablished after a few seconds.After 4 hours operation, the mean TBP flow rate is determined, together with the consunXption levels of HNO3 and H2904 relative to C, together with the OH consumption in the sodium trap with respect to C. The C quantity is determined on the basis of the introduced TBP quantity. The results obtained are given in table 1.

Carparative exanple 1 This example follows the sane operating procedure as in example 1, except that a visual check is carried out on the reaction and the tributyl phosphate supply is started and stopped manually. The results obtained under these conditions are given in table 1.

On capparing these results with those of example 1, it can be seen that optical sensor control permits a treatment capacity gain, a nitric and sulphuric acid consumption reduction and a reagent consumption reduction in the case of alkaline washing of gases.

Eanples 2 to 6: Tributyl phosphate treatment These examples study the influence of the nitric acid flow rate on the treatment capacity of the reactor. Use is mede of a 1 1 reactor kept at 250"C and containing 0.5 1 of 18 M sulphuric acid. The 14 N nitric acid is introduced at a flow rate varying fran 208 to 300 ml/h, as a function of the examples. For the tributyl phosphate supply use is mede of a flow rate of 100 ml/h and this supply is stopped as in exarqple 1 when the detector 21 detects an optical density above that corresponding to 1 g/l of cabin.

The nitric acid flow rates, the mean tributyl phosphate flow rates and the nitric acid consumption with respect to C obtained under these conditions are given in table 2. The latter shows that for a 1 1 reactor, the optinun value for the nitric acid flow is 250 ml/h, because beyond the said value the treatment capacity gain is zero.

Examples 7 to 10 These examples involve the treatment of different organic solvents operating with a constant nitric acid flow rate and solvent flow rates stopped by the optical sensor as in example 1. The results and treatment conditions are given in table 3.

The table shows that the capacity is significantly lower when the solvent contains trilauryl amine. Thus, the long carbon chains of trilauryl gamine are more resistant than those of tributyl phosphate.

The results of example 8 demonstrate that it is preferable to treat trilauryl amine mixed with tributyl phosphate.

TABLE 1 Test No. Comparative Example Example 1 HNO3 rate ml.h-1 195 199 Mean TBP rate ml.h 39 73 HNO3 consumption HNO3 1.75 1 C (1) H2SO4 consumption H2S04 0.6 0.4 C (1) OH consumption (sodium trap) (2) 2 0.9 OH C (1) (1) The C quantity is calculated on the basis of the chemical fonnula of the TBP introduced.

(2) The sodium trap stops HNO3, NO2 and H2SO4 in trace form and all the OD2 The soda is replaced on reaching a pH of approximately 8. In addition, the CO is trapped in the form of HCO3 - and only 3 OH is consumed by CO2 TABLE 2 Example 2 3 4 5 6 14N HNO3 rate ml.h-1 208 216 250 272 300 Mean TBP rate ml.h 1 76 74 80 80 81 HNO3 consumption Molar ratio HNO3 C (1) 0.9 1 1.1 1.2 1.3 (1) The C quantity is calculated on the basis of the chemical formula of the TBP introduced.

TABLE 3 Example No. 7 8 9 10 Solvent type to be TBP TBP:TIA volume TLA TLA 98% mineralized ratio 1:1 Hyfrane 2% Solvent rate ml.h 1 88 82 46 60 Carbon rate mol.h 1 3.9 4.1 2.6 3.4 HNO3 rate ml.h 1 272 291 238 251 Consumption 3 C (1) 1.04 1 1.4 1.2 Consumption H2SO4 C (1) 0.13 0.11 0.12 0.17 Consumption C (1) 1.03 1.2 1.4 1 TBP = tributyl phosphate TLA = trilauryl amine (1) The C quantity is calculated on the basis of the chemical formulas of the TBP and/or TLA introduced.

Claims (9)

1. Process for the continuous treatment of a liquid organic waste, characterized by introducing into a reactor kept at a tençerature of at least 1500C and containing concentrated sulphuric acid, a) the liquid organic waste to be treated and b) nitric acid and/or hydrogen peroxide at flow rates such that in the reactor is brought about a carbonization by sulphuric acid of the liquid waste introduced and an oxidation of the carbon and SO 2 formed during carbonization by sirtultaneously regenerating the sulphuric acid, measuring continuously the optical density of the liquid medium present in the reactor and regulating the liquid organic waste flow rate and/or the flow rate of nitric acid and/or hydrogen peroxide introduced as a function of time measured optical density.

2. Process according to claim 1, characterized in that fixing takes place to given values of the flow rates of the organic liquid waste, the nitric acid and/or the hydrogen peroxide introduced into the reactor and the introduction of liquid waste into the reactor is stopped when the measured optical density is abave a set level.

3. Process according to either of the claims 1 and 2, characterized in that the liquid organic waste is an organic solvent.

4. Process according to claim 3, characterized in that the organic solvent carprises at least one canpound chosen fran among tributyl phosphate and trilauryl anine.

5. Process according to either of the claims 3 and 4, characterized in that the organic solvent cerises an organic diluent.

6. Process according to any one of the claims 3 to 5, characterized in that the organic solvent undergoes a pretreatment consisting of firstly neutralizing said organic solvent to eliminate the acids and nitrate ions possibly present and then concentrating the neutralized organic solvent ty distillation in order to eliminate the very volatile products prior to introducing it into the reactor.

7. Process according to any one of the claims 1 to 6, characterized in that the reactor is kept at a tanperature of either 150 to 2000C or at 220 to 270"C, as a function of the oxidant chosen.

8. Apparatus for the treatment of liquid organic waste comprising a reactor (1), reactor heating means (14), means (7, 9) for introducing into the reactor the organic liquid waste to be treated and means (11, 13) for introducing into the reactor the nitric acid and/or hydrogen peroxide, characterized in that it also cawrises means (21, 23) for measuring the optical density of the liquid medium present in the reactor and means (27, 9) for regulating the flow rate of the organic liquid waste and/or the flow rate of the nitric acid and/or hydrogen peroxide introduced, as a function of the measured optical density.

9. Apparatus according to claim 8, characterized in that the optical density measuring means comprise a first light guide and a second light guide positioned with respect to one another in such a way that a light bean guided in the first light guide can be guided in the second light guide after having performed a path in the liquid medium of the reactor, means for introducing a light beam into said first light guide and means for measuring the intensity of the light bean leaving the second light guide.