IE920042A1 - Process for the purification of aqueous solutions polluted¹by nitrate ions - Google Patents

Abstract

Process for the purification of aqueous solutions contaminated with nitrate ions. This process is characterised in that it consists in precipitating double or mixed hydrated calcium nitroaluminates, such as especially calcium mononitroaluminate hydrate, by addition of at least one agent introducing the element aluminium and at least one agent introducing the element calcium into the solutions to be treated, the overall molar ratio of the element aluminium to nitrate, Al/NO3, being higher than 1, the overall molar ratio of calcium to nitrate, Ca/NO3, being greater than 2, and in that the precipitation reaction is performed with stirring and at basic pH, preferably higher than 10.5. The process according to the invention is intended especially for the treatment of polluted waters such as domestic sewage and industrial effluents and for the treatment of liquid manure.

Description

LAFARGE FONDU INTERNATIONAL, a Company organised and existing under the laws of France, of 157 Avenue Charles de Gaulle, 92200 Neuilly sur Seine, France 50J -1The invention relates to a process for the purification of aqueous solutions polluted by nitrate ions.

The invention also relates th the recycling of the 5 solid compounds emanating from the above-mentioned purification process, preferably in the field of fertilisers when the bound nitrate content is sufficient.

Nitric acid is one the most widely used mineral acids. Nitric acid is used on a very large scale in the chemical industry, predominantly for the manufacture of artificial fertilisers, in the form, in particular, of ammonium nitrate. Nitric acid, in addition, is of very great importance in the synthesis of many explosives.

The salts of nitric acid, nitrates, are also well known and widely used in the chemical and metallurgical industry.

The use of nitric acid or its salts is accompanied, in most cases, by discharges of effluents containing nitrate ions. Industry also discharges various other forms of nitrogen which may be converted to nitrates : ammoniacal nitrogen is discharged, in particular by coking plants, and nitrogenous organic compounds are discharged, in particular, by the agri-foodstuffs industries.

Nitrates can have a natural origin, such as in the form of deposits of salts, generally of sodium nitrate.

Nitrates can also emanate from the conversion of ammoniacal nitrogen by biochemical nitrification. Liquid manure constitutes one of the raw materials richest in ammoniacal nitrogen : approximately half the nitrogen present in liquid manure is in organic form, and approximately half in ammoniacal form (that is to say of the order of 2 to 3.5 g/1 of NH4+) . In smaller proportions, municipal waste water also contains ammoniacal nitrogen which can likewise be converted to nitrates biochemically.

To accomplish nitrification biochemically, it can be sufficient to aerate the liquid manure when the latter naturally contains the appropriate bacteria. The nitrogen present in liquid manure is modified to only a very small -2extent during storage of the liquid manure; at the very most, a slight loss takes place through outgassing of ammonia. Spreading on the ground causes a mineralisation of the readily decomposable organic matter and a nitrification of NH4* . The nitrates produced may be used by plants, assimilated by microorganisms or leached in depth into the soil beyond the root absorption zone; they are then lost to the plant and contribute in the longer or shorter term to the pollution of groundwater.

Whether of synthetic or natural origin and irrespective of the chemical nature of the counter anion, nitrates are, for the most part, soluble in aqueous media in high proportions. As an example which may be given, solubility in water of calcium nitrate, Ca(NO3)2, reaches 1.212 kg per litre at 18°C and that of sodium nitrate, NaNO3, reaches 0.921 kg per litre at 25°C.

The presence of ever increasing amounts of nitrates in the natural environment currently constitutes a matter of concern. It is known that, taken in repeated small doses, nitates are potentially toxic on account of the possible conversion of nitrates to nitrites, which can cause a methaemoglobinaemia which can be fatal in young children, and to nitrosamines which are reputedly carcinogenic. For this reason, doctors recommend giving only water free from nitrate ions to young children. In point of fact, as a result of their high solubility in aqueous media, the concentration of nitrates, whether they originate from discharges of industrial effluents, from municipal waste water or from chemical or natural fertilisers (spreading of liquide manure), is gradually increasing in surface water and groundwater which, in some instances, has become unfit for human consumption.

The nitrate concentration is limited to 50 mg per litre in drinking water in France and in the European Community in general. However, as a result of the lack of a purification plant suited to the removal of nitrates on French territory, nitrate ion concentrations which can range up to 100 mg per litre are to be found in some drinking water supplied to the mains network. -3 To solve this problem, various purification processes are currently in use for the treatment of water intended for human consumption. However, no economically advantageous process capable of industrialisation exists for the treatment of effluents highly polluted with nitrates, such as liquid manure.

The first type of process used for obtaining drinking water by physicochemical means proposes the removal of nitrate ions by denitratation with an ion exchange resin.

This process requires water possessing a content of suspended solid matter of less than l mg per litre, otherwise the water to be treated has to be subjected beforehand to a sieving followed by one or more flocculation operation(s) and filtration operation(s) and/or settling and physical separation operation(s).

The ion exchange resin retains the nitrate ions as well as sulphate ions, chloride ions and hydrogencarbonate ions. One main drawback of this process lies in the fact that it cannot be used if there is an excessive amount of sulphate ions and chloride ions in the water, which have an affinity which is, respectively, very much greater than and similar to that of nitrates for the resins. Since regeneration of the resins is generally carried out with sodium chloride, a replacement of the nitrate ions by chloride ions takes place during the purification process. This results in an increase in the chloride ion content in the water treated by this process.

Another major drawback of this process lies in the purpose of the eluates obtained after regeneration of the resins. As a matter of fact, what is to be done with these eluates rich in chlorides, nitrates and often sulphates ? If the concentrations are not too high, they go into the nearest stream. In the other cases, they are either sent to the nearest biological purification plant or stored.

Another type of biological process proposes removal of the nitrate ions using bound bacteria capable of metabolising these ions. Usually, heterotrophic bacteria drawing their energy from a carbonaceous nutrient such as -4ethanol or acetic acid are used. By this process, the nitrate ions are converted to gaseous nitrogen. Proliferation of the bacteria leads to an excess of biomass or sludge, which can be treated with municipal sludges or be incinerated.

This process can be used only at temperatures above 8°-10°C. In addition, these temperatures must be stable during the treatment, otherwise control of the process is complex. This process is consequently relatively expensive to carry out.

An aim of the present invention is to propose a process for the purification of aqueous solutions polluted by nitrate ions which is simple, effective and relatively inexpensive to carry out.

Another aim of the invention is recycling of the compounds containing nitrate ions and resulting from this process, preferably in the fertiliser sphere, without risk of pollution of the environment.

According to the invention, to achieve these aims, a process is provided for the purification o aqueous solution pollyted by nitrate ions, characterised in that it consist in precipitating hydrated double or mixed calcium nitroaluminates, such as, in particular, hydrated calcium mononitroaluminate - 3CaO.Al2O3.Ca(NO3)2.nH2O - by adding to the solutions to be treated at least one agent supplying the element aluminium, this agent being referred to a ’’active alumina", and at least one agent supplying the element calcium, the overall mole ratio of the element aluminium to nitrate, A1/NO3, superior to 1 and the overall mole ratio of calcium to nitrate, Ca/NO3 , superior to 2, in that the precipitation reaction is performed with stirring at basic pH, preferably above .5, and in that, where appropriate, the precipitate obtained is removed, for example, by flocculation followed by one or more settling and physical separation and/or filtration operation(s).

In spite of variety of the compounds containes in the solutions to be treated, and therefore the reactions that can compete with the formation of hydrated calcium -5nitro-aluminate, the elimination of the nitrate ions, with the help of the invention process is wholly satisfactory.

The process according to the invention is advantageous in more than one respect: apart from a trapping of a large proportion of the nitrate ions in the form of hydrated double or mixed calcium nitroaluminates, this process makes it mossible to remove a large part of the undesirable cations in hydroxide form, as well as undesirable anions forming insoluble precipitates in the presence of the element calcium, or forming complex calcium aluminate such as carbonates, sulphates and chlorides.

Advantageously, the process according to the invention is carried out at room temperature. That is to 15 say at values inferior to 20°C or approaching 30 to 40°C according to seasons. Preferably the process according to the invention is implemented at a temperature of about 20°C, and more preferably at a temperature inferior to 20°C : under those conditions although the starting kinetic of the reaction is slower, on the other hand the yields at those temperatures are better.

Where appropriate, prior to the separation of the nitrate ions, various chemical treatments may be performed to remove other undesirable compounds included in the 25 effluent to be treated.

In the case where the effluents to be treated comprise suspended solid matter, it is possible to perform one or more precipitation (s) of this matter by flocculation, for example using an addition of ferric 30 chloride and a base, or using aluminium sulphate, or alternatively using an organic flocculating agent such as polyacrylamides, this precipitation being followed by one or more filtration operation(s) and/or settling and physical separation operation(s).

In the case where the effluents to be treated comprise undesirable anions insoluble calcium salts, a components is advantageously performed by adding an agent supplying the element calcium, and the precipitated phases or components forming precipitation of these -6obtained are then removed before carrying out the removal of the nitrates.

Preferably, this precipitation is performed using calcium hydroxide. This operation results in a lowering of the concentration of anions which are only sparingly soluble in the form of calcium sals, such as sulphates, phosphates, carbonates, fluorides, and the like. The removal of these anions makes it possible to limit the formation of other complex aluminates, concomitantly with the hydrated calcium nitroaluminates.

In the case where the effluents to be treated comprise undesirable cations or components forming hydroxides which are only sparingly soluble or insoluble, a precipitation of these components is advantageously performed by adding a basic agent. Preferably, this precipitation is performed using calcium hydroxide. This operation results in a lowering of the concentration of anions which are only sparingly soluble in the form of calcium salts, as well as that of cations which are only sparingly soluble, such as lead, etc., in hydroxide form. After removal of the precipitated phases obtained, the removal of the nitrates is carried out.

The process according to the invention may be performed using various sources of active alumina, employed alone or in combination. As an example, there may be mentioned aluminates, and in particular alkali metal aluminates, calcium aluminates constituting certain cements, such as CA2, CA, C12A7, and C3A, and calcium aluminoferrites. In the various formulae mentioned above and in the description which follows : C represents CaO A represents A12O3 H represents H2O.

Among calcium aluminates, those obtained by a sintering process rather than those obtained by a fusion process are preferably used. In fact, the calcium aluminates emanating from a sintering process generally possess a better capacity for being ground as well as greater reactivity . -7Hydrated calcium aluminates such as C4AH13 can also lead to the formation of complex aluminates by ion exchange. Aluminous cements bearing the trade names FONDU, SECAR, LDSF as well as cubic-T are also suitable : the 5 latter is a cubic calcium aluminate C3AH5 decomposed thermally at approximately 300°C to C12A7 and Ca(OH)2. This product possesses directly the stoichiometry needed for the formation of complex aluminates (C/A=3) in the case of a reaction with a calcium salt.

Preferably also, products comprising C3A and/or C12A7 are used, employed alone or in combination with other compounds with which greater reactivities as well as high yields in short periods of time, of the order of 1 to 3 hours, have been observed.

The yields are improved again when employing as reagents C3A and Cao. The best yields and reactivities have been obtained using the reagent CHA which corresponds to the product of the making of C3A with too much lime.

It is preferable to work close to this theroretical stoichiometry in the first moments of formation of the calcium nitroaluminates. This stoichiometry corresponds to C/A = 4 in the general case, and to C/A = 3 for the treatment of a solution of a calcium salt.

The agent supplying the element calcium can be calcium hydroxide, or a calcium aluminate supplemented, if necessary, with a second agent supplying the element calcium, such as calcium hydroxide.

Advantageously, the reaction of aqueous solution polluted by nitrates according to the invention is initiated by means of addition of hydrated double or mixed calcium nitroaluminate at the beginning of the reaction in the proportion of approximately 10 to 50 % by weight relative to the weight of calcium aluminate introduced.

Under these conditions, an improvement has, as a matter of fact, been observed in the rate of formation of hydrated double or mixed calcium nitroaluminates, and principally during the initial phase of this reaction, as well as an improvement in the precipitation yield of the nitrates. The hydrated double or mixed calcium -8nitroaluminate which is mainly to be formed during the reaction is advantageously used as a seed.

The process according to the invention, as well as the different variants of this process presented above, find an especially advantageous field of application for treating water loaded with nitrates and possibly with other inorganic compounds, such as industrial effluents and municipal waste water, as well as media loaded with ammoniacal nitrogen, the nitrogen being liable to be converted to nitrates such as liquid manure.

When the solution to be treated is a solution rich in nitrogen in ammoniacal form and in organic form, the present invention enables the pollutant power of these solutions to be reduced considerably or even abolished.

This applies in particular to raw materials such as liquid manure which generate solutions rich in nitrates through nitrif ication.

Liquid manure, in particular, is at present simply spread in the crude state; its high content of soluble nitrogen, chiefly in ammonium form, leads o risks of substantial pollution, especially when it is spread outside the periods of the year in which plants assimilate large amounts of nitrates needed for their growth. By means of a simple treatment of the liquid manure, prior to spreading and in accordance with the process according to the invention, the fertiliser qualities of the liquid manure are retained and/or improved while advantageously abolishing its pollutant power.

According to a first preferred embodiment of the invention, which relates especially to the treatment of solutions or suspensions comprising nitrogen in ammoniacal form, such as liquid manure, the following steps are performed successively: 1) the ammoniacal nitrogen present in the solutions to be treated is converted to nitrates biochemically in the presence of oxygen. For this purpose, the following procedure may be adopted : the solutions rich in ammoniacal nitrogen, such as liquid manure, are aerated -9and, if necessary, these solutions are inoculated with nitrifying microorganisms to induce nitrifications. 2) then, once the majority of the ammoniacal nitrogen is converted to nitrates, one or more calcium aluminates is/are added to the solution to be treated, so that the overall mole ratio of the element aluminium to nitrate, A1/NO3, be superior to 1 and that the overall mole ratio of the element calcium to nitrate be superior to 2 and, if necessary, calcium hydroxide and/or another basic agent to adjust the pH to a basic value, preferably to a value above 10.5, and the precipitation reaction is performed with stirring and at room temperature. 3) after separation, for example after settling has taken place or by filtration, a thick suspension or a solid are collected, which product is advantageously used as an enriching agent and fertiliser for agricultural soils. 4) finally, a liquid effluent depleted in nitrates, the pollutant nature of which is greatly reduced in comparison with the initial liquid manure, is collected. Thus, its discharge into the environment can be safety envisaged.

Apart from a removal of a large part of the nitrate ions, as well as of ions forming insoluble hydroxydes and isoluble calcium salts, the process according to the invention, in contrast to known processes, when applied to the treatment of liquid excrement such as liquid manure, has the advantage of producing clear, decolorised and deodorised liquid effluents.

In point of fact, liquid excrement includes organic compounds of biliary origin, which are highly coloured and whose coloration varies between yellow and brown hues and which, moreover, biodegrade with difficulty. It may hence be supposed that these coloured organic compounds are bound in precipitate advantageously the hydrated and/or the accompany calcium mononitroaluminate other precipitates which the formation of this precipitate, namely insoluble hydroxides and insoluble calcium salts.

The alkalinity of the suspensions or solids obtained at the end of step (3) can enable an acid soil to be corrected. In addition, their content of sparingly soluble nitrate enable the nitrate fertiliser to be supplied at a rate proportional to its consumption by the plants, and thereby eliminates a source of pollution of groundwater and stream water.

The nitrification according to step (1) consists in converting ammoniacal nitrogen to nitrate biochemically in the presence of nitrifying bacteria and oxygen, preferably supplied by aeration. This conversion is performed according to conventional working conditions.

It is generally accomplished in two stages by atutorphic microorganisms: - oxidation of ammoniacal nitrogen to nitrite by suitable microorganisms. As an example, microorganisms of the genus Nitrosomonas may be mentioned. - followed by an oxidation of the nitrites to nitrates by suitable microorganisms. As an example, microorganisms of the genus Nitrobacter may be mentioned.

The nitrifying microorganisms may be found in water purification plant sludges or alternatively in partially or completely nitrified liquid manure.

The rate of nitrification depends, in particular, on the temperature, on the pH, which is preferably maintained at between 7.2 and 8, and on the amount of oxygen which may be available to the microorganisms. It is also -11important to take care that the nitrification reaction medium does not contain organic or inorganic compounds capable of modifying or even inhibiting the growth of the microorganisms. For further details regarding the conditions necessary during nitrification, reference may be made to the work entitled "M6mento technique de 1’eau (Technical handbook on water), 9th edition, volume l, pp. 300-301, published by DEGREMONT.

Advantageously, the adjustment to a basic pH for the requirements of the precipitation reaction of step (2) is performed using potassium hydroxide, in order to yield residual sludges rich in potassium, increasing their value as fertiliser.

Advantageously, before the precipitation of hydrated double or mixed calcium nitroaluminates (step (2) of the first embodiment), either before or after the nitrification step (1) , the solid matter included in the liquid manure is separated at least partially, generally by flocculation followed by a filtration and/or settling and physical separation operation. Among the solid matter, protein-rich matter, phosphates and nitrogen compounds which are only sparingly soluble in water are recovered.

Preferably, the separation of the solid matter included in the liquid manure is performed before the nitrification step (1) . If this is not done, the solid matter, which possessed a certain biochemical oxygen demand (BOD), competes with the nitrification reaction and naturally places it at a disadvantage. The filtrate constituting the clarified liquid manure is then subjected: - to the nitrification treatment according to step (1) in the case where this treatment has not been carried out beforehand; - and then to the treatment for precipitation of the nitrates in accordance with steps (2) and (3).

The best precipitation yields of the nitrates have been obtained under these conditions. -12Where appropriate, the solid matter is separated from the suspension obtained at the end of step (3), for example by flocculation followed by a filtration and/or settling and physical separation operation. There are collected, on the one hand a filtrate depleted in nitrates, which takes the form of a clear and practically odourless solution capable of being spread or of being used for irrigation, and on the other hand solid matter containing hydrated double or mixed calcium nitroaluminates, which find application as a fertiliser. It has, in effect, been shown by means of agronomic experiments that the nitrate ions are temporarily trapped in the solid matter in the state of hydrated double or mixed calcium nitroaluminates, but remain available to the plants and assimilable by the latter.

The practical application of the process according to the invention to the treatment of liquid manure may be envisaged according to the following example of an installation arranged on a pig farm.

Installation of the process This consists in adapting and supplementing the existing installation by partitioning the liquid manure storage tank. After adaptation and supplementation, the new installation comprises a first compartment consisting of a nitrification tank N.

The nitrification tank N is equipped with a microbubble aeration system (production of air + diffusion).

This installation also comprises a reactor for the precipitation reaction of the nitrate ions in accordance with the process according to the invention, and a salt/effluent separation system (sieve type).

The installation may be supplemented with a pretreatment of the liquid manure before nitrification (settling and physical separation, or screening and/or flocculation) with the object of reducing the BOD and hence the oxygen consumption of the medium. The - 13flocculation may be carried out using a weakly cationic polyacrylamide.

The second compartment of the existing storage tank, referred to as tank S, is used for storing the effluent obtained after separation of the nitrate ions.

Operation The average input to the installation, in m3 of crude liquid manure per day, is dependent on the number of pigs.

The pretreatment before nitrification produces : - organic sludges having a high percentage of dry matter. These sludges contain organic nitrogen and virtually all of the phosphorus; they are used as a humous enriching agent on the farm. - the liquid manure to be treated having a small solid matter in suspension content.

Nitrification Feeding the nitrification tank N is carried out using liquid manure cleared of organic sludges.

The residence time is matched to the rate of nitrification.

Treatment The sequences of the treatment are as follows: At regular intervals, a certain volume VI of nitrified liquid manure is withdrawn from the tank N to the reactor. At the same time, a certain volume V2 of nitrate-depleted final effluent may be transferred from the effluent storage tank S to the nitrification tank N in order to maintain a constant volume in the latter. The volumes VI and V2 to be treated are then calculated so as to maintain a constant N03 concentration in the tank N and a constant level in the tank N; it enables an amount of nitrates equivalent to that formed during the period in question to be extracted from this tank. - 14In the reactor, the liquid manure to be treated is stirred and a suitable dose of active alumina and, if necessary, of calcium hydroxide is introduced.

The medium is kept stirring during the reaction. 5 After the reaction, ths suspension of nitrate salt is extracted from the reactor to a sieve in order to separate the salt from the effluent. It is also possible to separate the salt by a simple settling and physical separation operation.

The salt is stored while awaiting its use as a nitrogenous fertiliser. The effluent, depleted in nitrate (it generally does not contain more than 0.3 to 0.5 g/1 of NOj) and containing virtually all of the potassium in the liquid manure, is stored in the tank S; it is used as a fertiliser by sprinkling or by spreading.

According to a second embodiment of the invention, which is directed towards treatment of aqueous solutions comprising a high to moderate content of nitrate ions and small amounts of organic compounds such as industrial effluents or municipal waste water or regeneration solution of exchanger resins used in sewerage plants, the following second variant of the process according to the invention is advantageously used: 1) one or more calcium aluminate (s) and, if necessary, calcium hydroxide and/or another basic agent to adjust the pH to a basic value, preferrably to a value above 10.5, is/are added to the solution to be treated. 2) the precipitation reaction is performed with stirring and at room temperature. 3) at the end of this reaction, the suspension obtained is subjected to a settling and physical separation and/or filtration operation, after the addition of a flocculating agent where appropriate. -154) then, if necessary, the filtrate obtained is neutralised, which also has the effect of precipitating the alumina in solution in the form of an insoluble hydroxide. ) lastly, the new suspension obtained is subjected to a settling and physical separation operation and/or filtration operation to obtain a purified water which can be discharged into the environment without risk of pollution.

If the nitrate content of this purified water is less than 50 mg per litre, and if it does not include other components or compounds at levels greater that those laid down in regulations, this water can be fed into drinking water distribution networks.

If the solution to be treated is a polluted water, such as a water originating from aquiculture or pisciculture or a municipal waste water containing ammoniacal nitrogen, an operation of nitrification of this solution, of the type presented above, is advantageously arranged.

During step (1) , sodium hydroxide, potassium hydroxide or calcium hydroxide may be used as a basic agent.

Advangateously, the addition of a basic agent is performed prior to the addition of calcium aluminate. This results in a decrease in the concentration of cation which are only sparingly soluble in the form of insoluble hydroxides.

Advantageously, prior to the addition of calcium aluminate, an agent supplyint the element calcium other than calcium aluminates is added to the solution to be treated. This results in a decrease in the concentration of anions which are only sparingly soluble in the form of calcium salts, such as sulphates, phosphates, carbonates, fluorides and the like. -16Advantageously also, calcium hydroxide is chosen at the basic agent and agent supplying the element calcium, and is introduced prior to the calcium aluminate.

The solution then undergoes the treatment of removal 5 of nitrate ions by precipitation after removal of the precipitated phases. These process variants enable the formation of other complex aluminates concomitantly with the mononitroaluminate to be limited.

In the precipitation steps, flocculating agents of 10 natural origin such as alginates or xanthan gums are preferably chosen. However, salts such as ferric chloride or aluminium sulphate as well as polymers such as polyacrylamides are also suitable for the invention.

Advantageously, the residual sludges obtained in 15 this variant of the process according to the invention at the end of step (3), when they are rich in calcium nitroaluminates, may be used as an enriching agent and fertiliser for agricultural soils. In effect, their content of sparingly soluble nitrate enables the nitrate fertiliser to be supplied at a rate proportional to its consumption by the plants, and thereby eliminates a source of pollution of groundwater and stream water. In addition, the alkalinity of these sludges can enable an acid soil to be corrected.

In the case where the filtrate is intended for the preparation of drinking water or for discharging into the environment, the filtrage [step (4)] is advantageously neutralised using CO2 in order to precipitate calcium carbonate simultaneously with the aluminium hydroxide.

In accordance with the process according to the invention, apart from the agents needed for precipitation of the nitrates in solution, the solutions to be treated can receive crystallising agents having the property of influencing the kinetics of formation of the double or mixed nitroaluminates. These crystallising agents consist, for example, of at least one of the precipitated double or mixed nitroaluminates, taken alone or in combination.

Other features and advantages of the invention will become apparent on reading the examples which follow, - 17given for the purpose of illustration and without implied limitation of the invention.

Example 1 to 3 An aqueous solution containing calcium nitrate Ca(NO3)2 is introduced into a chemical reactor equipped with a magnetic stirrer: the initial NO3* concentration varies between 500 mg per litre and 125 mg per litre. In these examples, the active aluminal is calcium aluminate CA in aqueous solution. The additional supply of calcium is obtained by adding calcium hydroxyde Ca(OH)2.

The treatment is carried out by simultaneously adding CA and Ca(OH)2. The treatment is carried out with stirring. The suspension obtained is filtered or treated with a flocculating agent and subjected to a settling and physical separation operation. Nitrate analysis is then performed with a specific electrode after adding a buffer and adjusting the pH to 5.5.

The other working conditions and the results for Examples 1 to 3 are recorded in Table 1 below.

Table I EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 Initial NO3 concentration (mg/1) 500 125 125 Amount of CA in aqueous solution (mg/1) 2844 711 355 Amount of Ca(OH)2 (mg/1) 3696 925 425 Overall mole ratio A1/NO3 4.46 4.46 2.23 Overall mole ration Ca/NO3 8.93 8.92 4.71 Treatment period (min.) 15 15 15 Residual N03, concentration (mg/1) 123 81 96 Yield (%) 75 35 23 - 18Examples 4 to 6 The working conditions in these examples are identical to those in Examples 1 to 3, except for the agent supplying the element aluminium, which is in the form of C3A.

The other working conditions and the results for these examples are given in Table II below: Table II EXAMPLE 4 EXAMPLE 5 EXAMPLE 6 Initial NO3 concentration (mg/1) 500 250 2300 Amount of C3A (mg/1) 4899 5000 5000 Amount of Ca(OH)z 0 0 0 Overall mole ratio A1/NO3 4.5 9.19 1.0 Overall mole ratio Ca/NO3 7.25 14.28 2.0 Treatment period (min) 350 (220) 160 1100 Residual NO3 concentration (mg/1) 86 (96) 98 480 Yield (%) 83.4 (80.8) 60.8 79.1 Examples 7 to 9 The working conditions in these examples are identical to those of Examples 1 to 3, except for the agent supplying the element aluminium, which is in the form of SECAR 71, an industrial refractory cement produces by LAFARGE FONDU INTERNATIONAL. SECAR 71 is composed of approximately 70% of A12O3, approximately 29% of CaO and various other oxides. The other working conditions and the results for these examples are given in Table III below: Table ill EXAMPLE 7 EXAMPLE 8 EXAMPLE 9 Initial NO3 concen- tration (mg/1) 125 500 500 Amount of SECAR 71 (mg/1) 655 2620 2620 Amount of Ca(OH)z (mg/1) 1000 4000 2970 Overall mole ratio A1/NO3 4.46 4.46 4.46 Overall mole ratio Ca/NO3 8.91 8.91 7.19 Treatment period (min) 300 1020 1020 Residual NO3 concentration (mg/1) 78 102 169 Yield (%) 37.6% 79.6 66.2 Examplas 10 and 11 The working condition in these examples are identical to those in Examples 1 to 3, except for the agent supplying the element aluminium, which is in the form of a product composed of 36 % by weight of C3A, 60 % of C1ZA7, and 4% of calcium titanate: this product will be identified by M in the examples which follow.

The other working conditions and the results for these examples are given in the following Table IV: Table IV EXAMPLE 10 EXAMPLE 11 Initial N03 concentration (mg/1) 500 500 Amount of M (mg/1) 4390 4390 Amount of Ca(OH)z (mg/1) 0 965 Overall mole ratio Al/NO3 4.45 4.45 Overall mole ratio Ca/NOj 5.60 7.21 Table IV (continued) EXAMPLE 10 EXAMPLE 11 Treatment period (min.) 270 75 Residual NO3 concentration (mg/1) 186 82 Yield (%.) 62.8 83.6 It is observed that the increase in the overall mole ratio Ca/NO3 by adding calcium hydroxide enables the yield to be substantially improved.

Examples 12 and 13 The working conditions in these examples are identical to those in Examples 1 to 3, except for the agent supplying the element aluminium, which is a product composed of 37% by weight of C3A, 55 % of C12A7 and 8% of calcium titanate. This product will be identified by N in the examples which follow.

The other working conditions and the results for 15 these examples are given in the following Table V.

Table V EXAMPLE 12 EXAMPLE 13 Initial NO3 concentration (mg/1) 500 500 Amount of N (mg/1) 4390 4390 Amount of Ca(OH)2 (mg/1) 0 965 Overall mole ratio A1/NO3 4.49 4.49 Overall mole ratio Ca/IR^ 5.58 7.19 Treatment period (min.) 540 100 Residual NO3 concentration (mg/1) 187 98 Yield (%) 62.6 80.4 It is observed that the increase in the overall mole ratio Ca/NOj by adding calcium hydroxide enables the yield to be substantially improved. -21Example 14 This example relates to the treatment of liquid manure.

The test liquid manure is a porcine liquid manure.

Nitrification is performed by aeration of the manure for 72 hours, by bubbling air through it in the presence of nitrifying microorganisms.

After nitrification, this liquid manure comprises an initial nitrate concentration equal to 3000 mg/litre. The suspended solid matter is precipitated by adding aluminium sulphate in the proportion of 0.3 g/litre of manure. After separation when settling has taken place, the supernatant liquid is treated by adding an active alumina and calcium hydroxide Ca(OH)2 to this manure with stirring during 30 minutes. The suspension obtained is filtered and the residual nitrate content is measured on the filtrate after adding a buffer of ionic strength and adjusting the pH to .5.

The other working conditions and the results obtained are recorded in the following Table VI: Table VI EXAMPLE 14 Initial NO3 concentration (mg/1) 3000 Amount of Cubic - T(mg/1) 14,182 Amount of Ca(OH)2 (mg/1) 3582 Overall mole ratio A1/NO3 2 Overall mole ratio Ca/NO3 3.5 Treatment period (min.) 30 Residual NO3 concentration (mg/1) 600 Yield (%) 80 A prior precipitation of the suspended matter, especially that of a proteinaceous nature, enabled a good denitration yield to be obtained. -22Example 15 To an aqueous solution containing calcium nitrate in the proportion of 2000 mg/1 of nitrate ions, 8.78 g of the calcium aluminate identified by the letter N in Examples 12 and 13 and 1.93 g of Ca(OH)z are added per litre of solution.

Under these conditions, the overall mole ratio Al/NOj is equal to 2.25 and the overall mole ratio Ca/NO3 is equal to 3.6.

The change in the pH and in the concentration of nitrates in solution is monitored as a function of the reaction time using a specific electrode after adding a buffer and adjusting the pH to 5.5.

The results of these measurements are recorded in the following Table VII: Table VII - EXAMPLE 15 Time (min.) (NO3) (mg/1) PH 15 1251 12.00 30 1065 12.03 45 1005 12.04 75 754 12.00 105 579 11.87 135 521 11.82 The yield of the precipitation reaction of the nitrate ions after 135 minutes' reaction is 75 %.

Example 16 To an aqueous solution containing 1608 mg per litre of nitrate, 8.78 g per litre of the calcium aluminate identified by the letter N in Examples 12 and 13 and 1.93 g per litre of Ca(OH)2 are added.

After five minutes' reaction, 5 g of hydrated calcium mononitroaluminate - 3CaO.Al2O3. CA(NO3)2 . nH2O containing 46% by weight of dry extract are added. -23Under the conditions of this test, the overall mole ratio A1/NO3 is equal to 2.8 and the overall mole ratio Ca/NO3 is equal to 4.5.

The change in the pH and in the nitrate 5 concentration is monitored as a function of the reaction time according to the method of measurement used in the previous example.

The results are recorded in the following Table VIII: Table VIII - EXAMPLE 16 Time (min.) (NO3) (mg/1) PH 15 438 — 30 289 11.82 45 241 11.85 60 171 11.79 90 123 11.80 150 93 11.75 The yield of the precipitation reaction of the nitrate ions is equal to 94% after 150 minutes.

Example 17 This example relates to a treatment of porcine liquid manure.

In a first step, a clarification of the manure is carried out by removing the solid matter. For this purpose, a flocculation is performed by adding a polyacrylamide of trade name PROSEDIM CS 284 in the proportion of 75 mg/1 to precipitate the proteinaceous matter and phosphates, and the solid matter is removed after settling has taken place. The clarified supernatant is then nitrified.

Nitrification is carried out by aeration and inoculation of the liquid manure to be treated by adding a liquid manure undergoing nitrification and hence containing nitrifying microorganisms. -24After nitrification, this manure includes an initial nitrate concentration equal to 2737 mg/1.

The nitrates are precipitated in the form of calcium mononitroaluminate by adding calcium aluminate identified by the letter "NM in Examples 12 and 13, as well as Ca(OH)2.

At regular intervals during the reaction, the suspension obtained is filtered and the residual contents of nitrate, chloride ions, phosphate ions HPO42 and io sulphate ions SO42' are measured on the filtrate by ion chromatography.

To carry out the precipitation, 11853 mg/1 of calcium aluminate designated by the letter N and 4215 mg/1 of Ca(OH)2 are added. Under these conditions, the overall mole ratio Al/NOj is equal to 2.25 and the overall mole ratio Ca/NO3 is equal to 3.6.

The results of this test are given in the following Table IX : Time (min Cl~ (mg/1) NO3- HPOf so4= Yield (%) 0 384 2737 204 506 - 15 342 2400 0 84 12 30 469 2018 0 0 26 45 324 1515 0 153 45 60 455 1129 0 59 59 75 448 934 0 32 66 90 262 752 0 0 72 105 405 688 0 0 75 135 224 553 0 0 80 165 399 520 0 12 81 195 213 501 0 0 82 225 195 470 0 0 83 The results of chemical analyses of the nitrified liquid manure before and after precipitation of the nitrates are given in the following Tabla X: CHEMICAL ANALYSES (mg/1) Nitrified liquid manure before separation of the nitrates by prec ip itat ion Final effluent ob- tained after precipitation of the nitrates and separation of the precipitate Al 5.3 31.7 Ca 427.6 191.6 K 831.8 760.8 Na 148.4 150.8 Cu 3 0.035 Zn 12.5 nd* Ni 0.21 0.01 Pb 0.09 nd* W 0.09 nd* Co 0.06 nd* CD 0.06 nd* no3 2737 470 nh4 nd* nd* Cl 384 195 P2O5 89 nd* so4 506 nd* PH 5.8 12.2 Analysis of the precipitate (weight %) A12O3 18.00 CaO 39.71 K2O 0.26 Na2O nd* NO3 9.14 H20 31.71 * nd means non detectable Apart from a substantial removal of the nitrates by means of the process according to the invention, it will be noted that the proportions of the elements Cu and Zn in the effluent are quite negligible. These elements are, in effect, advantageously bound in the precipitate. -26The final effluent, depleted in nitrates, may be spread in nature without risk of pollution. The nitraterich precipitate is advantageously used as fertiliser, and spread at the time when the growth of the plants requires a supply of nitrogen.

Example 18 and 19 An aqueous solution containing calcium nitrate Ca(NO3)2 at a concentration of 125 mg/1 of nitrate ions is introduced into a chemical reactor equipped with a magnetic stirrer. The agent supplying the element aluminium is sodium aluminate NaA102 in aqueous solution.

The additional supply of calcium is provided by adding calcium hydroxide Ca(OH)2.

The treatment is carried out by simultaneously adding NaAlO2 and Ca(OH)2. The treatment is carried out with stirring. The suspension obtained is filtered or treated with a flocculating agent and subjected to a settling and physical separation operation. The nitrate analysis is performed with a specific electrode.

The other working conditions and the results from these examples are given in the following Table XI.

Table XI EXAMPLE 18 EXAMPLE 19 Initial NO3 (mg/1) 125 125 NaAlO2 in the solution (mg/1) 2950 1475 Ca(OH)2 (mg/1) 5255 2590 Mole ratio Ca/NO3 35 17.5 Mole ratio Al/NO3 18 9 Treatment period (min) 15 15 Residual NO3 (mg/1) 62 114 Yield (%) 50 8 -27Examp lea 20 to 21 A reagent to eliminate nitrate ions in the state of hydrated calcium nitroaluminate, double or mixed, is addet to an aqueous solution containing 300 mg/litre of nitrate and corresponding to the supernatant liquid obtained after nitrification and precipitation of the suspended solid matter of the manure presented in Example 14.

After reaction, the obtained suspension filtered and the residual nitrate content is measured on the filtrate.

The other working condition and the obtained results are reported in the following Table XII.

Table XI EXAMPLE 20 EXAMPLE 21 Amount of added pure C3A (mg/1) 29,7 0 Amount of added Ca(OH)2 (mg/1) 3,5 0 Amount of added C4A(C3A.CaO) 0 35.3 Overall mole ratio ai/no3 2 2 Overall mole ratio Ca/N03 3.5 4 Treatment period (min) 180 90 Residual NO3 concentration(mg/1) 572 410 Yield (%) 81 86 Examples 22 to 24 Aqueous solution polluted by nitrate ions and other ions are purified by using the reagent identified by M in examples 10 and 11. -28These examples 22 to 24 have been made with aqueous solution containing, before treatment, a majority of 3 g/1 of N03.

The· purification method is the same for each of these ex., that is to say : - overall mole ratio A1/N03 = 2 - overall mole ratio Ca/No3 = 4 In the following Table ZZII habe been reported the other ions which are in high proportion be treated as well as the purification in the results. solution to Example Nature of Amount Treat- Yield Yield of the ions Y of ment of epura- in high proportion Y (mg/1) period (mg/1) epuration in NO3 tion in Y 22 Chloride 1000 120 70 % 52 % 23 Phosphate 1000 120 78 % 100 % 24 Sulphate 1000 120 68 % 99 %

Claims (17)

1.CLAIMS 1. Process for the epuration of aqueous solutions 5 polluted by nitrate ions, characterised in that it consists in precipitating hydrated double or mixed calcium nitroaluminates, such as, in particular, hydrated calcium mononitroaluminate, by adding to the solutions to be treated, at least one agent supplying 10 the element aluminium and at least one agent supplying the element calcium, the overall mole ration of the element aluminium to nitrate, A1/NO3, being superior to 1 and the overall mole ratio of calcium to nitrate, Ca/NO3, being superior to 2, and in that the 15 precipitation reaction is performed with stirring at basic pH.

2. Process according to Claim 1, characterised in that the precipitation reaction is performed at room 20 temperature.

3. Process according to Claim 1 or 2, characterised in that the precipitation reaction is performed at a pH above 10.5.

4. Process according to one of Claims 1 to 3, characterised in that the agent supplying the element aluminium is chosen from alkali metal aluminates. 30

5. Process according to one of Claim 1 to 3, characterised in that the agent supplying the element aluminium is chosen from calcium aluminates.

6. Process according to Claim 5, characterised in that 35 the agent supplying the element aluminium is supplied by products comprising C3A and/or C12A7, employed alone or in combination with other compounds. -307.

7.Process for the treatment of solutions and suspensions containing nitrogen in ammoniacal form according to one of Claims 1 to 6, characterised in that the following steps are performed successively : 5 1) the conversion to nitrates of the ammoniacal nitrogen present in the solutions to be treated is performed biochemically in the presence of oxygen. io 2. ) then, once the majority of the ammoniacal nitrogen is converted to nitrates, one or more calcium aluminate(s) is/are added to the solution to be treated, so that the overall ole ratio of the element aluminium to nitrate, A1/NO3 be superior to 1 and that the overal mole ratio of the element calcium to nitrate be superior to 2, and, if necessary, calcium hydroxide and/or another basic agent to adjust the pH to a basic value, preferably to a value above 10.5, is/are added and the precipitation reaction is performed with stirring and at room temperature. 3. ) after separation, a thick suspension or a 25 solid is collected, which is advantageously used as an enriching agent and fertiliser for agricultural soils. 4. ) finally, a liquid effluent nitrates is collected. depleted in

8. Process according to Claim 7, characterised in that the solution to be treated is brought to a basic pH by adding potassium hydroxide. 9. Process according to Claim 7 or 8, characterised in that, before the precipitation of hydrated double or mixed calcium nitroaluminates, either before or after the nitrification step (1), the solid matter included -31 in the solution to be treated is separated at least partially, generally by flocculation followed by a filtration and/or settling and physical separation operation.

9. Process according to Claim 7 or 8, characterised in that, before the precipitation of hydrated double or mixed calcium nitroaluminates, either before or after the nitrification step(l), the solid matter included 10 in the solution to be treated is separated at least partially, generally by flocculation followed by a fioltration operation and/or settling and physical separation operation. 15

10. Process according to Claim 9, characterised in that the separation of the solid matter is performed before the nitrification step (1).

11. Process according to one of Claims 1 to 6, 20 characterised in that the solution to be treated is a water polluted with nitrates, such as an industrial effluent or a municipal waste water, and in that : 1) One or more calcium aluminate (s) and, if necessary, calcium hydroxide and/or another 25 basic agent to adjust the pH to a basic value, preferably above 10.5, and such that the mole ratio of the element aluminium to nitrate, A1/N03, is superior to 1 and that the overall mole ratio of calcium to nitrate, Ca/NO3, is 30 superior to 2, is/are added to the solution to be treated ; 2) the precipitation reaction is performed with stirring and at room temperature ; 3) at the end of this reaction, the suspension obtained is subjected to a settling and physical separation operation and/or -32filtration operation, after the addition of a flocculating agent where appropriate; 5 4) then, if necessary, the filtrate obtained is neutralised to precipitate the alumina in solution j hydroxide; Ln the form of an insoluble 5) lastly, the new suspension obtained is 10 subj ected to a settling and physical separation and/or filtration operation to obtain a purified water which can be discharged into the environment without risk of pollution.

12. Process accordin to Claim 11, characterised in that the solution to be treated is a polluted water such a water originating from aquiculture or a municipal waste water containing water-soluble compounds 20 containing ammoniacal nitrogen, and in that, prior to removal of the nitrates, an operation of nitrification of this solution is arranged.

13. Process according to Claim 11 or 12, characterised in 25 that, prior to the addition of calcium aluminate, a basic agent is added to decrease the concentration of cations which are only sparingly soluble in the form of hydroxides. 30 14.

14.Process according to one of Claims 11 to 13, characterised in that, prior to the addition of calcium aluminate, an agent supplying the element calcium is added to decrease the concentration of anions which are only sparingly soluble in the form of 35 calcium salts.

15.Process according to one of Claims 11 to 14, characterised in that the anions which are only sparingly soluble in the form of calcium salts and the 15. -33cations which are only sparingly soluble in the form of hydroxides are decreased simultaneously by adding calcium hydroxide. 5

16. Process according to one of Claims 11 to 15, characterised in that the filtrate obtained at the end of step (4) is neutralised using CO2.

17. A process for the purification of aqueous solutions polluted by nitrate ions substantially as hereinbefore described by way of Example.