WO2009087507A1

WO2009087507A1 - Process for reducing the nitrogen content of zootechnic refluents

Info

Publication number: WO2009087507A1
Application number: PCT/IB2008/055105
Authority: WO
Inventors: Matteo Lombardi; Giovanni Bonometti
Original assignee: Nitrostop Services S.R.L.
Priority date: 2007-12-31
Filing date: 2008-12-04
Publication date: 2009-07-16
Also published as: ITMI20072448A1

Abstract

A process for reducing the nitrogen content of a zootechnic refluent comprises the following operative phases: placing the refluent in contact with a basifying agent in a reactor; generating a gaseous flow above the refluent and specifically above the free surface of the refluent drawing out of the reactor the gaseous current comprising the ammonia; recovering the ammonia from the gaseous current coming out of phase b) with formation of a purified gaseous current.

Description

DESCRIPTION

"Process for reducing the nitrogen content of zootechnic refluents"

[0001] The present invention relates to a process for reducing the nitrogen content of zootechnic refluents .

[0002] The present invention falls within the technical sector of treatment of zootechnic refluents containing animal dung deriving from livestock breeding of pigs, cows, chickens and other animals. [0003] With the passage of time, national and international environmental regulations have imposed and impose increasingly restrictive limits to the quantity of nitrogen which can be distributed in the environment by the spreading of animal dung on farmland. Specifically, the limits imposed on the spreading of dung in those farming areas where the concentration of nitrates in the ground have reached such high levels as to be defined "Nitrate Vulnerable Areas" are increasingly more restrictive . [0004] The quantity of dung produced and its nitrogen content remaining unchanged, the total area of land available for spreading it on is thus becoming progressively insufficient to ensure the disposal of the dung unless the nitrogen content is reduced. [0005] In the known state of the art various treatment processes of zootechnic refluents are known of and used which completely or at least partially remove the contents of nitrogen which in a refluent coming from a pig farm, for example, is present in concentrations variable from 1000 to, 6000 mg/1, depending on the specific diet provided at the farm and the dilution caused by the more or less frequent washing of the pigsty. [0006] A medium-sized pig farm (3000-5000 animals) produces a quantity of such refluent varying from 30 to 50 m³/day. [0007] An almost total removal of the nitrogen (98-99%) may be obtained using refluent treatment processes of an organic type. These processes, which comprise at least a phase of organic oxidation of the refluent followed by a phase of denitrification are however v&τγ complicated to manage as well as requiring the installation of rather costly systems .

[0008] Since the final destination of zootechnic refluents remains mainly spreading on farmland, the removal of the nitrogen does not necessarily have to be complete or, in any case, very high. In fact a reduction of the nitrogen content of a refluent of approximately 50% in weight compared to the total nitrogen would be sufficient to permit spreading of double the quantity of refluent on the same farmland (compared to the original refluent) . Just as import moreover, is the economic aspect of the treatment, that is the relation between the cost of the treatment and its efficacy.

[0009] From this point of view, a type of refluent treatment process enabling reduction of the nitrogen to acceptable levels and, at the same time, at a limited cost compared to the known state of the art processes, is one based on the stripping of the nitrogen contained in the refluent in the form of ammonia. This type of process, typically performed in a plate tower or Raschig rings, foresees stripping of the ammoniacal nitrogen by means of insufflation of a current of air (stripping current) in counterflow to the descent of the liquid refluent with consequent removal of the gaseous ammonia. To encourage the production of the gaseous ammonia, the refluent is generally heated and basifying agents are added, such as NaOH.

[0010] Usually the refluent must also be graded or filtered, if necessary after the addition of flocculants, since the presence of solids, even if of a small size, may obstruct the holes in the plates or dirty the Rashig rings, causing the system to block. In addition, to enable an acceptable extraction speed the refluent must be heated to a temperature of over 8O⁰C with a very high energy consumption. [0011] The stripping current containing the gaseous ammonia is generally treated in scrubbers with acid solutions. Once the gaseous ammonia has been absorbed in the washing solution, the gaseous current free of nitrogen is released into the atmosphere.

[0012] The treatment based on stripping, although sufficiently effective in the removal of the nitrogen contained in the refluent, entails considerably high energy costs and significant costs connected with the use of chemical additives. In fact, to conduct the process large quantities of basifying agents and flocculants as well as the energy to move the mass of air of the stripping current and to heat the refluent are needed. All this makes the construction of treatment plants on farms, especially those of a small size, hardly worthwhile economically.

[0013] The purpose of the present invention is to identify a process for reducing the nitrogen content of zootechnic refluents which overcomes the drawbacks of the known technique and, specifically, which proves more viable both from an energetic and economic point of view. [0014] The object of the present invention is therefore a process for reducing the nitrogen content of zootechnic refluents comprising the following operative phases: [0015] a) placing the refluent in contact with a basifying agent in a reactor;

[0016] b) generating a gaseous flow above the refluent and specifically above the free surface of the refluent and drawing out of the reactor a gaseous current comprising ammonia;

[0017] c) recovering the ammonia from the outgoing gaseous current of phase b) with formation of a purified gaseous current . [0018] Preferably, the process is conducted by using an ingoing gaseous current in phase b) composed of one or more gases and/or vapours chosen from: air, nitrogen, argon, carbon dioxide, helium, hydrogen, oxygen, water vapour and/or their mixtures . [0019] The process according to the present invention applies to refluents coming from farms breeding various animals, such as pigs, cows, chickens etc and more generally to all refluents containing nitrogen in ammoniacal form, such as those remaining after anaerobic digestion treatment. [0020] The refluents collected from a single farm or from several farms are stored in a collection tank. They are generally in the form of a liquid-sludge such as to be agitated by means of the pump systems typically used in the sector. Sometimes, depending on the nature of the refluents and the characteristics of the farm producing them, before being subjected to treatment, the refluents may be mixed together and/or diluted with water so as to acquire sufficient fluidity to be agitated using the aforesaid pumping systems . [0021] The process according to the present invention is generally conducted in a single reactor which is filled with the refluents by means of pumping systems. In one preferred embodiment of the invention, the reactor is filled with the refluents in a fully automatic manner. [0022] Phase a) of the process according to the present invention preferably consists of placing the refluent, characterised by a neutral pH, in contact with a quantity of basifying agent able to raise the pH value up to a value varying from 7.2 to 13, preferably from 10.0 to 13. [0023] The Applicant has observed that the basifying effect is preferably performed by a solid basifying agent chosen from CaO, MgO and/or their mixtures or chosen from watery solutions of Ca (OH)_2/ Mg (OH)₂ and/or their mixtures. Preferably, the basifying agent is in a solid, granular form. Unlike the basifying agents used in the state of the art processes (e.g. NaOH) , these additives have a significantly lower cost .

[0024] The preferred basifying agent is quicklime, that is a CaO/MgO mixture. [0025] The present invention may however be realised with the same additions, albeit in a limited form, with other basifying agents such as Na₂O, K₂O, NaOH, KOH, Na₂CO₃, K₂CO₃ and/or their solid mixtures or with the respective watery solutions and/or their mixtures . [0026] In general, the basifying agent may be added to the refluent to be treated in solid form or as a watery solution. Typically, when the basifying agent is added in solid form, its dosage varies from 5 to 20 kg of agent per m3 of refluent to be treated. When added in the form of a watery solution, typically at 30% in weight, the dosage varies from 30 to 50 litres/m³.

[0027] As a result of raising the pH, the ammoniacal nitrogen contained in the refluent, (in which it is present mainly in the form of NH₄ ⁺ ions) , tends to separate from the refluent in the form of gaseous NH3.

[0028] The concentration of gaseous ammonia in the liquid refluent increases until, once it has exceeded the saturation level typical of the specific operating conditions of temperature and pressure, tends to spontaneously come out of the refluent until a condition of balance is reached.

[0029] Generating a gaseous flow inside the reactor which removes the ammonia evolving from the refluent, this balance is altered and new ammonia is produced in the refluent which will tend to come out of it to re- establish the balance.

[0030] It has been observed that such phenomenon is particularly accentuated if the gaseous flow is generated by introduction of a gaseous current over the free surface of the refluent, since this way the outcoming ammonia is promptly drawn away, encouraging the rapid evolution of new ammonia from the refluent. [0031] The ammonia extraction process continues until the desired quantity of airanoniacal nitrogen has been extracted.

[0032] Specifically the gaseous current introduced is composed of one or more gases and/or vapours chosen from the group comprising: air, CO₂, N₂, Ar, He, H₂, O₂, water vapour and/or their mixtures. Preferably air is used. [0033] ] The pressure of the gaseous flow generated to extract the ammonia is variable from -100 mbar (negative pressure reactor - aspirated gaseous current) to 100 mbar (gaseous current introduced) , in other words the gaseous flow which enables extraction of the gaseous current comprising ammonia may be generated either by introducing a gaseous current which runs over the free surface or by placing the reactor in negative pressure . In one preferred embodiment the pressure of the gaseous flow is chosen from -10 to +10 mbar. [0034] In both operating modes, that is either using a gaseous current introduced or by placing the reactor in negative pressure, the gaseous flow generated is present above the surface of the refluent without bubbling through it . [0035] By not allowing huge masses of air to bubble through the refluent, as usually happens in the state of the art techniques, the formation of foam is prevented with a consequent reduction of costs deriving from the use of anti- foaming products. [0036] The gaseous flow draws the ammonia which gradually separates from the refluent, outside the reactor. The gaseous current comprising the ammonia is then directed to the subsequent phase c) of the process. [0037] It has been observed how the process of nitrogen reduction proceeds more rapidly when the gaseous phase removal is produced by means of introduction of a gaseous current rather than placing the inside of the reactor in negative pressure. [0038] It has also been observed that the extraction of the ammonia from the refluent is faster in reactors of such a shape as to provide an ample exchange surface between the refluent and the gaseous current.

[0039] The gaseous flow generated over the refluent and especially on the free surface of the refluent inside the reactor, being produced by means of a gaseous current introduced in low pressure or by placing the reactor in slight negative pressure entails a modest consumption of energy. The process which the present invention relates to therefore enables the removal of the nitrogen from the refluent at favourable economic conditions compared to the state of the art stripping processes known of, which, use gaseous currents at significantly higher pressure. In addition, to set up the process according to the present invention, gas and/or vapour pumping systems of modest dimensions easily found for sale at a limited cost, are sufficient .

[00403 To encourage the nitrogen extraction process from the refluent and to reduce the duration of the treatment, the refluent may be heated and kept at a temperature of about 50⁰C, for example, by means of special heating elements, such as coils immersed in the body of the refluent. At this temperature the formation of foam caused by the decomposition of the bicarbonates is avoided, as is the precipitation of the insoluble carbonates with consequent formation of calcareous incrustations on the walls of the reactor and on the heating coils. Heating may be particularly useful in the winter period when the refluents collected are at an average temperature of 15-20⁰C, while in the summer period the temperature of 40-50⁰C of the refluent is easy to reach. In addition, as a result of the addition of the basifying agent there is a rise in temperature of 4 -5⁰C due to the exothermicity of the dissolution reaction. [0041] The extraction process of the nitrogen from the refluent according to the present invention proceeds at an acceptable speed even in the absence of agitation of the refluent. The gaseous ammonia, in fact, evolves from the surface of the refluent by effect of the chemical action of the basifying agents and by the continual draw of the gaseous phase. However, to make treatment faster, the refluent may be kept agitated, for example, by mechanical means such as paddle mixers.

[0042] Optionally, the agitation of the refluent may be performed by insufflation into the body of refluent of at least a part of the gaseous current coming out of the reactor (non-purified current coming out of phase b) or from the gaseous current (purified current coming out of phase c) . [0043] Alternatively, agitation may also be obtained by the application of ultrasounds to the refluent.

[0044] Agitation may also be obtained by the recirculation of a part of the refluent itself, taking it from the bottom of the reactor and re-introducing it in a second point of the reactor, situated above the free surface of the refluent, so as to create turbulence inside it. Typically the recirculation capacity of the refluent varies from 0.2 to 3.5 recirculation cycles/hour, in other words such as to recirculate from 0.2 to 3.5 times in one hour the volume of refluent subject to treatment (e.g. if the volume of refluent to be treated is 5000 litres, 2 circulation cycles/hour correspond to a recirculated volume of 10,000 litres/hour) .' [0045] In a further embodiment of the invention, in addition to the effect of the aforesaid recirculation, the refluent may be kept agitated by insufflation of air drawn from the outside.

[0046] The agitation of the refluent according to the methods described above make it possible to increase the efficiency of the process increasing the speed of extraction of the ammonia.

[0047] In order to reduce the overall energy consumption of the process and to limit the emissions into the atmosphere deriving from the same, at least a part of the gaseous current coming out of the reactor, once purified of the ammonia in phase c) (even only partially) , may be used as gaseous current introduced in phase b) . Preferably, all the gaseous current is purified and recycled in phase b) of the process so as to create a closed circuit of recovery. [0048] At the end of the process the refluent is discharged from the reactor and stored in tanks before being spread on farmland and/or used in part to pre-basify the new refluent to be treated. If the refluent is spread on farmland a correction of its pH may be needed so as to make it conform to the criteria established by environmental legislation on the matter.

[0049] In one preferred embodiment of the invention, the end of treatment is determined automatically by means a system of continuous analysis which by continuously determining the concentration of nitrogen still present in the refluent (such as the ammonium ions) or extracted during the gaseous phase (such as gaseous ammonia) , is able to indicate the level of nitrogen reduction achieved by the process . [0050] The process according to the present invention described above is preferably performed in a batch mode, in other words, treating pre-defined quantities of refluent inside a single reactor until completion of the process. However, the process may also be performed in continuous mode by connecting two or more reactors in series . In this configuration the gaseous current coming out of the first reactor, once purified of the ammonia, can be introduced into the subsequent reactor, increasing the speed of nitrogen removal from the refluent compared to the process conducted in a single reactor (with the same quantity of refluent treated) at the same time reducing the specific energy consumption of the process. [0051] The number of reactors and their size are chosen by a person skilled in the art so as to obtain an optimal "evaporation surface: volume of refluent" ratio, maximise the yield of ammonia extraction and minimise the energy costs and costs of chemical additives to be used, depending on the specific chemical composition of the refluent . [0052] The reactors may be made from plastic or composite material, stainless steel, rust-resistant steel alloy or waterproof concrete .

[0053] The saving of energy and of materials (chemical additives) permitted by the process according to the present invention prove even more evident if one considers that the state of the art processes involve the insufflation of considerable masses of clean air in the refluent, which must necessarily be purified before they can be released into the atmosphere at the conditions foreseen by environmental legislation, with a decidedly high cost in terms of energy and of materials . [0054] The recovery of the gaseous ammonia in phase c) of the process according to the present invention may be achieved using various techniques. For example the gaseous current coming from phase b) may be made to bubble through an acid solution (neutralisation) . Alternatively, the gaseous current is sent to a scrubber which uses a similar acid solution as washing liquid. [0055] In a preferred embodiment of the present invention the gaseous current coming from phase b) is made to pass over the free surface of an acid solution which captures the ammonia and neutralises it, forming the corresponding ammonium salt. [0056] According to the present invention it is preferable to use watery solutions of an acid chosen from sulphuric acid (H₂SO₄) , phosphoric acid (H₃PO₄) , nitric acid (HNO₃) , carbonic acid (H₂CO₃) and/or their mixtures. The solution resulting at the end of the ammonia recovery operation

(bubbled or saturate washing solution) is a watery solution containing one or more ammonium salts, depending on the mixture of acids used for neutralisation (for example using H₂SO₄ one gets (NH₄) ₂SO₄, using HNO₃ one gets NH₄NO₃, using H₂CO₃ one gets ammonium carbonate and/or bicarbonate) . The ammonium salts solution may be effectively used as a fertiliser. Typically, the concentration of ammonium salt varies (depending on the type of acid used) from 150 to 350 g/1 and the quantity of solution which can be obtained varies from 1500 to 2000 litres a day, implementing for example the process according to the present invention in a farm of 10,000 pigs .

[0057] To recover the ammonia in phase c) it is also possible to bubble the gaseous current coining out of the reactor in water and/or subject it to washing in water scrubbers . The product obtained with this type of recovery is ammonium hydroxide.

[0058] According to a further alternative, it is possible to condense the ammonia by means of condensation systems (e.g. condensation towers) to obtain anhydrous ammonia. [0059] It is also possible to foresee the combustion of the gaseous current coming out of the reactor and containing the ammonia. The current can be used as is or mixed with other combustibles. Preferably, a fraction of such current is sent for combustion and the remaining fragment is used as a reagent to neutralise the nitrogen oxides (NOx) produced by the combustion of the first fraction. [0060] It is also possible to subject the gaseous current coming out of the reactor to a process of dissociation of the ammonia with formation of H₂ and N₂, then storing the H₂ appropriately, for example using suitable compounds in a solid state.

[0061] A further alternative for the recovery of the ammonia extracted from the refluent is that of liquefaction. Compressing the gaseous current containing the ammonia, after dehydration and in appropriate temperature and pressure conditions, it is possible in fact to obtain the separation of the ammonia in liquid form. To dehydrate the gaseous current, CaO or MgO and/or their solid mixtures may be used. Performing the process according to the present invention on a farm of 10,000 pigs for example, it is possible to obtain about 150÷200 kg/day of liquid ammonia.

[0062] Unlike neutralisation, the recovery of the ammonia by means of its liquefaction permits the production of a highly commercial product, of decidedly higher value than that of liquid fertiliser and/or ammonium hydroxide, in that liquid ammonia can be used as a raw material in other industrial processes . [0063] The process according to the present invention presents various advantages compared to processes for the reduction of the nitrogen content of zootecnic refluents known of in the state of the art .

[0064] It enables, first of all₇ reductions of the nitrogen content of refluents equal to the content of nitrogen in ammoniacal form, thus making spreading on farmland certified as "nitrate vulnerable areas" possible. Typically, in a refluent of pig origin the ammoniacal nitrogen amounts to about 50% of the total nitrogen present . [0065] The process according to the present invention is also economically advantageous given the reduced costs of the basifying agents used, especially of quicklime. [0066] In addition, since the process which the present invention relates to is based on the generation of a low pressure gaseous flow over the free surface of the refluent, it is possible to achieve high speeds of treatment of the refluent against limited energy costs required to generate such flow. [0067] The recovery of the ammonia from the gaseous current coming out of the reactor according to the methods described also permits a significant reduction of pollutant emissions and of energy consumption of the process compared to the known state of the art processes using traditional purification systems (such as filtration on biofilter) .

[0068] The possibility of keeping the refluent agitated by means of recirculation of the gaseous current coming out of the reactor, whether purified or not or through the recirculation of the refluent itself makes it possible to further increase the efficiency of the process. This recirculation in fact, increases the efficacy of the separation of the ammonia from the refluent without using the insufflation of clean air from the outside, which would have to be specially purified before being released into the atmosphere. [0069] In addition, the recirculation of the gaseous current coming out of the reactor, purified or not of the ammonia, to keep the refluent in agitation, allows a limitation of the diffusion of foul odours in the surroundings of the plant .

[0070] A further advantage of the process according to the present invention is provided by the possibility of using the process by-product, that is the ammonia recovered in the form of ammonium salts in a watery solution, liquid ammonia or solution of ammonium hydroxide, as farm fertiliser or as a raw material for other industrial processes .

[0071] These advantages, combined with the fact that the realisation of the equipment to perform the process entails modest outgoings and its running can be fully automised, make the process according to the present invention particularly suitable for industrial use in the sphere of breeding pigs, cows, chickens and other animals . [0072] The equipment needed to perform the process according to the present invention is of limited size and can therefore also be transported on a moving vehicle such as a lorry or articulated lorry with the obvious logistic advantages deriving from a mobile system. The equipment can also be inserted inside a "container" with airtight walls which considerably reduces the risk of contamination of water and air in the event of malfunctioning. In addition, even the acoustic pollution deriving from the functioning of the machine is considerably reduced.

[0073] One possible form of enactment of the process according to the present invention is shown in the block diagram in figure 1, showing the various phases of the process as indicated below: [0074] (1) filling the reactor with the refluent to be treated;

[0075] (2) addition of the basifying agent and heating of the refluent, where applicable; [0076] (3) generation of the gaseous flow over the refluent and specifically over the free surface of the refluent

[0077] (3a) recirculation of the gaseous current purified in phase b) and, optionally, to keep the refluent agitated; [0078] (3b) recirculation of the non purified gaseous current to keep the refluent agitated;

[0079] (3c) internal recirculation of the refluent subject to treatment to keep the same refluent agitated; [0080] (4) treated refluent [0081] (4a) recirculation of a part of the treated refluent to pre-basify the new refluent sent for treatment;

[0082] (5) recovery of the ammonia from the gaseous current coming out of phase b) . [0083] Figure 2 shows a schematic representation of the various options for recovering the ammonia:

[0084] (A-I) recovery of the ammonia by means of neutralisation by bubbling through an acid solution or scrubber with acid solution; [0085] (A-2) storage of the saturate bubbled solution or of the washing solution containing the ammonium salt or salts produced in (A-I) ;

[0086] (B-I) dehydration of the gaseous current coming out of the reactor; [0087] (B-2) recovery of the ammonia from the dehydrated gaseous current coming from (B-I) by means of compression

(liquefaction) ;

[0088] (B-3) storage of the liquid ammonia obtained from

(B-2) ; [0089] (C-I) recovery of the ammonia from the gaseous current by means of absorption by bubbling through water or in water scrubbers ;

[0090] (C-2) storage of the bubbled solution or of the washing solution containing ammonium hydroxide; [0091] (D-I) recovery of the ammonia from the gaseous current by means of condensation;

[0092] (D-2) storage of anhydrous ammonia;

[0093] (E-I) combustion of a first fraction of the gaseous current containing ammonia ; [0094] (E-2) neutralisation of the NOx produced by combustion (E-I) by means of a second fraction of gaseous current containing ammonia ;

[0095] (F-I) dissociation of the ammonia present in the gaseous current into H2 and N₂; [0096] (F-2) recovery and storage of 'H₂.

[0097] (G-I) recovery of the ammonia from the gaseous current by means of absorption of the latter through the passage of an acid solution over the free surface; [0098] (G-2) storage of the corresponding ammonium salt; [0099] The following examples of embodiment are provided merely for illustrative purposes of the present invention and should not be interpreted as a limitation of the sphere of protection defined by the appended claims . [00100] Example 1 [00101] 4000 litres of refluent coming from a pig farm and containing 4500 mg/l of nitrogen in ammoniacal form, (equal to 68% of the total nitrogen of the refluent) , corresponding to a total quantity of 18 kg of nitrogen in ammoniacal form were treated for 2.5 hours in a single reactor system. 40 kg of a mixture of CaO and MgO in granules were added to the refluent to bring the pH to a value of 11.2. The pressure of the gaseous current introduced was 5 mbar and the flow rate was 4000 Nmc/h. [00102] The flow rate of recirculation of the refluent was set at 800 litres/hour. The gaseous current containing the ammonia coming out of the reactor was made to flow on the free surface of a solution (having a volume of 500 litres) of H2SO4 at 9.14% in weight. [00103] At the end of treatment the concentration of the ammonium in the refluent was approx. 300 mg/l; this means that 4200 mg/l of ammonium were extracted from the refluent, giving rise to 500 litres of solution at 12.5% in weight of ammonium sulphate . [00104] Example 2 [00105] 8000 litres of refluent coming from pig farms and containing 3000 mg/l of nitrogen in ammoniacal form,

(equal to 60% of the total nitrogen of the refluent analysed) , were treated for 2.5 hours in single reactor system. 90 kg of a mixture of CaO and MgO in powder were added to the refluent to bring the pH to a value of 11.9. The pressure of the gaseous current introduced was 40 mbar and the flow rate was 7000 Nmc/h.

[00106] The flow rate of recirculation of the refluent was set at 2000 litres/hour. The gaseous current containing the ammonia coming out of the reactor was washed in a scrubber with an acid solution for sulphuric acid at 20% (volume of the washing tank of the scrubber 1000 litres) . At the end of treatment the final concentration of the ammonium ions in the refluent was approx. 500 mg/1, corresponding to an extraction of 20kg in all of ammonium which produced 74kg of ammonium sulphate in acid solution. The final concentration of sulphuric acid in the washing tank, after treatment of the refluent, was 14.6% in weight. Such residual concentration of the acid, permits treatment of a further 21,600 litres (approx.) of the same refluent, before reaching the complete neutralisation of the sulphuric acid and formation of a salt (ammonium sulphate) utilisable as agricultural fertiliser. [00107] Example 3 (comparative)

[00108] To make air bubble through a slurry having a hydrostatic free surface of one metre a head of over 100 mbar is needed. [00109] To generate a gaseous current having a flow rate of 500 m³/h and a pressure of 150 mbar an energy commitment of 5.5 kW equal to 11 Wh/ (m³ air) is required. [00110] To create a gaseous flow on the free surface of the refluent a pressure of a few mbar is sufficient. To generate a gaseous current having a flow rate of 8000 m3/hour and a pressure of 5 mbar an energy commitment of 1.5 kW equal to 0.1875 Wh/ (m³ air) is required. [00111] The process according to the present invention, requiring the generation of a gaseous current on the free surface of the refluent enables improved performance in terms of power (Wh/ (m³air) of a factor of 58.6.

[00112] According to one embodiment variation of the present invention, a process to reduce the nitrogen content of a zootechnic refluent comprising the following operative phases: [00113] a) placing the refluent in contact with a basifying agent in a reactor in such a quantity as to bring the pH to a value varying from 7.2 to 13; [00114] b) keeping the refluent deriving from phase a) agitated; [00115] c) recovering the ammonia from the gaseous current which is generated in phase b) ; wherein [00116] - the refluent is kept agitated in phase b) by insufflating all or a part of the gaseous current purified or not of the ammonia deriving from phase c) ; and/or

[00117] - the refluent is kept agitated in phase b) by recirculating at least a part of the same treated refluent . [00118] In such variation, phase a) of the process consists of placing the refluent, generally pH neutral, in contact with a quantity of basifying agent able to raise the pH value to a figure varying from 7.2 to 13 and preferably from 10.0 a 13. [00119] In phase b) of the process according to the present variation, the refluent is kept in constant agitation so as to support the aforesaid process, facilitating stripping of the gaseous ammonia. [00120] The gaseous ammonia which develops from the refluent during agitation mixes with the air present in the reactor, creating a gaseous current rich in ammonia which collects in the upper part of the said reactor. The current is then subjected to phase c) of the process according to such variation, consisting in the recovery of the ammonia from said current . [00121] In order to reduce the energy consumption of the operation of agitating the refluent, once it is at least partially purified of the ammonia at least a part of the gaseous current is recycled in phase b) of the process and insufflated into the reactor. Preferably, all the gaseous current, purified or not is recycled at phase b) of the process according to the present variation, so as to create a closed circuit of recovery. For the same purpose, contemporarily, a part of the refluent being treated is extracted from a point situated in the lower part of the reactor by means of pumping and re- introduced in another point situated in the upper part of the reactor, above the free surface of the refluent, so as to create further turbulence and encourage stripping of the ammonia. Typically, the flow rate of recirculation of the refluent varies from 0.2 to 3.5 recirculation cycles/hour, in other words such as to recirculate from 0.2 to 3.5 times in one hour the volume of refluent subject to treatment (e.g. if the volume of refluent to be treated is 5000 litres, 2 circulation cycles/hour correspond to a recirculated volume of 10,000 litres/hour) .

[00122] In a preferred embodiment of such variation, optionally, the agitation in phase b) may be made even more efficient by using, in addition to the recirculation of the air, purified or not, and of the refluent treated, mechanical means of agitation and/or mixing (e.g. paddle mixer) .

[00123] In a further preferred embodiment of the invention, in addition to the effect of the aforesaid recirculation, the refluent may also be kept agitated by insufflation of the air captured from outside. [00124] One preferred form of enactment of the process according to the present variation is shown in the block diagram in figure 3, showing the various phases of the process as indicated here: [00125] (1) filling the reactor with the refluent to be treated;

[00126] (2) addition of the basifying agent and heating of the refluent, where applicable; [00127] (3) agitation of the refluent by means of recirculation of the gaseous current purified or not of the ammonia and/or of the refluent subject to treatment ;

[00128] (4) collection of the gaseous current containing the ammonia in the upper part of the reactor; [00129] (5) discharge of the treated refluent or re-use by mixing with non-treated refluent for pre-basifying;

[00130] (A-I) recovery of the ammonia from the gaseous current coming from (4) by means of neutralisation by bubbling through an acid solution or scrubber with acid solution;

[00131] (A-2) storage of the saturate bubbled solution or of the washing solution containing the ammonium salt or salts produced in (A-I) .

[00132] Alternatively to phases (A-I) and (A-2) the process may entail the phases :

[00133] (B-I) dehydration of the gaseous current (4) ;

[00134] (B-2) recovery of the ammonia from the gaseous current coming from (4) by means of compression (liquefaction) ; [00135] (B-3) storage of the liquid ammonia obtained from (B-2 ) .

[00136] The following examples of embodiment are provided merely for illustrative purposes of the present invention and should not be interpreted as a limitation of the sphere of protection defined by the appended claims .

[00137] Example 4

[00138] 5000 litres of refluent coming from a pig farm and containing 1500 mg/1 of nitrogen in ammoniacal form, equal to 48% of the total nitrogen, were treated for 2 hours in a single reactor system. 50 kg of a mixture of CaO and MgO in granules were added to the refluent to bring the pH to a value of 12.3. The recirculation flow rate of the refluent was set at 5000 litres/hour. The ammonia in the stripping current was made to bubble through 100 litres of a watery solution at 20% in weight of H₂SO₄.

[00139] At the end of treatment the complete removal of the ammoniacal nitrogen of the refluent was achieved and 100 litres of solution of ammonium salt at 27.9% in weight of ammonium sulphate utilisable as agricultural fertiliser was produced. [00140] Example 5 [00141] 10000 litres of refluent coming from a pig farm and containing 1900 mg/1 of nitrogen in ammoniacal form, equal to 50% of the total nitrogen, were treated for 2 hours in a single reactor system. 100 kg of a mixture of CaO and MgO in granules were added to the refluent to bring the pH to a value of 12. The recirculation flow rate of the refluent was set at 10,000 litres/hour. The stripping current containing the ammonia was made to bubble through 200 litres of a watery solution at 15% in weight of H2SO4 and 14% in weight of HN03. [00142] At the end of treatment the complete removal of the ammoniacal nitrogen of the refluent was achieved and 200 litres of solution of ammonium salt composed of 36.9% in weight of ammonium sulphate and 18.2% in weight of ammonium nitrate utilisable as agricultural fertiliser was produced .

Claims

1. Process for reducing the nitrogen content of a zootechnic refluent comprising the following operative phases : a) placing the refluent in contact with a basifying agent in a reactor; b) generating a gaseous flow above the refluent and specifically above the free surface of the refluent and drawing out of the reactor a gaseous current comprising ammonia; c) recovering the ammonia from the outgoing gaseous current of phase b) with formation of a purified gaseous current .

2. Process according to claim 1 wherein the gaseous flow in phase b) is generated by means of a gaseous current introduced.

3. Process according to claim 1 or 2 wherein the gaseous current introduced is composed of one or more gases and/or vapours chosen from the group comprising: air, CO₂, N₂, Ar, He, H₂, O₂, water vapour and/or their mixtures .

4. Process according to any of the previous claims wherein the gaseous current introduced has a pressure between -100 and 100 mbar.

5. Process according to claim 4 wherein the gaseous current introduced has a pressure varying from -10 to +10 mbar .

6. Process according to any of the previous claims wherein the basifying agent is added in such quantity as to bring the pH to a value varying from 7.2 to 13.

7. Process according to claim 6 wherein the basifying agent is added in such quantity as to bring the pH to a value varying from 10.0 to 13.

8. Process according to any of the previous claims wherein the basifying agent is chosen from Na₂O, K₂O,

NaOH, KOH, Na₂CO₃, K₂CO₃ and/or their solid mixtures and/or the respective watery solutions and/or their mixtures, preferably quicklime, that is a mixture of CaO/MgO .

9. Process according to any of the claims from 2 to 8 wherein the purified gaseous current is totally recirculated as gaseous current introduced in phase b) .

10. Process according to any of the previous claims wherein the refluent is kept agitated in the reactor by means of mechanical agitators.

11. Process according to any of the previous claims wherein the refluent is kept agitated in the reactor by means of the application of ultrasounds.

12. Process according to any of the previous claims wherein the refluent is kept agitated in the reactor by means of air insufflated into the body of the refluent.

13. Process according to any of the previous claims wherein the refluent is kept agitated in the reactor by insufflating all or part of the purified gaseous current in the reactor in the body of the refluent.

14. Process according to any of the previous claims wherein the refluent is kept agitated by extracting a part of said refluent from the reactor and re-introducing it in a second point of the reactor above the free surface of the refluent .

15. Process according to claim 14 wherein the flow rate of extraction of the refluent from the reactor and of re- introduction above the free surface of the refluent varies from 0.2 to 3.5 recirculation cycles/hour.

16. Process according to any of the previous claims wherein the refluent is kept at a temperature of approx. 50⁰C.

17. Process according to any of the previous claims characterised by the fact that it is performed contemporarily in two or more reactors.

18. Process according to claim 17 wherein the purified gaseous current coming out of a first reactor is used as the gaseous current introduced to the process conducted in the subsequent reactor.

19. Process according to any of the previous claims wherein the recovery of the ammonia in phase c) is performed by subjecting the gaseous current coining out of phase b) to one or more stages of bubbling in an acid solution or in water.

20. Process according to any of the previous claims wherein the recovery of the ammonia in phase c) is performed by subjecting the gaseous current coming out of phase b) to one or more stages of washing with an acid solution or with water in a scrubber.

21. Process according to any of the previous claims 1-19 wherein the recovery of the ammonia in phase c) is performed by making the gaseous current coming from phase b) pass over the free surface of an acid solution.

22. Process according to any of the previous claims 19 - 21 wherein the acid solution is a watery solution of an acid chosen from H₂SO₄, H₃PO₄, HNO₃ H₂CO₃ and/or their mixtures .

23. Process according to any of the previous claims 1-19 wherein the recovery of the ammonia in phase c) is performed by liquefaction of the ammonia present in the gaseous current coming out of phase b) , after dehydration .

24. Process according to any of the previous claims 1-19 wherein the recovery of the ammonia in phase c) is performed by condensation of the gaseous current coming out of phase b) .

25. Process according to any of the previous claims 1-19 wherein the recovery of the ammonia in phase c) is performed by combustion of the gaseous current coining out of phase b) .

26. Process according to any of the previous claims 1-19 wherein ammonia recovered coming out of phase c) is subjected to a further process of dissociation in H₂ and N₂.

27. Process according to any of the previous claims wherein part of the refluent treated is used to pre- basify the refluent to be treated.

28. Process to reduce the nitrogen content of a zootechnic refluent comprising the following operative phases: a) placing the refluent in contact with a basifying agent in a reactor in such quantity as to bring the pH to a value varying from 7.2 to 13 ; b) keeping the refluent deriving from phase a) agitated; c) recovering the ammonia from the gaseous current generated in phase b) ; said process being characterised by the fact that: - the refluent kept agitated in phase b) by insufflating all or part of the gaseous current deriving from the same phase b) and/or all or part of the gaseous current deriving from phase c) ; and/or the refluent is kept agitated in phase b) by recirculating at least a part of the refluent extracted from phase b) .

29. Process according to claim 28, wherein said basifying agent is added in such quantity as to bring the pH to a value varying from 9 to 13.

30. Process according to claim 28 wherein the basifying agent is chosen from Na₂O, K₂O, NaOH, KOH, Na₂CO₃, K₂CO₃ and/or their solid mixtures and/or from the respective watery solutions and/or their mixtures, preferably quicklime, that is a mixture of CaO/MgO.

31. Process according to claim 28, wherein all the gaseous current deriving from phase c) is recirculated in phase b) .

32. Process according to claim 28, wherein all the gaseous current deriving from phase b) is recirculated in phase b) .

33. Process according to claim 28, wherein the recirculation flow rate of the treated refluent varies from 0.5 to 3.5 recirculation cycles/hour.

34. Process according to claims 28, 31 or 32, wherein the refluent is kept agitated also by means of mechanical agitators .

35. Process according to claims 28, 31 or 32, wherein the refluent is kept agitated also by means of insufflated air.

36. Process according to claim 28, wherein the recovery of the ammonia contained in the gaseous current deriving from phase b) occurs by subjecting said current to one or more stages of bubbling in an acid solution or in water.

37. Process according to claim 28, wherein the recovery of the ammonia contained in the gaseous current deriving from phase b) occurs by means of one or more stages of washing said current with an acid solution or in water in a scrubber.

38. Process according to claims 35 and 36, wherein the acid solution is a watery solution of an acid chosen from H₂SO₄, H₃PO₄, HNO₃ and/or their mixtures or water.

39. Process according to claim 28, wherein the recovery of the ammonia contained in the gaseous current deriving from phase b) occurs by means of liquefaction of the ammonia present in the gaseous current, after dehydration.

40. Process according to claim 28, wherein phase b) is conducted at a temperature variable from 10 to 55°C.